Shelf-margin deltas: their stratigraphic significance and relation to deepwater sands

Abstract

Shelf-margin or shelf-edge deltas are a common constituent of Quaternary shelves, where their genetic link to falling and lowstand sea level has been well established, but they are rarely reported from older successions. Recognition of this delta type in the ancient record is nevertheless important, because (1) it helps to position the fossil shelf edge, (2) it provides insight into how sand budgets were partitioned between shelf edge, slope and basin-floor settings, and (3) it can contribute to the debate concerning three versus four systems-tract systematics and the positioning of a sequence boundary.

Full text

Shelf-margin deltas: their stratigraphic significance and
relation to deepwater sands
Szczepan J. Pore˛bski
a,
*, Ronald J. Steel
b
a
Polish Academy of Sciences, Institute of Geological Sciences, Krako
´w Research Centre, Senacka 1, 31-002 Krako
´w, Poland
b
University of Wyoming, Department of Geology and Geophysics, Laramie, WY 82071, USA
Received 5 November 2001; accepted 12 November 2002
Abstract
Shelf-margin or shelf-edge deltas are a common constituent of Quaternary shelves, where their genetic link to falling and
lowstand sea level has been well established, but they are rarely reported from older successions. Recognition of this delta type
in the ancient record is nevertheless important, because (1) it helps to position the fossil shelf edge, (2) it provides insight into
how sand budgets were partitioned between shelf edge, slope and basin-floor settings, and (3) it can contribute to the debate
concerning three versus four systems-tract systematics and the positioning of a sequence boundary.
Shelf-margin deltas create wide (tens of kilometers), high (hundreds of meters) and steep (3 –6j) clinoforms, because they
build across the relatively deepwater shelf margin. Such bodies tend to form strike-elongate wedges that initially thicken
basinwards, attaining a maximum thickness of 50 –200 m just outboard of the shelf edge, then thin on the middle/lower slope,
whereas they commonly pinch out landwards by lapping back onto shelf shales. Prior to reaching the shelf edge, small-scale
(tens of meters), tangential foresets (usually < 3jslope) of the shelf delta downlap onto the preexisting outer-shelf surface,
whereas below the shelf edge, the shelf-margin deltaic foresets become longer, steeper (3– 6j) and more turbidite prone, and
they downlap onto the preexisting slope of the shelf margin. The top of the shelf-edge delta package is a sharp, planar to incised
surface that truncates the delta increasingly landwards, merging eventually with the basal downlap surface. The clinoform series
created by this transition from outer-shelf to shelf-margin deltas tends to have a horizontal-to-downward trajectory and,
eventually, in the most distal bundles, an aggrading to backstepping trajectory as relative sea level begins to rise.
The key facies association in shelf-edge deltas is the mouth bar-to-delta front association. Mouth-bar facies, landwards of the
shelf edge, consist mainly of thick, clean, flat to low-angle and ripple-laminated medium to fine sands. Basinwards from the
shelf edge, such sands commonly alternate with heterolithic slumped unit, creating characteristic slumped to laminated couplets.
Because the delta front is superimposed on a preexisting, steep and extended shelf margin, it commonly contains (beyond the
mouth bars) thick successions (up to many tens of meters) of sandy, slope turbidites. Shelf-margin deltas differ from inner shelf
deltas in showing: (1) an order of magnitude higher clinoforms (hundreds instead of tens of meters), and strike-elongated,
locally pod-like sand bodies commonly affected and augmented by growth faulting; (2) paleoecological evidence of abrupt
shallowing (foreshortened stratigraphy); (3) turbidite-prone delta-fronts; (4) larger scale and greater abundance of slope-
controlled soft-sediment deformation and (5) the general absence of a paralic ‘tail’ along the trailing edge of the delta front.
Shelf-margin deltas can be classified into two types. Stable shelf-margin deltas are usually tens of meters thick and are not
associated with shelf-edge incision or major slope collapse/disruption features. Slope turbidites are common and occur as
unconfined sheets and lobes. Such deltas form when relative sea level falls no lower than the level of the shelf platform or, after
0012-8252/03/$ - see front matter D2003 Elsevier Science B.V. All rights reserved.
doi:10.1016/S0012-8252(02)00161-7
* Corresponding author. Fax: +48-12-422-16-09.
E-mail addresses: ndporebs@cyf-kr.edu.pl (S.J. Pore˛bski), rsteel@uwyo.edu (R.J. Steel).
www.elsevier.com/locate/earscirev
Earth-Science Reviews 62 (2003) 283 – 326

longer sea-level falls, when rivers reestablish at the shelf edge on the rise. Unstable shelf-margin deltas are associated with
large-scale slope collapse, listric growth faults and sometimes with salt diapirs. Slides are common on the upper slope, whereas
imbricated sediment packages and compressional ridges affect the slope toe. Slope turbidites can be ponded within structurally
controlled mini-basins on the slope. Such deltas are often related to unusually great sand influx to the shelf edge, or simply to
prolonged or large fall of relative sea level below the shelf edge.
The diachronous erosional unconformity atop the shelf-to-shelf-edge deltaic wedge is viewed by some researchers as the
most easily recognizable and persistent surface on the shelf and slope. This surface contrasts with the downlap erosional surface
developed from the beginning of sea-level fall, which is viewed by other researchers as the key boundary surface within this
complex. It should be noted that sea level can fall for a long period of time (tens of thousands of years) before deltas even reach
the shelf margin and, therefore, before significant volumes of sand are delivered across the shelf break. Hence, the time of
incision of the shelf edge and emplacement of deepwater sand is commonly long after the initial fall so the time of (maximum)
relative lowstand of sea level may be a more practical choice for the timing of the sequence boundary.
Recognition of shelf-margin deltas and analysis of their architecture help in the prediction of presence or absence of basin-
floor fans. Deltaic complexes that are aggradational to backstepping, downlap onto disrupted or complex slopes, and overlie an
incised shelf and shelf edge, predict that there should be basin-floor fans present. In contrast, delta complexes that show a
prolonged and preserved progradational to downstepping architecture implies the presence of turbidite accumulations on the
slope but not on the basin floor.
D2003 Elsevier Science B.V. All rights reserved.
Keywords: Shelf-margin delta; Forced regression; Depositional systems tract; Sequence boundary; Basin-floor fan
1. Introduction
There is a range of delta types that develop across
different segments of the shelf during a cycle of fall
and rise of relative sea level (Pore˛ bski and Steel,
2001), particularly when the fall exposes the former
shelf surface. The shelf-edge or shelf-margin delta is
an end-member of that spectrum that becomes prom-
inent during the falling stage of relative sea level on
the shelf, but may also result from an unusually high
supply of sediment, at other stages in the cycle. There
has been much interest in shelf-margin deltas,
because: (1) they form at the morphologic shelf break,
the boundary between major facies belts at continental
margins, as well as at the margins of other types of
basins; (2) they probably represent the primary incre-
ments by which shelf margins grow (Morton and
Suter, 1996); (3) they are the potential staging areas
for supply of sand to the deep sea (e.g., Curray and
Moore, 1964; Winker and Edwards, 1983; Suter and
Berryhill, 1985; Mayall et al., 1992; Morton and
Suter, 1996; Sydow and Roberts, 1994), even though
this potential may not be realized unless the shelf-
edge deltas become incised by their own driving rivers
(Steel et al., 2000); and (4) they commonly are prolific
reservoirs for hydrocarbons (e.g., Mayall et al., 1992;
Hart et al., 1997).
Interest in shelf-margin deltas has grown with
the increased attention paid to the stratigraphic
response of depositional systems to falling relative
sea level (Plint, 1988; Posamentier et al., 1992;
Mellere and Steel, 1995; Plint and Nummedal,
2000). By definition, a shelf-margin delta requires
its fluvial distributary channels to empty near the
shelf break. This involves shoreline regression
across the entire shelf, and commonly implies
forced regression—i.e., a regression that is forced
by active fall in relative sea level and, thus, is
independent of sediment supply (Posamentier et al.,
1992). Although unusually high sediment flux,
combined with low-energy coast conditions, may
bring deltas close to the shelf break during high-
stands (e.g., the Balize Lobe of the present-day
Mississippi Delta), many shelf-margin deltas have
been proven to be forced-regressive and lowstand
features (Posamentier et al., 1992; Tesson et al.,
1993; Sydow and Roberts, 1994; Herna
´ndez-Molina
et al., 2000; Kolla et al., 2000).
There are a number of issues regarding these deltas
that remain unsolved or controversial. (1) Why have
pre-Pleistocene deltas been reported from the litera-
ture so rarely? (2) To what extent is the identification
of deltas at the shelf edge a predictor of coeval
deepwater sand on the slope or basin floor? (3)
S.J. Pore˛bski, R.J. Steel / Earth-Science Reviews 62 (2003) 283–326284

Because shelf-edge or perched-slope deltas result
from sea-level fall to the shelf platform or below,
can they also help resolve the debate around the
positioning of the sequence boundary, above or below
the falling-stage or early lowstand tract?
In the present work, our objectives are: (1) to
review the characteristics of shelf-margin deltas, and
to show that they develop as one member in a
spectrum of delta types developed from sediment
delivery during a cycle of relative sea-level fall and
rise on the shelf; we suggest that they are common in
the pre-Pleistocene record, though rarely termed
‘shelf-margin deltas’ in the literature; in order to help
identify them in ancient successions, we provide
criteria with which to distinguish this member of the
delta family from inner-shelf, highstand deltas; (2) to
evaluate the connection, or lack of one, between the
presence of shelf-edge deltas and the transport of sand
down into deepwater areas. This point is significant if
deltaic facies at paleoshelf edges are to be used as a
predictor of the presence or absence of deepwater
sands in basinal areas.
2. Delta classification, sea-level change and
shelf-margin growth
Deltas have been classified traditionally in terms of
depositional process – product reaction, as emphasized
in facies modelling. Fisher et al. (1969) categorized
deltas as high-constructive and high-destructive sys-
tems, depending on relationships between the fluvial
input and the degree of reworking by marine pro-
cesses. Galloway (1974) expanded this approach in
his ternary classification, based on Holocene (i.e., sea-
level highstand) examples, in which delta type is
defined by the relative contribution of fluvial, wave
or tidal energy flux that was dominant during depo-
sition at the seaward edge of the delta. In other
classifications of deltas, an emphasis has also been
put on foreset/topset geometries (Gilbert, 1890; Fisk,
1961), sediment grain size (Orton and Reading, 1993)
and delivery system type (Holmes, 1965; Chough et
al., 1990; Friedman et al., 1992).
In these classifications, the overall delta regime
reflects the sum of the environmental controls acting
Fig. 1. Cartoonal representation of inner-shelf delta versus shelf-margin delta based on the Paleocene– Eocene Upper Wilcox Rosita delta
system (after Edwards, 1981). Note the expansion of the clinoform height and the mouth-bar thickness in the shelf-margin delta. The model
implies the contemporaneity of both types of deltas, as exemplified by the modern shelf off the Louisiana coast. However, it is now well proved
that inner-shelf deltas tend to be predominantly highstand features while shelf margin deltas form during forced regression and early lowstand.
S.J. Pore˛bski, R.J. Steel / Earth-Science Reviews 62 (2003) 283–326 285

Fig. 2. Classification of shelf deltas in terms of relative sea-level change (based on Pore˛bski and Steel, 2001).
S.J. Pore˛bski, R.J. Steel / Earth-Science Reviews 62 (2003) 283–326286

on the deltaic system, which otherwise remains in a
steady state, only incidentally affected by relative sea-
level changes. More recent research has now empha-
sized how sea-level change (increased rate of sea-level
rise) may generally change regressive deltaic shore-
lines into transgressive estuarine or barrier shorelines
(Penland et al., 1988; Boyd et al., 1992), but there has
been little attempt to show the effect of sea-level
change on the deltas themselves (but see, Postma,
1995; Kolla et al., 2000; Pore˛bski and Steel, 2001).
3. Deltas and relative sea-level change
Deltas can prograde to any location across the
shelf, and some transit to the shelf margin itself, so
that two principal types have been distinguished: (1)
shoal-water or inner-shelf deltas; and (2) shelf-margin
or shelf-edge deltas (Edwards, 1981; Suter and Berry-
hill, 1985; Reading and Collinson, 1996, p. 181).
Although deltas formed on the middle and inner shelf
are different from those sited near the shelf edge
(Edwards, 1981; Winker, 1982; Elliot, 1989),the
importance of this has not been well appreciated so
far among facies modellers and sequence stratigra-
phers. On the basis of his work in the Paleocene–
Eocene Rosita delta system in the Gulf of Mexico,
Edwards (1981) suggested that inner-shelf deltas tend
to be ‘‘horsetail’’ and develop a wide delta-front sheet
sand, whereas those that reach the shelf break are
characteristically locally thickened in response to on-
growth faulting (Fig. 1).Suter and Berryhill (1985)
Table 1
Main distinctive characteristics of shelf deltas (see also Kolla et al., 2000, and references therein)
Bayhead deltas Inner-Shelf Deltas Mid-Shelf Deltas Shelf-Margin Deltas
Shape Confined,
funnel-shaped
Birdfoot, lobate to
cuspate
Erosional pods common Lobate to strike-elongate
Clinoform slope ? Usually < 0.001* < 0.5j, but can be as
much as 8j**
Up to 8j, usually 3 – 6j
Clinoform height Several meters Few tens of meters Few tens of meters Several hundreds of meters
Thickening trend Landwards Landwards Seawards, or no distinct trend Seawards, with maximum
near shelf break
Dominant energy
flux at delta front
Fluvial; can be
tide influenced
Fluvial, wave or tide Fluvial or wave Fluvial or wave; tide influence
may become important during
late lowstand
Delta-top facies Fluvial
distributaries
Delta plain with
thick distributary
channel sand
Thin to absent delta plain; thick
distributary channel sand below
incised valley systems; erosional
bypass in places
No delta plain; thin fluvial
distributaries where delta
perches below shelf edge.
Erosional bypass common
Delta-slope facies Heterolithic,
ripple-laminated
foresets
Varies depending on
delta front regime
Sandstone to heterolithic foresets;
rare turbidite sands
Sandstone to heterolithic
foresets; slumped mouth-bar
sand embedded in prodelta
shales; common hyperpycnal
sand turbidites. Growth faults
and diapirs
Updip termination Intertonguing
with paralic
deposits
Intertonguing with
paralic deposits
Erosional pinchout within
mid-shelf shales
Erosional pinchout within
outer-shelf or upper
slope shales
Downdip
termination
Pinchout within
central estuarine-
basin heteroliths
Pinchout with
inner-shelf to
mid-shelf shales
Pinchout within
outer-shelf shales
Pinchout within slope shales
Clinoform
breakpoint
trajectory
Landward rising
to basinward
rising
Basinward rising Horizontal to basinward falling Basinward falling, aggraded
to landward rising
Systems tract Transgressive Highstand Falling stage Falling stage to lowstand
* See Table 6.2 in Reading and Collinson (1996).
** Posamentier and Morris (2000).
S.J. Pore˛bski, R.J. Steel / Earth-Science Reviews 62 (2003) 283–326 287

demonstrated, in the Pleistocene deposits of Gulf of
Mexico, that prograding deltaic clinoforms sited on
the middle shelf tend to be thin, patchily developed
and show low-angle clinoforms, whereas those lo-
cated near the shelf margin and on the upper slope
produce thick, localized, strike-oriented wedges of
steeply dipping strata, commonly bearing signs of
slope failure. These differences are believed to reflect
increased accommodation, when the deltaic shoreline,
having transited the shelf during falling sea level,
meets deeper water near the shelf break (Suter and
Berryhill, 1985). At the opposite extreme, a regional
sea-level rise that transforms the deltaic shoreline into
an estuary will eventually result in small, dip-elon-
gated and often tide-dominated bayhead deltas formed
at the landward end of flooded valleys (Dalrymple et
al., 1992; Nichol et al., 1996). Inner-shelf (platform or
shoal-water) deltas, in turn, tend to be wide and
backed by thick paralic facies, reflecting their aggra-
dational-to-progradational (highstand) delta-front tra-
jectory during formation.
Thus, for a given tectonic setting and sufficient
sediment supply, it is the rate and direction of relative
sea-level change that produces different delta types,
each of them displaying a fairly distinguishable set of
stratigraphic characteristics (Fig. 2,Table 1). For a
given sea-level cycle, these types are likely to form a
predictable evolutionary pattern in which they either
form intergradational transitions, or occur as distinct
entities separated in space and time by major discon-
tinuities.
4. Shelf generation and shelf-margin accretion
Although the term shelf is commonly used to
denote the fringe of a continent—the continental
shelf—we suggest that the same term is fully appro-
Fig. 3. Distinction between clinoforms generated by (A) shelf-margin accretion, and by (B) prograding shelf deltas. Note that the two types
coincide only when deltas reach the shelf edge and drape partly down the slope.
Fig. 4. Example of a shelf-edge delta (labelled 2) draping halfway down a shelf-margin slope on Storvola, Spitsbergen. The sandstone bench
below represents basin-floor fans of an older clinoform that peeled off the shelf at the left end of the mountain. The sandstone bench above has
its shelf edge (labelled 3) a few kilometers to the SE. The five clinoform complexes migrated successively to the SE. The overall trajectory of
the shelf edge (see successive shelf-edge locations) also reveals an aggradational component. Map shows this and other localities mentioned
further in the text with respect to Eocene shelf-margin deltas in Spitsbergen.
S.J. Pore˛bski, R.J. Steel / Earth-Science Reviews 62 (2003) 283–326288

S.J. Pore˛bski, R.J. Steel / Earth-Science Reviews 62 (2003) 283–326 289

priate for a shallow-marine platform located around
the margin of a deeper basin irrespective of the basin’s
tectonic setting. This is a morphological shelf. Such
shelves, commonplace at the edges of marine basins,
can vary in their width from a few kilometers to
several hundred kilometers, and generally slope basin-
wards at less than 0.1j(Posamentier and Allen, 1999,
their table 2). A characteristic feature of the outer edge
of the shelf is a break or ‘rollover’ in the gradient,
leading down onto a slope most commonly 2– 7j
steep. In our experience, this slope break occurs
(and the use of the term shelf is warranted), if the
maximum relief from shelf-platform edge to basin
floor exceeds 150– 200 m (Fig. 3A). During sea-level
highstand, the shelf can support a water column of up
to several hundred meters at its outer edge.
Such basin-margin platforms, facilitated by a
preceding period of low sediment flux and/or high
subsidence, are constructed by the long-term balance
between sediment accommodation and sediment by-
pass, and the gradual extension or progradation of
the front margin of this platform into the basin. A
coastal sedimentary prism is thus constructed, and
this commonly has its maximum seaward extent
during falling stage and lowstand of relative sea
level, when sediment supply is high and accommo-
dation is decreasing (Posamentier and Allen, 1999).
At this time, the shelf-margin delta drapes across the
shelf edge, and the delta slope and the slope below
the preexisting shelf break coincide, though the
former is still somewhat steeper than the latter (Fig.
3B). Other than at this particular time, the smaller
scale delta clinoforms should not be confused with
the shelf-edge clinoforms. The platform thus con-
structed then becomes a shelf during subsequent rise
of relative sea level, as the former subaerial platform
drowns and the shore zone retreats landwards. This
process repeats during successive cycles of relative
sea-level fall and rise. When the deltaic or other
shore zone system manages to deliver sediment as far
out as the shelf edge, there occurs accretion of the
shelf margin and the generation of deepwater clino-
forms (Figs. 3 and 4). The clinoforms have a height
approximately equal to the height of the deepwater
slope, connecting the shelf edge with the basin floor,
though if the water depth is great and the slope
morphology complex, as it is on many passive
margins, the clinoforms are muddy long before they
reach the basin floor, or the clinoform toes them-
selves downlap long before the basin floor is
reached. Because of the water depth and accommo-
dation increase beyond the shelf edge, the prograda-
tional distance of any set of clinoforms beyond the
older shelf edge is somewhat limited. In this way,
shelf margins prograde mainly during sea-level still-
stand and fall, whereas they aggrade mainly during
relative sea-level rise. Shelf deltas, because of their
powerful fluvial drive, play a prime role in this shelf
aggradation and shelf-margin accretion (Fig. 4)
(Morton and Suter, 1996).
5. General characteristics of shelf-margin deltas
5.1. Data base
Much of our knowledge about shelf-margin deltas
derives from Quaternary continental shelves, and is
primarily based on high-resolution seismic images
(Table 2). This includes Costa de Nayarit, Mexico
(Curray and Moore, 1964), Gulf of Mexico (Lehner,
1969; Winker, 1982; Suter and Berryhill, 1985; Mor-
ton and Price, 1987; Suter et al., 1987; Kindinger,
1988; Thomas and Anderson, 1991; Sydow et al.,
1992; Sydow and Roberts, 1994; Winn et al., 1995,
1998; Morton and Suter, 1996; Hart et al., 1997; Kolla
et al., 2000), off eastern United States coast (Mat-
teucci and Hine, 1987), West Africa (McMaster et al.,
1970; Pegler, 1999), and Mediterranean Sea (Aksu
and Piper, 1983; Aksu et al., 1987; Farra
´nand
Maldonado, 1990; Tesson et al., 1990, 2000; Trincardi
and Field, 1991; Posamentier et al., 1992; Chiocci,
1994; Herna
´ndez-Molina et al., 2000). In contrast to
this Pleistocene data, there is relatively little docu-
mentation of older shelf-margin deltas. Notable
exceptions include, from passive margin settings, a
Cretaceous shelf edge adjacent to the Baltimore Can-
yon (Poag et al., 1990), Tertiary of Gulf of Mexico
(Edwards, 1981; Winker, 1982; Galloway, 1989,
1990; Mayall et al., 1992; Xue and Galloway,
1995), and Miocene clinoforms offshore New Jersey
(Fulthorpe and Austin, 1998; Fulthorpe et al., 1999),
from transpressional basins, Eocene of Spitsbergen
(Steel et al., 2000; Plink-Bjo¨rklund et al., 2001), from
foreland basins, Miocene of the Carpathian Foredeep
Basin in Poland (Pore˛bski, 1999), and from intra-
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(continued on next page)
Table 2
Summary characteristics of Pleistocene shelf-margin deltas
Location Gulf of Mexico Izmir Bay,
East Mediterranean Sea
Rho
ˆne Shelf,
West Mediterranean Sea
Guinea – Sierra
Leone Shelf,
West Africa
Delta regime Fluvial dominated Fluvial dominated Wave dominated Wave-dominated
Tectonic setting Passive margin; basinwards
downwarping due to
sediment load
Active strike– slip-
related extension
Passive margin;
basinwards downwarping
due to sediment load
Passive margin
Physiographic setting
Shelf width 70 – 140 km 40 – 60 km 75 km 35 – 190 km
Shelf gradient < 0.05j1–3j, < 0.1j
near the shelf edge
V1j
Depth of
shelf break
75 – 100 m 110– 120 m 120 – 135 m 85 – 115 m
Slope gradient 0.6 – 1.5j, maximum 3jGentle < 10j4j
Delta geometry
Shape Multilobate to
strike-elongate
Lobate to strike-
elongate
Lobate
Length* 25 – 60 km < 60 km 40 – 50 km 30 – 40 km
Width 28– 120 km ? 90 km and more
Maximum
thickness
60 – 90 m, locally
above 180 m
5 – 50 ms, controlled
by syndepositional
block faulting
50 m 30 –95 to
above 130 m
Clinoform
Dip 3– 6j, maximum 8j4–6j1.5j(?) 4 – 6j
Seismic
expression
Oblique – tangential reflectors
steepening basinwards, with toplap
terminations below planar scour
surface. Sigmoidal forms in the
most seaward bundles. Offlap
associated with downstepping
ensued by backstep in youngest
bundles. Common internal downlap
surfaces. Progradation centres
defined by bidirectional downlap
Oblique reflectors
terminating updip
by toplap and
changing downwards
into gently dipping
shingled forms.
Transparent facies
in distal delta
Oblique – tangential
reflectors showing
downward decreasing
amplitude, intercalated
with hummocky reflection
packages. Individual
bundles separated by
steeper erosional
truncations of regional
extent. Clinoform
bounded at top by
channeled surfaces with
toplap terminations below.
Overall downstepping to
backstepping pattern in time
Offlapping stack
of thinning-up
clinoforms, each
above major
erosional
unconformity
with toplap.
Terminations
below and
downlap above
Facies
Upper foresets Upward coarsening from distal to
proximal delta front. Distal delta
front: fauna poor, weakly burrowed
thinly interbedded ripple and flat
laminated fine sand and silt.
Proximal delta front: 15 – 20 m of
medium-grained sand with silt
partings; weakly to non-burrowed,
massive or flat laminated, rare
ripple lamination, dispersed plant
matter, very low fossil content
Medium-grained
sand, numerous
broken shells
Centimeter- to decimeter-
thick beds of fine sand to
silt alternating with silty
clay laminae; microfauna
of lower shoreface to
mid-shelf aspects
S.J. Pore˛bski, R.J. Steel / Earth-Science Reviews 62 (2003) 283–326 291

cratonic basins, Namurian of County Clare, Ireland
(Pulham, 1989; Collinson et al., 1991).
The bias towards overrepresentation of Pleistocene
shelf-margin deltas in the data set reflects the fact that
(1) multiple glacieustatic oscillations left a remarkably
clear and fairly accessible record of long-distance
shoreline shifts, across the present-day continental
shelves, and that (2) high-magnitude (100 – 130 m),
high-frequency (10’s to few 100’s of thousands of
years) and strongly asymmetric sea-level fluctuations
were particularly instrumental in producing this type
of delta. However, the remarkable scarcity of shelf-
margin deltas reported otherwise justifies the develop-
ment of criteria that can be used to recognize shelf-
margin deltas in outcrop.
It was suggested by Steel et al. (2000) that the key
to recognizing older shelf-margin deltas is in the
correct identification of the ancient shelf edge, i.e.,
Table 2 (continued)
Location Gulf of Mexico Izmir Bay,
East Mediterranean Sea
Rho
ˆne Shelf,
West Mediterranean Sea
Guinea – Sierra
Leone Shelf,
West Africa
Facies
Lower foreset Mud, silt and clay, locally highly
bioturbated, moderately fossiliferous;
thin beds of laminated fine sand;
pyritised microfaunal burrows,
carbonate concretions; layered plant
matter concentration
Muds an silt with
sparse benthonic
fauna, rare
planktonic forams
Mud, silt and clay, locally
highly bioturbated,
moderately fossiliferous;
thin beds of laminated
fine sand; pyritised
microfaunal burrows,
carbonate concretions;
layered plant matter
concentration
Soft-sediment
deformation
Growth faults and associated slope
basins; common contorted and
sheared intervals. Extensive
wedge-shaped, mounded to chaotic
seismic zones interpreted as slumps
and slides; slide scars filled with
oblique progradational facies. Shale
and salt diapirs at lower slope
Imbricated slump
blocks in lower
prodelta slope
Retrogressive slumping
and faulting at shelf edge
Delta top Seaward shallowing channels,
1 – 5 km wide and 30 – 40 m deep,
merging landwards into incised
valleys up to 30 km wide and
60 m in relief. Channel fills of
medium to coarse, locally pebbly
sand; trough cross-bedding,
minor parallel lamination
Channels, several hundreds
of meters wide and
10 – 15 m deep, filled with
hummocky to subparallel
seismic facies, interpreted
as passive infill of slump
scars. U-shaped troughs,
>50 m deep, interpreted as
canyons grading updip into
incised valleys
Incised valley
systems, 15 m
deep in outer
shelf and up to
90 m inshore,
passing seawards
into cemented
surface of marine
erosion
Incision of
delta edge
No major fluvial incisions except
off Mississippi River Delta;
common troughs initiated by
retrogressive slope failure
Slump-related canyon
heads and gullies
No major
incisions
Sources Lehner, 1969; Sangre et al., 1978;
Winker, 1982; Winker and Edwards,
1983; Suter and Berryhill, 1985;
Suter et al., 1987; Sydow et al., 1992;
Sydow and Roberts, 1994; Morton
and Suter, 1996; Hart et al., 1997;
Winn et al., 1995, 1998
Aksu and Piper,
1983; Aksu
et al., 1987
Tesson et al., 1990;
Posamentier et al., 1992;
Tesson et al., 1993, 2000
McMaster et al.,
1970; Pegler,
1999
* Distance from shelf break to landward pinchout onlap.
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the recognition of shelf-slope clinoforms that require
exceptionally large outcrops (more than 10-km out-
crop length is required to see shelf edge connecting
with basin floor, if slope gradient is 3jand relief from
shelf edge to basin floor is >500 m). In the Eocene
examples of Steel et al. (2000) and Plink-Bjo¨rklund et
al. (2001), shelf-edge deltas are common features of
the shelf margin, especially during time intervals
when the shelf edge and upper slope areas were not
canyonised. It is likely that many other ancient
deltaic units, and particularly those showing tens to
hundreds of kilometers of regressive transit across
shallow shelves (e.g., some of those at the youngest
levels in the Cretaceous Western Interior Basin,
USA—Asquith, 1970), are also of shelf-edge type.
They have simply not been referred to in these terms
possibly because of an assumed ramp-type setting, or
because their forced regressive or lowstand character
has not been documented (a notable exception being
the Panther Tongue Member of the Star Point For-
mation—Posamentier and Morris, 2000). Some of the
Western Interior Basin’s shelf-ridge sand bodies have
now been reinterpreted as examples of lowstand shelf
deltas, though not developed across a shelf-slope
break (Mellere and Steel, 1995; Bhattacharya and
Willis, 2001).
5.2. Overall geometry
Isopach maps show shelf-margin deltas as multi-
lobate, arcuate to lunate bodies (Suter and Berryhill,
1985). However, a striking feature is that the bodies
show a considerable elongation along the shelf edge,
and can be 30– 90 km wide in this direction (Figs. 5
Fig. 5. Geometry and dimensions of Wisconsinan shelf-margin deltas in the northwest Mexican Gulf off the Texas coast (A and B), and the
ancestral Mississippi (C) and Rio Grande (D) rivers (modified after Suter and Berryhill, 1985). Deltaic depocentres are dotted; shaded areas
correspond to continental slope.
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and 6). This strike-elongation is associated with the
abrupt increase of accommodation outboard of the
shelf edge (stalling the regression, as it were), reflect-
ing an increased influence of tectonic and sediment
load-induced subsidence rather than any particular
hydraulic regime of the outer shelf. The highest
subsidence rate near the shelf edge is promoted by
the gravity spreading of the associated slope whose
instability increases with an increased sediment sup-
ply (Winker, 1982). In dip cross-sections, a shelf-
margin delta complex appears as a clinoformed
wedge that thickens towards the shelf edge, attaining
a maximum thickness of 50 – 200 m at or just below
the shelf edge, and then thins gradually down onto
the upper to middle slope (Figs. 7 and 8). In areas
affected by growth faulting, the thickest part of the
wedge tends to be located slightly landwards from
the physiographic shelf break (Edwards, 1981; Mor-
ton and Suter, 1996). Farther landwards, the wedge
commonly pinches out by onlap, and the pinchout
zone lies some 25– 60 km behind the shelf break
(Aksu and Piper, 1983; Lehner, 1969; McMaster et
al., 1970; Suter and Berryhill, 1985; Posamentier and
Allen, 1999).
5.3. Bounding surfaces
The base of outer-shelf and shelf-edge deltaic clino-
form complexes downlaps onto the preexisting shelf
surface; however, consecutive reflections tend to show
asymptotic lower ends rather than an abrupt downlap
onto this surface (Morton and Suter, 1996; Posamentier
Fig. 6. Clinoform thickness and net-sand isopach map showing the subsurface geometry of Badenian shelf-margin delta that is aligned along and
extends downdip (northwards) beyond the former shelf break, Carpathian foredeep, southeast Poland (Pore˛bski et al., in press). Inset map shows
the location of studied succession within the geological framework of the Carpathians.
S.J. Pore˛bski, R.J. Steel / Earth-Science Reviews 62 (2003) 283–326294

et al., 1992; Tesson et al., 2000). At the landward
reaches of the shelf-edge delta complex, this basal
surface invariably truncates the underlying succession,
emphasized by a toplap truncation of older shelf, shelf-
edge or slope reflections (Sydow and Roberts, 1994)
(Fig. 9). Basinwards of the shelf margin the same
Fig. 7. Dip cross-section through Late Pleistocene shelf-margin delta formed during falling stage to early sea-level rise. Note the seaward
steepening and thickening of outer-shelf delta clinoform complex and the thickening of mouth-bar sands. Part of the fluvial sands is time
equivalent to the youngest, backstepping clinoform bundle. Internal downlap unconformities, where of regional persistence, are believed to
record the stepwise nature of sea-level fall (modified from Sydow and Roberts, 1994). See Fig. 12A, for core log from the MP303 boring.
Fig. 8. Interpreted line drawing from seismic dip-section through Badenian shelf margin in the Carpathian foredeep (southeast Poland), showing
the distribution of sand bodies (Pore˛bski et al., in press). Note that the sand bodies thin and pinch out landwards (to the south), and form the
thickest accumulation near the shelf edge which is constructed mainly of mouth-bar and slope turbidite sandstones. See Fig. 19B, for location of
section.
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downlap surface persists, but the downlapping seismic
reflections eventually flatten and approach the gradient
of the slope, and the surface eventually becomes
conformable into the basin. The upper surface of the
shelf-margin delta complex tends to be a high-ampli-
tude reflection. It displays a planar to incised geometry
along the shelf reaches of the complex (Fig. 10),
truncating the internal clinoforms of the delta increas-
Fig. 9. A series of four clinoform complexes (2– 5) on Storvola, Spitsbergen (see also Fig. 4). Clinoform complex 3 (see Fig. 4) represents a
shelf-margin delta that has a sharp and incised base. It truncates toplapping, older slope clinoforms (shaly), because of its development during
falling relative sea level. Note that clinoform complexes 4 and 5 are gradationally based, showing an upward coarsening. They occupy an inner
to mid-shelf (highstand [?]) position. At the level of clinoform 2, the distal tip of a late lowstand, shelf-margin delta can be seen to downlap onto
lower slope turbidite channels.
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ingly landwards, and eventually amalgamating with the
basal downlap surface described above. Along the
shelf-edge to upper slope reaches of the system, this
upper surface becomes conformable with the slope-
elongated clinoforms (Morton and Suter, 1996). Strati-
graphically, this surface bears signs of the erosion
associated with the basinward propagation of a fluvial
distributary channel feeder, and its subaqueous-equiv-
alent erosion surface, possibly induced by hyperpycnal
flows (Plink-Bjo¨ rklund et al., 2001), although this
evidence can be removed by a subsequent transgressive
ravinement. This ravinement surface is likely to amal-
gamate or erode through the regressive erosion surface
at the top of the delta in the region of the shelf edge.
Farther landwards, it is likely to diverge from the
regression surface, separated from it by estuarine or
coastal deposits, if sediment supply is still high during
transgression.
5.4. Clinoform bundling and slopes
Clinoform geometry tends to change basinwards as
the deltas prograde from the outer shelf, across the
shelf edge and onto the upper-slope reaches of the
system (Fig. 7). While the deltas are prograding on the
shelf, the visible, small-scale clinoforms (low-angle
oblique forms) correspond with the accreting delta
front, and the slope angles are consistent with this. On
reaching the shelf edge, however, the front of the delta
then progrades onto a preexisting slope that can be as
steep as 3– 5j. The visible clinoforms at and just
below the shelf edge thus become steep (4 – 7j)
reflections with long tangential ends and eventually
sigmoidal forms with strongly asymptotic toes on the
slope (Kolla et al., 2000; see also examples in Pos-
amentier and Allen, 1999). The seaward steepening of
the clinoform slope tends to be associated with the
basinward segregation of sand (Sydow and Roberts,
1994) that attains maximum net values at the newly
formed shelf break (Fig. 7). This pattern of steepening
of deltaic clinoform slopes at the shelf margin,
accompanied by a sand body thickness change at this
point(from20–30upto70m),hasalsobeen
described from Eocene (Plink-Bjo¨ rklund et al.,
2001) and Miocene (Pore˛bski et al., in press) shelf
edges (Fig. 8).
Fig. 10. Distributary channel truncation down into top of 60-m-thick shelf-edge delta succession. Note the slight discordances between
successive delta-front lobes. Except for the distributary channel fill, all sand beds are turbidites; Litledalsfjellet, Spitsbergen.
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The progradational trajectory of the entire shelf-
margin delta complex is commonly interrupted by
internal downlap surfaces (Fig. 7) that partition the
complex into individual, stacked and laterally offset
clinoform bundles (Sydow and Roberts, 1994; Tes-
son et al., 2000). Such breaks presumably reflect
either slight changes in progradational direction or
in the process regime affecting the deltas. In their
distal reaches, such internal discontinuities can be
associated with a high-amplitude reflection that can
be traced in cores to nodular or layered carbonate
concentrates (Sydow and Roberts, 1994). Strike
sections through a single clinoform bundle may
show a bidirectional downlap indicative of the core
of a prograding delta lobe (Sydow et al., 1992).
Clinoform offlap breaks that record successive
positions of the prograding shelf margin, tend to be
preserved in the most basinward bundles. Near the
termination of the shelf-margin delta trajectory, the
offlap break tends to shift downwards and basinwards
because of continued relative sea-level fall (Fig. 11).
The offlap break tends to acquire an upward aggrad-
ing and eventually backstepping trajectory within the
youngest bundles, as relative sea-level rise produces a
prograding lowstand wedge and eventually a trans-
gressive systems tract (Sydow and Roberts, 1994;
Kolla et al., 2000; Plink-Bjo¨ rklund et al., 2001).
Increasingly, transgressive systems tract deposits
overlying lowstand deltas are being correctly identi-
fied in terms of tidally dominated, shelf-margin estua-
ries (Rabineau et al., 1998; Berne
´et al., 1998;
Schellpeper, 2000) and/or shelf ridges (Posamentier,
2002).
5.5. Facies spectrum in shelf-margin deltas
In contrast to the well-documented geometry of
shelf-margin deltas from seismic data, there is a marked
deficiency of detailed facies information available for
these systems. The situation has been improved
recently by lithofacies, biostratigraphic and pale-
obathymetric data from a series deep borings that
penetrated the Late Pleistocene Lagniappe Delta (Kin-
dinger, 1988), east of the present Mississippi Delta
(Sydow and Roberts, 1994; Winn et al., 1995, 1998;
Scott et al., 1998; Kolla et al., 2000), and from
spectacularly exposed shelf-margin deltas in the Early
Eocene foreland basin of Spitsbergen (Steel et al.,
2000; Plink-Bjo¨rklund et al., 2001), as well as in the
Namurian of County Clare, Ireland (Pulham, 1989;
Collinson et al., 1991). The data from the Lagniappe
Delta document mud-prone to thin-bedded heterolithic,
Fig. 11. Concept of offlap break trajectory (partly based on Helland-Hansen and Martinsen, 1996) and architectures of shelf-margin deltas.
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locally slumped character of the shelf-edge delta front.
Mayall et al. (1992) documented alternations of heter-
olithic slumps with clean, thick-bedded sands in the
Pliocene of the Mississippi Canyon area. On the other
hand, evidence from Eocene and Miocene shelf-margin
deltas in foreland basins of Spitsbergen and Poland
show that similar mud-prone delta-front sets can alter-
nate with those dominated by thick-bedded turbidite
sandstones (Steel et al., 2000; Plink-Bjo¨rklund et al.,
2001; Pore˛bski, 1999; Pore˛ bski et al., in press).Xue
and Galloway (1995) also mapped several thick sand-
rich pods beyond the shelf margin in the Middle
Paleocene Wilcox subgroup, offshore Texas, and
ascribed their origin to slump lobes or turbidite resedi-
mentation of shelf-edge mouth-bar facies. All the
above examples share a characteristic feature, i.e.,
slope deformation and collapse, whereas differences
in the sand percentage on the delta-front deposits reflect
mainly variations in the calibre and sediment volume
delivered to the slope. Although the tectonic setting of
the host basin strongly influences sediment supply and
large-scale stacking patterns, it probably has relatively
little influence on the internal character of individual
shelf-edge deltas. Deltas form and generally are able to
regressively transit the entire shelf width in less than
100,000 years (Burgess and Hovius, 1998; Muto and
Steel, in press)—a time scale within which they will be
only moderately affected by tectonics.
If the above examples from Gulf of Mexico, Spits-
bergen, Carpathians and Ireland seem few, on which to
base our knowledge on shelf-edge delta facies, it is
because it is largely these literature examples that have
been examined and documented in a shelf-edge posi-
tion.
5.6. Facies landwards of the shelf edge
Along the shelf traverse, the upper parts of the
deltaic succession (steepest part of delta clinoforms)
are dominated by sandy mouth bars (Mayall et al.,
1992; Hart et al., 1997; Plink-Bjo¨rklund et al., 2001)
or proximal delta-front deposits (Fig. 12A) (Sydow
and Roberts, 1994; Winn et al., 1995, 1998). Mouth-
bar sands are fine to medium grained, often clean,
well sorted and bedded on a decimeter to meter
scale. They form up to 20-m-thick units that show a
‘‘blocky’’ to slightly serrated, coarsening-upward
gamma-ray well-log response. Beds are non- to
weakly bioturbated and poorly to non-fossiliferous,
although isolated mollusc shells, shell debris and
dispersed plant matter can locally be present. Parallel
laminated sandstone beds intercalate with current-
rippled and ‘‘structureless’’ sandstone beds, and with
siltstones. The sandy mouth-bar unit grades basin-
wards and downwards into a more heterolithic suc-
cession attributable to delta-front deposition (Fig.
12A). It comprises non- to weakly burrowed, silty
sand and sandy silt, locally rich in plant matter,
which form a host for thin beds/bedsets of well-
sorted, plane-parallel laminated and graded-lami-
nated fine- and very fine-grained sandstone. The
fossil content is low, though can be higher than in
the mouth-bar/proximal delta-front sand (Sydow and
Roberts, 1994). Long tangential to parallel toeset
units correspond to distal delta front and prodeltaic
sediments which are chiefly silty clay and clay in
grade. These sediments are moderately fossiliferous
and locally strongly bioturbated. Fossils are of
neritic to bathyal affinity. Thin beds of parallel
laminated sand and thick, unburrowed, structureless
mud beds are commonly interpreted as delta-front
turbidites (Morton and Suter, 1996; Winn et al.,
1998).
The vertical facies succession described above
apparently does not differ substantially from that
found in an inner-shelf, river-dominated delta. How-
ever, the overall upward-coarsening trend in shelf-
margin deltas is associated with paleoecological evi-
dence of an extremely rapid shallowing. The close
proximity of shallow- and deep-marine biotic indica-
tors is common in shelf-margin deltaic successions
(Winker, 1982). One of best example of this is
provided by the VK774 well drilled through the
Pleistocene Lagniappe Delta (eastern Gulf of Mex-
ico), where 20 – 30 m of sediments records a transition
from upper bathyal to middle neritic depths, i.e., 100 –
200 m of decreasing water depth (Winn et al., 1998;
see also Kolla et al., 2000). This phenomenon,
referred to as foreshortened stratigraphy (Posamentier
and Morris, 2000, p. 36), together with the general
absence of delta-plain facies appear to be one of the
main distinctive features of shelf-margin deltas. The
absence of delta-plain facies and the foreshortening of
the vertical succession result from relative sea-level
fall and decreasing accommodation during delta pro-
gradation on the outer shelf.
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5.7. Facies basinwards of the shelf edge
As a delta reaches the shelf edge and begins to
prograde onto the upper slope, there is a significant
change in facies development, due to the increased
gradient of the substratum that promotes mass-gravity
deformation and flow (Mayall et al., 1992; Winn et
al., 1998). Over half of heterolithic delta-front units in
the inferred shelf-margin deltas in the Namurian of
County Clare, Ireland, reveal evidence of growth
faulting, slumping and sliding (Pulham, 1989), and
such phenomena are even more pronounced in the
underlying turbiditic slope deposits of the Gull Island
Formation (Fig. 12C) (Martinsen and Bakken, 1990;
Collinson et al., 1991).
In the Spitsbergen examples, which provides a
considerable insight into the 3D facies geometry of
a fossil deltaic shelf margin (from outer shelf to basin
plain—Figs. 4 and 9), the sand-prone mouth bars
consist of both thick, massive sandstones, scours filled
with single, cross-stratified sets and flat to low-angle
laminated sandstones. In addition, however, units of
soft-sediment deformed sandy mudstones/siltstones
now abound (Fig. 13). If falling sea level causes the
mouth-bar system to perch at successively lower
levels on the upper slope, a series of erosional terraces
can develop that cut back onto the shelf edge (Plink-
Bjo¨rklund and Steel, 2002). The delta-front facies
basinwards of the shelf edge tends to be heterolithic
as above, but clean, plane-parallel to ripple-laminated,
or massive sand beds now spectacularly alternate
vertically with slumped, sandy mudstone units, up to
several meters thick (Fig. 14).Mayall et al. (1992)
provided detailed core descriptions of such hetero-
lithic slumps, 6 – 15 m thick, each capped by 3 – 5 m of
clean, structureless to parallel-laminated sand from the
upper slope of a Pliocene shelf-margin delta in the
Mississippi Canyon Block 109 (Fig. 12B). The occur-
rence of such repeated, heterolithic slump/clean sand-
stone couplets appears as one of the clearest criteria
for identifying an outcrop location as being below the
shelf edge (the steeper slope angle may not be obvious
close up on the outcrop).
The clean, sharp-based sandstone beds described
above that are structureless or flat laminated and ripple
capped are more clearly identified as turbidites (Fig.
15) than the analogous delta-front beds on inner-shelf
deltas. In the Spitsbergen examples, such beds can be
seen to stack vertically to form upward-thickening
Fig. 12. Examples of vertical facies successions in shelf-margin deltas. (A) Core log from borehole MP303 through the Pleistocene Lagniappe
Delta, Gulf of Mexico (modified from Sydow and Roberts, 1994, and Kolla et al., 2000, with interpretation after Sydow and Roberts, 1994). The
well is located landwards of the former shelf break (comp. Fig. 7). (B) Core log from well MC109-3(OH), Middle Pliocene, Mississippi Canyon.
Block 109, showing repetitions of slump/sand-turbidite couplets (modified from Mayall et al., 1992). (C) Composite section for the lower
Central Clare Group, Ireland (modified from Elliot and Davies, 1996, and based partly on Pulham, 1989; Martinsen, 1989; Collinson et al.,
1991).
Fig. 13. Alternations of slumped beds with scour-based, single sets
of cross strata, occurring in the distal mouth-bar succession just
basinwards of a shelf edge. Slumped bed in centre is 25 cm thick;
Storvola, Spitsbergen.
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Fig. 14. Mouth-bar succession (20 m thick), showing multiple slumped beds. The slump in centre of succession is 3 m thick, and originated from
a slump scar that can be seen halfway along the cliff. The slumped beds are mud-rich sandstones, whereas the intervening distal mouth-bar beds
are clean, laminated sandstones; Storvola, Spitsbergen.
Fig. 15. Thin-bedded, ungraded sandy turbidites typical of the slope association when shelf-edge deltas drape across the shelf margin; Storvola,
Spitsbergen.
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slope lobes, up to 20 m thick, and are frequently cut by
small turbidite-filled channels or chutes that extend
down the slope for several kilometers (Mellere et al., in
press). Such chute- or channel-topped lobes are clearly
progradational on the slope, but tend to systematically
thin downslope and interfinger gradually with thicker
units of rippled siltstones and mudstones. The exis-
tence of similar slope channels, or chutes, on delta-
slope lobes have been inferred from well-log responses
in the sand-prone delta-front deposits in the middle
Wilcox subgroup shelf-edge deltas in Texas (Xue and
Galloway, 1995).
On the Spitsbergen Eocene deltaic shelf-margin
successions, sandy turbidites are extremely abundant
(Fig. 16) , but it is not always clear if the turbidites
derive from slumps or from sediment-laden currents
directly output from the delta (hyperpycnal flows).
However, the decelerating/waning nature of these
delta-front flows, inferred from their thinning and
termination on the slope, suggest relatively low-den-
sity, non-ignitive flows. Such sandy turbidites make
up the bulk of sand-prone clinothems in which they
are connected updip to mouth-bar sandstones.
Thick, massive to crudely laminated sand beds
dissected by upflow-dipping shear planes and show-
ing convoluted to locally wave-rippled tops are com-
mon constituents in the delta-front/slope deposits off
Badenian shelf-margin deltas in the Carpathian Fore-
deep of Poland (Fig. 17). On well logs, such sands are
commonly identifiable as ‘‘blocky’’ units 5– 30 m
thick that, together with intervening heterolithic inter-
vals, form sand-prone accumulations up to 120 m
thick (Fig. 18). The latter are parallel to the shelf
break and extend for 14– 30 km along strike and 2– 4
km down on the slope (2 – 4j)(Fig. 19). On seismic
sections, the landward ends of thicker sand bodies
appear either to pinch out within slope heteroliths in a
shingled fashion, or onlap onto an unconformity that
is traceable further up either into thin mouth-bar
deposits or directly below transgressive shelf shales.
Where there is poor seismic control and/or only
discontinuous core data, the repeated occurrence of
‘‘blocky’’ sands pinching out updip within shales of
outer neritic to bathyal affinities strongly implies
proximity to a fossil shelf edge.
5.8. Soft-sediment deformation
Common features of clinoform seismic images,
particularly at or below the shelf margin, are discon-
Fig. 16. Schematic drawing of the turbiditic slope association commonly developed when shelf deltas reach the shelf margin (modified from
Plink-Bjo¨ rklund et al., 2001).
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tinuous, discordant to wedge-shaped units of chaotic,
mounded and transparent seismic facies, interpreted as
the product of slope-gravity collapse (e.g., Lehner,
1969; Sangree et al., 1978). The combination of mud-
rich upper slope and high sediment influx makes
shelf-margin deltas particularly susceptible to grav-
ity-driven soft-sediment deformation on a variety of
scales. Extensional strain dominates the shelf edge
and uppermost slope, and this commonly results in
regional down-to-basin, listric growth faults that are
subparallel to and centred at the shelf break, whereas
compression lower on the slope generates imbricated
folds and thrusts, piled up slides and diapiric intru-
sions (Winker, 1982; Galloway, 1989). Sediment-load
subsidence, when acting together with growth fault-
ing, expands delta thickness significantly and results
in the predominant strike alignment of shelf-edge
depocentres irrespective of the delta-front regime
(e.g., Edwards, 1980; Xue and Galloway, 1995). Evi-
dence of large-scale slumping and sliding is wide-
spread, and includes both contorted and imbricated
sediment packets seen in cores, as well as chaotic to
mounded seismic facies best interpreted as failure
deposits. Such deposits are particularly common in
base-of-slope areas, but they can also be present high
on the slope (Mayall et al., 1992; Martinsen and
Bakken, 1990; Collinson et al., 1991).
Magnificent examples of shelf-edge rotational sli-
des and related scars filled with sandy debrites and
high-density turbidites were reported from an Early
Cretaceous shelf-margin in eastern Spitsbergen (Ne-
mec et al., 1988). These same Cretaceous shelf-edge
collapse features are later healed over by a 60 – 80-m-
thick series of deltas that reestablished at the shelf edge
when sea level reattained that position (Fig. 20).It
seems likely that a mixture of rotational slides and
cohesive slumps overlain by remobilised sand that
filled the post-collapse hummocky relief can produce
chaotic seismic facies, whereas more transparent zones
intercalated with parallel reflections can be interpreted
in terms of thicker, sandier intervals (cf., Morton and
Price, 1987).
As exemplified by parts of the Tertiary succession
of the northern Gulf of Mexico (Edwards, 1980, 1981;
Winker, 1982; Galloway, 1989; Morton, 1993) and of
the Namurian of County Clare (Collinson et al.,
1991), large-scale slope-gravity spreading and gliding
are almost certainly triggered and facilitated by a large
sediment supply (and accompanying sediment-load
subsidence) to the shelf margin by deltas. This defor-
mation, particularly when enhanced by salt with-
drawal, can result in the delta-fed slopes having a
complex topography, with semicircular to strike-elon-
Fig. 17. Features of thick-bedded, massive sandstones deposited
within a perched mouth bar. Zabawa, Carpathian foredeep (modified
from Pore˛ bski and Oszczypko, 1999). (a) Listric shear planes
(arrowed) verging downflow (to the left) and merging downwards
into the intrabed, flat decollement surface. (b) Water-escape
convolutions near top of massive sand bed. (c) Symmetrical ripple
forms (arrowed) in a centimeter-thick siltstone parting at the contact
of thick, massive sandstone beds.
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gated elliptical slope basins that may or not be
connected by dip-trending gullies and channels in a
‘‘fill-and-spill’’ style (Prather et al., 1998).These
perched basins have onlapping fills containing sand-
rich turbidites, whereas the linking channels may
show a local levee development (Badalini et al.,
1999; Prather et al., 1998). Obviously, collapsing of
a shelf-margin delta into a series of rotated and tilted
blocks, as demonstrated for example in the Ubit Field,
offshore Nigeria (Clayton et al., 1998), will create a
disrupted slope topography that would promote the
trapping of delta-derived, resedimented sands within
irregular ‘basins’ relatively high on the slope itself.
This would contrast with a predominantly rilled to
gullied slope topography, as shown by Miocene clino-
forms off New Jersey (Fulthorphe and Austin, 1998;
Fulthorphe et al., 1999) and the Pleistocene, wave-
dominated shelf margin off the Rho
ˆne Delta (Tesson
et al., 1990, 2000). As observed in modern delta-fed
slopes, gullies or chutes tend to form on the slope
immediately off major distributaries, apparently in
some relationship to enhanced sediment flux (Cole-
man and Prior, 1988). The gullies are believed to be
initiated either as slide/slump scars (Prior and Cole-
man, 1978), or as erosional rills carved by sediment-
gravity flows on an oversteepened slope (Pratson and
Coakley, 1996) that, once formed, can propagate
upslope and may intersect the shelf edge. Gullies are
modified by erosion imposed by sediment-gravity
flows and may possibly evolve into canyons (Pratson
and Coakley, 1996). This type of disrupted slope
topography is likely to signify sediment by-pass on
the slope, causing sand transfer to deepwater areas
beyond the base of slope. Consequently, slope facies
can be expected to be rich in cohesive slumps, sandy
debris-flow deposits and high-density turbidites, most
of them derived directly or indirectly (slumped) from
mouth-bar deposits (cf., Mayall et al., 1992). Such
Fig. 18. Dip correlation through Badenian deltaic shelf margin in the Carpathian foredeep (modified from Pore˛bski, 1999). See Fig. 19B, for
location of section.
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gullied slopes may reflect either a mature stage in
slope evolution, due to diminished sediment flux, or
development of a more wave-dominated regime along
the shelf edge.
In summary, although there is practically no lower
limit for the slope inclination that can induce gravity
collapse (e.g., Field et al., 1982), shelf-margin delta
slopes appear to be subject to deformation, slumping
and collapse on a much greater scale than are the
slopes of inner-shelf deltas. The slope oversteepening
created by anomalously great sediment flux to the
shelf margin promotes mass wasting and efficient
sand transport to the base-of-slope setting. Once the
deltaic wedge attains a critical weight, it will start to
deform internally by growth faulting and the resultant
slope basins may become efficient sediment traps.
6. Fluvial feeder to the shelf-margin deltas
With good coverage of seismic data, shelf-margin
deltaic clinoforms commonly can be seen to merge
landwards into a buried network of fluvial channels
(Fig. 21). This has been particularly well documented
Fig. 19. Palaeogeographic interpretation of the falling-stage and lowstand systems tracts in Badenian deltaic shelf margin in the subsurface of
the Carpathian foredeep (modified from Pore˛bski et al., in press).
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in the Quaternary shelf of the Gulf of Mexico (Suter
and Berryhill, 1965; Morton and Price, 1987; Kin-
dinger, 1988; Suter et al., 1987). Although most of
these channel systems have been interpreted as incised
valleys, it is probably more correct to assume that
channels that merge basinwards into thick shelf-margin
deltas are the deltaic distributary channels. The for-
mation of incised valleys on the shelf requires the shelf
to be fully emerged, i.e., accompanied by the relative
sea-level fall below the former shelf edge (Steel et al.,
2000; Posamentier, 2001, p. 1789). Such conditions
would promote incision and by-pass of the shelf margin
rather than its accretion through deltaic deposition.
Near the shelf edge, the feeder or distributary
channels can be hundreds of meters to several kilo-
meters wide and 30 – 40 m deep (Suter and Berryhill,
1965; see also examples in Posamentier and Allen,
1999). Channels tend to show composite infills (Fig.
22), and the infill-lithology varies from clean, cross-
bedded medium-grained, occasionally pebbly sand
(Sydow and Roberts, 1994) to lithologies that are
heterolithic and sometimes show upward-fining
grain-size trends (Morton and Suter, 1996). The bases
of channels incise the substrate and can in places be
seen to truncate the underlying clinoforms. The latter
is a signal of some deepening incision of the distribu-
tary channels, possibly during relative fall of sea level.
However, the filling in some channels is thought to
have been concomitant with the deposition of the
most distal, aggrading clinothems (Fig. 7), usually
after the initial rise of sea level (Morton and Suter,
1996; Sydow and Roberts, 1994), although there is a
growing evidence that fluvial deposits can be formed
and preserved during falling stage, and can in fact be
more common than assumed so far (Blum, 1990;
Posamentier, 2001). The top of the channelised strata
is a flat, landward-rising truncation surface that
extends to inter-channels areas and is invariably
interpreted as a transgressive ravinement surface.
Farther landwards, the erosional relief on top of the
clinoform sets increases and distributary channels
merge into a network of incised fluvial valleys that
can be up to 20 km wide and 60 m in relief, as in the
southwestern Louisiana shelf (Suter et al., 1987).
Suter and Berryhill (1985) emphasized that such an
association is one of the main features that allow
shelf-margin deltas to be distinguished from other
types of shelf-edge progradation. In the absence of
evidence of a connecting fluvial drainage system,
clinoforms at the shelf-margin have sometimes been
interpreted in terms of either prograding shelf-edge
shorefaces (Hovland and Dukefoss, 1981; Trincardi
Fig. 20. Features of a falling-stage, Barremian shelf-edge collapse and upper canyon (Kvalvagen, Spitsbergen). Fluvial blocks 100 m long have
fallen down into canyon head. Turbidite beds are present in canyon head at left end. Shelf-edge deltas are healing across earlier collapse area,
after sea level again rose above the shelf edge (modified from Nemec et al., 1988; Steel et al., 2000).
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Fig. 21. Geometry of fluvial channel system associated with Late Pleistocene Lagniappe Delta, outer Mississippi – Alabama shelf (modified after
Kindinger, 1988).
Fig. 22. Transverse seismic section through Late Pleistocene fluvial channel in outer Louisiana shelf, showing compound infill (after Suter and
Berryhill, 1985).
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and Field, 1991; Tesson et al., 1993; Berne
´et al.,
1998; Trincardi and Correggiarri, 2000). However,
where a fluvial feeding system is absent, care should
be taken in making a non-deltaic interpretation. One
of the central and inherent aspects of shelf-edge delta
progradation is precisely the ability of the delta to ca-
nnibalise and remove evidence of its updip coastal
plain during falling relative sea level. Moreover,
subsequent transgression can certainly remove this
evidence (e.g., 6– 15 m depth of Holocene ravinement
on the Gulf Coast—Rodriguez et al., 2001, p. 850)
causing even greater exaggeration of a ‘detachment’
of the delta (cf., Ainsworth and Pattison, 1994). In
addition, it is relatively difficult to transport large
quantities of medium- or coarse-grained sand out to
the shelf margin without fluvial feeders, except in
cases of very narrow ( < 20 –30 km) shelves.
In other cases, the absence of fluvial feeders turns
out to be merely apparent, due to the subtlety of shelf-
drainage features coupled with the low resolution of
seismic data. This is exemplified by Miocene clino-
forms on the New Jersey shelf (Fulthorpe and Austin,
1998). Only when these clinoforms were analysed on
images with a 5-m vertical resolution was the pres-
ence of distributary channels (100 – 400 m wide, 30 m
deep and 10– 15 km in spacing) revealed (Fulthorpe et
al., 1999). Channel-like erosional forms having fills
characterized by both concave-up and chaotic reflec-
tions have been observed at the top of prograding
clinothem units of the western Rho
ˆne shelf (Tesson et
al., 2000). These features were interpreted as shelf-
edge slide/slump scars because of their in-filling style
and their apparent restricted extent inboard of the
clinoform offlap breaks (Tesson et al., 2000). How-
ever, distributary-channel origin was invoked for
similar styles of infill in the Upper Pleistocene
shelf-margin delta in the Eugene Island, offshore
Louisiana (Hart et al., 1997).
There is also another way in which some of the
fluvial feeder infills have been reinterpreted recently.
Although the upper, channelised erosion surface of
shelf-edge deltas is indeed likely to have been the
feeder conduits for the delta-front growth (and this is
seen especially in the youngest clinoforms of the set, as
noted above), the in-filling of the irregular relief on this
surface commonly happens later during transgression.
During transgression, the earlier deltas become trans-
formed into estuaries or embayments that overlie the
axis of the previous distributary system (cf., Boyd et al.,
1992). Shelf-edge delta/shelf-edge estuary couplets are
now being recognised as the normal components of
many ‘‘shelf’’ sand tongues, and this is confirmed by
the strong tidal signatures and landward-directed pale-
ocurrents now identified in the strata filling some of the
so-called distributary channels (Schellpeper, 2000).
Pleistocene examples of shelf-edge deltas transforming
to shelf-edge estuaries during turnaround and trans-
gression have been described by Berne
´et al. (1998).In
all these cases, the erosive or incised tops of the
regressive tongues or wedges testify to the former
presence of fluvial feeders, even though the deposits
in the upper part of the tongues are of transgressive
origin.
7. Generation of shelf-margin deltas
7.1. Importance of sediment supply
Deltaic progradation out across a shelf to its margin
can be caused by either (1) anomalously great silici-
clastic sediment flux with fairly stable relative sea level
(though there may be a problem with great water depth
on the outer shelf) or (2) normal sediment flux with
forced regression during relative sea-level fall. For
example, Paleocene regional uplift within the conti-
nental interior of North America resulted in the gen-
eration of delta-driven shelf-margin accretion onto the
inherited Cretaceous shelf edge in the Gulf of Mexico
(Edwards, 1981; Galloway, 1989, 1990). Climatic
cooling combined with uplift of the Appalachians
caused a tenfold increase in clastic sediment input to
the New Jersey shelf margin during the Miocene (Poag
and Sevon, 1989). This resulted in delta-driven clino-
forms offlaping onto the former, shallow-dipping car-
bonate ramp (Fulthorpe and Austin, 1998; Olsson et al.,
2002). Although controlled by extrabasinal factors,
great clastic influx and the resulting shelf-margin
accretion need to be neither synchronous basinwide
(Winker, 1982; Galloway, 1989, 1990) nor dependent
on eustatic sea-level change. The latter is exemplified
by the Balize Lobe of the modern Mississippi Delta,
which during the Holocene transgression and subse-
quent highstand has prograded over a low-energy,
storm-dominated shelf to its present location within
30 km of the shelf edge (Frazier, 1967). However, the
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Mississippi River, with its continental-wide drainage
and former access to continental ice sheets, is a river in
a class of its own.
7.2. Evidence of sea-level fall during shelf-margin
growth
In contrast, Pleistocene shelf-margin accretion on
the Gulf Coast, though also delta-driven, appears to
have been controlled by eustatic changes. There is a
general acceptance that shelf-edge deltas commonly
prograded during falling (though not necessarily much
below the shelf edge) and lowstand of sea level.
Supportive arguments include (see also Posamentier
and Morris, 2000)
landward delta termination by onlap-pinchout, so
that the delta is separated by a zone of sediment
by-pass from the highstand shoreline (Morton and
Suter, 1996; Tesson et al., 2000);
rapid upward-shoaling trend (Sydow and Roberts,
1994; Winn et al., 1998; Kolla et al., 2000) leading
to foreshortened stratigraphy (Posamentier and
Morris, 2000);
seaward descending elevation and trajectory of
sigmoidal clinoform tops (Curray and Moore,
1964) (that changes commonly to an ascending
pattern in the youngest clinoform bundles);
erosional truncation of clinoforms at their land-
ward side (Winn et al., 1998), including the cutting
of valleys on the shelf;
chronostratigraphic evidence, based on oxygen-
isotope ratios, showing that this upper uncon-
formity surface at its final stage corresponds to the
maximum sea-level lowstand (Kolla et al., 2000;
Scott et al., 1998).
7.3. Shelf-margin delta growth during fall-to-rise
cycle of relative sea level
A shelf-margin delta system comprises a clinoform
set whose upper bounding unconformity represents the
trace of either the fluvial feeder, or a combined fluvial-
erosion/transgressive ravinement surface. As modelled
by Sydow and Roberts (1994), eustatic sea-level fall
forced both the fluvial feeder and the clinoforms across
the shelf, with erosion and deposition prevailing at the
trailing edge and leading edge of the system, respec-
Fig. 23. Edge of a small valley incision (12 m deep) cut into the outer shelf-edge delta deposits of an Eocene shelf-margin clinoform; Storvola,
Spitsbergen.
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tively. This process results in cannibalisation and
incision of feeder channels into the former proximal
clinoform tops (Fig. 23) and leads to an increased
segregation of sand within more distal bundles (Fig.
7). The latter is consistent with the commonly observed
seaward steepening of the upper foreset segments of the
clinoforms (Lehner, 1969). The sigmoidal shape of the
youngest descending clinoforms indicates that the
shoreline did not drop below the shelf break (cf.,
Trincardi and Field, 1991). The latest stages of accre-
tion at the leading edge of the system show an active
up-building, with well-preserved sigmoidal clino-
forms, indicative of early rise of sea level. Internal
downlap surfaces within the clinoform body reflect
either (1) lobe abandonment followed by reactivated
delta growth (Sydow et al., 1992) or, (2) when laterally
persistent, short periods of stillstand or small rises in
relative sea level, superposed on an overall falling trend
(Posamentier et al., 1992; Tesson et al., 2000). The
aggradational stacking pattern of the youngest deltaic
clinoforms, commonly taken as evidence of sea-level
turnaround (fall to rise—Fig. 7), may in some cases
reflect only a local rise in relative sea level. This is
exemplified by the rising clinoforms break-point tra-
jectory in the late Pleistocene Mississippi Delta (Fig.
24), due to sediment loading and subsidence behind a
growing diapir (Suter and Berryhill, 1985).
8. Criteria for distinguishing shelf-margin deltas
from other shelf deltas
There is a rich literature on the geometry, facies
character and variability of inner mid-shelf deltas
(Morgan, 1970; Galloway, 1974; Elliot, 1986; Cole-
man, 1988; Whateley and Pickering, 1989; Friedman
et al., 1992; Orton and Reading, 1993; Reading and
Collinson, 1996). With the advent of sequence strat-
igraphy, shelf-margin deltas became an important
member of the delta family, and some of the early
researchers (e.g., Edwards 1981; Suter and Berryhill,
1985) made a point of drawing some key distinctions
between shelf-margin and inner-shelf delta types.
Since that time, however, the fact these different delta
types formed under differing sea-level regimes, and
the consequence of this for their geometry, internal
architecture and facies has been largely ignored. We
summarize below the key differences.
The main distinctive criteria of shelf-margin deltas,
in contrast to inner-shelf deltas include (Table 1)
the very large scale of the clinoforms, that can be
an order of magnitude greater in amplitude than
that of mid- and inner-shelf deltas;
the resultant thick, strike-elongated isopach image
of shelf-margin deltas is commonly augmented by
growth faulting;
sigmoidal dip cross-sectional shape, with the
thickest part located near the offlap break of a
preexisting shelf margin;
the landward pinchout by onlap onto shelf shales
and the basinward pinchout by downlap within
hemipelagic shales;
evidence of foreshortened stratigraphy;
the characteristic turbidite-prone nature of the
delta-front/prodelta segment;
the abundance and a large scale of slope-controlled
soft-sediment deformation;
the absence of a paralic ‘‘tail’’ along delta’s trailing
edge.
Fig. 24. Minisparker seismic section through late Wisconsinan Mississippi delta, showing the progradation and aggradation of deltaic shelf edge
due to relative sea-level rise generated by local, diapir-related subsidence (after Suter and Berryhill, 1985).
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Shelf-margin deltas show by far the largest clino-
forms among all shelf delta types, because they build
out into deep water beyond the shelf break. As soon
as a delta reaches the former shelf edge, the initially
thin-bedded, heterolithic delta front will become
increasingly turbiditic in character because the fluvial
feeder will empty directly onto a longer (minimum
2– 4 km in length, if water depth exceeds 200 m)
and steeper slope, promoting hyperpycnal discharges.
Although some mid-shelf, wave-dominated deltas
can build up steep (2 – 4j, maximum 8j) turbidite-
rich slopes (see, Panther Tongue example in Posa-
mentier and Morris, 2000), the resultant clinothems
are an order of magnitude smaller (up to 15 m in the
above example) and turbidite beds are few and thin,
because of shallow water depth and the consequent
short slope length.
In inner-shelf deltas, the combination of high-
suspended-load yield with low slopes promotes the
development of heterolithic, thin-bedded delta-front
deposits in which, depending on delta-front regime,
wave-modified or purely traction to suspension-fall-
out structures dominate (Elliot, 1986; Friedman et
al., 1992; numerous papers in Morgan, 1970, and in
Whateley and Pickering, 1989). Such conditions
promote also chiefly vertical soft-sediment deforma-
tion, such as mud diapirs and localized bar front
slumps (Fisk, 1961; Coleman and Prior, 1988) rather
than large-scale slope wasting. Invariably, inner-shelf
deltas develop thick delta-plain deposits (e.g., Elliot,
1986; Coleman, 1988; Penland et al., 1988), because
they are formed during conditions of slightly rising
relative sea level (highstand systems tract). This
would contrast with falling-stage shelf-margin deltas
that do not have a paralic tail along their trailing
edge, because such a tail either barely develops
during forced regression due to the decreasing
accommodation behind the delta front, or is entirely
removed during subsequent transgressive ravine-
ment.
9. Types of shelf-margin delta
As seen from the preceding review, two types of
shelf-margin delta can be distinguished on the basis of
the style and scale of internal gravity-induced defor-
mation.
9.1. Sequence-stratigraphic context
During the development of sand-rich lowstand
complexes beyond and below the shelf platform,
shelf-edge deltas basically develop at two distinct
times: during the fall of relative sea level that forces
deltas to migrate out to the shelf edge, and during the
subsequent rise of relative sea level back to a level
above the shelf platform (provided sea level dropped
below this platform to allow the development of a
full-scale lowstand complex).
The deltas that develop during fall of relative sea
level (with variable potential for subsequent preser-
vation) would be variously classified as deposits
within the early lowstand systems tract (Posamentier
and Vail, 1988), within the late highstand systems
tract (Van Wagoner, 1995) and within the forced
regressive or falling-stage systems tract (Hunt and
Tucker, 1992, 1995; Helland-Hansen and Gjelberg,
1994; Plint and Nummedal, 2000). Shelf-margin del-
tas developed at this time in the cycle have the
potential to be severely or entirely cannibalised by
valley incision on the shelf and canyon incision on the
upper slope, as sea level continues to fall. However, it
is important to be aware that they do develop at this
time because significant remnants of these deltas can
be preserved on the outer shelf and upper slope
between and below zones of incision.
The deltas that reestablish at the shelf margin
during rise of relative sea level, after a period of
lowstand below the shelf platform, have been referred
to as the lowstand wedge prograding complex (Vail,
1987), as the late lowstand wedge (Posamentier and
Vail, 1988), the late lowstand systems tract (Normark
et al., 1993) and the late lowstand (Kolla, 1993). The
nature and timing of this late accretion has been well
illustrated by Posamentier et al. (1991) and is shown
in Fig. 25.
Although we have emphasized the role of changing
relative sea level in the generation of these deltas, high
supply of sediment can be additionally critical for their
creation, as noted below, where we outline the charac-
teristics of the two main classes of shelf-margin deltas.
9.2. Unstable shelf-margin deltas
Unstable shelf-margin deltas (Edwards, 1980) are
characterized by great sediment thicknesses (up to
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several hundreds of meters) and associated listric
growth faults and rollovers along the shelf edge
(Fig. 26A). Rotational slides are common on the
upper slope, whereas compressional ridges affect the
slope toe (Winker, 1982). Where the slope is long, the
above irregularities tend to pond sand turbidites and
heterolithic facies on the slope. Such deltas develop
due to a combination of great accommodation space
available in the deep water beyond the shelf platform,
with great sediment flux to the shelf edge, either due
to tectonically active source areas or anomalously big
rivers. High sediment supply and relative sea-level fall
below the shelf edge promote development of can-
yons which once connected with shelf valleys, will
cannibalise the former deltaic shelf margin and bypass
much of the sediment directly to the basin floor to
build basin-floor fans. The formation of unstable
shelf-margin deltas appears to be related commonly
to very high sediment supply, as well as to relative fall
of sea level, and can be facilitated by the flow of
evaporates in the underlying slope sediment that
results in additional accommodation. Tertiary deltas
of the Gulf Coast belong to this type (Edwards, 1980,
1981; Winker, 1982; Winker and Edwards, 1983; Xue
and Galloway, 1995; Prather et al., 1998).
9.3. Stable shelf-margin deltas
These are typified by smooth shelf-margin mor-
phologic profile and little to no growth faulting along
the shelf edge (Fig. 23B). Large-scale sliding/slump-
ing is not common, although it may locally be
voluminous (e.g., Lehner, 1969), and it is essentially
limited to collapsing mouth-bars. The slope tends to
Fig. 25. Block diagrams of the lowstand systems tracts (modified from Posamentier et al., 1991; Normark et al., 1993). Note that the deposition
of basin-floor fan is promoted when incised valley becomes linked to canyon on the slope during relative sea-level fall (A), while shelf-margin
delta is the dominant component of the prograding complex that is formed during early rise (C).
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be smooth, rilled to gullied with neither major can-
yons cut in it nor any incised valleys breaching the
shelf edge. The delta-front/slope facies vary from mud
to sand prone. In the latter case, it can contain thicker
turbidite sands of hyperpycnal origin (they fail to
ignite on the slope, and therefore die on the slope).
These sands tend to form strike-oriented to lobate
bodies of limited lateral extent that can reach down to
the middle or even the lower slope. Stable shelf-
margin deltas appear to be associated with (1) small
Fig. 26. Cartoonal representation of unstable (A) and stable (B) shelf-margin deltas. Because of an association with sea-level fall below the shelf
edge, both types of shelf-margin deltas, during their main regressive transits, tend to have their delta plain and upper delta front removed by
erosion.
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to modest rivers that reestablish at the shelf edge after
sea level rises back to this level, just prior to trans-
gression. Such deltas can also form with (2) modest
relative sea-level falls to the level of the shelf plat-
form, but not below. Many Quaternary shelf-margin
deltas belong to this category.
10. Implications for sequence stratigraphy
10.1. Sea-level drop and shelf-edge incision
Although submarine canyons form in a variety of
ways (Pratson et al., 1994), they seem to share one
feature, i.e., an association with high sediment supply
to the shelf edge and upper slope, whether or not they
are visibly backed by fluvial valleys (May et al., 1983).
Such conditions promote slope loading, slope defor-
mation and headward erosion by retrogressive mass
wasting and turbidity currents, and appear to be
responsible for the initiation of most canyons located
off major deltas (Coleman et al., 1983). With relative
sea-level fall below the shelf edge, such canyons are
able to capture river inflow and become connected to
incised valleys on the shelf (Figs. 23 and 25), as well
as to a coeval basin-floor fan (Posamentier and Vail,
1988). Well-documented examples of submarine can-
yons that are directly connected to fluvial erosional
systems at their shelf side include the Rho
ˆne and Nile
canyons that were generated during a spectacular sea-
level drop associated with the Messinian salinity crisis.
Examples of similar linkage between canyons and
fluvial systems have been shown from the Cretaceous
and Eocene by Nemec et al. (1988) (Fig. 20),Steel et
al. (2000) and by Mellere et al. (in press). A similar
origin has also been proposed for the Zaire Canyon,
whose head lies well within the Zaire River estuary
(Droz et al., 1996). As implied by classic sequence
stratigraphic models, establishment of such a direct
fluvial-valley/canyon connection enhances the sedi-
ment transfer directly to the basin floor (see discussion
in Posamentier and Allen, 1999). This incision at the
shelf edge tends to cannibalise, partly or completely,
the earlier formed falling-stage deltas, and precludes
the generation of new deltas until sea level reestab-
lishes above the shelf edge during later rise.
As shown by Quaternary examples, falling sea level
generates a prograding deltaic-shelf margin which is
fed by distributary channel discharge plus the products
of erosion of the increasingly emergent inner shelf. The
latter is overridden by subaerial incision, cut progres-
sively basinwards by the fluvial feeder (Fig. 27). In this
way, the shelf margin progrades through the welding of
successive deltaic clinoforms, and is apparently never
overtaken by the zone of fluvial erosion, except where
sea level falls significantly below the shelf edge, and
fluvial systems and slope canyons link directly with
each other. Talling (1998) predicted that during sea-
Fig. 27. Stratigraphic architecture generated at shelf edge during prolonged, stepped fall followed by rapid rise in the sea level (isotopic stages 5
to 1), as exemplified by Late Pleistocene Lagniappe Delta, Gulf of Mexico (after Kolla et al., 2000, slightly modified). The stepwise fall resulted
in a series of downstepping shelf-edge deltaic increments, each based by minor unconformity attributed to a high-order sequence boundary. The
pronounced unconformity atop of forced regressive deltaic wedge is taken as the master sequence boundary.
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level falls not extending beyond the shelf break, inci-
sions will be restricted to the inner shelf, with a
maximum relief of 20 – 70 m centred at the former
highstand coastal shoreline, decreasing both land- and
seawards in relief (see also Posamentier and Allen,
1999). This seems to be the case for most of the incised
valley feeders of Pleistocene shelf-margin deltas. In
fact, evidence of dissection of topographic deltaic-shelf
edge by incised valleys is at least equivocal at this time.
Farra
´n and Maldonado (1990) recognized several can-
yons incised into Quaternary shelf-margin deltaic units
off the Ebro Delta, but interpreted them as submarine
incisions. Tesson et al. (2000, p. 137) described prom-
inent U-shaped troughs extending 5 – 10 km landwards
of the shelf break and showing sagging reflections.
They interpreted this type of trough as ‘‘canyon grading
updip in some instances into an incised valley’’, but did
not elaborate further on this subject. Weimer (1990)
noted that 10– 15 Pleistocene canyons on the Louisiana
slope had updip connections to incised valleys. How-
ever, except for the ancestral Mississippi River-canyon
system, no such a connection was proved in the north-
ern Gulf of Mexico (Suter et al., 1987; Sydow and
Roberts, 1994; Morton and Suter, 1996).Posamentier
(2001) noted the scarcity of incised valley systems in
the Pleistocene of offshore Java, and concluded that
their formation would be limited only to those very
short periods of the maximum lowstand when sea-level
fall exceeded 110–120 m.
Rate of sea-level fall can be classified as rapid or
slow, when it is greater or less, respectively, than the
rate of subsidence at the shelf edge (Posamentier and
Vail, 1988). Rapid fall below the shelf edge results in
dissection of the shelf by fluvial erosion. However,
the effects of Pleistocene glacieustatic sea-level falls
demonstrate, at least on passive margin shelves, that
even large magnitude (120– 150 m) falls that are
associated with high-frequency, asymmetrical sea-
level changes should not a priori be expected to result
in fluvial dissection of the shelf break (McMaster et
al., 1970; Trincardi and Field, 1991; Morton and
Suter, 1996). Such falls are apparently not fast
enough, probably because they are interrupted by
stillstands and small rises, so that during much of
the period of overall sea-level fall, the shelf was never
fully exposed (Posamentier, 2001).
Sydow and Roberts (1994) noted that the water
depth of the present-day shelf break on passive margins
(75– 135 m, Table 2) is close to the sea-level minimum
estimates (120 m) for the last glacial, thus, promoting
shelf-edge accretion rather than dissection during sea-
level fall. During this glacial period, sea level was
falling at ca. 1.2 m/ka, whereas Mississippi Delta lobes
are presently subsiding locally at a rate of up to 10 m/ka
(Nummedal, 1983). These figures highlight the impor-
tance of subsidence in deltaic shelf-margin settings
(Winker, 1982; Galloway, 1989; Tesson et al., 1993),
which is perhaps the most important single factor
preventing the extension of incised valleys through to
the deltaic shelf edge.
Morton and Suter (1996) suggested that the for-
mation of linked incised valley-submarine canyon
systems could be facilitated by the presence of a river
mouth near the shelf margin when sea level begins to
fall, i.e., at the peak of highstand. This would create
particularly steep shelf-to-slope gradients so that fall
of sea level below the shelf break would effectively
cause the entrenchment of the fluvial system and
cannibalisation of former deltas at the shelf margin.
Burgess and Hovius (1998) have calculated the time
necessary for 24 modern deltas to arrive at the shelf
edge in present-day highstand (stillstand) conditions.
This required transit times that varied between 8.5 and
116.5 ka. However, these calculations did not take
into account the effect of rising relative sea level that
is an integral part of the highstand systems tract.
Using a 2-m/ka eustatic rise that is typical for the last
7ka(Fairbanks, 1989), these minimum times would
have to be increased by a factor of 1.09 – 1.69 (1.30 on
average) and, in some cases, the deltas would never
reach the shelf edge (see Muto and Steel, in press).
This necessary transit time greatly exceeds the dura-
tion of fourth-order glacieustatic sea-level highstand
intervals (for example, ca. 18 ka for the last 120-ka
sea-level oscillation), and so, not many of the 24
modern deltas would be able to deliver sand to the
shelf edge or to the adjacent deepwater slope during
even slightly rising relative sea level. We should add
that the necessary transit time above probably also
exceeds the regressive transit time of fourth-order
regressions during non-glacial periods.
Pleistocene glacieustatic sea-level cycles are stro-
ngly asymmetrical, because they reflect periods of slow
cooling and rapid warming (Shackleton, 1987). This
results in prolonged conditions of stepped sea-level fall
and very short periods of sea-level rise, as also reflected
S.J. Pore˛bski, R.J. Steel / Earth-Science Reviews 62 (2003) 283–326316

in a characteristic asymmetry of forced regressive
deposits versus transgressive and highstand deposits
(e.g., Chiocci, 1994; Herna
´ndez-Molina et al., 2000).
To the extent that the Pleistocene sea-level curve can be
used as analogue to glacieustatic oscillations in the
more distant past, it seems very unlikely that glacieu-
static control alone is able to bring highstand deltas
consistently to the shelf margin, even during third-
order cycles (cf., Haq et al., 1987). Other limiting
factors include an increasing tendency for rivers to
switch in an autocyclic manner as they lengthen, and
the inherent tendency of prograding deltas, during even
modest rates of rising sea level, to be subject to auto-
retreat (Muto and Steel, 1992, 2000).
10.2. Systems tract assignment
There is a general consensus that Pleistocene deltas
prograded across the shelf and located at the shelf-
margin during relative sea-level fall. However, the
systems tract assignment of such forced regressive
deposits and the stratigraphic position of the associated
sequence boundary are still a matter of controversy.
Such deltas have been assigned to early lowstand
(Posamentier and Vail, 1988), stable highstand (e.g.,
Torres et al., 1995), highstand (Sydow and Roberts,
1994), late highstand (Van Wagoner, 1995), early low-
stand (Van Wagoner et al., 1990; Posamentier and
Allen, 1993; Posamentier and Morris, 2000), detached
lowstand (Ainsworth and Pattison, 1994) and to the
forced regressive systems tract (Hunt and Tucker,
1995). Clearly, this reflects conflicting tendencies, on
the one hand, to describe stratigraphic architecture in
terms of classic sequence stratigraphy, and on the other,
to modify and extend existing terminology in the light
of new understanding of stratigraphic patterns (cf.,
Wilson, 1998; Friedman and Sanders, 2000). This
debate is not merely a matter of nomenclature; it has
consequences for facies predictions.
Shelf-margin deltas that form during relative sea-
level fall are, by definition, forced regressive deposits,
and are directly or indirectly the feeders to sharp-
based shoreline sandstones. Such deltas, when occur-
ring below the shelf platform to perch on the slope,
have also been used to provide evidence in the debate
regarding the three- versus four-systems-tract se-
quence terminology (Hunt and Tucker, 1992, 1995;
Kolla et al., 1995). However, it should be noted that
shelf deltas also form during rising relative sea level,
as when they reestablish at late lowstand, as sea level
rises to back above the shelf margin. This delta type
has been designated as the ‘late prograding complex’
of the classic lowstand systems tract (Posamentier et
al., 1991). There are significant differences in archi-
tectural style between these two cases of shelf-edge
delta. The one shows stepped fall during progradation,
in contrast to the other that shows aggradation during
progradation (Figs. 11 and 27).
The erosional unconformity along the top of a
shelf-margin deltaic body begins to form already
during initial relative sea-level fall (Sydow and Rob-
erts, 1994). Initially, fluvial channels incise vertically,
but their lateral migration increases when the rate of
fall slows (Hart and Long, 1996). Thus, the uncon-
formity continues to be generated throughout the
period of stepped fall to the point of lowest relative
sea level (Suter and Berryhill, 1985),anditthen
passes into a conformable surface at the turnaround
to relative sea-level rise (Hart and Long, 1996; Kolla
et al., 2000). Below the initial shelf break, this
unconformity becomes an onlap surface that separates
the downstepping forced regressive deposits formed
during the sea-level fall from the aggrading-to-back-
stepping deltas formed during the early rise in relative
sea level. The initial forced regressive surfaces are
unlikely to be of basinwide extent, whereas the
erosion surface that continues to be generated across
the tops of the delta all during the fall to the minimum
sea-level position is likely to be widespread. This
mitigates against including the forced regressive
deposits within the lowstand systems tract.
10.3. Sequence boundary
Exxonian depositional sequences are bounded by
unconformities that are formed during relative-sea-
level falls (Vail et al., 1984). A stepped fall results in a
series of downstepping wedges, each based by a
regressive surface of marine erosion, and cumula-
tively forming a basal downlap surface. The high-
frequency erosion surfaces at the base of each down-
stepping wedge record basinwards facies shift and
represent higher order sequence boundaries (Posa-
mentier et al., 1992). Such boundaries merge upwards
and landwards into a major erosional unconformity
that reflects subaerial incision often modified subse-
S.J. Pore˛bski, R.J. Steel / Earth-Science Reviews 62 (2003) 283–326 317

quently by transgressive ravinement. A debate has
now centred as to whether the master sequence
boundary should be placed at (1) the first downlap
surface separating normal (highstand) regression from
forced regressive deposits (Posamentier et al., 1992;
Tesson et al., 2000; Posamentier and Morris, 2000),or
Fig. 28. (A and B) Stratigraphic architecture of shelf margin through different physiographic settings in a basin affected by the same relative sea-
level fall, showing different placement of potential master sequence boundary (modified from Posamentier and Morris, 2000). In (A), highstand
shoreline reached the shelf edge and mass wasting to the deep-water area both began already at time 3, while in (B), this had not started until
time 7. Posamentier and Morris (2000) argue that isochronous though largely cryptic surface X is a better candidate for the sequence boundary
than surface Y, because the latter gives a problematic correlation between the locations (A) and (B). (C and D) Alternative interpretation of the
same sections, envisaging two sequence boundaries. Sequence boundary Y, though diachronous and polygenetic, is well recognizable across the
entire basin, because it corresponds at its final stage to the maximum sea-level fall that coincides at both locations with enhanced mass-flow
deposition (dotted).
S.J. Pore˛bski, R.J. Steel / Earth-Science Reviews 62 (2003) 283–326318

at (2) the erosional unconformity that is perpetuated
across the top of the forced regressive wedge (Hunt
and Tucker, 1992, 1995; Helland-Hansen and Gjel-
berg, 1994; Plint and Nummedal, 2000).
An increasing use of the second choice has been
based on the fact that this surface commonly forms
the most prominent and easily recognizable disconti-
nuity on the shelf. This approach has recently been
criticized by Posamentier and Morris (2000; see also
Posamentier and Allen, 1999), who point out that this
surface is diachronous and its location will vary
depending on variations in local physiography and
on the extent of database available (data window);
hence, paleogeographic maps based on such an
assignment will be meaningless. However, it should
be stressed that this is a weakness of all prominent
discontinuities, as most are diachronous. Posamentier
and Morris (2000) (Fig. 28A and B) propose to locate
the master sequence boundary at the base of the first
forced regressive wedge and its correlative conform-
ity into the basin. They argue that such a choice will
not only provide the necessary isochroneity to the
sequence boundary but it will also help resolve
stratigraphy within a basin in which the same relative
sea-level fall affected the highstand shoreline that is
near the shelf edge along one part of the coast, but far
inboard of the edge in another part.
The main problem with the sequence boundary so
defined is that for much of its extent it remains
cryptic and difficult to identify. Moreover, although
on cartoon drawings and conventional seismic sec-
tions, such a surface may appear as a downlap
unconformity (Vail et al., 1977), it is not necessarily
so when traced downdip. This boundary continues
downdip as a correlative conformity that, however,
can be difficult to pinpoint within a wide zone of
ascending downlapping contacts. Furthermore, Pos-
amentier and Morris (2000) omitted to indicate on
their drawing (Fig. 28A and B) that in both coastal
segments surface Y, atop the forced regressive
wedge, forms another sequence boundary—one that
actually corresponds to the maximum sea-level drop
within the portrayed stratal architecture (Fig. 28C and
D). Thus, we conclude that this architecture shows
two sequence boundaries: one, located between
wedges 2 and 3, running atop the highstand and
below the first forced regressive wedge, corresponds
to an initial sea-level fall that affected only a small
portion of the shelf, albeit producing a basin-floor fan
offshore location B. The other surface (Y) that is
located between wedges 6 and 7, running atop the
forced regressive body, corresponds to the final fall
(the sum of falls between wedges 2 –3 and 6 – 7) that
affected the entire shelf area and can be associated
with basin-floor fans across the entire basin plain
(Fig. 28C and D).
Nonetheless, we feel that choices (1) versus (2)
should not be set up as alternatives. In some situations,
particularly in ramp settings where a seaward part of
the forced regressive deposits is detached from their
proximal part, the master sequence boundary should
occur below the detached segment as the latter may
actually correspond to lowstand systems tract devel-
oped during initial sea-level rise (e.g., Mellere and
Steel, 1995). In the presence of a shelf break, the upper
unconformity appears to be a more correct choice. In
all cases, the available data window will affect any
choice; this cannot be remedied by putting the
sequence boundary at more or less cryptic surfaces.
11. Concluding remarks: architecture of
shelf-margin deltas as predictor for slope and
basin-floor turbidites
It is clear from the preceding discussion that shelf-
margin deltas record essentially periods when the
sediment is stored at the shelf margin or when sedi-
ment contributes to the accretion of the shelf margin,
rather than necessarily periods when sand is delivered
into the basin floor. This implies that shelf-margin
deltas and basin-floor fans do not generally or auto-
matically form temporally linked entities, though a
genetic link between shelf-edge deltas and deepwater
systems nonetheless is very likely.
In dip section, some shelf-margin delta complexes
reveal a bipartite stratal architecture, with a lower and
older part composed of progradational and down-
stepping increments, and an upper and younger part
with an aggradational to backstepping stacking pat-
tern (Fig. 27). There may or may not be significant
erosion between these two parts. This pattern records
deposition during relative sea-level fall and subse-
quent early rise, with the downdip length of each
segment reflecting the duration of forced regression
(or early lowstand) and late lowstand, respectively. At
S.J. Pore˛bski, R.J. Steel / Earth-Science Reviews 62 (2003) 283–326 319

least three stratigraphic scenarios portraying either
fully developed, base-missing, or top-missing pat-
terns can be envisaged (Fig. 29).
1. The presence of a marked aggrading-to-backstep-
ping shelf-margin deltaic wedge, overlying a
widespread erosional surface on the shelf and
downlapping onto an irregular or disrupted slope
implies conditions of shelf-margin accretion that
were preceded by a period of significant sea-level
fall below the shelf edge, shelf-edge incision and
shelf-edge delta cannibalisation (Fig. 29A). This
situation is the one portrayed in the classical
Exxonian depositional sequence model associated
with Type 1 sequence boundary (Posamentier and
Vail, 1988). This scenario predicts the downdip
Fig. 29. Stratigraphic architectures of deltaic shelf margin as potential predictors for the presence or absence of basin-floor fans. (A) Aggrading-
to-backstepping complex overlying a growth faulted/disrupted slope and unconformity on the shelf (falling-stage delta deposits cannibalised
during sea-level fall below the shelf edge). This scenario broadly corresponds to Exxon’s Type 1 sequence. (B) Prograding-to-aggrading-to-
backstepping shelf-margin delta complex, without internal unconformity (Exxon’s Type 2 sequence). (C) Prograding shelf-margin delta complex
overlain by transgressive shales—this scenario may reflect either reduced sediment supply during progradation, or relative sea-level fall turning
abruptly to rise.
S.J. Pore˛bski, R.J. Steel / Earth-Science Reviews 62 (2003) 283–326320

presence of basin floor-fans, as documented in the
Eocene deltaic shelf-margin of Spitsbergen (Steel
et al., 2000; Plink-Bjo¨rklund et al., 2001).
2. Fully developed shelf-margin delta complexes
signify optimum shelf-margin accretion, and con-
sist of a fully preserved, falling stage regressive
segment, an overlying aggrading segment and a
retrogressive cap (Fig. 29B). There will be an
unconformity above the falling-stage deposits on
the shelf, but no unconformity on the slope. In such
cases, it is likely that the relative sea level fell as
the deltas prograded to the shelf edge, but the fall
did not persist below the shelf edge. The complex
continued to accrete during sea-level rise and
transgression. It is unlikely that there was signifi-
cant shelf-edge incision, and no basin-floor fans
are predicted. This type of deltaic complex re-
sembles the Exxon Type 2 sequence (Posamentier
and Vail, 1988).Posamentier (2001) suggested that
even for large-magnitude (>100 m) sea-level
oscillations, it is mainly the duration of fall relative
to the duration of rise that determines whether or
not valley incision will extend across the entire
shelf. His data shows that the longer the fall takes
to gradually expose the shelf, the smaller is the
chance for dissection of the shelf edge and, if this
happened, it would be limited to a very short
period of maximum lowstand. We might then
predict that the likelihood of getting a basin-floor
fan would increase for relatively small lengths of
the progradational-to-downstepping segment and
the fan, if developed at all, should be looked for on
a surface correlative to the turnaround surface from
the base level fall to rise and not on a downlap
surface marking the initial sea-level drop.
3. The presence of a shelf-margin delta complex with
only a progradational-to-downstepping segment
overlain by transgressive shales, and no evidence
of shelf-edge incision by the delta’s distributaries
implies conditions of prolonged forced regression,
but without sea-level fall below the newly formed
shelf edge. The rapid turnaround to capping
transgressive shales suggests decreased sediment
supply and rapid retrogradation of the deltaic
system, without any significant intervening aggra-
dational phase. In such a case, the deltaic shelf-
margin accreted by the broad dispersal of deltaic
mouth bars. However, the lack of incision and lack
of consequent focusing of sediment delivery
through a valley-canyon system, predicts that little
or no sand escaped to the basin floor (Fig. 29C).
The above architectural features at the shelf margin
could be used as an initial predictor of the likely
presence or absence of time-equivalent basin-floor
fans. If, in addition, valley incision on the shelf and/
or canyon incision on the slope can be imaged, or if
some of the other features of unstable shelf-margins
that are described above can be documented, then our
ability to predict deepwater sand on the basin floor is
even more likely.
Acknowledgements
RJS thanks the WOLF-consortium members
(Norsk Hydro, Conoco, Phillips, BP and PDVSA)
for continued and enthusiastic support for the shelf-
margin research work in Spitsbergen. SJP thanks
Institute of Energy Research and University of
Wyoming for support during Spring 2000 when much
of this review was undertaken. The study was also
supported by a State Committee of Scientific Re-
search (KBN) Grant T12B05218 to SJP, and by
Landmark Graphics via the Landmark University
Grant Program (grant to Institute of Geological
Sciences, Polish Academy of Sciences). The final
manuscript benefited greatly from critical and in-
sightful review by Henry W. Posamentier, Mike
Mayall and Gerald M. Friedman.
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