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PALAIOS, 2009, v. 24, p. 290–302
Research Article
DOI: 10.2110/palo.2008.p08-080r
SEQUENCE STRATIGRAPHIC CONTROL ON PRESERVATION OF LATE EOCENE WHALES AND
OTHER VERTEBRATES AT WADI AL-HITAN, EGYPT
SHANAN E. PETERS,
1
* MOHAMMED SAMEH M. ANTAR,
2
IYAD S. ZALMOUT,
3
and PHILIP D. GINGERICH
3
1
University of Wisconsin–Madison, Department of Geology and Geophysics, Madison, Wisconsin, 53706, USA;
2
Egyptian Environmental Affairs Agency,
Wadi Al-Hitan World Heritage Site, Fayum, Egypt;
3
University of Michigan, Museum of Paleontology and Department of Geological Sciences,
Ann Arbor, Michigan, 48109, USA
e-mail: peters@geology.wisc.edu
ABSTRACT
Biological and physical factors govern the distribution of fossils, but
it is not always clear which is more important. The preservation of
late Eocene vertebrates at the UNESCO World Heritage site of Wadi
Al-Hitan, Western Desert of Egypt, is controlled primarily by the
physical processes responsible for sequence stratigraphic architecture
on a siliciclastic shelf. Three types of stratigraphic surface, each char-
acterized by a taxonomically and taphonomically distinct fossil as-
semblage, yield most of the known vertebrate fossils. Complete, par-
tially articulated whale skeletons, primarily Basilosaurus isis, are
abundant in offshore marine flooding surfaces (MFS) in the late
transgressive systems tract (TST) of the first Priabonian sequence
(TA4.1), where low net sedimentation rates and environmental av-
eraging in offshore environments promoted the accumulation of car-
casses on traceable stratigraphic surfaces. Complete, well-articulated
whales, primarily Dorudon atrox, are more widely scattered on minor
erosion surfaces in rapidly accumulating shoreface sediments of the
overlying falling stage systems tract. Fragmented and abraded ver-
tebrate remains are abundant and diverse in a discontinuous con-
glomerate that marks the first sequence boundary above the base of
the Priabonian (Pr-2), which has not been previously recognized in
Egypt, but which formed incised valleys with at least 45 m of total
relief. Fossils in this variably thick lag conglomerate include skeletal
elements reworked by rivers from underlying marine deposits and
bones of terrestrial animals living in the fluvial environment. Mar-
ginal marine vertebrates, primarily dugongs, occur on shelly marine
ravinement surfaces above Pr-2, in the early TST of the second Pria-
bonian sequence. Most vertebrate remains in Wadi Al-Hitan occur
in condensed stratigraphic intervals and taxonomic composition
changes with sequence position, both important considerations in in-
terpretation of paleobiological patterns.
INTRODUCTION
Sedimentation and fossil preservation are both influenced by the balance
between sediment supply and the formation and destruction of sediment
accommodation. For this reason, the processes that control sequence
stratigraphic architecture often determine apparent timings of biological
origination and extinction in the fossil record (Holland, 1995; Brett, 1998;
Holland and Patzkowsky, 1999), as well as the abundance, distribution,
composition, and taphonomy of fossils within individual stratigraphic sec-
tions and sequences. Although a number of field studies and conceptual
literature reviews have examined the relationship between sedimentary
processes and the preservation and character of shelly marine macroin-
vertebrates (e.g., Kidwell, 1985, 1991, 1997; Banerjee and Kidwell, 1991;
Rogers 1994; Brett, 1995; Courville and Collin, 2002; Scarponi and Ko-
walewski, 2004; Crampton et al., 2006), few studies have explored marine
* Corresponding author.
vertebrate preservation in the context of sequence stratigraphy (Rogers
and Kidwell, 2000). The extent to which processes that govern macro-
invertebrate preservation can be extrapolated to vertebrates is uncertain
because shelly macroinvertebrates differ substantially from marine ver-
tebrates in chemical composition, size, and skeletal durability, as well as
in life history modes and standing population sizes.
Here we document sequence stratigraphic architecture and the preser-
vation of marine and terrestrial vertebrates in Priabonian (late Eocene)
siliciclastic deposits at Wadi Al-Hitan, Valley of the Whales, in the West-
ern Desert of Egypt (Fig. 1). Archaeocete whales and other fossil verte-
brates were first documented from Wadi Al-Hitan more than a century
ago (Beadnell, 1905), but the full extent and importance of this site has
only recently come to light. An ongoing program of mapping and study
initiated in 1983 (Gingerich, 1992) has documented more than 1,400
complete or partial vertebrate skeletons from an area of
⬃
200 km
2
.
Whale fossils found in Wadi Al-Hitan, including Basilosaurus isis,Do-
rudon atrox, and other recently described and related taxa (e.g., Ginger-
ich, 2007), are fully aquatic but retain external hind limbs, providing
anatomical evidence that helps to chronicle a profound land-to-sea evo-
lutionary transition (Gingerich et al., 1990; Uhen, 2004). Wadi Al-Hitan
was designated a United Nations Educational, Scientific and Cultural Or-
ganization (UNESCO) World Heritage site in 2005 in recognition of its
paleontological importance and natural desert beauty.
Ongoing GPS mapping of traceable stratigraphic beds and fossil sites
in Wadi Al-Hitan demonstrates that vertebrate remains are most abundant
in discrete stratigraphic intervals (Gingerich, 1992). Several questions
arise from this observation. Is the prevalence of vertebrate remains in
certain stratigraphic horizons indicative of such biological events as mass
mortality driven by environmental perturbation (e.g., Hogler, 1992), or is
vertebrate fossil preservation controlled primarily by such physical fac-
tors as sedimentological processes that might also be reflected in sequence
architecture? If under physical control, what sedimentary processes are
most important in determining the distribution and preservation of ver-
tebrate fossils in Wadi Al-Hitan, and how do these processes compare to
those that have been shown to influence the fossil record of shelly marine
macroinvertebrates?
GEOLOGIC CONTEXT
The principal synthesis of Eocene stratigraphy and paleogeography in
northern Egypt is provided by Salem (1976), who recognized that
Ypresian-middle Bartonian strata are dominated by nummulite-bearing
carbonates deposited on a complicated and irregular structural topography
generated primarily during Late Cretaceous tectonic shortening (Guiraud
and Bosworth, 1999). The overlying late Bartonian and Priabonian strata
consist primarily of fine-grained siliciclastics deposited at the edge of a
northwestward-prograding shoreline. Continental, fluvio-lacustrine strata
of Oligocene age are exposed west of the study area. Thus, the Eocene-
Oligocene interval in the western desert of Egypt records an overall re-
gressive sedimentary succession, from a time of maximum transgression
PALAIOS 291
EOCENE SEQUENCE STRATIGRAPHY
FIGURE 1—Late Eocene paleogeography of northern Egypt and map of study area.
A) Paleogeography and paleobathymetry of northern Egypt in the late Bartonian–
Priabonian (from Salem, 1976). Red box shows Wadi Al-Hitan World Heritage Site;
contours show 50-m isopachs on late Bartonian and Priabonian strata; asterisk
⫽
location of cores used by Salem (1976) to reconstruct paleogeography. B) Detail of
the study area in Wadi Al-Hitan, with measured current directions (blue arrows),
fossil vertebrate locations (dots), and stratigraphic boundaries (inset) shown. SB
⫽
Pr-2 sequence boundary; IVF
⫽
incised valley fill; red line
⫽
widely traceable
Carolia bed marking traditional boundary between Birket Qarun (B.Q.) and Qasr el-
Sagha (Q.S.) Formations; dotted lines indicate section traces A–D.
in the middle Eocene to maximum regression in the early Oligocene. This
history of relative sea level in Egypt is consistent with published esti-
mates of global second-order (10
7
years) eustatic sea level (Haq et al.,
1987; Miller et al., 2005; Mu¨ller et al., 2008), suggesting that there may
be a strong eustatic component in the second-order Eocene–Oligocene sed-
imentary record of northern Egypt. Glacio-eustatically driven changes in
sea level on both Milankovitch-band and longer wavelengths are expected
during this time because the Eocene witnessed the initial buildup of con-
tinental Antarctic ice and a corresponding transition from a hothouse to
an icehouse world (Katz et al., 2008; Lear et al., 2008).
Priabonian strata in Wadi Al-Hitan are composed of very fine grained
sands, silts, muds, and sandy skeletal coquinas traditionally assigned to
the lithostratigraphic Gehannam, Birket Qarun, and Qasr el-Sagha for-
mations (Gingerich, 1992). Shell beds, consisting primarily of oysters,
gastropods (turritellids), the extinct anomiid bivalve Carolia, and large
benthic foraminifera (nummulitids), form prominent, carbonate-cemented,
sandy coquina ledges that cap some coarsening-upward mudstone-sandstone
and very fine grained sandstone packages. Most of the sands in Wadi Al-
Hitan are heavily bioturbated, most conspicuously by Thalassinoides.
These burrows, along with a wide variety of other trace fossils, obscure
nearly all of the sedimentary structures in most of the marine deposits in
Wadi Al-Hitan; however, ichnology provides important information about
paleoenvironments and sedimentary processes (e.g., McIlroy, 2004).
Environmentally, Priabonian strata in the study area represent laterally
heterogeneous, offshore shelf to shoreface environments. Water depths
ranged from below maximum storm wave base (SWB) to above normal
wave base (NWB), and environments ranged from offshore shelves to
protected estuaries and embayments, as well as to continental settings.
Lateral facies associations, paleocurrent directions within associated
fluvial-tidal deposits (Fig. 1B; see below), and borehole data compiled
for the whole of northeastern Egypt (Salem, 1976) show that Priabonian
shorelines were located near and to the south and east of Wadi Al-Hitan
and that the broader region occupied a protected shelf with numerous,
smaller embayments (Fig. 1A). An offshore island is thought to have
been located
⬃
50 km to the north of the study area (Fig. 1A; Salem,
1976), which would have contributed to the sheltered, semi-enclosed, and
protected character of the Wadi Al-Hitan shelf.
LITHOFACIES, SEQUENCE STRATIGRAPHY, AND
VERTEBRATE TAPHONOMY
Haq et al. (1987) recognized three, third-order (10
6
years) marine se-
quences in the Priabonian—TA4.1 through TA4.3—defined by four se-
quence boundaries, Pr-1 through Pr-4 (Fig. 2; Haq et al., 1987; Hardenbol
et al., 1998). We find that this same third-order architecture is closely
reproduced in the Priabonian of Wadi Al-Hitan, corroborating the eustatic
interpretation of Haq et al. (1987). All four sequence boundaries are pres-
ent at Wadi Al-Hitan, but here we focus on the interval around the second
sequence boundary (SB), Pr-2, which separates early Priabonian sequence
TA4.1 from middle Priabonian sequence TA4.2 (Fig. 2). Age assignment
is based on overall sequence stratigraphy and, more importantly, on cal-
careous nannofossil biostratigraphy indicative of an NP18 age (see later
discussion). Here we focus on sequence stratigraphy and vertebrate pres-
ervation in early Priabonian sequence TA4.1 for several reasons: (1) this
sequence is exposed at the World Heritage site, (2) a prominent sea-level
fall responsible for generating a stratigraphically important surface of
subaerial erosion and large-scale fluvial incision (Pr-2; see below) has
not previously been recognized in Egypt, and (3) this part of the succes-
sion has yielded all of the whale fossils that have been described so far
from this World Heritage site. It should be noted, however, that whale
fossils are known from underlying carbonate-rich Bartonian strata and
that we have also observed whale remains in the younger TA4.2 se-
quence.
Figure 3 summarizes the measured stratigraphic-section data and our
sequence stratigraphic interpretations for the Priabonian strata in Wadi
Al-Hitan. Figure 4 provides an outcrop panorama with major stratigraphic
features and two measured section traces identified. Each of the major
lithofacies, their associated vertebrate remains, and our environmental and
sequence stratigraphic interpretations are described below in ascending
stratigraphic order.
Mudstone-Sandstone Parasequences of the Late Transgressive
Systems Tract (TST)
The base of the measured interval is within the Birket Qarun Formation
(Gingerich, 1992) and consists of three,
⬃
3–5-m-thick mudstone-sandstone
packages that coarsen upward from clean, light purplish-gray mudstone
with rare body fossils to silty or sandy, brownish, bioturbated mudstone
with small nuculid bivalves and gastropods. These silty mudstones coars-
en upward and are capped by ledge-forming, tan, burrowed, very fine
grained calcareous sandstones that are traceable continuously over the
292 PALAIOS
PETERS ET AL.
FIGURE 2—Late Eocene–Oligocene timescale, inferred cycles in eustatic sea level,
and third-order sequence boundaries (modified from Hardenbol et al., 1998). Studied
interval shown (approximately) by hatched area. CN
⫽
calcareous nannoplankton
zones; PF
⫽
planktonic foraminfera zones; SB
⫽
sequence boundary.
entire study area (Fig. 1B) and over the greater Wadi Al-Hitan region
(
⬃
225 km
2
). Bedding and sedimentary structures in the capping sand-
stones are obscured by bioturbation and, more problematically, disrupted
by late diagenetic gypsum; however, there is evidence for hummocky
cross stratification (HCS) and wave ripples at some locations. The sand-
stone ledges have sharp upper contacts with the light purplish-gray mud-
stones that constitute the base of each cycle (Fig. 3).
Invertebrate fossils are not prominent anywhere throughout this part of
the measured section, but a low-diversity assemblage of small (
⬃
1 cm)
nuculid bivalves and gastropods is present in the yellowish-tan, calcare-
ous, silty mudstones of each cycle. Calcareous nannoplankton indicative
of an NP18 age, including Discoaster tani nodifer and Chiasmolithus
oamaruensis, are abundant and well preserved in calcareous silty mud-
stones, indicating that the shelf environment was well connected to the
open marine system.
Vertebrate fossils in this part of the section consist primarily of Basi-
losaurus, although a few specimens of Dorudon have also been recovered
(Table 1). Nearly all of the skeletons are found on or within the tops of
the coarsening-upward mudstone-sandstone packages. It is difficult to de-
termine the exact position of large whale skeletons located in weathered
outcrops—individual vertebrae are up to 35 cm long (Fig. 5)—because
the very fine grained sandstones that cap each package are thin (
⬍
60
cm). Nevertheless, mapping shows that most of the whale remains are
located within 0.5 m of the sharp sandstone-mudstone contact that defines
each package boundary. One skeleton has, however, been found in muddy
deposits just below the capping sandstone beds, and this specimen ap-
pears to be better preserved than most of the whales in this interval.
Although whale skeletons are concentrated at the top in each of the three
sandstone packages, none of the sandstones can be considered a bone bed
because the bones are rather widely scattered. Only 32 vertebrate fossil
sites were encountered during the mapping of
⬃
3.6 linear km of nearly
continuous exposure of all three packages (Table 1).
Many of the whale skeletons in this part of the section are complete
or nearly so (Fig. 5A–C), but skeletons within the same sandstone bed
vary in the extent to which they are articulated (i.e., bones located in
anatomically correct positions). Most complete specimens clearly pre-
serve the anterior-posterior (AP) axis and the relative positions of major
skeletal elements along this axis, even though the AP-axis itself is often
curved into a semicircular shape (Fig. 5D). Small bones and bones that
are easily transported (e.g., ribs and scapulae) tend to be more widely
dispersed about complete skeletons than large or heavy bones such as
vertebrae (Fig. 5B). Figure 5A–C shows typical states of preservation for
skeletons in this interval. Individual, isolated whale bones have been
observed within the sandstone capping beds, but the majority of remains
appear to derive from complete or nearly complete skeletons that were
disarticulated on the sea floor prior to final burial.
Interpretation.—We interpret this part of the section to represent three
shallowing-upward parasequences that record an overall minor deepening
during the latest TST of sequence TA4.1 (Fig. 2). Evidence for continued
deepening hinges primarily on the thickness and character of the over-
lying lithofacies (see below), and, given the similarity of these parase-
quences to each other and to the overlying deposits, it is possible that
this part of the section is within the highstand systems tract (HST). Lack
of rock exposure in the study area prevented the reliable downward ex-
tension of the section, but regional exposures suggest that the base of the
measured sections may be within
⬃
20 m of the argillaceous carbonates
(marls) and nummulitic limestones of Bartonian sequence TA3.6 (Fig. 2).
Water depths in this part of the section are inferred to have ranged from
below maximum SWB (mudstone at the base of parasequences) to above
average SWB (very fine-grained sandstone at parasequence tops).
The very fine sandstone that caps each parasequence represents amal-
gamated and winnowed storm deposits that received their final overprint
during marine flooding and sediment starvation. Sedimentologic evidence
for sediment starvation includes (1) a sharp mudstone-sandstone contact
that marks a lithological discontinuity in otherwise rather uniformly
coarsening-upward mudstone-sandstone packages; (2) the amalgamation
of storm beds in the capping sandstones; and (3) intense bioturbation of
capping sandstones that stand in contrast to poorly bioturbated or undis-
turbed overlying mudstone.
Whale skeletons occur with higher frequencies on the marine flooding
surface (MFS) in this part of the section as a result of environmental and
temporal condensation during periods of lower net rates of sediment ac-
cumulation. Taphonomy and patterns of bone dispersal indicate that storm
currents may have played an important role in disarticulating complete
whale skeletons and dispersing bones on the sea floor, although scaveng-
ing by sharks and other whales is also likely to have been an important
disarticulation and dispersal mechanism. We have not yet observed any
direct evidence for the scavenging of whale carcasses. Whale skeletons
on the MFS vary in their degree of disarticulation, and this may reflect
differential arrival times of carcasses to the sea floor and resultant variation
in exposure times, which is consistent with sedimentological evidence for
low net rates of sediment accumulation. It is somewhat surprising that
no direct evidence for colonization of whale bones by epibenthic organ-
isms has been observed (e.g., Allison et al., 1991), although an exhaus-
tive, targeted search for such remains has not yet been conducted. The
general paucity of macroinvertebrates in this part of the section does not
make the absence of colonizing macroinvertebrates particularly unusual
when observing whale remains in the field.
Thick Mudstone-Sandstone Parasequence of HST
The upper sandstone ledge of the late TST, described above (Fig. 3)
contains disarticulated whale remains and has a sharp contact with a thick,
overlying purplish-gray mudstone that is mostly devoid of body and trace
PALAIOS 293
EOCENE SEQUENCE STRATIGRAPHY
FIGURE 3—Measured stratigraphic sections, stratigraphic position of vertebrate remains, and sequence stratigraphic interpretation of study area. Location of section traces
A–D is shown in Fig. 1B. Section trace E is located 1.2 km north of area shown in 1B. Note that lenticular bedforms indicated within sections contained by the shaded
area (labeled IVF) represent clinoforms. Inclined cobbles indicate weak imbrication. SB
⫽
sequence boundary Pr-2; HST
⫽
highstand systems tract; MMFS
⫽
maximum
marine flooding surface; MFS
⫽
marine flooding surface; TS
⫽
transgressive surface; TST
⫽
transgressive systems tract; FFST
⫽
falling stage systems tract; IVF
⫽
incised valley fill.
FIGURE 4—Outcrop panorama of Birket Qarun and Qasr el-Sagha formations. A) Original image mosaic. B) Image mosaic with important stratigraphic features labeled;
section traces for Section B and Section C (Fig. 3) are shown by white dashed lines and location of Figure 6 indicated by box. Abbreviations as in Fig. 3.
294 PALAIOS
PETERS ET AL.
TABLE 1—Vertebrate taphonomy and abundance in relation to stratigraphic position. Each value shows the number of mapped fossil sites. Average taphonomic condition of
the vertebrate fossils at each of these sites is summarized by the row titled typical preservation. Average number of sites per linear distance surveyed is given by sites per km,
which provides an indication of the approximate absolute abundance and relative abundance of vertebrates in each stratigraphic position. Here, a vertebrate site (specimen)
constitutes an occurrence of taxonomically identifiable vertebrate remains, excluding shark and ray teeth.
Sequence TA4.1
L-TST PS† (3.6 km)# FSST (4.6 km)#
Sequence TA4.2
SB Pr-2 (1.6 km)# E-TST PS§ (1.2 km)#
Discontinuity marine flooding surface: off-
shore mudstone on offshore-
transition zone sandstone
(n
⫽
3)
marine erosion surface: shore-
face sandstone on shoreface
sandstone (n
⫽
15)
basal scour: fluvial channel on
shoreface to offshore sand-
stone and mudstones (n
⫽
1)
marine flooding surface: shore-
face sandstone on estuary
mudstone or shoreface sand-
stone (n
⫽
2)
Fossils*
Barytherium -- 3-
Basilosaurus 29 2 3 -
Crocodilian - - 2 -
Dorudon 3 4 11 -
Dugong - - 6 3
Sawfish - - 2 1
Sea Snake - - 4 -
Turtle - - 2 -
Invertebrates rare to absent common to abundant rare abundant
Vertebrate taphonomy
Sites per km 8.9 1.3 20.6 3.3
Typical Complete Complete Incomplete Variable
preservation disarticulated unfragmented articulated unfragmented isolated fragmented disarticulated variable
* Number of mapped sites (specimen); shark and ray teeth are abundant in SB Pr-2, some FSST PS, and the E-TST but were not counted; macroinvertebrates were not mapped
so abundances are qualitative.
† Late TST located below Pr-2, including maximum MFS (Fig. 3).
§ Early TST above Pr-2, including ravinement surface (Fig. 3).
# Approximate linear outcrop distance traversed to map vertebrate fossil sites.
fossils. This thick, muddy interval, also assignable to the lithostratigraph-
ic Birket Qarun Formation (Gingerich, 1992), is lithologically similar to
the thinner, muddy intervals that form the base of each parasequence in
the underlying TST. Like the underlying parasequences, this muddy in-
terval coarsens upward, becoming noticeably silty at
⬃
5 m above the
base. Scattered, reddish brown, sideritic nodules are present throughout
the lower two-thirds of the interval and a low-diversity assemblage of
small nuculid bivalves and gastropods, similar to those found in the un-
derlying calcareous mudstones of the late TST, occurs beginning several
meters above the basal mudstone-sandstone contact.
The most important sedimentological features in this thick, dark, mud-
dy interval are thin (1–30 cm) interbeds of poorly cemented, very fine
grained, white sandstone (Fig. 6A). These prominent sandstone beds,
most of which vary in thickness laterally, have sharp upper and lower
contacts, are absent in the lower two-thirds of the unit, and become thick-
er and more closely spaced up-section. The majority of the sandstone
beds exhibits well-preserved and unambiguous hummocky cross stratifi-
cation (HCS; Fig. 6B) or wave ripples. Gutter casts filled by laminated,
very fine grained sandstone are also present at the base of some of the
HCS sandstone beds or occur as isolated sandy stringers. A minority of
the sandy interbeds lack sedimentary structures because they have been
completely homogenized by bioturbation with the surrounding mudstone,
primarily by Thalassinoides.
Outcrop characteristics produced by the overlying thick mudstone typ-
ically include a broad, flat contact between the mudstone and underlying
sandstone ledge of the TST that bears whale remains (Fig. 3). This broad
surface has favored the accumulation of modern, massive caliche rinds
and desert pavements, thereby making reliable estimates of vertebrate
abundance difficult. No vertebrate remains have been recovered from the
thick mudstone itself, although some weathered bones with concretionary
rinds have been found in float. Some of these remains may derive from
the scattered concretions that occur in the lower half of the mudstone.
Interpretation.—The base of the thick, coarsening-upward, widely
traceable mudstone that overlies the last parasequence of the late TST is
interpreted to represent the time of maximum sediment accommodation
and an expanded parasequence within the HST of sequence TA4.1 (Fig.
3). The maximum MFS is interpreted to occur just above the last sandy
parasequence of the underlying TST, but there is little evidence that this
surface experienced significantly more sediment starvation or environ-
mental condensation than the three flooding surfaces in the underlying
TST. Instead, a slowing rate of sea-level rise and the release of abundant
sediment from shoreface sediment traps appear to have resulted in the
accumulation of a thick parasequence or parasequence set on the maxi-
mum MFS.
The HST mudstone was initially deposited below maximum SWB in
hypoxic bottom waters, as evidenced by relatively clean, purplish mud-
stone that lacks body fossils or prominent bioturbation. During the de-
position of this mud, the sea floor was located below the wind-mixed
surface layer and bottom circulation may have been restricted by a pyc-
nocline. Water depth appears to decrease uniformly up-section, with
storm-deposited, very fine grained sandstone interbeds providing a clear
indication of an overall shallowing from below to above maximum SWB.
It is also possible that sandy interbeds interpreted here to represent single
storm events (Fig. 6B) might also cap parasequence-scale, shallowing-
upward cycles that have relatively cr yptic signatures within the surround-
ing mudstones. A minority of the storms responsible for the interbedded
HCS sands appears to have resulted in the temporary oxygenation of
offshore bottom waters and the transient colonization of the sea floor by
crustaceans that subsequently bioturbated the sandy storm beds, possibly
as doomed pioneers (sensu Grimm and Foellmi, 1994). Net sediment
accumulation rates were high enough and water depths great enough to
prevent the amalgamation of most storm deposits.
Whale remains are preserved on the maximum MFS, but there is a
paucity of vertebrate fossils in the HST muddy interval. This is attributed
to a large increase in average rates of sediment accumulation (i.e., to a
dilution effect) and to the lack of sedimentologically well-defined MFS
that would have resulted in a greater degree of temporal and spatial av-
eraging. It is also possible that whales were less frequent in the most
PALAIOS 295
EOCENE SEQUENCE STRATIGRAPHY
FIGURE 5—In situ whale remains in Birket Qarun Formation. A) Basilosaurus vertebral column within sandy parasequence top of TST; scale nearly the same as in B. B)
As in A, but showing semi-articulated rib fragments; skeleton abraded by wind-blown sands; scale bar shows 10-cm increments.C) Map view of complete Basilosaurus
from a parasequence top in TST. D) Complete and articulated Dorudon from within the ver y fine grained sandstone of FSST. Many of the articulated Dorudon skeletons in
Wadi Al-Hitan have a strongly curved anterior-posterior (AP) axis; this specimen has its head juxtaposed against the posterior thorax.
FIGURE 6—Contact between HST mudstone and FSST very fine grained sandstone in Birket Qarun Formation. A) Contact between upper part of purplish-gray silty
mudstone and FSST yellowish-tan sandstone; SB Pr-2 and overlying IVF are visible near the top of the photograph. B) Large convex-up hummock of very fine sand within
the top of the HST mudstone; see Fig. 4 for photo location.
296 PALAIOS
PETERS ET AL.
FIGURE 7—Outcrop view of FSST and underlying slopes of HST mudstone in
Birket Qarun Formation. Note laterally continuous ledges in FSST sandstone. Break
in slope near top of cliff reflects increasing mud content up-section and is within
the Qasr el-Sagha Formation.
offshore environments preserved in Wadi Al-Hitan and that the flux of
whale carcasses to the sea floor was much lower in these offshore envi-
ronments than it was in the shallower water environments represented by
the rest of the section.
Sandy Cycles of the Falling Stage Systems Tract (FSST)
The coarsening-upward HST mudstone has a very sharp and widely
traceable contact with overlying yellowish-tan, burrow-homogenized,
very fine grained sandstone. The contact between the mudstone and the
sandstone is riddled with prominent sand-filled Thalassinoides that extend
up to 50 cm into the underlying silty HST mudstone. A complete suc-
cession of this part of the TA4.1 sequence is only preserved at section
A (Fig. 3), though portions of the sandstone occurring elsewhere are
nearly identical. In the study area, the sandstone consists of
⬃
3–10-m-
thick cycles of homogenous, very fine grained, yellowish-tan sandstone
(Fig. 7) with variable amounts of invertebrate shells and shell fragments.
Cycle boundaries in the basal 20 m are defined primarily by changes in
the style and increases in the intensity of bioturbation and, in the upper
20 m, by both bioturbation and an increase in the abundance of inver-
tebrate fossils and calcite cementation. The latter is due to the dissolution
of aragonitic shells and their reprecipitation as porosity-filling calcite. No
physical sedimentary structures have been observed in the sandstone due
to pervasive bioturbation.
Invertebrate diversity and abundance increase up-section. A prominent,
densely packed bed of the anomiid bivalve Carolia, marking the tradi-
tional boundary between the Birket Qarun and Qasr el-Sagha formations,
occurs near the top of the interval (Fig. 1B; Fig. 3, section A and E). In
the upper quarter of section A, within the Qasr el-Sagha Formation,
greenish-gray and buff-colored mudstone is interbedded with densely
packed sandy coquinas consisting primarily of nummulitid foraminifera,
turritellid gastropods, and oysters, though individual beds tend to be dom-
inated by one taxon. Some of the muddy and sandy packages near the
top of this interval contain localized pycnodontid oyster clusters with
articulated, extraordinarily thick-shelled individuals up to 18 cm wide.
Large Thalassinoides are present throughout the entire interval but are
prominent at the basal mudstone-sandstone contact and at cycle bases,
where sand-filled and shell-fragment–filled burrows penetrate into under-
lying strata.
Vertebrate remains are relatively rare in this part of the section (Table
1). The few whale fossils that have been found occur in the yellowish,
very fine grained sandstone of the Birket Qarun Formation, and these
skeletons are typically complete, articulated, and very well preserved
(Fig. 5D). Dorudon is the most commonly recovered whale in the sand,
though Basiliosaurus skeletons have also been found (Table 1). A locality
near section A, in FST sands of the Birket Qarun Formation, yielded a
complete and articulated Dorudon atrox specimen that provided casts
now on display in many museums around the world. With few exceptions,
vertebrate skeletons are located at the tops of burrow- and shell-defined
cycles that form subtle ledges on outcrop (Fig. 7). One Dorudon skeleton
found within the middle of a sandy cycle is exceptionally pristine, with
only minor displacement of loosely articulated bones (Fig. 5D). Only
dugongs have been found in the greenish-gray mudstones and sandy skel-
etal coquinas of the Qasr el-Sagha Formation, in the upper part of this
interval (Fig. 3).
Interpretation.—The sharp contact between the silty mud of the HST
and the overlying very fine grained sandstone (Fig. 6–7) is interpreted to
represent a regressive surface of marine erosion followed by a rapid influx
of very fine sand during the initiation of a fall in sea level. The resultant
decrease in accommodation prompted the rapid offshore export of very
fine grained sandstone and the emplacement of a thick, sandy FSST above
offshore HST silty mudstones. Sedimentation rates were comparatively
high throughout most of the FSST, particularly in the lower half of the
deposit, but shallowing resulted in the oxygenation of bottom waters and
the colonization of the sea floor by benthic animals, particularly biotur-
bators. Thalassinoides occurring at the HST-FSST contact may reflect
marine erosion of water-saturated mud and the establishment of a firm-
ground on offshore-transition zone mud. Alternatively, the burrows could
have originated from a surface within the sands above the mud-sand
contact and only penetrate downwards into the mud.
The thickness of the FSST relative to the HST in the study area (Fig.
3) might seem unusual, and in up-dip directions the FSST is expected to
be absent. Regional mapping, however, shows that the study area is lo-
cated just offshore from the slope break formed by the progradational
HST clinoform (results not presented in this report). The study area is,
therefore, optimally positioned along the shelf to preserve a thick FSST
and a comparatively thin HST. Up dip, the FSST amalgamates with a
composite surface that marks a SB and the ravinement surface (RS) of
the overlying sequence. The FSST in the study area is similar in thickness
and in character to FSSTs from the Cretaceous Interior that abruptly jux-
tapose middle-shoreface sandstones on transition-zone silty mudstones
(e.g., Ferron Sandstone; Edwards et al., 2005).
Cycle boundaries within the FSST reflect minor temporal and spatial
condensation, probably as a result of forced regression, but this minor
condensation was sufficient to permit accumulation of a greater number
of invertebrate shells and vertebrate remains than is typically found within
the surrounding sediment. Further higher-resolution analysis is required
to ascertain the specific environmental signature of the cycle boundaries.
The high quality of preservation of vertebrate remains throughout the
FSST is attributed to relatively high sedimentation rates overall. Rarity
of vertebrate remains in general (Table 1) is attributed to a shift in en-
vironment up-section from an offshore shelf to a shallow, protected em-
bayment and to higher rates of sedimentation that resulted in the dilution
of skeletal input. The presence of rare but exceptionally well-preserved
skeletons within cycles, rather than at cycle tops, provides additional
evidence for rapid sediment accumulation and minor condensation along
cycle boundaries.
The transition from sand-dominated cycles to mixed mud-sand cycles
is interpreted to record the rapid infilling of the basin combined with
continued sea-level fall. This sea level fall resulted in ponding, or the
formation of a broad, protected shallow embayment with no analogue
lower in the section. Continued sea-level fall reduced this remnant seaway
further and eventually drained it completely, resulting in subaerial ex-
posure and erosion or nondeposition (discussed in the following section).
Falling sea level, thus, did not simply result in the seaward migration of
a static shoreface, but it resulted in the development of a qualitatively
PALAIOS 297
EOCENE SEQUENCE STRATIGRAPHY
FIGURE 8—SB Pr-2 and IVF in Birket Qarun Formation. A) Cutbank developed on erosional sequence boundary; massive lunate exposure on left, with Gingerich leaning
against it, is an erosional remnant of very fine grained sandstone from the FSST; on the right, thinly bedded, ripple-laminated very fine grained sandstone and mudstone
abuts concave erosional surface. A large slump block of ripple-laminated sandstone and mudstone has foundered from above the erosional surface and toppled into a scour
fill formed within the IVF. B) Imbricated pebble conglomerate developed on the SB. C) Molar tooth and jaw fragment of land mammal Barytherium in SB conglomerate.
D) Sirenian rib fragment in thin SB conglomerate.
different shelf geomorphology from that characterizing the underlying
TST and progradational HST.
Fluvial Conglomerate on SB Pr-2 (LST)
The most important stratigraphic feature in Wadi Al-Hitan is a discon-
tinuous, variably thick (0–70 cm) conglomerate consisting of pebbles and
cobbles of platy, limonite-goethite–cemented, very fine grained sandstone
(Fig. 8). Where the conglomerate is thick, the platy sandstone cobbles
are typically imbricated and indicate current flow to the west-northwest
(Fig. 8B). The contact between the conglomerate and the underlying strata
is always very sharp and exhibits considerable relief that truncates hori-
zontal bedding in underlying marine strata on many different scales (Fig.
3, 8A). All of the beds below the conglomerate are laterally continuous
in the study area, except where they are interrupted by the conglomerate-
bearing surface. For example, the prominent Carolia bed, marking the
traditional boundary between the lithostratigraphic Birket Qarun and Qasr
el-Sagha formations, occurs at the same height above the MFS in sections
A and E but is absent in sections B–D where the conglomerate occurs
lower in the section. In some places (e.g., near section C), the sandstones
of the thick FSST are completely absent, but sand-filled Thalassinoides
characteristic of the HST-FSST contact remain in the few centimeters
beneath the conglomerate, thereby providing conclusive evidence that the
FSST sandstone previously existed below the conglomerate but was re-
moved by erosion.
Continuous physical tracing and measured sections (Fig. 3) demon-
strate that the conglomerate surface has more than 45 m of vertical relief
over the entire study area and up to3mofrelief at the scale of individual
exposures. For example, Figure 8A shows a vertically oriented contact
between the conglomerate surface and flat-lying FSST sands. This contact
can be traced continuously over
⬃
10 m into a sharp horizontal contact
with a thick, imbricated pebble conglomerate bed (Fig. 8B). Such topo-
graphically irregular contacts (Fig. 8A) are common on this surface,
which is often characterized by a discontinuous iron oxide rind.
Vertebrate remains are abundant and highly diverse in the conglomerate
(Table 1, Fig. 1), but they consist of isolated bones and teeth, most of
which are fragmented and abraded to various degrees (Fig. 8D). Dorudon
vertebrae are the most common whale remains, but isolated bones of
Basilosaurus, dugongs, crocodilians, turtles, and a variety of other ver-
tebrates have also been recovered (Table 1). Shark and ray teeth are also
very abundant in this conglomerate. Most notable among the fossils found
in the conglomerate are several specimens of the semi-aquatic, but en-
tirely continental, proboscidians Moeritherium and Barytherium. Remains
of Barytherium, which have not been found at any other stratigraphic
level in Wadi Al-Hitan, include a tooth and jaw fragment (Fig. 8C), as
298 PALAIOS
PETERS ET AL.
FIGURE 9—IVF and transgressive RS. A) Outcrop view showing large clinoforms (bedding along inclined dashed lines) truncated by RS; the first parasequence of the
TST in sequence TA4.2 (Fig. 2) is within the sandy ledge above the IVF; the second parasequence is thicker and has greenish-gray mudstone at its base. B) Down-dip
slump folds on a clinoform within the IVF. C) Asymmetric, out of phase linguloid ripples in an unusually well cemented, fine-grained sandstone within the IVF; beds were
measured for current directions in Fig. 1B, D) Climbing, flaser-bedded ripples in IVF.
well as a partially articulated lower hind limb. The partial Barytherium
limb is the only semi-articulated fossil recovered from the conglomerate.
Interpretation.—We interpret the thin, discontinuous, imbricated con-
glomerate and associated vertebrate remains to indicate fluvial incision
and physical reworking of FSST and HST marine sediments during a fall
in sea level that resulted in subaerial exposure. The resultant surface of
fluvial erosion constitutes a SB interpreted to be correlative to Pr-2 (Fig.
2) of Haq et al. (1987), based on calcareous nannoplankton biostratig-
raphy and large-scale sequence architecture in the Wadi Al-Hitan region.
The irregular paleotopography developed on the SB, including clearly
defined erosion surfaces within FSST marine sandstones (Fig. 8A), in-
dicates exhumation of underlying marine deposits and the development
of an incised river system that indicates a minimum base level fall of 45
m. The conglomerate and underlying SB can be continuously traced into
an interfluve in section A. Although we would expect the development
of paleosols on the interfluve, the SB in section A is cryptic because all
evidence of subaerial exposure has been scavenged during subsequent
marine transgression (explained later).
Most of the vertebrate remains in the conglomerate were eroded from
marine deposits and transported short distances as sedimentary clasts in
the fluvial system. The marine remains are, thus, allochthonous and rep-
resent reworked and eroded fossils left behind as erosional lags at the
base of fluvial channels. Continental vertebrate remains derive directly
from animals that lived in and adjacent to the fluvial system and were,
therefore, subject to less taphonomic and diagenetic modification than
marine remains. Platy, iron-cemented sandstone cobbles were similarly
derived from the exhumation and reworking of Thalassinoides, which
tend to be better cemented by iron oxides than the surrounding FSST
sandstones. The iron cementation prevalent in the sandstone clasts, and
locally on the SB itself, may indicate subaerial exposure and meteoric
diagenesis during subaerial exposure.
Flaser-Bedded and Rippled Sand of Incised Valley Fill (IVF) of
Initial TST
A succession of flaser-bedded, cross-laminated, very fine grained to
fine-grained, white, poorly cemented sandstone with thin, greenish-gray
mud interbeds (Fig. 9) overlies the imbricated pebble conglomerate wher-
ever it is found. This flaser-bedded interval is lens-shaped in large-scale
geometry (Fig. 3). It is, however, remarkably homogeneous lithologically.
Ripples within the flaser-bedded sand consist primarily of asymmetric,
out-of-phase, linguoid sand waves (Fig. 9C) that are often climbing and
that typically have thin laminae consisting of alternating mud-sand cou-
plets (Fig. 9B). Measurement of the mean transport direction on 22 well-
exposed bedding planes indicates a consistent west-northwest direction
across the study area (Fig. 1B). This transport vector is similar to the
PALAIOS 299
EOCENE SEQUENCE STRATIGRAPHY
flow direction indicated by imbricated cobbles in the underlying Pr-2
conglomerate.
Large-scale clinoforms more than 8-m high are clearly visible within
the flaser-bedded interval (Fig. 4, 9A). The beds within these clinoforms
consist of finely ripple-laminated and flaser-bedded sand that indicate
similar transport vectors regardless of dip direction, which is variable,
but generally west of north-south. Decimeter-scale, ripple-laminated ho-
rizons within the clinoforms show evidence of down-dip, bedding-parallel
slippage and soft sediment deformation (Fig. 9B), and growth faults have
been observed. Broad (5–10-m-wide), low-angle scours are common
within the flaser-bedded sandstone and are filled with either tabular cross-
bedded, very fine grained, white sandstone or flaser-bedded sandstone
similar to those found throughout the rest of the interval. The subaqueous
erosive channels responsible for the broad scour-and-fill structures re-
worked the flaser-bedded sand, as evidenced by greenish-gray mud rip-
up clasts and by foundered, large blocks of flaser-bedded sandstone (Fig.
8A).
Bioturbation and invertebrate body fossils are rare in this interval. The
rostra of two sawfish, which are known to swim far upstream in modern
rivers, are the only vertebrates known, and no invertebrate fossils have
been recovered. Well-preserved, whole dicot plant leaves are abundant in
finer-grained beds throughout the interval. Organic-rich beds with abun-
dant plant debris are also present.
Lithostratigraphic definitions for the Birket Qarun and Qasr el-Sagha
formations have not satisfactorily accounted for this unit, and it has,
therefore, been inconsistently assigned to both formations, primarily
based on its variable stratigraphic position relative to the oldest Carolia
bed in a given section (Fig. 1, 3).
Interpretation.—We interpret the flaser-bedded sandstone interval to
represent infilling of an incised river valley during the initial shift in the
balance between sediment supply and the formation of accommodation,
probably during the initial transgressive phase of sequence TA4.2 (Fig.
2). Bundling of mudstone-sandstone packages at the scale of ripple lam-
inations (Fig. 9D) and decimeter-scale bundles (visible in Fig. 9A-B) are
interpreted as tidally generated rhythmites. The sandstones and thin mud-
stone drapes in this IVF are, thus, thought to have been deposited in a
river-dominated estuary that experienced variation in the magnitude of
unidirectional flow due to ebb-flood and spring-neap tidal cycles. The
large clinoforms, which appear to lack a consistent orientation, are inter-
preted to represent rapidly building, broad, subaqueous sediment lobes or
kinematic waves that formed in response to rapid deposition and aggra-
dation of river transported sediment. Accommodation formed quickly as
a result of sea level rise and was closely matched or slightly exceeded
by sediment delivery, resulting in rapid aggradation.
Rapid sediment buildup during the deposition of the IVF resulted in
the development of subaqeous distributary-tidal channel systems that
scavenged the flaser-bedded sandstone. The lack of bioturbation, an abun-
dance of well-preserved plant fossils, some of which are indicative of
fresh or brackish water, the presence of sawfish, and the prevalence of
asymmetric ripples yielding unimodal current orientations (Fig. 1B) in a
lenticular sand body (Fig. 3–4), all indicate rapid sedimentation in a
fluvial-dominated, fresh- or slightly brackish-water tidal estuary. The ap-
parent lack of any significant aggradation of a fully fluvial system during
the initial stages of sea-level rise is somewhat surprising given the evi-
dence for high sediment supply. This may provide evidence for a very
rapid initial sea-level rise that abruptly transitioned a fluvial system into
an estuary. Isolated evidence for fluvial channels, however, occurs near
the base of the IVF (Fig. 8D) and may reflect minor aggradation of the
fluvial system during initial transgression.
Conglomerates and Mudstone-Coquina Parasequences of Early TST
The top of the IVF, including the tops of steeply dipping clinoforms,
is truncated (Fig. 9A) by a thin (
⬍
5 cm), matrix-supported conglomerate
with abundant pebbles and cobbles of very fine grained sandstone as well
as shark teeth and fragmented vertebrate bones, primarily from dugongs,
and marine macroinvertebrates (Table 1). The conglomerate is continuous
in thickness and in character over the entire study interval, including
where the IVF is absent (section A, Fig. 3). The conglomerate is overlain
by a
⬃
3-m-thick, widely traceable, very fine grained, tan sandstone that
contains abundant, fragmented marine invertebrate shells indicative of
normal-marine conditions, including oysters, gastropods, corals, echi-
noids, and crustaceans. These sandstones are thoroughly bioturbated, and
Thalassinoides boxworks penetrate down into the top 1–1.5 m of the
underlying IVF. Locally, large-scale (1–3 m) ball-and-pillow structures
with clear dish and pillar structures are present in the sandstone imme-
diately overlying the conglomerate, which is often injected vertically be-
tween the pillows.
The top of the shelly sandstone that overlies the conglomerate is unique
in having small scleractinian coral colonies and a great abundance of the
enigmatic calcareous fossil Kerunia, which occurs nowhere else in the
section (Fig. 3). The shelly sand bed is overlain by a
⬃
4-m thick,
greenish-gray mudstone that coarsens upward to silty, lighter greenish-
gray mudstone (Fig. 3; Fig. 9A). This mudstone is overlain by a thin
conglomerate and skeletal coquina that is similar to the sandy bed that
caps the underlying IVF, but with larger, more complete invertebrate
shells and abundant Carolia. Dugong skeletons are present near the top
of the second skeletal bed (Table 1), which is complex and exhibits ev-
idence for scour-and-fill structures to form broad (6 m), shallow troughs.
The measured sections are capped by a densely packed ostreiid oyster
bed that contains rare dugong skeletons. Traditional lithostratigraphic def-
initions place the entire interval within the Qasr el-Sagha Formation.
Vertebrates in this part of the section, albeit rare in the upper quarter
of the underlying upper FSST, are consistently located on two shelly,
conglomeratic surfaces. No whales have been found in this interval, but
dugongs, sharks, and sawfish are present (Table 1). Skeletons vary in
their degree of articulation and completeness, with remains on the first
RS being mostly incomplete and fragmentary and remains on the second
surface being better preserved, on average.
Interpretation.—The top of the IVF is erosively truncated by a marine
RS that resulted in the formation of a thin, widely traceable transgressive
lag of matrix-supported conglomerate that consists of sedimentary clasts
(iron-cemented, very fine grained sand, probably Thalassinoides burrow
fills) and vertebrate remains, primarily from dugongs and sharks. Normal
marine conditions were established during the initial transgression, as
evidenced by the presence of fragmented corals and echinoids in a con-
densed transgressive sandstone overlying the RS. The lower 1.5 m of the
early TST in sequence TA4.2 appears to be condensed, but localized,
large-scale ball-and-pillow structures that deform the RS conglomerate
suggest sediment destabilization, possibly during a single storm that rap-
idly deposited sand over the newly created, broad shelf.
Another marine flooding in the early TST,
⬃
2.8 m above the initial
RS, resulted in the formation of a clear-water, normal marine, shallow
shelf that permitted scleractinian corals and abundant Kerunia to colonize
a sandy skeletal seafloor. The overlying parasequence of the early TST
is thicker, contains greenish-gray mudstone at its base, and records con-
tinuing transgression during the TST. An erosive marine RS with Thal-
assinoides; a conglomerate; and a densely packed, sandy skeletal lag
dominated by oysters and anomiid bivalve Carolia also caps this
coarsening-upward parasequence. This ravinement is very similar to the
conglomerate developed on the underlying RS, but the invertebrate shells
and vertebrate remains are typically less fragmented. The early TST, thus,
records at least two episodes of prominent marine ravinement during an
overall deepening succession.
The mudstone, sandy coquinas, and densely packed oyster bed that
caps these sections record the formation of a broad, muddy shelf or em-
bayment similar to the embayment that existed during the late stages of
the underlying FSST at section A. On the interfluve (section A), deposits
above and below the Pr-2 SB are, therefore, lithologically and environ-
mentally similar, despite the fact that they are from different third-order
300 PALAIOS
PETERS ET AL.
sequences. Without the aid of a clearly exposed incised valley and an
easily traceable RS, the Pr-2 sequence boundary, which represents a
ⱖ
45-m-fall in sea level and subaerial exposure of the region, would be
cryptic and is very easily overlooked at section A. The lack of clear
evidence for subaerial exposure within the interfluve at section A appears
to be the result of substantial erosion during the formation of the first RS
in sequence TA4.2.
DISCUSSION
Relationship to Previous Stratigraphic Research in Egypt
Several kilometers northwest of the study area, and higher in the Qasr
el-Sagha Formation, a flaser-bedded, ripple-laminated sand with large-
scale clinoforms overlies a vertebrate-bearing conglomerate. This interval
constitutes the cross-bedded siltstone and shale with gypsum and carbo-
naceous shale from 163–172 m in Gingerich’s (1992) Minqar Abyad sec-
tion, and it is lithologically very similar to IVF and basal conglomerate
described here on top of the Pr-2 sequence boundary. We interpret this
up-section repetition of a lithofacies assemblage that is diagnostic of the
IVF of Pr-2 to mark the overlying SB, Pr-3 (Fig. 2). The IVF above SB
Pr-3 is overlain by a shelly sandstone that is nearly identical to the trans-
gressive RS described here in the TA4.2 sequence, including the occur-
rence of Kerunia. Both the Pr-2 and the Pr-3 sequence boundaries in the
Wadi Al-Hitan region are, thus, represented by conspicuous and similar
facies assemblages that formed in response to subaerial exposure and
subsequent transgressive ravinement. The Eocene-Oligocene boundary in
the Wadi Al-Hitan region is characterized by the permanent withdrawal
of the sea and a transition to fully continental sedimentation. This final
Eocene sea-level fall marks the Pr-4 SB (Fig. 2).
The flaser-bedded sandstone facies that constitutes the IVF of sequence
TA4.3 at Wadi Al-Hitan is the westward and basinward extension of the
interbedded claystone, siltstone, and quartz sandstone facies (Vondra,
1974) as well as the giant, cross-bedded sandstone (Bown and Kraus,
1988) described near Qasr el-Sagha, some 60 km east-northeast of Wadi
Al-Hitan. Vondra (1974) interpreted this conspicuous, upper, cross-
bedded unit as a prograding delta front, whereas Bown and Kraus (1988)
thought it represented lateral accretion deposits formed within stream
channels. Given the more proximal, landward location of these previously
described sections (Fig. 1), we would expect the IVFs to have a more
fluvial character than the IVF in Wadi Al-Hitan.
The lower part of the Qasr el-Sagha Formation is less well exposed
near Qasr el-Sagha, but it is probable that the Pr-2 SB and overlying IVF
is also present southeast of Qasr el-Sagha. For example, Seiffert et al.
(2008) report fragmentary remains of the proboscideans Moeritherium
and Barytherium, as well as sirenians, whales, and an associated BQ-2
fauna of terrestrial micromammals, from the Umm Rigl Member of the
Birket Qarun or Qasr el-Sagha Formation. The precise stratigraphic po-
sition of BQ-2 and its correlation to strata in Wadi Al-Hitan is uncertain
at this time because the section at BQ-2 is comparatively poorly exposed.
The Pr-2 IVF that we describe at Wadi Al-Hitan, however, provides the
earliest evidence of land mammals (Moeritherium and Barytherium)in
the Wadi Al-Hitan section, the stratigraphic setting below Pr-3 is similar,
and the Pr-2 IVF is probably correlative with the BQ-2 terrestrial mammal
locality.
Implications for Vertebrate Taphonomy and Ecology
In the absence of a sequence stratigraphic framework, it would be
tempting to interpret the prevalence of complete whale skeletons in a few
stratigraphic intervals as evidence for episodic mass mortality during
breeding periods, biotic responses to environmental change, or some other
unusual biological events. The consistent occurrence of whales and other
vertebrates on distinct stratigraphic surfaces that are expected to have
experienced enhanced environmental and temporal averaging, however,
suggests that such interpretations are unjustified. Instead, the distribution
of whale skeletons in Wadi Al-Hitan is consistent with a relatively steady
flux of carcasses to the sea floor, with periods of reduced sedimentation
and environmental averaging during marine flooding, affording greater
opportunity for remains to accumulate on widely traceable stratigraphic
surfaces. There is often little transport or loss of remains following the
emplacement of vertebrate remains on active depositional surfaces be-
cause they have large, durable bones that can eventually act as large
sedimentary clasts (Fig. 5) in relatively low-energy marine environments.
Rapid burial is, thus, not required in order for marine vertebrates to occur
as complete and semi-articulated to articulated skeletons on traceable
stratigraphic surfaces.
Superimposed on variation in the concentration and state of preser-
vation of vertebrate remains in Wadi Al-Hitan is a shift in the dominant
taxon from Basilosaurus at the base of the section to Dorudon remains
in the middle, and to dugong-dominated assemblages at the top (Table
1). It is unlikely that taphonomic processes in subtidal marine environ-
ments studied here have strongly biased the relative abundance of ver-
tebrates, based on a study of a modern beach with marine vertebrate
bones that accurately reflect local community composition (Liebig et al.,
2003). The vertical succession in relative abundance observed in Wadi
Al-Hitan, however, may not be indicative of a true temporal biological
trend. Instead, this up-section change in dominant taxa (Table 1) could
be controlled by a sea-level-forced environmental stacking pattern (Fig.
3) that resulted in the vertical stratigraphic juxtaposition of an original
onshore-offshore faunal gradient (i.e., habitat tracking; Brett, 1998). Spe-
cifically, the stratigraphic distribution of remains suggest that Basilosau-
rus, the largest-bodied whale in Wadi Al-Hitan, may have been more
prevalent than Dorudon in deeper water, offshore environments, even
though both whales occur across the same range of environments (Table
1). The dominance of dugong remains in shoreface and embayment en-
vironments (Table 1) is consistent with their subsistence on sea grasses,
which have been found as fossils in dugong-bearing strata in Wadi Al-
Hitan.
The possibility of an onshore-offshore gradient in faunal dominance,
with Basilosaurus dominant in the deeper water, transgressive phase of
sequence TA4.1, does not support the hypothesis that Basilosaurus was
a shallow marine specialist. Instead, Basilosaurus may have been gen-
erally adapted to the expansive, epeiric sea environments in which whales
first evolved (Gingerich, 1983). A sea-level fall, through the elimination
or modification of shallow shelf habitats, could cause a true decline in
the abundance of Basilosaurus as well as a coordinated environmental
trend up-section. Our observations do not allow this common-cause
mechanism to be adequately tested at this time, but they do constrain the
range of biological hypotheses that should be entertained and provide the
necessary stratigraphic framework for further consideration of a common
cause hypothesis. For example, the results of this study indicate that
stratigraphic patterning is important in determining the stratigraphic dis-
tribution of vertebrate fossils, but they also suggest that there are quali-
tative differences in coastal geomorphology and shallow shelf environments
between transgressive and regressive phases that could have evolutionary
and ecological consequences. Such effects might cause elevated rates of
morphological evolution or extinction and cladogenesis during inflection
points in relative sea level that transcend those expected by sampling
effects imposed by sequence stratigraphy (e.g., Holland, 1995; Brett,
1998; Hannisdal, 2006).
Notwithstanding the possible evolutionary consequences of rising and
falling sea level, sedimentary processes operating within a third-order
sequence exert a dominant control on vertebrate preservation and ta-
phonomy in Wadi Al-Hitan. It is, therefore, probable that physical sedi-
mentological processes similarly structure other marine vertebrate assem-
blages. Insofar as eustatic sea level is an important component, sequence
stratigraphic control on marine vertebrate preservation could be useful in
more precisely correlating distant fossil localities. For example, the
ⱖ
45
m sea-level fall recorded by the Pr-2 SB in Wadi Al-Hitan is identified
PALAIOS 301
EOCENE SEQUENCE STRATIGRAPHY
elsewhere (Haq et al., 1987) and does not appear to be under local tec-
tonic control. We, therefore, anticipate that eustatic sea-level changes, in
conjunction with the bio- or chemostratigraphic control necessary to align
sequence stratigraphic records, will prove useful for correlating whale
fossils globally and locally, and that sequence stratigraphy will provide
additional control on the timing and environmental context of the land-
to-sea transition in whale evolution.
As a tangible example of the potential of sequence stratigraphy as an
additional correlation tool for whale fossils, Basilosaurus isis in Egypt is
found principally in the Birket Qarun Formation, below SB Pr-2, in lower
Priabonian sequence TA4.1 (Fig. 2). In North America, a closely related
whale species, Basilosaurus cetoides, is found in the Pachuta Marl and
Shubuta Marl members of the Yazoo Formation of Alabama (and cor-
relative strata in nearby states). The Pachuta Marl contains SB Pr-3, and
virtually all B. cetoides are found in the Pachuta and Shubuta members
in upper Priabonian sequence TA4.3 (Miller et al., 2008). On the basis
of sequence stratigraphy, B. isis in Egypt (c. 36.5 Myr) is, thus, expected
to be on the order of 2.5 million years older than B. cetoides in Alabama
(c. 34 Myr). In this case, calcareous nannofossil biostratigraphy provides
the temporal constraints necessary to properly align a sequence-stratigraphy–
based correlation scheme, but enhanced resolution within individual bio-
stratigraphic zones may be facilitated by eustatically controlled SBs (Fig.
2), both within and between sedimentary basins.
Comparison to Other Skeletal-rich Marine Records
In terms of vertebrate preservation, one of the studies that is most
closely related to the present is that of Rogers and Kidwell (2000), which
documented the sequence stratigraphic distribution of vertebrates along
an onshore-offshore transect in the Cretaceous of Montana. In that area,
no consistent relationship was found between the inferred durations of
erosional-omissional hiatuses and the degree of concentration or state of
preservation of vertebrate remains. Instead, vertebrate-rich beds occurred
as lag concentrations, with taxonomic composition and skeletal abun-
dance controlled primarily by the fossil contents of facies that underlie
and that are adjacent to ravinements and fluvial channels. In Wadi Al-
Hitan, a similar example of a vertebrate lag concentration occurs on the
Pr-2 SB within the incised valley. In this case, terrestrial animals living
near the active fluvial environment contributed a minority of remains, but
a majority were reworking from underlying FSST mudstone and sand-
stone. Despite the prevalence of macroinvertebrates in the reworked sed-
iments, only limited numbers of the most durable (primarily ostreiid oys-
ters) occur as fragments in this lag. The abundance of bone is the direct
result of differences in hard-part durability between macroinvertebrates
and vertebrates.
The stratigraphic distribution and taphonomy of marine macroinver-
tebrates has been well documented for third- and fourth-order sequences
throughout the Phanerozoic (e.g., Kidwell, 1991, 1993; Brett and Baird,
1993). For example, Kidwell (1989) examined shell concentrations as-
sociated with Thalassinoides-burrowed, transgressive surfaces that are ei-
ther merged with an underlying SB or separated from it by an IVF. Tax-
onomically diverse and internally complicated coquinas overlie these
transgressive surfaces and consist of shells that were produced locally
during deepening. The taphonomic signatures of these concentrations are
typically commensurate with the duration of the inferred hiatus (Kidwell,
1993). Very similar patterns of macroinvertebrate skeletal accumulation
occur in association with MFSs in Wadi Al-Hitan, particularly in the early
TST of sequence TA4.2. Outside of the incised valley, the distribution
and taphonomic conditions of vertebrates closely follow the expectations
of hiatal concentrations developed on the basis of marine macroinverte-
brates (Kidwell, 1991, 1993).
Despite notable similarities between the marine vertebrate and mac-
roinvertebrate fossil records, there are several important differences. First,
fossils of marine vertebrates are much less common than those of mac-
roinvertebrates and, with the possible exception of locally rich lag con-
centrations on the Pr-2 SB, no horizon in Wadi Al-Hitan can be consid-
ered a true bone bed, even though skeletons are unambiguously associated
with MFS. The widely scattered nature of vertebrate remains in Wadi
Al-Hitan is probably due both to comparatively high rates of net
sedimentation—average accumulation rate is
⬃
100 m/Myr, or within the
moderate subsidence range of Kidwell (1993)—and to a comparatively
low flux of skeletal material to the sea floor. For these reasons, the ac-
cumulation of marine vertebrates in Wadi Al-Hitan did not change the
physical properties of the sea floor during the formation of a hiatus, as
macroinvertebrates often do. The second difference is that marine ver-
tebrate bones and teeth are much more durable and less easily transported
than most macroinvertebrates. Complete skeletons can, therefore, remain
articulated even when they are exposed on the sea floor for long periods
of time. In contrast to the typical situation for macroinvertebrates, artic-
ulation, therefore, need not indicate rapid burial. Nevertheless, more de-
tailed aspects of quality of preservation do appear to vary with inferred
hiatal durations (Table 1).
CONCLUSIONS
1. Strata at Wadi Al-Hitan record the transgressive and regressive
phases of third-order Priabonian sequence TA4.1, as well as the initial
transgressive phase of Priabonian sequence TA4.2. Environments at Wadi
Al-Hitan spanned a range of water depths—from below SWB in a muddy,
hypoxic offshore shelf, to above NWB in a sandy shoreface, and to pro-
tected, muddy-bottomed embayments with oyster shoals and nummulitid
banks.
2. A fall in sea level of at least 45 m resulted in the subaerial exposure
of the Wadi Al-Hitan shelf, and the formation of incised river valleys.
An imbricated fluvial conglomerate characterizes the Pr-2 SB in the in-
cised valley and is overlain by flaser-bedded and ripple-laminated, tidal
estuary sandstones with a multi-km lenticular geometr y. Subsequent ma-
rine transgression likely resulted in the erosion of continental soils or
sediments on interfluves and the formation of a cryptic SB, merged with
a marine RS, that juxtaposes regressive marine strata of sequence TA4.1
on transgressive marine strata of sequence TA4.2.
3. Second- and third-order sequence stratigraphy in Wadi Al-Hitan is
consistent with Haq et al. (1987), suggesting a eustatic sea level com-
ponent to late Eocene sedimentation in the western desert of Egypt. Eu-
static sea level may have been driven by the initiation of widespread
Antarctic glaciation at the end of the Eocene. The overall thickness of
the Priabonian section in Wadi Al-Hitan (
⬃
110 m) also suggests an im-
portant role for passive margin subsidence.
4. The preservation of vertebrates at Wadi Al-Hitan is closely linked
to sequence stratigraphic architecture. The majority of vertebrate remains
are found as complete, articulated to semi-articulated skeletons on MFSs
or as fragmented, isolated elements in an erosional lag concentration on
the Pr-2 SB. Terrestrial mammals, the oldest known from Egypt, are also
found in the Pr-2 lag. There is no direct evidence for biological events
as fossil concentration mechanisms at Wadi Al-Hitan. The general abun-
dance of whale remains in the Wadi Al-Hitan region may, however, reflect
ecologically favorable conditions within a protected shelf setting. The
scarcity of whales in the most-offshore marine environments in Wadi Al-
Hitan may indicate the preferential congregation of whales in shallower
water environments.
5. Basilosaurus is the most common whale in the deepest-water en-
vironments at Wadi Al-Hitan, and Dorudon, a smaller-bodied archaeocete
whale, is most abundant in lower to middle shoreface environments,
though both whales occur across the same spectrum of shelf settings.
Dugong remains predominate in the shallowest water environments, such
as protected embayments at the top of sequence TA4.1 and in the early
transgressive shelf environments of sequence TA4.2. The elimination of
broad, shallow shelves during eustatic sea level fall, as well as correlated
302 PALAIOS
PETERS ET AL.
changes in coastal-shelf geomorphology, may have influenced marine
vertebrate ecology and evolution during the late Eocene transition to an
icehouse world.
ACKNOWLEDGMENTS
Research at Wadi Al-Hitan was encouraged by Dr. Mostafa M. Fouda,
Director of the Nature Conservation Sector of the Egyptian Environmen-
tal Affairs Agency. We thank James Lamb of the McWane Science Center,
Birmingham, Alabama, and Doug Jones of the University of Alabama
Museum of Natural History, Tuscaloosa, Alabama, for information about
the stratigraphic distribution of North American Basilosaurus. We also
thank G. Abuelkhair, A. Carroll, C. Gee, B. Hannisdal, P. McLaughlin,
M. Sander, and A. Strougo for discussion. C.E. Brett and R. Rogers pro-
vided helpful reviews of an earlier version of this manuscript. C.E. Brett,
S. Hasiotis, and S. Kidwell provided insightful reviews that greatly im-
proved the language and arguments herein. Research was supported by
the Egyptian Environmental Affairs Agency, Egyptian Mineral Resources
Authority, Cairo Geological Museum, American Chemical Society Petro-
leum Research Fund, National Geographic Society (7726-04), and Na-
tional Science Foundation (EAR 0517773, OISE 0513544).
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ACCEPTED NOVEMBER 19, 2008
