SHORT COMMUNICATION: Sparsely encrusted hardground in the Darriwilian calcareous sandstone of Cape Pakri, NW Estonia (Baltica)

Abstract

The occurrence of echinoderm and ptilodictyid bryozoan holdfasts on the surface of Darriwilian calcareous sandstone in northwestern Estonia indicates that it was lithified before encrustation. Pelmatozoans outnumber the bryozoans and cover a larger area of the hardground although both are very sparse. The hardground is very sparsely encrusted (0.37% of the total area studied) and lacks signs of bioerosion.

Full text

Estonian Journal of Earth Sciences, 2015, 64, 3, 249–253 doi: 10.3176/earth.2015.31
249
SHORT COMMUNICATION
Sparsely encrusted hardground in the Darriwilian calcareous sandstone
of Cape Pakri, NW Estonia (Baltica)
Olev Vinn
Department of Geology, University of Tartu, Ravila 14A, 50411 Tartu, Estonia; olev.vinn@ut.ee
Received 17 February 2015, accepted 13 May 2015
Abstract. The occurrence of echinoderm and ptilodictyid bryozoan holdfasts on the surface of Darriwilian calcareous sandstone
in northwestern Estonia indicates that it was lithified before encrustation. Pelmatozoans outnumber the bryozoans and cover a
larger area of the hardground although both are very sparse. The hardground is very sparsely encrusted (0.37% of the total area
studied) and lacks signs of bioerosion.
Key words: hardgrounds, encrustation, cementation, Middle Ordovician, Baltica.
INTRODUCTION
Hardgrounds form excellent attachment surfaces for
encrusting and bioeroding organisms that otherwise
must dwell on carbonate cobbles or the exoskeletons
of various invertebrates. Carbonate hardgrounds are
synsedimentarily cemented carbonate layers that have
been exposed on the seafloor (Wilson et al. 1992). They
were relatively common in calcite seas, such as those
of the Ordovician, due to favourable conditions for
early cementation of carbonate sediments in the seafloor
(Wilson & Palmer 1992). Upper Ordovician hardground
faunas were dominated by bryozoans and echinoderms,
but cornulitid tubeworms and sphenothallids were also
common (Wilson 1985; Taylor & Wilson 2003). The
Ordovician hardground faunas of Baltica have been
described both from Russia and Sweden (Vishnyakov &
Hecker 1937; Rozhnov 2000; Blake & Rozhnov 2007;
Eriksson et al. 2012). Trypanites borings have been
reported from the Ordovician hardgrounds in Estonia
(Orviku 1940, 1960; Einasto 1964; Põlma 1984; Saadre
1992, 1993). The earliest hardground faunas of Baltica
include Trypanites, Gastrochaenolites? and Balanoglossites
from the latest Early Ordovician (Vinn & Wilson 2010;
Knaust & Dronov 2013). Ripple marks, associated
with the hardground in the Vasalemma quarry, were
described in detail by Hints & Miidel (2008). They also
mentioned the occurrence of Trypanites borings and the
edrioasteroid echinoderm Cyathocystis.
The aims of this paper are (1) to describe for the first
time a hardground echinoderm–bryozoan community
from the Darriwilian of Baltica and (2) to compare the
hardground fauna from the Darriwilian of Estonia with
other Ordovician examples.
GEOLOGICAL BACKGROUND AND
LOCALITY
The area of modern Estonia (Fig. 1) was covered by
a shallow epicontinental sea in the Ordovician. The
Ordovician sequence of North Estonia is represented
mostly by carbonate rocks except for the terrigenous
Lower Ordovician part. Ordovician rocks, mainly lime-
stones, are exposed in northern Estonia as a wide belt
from the Narva River in the east to Hiiumaa Island in
the west (Mõtus & Hints 2007). Drift of Baltica from
the southern high latitudes to the tropical realm caused
a drastic climatic change. The warming of the climate
increased the sedimentation rate of carbonates. Deposits
characteristic of arid and subtropical climate appeared
in the Late Ordovician (Mõtus & Hints 2007; Dronov &
Rozhnov 2008). In the Early and Middle Ordovician,
when the Baltic Basin was situated in the temperate
climate zone, these types of tropical deposits were
completely lacking (Jaanusson 1973).
Pakri Cliff is situated on the western coast of the
Pakri Peninsula in NW Estonia (Fig. 1). The hardground
was studied at the northern border of the town of Paldiski.
It is the second hardground from the top of the Pakri
Formation and forms a surface within the formation
(Fig. 1), which is detectable throughout the several
kilometres long Pakri Cliff section. The Pakri Formation
is about 1 m thick at Pakri Cliff and is composed of
carbonate-cemented sandstone and sandy limestone. It
represents the most onshore environment known from
the carbonate part of the Ordovician of Estonia. The
calcareous sandstones of the formation formed in a
very shallow subtidal environment close to the ancient
coastline (Põldsaar & Ainsaar 2014). These sandstones

Estonian Journal of Earth Sciences, 2015, 64, 3, 249–253
250
are rich in gastropods, cephalopods, brachiopods and
trilobites. The calcareous sandstones of the Pakri Formation
are overlain by limestones of the Kandle Formation,
which have a thickness of about 0.10 m (Põldsaar &
Ainsaar 2014).
MATERIAL AND METHODS
The encrusted hardground was discovered during
extensive field work at Pakri Cliff in 2013. Several
kilometres of coast below the cliff were carefully searched
Fig. 1. A, location of Pakri Cliff in NW Estonia. B, Pakri Cliff with the studied hardground in the Pakri Formation (Darriwilian).
Modified after Hints et al. (2008). C, the Middle Ordovician in Estonia. Location of the hardground within the Pakri Formation.
Modified after Hints et al. (2008).

O. Vinn: Encrusted hardground from the Darriwilian
251
for hardground surfaces exposed on boulders broken
from the cliff. The total hardground surface available
for the study measures 1260 cm2. The hardground surface
in the cliff is very hard to reach due to heavy cementation
and falling rocks. The cliff is under natural protection
and the removal of large parts of it to reach the hard-
ground surface is not permitted. The hardground surface
was cleaned with a brush and water and photographed
in sunlight with a scale bar. Macroscopic encrusters
were counted and identified. Signs of bioerosion and
small encrusters were searched for using a 10  magni-
fication lens. Only two echinoderms and a bryozoan
holdfast were collected due to the hard rock matrix.
They are deposited at the Natural History Museum
(Geological Museum), University of Tartu (TUG),
with specimen numbers TUG 1681-1, TUG 1681-2 and
TUG 1681-3.
RESULTS
Five discoidal echinoderm holdfasts and a ptilodictyid
bryozoan holdfast are cemented to the hardground surface
within the Pakri Formation (Darriwilian) (Fig. 2). The
hardground surface is darker in colour than rest of the
rock matrix due to pyrite impregnation. It is not smooth
and has a relatively low and irregularly shaped micro-
relief (Fig. 2). It lacks obvious signs of bioerosion and is
sparsely encrusted. The encrusters cover only 4.65 cm2
(0.37%) of the total hardground surface studied
(1260 cm2). Echinoderms cover 3.45 cm2 and bryozoans
1.2 cm2 of the hardground surface. Pelmatozoans out-
number bryozoans and cover a larger area of the
hardground.
A single large discoidal holdfast is 26 mm wide and
has a circular outline (Fig. 2A). It is strongly elevated
(up to 3.0 mm) above the hardground surface. The
holdfast has a relatively smooth surface. A few weakly
developed short shallow radial furrows are found at the
periphery of the holdfast in some places. The holdfast
has a large shallow central depression (about 9 mm
in diameter) that is surrounded by a slightly elevated
narrow rim. The holdfast is assigned to echinoderms
because of its circular outline, relatively smooth surface
and shallow central depression (Wilson et al. 1992).
Five small discoidal holdfasts have a circular outline
(Fig. 2B). They have a mostly unornamented surface
with weakly developed irregular relief in some places.
Their holdfasts are relatively flat, not much elevated
above the surface of the hardground. One small discoidal
holdfast shows a weakly developed central pit which
is about 1/4 as wide as the holdfast. The holdfasts are
assigned to echinoderms because of their circular outline,
relatively smooth surface and lack of radial ornamentation
(Wilson et al. 1992).
The single large semi-circular holdfast is 13.0 mm
wide and covered with radial ridges that bifurcate and
intercalate (Fig. 2B). About two ridges per 1 mm are
found along the periphery. The surface of the holdfast is
only slightly elevated above the hardground surface.
The holdfast is assigned to ptilodictyid bryozoans because
of its semi-circular outline and radial ornamentation
composed of bifurcating and intercalating ridges (Wilson
et al. 1992).
Fig. 2. A, hardground surface; Ech, large echinoderm holdfast, Pakri Formation, Darriwilian, Middle Ordovician, Pakri Cape,
N
W Estonia. TUG 1681-1. B, hardground surface; a ptilodictyid bryozoan holdfast (br) (TUG 1681-2) and small echinoderm
holdfast (ech) (TUG 1681-3) cemented to the hardground from the Pakri Formation, Darriwilian, Middle Ordovician,
N
W Estonia.

Estonian Journal of Earth Sciences, 2015, 64, 3, 249–253
252
DISCUSSION
Sedimentology
The occurrence of echinoderm and bryozoan holdfasts
indicates that a surface of calcareous sandstone within
the Pakri Formation was lithified before encrustation.
The hardground seems to be slightly abraded and has
few cryptic surfaces and broken off cobbles. The darker
colour of the hardground surface is presumably caused
by post-sedimentation mineralization by iron minerals,
probably pyrite. The hardground formed in normal marine
conditions in relatively shallow water as it was close to
the supposed shoreline to the north (Jaanusson 1973).
Preservation
The Pakri Cliff holdfasts show signs of slight erosion.
No echinoderm stem fragment was found attached to the
holdfasts. This indicates that there was some hydro-
dynamic activity above the hardground surface.
Encrustation and diversity of the association
The Pakri hardground is very sparsely encrusted (Brett
& Liddell 1978). The slightly older hardground fauna
from the Middle Ordovician of Utah (Wilson et al.
1992) has similar diversity. The nearly equal areas of
the Utah hardground were covered by bryozoans and
echinoderm holdfasts (Wilson et al. 1992). The Pakri
hardground bryozoans are among the earliest bryozoans
known from the hardgrounds of Baltica or elsewhere
(O. Vinn pers. obs.). Bryozoans are known from the
Billingen and Hunneberg of Baltica (Koromyslova 2011).
The lack of cornulitids in the Pakri association is most
striking, but their earliest representatives in Baltica are
known from the latest Darriwilian. Thus, the lack of
cornulitids can be explained by their later appearance
in the Ordovician of Baltica. Microconchids, which are
characteristic of many Early Palaeozoic hard substrate
faunas that appeared in the Late Ordovician (Zatoń &
Vinn 2011), are similarly absent in the Pakri association.
Bryozoans usually form an important part of Ordovician
hard substrate faunas (Taylor & Wilson 2003) and they
also occur in the Pakri fauna. The Pakri hardground
environment may have been atypical for the Ordovician
shallow sea carbonate hardgrounds. It presumably
represents a very shallow subtidal environment (Põldsaar
& Ainsaar 2014) with a lithified sand bottom (with
carbonate cement), unlike most other Ordovician lime-
stone hardgrounds. According to Orviku (1960),
Trypanites borings occur in the Pakri Formation. It is
surprising that the described hardground does not
contain Trypanites.
SUMMARY
 A hardground in the calcareous sandstone of the
Pakri Formation is encrusted by bryozoans and
echinoderms. The association is dominated by
echinoderms.
 The Pakri hardground differs from many other
Ordovician hardgrounds of Baltica by the lack of
bioerosion. It is possible that calcareous sandstone
may have been an unfavourable substrate for boring
organisms.
Acknowledgements. I am grateful to Mark A. Wilson for
advice on the identification of the holdfasts. This paper is a
contribution to IGCP 591 ‘The Early to Middle Palaeozoic
Revolution’. I am also grateful to Ingrid Vinn for help during
field work. Financial support was provided by the Palae-
ontological Association Research Grant, and Estonian
Research Council projects ETF9064 and IUT20-34. I am
grateful to journal editor D. Kaljo for useful remarks and to
C. E. Brett and A. V. Dronov for the constructive reviews.
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Hõredalt asustatud muistne tsementeerunud kihipind Pakri poolsaare Ordoviitsiumist
Olev Vinn
Pakri pangal paljanduvast Kunda lademe lubi-liivakivist leiti kunagine tsementeerunud kihipind, mis oli hõredalt
asustatud sinna külge kinnitunud sammalloomade ja okasnahksetega.