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UNCORRECTED PROOF
The Port Morant Formation (Upper Pleistocene, Jamaica):high resolution sedimentology and paleoenvironmental analysis
of a mixed-carbonate-clastic lagoonal succession
Simon F. Mitchella,*, Ron K. Pickerillb, Thomas A. Stemanna
aDepartment of Geography and Geology, University of the West Indies, Mona, Kingston 7, JamaicabDepartment of Geology, University of New Brunswick, Fredericton, New Brunswick, Canada E3B 5A3
Received 4 April 2000; accepted 31 October 2000
Abstract
The Port Morant Formation consists of a mixed clastic-carbonate sedimentary sequence that was deposited as a lagoon ®ll
during the Sangamonian interglacial. Ten sedimentary facies are recognised and sequence stratigraphic analysis indicates the
presence of transgressive and highstand systems tracts. The transgressive systems tract consists of a basal transgressive
conglomerate (facies I), crustose coralline algal bindstones-boundstones (II) and 2 m high Solenastrea coral heads (III). The
highstand systems tract is represented by sediments of a braid delta/fan-delta prograding into the lagoon (IV and V), marine
pebbly sandstones deposited adjacent to mangrove swamps (VII), more distal algal mudstones (VIII), and sheet-like (VI) and
channelized (IX) conglomerates ®lling delta-top distributary channels. A barrier and/or fringing reef is present (X), but its
relationship with the lagoon-®ll sediments is obscure due to poor exposure. Carbonates are restricted to the transgressive
systems tract and the barrier/fringing reef (transgressive and/or highstand systems tract). Two transgressive events are recog-
nized, the transgressive systems tract (facies I to III) and facies VII. The latter either a second sea-level rise or due to delta
abandonment. A single coral date from facies VII gave an age of 132 ^ 7 kyr. This indicates that the upper transgressive event
(facies VII) belongs to the early highstand that has been recognized in isotope substage 5e. The lower transgressive event (facies
I to III) in the Port Morant Formation is therefore either also of this age, or older. q 2001 Elsevier Science B.V. All rights
reserved.
Keywords: Sequence stratigraphy; Pleistocene; Lagoon sedimentation; Coral reefs; Mixed clastic-carbonate sedimentology
1. Introduction
Mixed clastic-carbonate depositional systems are
rare in the Caribbean where sedimentation is either
dominated by carbonate deposition (eg, Jones and
Pemberton, 1989; Blanchon et al., 1997) or clastic
deposition (eg, Westcott and Etheridge, 1980, 1983).
This is also true of Late Pleistocene strata in Jamaica
(eg, Liddell and Ohlhorst, 1987; Boss and Liddell,
1987) that are largely dominated by carbonates of
the Falmouth Formation, deposited as a fringe around
the island during high sea-level stands between 135
and 120 kyr (Moore and Somayajulu, 1974). In
Jamaica, clastic deposits of this age are rare and
limited to the Port Morant Formation (Mitchell et
al., 2000), which is restricted to the south-eastern
part of Jamaica. In the vicinity of the village of Old
Pera (Fig. 1), the Port Morant Formation exhibits
a mixed clastic-carbonate style of sedimentation.
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Sedimentary Geology 000 (2001) 000±000
SEDGEO2853
0037-0738/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved.
PII: S0037-0738(01)00101-4
www.elsevier.nl/locate/sedgeo
* Corresponding author.
E-mail address: smitchell@cwjamaica.com (S.F. Mitchell).
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UNCORRECTED PROOF
Previously, body fossils (eg, echinoids: Donovan et
al., 1994; crabs: Collins et al., 1996; Collins and
Donovan, 1997) and trace fossils (Pickerill et al.,
1998a) have been described.
In this paper we present a detailed description
and interpretation of the lithofacies present in the
Port Morant Formation. We use these facies to inter-
pret the depositional environment and to suggest a
model that is applicable for similar mixed clastic-
carbonate depositional systems. The sea-level history
of the Port Morant Formation is discussed in
relation to recent work on sea-level highstands in
the Sangomonian.
2. History of research
The name Port Morant Formation was introduced
by Robinson (1969) in a ®eld guide to Neogene
sections in Jamaica. The formation rests unconform-
ably on the latest Pliocene to earliest Pleistocene Old
Pera beds of the Manchioneal Formation (Robinson,
1969; Donovan, et al., 1994; Budd and McNeill,
1998). Donovan et al. (1994); Collins et al. (1996)
informally divided the Port Morant Formation exposed
on the coast at Old Pera into three units. These
consisted of a basal unit of boulder conglomerate
with bored clasts and large in situ coral colonies grow-
ing from the upper surface of this conglomerate, a
middle unit consisting of heavily bioturbated sili-
ciclastics with some `marlier' horizons, and an
upper unit that had several thin conglomerate horizons
and was highly fossiliferous (Donovan et al., 1997).
3. Methodology
The Port Morant Formation is exposed in sea cliffs
around the coastline to the south of Pera Point (Fig. 1).
The cliffs are relatively low, reaching a maximum
height of about 6 m near Pera Point. To the south
the height of the cliffs gradually decreases so that at
Canoe Bay they are only 1.5±2.0 m high. The cliff-
foot can be accessed at numerous points and the whole
of the succession studied by walking along the beach
or wading.
Initially detailed mapping was undertaken to deter-
mine the distribution of facies and sections suitable for
logging. Mapping was at a scale of 1:12,500 using the
standard 1:12,500 base sheets published by the Survey
Department, Jamaica. Once the distribution of facies
was determined, it became clear there were signi®cant
lateral and vertical transitions in facies. The approach
taken was to use graphic logs to show the vertical
succession, and mapping to show the lateral variation
of facies. Photomontages of the cliff line were not
used ± extensive mangroves (which are protected)
grow at high water mark and obscure much of the
cliff-line. About 20 sections were logged with the
spacing between sections ranging from 3 m up to
several hundred metres depending on the complexity
of the facies distribution. Each facies recognised in
the mapping exercise was described in detail from
®eld exposures. The sediment composition, hetero-
geneity and the orientation of bedding were recorded.
Sediment samples were collected for analysis in the
laboratory, but thin sections were not prepared
because most samples were poorly lithi®ed and the
high clay content made section cutting impossible.
Trace fossil relations within and between facies
were recorded in detail, and extensive collections of
fossils were made from fossiliferous units. Fossils are
deposited in the Geology Museum, University of the
West Indies, Kingston, Jamaica (numbers preceded by
UWI GM).
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Fig. 1. Map showing locations (1±15) mentioned in the text in the
vicinity of Old Pera, eastern Jamaica.
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UNCORRECTED PROOF
4. Description and interpretation of sedimentaryfacies
Ten facies are recognised (Table 1), which have
complex lateral and vertical relationships. The facies
recognised here are de®ned primarily on their sedi-
mentary characteristics. However, the ®ner grained
clastic units are further divided according to the domi-
nant sediment-producing elements present, that is
free-living algae, corals and molluscs.
4.1. Facies I: Basal conglomerate
The basal conglomerate is exposed on the coast for
a distance of approximately 100 m immediately to the
south of Pera Point (Fig. 1, localities 1±4). It has
previously been brie¯y described by Donovan et al.
(1994, 1997) and Collins et al. (1996).
The base of the Port Morant Formation is marked
by a distinct erosion surface, which truncates the
gently southerly dipping Old Pera beds of Robinson
(1969) (Figs. 2 and 3). The unconformity surface is
irregular and largely controlled by the distribution of
strongly cemented sandstone beds in the underlying
Old Pera beds. The surface itself does not appear to
show any boring, although the abundance of reworked
blocks overlying the surface indicates that the sand-
stones of the Old Pera beds were strongly lithi®ed
prior to the cutting of the erosion surface. There is
no evidence of subaerial exposure of the surface.
The basal erosion surface is overlain by a coarse-
grained (cobble to boulder sized) conglomerate that
is 25±57 cm thick. The conglomerate is largely
composed of blocks up to 50 cm long reworked
from the underlying Old Pera beds. The blocks vary
from angular to strongly rounded. Flat blocks immedi-
ately above the basal erosion surface are orientated
parallel to the surface; however, stratigraphically
upwards the orientation of blocks becomes more vari-
able. The abundance of the hard substrate trace fossil
Gastrochaenolites cf. cluniformis Kelly and Bromley
(Pickerill et al., 1998a) indicates important boring
activity of bivalves. In the upper part of the conglom-
erates, extrabasinal material is represented by pebble
sized clasts (1 ± 5 cm in maximum dimension) that
consist of igneous and volcaniclastic rocks, quartz and
chert. These clasts are rarely bored by Entobia isp.
(Pickerill et al., 1998a)
Relatively few body fossils are directly associated
with the basal conglomerate, although a test of the
regular echinoid Echinometra viridus A. Agassiz has
been recorded and spines of Eucidaris tribuloides
(Lamarck) are fairly common (Donovan et al.,
1994). Donovan et al.(1994) also recorded Conus
sp., Cerithium sp., and Chione sp.
Interpretation. The basal conglomerate rests on a
distinct erosion surface cut into the underlying Old Pera
beds, with many of the clasts clearly derived from these
strata. Bored conglomerates, where the clasts are
clearly derived from the underlying sedimentary unit
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Table 1
Facies recognized in the Port Morant Formation
Facies Description Interpretation Systems tract
I Basal conglomerate Transgressive shoreface TST
II Coralline algal bindstone intertidal to shallow subtidal
stabilization
TST
III Solenastrea boundstone Coral heads TST
IV Bioclastic poorly sorted
sandstone
Sea grass communities
between coral heads
HST
V Poorly sorted sandstone Delta foresets HST
VI Erosively based sheet
conglomerate
Distal distributary channels HST
VII Shelly pebbly sandstone Bioturbated shallow marine
sandstones
HST
VIII Algal mudrock Open lagoon sediments HST
IX Channelized conglomerate Proximal distributary channels HST
X Coral framestone-boundstone Reef crest coral assemblage ?TST-HST
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UNCORRECTED PROOF
are frequently attributed to rocky shoreline beach
deposits (eg, Johnson, 1988, 1992). We attribute the
basal facies to a beach and immediate offshore zone
where the large clasts were bored by gastrochaenid
bivalves. The exotic pebbles of igneous rock indicate
a secondary source, either direct riverine input of
material eroded from the Blue Mountains, or second
generation sediments derived from part of the earlier
Coastal Group (eg, the Bowden Formation: Pickerill
et al., 1998b).
The basal erosion surface is interpreted as a trans-
gressive surface of marine erosion that overlies a
sequence boundary. The bored blocks derived from
the Old Pera beds were either formed during subaerial
exposure during lowstand conditions, or during
erosion due to the landward migration of the wave
breaker zone.
4.2. Facies II: Coralline algal bindstone facies
This unit is exposed for approximately 100 m
immediately to the south of Pera Point (Fig. 1, local-
ities 1±3). Although the large in situ corals, which
grow from the top of this unit, are well-known (eg,
Donovan et al., 1994, 1997), the importance of crus-
tose coralline algae in binding the conglomerates has
only been reported by Pickerill et al.(1998a).
The basal conglomerate is overlain and stabilized
by a variably developed carbonate bindstone-frames-
tone unit that is 0±30 cm thick (Fig. 2). The boulders
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Fig. 2. Detailed graphic logs showing the lower part of the Port Morant succession at localities 1±4, and the relationship between facies. See
text for description of facies. Scale bar in metre intervals.
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UNCORRECTED PROOF
of the basal conglomerate are coated and cemented
together by a veneer of red crustose coralline algae.
The algal layer is typically only a few mm to 1 cm
thick; locally it is absent (Fig. 2, locality 4). Elsewhere
it is up to 30 cm thick and also contains many other
cementing organisms such as small corals [Siderastrea
radians (Pallas) and S. siderea (Ellis and Solander)]
and chamid bivalves. The algae coat boulders that
have G. cf. cluniformis borings, but they show no
sign of boring themselves.
Interpretation. The coralline algal bindstone facies
records a period of stabilization of the sea ¯oor during
which there was a lack of clastic sediment in¯ux.
Modern crustose corallian algae thrive in the upper
7 m of the water column where high wave energy
restricts coral growth and limits grazers (Adey,
1975). This facies is therefore interpreted as shallow
marine and records evidence for continued marine
transgression and high wave energy. It represents
the start-up phase of carbonate sedimentation
(Kendall and Schlager, 1981).
4.3. Facies III: Solenastrea boundstone facies
The upper surface of the algal bindstone has spor-
adic growths of large coral colonies that are up to
1.8 m high. These are clearly visible growing from
the basal surface between localities 1 and 4 (Figs. 2
and 3), but the tops of similar coral heads are also
visible protruding through the Recent beach sands at
various localities (eg, near localities 5, 6 and 7, Fig.
1). The most common coral present is Solenastrea
bournoni Edwards and Haime, but other species
include S. siderea, S. radians, Montastraea cavernosa
(Linnaeus) and Diploria strigosa (Dana). Secondary
frame builders associated with these corals include
crustose coralline algae, chamid bivalves and serpu-
lids. The bases of many of the corals display extensive
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Fig. 3. Photograph of the lower contact of the Port Morant Formation at locality 3. The basal erosion surface cuts into the Old Pera beds and is
overlain by the basal conglomerate (I). A single Solenastrea coral colony (III) grows from the upper stabilized surface of the conglomerate and
is buried by the sandstones of facies V. Hammer for scale.
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UNCORRECTED PROOF
borings, Gastrochaenolites cf. torpedo Kelly and
Bromley and G. cf. cluniformis, the latter occasionally
containing the gastrochaenid bivalve Rocellaria
(Gastrochaena) hians Gmelin (Pickerill et al.
1998a). Most of the secondary frame builders are
attached to the coral colonies.
Interpretation. The growth of the corals in this
facies represents a time of clear, relatively quiet
water of at least near normal marine salinity. Recent
S. bouronia occurs in sandy areas in water depths of
up to 15 m (Hudson et al., 1989). The restriction of the
boring bivalve crypts and secondary encrusters to the
bases of colonies indicates that these were exposed to
sea water up to the time of the corals death.
4.4. Facies IV: Bioclastic poorly sorted sandstone
facies
Immediately overlying the top surface of the algal
coating on the basal conglomerate between the coral
colonies of facies III is a bioclastic, weakly to moder-
ately cemented, poorly sorted muddy (clayey) sand-
stone up to 20 cm thick (Fig. 2), which contains a rich
and diverse fauna of corals and associated faunal
elements. This facies contrasts with the overlying
facies V (poorly sorted sandstone facies) by having
abundant corals; the overlying facies lacks corals. The
corals present include Undaria agaricites (Linnaeus),
S. radians, S. siderea, Manicina areolata (Linnaeus)
and Oculina diffusa (Lamarck). Other faunal elements
include spines of the echinoid E. tribuloides, and
bivalves.
Interpretation. The fauna found in the bioclastic
poorly sorted sandstone facies includes the free-living
forms of the small coral M. areolata. This species
commonly occurs in Recent sea grass communities
(Goreau, 1959, p. 73 ). We interpret the bioclastic
poorly sorted sandstone facies to be a distal equivalent
of the poorly sorted sandstone facies (facies V), and to
have been deposited around the Solenastrea bound-
stone facies (facies III). We envisage the Solenastrea
coral heads surrounded by sea grass meadows.
4.5. Facies V: Poorly sorted sandstone facies
This facies is extensively exposed along the
western coast to the south of Pera Point (Localities
1±6), and is also exposed beneath facies VII, the `crab
beds' (locality 8).
The bioclastic poorly sorted sandstone facies
(facies IV) is overlain by poorly sorted sandstone
forming beds from 20 to 60 cm thick. Boundaries
between beds can be picked out by stringers of exotic
pebbles (mainly andesite clasts), weakly calci®ed
concretionary layers or omission surfaces from
which burrow networks descend (Figs. 2 and 4). On
NW-SE trending stretches of the coastline (Localities
1±5, Fig. 1) the bedding has an apparent dip ranging
from very low angle to horizontal (Fig. 4); on N-S
trending stretches of the coastline (Locality 6, Fig.
1) the depositional surfaces have an apparent dip of
,108 to the south (Fig. 5). Both up-dip and down-dip
the depositional surfaces become horizontal, but lose
their identity in the heavily bioturbated parts of the
succession. Locally the up-dip extensions of depo-
sitional surfaces are truncated by erosively based
sheet conglomerates, by incised channelled conglom-
erates (Fig. 4), or to the south, by the `crab beds'. The
lower part of individual depositional surfaces is
destroyed due to extensive bioturbation by Thalassi-
noides paradoxicus (Woodward). The T. paradoxicus
burrow networks have been preferentially cemented
and where recent wave action has eroded the softer
sediment, a highly nodular fabric is revealed (Fig. 6).
T. paradoxicus is also abundant on the lower portion
of the depositional surfaces, but is progressively
replaced in the higher part of the depositional
surfaces by black-coloured Ophiomorpha nodosa
Lundgren (Fig. 4). Many of the Ophiomorpha
burrows are ®lled with pebbles piped-down from the
unit above. Beneath the `crab beds' at locality 8 (Fig.
1), numerous O. nodosa are present in the upper part
of the poorly sorted sandstone facies beneath the
erosion surface which truncates the depositional
surfaces.
The mudrocks are relatively poor in body fossils.
They are also partially decalci®ed, but a few layers
contain thin indeterminate bivalves.
Interpretation. These poorly sorted sandstones,
with their 4-5 m thick inclined depositional surfaces
(foresets), are interpreted as a braid delta (or distal
fan-delta) that prograded into the shallow marine
environment. The distribution of trace fossils indi-
cates a shallower water Ophiomorpha-dominated
assemblage passing laterally into a deeper water
Thalassinoides-dominated assemblage. The conglom-
erate stringers at the bases of certain layers are
S.F. Mitchell et al. / Sedimentary Geology 00 (2001) 000±0006
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UNCORRECTED PROOF
interpreted as river ¯ood events when pebbles were
carried onto the delta by ¯ood waters.
4.6. Facies VI: Erosively based sheet conglomerates
These are best exposed in the 200 m of cliff imme-
diately to the south of Pera Point (localities 1±4, and
just southeast thereof, Fig. 1). Donovan et al. (1994)
took the lowest of these conglomerate beds as the base
of their upper unit of the Port Morant Formation.
The conglomerates and minor interbedded coarse-
grained sandstones form bodies with distinctly erosive
bases with relief of up to 50 cm (Figs. 2 and 4). They
occur in beds 8 cm to 1 m thick and individual layers
can be traced laterally for distances of up to 50 m.
The conglomerates therefore have a `sheet-like' form.
The clasts range in size from granules to pebbles,
have a clast-supported fabric and a sandy matrix.
Clast types include variably altered porphyritic and
non-porphyritic andesite. Medium- and small-scale,
unidirectional tabular cross-bedding is present. The
interbedded units of coarse-grained sandstone contain
ripple cross-lamination. A few Thalassinoides burrows
are present in the conglomerates.
Interpretation. These conglomerates and associated
sandstones have unidirectional cross-bedding, and
lack fossils. They are interpreted as ¯uvial channel-
®ll deposits. Their occurrence as incised broad
channels cut into the poorly sorted sandstone facies
suggests that they represent distal distributary
channels on the proximal portion of the delta front.
4.7. Facies VII: Shelly pebbly sandstone facies (`crab
beds')
This facies is well-exposed at locality 8 (Fig. 1)
where large ex situ blocks are scattered across the
foreshore. The low cliff behind shows the facies in
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Fig. 4. Poorly sorted sandstones (facies V) overlain by erosively based sheet conglomerates (facies VI) at locality 4. Facies V contains nodular
Thalassinoides burrows and calcareous cemented layers indicating apparent horizontal bedding. Towards the higher part of the facies, dark
coloured Ophiomorpha burrows predominate. Facies V is overlain erosively and interdigitates with the sheet conglomerates of facies VI.
Sedimentary Geology ± Model 3 ± Ref style 2 ± AUTOPAGINATION 2 Alden23-04-2001 14:33 all KM
UNCORRECTED PROOF
S.F. Mitchell et al. / Sedimentary Geology 00 (2001) 000±0008
673
674
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766
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768Fig
.5
.R
elat
ion
ship
bet
wee
nfa
cies
inth
eP
ort
Mo
rant
Form
atio
nat
loca
lity
6.In
clin
edfo
rese
tsof
faci
esV
show
exte
nsi
ve
bio
turb
atio
nin
thei
rupper
par
tan
dar
eover
lain
by
the
shel
lysa
nd
sof
faci
esV
II.
Both
faci
esar
ecu
ter
osi
vel
yby
the
chan
nel
ized
conglo
mer
ates
(fac
ies
IX).
Note
ver
tica
lex
agger
atio
nof
scal
e.
Sedimentary Geology ± Model 3 ± Ref style 2 ± AUTOPAGINATION 2 Alden23-04-2001 14:33 all KM
UNCORRECTED PROOF
situ and the contact with the underlying facies V
(poorly sorted sandstone facies). The shelly pebbly
sandstone facies may also be seen as fallen blocks
on the foreshore south of locality 6, and in the cliff
at localities 6 and 7 (Fig. 1). This is the unit from
which Collins et al. (1996) described extensive crust-
acean faunas and from which Pickerill et al. (1998a)
recorded abundant hard substrate borings.
This facies consists of very poorly sorted, fossili-
ferous pebbly sandstones. They are erosively based,
truncating the sediments (Fig. 5) of the poorly sorted
sandstone facies (facies V). The pebbles, up to 2 cm
long, are well-rounded and consist of ma®c volcanics,
andesitic volcanics, and jasper. The ichnofabric is
dominated by abundant T. paradoxicus, which are
obvious where there are signi®cant lithological
contrasts.
Body fossils are abundant and include assemblages
dominated by bivalves, gastropods, and crustaceans.
The bivalves include numerous Chione sp., which is
particularly abundant in the lower part of the bed,
together with `Ostrea' sp. which has attachment
scars carrying imprints of mangrove roots. Some
Chione sp. are bored by Oichnus ispp. (Pickerill et
al., 1998a). The gastropods include: Strombus gigas
(LinneÂ) (Pickerill and Donovan, 1997), and Bulla cf.
striata (BruguieÁre), the former commonly encrusted
by the coral S. radians and bored by annelids leaving
the trace fossil Caulostrepsis isp. (Pickerill et al.,
1998a). Other groups represented include abundant
crustaceans (see Collins et al., 1996; Collins and
Donovan, 1997) and serpulids. Corals are fairly
diverse, but relatively rare: they include Stephano-
coenia intersepta (Lamarck), Acropora cervicornis
(Lamarck), U. agaricites, S. radians, S. siderea,
Porites porites (Pallas), Favia fragum (Esper), M.
areolata, O. diffusa, S. bournoni and Eusmilia fasti-
giata (Pallas).
Interpretation. The presence of oysters with attach-
ment scars of mangrove roots indicates that
mangroves grew on the shore zone adjacent to the
delta. The remaining fauna (corals, diverse crabs
and molluscs) indicates relatively normal marine
salinities. The assemblage clearly suggests an open
marine environment. The erosive base of the unit
and its transgressive nature indicates that the base in
all likelihood represents a ¯ooding event. This facies
can be interpreted in two ways. It may relate to a
period of inactivity of this portion of the delta during
which wave energy cut an erosion surface and the
S.F. Mitchell et al. / Sedimentary Geology 00 (2001) 000±000 9
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795
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798
799
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804
805
806
807
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811
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813
814
815
816
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819
820
821
822
823
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826
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828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
Fig. 6. Lower part of the poorly sorted sandstones (Facies V) at locality 4, showing extensive nodular texture due to extensive bioturbation
(Thalassinoides) which becomes less nodular upwards. Hammer for scale.
Sedimentary Geology ± Model 3 ± Ref style 2 ± AUTOPAGINATION 2 Alden23-04-2001 14:33 all KM
UNCORRECTED PROOF
delta margin was stabilized by mangroves, or it may
mark a second cycle of sea-level rise in the Port
Morant Formation.
4.8. Facies VIII: Algal-mudrock facies
This facies is well-exposed on the small islands at
localities 9±11 (Fig. 1). It is also intermittently
exposed in the low-lying cliffs to the east of the
`crab beds' exposure (locality 8).
The facies is characterised by unstrati®ed, poorly
sorted mudrocks and sandy mudrocks that contain
common to abundant skeletal fragments of calcareous
algae. The algae are dominated by Halimeda and/or
Amphiroa. Accordingly, two subfacies are recognised.
Facies VIIIa: Amphiroa mudstone facies. This facies
is exposed on two small islands (localities 9 and 10,
Fig. 1) and comprises poorly sorted sandy mudstone,
which develops a knobbly weathering texture due to
preferential cementation of Thalassinoides burrows.
On weathered surfaces abundant small `sticks' of
the calcareous red alga Amphiroa are abundant, and
scattered rhodoliths are present. At locality 9,
molluscs are rare, being represented by Cerithium
sp. and some bivalves, while scattered corals include
U. agaricites, Undaria sp. S. radians, Porites furcata
Lamarck, F. fragum and M. areolata. Towards the
south (locality 10: Fig. 1), bivalves and chelae of
callianassid shrimps become more common. The scat-
tered corals include S. radians, D. strigosa, S. bour-
noni, U. agaricites, and a single branch of Acropora
palmata (Lamarck).
Subfacies VIIIb. Halimeda-Amphiroa mudstone
facies. This subfacies is exposed on a small island
(locality 11, Fig. 1) and is characterised by poorly
sorted, sandy mudstone containing abundant Hali-
meda plates (Fig. 7) and Amphiroa`sticks'. Also
present are numerous small oyster shells, echinoid
spines, isolated barnacles and callianassid chelae.
The sediment has been extensively bioturbated
S.F. Mitchell et al. / Sedimentary Geology 00 (2001) 000±00010
865
866
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870
871
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873
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878
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881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
Fig. 7. Detail of algal mudrock facies at locality 11 showing disarticulated remains of the green alga Halimeda. Coin has a diameter of 2 cm.
Sedimentary Geology ± Model 3 ± Ref style 2 ± AUTOPAGINATION 2 Alden23-04-2001 14:33 all KM
UNCORRECTED PROOF
by presumed decapod crustaceans producing Thal-
assinoides. Locally a rich coral fauna is present
including A. cervicornis, S. siderea, Porites astreoides
(Lamarck), P. furcata, P. porites, F. fragum, M. areo-
lata, Colpophyllia natans (MuÈller), O. diffusa, Mean-
drina meandrites (Linnaeus) and E. fastigiata.
Interpretation. The facies, found seaward of the
`crab beds' (facies VII), is regarded as a lateral
equivalent. It is interpreted to represent a more
open lagoon environment with a soft bottom faunal
assemblage. The facies was probably dominated by
sea grass since small sea-grass corals such as M. areo-
lata and S. radians are present. The algae Halimeda
and Amphiroa commonly grow in sea grass communi-
ties (Goreau, 1959, p. 81).
4.9. Facies IX: Channelized conglomerate facies
The facies is only exposed at localities 6, 7 and 12
(Fig.1). It erosively cuts into the following facies:
facies V and VII at locality 6 (Fig. 5); facies VII at
locality 7; and facies VIII at locality 12.
The facies consists of clast-supported pebble
conglomerates and coarse-grained sandstones, which
®ll well-de®ned channels with relief of up to 3 m
(Figs. 5 and 8). The pebbles consist of well-rounded
clasts of ma®c and felsic volcanic rocks (including
andesite). The conglomerates lack body fossils and
ichnofossils.
Interpretation. The strongly channelized conglom-
erates of this facies are interpreted as distributary
channels cut by rivers on the delta top. This is
supported by the lack of trace and marine body fossils.
It is likely that this facies passes seawards into facies
VI (erosively based sheet conglomerates).
4.10. Facies X. Coral framestone-boundstone facies
At Canoe Bay (locality 15, Fig. 1), and on the small
island (Locality 14) and headland (Locality 13)
S.F. Mitchell et al. / Sedimentary Geology 00 (2001) 000±000 11
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979
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986
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989
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991
992
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998
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1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
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1028
1029
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1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
Fig. 8. Channelized conglomerates (facies IX) cutting erosively into the poorly sorted sandstones (facies V) at locality 6. The foresets in Facies
V gently dip down towards the right, and 1 m erosive relief is visible on the base of the conglomerates.
Sedimentary Geology ± Model 3 ± Ref style 2 ± AUTOPAGINATION 2 Alden23-04-2001 14:33 all KM
UNCORRECTED PROOF
nearby, extensive exposures of this facies are visible.
The relationship between this facies and facies I to IX
is unknown due to lack of exposure.
The limestones around the coast of the small island
(Locality 14) are heavily karsti®ed, making obser-
vation dif®cult for the most part. However, on the
seaward side of the island, there are extensive areas
dominated by large in situ coral colonies. A. palmata
accounts for t90% of the coral cover at this locality.
Other important coral species include large round
colonies of D. strigosa and columnar Montastraea
annularis (Ellis and Solander) (Fig. 9). Also notable
are Thalassinoides and the abundant mouldic remains
of S. gigas.
At Canoe Bay (Locality 15) the succession can be
divided into a lower and upper part. Both parts are rich
in corals and have subhorizontal bedding. The lower
part of the succession consists of abundant large
corals set in a hard cemented sandy matrix. The
fauna in this lower portion is dominated by massive
C. natans, M. cavernosa, M. annularis, D. strigosa
and P. astreoides. Other corals include rare A.
palmata and slightly more common agariciids, P.
porites, P. furcata, Diploria labyrinthiformis
(Linnaeus), M. areolata, Meandrina sp., Mycetophyl-
lia ferox Wells, M. lamarckiana (Edwards and Haime)
and E. fastigiata. The upper part of the succession
consists of less-indurated sandstones, which contain
smaller corals including U. agaricites, U crassa
(Verrill), S. siderea, S. radians, P. astreoides, P.
divaricata Lesueur, P. furcata, F. fragum, D. laby-
rinthiformis, M. areolata, M. annularis, M. faveolata
(Ellis and Solander), Scolymia cubensis (Edwards and
Haime) and Mussa angulosa (Pallas).
Locality 13, while not as well-exposed as localities
14 and 15, yielded a number of small massive corals
including S. siderea, Dichocoenia stellarisEdwards
and Haime, and some branched Porites sp.
S.F. Mitchell et al. / Sedimentary Geology 00 (2001) 000±00012
1057
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1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
Fig. 9. Example of the columnar variety of the coral Montastraea annularis (Ellis and Solander) at locality 14. This morphology is typical of
those found on the exposed, wave-washed reef crest zone of modern Caribbean reefs. Hammer for scale.
Sedimentary Geology ± Model 3 ± Ref style 2 ± AUTOPAGINATION 2 Alden23-04-2001 14:34 all KM
UNCORRECTED PROOF
Interpretation. The coral framestone-boundstone
facies were deposited as part of a fossil fringing reef
trending roughly parallel to the present shoreline and
extending for approximately 1.5 km (Fig. 1). The
shallow marine facies described above apparently
formed in relatively sheltered environments behind
this reef. The abundant in situ A. palmata at locality
14, in association with numerous D. strigosa and M.
annularis, indicates that this locality represents an
exposed, wave-washed reef crest zone similar to
those described on modern and fossil reefs (Geister,
1975; Graus, et al., 1984; Greenstein and Pandol®,
1997). On modern reefs, the A. palmata zone is found
on high-energy reef crests usually at depths of less than
6 m (Goreau, 1959; Geister, 1975; Graus, et al., 1984).
The assemblage of massive and some smaller branched
corals at locality 15, Canoe Bay, and to some extent
locality 13 are similar to the mixed coral zone of
Graus et al. (1984). Based on their inferred position
on the fringing reef (Fig. 1) and the lack of deposi-
tional dip that would suggest a reef front environment,
these localities were probably part of a shallow mixed
coral zone just behind the wave resistant A. palmata
zone. Outcrop conditions do not allow us to identify
the substrate upon which the corals are growing.
5. Stratal architecture and sequence stratigraphy
The known lateral continuity and the vertical tran-
sition in the facies described herein is shown
schematically in Fig. 10. The transgressive systems
S.F. Mitchell et al. / Sedimentary Geology 00 (2001) 000±000 13
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1177
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1186
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1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204
1205
1206
1207
1208
1209
1210
1211
1212
1213
1214
1215
1216
1217
1218
1219
1220
1221
1222
1223
1224
1225
1226
1227
1228
1229
1230
1231
1232
1233
1234
1235
1236
1237
1238
1239
1240
1241
1242
1243
1244
1245
1246
1247
1248
Fig. 10. (A) Lateral facies relationships deduced for Facies I to X in the Port Morant Formation at Old Pera (approximate scales indicated). (B)
Sequence stratigraphic interpretation of facies shown in A showing chronostratigraphic relationships and interpreted systems tracts.
Sedimentary Geology ± Model 3 ± Ref style 2 ± AUTOPAGINATION 2 Alden23-04-2001 14:34 all KM
UNCORRECTED PROOF
tract is represented by facies I to III. These record
progressive deepening of the marine succession.
Facies I represents a transgressive shoreface that
rests erosively on the Old Pera beds. The offshore
shelf sandstones of the Old Pera beds range in age
from late Pliocene to earliest Pleistocene (Budd and
McNeill, 1998). There have therefore been many
cycles of sea-level rise and fall between the deposition
of the Old Pera beds and the Port Morant Formation
during which the former was subject to subaerial
exposure. Consequently, the base of facies I represents
a transgressive surface overlying a sequence bound-
ary. The conglomerates of facies I may represent
material eroded during the lowstand in a subaerial
environment or formed by erosion during shoreline
transgression. Facies II records the stabilization of
the shoreface conglomerates by calcareous algae
(the start-up phase of Kendall and Schlager, 1981),
and facies III the growth of coral heads in the lagoon
with little clastic in¯ux. We envisage the paleoenvir-
onment during the transgressive system tract as a shal-
low marine embayment with the growth of isolated
coral heads with sea grass meadows between and a
high-energy rocky shoreline. The high energy depos-
its of facies I suggest that there was high wave energy
at the shoreline and that a well-developed reef barrier
was absent, patchily developed, or had not grown up
to sea level.
The maximum ¯ooding surface is placed at the top
of facies III, and represents the most landward shift in
facies. This is succeeded by the highstand systems
tract. The highstand systems tract is represented by
the progradational (downlapping) deltaic deposits of
facies V, the shelly sands of facies IV and the sheet
conglomerates of facies VI. Modern clastic deposits
associated with Caribbean islands with non-carbonate
highlands, such as Jamaica, are deposited as fan-
deltas adjacent to uplifted areas (Westcott and Ethe-
ridge, 1980, 1983) or braid deltas where braided rivers
reach the sea (McPherson et al., 1987). The clasts in
the terrestrial deposits of the Port Morant Formation
(facies VI and IX), match the lithologies found in
the Cretaceous and Tertiary succession in the Blue
Mountains. This indicates that during Sangamonian
time a signi®cant catchment existed that drained the
Blue Mountain Block and entered the sea as a fan- or
braid delta at Old Pera. It is the marine portion of this
delta that represents the progradational clastic wedge
of the Port Morant Formation. No such river system,
carrying coarse-grained detritus, is active at the
present time, indicating subsequent river capture.
The erosion surface at the base of the crab beds
(facies VII) is interpreted as a marine ¯ooding
surface, which was succeeded by continued progra-
dation of the shoreline. This erosion surface might
represent a second pulse of sea-level transgression
following a small sea-level fall. Alternatively, the
surface might represent a transgression related to the
abandonment of the delta. The latter hypothesis is
illustrated in Fig. 10. The presence in facies VII of
oysters with mangrove root impressions indicates the
presence of mangroves growing around the delta
front. Mangroves are widespread along the modern
coastline at Port Morant, and clearly were also import-
ant in stabilizing the delta front during the Sanga-
monian. The channelized conglomerates of facies IX
are also part of the highstand systems tract represent-
ing distributary channels on the delta top.
Facies X is dif®cult to relate to the other facies as it
is separated from them by a signi®cant unexposed
interval (Fig. 1). Furthermore the base of the facies
is unexposed. It might be equivalent to the transgres-
sive systems tract (facies I to III) or facies VII of the
highstand systems tract. In Fig. 10 it is tentatively
attributed to the transgressive and early highstand
systems tracts with the maximum ¯ooding surface
tentatively placed between the two coral assemblages
recognised at Canoe Bay. This surface corresponds to
an increase in sand content and may be related to a
seaward shift in depositional facies.
6. Discussion
The Port Morant Formation clearly represents an
important episode of deposition of mixed clastic-
carbonate facies in the Sangamonian. The carbonate
deposits are associated with two major settings. They
predominate as algal bindstones/framestones and as
isolated large coral heads in the transgressive systems
tract when the carbonate system was in start-up mode
after the major transgression at the base of the Port
Morant Formation. At this time new accommodation
space was created at a high rate, outpacing sediment-
ation. Consequently clastic deposition was pushed
landward and probably became restricted to drowned
S.F. Mitchell et al. / Sedimentary Geology 00 (2001) 000±00014
1249
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1262
1263
1264
1265
1266
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1268
1269
1270
1271
1272
1273
1274
1275
1276
1277
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1279
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1284
1285
1286
1287
1288
1289
1290
1291
1292
1293
1294
1295
1296
1297
1298
1299
1300
1301
1302
1303
1304
1305
1306
1307
1308
1309
1310
1311
1312
1313
1314
1315
1316
1317
1318
1319
1320
1321
1322
1323
1324
1325
1326
1327
1328
1329
1330
1331
1332
1333
1334
1335
1336
1337
1338
1339
1340
1341
1342
1343
1344
Sedimentary Geology ± Model 3 ± Ref style 2 ± AUTOPAGINATION 2 Alden23-04-2001 14:34 all KM
UNCORRECTED PROOF
river valleys. This allowed an open marine bay with
clear waters to form allowing the growth of the coral
heads. Carbonate deposition also occurs in facies X
which might represent either a reef barrier to the open
marine lagoon that developed during the transgressive
systems tract, or a fringing reef that developed sea-
ward of the prograding delta front. Unfortunately, the
relationships between facies X and the rest of the Port
Morant Formation are poorly constrained due to
poor exposure, however both alternatives are equally
plausible. Clastic deposition in the investigated
portion of the Port Morant Formation is largely
restricted to the highstand systems tract (although a
transgressive conglomerate is developed at the base
of the transgressive systems tract) when the braid/fan-
delta was able to prograde into the shallow lagoon. At
this time, the creation of new accommodation space
was reduced and sediment supply was high leading to
the rapid in®lling of the embayment. The Port Morant
Formation therefore represents an eloquent model for
the formation of mixed clastic-carbonate depositional
systems in tropical environments.
The sequence stratigraphic interpretation of the
Port Morant Formation clearly indicates that it
contains a signi®cant record of Sangamonian sea-
level ¯uctuations. Two sea-level highstands separated
by a short sea-level fall have been demonstrated in
early oxygen isotope substage 5e in Bermuda and
the Bahamas (Hearty and Kindler, 1995; Neumann
and Hearty, 1996; White et al., 1998; Wilson et al.,
1998), Hawaii (Sherman et al., 1993), Italy (Kindler et
al., 1997) and South Carolina (Hollin and Hearty,
1990). These studies suggest high sea-level stands
from 135-125 kyr and 124±122 kyr, separated by a
short sea-level fall. Two coral samples from the Port
Morant Formation have been dated using electron
spin resonance (Mitchell et al., 2000). A coral sample
from facies III gave an age of 125 ^ 7 kyr, but
showed some dissolution of the primary coralline
aragonite as well as secondary mineral precipitation
within pore space. The age can therefore be only
considered a minimum age. A coral sample collected
from facies VII yielded an age of 132 ^ 7 kyr that was
believed to be accurate. This suggests that facies VII
represents the earliest sea-level highstand recognised
in isotope substage 5e. There are therefore two possi-
bilities for an interpretation of the two transgressive
episodes (Facies I to II and facies VII) in the Port
Morant Formation. Either, the whole of the Port
Morant Formation belongs to the sea-level highstand
in early substage 5e, and facies VII represents a trans-
gressive surface related to the abandonment of the
delta, or facies VII to IX represent the early sea-
level highstand in substage 5e, and facies I to VII
represent a sequence of pre-substage 5e age. Further
dates from unaltered corals (which have not yet been
identi®ed) are required to distinguish between these
alternatives.
7. Conclusions
Ten sedimentary facies (I±X) are recognised in the
mixed clastic-carbonate depositional system in the
Port Morant Formation. The transgressive systems
tract is represented by a transgressive conglomerate
(facies I), coralline algal bindstone (facies II) and
coral heads (facies III). The highstand systems tract
shows the progradation of a delta (facies IV to VI). A
second transgressive facies is present represented by
fossiliferous pebbly sandstones (facies VII), and
succeeded by algal mudstones (facies VIII) and ¯uvial
incised conglomerate channels (facies IX). A coral
reef crest assemblage (facies X) is present, but its
exact relation with the other facies is unknown due
to poor exposure. The Port Morant Formation is inter-
preted as a lagoonal-®ll succession and can be used as
a model for tropical mixed clastic-carbonate depo-
sitional systems.
An electron spin resonance date from a coral in
facies VII of the Port Morant Formation gave an age
of 132 ^ 7 kyr indicating the early high sea-level
stand in isotope substage 5e. This suggests two possi-
bilities for the two transgressive events in the Port
Morant Formation, (i), the whole of the formation
belongs to substage 5e, the upper transgressive event
being related to delta abandonment, (ii) the upper
transgressive event belongs to early substage 5e, and
the lower Port Morant Formation is of pre-substage 5e
age.
Acknowledgements
We thank Steve Donovan (BMNH) for help in the
®eld. RKP gratefully acknowledges the ®nancial
support of NESRC. Comments by Andre Strasser
S.F. Mitchell et al. / Sedimentary Geology 00 (2001) 000±000 15
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UNCORRECTED PROOF
and an anonymous reviewer greatly improved the
original manuscript.
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