The Port Morant Formation (Upper Pleistocene, Jamaica ... · high resolution sedimentology and paleoenvironmental analysis of a mixed-carbonate-clastic lagoonal succession Simon F

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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: [email protected] (S.F. Mitchell).

Sedimentary Geology ± Model 3 ± Ref style 2 ± AUTOPAGINATION 2 Alden23-04-2001 14:32 all KM

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).

S.F. Mitchell et al. / Sedimentary Geology 00 (2001) 000±0002

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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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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

S.F. Mitchell et al. / Sedimentary Geology 00 (2001) 000±000 3

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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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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.


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.


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


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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.


UNCORRECTED PROOF


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.5

.R

elat

ion

ship

bet

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nfa

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inth

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rant

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the

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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


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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.


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


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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.


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)


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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.


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.


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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.


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


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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.


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


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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 AndreÂ Strasser


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UNCORRECTED PROOF

and an anonymous reviewer greatly improved the

original manuscript.

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The Port Morant Formation (Upper Pleistocene, Jamaica ... · high resolution sedimentology and paleoenvironmental analysis of a mixed-carbonate-clastic lagoonal succession Simon F