Foraminiferal paleoecology of the Montpelier and lower coastal groups (eocene-miocene), Jamaica, West Indies

Palaeogeography, Palaeoclimatology, Palaeoecology, 16( 1974)217--242 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

FORAMINIFERAL PALEOECOLOGY OF THE MONTPELIER AND LOWER COASTAL GROUPS (EOCENE--MIOCENE), JAMAICA, WEST INDIES 1

P. LEWIS STEINECK

Division of Natural Sciences, State University College at Purchase, Purchase, N.Y. (U.S.A.) (Submitted for publication July 4, 1973; accepted June 27, 1974)

ABSTRACT

Steineck, P. L., 1974. Foraminiferal paleoecology of the Montpelier and Lower Coastal Groups (Eocene--Miocene), Jamaica, West Indies. Palaeogeogr., Palaeoclimatol., Palaeoecol., 16:217--242. Two domains of carbonate deposition characterized mid-Tertiary Jamaica. After latest Cretaceous to Paleocene orogeny, submergence of insular paleo-Jamaica accompanied the strike-slip or extensional faulting associated with the formation of the Cayman Trench to the north. Differential subsidence along a series of peripheral subsea escarpments (DuanvaleWagwater escarpment) produced relief in excess of 2000 m by the Late Eocene. Shoalwater limestones covered the slowly subsiding Cornwall--Middlesex platform, thus ending the supply of clastics to deep-sea bottoms north and east of the escarpment where contemporaneous planktonic-foraminiferal pelagites accumulated. Middle Eocene to Middle Miocene carbonate rocks deposited in the deep-sea represent a distinctive lithogenetic unit termed the Montpelier Group. A preponderance of globigerinacean and radiolarian tests characterizes lower Montpelier microfossil assemblages. Dominant benthonic forms include Melonis pompilioides, Fontbotia wuellerstorfi and species of Stilostomella and Pleurostomella. Available faunal criteria including the assemblage composition, depth preferences of extant species and recurrent morphologic--ecologic patterns suggest abyssal (2000 m) paleo-depths at the site of accumulation on a sediment apron near the base of the Duanvale--Wagwater escarpment. Computed from inferred paleodepth and estimated sedimentary thickness, Middle Eocene to Lower Miocene subsidence totals 2800 m. Biostratigraphic and paleoecologic data do not support the prevalent concept of a regional unconformity within the Montpelier. In the Middle Miocene, regional uplift led to the emergence of the Cornwall--Middlesex platform and to pronounced shoaling of marginal sea bottoms. Here, hemipelagic sedimentation resumed during the later Middle Miocene after the carbonate veneer on the adjacent platform was sufficiently eroded so as to expose noncarbonate rocks.

l See NAPS document No. 02395 for 67 pages of faunal reference lists, maps of sample locations and tables of foraminiferal occurrences. Order from ASIS/NAPS, c/o Microfiche Publications, 305 East 46th St., New York, N.Y., U.S.A. 10017. Remit with order ~9.05 for photocopies or $5.55 for microfiche. Make checks payable to Microfiche Publications.

218 INTRODUCTION

Recognizing that the origin and evolution of exposed fold and tectonic systems are closely linked to an adjacent ocean basin is a significant implication of the current synoptic theories on sea-floor spreading and plate tectonics. The geosynclinal cycle, formerly the Ptolemaic center of geotectonic synthesis, is held to be a predictable and varying manifestation of a Copernican system centered on ocean-basin tectonics: the generation of new crust at oceanic rises and its eventual consumption at trenches. Although mobilist theories receive widespread acceptance, even enthusiastic supporters have recognized that many aspects of the development of small ocean basins such as the Caribbean (Fig.l) are open to various interpretations (MacGillivray, 1970; Edgar et al., 1971; Malfait and Dinkelman, 1972; Walper and R o w e t t , 1972; Moore and Del Castillo, 1974). In particular, considerable controversy remains over the relationship between the Caribbean region and the evolution of the m o d e m Atlantic Ocean. Progress in the reconciliation of

Fig.1. Generalized location map of the Caribbean region.

219 contrasting opinions must be based on the geophysical surveying of the deepsea basins and on the geological reinterpretation of exposed stratigraphic sequences in and around the Caribbean. Towards this aim, studies of exposed deep-sea sediments are of signal value because these sequences bear the direct imprint of oceanic tectonics and can be resampled extensively. Available data on Jamaican geology indicate that high-angle normal faults of the D uanvale--Wagwater system (Wright, 1971 ) separated mid-Tertiary Jamaica (Fig.2) into two major depositional realms characterized by distinctive lithogenetic rock-sequences. Seaward of the fault system calcareous pelagites and hemipelagites accumulated in an oceanic environment while at the same time, b u t with p r o f o u n d environmental contrast, platform carbonates were deposited in shoaling waters on the adjacent, slowly subsiding Cornwall-Middlesex block. Hill (1899), noting the unique nature of the marginal Eocene to Miocene sediments of Jamaica, first suggested an oceanic milieu for their genesis. In an a t t e m p t to substantiate this view the paleoecologic and biostratigraphic implications of foraminiferal assemblages in over 150 samples from the deeper-water sequence are recorded below. The paleogeographic reconstruction elucidated in this study contributes to the understanding of the development of Jamaica, and ultimately, the geotectonic evolution of this part of the Caribbean. METHODS As a result of post-Middle Pliocene block faulting and folding (Zans et al., 1963) exposures in Jamaica of Early and Middle Tertiary sediments are fragmental and incomplete. Study of these rocks is hindered further because available outcrops are obscured by deep subaerial weathering and dense vegetation. To provide regional control, six isolated sections were sampled (Fig.2). This collection was supplemented by splits of samples collected by E. Robinson, University of West Indies (Robinson, 1969a,b) from five additional locations. The generally indurated nature of the carbonate rocks c o m p o u n d e d the problem of faunal analysis as suitable assemblages were recovered only from intercalated marly layers. Through trial and error, a method involving repeated soakings in the surfactant Q u a t e r n a r y - 0 (Geigy Chemical Corp., Ardsley, N.Y.) was found to yield the richest and best-preserved foraminiferal residues. Biostratigraphic and paleoecologic data were obtained from each sample by noting the foraminiferal faunas present. Wherever possible a minimum of 200 specimens of both planktonic and benthonic forms were picked from each sample which was then scanned for any rare planktonic species that might be useful for biozonation. Many samples, particularly from the Bonny Gate Formation, yielded fewer than the desired number of specimens of either foraminiferal group. These samples are included in the present study only when

220 DISCOVER'*( B A Y M O N T P E L q E R - HOP,ETON

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Fig.2. Mal~ of Jamaica showing areas from which samples have been collected.

the recovered faunas were considered diagnostic for either biostratigraphic or paleoecologic assignment. Biostratigraphic zonation and correlation of the Cenozoic strata of Jamaica was enhanced after attention was paid to the planktonic foraminifers (compare Zans et al., 1963, with Robinson, 1967b). The stratigraphic distribution of planktonic species provided correlations between the disjunct sections studied in this report. For the Eocene to Middle Miocene interval, zones proposed by Blow (1969) are readily recognized in the Jamaican chalks. In his zonation, however, Upper Miocene zones are based on subtle evolutionary changes in the genus Globorotalia which the writer found to be subjective and difficult to apply in contrast to more objective and readily recognizable zonal definitions for the Upper Miocene in Lamb and Beard (1972). Accordingly correlation of Upper Miocene sediments was accomplished through the later zonation. The present author's synecologic analysis of the Jamaican chalks utilized the tripartite method developed by Orville Lee Bandy (Bandy, 1953a,b; Bandy and Arnal, 1960, 1969). Three major independent but mutually corroborating lines of evidence yielded data for interpretation including: (1) quantitative faunal parameters; (2) depth preferences for extant species; and (3) recurrent ecologic--morphologic relationships. GEOLOGIC FRAMEWORK The interested reader is referred to Arden (1968), Robinson (1969a) and Robinson et al. {1971) for modern summaries of the geology of Jamaica. The discussion presented below focuses on the mid-Tertiary chalks and coeval shallow-water units (see Fig.3).

221 LITHOFACIES

LITHOSTRATIGRAPHY

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CRETACEOUS UNITS VOLCANICLASTIC REEFAL C A R B O N A T E

EXPOSED

IN

INLIERS

Fig.3. Generalized stratigraphic column for Jamaica.

Cre t a c e o u s - - L o wer Tertiary

Jamaica lacks a basement complex of Paleozoic age (Zans et al., 1963). The oldest rocks exposed are the Westphalia Schists in the Blue Mountain region referred to as "indefinite pre-Cretaceous" by Robinson et al. {1971). Cretaceous rocks are thought to underlie much of Jamaica, as they presently crop out in disjunct inliers in the central part of the island and in the Blue Mountains (Zans et al., 1963). Coates (1969) and Arden {1968) concurred that these rocks record the formation of an andesitic volcanic archipelago and its later denudation by subaerial erosion. Maestrichtian volcanic activity presaged a period of latest Cretaceous to Paleocene orogenesis, during which granodioritic and ultrabasic magmas were intruded into the island-wide volcanic-sedimentary sequence, accompanied by extensive folding and thrusting. These events were part of a Caribbean-wide orogenic pulse (MacGillivray, 1970) which lead locally to the emergence of an insular land-mass.

222 Basal Tertiary strata are part of a thick clastic wedge deposited in a rapidly subsiding graben in the region of the present-day Blue Mountains whose western border was delineated by the northwest--southeast trending Wagwater Fault. This trough was filled by sediments derived from the andesitic and granodioritic highlands to the west and east. The Wagwater Group consists of poorly bedded purple to reddish conglomerates, sandstones and tuffaceous shales, which are no older than Lower Eocene (Robinson, 1969a); rocks of Paleocene age are apparently absent from Jamaica. Marginal (western) Wagwater beds contain sedimentary features indicating deposition in an alluvial plain or littoral environment (Burke and Robinson, 1965). To the east, in the main part of the Wagwater Trough, equivalent clastic units containing marine fossils formed in a shallow marine environment. The overlying Richmond Group is generally finer grained than the Wagwater beds and consists of brown to red well-bedded shales, sandstones and carbonaceous limestones. A transition from paralic lithofacies at the trough margin to fully marine beds containing algae, molluscs and foraminifers in the Wagwater Trough proper was noted by Robinson (1969a). Planktonic foraminifers recovered from mudstones near the top of the Richmond suggest a Lower Eocene age.

Mid-Tertiary The Yellow Limestone (Font Hill and Claremont Formations), White Limestone (Clarendon and Montpelier Groups), and Lower Coastal Groups of earlier usage (Zans et al., 1963; Robinson, 1969c) comprise the sedimentary expression of the complex Tertiary history of Jamaica. The stratigraphic nomenclature, correlations and lateral equivalence of these rocks are summarized in Fig.4. The diversity of lithologies present within the mid-Tertiary rocks of Jamaica is related to two factors. The first factor was the gradual subsidence and later emergence of the central Cornwall--Middlesex platform (Fig.2). During the early Middle Eocene, transgressive marine deposits onlapped Jamaica from the north and east (e.g. from the direction of the previously existing Wagwater Trough; Wright, 1971). The covering of this platform in the Lower and Middle Eocene by a veneer of limestones initiated a period of clastic-free deposition throughout Jamaica. Uplift and erosion of the Cornwall--Middlesex block during the late Middle Miocene is suggested by the fine clastics of the Lower Coastal Group. The second factor involves activity along the Duanvale--Wagwater fault system. These faults, circumscribing the Cornwall--Middlesex block, became active during the late Middle to early Late Eocene (Wright, 1971). From the Late Eocene to Early Pliocene these faults formed subsea escarpments resulting in the deposition of two coeval, but contrasting, mid-Tertiary lithofacies: shoal-water limestones of the Clarendon Group were deposited on the

223 Platform

Cornwall'Middlesex

SOUTH

CENTRAL

Upper

Augustown

Miocene

Fm Buff Bay Formation

Middle

~Subaerial

Miocene

Spring Garden Formation

"~rosion &

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Lower Miocene

COASTAL JAMAICA

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BrownstownFro.

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Oligocene

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Upper Middle Eocene

Chapleton Formation

Font Hill Formation

Fig.4. Stratigraphic n o m e n c l a t u r e of Mid-Tertiary Jamaica.

Cornwall--Middlesex platform, while the deeper-water chalks of the Montpelier Group accumulated on marginal ocean bottoms.

Clarendon Group The Clarendon Group (see Fig.4) contains two major biofacies which persisted through the Middle Tertiary. A nearshore or lagoonal peneroplid-miliolid--molluscan biofacies is developed in the Walderston, Troy and Newport Limestones. The coralgal--orbitoid-rich limestones of the Swansick and Brownstown Formations are thought to represent an open-water shelf-edge environment (Robinson, 1969c; Wright, 1971 ). These biofacies migrated across the block in response either to eustatic changes in sea level or warping of the underlying platform (Robinson et al., 1971). The Miocene N e w p o r t Limestone is restricted to the southern part of the island where deposition was partly contemporaneous with uplift, erosion and karstification of central Jamaica (Robinson, 1967b, 1969c).

Montpelier Group Robinson (1967a, 1969b) reviewed the nomenclatural history of the "Montpelier" and restricted its use to beds correlative with Hill's (1899)

224 type section south of Montego Bay. Here the Montpelier contains Miocene planktonic and larger foraminifers. Robinson (1965, 1967a) recognized a second and older deep-water carbonate unit in eastern Jamaica termed the Bonny Gate Formation that was subsequently shown to underlie the Montpelier throughout the outcrop belt of the deep-water chalks (Robinson, 1969a). As redefined in the present work, the Montpelier is raised to group status and used in the original sense of Hill (1899) to include all the deepwater Tertiary chalks of Jamaica. Two formations and an informal unit are recognized. (1) Bonny Gate Formation (upper Middle Eocene to Upper Oligocene): This formation is composed of chalky to indurated planktonic foraminiferal micrites conformably overlying the Yellow Limestone Group in eastern Jamaica. Strata underlying the Bonny Gate in western Jamaica are not exposed but are presumed to be a deep-water facies of the Yellow Limestone Group. The basal Lloyds Member in the Yallahs area contains interbedded clastic, noncarbonate and carbonate slide and olistostrome units (Robinson, 1967a); noncarbonate material is absent in higher parts of the formation. The ubiquitous presence of nodular to bedded brownish-red weathered chert separates the Bonny Gate from the superficially similar higher formations of the Montpelier. Graded bioclastic layers containing coral and orbitoid debris occur throughout the formation but are particularly common in the upper part in which large allochthonous blocks of Clarendon lithology are also found. Robinson (1965) reported a maximum thickness of 1000 ft. in the Yallahs area. The top of the Bonny Gate is not known to be exposed and previous authors postulated a regional unconformity at this level (Zans et al., 1963; Arden, 1968; Blow, 1969; Robinson, 1969c; Wright, 1971). (2) Sign beds (uppermost Oligocene to Lower Miocene): This unit includes soft, even-bedded radiolarian and planktonic foraminiferal-rich chalks referred to as the "cherty lower Montpelier" by Robinson (1969b,c). A typical outcrop of this informal unit is the series of low bluffs along road shoulders in the immediate vicinity of Sign crossroads, along the Montego Bay--Adelphi road in northwestern Jamaica (Fig.2). Chert-bearing Sign chalks also occur along the north coast near Falmouth, east of Buff Bay Village (Robinson, 1969a) and immediately south of Discovery Bay (Wallace, 1969). In extreme eastern Jamaica, E. Robinson (personal communication, 1973) noted the occurrence of Sign beds in the Yallahs and Ecclesdown districts. Stratigraphic thickness could not be determined because of the poorly exposed outcrops that prevail in the Sign region. Graded and bioclastic beds were not recognized, but most Sign samples analyzed contained shallow-water bioclastic debris. Although these beds are lithologically distinct from both the Bonny Gate and Spring Garden, and should be recognized as a distinct formation, the writer proposes to use the term "Sign" informally until an acceptable type section is discovered. (3) Spring Garden Formation (Middle Miocene): Robinson (1969c) designated the soft, even-bedded, chert-free chalks that crop out near Buff Bay

225 Village as the t y p e section for this formation which includes the youngest sediments placed in the Montpelier Group. The Spring Garden Formation is probably gradational both with the underlying Sign and overlying Buff Bay Formations. Lower Coastal Group Uplift of central Jamaica began in the Early Miocene (Wright, 1971). During the late Middle Miocene, the Clarendon carbonate blanket was eroded and noncarbonate rocks were exposed on the Cornwall--Middlesex platform. Subsequent to this event, fine silts and clays were deposited as hemipelagites on marginal sea b o t t o m s which had not received terrigenous detritus since the Middle Eocene. At Buff Bay Village, Spring Garden chalks grade upward into the marly claystones of the type Buff Bay Formation (Robinson, 1969b,c). The Buff Bay Formation is widely exposed on the north coast, in the Manchioneal and Bowden districts, and on the south coast near Yallahs. Fine-grained dark claystones exposed at San San Bay formerly distinguished as the San San Formation are retained in the Buff Bay as suggested by Robinson (1969a). In the Bowden and Arcadia Road districts (Fig.2), Buff Bay beds are partial lateral equivalents of the coarse-clastic and conglomeratic Augustown Formation, which contains a shallow-water foraminiferal and molluscan fauna.

BIOSTRATIGRAPHY A correlation of the isolated outcrops sampled in this study is presented in Fig 5. According to Robinson (1969c), Yellow Limestone sediments in the Wagwater Trough area contain the Hantkenina aragonensis through the Morozovella lehneri Zones of Bolli (1957) spanning the Lower to Middle Eocene. In its type area near Dressikie (Fig.2) the basal Bonny Gate is correlative with the Orbulinoides bectzmanni Zone or the upper part of the Middle Eocene (Robinson, 1965, 1967a). In this area Robinson (1969b) and Steineck (1973) recognized virtually all of the planktonic zones in the Upper Eocene to Lower Oligocene interval. Near the top of the exposed section north of Dressikie a well-developed Turborotalia opima (Bolli) fauna correlative with Zone N.1. of Blow (1969) was recovered. This assemblage is younger than any previously reported in the Bonny Gate. Most writers (Arden, 1968; Blow, 1969) placed a major unconformity at the t o p of the Bonny Gate that according to Blow (1969) spanned the N.1, N.2, and N.3 Zones. The recognition of N.1 faunas in the Dressikie section and the discovery of an N.3 fauna within Sign sediments along the Montego Bay--Adelphi road reduces this unconformity to Zone N.2 of Blow (1969). In summary, the Bonny Gate Formation in its type area ranges in age from the Middle Eocene to the Late Oligocene. Although well-exposed along the Montego Bay--Adelphi road section, the Bonny Gate is typically indurated and only a few well-preserved faunas were

226

recovered. Apparently, much of the exposed section here is younger than Eocene,'but older than Zone N.1. Assemblages from the Sign beds range from Zone N.3 to the uppermost part of Zone N.7 (with specimens transitional to Globigerinoides sicanus (de Stefani)). Robinson (1969a) suggested that the disappearance of chert in the "Montpelier" occurred island-wide near the end of Zone N.7 time as

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Fig. 5. Correlation chart o f the M o n t p e l i e r and L o w e r Coastal Group rocks f r o m the s e c t i o n s s t u d i e d in this report. Z o n e s in E o c e n e to Middle M i o c e n e interval based on B l o w ( 1 9 6 9 ) . Upper M i o c e n e z o n e s based o n species o f Sphaeroidinellopsis as suggested b y Lamb and Beard ( 1 9 7 2 ) .

Sign beds of this age occur near Buff Bay Village. However, Spring Garden beds at the base of the Ecclesdown section lack Globigerinoides sicanus and are tentatively correlated with Zone N.7. Therefore based on the presently available evidence, the Sign-Spring Garden transition is n o t a time-horizon but occurs within a narrow interval corresponding to the highest N.7 and lowest N.8 Zones. The Spring Garden Formation contains faunas correlated with the N.7 to N.13 Zones. The Spring Garden--Buff Bay transition corresponds closely with the base of Zone N.14 and the evolutionary appearance of Globigerina nepenthes Todd. This species is present in basal Buff Bay sediments throughout the outcrop belt. It is absent from the highest Spring Garden in the Buff Bay, Ecclesdown and San San sections, but typical specimens were recovered at Stony Hill. Thus, the Spring Garden--Buff Bay boundary is diachronous with respect to the planktonic zonation. The Buff Bay Formation ranges from the G. nepenthes Zone into the Pliocene but was not sampled above Sphaeroidinellopsis sphaero[des Subzone (Upper Miocene) of Lamb and Beard (1972).

227 PALEOBATHYMETRY

Discussion Bandy (1953a,b; 1964b), Bandy and Arnal (1960, 1969), and Ingle (1967a) presented excellent summaries of the paleoecologic method utilized in the study of Jamaican assemblages, but one aspect, the application of depth ranges of extant species, calls for further discussion. Ideally, recent species used for paleobathymetric interpretation should possess stable upper depth limits in modern oceans, but there is increasing evidence that for many species this level varies widely in different marine provinces (Bandy and Echols, 1964; Bandy and Chierici, 1966). In a survey of the distribution of seven cosmopolitan bathyal species, Steineck (1973, fig.16) found that f o u r - Fontbotia wuellerstorfi (Schwager), Laticarinina pauperata (Parker and Jones), Uvigerina auberiana d'Orbigny, Pullenia bulloides (d'Orbigny) -- exhibited strong variation in the reported upper depth limit. Three species -- Melonis pornpilioides (Fichtell and Moll) at 2000 m, Uvigerina rustica, Cushman and Edwards at 1200 m; Uvigerina peregrina d'Orbigny at 400 m - - h a d reasonably consistent upper depth limits. Consequently these latter species are useful for paleoecologic inference on a wide geographic scale. The former species can be used only when there is adequate Recent distributional data within a given marine province. It is important to note that this variation in the upper depth limit may involve a very small percentage of the total population of a species. A b u n d a n t occurrences of bathyal species often occur in similar depths throughout modern ocean basins. For example, Ingle (1967a,b) recorded an unusually shallow upper depth limit of 1500 m for M. pompilioides but noted that this species does not occur in abundance above 2000 m, which corresponds to the upper depth limit most often reported. Occurrence of this species in abundance or in dominance is characteristically 2200 m or deeper (Bandy, 1967; Caralp et al., 1970; Pujos-Lamy, 1973). Likewise, Fontbotia wuellerstorfi has a widely varying upper depth limit but is most characteristic of lower bathyal or abyssal depths (Parker, 1954; Bandy and Chierici, 1966; Pujos-Lamy, 1973). In summary, a reasonable estimate of paleodepth may be obtained when quantitative and presence--absence data are combined. The preceding discussions emphasize the need for regional distributional data to aid in the paleoecologic interpretation of extant species. The m o d e m bathymetric ranges for important bathyal species in the Gulf-Caribbean region as reported in Parker (1954) and Norton (1930) are summarized in Table I. There is general agreement on the restriction of Melonis pompilioides to abyssal depths, of Uvigerina auberiana to depths more than 1000 m, and of Fontbotia wuellerstorfi to depths greater than 750 m. The analysis of deep-sea cores from the Caribbean and adjacent waters by Phleger et al. (1954) provided a picture of species characteristic of, but n o t necessarily restricted to, lower bathyal and abyssal sediments for this region.

228 TABLE I Upper depth limits of selected species from northeast Gulf of Mexico (Parker, 1954) and the Caribbean Sea (Norton, 1930) Depth

750 m 1000 m

2000 m 3800 m

Species Gulf

Caribbean

Fontbotia wuellerstorfi (Schwager) Eggerella bradyi (Cushman) Gaudryina flinti (Cushman) Globobulimina affinis (d'Orbigny) Pyrgo spp. Robertina spp. Pullenia bulloides (d'Orbigny) most Cibicidoides Melonis pompilioides (Fichtel and Moll)

Fontbotia wuellerstorfi (Schwager) Laticarinina pauperata (Parker and Jones) Oridorsalis umbonatus (Reuss) Uvigerina auberiana (d'Orbigny)

Melonis pompilioides (Fichtel and Moll)

Species recovered from cores in the abyssal zone include M. pompilioides, Oridorsalis umbonatus (Reuss), Pullenia bulloides (d'Orbigny), F. wuellerstorfi, Cassidulina subglobosa (Brady) and Gyroidina soldanii (d'Orbigny). In the lower bathyal zone, the following were recorded: Laticarinina pauperata, Uvigerina auberiana, Pyrgo murrhyna (Schwager), Hoeglundina elegans (d'Orbigny) and Robertina spp. These are the only studies known to the author on the modern deep-sea foraminiferal faunas of the Caribbean. Regrettably these reports are somewhat inadequate as a base for refined paleoecologic interpretation of Caribbean Tertiary sediments. Therefore, tops of deep-sea cores taken from the Cayman Trench north of Jamaica were analyzed for their benthonic foraminiferal content (Table II). These data correspond strikingly to distributions given in Parker (1954) and indicate t h a t in this region b o t h Fontbotia wuellerstorfi and M. pompilioides are restricted to water depths in excess of 1980 m. However, the validity of these ranges for the Caribbean as a whole remains unsubstantiated. Analysis The overall similarity of Bonny Gate and Sign benthonic faunas permits t h e m to be analyzed jointly in Table III. An overwhelming preponderance of planktonic tests characterizes lower Montpelier samples. Forms with calcareous-perforate walls dominate the benthonic assemblage as porcellaneous and arenaceous species are rare, and it is important to note that Melonis pompilioides and Fontbotia wuellerstorfi are present in appreciable percentages in most samples. Other significant benthonic forms include species of Pleu-

Cassidulina crassa (d'Orbigny) Cassidulina su bglo bosa (Brady) Cibicidoides richardsoni ( B e r m u d e z ) Fontbotia wuellerstorfi (Schwager) Gyroidinoides altiformis ( S t e w a r t and Stewart) Hoeglundina elegans (d'Orbigny) Laticarinina pauperata (Parker and Jones) Martinottiella communis ( d ' O r b i g n y ) Melonis barleeanus (Williamson) Melonis pompilioides (Fichtel and Moll) Oridorsalis umbonatus (Reuss) Planulina ariminensis ( d ' O r b i g n y ) Pullenia bulloides (d'Orbigny) Pullenia quinqueloba (Reuss) Pyrgo murrhyna (Schwager) Robertina bradyi (Cushman and Parker) Siphonina pulchra (Cushman) Sphaeroidina bulloides ( d ' O r b i g n y ) Uvigerina auberiana (d'Orbigny) Uvigerina pygmaea (d 'Orbigny )

Species

x

x

x

× x

x

8542 950 m

x x

×

×

8543 550 m

x

x

×

x

c.f. × x

x

8537 1980m

x

c.f.

x

x

×

8544 1000m

Core n u m b e r and d e p t h

Benthonic foraminiferal species in deep-sea cores f r o m the C a y m a n Trench

TABLE II

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

8547 2900m

X

X

X

X

X

8535 2810m

X

X

X

X

X X

X

X

8546 2400m

X

X

8536 2400m

X

X

X

X

X

X

X

8534 4030m

b~

230 TABLE III Summary of the paleoecologic analysis of the Bonny Gate Formation and Sign beds Stratigraphic unit: Bonny Gate--Sign Faunal criteria Assemblage composition

planktonic/benthonic ratio = 1000 foram/ostracod ratio >/ 1000 species number 25 -~ 40; dominance = 1--5% foram/radiolarian ratio = 100 foram number = 1000's

Convergent ecologic morphologies

spinose and hispid Uvigerina and Rectuvigerina small striate Bulimina large and smooth-walled arenaceous species globular Pullenia

Reworked species

orbitoids, Pararotalia mexicana (Nuttall)

Extant species

Melonis pompilioides (Fichtel and Moll) Fontbotia wuellerstorfi (Schwager) Uvigerina auberiana (d'Orbigny) Cassidulina subglobosa (Brady) Pullenia bulloides (d'Orbigny) Eggerella bradyi (Cushman)

Dominant species

Gyroidinoides mauryae (Bermudez) Pleurostomella brevis (Schwager) Stilostomella abyssorum (Brady) Cassidulina tricamerata (Galloway and Hemingway) Oridorsalis umbonatus (Reuss) Osangularia renzi (Bronniman) Discanomalina japonica (Asano) Neoeponides campester (Palmer and Bermudez)

Ostracods

approximately 8--12 ostracod species smooth, unornamented, weakly hinged species Krithe dominant

Probable paleodepth

2000 m

rostomella, Stilostomella, K o r o b h o v e l l a p o m p i l i o i d e s ( G a l l o w a y a n d H e m i n g w a y ) , D i s c a n o m a l i n a j a p o n i c a A s a n o as well as papillate species o f Uvigerina. A l l o c h t h o n o u s e l e m e n t s o f t h e f a u n a include L e p i d o c y c l i n a a n d o t h e r orbit o i d s a n d Pararotalia m e x i c a n a ( N u t t a l l ) w h i c h are t h o u g h t t o h a v e b e e n derived f r o m t h e a d j a c e n t C o r n w a l l - - M i d d l e s e x p l a t f o r m b y g r a v i t y - i n d u c e d downslope movement along the Duanvale--Wagwater escarpment. To summarize Table III, faunal parameters, recurrent-ecologic morphologies, a n d species c o m p o s i t i o n w h e n c o m p a r e d t o m o d e r n values a n d d i s t r i b u t i o n a l t r e n d s , c o n s i s t e n t l y i n d i c a t e o c e a n i c p a l e o d e p t h s f o r t h e s e units. T h e

231 presence of appreciable percentages of Melonis pompilioides and Fontbotia wuellerstorfi is suggestive of paleodepths greater than 2000 m (Pujos-Lamy, 1973) which is considered here as the upper limit of the abyssal zone. Abundant radiolarian shells in well-preserved samples is important substantiating evidence of an abyssal paleodepth (Bandy and Arnal, 1957, 1960; Bandy and Rodolfo, 1964). Stilostomella--Pleurostomella dominated benthonic foraminiferal faunas similar to those developed in the Sign--Bonny Gate have been reported from the Middle and Upper Eocene of Panama (Bandy, 1970) and from the Oceanic Formation of Barbados (Beckmann, 1953). Beckmann (1953) initially recognized the deep-sea affinities of the Oceanic Formation, suggesting 1000 m as a possible paleodepth. Based on more complete comparative data, Frerichs (1970) reinterpreted that part of the Oceanic Formation which contains Melonis pompilioides as an abyssal sediment. Hofker (1968) described elements of the Stilostomella--Pleurostomella fauna from the Oligocene Chira Shale, Ecuador. Noting the high percentages of Pararotalia mexicana and orbitoids, he suggested a neritic site of accumulation, b u t is it not likely that these larger foraminifers are a u t o c h t h o n o u s in the Globigerina-rich Chira sediments as believed by Hofker. In the Oligocene of Jamaica similar artificial associations of larger foraminifers (including P. mexicana and Lepidocyclina cf. gunteri Cole) with planktonic oozes are accompanied by sedimentary features (including graded bedding) indicative of sediment transport and slumping. Bandy (1964a) described a modern analogue from the Gulf of Batabano, Cuba, where shelf-derived material constitutes an important part of slope sediments. Hofker (1968) may have overemphasized the shallow-water aspect of the fauna by ignoring the possibility of downward displacement and artificial association. The Chira Shale fauna is similar in most aspects to the other abyssal--lower bathyal faunas described above. Important support for the assignment of an abyssal site of accumulation for the Pleurostomella-- Stilostomella faunas enumerated above was provided by recent JOIDES research. Bader et al. (1971) proposed correlation of the outcropping Oceanic beds with s u b b o t t o m sediments in cores taken from the westernmost Atlantic abyssal hills. Formerly coextensive with s u b b o t t o m layers of the abyssal western Atlantic, the Oceanic beds were uplifted over 4000 m by tectonic movements associated with the formation of the Lesser Antilles Trench and adjacent islands. Paleogene deep-sea b o t t o m s appear to have been inhabited by a widespread and temporally persistent foraminiferal c o m m u n i t y dominated by species of Pleurostomella and Stilostomella. This c o m m u n i t y is known from the Lesser Antilles west to Panama and south to Peru and has been recognized in sediments of Late Eocene to Early Miocene age, ranging upward to Zone N.8 in Jamaica. Middle Miocene and younger abyssal assemblages are strikingly different from those of the Paleogene (Cushman, 1922, 1923, 1930, 1931; Hofker, 1927, 1930, 1951; Phleger et al., 1954; Todd, 1958; Ingle, 1967a; Bandy and Arnal, 1969). Both Pleurostomella and Stilostomella are u n c o m m o n

232

in modern assemblages although they are characteristically bathyal in their occurrence (Bandy, 1964b; Bandy and Rodolfo, 1964). It is apparent that the abyssal foraminiferal community underwent significant reorganization subsequent to Early Miocene time. These results agree with those of Bandy and Rodolfo (1964) who noted the hybrid nature of the modern abyssal foraminiferal fauna which contains recent immigrants as well as persistent ancient stocks. They further suggested that a modern foraminifera] fauna did not develop until at least the Middle Miocene. Menzies et al. (1973) reported similar faunal turnovers during the Early and Middle Miocene in a wide variety of abyssal animal groups. This phenomenon was attributed to cooling of the deep Atlantic ocean by polar waters of the Antarctic bottom current resulting from onset of the Late Tertiary refrigeration and glaciation of Antarctica. The presence of well-preserved abyssal faunas above and below the Bonny Gate--Sign "unconformity" precludes the possibility of any emergence-submergence or shoaling cycle at this interval as suggested by Zans et al., (1963) and Wright (1971). Robinson (1969a) recorded an extensive interval of Late Oligocene tectonism and dynamic metamorphism throughout Jamaica, which resulted in extensive bioclastic and olistostrome beds within the upper Bonny Gate Formation at its type section. At this time extensive slumping and sediment transport would have interrupted and altered the prevailing pelagic sedimentary regime. Although additional collections are needed to resolve this point, present evidence supports the view that marine processes such as erosion, slumping and sediment transport obscured sedimentary patterns during the Oligo-Miocene transition so that beds of this age are difficult to recognize by biostratigraphic analysis. Spring Garden microfaunas are generally similar to underlying ones although differing in several significant respects (Table IV). Melonis pompilioides is not consistently present and is never common; this pattern of occurrence in the Spring Garden is thought to be a feature of the upper feather-edge of its distribution at around 1500 m. The absence of radiolarian skeletons further mitigates against an abyssal paleodepth. Extant species present in the Sign include Uvigerina rustica, U. auberiana, Fontbotia wuellerstorfi and Pullenia bulloides, an assemblage characteristic of 1000-2000 m depths in the modern Gulf-Caribbean (Parker, 1954) and elsewhere (Pujos-Lamy, 1973). Thus, Spring Garden chalks were probably deposited in paleodepths approaching 1500 m. Hill (1899) directly compared the Montpelier to deep-sea calcareous pelagites but later workers ignored this essentially correct conclusion. Wright (1971) postulated an outer neritic open-sea environment for the Montpelier primarily based on the juxtaposition of shoal-water limestones of the Clarendon Group with Montpelier beds. This view, however, fails to recognize the significance of the Duanvale--Wagwater fault system as a Tertiary subsea escarpment which allowed attainment of oceanic depths in a short horizontal distance. Wallace (1969) concluded that Sign beds near Discovery Bay were

amphisteginids, Cellanthus

Reworked species

Minimum paleodepth

amphisteginids, peneroplids, miliolids

j_

species number = 60--~75; dominance = 1%

planktonic/benthonic ratio = 100

Buff Bay (north and east coast)

1500 m

1000 m

Melonis pompilioides (Fichtel and Moll) (rare) Chilostomella oolina (Schwager) Planulina ariminensis (d'Orbigny) Laticarinina pauperata (Parker and Jones) N Uvigerina rustica (Cushman and Edwards) U. auberiana (d'Orbigny) Fontbotia wuellerstorfi (Schwager) Planulina renzi (Cushman and Stainforth) Sigmoilopsis schlum bergeri (A. Silvestri ) Oridorsalis um bonatus (Reuss) Pullenia quinquelo ba (Reuss)

spinose and hispid Uvigerina flanged Pyrgo

Convergent ecologic morphologies

Extant Species

planktonic/benthonic ratio = 1000 foram number = 1000's species number ~ 40; dominance ~< 2% foram/ostracod ratio/> 100

Assemblage composition

Faunal criteria

Stratigraphic unit: Spring Garden

Summary of the (paleoecologic) analysis of the Spring Garden and Buff Bay Formations

TABLE IV

t~ O~ ¢O

234

deposited in waters no deeper than 1000 m. The faunal data summarized in Table III suggest that b o t h Wallace and Wright underestimated the depth of Montpelier deposition. Basal Buff Bay faunas are generally similar to highest Spring Garden assemblages (Table IV). Changes in faunal parameters and the absence of Melonis pompilioides in the middle and upper Buff Bay indicate significant shoaling of the depositional environment. The fauna of the Buff Bay contains many cosmopolitan bathyal species. An upper depth limit of 1000 m for Buff Bay deposition on the north and east coast is suggested b y the presence of Uvigerina rustica and U. auberiana which in the Caribbean have not been recorded in water shallower than 1000 m. On the southeast coast (Arcadia Road section, Fig.2) post-Neogloboquadrina acostaensis Zone sediments contain a fauna dominated b y striate Uvigerina, Cibicidina and other outer-shelf forms, from which Uvigerina rustica and U. auberiana are significantly absent. The Buff Bay in this area is laterally equivalent to the Augustown Formation which includes b o t h a proximal alluvial plain and a distal shallow marine shelf facies (Robinson, 1965). Apparently, large amounts of sediment derived from rapidly rising highlands in the vicinity of the present-day Blue Mountains shoaled paleodepths in the adjacent sea b o t t o m to the southeast. The Buff Bay in this region can be considered as the seaward termination of a prograding clastic wedge.

PALEOTECTONISM

Paleotectonic (subsidence and uplift) values can be c o m p u t e d when sedimentary thickness and paleodepth are k n o w n (Bandy, 1953b). For Jamaican sequences, total thickness of strata is difficult to determine because of isolated outcrops, inadequate exposures, and tectonic complications. In the calculations below, stratigraphic thickness was taken from Robinson (1969a) and minimum paleodepth was taken from Tables III and IV. Therefore, subsidence figures are conservative b u t should be roughly accurate. The Cornwall--Middlesex block remained a shoal-water carbonate platform from the Middle Eocene to the Late Oligocene when uplift restricted sedimentation to the southern third of the island (Wright, 1971). By analogy with the modern reef and shelf faunas of Jamaica, water depth probably did n o t exceed 100 m (Wright, 1971; C. H. Moore Jr., personal communication, 1971). Subsidence can be calculated as: Sedimentary thickness (500 m) + water depth (100 m) = 600 m total subsidence.

For the same interval of time, subsidence in the peripheral troughs is calculated: S e d i m e n t a r y t h i c k n e s s ( 6 0 0 m ) ÷ w a t e r d e p t h ( 2 0 0 0 m ) = 2 6 0 0 m o f p r e - M i o c e n e subsidence.

235 Lower Miocene (i.e. Sign) subsidence represents only the additional sedimentary thickness because inferred paleodepth is unchanged and accounts for an additional 200 m. In summary, conservative estimates of subsidence in peripheral troughs during the Middle Miocene totals 2800 m. Spring Garden uplift is calculated: Change in paleodepth (500 m) -- sedimentary thickness (100 m) = 400 m of Middle Miocene uplift. Uplift for the Late Miocene and Early Pliocene approaches 450 m. The postMiddle Pliocene uplift of marginal Kamaica must have been nearly 1000 m to account for the exposure of Buff Bay beds at their present elevation above sea level. PALEOGEOGRAPHY Fig.6, an inferred cross-section of Upper Oligocene Jamaica shows Montpelier sediments accumulating on an abyssal fan at the base of the Duanvale-Wagwater escarpment. This model more closely corresponds to the paleodepth interpretation presented above for Montpelier foraminiferal assemblages than those of Wallace (1969) and Wright (1971) who conceived of Montpelier deposition as occurring on a gently to moderate sloping "fore-reef area". In addition, it postulates a topographic setting that is prevalent in the modern Caribbean, for example at Tongue of Ocean--Bahama Bank, Gulf of Batabano, Cuba and the present-day north coast of Jamaica towards the Cayman Trench. In these areas, carbonate platforms pass rapidly into adjacent oceanic deeps across intervening nearly vertical and fault-originated subsea escarpments which reach 20 ° on the north coast of Jamaica (C. H. Moore, personal communication, 1973; Goreau and Land, 1974). Fig.7 shows the inferred Oligo-Miocene paleogeography of Jamaica in which the extreme eastern (Blue Mountain region) and western (Lucea block) parts of the island are considered as deep-sea bottoms. In this view, the pelagite cover of these areas was stripped from these regions by erosion during the later Tertiary. E. Robinson (personal communication, 1973) disagreed, suggesting that shallow platforms analogous to the Cornwall--Middlesex block existed to the east and west during the mid-Tertiary and that Montpelier deposition was restricted to the north coast belt, Wagwater Trough (Dressikie and Yallahs section of Fig.2) and to a narrow belt, south of Montego Bay between the Cornwall--Middlesex and Lucea blocks (Hopeton--Montpelier section). The Middle Eocene marginal subsidence that initiated Montpelier deposition in Jamaica appears to be related to the growth of the Cayman Trench, north of the island. During the Cretaceous the northern boundary of the Caribbean "plate" apparently extended from central Cuba eastward through Hisoaniola, an area that is n o w a complex overthrusted zone of ultramafics separating southern deep water from northern carbonate platform lithofacies (MacGill-

236 ~

40MILES

sw"

~

I~ Z II,i

5-10 MILES

=

&

NE

CORNWALL- MIDDLESEX ~BLOCK CAYMAN TRENCH MILIOLID B I O F A C I E S J O R B I T O I D

SEA

2000

BIOFACIES

[EVEL-~MONTPELIE

R PELAGITES

M

ooooo

~o@#$ooo

Fig.6. Inferred cross-section of Oligocene Jamaica.

ivray, 1970; Walper and R o w e t t , 1972; Moore and Del Castillo, 1974). Some time after the latest Cretaceous the northern Caribbean boundary fault assumed its present position along the strike of the Cayman Trench. Although the Cayman Trench is considered to be a region of predominantly strike-slip faulting, there is a significant extensional c o m p o n e n t as well (Bowin, 1968). Malfait and Dinkelman (1972) suggested that the Cayman Trench formed first to the west in the isthmian region and later extended eastward. Marine transgression across Jamaica and marginal subsidence during the Middle Eocene is consistent with the development of the Cayman Trench in the Jamaican region at this time. The nature of the lateral and vertical stratigraphic relationships b e t w e e n Montpelier and Clarendon rocks has engendered controversy particularly in areas of inadequate exposure or where precise faunal control is lacking. The morphotectonic model presented in Fig.6 implies that Montpelier sediments superpose only on the deeper water units of the Yellow Limestone. In contrast, some writers presented evidence for a Miocene transgression during which Montpelier beds were deposited u n c o n f o r m a b l y on the previously exposed

237

.

.

.

.

DUANVALE--WAGWATER

ESCARPMENT

Fig.7. Inferred paleogeographical map of mid-Tertiary Jamaica.

northern lip of the Cornwall--Middlesex block (Wallace, 1969; Wright, 1971). To support his view, Wallace (1969) assumed a gradational Brownstown-Sign contact in the tectonically complex Duanvale fault zone south of Discovery Bay, a relationship that has proven difficult to substantiate (E. Robinson, personal communication, 1971). Alternatively, at this locality C. H. Moore Jr. (personal communication, 1971), reported blocks (exceeding 15 m) of Brownstown lithology to be present as mega-clasts in a matrix of chert-bearing Sign chalks. Moore concluded that the Brownstown blocks represent semiconsolidated and coherent slip masses of platform material resedimented into an adjacent ocean b o t t o m . This situation parallels that of the modern reef edge in Jamaica where large slump-masses episodically break off the reef parapet, sliding downward to the base of the escarpment. While a final conclusion is n o t reached, the writer finds no compelling reason to accept Wallace's concept of a gradational Sign--Brownstown contact. In the western part of the island, contacts of Montpelier beds with units of the Cornwall--Middlesex blocks can be observed at the eastern end of the Adelphi--Montego Bay section (Brownstown--Bonny Gate) and south of the

238

town of Montpelier (Sign--Yellow Limestone). The rocks in these areas are not sufficiently well-exposed to completely reconstruct the three-dimensional relationships of the lithological units. Consequently, the generalized crosssections by Zans et al. (1963) and Wright {1971) do not require a particular interpretation. These writers infer a transgression with an unconformity between the Montpelier and other limestones. Two alternative explanations are consistent both with the stratigraphic relationships and with the morphotectonic model depicted in Fig.6. For one such explanation, along high-angle fault zones characterized by major uplift (such as the Duanvale--Wagwater faults), mega-breccias occur which contain large slivers that have been detached and moved large distances vertically (Vasquez and Dickey, 1972; Moody and Hill, 1956). Thus, slivers of platform limestones could have been detached from the edge of the Cornwall--Middlesex Platform during movement of the Duanvale--Wagwater fault and incorporated into the hanging wall block composed of Montpelier rocks. Secondly, considerable differences exist in subsidence-uplift values between the Cornwall--Middlesex block and the hanging wall block composed of Montpelier sediments. Strata of greatly differing age may be juxtaposed across the Duanvale--Wagwater fault as a result of relative uplift and level of erosion. This relationship could simulate an unconformity when, as in this case, exposures are minimal. SUMMARY

During a period of latest Cretaceous to Paleocene orogenesis, granodioritic and ultrabasic magmas intruded into Cretaceous sediments in Jamaica while folding and thrusting took place in the Blue Mountain range. Basal Tertiary (Lower Eocene) strata comprise a clastic wedge shed from adjacent highlands into a shallow, but rapidly subsiding graben. Complete submergence of insular Jamaica during the Middle Eocene accompanied tensional adjustments induced by the formation of the Cayman Trench to the north. Differential subsidence along a series of linear nearly vertical faults produced paleodepths approaching 2000 m seaward of the resulting subsea escarpments. Shoal-water limestones blanketed the more slowly subsiding Cornwall--Middlesex platform, ending the supply of clastic sediments to sea bottoms north and east of the platform where planktonic-foraminiferal pelagites accumulated contemporaneously. Thus, two facies of carbonate deposition characterized mid-Tertiary Jamaica: the Clarendon Group includes coralgal and miliolid limestones deposited on the Cornwall--Middlesex platform; the Montpelier Group comprises cherty or chert-free oceanic deposits formed seaward of the Duanvale--Wagwater escarpment. The preponderance of globigerinacean and radiolarian tests characterizes lower Montpelier (Sign and Bonny Gate) microfossil assemblages. Dominant benthic forms include Melonis pompilioides, Fontbotia wuellerstorfi and

239 species of Stilostomella and Pleurostomella. Available faunal criteria including assemblage composition, depth preferences of extant species, and recurrent ecologically controlled morphologies suggest that abyssal (below 2000 m) paleodepths prevailed at the depositional site on a sediment apron at the base of the Duanvale--Wagwater escarpment. Middle Eocene to Lower Miocene subsidence c o m p u t e d from inferred paleodepth and estimated sedimentary thickness totals 2800 m. Planktonic foraminiferal assemblages correlative with Zones N.1 and N.3 of Blow (1969) were d o c u m e n t e d from the highest Bonny Gate and lowest Sign respectively, limiting to one zone a putative regional unconformity between these units. Because well-preserved abyssal assemblages were recovered from above and below this formational boundary, it is suggested that the Montpelier represents an unbroken depositional sequence. General uplift occurred in the Middle Miocene. Spring Garden chalks were deposited at lower bathyal depths while the central part of Jamaica was emergent. The carbonate blanket on the Cornwall--Middlesex platform was eroded during the late Middle Miocene (essentially at the base of Zone N.14), thus renewing supplies of terrigeneous material to the deep sea. Buff Bay microfaunas contain many bathyal species such as Uvigerina auberiana, U. rustica, Hoeglundina elegans, Laticarinina pauperata and others which suggest 1000 m of paleodepth.

ACKNOWLEDGEMENTS Special appreciation is expressed to W. A. van den Bold, Louisiana State University, who suggested the problem, provided needed counsel, and in every way possible facilitated its completion. Edward Robinson, University of the West Indies,,acted as gracious host to the writer during his visits to Jamaica and discussed all aspects of this work with him. Dr. Robinson kindly permitted the writer to quote from his unpublished dissertation, an important d o c u m e n t which served as the basis for the present investigation. Donation of valuable samples by E. Ann Butler, Atlantic-Richfield Corporation and William Neal, Georgia Southern University, is acknowledged. John, Beard and James Lamb, Esso Production Research, kindly verified planktonic species identifications and made available a preprint of their important contribution on biostratigraphy. The Kaiser Aluminum Corporation transported the writer's collection from Jamaica to Baton Rouge. The writer expresses gratitude to Harold V. Anderson, George F. Hart, James P. Morgan and Clyde H. Moore for their criticisms of the manuscripts. Richard F. Inden and R o b e r t W. Pierce graciously reviewed earlier drafts. Financial support was provided by the Society of Sigma-Xi and b y the Department of Geology of the Louisiana State University for which the writer is grateful.

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