Miocene pelagic biogenic sediment production and diagenesis, St. Croix, U.S. Virgin Islands

Palaeogeography, Palaeoclimatology, Palaeoecology, 22(1977): 137--171 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

MIOCENE PELAGIC BIOGENIC SEDIMENT PRODUCTION D I A G E N E S I S , ST. C R O I X , U.S. V I R G I N I S L A N D S

AND

STANLEY H. FROST AND NANCY A. BAKOS

Department of Geology, Northern Illinois University, DeKalb, Ill. (U.S.A.) 7508 Clarewood, Apartment 245, Houston, Texas (U.S.A.) (Received June 25, 1976; revised version accepted November 9, 1976)

ABSTRACT Frost, S. H. and Bakos, N. A., 1977. Miocene pelagic biogenic sediment production and diagenesis, St. Croix, U.S. Virgin Islands. Palaeogeogr., Palaeoclimatol., Palaeoecol., 22: 137--171. The Middle Miocene Kingshill Marl of St. Croix, Virgin Islands, affords an opportunity to reconstruct ancient island-margin calcareous plankton communities and to determine their contribution to the accumulation of island-slope sediments. Because of the present outcrop pattern of this unit, both lateral and vertical changes in organism/sediment relationships may be investigated. Subsidence of a NE--SW trending grabenal structure on St. Croix during the latest Early Miocene produced the Kingshill Seaway, which was flanked on the northwest and southeast by island masses of Cretaceous volcanogenic sediments, and on the northeast and southwest by the insular shelf edges. Hydrographic conditions in the shallow Seaway promoted high rates of pelagic biogenic skeletal production, resulting in the accumulation of thick pelagic calcareous oozes composed of a framework of calcitic planktonic foraminiferal tests in a matrix of calcareous nannoplankton, planktonic foraminiferal debris and fine aragonite needles. Minor siliceous components included diatom frustules and sponge spicules. Turbidity currents and debris flows transported terrigenous detritus and reef-tract skeletal rubble into the Seaway from the shallow basin margins. Comparison of the pelagic chalks and marls of the Kingshill Marl with modern sediments accumulating on the northwest St. Croix island slope establishes valuable guidelines to infer the total biogenic composition of the original ooze accumulating on the Kingshill Seaway floor. Comparison of the diagenetic processes affecting island-slope calcareous oozes with those affecting their deep-sea counterparts underscores the necessity of considering the range and intensity of differential solution as a factor in the ooze -~ chalk diagenetic continuum. The major diagenetic event in the Kingshill Marl ooze -~ chalk process was the solution of aragonitic skeletal sediment, probably during flushing by fresh water.

INTRODUCTION The processes of production and accumulation of skeletal aragonite and calcite sediments by benthos of modern and ancient shallow-shelf and reef environments have been extensively described and interpreted in the literature (e.g., I l l i n g , 1 9 5 4 ; G i n s b u r g , 1 9 5 6 ; W a n t l a n d a n d P u s e y , 1 9 7 1 ; S t e i g l i t z , 1 9 7 2 ;

137

138 Neumann and Land, 1975). Moreover, there is rapidly increasing knowledge a b o u t deep-sea biogenic limestones and the processes of pelagic biogenic carbonate and opal skeletal production by which they are formed (e.g., Fischer et al., 1967; Garrison and Fischer, 1969; Schlanger et al., 1971; Schneidermann, 1973a; Berger, 1975). However, relatively little is known a b o u t carbonate skeletal production and accumulation in the transition between the shallow-shelf and deep-sea floor, especially on oceanic island slopes. In these environments carbonate sediment accumulates which is of b o t h benthic shallow-shelf and pelagic biogenic origin. The Miocene Kingshill Marl of St. Croix, U.S. Virgin Islands (Fig. 1A) offers an o p p o r t u n i t y to reconstruct fossil island-margin calcareous plankton communities and to determine their contribution to the deposition of island-slope sediments. This is because pelagic biogenic chalks of the Kingshill Marl were deposited across the island in an open seaway (the Kingshill Seaway of Multer et al., 1977) which closely approximated modern tropical island-slope environments at depths of a few hundred meters (Multer, 1972). Island-slope environments represent the transition between those of the deepest shelfmargin reefs and those of the middle- and deep-basin slopes. Because of the present Kingshill Marl outcrop pattern, both lateral and vertical changes in organism/sediment relationships may be investigated, especially the taxonomic diversity and sediment-producing potential of islandmargin calcareous planktonic communities. The objectives of this investigation are: (a) to examine the nature and origin of Miocene pelagic biogenic skeletal sediments deposited on St. Croix in the ancient analog of an island-slope environment; (b) to determine the relative abundance and diversity of the sediment-producing plankton communities; (c) to determine processes of sedimentation of pelagic skeletal sediment in the island-slope environment; and (d) to examine the stability of biogenic skeletal particles of benthic and pelagic origin in the ooze -~ chalk -+ limestone diagenetic transition. THE KINGSHILL MARL The Kingshill Marl crops out extensively over central and southwestern St. Croix (Figs. 1A and B). It was first named by Kemp (1926), who called the carbonate rocks underlying the Central Plain of St. Croix the Kingshill Series. These white and cream-coloured limestones, marls and chalks were renamed the Kingshill Marl by Cederstrom (1942), who investigated them by means of outcrops and well samples, and later estimated (1950, p. 21) their total thickness to be a b o u t 200 m. He (1950) concluded that the Kingshill Marl was deposited by a shallow sea in a valley subaerially eroded into the Cretaceous basement rocks. Subsequent workers (e.g., Cushman, 1946; Butterlin, 1956; Whetten, 1966) accepted Cederstrom's conclusions. Recent studies of the Kingshill Marl by Multer (1972), Curth et al. (1974), Multer et al. (1974, 1977) est3blish that the Kingshill Marl is dominantly an

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140 open basin deposit of pelagic biogenic sediment interbedded with turbidites and debris flows. CENOZOIC STRATA OF ST. CROIX Although the Kingshill Marl was considered by Kemp (1926) and other early workers to comprise the entire Tertiary stratigraphic section on the island, test wells drilled in 1938--39 demonstrated the presence beneath the Kingshill Marl of a thick sequence of dark montmorillonitic clays with minor limestone and conglomerate. These strata, named the Jealousy Formation by Cederstrom (1950), are at least 420 m thick and rest with angular unconformity on Cretaceous basement rocks. Whetten (1966, P1.1) mapped extensive outcrops of the Jealousy Formation along the western margin of the Central Plain although there appear to be none along the eastern margin. Vaughan (1923) considered all of the Jealousy Formation to be Oligocene on the basis of his identification of larger foraminifera, while Cushman (1964, p. 2) considered the upper part to be Lower Miocene and the lower part to be Oligocene on the basis of smaller benthic foraminifera. The Jealousy Formation is overlapped by the Kingshill Marl, accounting for the general lack of exposures, and details of its lithology or depositional environments are not well known. Outcrop samples examined by Van den Bold (1970) contained a brackish-water ostracod assemblage. Whetten (1968) reports thin oyster beds and layers of transported shallow marine fossils in outcrops along the northeast base of the Northside Range, although the bulk of the formation is assumed to be non-marine in character. Tertiary carbonates younger than the Kingshill Marl and older than Pleistocene terraces have recently been discovered along the southern edge of the Central Plain (Fuller, 1974, p. 292; Multer et al., 1977; Behrens et al., in prep.). In a deep highway cut east of Alexander Hamilton Airport (Fig. 1B), lagoonal carbonates with Pliocene large solitary hermatypic corals lie with apparent angular unconformity on beds of typical upper Kingshill Marl lithology. In addition, new cuts in and near the Amerada-Hess refinery complex expose a fringing reef containing certain species of reef-building corals which cannot be younger than Pliocene (Multer et al., 1977; Frost and Behrens, in prep. ). CRETACEOUS ROCKS Tuffaceous sandstones and mudstones of epiclastic volcanogenic origin comprise Cretaceous basement rocks outcropping in the highlands on the west and east sides of St. Croix (Figs. 1B, 2) and presumably underlie the Tertiary formations of the Central Plain at great depth (Cederstrom, 1950; Whetten, 1966). Terrigenous debris from the erosion of outcropping Cretaceous rocks was transported into the basin of deposition of the Kingshill Marl during the Miocene by turbidity currents and debris flows.

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Fig.2. Paleogeography and paleocirculation patterns, Middle Miocene St. Croix. Northeastward-flowing currents at intermediate depth are produced by the Ekman spiral effect on southwestward-flowing surface currents in the Seaway. BIOSTRATIGRAPHY OF THE KINGSHILL MARL A detailed s u m m a r y o f biostratigraphic age d e t e r m i n a t i o n s o f t h e Kingshill Marl m a y be f o u n d in Multer et al. (1977). We consider the Kingshill Marl to be early Middle Miocene (Globorotalia fohsi fohsi Z o n e ) based on analysis of the stratigraphic d i s t r i b u t i o n of p l a n k t o n i c f o r a m i n i f e r a (Table I). This corresponds t o an a b s o l u t e age o f r o u g h l y 14 m.y. b e f o r e present o n Berggren's ( 1 9 7 2 ) revised t i m e scale. This c o n c l u s i o n is largely s u p p o r t e d by Van den Bold (1970, p. 39), who depicts the Kingshill to range f r o m the Globorotalia fohsi fohsi i n t o the G. fohsi robusta p l a n k t o n i c f o r a m i n i f e r a l zones based on the stratigraphic o c c u r r e n c e of b o t h ostracods and p l a n k t o n i c foraminifera. PELAGIC ENVIRONMENTS OF THE KINGSHILL SEAWAY

The Central Plains Graben, St. Croix The Kingshill Marl was d e p o s i t e d in a n o r t h e a s t - - s o u t h w e s t t r e n d i n g seaway which was b o u n d e d on the n o r t h w e s t and s o u t h e a s t b y islands f o r m e d b y the e m e r g e n t N o r t h s i d e and East End ranges (Fig. 2). This Kingshill S e a w a y r e s u l t e d f r o m t h e f l o o d i n g o f the d o w n - d r o p p e d central p o r t i o n of the St. Croix island block. The resulting graben, which was a p p a r e n t l y f o r m e d during

142 TABLE I Stratigraphic distribution of the Kingshill Marl index planktonic foraminifera, plotted after Saunders et al. (1973) Planktonic Foraminifera

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the Early Tertiary, received a substantial thickness of basin fill (Shurbet et al., 1956). The Jealousy Formation comprises only the upper part of this basin fill, and Early Paleogene sediments may occur at depth. Although the basal units of the Kingshill Marl are of shallow marine origin (Multer et al., 1977), the major part of the formation provides evidence of

143 deposition under open-basin conditions. Subsidence of the St. Croix island block during the early Middle Miocene produced a progressive overlap of the Seaway margins on the lower slopes of the Northside and East End island masses. Thus, as water depth in the center of the Seaway increased, shallow marine biosparites and biomicrites were replaced by increasing proportions of calcareous pelagic biogenic skeletal chalk, while the littoral and neritic basin-margin environments migrated up the shallow slopes of the Northside and East End islands. The major portion of the volume of sedimentary rock comprising the Kingshill Marl is of open-basin origin.

Basin-margin facies Sediments and organisms occurring in shallow neritic and littoral environments at the margin of the Miocene Kingshill Seaway are discussed in detail by Multer et al. (1977). Their conclusions are summarized here in order to describe the basin-margin environments from which terrigenous debris and skeletal rubble of reef-facies organisms were transported by debris flows and turbidity currents onto the open basin floor. As the St. Croix island block subsided, the water volume of the Kingshill Seaway overreached the margins of the original grabenal structure and flooded the lower slopes of the bounding island masses {Fig.2). Thus, sediments of the Kingshill basin-margin facies can be seen to overlap the Jealousy Formation on the lower slopes of the Northside Range and rest directly on the Cretaceous basement rocks. These sediments contain a benthic large foraminiferal assemblage which establishes them to be contemporaneous with the basinal chalks. On the east side of the Seaway, Kingshill Marl basin facies sediments of mixed debris flows and pelagic chalks completely cover the Jealousy Formation, and evidence of extensive submarine sliding of soft basinal oozes and muds (Curth et al., 1974, figs.1 and 2) at Cathrines Rest suggests t h a t the East End island slope may have been steeper than that developed on the Northside Range island. Progressive lateral transgression of the sea up the flanks of the islands led to the erosion and transportation of substantial quantities of clay-size to boulder-size debris from the Cretaceous country rock. Much of this was eventually transported into the open basin. Although no fossil reef framework structures have as yet been found in situ, the great volume of skeletal rubble of reef-dwelling organisms that was transported into the open basin indicates that fringing reef tracts must have occurred at least sporadically on the landward margins of the Kingshill Seaway. Continued lateral migration of the littoral/shallow neritic basin-margin environments may have made it difficult for reef communities to persist at any one locality long enough to construct massive reef frameworks. Shallow sublittoral calcareous sandstones and sandy biosparites with abundant marine microfossils and macrofossils have been discovered recently by L. C. Gerhard {written communication) in the Judith Fancy area. Associated are coratline algal/miliolid foraminiferal biosparites that accumulated on shallow carbonate

144 shoals at depths of 1--5 m. Exposures such as these provide evidence of the basin-margin environments, organism comnmnities and sedimentary processes of the Seaway margins.

Basinal facies The rocks which comprise the basinal or open seaway facies of the Kingshill Marl underlie almost all of its present outcrop belt. In general, t h e y are a sequence of interbedded, pelagic biogenic chalks and allochthonous sediment gravity flows (Fig. 3). Because these rocks are easily eroded, extensive exposures of the Kingshill Marl are few, and those present are deeply weathered (Cederstrom, 1950). The extensive exposures recently created during excavation for the Ville la Reine shopping center thus provide an o p p o r t u n i t y to analyze a relatively thick stratigraphic sequence of basinal Kingshill sediments. The Ville la Reine section is located in the center of the Kingshill Marl outcrop belt (Fig. 1B) about 12 km ENE of Frederiksted. Here, gently southeastward-dipping chalks and marls occupy an exposure face which extends laterally for about 170 m, comprising 23.7 rh of stratigraphic section (Figs.3,4). In addition, a road cut located about 300 m southeast of the shopping center exposes a 10 m thick section of similar strata which lies stratigraphically above the Ville la Reine sequence. Although this latter section was also sampled, it was not plotted on Fig.4 because the exact stratigraphic interval between the top of the Ville la Reine section and the base of the road cut section could not be determined. On fresh exposures such as at Ville la Reine, Kingshill Marl basinal sediments consist of light buff to white, moderately indurated porous limestone interbedded with soft cream to tan sandstone and boulder-bearing marl. The limestones, in beds 10--30 cm thick, alternate rhythmically with sandstones and marls, the latter two weathering back to form reentrants in the exposure face (Fig. 4F). Detailed examination of the porous limestone indicates that it is a chalk formed from calcareous skeletal components of pelagic organisms. Analysis of the soft calcareous sandstone and conglomeratic marl establish t h a t these actually are turbidites and debris flows consisting of mixed terrigenous material and skeletal detritus of shallow marine organisms.

Chalk The chalk, which forms the more resistant layers in any given exposure, is composed of a framework of calcite planktonic foraminiferal tests and fragments of tests in a matrix made up of whole and disarticulated calcite calcareous nannoplankton, silt- and clay-size fragments of planktonic foraminifera, clay minerals and reprecipitated calcite chamber and vug fillings (Figs. 4A--E). Volumetrically minor components are arenaceous agglutinated and calcite tests of benthic foraminifera, rare casts and molds of pteropods (which were originally aragonite) and rare diatoms in which the original opaline

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benthic biogenic origin was present in the original ooze. The fossil assemblage is moderately well preserved, although varying degrees of solution of planktonic foraminiferal tests and calcareous n a n n o p l a n k t o n have taken place.

Allochthonous sediments Interbedded with the pelagic chalks are beds of allochthonous mixed carbonate and terrigenous detrital material deposited by sediment gravity flow or by soft-sediment gliding and slumping. Turbidite beds tend to be relatively thin, rarely being more than 12--15 cm in thickness, and occur in the Ville la Reine section at more or less regular stratigraphic intervals (Fig.3) indicating a crude cyclicity in the transport of rubble into the basin. The turbidites typically have a coarse sand or gritty texture and consist of weathered Cretaceous volcanogenic sedimentary rock mixed with similar size skeletal particles of coralline algae, benthic foraminifera, scleractinian corals, echinoids, mollusks and rare bryozoa. These beds contain varying a m o u n t s of intergrain calcite cement, but in general are friable and weather back in exposure faces (Fig.4F). Aragonitic and siliceous skeletal components are leached out, leaving internal and external molds. Debris flows occur sporadically t h r o u g h o u t the basinal sequence, and may be as much as 2.5 m thick, as in the two superposed flows of Unit 45 of the Ville la Reine section (Fig.4). These are masses of unsorted terrigenous and skeletal rubble with chaotic internal structure and which occupy channels eroded into the soft pelagic ooze and turbidite sand substrate. Boulders of Cretaceous tuffaceous sandstone as much as one meter in diameter, and rubble of massive scleractinian corals are abundant in the debris flows. PELAGIC BIOGENIC SEDIMENT PRODUCTION, KINGSHILL SEAWAY

Calcareous nannoplankton Coccoliths, discoasters and other nannofossils, or their disarticulation products, make up an estimated 40% of the fine matrix of pelagic Kingshill chalks. This

Fig.4. Stratigraphic sequence for the middle Kingshill Marl, open-basin facies. Brick pattern represents planktonic marl or chalk while stipple pattern depicts turbidite or debris-flow beds. A. Components of the chalk nannomatrix, x 400, unit 51;Discoaster deflandrei in the upper left, Coronocyclus? sp. in the lower mid-left and distal shield of Cyclicargolithus floridanus in the lower center. B. Nannomatrix, x 2000, unit 51 ; tabular calcite elements of calcareous nannoplankton and planktonic foraminifera, with Thoracosphaera in upper left and etched wall of Orbulina embryonic chamber, lower right center. C. Light micrograph, x 30 of unit 47. Framework grains of chalk are planktonic foraminiferal tests. D. Secondary calcite void fillings, x 1800, unit 51. E. Fracture surface of chalk, x 200, unit 49; with calcite microdruse coating test surface of Globigerina praebulloides and b l o c k y calcite void filling in lower right center. F. Outcrop of units 31 through 53, taken 10 m south of the photo in 3A.

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PLATE I

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corresponds to a b o u t 15% of the total chalk sediment volume. However, whole nannofossils comprise not more than 5% of the chalk matrix (Figs.4A and B) because the calcareous nannoplankton assemblage has apparently been subjected to pervasive mechanical disarticulation and possibly minor solution. Nannoplankton structures such as placoliths are composed of low-Mg calcite (Thompson and Bowen, 1969) and consist of proximal (Plate I, 3) (to cell cytoplasm) and distal (Plate I, 2) disk-like shields connected by an axial stem, ring or mesh-like structure. Each shield of the placolith is composed of tabular calcite elements overlapping each other in a helicoid pattern. Differences in size and number of elements in the shields and in proportion between the shields and axial stems produce a variety of structures such as stem-like rhabdoliths and club-like lopadoliths. Discoasters are composed of calcite skeletal elements arranged in a stellate pattern (Plate I, 5--7). These presumably covered the cell surfaces of a lineage o f extinct p h y t o p l a n k t o n in a manner similar to that of coccolith placoliths on modern coccolithophorids. Thoracospheres also occur in the Kingshill Marl. These are minute calcareous spheres composed of tabular calcareous platelets. A characteristic opening or "escape aperture" occurs at one of the poles of the spheroid (Plate I, 8). Disso lu tion

A number of investigators (e.g., Bramlette, 1958; Hay, 1970; McIntyre and McIntyre, 1971; Berger, 1973; Bukry, 1973; Schneidermann, 1973b; PLATE ! Kingshill Marl calcareous n a n n o p l a n k t o n . 1. Small p l a c o l i t h s (Coronocyclus?) in n a n n o m a t r i x , x 4 0 0 0 . S a m p l e f r o m r o a d c u t 300 m SE of Ville la Reine s e c t i o n , s t r a t i g r a p h i c a l l y a b o v e it. O t h e r n a n n o p l a n k t o n d e p i c t e d are f r o m t h e Ville la Reine s e c t i o n .

2. Cyclicargolithus floridanus ( R o t h a n d Hay). Distal shield, × 3 4 2 0 , w i t h c o r r o d e d c e n t r a l area. Lateral view of a c o m p l e t e p l a c o l i t h is in t h e u p p e r right. 3. Coccolithus eopelagicus ( B r a m l e t t e a n d Reidel), x 3 7 0 0 , distal surface of d i s i n t e g r a t i n g placolith. O u t e r cycle of e l e m e n t s of p r o x i m a l shield is m o s t l y gone. 4. C. eopelagicus, x 4 5 6 0 , distal shield. The c e n t r a l area is c o m p o s e d o f t h i n i m b r i c a t e laths. 5. Discoaster lani B r a m l e t t e a n d Riedel, x 4 5 6 0 . Rays are a p p a r e n t l y o v e r g r o w n with s e c o n d a r y calcite. 6. Discoaster? sp. cf. D. deflandrei B r a m l e t t e a n d Riedel, x 4 5 6 0 . S p e c i m e n has h e a v y calcite o v e r g r o w t h s . 7. D. deflandrei B r a m l e t t e a n d Riedel, x 3420. Slightly e t c h e d s p e c i m e n displaying typical ray m o r p h o l o g y . 8. Thoracosphaera heimi ( L o h m a n n ) , x 2 9 5 0 , w i t h typical " e s c a p e a p e r t u r e " .

150

Roth and Berger, 1975) have contended that the abundance and diversity of the calcareous plankton community of the upper layers of the oceanic water column is not accurately reflected in the calcareous ooze accumulating below on the deep-sea bottom. It has been generally assumed that the long residence time in the water column of settling skeletal particles having large surface area/volume ratios promotes dissolution of biogenic carbonate and opal. Honjo (1975) shows, though, that significant quantities of coccolith debris are rapidly transported through the water column in fecal pellets, thereby bypassing oceanographic filters such as selective dissolution and transport or winnowing. Therefore, if no selective grazing occurs at the species level, the thanatocoenose reaching the sea floor should reflect reasonably accurately the structure and diversity of the original skeleton-bearing community. However, selective dissolution of coccoliths and discoasters may occur after arrival at the sediment--water interface (McIntyre and McIntyre, 1971; Schneidermann, 1973b; Roth and Berger, 1975). Selective dissolution and overgrowths increase in pervasiveness and development during diagenesis of the sediment, decreasing the diversity and abundance of the nannoplankton present (Bukry, 1971; Roth, 1971; Ramsay, 1972; Schlanger et al., 1971; Adelseck et al., 1973). It has been established by Bukry (1971), Roth (1971), Ramsay (1972), Ramsay et al., (1973) and Koth and Berger (1975) that the various morphologies of calcareous nannoplankton display unequal resistance to dissolution; in general the larger the skeletal elements or crystallites in a structure, the more resistant it is to solution. Also, as dissolution advances, the more delicate parts, such as central mesh-like structures and loosely attached plates, are destroyed. Moreover, several authors contend (e.g., Hay, 1970) that calcareous nannoplankton are more resistant to dissolutionthan are planktonic foraminifera, although Roth and Berger found that in general the two groups are about equal in solution resistance. Large coccolith placoliths and species of D i s c o a s t e r are clearly the more durable of nannofossils, and comprise an increasing proportion of the nannoflora as dissolution progresses. Ramsay et al. (1973) employ the relative abundance of the solutionresistant discoasters to planktonic foraminifera in estimating carbonate compensation depths for deep-sea environments. In applying their estimates of Caribbean Miocene carbonate compensation depths to the effects of solution on calcareous nannoplankton in the Kingshill Seaway water column and at the sediment--water interface, it may reasonably be concluded that dissolution in the water column or at the sediment surface was most probably not a major factor in the relatively low detectable diversity of nannofossils in the Kingshill Marl. Ramsay and others conclude that the calcite lysocline lay at a depth of about 4200 m in the Middle Miocene Caribbean Sea. The bottom of the tropical Kingshill Seaway probably lay at depths substantially above the calcite lysocline. Moreover, a sensitive indicator of incipient dissolution is the planktonic foraminifer O r b u l i n a universa. Arrhenius (1952) reports that this species is among the least resistant to solution destruction. This has recently

151 been confirmed by Berger (1967). The rock-building abundance o f O. universa in Kingshill Marl pelagic sediments indicates that dissolution during settling in the water column must have been comparatively slight for planktonic foraminifera, and by inference, for the more resistant calcareous nannoplankton.

Calcareous nannoplankton assemblages As indicated above, the nannofossils of the Kingshill Marl show evidence of pervasive disarticulation. Of the roughly 5--7% of the nannomicrite chalk matrix, placoliths of Cyclicargolithus floridanus (Plate I, 2) are clearly the most a b u n d a n t nannofossils. These relatively large coccoliths are robustly constructed and are resistant to both mechanical disarticulation and to dissolution. Discoasters, which appear to be among the most durable of all calcareous plankton, are conspicuous by their rarity. Of thousands of SEM fields examined from dozens of samples, only a few specimens of Discoaster were found, with the large, robust species Discoaster deflandrei being the most c o m m o n of these (Plate I, 6 and 7). Samples of Miocene ooze with abundant discoasters from DSDP Site 285 were studied to serve as a control to assure t h a t their apparent rarity in Kingshill chalk was n o t due to lack of operator recognition. Coccolithus eopelagicus and Thoracosphaera heimi were determined to be roughly as a b u n d a n t as D. deflandrei. Coccolithus pelagicus Sphenolithus spp., Coronocyclus sp., and a small, five-rayed species of Discoaster (Plate I, 5) were rare. Thus, although W. W. Hay (written communication) identified as many as eleven species of nannofossils in specially sedimented strewn samples, only Cyclicargolithus floridanus could be said to be relatively abundant on the basis of our extensive SEM examination of fracture surfaces of the chalk matrix.

Planktonic foraminifera Planktonic foraminifera occur in rock-building abundance in basin-facies chalks of the Kingshill Marl. Whole tests and large fragments of tests contribute to the sediment as framework grains which comprise as much as 60% of the total sediment volume in some samples (Fig.4C). Smaller fragments derived by the mechanical breakdown or disarticulation of planktonic foraminiferal tests make up about 60% of the fine matrix of the chalk, or roughly about a fourth of the total sediment volume.

Test wall ultrastructure The porous tests of planktonic foraminifera consist of a basically radial microstructure composed of a palisade of subrhombic low-Mg calcite crystallites (Fig.5G) flanked on the inside (proximal to the cytoplasm) by a microgranular layer of anhedral granules each 0.2--0.5 pm in diameter (Fig. 5F) and on the outer test surface by a euhedral layer of terminated crystallites each

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Fig.5.

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10--20 pm long (Fig.5B--D) (B~ and Lott, 1964; B~ et al., 1975). Disintegration of planktonic foraminiferal tests by mechanical breakage, by progressive solution, or by a combination of the two, yield silt- and clay-size debris in the Kingshill Marl composed of pieces or whole tabular crystallites or clumps of them. With incipient dissolution, the outer euhedral layer of porous tests such as Globorotalia mayeri progressively disintegrates by "exfoliating" tiny fragments produced by the fracture of crystallites along planes transverse or oblique to their long axes (Fig.5H). Recognition of the exact origin of the finest skeletal particles of the Kingshill chalk matrix is difficult because the range in size of the smaller disintegration products of planktonic foraminifera overlaps that of the larger skeletal c o m p o n e n t s of nannofossil placoliths such as Cylicargolithus. Terminated euhedral crystals form the ultrastructure of nannofossils such as Sphenolithus: the largest of these are roughly the size of crystals in the euhedral layer of early-formed planktonic foraminifera chambers (Fig.5F). Eleven species of planktonic foraminifera have been recovered from the Kingshill Marl, and are listed in Table I. Orbulina universa is clearly the most abundant (Plate II, 2 and 3) and many comprise as much as 45--50% of the total sediment volume of relatively pure chalks. Indeed, many beds appear to be pseudo-oolitic upon casual inspection because of the abundance of spherical tests of O. universa 0.4--0.9 mm in diameter. Globose-chambered species of Globorotalia such as G. obesa and G. rnayeri, as well as Orbulina bilobata are next in terms of abundance (Plate II, 1; Plate III). Species of Globorotalia with flattened chambers such as G. fohsi fohsi and G. archeomenardii (Plate III) as well as Orbulina suturalis, Globoquadrina dehiscens, Globigerina praebulloides, Globigerinoides obliquus and Globigerinoides diminutus are volumetrically minor in terms of contribution to sediment volume. Orbulina suturalis may be actually somewhat more abundant than originally noted because some specimens may have been misidentified as Orbulina universa. In general, preservation of planktonic foraminifera ranges from poor to fair in basinal sediments of the Kingshill Marl. Although there is little evidence of current-induced breakage, disarticulation or winnowing of these in the pelagic chalks, planktonic foraminiferal tests in any of the more porous beds display a sugary or chalky surface texture due to apparent solution and concomitant recrystallization of the test wall microstructure (Fig. 4E). Fig.5. U l t r a s t r u c t u r e o f Kingshill Marl p l a n k t o n i c f o r a m i n i f e r a l tests. A. Surface, x 600, of Globorotalia archeomenardii, t e s t slightly e t c h e d . B. Globorotalia fohsi fohsi, x 700. Surface layer of test wall has b e e n r e m o v e d b y s o l u t i o n , e x p o s i n g the surface o f t h e m i d d l e l a y e r or palisade. C. G. fohsi fohsi, × 855. Surface of o u t e r or e u h e d r a l layer f o r m i n g test surface. Note t h e t e r m i n a t e d crystallites. D. Orbulina universa, s o l u t i o n - e t c h e d test surface, x 80C. E. Globorotalia sp. cf. fohsi, x 3000. F. I n n e r surface, × 9 0 0 0 , of Orbulina e m b r y o n i c c h a m b e r s . N o t e t h e rosette-like o r d e r i n g of t h e i n n e r l a y e r o f crystallites a r o u n d t h e pores. G. F r a c t u r e surface, x 1 6 0 0 , of Orbulina test wall s h o w i n g the basic palisade s t r u c t u r e o f t h e m i d d l e layer. The i n n e r surface is c o a t e d w i t h calcite m i c r o d r u s e . H. Globorotalia rnayeri. O u t e r test surface, x 6 0 0 0 , d e p i c t i n g s o l u t i o n e n l a r g e m e n t of test wall pores.

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PL ATE II

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Other planktonic microfossils Even though siliceous plankton such as diatoms, silicoflagellates, ebridians and radiolaria and aragonitic pteropods are important in tropical open-ocean plankton communities, skeletons of these organisms are now rare or absent as fossils in the Kingshill Marl. A few fragments of centric diatoms occur; these have been replaced by calcite (Fig.6). Rare casts and molds of what appear to have been pteropods are also present b u t no original aragonite skeletal material has been noted. P E L A G I C F O O D CHAIN, K I N G S H I L L S E A W A Y

Just as the carbonate sediments deposited in the Kingshill Seaway display evidence of both shallow tropical coastal and of tropical open-sea pelagic depositional processes, the total Miocene plankton c o m m u n i t y inhabiting the upper water layers of the Seaway might also be expected to have been of transitional character. Reference to standard works on open-ocean planktonic food chains (e.g., Raymont, 1963; Russell-Hunter, 1970; Steele, 1970) establishes that a substantial proportion of p h y t o p l a n k t o n biomass and an even greater proportion of zooplankton biomass is c o m p o s e d of species that lack mineralized skeletons. These make no contribution to the sediments accumulating on the sea floor below and thus lack the potential to leave a fossil record. On the other hand, coastal skeleton-bearing p h y t o p l a n k t o n assemblages of the tropics are dominated by diatoms instead of coccolithophorids and skeletonbearing zooplankton are relatively rare, so that an even smaller proportion of the plankton c o m m u n i t y would produce potentially fossilizable skeletal remains. We conclude that the Miocene plankton c o m m u n i t y of the Kingshill Seaway was neither of inshore coastal- nor of full open-ocean aspect. The chalks of the Kingshill Marl are dissimilar to shelf or coastal carbonate sediments, but in general resemble open-sea oozes or chalks in their overall biotic

P L A T E II

1. Orbulina suturalis Brgnnimann, x 80. 2, 3. Orbulina universa d'Orbigny, x 80. Specimen in 3 has test wall pores enlarged by solution etching.

5. Orbulina bilobata (d'Orbigny), x 74. 4, 6. Globigerina praebulloides Blow. 4, umbilical view, x 80; 6, spiral view x 85. 7. Globigerina sp. of. G. praebulloides, x90, view into aperture.

8. Globigerinoides diminutus Bolli, x 160, umbilical view. 9. Globoquadrina dehiscens (Chapman, Parr and Collins), x 100, umbilical view. 10. Globorotalia obliquus Bolli, x l 0 0 , umbilical view. Test surface is heavily etched.

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P L A T E III

5 6

9

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Fig.6. Siliceous plankton of the Kingshill Seaway. A. Centric diatom, probably Coscinodiscus, replaced by calcite (x 600). B. Detail, x 3990. Although sieve plates are destroyed and the pores have been filled by calcite, some surface ornamentation remains. c o m p o s i t i o n a n d t e x t u r e . H o w e v e r , t h e d i v e r s i t y o f t h e fossil n a n n o p l a n k t o n a n d p l a n k t o n i c f o r a m i n i f e r a l a s s e m b l a g e s is s u b s t a n t i a l l y l o w e r t h a n t h a t w h i c h m i g h t be e x p e c t e d f o r t h e i r o p e n - s e a c o u n t e r p a r t s . D i v e r s i t y o f calcareous p l a n k t o n As n o t e d a b o v e , a m a x i m u m o f eleven species o f c a l c a r e o u s n a n n o p l a n k t o n has b e e n i d e n t i f i e d in the Kingshill chalks, a n d o f t h e s e o n l y o n e species, Cyclicargolithus floridanus, c o u l d a c t u a l l y be c o n s i d e r e d t o b e a b u n d a n t . This c o n t r a s t s w i t h t h e richer o p e n - s e a n a n n o f l o r a s o f t h e s a m e age r e p o r t e d b y H a y a n d B e a u d r y ( 1 9 7 3 ) a n d b y B u k r y ( 1 9 7 3 ) f r o m D S D P Leg 15 sites in t h e V e n e z u e l a n Basin; t h e s e n u m b e r 20+ species, o f w h i c h d i s c o a s t e r s c o m p r i s e a l m o s t a third. T h e l o w - d i v e r s i t y species c o m p o s i t i o n o f Kingshill Marl n a n n o PLATE III 1--4. Globorotalia fohsi Cushman and Ellisor subsp, fohsi Cushman and Ellisor, all x 80. 1, 4, umbilical views; 2, 3, spiral views in which outer test surface has been partially stripped away. 5--6. Globorotalia archeomenardii Bolli. 5, spiral view, x 140; 6, umbilical view, x 90. 7. Globorotalia fohsi, specimen intermediate between the subspecies fohsi and lobata, umbilical view, x 100. 8--9. Globorotalia mayeri Cushman and Ellisor. 8, spiral view, x 90; 9, oblique umbilical view, x 95. 10. Globorotalia obesa Bolli, spiral view, x 110.

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fossil assemblages is comparable to Paleogene nannofloras of coastal aspect described by Gartner (1971) for the Blake Plateau. Kingshill Marl planktonic foraminiferal assemblages are also of limited diversity. Although Saunders et al. (1973) list a total of 34 planktonic foraminiferal species that are present in the Middle Miocene of the Caribbean area, only eleven species have been recovered from the Kingshill Marl. Of these eleven species, Orbulina universa is clearly several times more abundant than any of the other species, including globose-chambered Globorotalia, and it dominates the assemblage. Analysis of the modern pelagic biogenic oozes in samples collected b y the R/V Eastward from the St. Croix island slopes and floor of the Anegada Passage north of Canebay indicates that Holocene calcareous nannoplankton and planktonic foraminiferal assemblages in these island-slope environments are also of relatively low diversity when compared with open-ocean assemblages (Bakos, 1975). Saunders et al. (1973) list a total of 8 species of calcareous nannoplankton and 44 species of planktonic foraminifera which have been identified by the authors in the St. Croix Eastward samples. Although a few more species may be identified in these samples after continued search, the basic low-diversity nature of the calcareous plankton communities will und o u b t e d l y remain. WATER MASS CIRCULATION, KINGSHILL SEAWAY

There must have been a high nutrient supply over a long period to produce the pelagic biogenic sediments of the Kingshill Marl. First-order controls other than grazing that limit biomass production by p h y t o p l a n k t o n (and ultimately the entire pelagic f o o d chain) are the availability of nutrient c o m p o u n d s essential for photosynthesis (Russell-Hunter, 1970; Lipps and Valentine, 1970). Nutrient c o m p o u n d s such as nitrates and phosphates are used exhaustively b y p h y t o p l a n k t o n and, once used, are removed from further availability by being locked up in the biomass of dead plants and animals and in the excreta of animals, all of which settle through the water column o u t of the photic zone. Bacterial decay released nutrients from dead biomass, with concentration in t w o zones of bacterial activity, one directly beneath the photic zone and one in the water layers directly above the ocean floor. Although nutrient c o m p o u n d s are returned to solution, they are not available to plants until they are returned to the sunlit upper part of the water column, usually by the physical process of upwelling.

Kingshill Seaway paleocirculation It is somewhat difficult to visualize a water column circulation model b y which nutrients could be returned to the upper layers of the Miocene Kingshill Seaway water column. Instead of a water column ranging into the thousands of meters (as off modern St. Croix), the Kingshill Seaway did not reach depths

159 of more than a few hundred meters even at maximum flooding of the St. Croix island block (Multer et al., 1977). Taking into account the post-Miocene tectonic history of this island block (Whetten, 1966), it appears reasonable to infer that the tectonic configuration of the island block margin was largely the same during Miocene time as today, although the Kingshill Marl may be downwarped under the modern island shelf and is, of course, upwarped in the present-day island proper. We can thus reconstruct the Kingshill Seaway as a relatively shallow, narrow strait flanked on the west and east by two island masses and b o u n d e d on the north and south by sharply defined shelf breaks (Fig.2) flanking the open sea. The shelf margins, therefore, separated the relatively shallow Kingshill Seaway from much deeper-water conditions on the Miocene St. Croix island slope. This topographic/bathymetric configuration most likely exerted a funnelling effect on shallow or surface currents flowing into the Seaway, accelerating their velocity, and creating a fairly strong, unidirectional current flowing along the NE--SW axis of the trough. Fell (1967), L u y e n d y k et al. (1972) and Frakes and Kemp (1972) show that the dominant major wind and surface shallow ocean circulation in the northern end of the Lesser Antilles chain from the Late Cretaceous to the Early Neogene most probably flowed in a westward or slightly southwestward direction (trade winds belt). This is similar to the modern wind surface circulation patterns of St. Croix (Ogden, 1974). Major shallow and surface ocean currents flowed WSW down the axis of the Miocene equivalent of the deep Anegada Passage, while shallow surface curents diverted southeastward into the Kingshill Seaway would enter it at its constricted northeastern end. The currents therefore converged and accelerated as they passed through the narrowest part of the Seaway between the northeastern end of the Northside Range island mass and the northwestern end of the East End Range island mass (Fig.2). Where the Seaway broadens substantially, south of this constriction, currents would broaden and diverge, promoting the upwelling of nutrient-rich b o t t o m water masses. High biogenic productivity in the Seaway could thus be sustained over long periods of time because nutrients " p u m p e d " downward by sinking organic debris and by excreta would be retained in the relatively shallow water mass of the Seaway or on its bottom. Nutrients could then, u p o n release by bacterial decomposition, be efficiently resupplied to the surface water layers by relatively small-scale overturn of the Seaway water column. Moreover, the steep island slopes bounding the St. Croix block would also facilitate upwelling on its lee side, as water from intermediate oceanic depths rises in response to the lateral displacement of surface water by winds blowing persistently along or away from the coast (Fig.2). If Kingshill Seaway circulation occurred in the pattern described above, rates of biogenic skeletal production may have been higher along the southern margin of the Seaway adjacent to the shelf edge than in its central parts. Moreover, interaction of winds blowing from the ENE with south-southwestward moving surface currents in the Seaway would produce a net right-hand deflection or transport (Ekman transport) of the surface layers of water, resulting in the westward deflection of surface currents to

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curve around the southwestern end of the Northside Range Island (Fig.2) possibly promoting an additive impetus for upwelling along the lee (southern) sides of the St. Croix island block. Water masses below the surface current in the Seaway, however, would be further deflected in a right-hand or clockwise Ekman spiral, decreasing in effect with depth so that weak currents at intermediate depth would be moving in the opposite sense from the direction of surface currents at a velocity o f a b o u t 4% of that of the surface currents (Gross, 1972, pp. 225--226). These weak currents at intermediate depths might have served to transport nutrients from the steep southern shelf edge northward or northeastward back into the Kingshill Seaway. DIAGENETIC HISTORY OF KINGSHILL CHALKS

In order more fully to understand the nature of pelagic biogenic sediment production of the Kingshill Seaway, it is useful to reconstruct what must have been the original sediment composition as it was being deposited, by comparison with similar modern sediments.

Holocene analog sediments Multer (1972) and Multer et al. (1974) established that Holocene communities, sediments, and sedimentary processes along the northwest coast seaward o f Canebay serve as a highly satisfactory standard of comparison in reconstructing the origin of the Kingshill Marl, b o t h in the basin-margin and open-seaway facies. Thus, 66 samples of Holocene sediments were taken from cores # 2 1 4 4 5 , # 2 1 4 4 6 , # 2 1 4 6 7 , and # 2 1 4 7 2 made by the R/V Eastward on the northwest St. Croix island slopes and floor of the Anegada Passage. Cores # 2 1 4 6 7 (depth 2140 m), # 2 1 4 4 5 (depth 2500 m), and # 2 1 4 7 2 (depth 2875 m) were taken in channels crossing the lower island slope and island rise at distances of, respectively, 4.5 km, 5.5 km, and 8 km northwest of the t o w n of Canebay (Fig.lB). Sampled intervals of the cores are, respectively, 0--134 cm, 0--35 cm, and 0--134 cm. The sedimentary sequence penetrated consists mostly of turbidites which include: (1) carbonate sands of sand-size to pebble-size skeletal rubble of organisms characteristic of reefal and shallow-shelf environments; (2) pebbles and cobbles of igneous and sedimentary rocks; and (3) the winnowed coarser fraction of the pelagic biogenic sediment accumulating on the deep island slope, consisting of the larger tests of planktonic foraminifera and pteropods. The ultra-fine matrix, made up of coccoliths, fragments of planktonic foraminifera and fine aragonite needles, is sparse or missing in even the most poorly sorted of the turbidites, indicating that at least weak currents persisted in the areas of the channels for much of the time while the sediments were being deposited. Soft-sediment intraclasts incorporated into the turbidites in core # 2 1 4 4 5 indicate the type of sediment being deposited on the St. Croix island slope and rise in the areas between the channels. These clasts are composed of planktonic foraminiferal/pteropod/nannoplankton calcareous oozes

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which may in some units contain large quantities of fine aragonite needles derived from shallow shelf- and reef-tract animals and plants. These have settled from the water column, and might have been: (1) transported from the island margin in suspension to the open sea, where they survived settling through 2000--3000 m of the oceanic water column; (2) precipitated chemically from the upper layer of the open-sea water column; or (3) transported down the island slope by turbidity currents, where they settled after being suspended in the near-bottom water column. The third mechanism of transport, turbidity currents, is most likely in view of the comparatively shallow depth of the aragonite lysocline [estimated by Berger (1970) to be between 250 and 750 m ] , although much larger particles of aragonite (pteropod tests) have settled through the water column without being dissolved. Some units have been intensely reworked by borrowing infaunal organisms. In virtually all of the sampled units of core # 2 1 4 7 2 , the coarser winnowed fraction of the pelagic biogenic ooze has been finely comminuted in place, with planktonic foraminiferal and pteropod tests pulverized to silt- and mud-size fragments. Sediments accumulating on the floor of the Anegada Passage under deep-sea conditions were sampled by E a s t w a r d core # 2 1 4 4 6 , taken in 4300 m of water a b o u t 9 km north of the town of Canebay (Fig.lB). Samples from the 427 cm thick core indicate that sediments accumulating along the south side of the Anegada Passage are interbedded pelagic oozes and fine turbidite sands. The a u t o c h t h o n o u s sediments are gray, argillaceous, planktonic foraminiferal/ n a n n o p l a n k t o n / p t e r o p o d pelagic ooze with as much as 10% biogenic opaline silica in the form of hyalosponge and demosponge spicules and diatom frustules (Fig. 7). Radiolaria and silicoflageUates were found to be very rare or absent. Many of the ooze units contain significant amounts (5--10%) of acicular aragonite needles (Fig. 7C,~ derived from the mechanical disintegration of shallow marine animals and plants. These can be demonstrated on a morphological basis to be clearly of shallow-water origin rather than from the breakdown of aragonite pteropod and heteropod skeletons. The turbidite sands interbedded with the biogenic ooze are finer-grained than those of the St. Croix island slope and rise but many still contain appreciable quantities of fine to medium sand-size skeletal rubble and lithic fragments, both derived from the island shelf. Some of the coarser sandy oozes contain neither nannomatrix nor shallow-water detritus and appear to have been winnowed in situ by b o t t o m currents, thus being superficially similar to turbidite sands. Mineralogically, the oozes are composed of low-Mg calcite planktonic foraminifera and calcareous nannoplankton, aragonite pteropods and fine aragonite needles, and biogenic opal in the form of pelagic diatoms and benthic sponge spicules. The turbidites contain a wider range of skeletal mineralogy, being c o m p o s e d of: (1) aragonite from various calcareous green algae, scleractinian corals and mollusks; (2) calcite from foraminifera, calcareous sponge spicules, octocorals, bryozoans, rare brachiopods, bivalve mollusks, and coralline red algae; (3) Mg-calcite from miliolid foraminifera, some coralline algae, echinoids and asterozoan and ophiuroid ossicles. Turbidites, especially those

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Fig.7.

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on the lower island slope and Anegada Passage, also contain major quantities of calcite and aragonite pelagic sediment swept up as the turbidity current moved down slope. These Holocene sediments are thus a close counterpart of open-basin sediments deposited in the Miocene Kingshill Seaway. Although the proportions of the various biogenic c o m p o n e n t s may have differed from our Holocene analog, all of the various types of skeletal mineralogy and almost all of the sediment-producing groups of plants and animals must have been originally represented in Kingshill Seaway sediments. Before or during the diagenetic transition from pelagic ooze to chalk and from lime sands and muds to biosparite and biomicrite, much of the fossil evidence for the existence of several groups of organisms bearing aragonite or opaline silica skeletons has been removed.

Diagenesis of Kingshill pelagic sediments By comparison with data on deep-sea chalks, the Kingshill pelagic chalks are estimated to have had an original porosity of 70--75% as nannoplankton/ planktonic foraminiferal ooze. Pliocene through Quaternary pelagic deep-sea oozes and chalks from various DSDP sites in the Pacific have been determined to have porosities ranging from 72 to 96% (Gealy, 1971; Cook and Cook, 1972; Schlanger et al., 1973). Kingshill Marl pure chalks have porosities in the 60--65% range. In the Kingshill ooze -~ chalk diagenetic transition, porosity has been diminished by certain processes and increased by others. Schlanger et al. (1973) postulate that a reduction of roughly one-fourth in porosity accompanies the deep-sea ooze to chalk diagenetic transition, accomplished in the following stages: (1) compaction due to loading (important only in the first few tens of meters below the sediment surface); (2) partial dissolution of planktonic foraminiferal tests and solution of delicate nannofossils; (3) progressive loss of thin-walled planktonic foraminiferal species and formation of calcite overgrowths on nannofossils; (4) virtually complete dissolution of foraminifera, general recrystallization of the nannofossils, and the precipitation of secondary calcite as silt-size grains. They substantiate Gealy's (1971, p. 1100) observation that much of the porosity of planktonic foraminiferal/nannoplankton ooze results from the pore space within the planktonic foraminiferal tests. As much as 80% of the grain volume occupied by a planktonic foraminiferal test is made up of chamber interior voids, pores in the test wall, and foramina between chambers (Schlanger et al., 1973, p. 424). Fig.7. H o l o c e n e c a r b o n a t e s e d i m e n t f r o m t h e St. Croix island slope a n d t h e A n e g a d a Passage. A. C o c c o l i t h s Emiliania huxleyi ( L o h m a n n ) , l o w e r right; Cyclococcolilhina leptopora ( M u r r a y a n d B l a c k m a n ) , c e n t e r right; a n d Gephyrocapsa oceanica K a m p t n e r , u p p e r right. F r o m c o r e 2 1 4 4 6 - - 3 , interval 2 1 9 - - 2 2 1 c m , x 5 2 8 0 . B. S p o n g e spicule in n a n n o p l a n k t o n / a r a g o n i t e n e e d l e m a t r i x , x 1760. F r o m core 2 1 4 4 6 - - 1 , interval 9 6 - - 9 8 cm. C. H o l o c e n e o o z e s e d i m e n t assemblage, x 1760. F r o m core 2 1 4 4 6 - - 1 , interval 3 8 - - 4 0 cm. D. Detail of o o z e n a n n o m a t r i x , s h o w i n g m i x t u r e o f c o c c o l i t h s a n d a r a g o n i t e needles, x 3520. F r o m c o r e 2 1 4 4 6 - - 1 , interval 8 - - 1 0 cm.

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Thus, as long as tests of foraminifera in the ooze are only surficially attacked by dissolution, the supporting framework of test grains remains intact, and maintains a high sediment porosity. Schlanger et al. (1973, p. 425) postulate an initial 15% porosity reduction due to loading which may be the result of a combination of some test breakage plus the closer packing of the other largely intact tests. Further reduction of porosity results from the collapse of tests weakened b y solution and the concomitant closer inter-particle packing of the nannomatrix. There is no evidence to suggest that the Kingshill Marl was subsequently buried by a significant thickness of overlying sediments, even though a suprajacent veneer of Pliocene carbonates has been discovered along the southcentral coast of St. Croix (Multer et al., 1977). Lithification of the Kingshill Seaway sediments through compaction by deep burial was therefore n o t an important factor, with total loss of porosity by breakage and packing probably less than 10%. Moreover, the Kingshill Marl apparently retains a large proportion of its original planktonic foraminiferal intrabiotic volume (Fig.4C) because even highly soluble thin-walled species are intact, although many tests are surficially etched (Fig. 5). Pteropods also may have contributed to the initial grain-supported framework. Aragonite tests of these planktonic gastropods are less numerous than those of planktonic foraminifera, but because of their larger size contribute significantly to intra-grain porosity in the modern calcareous oozes accumulating on the St. Croix island slope. Original pteropod abundance in Kingshill chalks is difficult to reconstruct because no aragonitic skeletal material is n o w present. A few casts and molds of what appear to be straight pteropods (Cresis?) occur, but if pteropods were present in quantity in the original pelagic ooze, most were dissolved before the matrix was lithified sufficiently to form casts and molds around the skeletal fragments. In contrast to examples of Miocene ooze and chalk nannomatrix illustrated by Schlanger et al. (1973, figs.4--6 ) and Cretaceous chalk illustrated by Schneidermann (1973a, plate 3, fig.2) whole coccolith platelets are relatively rare in the Kingshill chalk matrix. Instead, the calcareous nannoplankton c o m p o n e n t is represented by finely disarticulated or comminuted skeletal fragments, thereby increasing the bulk porosity of the very fine particulate fraction of the matrix. The ooze -* chalk -~ limestone diagenetic model postulated by Schlanger and others (1973) is concerned only with the stability of calcite skeletal c o m p o n e n t s in its various phases; calculations of porosity values for their samples were based on the assumption that each is composed entirely of calcite with a density of 2.7 g cm -3 . While this assumption may be largely correct for deep-sea calcareous ooze in which aragonite and opal c o m p o n e n t s are absent or in minor quanity, the Kingshill ooze-~chalk diagentic system must have been more complex.

Solution of skeletal opal Kingshill basinal sediments must have initially contained several percent by

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grain volume of opaline silica diatom frustules and sponge spicules. Radiolaria and ebridians may also have been present although these appem: to be rare in the Holocene analog sediments, even in samples from the deep-sea floor of the Anegada Passage. Marine siliceous skeletons have, in general, a low preservation potential. For example, Hurd (1973) found that only 0.05--0.15% of opal plankton skeletons remained in the sedimentary record at test stations in the central equatorial Pacific. Opaline silica is attacked at all depths in the oceanic water column because marine water is undersaturated with respect to silica (Lewin, 1961; Hurd, 1972). In sea water of normal salinity and pH values of less than 9.5, temperature controls the rate of opal dissolution (Krauskopf, 1956; Siever, 1962; Hurd, 1973). Maximum dissolution rates of radiolarian tests at stations in the central Pacific occurred at 250 m (Berger, 1968), a shallow depth at which the water is relatively warm; a depth closely comparable to the open Kingshill Seaway water column. Although research by Schrader (1971) has demonstrated that fecal pellets of grazing arthropod zooplankton may transport opal diatom frustules through the water column to the sea floor with great efficiency, these may be readily dissolved by contact with sediment interstitial water or in the pellets themselves as incorporated organic matter decomposes. Diatom frustules are very rare in Kingshill chalks and the ones that do occur have been replaced by calcite (Fig.6A and B). We might thus also explain the apparent total absence of opal sponge spicules from the Kingshill Marl even though its paleoenvironmental setting and analogy with Holocene sediments of similar environments indicate that sponges most probably were present on the sea floor. Dissolution of the spicules must have taken place before the Kingshill sediments were even partially indurated, because solution of triaxon, tetraxon, or hexactine spicules would leave molds that could be readily identified by their distinctive morphology. Decomposition of the organic material occupying the axis of each spicule ray (Fig. 7B) may also have increased their dissolution rates. Preservation of spicules in the Holocene analog sediments may be due to the lower temperatures of b o t t o m and interstitial waters, thus slowing rates of silica dissolution.

Solution of skeletal aragonite Information regarding aragonite c o m p o n e n t s of the Kingshill calcareous oozes and turbidites has also been lost due to solution. A wide variety of shallow marine aragonite skeletal debris was introduced into the basin by debris flows and turbidities, perhaps totalling as much as 10--15% of the sedimentary volume. The original existence of large aragonite skeletons such as scleractinian corals and gastropods is confirmed by abundant casts and molds of these in the debris flows and coarser-grained turbidites. Fine aragonite needles derived either from the mechanical breakdown of biogenic skeletal debris or by precipitation from the uppermost layers of sea water (Alexanders-

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son, 1972) were u n d o u b t e d l y introduced into the open Kingshill basin in large quantities. In view of the postulated relative shallowness and warmth of the Seaway water column, it is reasonable to postulate that rates of aragonite dissolution were low. As noted above, Holocene sediments of the St. Croix deep island slope contain appreciable volumes of acicular aragonite even though they were deposited as much as 2000--3000 m below the 200--750 m depth estimated b y Berger (1970) to be the aragonite lysocline in the tropics. Moreover, there is some evidence to indicate that the aragonite c o m p o n e n t of the Kingshill ooze may have persisted well into the ooze -÷ chalk diagenetic process. Casts and molds retaining complex skeletal details of scleractinian corals in the debris flows indicate that the calcareous ooze or mud matrix was mostly indurated before the skeletons were removed by solution. A number of investigators (e.g., Land, 1967; James, 1974) have demonstrated that aragonitic megafossil skeletal material such as scleractinian corals and gastropods remains stable while still in contact with saline interstitial fluids, b u t that exposure to fresh water initiates a recrystallization and replacement sequence which culminates in calcite pseudomorphs after the original aragonite microstructure. Application of this to the Kingshill Marl diagenetic sequence suggests that aragonitic skeletal debris, even fine needles in the ooze matrix, may have persisted through ooze -~ chalk diagenesis and was not dissolved until the sediments were flushed with fresh water. Reprecipitation of dissolved aragonite or calcite as calcite cement in the highly porous chalk or in the interbedded turbidites and debris flows did n o t occur on a large scale. The diagenetic system was n o t carbonate conservative then, as in the Schlanger et al. (1971) model, because the carbonate loss by solution leaching far exceeded redeposition as calcite void-fillings or as cement. The bulk of the dissolved CaCO3 must have been transported o u t of the system.

Calcite stability Limited solution-etching of calcite skeletal materials is apparent. Large, solution-resistant calcareous nannofossils such as Cyclicargolithus and Discoaster have solution-etched sutures and some placoliths display serrate margins and dissolved axial structures (Plate I, 4). Planktonic and rare calcareous benthonic foraminifera show varying degrees of etching of the test wall microstructure, both in the outer euhedral layer and in the microgranular layer of the inner surface (Fig.5). In view of the hydrographic conditions that most probably existed in the Kingshill Seaway, it is unlikely that calcite skeletal c o m p o n e n t s would have undergone significant solution either in the water column or in sediments of the shallow sub-bottom. It is more likely that most o f the solution etching, with some concomitant redeposition as overgrowths or chamber fillings, occurred during initial flushing by vadose or phreatic fresh water. Continued exposure to fresh water has resulted in slow degradation of quality of p r e s e ~ a t i o n of the fossils, particularly the planktonic

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foraminifera, which appear to be undergoing micritization in some of the most porous units. SUMMARY

Subsidence of the St. Croix island block in the later part of the Early Miocene produced the Kingshill Seaway, which was flanked on the northwest and southeast by the island masses formed by the Northside and East End ranges, and on the northeast and southwest by the edges of the island shelf. Hydrographic conditions in the tropical Seaway, which at most was only a few hundred meters deep, p r o m o t e d high rates of pelagic biogenic skeletal production which were sustained for an appreciable period of time. Sediments accumulating in the open Kingshill Seaway consisted of pelagic calcareous oozes which were interbedded with turbidites and debris flows. The oozes were composed of a framework of planktonic foraminiferal tests (of which Orbulina universa was most abundant) and aragonitic pteropod tests; the fine matrix was composed of calcareous nannofossils, planktonic foraminiferal test debris, and fine aragonite needles. Comparison of the chalks of the Kingshill Marl with modern sediments accumulating on the northwest St. Croix island slope establishes guidelines to infer the total components of the original Miocene calcareous ooze accumulating on the floor of the Kingshill Seaway. Comparison of the diagenetic processes affecting shallow calcareous oozes {such as those lithified to form the chalks of the Kingshill Marl) with those affecting deep-sea calcareous oozes underscores the necessity of considering the range and intensity of differential solution as a factor in diagenesis. Thus the ooze -~ chalk transition under relatively shallow, warm conditions involves the solution destruction of a significant aragonitic component, while diagenesis of deep-sea oozes deals with a sediment that is virtually all calcite to begin with. Siliceous skeletal debris, while attacked by dissolution at all depths within the oceanic environment, dissolves relatively more rapidly in shallow, island-slope environments than in the deep sea. Undoubtedly the major event in the KingshiU ooze ~ chalk diagenetic process was the solution of aragonitic skeletal sediment when the Kingshill Marl was flushed by fresh water. ACKNOWLEDGEMENTS

The writers gratefully acknowledge the encouragement and support of the staff of The West Indies Laboratory, Fairleigh Dickinson University, St. Croix. Drs. H. G. Multer and L. C. Gerhard, both formerly of the W.I.L., facilitated field studies and supplied much vital stratigraphic and sedimentologic information. Many of the samples investigated by us were collected by a group study of the Ville la Reine exposure by D. Boucher, P. Curth, D. Lash and T. McCormick of Windham College under the supervision of H. G. Multer. Dr. Malcolm P. Weiss made the outcrop photographs of the Ville la Reine

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