Chapter 13 Evolution of the neogene kingshill basin of St. Croix, U.S. Virgin Islands

Chapter 13

Evolution of the Neogene Kingshill Basin of St. Croix, U.S. Virgin Islands

I V A N G I L L , P E T E R P. M C L A U G H L I N ,

JR. a n d D E N N I S K. H U B B A R D

The sedimentary rocks of the Neogene Kingshill basin of St. Croix record part of the evolution of the tectonically complex region at the eastern edge of the North American-Caribbean plate boundary zone. The Kingshill basin is a northeasterly oriented graben or half-graben that contains a thick section of Neogene carbonates bounded by fault blocks of Cretaceous siliciclastic and intrusive rocks. Significant details of basin development have been added by the inclusion of data from a drilling program that included fourteen test holes with cumulative footage exceeding 533 m and a maximum depth of 91 m. Additional information came from outcrop sampling over the ca. 80 km 2 basin, subsurface records, and samples from engineering and water wells donated to the project. Previous models of basin development suggest that the carbonate rocks of the Kingshill basin were deposited (1) in shallow water or (2) entirely within the confines of an insular graben system. These models assume an isolated insular basin with self-contained sediment source. Instead, subsurface evidence suggests that early Kingshill basin sedimentation started in deep marine conditions prior to faulting on the basin margins and includes incursions of coarse, reef-derived sediment from a nearby source. The period of pre-rift sedimentation is documented to extend into the early Middle Miocene, but probably extends into the Oligocene or earlier. The faulting that formed the basin margins was initiated no earlier than the late Middle Miocene. After rifting, the Kingshill basin underwent significant shallowing and uplift in Late Miocene to Early Pliocene time. Basin development culminated in the establishment of a Pliocene reef tract and several episodes of subaerial exposure. The Jealousy Formation, the lowest formation described, is an entirely subsurface Middle Miocene unit of dark marls deposited at middle bathyal depths. The Kingshill Limestone conformably and diachronously overlies the Jealousy Formation and is divided into two members. The La Reine Member is characterized by buff pelagic limestones and marls with an upward increasing proportion of intercalated shelf-derived sediment flows. It ranges from basal Middle Miocene to uppermost Miocene and exhibits a transition from middle bathyal to upper bathyal environments. The Mannings Bay Member is composed of skeletal debris-rich carbonate slope deposits and lies near the Miocene-Pliocene boundary. The Blessing Formation overlies the Kingshill Limestone and represents a reef system that existed on the south coast of St. Croix during the Early Pliocene. Stratal relations on the basin margins indicate that the Jealousy Formation and at least the lower part of the Kingshill Limestone were deposited prior to graben formation near the end of the Middle Miocene. Subsidence analysis of the Neogene section indicates that 400 m of vertical uplift occurred on St. Croix between 10.5 and 3.5 Ma. A right-lateral model of movement between St. Croix and the Puerto Rico platform has been suggested by several recent workers. This model is consistent with the geomorphology of the Virgin Islands Trough and the Anegada Passage with right-lateral strike-slip motion in the Anegada Passage opening the Virgin Islands Trough as a pull-apart basin. However, an older left-lateral model of island movement is consistent with the northeasterly orientation of the normal fault system of St. Croix and the St. Croix Ridge. In addition, left-lateral motion would locate pre-rift St. Croix south of the known extra-basinal sources of Cretaceous and Tertiary shelf sediment required by the timing of Kingshill basin sedimentation. In this model, the Puerto Rico platform area could act to disperse slip between the North American and Caribbean plates. A variety of models are possible, but each should take into account geologic details of the Kingshill basin development.

INTRODUCTION AND GEOLOGIC SETTING

d i s t i n c t f r o m the m a j o r i t y o f the p r i m a r i l y i g n e o u s i s l a n d s o f the L e s s e r A n t i l l e s . A t h i c k s e c t i o n o f

St. C r o i x is the s o u t h e r n m o s t o f the U.S. V i r g i n I s l a n d s , l o c a t e d at the e a s t e n d o f the G r e a t e r A n t i l l e s

Neogene

c a r b o n a t e s o c c u p i e s a c e n t r a l g r a b e n or

h a l f - g r a b e n , h e r e r e f e r r e d to as the K i n g s h i l l b a s i n

a n d the n o r t h w e s t e d g e o f the L e s s e r A n t i l l e s arc

(Fig. 2). T h i s b a s i n lies b e t w e e n fault b l o c k s o f

(Fig. 1). T h e i s l a n d is t e c t o n i c a l l y a n d g e o l o g i c a l l y

C r e t a c e o u s s i l i c i c l a s t i c a n d i n t r u s i v e r o c k s o f the

Caribbean Basins. Sedimentary Basins of the World, 4 edited by E Mann (Series Editor: K.J. Hsti), pp. 343-366. 9 1999 Elsevier Science B.V., Amsterdam. All rights reserved.

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Fig. 1. Location map of St. Croix, the Virgin Islands platform, and the Virgin Islands basin. Bahymetry and structure are after Houlgatte (1983). Inset: NOAM = North American plate; SOAM -- South American plate; CARIB = Caribbean plate (after Burke et al., 1984).

Fig. 2. Generalized geologic map of St. Croix from Whetten (1974). Exposed strata mapped as Jealousy Formation by Whetten (1966) are re-mapped as Kingshill Limestone in this paper.

Mt. Eagle Group that comprise the mountainous East End and Northside Ranges. The Neogene carbonate section, which is the focus of this paper, is divided into three forma-

tions (Fig. 3): the blue-gray marls of the Jealousy Formation; marls and limestones of the Kingshill Limestone; and reef limestones of the Blessing Formation (Gill et al., in press). The stratigraphy of this

EVOLUTION OF THE NEOGENE KINGSHILL BASIN OF ST. CROIX, U.S. VIRGIN ISLANDS basin provides clues to the tectonic evolution of the eastern end of the North American-Caribbean plate boundary zone (PBZ). The purpose of this paper is to trace the evolution of the Kingshill basin from the Miocene to Recent based on observations from test holes and outcrops in the central plain of the island. This investigation also evaluates the implications of these findings, tied to marine geology studies in the area, for plate tectonic models of the northeastern part of the Caribbean region. Up to this point, there have been few integrated biostratigraphic-stratigraphic studies that have incorporated subsurface information. Previous studies of the geology of St. Croix have, for the most part, been based solely on outcrop data. These studies considered the carbonates of the Kingshill basin to record deposition in an isolated Oligocene-Miocene graben system (Multer et al., 1977; Gerhard et al., 1978; Lidz, 1984a). Whetten (1966) produced a detailed geologic map of St. Croix and a particularly detailed description of its Cretaceous section; some of this work is summarized in Fig. 2. Multer et al. (1977) and Gerhard et al. (1978) provided modern models for the carbonate sedimentation, including the structural and sedimentological model of the basin. Gerhard et al. (1978) designated the type section and provided detailed petrologic descriptions of the Kingshill Limestone that are still pertinent today. Lidz (1982, 1984a, 1988) defined the biostratigraphic relationships within the basin, tied them to the basin model of Gerhard et al. (1978) and suggested ties to global eustasy. In the only work to include subsurface information, Cederstrom (1950) provided an early geologic map and a detailed description of early deep drilling work on the island. This work includes the type-section description of the Jealousy Formation. Most of these studies either did not address a wider tectonic framework, or have related the late Cenozoic tectonic evolution of St. Croix solely to vertical uplift. However, cores from a drilling program conducted in the 1980's furnish new subsurface data that, in conjunction with outcrop data, help to more clearly delineate the sedimentary and structural evolution of the Kingshill basin during the Neogene. The drilling program included fourteen test wells drilled to depths of up to 91 m as well as data from a number of private wells donated to the project (Fig. 4). These wells were logged during drilling, and samples were collected for sedimentologic, micropaleontologic, and geochemical analysis at intervals of 1.5 or 3 m in the wells (Gill and Hubbard, 1986, 1987; McLaughlin et al., 1995). Core material and logs from pre-existing wells provided additional data on the carbonate units underlying the southeastern portion of the central plain.

345

STRATIGRAPHY AND DEPOSITIONAL SETTING OF THE NEOGENE FORMATIONS OF THE KINGSHILL BASIN Jealousy Formation Lithology and distribution

The Jealousy Formation is a unit of blue-gray marls that underlies much of the central plain of St. Croix. The top of the formation is marked and abrupt in the subsurface; water-well drillers treat the top of this 'blue clay' as hydrologic basement and generally stop drilling when it is reached. The blue-gray marls are rich in planktonic foraminifera and other deep-water microfauna. The formation includes a number of conglomeratic limestone and thin limestone layers in the deep subsurface, below the reach of the drilling conducted for this study (Cederstrom, 1950). These coarse-grained beds are bracketed above and below by pelagic blue marls, so are considered allochthonous deposits of down-slope transported debris. Test well data indicate the Jealousy Formation is present in the subsurface throughout the central plain region, both inside and outside the fault boundaries of the Kingshill basin graben. The type section was defined by Cederstrom (1950) in the deepest of several test wells (Test Well 41) drilled by the Civilian Conservation Corps (CCC) in 1939, where a thickness of more than 426 m was encountered (Fig. 4). It is present in wells M1, M2, and M10 of this study, all west of the eastern fault boundary of the graben, and was reported by Cederstrom (1950) at 18 m below sea level in CCC Test Well C26, approximately 1 km east of this fault (Fig. 4). A maximum thickness of 450 m has thus far been recognized for this unit (Cederstrom, 1950). Although its base has never been reached in the center of the basin, gravity surveys indicate that more than 1800 m of Jealousy and older sedimentary rocks may underlie the central plain (Shurbet et al., 1956; R.C. Speed, written commun., 1994). Although the top of the Jealousy Formation exhibits considerable relief in the subsurface (Fig. 5), no apparent change in bulk mineralogy, microfauna, or grain size is observed across the Jealousy/Kingshill boundary. No hiatus or missing section is evident within the resolution of available biostratigraphic control. Extensive areas of Jealousy Formation outcrop exposures have been mapped in some previous studies (Cederstrom, 1950; Whetten, 1966). However, we recognize the Jealousy Formation as an exclusively subsurface unit and suggest that these outcrops are more correctly mapped as Kingshill Limestone, following Gerhard et al. (1978).

Fig. 3. Stratigraphic column, chronostratigraphic framework, and paleoenvironments of the Neogene section of the Kingshill basin (after McLaughlin et al., 1995). Planktonic foraminiferal zonation based on Bolli and Saunders (1985). Chronostratigraphy, coastal onlap curve, and eustatic cycles after Haq et al. (1988).

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EVOLUTION OF THE NEOGENE KINGSHILL BASIN OF ST. CROIX, U.S. VIRGIN ISLANDS

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Fig. 4. Locations of outcrops, test wells and water wells used in the stratigraphic cross-sections. A/P = Airport/Penitentiary; AQ -Airport Quarry; FC = Five Corners; HC = Hess Cut; MS = Morningstar; R/B -- Rattan/Belvedere; SR --- Salt River valley; VR = Villa La Reine and Fredensburg Quarry; WR = Work and Rest. Core sample locations designated with M are test holes drilled by Gill, the others are from previous studies. Cutting sample locations are noted for water well cuttings studied by the authors. Test holes drilled in 1939 by the Civilian Conservation Corps are designated by C.

Age and paleoenvironment Our recent micropaleontologic studies of borehole samples from the Kingshill graben place the Jealousy Formation in the lower part of the Middle Miocene, ranging from the P r a e o r b u l i n a g l o m e r o s a zone to the G l o b o r o t a l i a f o h s i f o h s i zone (McLaughlin et al., 1995). The Jealousy Formation has previously been referred to as Oligocene (Cushman, 1946; Cederstrom, 1950) and even as low as Middle Eocene (Lidz, 1984b). However, the Oligocene citations are based on older notions of the age significance of certain benthic macrofauna and larger foraminifera. The Middle Eocene citation is based on the planktonic foraminiferal fauna found by Lidz (1984b) in an allochthonous shale clast in the Kingshill Limestone that is presumed to have been derived from the Jealousy Formation. Although no in-place paleontologic evidence exists for an age any older than Miocene for the Jealousy Formation, the estimated 1800 m thickness of sedimentary fill in the Kingshill basin (Shurbet et al., 1956) leaves open the possibility that the Jealousy Formation and any underlying units could extend as far as the Oligocene or lower.

The Jealousy Formation is a dominantly hemipelagic, deep-water unit. The benthic foraminiferal fauna indicates deposition at 600 to 800 m water depth (McLaughlin et al., 1995). Most species recovered are generally associated with middle and upper bathyal environments; several species are present that indicate an environment no shallower than the middle bathyal zone. This differs from previous interpretations of the Jealousy Formation as an estuarine deposit (Van den Bold, 1970; Multer et al., 1977) based on outcrop samples previously mapped as Jealousy.

Source area and paleocurrents The coarse, shelf-derived carbonates in the Jealousy Formation are sandwiched between large intervals of foram-rich basinal sediments. The coarse carbonates are therefore allochthonous, and require a nearby shelf source. The thickness of Jealousy strata makes the uplifted horst blocks of St. Croix an unlikely source area (Gerhard et al., 1978) even if they had existed during Jealousy deposition. No data exist on presumed paleocurrent directions in the Jealousy Formation.

348

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Fig. 5. Geological crossections through the Kingshill basin. (A) North-south cross-section A-A'. Note that the Jealousy Formation surface roughly follows the topography of the Kingshill Limestone. (B) East-west cross-section B-B'. A normal fault (the Fairplain fault) forms the western boundary of the small graben on the south coast occurs between test wells M 1 and M4. The Jealousy Formation was not reached to the east of this fault.

Kingshill Limestone The blue-gray marls of the Jealousy Formation are succeeded upward by the more carbonate-rich succession of the Kingshill Limestone. The Kingshill Limestone crops out over large areas of the central

plain and can also be mapped in the subsurface based on well data. It is composed of limestones and buff pelagic marls with an upward-increasing proportion of shelf-derived sediment gravity-flows. Lithologic variations permit it to be divided into two members (Gill et al., in press): the interbedded

EVOLUTION OF THE NEOGENE KINGSHILL BASIN OF ST. CROIX, U.S. VIRGIN ISLANDS

Fig. 6. Interbedded planktonic foraminiferal packstones and sediment gravity flows of shallow-water debris at Villa La Reine outcrop, type section of the Kingshill Limestone (La Reine Member). Biogeographic control places this outcrop near the boundary of the Middle Miocene and Upper Miocene. This is denoted as location VR in Fig. 4.

marls and limestones of the La Reine Member and the benthic-foram-rich, burrowed limestone of the Mannings Bay Member.

Lower Kingshill Limestone- La Reine Member Lithology and distribution The La Reine Member is composed of interbedded planktonic foraminifera-rich marls and shallowmarine limestone debris beds with increasing proportions of downslope-transported material upsection. Typical lithologies of the La Reine Member are exposed at the type section of the Kingshill Limestone at Villa La Reine (Fig. 6). The outcrop is a rhythmically bedded alternation of polymictic packstones (Gerhard et al., 1978), some with boulder-sized coral heads, and deep-water planktonic foraminiferal chalks and marls. Similar lithologies occur at Fredensburg Quarry and Estate Work and Rest, but breccia beds at the latter include terrigenous material presumably derived from the Cretaceous Mt. Eagle Series. The lower part of the La Reine Member is similar but includes less transported debris (Five Corners,

349

Rattan/Belvedere, and Morningstar sections). In the subsurface, this interval is dominated by planktonic foraminiferal packstone, with less common lithic-pebble or foraminifera-rich wackestones. The boundary between it and the underlying Jealousy Formation is marked by a distinct color change from tan above to blue-gray below. However, the significance of this change is unclear; sedimentological and micropaleontological evidence reveal no notable change in lithology, mineralogy, or depositional environment, nor is any hiatus resolvable. The stratigraphically highest part of the La Reine Member is exposed in the Airport/Penitentiary section along the Melvin Evans Highway, where it is disconformably overlain by the Mannings Bay Member (Fig. 7). This interval is characterized by regularly bedded intercalations of softer, planktonic foraminifera-rich beds and more indurated, graded beds of shelf-derived debris. The quantity of shelfderived sand is greater than lower in the member, and burrowing appears to be more pervasive. The La Reine Member in the St. John's/Judith Fancy area includes beds of calcareous conglomerate composed of rounded terrigenous gravel and a fauna of shallow-water echinoids and benthic foraminifera. Previously, these and nearby outcrops in the Northside Range have been considered to be shelf and lagoon deposits or part of the Jealousy Formation, based on their similarity to conglomerates encountered in the CCC Test Well 39 (Gerhard et al., 1978; Lidz, 1982; Andreieff et al., 1986). However, because the conglomerate beds are overlain in outcrop, and underlain in Well M10 (Fig. 4), by planktonic foraminiferal packstones, we interpret them as allochthonous beds occurring within a succession of typical La Reine Member deep-water strata. In addition, structural relations support inclusion of these strata in the Kingshill Limestone rather than the Jealousy Formation. Because these exposures occur at elevations similar to outcrops of the La Reine Member only a few kilometers away (Fig. 4), faulting within the graben would be required to raise the stratigraphically lower Jealousy beds to the same elevation as the nearby Kingshill. However, there is no evidence of such faulting. The maximum thickness of the La Reine Member encountered in the test wells drilled for this study is approximately 140 m (Fig. 5A). Cederstrom (1950) reported a thickness range from 0 to 180 m for the Kingshill Limestone, the larger figure referring to extrapolated thickness in the carbonate highlands of the Rattan Hill area. Isopach patterns reveal thinning of the La Reine Member toward the north and northwest margins of the basin. The formation shows a pronounced thickening in the carbonate highlands close to the northern coast of St. Croix and less pronounced thickening toward the south, interrupted

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Fig. 7. Disconformable contact between the La Reine and Mannings Bay Members of the Kingshill Limestone at the Airport/Penetentiary outcrop, along Melvin Evans Highway, southern St. Croix. Disconformity occurs midway up the outcrop, approximately 7 m above road level. This is denoted as location A/P in Fig. 4. by post-depositional faulting along the south coast (Fig. 5). In general, these isopach patterns follow the trends of the top of the Jealousy Formation. If deformation is ignored, the Kingshill Limestone isopach patterns imply a basin opening to the south but with a deep area under the modem carbonate highlands in the north.

Age and paleoenvironment The La Reine Member extends from basal Middle Miocene to approximately the Miocene-Pliocene boundary (McLaughlin et al., 1995). The subsurface section sampled in our drilling program is the stratigraphically lowest, ranging from basal Middle Miocene (Praeorbulina glomerosa zone) to the medial Middle Miocene (Globorotalia fohsi robusta zone). The stratigraphically lowest outcrops are in the northern part of the island (Salt River Valley and Five Comers, Fig. 4), where the La Reine Member is placed in the lower part of the Middle Miocene (Globorotaliafohsifohsi zone, possibly to Praeorbulina glomerosa zone). The type section of the Kingshill Formation at Villa La Reine represents the middle part of the formation, with faunas indicative of the upper part of the Middle Miocene (Globorotalia mayeri zone, possibly to the Globorotalia menardii zone). The top of the member crops out on the south side of the island (Airport/Penitentiary section), where it is placed near the top of the Miocene (upper part of Globorotalia humerosa zone). The outcrops in the St. John area previously mapped as Jealousy Formation fall biostratigraph-

ically within the range of the La Reine Member, supporting the lithologic and structural arguments against their inclusion in the Jealousy. Van den Bold (1970, in Gill, 1989, and in McLaughlin et al., 1995) considers the ostracode fauna indicative of a position near the Lower Miocene-Middle Miocene boundary. The fauna is completely different from that of the subsurface Jealousy Formation, but contains several species in common with the lower part of the La Reine Member in the subsurface. Middle Eocene to Early Miocene foraminifera were described by Lidz (1984b) from a mud clast in the La Reine Member. Its presence in these deepwater strata indicates that older Tertiary sediments were being eroded and transported by sediment gravity flows during the deposition of the La Reine Member. This is consistent with the occurrence of pebbles in both the Jealousy Formation and Kingshill Limestone that are assumed to be derived from the Cretaceous strata presently exposed in the highlands of the basin-bounding fault blocks. The benthic foraminifera of the La Reine Member in Wells M1, M2, and M10 comprise a middle bathyal fauna (600-800 m water-depth) that differs little from that of the underlying Jealousy Formation (McLaughlin et al., 1995). No significant paleoenvironmental shift is evident at the boundary. Although there are significant numbers of shallower-water species in some of the stratigraphically higher outcrop samples from the La Reine Member, the faunas in the finer-grained beds that over- and underlie these samples are middle bathyal types, indicating that the

EVOLUTION OF THE NEOGENE KINGSHILL BASIN OF ST. CROIX, U.S. VIRGIN ISLANDS shallow-water forms are present due to downslope transport.

Source area and paleocurrents Paleocurrent indicators in the lower Kingshill Limestone at the Villa La Reine type section show west-southwest flow. Four measurements included a pebble halo around a boulder and orientations from cross-lamination and oscillation ripples (Gerhard et al., 1978). Clasts larger than 4 mm are concentrated in the northeast portion of the basin (Gerhard et al., 1978). None of the reef-derived material, including boulder-sized coral heads, is in-place, and this material must therefore be derived from a nearby shelf area.

Upper Kingshill Limestone: Mannings Bay Member Lithology and distribution The Mannings Bay Member of the Kingshill Limestone is characterized by channelized beds of grainstones rich in shelf debris, interbedded with softer wackestones and packstones. The grainstones contain abundant Operculinoides cojimarensis and Paraspiroclypeus chawneri (Behrens, 1976; Gerhard et al., 1978; S. Frost, oral commun., 1986). Many specimens show signs of transport or reworking, such as fracturing, abrasion, and imbrication. The foraminiferal wackestones and the softer packstones also include significant quantities of planktonic foraminifera. These lithologies are well exposed in the type section at the quarry on the southeast side of Mannings Hill (Gill et al., in press) and along

351

parts of Evans Highway, notably the upper part of the Airport/Penitentiary roadcut (Figs. 4 and 7). It was also examined in several of the test wells drilled for this project. The Mannings Bay Member rests disconformably on the La Reine Member, from which it was distinguished based on higher abundance of shallow-water carbonate material (Gill, 1989; Gill et al., in press). At the Airport/Penitentiary section, the disconformity is evident as a scour surface with more than 1 m of relief. This surface appears to have been scoured by partly channelized submarine flows of shelf-derived sediment (Lidz, 1984a; Gill, 1989; Gill et al., 1990). The Mannings Bay Member includes the strata referred to as a "benthic foraminiferal wackestone and grainstone facies" in the Kingshill Limestone by Gerhard et al. (1978) and those strata separated from the Kingshill Limestone as "postKingshill" limestones by Lidz (1982) and Andreieff et al. (1986). The Mannings Bay Member and the overlying Blessing Formation are best developed in a small graben on the south coast of St. Croix (Fig. 8). They can be difficult to differentiate from one another in core but together total more than 50 m thickness in some of the wells (Gill and Hubbard, 1986, 1987). The westernmost documented extent of the Mannings Bay Member is near the western fault boundary of this graben near Fairplain. The eastern edge of this graben is perhaps indicated by where the stream flow makes an abrupt southerly turn to the coast against the exposed Pliocene reef complex (Fig. 9).

Fig. 8. Disconformable contact between the Mannings Bay Member of the Kingshill Limestone (below) and the Blessing Formation (above) at the Airport Quarry outcrop. This is denoted as location AQ in Fig. 4.

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Fig. 9. Facies map for south coast industrial area. Dolomite in the vadose zone or exposed in outcrop is patchily distributed in an arcuate region following the Pliocene reef trend. Dolomite presently in the phreatic zone is found in off-shore facies. The western boundary of the small, south coast graben is well-defined by a normal fault. The northern and eastern boundaries are poorly known.

Age and paleoenvironment Planktonic foraminifera are generally uncommon and poor in the Mannings Bay Member. The basal part of the member, in the Airport/Penitentiary section, contains faunas suggestive of the lower Pliocene Globorotalia margaritae zone (McLaughlin et al., 1995), consistent with Lower Pliocene findings for this same section by Andreieff et al. (1986, 1987) and Lidz (1982). A subsurface sample from Well M4 yields a fauna indicative of an interval near the Miocene-Pliocene boundary, between the Globorotalia humerosa zone and the Globigerinoides trilobus fistulosus zone (McLaughlin et al., 1995). The foraminiferal control from above and below the disconformity at the base of the Mannings Bay Member suggests a general chronostratigraphic position at or near the terminal-Miocene (5.5 Ma) eustatic fall, but does not provide sufficient resolution to tie it exactly to this event, as has been proposed by Lidz (1982). The biostratigraphic control is also insufficient to determine whether a significant chronostratigraphic interval is missing at the disconformity. Smaller benthic foraminifera recovered from the Airport/Penitentiary section are a mix of outer ner-

itic species and forms associated with shallow-water carbonate environments (McLaughlin et al., 1995). In outcrop and core samples, the larger benthic foraminifera Operculinoides and Paraspiroclypeus are abundant; these forms were likely photic-zone inhabitants (S. Frost, oral commun., 1986). Other bioclasts that contribute to the facies are coralline algal crusts, rhodoliths, echinoid spines and plate fragments, coral fragments, and molluscan debris. These forms, the poorly developed planktonic fauna, and the evidence for transport in the larger foraminiferarich beds together indicate that the shallow-water material was carried into a deeper shelfal setting of approximately 100 m depth.

Source area and paleocurrents The beds of larger benthic forams show sorting and imbrication, the result of extensive current working. Sediment transport direction in the Kingshill basin was dominantly to the west-northwest, and ranged from southwest to north-northeast. This is based on 42 measurements of imbricated large benthic foraminifers on Mannings Hill, probably the same outcrop referred to here as the Airport/Penitentiary outcrop (Gerhard et al., 1978).

EVOLUTION OF THE NEOGENE KINGSHILL BASIN OF ST. CROIX, U.S. VIRGIN ISLANDS

Blessing Formation Lithology and distribution The highest stratigraphic unit, the Blessing Formation, represents a Pliocene reef tract that extended across the south and west coastlines of St. Croix. The reef tract consisted of interspersed reefs and shelf systems similar to the arrangement of reefs around the modem south coastline of St. Croix. The classic reef model with flanking fore- and backreef facies does not appear to apply here. Reef systems on St. Croix apparently formed planar deposits with little topographic relief. This planar geometry is apparently common in Caribbean Tertiary reef deposits (S. Frost, oral commun., 1986). The Blessing Formation was sampled in outcrops near the south coast and in cores. Based on subsurface data, its greatest thickness is in a small graben, just east of the Fairplain fault, where it may be up to 30 m thick. Scattered exposures of reef facies also occur west of the Fairplain fault along Evans Highway, a location near Fredericksted, and at an exposure described by Gerhard et al. (1978); its maximum thickness in that area is estimated to be between 10 and 20 m. The best exposures of this unit are in its type section at a road cut next to the Hess Oil refinery (Gill et al., 1990, in press) (Figs. 4 and 10). Reefal facies are predominantly composed of coralline boundstones characterized by external molds of scleractinians, gastropods and pelecypods, as well as skeletal debris. The scleractinians include species of extant genera such as Agaricia, Diploria, Montastrea, and Siderastrea, as well as the extinct forms Stylophora, Teliophyllia, and Thysanus (Behrens, 1976). Lagoonal facies include skeletal wackestones composed of shallowwater foraminifera, coralline algae and a wide variety of shallow-water invertebrates. The Hess outcrop is marked by several wellcemented undulatory layers (Fig. 10) distinguished by abrupt light stable-isotopic excursions, an onlap surface (Gill, 1989) and karstification (Lidz, 1984a; Gill, 1989). Nearby exposures were apparently marked by terra rosa beds within the Blessing Formation (Behrens, 1976) and underneath it (S. Frost, pers. commun., 1986; L. Gerhard, written commun., 1997). These surfaces indicate that the south coast reef trend of St. Croix was exposed several times during the Pliocene (Gill, 1989; Gill et al., 1990). Age and paleoenvironment The Blessing Formation is loosely placed by biostratigraphic data in the interval between the upper part of Lower Pliocene and the top of the Pliocene. This assignment is based on the occurrence of Globigerina nepenthes in the Hess refinery outcrop

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(Lidz, 1982), its position above the Lower Pliocene Mannings Bay Member (Andreieff et al., 1986; McLaughlin et al., 1995), and the occurrence of prePleistocene species of the scleractinian corals Teliophyllia, Stylophora, and Thysanus (Behrens, 1976). No planktonic foraminifera were recovered from our samples. The macrofaunal assemblages within the Blessing Formation represent co-existing reef, forereef, and lagoon environments that extended along the south and west coastlines of St. Croix.

Source area and paleocurrents Much of the exposed Blessing Formation appears to be in-place. The morphology of the reef tract mimics the present shoreline, and the reefs would have been affected by open-ocean conditions to the south and to the west.

EVOLUTION OF THE KINGSHILLBASIN In previous studies, the Kingshill basin was thought to have formed in the Oligocene as a result of vertical tectonic movement. Whetten (1966) characterized the carbonate section of the Kingshill basin as reefal and estuarine deposits that accumulated in a graben in the central part of the island following a period of low-rank metamorphism, faulting, folding, igneous intrusion, and uplift. He concluded that there was no significant evidence for strike-slip motion north of St. Croix, and that the Neogene section was affected only by vertical tectonics. More recent studies (e.g. Multer et al., 1977; Gerhard et al., 1978; Lidz, 1982) have accepted this structural framework; for example, Multer et al. (1977) envisioned the Northside Range and the East End Range as subaerially exposed horst blocks that provided both terrigenous and shelf-derived carbonate debris to the basin. However, these studies recognized that deposition took place in a deep-marine setting, which they envisioned as a seaway opening to the northeast and southwest in a basinal setting similar to modem deep basins north of St. Croix.

Basin-margin faulting and basin formation Subsurface stratigraphic evidence indicates that the Kingshill basin graben began to form no earlier than the latest part of the Middle Miocene, during deposition of the upper part of the La Reine Member. The Jealousy Formation and the lower part of the La Reine Member show no evidence of tectonic activity. These strata were deposited at water depths of approximately 600 m during the early part of the Middle Miocene (McLaughlin et al., 1995). Previous studies have suggested the existence of an active graben at this time with subaerially exposed mar-

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Fig. 10. Reefal facies of the Blessing Formation at the Hess Cut outcrop, southern St. Croix. HG = coral reef hardground, with onlap onto exposure surfaces (ES). CAL -- undulatory caliche layer under karst cavities. Backpack and hammer for scale at arrow. This is denoted as location HC in Fig. 4. gins. However, the existence of tentatively identified deep-water marls of the Jealousy Formation outside the graben in CCC Test Well C-26 (Cederstrom, 1950) indicates that faulting on the graben margins must post-date these early Middle Miocene deposits. In addition, exposed graben margins would require a marginal slope exceeding 45 ~ based on the measured distance from the test well samples in the basin to the hypothetical exposed margin on the northwest side of the graben. This angle is comparable to that of the modem slope north of St. Croix, which is dominated sedimentologically by input of shelf-derived material (Hubbard et al., 1981; Gill, 1983), including reef foraminifera such as Amphistegina (B. Sen Gupta, oral commun., 1984). Our samples from the Jealousy Formation and the lower part of the La Reine Member near this boundary do not show the major input of shelfal material that would be expected with deposition at the foot of a similarly steep slope. If a steep-sided Kingshill basin did not form before the middle Miocene, then the scattered conglomerates and shelf-derived sediments that do occur in the Jealousy Formation and lower part of the Kingshill Limestone must be derived from somewhere other than St. Croix. These lithologies suggest that St. Croix was close to a land mass capable of supporting reef growth and supplying clastic materials to the deep-water environment during the Middle Miocene. Puerto Rico and the Virgin Islands platform, to the northwest of St. Croix, and Anguilla and Saba to the northeast, are possible source areas;

either requires significant lateral translation of the St. Croix platform. We suggest that initiation of the St. Croix fault system occurred no earlier than in the latest part of the Middle Miocene (Fig. 11). The contacts between the lower Kingshill Limestone and the Cretaceous rocks on the east and west sides of the graben have been interpreted as faults by Multer et al. (1977) although they were mapped simply as stratigraphic contacts on the geologic map by Whetten (1966). The western contact, along the edge of the Northside Range, is mostly obscured by alluvial cover (Fig. 4). Gerhard et al. (1978) have suggested that displacement along the eastern fault boundary of the graben was greater than that along the western boundary. The eastern fault contact is sharp and characterized by offset of both Cretaceous and Kingshill strata. This age of graben formation is constrained by fault relations between the La Reine Member and the Cretaceous strata along the eastern boundary of the graben, which indicates that at least the lower part of the La Reine Member was deposited prior to basin faulting (Gill and Hubbard, 1986, 1987). Beds of coral debris and rounded pebble conglomerate exist low in the exposed section near Judith's Fancy and St. John and were penetrated by test hole M10. These materials were interpreted as in-place deposits by Gerhard et al. (1978) and Lidz (1982), but are interpreted as allochthonous deposits here (Gill, 1989). Similarly, the type-section at Villa La Reine (Gerhard et al., 1978) records the influx of shallow-marine and terrigenous debris in the basin.

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Fig. 11. Block models of the evolution of the Kingshill basin of St. Croix from the Early Miocene to Recent. The model reflects overall tectonic quiescence until near the end of the Middle Miocene when significant tectonism, uplift, and erosion of uplifted Jealousy Formation sediments apparently began. The Late Miocene to Plio-Pleistocene diagrams trace shoaling in the basin from bathyal to shallow-marine and reefal deposition. This section is placed near the Middle-Late Miocene boundary (Andreieff et al., 1986; McLaughlin et al., 1995), and includes a number of beds that contain large coral heads and siliciclastic material probably derived from exposed and eroded rock outside of the graben. Together these data suggest a shift from relative tectonic quiescence to uplift activity near the end of the Middle Miocene. Breccia beds occur in strata of approximately the same age at Estate Work and Rest along the eastern edge of the basin. These beds have been interpreted as syntectonic breccia by Gerhard et al. (1978) in an outcrop that could not be located for this study. In nearby exposures, including the Estate Work and Rest sec-

tion (Fig. 4), angular clasts of typical Cretaceous Mt. Eagle Group lithologies form breccia layers between beds of the La Reine Member. The Estate Work and Rest section is placed biostratigraphically near the Middle Miocene-Late Miocene boundary, between the Globorotalia mayeri and Globorotalia acostaensis zones (McLaughlin et al., 1995). This breccia may provide the earliest evidence for fault activity on the eastern boundary of the graben.

Eustatic events and basinal shallowing The effects of eustasy superimposed on tectonic uplift produced the disconformity separating the La

356 Reine and Mannings Bay Members at the Airport/ Penitentiary outcrop. This disconformity was interpreted by Lidz (1984a) as evidence of the Messinian eustatic fall of Haq et al. (1988). Our sedimentologic and paleontologic data indicate shoaling across this surface, from depths of approximately 200-300 m in the La Reine Member to 100 m in the Mannings Bay Member. There is no evidence of soil formation, dissolution, or karsting to indicate subaerial exposure. Based on this evidence, we believe this disconformity represents submarine erosion during the uplift of the island in the Late Miocene. The global sea-level fall associated with the Messinian event is of approximately the same age and could have triggered erosion via sediment gravity flows. However, biostratigraphic control (McLaughlin et al., 1995) cannot precisely tie the timing of this unconformity to the Messinian event, nor confirm a single event as its cause. The strata above the unconformity record the development of extensive foraminiferal-coralline algal bank environments on St. Croix during the early part of the Pliocene (Fig. 11). The environments served as shallow-water sources of the larger benthic foraminifera present in the Mannings Bay Member, in particular Operculinoides cojimarensis and Paraspiroclypeus chawneri (Gerhard et al., 1978). This environment is not present in modem St. Croix, where coral ecosystems predominate. It is likely that foram-algal ecosystems were supplanted by corals with the extinction of many larger foram groups in the Neogene (Frost, 1977). It is also possible that the relative lack of coral debris could be the result of (1) the upslope storage of coral reef sediments, with minor deposition only at sporadic intervals (e.g. Moore et al., 1976), (2) changes in circulation, nutrient or temperature conditions yielding competitive advantage to the foraminifera-algal community, or (3) rapid uplift and eustatic variation suppressing the establishment of coral reef systems. Fairplain fault

The tectonism that produced the shallowing across the boundary between the La Reine and Mannings May members is also reflected in faulting within the Kingshill basin. The Fairplain fault, which marks the western margin of the small graben on the south coast, cuts through the Mannings Bay Member and Blessing Formation, indicating that motion occurred on this fault during the Late Pliocene or later. The orientation of the Fairplain fault is roughly parallel to the orientation of the northeast-trending main basin boundary faults. It dips at least 20~ to the east, as indicated by the depth to the Jealousy Formation at the fault contact in several well sections: 29 m below sea level at Well M1 (Fig. 13); 53 m

I. GILL et al. below sea level in CCC Test Well 45a, located 60 m to the east of Well M1 (Fig. 4); and deeper than the 80 m below sea level bottom-hole depth of Well M4 less than 180 m to the east of M1. Minimum vertical fault displacement is 68 m, based on the occurrence of the La Reine Member-Mannings Bay Member contact at 24 m above sea level on Mannings Hill west of the fault, and approximately 44 m below sea level in Well M4, east of the fault. The presence of a fault at this location is also supported by surface features. Strata in nearby outcrops along Evan's Highway (Fig. 4) dip toward the fault line and ephemeral streams locally approximate the trend of the fault near the coastline (Fig. 9). The eastern edge of the presumed graben is not well marked. Stream flow north of Well M5 is from west to east, oddly parallel with the coastline. The eastern edge of this graben may be marked by the ephemeral stream drainage turning abruptly south where it meets the exposed Plio-Pleistocene reef trend (Fig. 9). Surface features also suggest that a northern hinge-line exists for this small graben, just north of the industrial areas along the coast. Together, these fault orientations suggest that normal faulting on the margins of this graben was produced by the same extensional tectonic regime that initiated faulting on the margins of the Kingshill basin during the latest part of the Middle Miocene. The stratal relations across the Fairplain fault suggest that the south-coast subsidiary graben existed as an entity during and after deposition of the Mannings Bay Member foraminiferal-algal facies. The fact that the greatest thickness of these deposits is preserved in the graben has two possible explanations: the graben formed a marine embayment along the south coast where these facies accumulated; or the strata were preferentially preserved within the subsiding graben during island uplift. The former alternative suggests that the faulting produced topographic relief prior to and during deposition of the foraminiferal-algal facies, whereas the latter alternative requires only post-depositional faulting. We believe that both processes are likely to have occurred. Pliocene reef tract and subaerial exposure

Continued tectonic uplift and shoaling of the Kingshill basin resulted in deposition of the Blessing Formation reef tract (Fig. 11). The greatest thickness of reef growth is found in the Krause Lagoon area on the south-central coastline where the arcuate distribution of reef and lagoonal facies suggest the existence of an embayment (Fig. 9). The size and shape of this embayment was probably controlled by faulting along the margins of the south coast graben. The Blessing Formation contains indications of more than one period of Pliocene subaerial exposure

EVOLUTION OF THE NEOGENE KINGSHILL BASIN OF ST. CROIX, U.S. VIRGIN ISLANDS along its southern coastline (described in a previous section), as well as an onlap surface near the Hess Oil Refinery. We suggest that eustatic variations superimposed on the overall tectonic-uplift trend account for the evidence of exposure noted in our field studies in St. Croix, the same as Lidz (1984a). The accumulation of the Pliocene sediments only on the south coast, the apparent extent of erosion/non-deposition in the northern central plain at the same time, and the general southerly dip of Neogene strata in the Kingshill basin suggest that Late Pliocene uplift preferentially raised the northern part of the island. Subsidence analysis

Subsidence analysis (Fig. 12) indicates that the majority of the Neogene shallowing in the Kingshill basin is due to tectonic uplift. The Kingshill basin shallowed from as much as 800 m of water depth in the Middle Miocene (ca. 10.5 Ma) to approximately 100 m in the Early Pliocene (ca. 3.5 Ma). Given a modem stratigraphic thickness of 250 m for the study interval, and correcting for the effects of sediment loading, water loading, and compaction, we calculate 400 m of tectonic uplift in this interval, which translates into a rate of 57 m/Ma.

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and coastline of the island are less steep and less suggestive of recent faulting. On submersible dives in DSRV Alvin along the south wall of the Virgin Islands basin off the north coast of the island, Dill (1977) encountered structures he interpreted as fault gouge in St. Croix basement rocks. In similar dives off the northwest coast, a vertical escarpment greater than 10 m in height was observed at a depth of greater than 2600 m (Hubbard et al., 1981; Gill, 1983). The escarpment was composed of dark, terrigenous rock similar in appearance to the Cretaceous Mt. Eagle Group rocks that form the east and west hills of St. Croix. If in-place, the face of the escarpment suggests the role of fault-offset in creating the north slope of St. Croix. Like the southern margin of the Virgin Islands basin off St. Croix, the northern margin off Vieques and St. Thomas is also characterized by very steep slopes (Fig. 13). If the Virgin Islands basin formed as a result of rifting, these slopes may represent scarps formed during the rifting event. If these scarps were initially juxtaposed, the present position of St. Croix could have been achieved by movement of the island south and east relative to its initial position in the Virgin Islands platform. This motion would require a combination of left-lateral movement and tensional separation, which is consistent with the oblique left-lateral strike-slip motion for the formation of the Virgin Islands basin. Fault orientation

Left-lateral oblique motion models

Stratigraphic and structural evidence indicates that the Kingshill basin graben began to form no earlier than the late Middle Miocene. St. Croix may have began to rift away from Puerto Rico during this period by oblique left-lateral faulting, movement that could have opened the Virgin Islands basin (Fig. 13). Similar left-lateral faulting may have also occurred between St. Croix and the Saba Bank area to the northeast, but structural and bathymetric relations in the intervening St. Croix basin (Fig. 1) are less clear. A left-lateral tectonic model is consistent with several lines of evidence. Bathymetry

Bathymetric profiles along the north coast of St. Croix are rugged and steep, sloping between 23 and 45 ~ to the center of the Virgin Islands basin and dropping off at nearly vertical angles near the shelf edge (Fig. 13). The northwestern shoreline is carved from cliffs of the Northside Range, prompting Meyerhoff (1927) to suggest relatively recent faulting and uplift for the northern coast, probably no older than the Pliocene. Gradients of the southern shelf

All documented Neogene faults on St. Croix are normal faults, including the graben-bounding faults, and strike in a northeasterly direction, oblique to the south margin of the Virgin Islands basin. On the St. Croix Ridge, seismic profiles and GLORIA imagery (Masson and Scanlon, 1991) indicate that the ridge is broken into a series of block-fault 'piano key' structures with the same northeasterly orientation as the St. Croix faults (Fig. 13). These structures are interpreted to be the products of normal faulting similar to those in the Neogene section of St. Croix (Holcombe, 1977). The northeasterly orientation of the apparently continuous set of tensional fractures is consistent with the type of deformation expected in a left-lateral wrench-fault zone aligned along the Anegada Passage. Such fractures tend to form parallel to the short axis of the strain ellipse in clay models (Wilcox et al., 1973). The northeasterly fault orientation is inconsistent with right-lateral strike-slip movement, which would produce normal faulting of the opposite orientation. Alternately, a model similar to that of Geist et al. (1988) may apply here. The consistent orientation of the St. Croix and St. Croix Ridge fracture system implies a tensional

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Fig. 12. Subsidence analysis for the Neogene of the Kingshill basin. Upward trend of tectonic subsidence curve indicates uplift beginning at approximately 10.5 Ma, coincident with initiation of formation of the Kingshill basin boundary faults. Tectonic subsidence is calculated based on stratigraphic thicknesses, age, and paleobathymetry of each unit, decompacted thickness of the strata based on lithology, and removing the effects of sediment and water loading. The sea floor line represents the position of the sea bottom based on paleobathymetry with eustatic contributions removed. origin under a consistent tectonic regime (Fig. 13). The sharply defined walls of the Virgin Islands basin, including the north slope of St. Croix, suggest relatively recent tectonic activity along this area. Assuming that the origin of this faulting is connected to movement in the Virgin Islands basin/Anegada Passage, the orientation of the fault system on St. Croix suggests left-lateral wrench faulting north of St. Croix beginning in the late part of the Middle Miocene. To the north of the Virgin islands basin, the islands of St. Thomas and St. John both show extensive faulting. Donnelly (1966) mapped a graben structure on both islands that also strikes northeasterly and displaces Cretaceous rocks. Left-lateral strike-slip displacement is apparent within the graben. Although these faults may pre-date St. Croix strata, they show a similar orientation.

Sediment sources for the Kingshill basin Because no uplifted horst blocks were available as sediment sources before the Middle Miocene formation of the Kingshill basin graben, an external source of coarse clastics is required to explain the significant quantities of conglomeratic deposits present in the type section of the Jealousy Formation (Cederstrom, 1950). The southeastern part of Puerto Rico contains exposures of Tertiary carbonates that extend eastward of Puerto Rico only as far as the southern coastline of Vieques Island (Khudoley and

Meyerhoff, 1971). This area is presently more than 60 km to the northwest of St. Croix. Given these distances, St. Croix was probably much closer to these potential sources of coarse clastics during the Middle Miocene than it is today. Such a reconstruction would require tens of kilometers of left-lateral movement to place St. Croix in its present position. A second potential sediment source is the Anguilla/Saba Bank area to the east. Shelf carbonates of the same age as the Jealousy Formation and the Kingshill Limestone exist in Anguilla (Van den Bold, 1970) and contain very similar ostracode faunas. Saba Bank is underlain by rocks interpreted to be early Tertiary carbonates and fluvio-deltaics (Nemec, 1980; Warner, 1989) with a possible sediment source on Puerto Rico. If St. Croix was originally juxtaposed with either of these areas, fault motion would also be left lateral, assuming movement crudely parallel to the Anegada Passage. Lateral movement from Anguilla to the present location of St. Croix would require a greater travel distance than would movement from Puerto Rico. Unfortunately, information on the basins between St. Croix and Saba Bank is too sparse to allow more thorough evaluation. Interestingly, the problem of a sediment source exists for Cretaceous St. Croix as well. Conglomeratic deposits of rudistid bivalves are found within deep marine sedimentary rocks on St. Croix (Whetten, 1966). The closest documented source for these

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Fig. 13. Migration model for Tertiary St. Croix and opening of the Virgin Islands basin showing hypothetical positions for St. Croix between the Early Miocene and Recent, assuming an initial position south of Vieques. The bottom diagram contrasts this model with an alternative model that assumes more of a north-south relative motion across the Virgin Islands basin. Position numbers for the same time periods are indicated with italics.

360 rudists of this type is Puerto Rico (H. Santos, pers. commun., 1998).

Seismicity Seismic activity today is detectable only in the shallow zones in the north wall from 0 to 50 km deep. These seismic events occur in swarms and are generally less than magnitude 3.2 (Frankel et al., 1980). Historic records indicate that the potential exists for much larger earthquakes in the Virgin Islands basin/Anegada Passage area. Two major earthquakes caused damage in both St. Croix and St. Thomas in 1867 and were calculated on the basis of tsunami arrival times to have originated in the north wall of the Virgin Islands basin (Reid and Tabor, 1920, cited in Frankel et al., 1980). In general, the Virgin Island basin and Anegada Passage are no more active than the Puerto Rico Platform, and do not show seismic patterns correlated with their bathymetry. To our knowledge, no fault-plane solutions have been calculated for the seismic events currently taking place in the Virgin Islands basin. For these reasons, present seismicity patterns do not support any one tectonic model or significant movement along the Anegada Passage today (J. Joyce, pers. commun., 1998). Tectonic context The Caribbean plate is interpreted to have an eastward present-day motion relative to the North American plate. This plate motion is generally manifested by sinistral slip along the northern Caribbean plate boundary, and by dextral slip zones along the southern boundary (Stephan et al., 1986). Active documented subduction in the northeastern Caribbean is presently taking place only along the Lesser Antilles arc. In the area near the Puerto Rico trench, the northern boundary of the Caribbean plate is characterized overall by slip but with some evidence for compression (Frankel et al., 1980; Burke et al., 1984). Given the modem left-lateral motion of the northern Caribbean plate boundary, it is reasonable to predict that the opening of the Virgin Islands basin between the Virgin Islands platform and St. Croix also reflects left-lateral movement. The consistency and simplicity of this model is perhaps the reason that left-lateral motion in the Anegada Passage was suggested by Hess (1933, cited in Whetten, 1966), Hess (1966), Burke et al. (1984, table 7), and in Case et al. (1984). Estimation of rifling rate For a model in which St. Croix moves from a position south of Vieques, we estimate that the Virgin Islands basin has opened with rate of lateral motion of approximately 8 mm/yr since the late Middle Miocene. This rate is based on an assumed

I. GILL et al. age of 11 Ma for initiation of motion and a distance of lateral movement of 91 km. This rate is somewhat slower than the estimated 20 m m / y r or greater rate of movement between the North American and Caribbean plates cited by Rosencrantz and Mann (1991). This slower rate is consistent with the idea of Puerto Rico and the Virgin Islands platform being decoupled from the Caribbean plate (McCann et al., 1987) and moving eastward with a slower relative motion. The resulting differential motion between the Virgin Islands platform and the main body of the Caribbean plate, including St. Croix, may have caused the opening of the Virgin Islands basin.

Rotating platelet models An alternative group of models call for counterclockwise rotation of a Puerto Rico microplate or terrane. The idea of a separate Puerto Rico platelet was proposed by McCann et al. (1987), who suggested that this platelet had a westward (left-lateral) motion relative to the main Caribbean plate. Lithgow et al. (1987) suggested that the Virgin Islands basin formed as a pull-apart in response to these relative motion differences.

Seismic profiling and sidescan sonar data Based on seismic profiles and GLORIA sidescan sonar surveys (EEZ Scan Scientific Staff, 1987), Scanlon and Masson (1988) proposed that the Puerto Rico microplate has undergone counterclockwise rotation, with a pole of rotation south of Puerto Rico at the juncture between the St. Croix Ridge and the Muertos Trough (Fig. 14C). This model does not address relative motion between the microplate and the Caribbean and North American plates. Based on paleomagnetic studies in Puerto Rico, Reid et al. (1991) have documented approximately 25 ~ of counterclockwise rotation of Puerto Rico relative to North America between 11 and 4.5 Ma. They concluded that the Puerto Rico microplate behaved as a 'roller beating' between the North American and Caribbean plates during this period as it either became uncoupled from Hispaniola or responded to changes in relative plate motions. They suggested that this rotation ceased as the Puerto Rico microplate detached from the Caribbean plate and transferred extensional stress to the Anegada Passage and Mona Canyon. Speed and Larue (1991) proposed that the northern Caribbean PBZ has been in left-lateral transtension for the last 15 to 20 m.y., with much of the motion dispersed by counterclockwise rotation of terranes within the PBZ such as the Puerto RicoVirgin Islands terrane. In their model, as in the others, this rotation caused extension in the Anegada Passage/Virgin Islands basin area.

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Fig. 14. Three models for the tectonic evolution of the northern Caribbean. (A) Plate tectonic model for the northern Caribbean with dextral slip in the Virgin Islands basin/Anegada Passage via a 'Puerto Rico Festoon' (after Stephan et al., 1986). Note that documented sinistral faults through Puerto Rico are not indicated in this diagram. The inset shows a mechanical analog for the formation of a 'Puerto Rico Festoon' with dextral slip to the east, i.e. Virgin Islands basin/Anegada Passage, and sinistral slip to the west (from Stephan et al., 1986). (B) Plate tectonic model for the northern Caribbean with dextral slip in the Virgin Islands basin/Anegada Passage. Note proposed triple junction to the southeast of Puerto Rico (after Jany et al., 1990). (C) Rotating microplate model of Scanlon and Masson (1988).

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Discussion With an axis of rotation situated to the southwest of St. Croix (Scanlon and Masson, 1988), counterclockwise rotation of a separate Puerto Rico platelet would produce a zone of extension in the Anegada fault zone that widens to the northeast. In fact, the Anegada Passage has narrow, subparallel walls until it empties into the Sombrero basin to the northeast and into the Virgin Islands basin to the southwest. For this reason, the bathymetry of the region does not support simple rotation alone, and if rotation did occur, it may have been coupled to other motion. In any case, rotational models do not preclude either right or left-lateral movement between St. Croix and the Virgin Island platform. The direction of slip would be dependent on relative motion between the rotating Puerto Rico microplate and the Caribbean plate. If Puerto Rico rotated counterclockwise relative to a fixed or slowly eastward-moving Caribbean plate, this motion would most likely produce rightlateral slip in the Anegada Passage and Virgin Islands basin. However, if the rotation of Puerto Rico is accommodating part of the left-lateral movement between the North American and Caribbean plates, as proposed by Speed and Lame (1991), then the Caribbean plate would have a faster eastward relative motion. The result would be a left-lateral sense of motion in the Anegada Passage and Virgin Islands basin. The extensional nature of the northeasterly trending structures in that area supports the latter model.

Right-lateral models An alternative model for the origin of the Virgin Islands basin proposes that motion in the Virgin Islands basin and Anegada Passage is fight-lateral (Fig. 14A) (Houlgatte, 1983; Stephan et al., 1986). Mauffret et al. (1986) and Jany et al. (1987) propose that motion along the Anegada Passage was originally sinistral, but reversed during the Pliocene or later.

Basin morphology Jany et al. (1990) suggested that the Virgin Islands basin shows a rhomboidal 'lazy Z' shape (Fig. 14B), a shape indicated by Mann et al. (1983) to be diagnostic of fight-lateral shear zones. If strikeslip motion is parallel to the Anegada Passage, the oblique orientation of the Virgin Island basin is consistent with a fight-lateral pull-apart basin. However, although the geometry of the Virgin Islands basin and Anegada Passage is clear, the extension of the southwestern part the basin is not. Depending on the placement of the strike-slip zone to the southwest of the basin, the shape of the Virgin Islands basin could suggest opening of the basin by left-lateral motion.

I. GILL et al.

Seismic profiling Based on north-northeasterly oriented seismic profiles, Mauffret et al. (1986) have interpreted a structure on the north side of the Virgin Islands basin as a northward-verging reverse-fault zone. They cite this faulting as evidence for fight-lateral slip during the formation of the basin. However, if this structure trends normal (east-southeasterly) to this seismic profile, fight-lateral slip should produce extension rather than compression. Therefore, this structure could actually represent sinistral rather than dextral slip. Jany et al. (1990) have interpreted a similar structure - - perhaps the same one on an intersecting north-northwesterly oriented seismic line as a 'flower structure' indicative of strike-slip faulting. This structure overlies what is interpreted as the Late Miocene sedimentary surface. Although strikeslip motion may be a reasonable interpretation for the structures on these two lines, there is no way to assess accurately the sense of strike-slip motion for structures normal to such generally northerly oriented transects. Several studies have noted the predominance of generally easterly to northeasterly oriented normal faulting in the marine basins off St. Croix. Jany et al. (1987) identified northeasterly oriented normal faults on a northwesterly oriented seismic cross-section across the St. Croix basin east of St. Croix. In addition, several other seismic traverses published by Houlgatte (1983) similarly document normal faulting within the Virgin Islands basin. The sense of movement on these faults appears to be dominantly dip-slip; the degree of strike-slip motion is difficult to document. Although these data were related to a model for right-lateral movement in the plate boundary zone, the occurrence of northeasterly oriented extensional features in an easterly oriented strike slip zone is more consistent with left-lateral movement.

GPS data Global Positioning data exist for stations on Puerto Rico and St. Croix. Present data, although still preliminary, indicates that Puerto Rico may be moving to the east-northeast slightly faster than St. Croix (Dixon et al., 1998). These data support fightlateral displacement between St. Croix and Puerto Rico. However, the measurement error is presently too large to make reliable conclusions (P. Jansma, pers. commun., 1998).

Discussion The dextral slip model suffers from several problems when incorporated into a regional model of tectonics. Primary among these is the difficulty in reconciling a dextral strike-slip fault in the Vir-

EVOLUTION OF THE NEOGENE KINGSHILL BASIN OF ST. CROIX, U.S. VIRGIN ISLANDS gin Islands basin with the compression established along the length of the Muertos Trough (McCann et al., 1987). This would require an extension of the Anegada Passage/Virgin Islands basin fault zone westward of its present termination, and there is no seismic or bathymetric evidence to support this. Jany et al. (1990) suggest a triple-junction south of Puerto Rico to accommodate right-lateral plate movements (Fig. 14B). However, the deformation front along the Muertos Trough can be seen in the GLORIA data to extend nearly to the longitude of St. Croix, well east of where Jany's triple junction is shown (K. Scanlon, written commun., 1997). Dextral motion along the Anegada fault zone would also require Puerto Rico to move eastward faster than the Caribbean plate, and a driving mechanism for this movement requires a complicated model. Similarly, it is difficult to reconcile right-lateral motion in the Anegada Passage with established left-lateral motion for the northern Caribbean plate boundary zone (Stephan et al., 1986; Burke et al., 1984). The structural geology of the islands in the area is more consistent with left-lateral than rightlateral motion; easterly oriented terrestrial faults of Puerto Rico and the northern Virgin Islands are mapped with left-lateral displacement and the normal faults on St. Croix and on the St. Croix Ridge are oriented northeasterly. If dextral faulting is occurring in the Anegada fault zone, it must post-date the faulting on the Puerto Rico/Virgin Islands platform and on St. Croix; such a change in the latest Neogene or Quaternary would require a major reversal of plate motion in the northeastern Caribbean. If such a reversal did occur, it apparently left no trace in the rocks exposed on St. Croix, which record deposition and faulting through at least the Early Pliocene. Discussion of tectonic models

An oblique sinistral model for the opening of the Virgin Islands basin and the Anegada Passage satisfies structural evidence on St. Croix such as the northeasterly orientation of the normal fault system. It also permits the Kingshill basin to be paleogeographically situated near a known extra-basinal sediment source. Left-lateral motion, if oblique, provides a mechanism for the opening of the Virgin Islands basin. Left-lateral transtension across the Virgin Islands basin may disperse the slip between the North American and Caribbean plates. Our estimate of 8 mm/yr of movement across the Virgin Islands basin is slower than the 20 mm/yr rates of motion between the North American and Caribbean plates estimated by Rosencrantz and Mann (1991). Dextral motion is consistent with most interpretations of present basin morphology assuming

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strike-slip movement directly along the Anegada Passage. In addition, recent GPS data may be more supportive of present fight-lateral displacement than with left-lateral models. In contrast, rotating platelet models are supported by paleomagnetic data, and are consistent with either right- or left-lateral movement in the Virgin Islands basin. We feel left-lateral motion is most consistent with the long-term evolution of the Virgin Islands basin area, but all the models have serious weaknesses. The only documented type of faulting between St. Croix and the Virgin Islands platform is extensional. Subsidence analysis in the Kingshill basin indicates that a period of rapid tectonic uplift took place between 10.5 and 3.5 Ma. The timing of this uplift is consistent with the relative plate motion changes between the Puerto Rico platelet and the Caribbean plate cited by McCann et al. (1987), the age of extension in the Virgin Islands basin noted by Lithgow et al. (1987), and tectonic changes north of Hispaniola (Dillon et al., 1992). The lithologies of the lower Kingshill Limestone suggest that uplift of the margins of the Kingshill basin graben may have begun within this time interval, and St. Croix may have rifted away from shelf areas of the Virgin Islands platform and Saba Bank. Further discussion of tectonic models involving the Virgin Islands platform is contained in van Gestel et al. (1998).

CONCLUSIONS

(1) St. Croix and the Virgin Islands basin were produced by transtension and associated uplift and subsidence along the northern Caribbean plate boundary zone during the Neogene. The stratigraphic and structural evolution of the Neogene Kingshill basin record a major tectonic event in this plate boundary zone near the Middle Miocene-Late Miocene transition. (2) The Jealousy Formation is a strictly subsurface unit of blue-gray marls deposited in a deepwater (600-800 m) environment before the onset of tectonism. All documented occurrences from well samples place this unit in the lower part of the Middle Miocene, although the formation may extend lower, perhaps into the Oligocene, below current depths of well penetration. (3) The lithologic transition between the Jealousy Formation and the Kingshill Limestone is abrupt and distinct, but is time-transgressive and does not appear to indicate any major mineralogic, faunal, or environmental change. (4) The Kingshill Limestone records the shallowing of the Kingshill basin and the initiation of uplift of basin-bounding horst blocks near the end of the Middle Miocene. There is an increase in

364 the abundance of coarse-grained debris in the middle of the formation and a progressive change to shallower-water microfaunas. Calculated uplift rates (backstripped, decompacted) for the Late Miocene to Early Pliocene are 57 m / M a . (5) St. Croix acquired its present shoreline configuration by the Pliocene, and an extensive reef and lagoon tract had established itself along the west and south shorelines of the island. (6) Structural control of the coastline in the form of a small graben allowed the accumulation and preservation of reef and platform Pliocene sediments along the south coast. Normal faulting has continued at least into the Pliocene. (7) The Virgin Islands basin is a structure formed by extension and sinistral faulting that rifted St. Croix away from Puerto Rico, the Virgin Islands platform and perhaps Saba Bank. Separation rates are estimated to have been 8 m m / y r . (8) Extension related to left-lateral plate motion is most consistent with the long-term evolution of the Virgin Islands basin area and St. Croix, but none of the regional tectonic models discussed is consistent with all the available data. The only solid tectonic evidence for formation of the Virgin Islands basin is extensive normal faulting. No definitive evidence for strike-slip motion in the Virgin Islands basin or Anegada Passage exists.

ACKNOWLEDGEMENTS

The authors thank Lee Gerhard, Nancy Grindlay, Barbara Lidz, Kathryn Scanlon, and Robert Speed for helpful, critical reviews of this paper, and Paul Mann for his constructive and patient editing. Pamela Jansma and James Joyce generously discussed various aspects of Caribbean tectonics and provided unpublished data. Clyde Moore served as major professor to the senior author throughout field and laboratory work. The aid and cooperation of the West Indies Laboratory, the U.S. Geological Survey Water Resources Division in San Juan, ER., and K. Eastman and the staff of Caribbean Geological Services in undertaking this study is gratefully acknowledged. The assistance of Marc Lowman with illustrations is also appreciated. Funding for St. Croix drilling and field work was provided to Gill by: the Virgin Islands Water Resource Center; SOHIO, Chevron, and Shell field research grants; Geological Society of America and American Association of Petroleum Geologists student grants; D. Eby and Champlin Petroleum; and the Applied Carbonate Research Program, the Department of Geology, and the Basin Research Institute at Louisiana State University. This work is part of a dissertation project of the senior author at Louisiana State University.

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