BIOSTRATIGRAPHY, PALEOENVIRONMENTS AND STRATIGRAPHIC EVOLUTION OF THENEOGENE OF ST-CROIX, US VIRGIN-ISLANDS

Analysis of the microfauna of the Jealousy Formation and Kingshill Lim estone from four cored wells, two water wells, and ten outcrops in the Kingshill basin of St. Croix allows a comprehensive reconstruction of the Neogene stratigraphic evolution of the island. This study results in the first precise biostratigraphic zonation and paleoenvironmental evaluation of these Neogene units in the subsurface as well as a re-e valuation of the age and correlation of outcropping strata. The Jealou sy Formation is an entirely subsurface unit of dark, plankton-rich mar ls which contains planktonic foraminifera indicative of the lower part of the middle Miocene (Praeorbulina glomerosa Zone to the Globorotali a fohsi fohsi Zone) and a middle bathyal benthic microfauna. The Kings hill Limestone conformably and diachronously overlies the Jealousy For mation and is subdivided into two members, both of which occur in outc rops and the subsurface. The lower of these, the La Reine Member, is c haracterized by buff pelagic limestones and marls with an upward incre asing proportion of intercalated shelf-derived debris. It ranges from the basal part of the middle Miocene (Praeorbulina glomerosa Zone) to the upper part of the uppermost Miocene (Globorotalia humerosa Zone), and exhibits a transition from middle bathyal (600-800m) to upper bath yal (200-300m) environments. The uppermost part of the Kingshill Limes tone, the Manning's Bay Member, is made up of skeletal debris-rich car bonate slope deposits and is placed near the Miocene-Pliocene boundary . Biostratigraphic control indicates activation of the St. Croix grabe n fault system and formation of the Kingshill basin at the beginning o f the late Miocene. The Neogene section records deposition in an open- marine setting at middle bathyal depths during the early middle Miocen e, shoaling gradually to upper bathyal depths in the late middle and l ate Miocene and more rapidly to a shallow-marine setting at the beginn ing of the Pliocene. Shoaling was accompanied by an increasing proport ion of transported shallow marine sediments. Subsidence analysis for t he section indicates more than 400 m of tectonic uplift between 10.5 a nd 3.5 Ma. Two depositional shifts in the Kingshill Limestone may be t ied chronostratigraphically to postulated major global sea-level fails . An interval of carbonate debris in the type section of the Kingshill Limestone may record a fall at 10.5 Ma, and a disconformity at the ba se of the Mannings Bay Member may be tied to a fall at 5.5 Ma.