Basinal carbonate accumulations on St. Croix, U.S. Virgin Islands, are composed of emergent open-basin biogenic deep-water facies of Tertiary age intercalated with coeval reefal and older terrigenous turbidite-slump deposits. Detailed biostratigraphic investigation using planktonic Foraminifera provided data for interpretation of the paleoecology, sedimentary relationships, and geologic history of the area. The massive and extensively leached hemipelagic deposits suggest that nutrient-rich conditions, characteristic of a sea-level high stand, prevailed for a substantial period of time. A dramatic increase in abundance of aberrant specimens and decrease in test size of the planktonic assemblage occurred approximately 5.7 to 5.4 m.y. B.P. and reflect the onset of an adverse environment; rapid eustatic fall of sea level began within 1.2 m.y. of the end of Miocene time. Micropaleontologic and sedimentologic evidence, as well as absence of sediments younger than early Pliocene, implies that emergence of the island probably occurred about 5 m.y. ago. These limestones form a hydrocarbon reservoir-rock model for study of subsurface oil- and gas-producing horizons of comparable age and origin elsewhere in the southern Caribbean.