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

Abstract

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.