Biostratigraphy, Paleoenvironments and Stratigraphic Evolution of the Neogene of St. Croix, U.S. Virgin Islands

Abstract

Analysis of the microfauna of the Jealousy Formation and Kingshill Limestone 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-evaluation of the age and correlation of outcropping strata. The Jealousy Formation is an entirely subsurface unit of dark, plankton-rich marls which contains planktonic foraminifera indicative of the lower part of the middle Miocene (Praeorbulina glomerosa Zone to the Globorotalia fohsi fohsi Zone) and a middle bathyal benthic microfauna. The Kingshill Limestone conformably and diachronously overlies the Jealousy Formation and is subdivided into two members, both of which occur in outcrops and the subsurface. The lower of these, the La Reine Member, is characterized by buff pelagic limestones and marls with an upward increasing 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 bathyal (200-300m) environments. The uppermost part of the Kingshill Limestone, the Manning's Bay Member, is made up of skeletal debris-rich carbonate slope deposits and is placed near the Miocene-Pliocene boundary. Biostratigraphic control indicates activation of the St. Croix graben fault system and formation of the Kingshill basin at the beginning of the late Miocene. The Neogene section records deposition in an open-marine setting at middle bathyal depths during the early middle Miocene, shoaling gradually to upper bathyal depths in the late middle and late Miocene and more rapidly to a shallow-marine setting at the beginning of the Pliocene. Shoaling was accompanied by an increasing proportion of transported shallow marine sediments. Subsidence analysis for the section indicates more than 400 m of tectonic uplift between 10.5 and 3.5 Ma. Two depositional shifts in the Kingshill Limestone may be tied chronostratigraphically to postulated major global sea-level falls. 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 base of the Mannings Bay Member may be tied to a fall at 5.5 Ma. INTRODUCTION St. Croix is the southernmost of the U.S. Virgin Islands, located on the crest of an east-west oriented submarine platform near the eastern end of the northern Caribbean plate boundary zone. The central plain of the island is underlain by a northeast-southwest trending sedimentary trough (Gerhard et al. 1978; Whetten 1966), here referred to as the Kingshill basin (text-fig. 1), and bounded by the mountainous East End and Northside Ranges. Most studies of the Kingshill basin have been based on outcrop data and considered the Tertiary carbonate succession to record deposition in an isolated graben system (Whetten 1966; Multer et al. 1977; Gerhard et al. 1978; Lidz 1984). These strata have been assigned a wide range of ages in past studies, ranging from middle Oligocene (Vaughan 1923) to early Pliocene (Lidz 1982) (Table 1). However, exposures are limited in number and predominantly represent the upper part of the section. A drilling program undertaken in the mid-1980's has allowed us to conduct a more comprehensive study of the Neogene stratigraphy of the island and better constrain the nature and timing of the tectonic events affecting the evolution of the Kingshill basin. This study focuses on foraminiferal and ostracode faunas present in cores collected during this drilling program and in outcrop samples collected during field studies. The goals of this study are to: (1) to establish an accurate age and environmental framework for the Neogene strata of St. Croix; (2) to resolve surface-subsurface correlations of these strata; and (3) to outline the stratigraphic evolution of the Kingshill basin and its implications for the Neogene tectonics of the structurally complex region at the eastern end f the northern Caribbean plate boundary zone.