Sequence development of a mixed carbonate-siliciclastic high-relief ramp, Mississippian, Kentucky, U.S.A

Abstract

The discovery of the supergiant oil fields of the Caspian region has focused attention on eustatic, tectonic, and climatic controls on Lower Carboniferous facies. This paper documents the development of a highly complex, mixed carbonate-siliciclastic ramp succession of the Lower Mississippian (Upper Osagean-Lower Meramecian) Fort Payne-Salem interval, Kentucky, that formed peripheral to an abandoned delta system (Borden Formation) following drowning of the paleoshelf, during a time of arid climate and tectonic quiescence. The antecedent Borden deltaic-marine paleoshelf stood 100 m above the starved basin plain to the southwest and had marginal slopes of less than 0.5°. The succession is a supersequence (0 to 150 m thick, approximately 5 to 10 My duration) composed of two Upper Osagean (Fort Payne) and two Lower Meramecian (Warsaw-Salem) depositional sequences which correlate with the global sea-level cycles of Ross and Ross (1988). The supersequence lowstand systems tract on the basin floor consists of the lower part of a unit green shale-and-mounds. Mounds growth appears to have continued during supersequence transgression, culminating in some mounds up to 60 m thick. On the Borden paleoshelf, the supersequence transgressive systems tract is a condensed glauconite horizon(s). The supersequence highstand tract consists of marine quartz sandstone and shale, peritidal carbonates, high-energy crinoidal-bryozoan grainstone banks, and deep ramp and slope siliceous calcisiltite. Third-order sequences have local lowstand-transgressive sand bodies; those in sequence 2 are clinoformed strike sand lobes up to 50 m thick, while those in the younger sequences are discontinuous sheetsdip oriented lobes. The siliciclastic units appear to be associated with a paleodrainage system off the Borden paleoshelf. In Upper Osagean sequences 1 and 2, third-order highstand systems tracts are aggradational on the paleoshelf and prograded to the southwest as a series of clinoforms into the starved basin. Lower Meramecian sequences 3 and 4 highstand systems tracts are relatively layer-cake, reflecting loss of accommodation during Late Osagean basin filling, and they prograded more to the west into the subsiding Reelfoot Graben-Illinois Basin. The extremely complex facies relations reflect the interaction of the antecedent shelf-slope-basin topography, the mixed carbonate and siliciclastic systems, mounded basin-floor topography, the highly channeled margin morphology, eustasy, and relatively stable tectonics.