Slope to Basin
The Trenton Limestone of West Virginia was deposited on a gentle carbonate ramp that sloped eastward into a deep foreland basin. Unlike many limestones this unit formed during an early stage of orogeny, is transgressive in nature, and accumulated during a large influx of shale. Initial uplift of an eastern fold-thrust belt, created as the North American plate collided with a volcanic-arc system, led to lithospheric flexure and downwarping of the foreland basin under the load of an accreted terrane. Basin subsidence was rapid, being fast enough to outpace sedimentation but not so fast as to suppress carbonate sedimentation under deep, anaerobic conditions. Thus, limestone deposition continued, although with a deepening-upward sequence. Introduction of terrigenous mud was discontinuous through time, which generated a periodicity to the limestone and shale interbedding. These muds did temporarily downgrade or eliminate skeletal lime production, especially to the east where shale influxes were greater. With each return to clear water, however, carbonate deposition resumed. Eventually plate convergence and uplift of the source area proceeded to the point where clastic deposition overwhelmed the carbonate ramp. Trenton sedimentation then gave way to the flysch wedge of the overlying Martinsburg Formation. Thus, the relationship in space and time between subsiding basin and rising orogenic landmass exerted a major influence on the Trenton’s internal stratigraphy and facies development.
Figures & Tables
Mixed Carbonate-Siliciclastic Sequences
The study of carbonate-siliciclastic mixed sequences has seen an increase in the number of investigations that focus on mixed settings as part of the continuum between the carbonate and clastic end members. Cyclic deposition in mixed basins by reciprocal sedimentation has become one of the foundation blocks for sequence stratigraphy. In addition, these mixed sequences have a variety of distinctive petroleum reservoir characteristics, important for both exploration and development programs. The emphasis now is on reevaluating ancient sequences in the light of a more dynamic understanding of spatial and temporal variations and controls on these sequences. Examples in this volume are subdivided under the following headings: Shelf Wide, Coastal and Inner Shelf, Middle to Outer Shelf, Slope to Basin and Paleokarst. Many mixed sequences have been described in the literature, but understanding the controls of these sequences from a process approach in now in an adolescent stage.