Outcrop Features and Origin of Basin Margin Unconformities in the Lower Chesapeake Group (Miocene), Atlantic Coastal Plain
Susan M. Kidwell, 1984. "Outcrop Features and Origin of Basin Margin Unconformities in the Lower Chesapeake Group (Miocene), Atlantic Coastal Plain", Interregional Unconformities and Hydrocarbon Accumulation, John S. Schlee
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This paper describes unconformable stratigraphic relations within basin-margin deposits from an outcrop rather than seismic perspective, emphasizing the physical characteristics, stratigraphic context, age, origin, and usefulness of unconformities in stratigraphic subdivision and correlation. Stratigraphic relations within the outcropping Miocene of the Salisbury Embayment, middle Atlantic Coastal Plain, are more complex than previously described. The Calvert (Plum Point Member) and Choptank formations are subdivided by a series of erosional surfaces into six depositional sequences traceable over a 9,000-sq-km (3,475-sq-mi) study area in Maryland and Virginia. The disconformities take the form of burrowed firmgrounds in outcrop, but exhibit up to 14 m (46 ft) of topographic relief locally and represent transgressive and regressive ravinement surfaces. One of the surfaces records sediment starvation in a distal marine environment. Internal facies relations within the depositional sequences are complex but basically cyclic, consisting usually of a basal condensed shell or bone deposit formed under prolonged conditions of reduced net sedimentation and grading upward into less fossiliferous and siltier facies in regressive sequence. Each sequence consists of two or more of the original lithologic zones of Shattuck (1904).
Diatom biostratigraphic data indicate that the disconformities are not measurably diachronous within the study area and represent less than one biozone except where erosion has enlarged the vacuity; they thus provide a basis for fine-scale chronostratigraphic subdivision and correlation within the Miocene outcrop belt. Diatom data published elsewhere identify a major, 2.5 m.y.-long hiatus within the Fairhaven Member of the Calvert Formation that encompasses the Burdigalian-Langhian stage boundary, where Vail and Hardenbol (1979) have identified a minor interregional unconformity in seismic profiles (16.5 m.y. event). The disconformity at the base of the Plum Point Member of the Calvert Formation corresponds in age to another minor reflector at the Langhian-Serravallian boundary (15.5 m.y. event), and a disconformity between the Choptank and St. Marys formations may correspond to Vail and Hardenbol’s (1979) mid-Serravallian reflector (13.0 m.y. event). Other disconformities within the studied section, including the disconformable contact of the Calvert and Choptank formations, are expressions of stratigraphic complexity relating to transgressive-regressive migrations of marginal marine and open marine strandlines within the basin margin record.
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Interregional Unconformities and Hydrocarbon Accumulation
The result of a session at the 1981 AAPG Annual Meeting, this volume attempts to document global age and magnitude of sea-level shifts, and ultimately, the cause of the short-term shifts. Twelve individual papers were published on topics such as: comparative anatomy of cratonic unconformities; relation of unconformities, tectonics, and sea-level change; outcrop features and origin of basin margin unconformities; significant unconformities and the hiatuses represented by them; regional unconformities and depositional cycles; relative sea-level changes during the Middle and Late Cretaceous; Late Oligocene-Pliocene transgressive-regressive cycles of sedimentation; oxygen-isotope record of ice-volume history; oceanic ridge volumes and sea-level change; Jurassic unconformities, chronostratigraphy, and sea-level changes; Cenozoic regional arosion of the Abyssal sea floor; and depositional sequences and stratigraphic gaps on submerged United States Atlantic margin.