Part Ⅰ Dinoflagellate cyst zonation and allostratigraphy of the Chesapeake Group

摘要

Ten dinoflagellate cyst interval zones are erected for the Miocene of the Salisbury Embayment, Atlantic Margin, U.S.A., based on analyses of classic Miocene outcrops in Calvert County, Maryland, plus outcrop and subsurface sections from New Jersey, Delaware, Maryland and Virginia. The zonation provides continuous coverage from the uppermost Oligocene to the uppermost Miocene, utilizing 62 dinocyst horizons, with average zonal durations of ca. 1.8 My. The zonation refines earlier biostratigraphies based on diatoms, radiolarians and mollusks and is integrated with calcareous nannofossil and planktonic foraminiferal stratigraphies from ODP Leg 150 sites offshore New Jersey. Comparisons with Miocene dinocyst assemblages indicate that the lowest occurrences of Labyrinthodinium truncatum and Unipontidinium aquaeductum, and the highest occurrences of Distatodinium biffii and Hystrichos-phaeropsis obscura, are important dinocyst horizons across the North Atlantic and Mediterranean. Regional endemism and paleoenvironmental parameters are responsible for significant differences in overall composition, as well as some species ranges, between Miocene assemblages from the Mediterranean and western North Atlantic. Although high latitude dinocyst assemblages from the Norwegian Sea and Baffin Bay are distinctive, there is reasonable correlation with mid-latitude assemblages from the Salisbury Embayment. Twelve informal allostratigraphic units are defined from outcrops that yielded dinocysts, in relation to Chesapeake Group lithostratigraphy and earlier statigraphic work in the Calvert Cliffs ourcrop belt. Dinocyst stratigraphy facilitates the extention of these units into the subsurface of eastern Virginia, Maryland, Delaware and New Jersey. The distribution of Miocene allostratigraphic units in the Salisbury Embayment is related to 2nd order eustasy and basin structure. Fifteen potential 3rd order depositional sequences are identified in the Salisbury Embayment and dated by dinocyst stratigraphy; 3 in the lower Miocene, 5 in the middle Miocene and 7 in the upper Miocene. These show excellent agreement with the 2nd order eustatic curve of the Exxon Cycle Chart but chronostratigraphic resolution does not permit detailed comparisons of the 3rd order cyclicity.