MIOCENE DINOFLAGELLATE STRATIGRAPHY AND SYSTEMATICS OF MARYLAND AND VIRGINIA .1. DINOFLAGELLATE CYST ZONATION AND ALLOSTRATIGRAPHY OF THE CHESAPEAKE GROUP

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 an d subsurface sections from New Jersey, Delaware, Maryland and Virginia . The zonation provides continuous coverage from the uppermost Oligoce ne to the uppermost Miocene, utilizing 62 dinocyst horizons, with aver age zonal durations of ca. 1.8 My. The zonation refines earlier biostr atigraphies based on diatoms, radiolarians and mollusks and is integra ted with calcareous nannofossil and planktonic foraminiferal stratigra phies from ODP Leg 150 sites offshore New Jersey. Comparisons with Mio cene dinocyst assemblages indicate that the lowest occurrences of Laby rinthodinium truncatum and Unipontidinium aquaeductum, and the highest occurrences of Distatodinium biffii and Hystrichosphaeropsis obscura, are important dinocyst horizons across the North Atlantic and Mediter ranean. Regional endemism and paleoenvironmental parameters are respon sible for significant differences in overall composition, as well as s ome species ranges, between Miocene assemblages from thr Mediterranean and western North Atlantic. Although high latitude dinocyst assemblag es from the Norwegian Sea and Baffin Bay are distinctive, there is rea sonable correlation with mid-latitude assemblages from the Salisbury E mbayment. Twelve informal allostratigraphic units are defined from out crops that yielded dinocysts, in relation to Chesapeake Group lithostr atigraphy and earlier statigraphic work in the Calvert Cliffs ourcrop belt. Dinocyst stratigraphy facilitates the extention of these units i nto the subsurface of eastern Virginia, Maryland. Delaware and New Jer sey. The distribution of Miocene allostratigraphic units in the Salisb ury Embayment is related to 2(nd) order eustasy and basin structure. F ifteen potential 3(rd) order depositional sequences are identified in the Salisbury Embayment and dated by dinocyst stratigraphy; 3 in the l ower Miocene, 5 in the middle Miocene and 7 in the upper Miocene. Thes e show excellent agreement with the 2(nd) order eustatic curve of the Exxon Cycle Chart but chronostratigraphic resolution does not permit d etailed comparisons of the 3(rd) order cyclicity.