Campanian-Maestrichtian and Paleocene dinoflagellate and acritarch assemblages from the Maryland-Delaware Coastal Plain

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Virginia Polytechnic Institute and State University


The taxonomy and biostratigraphy of Upper Cretaceous and Paleocene dinoflagellate and acritarch assemblages from the Maryland-Delaware Coastal Plain have been investigated from strata outcropping near Washington, D.C., to the eastern end of the Chesapeake and Delaware Canal, Delaware. Samples of the Cretaceous Mount Laurel Sand and Monmouth Formation (Red Bank equivalent) and the Paleocene Brightseat Formation yielded 51 genera containing 95 species and four subspecies, of which one genus and nine species are described as new; five new combinations are proposed. Twelve acritarch genera with 14 species and several informally-designated species, two species of Chlorophyta, and one species of Protozoa also occur in the section.

The Cretaceous-Tertiary boundary in Maryland is marked by the termination of ranges of eight species and one subspecies of dinoflagellates and one acritarch species, and by the first appearance of 13 species of dinoflagellates. An unconformity representing a time interval of unknown duration may separate Cretaceous and Tertiary strata in the study area in Maryland; lithologic evidence (Bennett and Collins, 1952) and evidence based on the ranges of dinoflagellates near Annapolis, Maryland (Benson, 1975), and the ranges of ostracodes (Hazel, pers. comm., 1975) supports this conclusion. Species with stratigraphic ranges restricted to the Mount Laurel, Monmouth, or Brightseat Formation indicate the potential usefulness of dinoflagellates in zoning Atlantic Coastal Plain strata and in correlating coastal plain and continental shelf strata.

A correlation of the Mount Laurel with Wilson's Zone I of the Maestricht section (Belgium-Netherlands), based on the ranges of species in common, indicates a Late Campanian age; megafossil evidence supports a Late Campanian age, and foraminiferal evidence, a Late Campanian-Early Maestrichtian age for the Mount Laurel (Owens et al., 1970. A correlation of the Monmouth Formation with Wilson's Zone II confirms a Maestrichtian age determination based on megafossils (Cooke, 1952; Glaser, 1971). Concurrent ranges of dinoflagellate species indicate a Paleocene age for the Brightseat Formation, confirming evidence based on ostracodes (Hazel, 1968, 1969) and Foraminifera (Nogan, 1964).

Species diversity and the ratio of gonyaulacacean to peridiniacean species (G/P ratio) are seemingly interrelated. Diversity varies with the G/P ratio from sample to sample through the section. On the basis of an overall high diversity and high G/P ratio, the section as a whole may represent deposition under open marine conditions (Harland, 1971).

Dominance by cavate cysts (e.g., Deflandrea) to dominance by chorate cysts (e.g., Areoligera) occurs progressively from Mount Laurel into the Brightseat Formation. Vozzhennikova (1967) and Downie et al. (1971) indicate that chorate cysts occur seaward of cavate cysts. Brightseat samples are dominated by chorate cysts. Water depth during deposition of the Brightseat was about 300 feet, according to Nogan (1964), who based his evidence on foraminiferal paleoecology. In the Monmouth, samples dominated by chorate cysts alternate with samples dominated by cavate cysts; the Monmouth appears to represent an environment of deposition nearer to shore than the Brightseat. The Mount Laurel Sand, dominated by cavate cysts, indicates deposition still nearer shore; this conclusion is supported by lithologic evidence (Owens and Sohl, 1969),