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Biostratigraphic and paleoenvironmental studies were carried out on radiolarians from over 50 core and outcrop localities from the U.S. mid-Atlantic coastal plain and continental margin. These deposits consist of sandy diatomaceous silts and clays, and represent depositional environments ranging from inner shelf (onshore localities) to upper continental slope (offshore localities).
The limited radiolarian assemblage (approximately 50 species) lacks many stratigraphically important low latitude forms but nevertheless allows the recognition of zones (i.e., those of Riedel and Sanfilippo in 1978). Radiolarians occur at onshore localities in New Jersey, Delaware, Maryland, and Virginia representing ages from early Miocene (Stichocorys wolfii zone) to middle Miocene (Diartus petterssoni zone). Late Miocene deposits are devoid of radiolarians at all the coastal plain localities studied. However, certain deposits of presumed Pliocene age in Virginia contain a sparse radiolarian assemblage. Offshore wells (such as COST B-3 and ASP 15) appear to have a longer Neogene radiolarian record, ranging from earliest Miocene (probable Cyrtocapsella tetrapera zone) to Pliocene age
Analyses of variations in the entire assemblage with time were conducted on samples from several onshore localities. These were conducted by counting more than 40 important family and generic groups. The results reflect the fluctuating role of oceanic processes in the Neogene shelf environments. Abundance and diversity increase (particularly in the rarer nassellarians and in apparently deeper dwelling forms) in the intervals interpreted to have been strongly influenced by oceanic processes (i.e., upwelling of subsurface waters). Abundance and diversity decrease, and dominance by certain spumellarians increases in the intervals believed to represent shallower, lower salinity and/or more terrestrially influenced environments, as interpreted from independent paleontological and sedimento ogical evidence.
These results are consistent with the idea that both coastal upwelling and deltaic progradation were significant processes in the Neogene development of the U.S. mid-Atlantic continental margin. Their relative importance at any given time or locality appears to have been determined by paleobathymetry and amount of terrigenous influx, resulting from effects of changes in sea level and local tectonic conditions.
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