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The AAPG/Datapages Combined Publications Database

AAPG Bulletin

Abstract


Volume: 69 (1985)

Issue: 9. (September)

First Page: 1443

Last Page: 1443

Title: Miocene Calvert and Choptank Formations in Inner Coastal Plain of Virginia: A Record of Marine Onlap and Late Cenozoic Deformation: ABSTRACT

Author(s): R. B. Mixon, G. W. Andrews, L. W. Ward, D. S. Powars

Article Type: Meeting abstract

Abstract:

The 200-ft thick section of lower and middle Miocene beds in the Maryland-Virginia coastal plain includes two diatomaceous sand-silt-clay sequences, the Calvert Formation (below) and the Choptank Formation. The Calvert has been mapped over much of eastern Maryland and Virginia and is commonly believed to represent the maximum marine transgression during the early and middle Miocene. In contrast, a much more restricted distribution for the Choptank was suggested because it was known only in southern Maryland and northeasternmost Virginia. Thus, until recently, a general picture of offlapping relationships of the Calvert and Choptank beds has prevailed.

Our field studies show that both the Calvert and the Choptank are widely distributed in the central and inner Virginia coastal plain. The sandy and silty Calvert beds contain diatom assemblages typical of diatom zones 3 and 4 of Andrews; the Choptank is lithologically similar, but its diatom assemblage correlates with diatom zones 6 and 7. In updip areas in Virginia, the formations are separated by an erosional unconformity that is equivalent, in part, to the time interval represented by diatom zone 5.

The Calvert is truncated by the Choptank beds well east of the Fall Line. The Choptank laps over the Calvert and successively overlies, from east to west, Eocene, Paleocene, and Cretaceous strata and crystalline rocks of the Piedmont. These relations indicate extensive marine onlap during Choptank deposition and record a major transgression of the Miocene sea. Choptank beds thicken and thin across coastal plain structures, a result of erosion over structural highs following deformation of the Choptank and preceding deposition of higher rock units.

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