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The AAPG/Datapages Combined Publications Database

AAPG Bulletin

Abstract


Volume: 53 (1969)

Issue: 9. (September)

First Page: 1923

Last Page: 1937

Title: Depositional Environments of Subsurface Potomac Group in Southern Maryland

Author(s): Harry J. Hansen (2)

Abstract:

The Potomac Group of Cretaceous age in the Baltimore-Washington, D.C., area has been recognized for many years as a product of fluvial and paludal sedimentation. Only within the past decade, however, have subsurface data from groundwater investigations been sufficient for delineation of the fluvial-deltaic lithofacies characterizing the group in the subsurface.

The location of Baltimore, near a landward bulge of the Baltimore Canyon trough, was the locus for fluvial sedimentation of braided or multistory sands during the time of Patuxent (Barremian) and Patapsco (Albian) deposition. Because of the thickness and permeability, those deposits give relatively high coefficients of transmissibility, between 25,000 and 75,000 gpd/ft. In the context of the classic delta form, the area is the braided upper floodplain lithofacies of a major axial river system. Southward, toward the Potomac River, there are lithofacies associated with a floodplain of meandering streams and a fringing swamp zone. Aquifers in those sediments have transmissibility values from less than 5,000 to 25,000 gpd/ft.

The sand content decreases southward in both the Patuxent and Patapsco Formations. In the Anne Arundel and northern Prince Georges County area (upper floodplain zone) those formations commonly are more than 45 percent sand; in Charles County (fringing swamp zone) they are less than 25 percent sand. The color of the interbedded clay, which is predominantly ocherous in the north, becomes more gray or greenish gray toward the south.

Because of the lack of core data, electric logs are used to distinguish between the deposits of braided and meandering streams. Ideally, the resistivity curve of a meandering-stream deposit shows a sharply defined base overlain by a fining-upward sequence. In contrast, the resistivity curve resulting from braided-stream deposits commonly has a massive profile and does not show a definite fining-upward sequence.

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