Reports

Geophysical log cross-section network of the Cretaceous sediments of Southern Maryland

1968, Hansen, H.J.

Report of Investigations 7

Abstract

Deep geophysically logged wells are used to demonstrate the lateral and vertical changes in lithology that characterize the nonmarine Cretaceous sediments of Southern Maryland. The correlation network consists of four cross-sections subparallel to the strike of the Cretaceous formations and 12 cross-sections subparallel to the dip. Correlation is based on geophysical (electric and gamma ray logs) and lithological data; selected well samples were studied palynologically.

This network suggests that groups of sands, such as those comprising the Patuxent Formation, can be correlated along strike in the Anne Arundel-Prince Georges County area for distances exceeding 20 miles. This occurrence is not fully compatible with a concept of individually isolated, channel or “shoestring” sands; rather the sand groups characteristic of the subsurface Potomac Group approximate the fluvial model of Visher (1965) (i.e., zones of blanket sands which are broadly correlative).

Sand counts indicate that both the Patuxent and the Raritan(?)-Patapsco Formation (undivided) exhibit a southerly decrease in sand percentage. In the Anne Arundel-northern Prince Georges County area these formations contain greater than 45 percent sand; in Charles County sand percentages characteristically are less than 30 percent. The mottled clays associated with these sediments change from predominantly red-brown in the north to predominantly gray-green in the south.

In the absence of core samples electric logs may be used to identify the different types of sediments associated with fluvial deposition. Ideally, a meander sequence exhibits a resistivity curve having a sharply defined erosive base overlain by a fining-upwards cycle of inchannel and point bar sediments. Capping this sequence are fine-grained deposits, the product of overbank sedimentation.