Abstract: Lithostratigraphy of Oak Grove Core, Northeastern Virginia

Juergen Reinhardt, Wayne L. Newell, Robert B. Mixon

A stratigraphic test hole drilled near the southern margin of the Salisbury Embayment about 45 km east of the Fall Line was cored continuously from a depth of 24 to 400 m. From 24 to 137.5 m, the core is composed of unconsolidated to partly cemented Tertiary sediments (> 95% recovered); the remaining 262.5 m consists of unconsolidated Lower Cretaceous sedimentary deposits (about 90% recovered). No Upper Cretaceous sediments are present in the core. On the basis of subsurface control in adjacent areas and lithologic and palynological data from this core, penetration was within 30 m of basement.

Stratigraphic units penetrated by the test hole include: upland gravels, undivided (0 to 20.5 m); Calvert Formation (20.5 to 60 m); Nanjemoy Formation (60 to 96 m); Marlboro Clay Member of the Nanjemoy (96 to 102 m); Aquia Formation (102 to 137.5 m); and Potomac Group, undivided (137.5 to 400 m). General lithology and contact relations are similar to those of the equivalent updip lithostratigraphic units.

The Calvert Formation (Miocene) consists of interbedded illite-vermiculite clays and bioturbated quartz silts. The lower Tertiary bioturbated greensands and illitic clays (Aquia and Nanjemoy) are separated by a massive, slightly burrowed kaolinitic clay (Marlboro). The nature of the sediments, the lack of primary sedimentary structures, and the faunas indicate sedimentation on an open-marine shelf well below wave base.

The Lower Cretaceous sediments (Potomac Group) are characterized by alternation of mottled, highly oxidized, and slickensided clay, laminated carbonaceous silt, and fine to coarse, poorly sorted feldspathic quartz sand. The overall fine-grained nature of the Potomac Group sediments (Aptian and Albian in this core), sparse burrows(?) and root structures, and abundant laminated intervals suggest deposition on an alluvial flood plain, probably close to a delta platform.

AAPG Search and Discovery Article #90968©1977 AAPG-SEPM Annual Convention and Exhibition, Washington, DC