Abstract: Geology of Jurassic-Triassic Culpeper Basin, North of Rappahannock River, Virginia

Ray C. Lindholm

In the Jurassic Culpeper basin, north of the Rappahannock River in Virginia, dip is generally westward, toward the border fault, but is by no means uniform. Local steepening of dip adjacent to the eastern margin, may indicate local faulting. In general, dips increase toward the northwest. In the southern part of the area several rather broad folds, with a wavelength of nearly 4 mi (6 km), plunge toward the west.

Most of the sedimentary rocks (sandstones plus fine-grained rocks account for 62% of total exposures in the basin) are fine grained and generally are bioturbated and mud cracked. Conglomeratic sequences (5% of total exposure) are scattered along the western margin of the basin. In the northern part, they are dominated by limestone and dolomite pebbles; quartzite and fine-grained metamorphic pebbles dominate elsewhere. Sandstones are interbedded with the conglomerates, and also are present on the eastern side of the basin. Paleocurrent data indicate that most of the sands were transported in an easterly direction.

Sedimentary rocks, adjacent to mafic plutons, are metamorphosed to dense and very hard black and green rocks (8% of total exposure) in aureoles up to 0.8 mi (1.3 km) wide. In addition to intrusive rocks, basalt flows account for 25% of total exposures. Approximately 2,800 ft (850 m) of basalt is present in five mappable units (several of which contain more than one flow), which provide excellent stratigraphic markers in the western part of the basin. The oldest flow has an outcrop length of nearly 40 mi (64 km), although much of it has been removed by Jurassic erosion.

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