Print this pageAbstract: Tectonic Control of Sedimentation in Triassic-Jurassic Culpeper Basin, Virginia
Movement along an east-dipping normal fault and westward tilting of the downthrown block were the major factors in determining sedimentation patterns in the Culpeper basin in Virginia. Coarse clastic materials were spread eastward during the first period of fault movement. Quiescence and erosion of the western uplands led to deposition of fine-grained sediments. This deposition was followed by renewed uplift and a second flood of coarse clastic deposits from the west, including most conglomeratic beds in the sequence.
Younger beds generally dip more steeply than successively older beds. Although this clearly indicates that most of the westward tilting of the downthrown block came after deposition, paleocurrent patterns indicate that minor tilting occurred relatively soon after the first period of faulting. Several other lines of evidence indicate additional tilting prior to cessation of deposition.
Seven basalt flows (in five mappable flow units) are sandwiched within the upper conglomeratic sequence. These flows are abnormally thick, averaging 124 m, with the thickest being 235 m. They probably were formed during a period when the downthrown block was inclined toward the west, causing lava to be ponded against the fault scarp on the west.
Within the uppermost conglomeratic unit are several thin calcareous black shales which contain fossil fish. These beds were deposited in a lake which formed by the same mechanism which caused ponding of the basalt flows. Slow, continuous deposition of the lacustrine beds was periodically interrupted by debris flows which poured down alluvial fans to the west.
AAPG Search and Discovery Article #90961©1978 AAPG Annual Convention and Exhibition, Oklahoma City, Oklahoma