Abstract

Major conglomeratic units of the Culpeper Basin occur in four members of the Bull Run Formation (Leesburg, Goose Creek, Cedar Mountain, and Barboursville Members) and in the Waterfall Formation. These rocks are for the most part muddy/sandy pebble conglomerates, and are interpreted as debris flow deposits. Isopleths of maximum clast size show a decrease in grain size toward the east. These data together with an easterly paleocurrent flow direction indicate that the conglomerates were deposited as alluvial fans at the foot of highlands that lay west of the Culpeper Basin. The border between the highland source and the alluvial fans was an east-dipping normal fault. Pre-Triassic rocks in the north-plunging Blue Ridge anticlinorium west of the basin were the source for gravel deposited on the fans. Spatial relationships between the eastern limb of the anticlinorium and the border fault greatly influenced the composition of these gravels. The pre-Triassic rock units strike more westerly than the border fault. Pre-Triassic units are cut by the fault such that progressively older rocks are exposed toward the south. Thus, from north to south, the conglomerates contain clasts produced by erosion of progressively older rocks exposed in the eastern limb of the anticlinorium.

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