Lacustrine rocks in the Culpeper basin were deposited in open lacustrine, marginal lacustrine, and mud-flat environments. Lateral and vertical facies relationships suggest that lacustrine deposition was controlled by a complex interplay of tectonic movement and climatic change. Lacustrine units thicken stratigraphically upward and toward the fault zone that bounds the basin on the west, suggesting tectonic control on lake formation. Some lacustrine sequences show an asymmetric arrangement of facies consisting of a diastem overlain by open lacustrine black shale, followed by marginal lacustrine and mud-flat deposits. This pattern suggests rapid deepening, possibly tectonic in origin, followed by gradual shallowing. Other lacustrine sequences consist of a symmetrical arrang ment of facies representing gradual deepening followed by gradual shallowing, possibly as a result of climatic change.

Preliminary work in the Deep River basin (Durham and Sanford subbasins) has shown that, at several localities, lacustrine units are present at the tops of fluvial fining-upward cycles. The lacustrine rocks include both shallow-water and mud-flat deposits. The lacustrine shales are overlain by massive, mottled units interpreted as paleosols, they coarsen upward into ripple cross-laminated and wavy-laminated siltstone and very fine-grained sandstone, or they are truncated at the top by fluvial channel scour.

Lacustrine units in the Culpeper and Deep River basins contain shallow-water and mud-flat deposits. Facies relationships show that, in Culpeper basin, relatively large lakes were present, whereas in Deep River basin, many lacustrine units were deposited in shallow flood-plain lakes.

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