Abstract

CONTRASTING FLUVIAL STYLES OF THE CATOCTIN AND WEVERTON FORMATIONS, CENTRAL VIRGINIA: SHOULD HYPERCONCENTRATED FLOWS BE MORE COMMON BEFORE THE ADVENT OF LAND PLANTS?

SIMPSON, E. L., and DILLIARD, K. A., Department of Physical Sciences, Kutztown University, Kutztown, PA 19530, dill5029@kutztown.edu and simpson@kutztown.edu. NOTO, R. C., Department of Geoscience, University of Nevada, Las Vegas, Las Vegas, NV 19154, notor@nevada.edu.

Braided-stream systems are reported to be more commonplace during pre-vegetation times and associated with diverse climatic conditions. The absence of plants increases the availability of loose sediment. Large fluctuations in discharge characterize fluvial systems in a pre-vegetation regime. Could these conditions that caused a dominance of braided-stream systems, also lead to an increase in the probability of generating hyperconcentrated flood flows? Lateral erosion of channel margins increases the suspended load, transforming a "normal" streamflow into a "hyperconcentrated" flood flow. Typical hyperconcentrated flood flows result in deposits that are massive with occasional crude horizontal stratification. Deposits of this nature are generally not produced by bedload transport. There is a paucity of hyperconcentrated flood flows recognized within pre-vegetation sequences; these deposits should be more abundant.

A study of the Catoctin and Weverton formation sediments, both pre-vegetation, illustrate the difference between a flash flood-dominated system versus a typical braided-stream system, respectively. The Catoctin Formation, of central Virginia, consists of a flood basalt sequence with intercalated fluvial sedimentary units. Whereas, the overlying Weverton Formation is considered to be dominantly a fluvial to marine sequence. Sediments within the upper Catoctin Formation are characterized by sheet geometry. Massive conglomeratic and pebbly sandstone beds contain occasional crude horizontal stratification, which is characteristic of hyperconcentrated flood flows. Trough crossbedding, found associated with massive beds within stratigraphic sequences, records deposition by "normal" streamflow tractional processes. The overlying Weverton Formation sandstones are also characterized by sheet geometry. In contrast to the Catoctin Formation, the lower Weverton Formation exemplifies structures generated by a typical braided stream system. Tabular planar and trough crossbedding deposited from bars reflect deposition from "normal" streamflow. As basaltic volcanism waned, the greater ratio of sediment to basalt allowed increased infiltration and decreased discharge hence typical braided-stream systems become better defined.

Kutztown University of Pennsylvania

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D.J. Moore
Oct. 2002
Last modified Sept. 2004
moore@kutztown.edu