CHARACTERIZATION OF FLUVIAL SYSTEMS WITHIN THE NEOPROTEROZOIC CATOCTIN FORMATION CENTRAL AND NORTHERN VIRGINIA

DILLIARD, K. A., and SIMPSON, E. L., 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.

The Neoproterozoic Catoctin Formation, of central Virginia, consists of a thick flood basalt sequence with subordinate interlayered sedimentary units. The Catoctin Formation overlies Grenville-age basement or the Swift Run Formation and is overlain by the Chilhowee Group. The Catoctin basalt flow and sediment record rifting associated with the opening of the Iapetus Ocean. The intercalated sedimentary units exhibit a change in depositional architecture from the lower to the upper Catoctin Formation.

The lower Catoctin Formation fluvial deposits are characterized by lens-shaped channel deposits that filled paleotopography on the basalt flows. Interflow units range in thickness from 0.1 to more than 2 meters. Well-exposed channels can be traced for more than 70 meters. Basal parts of channels often consist of cobble- to boulder-size basalt-clast conglomerates with a sandy matrix. Channel deposits are normally graded from the conglomerate to medium-grained sandstone. Trough cross-beds are developed in medium-grained sandstones. Limited paleocurrent data indicate north-directed flow. Basalt flows often incorporated unconsolidated sediment in the basal portion of flows. This vertical sequence is typical of a braided-fluvial system.

The upper Catoctin Formation fluvial deposits are characterized by sheet geometry. The frequency of sedimentary layers between basalt flows increases vertically. Contacts between sediment layers and basalt flows are often either sharp, baked, or erosive. Dune bedforms are preserved beneath one basalt flow. Interlayered conglomerates, pebbly sandstones, siltstones, and mudstones form fining-upward sequences between basalt flows. These fining-upward sequences may reflect gradual abandonment of the braided system or waning flood deposition. Conglomerates contain clasts of basalt, granite, and sometimes red phyllite. Some layers of conglomerates and coarse sands have thin horizontal layers of fine sand. These conglomerates record deposition from bars. Small- to medium-scale trough crossbeds, within sandstones, record the migration of three-dimensional bedforms. Paleocurrent data indicate southward transport. The sandy facies are interpreted to be bar-top and bar-edge deposits.