CHARACTERIZATION OF THE PRESERVED NEOPROTEROZOIC LANDSURFACE AND ASSOCIATED CATOCTIN FORMATION FLUVIAL DEPOSITS, SHENANDOAH NATIONAL PARK, 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, records rifting associated with the opening of the Iapetus Ocean and consists of a thick, flood basalt sequence with subordinate, interlayered sedimentary units. An inlier through the Catoctin Formation exposes the Grenville-age, Pedlar Formation, a granodiorite, in stream cuts near Big Meadows, Shenandoah National Park, VA. The study of the inlier contact, a nonconformity, and onlapping relationship of sediments and basalts provides an opportunity to gain insight into the characteristics of the paleotopography and the development of adjacent fluvial systems.

The contact elevation is corrected for the local structure and new elevations are contoured. The gradient of this currently north-facing slope is calculated to be ~20%. Using a distinctive porphyritic basalt flow, the minimum relief from the lowermost basalt flow to the top of the inlier is determined to be ~180 meters.

The Pedlar Formation beneath the nonconformity surface is in sharp contact with either sediment and basalt. No soil horizons are identified and probably did not develop due to the high gradient. In some areas, the Pedlar Formation surface is characterized by numerous cracks infilled with basalt, siltstone and fine-grained sandstone, or intermixed basalt and sediment. Crack dimensions average 1.5 cm in width with variable lateral continuity. Some cracks record complex sediment and basalt infill histories.

Sediment deposits, associated with the paleotopography, consist of granite with subordinate basalt clasts. Deposits range from very angular and poorly sorted breccia to rounded and sorted sandstone. The breccia is considered to represent colluvium that washed off the steep granodiorite highs. Breccias containing intermixed basalt clasts reflect minor reworking of the colluvium by fluvial processes and intermixing of basalt clasts from a distant source. The sorted sandstones indicate significant fluvial reworking of the colluvial material.