Wave-Dominated and Storm-Dominated Sedimentation in Lower Cambrian Hampton and Erwin Formations of Chilhowee Group, Central and Southern Virginia
Edward L. Simpson, Kenneth A. Eriksson
Siliciclastic sediments of the Chilhowee Group overlie Grenville basement, Mount Rogers group, or Catoctin Formation, and pass conformably upward into the Shady Dolomite. The Chilhowee Group consists of three formations (oldest to youngest): (1) Unicoi, (2) Hampton, and (3) Erwin. Basal parts of the Unicoi Formation are of fluvial origin, whereas the upper beds represent a transition into shelf environments of the Hampton and Erwin Formations. Examination of thrust sheets allows for the recognition of four different facies associations and enables a generalized reconstruction of the shelf for this interval.
Facies association 1 consists of massive to trough cross-stratified, Skolithos-bearing, medium to coarse-grained quartzarenites. This association is interpreted to be of nearshore origin and accumulated under the influence of fair-weather wave and longshore currents. Facies association 2 consists of massive, fine-grained quartzarenites; some beds are capped by large-scale wave ripples developed in coarse-grained arenite. This association is interpreted to be a nearshore to inner shelf deposit. The massive arenites were deposited from suspension with the tops reworked into wave ripples by onshore-directed waning storm or fair-weather waves. Facies association 3 consists of microhummocky and planar-laminated, fine-grained arenite with wave-rippled tops, and bioturbated, horizontally lam nated siltstones and shales. Facies association 4 consists of bioturbated, planar-laminated, and unidirectional cross-laminated, fine-grained arenites, and bioturbated, horizontally laminated siltstone and shale. Both facies associations 3 and 4 are the product of traction deposition from storm-generated currents followed by suspension settling, but differ in that association 3 exhibits evidence of wave modification. Association 3 represents an inner shelf deposit, whereas association 4 is of inner shelf to outer shelf origin.
This stratigraphic interval represents an overall regressive sequence that is interrupted by at least one regressive pulse within the Hampton Formation. Nearshore to outer shelf transitions are recognized vertically within these sequences as well as laterally across the different thrust sheets. Facies associations 1 through 4 are developed in the northwest, whereas associations 3 and 4 predominate in the southeastern outcrop belt.
AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.