Abstract

PROCESS INTERPRETATION OF TIDAL DEPOSITS WITHIN NEOPROTEROZOIC TO LOWER CAMBRIAN UNICOI FORMATION, CENTRAL BLUE RIDGE, VIRGINIA

SIMPSON, E. L., Department of Physical Sciences, Kutztown University, Kutztown, PA 19530; WALKER, D., North Carolina Geological Survey, P.O. Box 27687, Raleigh, NC 27611; NOTO, R. C.; GLEDHILL, H. M.; and HARRISON, M. J., Department of Physical Sciences, Kutztown University, Kutztown, PA 19530.

The Neoproterozoic to Lower Cambrian Unicoi Formation is the oldest formation of the Chilhowee Group and records the rift to passive margin development of the Iapetean margin. The Unicoi Formation can be subdivided into a lower rift-related marine unit characterized by quartz sandstones. Three-dimensional and bedding plane exposures of the upper Unicoi Formation along the James River in western Virginia provide a unique opportunity to reconstruct processes active along the newly developed shoreline of the Laurentian shelf.

The upper Unicoi Formation reflects development of a transgressive systems tract composed of three parasequences. Vertically with the parasequence set, deeper water progressively overlie the flooding surface of younger parasequences. Two types of architectural elements have been recognized, Type 1 (sheet-like) and Type 2 (channelized). The Type 2 elements are less common and downcut into Type 1 elements. Both element types can be traced across outcroppings (100 m). Type 1 elements range in thickness from less than one meter to a maximum of 1.5 meters, and are characterized by planar-tabular cross beds overlain by subordinate medium- to small-scale trough cross beds, which often can only be observed as bedding plane traces. Type 2 elements are greater then 2.0 meters in thickness and consist of planar-tabular cross beds overlain by trough cross beds in the upper portion or are completely composed of trough cross beds. When a Type 2 element directly underlies a flooding surface of a parasequence, opposed bidirectional bedforms are preserved.

Computer-aided bedform reconstruction employing orientation of cross beds and set boundaries indicates planar-tabular and trough cross bedding are variants of straight- and sinuous-crested bedforms respectively. Type 1 and Type 2 elements were produced by ebb-dominated tidal currents. Type 1 elements record shallow broad tidal channels characterized by low flow velocities in a subtidal setting. Type 2 elements reflect the effects of a larger tidal prism and higher flow velocities. Bedforms preserved below flood surfaces record a subtidal or low interitdal setting possibly on a tidal shoal. The interpreted tidal system is dominated by a broad shallow tidal channels that may be an important characteristic of the Laurentian shoreline.

Kutztown University of Pennsylvania

A member of the State System of Higher Education

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