A NEW WORKING MODEL FOR DEPOSITION AND STRATIGRAPHY OF THE CHILHOWEE GROUP IN THE BLUE RIDGE OF VIRGINIA

Rocks of the Early Cambrian Chilhowee Group in the Blue Ridge from GA north to PA were deposited in the early stages of the opening of the Iapetus Ocean. From northeastern TN north to southwestern VA, the stratigraphy is Unicoi, Hampton, and Erwin Formations; from central VA to PA it is Weverton, Harpers, and Antietam Formations. Olenellus trilobite in Antietam sandstone in central VA and PA has traditionally been used to establish an age younger than 542 Ma for the Group. Published depositional models for the rocks in southern VA correctly identified fluvial facies that grade upward into marine shelf and nearshore deposits in the upper part of the Unicoi. These rocks are overlain by deeper water marine shales of the Hampton Formation, followed by shallower marine sandstone of the Erwin Formation. New sedimentological data suggest that the traditional correlation of Unicoi to Weverton, Hampton to Harpers, and Erwin to Antietam is not valid. Multiple basalt flows, as much as 30m thick and continuous over 30 km, within fluvial deposits in the lower Unicoi are likely related to basalts of the ~571-563 Ma Catoctin Formation, making them Neoproterozoic in age. Clasts of Catoctin volcanic rocks within the lowermost Weverton Formation from central VA north to PA, however, support a basal unconformity post-563 Ma.

The Unicoi rocks are interpreted as post-rift, seaward-dipping deposits formed during the initial drift of continents from the spreading center. Similar deposits are recognized in seismic profiles (SDRs) through the outer shelf in modern settings, where they dip into oceanic crust and are overlain by a marine unconformity. A possible angular unconformity within the Hampton is underlain by turbidite-rich marine deposits and overlain by shales lacking turbidites. This unconformity is correlated northward to the lower Weverton Formation where wave-sorted conglomerates rest directly on Catoctin basalts or unconformably overlie fluvial conglomerates. The southern Chilhowee Group rocks are remnants of the outer shelf, whereas the northern Chilhowee Group rocks are inner shelf deposits. The similar successions of facies from north to south are attributed to similar responses to sea-level fluctuations superimposed at different times along the Laurentian rift margin.