WORKING MODEL FOR THE CHILHOWEE GROUP IN THE BLUE RIDGE PROVINCE: A PASSIVE VOLCANIC MARGIN SEQUENCE FROM THE OPENING OF THE IAPETUS OCEAN

The Early Cambrian Chilhowee Group crops out in the Blue Ridge from PA to GA. A consistent package of sedimentary rocks (commonly associated with basalts) represents a portion of a volcanic passive rift margin at least from central Tennessee to southern Pennsylvania. Anastomosing river deposits (Cochran and Unicoi Fms.) grade upward into marine shelf sandstones and turbidite-rich offshore deposits (Nichols Shale and Hampton Fm.). Unicoi fluvial deposits contain sheet-like, subaerial basalt flows that thicken and increase in number to the east. Rapid subsidence is inferred for the depositional conditions. These deposits are unconformably overlain by wave-dominated marine rocks in rhythmic packages consistent with lower subsidence rates (Nebo Quartzite, Murray Shale, Hesse Quartzite, Helenmode Fm. , upper Hampton Fm., and Erwin Fm.). These marine rocks are equivalent to the Weverton, Harpers, and Antietam Fms. to the north. Weverton Fm. rocks unconformably overlie turbidite-rich marine shale of the Loudoun Fm.,restricted to the east limb of the Blue Ridge anticlinorium to the north. Loudon Fm. rocks are interpreted as equivalents to the Nichols Shale and lower Hampton Fm. . In the northern part of the Blue Ridge, the Weverton Fm unconformably overlies mostly the Late Neoproterozoic Catoctin Fm that consists of sheet-like subaerial basalt flows that also thicken to the east. Unicoi Fm unconformably overlies Mesoproterozoic basement and glaciogenic Neoproterozoic Konnarock Fm. Cochran Fm is transitional above Walden Creek Group rocks.

We propose that the lower Chilhowee Group indicates rapid subsidence and the basalts are equivalent to the seaward dipping reflectors interpreted from seismic reflection profiles of modern volcanic rift margins formed during the initial attenuation of continental crust prior to rifting. Marine deposits reflecting lower subsidence rates are similar to sequences that overlie the post-drift unconformity in modern volcanic rift margins. We predict that the variation in thickness and character of deposits will primarily reflect lateral displacement by Paleozoic thrust faults and that correlative metamorphic rocks to the east (such as the Ashe and Alligator Back Fms.) are intercalated marine shales containing mafic volcanic rocks younger than Catoctin or Unicoi Formations.