Basal Cambrian Siliciclastic Shelf Deposits of Appalachians
Frederic L. Schwab
The Chilhowee Group and equivalents, an 800 to 2,300-m thick succession of alluvial(?) to shallow marine siliciclastic shelf deposits, extends along the northwestern margin of the Blue Ridge-Green Mountain anticlinoriums from Alabama through New England. Locally, it lies disconformably on discontinuous late Proterozoic alluvium, or nonconformably on older, Grenville-age basement. The Chilhowee is overlain by 2-3 km of cyclic, shallow-water, marine shelf carbonates. As such, the Chilhowee and related units mark the initial regional subsidence of an early Paleozoic passive continental margin developed after an earlier episode of rifting related to the opening of the proto-Atlantic (Iapetus) Ocean or a smaller, back-arc sea adjacent to that ocean.
Recent application of process-oriented sedimentologic analyses to the Chilhowee succession suggests systematic upward changes in depositional setting. Older sand-sized and pebble-sized detritus accumulated as fluvial channel and levee deposits developed along a coast characterized by wave-dominated deltas and adjacent coastal barrier-beach and strand-plain complexes. Overlying units consist of finer grained, laminated pro-delta(?) and/or back-barrier lagoonal siltstone and mudstone alternating with 5 to 20-m thick mappable barrier-beach bodies. The uppermost Chilhowee is a thick sequence of quartzarenite deposited as a complex of barrier islands, beaches, and beach ridges developed before the abrupt general subsidence of the continental margin that produced the overlying carbonate ban successions.