Volcanic rift margin model for the rift-to-drift setting of the late Neoproterozoic-early Cambrian eastern margin of Laurentia: Chilhowee Group of the Appalachian Blue Ridge

摘要

New data support a model of a volcanic rifted margin for eastern Laurentia and the breakup of the supercontinent Rodinia. Upper Neoproterozoic-lower Cambrian rocks of the Chilhowee Group in the Blue Ridge Province of eastern North America are subdivided into two facies assemblages separated by an unconformity. Historically, the rocks have been correlated as a tripartite division: 1) basal sandstone and conglomerate (Cochran, Unicoi, and Weverton Formations), 2) middle siltstone and shale (Nichols Shale, Hampton Shale, and the Hampton and Harpers Formations), and 3) an upper sandstone and shale (Nebo Quartzite, Murray Shale, Hesse Quartzite, and Helenmode, Erwin, and Antietam Formations). Sedimentary analyses show that boundaries of the newly defined facies assemblages transect the named stratigraphic units. Assemblage A consists of fluvial-lacustrine deposits with interbedded subaerial basalt flows overlain by marine deposits. Fluvial strata formed in anastomosing braided-river channels and are interbedded with mudstones deposited in shallow lacustrine plains. Overlying shallow marine deposits consist of upward-coarsening successions capped by over-thickened sandstones. Shale-rich turbidite deposits characterized by subdued bioturbation and common slump features occur at the top. Assemblage B consists of sandstone and shale deposited on a stable shelf. These upward-coarsening para-sequences are stacked to form transgressive-regressive system tracts reflecting long-term sea-level fluctuations. A working hypothesis is that these rocks are comparable to those present in seismic reflection profiles and drill cores of the volcanic rift margins of the Atlantic and Indian Oceans. Assemblage A deposits formed during rapid subsidence associated with the formation of seaward-dipping reflectors. Flood basalts within fluvial sandstones are the landward facies of more voluminous basalt flows in the opening basin. Overlying marine deposits of Assemblage A were coeval with basaltic volcanism and deeper-water deposits to the east Assemblage B unconformably overlies Assemblage A rocks and laps onto much older continental rocks. It was deposited on the craton into the late Cambrian. Much of the lateral lithologic variability and differences in character and thickness of the Chilhowee Group is attributed to the depositional setting that was influenced by the underlying rift architecture. It was further complicated by juxtaposition of rocks telescoped along numerous Paleozoic thrust faults. This model predicts that coeval rocks in outboard thrust sheets are finer-grained, mafic volcanic-rich marine sequences (Assemblage A), overlain by shelf-margin and basin deposits that lack volcanic rocks (Assemblage B). This model provides more refined sedimentary criteria for examination of other volcanic rift margins.