CHILHOWEE-EQUIVALENT OUTER PASSIVE-MARGIN SHELF DEPOSITS ALONG SOUTHEAST LAURENTIA

The most distially preserved strata of the Lower Cambrian Chilhowee Gp. occur in the S. Appalachian Talladega belt (TB)/western Blue Ridge (WBR) allochthon, and formed near the outer edge of rifted continental crust during the initial stages of passive-margin sedimentation and Iapetian seafloor spreading. Along the Alabama recess this sequence, the Kahatchee Mtn. Gp. (KMG), occurs immediately above the basal thrust of the TB, extending discontinuously for ~250 km from the Gulf Coastal Plain to Cartersville, Georgia. From there, across the Cartersville transverse zone, the sequence continues NE for another 175 km across the Tennessee salient in the WBR as the Nantahala and Brasstown Fms. (NBF) on the flanks of the Murphy syncline and Copper Hill anticline. In both regions the sequences are gradational upward into carbonate units, which, in Alabama, contain Early Cambrian archaeocyathids at the base (Jumbo Dolomite=Shady Dolomite), establishing the Chilhowee-equivalency of the underlying sequence. The WBR records the rift-to-drift transition, as the NBF is conformable above a thick (>10 km) Ocoee rift basin sequence. Along the Alabama recess to the SW, however, there is no evidence for rift-facies rocks and the KMG may have formed directly above basement. Throughout its extent, the KMG/NBF, which represents the initial coastal plain sequence along W. Iapetus, is dominated by thinly laminated carbonaceous metapelite at various metamorphic grades. Sandstones compose <20% of the sequence, and in the KMG, while dominantly quartz arenite to subarkose, are coarser-grained, less rounded, and more poorly sorted than Chilhowee sandstones. Because the KMG/NBF sequence was deposited palinspastically at a minimum of many 10's of km. SE of the easternmost foreland Chilhowee sequence, direct correlation of internal formation-level units between the two sequences may not be possible, even though both have many similarities in depositional environment. Most of the former sequence formed as transgressive open-marine middle to outer continental shelf deposits below wave base, whereas the foreland-Chilhowee formed as more proximal alluvial-fan and marginal marine deposits. The more outboard TB/WBR sequence is generally significantly thicker than the foreland Chilhowee (2,000 to >3,000 m vs. <1,000 to ~2,000 m).