TRANSGRESSIONS, REGRESSIONS, AND STABILIZATION OF THE CAMBRIAN PASSIVE MARGIN: THE CHILHOWEE GROUP IN SW VA

In the Appalachian mountains, clastic deposits of the Neoproterozoic to early Cambrian Chilhowee Group record the transition from the final stages of rifting of the supercontinent Rodinia to the establishment of a passive margin along the margin of Laurentia. The Chilhowee Group is exposed in the Valley and Ridge-Blue Ridge transition zone throughout much of Virginia; in SW VA it comprises the Unicoi (lower), Hampton (middle), and Erwin (upper) Formations.

Significant exposures of the Chilhowee Group occur within the Blue Ridge Scout Reservation in Pulaski County, VA. Although the Unicoi Fm. appears to be absent in this area, the Hampton and Erwin Fms. are well-represented. The Hampton Fm. can be divided into four facies: 1) a lower mudstone sequence of black shale and siltstone with minor thin-bedded sandstone; 2) turbiditic thin-bedded feldspathic sandstone and black mudstone, commonly displaying convolute layering, small-scale ripples and lenticular bedding; 3) quartz arenite and minor conglomerate with cross bedding and megaripples; and 4) an upper interval of turbiditic, thin-bedded sandstone and mudstone, locally containing Skolithos and Monocraterion vertical burrows, as well as horizontal burrows (Planolites?). This facies is capped by quartz sandstone that is gradational with mature quartz arenites of the Erwin Fm.

We interpret the lower Hampton (Facies 1) to represent quiet-water marine conditions punctuated by high-energy events, consistent with marine transgression as the continental margin evolved from rift to drift facies. Facies 2 represents turbidite deposition on the outer marine shelf. The abundance of angular grains and preservation of feldspars suggest that post-rifting erosion of the continental margin was still active during sea-level transgression. Facies 3 heralds a return to sea-level regression, as represented by thick-bedded quartz arenites. Moderate to poor sorting suggests a relatively energetic environment, although the abundance of quartz and lack of feldspar suggests significant reworking of sediment. Facies 4 transitions into the Erwin Fm., and represents the shift into a well-established passive margin. The common occurrence of Skolithos is consistent with shoreline or shallow marine environment of deposition.