Sedimentology of the Unicoi Formation in southern and central Virginia; evidence for late Proterozoic to Early Cambrian rift-to-passive margin transition

Few detailed-facies analyses of the rift-to-passive margin transition have been undertaken in exhumed orogenic belts. The Unicoi and lower Hampton Formations in the central and southern Appalachians record the transition from late Proterozoic to Early Cambrian rifting to initial opening of the Iapetus Ocean.The lower and middle Unicoi Formation consists of feldspathic sandstones, conglomerates, and basalts. Sandstones and conglomerates were deposited from hyperconcentrated flows and tractional currents in distal alluvial-fan and proximal and medial braid-plain environments. Extrusion of basalts was associated with influx of coarse-grained siliciclastics. The presence of thick alluvial-fan sediments, basalts, and abundant lithic clasts and the feldspathic nature of the sandstones are suggestive of a rift setting. Paleontological and geochronological data indicate that rifting continued into Early Cambrian time. The upper Unicoi Formation is composed predominantly of transgressive, quartzose sandstones which represent the incipient phase of passive margin sedimentation related to a second-order, sea-level rise. Differences in degree of crustal attentuation probably controlled the distribution of sedimentary environments during transgression. On most attenuated crust to the east, initial transgressive facies consist of tidal sand-wave and sand-ridge deposits intercalated with proximal and medial braid-plain deposits. As transgression progressed cratonward, tidal sedimentation was supplanted by tide- and wave-influenced sedimentation characterized by sand-wave complexes, tidal inlets, and longshore-directed bedforms. Drowning at the top of the Unicoi Formation is indicated by outer-shelf black mudstones containing Tabc, Tbc, and Tc beds and gravity-flow deposits at the base of the Hampton Formation. Deepening may have been enhanced by continued movement along listric faults throughout the incipient phase of passive-margin development. Most of the Hampton Formation and the overlying Erwin Formation and Shady Dolomite record pro-gradation of the passive margin which culminated in development of a carbonate-rimmed shelf. The rift-to-passive margin phases of sedimentation in the central Appalachians reflect a continuum from fault-influenced to thermothectonic subsidence.