Abstract

The Early Cambrian--Late Precambrian(?) middle unit of the Chilhowee Group in Virginia, the Harpers Formation, consists of four lithological facies: (1) quartz arenite, (2) lithic and feldspathic arenite, (3) wacke, and (4) shale. All four facies were derived primarily from the erosion of a cratonic source area located northwest of the depositional area at the margin of the Appalachian geosyncline. A southeasterly paleocurrent system, hypothesized on the basis of cross-bedding, resembles dispersal patterns previously proposed for the coarser, lower and upper units of the Chilhowee Group. The principal lithologies in the northwestern source terrain were silicious igneous rocks and coarse-grained metamorphic rocks (quartzite, gneiss, schist), but unmetamorphosed sedimentary rocks overlying the basement were also eroded. The specific depositional environments and the lateral relationships for all four Harpers facies can only be generally inferred because the Harpers outcrop belt parallels depositional strike and individual exposures are poor. Measured sections of the Harpers can most logically be interpreted as vertical repetitions of (A) shallow water sandstone and silt deposited in a slowly subsiding area of nearshore marine and nonmarine environments (facies 1 and 2), and (B) offshore, deeper water, rhythmically alternating turbidite sandstone and shale (facies 3 and 4). This vertical juxtaposition of usually contemporaneous facies probably is the result of repeated shifts in the position of the shoreline and the shallow water--deep water boundary (hinge line). A similar tectonic mechanism may explain large scale, longer term vertical lithological contrasts within the overall Chilhowee Group.

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