Abstract

This paper is concerned with the petrology and stratigraphy of the arenaceous beds of the Late Cambrian Conococheague formation in the northern Appalachian area of Virginia. The arenaceous beds are confined to two definite stratigraphic positions in the formation. The Big Spring Station member occurs at or near the base of the formation and a higher succession of arenaceous beds are near the top. Both sandy phases are laterally persistent throughout the area in the western and medial parts of the Appalachian Valley but only one thin phase is present on the easternmost edge of the Valley. The arenaceous beds are calcareous sandstones and orthoquartzites which are cemented with calcium carbonate or silica. The dominant constituent is quartz which is moderately well-rounded and sorted. The principal heavy minerals are well-rounded tourmaline and zircon and local floods of other minerals such as hornblende and topaz. Angular overgrowths on the tourmaline indicate such growths have developed in place, whereas abraded overgrowths denote sedimentary reworking of some material. Relatively angular and unstable minerals associated with very stable varieties suggests two distinct suites of minerals derived from different sources. In nearly every sample the quartz of the calcareous sandstones has been replaced around the edges by the calcite cement, and very irregular and etched grains have been produced. A southeasterly decrease in sand content of the Conococheague in the area studied and similar trends in equivalent Gatesburg beds in Pennsylvania and the Copper Ridge dolomite of northeastern Tennessee and southwestern Virginia suggest that the source of the detritus was to the northwest.

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