Paleocurrent and Petrographic Analysis of Imbricate Intraclasts in Shallow-Marine Carbonates, Upper Cambrian, Southwestern Virginia

Abstract

ABSTRACT The Conococheague Limestone and Copper Ridge Dolomite are coeval Upper Cambrian shallow-marine carbonates exposed in the Valley and Ridge Province of southwestern Virginia. Sheetlike conglomerates of probable storm origin showing well-developed intraclast imbrication are common in these units. Directional and petrographic data obtained from 48 such conglomerates and adjacent strata reflect the landward (Copper Ridge) versus seaward (Conococheague) depositional frameworks in which the two formations originated. Analysis of imbrication azimuths suggests that the Conococheague conglomerates were transported by multidirectional paleocurrents associated with middle- to outer-shelf storm activity. In contrast, the Copper Ridge imbricate intraclasts show much more consistent shoreward movement toward the northwest, probably the result of currents generated by storm waves refracted by shallower, middle-shelf to intertidal environments. At individual sample localities, apparent long-axis alignments of elongated clasts in the conglomerates show inconsistent relationships to transport directions deduced from imbrication, evidently because of high clast concentration and general lack of prolate grains. An overall shore-parallel trend of long axes, more evident in the Copper Ridge, may have developed fr m poststorm-tidal-current and wave activity and could serve as another shallow- versus deep-water discriminant. Paleoflow patterns derived from cross stratification in sandstones above and below the imbricate conglomerates reflect normal, day-to-day, tidal, longshore, and rip-current sedimentation. Petrographically, the intraclast conglomerates show a tripartite grouping strongly related to source sediments and their environments. Group I, found mainly in the Conococheague, formed in platform-margin ooid sand shoals; Group II, common in both formations, originated in subtidal middle-shelf to lower-intertidal inner shelf, peloid sand silt and mud flats; and Group III, present almost exclusively in the Copper Ridge, accumulated in inner-shelf intertidal environments where algal mats and carbonate mud were abundant. Paleocurrent directions determined from imbrication in the petrographic groups also reflect these differences in depositional environments. The importance of paleocurrent data from imbrication in shallow-marine carbonate conglomerates, which are abundant in the geologic record, has not been realized. This information can be synthesized with petrographic data to aid considerably in reconstructing ancient environments.