Sedimentation on a Passive Continental Margin (Late Cambrian-Early Ordovician), Central Appalachians: ABSTRACT

Abstract

Environmental reconstruction of rocks of the Richmond slice, the tectonically highest slice at the eastern end of the Hamburg klippe in Pennsylvania, indicates that these rocks were deposited on a northwest-facing passive continental margin in Late Cambrian to Early Ordovician time. Detailed sedimentologic studies suggest that they can be divided into four main lithologic types, each bearing the imprint of one or more processes involved in its deposition. These include: (1) thin to thick-bedded, massive to graded, parallel and cross-laminated grainstone or calcarenite (high density turbidity currents; fluidized flows); (2) thin to medium-bedded, massive to structureless, graded and cross-laminated black lime mudstone and wackestone rhythmically interbedded with very thin o thin-bedded black limy mudstone and shale (low density turbidity currents and suspension); (3) thinly laminated graphitic black shale interlayered with irregular (lag) concentrations of fine sand and silt (redistribution of sediment by bottom currents); and (4) very thick-bedded, sand-matrix carbonate-clast conglomerate (gravelly high density turbidity currents and cohesive debris flows). The proposed depositional processes form a continuum of mechanisms that were in operation in the slope environment. Regional stratigraphic studies suggest that the carbonate rocks of the Richmond slice were deposited on a depositional margin or ramp characterized by a gentle slope (1 to 2°) that decreased in gradient basinward. Only the lower slope portion of the continental margin has been preserved. End_of_Article - Last_Page 1171------------