ABSTRACT

The turbidites in the Rock Stream Member of the Sonyea Group (Upper Devonian of New York State) are flat lying, structurally undisturbed and have consistent sole mark orientations indicating current flow toward west-northwest. Measurements of internal sedimentary structures have been analyzed using the ABC percentage index recently proposed by Walker. It is hypothesized that values of 100 percent indicate fully proximal environments of deposition, with values gradually decreasing to zero percent in fully distal environments. In the Rock Stream Member, values were found to range from 96 to 3 percent. Graphs of sedimentary features against ABC index percentages show that with decreasing percentages, bed thicknesses decrease exponentially, and parallel lamination and cross- amination become more abundant. One graph shows that cross-lamination is abnormally abundant in geographically the most proximal area, and here, the cross-lamination vector mean directions differ significantly from sole mark directions. Ocean current reworking by a northwestward flowing current is indicated.

ABC index values calculated from successive outcrops in a downcurrent direction do not show gradually depending creasing percentages. It is concluded that individual beds are not continuous along the length of the basin. An alternative model of basin filling is proposed, in which turbidites are deposited in a series of wedges. The wedges are stacked imbricately one in front of another, and gradually build forward into the basin during sedimentation.