Sediments and Sedimentary Structures of a Beach-Ridge Complex, Southwestern Shore of Lake Michigan

Abstract

ABSTRACT Seven different sedimentary environments occur in the beach-ridge complex along the southwestern shore of Lake Michigan. Poorly sorted gravel and sandy mud form colluvium along the base of slopes. Cross-bedded sands occur as channel deposits, and interbedded sand, silt, and clay occur as overbank deposits in valleys cutting through the clay-till bluff to the west and in channels on the beach-ridge plain. Organic muds and peat are deposited in the marsh, and well-sorted sand in low-angle accretionary cross-sets is found both in dunes and on eolian sand plains. Beach-ridge sediments consist of sand in convex-upward cross beds dipping landward, and sand and gravel in low-angle accretionary cross beds dipping lakeward. Upper shoreface sands consist of trough crossbedded sand formed in lon shore troughs and low-angle planar laminae formed on longshore bars. Very fine-grained sands in wavy horizontal laminae are characteristic of the lower shoreface. These sediments have variable potentials of preservation and inclusion in the vertical sequence. Lower shoreface sands are well represented in both transgressive and regressive sequences, while subaerial facies would tend to be destroyed during periods of transgression. Erosion by migrating troughs apparently occurs during both transgression and regression, resulting in upper shoreface sequences consisting mainly of sediments deposited in longshore troughs.