Micro-Cross-Lamination in Upper Keweenawan Sediments of Northern Michigan

Abstract

ABSTRACT Micro-cross-lamination is the dominant type of stratification in the fine-grained sandstone and siltstone units in the Keweenawan sediments of northern Michigan. Except for its small size it is identical to trough or festoon cross-bedding and is therefore significantly different from ordinary ripple lamination. The average set of micro-cross-lamination is 2.5 inches wide, 0.5 inch thick and 8 inches long. This structure is also extremely abundant in other fine-grained sediments although it is often partly obscured by weathering or diagenesis and may not appear as typical cross-stratification. Its presence in apparently homogeneous sediments have been revealed by staining the iron or calcite cement. It is therefore quite likely that micro-cross-lamination represents a fundamental m chanism often associated with the deposition of fine grained sediments. Associated sedimentary structures clearly indicate that this micro-cross-lamination originates in a shallow water environment repeatedly subjected to subaerial erosion. Observations of similar structures in modern streams and flood plains suggest that it develops from migrating cusp ripples. Micro-cross-lamination constitutes an excellent criterion for paleocurrent direction. Its value is greatly enhanced by its abundance and small size since several complete sets can easily be collected in an oriented hand specimen, sawed in numerous sections and analyzed in the laboratory.