The Coincidence of Macroscopic Paleocurrent Indicators and Magnetic Lineation in Shales from the Precambrian Belt Basin

Abstract

ABSTRACT Paleocurrent flow direction in shale sequences can be inferred from sedimentary structures of associated lithologies, from orientation of fossils, from alignment of silt and sand-sized particles in shales, from mapping of scalar properties of a shale sequence, and from the orientation of concretions. Not all of these methods relate to shale properties directly, and most of them are very time-consuming and may even require fortuitous circumstances to be applicable. Consequently, even though shales constitute about 60% of all sediments, paleocurrent data from shale sequences are sparse. Thus, in order to improve our understanding of the evolution of sedimentary basins, it would be beneficial to have an accurate and conveniently applicable method of paleocurrent determination for shales. /P> The anisotropy of magnetic susceptibility (AMS) of a number of shale samples from the Precambrian Belt Supergroup carries a fabric-related magnetic lineation that coincides with paleocurrents indicated by cross-laminated silt beds within the shales. Magnetic foliation decreases as dolomite content increases because of diagenetic dolomite growth, but magnetic lineation is only slightly affected. These data indicate that primary flow direction as represented by AMS measurement of these samples is preserved and therefore the AMS method can provide a rapid and accurate way to determine current-flow systems in shale-dominated sedimentary basins.