Paleocurrents and Paleogeography

ABSTRACT The objective of this paper is to review the criteria used for direction of sediment transport and to outline the ways in which such knowledge can be used to make paleogeographic reconstructions. The criteria of transport direction which have been found to be most useful are cross-bedding, the various linear sole markings of sandstone beds, and the maximum pebble size of conglomeratic beds. Less generally used are ripple marks, sediment fabric (grain orientation, etc.), heavy minerals, and other compositional attributes. The prime objective is establishment of the sedimentary strike and initial dip of the deposits which should bear close relation to the paleoslope or depositional surface, the margin of the basin, the shoreline, and the bathymetric and structural axes of the basin. Sternberg’s Law of size decline governing maximum gravel sizes of alluviating streams has been used to estimate distance to the basin margin. The contours of equal grain size and the normals to current directions established from cross-bedding define the sedimentary strike. The shoreline, established at one point by paleontologic criteria, can be extrapolated parallel with the depositional strike. Sand bodies, accumulated in areas of maximum turbulence, may be transverse to the paleocurrent pattern (barrier beaches, submarine bars, etc.) or parallel to the ruling current system (fluvial channels and longitudinal bars). Problems of predicting orientation and location of sand bodies need further study. The relations between internal structures and the paleocurrent system will contribute to our understanding of these problems.