Down-Current Bed Thickness and Grain-Size Variations Along Graywacke Beds, Gault Formation, Eastern Alps: ABSTRACT

Abstract

Paleocurrent directions indicate that deposition of 52 continuous graywacke beds in the 200 m thick Gault Formation (Lower Cretaceous) was from a western source. Down-current decrease in bed thicknesses over 115 km along the Eastern Alpine flysch belt is slight and usually cannot be detected within individual layers. Bed-by-bed correlation between each section, however, provides a basis for mathematical treatment of the thickness data. If calculated as average deviations from a standard section, most individual sections show a systematic down-current decrease in graywacke bed thickness. The standard section was obtained by using the average thickness of each graywacke bed and each claystone layer and should, therefore, theoretically provide a medial section approximately midway between the proximal and the distal sections. The decrease in thickness appears more pronounced, if thickness ratios (thickness of the graywacke bed divided by thickness of the overlying claystone layer) are taken into account. The average deviation of these thickness ratios from the standard section thus provides an index for the proximality. Similarly slight down-current changes were observed in grain sizes. For instance, a feldspar-rich marker bed shows a nonuniform decrease of the median grain size at its base from 535µ in the west to 176µ at the easternmost section, 115 km east. End_of_Article - Last_Page 343------------