DIRECTIONAL-CURRENT STRUCTURES FROM THE PREALPINE FLYSCH, SWITZERLAND

Abstract

Gurnigel sandstone beds in the Paleocene-Eocene Flysch of the External Prealps, Switzerland, contain primary structures which display common orientation through many feet of strata. The linearity of some of these directional-current structures is easily measured in the field. The bottom of beds display groove-cast and load-cast primary-current lineations; within a layer are found clast lineations subdivided into grain and charcoal-fragment lineations, and parting lineations consisting of streaks on bedding-planes. Flute casts, torose load casts, current stratification, convolute bedding, and ripple marks also occur. These, and other factors, suggest that the sediments were laid down in a relatively sheltered basin by turbidity currents. In three sections of the Gurnigel Flysch 401 field measurements of the orientation of current structures show that during the Paleocene and Eocene a source for sediment lay not far to the northwest. This information, with published data from clast types and the distribution and facies of other Paleocene and Eocene units, implies that a land mass or island probably lay within the northwestern part of the Ultrahelvetic sedimentation region within which the Gurnigel Flysch accumulated, and near the border of the Helvetic region. The Oligocene and Pliocene culminations of the Alpine orogeny carried the Gurnigel Flysch far to the northwest within Ultrahelvetic nappes.