Downcurrent Changes in Sedimentary Structures in Ordovician Turbidite Greywackes

Abstract

ABSTRACT Sedimentary structures were studied in detail in eight turbidite greywacke beds from the Cloridorme Formation (Middle Ordovician), Gaspe, Quebec. The beds are exposed on the wave-cut platform near Grande Vallee and were traced for about two miles along the strike (276°), which is parallel to the average current direction (274°) as indicated by sole mark directions. The beds under study show the following systematic downcurrent changes along the strike from east to west: (i) Sole mark types change as follows: (1) Tool marks such as groove marks, prod marks and isolated flute marks, to (2) longitudinal ridges, longitudinal ridges with overlapping flute marks and closely spaced flute marks, to (3) poorly developed shallow longitudinal ridges with occasional cuspate crossing bars, to (4) smooth bottom with occasional groove marks. (ii) There is an increase in variance of sole mark directions in the downcurrent direction. The sole mark direction changes by nearly 90° as one of the beds is traced from east to west along the strike. (iii) Most beds are massive and are divided into two parts by a bedding joint. However, in the proximal regime, if a bed is unusually thin, the bedding joint is absent and the bed shows slightly wavy plane-laminated structure. It is considered that beds were deposited by relatively low concentration, highly turbulent currents. Deposition of the beds took place in two distinct phases separated by a small time interval and the depositing currents tended to meander greatly on slowing down in the distal region.