DOWNCURRENT TEXTURAL CHANGES IN ORDOVICIAN TURBIDITE GREYWACKES

Abstract

SUMMARY Longitudinal and vertical variation in grain orientation and size has been studied in eight turbidite greywacke beds from the Cloridorme Formation (Middle Ordovician), Gaspé, Quebec, Canada. The beds are exposed on a wave‐cut platform near Grande Vallée, Gaspé. These beds were traced for about two miles along the strike (276°), which is almost parallel to the average current direction (274°) as indicated by sole marks. Agreement between grain orientation and other internal current‐direction indicators such as graptolites and intra‐bed lineation shows that grain fabric is primary in nature and that grains are typically aligned parallel to the current direction. Grain orientations are generally statistically non‐significant in the upper massive part of the beds in the proximal region and significant in the rest of the beds. Most of the significant grain orientations are close to the sole direction in the proximal region and deviate from the sole direction by as much as 90° in the distal region. Grain orientations are increasingly divergent from the base towards the top of the bed. Current‐normal grain orientations are observed in a plane‐laminated part of a bed. The beds show a good grading in the proximal area and are ungraded or slightly reverse graded in the distal area. These grading characteristics are exhibited by both the mean and maximum grain size. Most beds show no appreciable variation in the mean grain size at the base of the bed along the strike. However, for one bed, the largest mean size at the base occurs in the middle of the outcrop and grain size decreases from this point both in the upstream and downstream directions.