ABSTRACT

The Tourelle Formation (Ordovician) consists of shales and sandstones of several different facies types deposited in the midfan region of a submarine fan complex. Dimensional orientation of quartz grains was measured on 93 core-samples taken from ten unusually thick, coarse sandstone layers, to investigate the mechanism of deposition of such layers. Samples were taken both vertically within a layer and horizontally along the strike (for distances up to 570 m) to study local variation in both grain orientation and imbrication.

The layers studied are of two facies types, which also differ in fabric. Both types generally show size grading, from about 1 mm at the base to about 0.2-0.5 mm near the top. Layers showing internal stratification have strong preferred orientation that is consistent both vertically and horizontally within a layer, with long axes of grains parallel to flow and imbricate upcurrent, commonly at angles greater than 20°. In some layers orientation is consistently rotated from sole marks. Within individual (reverse graded, cm-thick) stratification bands, vector magnitude is strongest in the finer part of the band. Layers lacking internal stratification have strongest orientation at the base and top. Some basal samples are bimodal. The middle of the layer is characterized by isotropic or bimodal fabrics. Grain orientation and imbrication are generally highly variable within each layer, although one structureless layer displays consistent orientation and upcurrent imbrication.

Stratified and most structureless layers were deposited from turbulent suspensions (turbidity currents): a few massive layers were probably deposited from sandy submarine debris flows. Fabric inconsistency, bimodality or isotropism appears to be related mainly to very rapid deposition from suspension or to deposition from sandy debris flow.