ABSTRACT

The Cow Head Group is an allochthonous Cambro-Ordovician sequence of deep-water limestone conglomerates, mudstones to grainstones, minor quartz sandstones, siltstones and shales. Platy-clast conglomerates and boulder-rich megaconglomerates as much as 100 m thick exhibit features similar to those described for subaerial debris flows; that is, flat, sharp bases with only minor evidence for erosion, poorly developed clast fabric with tilting of platy clasts at right angles to the flow direction, boulders floating within flows or projecting above the tops of flows, irregular flow tops indicating primary relief above the sea floor, and tapered flow margins (snouts).

Snout geometry and the extent of boulder projection both allow calculation of debris static strength. These results, combined with measured flow thickness, permit calculation of minimum and maximum paleoslope gradients. Original debris static strengths were in the range of 10³ to 10⁵ dynes/cm², similar to the observed range in subaerial flows. Calculations yield bottom gradients of l-l 8°, but independent field data suggest that the paleoslope dip was on the order of only a few degrees. This constraint suggests that debris strength was substantially reduced during flow, probably by excess pore-fluid pressures, thus permitting flow on very gentle slopes.

The paleoslope orientations used in this study are substantially different from those previously published by Hubert et at. (1977).