Abstract

Within the Middle Triassic Doig Formation of Western Canada there are thick sandstone bodies encased in muddier sediment — “anomalously thick sandstone bodies” (ATSBs). Interpretations vary from incised valleys, deltaic, barrier islands, turbidites in slope slumps, to shallow-water gravity-flow beds overlain by shoreface deposits in a slump or growth fault. Several authors have interpreted the uppermost beds in the sandstone bodies as shoreface deposits. The thick sandstone bodies were deposited during a period of regression and transgression but authors disagree as to whether or not the ATSBs and enclosing strata were deposited contemporaneously. Abrupt basal contacts, apparently abrupt lateral contacts, and stratigraphic relationships favour the sandstone bodies to be post-mudstone, formed during late regression.

The most probable explanation for the origin of ATSBs is that during late regression, structural depressions formed on a muddy, pre-existing shelf and were filled by sandy sediment. Initial deposits in the thicker sandstone bodies commonly are debris-flow and/or slump deposits that filled in the deeper parts of the depression. As the depression filled, sediment entered wave- and current-influenced water depths and more typical shoreface deposits began to form. As regression proceeded some sandstone bodies were truncated by a regressive surface of marine erosion. Finally, all ATSBs and adjacent sediment were capped by a thin veneer of transgressive sediment.