Exploration significance of healing-phase deposits in the Triassic Doig Formation, Hythe, Alberta

The Middle Triassic Doig and Halfway formations in the Hythe area of west-c entral Alberta comprise a siliciclastic progradational parasequence set. In areas proximal to the palaeoshoreline, most parasequence boundaries in thi s stratigraphic interval are marked by an erosional ravinement surface with a thin bioclastic sandstone lag. In distal settings, parasequence boundari es are marked by a marine flooding surface overlain by a condensed phosphat ic mudstone. However, one parasequence boundary at the top of the Doig Form ation in the Hythe area is marked by a 0-25 m thick unit of very-fine-grain ed sandy siltstone that rests conformably between a regressive portion of t he parasequence below and the flooding surface at the base of the overlying parasequence. This anomalous sandy siltstone unit (SSU) contains plane lam ination, hummocky cross-stratification, soft sediment deformation and rare oscillation ripple cross-lamination and was deposited in the marine offshor e-shoreface transition zone. Based on its vertical facies succession, the SSU would typically be interpr eted as a continuation of the underlying regressive shoreface parasequence. However, when the geometry, areal distribution and stratigraphic relations hip of the SSU to adjacent deposits are considered, such an interpretation becomes unsupportable. The SSU forms a shoreface-detached, seaward-thickeni ng wedge of sediment that onlaps the underlying parasequence, partly infill ing, or "healing over," the palaeobathymetry. The facies, geometry and dist ribution of the SSU point to an origin as transgressive healing-phase depos its. We suggest that development of significant sand-prone healing-phase de posits in the Doig Formation in the Hythe area is due to the presence of an adjacent, thick, growth-fault sandbody, which would have been a local sour ce of sandy sediment during transgression. Although the healing-phase depos its do not form an attractive exploration target, the presence of anomalous sandy siltstone in this interval may prove useful as an indicator of nearb y, up-dip, thick growth-faulted sandstones in the Doig Formation.