Abstract

The Middle Triassic Doig and Halfway formations in the Hythe area of west-central Alberta comprise a siliciclastic progradational parasequence set. In areas proximal to the palaeoshoreline, most parasequence boundaries in this stratigraphic interval are marked by an erosional ravinement surface with a thin bioclastic sandstone lag. In distal settings, parasequence boundaries are marked by a marine flooding surface overlain by a condensed phosphatic mudstone. However, one parasequence boundary at the top of the Doig Formation in the Hythe area is marked by a 0–25 m thick unit of very-fine-grained sandy siltstone that rests conformably between a regressive portion of the parasequence below and the flooding surface at the base of the overlying parasequence. This anomalous sandy siltstone unit (SSU) contains plane lamination, hummocky cross-stratification, soft sediment deformation and rare oscillation ripple cross-lamination and was deposited in the marine offshore–shoreface transition zone.

Based on its vertical facies succession, the SSU would typically be interpreted as a continuation of the underlying regressive shoreface parasequence. However, when the geometry, areal distribution and stratigraphic relationship of the SSU to adjacent deposits are considered, such an interpretation becomes unsupportable. The SSU forms a shoreface-detached, seaward-thickening wedge of sediment that onlaps the underlying parasequence, partly infilling, or “healing over,” the palaeobathymetry. The facies, geometry and distribution of the SSU point to an origin as transgressive healing-phase deposits. We suggest that development of significant sand-prone healing-phase deposits 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 source of sandy sediment during transgression. Although the healing-phase deposits do not form an attractive exploration target, the presence of anomalous sandy siltstone in this interval may prove useful as an indicator of near-by, up-dip, thick growth-faulted sandstones in the Doig Formation.

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