ABSTRACT

The continuous part of the Halfway Formation of Triassic age forms a wedgeshaped deposit of sandstone and minor amounts of carbonate which thickens southwestward from just west of the Milligan Creek area where it occurs in the subsurface towards the foothills where it crops out. It unconformably overlies the Doig Formation and is conformably overlain by the Charlie Lake Formation.

In the subsurface of the Milligan Creek area fine-grained, isolated, sandstone developments which are classed as 'upper' sandstones occur to the northeast of and separate from the continuous part of the formation. These 'upper' sandstones, which consist of two parallel trends of isolated, southeast-aligned developments, form the discontinuous part of the Halfway Formation. Fine-grained sandstone to coarse-grained siltstone developments which occur stratigraphically below the 'upper' sandstones are classed as 'lower' sandstones. These 'lower' sandstone developments are much more limited in extent and would appear to be a lithofacies of the Doig Formation rather than to belong to the Halfway Formation.

Before the deposition of the Halfway Formation, the Triassic sea regressed southwestward along a line a short distance northeast of the present-day foothills, and the Milligan Creek area was part of a gently sloping coastal plain. The Halfway Formation forms the basal formation of an onlap sequence that was deposited as the Triassic sea once again transgressed northeastward. The detrital material composing the Halfway Formation probably came from a northeasterly source and was carried southward by longshore drift. The 'upper' sandstone developments of the Beatton, Milligan, Willow, Wildmint and Bulrush trend were formed as longshore bars during a pause in the northeastward transgression, at a time when the rate of supply of detrital material was low. The 'upper' sandstone developments at Peejay and Currant which occur parallel to, and to the southwest of, the Beatton to Bulrush trend were initially formed as deeper-water longshore bars that ultimately evolved into barrier beaches during a limited marine regression.