Abstract

The dominant view of Middle Triassic stratigraphy in western Alberta and northeastern British Columbia includes westward regression, via offlapping parasequences, continuously from the Doig Formation upward through the Halfway and the lower part of the Charlie Lake Formation to the Coplin Unconformity. The Doig-Halfway contact is arbitrarily picked within a succession of stacked parasequences. Some writers portray the Charlie Lake Formation as mildly unconformable above the Halfway; others depict the contact as conformable and time-transgressive, as the Charlie Lake is interpreted as a mainly nonmarine sabhka facies overlying regressive Halfway beach sands. In the Umbach-Wargen area of British Columbia, few of these entrenched models seem to apply; instead, the Halfway appears to overlie unconformably a truncated Doig Formation, forming a succession of stacked retrogradational and/or transgressive parasequences, which onlap progressively towards the northeast a structural high on the Doig. In addition, the Charlie Lake Formation forms a succession of widespread, correlateable parasequences, more akin to those deposited in shallow marine conditions rather than nonmarine. These strata overlie the beach sands of the Halfway Formation as its coeval, marine offshore equivalent. If the Charlie Lake Formation is an offshore, basinward equivalent to some of the Halfway shoreline sands, then depositional models have to be re-examined.

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