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Regional Frasnian Stratigraphic Framework, Alberta Outcrop and Subsurface

Abstract

The latest Givetian to Frasnian succession of Alberta, Canada comprises a second-order depositional sequence. This sequence is bounded by major, regionally-correlatable unconformities. These surfaces have associated siliciclastic lowstand deposits, sourced from the Canadian Shield and the Peace River Arch Landmass. The bulk of the sequence consists of shallow water carbonates (reefs and reef complexes) and basin-filling carbonates and shales sourced from the east. A major marine, euxinic shale source rock, the Duvernay Formation, was deposited during the second-order maximum transgression. Maximum syndepositional relief in the basin, on the order of 200m, occurred at this time. Nine third-order depositional sequences make up the larger sequence, correlatable across the basin. An ancestral landmass, the West Alberta Ridge, extended southwest of the Peace River Arch, precluding deposition of the three oldest third-order sequences in the Rocky Mountain outcrops. Correlations from outcrop to basin suggest progressive onlap onto the Ridge, the bulk of which was transgressed by the sediments of the Woodbend Sequence One. This is corroborated by the presence of the transitans conodont zone in the outcrop belt and adjacent subsurface. The antecedent topography favoured the growth of carbonate build-ups in a northwest-southeast trend, intersected by major marine channels and embayments. The corresponding platform margins show different character, depending on paleogeography and stratigraphic position within the second order sequence. Several in-situ carbonate lowstands have been identified, notably at the base of the Woodbend Sequences One and Two, in the Cline Channel and on the southern margin of the Jasper Basin. Similar platform margin styles and third-order sequence architecture are seen in the Alberta subsurface as in the outcrop belt. Sequence stratigraphic correlations are supported by biostratigraphic (conodont) data. Comparisons to Tertiary/Modern carbonate platforms, such as the Great Bahama Bank and the Upper Miocene platforms of the Balearic Islands, show similar patterns of margin development, segmentation of platforms and filling of marine channels. Contrasts include the much thinner Alberta Frasnian, deposited over a longer time, compared to the Bahamian/Balearic platforms.