Abstract

Biostratigraphic analysis of conodont faunas from the Upper Devonian Wabamun Group, the Upper Devonian to Lower Mississippian Exshaw Formation and the Lower Mississippian Banff Formation in southern Alberta has shown that significant hiatuses may exist within the Late Devonian Big Valley and equivalent upper Costigan Member of the Palliser Formation and within the Early Mississippian lower Banff and Lodgepole formations. Minor hiatuses occur in the Late Devonian to Early Mississippian Exshaw Formation between the lower and upper members and between the equivalent lower black shale and middle sandstone members of the Bakken Formation. All of these hiatuses are interpreted to have been the result of forced regression due to uplifts. Biostratigraphic data and stratigraphic evidence derived from mapping suggest an erosional high existed at about the position of the Fifth Meridian in southwestern Alberta, as indicated by the repeated removal of Upper Devonian to Lower Mississippian strata in this area. The only other tectonic elements that may have influenced sedimentation and distribution patterns of these strata are possible deep-seated basement faults beneath the position of the Early Cretaceous Cutbank channel and the Sweetgrass Arch.

The lower part of the Big Valley Formation and upper Costigan Member may represent one major episode of deposition, whereas the upper part of these units, the Exshaw Formation and the lower and middle members of the Bakken Formation appear to represent another depositional event. At least two depositional events may be represented in the lower Banff and Lodgepole formations in the study area. The depositional events and hiatuses documented in the study area generally correlate with similar events in western North America and globally. However, discordance between events in our study area and elsewhere probably reflect the masking effect of local tectonism. Correspondingly, one transgressive-regressive sequence may be recognized in the lower part of the Big Valley and upper Costigan Member. The upper part of these units, the Exshaw Formation and the lower and middle members of the Bakken Formation are interpreted as a single transgressive-regressive sequence, contrary to previous interpretations. One complete sequence and a portion of another are recognized in the lower Banff and Lodgepole formations.

Many of the unconformities may be linked to Antler orogenesis, which also would have affected subsidence rates. Changes in subsidence rates interacting with eustatic sea level changes produced the stratigraphic framework and distribution of units observed in the study area.