Regional sedimentology, conodont biostratigraphy and correlation of Middle Cambrian-Lower Ordovician (?) strata of the Finnegan and Deadwood formations, Alberta subsurface, Western Canada sedimentary basin

Hein, F.J.; Nowlan, G.S.

Bulletin of Canadian Petroleum Geology 46(2): 166-188


ISSN/ISBN: 0007-4802
Accession: 019881062

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Detailed sedimentological and paleontological analysis of 187 cores from the Alberta subsurface allow for a better understanding of depositional settings and preserved stratigraphy of Middle Cambrian to possibly Lower Ordovician strata within the Western Canada Sedimentary Basin. Facies analysis indicates that the Cambrian successions were emplaced and reworked as mixed clastic and carbonate ramps or marine shoals within a broad, shallow, marine intracontinental basin, south of the Peace River Arch, and east of the West Alberta Arch, in an area that has been called the Lloydminster Embayment. Locally, pre-existing basement highs pierced the marine embayment, shedding coarse clastic debris, disrupting the stacking patterns of the sedimentary successions, and resulting in overstep relationships and condensed sections. Much of the history of Middle Cambrian to Early Ordovician in the Alberta subsurface is one of repeated cycles of erosion and deposition of both clastics and carbonates that resulted in a very fragmentary preserved stratigraphy. In the subsurface of Alberta and Saskatchewan the upper part of the Sauk II subsequence consists of the Deadwood Formation. In the Alberta subsurface, the Deadwood Formation is a transgressive, clastic-dominated, shelf-ramp succession. This is overlain by reworked platformal carbonates, with varying amounts of clastic debris, that have been traditionally termed the "Finnegan" Formation. Detailed core logging, facies and geophysical-log analysis of subsurface data during the present study indicates that, where distinctive geophysical log-markers or other marker horizons in core are absent. It is impossible to definitively differentiate "Finnegan" from Deadwood strata. Conodont biostratigraphy shows the presence of a paraconodont-dominated zone overlain by the Proconodontus, Eoconodontus and Cordylodus zones. The top of the succession is diachronous, ranging in age from latest Cambrian to possibly earliest Ordovician. Distinction of the Finnegan and Deadwood formations is very difficult in the Alberta Plains subsurface. It is proposed here that the term "Finnegan" be abandoned, and that all units previously assigned to the "Finnegan" unit now be included as part of the Deadwood Formation. Similarly, other stratigraphic units are difficult to separate in the subsurface without traceable geophysical or seismic markers. Future work should critically examine the applicability of existing stratigraphic nomenclature to the Alberta Plains subsurface.