Abstract

A one-to-fifty metre thick mixed carbonate-clastic interval in the subsurface of central and southern Alberta was investigated to determine the stratigraphic framework of Devonian-Carboniferous strata of the Alberta cratonic platform of western Canada. The platform experienced widely fluctuating subsidence rates superimposed on slowly varying eustatic sea-level, and paleoceanographic changes which altered depositional environments and caused large variability of strata. Intertidal evaporites of the Upper Famennian Stettler Formation are separated from the overlying open marine bioclastic packstones of the Big Valley Formation by a long-ranging hiatus. The hiatus, manifest as a reworked phosphatized hardground, is interpreted as a type 2 sequence boundary. The overlying Big Valley carbonates are part of a shelf margin wedge systems tract which onlapped paleobathymetric highs in Alberta and Saskatchewan during an Upper Devonian relative sea-level rise. The deepening-upward succession of Big Valley carbonates above the sequence boundary reflects an interval of rapid increase in accommodation space. Mudstone, glauconite, phosphate content increase upward and omission surfaces become more frequent toward the Big Valley-Exshaw contact. Big Valley carbonates pass into the overlying organic-rich, laminated distal shelf mudrocks of the Exshaw Formation which form a transgressive systems tract. Transition from Big Valley carbonates to Exshaw black mudrocks was the result of changing paleoceanographic circulation, leading to eutrophication of surface waters and reduced carbonate accumulation rates culminating with demise of the carbonate ramp. A phosphatic, pyritiferous lithic arkose, representing a hiatus event between the two formations, is interpreted as a condensed deposit formed by dissolution of the carbonate ramp during its final accumulating phase. The organic-rich mudrocks are abruptly to gradationally overlain by bioturbated shelf siltstones and cross-stratified nearshore marine sandstones of the upper Exshaw Formation. These deposits record an abrupt relative sea-level fall and are interpreted as a lowstand systems tract. The lack of evidence for development of a highstand systems tract in the Exshaw mudrocks suggests that any relative sea-level drop prior to development of the sequence boundary could not match rates of subsidence. The base of the bioturbated siltstones in Alberta is the conformable correlative to a type 1 sequence boundary that is well expressed in the Williston Basin to the east. The distal position of the sequence boundary in southern and western Alberta is reflected by minimal submarine erosion and the development of a conformable contact. The stratigraphic nature of this lowstand shoreface varied from an initial regressive to an aggradational phase. As subsidence rates gradually outpaced sedimentation rate and relative sea-level fall, the shoreface finally became retrogradational and formed a transgressive systems tract. The transgressive systems tract was overlain by black mudrocks of the Banff Formation, forming the base of a prograding carbonate succession, interpreted as part of a highstand systems tract.

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