Effective Wave Base Research Articles

Late Devonian Geologic History in Stettler Area, Alberta, Canada

Late Devonian history in the Stettler area is divisible into four sequences of deposition comprising a major evaporite cycle. Unconformities mark the base and top of this cycle. The sequences in increasing age are: (1) final normal marine, (2) biostromal and evaporitic, (3) marine-reefoid, and (4) initial normal marine. The initial normal marine sequence is represented by normal, fragmental or oolitic, locally dolomitized limestones and shaly limestones and calcareous shales constituting the Beaverhill Lake group. The southward thinning of this group and increasing import of fossiliferous-fragmental limestones and secondary dolomites south and southeast indicate decreasing negative tectonism from a northern basinal province to an unstable shelf at the south. The marine-reefoid sequence, represented by the Woodbend group, commenced with a marine limestone episode followed by a reef-growing episode. The marine limestone episode was characterized by deposition under shelf conditions of normal marine, fossiliferous-fragmental, partly dolomitized limestones, locally reefoid. Leduc, Duvernay, and Ireton formations (of the Woodbend group) were deposited during the reef-growing episode. Three phases of development of reef-growth are recognized: early, medial, and late phase. Increased subsidence in central and south-central Alberta occurred during the early phase of reef growth. Biohermal reef-growth commenced where organic shoals had become prolific near the end of Cooking Lake time. These biohermal structures had considerable effect on off-reef sedimentation. The subsidence rate was not sufficient to keep pace with reef-growth. Thus they acted as sills, impeding effective circulation of marine waters in the inter-reef areas where dark-colored bituminous shales and limestones were deposited. Lighter-colored limestones and shales formed in localities of relatively free circulation. The rate of subsidence in the basinal area increased during the medial phase, and biohermal growth continued. Marine conditions varying from nearly normal to restricted were established throughout most of the inter-reef areas. Deposition of clay from northwest of the Central Alberta basin into inter-reef areas reached an approximate maximum during this medial phase. The minor importance of carbonates and absence of agitated-water limestones and seconda y dolomites in off-reef areas are suggestive of contemporary subsidence such that reefs grew partly below effective wave-base. An approach to shelf deposition in the late phase of reef-growth is indicated by an increase of dolomites, with shaly beds, and normal marine to fragmental limestones, in the inter-reef areas. The arrival of relatively more stable conditions was partly instrumental in causing cessation of biohermal growth. The biostromal episode represents a continuation in the progressive reduction of rate of subsidence and tectonic differentiation initiated in the late phase of biohermal reef-growth. Biostromal structures End_Page 2500------------------------------ flourished in relatively shallow water depths. These organic shoals became instrumental in controlling the physical, chemical, and organic factors of sedimentation. Progressive shoaling of waters formed restrictive barriers, and evaporitic precipitation commenced in the inter-shoal areas. The precipitation of evaporites commenced earliest in the inter-shoal areas and later on-lapped the shoals. The biostromal episode was terminated by an approach to subaerial conditions. The evaporitic episode represented continuation of restricted conditions throughout most of the area. Evaporitic sedimentation extended southward into southern Alberta, but intensity of restriction was less severe. The return to normal marine conditions of deposition toward the end of Wabamun time marks completion of the major cycle initiated by Beaverhill Lake sedimentation.