Tectonic Control of Devonian Reef Sedimentation, Alberta: ABSTRACT

Abstract

Kaybob is one of a group of geologic reefs which underlie west-central Alberta and which comprise the Upper Devonian Swan Hills Formation of the Beaverhill Lake Group. Kaybob is a flat north-south elongate lens, 250 ft thick, 11 mi long, and 3 mi wide, built on the Slave Point Formation, a widespread platform carbonate. Detailed core study of sedimentary structure, texture, and constituents reveals that the carbonate sediments composing the can be grouped into 3 environmentally controlled facies groups: reef-slope, reef-margin, and shelf-lagoon facies. The reef-slope facies is a stromatoporoid-crinoid-brachiopod grainstone. The marginal deposits consist of a groundmass of massive stromatoporoid grainstone containing varied concentrations of different stromatoporoid growth forms (ranging from slender branching to large massive heads). The interior facies includes several rock types arranged in repeated vertical sequences. A basal unit of massive wackestone is overlain by combinations of massive, thin-bedded, and laminated mudstones, mudclast grainstones, and amphiporid carbonate conglomerate. Submarine and subaerial scoured surfaces are present within the upper units. The outer slope sediments intertongue with contemporaneous open-marine deposits seaward and with the reef-marginal facies on the inner side of the reef. This pattern changes laterally in vertical section, as the thickness of the changes from place to place. The upper half of the shows a marked westward displacement. Position of the various facies within the geologic mass, and comparison with Holocene carbonate sediments from several Caribbean localities, together provide a paleoenvironmental interpretation. The interior facies includes most of the sediment types which have been described from recent shallow shelf-lagoon environments. Sequential arrangements are the same and result from similar processes. The marginal realm includes many facies comparable with those observed in Holocene reef tracts, ranging from scattered coral growth on a sand bottom to coral constructed buttresses of ecologic reefs. Circumferential variations in geologic development, the westward displacement of the upper part, and a thick pile of open-marine deposits on the southwest are attributed to prevailing nor h winds during the Devonian. Tectonic control is exhibited at 4 levels. 1. The interior shelf-lagoon sequences are initiated by small-scale pulses of subsidence, perhaps complicated by eustatic sea-level changes. 2. Larger scale subsidence variations account for the thickening and thinning of the geologic body as a whole. 3. An orthogonal pattern of sharp elongate folds trending NE-SW and NW-SE is expressed clearly at the base of the reef. The folds are confined to the area of reefing, where they controlled initiation by forming mud mounds during upper Slave Point deposition. The pattern may reflect Slave Point block faulting. 4. This and others in the region, together with their associated carbonate-shelf deposits, fit into a well-expressed orthogonal tectonic pattern controlled by larger scale ba ement features. The basement features include a family of NE-SW-trending, relatively stable arches revolving about a NW-SE-trending major arch, the West Alberta ridge. They form part of the system of stable arches that provides the tectonic framework of the continent. End_of_Article - Last_Page 854------------