ARCHITECTURE AND SIGNIFICANCE OF THE LOWER TRIASSIC MCKENZIE DOLOMITE LENTIL IN THE WHITEHORSE WILDERNESS-MCLEOD RIVER AREA, ALBERTA

Lower Triassic sedimentary successions in northwestern Pangaea are dominated by fine-grained siliciclastic lithologies. Bioclastic accumulations are, in general, quite rare and limited to a few horizons. This is due, at least in part, to the demise of many carbonate-secreting organisms during the Permian-Triassic biotic crisis. Although fossil invertebrates do occur in siliciclastic units, fossils consist primarily of bivalve and ammonoid impressions on bedding planes, with preservation of neither primary nor replacement carbonate. Dissolution of the original carbonate shell material has been interpreted to represent syndepositional acidic ocean water conditions.

In the Western Canada Sedimentary Basin bioclastic units are limited to a few specific horizons, generally within the late Induan (Dienerian) and early Olenekian (Smithian) parts of the succession. The thickest and most pervasive of these bioclastic units is the Mackenzie Dolomite Lentil (MDL). The MDL is lithologically variable between areas. In most outcrop successions analyzed in this study the MDL consists of either a bivalve or gastropod-dominated bioclastic grainstone. In other sites the MDL is locally dominated by lingulide brachiopods. Correlation of the coquina units indicates that the thickest coquinas grade westward with fine-grained sandstone-dominated successions and eastward with thinly interbedded bioclastic grainstone and silty calcareous wackestone.

The MDL was deposited as a shallow marine beach ridge system analogous to coquina ridge complexes along the coast of Hamelin Pool in western Australia. Thick tabular (3 to 8 meter) coquina bedsets in the outcrop portion of the study area comprise the main beach ridge complex. Laterally equivalent cross-stratified sandstone units are interpreted as siliciclastic shoreface and foreshore deposits. Laterally equivalent interbedded bioclastic grainstone and calcareous wackestone units are interpreted as washover fan successions.

The MDL and equivalent Lower Triassic depositional units represent a temporary return to pre-extinction oceanic conditions (i.e. well-oxygenated and pH ~ neutral) and reflect a temporary partial recovery of mid-latitude carbonate depositional systems on the northwestern coastline of Pangaea.