Richard D. Trotter

Granite Wash is an informal term applied to the Paleozoic basal siliciclastic unit associated with the Peace River Arch, northwestern Alberta. The Granite Wash, which unconformably overlies Precambrian igneous and metamorphic rocks, consists of conglomeratic sandstone, sandstone, and mudstone. Examination of 50 cores and 650 geophysical well logs provided sufficient data to determine the lithofacies distribution and depositional history of the Granite Wash in the Utikuma and Red Earth field areas. In these areas, Granite Wash clastics interfinger with open marine carbonates of the Keg River Formation (Middle Devonian) and form elongate trends which correspond to Precambrian structural trends.

Initial Granite Wash sediments in the Utikuma area consists of red, poorly sorted, dominantly unstratified conglomeratic sandstones probably

deposited in alluvial fans. The overlying succession consists of stacked 10-20 m coarsening-upward sequences interpreted as the deposits of prograding fan deltas. The widespread lateral extent of these sequences suggests a probable allocyclic control on their development.

In the Red Earth area, initial Granite Wash sediments were confined to narrow, northeast-southwest trending paleovalleys which opened into a wider basin to the paleovalleys which opened into a wider basin to the northeast. Conglomeratic sandstones were deposited in braided fluvial channels, alluvial fans (?), and deltas with a possible tidal influence. Transgressive drowning of the valleys resulted in the deposition of a widespread black mudstone unit. Renewed progradation is indicated by the deposition of a 5-15 m coarsening-upward fluvio-deltaic sequence which is capped by a mudflat/tidal flat succession. The Granite Wash sequences are conformably overlain by shale, dolomite and anhydrite of the Muskeg Formation. The diminution in coarse clastic sediment supply reflects conditions of rising base level and/or tectonic quiescence

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Supervisor: F. J. Hein