Changing Depositional Environments in the Wapiabi-Belly River Transition (Upper Cretaceous) near Longview, Alberta

Abstract

<p> The transition from the Wapiabi Formation to the Belly River Formation was studied in two outcrops, Highwood 1 and 2, near Longview, Alberta. The lowest units in the stratigraphic sections consist of deep marine, storm-generated density flow deposits interbedded with shales. They are overlain by dominantly crossbedded sandstones deposited in a marine environment which was dominated by shallow water processes. At Highwood 1, the next deposits are those of a braided fluvial system, which consist of crossbedded sandstones and much conglomerate. There is no shale. At Highwood 2, the fluvial deposits consist of thick sandstone units separated by thick dominantly shale units, with some roots. The uppermost units are again marine sandstones and shales. This return to marine conditions has not been mentioned previously in the literature. </p> <p> Paleoflow directions indicate that regional paleoslope dipped northwest at the base of the sections, but northeast in the fluvial parts. It is suggested that the slow rate of deposition in the Coniacian and Santonian, coupled with slow subsidence, permitted topographic expression of a northwest trending trough between the emerging Cordillera and the Aptian Ridge. In the Campanian, the trough was filled in with Belly River sediments, so that the paleoflow swung toward the northeast. </p> <p> Petrographic studies show that these sediments are much like those in the Belly River in the Milk River area, studied by Ogunyemi and Hills (1977) . </p>