Depositional Environments in Lower Cretaceous Gates Member, Northeastern British Columbia: ABSTRACT

Abstract

The Lower Cretaceous Gates Member, which outcrops in the Rocky Mountain Foothills of northeastern British Columbia, consists of approximately 300 m of marine and nonmarine clastic sediments. The lower unit consists of several coarsening-upward marine cycles, which thicken and increase to the north. Thin coal seams at the top of some of these cycles thicken to the south and pinch out northward. Sediments in the middle unit of the Gates were deposited in a nonmarine environment and include several thick coal seams which are economically important. Fluvial sandstones and conglomerates in this interval were deposited in rivers which flowed in general to the north, sub-parallel to the tectonic strike and into a paleocoastline which was trending approximately east-west. Three m jor fluvial conglomerates are recognized and are interpreted as indicating three pulses of tectonic activity in the source area to the southwest. During the deposition of the upper unit of the Gates, a marine transgression occurred which reworked part of the underlying section and deposited locally a thin marine lag conglomerate. The overlying marine sandstones were deposited in a marine shelf and tidally influenced coastal environment. Coastal sand bodies include fining-up subaqueous channel deposits which grade laterally into coarsening-up sandstone units interpreted as marine shoals. The orientation of these sand bodies is at right angles to the paleocoastline. End_of_Article - Last_Page 557------------