Rates, Controls, and Sand-Body Geometries of Transgressive-Regressive Cycles: Cretaceous Moosebar and Gates Formations, British Columbia

Abstract

The Moosebar and Gates Formations (Lower Cretaceous) in northeastern British Columbia consist of seven transgressive-regressive sequence. Nonmarine sediments are thickest in the south and pinch out northward. The sequences are laterally persistent, extending east from the foothills for 240 km (150 mi). The orientation of maximum transgressive and regressive limits in five cycles occurs within an east-west-trending zone 30-40 km (19-25 mi) wide. A recurring, northward-bulging sediment depocenter near the Alberta-British Columbia border was fed by a series of stacked, northerly flowing rivers. The rivers were a major source of sediment for gravel beaches to the north and east. Isopach maps of marine sandstone and conglomerate show a series of west-northwest-east-southeast-o iented sand bodies 60-70 km (37-43 mi) wide, parallel with the shorelines. Each coastline prograded northward as a shoreline-attached strand plain and a series of arcuate or cuspate deltas. Gates Formation marine-nonmarine cyclicity is a series of fourth-order cycles superimposed on a major third-order cycle. The cyclicity and lateral extent of the sequences can be explained as a result of tectonic thrusting and loading superimposed on the trend of the Peace River arch. The influence of eustatic sea level variations and sedimentologic switching of point sources had minimal effect. Coastal retreat ranged from 437 to 875 m/1,000 years (1,433 to 2,780 ft/1,000 years). Coastal progradation ranged from 218 to 437 m/1,000 years (175 to 1,433 ft/1,000 years). The duration of several transgressive-regressive cycles was 103,000-275,000 years.