Early Mesozoic Silt-Dominated Coastal Successions on the Western Margin of the North American Craton

Abstract

The depositional dynamics of fine-grained coastlines received scant attention in the published literature until recently. The development of new technologies and the focus on unconventional hydrocarbon resources has shifted the focus from comparably small, coarse-grained clastic reservoirs to areally extensive, finer-grained intervals, both in basin-centered plays as well as shallow marine coastal successions. The recognition that many of the world's coastlines are mud-dominated resulted in a mudrock renaissance, with considerable attention focused on clay and mud-dominated successions. The shift in focus to mud-dominated deposition has focused primarily on successions dominated by shale. Published analyses of coastal successions dominated by silt / siltstone remain comparably rare despite their economic importance. The Sulphur Mountain Formation (outcrop equivalent of the Montney Formation) crops out in the Foothills and Front Ranges of the Canadian Rocky Mountains in Alberta and British Columbia. This succession is dominated by siltstone with subordinate sandstone. Clay, although present, is minor. This study focuses on Sulphur Mountain outcrop in the Kananaskis-Canmore-Banff area of western Alberta. Siltstone and very fine-sandstone successions in the southeastern part of the study area were deposited in a proximal clastic ramp succession. The prevalence of wave-ripples and common hummocky cross-stratification indicate deposition within a shoreface setting. Most bed surfaces, including those characterized by wave and current ripples, exhibit wrinkle structures indicating microbial stabilization of sediment surfaces and a paucity of grazing invertebrates in the early Triassic Ocean. Small-scale convolute horizons reflect storm loading and concomitant en masse bed movement of microbial-bound sediment. Large-scale (> meter-scale) convolute beds reflect tectonic influences on basin subsidence and sediment movement. Successions in the northwestern part of the study area were deposited in a distal clastic ramp succession. Interbedding of hummocky cross-stratified beds with turbidities units underscore the shallow nature of the Montney-Sulphur Mountain depositional basin. Even in the most distal localities, the interval is siltstone dominated and the clay component, although present, is subordinate. The unique grain-size distribution of the study interval is interpreted to reflect an arid sediment source area and mature sediments in the source area.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016