T. R. Wiseman, M. W. Tustian, and S. A. Reader
bp Canada Energy Company, 240 4th Ave. S.W., Calgary, AB., Canada, T3C 3J2
ABSTRACT: Sequence Stratigraphy of the Gething and Bluesky Formations, Western Canada Sedimentary Basin: Prediction of Systems Tract Development, Reservoir Performance and Deep Basin Conditions.
The Gething and Bluesky Formations of Alberta and northeast British Columbia record an early infill of the Cretaceous Western Canada Sedimentary Basin. The several hundred meter thick Gething Formation has low net to gross cross-bedded sandstones, coals, rooted mudstones, and brackish lacustrine mudstones, which are interpreted to be deposits of a low energy fluvial system. Channel sandstones are meters thick, several hundreds of meters in width and are poorly connected. Upsection, at the Gething and Bluesky transition, thicker, multi-storey, cross bedded sandstones abruptly occur, which have excellent connectivity along northwest-southeast oriented trends which are several kilometers in width and tens of kilometers in length. These erosively-based sandstones are interpreted to be incised valleys filled with fluvio-estuarine deposits. Individual fills may fine upwards to a capping laterally-extensive bioturbated mudstone. Downdip of an incised valley are complexly-organized progradational packages of Bluesky Formation sandstones, mudstones, and coaly sediments, which are interpreted as northward offlapping deltaic successions.
High frequency sequences in this succession are composed of a valley incised into the upper Gething, a downdip sandy progradational succession that comprises a segment of the defined Bluesky, a capping marine shale, and in some cases a transgressive succession of shallow marine deposits. There may also have been systematic changes in the depositional system through time associated with this long-term transgression. Regionally these sequences progressively backstep, recording a long-term southward transgression of the Boreal Sea. In any vertical transition, the Gething and Bluesky Formations are separated by a sequence boundary. Because the sequences are organised into a backstepping sequence set, sequence boundaries in individual sections become younger to the south. Within each sequence, lowstand deltas stack in an en echelon pattern along strike from east to west with these units becoming progressively younger toward the west. Early shorelines in Alberta appear to have been sandy and waveinfluenced, and valleys appear to have been wide embayments backfilled with fluvioestuarine sediments. Later shorelines tended to become progressively more conglomeratic westward into British Columbia. The Gething-Bluesky succession is capped by a basinwide unconformity at the base of the Wilrich Member of the Spirit River Formation. The entire Gething to Bluesky succession reflects the evolution from low energy, nonmarine conditions to estuarine/marginal marine and fully marine environments under the influence of rising base level
AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado