Robbins, Demian J.C.1, George Pemberton1
(1) University of Alberta, Edmonton, AB
ABSTRACT: Sedimentology and Ichnology in the Upper Triassic Baldonnel Formation (western Canada): Implications for Palaeoenvironmental Analysis
The Upper Triassic Baldonnel Formation of west-central Alberta and northeastern British Columbia is a mixed siliciclastic-carbonate unit that represents part of the last major episode of carbonate deposition in the Western Canada Sedimentary Basin. The Baldonnel represents the lower portion of a significant but gradual transgression that continued into the overlying, deeper-marine carbonates of the Pardonet Formation.
Baldonnel successions are dominated by locally dolomitized fine (~0.5 mm) to medium (2-4 mm) grained crinoid ossicle - brachiopod fragment wackestones and packstones that also serve as the primary reservoir facies. This carbonate succession is frequently interrupted by siliciclastic mudstones, siltstones and fine-grained sandstones.
Four major facies are recognized as predominant within the Baldonnel Formation. Dark, organic-rich mudstones incorporating chert and chalcedony nodules were deposited in restricted environments and/or during hiatal episodes. Sandstones, typically structureless but locally exhibiting wavy lamination and small-scale ripple- and cross-lamination, represent storm-reworked shallow shelf sand banks and bars. Crinoid-brachipod skeletal wackestones and packstones are the dominant facies. These represent persistent, large, detrital shoals and bars, presumably originating as storm-transported deposits subsequently reworked by fair-weather processes. Laminated algal carbonates, locally sandy, represent very shallow subtidal to possibly intertidal environments along the Baldonnel shoreline and possibly on top of emergent shoals.
Burrowed fabrics in the Baldonnel include opportunistic suites within storm-deposited sands, pervasive deposit-feeding structures within bioclastic wackestones, and persistent development of Glossifungites-type exhumation/hiatal surfaces.
Ichnological and sedimentological data together suggest the Baldonnel mixed carbonate-siliciclastic system was deposited on an open, storm-dominated shoreface subject to large storms and intermittent clastic input.
AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.