Abstract

The upper Liard Formation (Middle Triassic, Upper Ladinian) in the Peace River Foothills of northeastern British Columbia comprises a progradational succession of at least eleven parasequences deposited within a mixed siliciclastic-carbonate depositional system on the western margin of the North American craton. Sediments accumulated along a low gradient shoreface-continental ramp within an embayed/restricted portion of the northwestern Pangean continental margin. Thirteen sedimentary facies are recognized within the following three lithofacies successions: A) progradational clastic offshore/shoreface, B) progradational mixed-siliciclastic carbonate shoreface and C) mixed siliciclastic-carbonate marginal marine. Lithofacies association A is a coarsening-upwards offshore/lower shoreface succession, locally incised by tidal channels. Lithofacies association B consists of a coarsening-upwards, mixed siliciclastic-carbonate shoreface. Thick bioclastic accumulations within this unit are interpreted as terebratulid brachiopod-echinoid dominated reef mounds. Lithofacies association C consists of a mixed siliciclastic-carbonate intertidal-supratidal succession of mud flats, algal mats, evaporites and minor tidal channels. The terebratulid-echinoid reef mounds were initiated by allogenic taphonomic feedback. Storm-generated skeletal concentrations provided "islands" of comparably stable substrate. These islands provided a locus for colonization by rocky substrate-preferring organisms such as terebratulid brachiopods, cidaroid echinoids and articulate crinoids. The Liard reef mounds accumulated during periods of comparably low siliciclastic input to the shoreface, likely during the later stages of a highstand systems tract.

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