LITHOFACIES ASSOCIATIONS AND DEPOSITIONAL-ENVIRONMENTS IN A MIXED SILICICLASTIC-CARBONATE COASTAL DEPOSITIONAL SYSTEM, UPPER LIARD FORMATION, TRIASSIC, NORTHEASTERN BRITISH-COLUMBIA

Tile upper Liard Formation (Middle Triassic, Upper Ladinian) in the Pe ace River Foothills of northeastern British Columbia comprises a progr adational succession of at least eleven parasequences deposited within a mixed siliciclastic-carbonate depositional system on the western ma rgin of the North American craton. Sediments accumulated along a low g radient shoreface-continental ramp within an embayed/restricted portio n of the northwestern Pangean continental margin, Thirteen sedimentary facies are recognized within the following three lithofacies successi ons: A) progradational elastic offshore/shoreface, B) progradational m ixed-siliclastic carbonate shoreface and C) mixed siliciclastic-carbon ate marginal marine. Lithofacies association A is a coarsening-upwards offshore/lower shoreface succession, locally incised by tidal channel s. Lithofacies association B consists of a coarsening-upwards, mixed s iliciclastic-carbonate shoreface. Thick bioclastic accumulations withi n this unit are interpreted as terebratulid brachiopod-echinoid domina ted reef mounds. Lithofacies association C consists of a mixed silicic lastic-carbonate intertidal-supratidal succession of mud flats, algal mats, evaporites and minor tidal channels, The terebratulid-echinoid r eef mounds were initiated by allogenic taphonomic feedback, Storm-gene rated skeletal concentrations provided ''islands'' of comparably stabl e substrate, These islands provided a locus for colonization by rocky substrate-preferring organisms such as terebratulid brachiopods, cidar oid echinoids and articulate crinoids, The Liard reef mounds accumulat ed during periods of comparably low siliciclastic input to the shorefa ce, likely during the later stages of a highstand systems tract.