THE LEWES RIVER GROUP (TRIASSIC) ARC-FRINGING CARBONATE PLATFORM: SEDIMENTOLOGICAL EVIDENCE WHITEHORSE TROUGH WAS AN EAST-FACING FOREARC BASIN

Whitehorse trough in south-central Yukon, Canada, consists of approximately 7000 m of sedimentary and volcanic rocks referred to as the Lewes River Group (Triassic), Laberge Group (Jurassic) and Tantalus Formation (Jura-Cretaceous). The trough is located east of the Lewes River arc (or Stikinia), and has previously been interpreted as a backarc basin based on geochemistry (a calk-alkaline to alkaline trend in volcanic rocks) and structural geology (the ‘Tally Ho shear zone’), and as a forearc basin based on structural geology (the ‘Teslin suture zone’). However, the sedimentology of the basin fill provides important (and overlooked) evidence for the tectonic setting of Whitehorse trough. Specifically, the uppermost part of the Lewes River Group (i.e., the Hancock member of the Aksala formation) is an arc-fringing carbonate platform, 400 km long and 30 km wide, composed of numerous, discontinuous carbonate masses. These are elongate in plan view, approximately one kilometre wide and a few kilometres long, and up to several hundred metres thick. An east-to-west transect of the carbonate masses comprising the platform reveals the following lithofacies: 1) Transported carbonate breccia with outsized blocks (~ 3 m long in outcrop) occur in basin foreslope deposits to the east and demarcate the platform margin. 2) The interior of the platform is characterized by stacked, shallow-water, mixed matrix>skeleton reefs (~130 m thick); shallow-water, mixed skeleton>matrix reefs (~100 m thick); and inter-reef carbonates representing locally derived accumulations of small disarticulated organisms and reef-derived debris. 3) Microbial laminated peritidal dolostone and organic-rich lagoonal carbonates occur along the western edge of the platform and delimit the shoreline. A comparison of carbonate platforms found in forearc basins with those in backarc basins reveals that the size, architecture and lithofacies of the Lewes River Group arc-fringing carbonate platform is more typical of carbonates deposited in a forearc basin. The location of the platform margin to the east (i.e., outsized carbonate blocks) and the shoreline to the west (i.e., peritidal dolostone) indicates that the Lewes River Group carbonate platform was located on an east-facing arc (i.e., westward-directed subduction, present-day coordinates).