STREAM-DOMINATED ALLUVIAL-FAN AND LACUSTRINE DEPOSITIONAL SYSTEMS IN CENOZOIC STRIKE-SLIP BASINS, DENALI FAULT SYSTEM, YUKON-TERRITORY, CANADA

Ancient stream-dominated ('wet') alluvial fan deposits have received f ar less attention in the literature than their arid/semi-arid counterp arts. The Cenozoic basin fills along the Denali fault system of the no rthwestern Canadian Cordillera provide excellent examples of stream-do minated alluvial fan deposits because they developed during the Eocene Oligocene temperate climatic regime in an active strike-slip orogen. The Amphitheatre Formation filled several strike-slip basins in Yukon Territory and consists of up to 1200 m of coarse siliciclastic rocks a nd coal. Detailed facies analysis, conglomerate: sandstone percentages (C:S), maximum particle size (MPS) distribution, and palaeocurrent an alysis of the Amphitheatre Formation in two of these strike-slip basin s document the transition from proximal, to middle, to distal and frin ging environments within ancient stream-dominated alluvial-fan systems . Proximal fan deposits in the Bates Lake Basin are characterized hy d isorganized. clast-supported, boulder conglomerate and minor matrix(mu d)-supported conglomerate. Proximal facies are located along the fault ed basin margins in areas where C:S = 80 to 100 and where the average MPS ranges from 30 to 60 cm. Proximal fan deposits grade into middle f an, channelized, well organized cobble conglomerates that form upward fining sequences. with an average thickness of 7 m. Middle fan deposit s grade basinward into well-sorted, laterally continuous beds of norma lly graded sandstone interbedded with trough cross-stratified sandston e. These distal fan deposits are characteristic of areas where C:S = 2 0 to 40 and where the average MPS ranges from 5 to 15 cm. Fan fringe d eposits consist of lacustrine and axial fluvial facies. Palaeogeograph ic reconstruction of the Bates Lake Basin indicates that alluvial-fan sedimentation was concentrated in three parts of the basin. The larges t alluvial-fan system abutted the strike-slip Duke River fault, and pr ograded westward across the axis of the basin. Two smaller. coarser gr ained fans prograded syntaxially northward from the normal-faulted sou thern basin margin. Facies analysis of the Burwash Basin indicates a s imilar transition from proximal to distal, stream-dominated alluvial f an environments, but with several key differences. Middle-fan deposits in the Burwash Basin define upward coarsening sequences 50 to 60 m th ick composed of fine-grained lithofacies and coal in the lower part, t rough cross-stratified sandstone in the middle, and conglomerate in th e upper part of the sequence. Upward-coarsening sequences. 90 140 m th ick, also are common in the fan fringe lacustrine deposits. These sequ ences coarsen upward from mudstone, through fine grained, ripple-lamin ated sandstone, to coarse grained trough cross-stratified sandstone. T he upward-coarsening sequences are basinwide, facies independent, and probably represent progradation of stream-dominated alluvial-fan depos itional systems. Coal distribution in the Amphitheatre Formation is cl osely coupled with predominant depositional processes on stream-domina ted alluvial fans. The thickest coal seams occur in the most proximal part of the basin fill and in marginal lacustrine deposits. Coal devel opment in the intervening middle and distal fan areas was suppressed b y the high frequency of unconfined flow events and lateral channel mob ility.