Stream‐dominated alluvial fan and lacustrine depositional systems in Cenozoic strike‐slip basins, Denali fault system, Yukon Territory, Canada

Abstract

ABSTRACTAncient stream‐dominated (‘wet’) alluvial fan deposits have received far less attention in the literature than their arid/semi‐arid counterparts. The Cenozoic basin fills along the Denali fault system of the northwestern Canadian Cordillera provide excellent examples of stream‐dominated 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 and coal. Detailed facies analysis, conglomerate: sandstone percentages (C:S), maximum particle size (MPS) distribution, and palaeocurrent analysis of the Amphitheatre Formation in two of these strike‐slip basins document the transition from proximal, to middle, to distal and fringing environments within ancient stream‐dominated alluvial‐fan systems.Proximal fan deposits in the Bates Lake Basin are characterized by disorganized, clast‐supported, boulder conglomerate and minor matrix(mud)‐supported conglomerate. Proximal facies are located along the faulted 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 fan, channelized, well organized cobble conglomerates that form upward fining sequences, with an average thickness of 7 m. Middle fan deposits grade basinward into well‐sorted, laterally continuous beds of normally graded sandstone interbedded with trough cross‐stratified sandstone. These distal fan deposits are characteristic of areas where C:S = 20 to 40 and where the average MPS ranges from 5 to 15 cm. Fan fringe deposits consist of lacustrine and axial fluvial facies. Palaeogeographic reconstruction of the Bates Lake Basin indicates that alluvial‐fan sedimentation was concentrated in three parts of the basin. The largest alluvial‐fan system abutted the strike‐slip Duke River fault, and prograded westward across the axis of the basin. Two smaller, coarser grained fans prograded syntaxially northward from the normal‐faulted southern basin margin.Facies analysis of the Burwash Basin indicates a similar transition from proximal to distal, stream‐dominated alluvial fan environments, but with several key differences. Middle‐fan deposits in the Burwash Basin define upward coarsening sequences 50 to 60 m thick composed of fine‐grained lithofacies and coal in the lower part, trough cross‐stratified sandstone in the middle, and conglomerate in the upper part of the sequence. Upward‐coarsening sequences, 90–140 m thick, also are common in the fan fringe lacustrine deposits. These sequences coarsen upward from mudstone, through fine grained, ripple‐laminated sandstone, to coarse grained trough cross‐stratified sandstone. The upward‐coarsening sequences are basinwide, facies independent, and probably represent progradation of stream‐dominated alluvial‐fan depositional systems.Coal distribution in the Amphitheatre Formation is closely coupled with predominant depositional processes on stream‐dominated alluvial fans. The thickest coal seams occur in the most proximal part of the basin fill and in marginal lacustrine deposits. Coal development in the intervening middle and distal fan areas was suppressed by the high frequency of unconfined flow events and lateral channel mobility.