This is a multidisciplinary study which combines sedimentology and sedimentary petrology, palynology, organic petrology and structural analysis of the middle Cenozoic Amphitheatre Formation in the St. Elias Mountains, and documents syntectonic deposition in strike-slip basins along the Denali fault system. The outcrops of the Amphitheatre Formation can be divided into two discrete basins:: the northern Burwash basin and the southern Bates Lake basin.
Sedimentological analysis of the Burwash basin identified fault-controlled depocentres which allowed the development of several different types of non-marine depositional environments in close proximity. Palynology and organic petrology data indicate that the Amphitheatre Formation is diachronous and spans the Eocene-Oligocene boundary.
Light-mineral provenance studies of sandstones, clast-counts in conglomerates and paleocurrent analyses suggest that the Wrangellia and Yukon Crystalline Terranes were sources for the Amphitheatre Formation in the Burwash basin. In contrast, preliminary work suggests that Wrangellia and possibly the Gravina-Nutzotin Terrane may have been important sources for the Amphitheatre Formation in the Bates Lake basin.
Structural data combined with geologic mapping indicate a predominance of strike-slip deformation during and after deposition of the Amphitheatre Formation in the Burwash basin. The presence of syndepositional faults with subhoriziontal slickenside indicates that strike-slip deformation occurred during deposition of the Amphitheatre sediments. The Burwash basin contains structures indicative of both contractional and strike-slip deformation, whereas the Bates Lake basin contains structures indicative of both extensional and strike-slip deformation. The change in structural style between the two basins suggests that the Amphitheatre Formation may have been deposited in transpressional as well as transtensional tectonic settings along individual segments of the Denali fault system during the middle Cenozoic.
