Stratigraphic and provenance data from the Upper Jurassic to Upper Cretaceous Kahiltna assemblage of south-central Alaska

Abstract

Upper Jurassic to Upper Cretaceous marine sedimentary strata of the Kahiltna assemblage occur on the suture between the allochthonous Wrangellia composite terrane and the former Mesozoic continental margin of North America. The Kahiltna assemblage crops out in a northeast-southwest-trending belt that is about 800 km long and about 200 km wide in southern Alaska. Most previous studies described these strata as a monotonous, highly deformed flysch succession. We have measured a series of stratigraphic sections in the Kahiltna assemblage along a traverse from the south contact in the Clearwater and Talkeetna Mountains to the north contact in the Alaska Range. Our data suggest that the thousands of meters of Upper Jurassic to Upper Cretaceous strata which has been grouped together as the Kahiltna assem blage in south-central Alaska may actually represent several different sedimentary basins. Proximal-distal lithofacies trends along the southern part of our traverse in the Clearwater and Talkeetna Mountains indicate deposition by predominately mud rich submarine-fan systems that transported sediment northwestward. Locally, adjacent to the Wrangellia composite terrane, the Kahiltna assemblage consists of proximal submarine-fan conglomerate and sandstone. Compositional data from proximal conglomerates in this area show a dominance of volcanic (greenstone) and sedimentary clasts that probably reflect unroofing of the Wrangellia composite terrane carried in the hanging wall of the nearby Talkeetna thrust fault. Proximal-distal lithofacies trends in the Kahiltna assemblage along the northern part of our traverse suggest deposition by gravel- and sand-rich submarine-fan systems that transported sediment southwestward. Pebble-boulder conglomerates of the Alaska Range are dominated by sandstone, chert, and granitic clasts indicative of a source terrane rich in sedimentary and plutonic rocks. Possible source terranes for the Kahiltna assemblage of the Alaska Range are Paleozoic continental-margin strata north of the Denali Fault system. The Kahiltna assemblage of the Alaska Range is separated from the Kahiltna assemblage of the Clearwater and Talkeetna Mountains by a large thrust block of Paleozoic and Mesozoic strata known as the Chulitna terrane. Upper Jurassic to Upper Cretaceous sedimentary strata, also mapped in previous studies as the Kahiltna assemblage, crop out in the Chulitna terrane. These strata are characterized by massive black shale, chert, and inplace fossiliferous limestone that are distinct from lithofacies of the Kahiltna assemblage of the Talkeetna Mountains and Alaska Range. We interpret the lithofacies of the Chulitna terrane as representing deposition on a bathymetric high relative to submarine-fan lithofacies of the Kahiltna assemblage in the Talkeetna Mountains and Alaska Range.