THE DEADMAN AND CLARK CREEK FIELDS: INDICATORS OF EARLY TERTIARY VOLCANISM IN AN EXTENSIONAL TECTONIC ENVIRONMENT IN THE NORTHERN TALKEETNA MOUNTAINS, ALASKA

New geologic mapping has refined our understanding of early Tertiary volcanism in the northern Talkeetna Mtns, along and near the central Susitna River. The Clark Creek (>50 km2) and Deadman (>150km2) volcanic fields crosscut the Kahiltna overlap assemblage and provide an important link between Eocene volcanism cross-cutting the Wrangellia terrane and Paleocene volcanism (Cantwell Volcanics) in the Alaska Range.

Scoriaceous basalts and the thinning of basalt flows away from a cinder cone suggest a vent in the southern Deadman field. Thin basalt flows unconformably overlie steeply dipping, low-grade metamorphosed flysch of presumed Jurassic or early Cretaceous age, granitic stocks and migmatitic deformed flysch. Waterlain tuffs interlayered with intermediate(?) composition flows suggest that the northern Deadman field occupied a significant topographic low. Tertiary conglomerates overlying basalt suggest that a basin configuration continued after volcanism. Ar ^40/39 ages indicate an early Middle Eocene age (~48Ma) for basalt flows, but neither the oldest nor the youngest volcanic rocks have been dated.

In the Clark Creek area, volcanic rocks overlie a highly irregular basement of deformed, low-grade metamorphosed (argillite) to migmatitic flysch intruded by Paleocene (57-59Ma) granite and granodiorite. The lowermost map unit is a coarse, matrix-supported heterolithic tuff-breccia and tuffaceous conglomerate containing clasts of both granitoids and flysch. Above this are interlayered tuffs and flows of intermediate to felsic composition, locally cut by abundant hypabyssal dikes and plugs of intermediate composition..

Geologic and age relationships in the Deadman and Clark Creek fields suggests intrusion and contact metamorphism of flysch in Late Paleocene time, followed by rapid uplift and erosion prior to Middle Eocene volcanism. No volcanic activity younger than Eocene has yet been documented. Miocene to Oligocene terrestrial sediments in a half-graben near the Deadman field and moderate dips of some Clark Creek tuffs and flows suggest that tilting and regional uplift continued after volcanism ceased. These features suggest that extensional or transtensional tectonics dominated the area until at least the Neogene.