NEOGENE TRANSPRESSIONAL BASIN DEVELOPMENT ALONG THE DENALI FAULT SYSTEM IN THE EASTERN ALASKA RANGE: THE STRATIGRAPHY OF THE MCCALLUM BASIN AND IMPLICATIONS FOR DISPLACEMENT ALONG THE DENALI FAULT SYSTEM

Analysis of sedimentary and volcanic strata in the McCallum basin provides new insights into the tectonic evolution of the eastern Alaska Range and the Neogene displacement history of the Denali Fault system. The McCallum basin is located near the head waters of the Delta River within the regional footwall of a series of dextral-oblique reverse faults that parallel the south-side of the Denali fault. Our data set consists of detailed measured stratigraphic sections along with paleocurrent data, compositional data, detrital zircon geochronology from sandstone, U-Pb geochronology from igneous clasts in conglomerate, and ⁴⁰Ar/ ³⁹Ar geochronology on dacite clasts in conglomerate.

The basin is characterized by two stratigraphically distinct members. The lower member consists of organic rich mudstone, coal, and volcanic strata. This member has an estimated minimum thickness of 90 m. A previously dated tuff with a K-Ar date of ~ 5.4 Ma and several new unpublished ⁴⁰Ar/ ³⁹Ar tuff ages suggest that the lower member was deposited ~ 5 to 6 Ma. In some exposures, these ~5 Ma strata are folded with vertical and overturned bedding.

The upper member is dominated by cobble conglomerate that is interbedded with coarse sandstone and minor coal and tuff strata; this unit has an estimated minimum thickness of 215 m. The dominant clast type in the conglomerate is metabasalt. Metabasalt is a common lithology in the hanging walls of the nearby reverse faults. Preliminary detrital zircon geochronology from sandstone in the upper member record dominant peak ages of ~25 Ma, ~100 Ma, and ~155 Ma. Consistent paleocurrent indicators suggest southwest sediment transport. Beds in the upper member dip up to 60°.

Our working interpretation is that the McCallum basin records the development of a transpressional thrust belt/ foreland basin system that formed along the regional transition from strike-slip to compressional deformation along the Denali fault system. The steeply dipping strata in this basin indicate that the Denali fault system and adjacent splay faults in this area have been active in the past ~5 Ma. Exposed strata in the McCallum region provide an opportunity to link sedimentological, structural, and thermochronologic data to study in detail transpressional deformation along an active strike-slip system.