SEDIMENTARY AND VOLCANIC RECORD OF NEOGENE TRANSPRESSIONAL FORELAND BASIN DEVELOPMENT ALONG THE CENTRAL DENALI FAULT SYSTEM, EASTERN ALASKA RANGE

Analysis of sedimentary and volcanic strata of the Miocene-Pliocene McCallum Formation provides new insight into Neogene transpressional foreland basin development and strike-slip displacement along the central Denali Fault system (CDF). The McCallum basin is located within the regional footwall of a series of dextral-oblique reverse faults that parallel the south side of the CDF.

The McCallum Formation is defined by a two-part stratigraphy consisting of a lower and upper member with a minimum thickness of ~ 457 m. Our dataset consists of measured stratigraphic sections, paleocurrent data, clast composition data from conglomerate, U-Pb detrital zircon geochronology from sandstone, and ³⁹Ar/ ⁴⁰Ar geochronology on volcanic glass.

The lower member has a minimum thickness of ~181 m and is characterized primarily by laminated mudstone with minor pebble conglomerate, lignite, and tephra layers. Tephra ages in this member range from 6.01 to 5.07 Ma. Detrital zircon geochronology (n =1167) results indicate that the sources of these sediments have dominant ages of 5, 26, 49, 92, 123, 158, 193 and 356 Ma. The upper member has a minimum thickness of ~ 276 m and is characterized by cobble conglomerate with sandstone lenses interbedded with granular conglomerate and minor tephra. Tephra ages in this member range from 4.91 to 3.70 Ma. Detrital zircon geochronology (n = 904) results indicate that the sources of these sediments have dominant ages of 5, 26, 87, 94, 158, 189, 197 and 299 Ma.

We interpret the McCallum Formation to document the development of a Late Miocene transpressional foreland basin filled by a regional lacustrine system with adjacent peat swamps. Early Pliocene southward thrust sheet propagation is marked by the development of stream-dominated, alluvial fan systems. The preservation of tephras throughout the McCallum Formation suggests that basin subsidence was rapid. Detrital zircon results suggest that sediment was primarily derived from igneous sources located south of the CDF. These datasets provide the first detailed chronostratigraphic framework for the McCallum Formation and better define Neogene strike-slip displacement along the CDF.