Hatcher Pass, AK

Complex structural features within the Hatcher Pass Schist, recording evidence for a multistage uplift history

The Willow Creek area in the southernmost Talkeetna Mountains (Fig. 1) remains geologically enigmatic despite hosting one of the largest gold producing lode deposits in Alaska. Like most of AK, the region has seen no official mapping at scales less than inch to mile, and several issues pertaining to the interpreted bedrock geology in light of emerging datasets requires more detailed, and modern, bedrock geologic mapping. Establishing robust field constraints will aid in solving several outstanding issues in the geologic community with implications for: 1) understanding the development of the modern Cordilleran margin, 2) establishing timing and structural controls on critical metal mineralization, and 3) constraining the kinematic history of an active strike slip fault adjacent to highly populated regions in south central AK. Due to road access, superb exposure, and relative proximity to metropolitan areas, this region offers a great opportunity to safely engage a graduate student into modern geologic problems in a remarkably understudied portion of North America.

The bedrock geology of the Willow Creek area (Fig. 2) is composed of four basic units: 1) a suite of variably retrogressed chlorite-muscovite schists with km-scale serpentinite blocks (Schist of Hatcher Pass –HPS); 2) a series of late Cretaceous and mineralized plutonic bodies on the north of the HPS; 3) a late Cretaceous equigranular composite pluton; and 4) an undeformed package of latest Cretaceous conglomerates and feldspathic to lithic arenites (Arkose Ridge Formation; Trop et al., 2003). The Willow Creek area is the southern extent of the composite Wrangellia Composite Terrane (WCT), which was accreted to North America in the latter part of the Cretaceous. The southern Willow Creek area is delineated by the Castle Mountain Fault, which is a part of the larger Border Ranges Fault system with accreted marine deposits to the south. In addition to the relatively simple lithotectonic framework and broad implications for understanding seismic risks, the HPS and Willow Creek plutons are extensively cut by variably dismembered gold-bearing quartz veins, adding an economic component to the project.

Cobalt the geologizing dog standing on a low-angle detachment juxtaposing retrograded Hatcher Pass Schist adjacent to the undeformed Arkose Ridge Formation.