PROVENANCE ANALYSIS OF CHANNEL SANDS IN THE CHICKALOON FORMATION, MATANUSKA VALLEY, ALASKA

Most of southern Alaska’s modern Matanuska Valley lies north of the rocks in the Mesozoic Chugach accretionary wedge (MCAW) and south of mountainous exposures of the Jurassic Talkeetna Volcanic Formation (JTVC) in the Talkeetna Mountains. The MCAW south of the Border Ranges Fault consists of blueschist, mesomelange (altered shale, interbedded sandstone and siltstone, red and black chert, basaltic lava, and recrystallized limestone), and greywacke-conglomerate (Clift et al. 2012). The JTVC north of the Castle Mountain fault consists mostly of basaltic and andesitic lavas and tuffs, as well as volcaniclastic rocks (Clift et al. 2005). The Chickaloon Formation was deposited in the Paleocene and early Eocene within the Matanuska Valley-Talkeetna Mountain forarc basin (Neff et al. 2011), and is thought to have been sourced from these uplifted terrains. Detailed study of Chickaloon facies associations indicate lacustrine and low- sinuosity fluvial depositional settings, with tidally influenced fluvial and estuary systems in the western portion of the basin (Trop et al. 2003).

This study seeks to describe the extent to which the MCAW and JTVC have contributed to the sediment within the medial channel sands of the Chickaloon Formation. Twenty-eight samples of this lithofacies in the Chickaloon Formation were collected from the coarsest deposits at each study site throughout the Matanuska Valley along the east-west basin axis. The majority of these deposits are lithic graywackes and arenites with common calcite cement. Petrographic analyses of twenty-two of these samples reveal that the dominant lithic component of the sand-size fraction in all samples was chert, polycrystalline quartz of metamorphic origin, schist, and less commonly fine grained siliciclastic rock fragments. Volcanic lithic fragments did occur in some samples, primarily in those from the northeastern part of the sampling area, but not in significant amounts. This finding suggests that the Mesozoic Chugach accretionary complex was the dominant sediment source to the sampled Chickaloon Formation channel sandstones. This information provides useful knowledge for reconstructing the evolution of fluvial systems of the Chickaloon Formation, and may have implications for evaluating the amount and timing of motion along the Border Ranges Fault.