STRATIGRAPHIC RECORD OF PALEOGENE SPREADING RIDGE SUBDUCTION IN SOUTHERN ALASKA AND CENTRAL WASHINGTON

The Pacific Northwest, British Columbia, and southern Alaska are all associated with regional scale, dextral strike-slip faulting during Paleogene time. The Chumstick basin is one of multiple nonmarine Eocene-aged basins located within this zone, and formed in the complex forearc region adjacent to the precursor to the Cascadia arc during both a lull in arc-magmatism, and a period of robust forearc magmatism. Regional deformation, local strike-slip faulting, and forearc magmatism can be attributed to a well-documented event of spreading ridge subduction immediately west of the study area from ~60-40 Ma.

Strata within the basin, the Chumstick Formation, are 8-12 km thick, and contain 15 or more interbedded tuffs erupted from proximal arc and forearc volcanoes. Nonmarine strata were subdivided into 6 lithofacies associations, characterized by boulder-pebble conglomerate, arkosic sandstone, and organic-rich mudstone. Sixteen sandstone samples collected for detrital zircon age analyses and 16 conglomerate clast composition counts document variations in sediment routing systems and source terranes supplying sediments to the Chumstick basin throughout its evolution. Deposition took place after initial basin opening (~49.2 Ma) on braided-stream dominated alluvial fan systems along basin margins, and by meandering stream and lacustrine systems along the basin axis. The basin is a wedge-shaped strike-slip basin that initiated when strike-slip faulting accelerated on a regional scale.

Deposition of late Paleocene—Eocene sedimentary and volcanic strata of the Arkose Ridge Formation in the southern Talkeetna Mountains records fluvial-lacustrine deposition in a forearc basin modified by spreading-ridge subduction beneath southern Alaska. Between 61-50 Ma, the Kula-Resurrection ridge rapidly swept from west to east beneath southern Alaska as it was subducted. In contrast, between 60-40 Ma, the Resurrection-Farallon ridge was oriented perpendicular to the Pacific Northwest coastline as it was subducted. This study will allow for a better understanding of the variations in the timing and types of magmatism, deformation, and sedimentation patterns within sedimentary basins in both Washington and Alaska, as a result of two separate incidents of ridge subduction during the Paloecene to Eocene time.