STRATIGRAPHIC AND GEOCHRONOLOGICAL EVIDENCE FOR A TIME-TRANSGRESSIVE UNCONFORMITY AND SUBSEQUENT BASALT EXTRUSION EVENT ASSOCIATED WITH RIDGE SUBDUCTION, PALEOGENE ALASKA

A well-studied example of ridge subduction is recorded in the west-to-east time transgression of 61-50 Ma near-trench plutons in a 2100-km-long belt in the Paleogene forearc of southern Alaska (Bradley, et al., 2003, GSA Sp Pap 371, 1-49). We document additional evidence for the ridge subduction model by describing the presence of a coeval west-to-east time-transgressive unconformity developed in the Paleogene forearc. First, on the Alaska Peninsula, an overall west-to-east thickening of strata exists between a sequence bounding geosol (regionally correlative paleosol) and the time-transgressive unconformity. At Pavlov Bay, 150 km along the 2100-km-long transect, a rooted inceptisol with a pervasively burrowed and pyritized upper surface lies ~125 m below the unconformity, whereas ~300 km further east near Chignik Lagoon, a prominent coal seam with associated massively rooted sandstones overlain by pyritized inclined heterolithic strata, interpreted to represent tidal bundles, lies nearly 400 m below the unconformity. These observations are interpreted to indicate that subduction of a buoyant ridge led to uplift of the forearc seafloor to shallow depths above wave base where erosive forces cut the unconformity. Subsequently, basalt was extruded onto this surface in places, while to the east sediment accumulation continued at depths below erosive wave base in the forearc. The locus of erosion migrated along the forearc with the time-transgressing locus of ridge subduction-induced uplift. New U-Pb ages complement the stratigraphic evidence. On the Alaska Peninsula, the Paleogene Tolstoi Fm (shallow forearc) is unconformably overlain by the 60.6 +/- 3.6 Ma Meshik volcanics near Chignik Lagoon; this age closely matches the age of nearby near-trench plutons, and indicates that the unconformity must be older than ~60.6 Ma. In the Matanuska Valley, the Paleogene Chickaloon Fm (coal-bearing) is unconformably overlain by Wishbone Fm conglomerates, and in places, basalts. Age dating on the basalts is ongoing; a new date from the upper Chickaloon Fm (56 +/- 3.6 Ma) constrains the age of the unconformity as younger than ~56 Ma, a date which is supportive of the ridge subduction model. We propose further work to constrain the stratigraphic development of these features.