TIMING AND PALEOCLIMATIC SIGNIFICANCE OF QUATERNARY GLACIATION IN THE THREE-FINGERED JACK REGION, OREGON HIGH CASCADES

The Oregon Cascades preserve a long record of Pleistocene glaciation, and by extension, the paleoclimate conditions that fostered those glaciations. Until the recent availability of lidar data for much of the range, however, the deposits were difficult to map and interpret because of the dense forest cover obscuring many of the moraines. In our study, we use recently acquired lidar data (DOGAMI, 2010) to reevaluate and refine published mapping of the moraines near Three Fingered Jack (TFJ) in the Oregon High Cascades (Scott, 1977). Our preliminary mapping refines but largely supports Scott’s mapping indicating that the area preserves moraines related to at least four major periods of glaciation, mid-Pleistocene to late Holocene. The lidar data will allow us to reconstruct past glacial extents in detail and to assess quantitatively the paleoclimate conditions related to each of the major advances through mass-balance reconstructions. We will combine these reconstructions with age constraints from sediment cores and terrestrial cosmogenic nuclide (TCN) exposure dating of moraines associated with the major advances. We plan to collect sediment cores from shallow lakes and bogs associated with major moraine positions, dating rock-flour horizons related to the moraines using AMS ¹⁴C analysis of macrofossils in the cores. To date the moraines directly, we will collect suites of rock samples from large boulders perched on the crests of moraines related to the major advances. The dominant bedrock in the TFJ area is olivine- and pyroxene-bearing basalt and basaltic andesite, which represents an ideal lithology for ³He TCN dating. In addition to the exposure samples, we will collect shielded samples from roadcuts to evaluate the potential error related to non-cosmogenic production pathways (e.g., radiogenic, mantle). Our goal is to use the improved mapping, glacier reconstructions, and dating constraints to test regional correlations of glaciations in the Oregon Cascades to records in adjoining mountains of the western US. We will present our mapping and preliminary dating at the conference.