POSSIBLE ICE-BOUNDED LAVA FLOWS OF THE BEAR CREEK MOUNTAIN ERUPTIVE STAGE OF THE GOAT ROCKS VOLCANIC COMPLEX SOUTHERN WASHINGTON CASCADES

The large (6.6 km³) Tieton andesite (Qta 1, 1.64 Ma) is 74 km long and has features consistent with ice and water interactions, proximal and distal to the vent area at Bear Creek Mountain. This study examines ice + water interaction features in the Tieton andesite flow unit Qta1 as an example. Observations are supported by unusual flow thickness, features consistent with ice/water-contact, and emplacement during the time of the Stuck Drift (1.6 Ma) support the proposal the lava flowed beside and within a valley glacier that filled the ancestral Tieton River. Field evidence of ice/water-interaction include vertical basal columnar joints (30-50 cm diameter, <30 m in length) originated normal to lower contact, and basal breccias (0-10 cm), and a gradual to sharp transition to the flow exterior of thick entablature. All columns show pinch-and-swell features. The entablature groundmass is glassy. Flow interior is porphyritic, microcrystalline with glass. Flow margins are glassy sub-horizontal columns 20-40 cm in diameter, 2-3 m in length, and an outer margin 0-5 m wide which banks against and under the overlying basaltic andesite (Qba). The subsequent basaltic andesite melted the confining ice and banked against and overflowed the Tieton andesite. Given the evidence for lava and ice/water- interaction, the Tieton andesite shows a range of features with variable ice/water- interaction ratios along its length from Bear Creek Mountain to the confluence with the Naches River. Furthermore, the Tieton andesite Qta 1 has similar features to those described at Ruapehu volcano, New Zealand, and Mount Rainier volcano, USA.