THE PLEISTOCENE TIETON LAVA FLOW FIELD, BEAR CREEK MOUNTAIN ERUPTIVE STAGE OF THE GOAT ROCK VOLCANIC COMPLEX, SOUTHERN CASCADES, WA: EMPLACEMENT FEATURES

The Tieton andesite lavas located in Southern Washington Cascades consist of two lava flows (74 and 52 km in length), and a newly mapped flow field located on the SW portion of Pinegrass Ridge. Mapping has identified a compound flow field consisting of multiple flow lobes of Tieton andesite (Qta2) and basaltic andesites (Qba and Qob2). Mapping was also improved by using lidar. The goal of this study is to improve the resolution on previous mapping to understand the lavas associated with the andesites above Section 3 Lake (Qas31) which are related to Qta2. Lavas in the upper part of the sequence located in the SW portion of Pinegrass Ridge are related to Qas31 (Qta2) and have a flow direction to the NE. Older units comprising Qab and Qob2 have an average flow direction to the E. Physical characteristics of the flows consist of lava flow channels with massive cores surrounded by platy jointing on all sides and breccia levees and bases. In addition, similar lava sheet flows were identified. Breccias are sintered to welded with tongues of glassy lava often present. The morphology of lava flows can be used to determine whether a flow is a simple or compound flow field. However, several parameters need to be considered when using morphology such as, the sum of the effects of viscosity, eruption rate and duration. It has been shown that compound lava flows form by slow effusion, low viscosity magma, and high viscosity favors lavas with fewer flow units. The Pinegrass Ridge flow field appears to be between the two end members.