Paper No. 195-2
Presentation Time: 2:30 PM-6:30 PM
SURFICIAL GEOLOGY AND HYDROGEOLOGIC FEATURES OF PORT GAMBLE S’KLALLAM TRIBAL TRUST LAND, KITSAP COUNTY, WASHINGTON
New, detailed geologic mapping of the Port Gamble S’Klallam Tribe (PGST) land reveals a landscape dominated by glacial advance and recessional processes from several glacial cycles. The mapping was completed as part of a coastal management plan for the PGST which manages ~2.6 square miles of land on the Kitsap Peninsula of Washington State. Their land includes coastline abutting Port Gamble Bay where retreating bluffs have eroded into the living spaces of tribal members and threaten critically important cultural resources. For the mapping, I used field observations, laboratory analyses, geomorphic analysis, a few geotechnical reports and water well logs, and digital mapping techniques. My resulting map shows between two to eight times as many geologic units as previous maps, at a much finer scale (1:4,000 vs. 1:100,000), and includes hydrogeologic features. Through geomorphic analysis I identified at least 30 unique landforms including a distinct break in slope between uplands and lowlands at ~100 feet elevation which I interpret as a paleoshoreline of Glacial Lake Bretz. Glacial and interglacial deposits provide evidence for at least three distinct glaciations. Due to lack of age-control, I mapped deposits based on their relative stratigraphy and material properties and sorted them into four categories: Holocene deposits, Vashon stade deposits, Pre-Fraser glacial and nonglacial deposits, and Pre-Olympia glacial and nonglacial deposits. Vashon-age deposits drape older deposits and make up the constructional land surface. A major unconformity beneath Vashon-age deposits in the subsurface mimics modern topography suggesting paleotopography. The base of this unconformity likely has significant control on groundwater flow and storage. I also observed a repeating pattern of coarse- over fine-grained deposits which has negative implications for slope stability. My findings provide a record of baseline conditions for the PGST land and products derived from my work will help the tribe understand the implications of geologic mapping in their area and manage their resources for long-term health and enjoyment.