Investigation of a Pre-Vashon Interglacial Fine-Grained Organic-Rich Sedimentary Deposit, South Lake Union, Seattle, Washington
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his study evaluates a fine-grained interglacial deposit found in the subsurface of the South Lake Union (SLU) area, Seattle, Washington. The nearly one-square-km SLU study area is defined as north of Denny Way, south of Aloha Street, east of Aurora Avenue, and west of Interstate 5. The evaluation required an in-depth study of over 600 existing geotechnical and environmental boring logs found for the study area. My evaluation consisted of mapping the distribution, determining the depositional environment, and characterizing the organic-rich pre-Vashon deposits. The fine-grained organic-rich deposits correlate to the Olympia formation, which occurred prior to the last glaciation of the Puget Sound Lowland known as the Vashon. To characterize the subsurface conditions in SLU, I assigned the materials described on the boring logs to one of 3 basic layers: pre-Vashon, Olympia Formation (Qob), and Vashon. In the SLU, the Qob consists of grey silt with interbeds of sand and gravel, and has an abundance of organic debris including woody debris such as well-preserved logs and branches, fresh-water diatoms, and fresh-water aquatic deposits such as peat. Several other Qob deposits have been mapped including deposits in the South Puget Sound Lowland (Borden and Troost, 2001), and deposits in the Seattle region (Booth et al., 2003), but no current data exist on the Qob deposits located within the SLU area. Evidence of Qob deposits exists on Capitol Hill, and has been characterized and mapped during a project performed by Aspect Consulting. To evaluate the distribution of the Qob, I prepared a model using the software RockWorks 16 and visited excavation sites. I found that Qob in the SLU region is confined to the western edge of the project area along the Dexter Avenue corridor from John Street on the south to Mercer Street on the north. The constraints of Qob to the western portion of the project area can be attributed to several factors including localized unconformities and historical topography. The past advance and retreat of glaciers through the region likely removed evidence of Qob deposits beyond the valley-like feature found on the western side of the project area. This valley like feature was likely part of a Paleocene drainage to Puget Sound as evidenced by the strike and shallow dip of the valley configuration. The shallow nature of the valley, with a dip of 3.6 percent, was likely an area of wetlands feeding into the lake. Future work could include more observations of Qob, permitting a more detailed model to better understand this unit and the pre-Vashon topography.