Portland State University. Department of Geology.
Marvin H. Beeson
Date of Publication
Master of Science (M.S.) in Geology: Geohydrology
Geology -- Oregon -- Parrett Mountain Region, Structural Geology -- Oregon -- Parrett Mountain Region, Groundwater -- Oregon -- Parrett Mountain Region
1 online resource (3, xiv, 283 p.)
A geologic study of the Parrett Mountain area, located twenty miles (32 kms) south-southwest of Portland, Oregon, was initiated by the Oregon Water Resources Department. The main goal was to create a stratigraphic and structural model of Parrett Mountain in order to better understand the local basalt aquifers present there. Previous geologic studies of the area revealed the mountain to be composed of Columbia River basalt. Field mapping and hand lithologic and geochemical analyses allowed the basalt to be subdivided into eleven basalt flows. These flows are as follows: (from oldest to youngest) the Wapshilla Ridge (WpR) , the Ortley-Grouse Creek (undifferentiated) (OGC), the Umtanum (U), the Winter Water (WW), and the Sentinel Bluffs (SB) basalt units of the Grande Ronde Basalt and the Ginkgo (Gk) flow of the Frenchman Springs Member of the Wanapum Basalt. All the basalt flows were found to be laterally extensive throughout the entire area creating a stacked pancake-layered structure. Thickness variations in the Wapshilla Ridge, Ortley- Grouse Creek and Ginkgo basalt flows reflect paleotopographic relief present during the emplacement of the basalt flows. Sedimentary interbeds appear to be highly localized, occurring sporadically throughout the entire area stratigraphic column. Faults were identified based on geologic map compilation, cross section analysis, and topographic linear features. They trend N-S, E-W, NE and NW and are believed to dissect the entire basalt column. All faults have been designated with a normal sense of displacement, except those known to be thrust or r~verse. Cross-cutting relationships suggest the NS-trending faults are the youngest and the NWtrending faults are the oldest. The faulting created twenty-four separate basalt blocks, each represented by a distinct strike and dip. Flowtop morphology, stratigraphic layering, and the local geologic structures influence local groundwater systems. All flow boundaries yield water to at least one well on the mountain. The most commonly used aquifers are the U/OGC boundary, interflow zones within the OGC, the OGC/WpR boundary and permeable zones within the WpR. Faulting is believed to promote recharge of the groundwater systems by increasing the vertical infiltration of percolating precipitation through the highly fractured fault zone.
Brodersen, Brett Todd, "The Geology of Parrett Mountain, Oregon, and its Influences on the Local Groundwater Systems" (1994). Dissertations and Theses. Paper 5245.