Graduate Thesis Or Dissertation

 

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  • Neogene rocks of the Deschutes basin include the middle Miocene Columbia River Basalt Group and Simtustus Formation, and late Miocene to early Pliocene Deschutes Formation. Assignment of Prineville chemical-type flows to the Grande Ronde Basalt of the Columbia River Basalt Group is based upon correlation of these lavas from their type area, through the Deschutes basin, and onto the Columbia Plateau where they have been previously mapped as Grande Ronde Basalt. Simtustus Formation is a newly defined unit intercalated with and conformable upon these basalts and is unconformably overlain by Deschutes Formation. Burial of mature topography by middle Miocene basalts raised local base levels and initiated aggradation by low-gradient streams within the basin represented by the tuffaceous sandstones and mudstones of the Simtustus Formation. These sediments are enriched in pyroclastic constituents relative to contemporary Western Cascades volcanics reflecting preferential incorporation of easily eroded and more widespread pyroclastic debris in distal sedimentary sequences compared to epiclastic contributions from lavas. Following a 5 to 7 m.y. hiatus, aggradation was renewed at about 7.5 Ma when coarse-grained volcanogenic sediments, lava flows and ignimbrites from the early High Cascades entered the basin for 2 m.y. The proximal Deschutes Formation is primarily basalt and basaltic andesite lava flows but andesite to rhyolite ignimbrites are the primary volcanic constitutents in the sedimentary-dominated section farther east. Deposition on a broad, eastward-tapering alluvial plain was by debris flows, sheetfloods, and hyperconcentrated flood flows. Episodic aggradation correlates to periods of sediment influx following eruptions' of widespread pyroclastic debris and was separated by periods of incision. The abundance of basalts, combined with the paucity of hydrous minerals and FeO and TiO₂ enrichment in intermediate lavas characterize early High Cascade yolcanics as atypical for convergent-margin arcs. These petrologic characteristics are consistent with high-level fractionation in an extensional regime. Extension culminated in the development of an intra-arc graben which ended Deschutes Formation deposition by structurally isolating the basin from the High Cascade source area. Intra-arc extension may represent invasion of Basin and Range tectonism into the Cascades, or may relate to plate-margin processes, particularly decreasing convergence rate and highly oblique convergence vector.
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