Russell C. Evarts^1,*, Jim E. O’Connor², Ray E. Wells¹, Ian P. Madin³

1 U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025, USA
2 U.S. Geological Survey, Oregon Water Science Center, 2130 SW 5th Ave., Portland, Oregon 97201, USA
3 Oregon Dept. of Geology and Mineral Industries, 800 NE Oregon Street, Portland, Oregon 97230, USA

Metropolitan Portland, Oregon, USA, lies within a small Neogene to Holocene basin in the forearc of the Cascadia subduction system. Although the basin owes its existence and structural development to its convergent-margin tectonic setting, the stratigraphic architecture of basin-fill deposits chiefly reflects its physiographic position along the lower reaches of the continental-scale Columbia River system. As a result of this globally unique setting, the basin preserves a complex record of aggradation and incision in response to distant as well as local tectonic, volcanic, and climatic events. Voluminous flood basalts, continental and locally derived sediment and volcanic debris, and catastrophic flood deposits all accumulated in an area influenced by contemporaneous tectonic deformation and variations in regional and local base level.

Manuscript received 15 May 2009; accepted 27 May 2009.

doi: 10.1130/GSATG58A.1

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