1963/01/01 by Donald E. Trimble · 5 citations
Computer Science · Earth and Planetary Sciences · #Archaeology #Geochemistry and Geologic Mapping #Geography #Geological Modeling and Analysis #Geological Studies and Exploration #Geology #Mining engineering
paper · pdf · doi:10.3133/b1119
openalex publication_date 1963/01/01 · openalex created_date 2016/06/24 · openalex updated_date 2025/11/06
The geologic investigation and mapping of five 15-minute quadrangles embodying about 1,040 square miles within a radius of 25 miles of Portland, Oreg., are the basis for this report.The study has resulted in redefinition of the basic stratigraphy of the area, has yielded data on the nature of laterization and the time required for the formation of several profiles of weathering including fully developed laterite and has permitted a fuller understanding of late Quaternary history related to flooding of this area.A previously unknown laterite horizon was discovered during the geological investigation.The rocks of the Portland area range in age from late Eocene to Recent.The Tertiary rocks comprise four formations of volcanic origin: the Skamania volcanic series, the Columbia River basalt, the Rhododendron formation, and the Boring lava (Tertiary and Quaternary) ; the Silver Star granodiorite; the marine Scappoose formation; and two sedimentary formations of terrestial origin, the Sandy River mudstone and the Troutdale formation.The Scappoose formation, of late Oligocene and early Miocene age, is" the marine sedimentary equivalent, in part, of the upper part of the Skamania volcanic series, the oldest rocks in the area.The Skamania was intruded by the Silver Star granodiorite before the Columbia River basalt was erupted in middle Miocene.The Rhododendron formation, consisting primarily of volcanic inudflow breccia in this area, was erupted in late Miocene onto a slightly warped surface of the Columbia River basalt on the west flank of the growing Cascade Range in Oregon.The surface of the Rhododendron formation was later ized.The terrestrial sediments of the Sandy River mudstone and the Troutdale formation were deposited in early Pliocene in structural basins formed by the warping of the Columbia River basalt and the Rhododendron formation.The Boring lava was erupted discontinuously on a post-Troutdale erosional surface.Late Tertiary and Quaternary deposits include several formations of similar physical character and origin, but -distinguished by differences in relative topographic position and degree of weathering.These units are, from oldest to youngest: the Walters Hill, the Springwater, the Gresham, and tlie'Estacada formations.These formations all consist of mudflow deposits and fluvially deposited bouldery cobble gravel.A. loessial deposit, younger than Springwater but older than the Gresham, is distributed south and west of the valley of the lower Columbia River, and thins' away from the valley.Lacustrine deposits of late(?) Pleistocene age form an extensive fill in the valley of the lower Columbia River, the Willamette Valley, and the Tualatin Valley.These deposits were emplaced by flood waters of great volume that had their source in the sudden release of glacial Lake Missoula.The flood water was successively impounded behind several upstream channel constrictions and finally in the valley system of this area by hydraulic damming.