26 Structural Geology and Tectonics (Posters)

Wednesday, 6 April 2016
8:30 AM - 12:30 PM
Ballroom C

SEDIMENTOLOGY AND PROVENANCE OF THE EARLY EOCENE SWAUK FORMATION, CENTRAL CASCADES, WASHINGTON

Paper number: 26-8
Program: Technical Sessions
Time: 8:30 AM – 12:30 PM

Authors

Abstract

The Early Eocene Swauk Formation was deposited in a complex forearc setting during a regional shift from transpression to transtension associated with subduction of a spreading ridge beneath western North America. The basin forming mechanism for the Swauk Fm is not well understood; previous workers have suggested that it was a strike-slip basin, modified forearc basin, or a transtensional half graben. Swauk sediments were deposited unconformably on pre-Tertiary basement rocks of the Ingalls Complex, Mount Stuart Batholith, Easton Schist, and Swakane Gneiss. The basin is bound on the east by the Leavenworth fault zone, and by the Straight Creek fault (SCF) on the west. The Swauk and Chuckanut Formations were deposited contemporaneously in a single basin and later partitioned by ~150 km of dextral motion on the SCF. Post-basinal deformation produced NW trending folds and faults that may record the collision of the Siletzia terrane to the Pacific Northwest margin.

Thick (~8000 m) clastic sediments including arkosic sandstone, mudstone, and conglomerate filled the basin from <59.9 Ma to shortly after 51.3 Ma. Seven lithofacies and five lithofacies associations were characterized in the central and western part of the basin in this study and support previous work that the Swauk was deposited in a series of humid alluvial fans, braided and meandering streams, and deltaic-lacustrine systems.

To better understand the provenance and paleogeography of the Swauk Fm, 15 sandstone samples were collected for detrital zircon age analysis. Preliminary results from 7 samples (648 grains) reveal four prominent U-Pb age populations: Late Cretaceous (52%; 172 grains), Jurassic (14%; 91 grains), Early Paleocene (12%; 78 grains), and Early Cretaceous (8%; 53 grains). These results support the existence of a broadly integrated catchment that tapped shallow and deep structural levels of the exhumed ancient magmatic arc to the east and north, and the Siletzia collisional belt to the west.

Geological Society of America Abstracts with Programs. Vol. 48, No. 4

doi: 10.1130/abs/2016CD-274672

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