2003 Seattle Annual Meeting (November 2–5, 2003)

Paper No. 8
Presentation Time: 9:45 AM


ANDREW, Joseph E., Department of Geological and Environmental Sciences, Youngstown State Univ, Moser Hall 2120, Youngstown, OH 44555 and WALKER, J. Douglas, Univ Kansas, 120 Lindley Hall, Lawrence, KS 66045-2124, jeandr@wm.edu

We present new stratigraphic, age, structural, and geochemical data on Miocene to Pliocene volcanic rocks in the Slate and Argus ranges. Ar-Ar geochronology shows two periods of volcanism. Rocks in the Slate Range and eastern Argus Range yield Miocene ages between 14.5-13 Ma; volcanics in the axial Argus Range give Pliocene ages between 4.5-4 Ma. The general stratigraphy succession for the Miocene rocks is: basal sandstone and conglomerate overlain by local basalt and rhyolite lava flows; pumiceous epiclastics, andesitic to dacitic laharic deposits; and lava flows dominantly of basaltic-andesite with andesite and lesser volumes of basalt. The succession is capped locally by landslide deposits, which are in turn are locally capped by limestone. Although dismembered by extension, the Miocene rocks probably once formed a continuous volcanic field from the Argus Range to the Owlshead Mountains. Pliocene volcanics are more local in extent and mainly occur on the axial portion of the Argus Range. These volcanics are exclusively basalt lava flows, and are nowhere in contact with any of the Miocene units. The Miocene and Pliocene rocks are isotopically distinct with the younger rocks having more depleted signatures.

The volcanic sequences are cut by numerous faults, and similar sets of faults cut all units: this implies that deformation started after 4 Ma. Faulting roughly synchronous with extrusion of the Pliocene lava flows is recorded by local subvertical dips that probably require flow over fault scarps. This deformation produced tilting of the Miocene and Pliocene volcanic sequence to east-northeast dips. This deformation and tilting also probably led to the formation of the landslide deposits. Some deformation must have occurred just prior to or during the early stages of the Miocene volcanism based on large, abrupt thickness changes of the basal sedimentary and volcanic units. The basal sedimentary deposits vary from zero to over 30 m thickness, with the thickest sections deposited against a buttress unconformity with basement. This structure was a fault scarp, as confirmed by observations of slickensides along strike of this structure. The thickness changes of the epiclastic unit also records early deformation, as the thickest sections of this unit are on the down-dropped side of normal faults.