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31-10 Geologic Map of Part of the Shoestring Butte 7.5 Minute Quadrangle, South-Central Oregon: Characterization of a Possible Early Cascades Volcanic Suite

Session: Best Student Geologic Map Competition (Posters)

Poster Booth No.: 213

Presenting Author:

Andrew Bierbower

Authors:

Bierbower, Andrew Allen¹, Gray, Gary², Taylor, Devin³, Bruchmiller, Emily⁴, Johnson, Kenneth⁵

(1) Rice University, Houston, Texas, USA; University of Houston-Downtown, Houston, Texas, USA, (2) Rice University, Houston, Texas, USA, (3) University of Houston-Downtown, Houston, Texas, USA, (4) University of Houston-Downtown, Houston, Texas, USA, (5) University of Houston-Downtown, Department of Natural Science, Houston, TX, USA,

Abstract:

The Paisley Hills, in south-central Oregon, have long been an enigmatic part of Oregon's geologic past. Repetitively interbedded basaltic lava flows and volcanic mudflows suggests this section may be a previously unrecognized part of the Paleogene Cascades arc, or perhaps a continuation of the enigmatic John Day volcanic field. Three areas of the Paisley Hills were mapped at 1:24,000 scale as part of EDMAP award G24AC00191-00. This presentation documents the geologic relationships of a 7.5-minute section of the Shoestring Butte quadrangle. Mapping in this area identified eight primary stratigraphic units bounded by two, northwest-trending, west-dipping normal faults, along with phreatic surge deposits interspersed through the area. The Paisley Hills fault in the east and a smaller, unnamed fault in the west cause a repeating pattern of stratigraphy in the area and are constrained temporally to Basin and Range timing by the appearance of the surge deposits on both the footwall and hanging wall of the Paisley Hill fault. Stratigraphic sequences also dip more steeply the further east you traverse, indicating a listric faulting sequence. Geochronological data conducted on detrital zircons within the phreatic tuff-breccia are abundant with ~24.7 Ma aged zircons with a subordinate population of ~7.2 Ma zircon ages. We interpret this as a depositional age of ~7.2 Ma originating from the erosion of a ~24.7 Ma tuff. This area remains a priority for geologic mapping for the State of Oregon Department of Geology and Mineral Industries due to its potential for primary resources including nearby historic gold and uranium mining, lithium potential, geothermal potential, and environmental concerns.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

Geologic Map of Part of the Shoestring Butte 7.5 Minute Quadrangle, South-Central Oregon: Characterization of a Possible Early Cascades Volcanic Suite

Description

Session Format: Poster

Presentation Date: 10/19/2025

Presentation Room: Hall 1

Poster Booth No.: 213

Author Availability: 9:00–11:00 a.m.

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