Stratigraphic and Geochemical Evaluation of Distal Flows of the Columbia River Flood Basalts in the Greater Vale Area, Southeastern Oregon

Abstract

Exposures of Columbia River Basalt Group (CRBG) in southeastern Oregon are dominated by the four main phase CRBG units: Steens Basalt, Imnaha Basalt, Grande Ronde Basalt (GRB), and Picture Gorge Basalt (PGB). These units are best seen in stratigraphic sequence along the Malheur Gorge corridor of southeastern Oregon, between the towns of Vale and Juntura, with flows of Steens Basalt from the south overlain by flows of Imnaha and GRB from the north. Recently, PGB flows were found to be part of the eastern Malheur Gorge stratigraphy as well. In Malheur Gorge, local units of Birch and Hunter Creek Basalt have been correlated with GRB units though these units do not geochemically match the stratigraphy of GRB flows from the main outcrop area to the north. Additionally, Webb et al. (2018) found evidence for vent sites of Hunter Creek Basalt along Malheur Gorge and has called into question whether any of the GRB flows exposed in the Malheur Gorge are from the main eruption sites in northeastern Oregon. Furthermore, two other local units found in the Malheur Gorge, the lower Pole Creek and the upper Pole Creek flows, have been previously correlated with Steens and Imnaha Basalt flows, respectively. This study aims to reevaluate the unit identities of CRBG flows in the greater Vale area of southeastern Oregon located immediately north/northeast of Malheur Gorge. This study focuses on mapped CRBG lavas around latitude 44.3N, from near the town of Brogan, OR and east to Weiser, ID, located about 50-70 km north/northeast of Malheur Gorge. We sampled along three transects and include data collected by others from elsewhere in the area. Our new and published data of CRBG lavas for the Brogan-Weiser corridor indicate numerous samples have SiO2 values around 49-52 wt.%, but some are as silicic as local GRB units (Birch and Hunters Creek Basalt) with SiO2 values around 53 wt.%. Given the new CRBG data, we find that the lower silica content samples compositionally overlap with Imnaha and Picture Gorge Basalt. Higher silica samples are unlikely GRB due to stratigraphic constraints outlined herein and thus indicate a general lack of GRB flows in the study area. Our findings suggest that the GRB unit of Birch and Hunter Creek lavas of the Malheur Gorge are locally sourced rather than connected to distal lava flows from the north.