INVESTIGATION INTO POSSIBLE INTERFINGERING OF FLOOD BASALT LAVAS OF THE PICTURE GORGE BASALT AND STEENS BASALT IN SE OREGON

The Picture Gorge Basalt (PGB) and Steens Basalt are two of the main formations that make up the flood basalts of the Columbia River Basalt Group (CRBG). Lavas of the Steens Basalt were sourced from the Steens dike swarm cropping out along Steens Mountain and surroundings of SE Oregon. Steens Basalt lavas spread across SE Oregon with prominent outcrops along major fault blocks at Poker Jim Ridge (aka Hart Mtn) and Abert Rim to the west and have also been inferred to underlie younger volcanic deposits north from there within the Harney Basin. PGB lavas have been sourced from the Monument Dike swarm of the greater John Day valley area with the main outcrop area largely centered on the area of the dike swarm. Recent works (Cahoon et al., 2020, Geology, https://doi.org/10.1130/G47122.1; Cahoon et al. 2023, Geosphere, https://doi.org/10.1130/GES02508.1), however, have extended the distribution of PGB to farther south and southeast towards the Harney Basin near the town of Burns as well as into Malheur Gorge located between the towns of Juntura and Vale, respectively. These works also reassigned basalt outcrops initially associated with Steens Basalt along the north margin of the Harney Basin as PGB based on chemical correlation. In addition, a PGB-like basalt composition was identified as far south as the latitude of the Steens Basalt flows at Poker Jim Ridge. In addition, PGB and Steens Basalt lavas are now also considered to have started erupting nearly synchronously around 17 Ma.

The above is the impetus for reevaluating exposed basalt lavas at Poker Jim Ridge and Abert Rim with the aim to determine whether PGB lavas emplaced from the north may interfinger with Steens Basalt flows coming from the east. Such interfingering has been recently documented for basal basalt flows in Malheur Gorge by the works of Cahoon et al.

Our initial analysis is based on XRF and ICP-MS major and trace element data of select samples. Early results indicate basal basalt lavas at Poker Jim Ridge lithologically range from fine-grained without sizable phenocrysts to containing large plagioclase phenocrysts that are so typical for Steens Basalt. Furthermore, preliminary compositional evaluation does suggest that a subset of samples align better with PGB and hence suggest PGB lavas have been emplaced as far south as our sample collection sites.