Book Description

The Columbia River Basalt Group (CRBG) is the youngest and volumetrically smallest continental flood basalt exposed across the Pacific Northwest, USA. Similar to other large igneous provinces, the majority of material erupted during the initial 1 million years of activity, these lavas are subdivided into four main-phase units. The Picture Gorge Basalt (PGB) is the only main-phase unit of the CRBG whose age is not precisely known and understanding of PGB petrogenesis is largely based on a limited number of samples when compared with other main-phase units. It is suggested that a time gap of over 0.5 myr existed between eruptions of Steens Basalt and PGB, however my initial results suggest otherwise as I have identified PGB interstratified between Steens Basalt flows in added extent near the Malheur Gorge. This research identifies three primary hypotheses, 1. Lava flows and dikes of the PGB were emplaced earlier than previously recognized and have a longer eruptive duration based on observed stratigraphic relationships, 2. Newly identified exposures of volcanic material geochemically correlated to PGB suggests this CRBG unit erupted across a wide spatial footprint of eastern Oregon, and therefore reflects a larger volume of this continental flood basalt, and 3. There is more than one significant contributing mantle component which yields PGB, and isotopic differences suggest that contributing components were not all depleted. The broader significance of this work expands the initial magmatic footprint for CRBG eruptions and highlights two temporal pulses of eruptive activity in PGB volcanism, also demonstrated by all CRBG ages. The revised distribution area of PGB increases the total eruptive volume of this continental flood basalt and when coupled with ages illustrates a clearer picture of spatial and temporal relationships to other main-phase CRBG units. Geochemical signatures in PGB lavas indicate at least two mantle components which reflect fluctuations in their contributions through time.