A LITHOSTRATIGRAPHIC ANALYSIS OF THE CROOKED RIVER MASCALL FORMATION IN CENTRAL OREGON, USA

This project presents a comprehensive lithological and biostratigraphic record of the Miocene Mascall Formation deposits of the Crooked River Basin in Central Oregon. The Columbia River Basalt Group (CRBG) covered the Crooked River Basin and much of the Pacific Northwest in the middle Miocene (17-13ma), altering the landscape and ecosystem. An analysis of the depositional history of this region in the aftermath of the CRBG eruptions allows us to explore the impact of large scale basalt flows on subsequent basin evolution in a region that lacks extensive lithostratigraphic data. University of Oregon field crews have measured stratigraphic sections in several different locations across the Crooked River basin in order to quantify the differences in depositional history across the basin and reconcile stratigraphy across multiple field expeditions. The regions we have defined for a holistic representation of the region are Twin Buttes in the north, the South Fork of the Crooked River in the south, Cave Basin in the east, and Hawk Rim in the west. They are situated between the lower boundary CRBG and upper capping Rattlesnake Ash Flow Tuff (RAFT). For each region, we have chosen a representative stratigraphic column, and we correlate the depositional units between these disparate areas. The Mascall Formation in the Crooked River Basin is consistent with published descriptions of the Lower Mascall Formation: mostly fine siltstone and sandstone with diatomite, ash, and chert deposits and some tuff strata (Bestland, 1998). Each of the four sites share characteristics of the Lower Mascall which suggests similar depositional environments across the sites; however, the sections vary in the thickness and representation of individual identifiable strata, suggesting variation in where deposition was greatest through the half-million years of the section. This is the first comprehensive assessment of the stratigraphy of the Crooked River Basin, which will have significant implications for understanding landscape reorganization following large-scale basaltic volcanism as well as clarifying stratigraphic relationships between far-flung localities in the Mascall Formation.