Geology of Republic Graben and Implications for Eocene Sedimentation in North-Central Portion of the Columbia Basin: ABSTRACT

A sequence of potential hydrocarbon-producing volcaniclastic and siliciclastic Cenozoic sedimentary rocks is preserved in the Republic graben, the largest of a series of en echelon structural depressions that adjoin the Columbia River Basalt in north-central Washington. Gravity and structural trends suggest that the Republic graben and other analogous extensional basins may exist beneath the Columbia basin, thus making analysis of Republic graben sedimentary and stratigraphic trends of paramount importance. Sedimentary and volcanic fill in the 10-16 km by 80 km Republic graben exceeds 3000 m and includes deposits of the Eocene O'Brien Creek Formation, Sanpoil Volcanics, and the Klondike Mountain Formation. Organic-rich sedimentary rocks are concentrated in the upper Sanpoil Volcanics and Klondike Mountain Formation and include thick, highly fossiliferous, and rapidly deposited fluvial, deltaic, and lacustrine deposits interbedded with debris flows and pyroclastic flows and falls. Sedimentary facies in the Sanpoil Volcanics and Klondike Mountain Formation are difficult to trace laterally owing to pervasive intragraben faulting. However, available stratigraphic data suggest that filling was dominantly from the north and west. Organic-rich sedimentary rock in the Klondike Mountain Formation are 300-400 m thick and have total organic carbon values (1-4%) and production induces (0.14) that suggest their promise as hydrocarbonmore » sources. Availability of suitable hydrocarbon reservoir rocks is problematic but such sediments may intertongue with Klondike Mountain Formation equivalent strata in the basin.« less