NEW CONSTRAINTS ON THE TIMING OF NEOGENE FILLING AND INCISION OF THE DALLES BASIN, OREGON AND WASHINGTON

Ongoing geologic mapping near The Dalles, Oregon provides insight into the late Miocene to Quaternary history of the The Dalles basin, a depocenter in the backarc of the Cascade Range near the southwestern margin of the Columbia Plateau. The basin is bound to the north by the Columbia Hills anticline and to the south by less-pronounced folds, all related to north-south compression of the ~16-10 Ma Columbia River Basalt Group in the Yakima fold province. The basin was filled during Late Miocene time by volcaniclastic material shed from Cascade Range volcanoes to the southwest and by fluvial material derived from the Blue Mountains province to the east. Deposits from these two sources locally interfinger. The 5.4 Ma basalt of Fulton Ridge preserves a shallow paleocanyon in the Blue Mountain fluvial facies near the present-day Deschutes River, with a base about 300 m higher than modern river level. ⁴⁰Ar-³⁹Ar dating and geochemical analyses suggest the basalt of Fulton Ridge may be correlative to the Agency Plains flow of Smith (1986) in the Deschutes Basin near Madras, Oregon. At about 1 Ma, two local volcanic vents erupted basalts that entered the Deschutes and Columbia River canyons and are preserved with bases at elevations within 20 m of modern river levels. Average incision from 5.4 to 1 Ma was about 0.6 mm/yr. Early Pleistocene to modern loess deposits, chiefly derived from Columbia River sand and silt, are widespread and may locally be as thick as 30 m. Latest Pleistocene Missoula floods have left deposits in most valley bottoms and may have influenced the distribution of landslides. Ice-rafted erratics indicate Missoula floods inundated elevations as high as 345 m. Although the Columbia River is the major landscape feature at present in The Dalles basin, it first entered the basin sometime after the 5.4 Ma basalt of Fulton Ridge; deposits of quartzite-bearing gravels show that it previously followed a west-directed course north of the Columbia Hills.