Seismic facies, sedimentology, and significance of a lacustrine delta in Neogene Lake Idaho' deposits: Western Snake River Plain, Idaho and Oregon

The top of a buried fine-grained delta system of paleo- Lake Idaho' is detected by high-resolution seismic profiles, 300 m beneath the western Snake River Plain near Caldwell, Idaho. Characteristic 3--5[degree] dip of seismic reflectors in the prodelta-mud facies plus electrical-resistivity logs and cuttings from a 670-m well show a 150-m coarsening-upward prodelta sequence overlain by well-sorted fine sand and thin mud layers. Slope and vertical relief (compaction corrected) of prodelta clinoforms indicate the delta was prograding north into a lake basin 250-m deep. Elevation of the top of the delta front is a measure of a paleolake stand. The detected buried delta front is presently at 405 m elevation, about 500 m below what is regarded as the last high lake stand (elev. 850--975 m). The present low elevation of the delta front is partly explained by about 300 m of downward tectonic movement on faults and about 200 m of subsidence by basin-sediment compaction with respect to the basin margins. Distribution of lake deposits around the northwest end of the lake indicates the lake level reached 850--975 m elevation about 2 million years ago. Location of the former outlet of Lake Idaho' and the ancestral Snake River ismore » still a puzzle. Present geomorphology suggests that about 2 million years ago a southward-migrating, ancestral Hells Canyon tributary of the Columbia-Salmon Rivers system captured the lake drainage by overflow of a sill at Dead Indian Ridge near Weiser, Idaho. Hells Canyon and the enlarged Snake River subsequently cut down about 215 m to the present river elevation of 635 m at Dead Indian Ridge.« less