SEISMIC EXPRESSION AND GEOLOGICAL SIGNIFICANCE OF A LACUSTRINE DELTA IN NEOGENE DEPOSITS OF THE WESTERN SNAKE RIVER PLAIN, IDAHO

High-resolution seismic reflection profiles and well data from the wes tern Snake River plain basin are used to identify a buried lacustrine delta system within Neogene Idaho Group sediments near Caldwell, Idaho . The delta system is detected, 305 m (1000 ft) deep, near the center of the basin by progradational clinoform reflections having dips of 2- 5-degrees, a slope typical of prodelta surfaces of modem lacustrine de lta systems. The prodelta slope relief, corrected for compaction, indi cates the delta system prograded northwestward into a lake basin 255 m (837 ft) deep. Resistivity logs in the prodelta mud and clay facies a re characterized by gradual upward increase in resistivity and grain s ize over a thickness of about 100 m (300 ft). Lithology of the prodelt a is mostly calcareous claystone, with several layers of fine sand, so me of which fine upward, indicating a density-flow mechanism of transp ort and deposition. Delta-plain and front sediments are mostly very fi ne-grained, well-sorted sand separated by thin mud layers. These sedim ents produce several to five cycles of horizontal, high-amplitude refl ections with a toplap relationship to prodelta clinoforms. The sands h ave an abrupt lower contact with prodelta muds and have high resistivi ty on logs. In this study, permeable lacustrine sands within a predomi nantly mud and clay section are located by using high-resolution seism ic reflection data. Identification of a delta system in the Idaho Grou p provides insight into the history of Pliocene ''Lake Idaho. '' The p resent depth of the delta/prodelta facies contact of 305 m (1000 ft) i s 445 to 575 m (1460-1900 ft) below the lake deposits on the margins. Estimated subsidence from compaction is 220 m (656 ft), and the remain ing 225 to 325 m (740-1066 ft) is attributed to tectonic downwarping a nd faulting. The original lake area had been reduced to one third of t he original 13,000 km2 (5000 mi2) by the time the delta front prograde d to the Caldwell area. The original lake area may have been sufficien t to evaporate most of the inflow, and the lake may have only occasion ally spilled into other basins. Diminished area for evaporation later in the history of the lake, combined with reduced evaporation accompan ying onset of the ice ages, may have caused the lake to rise and overt op a basin sill about 2 Ma, and subsequently deepen Hells Canyon.