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Abstract: History of Neogene Lake Idaho from Geologic Mapping of the NE Margin and Subsurface Data of the Western Snake River Plain.

WOOD, SPENCER H., Geosciences, Boise State University, Boise, Idaho 83725; EDWARD SQUIRES, United Water Idaho, Boise, Idaho 83709

The western Snake River Plain has been a site of lacustrine deposition from about 10 Ma until 2 Ma. At greatest extent Pliocene Lake Idaho covered at least 13,000 km2 and was at least 250 m deep. The section of lacustrine sediment exceeds 1800 m in thickness. This paper illustrates outcrop and subsurface geology used to interpret key features of the basin history and a model of lacustrine deposition for the Neogene Idaho Group. The following model attempts to incorporate previous work of H.E. Malde, Smith et. al. (1982) Swirydczuk et. al (1982), Kimmel et. al., (1982), Clemens (1993) and Repenning et. al. (1994).

Much of the graben relief was probably generated between 11 and 8 Ma. The basal Poison Creek Formation is characterized by coarse arkosic fluvial sand interbedded with fine sediments similar to the overlying Chalk Hills Formation. The Chalk Hills Formation is generally of non-calcareous tuffaceous claystone and mudstone with thick silicic ash beds (0.1 to 15 m) and a few basalt lapilli layers that become thinner and finer upwards in the section. Gypsum occurs locally as veins and partings in some claystone intervals, but diverse fish fauna within the formation indicate mostly a fresh-water lake. The Chalk Hills lake apparently rose to an elevation of about 1000 m, and then fell a few hundred meters exposing tracts of lake sediment to erosion and soil formation; however, the lacustrine environment persisted. Topographic relations of Chalk Hills sediments are disturbed by downwarp and normal faulting, and this early lake history, prior to 6 Ma is poorly understood.

About 6 Ma the lake level began rising to a still higher level, somewhere between 1040 and 1160 m. Water of the rising lake reached high alkalinity favoring carbonate mud deposition and high-Mg oolitic shoreline sands - suggesting the lake did not ave an outlet during this rise. The lake was of low enough salinity to maintain a diverse fish fauna.

About 4 Ma the lake spilled over into the ancestral Hells Canyon - Columbia River drainage. Subsequent lowering of base level associated with the canyon deepening was probably slow (10 to 15 cm/1000 years). Much of the deposition of the Glenns Ferry Formation was probably associated with this declining base level. Progressive filling of the lake basin by delta and delta plain sediments of marginal streams and the main axial stream prograded to the northwest. These streams eroded sands previously accumulated on the margins of the lake and deposited prograding sand units intercalated with mudstone into the remaining lake basin, producing a distinctly more permeable sequence up to 500 m thick. This upper sandy sequence contains the principal aquifers of the western plain.

Between 2.6 and 1.7 Ma fluvial systems crossed the entire plain and reached the Hells Canyon outlet which was then at about 790 m elevation. These rivers systems deposited sandy cobble gravels typically 20 m thick. During the Quaternary the river systems have incised about 200 m, and the threshold of Hells Canyon is now at altitude 634 m.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah