--> ABSTRACT: Permeable Sands within Lacustrine Muds of the Westem Snake River Plain, Idaho, by S. H. Wood; #91021 (2010)

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Permeable Sands within Lacustrine Muds of the Westem Snake River Plain, Idaho

WOOD, SPENCER H.

Identification of major fluvial systems guided by tectonic physiography is key to finding permeable deposits in the predominantly mud sediment of a lacustrine rift basin. The sedimentary basin of the western Snake River Plain is a NW-SE trending continental rift 70 by 250 km that began subsiding about 10 Ma. Seismic sections show that graben faulting along the NE margin initially formed a 30-km wide basin with 1 km relief on the older Miocene plateau basalts. Subsequently the master fault shifted south and became the SW margin of the 70-km wide basin that accumulated a maximum of 2 km of sediment. Although the upper km of sediment buries the older basin relief, sand bodies over older intrabasin relief may be the result of shoaling and wave reworking of sediment. The final basin evolved into an asymmetric half-graben by faulting and downwarping with greatest subsidence on the SW side.

An axial river system flowed from the E to the NW into the lake basin from highlands with 2 kms of relief associated with the Yellowstone hot spot region. The river sediments prograded NW over the mud sediments of the deep lake producing an upper section of mostly fine-sand delta and floodplain deposits typically 400 m thick.

The region NE of the basin is a mountainous area with about 1.3 km of relief where present precipitation exceeds 1.0m and gives rise to rivers that delivered coarse arkosic sand from the Idaho batholith area. These rivers produced thick fan deposits and coarse-grained delta deposits extending up to 20 km into the basin from NE mountain-basin edge.

The region to the SW is of less relief above the plain (0.6 km), precipitation is less than 0.5 m and rivers tributary to the lake were small. This explains a general scarsity of coarse deposits within the thick mudstone sections along the SW margin except for those attributed to the river.

AAPG Search and Discovery Article #91021©1997 AAPG Annual Convention, Dallas, Texas.