--> Evidence for Climate Change in the Coal Bearing Goose Creek Basin, Cassia County, Idaho, by M. E. Brownfield and J. D. Cathcart; #90986 (1994).

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Abstract: Evidence for Climate Change in the Coal Bearing Goose Creek Basin, Cassia County, Idaho

Michael E. Brownfield, James D. Cathcart

During Miocene time, deposits of sandstone, shale, carbonaceous shale, lignite and tuff accumulated in the Goose Creek basin in a coalescing fluvio-lacustrine depositional setting. Sediments from pre-Miocene rocks, in the nearby highlands, accumulated in alluvial fans along the basin margins. Distal-fan sediments extended into shallow lakes and formed lacustrine fan deltas. Fine sediments and organic matter accumulated in the shallow-lake and lake-margin environments. Pyroclastic debris from a source located to the northwest of the Goose Creek Basin episodically covered the area. The basin was ultimately filled with thick ash-flow and ash-fall tuff deposits.

In the southwestern part of the basin, thick beds of air-fall, rhyolitic, vitric volcanic ash are found interbedded with lacustrine fan-delta deposits. The tuffaceous deposits contain a variety of authigenic minerals, including clinoptilolite, smectite (beidellite), mixed-layer clay (smectite-illite), calcite, gypsum, and pyrite. Clinoptilolite is the primary mineral in the altered tuffs. Above the altered tuffaceous deposits, the stratigraphic section contains diatom-bearing, non-zeolitized tuff, carbonaceous shale, and lignite.

The presence of the zeolites and smectitic clays in the lower tuffaceous unit indicates an alkaline environment, which suggests that evaporation in the basin exceeded rainfall. The overlying diatom-bearing, non-zeolitized tuff and lignite indicate a subsequent freshening of the lake when rainfall exceeded evaporation.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994