Nonmarine Carbonates of Neogene Lake Idaho in Utah: ABSTRACT

Abstract

Nonmarine carbonate sequences are not well known in Neogene rocks. Best known, perhaps, are two lacustrine sequences recently described in the Pliocene of western North America. One, in the Ridge Basin of southern California, contains numerous beds about one meter thick of stromatolitic and oolitic limestone interbedded with non-carbonate clastic rocks. The other, in southern Idaho, is the Glenns Ferry oolite, a 10-m thick carbonate sand unit exposed for 45 km along the western margin of the Snake River Plain. The latter unit is a deposit of the extensive Miocene-Pliocene lake system called Lake Idaho, and is considered the largest freshwater lacustrine oolitic carbonate sequence known in the rock record. New End_Page 955------------------------------ evidence from fossil fishes indicates that Lake Idaho extended into northern Utah, where it is recorded in an uninterrupted sequence 60 m thick of diverse carbonate facies in the fluviolacustrine Salt Lake Formation. In Utah, the lake carbonates are best exposed in a landslide scarp in the Junction Hills, near the northern end of the Wasatch Range. Distinctive carbonate facies in the scarp and nearby exposures include: (1) cross-stratified oolitic calcarenite, in which ooids range from 0.1 to 4.0 mm in diameter, in foreset units from a few cm to 17 m thick; (2) algal stromatolites in cabbage-shaped heads and laterally linked hemispheres as high as 50 cm; (3) skeletal carbonate beds composed predominantly of tests of ostracods and mollusks; (4) convoluted bodies of contemporaneously slumped carbonate sand up to 3 m thick; and (5) a 55-m thick unit of lithographic limestone overlying the scarp sequence. These lake beds appear to represent the thickest and most diverse succession of nonmarine Neogene carbonates known in North America. End_of_Article - Last_Page 956------------