SEDIMENTARY GEOLOGY OF THE NORTHERN FISH LAKE PLATEAU, CENTRAL UTAH

A heterogeneous, 700-1000 m thick sedimentary sequence is exposed beneath the Oligocene-Miocene volcanics that cap the Fish Lake Plateau in south-central Utah. These units range in age from late Cretaceous to Eocene and include the Price River, North Horn, Flagstaff, and Colton formations. This study aims to describe the distribution and environmental significance of these strata.

The Cretaceous Price River Formation (180-300 m) is a fine-to-coarse, buff sandstone with minor sandy shale and organic-rich shale. It varies from thinly bedded to massive, with local high-angle cross-bedding. The late Cretaceous-Paleocene North Horn Formation (215-300 m) consists of variegated shale and mudstone, gray to brown sandstones, local chert pebble conglomerate, and yellow-gray thinly bedded carbonate. The Paleocene-Eocene Flagstaff Formation (245-365 m) consists of light-gray and yellow carbonate that ranges from crystalline to argillaceous in thin to thick beds, but is locally a massive cliff-former. It is interbedded with gray-green shale and red calcareous siltstone, and has local algal nodules and oncolites. The Eocene Colton Formation (60 m) consists of deep-red to variegated shale and yellow-buff sandstone with minor carbonate mudstone and bentonite. A massive to cross-bedded chert pebble conglomerate with minor lithic sandstone sits above the Colton in some locations, and may be correlative to the Eocene Crazy Hollow Formation that crops out to the north.

These units represent a transition from high energy fluvial to lacustrine environments. The Price River Formation was deposited in a fluvial/marine environment, and the North Horn a fluvial/lacustrine environment. The Flagstaff and Colton formations both are interpreted as lacustrine deposits, that vary between freshwater and highly saline carbonate lakes. The lacustrine nature of this sequence is confirmed by the presence of fossil ostracods and mollusks, pollen from the Taxodiaceae, oncolites, and siliclastic and carbonate composition. Variation in these units reflects regional tectonic uplift and climate change during the Paleogene.