Abstract

Dark organic-rich starved-basin sediments of the basal, phosphatic member of the Deseret Limestone and equivalents were deposited west of a westward-prograding carbonate platform in late Tournaisian to early Visean (Osagean to early Meramecian) time. The rocks comprise mainly pelletal, peloidal, oolitic, and conglomeratic phosphorite; phosphatic shale enclosing large calcareous concretions; bedded spiculitic and radiolarian chert; cherty micritic limestone; silt-stone; and mudstone. The starved basin has been recognized to extend for more than 700 km (434 mi) from southeastern Nevada to southeastern Idaho. Its possible true northward extent into western Montana has been obscured by regional thrust faulting.

The biota of the basinal rocks comprises mainly planktic radiolarians, nektic goniatites and conodonts, benthic agglutinate foraminiferans and sponges, and infaunal traces of burrowing organisms. The sparse shelly fauna consists mainly of small solitary corals and the brachiopod Leiorhynohoidea. The bathymetry of the foreslope and shelf, considered together with the character and biota of the basinal rocks, suggests that the floor of the central basin lay in the dysaerabic zone at a depth of about 300 m (984 ft). If the corals and brachiopods prove to be epiplanktic, deposition could have been in the anaerobic zone at even greater depths.

Rate of sedimentation of starved-basin sediments is calculated from the conodont zonation to be about 10 m (32.8 ft)/m.y. Slope rocks that intertongue westward with basinal rocks and eastward with carbonate-platform rocks consist mainly of thin, evenly bedded dark micritic limestone with some lenticular crinoidal interbeds. The sediments were originally deposited on a gentle foreslope of 5° or less. The rate of slope-limestone sedimentation is calculated to be 16-18.5 m (52.5-60.7 ft)/m.y., nearly twice the rate of starved-basin sedimentation, but only about one-seventh the rate of adjoining carbonate-platform sedimentation.