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The AAPG/Datapages Combined Publications Database
The Mississippian Delle Phosphatic Member forms the basal member of seven different formations from southeastern Nevada, through Utah, to southeastern Idaho. The Delle is comprised of mainly dark organic-rich phosphatic mudstone containing large micritic limestone concretions, and bedded radiolarian chert, peloidal phosphorite, and cherty micrite. It was deposited in Osagean to Meramecian time within the eastern half of the Antler foreland trough in dysaerobic settings of the Deseret starved basin and adjacent lower foreslope of a carbonate platform. The Delle Phosphatic Member lacks conspicuous shelly megafaunas but contains a rich microfauna, consisting most importantly of conodonts, radiolarians, and agglutinate foraminiferans. Also present are fewer large planktonic, epiplanktonic nektonic, and benthic animals, and floating marine and transported land plants. Knowledge of this diverse biota, recovered mostly from the large micritic limestone concretions, is important to an understanding of the source of hydrocarbons in the organic-rich sequence. Organic carbon values are inversely related to conodont alteration index (CAI) values, and comparison of maps of these two sets of values suggests areas where hydrocarbons are either overmature or at optimum maturation for petroleum generation. Models for hydrocarbon accumulation and maturation in the Delle are applicable worldwide to other dysaerobic deep-basinal dark rocks containing similar concretions--for example, the Devonian and Mississippian Bakken-Exshaw-Sappington-Leatham depositional complex and Permian Phospho ia Formation of the western United States, the Devonian Huron Shale Member of the Ohio Shale in Ohio, and the Upper Devonian sequence of western New York.
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