The sedimentary petrography and sedimentation of the Deadwood Formation in the Black Hills, South Dakota



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The Cambrian (Dresbachian to Trempealeauan) Deadwood Formation crops out in a nearly continuous oval ring around the Precambrian core of the Black Hills. It nonconformably overlies the Precambrian basement complex and is overlain dis conformably by either Ordovician or Mississippian units. Five distinct members make up the Deadwood Formation: (1) a Basal Conglomerate and Granular Quartzarenite Member; (2) a Lower Laminated Subarkose, Shale and Intramierudite Member; (3) a Shale and Intrasparrudite Member; (4) an Upper Laminated Subarkose, Shale and Intrasparrudite Member; and (5) a Massive Quartzarenite, Intramierudite and Glauconitic Dolomite Member. Two transgressive-regressive depositional cycles consist of an initial transgression during Cedaria time; a regression during upper Crepicephalus and Aphelaspis time; a rapid inundation during Elvinia time with a slow regression during Ptychaspis-Prosaukia and Saukia time. Initial source areas were close to sites of deposition as suggested by boulders, cobbles, pebbles and granular quartzarenites which were derived from underlying Precambrian rocks. Later deposits of sand- to silt-sized, rounded grains suggest source areas far removed from sites of deposition. Textural maturity increases upwards in this section from immature basal sediments to mature or supermature sediments near the top of the formation. After deposition of basal conglomerates and granular quartzarenite, the presence of burrowed, glauconitic quartzarenites suggests the formation of tidal flats, whereas evidence of massive cross-bedded quartzarenites represents barrier islands or offshore bars. Seaward of the intertidal zone, very fine sand-sized: submature, highly glauconitic quartzarenites; glauconitic trilobite biosparrudites and glauconitic dolomites were deposited during periods of slow sedimentation or by-pass. [...]



South Dakota