Late Middle-Late OrdovicianSwan Peak Formation, Bear River Range: Further Evaluation of Provenance and Depositional Controls

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The Bear River Range in Northern Utah is composed of a thick succession of Paleozoic strata. Limestone and Dolomite almost entirely comprises the Paleozoic sequence. A siliciclastic sequence is seen in the middle of the Ordovician strata, the Swan Peak Formation (SPF). The SPF is a coarsening upwards series of sequences: first shale, then a purple to buff quartzite, and finally a supermature white quartzite. Petrography, detrital zircons, carbon and oxygen isotopes, sequence stratigraphy, and biostratigraphy were used to determine provenance, and depositional controls of the SPF, such as whether the SPF was deposited in response to changes in climate such as glaciation. During Middle to Late Ordovician time, regressions exposed the Canadian Shield to extreme tropical weathering. The consequent weathering of the metamorphic and igneous provinces produced vast amounts of quartz sand and clay into riverine systems and ultimately the oceans. The resulting detritus was swept by northern trade winds and riverine processes to the oceans where it was transported via alongshore currents southward to where it was deposited as the Swan Peak Formation in northern Utah. Detrital zircon analyses from both the middle and upper member show bimodal ages of 2.7 Ga and 1.9 Ga. Showing the detrital zircons were most likely derived from the Canadian Shield and the Wyoming Craton Province. _13 Ccarb and _18Ocarb analyses from the lower member of the SPF and upper Garden City Formation range from -2.50/00 to -5.80/00 (vs. PDB) and -70/00 to -130/00 (vs. SMOW) respectively. The lower member was deposited in a somewhat restricted shelfal environment, with decreased bioproductivity, and oxidized organic carbon. These and other factors likely drove down _13 C values. It is postulated that unconformities between the middle and upper member as well as the upper member and the Fish Haven Dolomite are periods of glaciation. Consequently it is also argued that Swan Peak Formation was deposited in response to the onset and subsequent intervals of those glaciations.

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