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Carbon and oxygen isotope stratigraphy of the Lower Mississippian (Kinderhookian–lower Osagean), western United States: Implications for seawater chemistry and glaciation

¹ Department of Geological Sciences, Ohio State University, Columbus, Ohio 43210, USA

A positive carbon isotope (¹³C) excursion has been recognized in upper Kinderhookian and early Osagean carbonates in three sections in southeast Idaho and Nevada. The oldest ¹³C peak (+7) is dated to the isosticha conodont zone, and a younger peak occurs in the typicus Zone. The shifts are recorded in a range of carbonate lithofacies representing various water depths along the shelf. Lithofacies sampled for ¹³C and ¹⁸O at the Samaria Mountain section in southeast Idaho record the shallowest-water conditions, indicated by cross- bedded skeletal and peloidal grainstones. The deepest water conditions are present in the Pahranagat Range section in eastern Nevada, which consists mainly of bioturbated lime mudstone and skeletal wackestone. The ¹³C values from these widely separated sedimentary basins show a consistent trend that correlates with Early Mississippian curves generated from brachiopod calcite in western Europe and the Midcontinent of North America, as well as dolomites in Utah and Wyoming. ¹⁸O values become more positive up section, generally paralleling the positive trend in ¹³C during the late Kinderhookian. No subaerial exposure surfaces are recognized in the sections examined in southeast Idaho and Nevada, and at least the ¹³C trends are interpreted as primary seawater fluctuations. Sea-level changes occurred near the beginning of the late Kinderhookian ¹³C shift (early to middle parts of the isosticha Zone) and within the peak of the ¹³C excursion (Kinderhookian-Osagean boundary), although tectonic changes associated with the Antler orogeny have likely modified the eustatic signature.

Key Words: carbon-13 • conodont • glaciation • Kinderhookian • Mississippian • oxygen-18

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