Carbon and oxygen isotope stratigraphy of the Lower Mississippian (Kinderhookian-lower Osagean), western United States: Implications for seawater chemistry and glaciation
A positive carbon isotope (δ 1 3 C) excursion has been recognized in upper Kinderhookian and early Osagean carbonates in three sections in southeast Idaho and Nevada. The oldest δ 1 3 C peak (+7‰) is dated to the isostichaconodont 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 δ 1 3 C and δ 1 8 O 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 δ 1 3 C 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. δ 1 8 O values become more positive up section, generally paralleling the positive trend in δ 1 3 C during the late Kinderhookian. No subaerial exposure surfaces are recognized in the sections examined in southeast Idaho and Nevada, and at least the δ 1 3 C trends are interpreted as primary seawater fluctuations. Sea-level changes occurred near the beginning of the late Kinderhookian δ 1 3 C shift (early to middle parts of the isosticha Zone) and within the peak of the δ 1 3 C excursion (Kinderhookian-Osagean boundary), although tectonic changes associated with the Antler orogeny have likely modified the eustatic signature.