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Extensive, uplift-related and non-fault-controlled spar precipitation in the Permian Capitan Formation


With time, unlithified grains in sediments become cemented and eventually lithified to form sedimentary rocks. Sedimentary rocks of all ages, lithologies and depositional settings exhibit cements. The timing of cementation within a given sedimentary unit, however, is generally poorly constrained. The formation conditions of the youngest of cement generations are particularly difficult to characterize. Typically, traditional carbonate carbon (δ13Ccarb) and oxygen (δ18Ocarb) isotope analyses are used to characterize precipitation timing and environment. However, ambiguities associated with the interpretation of δ18Ocarb data lead to conflicting hypotheses. The Permian Capitan Formation is one of the most widely studied carbonate sequences and contains extensive calcite cements that have been interpreted to form across a range of diagenetic environments through δ18Ocarb analyses. Here, we present new and previously reported clumped isotope data from calcite spars of Capitan fore-reef slope and equivalent shelf facies (Tansill Formation) in order to constrain mineralization temperatures, provide previously unattainable information concerning precipitation environment, and explore the spatial extent of precipitation events. Spar precipitation temperatures range from ~30 to 75°C and show positive correlation with reconstructed pore water δ18O values, indicating rock-buffered behavior. Evaluation of the data using a simple water-rock model indicates that the fluid(s) involved in diagenesis must have had a significant meteoric component, exhibiting fluid δ18O values approaching -12‰ (VSMOW). These new data along with previously reported outcrop and core relationships indicate that spar precipitation occurred well after deposition of the Capitan Formation and likely during Tertiary uplift when fluids with such light isotopic signatures would have infiltrated the basin, and not during burial as generally assumed. The meteoric fluids responsible for spar precipitation may have been delivered locally through fracture networks, but also penetrated less fractured facies and produced extensive spar cements. © 2013 Elsevier B.V.

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