Hypogene Calcitization: Evaporite Diagenesis in the Western Delaware Basin

Evaporite calcitization within the Castile Formation of the Delaware Basin is more widespread and diverse than originally recognized. Coupled field and GIS studies have identified more than 1000 individual occurrences of calcitization within the Castile Formation outcrop area, which includes both calcitized masses (limestone buttes) and laterally extensive calcitized horizons (limestone sheets). Both limestone buttes and sheets commonly contain a central brecciated zone that we attribute to hypogene dissolution. Lithologic fabric of calcitized zones ranges from little alteration of original varved laminae to fabrics showing extensive laminae distortion as well as extensive vuggy and open cavernous porosity. Calcitization is most abundant in the western portion of the Castile outcrop region where surface denudation has been greatest. Calcitization often forms linear trends, indicating fluid migration along fractures, but also occurs as dense clusters indicating focused, ascending, hydrocarbon-rich fluids. Native sulfur, secondary tabular gypsum (i.e. selenite) and hypogene caves are commonly associated with clusters of calcitization. This assemblage suggests that calcium sulfate diagenesis within the Castile Formation is dominated by hypogene speleogemesis

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

Patterns of local and global redox variability during the Cenomanian–Turonian Boundary Event (Oceanic Anoxic Event 2) recorded in carbonates and shales from central Italy

Careful evaluation of the local geochemical conditions in past marine settings can provide a window to the average redox state of the global ocean during episodes of extensive organic-carbon deposition. These comparisons aid in identifying the interplay between climate and biotic feedbacks contributing to and resulting from these events. Well-documented examples are known from the Mesozoic Era, which is characterized by episodes of widespread organic-carbon deposition known as Oceanic Anoxic Events (OAEs). This organic-carbon burial typically leads to coeval positive carbon-isotope excursions. Geochemical data are presented here for several palaeoredox proxies (Cr/Ti, V, Mo, Zn, Mn, Fe speciation, I/Ca and sulfur isotopes) from a section exposed at Furlo in the Marche–Umbrian Apennines of Italy that spans the Cenomanian-Turonian boundary. Here, OAE 2 is represented by a ~1-m thick radiolarian-rich millimetre-laminated organic-rich shale known locally as the Bonarelli Level. Iron speciation data for thin organic-rich intervals observed below the Bonarelli Level imply a local redox shift going into the OAE, with ferruginous conditions (i.e., anoxic with dissolved ferrous iron) transiently developed prior to the event and euxinia (i.e., anoxic and sulfidic bottom waters) throughout the event itself. Pre-OAE enrichments of elements sensitive to anoxic water columns were due to initial development of locally ferruginous bottom waters as a precursor to the event. However, the greater global expanse of dysoxic to euxinic conditions during the OAE greatly reduced redox-sensitive trace-metal concentrations in seawater. Carbonate I/Ca ratios were generally low, suggesting locally reduced bottom water oxygen conditions preceding the event and relatively increased O2 concentrations post-event. Combined, the Furlo geochemical data suggest a redox-stratified water column with oxic surface waters and a shallow chemocline overlying locally ferruginous bottom waters preceding the event, globally widespread euxinic bottom waters during the OAE, followed by chemocline shallowing but sustained local redox stratification following the event