38 research outputs found
Diagenetic controls on the isotopic composition of carbonateāassociated sulphate in the Permian Capitan Reef Complex, West Texas
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Origin of blocky aragonite cement in Cenozoic glaciomarine sediments, McMurdo Sound, Antarctica
Inorganic aragonite occurs in a wide spectrum of depositional environments and its precipitation is controlled by complex physioāchemical factors. This study investigates diagenetic conditions that led to aragonite cement precipitation in Cenozoic glaciomarine deposits of McMurdo Sound, Antarctica. A total of 42 sandstones that host intergranular cement were collected from the CIROSā1 core, located proximal to the terminus of Ferrar Glacier. Standard petrography, Raman spectroscopy and electron microprobe analysis reveal a prominent aragonite cement phase that occurs as a poreāfilling blocky fabric throughout the core. Oxygen isotope compositions (Ī“18OĀ =Ā ā30Ā·0 to ā8Ā·6ā° Vienna PeeāDee Belemnite) and clumped isotope temperatures (TĪ47Ā =Ā 13Ā·1 to 31Ā·5Ā°C) determined from the aragonite cements provide precise constraints on isotopic compositions (Ī“18Ow) of the parent fluid, which mostly range from ā10Ā·8 to ā7Ā·2ā° Vienna Standard Mean Ocean Water. The fluid Ī“18Ow values are consistent with those of pore water, previously identified as cryogenic brine in the nearby ANDā2A core. Petrographic and geochemical data suggest that aragonite cement in the CIROSā1 core precipitated from a similar brine. The brine likely formed and infiltrated sediments in flooded glacial valleys along the western margin of McMurdo Sound during the middle Miocene Climatic Transition, and subsequently flowed basinward in the subsurface. Consequently, the brine forms as a longstanding subsurface fluid that has saturated Cenozoic sediments below southern McMurdo Sound since at least the middle Miocene. Aragonite cementation in the CIROSā1 core is interpreted to reflect its proximal position to sites of brine formation and greater likelihood of experiencing brines with sustained high carbonate saturation states and Mg/Ca ratios. This unusual occurrence expands the range of known natural occurrences of aragonite cement. Given the potential for cryogenic brine formation in glaciomarine settings, blocky aragonite, as the end member of the spectrum of aragonite cement morphology, may be more widespread in glaciomarine sediments than currently thought
Lateāstage calcites in the Permian Capitan Formation and its equivalents, Delaware Basin margin, west Texas and New Mexico: evidence for replacement of precursor evaporites
Comparison of Upper Guadalupian foreāreef, reef and backāreef strata from outcrops in the Guadalupe Mountains with equivalent subsurface cores from the northern and eastern margins of the Delaware Basin indicates that extensive evaporite diagenesis has occurred in both areas. In both surface and subsurface sections, the original sediments were extensively dolomitized and most primary and secondary porosity was filled with anhydrite. These evaporites were emplaced by reflux of evaporitic fluids from shelf settings through solutionāenlarged fractures and karstic sink holes into the underlying strata. Outcrop areas today, however, contain no preserved evaporites in reef and foreāreef sections and only partial remnants of evaporites are retained in backāreef settings. In their place, these rocks contain minor silica, very large volumes of coarse sparry calcite and some secondary porosity. The replacement minerals locally form pseudomorphs of their evaporite precursors and, less commonly, contain solid anhydrite inclusions. Some silicification, dissolution of anhydrite and conversion of anhydrite to gypsum have occurred in these strata where they are still buried at depths in excess of 1 km; however, no calcite replacements were noted from any subsurface core samples. Subsurface alteration has also led to the widespread, lateāstage development of largeā and smallāscale dissolution breccias. The restriction of calcite cements to very nearāsurface sections, petrographic evidence that the calcites postādate hydrocarbon emplacement, and the highly variable but generally ālightācarbon and oxygen isotopic signatures of the spars all indicate that calcite precipitation is a very late diagenetic (telogenetic) phenomenon. Evaporite dissolution and calcitization reactions have only taken place where Permian strata were flushed with meteoric fluids as a consequence of Tertiary uplift, tilting and breaching of regional hydrological seals. A typical sequence of alteration involves initial corrosion of anhydrite, one or more stage