Covariant Trend Research Articles

Isotopic Evidence for the Early Meteoric Diagenesis of the Reef Facies, Permian Reef Complex of West Texas and New Mexico

ABSTRACT Multiple isotopic analyses of former aragonitic marine cements from a single sample of Permian Reef Complex reef facies define a linear covariant trend with a positive slope. Each individual analysis represents a different mixture of two isotopically distinct phases, which are too finely intergrown for separate analysis. In order to assess the systematic variation of the isotopic composition of these calcite phases over a paleobathymetric range, samples were collected from four sites, A through D, along a single Upper Capitan paleoslope reflecting the transition from reef-massive through reef-foreslope facies. On a plot of 13C versus 18O, the convergence f four marine-cement trends from these four sites indicates that the isotopically enriched phase is invariant, whereas the depleted phase, a luminescent calcite intergrowth along original marine-cement intercrystalline boundaries, varies systematically with position on the paleoslope. The emplacement of the calcite intergrowth in the marine cements is intimately related to the earliest episodes of equant calcite-spar precipitation. At the shallowest paleodepths (sites A through C), the compositions of the first generation of calcite spar (Spar I) clusters around a mean 18O of -8.2 and is progressively less depleted in 13C from site A to site C. As this variation is also present in the marine-cement trends, it is concluded that both the intergrowth and Spar I represent successive episodes of calcite precipitation during a single diagenetic event. At the deepest paleodepth (site D), Sp r I is absent, and the calcite intergrowth is interpreted to have the same composition as the second generation of spar (Spar II), -12.3 18O and 0.0 13C, which can be found throughout the reef facies. On the basis of its relatively constant oxygen composition, the systematic enrichment of its carbon composition with increasing paleo depth, and its absence from the deepest site on the paleoslope, we conclude that the Spar I diagenetic event represents the establishment of a meteoric-phreatic system in the reef facies, characterized by well-defined, downdip gradients in rock-water interaction. Furthermore, isotopic evidence of fresh-water alteration of allochthonous, massive-facies reef debris, deposited in foreslope strata near site D and below the limit of the Spar I diagenetic event, suggests that this meteoric system was active contemporaneously with the growth of the reef complex. Interpretation of the Spar II diagenetic event is less clear cut. The associated diagenetic fluid probably did not have marine or hypersaline oxygen signatures, as this would require the persistence of aragonite mineralogies to temperatures over 70°C and burial depths of at least I kin. Spar Il was probably precipitated from meteoric water, with a 18O similar to that of the Spar I meteoric water, at temperatures between 30°-65°C. Unlike the Spar I diagenetic event, the Spar II event shows no association with depositional systematics; hence, we tentatively suggest that Spar II may have been related to a presumed Ochoan (or younger) regional meteoric or shallow-burial phreatic system.

Derivation of the Original Isotopic Composition of Permian Marine Cements

ABSTRACT Diagenesis of former aragonite and magnesian calcite marine cements of the Permian Reef Complex has resulted in a fine-scale intermixture of two distinct secondary phases, a luminescent and a nonluminescent calcite. Multiple carbon and oxygen isotopic analyses of these marine cements from any single sample result in a linear covariant trend, reflecting random proportions of the two calcite phases contained in individual analyses. The convergence of four trends derived from samples of former aragonite marine cement collected at four sites along a single Upper Capitan paleoslope fixes the isotopic composition of the nonluminescent calcite at a value of -2.518O, +5.313C. Likewise, covariant trends from former aragonite cements in the Lower Capitan and the youngest Upper Capitan yield nonluminescent calcite compositions of -2.818O, +5.213C and -0.718O, +5.813C, respective y. The single covariant trend generated from former magnesian calcite cements does not converge with equivalent former aragonite covariant trends, indicating that the precursor mineralogy influences the isotopic composition of the nonluminescent calcite. Considered as a whole, the invariance of the nonluminescent calcite signature, the timing of luminescent calcite emplacement, and the fine scale of the fabric retention suggest that the precipitation of the nonluminescent calcite occurred while isolated from effective isotopic exchange with external diagenetic fluids. A direct consequence of this low water/rock ratio, closed system is that the isotopic signature of the nonluminescent calcite also represents the original isotopic composition of the marine cement. This is further support d by the preservation of primary differences between the original isotopic compositions of aragonite and magnesian calcite cements. The constancy of the marine isotopic signature throughout the reef massive and foreslope facies requires that the Delaware basin water mass was relatively well mixed. Comparison of marine compositions throughout the range of the Capitan samples indicates that, although the oxygen composition of the marine cements was apparently constant through much of the Guadalupian, a substantial enrichment in 18O occurred at the very end of the Guadalupian. This relatively abrupt change, recording a concomitant change in the oxygen composition of the Delaware Basin water mass, was probably the result of increased restriction of the Delaware Basin, and marked the first step in the sequence of events which led to the deposition of the deep-basin evaporites of the Castile formation.

Diagenetic Evolution of a Silurian Limestone Reef: Geochemical Documentation of Mixed-Water Dolomitization: ABSTRACT

Examination of a partially dolomitized Silurian platform reef (Pipe Creek Jr.) indicates that the timing of dolomitization is tightly constrained within the calcite diagenetic sequence. Cathodoluminescence and isotopic analysis of calcite diagenetic components define the diagenetic environment and fluid chemistries at the time of dolomitization. Abundant syntaxial and fibrous calcite marine cements suggest synsedimentary lithication of the reef complex. Multiple episodes of subaerial exposure are recorded both in the petrologic fabric as well as in the geochemical signatures of altered marine components and equant calcite cements. In the upper reef, marine components are corroded, and resulting porosity is partially infilled by red geopetal vadose silts. Resubmergence and continued marine cementation establishes a Silurian timing for this initial episode of exposure. Evidence for subsequent exposure is contained in the isotopic and cathodoluminescent patterns of later marine cements and overlying equant calcite spars. Marine cement isotopic compositions deviate from an initial marine composition (-4.0 ^pmil ^dgr18O; + 2.0 ^pmil ^dgr13C PDB) along covariant trends that reflect alteration by diagenetic pore waters. Equant calcite cements are restricted to the upper reef and partially occlude primary porosity. These cements exhibit invariant oxygen compositions between -7.0 to -8.0% and variable carbon which shows progressive depletion toward the upper reef. On this basis we interpret these cements as having formed within a shallow meteoric phreatic lens with light carbon derived from soil-gas CO2 at the exposure surf ce. Cathodoluminescent patterns and sulfide mineralization indicate progressive reduction of pore waters in the meteoric lens. The latest stage of phreatic cementation records anaerobic fermentation during which the oxygen composition remains unchanged with carbon varying up to + 7.0 ^pmil following precipitation of pyrite. Dolomitization occurs within this sequence of meteoric phreatic diagenesis. Early zoned meteoric spars, reflecting initial fluctuations in reducing conditions, are corrosively overlain by dolomite. The earliest formed dolomite is luminescent, signifying reduction of manganese, and is overlain by an outer ferroan dolomite. A progressive increase in reducing conditions is further indicated by the reduction of sulfate and concomitant precipitation of pyrite. The heavy carbon equant calcites, which postdate pyrite, suggest anaerobic fermentation during the final stages of phreatic cementation. Because dolomitization is bracketed by events of meteoric phreatic cementation and diagenesis, and its chemical evolution mimics the progressive reduction of the pore-water system, we suggest a mechanism of meteoric-marine water mixing to account for partial dolomitization of the Pipe Creek Jr. reef. This does not, however, imply a similar mechanism for the pervasive dolomitization of adjacent shelf reefs. Rather, this study emphasizes the complex early diagenetic history of shelf sequences with implications for late Silurian sea level fluctuations. End_of_Article - Last_Page 457------------