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.