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Stable Isotope and Elemental Geochemistry Reveal Environmental Controls and Diagenetic Modification of Microbialite Facies Within a Sequence Stratigraphic Framework, Upper Cambrian Point Peak Formation, Llano River and Mill Creek, Mason County, Central Texas


Upper Cambrian microbial reef complexes exposed in Mason, TX serve as useful analogues to study reservoir characteristics of similar systems found in hydrocarbons rich systems offshore Brazil. Microbialites occur within the transgressive phases of depositional sequences in a mixed carbonate siliciclastic ramp of the Point Peak Fm. Siliciclastics were delivered into shoreline and shallow marine environments during regression. Increasing carbonate proportions represent transgression. Depositional sequences and higher frequency depositional cycles are reflected in spectral gamma ray logs, carbon isotopes and concentration of trace metals. δ13C shows excursions of +1-2‰ in transgressive carbonates containing microbialite and reaches values of -2‰ in maximum regression. Al, Si, K concentrations reflect greater siliciclastic flux in regressive component of sequences. Redox proxies such as U show no correlation with sequences. The microbial reef complexes in the upper Point Peak consist of 3 geometrical phases consisting of: phase 1 with sharp walled microbial mounds and intermound skeletal grainstones that are onlapped by siltstone, phase 2 that prograded over and intertongued with intermound carbonate and siliciclastic facies, and phase 3 with sharp walled microbial mounds onlapped by siltstone. The phasing is interpreted to result from either sea-level fluctuation or pulsing of siliciclastic input. Isotopes remain relatively stable through the microbial reef interval δ13C ~1‰ δ18O -5.5‰ with a subtle zone of scattered δ13C values of 0.5-2.0‰ at the boundary between phase 1 and 2, a subtle positive shift in δ13C +1‰ in the upper part of phase 2 and a slight decrease in phase 3. Isotope values and petrographic observations of phase boundaries do not support subaerial diagenesis of phase boundaries. The upper boundaries of phases 1 and 2 however, contain elevated levels of proxies for siliciclastic flux concentrated along stylolites. This indicates the phase geometry was likely controlled by pulses of siliciclastic flux perhaps linked with oscillation in sea level. Patterns in Mg, Fe (and inversely correlated Sr concentrations) reflect degree of ferroan dolomitization likely formed during burial diagenesis. Vertical facies from a standard core through the reef would not reveal the phasing of the 3-d geometry. However, the vertical patterns in geochemistry identify phases in the architecture.