Abstract

Stable isotope ratios, strontium 87/86 ratios, and magnesium, strontium, and iron contents have been measured on carbonate matrix and on bulk-rock biomicrites in Early Pennsylvanian limestone (Marble Falls Formation) in central Texas. Patterns have been found which relate to primary conditions in the depositional environment, such as salinity and mineralogy, and to early diagenesis. Oxygen isotopes disclose differences of salinity across the ancient Llano Platform that extended from the Fort Worth Basin (east) to the Concho Arch (west). Open-marine circulation which existed basinward of the platform involved seawater with a possible delta 18 O of -1.5 per mil (SMOW), whereas 18 O enrichment in "red algal" biomicrites behind platform margin shoals reveal poor circulation and evaporation of seawater to a delta 18 O of at least +1 (SMOW) in that environment. The platform margin shoals are composed of oosparites and biosparites with moderately low bulk-rock delta 18 O values that were caused by meteoric-water cementation of marine ooids. Sponge-phylloid algal mounds of the protected Llano Platform are also depleted in 18 O, which suggests they also underwent meteoric-water diagenesis when the interior platform was exposed to rainfall during sea-level lowstands. Deductions about original mineralogy are based on several indirect lines of evidence. For example, high Sr:Mg ratios in several of the interior platform facies may reflect local concentrations of precursor aragonite. Celestite inclusions in microspar and chert, fluorite inclusions in microspar, and "needle-mud" pseudomorphs (calcite after aragonite) in matrix are additional by-products of the aragonite-to-calcite transformation in the sediment in the interior-platform environment. Microspar is common in sponge-algal biomicrites of the interior platform where an aragonitic sediment precursor underwent varying amounts of meteoric-water diagenesis. The microspar grades texturally into isotopically depleted spar which is interpreted to have had a meteoric origin. The microspar formed without a micrite precursor by aragonite solution and calcite precipitation in aragonite muds. This occurred in relatively fresh, but highly exchanged and reducing fluids. The replacement of aragonite by microspar was really a dissolution-reprecipitation process in which strontium was retained in the bulk sediment. Micrite, lacking strontium, may have formed more slowly, even after stabilization was complete. The Marble Falls escaped burial and later diagenesis.

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