One major goal of a depositional model is to portray variations in salinity across an ancient facies tract. Qualitative determination of paleosalinity gradients with whole-rock, oxygen isotope analyses of limestones is usually hampered by the effects of cementation and reequilibration with formation water at depth. Numerous characteristics of the Marble Falls Limestone make it ideal for facies analysis using isotopes. Burial did not exceed 1 km and the low vitrinite reflectance, averaging 0.30%, indicates cool temperatures. A closed system during stabilization is manifested in high strontium levels in calcite (2,000 to 5,000 ppm).

The Marble Falls was deposited on the Llano platform which was bordered on the east by the Fort Worth basin. An east-west sediment profile consisted of black spiculite, algal boundstone, oolitic grainstone, spiculitic wackestone, and calcareous shale. Carbonate mud within spiculite and boundstone of the foreslope environment is isotopically "marine" (^dgr¹⁸O = -2.52 + 0.55 per mil PDB, n = 12). The whole-rock composition of grainstone (^dgr¹⁸O = -4.00 ± 0.56, n = 10) is in accord with a two-component mixture consisting of marine ooids and meteoric cement, supporting the contention that the platform rim was exposed. The high standard deviation of the del values for spiculitic wackestone from the platform interior (^dgr¹⁸O = -3.76 ± 1.24, n = 2 ) is due to changing amounts of runoff from the Concho arch. Calcite within the shale, a marsh deposit, is depleted (^dgr¹⁸O = -5.10 ± 2.26, n = 2).

It is rare to find limestone sequences with facies-specific oxygen isotopes because most have undergone complex cementation histories or burial-related recrystallization. In limestones known to have escaped burial, oxygen isotopes should be of great utility for paleosalinity determinations, when combined with other types of data.

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