San Andres Carbonates in the Texas Panhandle: Sedimentation and Diagenesis Associated with Magnesium-Calcium-Chloride Brines

The San Andres evaporitic sequence in the Palo Duro Basin comprises several thick carbonate units in its lower part and many thin units in its upperpart. To the south, across the Northern Shelf of the Midland Basin, evaporites pinch out and carbonates predominate. Six lithofacies were differentiated in the Palo Duro and Northern Shelf carbonates: dolomudstone, pellet-oolite packstone-grainstone, filamentous (Girvanella-like) grainstone, sponge spicule packstone, wispy-laminated crinoid packstone, and skeletal packstone-grainstone. Facies distribution was controlled by water-body salinity, which increased from south to north. Within the Palo Duro Basin, the carbonates in the upper part of the sequence differ from those in the lower part in that the former lack skeletal lithofacies and have higher manganese, iron, and terrestrial organic matter content. Bromide (Br) content in halite in the lower part of the sequence is consistently high, whereas halite in the upper part is mostly depleted in bromide. Strontium (Sr) in dolomite, calcite and anhydrite, δ18O, δI3C values, and early diagenetic oxidizing conditions deduced from high pristane/phytane ratios are about the same throughout the entire San Andres Formation in the Palo Duro Basin. Depleted δI3 values in dolomites associated with low pristine/phytane ratios in the Northern Shelf formed under more reducing conditions in which organically derived carbon in the carbonates increased because of sulfate-reducing bacterial activity. Sodium/chloride and potassium/chloride ratios attributed to liquid inclusions in almost all carbonates are characteristic of marine brines evaporated beyond the level of halite saturation. Sodium content in the dolomite lattice is generally low and increases from north to south at the same stratigraphic levels. Varied sedimentologic and geochemical properties of the rocks throughout the area reflect different primary depositional regimes. Properties that do not vary are attributed to diagenetic modification of the rocks in contact with brines having similar compositions. The lower part of the formation was deposited in a broad shelf basin or lagoon sufficiently deep to maintain long periods of steady-state circulation. During these periods neither halite dissolution nor potash-magnesia mineral precipitation occurred. The upper part of the formation was deposited in smaller water bodies sensitive to inflow fluctuations. Increased proportion of meteoric water in the depositional environment during this period is evidenced by high content of manganese, iron, and terrestrial organic matter, and the meteoric water was a source of dissolved carbonate for the deposition of many of the thin carbonate units. Diagenesis of the San Andres carbonates occurred in contact with saline magnesium-calcium-chloride brines, which evolved from seawater by anhydrite and halite precipitation. Skeletal mold formation and subsequent anhydrite cementation, dolomitization, and high-strontium calcite cementation associated with celestite precipitation are all cogenetic processes controlled by this brine-rock interaction. The δ18O composition of dolomite, calcite, and chert indicates apparent equilibrium relations with the same solution. Possible low temperatures of 40° to 450° C (105° to 110° F) imply δ18O of such a solution to be about 2 to 3‰.The somewhat light δ18O composition of the proposed halite-saturated brine may have resulted from the reversal in the positive correlation between δ18O and increased evaporation in highly saline brines. San Andres carbonates in the Palo Duro Basin that were diagenetically altered in a halite-saturated magnesium-calcium chloride brine were plugged by precipitating salt and remain unchanged and isolated in a closed sedimentary basin. The Northern Shelf carbonates were modified by similar brines intermittently undersaturated with respect to halite because of mixing with seawater. As a result, some original porosity remained, and pressure solution occurred in the more deeply buried and more skeletal-rich sequence.