ABSTRACT

Pennsylvanian to Lower Permian cyclic platform carbonates from the Southwest Andrews area of West Texas are now 3 km below the land surface. Isolated samples of brachiopod shell, zones of calcite cement, and saddle dolomite from limestone cores of the Southwest Andrews oilfields have been analyzed isotopically to determine if: (1) diagenesis is dominated by early, meteoric processes, as suggested by bulk rock isotopes, (2) whether significant burial diagenesis occurred in hypersaline waters, and (3) whether most burial cements are allochthonous or internally derived.

Bulk isotopic analyses of limestones show systematic variation in ¹³C and ¹⁸O according to length of subaerial exposure and stratigraphic position. Chemically zoned calcite cements show four growth stages. Stage 1 and 2 cements have average ¹³C values of -4.6o/oo and average ¹⁸O of -5.0o/oo, and most are interpreted to have precipitated during subaerial exposure in meteoric water. A few stage 1 cements with average ¹³C values of -4.7o/oo and ¹⁸O of -1.4o/oo apparently precipitated below the sediment surface from modified seawater in a sulfate reduction zone.

Stage 3 and 4 calcite cements precipitated after compaction and compose up to 25% of the total rock volume. They have ¹³C values (-0.2o/oo to -4.4o/oo) that vary according to stratigraphic position, and are similar to bulk rock samples of the host limestone. The ¹³C values indicate local derivation by dissolution of adjacent rock. The ¹⁸O values of stage 3 and 4 cement (-5.5o/oo to -3.5o/oo) are indistinguishable from stages 1 and 2 cements precipitated from meteoric water. The decrease in ¹⁸O expected from elevated temperatures during burial (present formation temperature of 55°C) is negated by precipitation from a late Permian brine with ¹⁸O values of +5o/oo.

Stages 3 and 4 calcite and saddle dolomite cements have ⁸⁷Sr/⁸⁶Sr values that are lower than ⁸⁷Sr/⁸⁶Sr values for contemporaneous marine carbonate ( 0.70830). This is probably due to incorporation of Sr with low ⁸⁷Sr/⁸⁶Sr values (0.70672 to 0.70682) also from the late Permian hypersaline brine.

The Southwest Andrews area has several diagenetic features that are probably more common in ancient limestone than previously recognized. (1) Burial cements have ¹⁸O values similar to meteoric cements when precipitated from hypersaline waters with ¹⁸O values 5o/oo greater than contemporary seawater. (2) Deep-burial cements have ¹³C values similar to the bulk rock because carbon is derived from pressure dissolution of nearby carbonate. (3) Bulk-rock isotope values are characteristic of marine limestones subjected to meteoric diagenesis, even though they have suffered substantial deep burial diagenesis. This study clearly illustrates the value of analyzing diagenetic components to accurately understand porosity evolution in limestones.