Isotopic and Petrographic Record of Phreatic Diagenesis: Lower Cretaceous Sligo and Cupido Formations

Abstract

ABSTRACT Equant calcite results from a complex spectrum of processes in shallow to burial phreatic environments. Variations in temperature, composition of pore fluids, and redox potential of the environment may give rise to successive generations of equant spar which are not easily characterized through conventional petrographic approaches. Because petrographic criteria alone are not conclusive as diagenetic indicators, it is essential to combine petrographic and geochemical techniques to obtain a more complete understanding of the environmental setting of phreatic diagenesis. Integration of petrographic and stable isotopic analyses of equant cements within reefal facies of the Lower Cretaceous Sligo and Cupido formations, south Texas and northeastern Mexico, indicate multiple phases of cementation in phreatic environments and attest to the chemical evolution of the precipitational environment through time. Two isotopically distinct early generations of clear equant calcite, stages I and 2, with variable luminescence, occur immediately below intervals of wholesale dissolution and exhibit oxygen isotopic signatures approximately 1 and 3 lighter than the estimated original isotopic composition of Cretaceous marine carbonate. Moreover, both stages of these c ments yield lighter carbon isotopic signatures, relative to marine carbonate compositions, which reflect the addition of vadose-derived bicarbonate during recharge to the phreatic lens. On the basis of combined petrographic and isotopic evidence, these two early stages of cementation are interpreted to have formed in a shallow, meteoric, phreatic environment. A third generation of brightly luminescent equant calcite, stage 3, exhibits significantly lighter oxygen and invariant carbon isotopic compositions and occurs in fractures which postdate earlier stages of diagenesis. Two-phase fluid inclusions and the lighter oxygen isotopic composition indicate precipitation at elevated temperatures. The fourth cement, stage 4, is a coarsely crystalline calcite of variable luminescence which occur in fractures, moldic pores, and stylolitic seams, postdates the third stage of cementation, and contains abundant two-phase fluid inclusions and inclusions of bitumen. These petrographic criteria indicate precipitation at greater depths and elevated temperatures, and, as such, stage 4 cements should exhibit lighter oxygen isotopic signatures. What is observed, however, is that the isotopic compositions of this calcite are approximately 6 heavier in 18O than those of stage 3. Such isotopic values are in marked contrast to the progressive trend towards lighter carbon and oxygen exhibited by the first three generations of cement and must therefore reflect precipitation from fluids enriched in 18O, which counteracts t e effect of elevated temperatures. By combining isotopic and petrographic data we have succeeded in identifying four distinct episodes of phreatic cementation. The isotopic trends and the corresponding petrographic characteristics indicate that pore fluids evolved from a meteoric to a connate composition throughout the diagenetic history of the Sligo-Cupido sediments. Furthermore, based on the trends exhibited at the four locations analyzed, it appears that the marginal facies of these formations underwent comparable subsidence and diagenetic histories.