Wilcox sandstone diagenesis, Texas Gulf Coast: a regional isotopic comparison with the Frio Formation

Abstract

Summary The burial diagenesis of Eocene Wilcox sandstones differs significantly from the burial diagenesis of other Gulf Coast terrigenous clastic formations. Differences between the onshore Wilcox and Oligocene-Miocene Frio Formations are larger than regional variations within either formation. Younger, offshore units have undergone less diagenetic alteration than either of the older, onshore units, whereas Mesozoic sandstones are generally more extensively altered. Cementation by quartz and calcite was the first diagenetic modification of volumetric significance to affect either the Wilcox or Frio Formations. The average δ 18 O of Wilcox quartz cement is approximately +25‰ SMOW, in contrast to + 31‰ for the Frio. In both formations, quartz cement is more abundant in the overpressured zone. Calcite has an average δ 18 O of −10.8‰ PDB in the Wilcox Formation (compared to −7.2‰ for the Frio). 87 Sr/ 86 Sr values suggest that calcite was derived from coeval nannofossils, carbonate rock fragments of Mesozoic age, or by local input of fluids from underlying Mesozoic carbonates. Most calcite cement (and therefore quartz which pre-dates calcite cement) was apparently emplaced prior to extensive alteration of detrital silicates which would have released strontium with a high 87 Sr/ 86 Sr ratio. The most common carbonate cement in the Wilcox Formation is ankerite which replaces calcite with increasing depth. The δ 13 C of ankerite is essentially identical to that of calcite at the same depth, and is also similar to Frio and younger carbonate cements. In contrast, δ 18 O values of Wilcox ankerite (average −9.8‰) indicate emplacement at higher temperature or from more depleted water than was true of Wilcox calcite. 87 Sr/ 86 Sr values for Wilcox ankerite are very radiogenic (>0.7100), indicating that ankerite emplacement occurred during or after active silicate diagenesis. Both ankerite and dolomite are very uncommon phases in Frio and younger sandstones despite massive smectite stabilization, suggesting that the conversion of smectite to illite was not the source of iron and magnesium for the late cements. The volume of secondary porosity is similar in both Wilcox and Frio sandstones. Albitization and K-feldspar removal from both formations are essentially complete below 3000 m and essentially no unaltered detrital feldspar occurs below that depth. Differences between the Gulf Coast formations are attributed to different geothermal gradients, differences in the basinal sediments over which the units prograded, and to the changing nature of connate fluids in the units themselves. Diagenesis of the Wilcox Formation, like other Gulf Coast terrigenous wedges, is understandable only in terms of interaction with underlying units during the large-scale evolution, in both time and in space, of the Gulf Coast diagenetic system.