James R. Boles

Clay Diagenesis in Wilcox Sandstones of Southwest Texas: Implications of Smectite Diagenesis on Sandstone Cementation

ABSTRACT Sandstones and shales of the Wilcox Group (lower Eocene) in southwest Texas were examined by X-ray powder diffraction, electron microprobe, and petrographically to interpret their diagenetic history. Samples analyzed are from depths of 975 to 4650 m, representing a temperature range of 55°C to 210°C. No consistent trend of depositional environments is recognized with increasing depth, and mineralogic changes observed are interpreted as diagenetic. Major mineral distribution patterns are (1) disappearance of discrete smectite at temperatures >70°C, (2) gradation of mixed-layer illite/smectite to less expandable (more illitic) illite/smectite over the entire temperature range, (3) disappearance of kaolinite from 150-200°C accompanied by an increase in chlorite, and (4) replacement of calcite cement at about 117 120°C by ankerite. Calculations based on data of Hower and others (1976) indicate that the stability of smectite layers may be a function of composition. Smectites with high ratios of octahedral (Fe + Mg)/Al appear to resist conversion to illite until temperatures high enough to produce ordering are attained. A diagenetic model is proposed which involves the breakdown of detrital K-feldspar and of some smectite layers in illite/smectite to convert other smectite layers to illite. Silica and calcium released by the illitization of smectite is transferred from shales to sandstones to produce quartz overgrowths and calcite cements at temperatures as low as 60°C. Iron and magnesium released by the illitization reaction are transferred from shales to sandstones at temperatures >100°C and react with kaolinite to produce high-alumina chlorite and/or with calcite to produce ankerite.

Active ankerite cementation in the subsurface Eocene of southwest Texas

In subsurface samples of Wilcox (Eocene) sandstones, calcite cements occur above 2315 m depths, whereas ankerites occur at depths from 2560 m (temperatures 125 ° C) to at least 4650 m (temperatures 210 ° C). Microprobe analyses indicate that some shallow ankerites have appreciable excess calcium, analogous to protodomites. Ankerites at depths greater than 3200 m have compositions of about CaMg0.5Fe0.5(CO3)2.Oxygen isotope data suggests that the ankerites are similar to low temperature hydrothermal dolomites and that they have probably formed in pore fluids with higher O18/O16 ratios than sea water. The isotopic data also suggest that the ankerites have formed over a more limited temperature interval than they occur today.The ankerite is believed to have formed from calcite by the reaction 4CaCO3+Fe2++Mg2+=2CaMg0.5Fe0.5(CO3)2 +2Ca2+. Iron and magnesium for this reaction was apparently released by the breakdown of smectite to illite in mixed-layer clays. Bulk chemical analyses suggest that some iron was transferred from shales into sandstones. Mass balance and chemical considerations are compatible with this model.

Structure, stratigraphy, and petrology of mainly Triassic rocks, Hokonui Hills, Southland, New Zealand

Abstract About 260 km2 of mainly Triassic rocks on the north limb of the Southland Syncline, Hokonui Hills, Southland, consist of a thick sequence of steeply dipping, mainly marine, volcanogenic sedimentary rocks. Changes in strike and numerous faults with a conspicuous strike-slip component indicate the area has undergone a compressive stress in a NW-SE direction. The compressive stress post-dates the folding of the Southland Syncline. The North Range Group of the Murihiku Supergroup (Campbell & Coombs 1966) varies bewteen 3 and 3.5 km thick and ranges in age from Lower Triassic (Scythian) to Middle Triassic (Anisian). It is subdivided into six new formations: Fairplace Formation, Gavenwood Tuffs, Glenure Formation, Crosshill Gully Siltstone, North Peak Formation, and Stag Siltstone, in order of decreasing age. The Taringatura Group (Middle and Upper Triassic), which overlies the North Range Group, is not subdivided here, but at least one unit, the Bare Hill Tuff Zone, is continuous across the area. The ...