SURFICIAL WOODBINE SANDSTONES IN NORTH TEXAS: THE INFLUENCE OF SHALLOW BURIAL DIAGENESIS AND SUBSEQUENT EXPOSURE-RELATED CEMENTATION

The outcropping, fluvial, deltaic, sandstones of the Woodbine are very well sorted, sub-angular to sub-rounded, fine-grained, mature, quartz arenites. They have an average composition of G ₅₆C₂₅M₁P₁₈. Outcropping Woodbine sandstones are anomalous with respect to other Cretaceous formations in central and northern Texas because they are not calcite cemented, but rather, are cemented by iron oxides and clays; a product of meteoric water infiltration and shallow groundwater flux following uplift. Exposure-related cementation in several samples has resulted in well-indurated sandstones and some ferricretes. In contrast, burial diagenesis appears to play only a minor role in cementation; therefore many samples remain friable and porous. Finally, hollow, encrusting, hematite cements that are pseudomorphed after wheat-seed siderite, represent a continuum of diagenetic events that occurred during burial and reexposure.

Outcropping Woodbine sandstones were buried between 1 to 1.5 km and primary porosity was reduced by compaction to about 30% as measured in uncemented, matrix free samples. When all samples are considered, primary porosity averages 16.6% and secondary porosity averages 1.6%. Secondary porosity is attributed to the weathering and dissolution of detrital feldspars. IGV averages 41%, but this value is skewed by heavily iron-cemented samples that have IGV values up to 75%. Such large IGV values may be caused by unrecognizable grain replacement, the opaque nature of hematite, or expansive crystal growth.

There is no evidence of early cementation in Woodbine sandstones. During burial, small amounts of quartz overgrowths formed followed by wheat-seed siderite and rare occurrences of kaolinite. After uplift and reexposure, these siderite cements were both dissolved and pseudomorphed by hematite. Reexposure also resulted in other iron oxide cements and mechanically infiltered iron-rich and iron-stained smectitic clays, which are the primary intergranular materials in Woodbine sandstones. Grain dissolution occurred throughout burial and reexposure and finally, hydrocarbons migrated into these samples some time after reexposure.