The influence of diagenesis upon surface textures in the St. Peter Sandstone
Caraher, Michael F.
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Previous theories of diagenesis do not fully explain the surface textures observed in the St. Peter Sandstone. The purpose of this study is to demonstrate the application of surface textural analysis in the interpretation of the diagenetic history from the study of grain surface features and associated mineralogic data. This work also considers diagenesis in recognizing the relative influence of difference sources of authigenic minerals, changes in pore fluid compositions and the extent of rock-water interactions throughout the diagenetic history of the St. Peter. Seven cores from eastern Iowa and southwestern Wisconsin were sampled for this study. These core samples are composed of the middle Tonti Member of the St. Peter which exhibits extreme compositional and textural maturity. The composition of the clay and silt fractions was identified using X-ray diffraction. The clay composition of the St. Peter in Iowa was described in detail to supplement previous studies. Scanning electron microscopy followed for analysis of mineral and surface textural relationships. The authigenic minerals identified were illite, smectite, chlorite, kaolinite, potassium feldspar, quartz, pyrite, calcite and dolomite. Illite and mixed-layer clay were dominant in all cores. The clay mineral suite of the Iowa cores was slightly more diverse than in the Wisconsin cores. Both frosted and sugary surface textures are recognizable on St. Peter sand grains. The sugary textures are caused by the dominance of terminated overgrowths on grain surfaces. This study has documented that the frosted surface texture is produced by encrusting quartz overgrowths. In fact, St. Peter grains show a complete dominance of precipitation- related features. A paragenetic sequence, previously described by Odom, Willand, and Lassin in 1979, was generally observed. It indicates a trend of increasingly acidic conditions throughout the diagenetic history. Associated lithologies are interpreted to have contributed saline fluids to the St. Peter, which provided much of the source material for authigenesis.