ABSTRACT

High Na-HCO₃ pore fluids present in many areas of the shallow Gulf Coastal Plain are the product of pervasive, meteoric diagenesis of siliciclastic host sediments. This diagenesis is driven in part by the subsurface generation of CO₂ and the reaction of carbonic acid with Na-bearing silicates. While a significant body of water analyses exists, there has been comparatively little study of the mineralogy and petrography of the altered sediments themselves. An analysis of sediment samples from shallow boreholes and wells along a 100-km hydrologic flow path in Upper Cretaceous sands and clays in northeastern Mississippi reveals the presence of spatially systematic variations in sediment alteration and clay mineral diagenesis. Kaolinite is the dominant clay mineral phase in the updip, upgradient portion of the flow system and has apparently been produced in part by the in situ breakdown of feldspars. The relative proportion of kaolinite progressively decreases downgradient, and smectite becomes the dominant clay mineral in the distal portions of the flow system. Smectite is most abundant where meteoric waters are mixing with high-NaCl fluids migrating up from depth. Siderite is a common authigenic cement in the updip portions of the system, and pyrite is present in some downgradient samples. The results of the study are thus in general agreement with the earlier work of Lee (1985), who inferred from systematic variations in the chemistry of waters along this same flow path that mineral diagenesis involving aluminosilicates and Fe-bearing phases exerts an important control on the geochemical evolution of these fluids. Of interest in deeper basinal studies is the role that pervasive, shallow diagenetic precipitation of carbonate cements and authigenic clays, such as we have documented, may play in the subsequent porosity and permeability development of Gulf Coast sediments during deep burial.