Clay-Mineral Transformation as an Indicator of Depositional and Diagenetic Conditions in Pennsylvanian Black Shales, Paradox Member of the Hermosa Formation, Utah and Colorado

Abstract

The dominant clay minerals in the Paradox basin black shale facies include a detrital, aluminum-rich illite/smectite and a series of magnesium-rich, interstratified chlorite/smectite minerals. Black shales from the Gothic and Chimney Rock layers of the Paradox Member of the Hermosa Formation were sampled from 13 wells in southwestern Colorado and southeastern Utah. X-ray powder diffraction (XRD), scanning electron microscopy (SEM), Inductively-Coupled Plasma (ICP), and Rock-Eval pyrolysis were used to determine the nature of a reaction believed to consume aluminum-rich detrital clays to precipitate magnesium-rich authigenic clays. The potential for magnesium enrichment in the clay assemblage was found to be influenced by the following processes: particle size as a function of distance from detrital source area, paleosalinity as a function of transgressive-regressive stage, and the presence of large amounts of organic matter. The basic reaction that forms chlorite/smectite or corrensite (perfectly ordered chlorite/smectite with 50% smectite layers) from detrital clays is Illite/Smectite (detrital) + Mg(aq)[sup 2+] + (OH)[sup [minus]] [yields] Corrensite + K[sup +] + Al[sup 3+]. Farther from detrital source areas, this recreation proceeds easily. Within this constraint, the remaining variables either aid or inhibit the proposed reaction from taking place. Higher salinity during deposition results in more chlorite layersmore » within the chlorite/smectite structure. Large amounts of organic matter strongly inhibit the reaction from occurring, possibly due to pH effects from the production of organic acids during burial.« less