MINERALOGY OF LATE CRETACEOUS METHANE COLD-SEEP CEMENTS OF THE PIERRE SHALE, SOUTH DAKOTA

The stage of maturity of methane cold seeps can be determined by an examination of the mineralogy of seep cements. Modern methane cold-seeps contain differing mineralogies at varying stages of development, spanning early and late stages of maturity. Previous investigations of ancient methane seeps have also shown shifts in mineralogy depending on the stage of maturity. These previous investigations of ancient seeps have only established early and late stage mineralogies. Within the Pierre Shale, South Dakota, cold seeps contain an even greater disparity in mineralogical assemblages than those already documented. A total of 34 seep deposits were analyzed using electron dispersive spectrometry to determine their mineral assemblages, and then the data was entered into ArcGIS to document the existence of spatial patterns. There are four distinct mineralogies present: smectite-rich, kaolinite-rich, calcite-rich and Mg-calcite-rich. These mineralogies reflect the shift from a soft, clay-based substratum to a hard, carbonate dominated substratum as the seep matures and carbonate pavements are formed through microbially mediated processes. Through this investigation four distinct stages of seep development have been established ranging from incipient to moribund. Furthermore, there are distinct spatial patterns suggesting seep fluids escaped along fault lines, as earlier investigations have suspected. The use of mineralogic analysis can be used in the field to 1) determine the stage of maturity of a seep and 2) verify ancient methane seep source localities.