Pennsylvanian Silicification of Shallow Marine Fossils in the Jemez Mountains, New Mexico

The frequently red silicification of northern New Mexican Pennsylvanian fossils, especially brachiopods, is well-known to collectors but, as of yet, has not been described in detail or explained in the geological literature of New Mexico. In general, the process is interpreted as early, shallow burial, replacement diagenesis produced in calcite skeletons surrounded by marine water enriched in dissolved silica. Usually, fossil shells in limestones are silicified, while the enclosing calcite matrix is not. This is because replacement silicification is favored in the microscopic environments in the shell that contain organic matter, which upon decay, produces the more acidic conditions necessary for silica precipitation concomitant with calcite dissolution. Pore-water enrichment is ascribed to episodic increases in dissolved silica, principally produced either by variations in the delivery of primary silica by rivers or by changes in the delivery of wind-blown dust, which could be climatically controlled. Two occurrences that have been linked to red silicfication at other localities are not petrographically abundant in these rocks, namely sponge spicules or pyrite.