Print this pageOrigin of Chert in Mississippian Monte Cristo Limestone Formation, Southern Nevada, and Its Relationship to the Antler Orogeny
Lower Mississippian carbonate rocks in southern Nevada were deposited along the proto-North American continental shelf in environments ranging from evaporative supratidal to subtidal. Within the Lower Mississippian succession, abundant stratabound nodular chert deposits first appear within the Kinderhookian to Osagean Monte Cristo Formation. Field and petrographic evidence at Sloan, located ten miles south of Las Vegas, indicates an early formation of the chert in a marine environment, possibly occurring penecontemporaneously with dolomitization. Silicification was controlled by the distribution of porosity, permeability, faunal and evaporite content of the pre-existing sediment. Replacement of algal structures by chert is particularly common in the Anchor member of the Monte Cristo Formation. The origin of silica and the mechanisms that lead to the formation of chert are poorly understood in this area, as are many other Paleozoic shelf carbonate-chert successions in the western Cordillera.
In the Monte Cristo Formation, geochemical analyses strongly suggest a non-biologic origin of the chert, ruling out sponge spicules or radiolarian as the primary silica source. We hypothesize that the principal source of the silica is related to a combination of processes associated with the ongoing Antler orogeny to the west; including direct input of volcanic ash, shedding of siliceous arc sediment, localized sea level changes due to tectonic loading allowing influx of meteoric fluids. Understanding the relative influence of orogenic events on the origin and properties of chert in this area will provide a model that can be applied to nodular chert - carbonate associations in other regions.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009