Tripolitic Chert Development in the Mississippian Lime: New Insights from SEM
University of Arkansas
The Lower Mississippian succession in the southern midcontinent is an unconformity-bounded, third order, eustatic cycle with superimposed fourth and fifth order cycles. Lithostratigraphically, in Missouri/Oklahoma and Arkansas, the TST is referred to the Bachelor, Compton, Northview and Pierson Formations = St. Joe Limestone; the MFI is referred to the Reeds Spring Formation = Lower Boone Formation; and the HST/RST is referred to the Elsey, Burlington-Keokuk Formations = Upper Boone Formation. The MFI is characterized by penecontemporaneous chert – black to dark gray, compaction phenomena/disruption of bedding, shrinkage fractures, lack of included macrofossils, low carbonate content. The HST/RST is characterized by later diagenetic chert – white to cream, obvious silicification fronts and replacement along limestone bedding planes, replication of original textures, favors finer grained lithologies, commonly fossiliferous, high carbonate content – youngest portions of RST lack chert. Late diagenetic chert is a groundwater phenomenon and was already in place by the beginning of the Chesterian deposition.
Tripolitic chert forms by decalcification – removal by groundwater solution of carbonate grains, mostly bioclastic, included within the chert. There is not a high enough percentage of carbonate in typical Reeds Spring penecontemporaneous chert to produce tripolitic chert. In the Lower Mississippian succession of the southern midcontinent, the only chert that contains enough carbonate to be leached and form tripolitic chert occurs in the Elsey, Burlington-Keokuk Formations = Upper Boone Formation. In the Ouachita region, tripolitic chert development in the Arkansas Novaculite is restricted to its upper division, which also turns out to be calcareous.
The tripolitic chert interval experienced a second invasion by silica-bearing, hydrothermal water that allowed euhedral, quartz crystal druse to form in voids produced by the earlier decalcification. This water is probably the same medium that deposited the lead and zinc in the tri-state region suggesting that tripolitic chert formation is not older than Paleozoic age.
AAPG Search and Discovery Article #90201 © AAPG Education Directorate Mississippi Lime Forum, February 20, 2014, Oklahoma City, Oklahoma