ORIGIN OF THE CHERT IN THE BOONE FORMATION, LOWER MISSISSIPPIAN, TRI-STATE AREA, ARKANSAS, MISSOURI AND OKLAHOMA

The Lower Mississippian (Osagean) Boone Formation in the tri-state region, southern midcontinent, Arkansas, Missouri and Oklahoma, represents the maximum flooding, highstand and regressive intervals of a single, third order transgressive-regressive carbonate cycle bounded by regional unconformities. The lower Boone represents maximum flooding conditions and consists of calcisiltites with interbedded dark, nodular chert. This chert exhibits compaction phenomena that disrupt the bedding and shrinkage fractures from the loss of water in the change from Opal-A to Opal-CT to microcrystalline quartz. The phenomena suggest a penecontemporaneous origin, from reorganization of silica immediately below the sediment-water interface. The upper Boone represents highstand and regression during the late Osagean, and consists primarily of carbonate grainstones and packstones. This interval contains white, later diagenetic chert that exhibits an obvious groundwater replacement signature of the coarser carbonate along its bedding planes.

The source of the silica producing the chert in the lower and upper Boone Formation has been historically assigned a biogenic origin, but recent studies suggest a volcanogenic source. Chemical analyses of composite grains found within the insoluble fraction of the upper Boone carbonates indicate a felsic composition consistent with a quartz-rich granitoid igneous lithology. Elemental data determined by energy dispersive x-ray (EDX) of both lower and upper Boone chert include a significant weight percentage of both aluminum and potassium compositionally consistent with volcanic ash, and unlikely in biogenic silica. Further bulk geochemical and trace elemental evidence of a volcanogenic source of the silica for both penecontemporaneous and later diagenetic chert, which represent entirely different processes of formation, is provided by analyses utilizing a Quadrupole Inductively Coupled Plasma Mass Spectrometer (Q ICP-MS). The volcanogenic silica source is likely an island arc system associated with the Ouachita Mountain chain parallel to the Laurasian cratonic margin. This volcanogenic silica also produced the Arkansas Novaculite that has an equally high weight percentage of aluminum and potassium, and certainly does not represent a biogenic silica deposit.