DEPOSITIONAL SETTING, LITHOLOGIC CHARACTER, AND ORIGIN OF CHERT IN THE BOONE FORMATION (LOWER MISSISSIPPIAN), NORTHERN ARKANSAS

The Boone Formation of northern Arkansas is a sequence of various carbonate lithologies interbedded with chert that reflect maximum flooding and highstand conditions of a single, third-order, eustatic Lower Mississippian (Osagean) cycle. Lower Boone maximum flooding produced calcisiltites interbedded with dark gray to black chert associated with compaction phenomena that disrupt bedding. This interval is interpreted as penecontemporaneous carbonate replacement in a deep water setting below the sediment-water interface. Upper Boone highstand conditions returned shallow platform environments to the region producing grain-dominated carbonates associated with white chert that exhibits an obvious groundwater replacement signature along bedding planes. A type 1 sequence boundary occurs at the top of the Boone in northern Arkansas. Succeeding Chesterian carbonates of the Hindsville Formation develop a basal chert breccia obviously derived from the upper Boone Formation, and constraining the time of silicification to the middle Mississippian. Origin of the silica producing the Boone chert is problematic, but historically assigned a biogenic origin. Insoluble residue of the upper Boone from northern Arkansas contains an abundant sand size fraction comprising composite grains of platy minerals that resemble, and presumably represent, volcanic ash. Chemical analysis of these grains indicates a felsic composition consistent with a quartz-rich granitoid. We believe that an origin as reorganized volcanic ash is a better explanation of the extensive, seemingly blanket regional distribution of Boone chert development, its association with a variety of carbonate facies and depositional settings, and its correlation with the Arkansas Novaculite in the Ouachita Mountain orogen.