Algal-Bryozoan Carbonate Buildups Within the Pitkin Limestone (Mississippian-Chesterian), Northwest Arkansas

More than 14 biohermal buildups have been recognized within the Pitkin Limestone (Mississippian-Chesterian) in eastern Washington and western Madison Counties, northwest Arkansas. These buildups resemble previously described algal mounds in upper Pennsylvanian strata of the Midcontinent region, but differ in their faunal and floral constituents. The mounds are composed of calcilutite with variable amounts of spar and fossil allochemical grains. Associated flanking facies consist of mixed biosparite near the core, grading outward into oosparite. A few zones of shaly, poorly washed biomicrite containing rounded clasts bearing Archimedes fragments are interbedded with the flanking mixed biosparite facies. These clasts appear to be fragments of the mound facies, and suggest that the lithified mounds were attacked by wave activity. The mounds developed from the entrapment of carbonate mud by cyanophytic algae (blue-green) and cryptostomous bryozoans. The mounds and flank facies appear to have originated in an area of relatively great turbulence, as indicated by oolite development, and thus were restricted inlateral expansion. Coincidence of lateral expansion of the mounds with deposition of an extensive mixed biosparite facies and an absence of oosparite development suggests less turbulent conditions