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Shoaling-upward carbonate shelf cycles are known to occur in many parts of the world and throughout the geologic record. A typical shoaling carbonate cycle begins with deep-shelf, interbedded, lime mudstones and shale. These are succeeded by open-shelf bioclastic packstones and grainstones, and finally in many places by oolite, the latter representing the culmination of the shoaling process. Subsequent cycles repeat this basic pattern. When correlated across a shelf, the cycles are thought to represent the episodic progradation of a carbonate sand platform that built up to wave base, subsided, and built up again.
Upper Mississippian (Chesterian) strata in northern Arkansas consist of alternating limestones and shales that include a set of shoaling-upward cycles. These cycles, occurring within the dominantly carbonate Pitkin Formation, differ from the pattern described above in that they contain a significant terrigenous component. This material exists both as fissile black shale within lower-cycle lime mudstone, and as quartz sand admixed within upper-cycle oolite grainstones. This dual occurrence is thought to be related to movement of the shoreline that accompanied the repeated outbuilding of a carbonate platform. At the beginning of a cycle, the shoreline was distinct from the deeply submerged shelf and lime and terrigenous mud accumulated in a quiet-water setting. Thickness and facies anal sis of the Pitkin indicate an east-west trending shelf with a shoreline located in southern Missouri, perhaps curving south around the Ozark uplift. Eastward increase in shale percentage in the Pitkin suggests transport of fine clastics around the southeast flank of this uplift. The Illinois basin was an active Chesterian depocenter and during periods of maximum transgression fine clastics may have passed through a seaway southwest into northern Arkansas. As shoaling proceeded, the shoreline regressed south with the advancing carbonate sand sheet, cutting off the avenue of fine clastics. Quartz sand, probably derived from a thick sequence of lower Paleozoic sandstones exposed along the uplift by the retreating sea, became admixed with oolite across the newly built platform. These shoalin cycles are the product of slow, steady, subsidence along a northern Arkansas shelf punctuated by episodes of relatively rapid carbonate sand sedimentation and clastic influx. A similar tectonic and depositional setting must have given rise to other such deposits that occur in the geologic record.
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