Abstract

The Fayetteville Shale (Miss.) is a black marine shale that was deposited on a southward-deepening ramp in northern Arkansas. It is conformably overlain by skeletal-oolitic grainstones of the Pitkin Limestone, and together, they form a shoaling-upward succession of 1) "deep" muddy shelf, 2) storm-dominated muddy shelf, and 3) ooid-skeletal shoal and shoreface facies. Rhythmically bedded limestones of the upper Fayetteville Shale and some lower Pitkin ooid grainstones are interpreted to be shelf-storm deposits because of their stratigraphic position and bedding sequences. Upper Fayetteville limestone beds are transitionally positioned between cephalopod-bearing black shales ("deep" muddy shell) underneath and cross-bedded ooid grainstones (shoal and shoreface) above. Fayetteville and Pitkin beds are similar to other reported shelf storm deposits in that they contain hummocky cross-stratification (HCS) and unidirectional, climbing-ripple cross-stratification thought to have been laid down by combined flows. Six principal types of bedding sequences were noted, in order of increasing water depth and/or distance from the inferred shore: 1) cross-bedded and planar-laminated beds with some keystone vugs--beach; 2) graded units of coarse skeletal-ooid grainstone--shoreface; 3) planar-laminated and hummocky cross-stratified limestones--lower shoreface; 4) hummocky cross-stratified limestones and thin shales--storm-dominated muddy shelf; 5) fine grainstones with parallel laminae in lower half followed above by climbing-ripple cross-stratification--storm-dominated muddy shelf, and 6) structureless lime mudstones and black shale--"deep" muddy shelf. Reconstruction of the ancient shelf suggests that it sloped southward at 0.08 to 0.14 degrees , and the water depth at 30 km (estimated width of storm-dominated shell) was 40 to 70 m. The paleolatitudinal position of northern Arkansas (5-15 degrees S) indicates that strong tropical storms or hurricanes, rather than intense winter storms, were responsible for eroding carbonate sediment from lower-shoreface environments and depositing it as much as 30 km offshore in the muddy shelf.

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