Five major tuff sequences (8-120 ft thick) are interbedded with marine graywacke and shale in the 11,000 ft-thick Mississippian Stanley Group. They are present in the basal 1,500 ft and upper 350 ft of the highly folded flysch group. These widespread sequences are thickest and best exposed in the southern Ouachitas, but are traceable to the central Ouachitas.

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Three tuff sequences are composed of massive and bedded crystal tuff; 2 are composed of massive and bedded pumiceous vitric-crystal tuff. All five sequences have massive and sometimes laminated fine-grained vitric upper portions. Crystal-rich and pumiceous tuff sequences probably reflect different settling and/or eruptive histories.

Crystal tuff sequences originated from crystal-rich magmas or from crystal enrichment by gravity sorting of pyroclastic debris settling through long water columns, possibly as a result of vulcanian-type submarine eruptions. Bedded crystal tuff was deposited from a series of ash falls and tuffaceous turbidites. Widespread slumping of bedded crystal tuff produced massive crystal tuff.

Pumiceous tuff sequences probably formed from Katmai-like eruptions. Thick, nonwelded pumiceous vitric-crystal tuffs commonly overlain by thin-bedded pumiceous tuffs were produced from submarine pyroclastic flows covered by contemporaneous ash falls.

Fine-grained vitric tuff formed from slow settling of very fine ash. The ash was possibly the finest size remnants suspended in settling columns after major eruptions and/or was produced by weaker ash falls. Rare cross bedding is evidence for some current reworking. Tuff thickness, grain-size trends, paleocurrent indicators, and paleogeography suggest a southern volcanic source, possibly the buried Luling overthrust front in Texas.

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