Abstract

Argillized vitric-crystal tuff beds are present in a sequence of beds 18 m thick approximately 1200 m from the top of the Tesnus Formation in roadcuts along U.S. Highway 90. Tuffaceous material occurs in beds up to 5 m thick that include one graded turbidite and two structureless mass-flow deposits. The mass-flow deposits lack stratification, have a chaotic orientation of micas and plant fragments, and weather spheroidally. The upper part of each tuff bed contains discontinuous nodules, lenses, and disrupted beds of greenish-gray to black siliceous shale that were rafted into place in the mass flows. Tuff beds are separated by hemipelagic, green, fissile clay-shale beds up to 2 m thick.

Tuffaceous material is mixed with 5 to 80% terrigenous silt and clay, typical of Tesnus hemipelagites and turbidites. Tuffaceous components are now composed of 90% clay and 10% alkali feldspar. Shard and pumice ghosts comprise 40 to 60 percent of the clay and crystal ghosts comprise up to 10 percent of the clay. The < .5 μm clay is chiefly illite, ordered ISI, mixed-layer smectite-illite and minor chlorite, kaolinite, and celadonite(?). The < 0.1 μm clay fraction yields a Permian Rb/Sr age, which is the inferred age of authigenic clay formation.

The radiolarian assemblage from siliceous shale in the Tesnus tuff beds indicates a Chesterian through early Morrowan age, essentially the same age as the Hatton and Beavers Bend tuffs in the Ouachita Mountains.