Genetic Stratigraphy of Uranium Host Facies, Tordilla Sandstone Member, Upper Jackson Group, Panna Maria, Texas: ABSTRACT

Abstract

Uranium mineralization in the Panna Maria, Texas, area occurs in the Tordilla Sandstone Member of the Eocene upper Jackson Whitsett Formation on the reduced side of a linear alteration front that extends more than 3 mi (5 km) along strike. Two open-pit mines at Panna Maria expose parts of a strike-oriented sand belt associated with a barrier island-tidal inlet system. The east pit exposes (1) a tidal inlet-embayment entrance, overlain by (2) tidal-channel, and (3) tidal-flat facies. The inlet facies, the uranium host, is dominantly fine to very fine sand End_Page 759------------------------------ with multidirectional trough and planar cross-bedding, Ophiomorpha sp., rounded mud clasts, and mud blocks. The tidal channel cuts through tidal-flat deposits, is approximately 40 m wide and 11 m thick, and contains gently dipping accretion beds of fine to very fine sand, including small-scale cross-stratification, and upwardly decreasing sand bed thickness. The lithology and sedimentary structures of facies 3 strongly resemble classic tidal-flat deposits of the Wadden Sea. The west pit exposes a regressive sequence of (1) tidal-inlet fill, (2) a poorly represented upper shoreface-beach-barrier flat facies, overlain by (3) bay-lagoon deposits. The inlet facies of fine to very fine sand includes large-scale, low-angle planar cross-beds, Ophiomorpha sp., bipolar, planar cross-beds, and has a scoured base. Some zones are highly burrowed. Facies 2 consists of trough and planar cross-bedded fine to very fine sand, with low-angle parallel bedding, root traces, and woody fragments near a hummocky upper surface. Facies 3 includes an ashy, burrowed lignite overlain by a massive, burrowed clayey siltstone containing plant debris. Facies 1 and 2, as well as the base of the lignite bed, are uranium bearing. End_of_Article - Last_Page 760------------