The Catahoula Tuff of Oligocene to early Miocene(?) age is composed almost entirely of volcanic rock debris or its alteration products. Pastel-colored tuffaceous clay predominates, but sandstone, conglomerate, bentonite, vitric tuff, and ash also are present. The rocks studied in outcrop have a maximum thickness of approximately 900 feet, and were deposited largely on a coastal plain not far from shore. Bed geometry and sedimentary textures and structures provide evidence that the coarse clastic sediments and some tuffaceous clay beds were deposited as mudflows. One bed of air-fall ash has been recognized by its geopetal fabric: conical piles of glass dust rest on flat shards.

Vitric tuff and tuffaceous clay beds deposited as mud flows are characterized by massive (structureless) beds 1-4 feet thick, lack of sorting in beds that have particles ranging from fine ash to pebble-size tuff intraclasts, orientation of shards in random or swirl patterns, and desiccation polygons. One variety of tuff is characterized by moderate induration and the presence of tuff pisolites. The pisolites are generally 1-5 mm. in diameter and comprise as much as 30 per cent of a rock. Pisolites differ from their matrix by having different amounts or different sizes of shards and, in places, an opaque rind. A few pisolites grade imperceptibly into matrix of identical composition and texture where part of the rind is absent. The pisolites developed in tuffs by soil-forming processes, probably in an arid climate with seasonal rainfall. Many pisolites were reworked into successive mud flows.

Another variety of tuff is well indurated and pervaded by sinuous tubules (up to 2 mm. in diameter) probably formed by plant roots. The tubules trend in all directions but are predominantly vertical. Tubules in some beds are filled with zeolite (heulandite group) or montmorillonite.

Stream-deposited tuff is recognized by faint horizontal laminations and cross-bedding and by moderate sorting and sub-parallel orientation of elongate shards and tuff intraclasts. This type of tuff is friable, non-pisolitic, and lacks tubules. Pores in many of these tuffs are now reduced in size or filled by clay skins of montmorillonite that coat framework grains.

Bentonite is free of crystal fragments, has no relic texture, and has a random orientation of montmorillonite particles. The lack of orientation of clay particles suggests that they crystallized during the in situ argillation of beds of glass dust rather than by sedimentation of clay particles in quiet water.

Clay dikes from 1 mm.-2 cm. wide trend vertically through tuff and sandstone at some localities. Montmorillonite grains are well-oriented parallel with the dike walls, suggesting particle-by-particle deposition from injected slurries.

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