ABSTRACT

Pisoids in the Upper Jurassic Smackover Formation of the Conecuh Embayment (Alabama-Florida) exhibit textural characteristics of marine vadose diagenesis. They occur in prograding, repetitive shoaling-upward sequences which are separated by truncations reflecting transgressive episodes. At least eight such prograding cycles recur. Each sequence consists of a lower unit composed of fenestral-textured pelletal mudstone and an upper unit of vadose pisolite.

The pisoids exhibit features indicative of non-algal, in-place accretionary growth, including isopachous multicoatings, directional growth, perched inclusions, reverse grading, leaching, and brecciation and fracturing interpreted to be a result of subaerial exposure. The pisoids formed at the expense of pre-existing pelletal mudstone as indicated by progressive alteration and replacement of the original mudstone texture. In interpisoid pores a fibrous cement coats the pisoids (a first-generation cement). Equant crystals of dolomite line the fibrous cement, and the center of the former interparticle pore is filled with drusy crystals or megaspar of calcite.

These textures recall Quaternary analogs in the Persian Gulf and the Permian Capitan Reef complex of west Texas-New Mexico. In the Persian Gulf these features form in a hypersaline vadose diagenetic setting. By comparison, origin in an arid, Persian Gulf type of environment is inferred.