Paleoecology and diagenesis of Parafavreina ziczac isp. nov., a crustacean microcoprolite from the Upper Jurassic (Oxfordian) Smackover Formation of Alabama

Crustacean microcoprolites, Parafavreina ziczac isp. nov., are among the most common fossils in the upper Smackover Formation (Upper Jurassic) of southwest Alabama. The new ichnospecies, Parafavreina ziczac, is characterized by its zigzag arrangement of canals having a triangular outline in transverse section. Associated fossils include photosynthesizing microbial buildups, calcareous benthic foraminifera, sponges, and echinoid ossicles along with rare to uncommon bivalves and gastropods. Other crustacean microcoprolites, Palaxius and Helicerina, are also present. The salinity is thought to have been about 50–60 ‰ to account for the very low diversity of organisms and dominance by thrombolites and stromatolites. The inferred paleoenvironment consisted of shallow, restricted basins with clear, well lit, relatively quiet, hypersaline water in an arid tropical climate. The taphonomy and diagenesis of the microcoprolites were complex. Some, but not all, pellets were leached of organic material. Most were hardened sufficiently that they underwent no noticeable distortion through burial-related compaction. A mosaic of diagenetic pathways, heterogeneous at scales down to that of individual particles, led to three end-member classes for P. ziczac. These are 1) moldic pores, 2) dolomitized pellets, and 3) microcrystalline carbonate little altered from the pellets’ depositional texture. P. ziczac is widespread in the Smackover Formation in rocks that contain few other fossils. This suggests that it might be a useful environmental indicator of hypersaline conditions. The geographic distribution of P. ziczac outside Alabama is not well known, but it has some potential as a biostratigraphic marker, even though it may be endemic to marginal basins along the Gulf of Mexico.