EARLY MISSISSIPPIAN ENCRINITES: AN EXTREME CASE OF PELMATOZOANS DOMINATING THE CARBONATE DEPOSITIONAL SYSTEM AND AN EXAMINATION OF FACTORS CONTROLLING THEIR ABUNDANCE
During the Early Mississippian, echinoderms dominated the carbonate depositional system as no other clade before or since. This domination is most easily recognized in the numerous coarse-grained regional encrinites (accumulations of pelmatozoan debris that exceed 5 meters in thickness and extend over 500 km2) that developed on carbonate shelves and ramps during this time (Ausich, 1997). Although these large-scale deposits clearly represent a tremendous input of echinoderm grains into the sediment supply, analysis of the Joana and Lodgepole limestones of the western U.S. demonstrate that most of the carbonate sediments, from shallow near-shore to deep shelf, are dominated by echinoderm material ranging from coarse grained sands to fine-grained mud respectively. Echinoderm rich deposits within these formations include millimeter thick pavements, pods and lenses, sands, graded coarse-grained beds, pelmatozoan-rich wackestones and regional encrinites.
While encrinites are comparable to other depositionally formed biofabrics such as diatomites, oozes and chalks, encrinites are unique in that they occur in virtually all marine depositional environments and are a product of benthic invertebrates.
Although encrinites do occur throughout much of the Phanerozoic their ubiquity in Mississippian rocks requires explanation. Clearly pelmatozoans were one of the dominant invertebrate clades of the time. However, their abundance is likely due to a combination of intrinsic and extrinsic factors. Intrinsic factors may have included such adaptations as the development of holdfasts suitable for life on an episodically mobile substrate, or the rise of new species uniquely adapted to conditions present at this time. Extrinsic factors may have included a variety of conditions such as the presence of calcite seas, or a lack of rimmed shelves (e.g. no substantial reef formation). How these factors combined remains somewhat enigmatic but the clear result is the pelmatozoan domination of many Early Mississippian marine environments.