SIZE FREQUENCY OF ECHINODERM BIOCLASTS IN UPPER MISSISSIPPIAN, SAND-SIZED SEDIMENT

Encrinites, a common Mississippian lithology, are rocks composed largely of echinoderm bioclastic sediment formed from debris generated by the disarticulation of crinoids, blastoids, and other echinoderms. Most encrinites are coarse sand grainstones/rudstones and composed of large stem and calyx elements. However, the vast majority of echinoderm skeletal elements are fine sand-sized suggesting that a large portion of Mississippian fine sand-sized sediments may also be derived from echinoderms. To test this supposition, sediment was collected from a single horizon of the Sloan Valley Member of the Pennington Formation near Berea, Kentucky. The site was selected because the sediments were deposited in quite lagoonal water as indicated by the articulation of delicate fossils, the lack of current indicators in the sediment, and the deposition of clay sediments. In this environment, echinoderm carcasses disarticulated in situ and were buried without differential transport of skeletal debris. Sediment was collected from a single bedding horizon and prepared by disaggregating and decanting the clay-size fraction to isolate sand-sized particles. The remaining bioclastic residue was sieved and each size fraction was weighed. A representative subsample from each size fraction was randomly chosen and point counted to yield a percentage of echinoderm bioclasts in that sample. Percent mass of echinoderm bioclasts by size fraction was as follows: greater than -3φ, 11.9%; -2φ to -3φ, 20.5%; -1φ to -2φ, 38.1%; 0φ to -1φ, 45.1%; 1φ to 0φ, 45.5%; 2φ to 1φ, 41.7%; 3φ to 2φ, 36.5%; 4φ to 3φ, 29.0%. In 1.0 kg of sand-size sediment, the mass of echinoderm bioclasts per size fraction is as follows: greater than -3φ, 4.9g; -2φ to -3φ, 19.7g; -1φ to -2φ, 59.5g; 0φ to -1φ, 58.6g; 1φ to 0φ, 70.5g; 2φ to 1φ, 64.0g; 3φ to 2φ, 62.5g; 4φ to 3φ, 28.1g, and echinoderms comprise 36.8% of the sand-size sediment. Echinoderms make up a large percent of the fine sand fraction in terms of both percent and total mass. It is therefore reasonable to infer that echinoderms were a major producer of fine sand-sized sediments in the Paleozoic.