TAPHONOMIC EPIBOLES GENERATED BY DISTAL TEMPESTITES FROM THE MAQUOKETA SHALE (RICHMONDIAN) OF PIKE COUNTY, MISSOURI

Cyclical deposits of alternating silty, lime-mudstones and bluish-gray shale occur in the lower part of the Maquoketa Shale in northeastern Missouri. These beds are equivalent to the Elgin Member of the Scales Formation, Maquoketa Group of Iowa and northern Illinois.

Cyclic obrution deposits containing exceptionally well preserved specimens of Isotelus iowensis (Owen) have been observed from 1 to 6 m above the contact with the Kimmswick Limestone. This series of beds fits the definition of a taphonomic epibole proposed by Brett and Baird. In some instances, preservation of the carbonized alimentary canal-wall is observed. Biostratinomy of the trilobite carcasses is random in certain beds: cephalic direction yields all points of the compass, some specimens are enrolled, others out-stretched upside down or right side up or at some angle to bedding. A few specimens were found in situ at 90 degrees to bedding and were subsequently deformed and folded along the axial lobe due to compaction.

Anoxic muds, near storm wave base were possibly stirred by massive paleo-hurricanes. As a result, fine sediment moved down gradient as a density “cloud” transporting some organisms and burying others. Sedimentological evidence include: scoured base at the beginning of the cycle, graded bedding, grain size ranges from coarse silt to fine silt to clay and taphonomy of the organisms. Besides articulated isotelids, other whole organisms are rare but include: delicate ramose bryozoans, crinoids, conularids, other trilobite species, graptolites, and brachiopods. Not all beds yield the articulated fauna. Some beds may have been generated by proximal tempestites, tsunamites and others show bioturbation. However, no burrowing has been found in beds where the well preserved trilobite fauna occur.