PALEONTOLOGY OF THE UPPER SNYDERVILLE SHALE MEMBER (OREAD FORMATION, SHAWNEE GROUP, VIRGILIAN STAGE ) I-229, ANDREW COUNTY, MISSOURI

The upper 36 cm (14 in.) of the Snyderville Shale Member of the Oread Formation (Shawnee Group, Virgilian) located on I-229 northwest of St. Joseph, Andrew Co., Missouri, represents deposits of the early transgressive sysytems tract (TST) resting upon the marine flooding surface, and lower sequence boundary, of the overlying Plattsmouth cyclothem. The lower part of this interval is essentially a myalinid shell bed within a light-gray shale, with discontinuous lenses of intraclastic biomicrite. The grains in the intraclastic biomicrite are mostly dark-gray micritic intraclasts, myalinid clams, microgastropods and clams, Edmondia, Aviculopecten, Derbyia, productid brachiopod fragments, and ostracodes. Myalina and Septamyalina dominate the shell bed fauna with lesser numbers of Aviculopecten, bellerophontid gastropods, astartellid clams, and orthoconic nautiloid cephalopods. Most of the myalinids are complete and unbroken, with many still articulated in probable life position. Most of the myalinids have been heavily encrusted by algae and serpulopsid foraminifers along with rare leptalosiid brachiopods, bryozoans and Phosphannulus. The most common borer is acrothoracian barnacles. Disarticulated myalinids are also encrusted, but seldom bored, on the interior. The upper part of the shale contains a fauna dominated by brachiopods Juresania, Derbyia, and Linoproductus, with lesser numbers of bryozoa, crinoids, echinoids, ostracodes, microgastropods, and ammodiscoid foraminifers. We propose that the myalinids accumulated, as the lower part of the shale was deposited in a shallow subtidal environment of a relatively small, restricted lagoon, with the intraclastic biomicrite deposited as lags during storm events. The upper part of the shale with its more diverse fauna was deposited in more open marine conditions as the sea transgressed over this area.