MICROFOSSILS FROM THE MIDDLE ORDOVICIAN ICEBOX MEMBER OF THE WINNIPEG FORMATION, NORTHERN BLACK HILLS, SOUTH DAKOTA: COMPARISON TO 'BUTTER SHALES' OF THE CINCINNATI ARCH REGION

The Middle Ordovician Icebox Member of the Winnipeg Formation is a distinctive mudrock unit that crops out sporadically throughout the northern Black Hills region of west-central USA. The lithologic characteristics of this unit, notably the unlithified character, clay-dominated grain size, and apparent internal consistency, are comparable to units known as ‘butter shales’ throughout the Upper Ordovician (Cincinnatian) succession of the Cincinnati Arch region. Recent work has shown that ‘butter shales’ contain a rich and diverse microfossil assemblage, dominated by chitinozoans, monaxial siliceous sponge spicules, scolecodonts, rhynchonelliform brachiopods, ramose trepostome bryozoans, and crinoid brachials and columnals. The present study seeks to determine whether the Icebox Member is similar in microfossil content in addition to lithologic properties. Samples were collected from an outcrop near Deadwood, South Dakota and processed using methods identical to those used to extract microfossils from Cincinnatian ‘butter shales.’ Although nearly identical in lithological properties, the Icebox Member is very different in fossil content from ‘butter shales,’ being characterized by a very sparse fossil fauna consisting entirely of organic-walled microfossils (chitinozoans) found exclusively within the finer (0.074 mm) residual fraction. No minute calcareous or siliceous skeletal elements were recovered and no fossils in the 0.125 mm residual fraction were identified, in stark contrast to ‘butter shales.’ These results indicate that significant paleoenvironmental differences existed between the Winnipeg Formation in the northern Black Hills and ‘butter shales’ of the Cincinnati Arch region, most likely related to diminished benthic oxygenation in the Icebox Member. Further, this suggests that lithologic motifs such as widespread, thin, clay-dominated units may occur variously throughout the stratigraphic record but nevertheless may contain a diversity of biofacies and taphofacies.