North-Central Section - 50th Annual Meeting - 2016

Paper No. 29-20
Presentation Time: 8:00 AM-12:00 PM


GABEL, Vanessa1, WITTMER, Jacalyn M.1, HEADS, Sam2 and GUENSBURG, Thomas E.3, (1)Geology, University of Illinois at Urbana - Champaign, 605 E. Springfield St, Champaign, IL 61820, (2)Illinois Natural History Survey, University of Illinois at Urbana-Champaign, Forbes Building, 1816 S. Oak Street, MC-652, Champaign, IL 61820, (3)Physical Science Division, Rock Valley College, Rockford, IL 61114,

A crinoid-stromatoporoid reef complex has recently been discovered within the Kimmswick Formation (Middle Ordovician) of Jefferson County, Missouri. This represents the first true reef of the Kimmswick, as well as an extremely rare fossil assemblage, including potentially new species of edrioblastoids, algae, and stromatoporoids. Prior to the discovery of this locality, the Kimmswick has been described as a monotonous, extensive crinoid grainstone; this study identifies a source for the grainstone, changing previous paleoenvironmental interpretations of this region.

Based on Wilson’s Model of Carbonate Ramp Facies, this complex comprises three distinct shallow carbonate facies: (1) Foreslope dominated by bryozoan-crinoid rudstone and floatstone; (2) Organic reef build-up composed of edrioblastoids, cystoids, stromatoporoids, crinoids, bryozoans, tabulate coral, and an undetermined binding organism; and (3) Massive crinoid grainstone shoaling off the reef. The organic build-up suggests that this is a true reef, rather than a bioherm, due to the in situ growth of the framework organisms.

The scope of this study is to characterize the Kimmswick reef, primarily by investigating the paleoecological roles of the edrioblastoids and the unknown binding organism. Field methodology includes constructing detailed stratigraphic sections from the reef site to help define facies transitions; samples taken from the site will be further studied in thin section in order to describe the internal structures of the edrioblastoids and the binding organism, thus giving them taxonomic classification.

Comparable carbonate systems with similar faunal compositions may be found in eastern New York and Ontario as a result of oceanic circulation from the Sebree Trough; however, these occurrences formed bioherms, rather than reefs. Correlations between the organisms comprising the reef of this study, and similar occurrences of these organisms world-wide, will narrow their evolutionary window and shed light on how this circulation may have affected evolution in the mid-continent during this time. This data will help determine if this reef is isolated, or a component of a more extensive system, and will lend insight to the paleoenvironment and paleoecology of the Midwest throughout the Middle Ordovician.