A comparison of coral zones from within the Devonian Lingle (St. Laurent) Formation from the Illinois Basin

Corals show differences in the amount of deformation between two coral zones from the Middle Devonian Lingle Formation (St., Laurent) in the Illinois Basin. The two zones studied occur between 189 and 208.4 feet below the surface of Illinois State Geological Survey core no. 14845 (Douglas County, Illinois). The dominant lithology of the beds containing these zones is a grey-brown limestone (Lasemi and Berg, 2001). The two coral zones are relatively taxonomically uniform with abundant Heliophyllum specimens. Identifying the specimens in core presented some challenges, as only one view is typically available. At least one colonial coral, Hexagonaria potterensis(?), occurs in the upper coral zone. The taphonomy of specimens within the two zones differs quite markedly, with the upper one exhibiting coral specimens that are quite deformed in contrast to the lower zone. Methods outlined in a paper by Cooper (1990) were used to determine the strain rate of the deformed corals in each zone. Both the short axis and the long axis of the rugose corals were measured. The strain rate was then calculated by dividing the length of the long axis by the short axis. Eighty-nine specimens were measured in the upper bed; thirty-seven were measured in the lower bed. Statistics have been run on the measurements to determine if they are statistically significant. These rocks, deposited as part of the Kaskaskia Sequence, were probably influenced by tectonics resulting from the Acadian Orogeny. That may have deformed the corals in the upper coral zone. However, we are puzzled by why the lower coral zone is not deformed. Another possible cause of the deformation would be overburden, but again, we would expect both the upper and lower corals zones would be similarly deformed. The upper coral zone corals are shortened vertically, being elongated in the horizontal direction. While we see no discontinuity between the two coral zones within the core, we believe that slumping might have occurred in the upper bed causing the deformation.