CARBON AND OXYGEN ISOTOPE ANALYSIS OF A CARBONATE MUD-MOUND IN SOUTHWEST MISSOURI

Carbonate mud-mounds are a classic sedimentary feature of the Paleozoic and have been discovered throughout the world. These mounds appear in a variety of different shapes and sizes and many localities have been studied in great detail. However, the origin of these mounds remains controversial, with paleogeographic controls (light and water chemistry), structural controls (faults, vents, and sliding), and biogenic controls (baffling organisms and hydrocarbon fed microbial production) all having been proposed. Distinguishing which mechanism was responsible for the development of an individual set of mounds has proven difficult, and complications are further inflated with the likelihood that more than one mechanism may have been the cause.

Carbon and oxygen isotopic analysis of a carbonate mud-mound in southwest Missouri was performed to distinguish whether or not the mound itself is isotopically unique when compared with a coeval section that does not contains mounds with the goal to determine whether or not isotopic analysis could help confirm or eliminate some origin mechanisms. The mud-mound near Jane, Missouri is 4 m thick and exposed in cross-section. It is predominantly fine-grained peloidal limestone, and is the largest of many exposed along road cuts in the lower Mississippian (Kinderhookian Series) Compton Limestone. Mean carbon isotopic values for both sets of samples vary by only 0.2 ‰ (vs VPDB), however stratigraphic variation is nearly 2 ‰. Stratigraphic variation is 1.5 – 2 ‰ for δ¹⁸O (vs VPDB) values, however mean oxygen isotopic values for the mound are offset by nearly 1 ‰ from the section without mounds. When plotted on a δ¹³C vs δ¹⁸O diagram the samples taken from the mound represent an isolated population from the non-mound bearing section. This divergence in δ¹⁸O could be the result of a period of warming (or equivalently shallower depth) during which the mound grew, or the transport of a dislodged mound or slide block that accumulated in an upslope setting. The latter seems less likely as both sections indicate similar changes in δ¹⁸O at the same stratigraphic level. Future work will establish higher resolution intra-mound variation in δ¹³C and δ¹⁸O and to try to assess whether or not this variation is mirrored in other mounds in the region.