FIELD INVESTIGATION OF A CARBONATE TEMPESTITE THAT DEFIES EASY ENVIRONMENTAL INTERPRETATION: HELPING STUDENTS APPLY THE METHOD OF MULTIPLE WORKING HYPOTHESES

An outcrop in southern Illinois has features that do not seem to match known sedimentary environments. Sedimentology and Stratigraphy students study this site on a field trip. They are challenged to use the method of multiple working hypotheses (Chamberlin, 1890) to interpret the environment of deposition of a particular bed in the Joppa Member, Ste. Genevieve Formation, Valmeyeran Series, Mississippian Period. Many students quickly assume it is cross-bedded quartz sandstone. Closer observation with a hand lens and dilute acid reveal that the rock is limestone composed of invertebrate fossil fragments and ooids. Thin clastic laminae are also present. Confusion typically ensues. Many hypotheses are rejected and new ones are generated. Students inspect the top of the carbonate bed and find it very flat, truncating sedimentary structures within the bed. The base of the unit displays scour and loading features. Spatially, the bed seems limited to the local outcrop. As more observations are made, hypotheses appear and disappear. Students investigate the stratigraphic succession and find that muddy units below and above the carbonate are identical. Features in the mudstones point toward lagoon or estuarine environments. Students are faced with the puzzle of how the carbonate was inserted into a lagoon or estuary. Cross-bedding is unidirectional. As different students make key observations at different times, discussions occur as they compare notes. The exercise provides key elements of sedimentary field study: spatial distribution, vertical succession, textural composition, sedimentary structures, and environmental interpretation. The uniqueness of the outcrop adds the useful element of uncertainty. Students come away with the realization that geologic studies do not always provide neatly packaged answers. The experience is an excellent activity in using the method of multiple working hypotheses. At the conclusion of the exercise, students learn that published studies suggest this is a storm deposit. Miller and Reams (2011) specifically suggest this bed may be the result of erosion and deposition by a rogue wave.