RECUMBENT FOLDS IN PENNSYLVANIAN SANDSTONE: SEISMITES IN MUSCATINE COUNTY, EASTERN IOWA

The Spoon Formation sandstone of Muscatine County, Iowa consists of near-shore marine, fluvial deposits from the Pennsylvanian period (spanning the Atokan – Desmoinesian boundary). At Wildcat Den State Park these sandstones are cross-bedded, showing southwesterly paleoflow, with abundant secondary iron oxide deposits. Cliffs south of "Devil's Punchbowl" contain deformed cross bedding in the shape of closed recumbent folds, with hinges pointing upstream. Field observations and lab analyses indicate these folds originated penecontemporaneously by soft sediment deformation, possibly due to seismic shaking.

At the outcrop scale, the deformed structures occur in three distinct beds. The zone of deformation ranges horizontally from 1-5 m and is surrounded by undisturbed cross bedding on either side. In hand sample, the fold laminae are obscured by the convoluted nature of the Liesegang bands. However, in thin section the fold laminae become visible. Across the laminations, the average grain size decreases from fine sand (1/4 - 3/8 mm) to coarse silt/very fine sand (1/20 - 1/8 mm) and then increases back to fine sand. The grains also show an increase in angularity; changing from sub-rounded to angular and then back to sub-rounded.

The presence of sedimentary laminations indicates that these structures are more than just unusual looking Liesegang bands; they are recumbent folds possibly caused by liquefaction induced by seismic shaking. The liquefied sediment was apparently overturned by the current of the river. Additional field observations show that these features fit the criteria for seismic origin. The disturbed beds are confined to single, horizontal layers and do not penetrate bedding planes. The folds were presumably deformed at the surface, and, as the work of others has demonstrated, the folds formed quickly, making a seismic origin plausible. Most importantly, the area is known to have been seismically active during the Pennsylvanian.

Future research in this subject may be able to correlate the occurrence of similarly aged structures throughout Iowa, which may lead to further conclusions about mid-continent paleoseismicity during the Pennsylvanian.