STRATIGRAPHY AND POSSIBLE TECTONIC INFLUENCES ON THE LOWER MISSOURIAN, MARMATON AND UPPER CHEROKEE GROUPS IN A CORE FROM THE UPLIFTED BLOCK OF THE MIDCONTINENT RIFT, IOWA

In 2013 a 500-foot long core, which was drilled near the top of the northwest limb of a syncline in southwest Iowa, was made available for study. This asymmetrical syncline is situated on the uplifted block of the Midcontinent Rift, and is located about 6 miles (10 km) northwest of the southern boundary fault. This is one of a series of small synclines and anticlines, trending northeast-southwest along the uplifted block, from southwest Iowa to north-central Iowa. Dips on the northwest limb of the syncline approach 30 degrees to the southeast, while dips on the southeast limb are only about 5 degrees to the northwest. Most of the late tectonic movement on the uplifted block is inferred to have occurred after deposition of the Late Pennsylvanian, but before the Dakota Formation of the Cretaceous, with over 220 feet of stratigraphic offset. There is some evidence to support relatively minor movement on the southern boundary fault during the late Cherokee (deposition of Floris Formation). Cyclothems in the lower Missourian and upper Marmaton groups of this core show little change compared to cyclothems seen in a core from the down-thrown block to the southeast. Unusual development of cyclothems in this core, in the lower Marmaton and upper Cherokee, suggests that there may have been some movement of the uplifted block during this time. More deep cores from the uplifted block need to be examined to substantiate these inferences.