R.R. Anderson, B.J. Witzke, and B.J. Bunker

The Geological Society of America
31st Annual North-Central Section Meeting
Madison, WI, May 1-2, 1997
1997 Abstracts with Programs, v. 29, no. 4, p. 2


The Midcontinent Rift System (MRS) is a Middle Proterozoic (ca 1100 Ma) failed rift that stretches southwestward from eastern Lake Superior, across north-central to southwest Iowa, and into central Oklahoma. Over most of its length the MRS is characterized by an axial horst flanked by clastic-filled basins. During its development, the horst began as an axial graben which filled with extrusive volcanic rocks that were subsequently uplifted as much as 10 km or more in Iowa to form the Iowa Horst. Although structural movements in the Phanerozoic were less dramatic, there is abundant evidence of recurring reactivation of various parts of the MRS in Iowa. Sauk isopachs delineate the Hollandale Embayment in Iowa and Minnesota, primarily developed east of the Iowa Horst. By the onset of Cambrian sedimentation, portions of the MRS in Iowa were apparently active. For example, the sequence of Paleozoic rocks in the Adair town well (southwest Iowa above the Iowa Horst) is missing the basal 50 m of Cambrian strata (Dresbachian-Franconian), the St. Lawrence Fm is thinned by 55%, and the Prairie du Chien Gp is thinned by 48%. By Tippecanoe time, the North Kansas and East-Central Iowa basins developed as regional depocenters. The axis of the trough that connected these basins lay directly over the eastern horst-flanking basins of the MRS in central Iowa, marking renewed subsidence. Silurian strata preserved in this trough show displacements of as much as 120 m along the Thurman-Redfield Structural Zone (TRSZ), the eastern bounding fault of the Iowa Horst. Younger Paleozoic strata are not displaced as much, indicating pre-Devonian reactivation and displacement of Silurian strata. During the Devonian, the area of the Silurian trough in central Iowa became a regional depocenter, the Iowa Basin, with maximum subsidence centered over the Iowa Horst. By the Mississippian, this depocenter persisted in southwestern Iowa (Massena Basin), straddling the TRSZ. Pennsylvanian strata are deformed and displaced along the trend of the TRSZ, and a series of domes are recognized. The Glenwood Syncline, a small Pennsylvanian structure, developed directly above the MRS Mineola Basin on the Iowa Horst in southwest Iowa. In north-central Iowa, the Fort Dodge Graben and associated faults are probably associated with reactivation of the Northern Iowa Fault Zone which bounds the Iowa Horst on the west; maximum displacements are identified in Mississippian strata, with lesser offset of Jurassic units, indicating recurring movements into the Mesozoic. Various features of the MRS in Iowa, including Proterozoic faults, grabens, and basins, served as loci of complex and recurring structural movements throughout the Phanerozoic, with varying senses of movement and subsidence expressed in a complex structural history.