THE PLUM RIVER FAULT ZONE AND THE STRUCTURAL AND STRATIGRAPHIC FRAMEWORK OF EASTERN IOWA
B.J. Bunker, G.A. Ludvigson, B.J. Witzke
Iowa Department of Natural Resources, Geological Survey Bureau,
Technical Information Series 13, 1985, 126 p.
The Plum River Fault Zone is a 112 mile (180 km) long, east-west trending zone of high-angle faulting in east-central Iowa and northwest Illinois. The north side of the fault zone is downthrown, with documented net vertical displacements of Silurian strata up to 270 feet (70 m). Detailed geologic field mapping has shown that the internal structure of the Plum River Fault zone is characterized by a complex pattern of intersecting high angle faults which have graben and horst fault-block relationships. Major faults within the Plum River Fault Zone are recognized by the occurrence of zones of brittle cataclastic deformation, and the maximum known width of the fault zone in Iowa is 3,900 feet (1.2 km). Within the fault zone, vertical displacements of up to 500 feet (150 m) have been interpreted from Paleozoic rocks exposed in adjacent fault blocks, and vertical displacements on the Precambrian basement surface of up to 1,100 feet (335 m) have been estimated from combined gravity and magnetic traverses across the fault zone.
Recently completed investigations and revisions of the Paleozoic stratigraphy of eastern Iowa, along with detailed investigations of the Plum River Fault Zone, have led to a comprehensive reevaluation of the structural geology and Phanerozoic tectonic history of eastern Iowa. Descriptions and criteria for the recognition of exposed Phanerozoic stratigraphic units are presented in this report, and a new Silurian unit, the Scotch Grove Formation, is formally proposed. A newly recognized regional structure, the Fayette Structural Zone of northeast Iowa, is also defined.
The Phanerozoic tectonic history of eastern Iowa is reviewed within the context of Sloss' (1963) unconformity-bounded cratonic sedimentary sequences. During the deposition of the Sauk Sequence (Upper Cambrian-Lower Ordovician) in eastern Iowa, sediments accumulated in a north-south oriented, southward plunging trough termed the Hollandale Embayment. Significant structural reorganization in the central midcontinent region occurred during the deposition of the Tippecanoe (Middle Ordovician-Silurian) Sequence. The earlier north-south structural grain in eastern Iowa was supplanted by an east-west structural grain, and subsidence was initiated in the East-Central Iowa Basin, whose axis paralleled the Plum River Fault Zone. During the Early Silurian, maximum subsidence in this structural and depositional basin was 18.6 miles (30 km) to the north of the Plum River Fault Zone, whereas during the Middle Silurian maximum subsidence occurred along the north edge of the fault zone. Earliest Kaskaskia (Middle Devonian-Mississippian) deposition reoccupied the East-Central Iowa Basin, with penecontemporaneous faulting along the Plum River Fault Zone. A significant structural reorganization in the midcontinent region began in the Late Devonian, and prior to initial deposition of sediments of the Absaroka Sequence (Pennsylvanian), the East-Central Iowa Basin was uplifted and deeply eroded. Pennsylvanian deposition in east-central Iowa was preceded by major faulting along the Plum River Fault Zone, uplift of the Savanna-Sabula Anticlinal System, and the development of the present regional structural geometry. The physical relationships of Pennsylvanian deposits to the Plum River Fault Zone are not known with sufficient precision to preclude up to 33 feet (10 m) of post-Pennsylvanian displacement. Historic seismic data are inadequate to evaluate the potential for seismic hazard associated with the Plum River Fault Zone.
Sloss, L.L., 1963, Sequences in the Cratonic Interior of North America: Geological Society of America Bulletin, v. 74, p. 93-114.