RETROSPECTIVE VIEW ON A DECADE OF RESEARCH ON THE PALEOZOIC PLUM RIVER FAULT ZONE IN EASTERN IOWA
G.A. Ludvigson and B.J. Witzke
The Geological Society of America
31st Annual North-Central Section Meeting
Madison, Wisconsin, May 1-2, 1997
1997 Abstracts with Programs, v. 29, no. 4, p. 32
The Plum River Fault Zone (PRFZ) is a 180 km-long east-northeast trending structure that cuts across the Upper Mississippi Valley. Combined structural and stratigraphic studies in the late 1970s and early 1980s showed that the PRFZ experienced recurrent deformation during deposition of Silurian and Devonian units, and that most fault displacement predated regional deposition of Pennsylvanian strata. In the mid 1980s, petrographic and geochemical studies of fault rocks from deformed Paleozoic units in the PRFZ were inaugurated to investigate fluid-rock interactions during fault movement. None of these Paleozoic units had previously been the subject of diagenetic studies integrating petrographic and stable isotopic techniques. In all instances, the multiple generations of fault-healing minerals recognized in the PRFZ precipitated from low temperature shallow groundwaters that infiltrated exposed continental surfaces and circulated in flow systems completely contained within the Paleozoic section. Two major episodes of regional fault displacement and mineral cementation were recognized: (1) silicification by vein-filling and dolomite replacement in the Silurian Gower and Scotch Grove formations by fluids that circulated beneath the Tippecanoe-Kaskaskia Megasequence boundary; and (2) pervasive vein-filling and replacement by calcite, iron and manganese oxide, and pyrite cements in early-mid Paleozoic units by fluids that circulated beneath the Kaskaskia-Absaroka Megasequence boundary. Pioneering studies in the PRFZ played a major role in developing two new ideas regarding the sedimentary geochemistry/diagenesis of mid-Paleozoic carbonates in Iowa: (1) successive T-R cycles in the Devonian Cedar Valley Group were each chemically stabilized by unique episodes of meteoric-phreatic diagenesis coinciding with subaerial exposure of northern peritidal inner-shelf facies; and (2) as in many other parts of North America, Silurian dolostones in Iowa record an enigmatic bimodal distribution in d13C values, a 13C-depleted mode (-0.5 to 1.5 ) and a 13C-enriched mode (3.5 to 5.6 ). Questions about whether these 13C modes reflect original depositional signals or diagenetic overprints are not yet resolved.