Abstract In the three-corner area of Indiana, Michigan, and Ohio, much of the 500-ft (150 m) Silurian section younger than the Salamonie Dolomite is a facies of the reef-bearing rocks of the Wabash platform areas of Illinois, Indiana, and Ohio. Rocks of the Salina Group and stratigraphical-ly equivalent rocks of the platform area do not contain salts and anhydrites, but they reflect the Michigan-basin cyclic sedimentation as far south as central Indiana in the form of transgressive-regressive facies within named rock units. No major unconformity is in the section, and the up-dip carbonate rocks probably are lateral equivalents of salts in the basin. Approximate correlation of Wabash-platform rocks in Indiana with units of the Salina in the Michigan basin is: (1) Limberlost Dolomite—lowest part of A unit; (2) Waldron Formation through Louisville Limestone—much of remainder of A unit, especially A-1 carbonate; (3) Wabash Formation—from upper part of A unit (B unit in some areas) through uppermost Salina; (4) Kokomo Limestone Member (Salina Formation)—D unit and possibly younger; and (5) Kenneth Limestone Member (Salina)—probably younger than D. Three reef-start episodes on the platform were coordinated with periods of more normal salinity during late deposition of the Salamonie, late deposition of the Louisville, early in deposition of the Mississinewa, and during deposition of the Kenneth. Some of the earliest reefs aborted during A-unit periods of above-normal salinity, including periods represented by part of the Limberlost and middle Louisville rocks, but many reefs grew during all the time of Salinan cyclic deposition. Even in the northern platform area, where the upper part of Silurian rocks has been eroded, a complete platform buildup that included reefs could have continued to accrete during deposition of the uppermost parts of the Salina Group. These interpretations do not readily favor some current ideas on thick sabkha evaporites, hundreds of feet of drawdown, and near-desiccation in the proto-Michigan basin—nor do they favor regional development of a so-called “Niagaran-Cayugan unconformity.”

ABSTRACT Pinnacle reef tracts are geomorphic features of carbonate systems that originated in the early Silurian and display an episodic distribution into the Cenozoic. Detailed study of Silurian pinnacle reefs of the United States midcontinent demonstrates repeated motifs, but most enigmatic is the coincidence of carbonate carbon isotope (δ 13 C carb ) excursions and reef pulses. Silurian δ 13 C carb excursions were associated with environmental changes and extinctions, and reefs appear to mark a resurgence of conditions favorable to biomineralizers following those extinction events. Previous workers in the region identified six discrete reef origination events in the United States midcontinent during the Silurian. Our reevaluation of outcrops and cores, conodont collections, and the generation of considerable new chemostratigraphic data across the region are clarifying the age relations of these events and their relationships to perturbations of the global carbon cycle.

ABSTRACT The extremely important role of groundwater has been largely overlooked in studies of meteorite and comet impact processes. Beyond the radius of plasma generation, impacts can produce massive shattering in saturated porous rocks. Fluid pressure rise reduces rock strength and facilitates hydrofracture, to produce intraformational monomict breccias, faulting, and generation of mobile polymict breccia slurries. Decompression of a deep “transient” crater accounts for complex central uplift and gravitational collapse of tremendous slide blocks that in turn cause injection and ejection of fluidized breccia. As pore fluid pressures equilibrate, frictional strength increases, and the structural form is locked into stability. Evidence is reported here for Kentland, Indiana, where quarry rocks display relatively low pressure-temperature (elastic to ductile transition, 100 kb–100 °C) impact phases of the model of D. Stöffler. Breccias include monomict, polymict, mixed polymict-fault, and conventional fault types. The monomict breccias are associated with aquifer beds and formed by pervasive shockwave transmission on impact. Polymict breccias are derived from all rock types and formed from late stage injection-ejection pseudoviscous slurries. These processes can apply to similar impacts like Wells Creek, Flynn Creek, Decaturville, Sierra Madre, and many others.