HIGH-RELIEF PALEOTOPOGRAPHY AS A CHARACTERISTIC OF EASTERN GRANITE-RHYOLITE PROVINCE CALDERA COMPLEXES: COMPARING 2D SEISMIC LINES IN INDIANA AND MODELING OF THE ST. FRANCOIS MOUNTAINS

Ten 2D seismic lines collected by CountryMark near Wabash, Indiana, image the Precambrian unconformity above the Eastern Granite-Rhyolite Province (EGRP). The Precambrian unconformity interpreted from the unmigrated stacked seismic sections appears at ~0.5s two-way travel time and is characterized by undulations and bowtie artifacts commonly associated with high relief reflecting surfaces. Zero offset seismic models constructed using profiles of the exposed Precambrian unconformity across the St. Francois Mountains of Missouri, the only locality where the EGRP is exposed in the midcontinent, feature the same seismic expressions, and support the interpretation that the seismic expression observed in the CountryMark data represents high relief paleotopography on the unconformity. The modeled profiles are restricted to regions of the St. Francois Mountains dominated by sub-volcanic lithologies to provide a direct comparison. Potential field data in the Wabash area exhibit circular or ring patterns suggesting caldera complexes are also analogous to the geology observed in the St. Francois Mountains. Consequently, we infer that a high-relief unconformity may be characteristic of EGRP caldera complexes in contrast with a flat or low-relief unconformity observed in regions of sedimentary or volcanic EGRP rocks.