SHALLOW SEISMIC-REFLECTION PROFILES AND GEOLOGICAL STRUCTURE IN THE BENTON HILLS, SOUTHEAST MISSOURI

During late May and early June of 1993, we conducted two shallow, high -resolution seismic reflection surveys (Mini-Sosie method) across the southern escarpment of the Benton Hills segment of Crowleys Ridge. The reflection profiles imaged numerous post-late Cretaceous faults and f olds. We believe these faults may represent a significant earthquake s ource zone. The stratigraphy of the Benton Hills consists of a thin, l ess than about 130 m, sequence of mostly unconsolidated Cretaceous, Te rtiary and Quaternary sediments which unconformably overlie a much thi cker section of Paleozoic carbonate rocks, The survey did not resolve reflectors within the upper 75-100 ms of two-way travel time (about 60 -100 m), which would include all of the Tertiary and Quaternary and mo st of the Cretaceous. However, the Paleozoic-Cretaceous unconformity ( Pt) produced an excellent reflection, and, locally a shallower reflect or within the Cretaceous (K) was resolved, No coherent reflections bel ow about 200 ms of two-way travel time were identified. Numerous fault s and folds, which clearly offset the Paleozoic-Cretaceous unconformit y reflector, were imaged on both seismic reflection profiles. Many str uctures imaged by the reflection data are coincident with the surface mapped locations of faults within the Cretaceous and Tertiary successi on. Two locations show important structures that are clearly complex f ault zones. The English Hill fault zone, striking N30 degrees-35 degre es E, is present along Line 1 and is important because earlier workers indicated it has Pleistocene Loess faulted against Eocene sands. The Commerce fault zone striking N50 degrees E, overlies a major regional basement geophysical lineament, and is present on both seismic lines a t the southern margin of the escarpment. The fault zones imaged by the se surveys are 30 km from the area of intense microseismicity in the N ew Madrid seismic zone (NMSZ). If these are northeast and north-northe ast oriented fault zones like those at Thebes Gap they are favorably o riented in the modern stress field to be reactivated as right-lateral strike slip faults. Currently, earthquake hazards assessments are most dependent upon historical seismicity, and there are little geological data available to evaluate the earthquake potential of fault zones ou tside of the NMSZ. We anticipate that future studies will provide evid ence that seismicity has migrated between fault zones well beyond the middle Mississippi Valley. The potential earthquake hazards represente d by faults outside the NMSZ may be significant.