In 1988 a stratigraphic test core drilled in southwestern Ohio penetrated a previously unknown structure and sedimentologic unit. Core analyses disclosed a sequence of lithic arenite and siltstone of probable pre-Phanerozoic age, deposited within an alluvial-fluvial environment. A seismic reflection profile across the core site shows a sequence of strong, horizontal Paleozoic reflectors unconformably overlying eastward dipping, layered units of poor reflectivity. Beyond the core hole total depth, a regime of excellent reflectors is seen. The pre-Mount Simon (Upper Cambrian) sedimentary sequence encountered has been defined as the type section of the Middle Run Formation. Point counter analyses of typical Middle Run Formation samples indicate that this sequence is similar in composition to middle Proterozoic units associated with the Midcontinent Rift Systemin the Lake Superior region. The newly discovered structure in Ohio is also similar to half-graben structures composing the Lake Superior Basin portion of the Midcontinent rift. On the basis of similarities in lithology, stratigraphy, structure, and proximity to regional gravity anomalies, the Middle Run Formation and its apparent half-graben basin is proposed as evidence for the probable extension of the Midcontinent Rift System southward, from its normally accepted termination in southeastern Michigan, into southwestern Ohio. Hinze, 1985; Dickas, 1986]. The western arm of this system, termed the Midcontinent Gravity High by Thiel [1956], is identified on the gravity anomaly map of the United States [Lyons and O'Hara, 1982] as a sharply defined trend of positive anomalies and associated flanking gravity lows extending from western Lake Superior to central Kansas (Figure 1). Subsequently, King and Zietz [1971] equated this gravity trend, and its related magnetic signature, with a rift system and named it the Midcontinent Rift System. Additional gravity and aeromagnetic surveys [Lyons, 1959; Yarger, 1983] suggest an extension can be traced further southwest to the Kansas-Oklahoma border. The eastern arm of the Midcontinent Rift System is defined by the Mid-Michigan Gravity High [Rudman et al., 1965], traced across the eastern axis of Lake Superior and then south into the Lower Peninsula of Michigan. This anomaly is more subdued in amplitude than its western counterpart due to burial beneath 4000 m of Phanerozoic strata that fill the Michigan Basin. The southern terminus of the Mid-Michigan Gravity High is commonly placed in the vicinity of Detroit, Michigan [Halls, 1978; Klasner et al., 1982]. Various potential field anomalies have been used by Lyons [1970], Halls [1978], Keller et al. [1982, 1983], Wheeler [1983], and Van Schmus and Hinze [1985] as evidence that this rift system extends to the south of Michigan. These extensions, named the East-Continent Gravity High [Bryan, 1975; Keller et al., 1982], are based on limited crustal refxaction seismology, gravity and magnetic data, and basement petrographic data [Fox, 1988]. The basement rocks associated with thi...
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