Abstract

Regional geologic, stratigraphic, geochemical, remote sensing, and geophysical studies of the Rolla 1 degrees X 2 degrees quadrangle, Missouri, are in progress by the U. S. and Missouri Geological Surveys. Reconnaissance gravity data are available throughout the quadrangle and aeromagnetic data are available over its eastern part. A prominent north-west-trending gravity gradient, attributed in part to deep crustal structure, crosses the entire map. Otherwise, the major geophysical anomalies are subcircular features 10 to 40 km in diameter. There is no expression geophysically of gridded fault block structure.A positive gravity anomaly near Salem indicates a shallow intrabasement mafic or ultramafic body (density 3.0-3.2 g/cm 3 ) about 13 km in diameter and 2 to 4 km thick. Prominent gravity and magnetic lows near Belleview and Hawn State Park are attributed to granite plutons. Gravity lows of unknown but probably intrabasement origin occur near Steelville and Spring Valley. Model studies of the Hawn State Park anomalies lead to a doughnut-shaped anomalous mass distribution suggestive of a ring dike and associated resurgent cauldron about 25 km in diameter. On the other hand, the Taum Sauk "caldera," a feature inferred from geological field studies, shows no gravity or magnetic expression of an associated pluton, suggesting the feature may be a volcano-tectonic depression but probably is not a caldera.

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