Southeast Missouri Geology: Details from Annapolis Area and Ketcherside Gap, Missouri: GEOLOGICAL NOTE

New details on the Precambrian rhyolites and Upper Cambrian sedimentary rocks in the Annapolis area and Ketcherside Gap, Iron County, Missouri, indicate that the tectonic style of the St. Francois Mountain area most probably can be compared with plateau (cratonic) areas where germano-type tectonic patterns are present, and not with orogenic areas where thrust faults might be expected as recently proposed by Wheeler (1965). Thick geosynclinal sections, volcanic rocks, lava, metamorphism, schistosity, lineation, folding, and thrust faults are absent. The great areal extent of Precambrian porphyritic rocks is evidence for the widespread volcanism. Later movements were minor as shown by the local occurrence of normal faults. All indications suggest that, in general, the porph ritic rocks are in much the same position as when they formed. Also the distribution of the igneous rocks and rejuvenated movements around the St. Francois Mountain area indicate that large-scale lateral crustal movements have not occurred. The tectonic style of orogenic belts can hardly be transferred to plateau (cratonic) areas, where vertical (germano-type) movements are the rule. As described previously (Zimmermann and Amstutz, 1965), polygonal patterns, diatremes, and volcanic plugs typically accompany these vertical movements and commonly occur at intersections of major faults. The depositional and lithologic features of the sediments across the pronounced Precambrian topographic relief suggest that the sediments were deposited around and on the knobs of Precambrian porphyry without later horizontal movements. The features which suggest this include composition and grain sizes of the dolomites, porphyry fragments and pebbles in the sediments on knob margins, and the galena deposits. Joint patterns in the sediments indicate that the joints formed during compaction.