Abstract This road log runs from the southern edge of Ironton-Arcadia, Missouri, east across the southern portion of the st. Francois Mountain igneous terrane to Cottoner Mountain, a few miles east of the village of Cherokee pass on u.s. Highway 67 south of Fredericktown, Missouri (Fig. 1). All stops are either roadcuts or National Forest lands and are fully accessible. The route of the road log emphasizes the diversity of the terrane and includes rhyolitic ash flows and lavas, andesitic (trachyandesite) lavas, and both mafic and granitic dikes. Stratigraphically, the volcanics are older to the east although individual stops are not arranged in exact descending order. With the exception of the Lake Killarney and Grassy Mountain tuffs (Sides, 1978), all rock unit names are informal and reflect our recent remapping of the region. Only the Des Arc NE quadrangle geologic map, stops 1-3, has been published (Brown, 1988). The Rockpile Mountain, Cherokee Pass and Marquand quandrangle maps will be compiled and released through the Geological Survey, Missouri Department of Natural Resources, over the next 2-3 years (see Brown, this volume, fig. 5., p. 104), and others in the more southern portion of the mountains will follow. A formal stratigraphy will be compiled at a later date.

ABSTRACT The Saint Francois Mountains are the physiographic expression of the central part of the Ozark Dome of southeastern Missouri. The mountains are made up of a quaquaversal-dipping series of Paleozoic units cored by the Mesoproterozoic-aged rocks of the broader Saint Francois Mountains terrane. The Saint Francois Mountains terrane lies within the Eastern Granite-Rhyolite province along the eastern margin of Laurentia and contains at least four mapped caldera complexes (Eminence, Lake Killarney, Butler Hill, and Taum Sauk), associated volcanic and volcaniclastic rocks, and four distinct types of intrusive units. The Mesoproterozoic rocks represent two major pulses of magmatic activity: (1) an older 1.48–1.45 Ga episode of caldera-forming volcanism and associated subvolcanic to massif-type granitic intrusions; and (2) a younger 1.33–1.28 Ga episode of bimodal intrusions. Volcanism included primarily high-silica rhyolite and volcaniclastic sediments associated with caldera-forming volcanism with lesser amounts of basalt and basaltic andesite that formed as flows and subvolcanic intrusions. The older (ca. 1.4 Ga) intrusive rocks can be divided into three broad categories: (1) granite massifs including the Butler Hill/Breadtray massif-type granites, (2) caldera ring–type granites such as the Silvermine Granite, and (4) mafic- to intermediate-composition intrusive rocks such as the Silver Mines Mafic Series. The younger (ca. 1.3 Ga) bimodal intrusions are represented by the highly evolved felsic Graniteville-types granites and the gabbros of the Skrainka Mafic Group. This field guide provides an overview of the magmatic history of the Mesoproterozoic rocks exposed in the eastern Saint Francois Mountains. Field-trip stops are divided into two days, highlighting well-known stops and lesser-known localities that illustrate the magmatic activity of one the premier igneous locations in the midcontinent region. The field trip is focused on two main areas. Day 1 focuses on the rhyolite sequence and associated caldera-forming eruption of the Taum Sauk caldera. Day 2 focuses on the volcanic rocks and granitic intrusions related to the Butler Hill caldera and ends with a visit to one of the youngest granitoids in the terrane, the Graniteville Granite. The field guide presents a summary of the volcanic history and petrogenesis of the Saint Francois Mountains rhyolites and granites.