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.

The origin of clays and clay minerals in the Paleocene Clayton (CF) and Porters Creek (PCF) Formations within the Mississippi embayment of central North America has been debated for more than 50 years. X-ray diffraction and petrographic analysis of samples of the CF and PCF from a mine in southeastern Missouri are used to evaluate contributions from Cretaceous-Paleogene (K-Pg) impact debris and the role of sediment diagenesis in the fine-grained sediment. Expandable clay minerals increase in abundance relative to illite and kaolinite above the K-Pg unconformity in the CF and PCF, and include dioctahedral smectite, vermiculite, and minor mixed-layered clay components, along with trioctahedral smectitic clays in the CF and lowermost PCF. Additional diagenetic phases include clinoptilolite (in the CF and lower PCF), pyrite, siderite, and opal CT (mainly in the PCF). The results of the petrographic analysis show no evidence for volcanic ash contributing directly to the sediment in the PCF. The detrital silicate minerals are mainly quartz, muscovite, biotite, and metamorphic minerals, consistent with an ancestral Appalachian Mountains source rather than volcanic ash or a Cretaceous western interior sediment source. The illite, kaolinite, dioctahedral smectite, and ordered illite-smectite mixed-layered clays are present in varying quantities in Cretaceous through Paleogene marine and nonmarine mudstones from the Mississippi embayment, and appear to be detrital in origin. Trioctahedral smectite and clinopti-lolite in the CF and lowermost PCF are argued to derive from alteration of glassy impact debris; clasts in the basal CF contain microtektites replaced by trioctahedral smectitic mixed-layered clay with randomly interstratified illite. The X-ray diffraction characteristics of the vermiculite in the PCF indicate a hydroxyl-interlayered aluminous variety that is argued to have a diagenetic origin, formed by clay mineral reactions under variably anoxic conditions in the PCF sediments during early diagenesis. Throughout the CF and PCF, pyrite precipitated under reducing conditions during diagenesis, locally along with siderite replacement of micritic carbonate. In addition, opal-CT precipitated as a result of silicate reactions and dissolution of diatoms under alkaline conditions and replaced matrix and calcitic microfossils throughout much of the upper PCF.