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.

ABSTRACT The four largest spring systems in the mid-continent receive recharge through large interconnected voids in fractured and solution-weathered dolostones of the Ordovician and Cambrian systems. Cumulative thickness of the carbonate bedrock aquifer ranges up to 700 m in the Ozark region. Recharge from the surface occurs through weathered overburden, sinkholes, and losing streams and has been traced up to 60 km (straight-line horizontal distance) using fluorescent dyes. Mean discharge of the combined flow of these four spring systems is ~1400 cubic feet/second (ft 3 /s) or 40 m 3 /second (m 3 /s). All four spring systems will be visited while discussing the karst terrane that recharges them. Environmental and engineering challenges in the region will be discussed, such as wastewater treatment systems, solid waste disposal, and failed reservoirs. Hodgson Mill Spring represents a branch of the Rainbow/North Fork/Hodgson Mill System. While it receives base flow from the main system, it also receives local recharge that Rainbow and North Fork springs do not. A portion of the Mammoth Spring recharge system will be viewed at Grand Gulf State Park in Missouri, where a cave collapse has created cliffs and a natural bridge and exposed a small losing tributary that flows into a cave that has been traced to the spring. Mammoth Spring State Park in Arkansas offers a historical perspective of the development and use of large springs. Greer Spring in Missouri was used as a power source for grist, flour, and lumber mills, but has now largely returned to its predevelopment state and is managed by the U.S. Forest Service. Big Spring, featured in a former state park in Missouri, is now part of the Ozark National Scenic Riverways.

ABSTRACT A succession of Ordovician and Mississippian carbonates, separated unconformably, is exposed across the southern flank of the Ozark Dome in southwest Missouri. Deposits of both periods exemplify typical facies of the Midwestern United States: carbonate tidal-flat assemblages for the Early Ordovician and carbonate shelf environments for the Early–Middle Mississippian. The basic stratigraphic sequence of these deposits has been known for over a century, but interesting features remain to be addressed. Thin discontinuous sandstones are present within the Early Ordovician Cotter Dolomite, but the informal Swan Creek sandstone member seems anomalous. This sandstone can exceed 5 m in thickness and is fairly continuous across southwest Missouri. Most Ordovician sandstones in Missouri mark major transgressions above regional unconformities, but not the Swan Creek, and there is no obvious source of the sand. Therefore, we hypothesize that the Swan Creek represents reworked eolian dunes blown across the broad peritidal environment. Clastic sandstone dikes, apparently sourced from the Swan Creek, cut across beds of Cotter Dolomite near faults. We propose that these dikes are evidence of local faulting and seismicity during the Early Ordovician. Early and Middle Mississippian limestones comprise a sequence of shelf deposits, although mud mounds and other facies changes near the Missouri-Arkansas line mark the edge of the Mississippian shelf and the transition to a ramp setting. Early Mississippian carbonate deposition was interrupted by a short and localized influx of siliciclastic sediment comprising the Northview Formation. The Northview has additional characteristics consistent with a river-dominated deltaic deposit, which we suggest as its origin. If correct, this hypothesis implies that the history of tectonic features in the Midwest is more complicated than yet known. Finally, facies changes within and between the local Mississippian formations may record an early crustal response to the impending Ouachita orogeny farther to the south.

The 2021 GSA Northeastern, Southeastern, joint North-Central/South-Central, and Cordilleran Section Meet-ings were held virtually in spring 2021 during continued restrictions on travel and large gatherings due to COVID-19. Eleven groups put together field guides, taking participants on treks to states from Connecticut to Nevada in the United States, to Mexico, and to Italy, and covering topics as varied as bedrock geologic map-ping, geochemistry, paleodrainage, barrier islands, karst, spring systems, a southern Appalachian transect, Ordo-vician and Mississippian stratigraphy, high-energy events, Cretaceous arc granites and dextral shear zones, and Mesoproterozoic igneous rocks. This volume serves as a valuable resource for those wishing to discover, learn more about, and travel through these geologically fascinating areas.