ABSTRACT Three Silurian basin fills, the Llandovery–Wenlock Croagh Patrick and Killary Harbour–Joyce Country successions and the Ludlow–Pridoli Louisburgh–Clare Island succession, overstep the tectonic contacts between elements of the Grampian (Taconic) accretionary history of the Caledonian-Appalachian orogeny in western Ireland. New U-Pb detrital zircon data from lower strata of these Silurian rocks provide insight into basin evolution and paleogeography. The shallow-marine Croagh Patrick succession unconformably overlies the Clew Bay Complex and the northern part of the Ordovician South Mayo Trough. Two samples have zircon populations dominated by Proterozoic grains typical of the Laurentian margin, with few younger grains. Up to 13% of the grains form a cluster at ca. 950–800 Ma, which is younger than known Grenville magmatism on the local Laurentian margin and older than known magmatism from Iapetan rifting; these may be recycled grains from Dalradian strata, derived from distal Tonian intrusions. The Killary Harbour–Joyce Country succession overlies the structural contact between the Lough Nafooey arc and the Connemara Dalradian block and records a transgressive-regressive cycle. Four samples of the Lough Mask Formation show contrasting age spectra. Two samples from east of the Maam Valley fault zone, one each from above Dalradian and Nafooey arc basement, are dominated by Proterozoic grains with ages typical of a Laurentian or Dalradian source, likely in north Mayo. One sample also includes 8% Silurian grains. Two samples from west of the fault overlie Dalradian basement and are dominated by Ordovician grains. Circa 450 Ma ages are younger than any preserved Ordovician rocks in the region and are inferred to represent poorly preserved arc fragments that are exposed in northeastern North America. Cambrian to late Neoproterozoic grains in association with young Ordovician ages suggest derivation from a peri-Gondwanan source in the late stages of Iapetus closure. The Louisburgh–Clare Island succession comprises terrestrial red beds. It unconformably overlies the Clew Bay Complex on Clare Island and is faulted against the Croagh Patrick succession on the mainland. The Strake Banded Formation yielded an age spectrum dominated by Proterozoic Laurentian as well as Ordovician–Silurian ages. Although the basin formed during strike-slip deformation along the Laurentian margin in Ireland and Scotland, sediment provenance is consistent with local Dalradian sources and contemporaneous volcanism. Our results support ideas that Ganderian continental fragments became part of Laurentia prior to the full closure of the Iapetus Ocean.

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 Here we present an overview of the geology of the Manhattan Prong and a specific guide for field stops in northern Central Park. This guide is intended to provide a brief introduction to these complex rocks for researchers, undergraduate students, and teachers. Given the easy access to Central Park and numerous schools and institutions nearby, these outcrops provide ideal teaching outcrops for students of all levels. We also present new geochemical and isotopic results for the Manhattan and Hartland Schists. Previous work has focused primarily on field mapping, structural relationships, or infrastructure-related mapping, whereas our new geochemistry data allow for more detailed discussions of provenance and overall tectonic history of these rocks. Our results suggest that all of the rocks in northern Central Park (regardless of mapped unit) are derived from Laurentia.

ABSTRACT Georgia’s coastline is composed of a series of short, wide, mixed-energy (tide-dominated) barrier islands, each backed by extensive marsh, topped with mobile dunes, and flanked by deep inlets. Many of the islands, particularly those along the southern Georgia coast, consist of Pleistocene cores surrounded by mobile deposits that attached during the Holocene sea-level transgression. Positioned within the head of the funnel-shaped South Atlantic Bight, tidal ranges here commonly reach ~2–3 m. As a result, inlets are numerous and the back-barrier environment hosts nearly 400,000 acres of salt marsh. Today, many of the barriers are transgressive, and hard structures such as revetments and groins are becoming increasingly more common to stabilize shorelines along the four developed islands. This field guide presents evidence of island formation, modern ecologic function, and likely future changes for three island groups: (1) Blackbeard, Cabretta, and Sapelo Islands; (2) Sea Island and St. Simons Island; and (3) Jekyll Island. The field trip provides evidence of the Pleistocene-age island cores, the natural southward migration of the mobile Holocene-age sandy shorelines, and the impacts of storm erosion and hard structures built to combat that erosion. This field guide serves as the static, print companion to an online virtual field trip ( https://storymaps.arcgis.com/stories/0aa3fd921cc4458da0a19a928e5fa87c ).

ABSTRACT This guide presents an eight-hour, in-person tour of intersecting geologic and human history in western Nevada, USA. A 25 megaton phreatomagmatic blast created a mile-wide (1.6-km-wide) maar, now filled by Soda Lake. The magnitude 7 Dixie Valley earthquake ripped along more than 45 km of the Stillwater Range front in 1954. The 12 kiloton Shoal nuclear test in 1963 created a 50-m-wide cavity in solid granite.

ABSTRACT Rocks of the Upper Cretaceous Tuscaloosa Formation (Cenomanian) and Eutaw Formation (Santonian) in southwestern Georgia and southeastern Alabama record an interval of fluvial and nearshore marine deposition. In the vicinity of Columbus, Georgia, basal units of the Tuscaloosa Formation consist of a residual paleosol built on crystalline rocks of the Appalachian Piedmont covered by conglomeratic sandstones deposited in braided stream systems flowing across the mid-Cenomanian Coastal Plain unconformity. The unconformity, which separates Cretaceous detrital rocks from underlying metamorphic rocks and residual paleosols built on those metamorphic rocks, lies primarily within the Tuscaloosa Formation in this region and is marked at the modern surface by the geomorphic Fall Line. Mapping of the unconformity across the region reveals areas of significant paleorelief associated with a number of distinct paleovalleys incised into the mid-Cenomanian surface. The most distinct of these lie immediately east of the Alabama-Georgia state line, within 15 km of the modern Lower Chattahoochee River Valley. Spatially, these distinct paleovalleys lie immediately north of a Santonian estuarine environment recorded in the Eutaw Formation, disconformably above the Tuscaloosa Formation. Collectively, paleo-valleys in the mid-Cenomanian surface, the fluvial nature of the Tuscaloosa Formation in southwestern Georgia and southeastern Alabama, and the estuarine environment in the younger Eutaw Formation suggest a persistent (~10 m.y.) paleodrainage system that may be a forerunner to the modern Chattahoochee River.

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 This field trip traverses the Sahwave and Nightingale Ranges in central Nevada, USA, and northward to Gerlach, Nevada, to the Granite, northern Fox, and Selenite Ranges. Plutonic bodies in this area include the ca. 93–89 Ma Sahwave nested intrusive suite of the Sahwave and Nightingale Ranges, the ca. 106 Ma Power Line intrusive complex of the Nightingale Range, the ca. 96 Ma plutons in the Selenite Range, and the ca. 105–102 Ma plutons of the Granite and Fox Ranges. Collectively these plutons occupy nearly 1000 km 2 of bedrock exposure. Plutons of the Sahwave, Nightingale, and Selenite Ranges intrude autochthonous rocks east of the western Nevada shear zone, while plutons of the Granite and Fox Ranges intrude displaced terranes west of the western Nevada shear zone. Integrated structural, geochemical, and geochronological studies are used to better understand magmatic and deformation processes during the Early Cretaceous, correlations with Cretaceous plutons in adjacent areas of Idaho and California, and regional implications. Field-trip stops in the Sahwave and Nightingale Ranges will focus on: (1) microstructure and orientation of magmatic and solid-state fabrics of the incrementally emplaced granodiorites-granites of the Sahwave intrusive suite; and (2) newly identified dextral shear zones hosted within intrusions of both the Sahwave and Nightingale Ranges. The Sahwave intrusive suite exhibits moderate to weak magnetic fabrics determined using anisotropy of magnetic susceptibility, with magnetic foliations that strike NW-NE and magnetic lineations that plunge moderately to steeply. Microstructural analysis indicates that these fabrics formed during magmatic flow. The older Power Line intrusive complex in the Nightingale Range is cross-cut by the Sahwave suite and contains a N-S–trending solid-state foliation that reflects ductile dextral shearing. Field-trip stops in the plutons of the Gerlach region will focus on composition, texture, and emplacement ages, and key differences with the younger Sahwave suite, including lack of evidence for zoning and solid-state fabrics. The field trip will utilize StraboSpot, a digital data system for field-based geology that allows participants to investigate the relevant data projects in the study areas.

ABSTRACT Discoveries in the 1980s greatly expanded speleologists’ understanding of the role that hypogenic groundwater flow can play in developing caves at depth. Ascending groundwater charged with carbon dioxide and, especially, hydrogen sulfide can readily dissolve carbonate bedrock just below and above the water table. Sulfuric acid speleogenesis, in which anoxic, rising, sulfidic groundwater mixes with oxygenated cave atmosphere to form aggressive sulfuric acid (H 2 SO 4 ) formed spectacular caves in Carlsbad Caverns National Park, USA. Cueva de Villa Luz in Mexico provides an aggressively active example of sulfuric acid speleogenesis processes, and the Frasassi Caves in Italy preserve the results of sulfuric acid speleogenesis in its upper levels while sulfidic groundwater currently enlarges cave passages in the lower levels. Many caves in east-central Nevada and western Utah (USA) are products of hypogenic speleogenesis and formed before the current topography fully developed. Wet climate during the late Neogene and Pleistocene brought extensive meteoric infiltration into the caves, and calcite speleothems (e.g., stalactites, stalagmites, shields) coat the walls and floors of the caves, concealing evidence of the earlier hypogenic stage. However, by studying the speleogenetic features in well-established sulfuric acid speleogenesis caves, evidence of hypogenic, probably sulfidic, speleogenesis in many Great Basin caves can be teased out. Compelling evidence of hypogenic speleogenesis in these caves include folia, mammillaries, bubble trails, cupolas, and metatyuyamunite. Sulfuric acid speleogenesis signs include hollow coralloid stalagmites, trays, gypsum crust, pseudoscallops, rills, and acid pool notches. Lehman Caves in Great Basin National Park is particularly informative because a low-permeability capstone protected about half of the cave from significant meteoric infiltration, preserving early speleogenetic features.

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.

ABSTRACT The Appalachian Mountains expose one of the most-studied orogenic belts in the world. However, metamorphic pressure-temperature-time ( P-T-t ) paths for reconstructing the tectonic history are largely lacking for the southernmost end of the orogen. In this contribution, we describe select field locations in a rough transect across the orogen from Ducktown, Tennessee, to Goldville, Alabama. Metamorphic rocks from nine locations are described and analyzed in order to construct quantitative P-T-t paths, utilizing isochemical phase diagram sections and garnet Sm-Nd ages. P-T-t paths and garnet Sm-Nd ages for migmatitic garnet sillimanite schist document high-grade 460–411 Ma metamorphism extending south from Winding Stair Gap to Standing Indian in the Blue Ridge of North Carolina. In the Alabama Blue Ridge, Wedowee Group rocks were metamorphosed at biotite to staurolite zone, with only local areas of higher-temperature metamorphism. The Wedowee Group is flanked by higher-grade rocks of the Ashland Supergroup and Emuckfaw Group to the northwest and southeast, respectively. Garnet ages between ca. 357 and 319 Ma indicate that garnet growth was Neoacadian to early Alleghanian in the Blue Ridge of Alabama. The P-T-t paths for these rocks are compatible with crustal thickening during garnet growth.

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.

ABSTRACT On this field trip we visit three sites in the Salt Lake Valley, Utah, USA, where we examine the geomorphology of the Bonneville shoreline, the history of glaciation in the Wasatch Range, and shorezone geomorphology of Great Salt Lake. Stop 1 is at Steep Mountain bench, adjacent to Point of the Mountain in the Traverse Mountains, where the Bonneville shoreline is well developed and we can examine geomorphic evidence for the behavior of Lake Bonneville at its highest levels. At Stop 2 at the mouths of Little Cottonwood and Bells Canyons in the Wasatch Range, we examine geochronologic and geomorphic evidence for the interaction of mountain glaciers with Lake Bonneville. At the Great Salt Lake at Stop 3, we can examine modern processes and evidence of the Holocene history of the lake, and appreciate how Lake Bonneville and Great Salt Lake are two end members of a long-lived lacustrine system in one of the tectonically generated basins of the Great Basin.

ABSTRACT The Ottawa aulacogen/graben on the NE US—Canadian (SW Quebec and eastern Ontario) border is a long ENE-trending structure formed with initial late Neo proterozoic rifting of the Rodinia supercontinent. This rifting formed the active spreading arms (New York Promontory and Quebec Reentrant) along the (presently) NE margin of the new Laurentia paleocontinent, with the Ottawa aulacogen commonly regarded as a failed arm of the rifting. However, no sediment accumulation in the aulacogen is recorded until the late early Cambrian subsidence of a SE- trending belt that includes the aulacogen and its extension, the Franklin Basin, in NW Vermont. Late early Cambrian marine onlap (Altona Formation) followed by more rapid late middle Cambrian subsidence and deposition of fluviatile arkoses (Covey Hill Formation of SW Quebec and Ausable Formation/Member of eastern New York) record rapid foundering of this “failed arm.” Subsequent deposition (latest middle Cambrian–Early Ordovician) in the Ottawa aulacogen produced a vertical succession of lithofacies that are fully comparable with those of the shelf of the New York Promontory. One of the greatest challenges in summarizing the geological history of the Ottawa aulacogen is the presence of a duplicate stratigraphic nomenclature with lithostratigraphic names changing as state and provincial borders are crossed. RÉSUMÉ L’aulacogène/graben d’Ottawa, situé sur la frontière entre le NE des États-Unis et le Canada (SW du Québec et est de l’Ontario), est une longue structure d’orientation ENE formée au Néoprotérozoïque tardif durant le rifting initial du supercontinent Rodinia. Ce rifting a aussi mené à la formation de segments à expansion active (promontoire de New York et réentrant de Québec) le long de la marge NE (coordonnées actuelles) du nouveau paléo-continent Laurentia, avec l’aulacogène d’Ottawa qui est généralement considéré comme un segment de rift avorté. Toutefois, aucune accumulation de sediments n’est documentée au sein de l’aulacogène avant la fin du Cambrien précoce, période durant laquelle une ceinture d’orientation SE, representée par l’aulacogène et son prolongement dans le bassin de Franklin vers le NW du Vermont, a subi une subsidence. La sedimentation marine de la fin du Cambrien précoce (Formation d’Altona) a été suivie d’une subsidence rapide à la fin du Cambrien moyen et de la déposition d’arkoses fluviatiles (Formation de Covey Hill dans le SW du Québec et la Formation/Membre d’Ausable dans l’est de l’état de New York) qui ont enregistré un affaissement rapide de ce “bras avorté.” La sédimentation subséquente (Cambrien moyen tardif–Ordovicien inférieur) au sein de l’aulacogène d’Ottawa a produit une succession verticale de lithofaciès qui sont comparables à ceux de la plate-forme du promontoire de New York. Un des principaux défis dans la synthèse de l’histoire géologique de l’aulacogène d’Ottawa demeure la duplication des termes stratigraphiques de part et d’autre des frontières interprovinciales et entre les différents états.

ABSTRACT This three-day field trip focuses on the stratigraphy and the structural characteristics of the late- and post-Taconian sedimentary basins of the southern Québec Appalachians, with a particular emphasis on N-to-S and W-to-E structural and lithological variations. In order to discuss various aspects of the regional structural evolution of these basins, we will visit a series of key outcrops following three sections, the Beauce/Thetford-Mines sections, the Sherbrooke section, and the Coaticook section. RÉSUMÉ Cette excursion de trois jours se concentre sur la stratigraphie et les caractéristiques structurales des bassins sédimentaires tardi- et post-Taconien des Appalaches du sud du Québec, en mettant l’accent sur les variations structurales et lithologiques du nord au sud et d’ouest en est. Afin de discuter des divers aspects de l’évolution structurale régionale de ces bassins sédimentaires, nous visiterons une série d’affleure ments clés en suivant trois sections, soient les sections de Beauce/Thetford-Mines, de Sherbrooke, et de Coaticook.

COVID-19 made for a highly unusual year as it affected almost every facet of life. The pandemic made gathering and visiting the field nearly impossible as we quarantined and moved into virtual spaces. Three groups submitted guides for publication during the height of the pandemic: two for trips that would have taken place during the GSA Annual Meeting in Montréal, Canada, and one from the Rocky Mountain Section Meeting in Provo, Utah, USA. Readers will enjoy these journeys to the Ottawa aulacogen/graben on the Northeast U.S.–Canadian border; the southern Québec Appalachians; and Lake Bonneville, the Wasatch Range, and Great Salt Lake in Utah.