ABSTRACT The timing and kinematics of Paleozoic peri-Gondwanan terrane accretion along the southern and central Appalachian margin have long been debated. The Silurian–Devonian Concord plutonic suite intruded the western flank of the Carolina superterrane, suggesting east-dipping subduction of ocean crust beneath the Carolina superterrane just prior to accretion, based on Devonian–Mississippian plutonism and metamorphism in the adjacent Laurentian terranes. Geochemical and isotopic data support a subduction-related origin for the Concord plutonic suite, and our geochronologic data reveal the main pulse of plutonism occurred ca. 405 Ma. Our new sensitive high-resolution ion microprobe (SHRIMP) geochronologic data identify a suite of mafic plutons from the Carolinas to central Georgia that also belong to the Concord suite. These gabbros have U-Pb zircon ages of 372 ± 2 Ma (Gladesville contact aureole), 386 ± 5.7 Ma (Buffalo), 403.8 ± 3.7 Ma (Highway 200), 404.9 ± 6.9 Ma (Mecklenburg), and 416 ± 6.9 Ma (Calhoun Falls). The Ogden Gabbro has a U-Pb age from baddeleyite of 411.91 ± 0.25 Ma. In this study, we identified a previously unrecognized Alleghanian (Pennsylvanian) gabbro suite with U-Pb zircon ages of 308.2 ± 6.2 Ma (Farmington), 311 ± 6.2 Ma (Dutchman’s Creek), and 311 ± 6.5 Ma (Mount Carmel). These gabbros should henceforth not be included in the Concord suite. The ages of Concord suite plutons slightly predate the main phase of plutonism in the Cat Square terrane to the west, which we suggest represents the product of B-type subduction of ocean crust beneath the Carolina superterrane between 415 and 400 Ma. Arc-related magmatism terminated because of the switch to A-type subduction of the eastern Laurentian margin. Prograde upper-amphibolite- to granulite-facies metamorphism, wholesale migmatization, and extensive anatectic plutonism in the eastern Inner Piedmont occurred from Late Devonian into Mississippian time, shortly after cessation of Concord plutonic suite plutonism, which also supports this proposed model. These data, combined with the timing and geometry of foreland clastic wedges, provide compelling support for Devonian–Mississippian accretion of the Carolina superterrane via dextral transpressive obduction above the eastern Laurentian margin.

Abstract The Haile gold mine is located in southern Lancaster County, South Carolina, near the town of Kershaw. Gold was discovered at the site in 1827, and four periods of mining have yielded 360,000 ounces of gold. The mine is located between the past producing Ridgeway and Brewer mines that, when all are combined, constitute a significant amount of historical gold production in the southeastern United States. These mines are hosted within Neoproterozoic to lower Cambrian Carolina terrane rocks and are dominated by volcanic and epiclastic units that have experienced greenschist facies metamorphism. Saprolitic weathering is present in the near-surface portions of the deposit and is locally covered by Cretaceous-aged Coastal Plain sediments. The gold mineralization at the Haile mine is hosted within silicified meta-sediments containing fine-grained disseminated pyrite and pyrrhotite and is a replacement type-epithermal deposit. Re-Os ages from molybdenite associated with the mineralization indicate that the deposit formed shortly after major, arc-related volcanic activity. Haile currently has a measured and indicated resource of 4.03 million ounces at an average grade of 1.77 g/t Au with an additional inferred resource of 801,000 ounces at an average grade of 1.24 g/t Au. Included in the resource is a reserve of 2.02 million ounces of gold at an average grade of 2.06 g/t. Mine construction began in May 2015, and gold production is expected by the end of 2016. The construction cost is expected to be US$380 million. Ore will be extracted from eight open pits with mill extraction and the current mine life is 14 years.

Abstract This two-day field trip focuses on the geology and geomorphology of the Carolina Sandhills in Chesterfield County, South Carolina. This area is located in the updip portion of the U.S. Atlantic Coastal Plain province, supports an ecosystem of longleaf pine ( Pinus palustris ) and wiregrass ( Aristida stricta ), and contains three major geologic map units: (1) An ~60–120-m-thick unit of weakly consolidated sand, sandstone, mud, and gravel is mapped as the Upper Cretaceous Middendorf Formation and is interpreted as a fluvial deposit. This unit is capped by an unconformity, and displays reticulate mottling, plinthite, and other paleosol features at the unconformity. The Middendorf Formation is the largest aquifer in South Carolina. (2) A 0.3–10-m-thick unit of unconsolidated sand is mapped as the Quaternary Pinehurst Formation and is interpreted as deposits of eolian sand sheets and dunes derived via remobilization of sand from the underlying Cretaceous strata. This unit displays argillic horizons and abundant evidence of bioturbation by vegetation. (3) A <3-m-thick unit of sand, pebbly sand, sandy mud, and mud is mapped as Quaternary terrace deposits adjacent to modern drainages. In addition to the geologic units listed above, a prominent geomorphologic feature in the study area is a north-trending escarpment (incised by headwater streams) that forms a markedly asymmetric drainage divide. This drainage divide, as well as the Quaternary terraces deposits, are interpreted as evidence of landscape disequilibrium (possibly geomorphic responses to Quaternary climate changes).

We present a newly compiled geologic map of the Pine Mountain window based on available 1:24,000 (and smaller) scale geologic maps; this map provides an improved basis to reconcile long-standing issues regarding tectonic evolution. We integrate sensitive high-resolution ion microprobe (SHRIMP) single-grain U-Pb ages of igneous, metamorphic, and detrital zircons from Grenville basement rocks, associated metasedimentary units, and cover rocks to help clarify the pre-Appalachian history and to better delimit the distribution of Laurentian versus peri-Gondwanan and Gondwanan units along the southeast flank of the window. U-Pb results indicate that some units, which earlier had been correlated with Neoproterozoic to Early Cambrian Laurentian rift deposits of the Ocoee Supergroup (i.e., Sparks-Halawaka Schist), actually are supracrustal rocks deposited prior to ~1100 Ma that were intruded and metamorphosed during the Ottawan phase of the Grenville orogeny. Zircons from the Phelps Creek Gneiss are 425 ± 7 Ma and overlap in time with plutons that intruded rocks of the Carolina superterrane during the Silurian (i.e., the Concord-Salisbury suite). The host units to the Phelps Creek Gneiss had also previously been interpreted as Sparks-Halawaka Schist, but field relations combine with the Silurian intrusive age to suggest that they rather belong to the peri-Gondwanan Carolina superterrane, helping to refine the position of the Central Piedmont suture in its most southern exposures. Results suggest that the Pine Mountain window is not framed by a single fault, but by Alleghanian faults of different timing, rheology, and kinematics, some of which were reactivated while others were not. The new map and U-Pb dates reveal that the southwesternmost exposures of the Central Piedmont suture are located farther northwest, so the width of the Pine Mountain window narrows from 22 km wide in central Georgia to only 5 km in Alabama. At its narrowest, the flanks of the Pine Mountain window are marked by two relatively thin normal faults (the Towaliga and Shiloh faults, northwest and southeast, respectively) that have excised the wider, earlier-formed mylonite zones. All of the Alleghanian faults are cut by later high-angle, normal and left- and right-slip brittle faults (Mesozoic?), which also influenced the present configuration of the window.

A prominent unconformity, present across shallow shelf areas of the Euramerican paleoequatorial basins, is used to demark the boundary between the Mississippian and Pennsylvanian subsystems. This unconformity, the mid-Carboniferous eustatic event, is generally attributed to a major glacio-eustatic sea-level fall. Although a Mississippian-Pennsylvanian unconformity is recognized throughout most of the Appalachian region, the record of the mid-Carboniferous eustatic event in the structurally deepest part of the basin has been controversial. Based on early reports that suggested the most complete Pennsylvanian section was present in southern West Virginia, various conceptual depositional models postulated continuous sedimentation between the youngest Mississippian Bluestone Formation and the oldest Pennsylvanian Pocahontas Formation. In contrast, tabular-erosion models envisioned axial drainage systems that evolved in response to changing basin dynamics. These models predicted a Mississippian-Pennsylvanian unconformity. All these models suffered from a lack of biostratigraphic control. The presence of a sub-Pocahontas paleovalley, herein named the Lashmeet paleovalley, has been confirmed in southern West Virginia. The Lashmeet paleovalley was incised over 35 m into Bluestone strata and filled by lithic sands derived from the Appalachian orogen to the northeast and east. The polygenetic Green Valley paleosol complex marks the Bluestone-Pocahontas contact on associated interfluves. Together, these features indicate a substantial period of subaerial exposure and argue strongly in favor of a Mississippian-Pennsylvanian unconformity. Paleontologic data from the Bluestone Formation, including marine invertebrates and conodonts from the marine Bramwell Member and paleofloral data, support a late, but not latest, Arnsbergian age assignment. Marine fossils are not known from the Pocahontas Formation, but macrofloral and palynomorph taxa support a Langsettian age for most of the Pocahontas. The biostratigraphic, sedimentologic, and paleogeographic data support the presence of an early Pennsylvanian (middle to late Namurian) disconformity in the Appalachian Basin that corresponds to the mid-Carboniferous eustatic event.