American Midcontinent Research Articles

Detrital zircons from crystalline rocks along the Southern Oklahoma fault system, Wichita and Arbuckle Mountains, USA

Detrital zircons with ages of 535 ± 10 Ma in many North American Midcontinent sandstones are commonly attributed to sources in Cambrian synrift igneous rocks along the Southern Oklahoma fault system. New analyses are designed to test the characteristics of proximal detritus from the Wichita and Arbuckle uplifts. Detrital zircons from a sandstone (Lower Permian Post Oak Conglomerate) directly above an unconformable contact with the Wichita Granite Group in the Wichita Mountains have strongly unimodal U-Pb ages of 540–520 Ma and eHft values of +4.7 to +10.1. In contrast, two sandstone samples (Upper Pennsylvanian Vanoss Conglomerate) in the onlapping succession above an angular unconformity on Paleozoic strata on the flank of the Arbuckle anticline have detrital zircons with U-Pb ages that correspond domi­nantly to the Superior (∼2700 Ma) and secondarily to the Granite-Rhyolite (1480–1320 Ma) and Grenville (1300–970 Ma) provinces of the Laurentian craton. The Vanoss zircons indicate recycling from quartzose sandstones within the Middle Ordovician platform carbonates in the Arbuckle passive-margin cover succession. A stratigraphically higher sandstone (Permian Wellington Formation) above the onlapping conglomerates has a more diverse detrital-­zircon population, indicating that sediment dispersal from external sources overwhelmed the proximal detritus in the immediate cover of the Wichita and Arbuckle uplifts. The distinctive eHft values of the proximal detritus from the Cambrian synrift igneous rocks offer potential discrimination from zircons of the same age from Gondwanan accreted terranes, which are represented in the Wellington sample.

Nanoscale constraints on porosity generation and fluid flow during serpentinization

Research Article| February 01, 2016 Nanoscale constraints on porosity generation and fluid flow during serpentinization Benjamin M. Tutolo; Benjamin M. Tutolo * 1Department of Earth Sciences, University of Oxford, Oxford OX1 3AN, UK2Department of Earth Sciences, University of Minnesota, Minneapolis, Minnesota 55455, USA *E-mail: benjamin.tutolo@earth.ox.ac.uk Search for other works by this author on: GSW Google Scholar David F.R. Mildner; David F.R. Mildner 3National Institute of Standards and Technology Center for Neutron Research, Gaithersburg, Maryland 20899, USA Search for other works by this author on: GSW Google Scholar Cedric V.L. Gagnon; Cedric V.L. Gagnon 3National Institute of Standards and Technology Center for Neutron Research, Gaithersburg, Maryland 20899, USA4Department of Materials Science and Engineering, University of Maryland, College Park, Maryland 20742, USA Search for other works by this author on: GSW Google Scholar Martin O. Saar; Martin O. Saar 2Department of Earth Sciences, University of Minnesota, Minneapolis, Minnesota 55455, USA5Department of Earth Sciences, ETH-Zürich, 8006 Zurich, Switzerland Search for other works by this author on: GSW Google Scholar William E. Seyfried, Jr. William E. Seyfried, Jr. 2Department of Earth Sciences, University of Minnesota, Minneapolis, Minnesota 55455, USA Search for other works by this author on: GSW Google Scholar Author and Article Information Benjamin M. Tutolo * 1Department of Earth Sciences, University of Oxford, Oxford OX1 3AN, UK2Department of Earth Sciences, University of Minnesota, Minneapolis, Minnesota 55455, USA David F.R. Mildner 3National Institute of Standards and Technology Center for Neutron Research, Gaithersburg, Maryland 20899, USA Cedric V.L. Gagnon 3National Institute of Standards and Technology Center for Neutron Research, Gaithersburg, Maryland 20899, USA4Department of Materials Science and Engineering, University of Maryland, College Park, Maryland 20742, USA Martin O. Saar 2Department of Earth Sciences, University of Minnesota, Minneapolis, Minnesota 55455, USA5Department of Earth Sciences, ETH-Zürich, 8006 Zurich, Switzerland William E. Seyfried, Jr. 2Department of Earth Sciences, University of Minnesota, Minneapolis, Minnesota 55455, USA *E-mail: benjamin.tutolo@earth.ox.ac.uk Publisher: Geological Society of America Received: 10 Sep 2015 Revision Received: 25 Nov 2015 Accepted: 30 Nov 2015 First Online: 09 Mar 2017 Online Issn: 1943-2682 Print Issn: 0091-7613 © 2016 Geological Society of America Geology (2016) 44 (2): 103–106. https://doi.org/10.1130/G37349.1 Article history Received: 10 Sep 2015 Revision Received: 25 Nov 2015 Accepted: 30 Nov 2015 First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Benjamin M. Tutolo, David F.R. Mildner, Cedric V.L. Gagnon, Martin O. Saar, William E. Seyfried; Nanoscale constraints on porosity generation and fluid flow during serpentinization. Geology 2016;; 44 (2): 103–106. doi: https://doi.org/10.1130/G37349.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Field samples of olivine-rich rocks are nearly always serpentinized—commonly to completion—but, paradoxically, their intrinsic porosity and permeability are diminishingly low. Serpentinization reactions occur through a coupled process of fluid infiltration, volumetric expansion, and reaction-driven fracturing. Pores and reactive surface area generated during this process are the primary pathways for fluid infiltration into and reaction with serpentinizing rocks, but the size and distribution of these pores and surface area have not yet been described. Here, we utilize neutron scattering techniques to present the first measurements of the evolution of pore size and specific surface area distribution in partially serpentinized rocks. Samples were obtained from the ca. 2 Ma Atlantis Massif oceanic core complex located off-axis of the Mid-Atlantic Ridge and an olivine-rich outcrop of the ca. 1.1 Ga Duluth Complex of the North American Mid-Continent Rift. Our measurements and analyses demonstrate that serpentine and accessory phases form with their own, inherent porosity, which accommodates the bulk of diffusive fluid flow during serpentinization and thereby permits continued serpentinization after voluminous serpentine minerals fill reaction-generated porosity. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Geometric morphometric analysis and taxonomic revision of the Gzhelian (Late Pennsylvanian) conodont Idiognathodus simulator from North America

A new morphometric approach was developed to study morphological variation within P1 elements commonly referred to as Idiognathodus simulator, which was selected to be the biostratigraphic marker for the base of the global Gzhelian Stage (Carboniferous). This new approach combines landmark-based geometric morphometrics with eigen analyses to analyze shape variation within P1 elements of the I. simulator group, and could be used to analyze shape variation in other morphologically similar conodont groups. Specimens analyzed were obtained from three sections of the early Gzhelian Heebner Shale of the Oread cyclothem in the North American Midcontinent region, the cyclothem from which I. simulator was originally named. This analysis shows that the I. simulator group comprises a set of at least five species with asymmetrical P1 element pairs, relatively short adcarinal ridges, and a variably developed eccentric groove. Species discrimination is based on the presence of caudal and rostral lobes, character of the...

Crustal signatures of the tectonic development of the North American midcontinent

The stable eastern portion of the North American continent offers an excellent environment to study the tectonic development of intra-continental structures. The midcontinent of North America formed by the accretion of Proterozoic terranes, and has since experienced episodes of deformation during the subsidence of the Illinois Basin and uplift of the Ozark Plateau. Rifting also initiated in eastern North America, but extension did not continue and arms of failed rifts extend across the region. The New Madrid Seismic Zone, situated within a portion of the failed Reelfoot Rift, represents an active zone of intraplate seismicity. Analyzing the structure of the crust and upper mantle within the midcontinent will therefore provide insight into the factors that lead to intraplate deformation. Using data from over 180 Transportable Array seismic stations, we calculate receiver functions to investigate the crust and upper mantle of the midcontinent. At close to 40 km thick, the crust of the New Madrid Seismic Zone is thinner than in the surrounding areas outside of the Reelfoot Rift and Rough Creek Graben. The Illinois Basin cannot be characterized by a single crustal structure, as crust near 50 km thick in the central portion of the basin thins to between 40 and 45 km thick towards the northern and southern portions of the basin. Discontinuities within the crust and upper mantle are prominent in and around the New Madrid Seismic Zone and mark locations of crustal modification and underplating. Comparing changes in crustal structure to the distribution of Bouguer gravity anomalies, the presence of positive gravity anomalies suggests that higher density crust plays a role in maintaining low surface elevations within the Reelfoot Rift. Conversely, a negative gravity anomaly in an area of thinner crust within the Ozark Plateau supports the need for low-density crustal material to influence the uplift of the plateau.

Conodont biostratigraphy and paleoecology of the marine sequence of the Tapajós Group, Early-Middle Pennsylvanian of Amazonas Basin, Brazil

This study was undertaken in the south and western regions of the Amazonas Basin to describe the conodont biostratigraphy and paleoecology of Pennsylvanian carbonate rocks of the marine portion of the Tapajós Group comprising the upper Monte Alegre, Itaituba, and lower Nova Olinda formations.The analyzed area includes one outcrop along the Tapajós river (TAP), two carbonate quarries (QI, QII), and 18 wells (dots 1–18). The conodont fauna is dominated by Idiognathoides sinuatus and Neognathodus symmetricus in the Monte Alegre Formation, followed by Idiognathodus incurvus, Diplognathodus coloradoensis and Neognathodus bassleri in the Itaituba and Nova Olinda formations. The conodont association suggests an Early to Middle Pennsylvanian age to the analyzed section. Relative ages attributed to the three lithostratigraphic units using conodonts, palynomorphs, and foraminifers are consistent.Herein are proposed one local taxon-range zone of Idiognathodus incurvus in the Itaituba and lower part of the Nova Olinda Formation and one local taxon-range subzone of Diplognathodus coloradoensis in the Itaituba Formation, suggesting a late Bashkirian – Moscovian (Atokan – early Desmoinesian) age to these strata.The Itaituba Formation marks the establishment of large Pennsylvanian marine conditions in the Amazonas Basin and is composed primarily of marine carbonates of abundant fossil content, tidal flat evaporites and siliciclastic thin intervals. Its lower limit, with the Monte Alegre Formation, is characterized by the predominant occurrence of fluvial-deltaic sandstones superimposed on an extensive sequence of aeolian sandstones, siltstones and shales intercalated with the interdune and lakes. From the upper strata of Itaituba Formation the faunal and lithological characteristics indicate the occurrence of a regressive phase culminating in a restricted environment, arid which indicates the Nova Olinda Formation. This is characterized by the occurrence of evaporites increasingly abundant to the top of carbonates, also shales and siltstones intercalated with the sabkha plain. The carbonate strata have similar marine fossils to those of Itaituba Formation, however, the fauna becomes impoverished in abundance and diversity. The conodont fauna of Early-Middle Pennsylvanian of Amazonas Basin suggests similarities of species with the North American Midcontinent region and the Illinois Basin indicating their probable cosmopolitism; however in a slightly different paleogeographic context, probably due to latitudinal differences, coal deposits are present in North America, whereas widespread evaporitic deposits are registered to the Tapajós Group, Amazonas Basin.

Salinity contrast in the US Midcontinent Sea during Pennsylvanian glacio-eustatic highstands: Evidence from conodont apatite δ 18 O

Abstract Pennsylvanian phosphatic black shales are widely interpreted to have been deposited in waters deep enough to produce a pycnocline that would have developed as a consequence of increased riverine runoff into the largely landlocked North American Midcontinent Sea during the more humid interglacial intervals of the Late Palaeozoic Ice Age. We tested whether a gradient in surface water salinity existed in the North American Midcontinent Sea during times of maximum flooding by analyzing oxygen isotopes of conodont apatite from correlative Pennsylvanian black shales of the Midland, Midcontinent, Illinois and Appalachian basins. In the Midland and Midcontinent basins, average δ 18 O values for conodonts from the Oakley (middle Desmoinesian/upper Moscovian), Hushpuckney (lower Missourian, lower Kasimovian) and Heebner black shales (lower Virgilian/lower Gzehlian) are 18.8 ± 0.3‰, 19.1 ± 0.2‰, and 18.9 ± 0.1‰, respectively. Oxygen isotope ratios of conodonts from the Nuyaka Creek black shale (uppermost Desmoinesian/upper Moscovian) of the Midcontinent Basin are 19.8 ± 0.3‰, and thus significantly higher in comparison to the other black shales. These higher values support previous interpretations that the Lost Branch is a minor cyclothem, with south polar regions having been less deglaciated than during the interglacial phases of the more widespread major cyclothems. Conodonts from Oakley shale equivalents from Ohio and Pennsylvania are depleted in 18 O on average by 2.5‰ in comparison to conodonts from the Midcontinent Basin. This pattern is also evident for conodonts from the Heebner and Hushpuckney shales, although conodonts from the Appalachian Basin show only a 0.4–0.7‰ depletion in 18 O compared to conodonts from the Midcontinent Basin. The differences in δ 18 O are interpreted as reflecting primarily salinity, assuming that temperature was relatively constant across the palaeotropical Midcontinent Sea. Lower salinities in the Appalachian Basin and a significant salinity gradient in the Midcontinent Sea were reconstructed for the Desmoinesian Oakley shale, supporting the idea that freshwater discharge resulted in a salinity-stratified water column during times of “everwet” tropical palaeoclimate, at least in the eastern part of the epeiric sea. However, the salinity gradient is considerably reduced during the subsequent Missourian and Virgilian periodically dry interglacial periods (Hushpuckney and Heebner shales). This suggests that surface waters in the Midcontinent Sea were not significantly influenced by continental runoff and calls into question whether a halocline was necessary for the formation of phosphatic core shales.

Geochemical Stratigraphy of the Keweenawan Midcontinent Rift Volcanic Rocks with Regional Implications for the Genesis of Associated Ni, Cu, Co, and Platinum Group Element Sulfide Mineralization

The development of the North American Midcontinent rift can be understood in the context of a tectonic and magmatic event in which the far-field effects of continental drift influenced magmatism produced by a mantle plume. We report and interpret new data for samples collected from stratigraphically controlled sequences of basaltic rocks in the Osler Volcanic Group and the Mamainse Point Volcanic Group in Ontario, Canada. We confirm that the earliest phase of rift development involved primitive melts that produced basalts in the Lower Formation of the Osler Volcanic Group with Fo74–80 olivine (1,490–2,280 ppm Ni). The earliest basalts are primitive in both the Mamainse Point Volcanic Group and Osler Volcanic Group sections, with elevated MgO, Cr, and Ni; these rocks also have high TiO2 and high Gd/Yb, which are features that indicate low degrees of melting of a garnet-bearing lherzolite mantle. The Central Formation A and B basalts of the Osler Volcanic Group that overlie the Lower Formation lavas record an upward increase and then decrease in the degree of contamination of basaltic rocks with normal Ni and platinum group element (PGE) abundance levels that require no sulfide saturation event to accompany contamination. The Upper Formation of the Osler Volcanic Group is characterized by a limited number of PGE-depleted flows; although these basalts are highly fractionated and typically develop textures with stellate to laminated plagioclase feldspar, they are usually not significantly contaminated. The Lower and Upper formations of the Mamainse Point Volcanic Group show a diversity in geochemistry consistent with contributions from different mantle sources, but contamination is restricted to a small number of flows with normal Ni and PGE abundance levels. There is no indication of a regional metal depletion signature on the scale of the Nadezhdinsky Formation at Noril’sk, but there is evidence that local S saturation events were triggered in the unexposed conduits feeding some of the upper sequence flows in response to fractionation of the magma. Massive to heavy disseminated Ni-Cu-PGE sulfide mineralization in the Midcontinent rift is hosted by small primitive intrusions that were formed early in the evolution of the rift and are typically associated with transtensional structures in the cratonic blocks adjacent to the main rift. Examples include the Eagle deposit in Michigan, the Tamarack mineralization in Minnesota, and the Current Lake Complex in Ontario. The American examples are associated with splays of the Great Lakes tectonic zone and the Canadian example is associated with the Quetico fault zone. The intrusions share the geochemical fingerprints of the more primitive lowermost volcanic rocks of the Midcontinent rift and they are broadly contemporaneous with the development of the lower stratigraphy of the Osler Volcanic Group and Mamainse Point Volcanic Group. Although the low-grade but large-tonnage Cu-Ni-PGE deposits of the Duluth were formed significantly later than the small primitive intrusions, their host rocks share the same lithophile trace element signature as these early intrusions. The parental magmas that produced all of these intrusions were derived from deeper mantle depths, where garnet was a stable phase, than the magmas that produced the Central Formation basalts and some of the Upper Formation basalts investigated in this study.

Evolution of the Main Zone at the Marathon Cu-PGE Sulfide Deposit, Midcontinent Rift, Canada: Spatial Relationships in a Magma Conduit Setting

Various ultramafic Ni-Cu-platinum group element (PGE) deposits associated with the North American Midcontinent rift have been attributed to formation in a magma conduit setting, whereby PGE concentration is controlled by various fluid dynamic processes. The Marathon Cu-PGE sulfide deposit located within the Midcontinent rift-related Coldwell Alkaline Complex has been classified as a gabbro-associated contact-type deposit; however, both magmatic and hydrothermal processes have been proposed to account for the significant concentration of PGE. In light of the growing field of evidence for magma conduit-type settings, this study comprised a comprehensive geochemical investigation of the complicated crosscutting gabbroic to ultramafic units in the immediate vicinity of the Marathon deposit; and a thorough three-dimensional investigation of the distribution of Cu and Pd within the Main mineralized zone. The main objectives of this study were to test the applicability of the magma conduit deposit model to the Marathon deposit and to identify key exploration criteria for use elsewhere in the Coldwell Alkaline Complex. Mineralization is hosted by the Two Duck Lake gabbro, a 4-km-long and 250-m-thick unit of the Marathon Series. The Marathon Series is the latest of three magmatic series that make up the 1- to 2-km-thick Eastern Gabbro Suite, which wraps around the eastern and northern margin of the Coldwell Alkaline Complex. The three magmatic series are shown here to have distinct trace element signatures that enable reliable discrimination of potentially sulfide and PGE-bearing units of the Marathon Series from the barren rocks of either the Fine-Grained or Layered Series. At the Marathon deposit, sulfides consist of disseminated chalcopyrite, pyrrhotite, and minor bornite and occur within the Main, Footwall, and Hanging-wall zones and in the PGE-enriched W Horizon. This paper focused on sulfides located within the Main zone, including the keel-shaped feeder channel that continues downdip to over 550-m depth. The spatial distribution of Cu, Pd, and Cu/Pd were examined in relation to a three-dimensional surface model for the footwall contact; in a vertical profile through the Main zone; and in a longitudinal section that cuts the feeder channel. There are three important observations: (1) trends for elevated Cu and Pd are parallel to numerous troughs and ridges in the footwall, (2) Cu, Pd, and Cu/Pd varies up section in a saw-toothed pattern from high to low values, and (3) the proportion of high Cu/Pd sulfides is greatest within the thickest accumulations of sulfides within the feeder channel. Evaluation of interelement relationships between Cu and Pd and between Pd and Ir, Rh, Pt, and Au for mineralization within the Main zone indicate positive associative, but nonlinear behavior for all elements. Briefly, the data show nonlinear correlations between Cu and Pd in which Cu/Pd decreases with increasing Pd; and coherent but nonlinear behavior for Ir, Rh, Pt, and Pd in which Pd/Ir, Pd/Rh, Pd/Pt, and Pd/Au all increase with increasing Pd. The observed variation in Cu/Pd is consistent with a magmatic model calculated by others for deposits in the Duluth Complex, in which sulfides accumulated in a closed system from a melt with mantlelike Cu/Pd and an elevated silicate to sulfide ratio. The observed variations in Pd/Ir, Pd/Rh, and Pd/Pt are consistent with R factor fractionation related to differences in the relative partition coefficients between sulfide and silicate melts, and rule out the possibility that processes such as fractionation of sulfide melt by monosulfide solid solution (mss) or redistribution of metals during hydrothermal alteration played a significant role in the mineralizing event. The Two Duck Lake gabbro and associated sulfides of the Marathon deposit are proposed to have formed by multiple injections of plagioclase crystal mush that carried droplets of sulfide liquid along a conduit system that was controlled by radial and ring fault structures in the Coldwell Alkaline Complex. The accumulation of sulfides was controlled by flow dynamic processes within the magma channels, but Cu/Pd was controlled by local proportions of silicate melt to sulfide liquid. Key characteristics of the deposit that are critical to exploration elsewhere in the Coldwell Alkaline Complex include the following: (1) the recognition that gabbroic to ultramafic intrusions of the Marathon Series are the host for Cu and PGE mineralization, (2) the distribution of Cu/Pd data within sulfide occurrences are useful as vectors toward the feeder channel, (3) topographic lineaments are indicators of potential mineralized feeder zones, and (4) oxide- and apatite-rich, irregularly shaped gabbroic to ultramafic pods are potential indicators of an underlying feeder channel.

Floian (Early Ordovician) conodont-based biostratigraphy and biogeography of the Australasian Superprovince

The Australasian Superprovince, a biogeographical entity defined by shallow water conodont faunas that mainly inhabited Ordovician tropical zones of eastern Gondwana and peri-Gondwanan continental blocks and terranes, is characterized by the occurrence of serratognathids in the Floian. Biostratigraphic correlation of the Floian Stage within the Australasian Superprovince is based primarily on species of Serratognathus, Prioniodus and Oepikodus, which are distributed across South China, North China, the Tarim Basin, Korea, Indochina, Sibumasu, Kazakhstan and Australia. Conodont biozones from the shallow water Yangtze Platform succession of South China, from oldest to youngest the Serratognathus diversus, Prioniodus honghuayuanensis, Oepikodus communis, and O. evae biozones, correlate respectively with the Serratognathus bilobatus, Prioniodus elegans, and O. evae biozones recognized in the succession of the deeper-water Jiangnan Slope. Paroistodus proteus, a zonal index species of late Tremadocian to earliest Floian age in Baltoscandia, occurs abundantly in the slope setting of South China and in outer-shelf settings in Western Australia. Other zonal index species of the Baltoscandian middle and late Floian succession that are recognized in South China include P. elegans (only reported from the Jiangnan Slope) and O. evae, which is widespread on the slope and deeper parts of the Yangtze Platform. O. evae is, however, confined to deep water basin successions in Australia. O. communis (zonal index species for the middle Floian of the Ibexian Series in the North American Midcontinent fauna) is common in the middle-upper Floian Stage in Australia, but is relatively rare in South China.

Stable carbon‐isotope record of shallow‐marine evaporative epicratonic basin carbonates, Ordovician Williston Basin, North America

AbstractSecular variations in stable carbon‐isotope values of marine carbonates are used widely to correlate successions that lack high‐resolution index fossils. Various environmental processes, however, commonly may affect and alter the primary marine carbon‐isotope signal in shallow epicratonic basins. This study focuses on the marine carbon‐isotope record from the carbonate–evaporite succession of the upper Katian (Upper Ordovician) Red River Formation of the shallow epicratonic Williston Basin, USA. It documents the carbon‐isotope signal between the two major Ordovician positive shifts in δ13C, the early Katian Guttenberg and the Hirnantian excursions. Eight δ13C stages are identified based on positive excursions, shifts from positive to negative values and relatively uniform δ13Ccarb values. A correlation between carbon‐isotope trends and the relative sea‐level changes based on gross facies stacking patterns shows no clear relation. Based on the available biostratigraphy and δ13C trends, the studied Williston Basin curves are tied to the isotope curves from the North American Midcontinent, Québec (Anticosti Island) and Estonia, which confirm the Late Katian age (Aphelognathus divergens Conodont Zone) of the upper Red River Formation. The differences in the δ13C overall trend and absolute values, coupled with the petrographic and cathodoluminescence evidence, suggest that the carbon‐isotope record has been affected by the syndepositional environmental processes in the shallow and periodically isolated Williston Basin, and stabilized by later burial diagenesis under reducing conditions and the presence of isotopically more negative fluids.

Brachiopoda Taxonomy and Biostratigraphy of the RedWall Limestone (Lower Mississippian) of Arizona

Forty-six species, assignable to 36 brachiopod genera, are recognized, described, and illustrated from the Lower Mississippian Redwall Limestone of northern Arizona. Seven new species are recognized, four of which are named. Named species are: Spinocarinifera (Seminucella) costatula, new species; Magnumbonella ampla, new species; Setigerites gutschicki, new species; and Spirifer redwallensis , new species. The remaining three newly recognized species remain in open nomenclature because study material was too poorly preserved to justify naming. The majority of brachiopod species studied were recovered from the Thunder Springs and Mooney Falls members near the middle of the formation. The basal Whitmore Wash Member and uppermost Horseshoe Mesa Member contain only sparse and poorly preserved brachiopod material. The spotty stratigraphic distribution of collections, which were recovered from largely geographically disparate locations, resulted in the creation of a stratigraphic range chart that exhibits no recognizable segregation into any potential brachiopod zones.Many of the Redwall Limestone's brachiopod species are known from contemporaneous formations elsewhere in the Cordillera or central United States. Biostratigraphically key species such as Marginatia fernglenensis (Weller, 1909), Marginatia burlingtonensis (Hall, 1858), Stegacanthia bowsheri Muir-Wood and Cooper, 1960, Fernglenia vernonensis (Swallow, 1860), Voiseyella novamexicana (Miller, 1881), and Punctospirifer subtexta (White, 1862), indicate that much of the Thunder Springs and Mooney Falls members is correlative with latest Kinderhookian (late Tournaisian) through latest Osagean (early Viséan) formations of the American Midcontinent. These correlations indicate that the Redwall Limestone is temporally equivalent to the Fern Glen-Burlington formations of the central United States. These correlations are consistent with other Redwall forms that are biostratigraphially useful, such as foraminifers.

Confirmation of progressive plate motion during the Midcontinent Rift's early magmatic stage from the Osler Volcanic Group, Ontario, Canada

AbstractAs the supercontinent Rodinia was assembling ca. 1.1 billion years ago, there was extensive magmatism on at least five Proterozoic continents including the development of the North American Midcontinent Rift. New paleomagnetic data from 84 lava flows of the Osler Volcanic Group of the Midcontinent Rift reveal that there was a significant and progressive decrease in inclination between the initiation of extrusive volcanism in the region (ca. 1110 Ma) and ca. 1105 ± 2 Ma (during the “early stage” of rift development). Paleomagnetic poles can be calculated for the lower portion of the reversed Osler Volcanic Group (40.9°N, 218.6°E, A95 = 4.8°, N = 30) and the upper portion of the reversed Osler Volcanic Group (42.5°N, 201.6°E, A95 = 3.7°, N = 59; this pole can be assigned the age of ca. 1105 ± 2 Ma). This result is a positive test of the hypothesis that there was significant plate motion during the early stage of rift development. In addition to being a time of widespread volcanism on Laurentia and other continents, this interval of the late Mesoproterozoic was characterized by rapid paleogeographic change.

Species-level phylogenetic revision of the Ordovician orthide brachiopod Glyptorthis from North America

Species of the orthide brachiopod genus Glyptorthis occur as common constituents of Late Ordovician benthic marine faunas, particularly in Laurentia. A phylogenetic analysis of 23 North American species of Glyptorthis was conducted to inform a systematic revision and ascertain evolutionary relationships amongst species. Both discrete and continuous characters, from which character states were determined using statistical methods, were utilized within a maximum parsimony approach. Glyptorthis maquoketensis is recognized as a distinct species of Glyptorthis rather than a subspecies of G. insculpta. Glyptorthis virginica is herein synonymized with G. uniformis. Several species of Glyptorthis became widely distributed via dispersal events during the evolution of the clade. The revised phylogenetic topology provides structure to interpret biogeographical patterns within the genus. For example, Glyptorthis insculpta participated in the Late Ordovician Richmondian Invasion into the Cincinnati Basin. The recovered phylogenetic topology suggests that G. insculpta may have invaded the Cincinnati Basin from a low latitude subtropical region of Laurentia, such as the American mid-Continent.

Carbon isotope (δ13Ccarb) stratigraphy of the Lower–Middle Ordovician (Tremadocian–Darriwilian) in the Great Basin, western United States: Implications for global correlation

New stable carbon isotope data (δ13Ccarb) from Lower–Middle Ordovician (Tremadocian to Darriwilian) carbonate mudstone and wackestone rocks of the Pogonip Group are presented from two sections in the Great Basin region (USA) — Shingle Pass (east-central Nevada) and the Ibex area (western Utah). The Pogonip Group is a succession of mixed carbonate and siliciclastic rocks that accumulated on a carbonate ramp under normal marine conditions during the Late Cambrian (Furongian) to Middle Ordovician (Darriwilian). The Shingle Pass and Ibex area sections have been previously studied for their conodont biostratigraphy and contain a North American Midcontinent conodont fauna that range from the Cordylodus intermedius Zone (uppermost Cambrian) to the Phragmodus polonicus Zone (Darriwilian). The δ13C trend has four distinct characteristics recognized in both Great Basin sections: 1) a drop in δ13C from +1‰ at the base of the Ordovician (Tremadocian) to −0.7‰, 2) a 1 to 2‰ positive δ13C shift in the uppermost Rossodus manitouensis Zone during the late Tremadocian, 3) a gradual δ13C increase from −2‰ to ca. 0‰ during the end of the Early Ordovician (Floian), and 4) a steady δ13C decrease from 0‰ to −4 to −5‰ during Middle Ordovician (Dapingian–Darriwilian).In the Lower Ordovician, δ13C trends reported here from the Great Basin are not consistent with a causal mechanism involving sea level change and the migration of isotopically distinct water bodies. Instead, these Lower Ordovician isotope data most likely reflect primary seawater chemistry and changes in δ13C on a global scale. This interpretation is supported by the excellent correlation of δ13C in the Lower Ordovician to other δ13C trends reported from the sections in the Argentine Precordillera (La Silla and San Juan formations) and in western Newfoundland (St. George and Table Head groups). These correlations using δ13C are consistent with published biostratigraphic data and provide an integrated and high-resolution chemo-biostratigraphic framework for the Lower Ordovician sedimentary record of the Laurentian margin. The Middle Ordovician portion of the δ13C curves in the Great Basin represented by the Kanosh and Lehman formations shows significant isotopic depletion relative to the section in Argentina. Thus, although there is some indication that minima and maxima in the Middle Ordovician curves can be correlated, the Great Basin sections show clear evidence of overprinting by local variables related to both diagenesis (dolomitization) and platform restriction.

Late Palaeozoic South American pectinids revised: biostratigraphical and palaeogeographical implications

Neves, J.P., Anelli, L.E., Pagani, M.A. & Simões, M.G., 2014. Late Palaeozoic South American pectinids revised: biostratigraphical and palaeogeographical implications. Alcheringa 38 ISSN 0311-5518.A revision of the late Palaeozoic South American pectinid Heteropecten multiscalptus (Thomas) and the establishment of Heteropecten paranaensis sp. nov. have important implications for the relationship between faunal realms within South America. Late Palaeozoic bivalve faunas occur in three distinct realms in South America: a Central Gondwanic Realm with endemic taxa showing affinities to Australia, New Zealand, South Africa and Namibia, a cold Perigondwanic Realm, and a warm Extragondwanic Realm with tethyan-like affinities similar to faunas of the American Midcontinent. In South America, faunas east of the southern Andes belong to the first two realms and previous interpretations of bivalve faunas suggested biocorrelations with those of the Extragondwanic Realm because they shared the taxon Heteropecten multiscalptus (Thomas). A revision of the Peruvian and Brazilian material does not confirm this. Instead, a re-analysis suggests that two species are present, rather than one: Heteropecten multiscalptus in the Cerro Prieto Formation, Amotape Mountains (Peru; Extragondwanic Realm), and Heteropecten paranaensis sp. nov. in the upper part of the Itararé Group, Paraná Basin (Brazil; Central Gondwanic Realm). Thus, the correlation between the late Palaeozoic faunas of the Central Gondwanic and Extragondwanic Realms in South America can no longer be supported. Heteropecten paranaensis sp. nov. lived in a siliciclastic-dominated, cold, epeiric sea of Brazil and Argentina, and is morphologically similar to some Australian species, whereas the Peruvian H. multiscalptus thrived in the warm seas of the Extragondwanic Realm.Jacqueline P. Neves [nevesjp.unesp@gmail.com], Instituto de Geociências e Ciências Exatas, Universidade Estadual Paulista, Rio Claro campus, SP, Brazil, 13506-900; Luiz E. Anelli [anelli@usp.br], Instituto de Geociências, Universidade de São Paulo, 05508–080, Brazil; M. Alejandra Pagani, [apagani@mef.org.ar], CONICET-Museo Paleontológico Egidio Feruglio (MEF), U9100GYO, Chubut, Argentina; Marcello, G. Simões [profmgsimoes@gmail.com], Instituto de Biociências, Universidade Estadual Paulista, Botucatu campus, SP, 18618-970, Brazil. Received 19.7.2013; revised 12.11.2013; accepted 19.11.2013

The changing role of hydropower: from cheap local energy supply to strategic regional resource

The role of hydropower has evolved from being a local, low-cost energy source to being a flexible resource offering a variety of ancillary services including regional frequency control and energy storage for large interconnected power systems. This paper explores this development as it relates to a North American midcontinent electrical power region. It reviews traditional benefits from hydropower in a thermal-power-dominated region and traditional efforts to quantity those benefits. With the challenges of integrating increasing quantities of variable generation such as wind and solar power, new benefits from hydropower have been identified and new methods to quantify those benefits have been developed. Recent results of a major study of the sub-hourly behaviour of a hydropower system in a Midwestern United States electrical market are reviewed.

Katian (Upper Ordovician) conodonts from Wales

AbstractMiddle and upper Katian conodonts were previously known in the British Isles from relatively small collections obtained from a few localities. The present study is mainly based on 17 samples containing more than 17 000 conodont elements from an approximately 14‐m‐thick succession of the Sholeshook Limestone Formation in a road cut near Whitland, South Wales, that yielded a diverse fauna of more than 40 taxa. It is dominated by representatives of Amorphognathus, Aphelognathus/Plectodina and Eocarniodus along with several coniform taxa. Representatives of Decoriconus, Istorinus and Sagittodontina are reported from the Ordovician of UK for the first time. The fauna is a typical representative of the British Province of the Atlantic Realm and includes a mixture of taxa of North American, Baltoscandic and Mediterranean affinities along with pandemic species. Based on the presence of many elements of Amorphognathus ordovicicus and some morphologically advanced specimens of Amorphognathus superbus, the Sholeshook Limestone Formation is referred to the lower A. ordovicicus Zone. Most of the unit is also coeval with Zone 2 of the Cautleyan Stage in the British regional stage classification, and stage slice Ka3 of the middle Katian Stage in the global stratigraphical classification, an age assignment consistent with data from trilobites, graptolites and chitinozoans. The unusually large collection of M elements of Amorphognathus provides insight into the complex morphological variation in this element of some Katian species of this genus. The Sholeshook conodont fauna is similar to those of the Crûg and Birdshill limestones, but differs in several respects from the slightly older ones from the Caradocian type area in the Welsh Borderland. Although having some species in common, the Sholeshook conodont fauna clearly differs from coeval Baltoscandic faunas and is even more different in composition compared with equivalent North American Midcontinent faunas.

Halomonas sulfidaeris‐dominated microbial community inhabits a 1.8 km‐deep subsurface Cambrian Sandstone reservoir

A low-diversity microbial community, dominated by the γ-proteobacterium Halomonas sulfidaeris, was detected in samples of warm saline formation porewater collected from the Cambrian Mt. Simon Sandstone in the Illinois Basin of the North American Midcontinent (1.8 km/5872 ft burial depth, 50°C, pH 8, 181 bars pressure). These highly porous and permeable quartz arenite sandstones are directly analogous to reservoirs around the world targeted for large-scale hydrocarbon extraction, as well as subsurface gas and carbon storage. A new downhole low-contamination subsurface sampling probe was used to collect in situ formation water samples for microbial environmental metagenomic analyses. Multiple lines of evidence suggest that this H. sulfidaeris-dominated subsurface microbial community is indigenous and not derived from drilling mud microbial contamination. Data to support this includes V1-V3 pyrosequencing of formation water and drilling mud, as well as comparison with previously published microbial analyses of drilling muds in other sites. Metabolic pathway reconstruction, constrained by the geology, geochemistry and present-day environmental conditions of the Mt. Simon Sandstone, implies that H. sulfidaeris-dominated subsurface microbial community may utilize iron and nitrogen metabolisms and extensively recycle indigenous nutrients and substrates. The presence of aromatic compound metabolic pathways suggests this microbial community can readily adapt to and survive subsurface hydrocarbon migration.

On the integration of Ordovician conodont and graptolite biostratigraphy: new examples from Gansu and Inner Mongolia in China

Wang, Z.H., Bergström, S.M., Zhen, Y.Y., Chen, X. & Zhang, Y.D., 2013. On the integration of Ordovician conodont and graptolite biostratigraphy: New examples from Gansu and Inner Mongolia in China. Alcheringa 37, 510–528. ISSN 0311-5518.Few Ordovician successions in the world contain both biostratigraphically highly diagnostic conodonts and graptolites permitting an integration between standard biozones based on these fossil groups. The Sandbian Guanzhuang section in the vicinity of Pingliang in the Gansu Province has an outstanding graptolite record through most of the Nemagraptus gracilis and Climacograptus bicornis graptolite biozones. Calcareous interbeds in the succession yield biostratigraphically important conodonts, including some species used for biozonations in Baltoscandia and the North American Midcontinent. Likewise, the middle–upper Darriwilian Dashimen section in the Wuhai region of Inner Mongolia hosts both diverse graptolites of the Pterograptus elegans, Didymograptus murchisoni and lowermost Nemagraptus gracilis biozones, and conodonts of Midcontinent and Baltoscandic types. The distribution patterns of these index fossil groups provide an unusual opportunity to closely correlate conodont and graptolite biozones in the middle to upper Darriwilian to Sandbian interval. For instance, the base of the C. bicornis Biozone is approximately coeval with the base of the Baltoscandic B. gerdae Subbiozone and a level near the middle of the North American P. aculeata Biozone.Zhi-hao Wang [zhwang@nigpas.ac.cn] Xu Chen [xu1936@gmail.com], and Yuan-dong Zhang [ydzhang@nigpas.ac.cn], Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing 210008, China; Stig M. Bergström [stig@geology.ohio-state.edu], School of Earth Sciences, Division of Earth History, The Ohio State University, Columbus, OH 43210, USA; Yong Yi Zhen [yongyi.zhen@austmus.gov.au], Australian Museum, 6 College Street, Sydney NSW 2010, Australia.

The Okmulgee, Oklahoma fossil flora, a Mazon Creek equivalent: Spatial conservatism in the composition of Middle Pennsylvanian wetland vegetation over 1100 km

Temporal compositional conservatism of late Middle Pennsylvanian wetland vegetation and persistence of its dominance–diversity structure through time spans of millions of years have been documented from many places in North America and Europe. This conservatism occurs within the context of glacial–interglacial fluctuations that force spatial migration of that vegetation or its restriction to refugia during periodic sea-level changes on the craton. Although known from long temporal sequences, there have been few studies of spatial variability in these wetland floras over more than relatively small distances, generally much less than a km. Here we report a late Middle Pennsylvanian (Desmoinesian) flora from above the Henryetta Coal, near Okmulgee, Oklahoma, in the American Midcontinent. This flora is exactly correlative with the Mazon Creek flora, above the Colchester Coal of northern Illinois; the Colchester Coal and its equivalents may have been the largest, contiguous peat accumulating swamp of the Pennsylvanian. Okmulgee and Mazon Creek are separated by 1100km, but the depositional context of the two floras is the same and they can be considered isotaphonomic. Though a much smaller sample, and thus of lower overall biodiversity, the known composition of and dominance–diversity structure of the Okmulgee flora is convergent with Mazon Creek. The overwhelmingly dominant elements of both the Okmulgee and Mazon Creek floras are pecopterid tree-fern foliage mostly attributable to Lobatopteris vestita (Lesquereux) Wagner and the pteridosperm Macroneuropteris scheuchzeri (Hoffman) Cleal, Shute and Zodrow, Both of these floras likely represent wetlands that were being drowned during the early phases of sea-level rise associated with ice melting. Each sampled a broad area, and a variety of subhabitats that fringed the Pennsylvanian coastline.