Warren D. Huff

Nomenclature, stratigraphy, chemical fingerprinting, and areal distribution of some Middle Ordovician K‐bentonites in Baltoscandia

Abstract Based on biostratigraphic position, chemical fingerprinting, and lithic characteristics such as relative thickness, several of the numerous K‐bentonite beds, or complexes of beds, in the Middle Ordovician of Baltoscandia are shown to be traceable over large areas. Because the type of volcanic eruptions that produced such widespread ash beds lasts only a short time (a couple of weeks, or less), the individual ash layers represent as close equivalents of time planes as one is likely to find in the Lower Paleozoic stratigraphic record. Although these clay beds were recorded in section descriptions as far back as in the 1880s, their volcanic nature was recognized in Baltoscandia only in the mid‐1940s. Subsequent research has added important data on the occurrence and chemical composition of the Middle Ordovician K‐bentonites, but the present study is the first detailed regional investigation of these beds across their entire distribution area in Baltoscandia. Four beds, or complexes of beds (the Gr...

Large-magnitude Middle Ordovician volcanic ash falls in North America and Europe: Dimensions, emplacement and post-emplacement characteristics

Abstract Middle Ordovician K-bentonites represent some of the largest known fallout ash deposits in the Phanerozoic Era. They cover minimally 2.2 × 106 km2 in eastern North America and 6.9 × 105 km2 in northwestern Europe, and represents the coeval accumulation of plinian and co-ignimbrite ash on both Laurentia and Baltica during the closure of the Iapetus Ocean. The three most widespread beds are the Deicke and Millbrig K-bentonites in North America and the Kinnekulle K-bentonite in northwestern Europe. The vents were located near the Laurentian margin of Iapetus on an arc or microplate undergoing collision with Laurentia. The volume of ash preserved in the stratigraphie record converted to dense rock equivalent (DRE) of silicic magma is minimally estimated to be 943 km3 for the Deicke, 1509 km3 for the Millbrig and 972 km3 for the Kinnekulle. The Millbrig and Kinnekulle beds are coeval and possibly equivalent, yielding a combined DRE volume of nearly 2500 km3. Some unknown but probably large amount of additional ash fell into oceanic regions of the Iapetus, but these areas became subducted and the ash is not preserved in the geologic record. The symmetry of the thickness contours is suggestive that one or more ash clouds interacting with equatorial stratospheric and tropospheric wind patterns dispersed pyroclastic material to both the northwest and southeast in terms of Ordovician paleogeography. Based on grain size measurements and thickness/area 1 2 plots we conclude the three beds were each formed from co-ignimbrite or possibly phreatoplinian eruption columns. Analyses of melt inclusions in primary quartz crystals indicate the parental magma contained approximately 4% dissolved water at the time of the eruption. This water provided the explosive energy during the initial gas thrust phase. The implied fragmentation pressure on the magma would have reduced much of the ejecta to small particles, forming a deposit composed largely of single crystals and glassy dust. Conversion of the ash to K-bentonite resulted in a mass loss of approximately 35%, mostly in the form of Si with lesser amounts of Na and K.

The Lower Silurian Osmundsberg K-bentonite; Part II, Mineralogy, geochemistry, chemostratigraphy and tectonomagmatic significance

The Lower Silurian Osmundsberg K-bentonite is a widespread ash bed that occurs throughout Baltoscandia and parts of northern Europe. This paper describes its characteristics at its type locality in the Province of Dalarna, Sweden. It contains mineralogical and chemical characteristics that permit its regional correlation in sections elsewhere in Sweden as well as Norway, Estonia, Denmark and Great Britain. The < 2 µm clay fraction of the Osmundsberg bed contains abundant kaolinite in addition to randomly ordered (RO) illite/smectite (I/S). Modelling of the X-ray diffraction tracings showed the I/S consists of 18 % illite and 82 % smectite. The high smectite and kaolinite content is indicative of a history with minimal burial temperatures. Analytical data from both pristine melt inclusions in primary quartz grains as well as whole rock samples can be used to constrain both the parental magma composition and the probable tectonic set- ting of the source volcanoes. The parental ash was dacitic to rhyolitic in composition and originated in a tec- tonically active collision margin setting. Whole rock chemical fingerprinting of coeval beds elsewhere in Baltoscandia produced a pronounced clustering of these samples in the Osmundsberg field of the discriminant analysis diagram. This, together with well-constrained biostratigraphic and lithostratigraphic data, provides the basis for regional correlation and supports the conclusion that the Osmundsberg K-bentonite is one of the most extensive fallout ash beds in the early Phanerozoic. The source volcano probably lay to the west of Baltica as part of the subduction complex associated with the closure of Iapetus.

40Ar/39Ar dating of Ordovician K-bentonites in Laurentia and Baltoscandia

Several Ordovician K-bentonites occurring widely in eastern North America and western Europe were dated using the 40Ar/39Ar technique to test previously suggested inter-continental correlations. The three thickest and most widespread bentonites – Deicke, Millbrig (North America) and Kinnekulle (Sweden and Denmark) – were examined. Single-grain analyses of phenocrystic biotites yield 100% concordant plateau ages of 449.8±2.3 Ma (2σ internal error) (Deicke), 448.0±2.0 Ma (Millbrig) and 454.8±2.0 Ma (Kinnekulle in Sweden). The altered biotites from the Denmark Kinnekulle sample show discordant age spectra with highly variable plateau ages being most likely caused by recoil redistribution of Ar isotopes between biotite and interlayer secondary alteration products during neutron irradiation. Such recoil artifacts are likely evident in previously published 40Ar/39Ar data for these units, causing bias towards spuriously old ages. In such cases where intragrain alteration is evident and discordant age spectra are common, geologically meaningless plateau ages may be obtained which are less accurate than integrated ages, hence a 100% concordance criterion for acceptance is useful. The inconsistent 40Ar/39Ar ages and geochemical contrasts between the North American and Swedish bentonites imply distinct volcanic episodes, and preclude a specific correlation between the Millbrig and Kinnekulle K-bentonites. The precise 40Ar/39Ar ages indicate a minimum duration of 7±3 Myr for the Diplograptus multidens graptolite biozone. In comparison with previous U/Pb data, the results are useful for quantifying the bias (ca. 1%) between current calibrations of the 40Ar/39Ar and U/Pb geochronometric systems. This bias is most likely due to errors related to 40K decay constants and neutron fluence monitors used in 40Ar/39Ar dating.

The Ordovician Sebree Trough: An oceanic passage to the Midcontinent United States

The Sebree Trough is a relatively narrow, shale-filled sedimentary feature extending for several hundred kilometers across the Middle and Late Ordovician carbonate platform of the Midcontinent United States. The dark graptolitic shales within the trough stand in contrast to the coeval bryozoan-brachiopod-echinoderm– rich limestones on the flanking platforms. We infer from regional stratal patterns, thickness and facies trends, and temporal relations established by biostratigraphy and K-bentonite stratigraphy that the Sebree Trough initially began to develop during late Turinian to early Chatfieldian time (Mohawkian Series) as a linear bathymetric depression situated over the failed late Precambrian–Early Cambrian Reelfoot Rift. Rising sea level and positioning of a subtropical convergence zone along the southern margin of Laurentia caused the rift depression to descend into cool, oxygen-poor, phosphate-rich oceanic waters that entered the southern reaches of the rift from the Iapetus Ocean. The trough apparently formed in a system of epicontinental estuarine circulation marked by a density- stratified water column. Trough formation was accompanied by cessation of carbonate sedimentation, deposition of graptolitic shales, development of hardground omission surfaces, substrate erosion, and local phosphogenesis. The carbonate platforms on either side of the trough are dominated by bryozoan-brachiopod- echinoderm grainstones and packstones that were deposited in zones of mixing where cool, nutrient-rich waters encountered warmer shelf waters. Concurrently, lime mudstone and wackestone were deposited shoreward (northern Illinois, Wisconsin, Iowa, Minnesota, Michigan) in warmer, more tropical shallow seas. Coeval upward growth of the flanking carbonate platforms sustained and enhanced development of the trough shale facies. Five widespread diachronous late Mohawkian and Cincinnatian omission surfaces are present in the carbonate facies of the Midcontinent. These surfaces include sub-Deicke K-bentonite, DS1; top of Black River Limestone, DS2; base and top of the Guttenberg Limestone Member of the Decorah Formation, DS3 and DS4; and top of the Trenton Limestone, DS5. Some of the surfaces correspond to previously described depositional sequence boundaries. All five surfaces, which embody deepening phases on top of highstand-systems tracts, converge in the Sebree Trough, indicating that the trough was a long-lived feature and was the source of eutrophic waters that episodically spread across the adjacent platforms, terminating carbonate production. Late Turinian and early Chatfieldian incipient drowning episodes were followed by a final drowning event that began in the Sebree Trough during the late Chatfieldian ( Climacograptus spiniferus Zone) and reached southernmost Minnesota and other regions far within the platform interior by Richmondian time ( Amorphognathus ordovicicus Zone).

Chemical correlation of K-bentonite beds in the Middle Ordovician Decorah Subgroup, upper Mississippi Valley

Strata of the Champlainian (Middle Ordovician) Decorah Subgroup in the upper Mississippi Valley region have been correlated on the basis of the chemical composition of K-bentonite beds in widely distributed outcrops and cores. The four principal K-bentonite beds in the Decorah—the Deicke, Millbrig, Elkport, and Dickeyville—can be differentiated by their unique chemical fingerprints, which were established using a linear discriminant function analysis. The elements that served as the best discriminators of differences between beds were, in order of atomic number, Na, Sc, Ti, Zr, Sm, Eu, Tb, Dy, Yb, Lu, Hf, Ta, and Th. Although no one element serves to delineate a K-bentonite bed completely from others, a combination of elements can do so. The chemical signatures of the Deicke and Millbrig K-bentonite Beds, the two thickest and most widespread K-bentonites in the Mississippi Valley, were recognized in outcrop and subsurface from southern Minnesota to southeastern Missouri, a distance of about 900 km. The Elkport and the Dickeyville K-bentonites were chemically identified in a limited area in northern Illinois, southwestern Wisconsin, and northern Iowa. The Decorah consists of widespread lithologic units that are approximately parallel to K-bentonite beds in some areas, but in other areas lateral gradation of lithologies, as shown by K-bentonite correlations, indicates contemporaneity of Decorah lithofacies.

Chemical characteristics and origin of Ordovician K-bentonites along the Cincinnati Arch

K-bentonites of the Middle Ordovician High Bridge Group along the Cincinnati arch are characterized by interstratified illite/smectite (I/S) clays with rectorite-type ordering. Approximately 20% of the layers are expandable. They are structurally similar to I/S formed at temperatures exceeding 100°C during burial diagenesis, however stratigraphic evidence and a color alteration index of < 1.5 for conodonts in associated carbonates reveals they have never been deeply buried or subjected to temperatures greater than 80°C.Whole-rock samples of K-bentonites contain ∼8% K2O and ∼4% MgO, whereas the <0.1-µm size fraction contains 6–7% K2O and 5% MgO. By comparison with a hypothetical parent ash, these values represent a net gain of K and Mg and a net loss of Si, Fe, Ca, and Na during post-depositional alteration. K-fixation is accounted for by a layer charge imbalance arising primarily out of octahedral substitution of Mg+2 for Al+3, indicating that the interstratification evolved from a montmorillonite precursor. The chemical characteristics of I/S layers in K-bentonites developed early during the alteration of volcanic ash to montmorillonite. Relatively high contents of K and Mg probably reflect both seawater and parent material composition at the time of formation. The composition and ordered stacking in K-bentonites was determined by the composition of the original smectite rather than by the pressure-temperature conditions of burial diagenesis.РезюмеК-бентониты из Группы Средней Ордовикской Высокий Мосе вдоль Синсинатской дуги характеризируются наличием переслаивающихся иллито-смектитовых (И/С) глин, упорядоченных по типу ректорита. Приблизительно 20% слоев способно расширяться. По структуре они схожи с И/С, формированными при температурах, превышающих 100°С во время глубинного диагенеза. Однако стратиграфические данные и показатель изменения цвета <1,5 для конодонтов в ассоциированных карбонатах показывают, что они никогда глубоко не залегали или не подверголись воздействию температур выше 80°С.Цельные образцы скальной породы К-бентонитов содержат ~8% K2O и 4% MgO в то время, как фракция размером <0,1 µm содержит 6–7% K2O и 5% MgO. По сравнению с гипотетическим пепелом эти величины говорят о приобретении K и Mg и о потере Si, Fe, Са, и Na в результате послеосадочных изменений. Фиксация К рассчитивалась по увеличению дисбаланса заряда слоя, в основном, путем октаэдрического замещения ионов Аl3+ ионами Mg2+, указывая на то, что промежуточные напластование развивалось от монтмориллонитового предшественника. Химические характеристики И/С слоëв в К-бентонитах формировались ранее во время преобразования вулканического пепела в монтмориллонит. Относительно высокое содержание К и Mg, возможно, является отражением как морской воды, так и состава исходного материала во время образования. Состав и упорядоченная укладка в К-бентонитах определялась скорее составом первоначального смектита, чем условиями температуры и давления при диагенезе. [Е.С.]ResümeeK-Bentonite der mittel-ordovizischen High Bridge Gruppe entlang des Cincinnati-Bogens sind durch Illit/Smektit-Wechsellagerungen (I/S) mit einer Ordnung vom Rektorit-Typ charakterisiert. Ungefähr 20% der Lagen sind expandierbar. Sie sind strukturmäßig den I/S-Wechsellagerungen ähnlich, die bei Temperaturen über 100°C während der Versenkungs-Diagenese gebildet wurden. Die Stratigraphie und ein Farb-Umwandlungs-Index von < 1,5 füt Konodonten in benachbarten Karbonaten zeigen jedoch, daß sie niemals tief versenkt oder einer Temperatur über 80°C ausgesetzt wurden.Gesamtgesteinsproben der K-Bentonite enthalten ∼8% K2O und ∼4% MgO, während die Kornfraktion <0,1 µm, 6–7% K2O und 5% MgO enthält. Durch den Vergleich mit einer hypothetischen Ausgangsasche bedeuten diese Werte einen Nettogewinn yon K und Mg und einen Nettoverlust von Si, Fe, Ca, und Na während der Umwandlung nach der Ablagerung. Die K-Fixierung erklärt sich aus einem Ladungsungleichgewicht der Lagen, das vor allem durch die oktaedrische Substitution von Mg für Al hervorgerufen wird. Dies deutet darauf hin, daß die Wechsellagerung aus einem Montmorillonit-Vorläufer entstanden ist. Die chemischen Charakteristika der I/S-Lagen in den K-Bentoniten entwickelten sich zu Beginn der Umwandlung der vulkanischen Asche zu Montmorillonit. Relativ hohe Gehalte an K und Mg spiegeln wahrscheinlich sowohl die Zusammensetzung des Meerwassers als auch die der Ausgangssubstanz zur Zeit der Bildung wieder. Die Zusammensetzung und die regelmäßige Anordnung in den K-Bentoniten wurde eher durch die Zusammensetzung des ursprünglichen Smektit bestimmt als durch die Druck-Temperatur-Bedingungen einer Versenkungs-Diagenese. [U.W.]RésuméDes bentonites-K du groupe Ordovicien Moyen High Bridge le long de l’arche de Cincinnati sont caractérisées par des argiles interstratifiées illite/smectite (I/S) avec un rangement du type rectorite. Approximativement 20% des couches sont expansibles. Structuralement, elles sont semblables aux I/S formées à des températures excédant 100°C pendant la diagénèse d’ensevelissement, l’évidence stratigraphique, cependant, et un indexe d’altération de couleur < 1,5 pour les conodontes dans des carbonates associés révèlent qu’elles n’ont jamais été profondément enterrées ou soumises à des températures plus é1evées que 80°C.Des échantillons de roche entière de bentonites-K contiennent ∼8% K2O et ∼4% MgO, alors que la fraction de taille <0, l-µm contient 6–7% K2O et 5% MgO. En comparaison avec une cendre hypothétique apparentée, ces valeurs représentent un gain net de K et Mg et une perte nette de Si, Fe, Ca, et Na pendant l’altération produite après déposition. La fixation de K est expliquée par un déséquilibre de charge de couche produit par la substitution octaèdre de Mg +2 à Al +3, indiquant que l’interstratification avait évolué d’un précurseur montmorillonite. Les caractéristiques chimiques des couches I/S dans les bentonites-K se sont développées tôt pendant l’altération de la cendre volcanique en montmorillonite. Des contenus relativement é1evés en K et Mg réflétent probablement à la fois l’eau de mer et la composition de la matière parente au moment de la formation. La composition et l’ordre d’empilement dans les bentonites-K étaient déterminées par la composition de la smectite d’origine plutôt que par les conditions de pression et de température pendant la diagénèse d’enterrement. [D.J.]

Ordovician K-bentonites in the Argentine Precordillera: relations to Gondwana margin evolution

Abstract Ordovician K-bentonites have now been recorded from >20 localities in the vicinity of the Argentine Precordillera. Most occur in the eastern thrust belts, in the San Juan Limestone and the overlying the Gualcamayo Formation, but a few ash beds are known also from the central thrust belts. The oldest occur in the middle Arenig I. victoriae lunatus graptolite (Oe. evae conodont) Zone, and the youngest in the middle Llanvirn P. elegans (P. suecicus) Zone. Mineralogical characteristics, typical of other Ordovician K-bentonites, include a matrix of illite/smectite mixed-layer clay and a typical felsic volcanic phenocryst assemblage: biotite, beta-form quartz, alkali and plagioclase feldspar, apatite, and zircon, with lesser amounts of hornblende, clinopyroxene, titanite and Fe-Ti oxides. The proportions of the mineral phases and variations in their crystal chemistry are commonly unique to individual (or small groups of) K-bentonite beds. Glass melt inclusions preserved in quartz are rhyolitic in composition. The sequence is unique in its abundance of K-bentonite beds, but a close association between the Precordillera and other Ordovician sedimentary basins cannot be established. The ash distribution is most consistent with palaeogeographical reconstructions in which early Ordovician drifting of the Precordillera occurred in proximity to one or more volcanic arcs, and with eventual collision along the Andean margin of Gondwana during the mid-Ordovician Ocloyic event of the Famatinian orogeny. The Puna-Famatina terrane northeast of the Precordillera might have served as the source of the K-bentonite ashes, possibly in concert with active arc magmatism on the Gondwana plate itself.

Nature and regional significance of unconformities associated with the Middle Ordovician Hagan K-bentonite complex in the North American midcontinent

Stratal patterns of the Middle Ordovician Hagan K-bentonite complex and associated rocks show that the Black River–Trenton unconformity in the North American midcontinent formed through the complex interplay of eustasy, sediment accumulation rates, siliciclastic influx, bathymetry, seawater chemistry, and perhaps local tectonic uplift. The unconformity is diachronous and is an amalgamated surface that resulted from local late Turinian lowstand exposure followed by regional early Chatfieldian transgressive drowning and sediment starvation. The duration of the unconformity is greatest in southern Wisconsin, northern Illinois, and northern Indiana, where the Deicke and Millbrig K-bentonite Beds converge at the unconformity. On the basis of published isotopic ages for the Deicke and Millbrig beds, it is possible that in these regions erosion and nondeposition spanned a period of as much as 3.2 m.y. Two broad coeval depositional settings are recognized within the North American midcontinent during early Chatfieldian time. (1) An inner shelf, subtidal facies of fossiliferous shale (Spechts Ferry Shale Member and Ion Shale Member of the Decorah Formation) and argillaceous lime mudstone and skeletal wackestone (Guttenberg and Kings Lake Limestone Members) extended from the Canadian shield and Transcontinental arch southeastward through Minnesota, Wisconsin, Iowa, and Missouri. (2) A seaward, relatively deep subtidal, sediment-starved, middle shelf extended eastward from the Mississippi Valley region to the Taconian foreland basins in the central and southern Appalachians and southward through the pericratonic Arkoma and Black Warrior basins. In the inner shelf region, the Black River–Trenton unconformity is a composite of at least two prominent hardground omission surfaces, one at the top of the Castlewood and Carimona Limestone Members and the other at the top of the Guttenberg and Kings Lake Limestone Members, both merging to a single surface in the middle shelf region. The inner and middle shelves redeveloped later in approximately the same regions during Devonian and Mississippian time.

Chemostratigraphy, K-Ar ages and illitization of Silurian K–bentonites from the Central Belt of the Southern Uplands–Down–Longford terrane, British Isles

The Central Belt of the Southern Uplands Terrane, in both Scotland and Ireland, is a faulted and imbricated sequence of Caradoc to Llandovery shales and Llandovery turbidites with numerous interbedded K-bentonites. The bentonites are composed dominantly of R3-ordered mixedlayer illite/smectite (I/S) containing 90–95% illite, and represent the product of illitization during low-grade metamorphism. K-Ar age determination on the <0.5 μm size fraction gave a range from 379 ± 10 to 406 ± 10 Ma. The fixed K is thought to have originated in the precursor ash, and was remobilized during the transformation of smectite to I/S. K-Ar ages record a retrograde thermal event that post-dates Wenlock prehnite—pumpellyite facies metamorphism and is contemporaneous with cooling and uplift during the end-Silurian—early Devonian collision of Laurentia with the East Avalonian terrane. Differences in Rb and other trace elements between the K–bentonite beds are due, in part, to differences in original ash composition, and can be used to group the beds within biostratigraphically-defined boundaries. The chemical identification of groups of K-bentonite beds offers additional criteria for their stratigraphical correlation on a regional scale.