Air-fall Volcanic Ash Research Articles

Formation of columnar/tabular and vermicular kaolinite in the crystal tonsteins within the Late Carboniferous Coal in the Datong Coalfield, Shanxi Province, North China

The Datong Coalfield, Shanxi Province, North China, is rich in high-grade coal-measure kaolin, primarily occurring as partings within the Late Carboniferous–Early Permian coal seams. There is a type of kaolinite-dominant coarse-grained parting similar to sandstone in appearance, whereas the phanerocrystalline minerals are columnar/tabular and vermicular kaolinite rather than terrigenous detrital minerals. The origins of the coarse-grained partings and the formation mechanism of the columnar/tabular and vermicular kaolinite remain ambiguous. Herein, petrographical, mineralogical, whole-rock geochemical, and zircon U–Pb–Hf isotopic studies of the coarse-grained partings were conducted to explore these issues. The zircons in the coarse-grained partings show typical features of magmatic origin and yield a single age group of very latest Carboniferous at ca. 300 Ma, indicating they were transformed from air-fall volcanic ash. Therefore, the coarse-grained partings are essentially crystal tonsteins. The wide range of negative εHf(t) values and arc-related tectonic affinity of the zircons genetically link the primary volcanic ash to the Late Paleozoic continental arc volcanoes that existed in the Inner Mongolia Paleo-Uplift, related to the southward subduction of the Paleo-Asian oceanic plate beneath the North China Block. Two orthogonal cleavages are generally observed on the columnar/tabular and vermicular kaolinite in the crystal tonsteins, contradicting the crystalline structure of kaolinite that belongs to a 1:1 type phyllosilicate with only one cleavage along its (001) lattice plane. These columnar/tabular and vermicular kaolinite are considered to be pseudomorphic kaolinite after plagioclase in the crystal-rich volcanic ash. When the crystal-rich volcanic ash fell into open and acidic peat-forming environments (mires), the low-pH water containing abundant organic acids and good water circulation conditions facilitate dissolution of plagioclase along the chemically unstable (001) lattice planes and leaching out of Na+, Ca2+, and much of silica. Simultaneously, the Al3+ or hydroxides of aluminum and silica recrystallized along the (001) lattice planes of plagioclase as the substrate, constraining layer-by-layer growth of large kaolinite platelets to eventually form pseudomorphic kaolinite. The cleavages perpendicular to the exclusive (001) cleavage of pseudomorphic kaolinite should be inherited from the twin planes and (010) cleavage cracks (if present) of plagioclase.

Provenance of multi-stage volcanic ash recorded in the Late Carboniferous coal in the Jungar Coalfield, North China, and their contribution to the enrichment of critical metals in the coal

The origins of partings within the Late Carboniferous No. 6 coal in the Jungar Coalfield, North China, and their relationship to the enrichment of critical metals in the coal, is poorly understood. This study combines UPb ages and LuHf isotope data of zircon, whole-rock geochemistry, LA–ICP–MS elemental mapping, and selective chemical extraction experiments to investigate the provenance of the partings and the occurrence mode of critical metals in the partings collected from the exploration area in the northwest of the Jungar Coalfield. It was found that the zircons in the kaolinite-dominant partings yielded a single age group of ca. 300 Ma at the latest Carboniferous (Gzhelian), indicating that the partings are alteration products of air-fall volcanic ash (tonsteins). The zircons are characterized by negative εHf(t) values and a continental arc tectonic setting, suggesting a genetic connection between the volcanic ash and the Late Paleozoic continental arc that existed in the Inner Mongolia Paleo-Uplift, related to the southward subduction of the Paleo-Asian oceanic plate beneath the northern margin of the North China Block. During the devitrification of volcanic glasses to kaolinite in the mires, the incompatible elements, such as Nb, Ta, Zr, Hf, and rare earth elements (REEs and Y, REY), preserved in the volcanic glasses were leached out as they are incompatible with the crystal lattice of kaolinite. Niobium and Ta, similar to Ti in ionic radii, were preferentially incorporated into fine-grained Ti oxides as reprecipitated products in the alteration process of volcanic ash. The enrichment of Nb, Ta, and Ti in organic matter in the tonsteins suggests that they have some mobility during alteration of volcanic ash and could be, to some extent, transported into the enclosing peats. Zirconium, Hf, and REY were fixed by organic matter admixed with the volcanic ash after their deposition in the mires. Meanwhile, they could also be transported into the enclosing peats, as no secondary minerals formed could substantially retain them. Although Li and Ga were able to be incorporated into the crystal lattice of kaolinite during their crystallization, they could also be transported into the enclosing peats due to their fluid-mobile nature. Both the leaching of a voluminous amount of volcanic ash contacting with the peats and the input of the volcanic ash as an admixture into the peats contributed to the enrichment of critical metals in the Late Carboniferous coal in the Jungar Coalfield.

Organic petrology and geochemistry of Lower Cretaceous lacustrine sediments in the Chaoyang Basin (Liaoning Province, northeast China): Influence of volcanic ash on algal productivity and oil shale formation

Airfall volcanic ash deposited in the form of tuff layers is widely distributed in oil-shale bearing basins in western Liaoning Province, northeast China. In order to investigate the impacts of volcanic ash on algal productivity and the formation of organic-rich oil shales, the organic petrology and organic geochemistry of lacustrine sediments of the Lower Cretaceous Jiufotang Formation were investigated. Molecular geochemical data (the pristane/phytane ratio, the gammacerane index and the relative amount of β-carotane) indicate that they were formed in suboxic/anoxic depositional environments, with salinity fluctuations from freshwater to saline. Oil shales in the semi-deep to deepwater lake facies were mainly deposited under still and anoxic bottom water conditions, concomitant with water column salinity stratification. The relative abundances of diterpenoids and regular steranes indicate a limited terrigenous conifer plant contribution and a predominantly algal contribution to the organic matter. Blooms of algae contributing to high primary productivity, and anoxic depositional environments contributing to organic matter preservation, were key to organic-rich oil shale formation. Fish bones, ostracod shells, and alginite are present below the thick tuff layer. Alginite richness gradually increased from very low just above the tuff layer, to a higher concentration further above the tuff layer, indicating that during major volcanic eruptions, large quantities of volcanic ash led to water column poisoning, causing the sudden mass mortality of some aquatic organisms that were sensitive to turbid, acidified and toxic water. The increased proportion of brackish/saline-water dinoflagellates, which is corroborated by the high concentration of telalginite, abundant dinoflagellate-specific biomarkers (C31 4,22,23,24-tetramethylcholestanes), high values of the C27/C29 ααα-20R and C28/C29 ααα-20R sterane ratios, 4-methylsterane index, dinosterane index, and tetramethylsterane index, suggest that brackish/saline-water dinoflagellates show a positive response to volcanic ash input. Some specific brackish/saline-water dinoflagellates are present in organic-rich tuff laminae, indicating that during periods of minor volcanic eruptions or in intermissions between major eruptions, a minor volcanic ash input was more likely to result in rapid and efficient volcanic fertilisation effects and high-quality oil shale formation. Although the lacustrine oil shale system of the Jiufotang Formation shares some similar petrological and geochemical characteristics with the classical saline lacustrine oil shale system of the Green River Formation in the United States of America, the significant influence of airfall volcanic ash on algal productivity makes the formation mechanism of oil shales within the Jiufotang Formation strata quite different from that of oil shales within the Green River Formation.

Detrital zircons and sediment dispersal in the eastern Midcontinent of North America

Abstract Results of detrital-zircon analyses (U-Pb ages and initial Hf values, εHft) of Mississippian–Pennsylvanian sandstones in the Michigan, Illinois, and Forest City basins are remarkably similar to data for coeval sandstones in the Appalachian basin, indicating dispersal of sediment from the Appalachian orogen through the Appalachian basin to the eastern Midcontinent during the late Paleozoic. The similarities of results include matches of the two most prominent age groups (1300–950 Ma and 490–350 Ma), as well as matches of the less abundant age groups. Comparisons of the data are from observations of probability density plots and multidimensional scaling of U-Pb age data and of εHft values. Despite the dominance of an Appalachian signature in all samples, some samples contain grains with ages that suggest intermittent additional sources. Four samples (three ranging in depositional age from Morrowan to Atokan–Desmoinesian in the Illinois basin, and one of Desmoinesian age in the Forest City basin), in addition to typical Appalachian age distributions, have prominent age modes between 768 and 525 Ma, corresponding in age to Pan-African/Brasiliano rocks in Gondwanan accreted terranes in the Appalachian orogen, suggesting intermittent dispersal from the Moretown terrane of the northern Appalachians. Sandstones in the Appalachian basin and those in the Midcontinent basins have very few grains with ages that correspond to the Alleghanian orogeny in the Appalachian orogen. Nevertheless, three sandstones each in the Illinois basin and Forest City basin with depositional ages of 312–308 Ma have a few zircon grains in the age range of 321 ± 5 to 307 ± 4 Ma. The nearly identical crystallization and depositional ages suggest reworking at the depositional sites of air-fall volcanic ash from the Alleghanian orogen, rather than fluvial transport from the orogen. The basal Pennsylvanian sandstones lap onto a regional unconformity around the northern rims of the Illinois and Forest City basins, suggesting sources for recycled grains. Along the northern edge of the Illinois basin, Ordovician sandstones beneath the unconformity may have contributed minor concentrations of Superior-age zircons in the basal Pennsylvanian sandstones. Basal Pennsylvanian sandstones in the Forest City basin lap onto Mississippian strata, suggesting possible recycling of zircons from eroded Mississippian sandstones.

The Undrained Behaviour of an Air-Fall Volcanic Ash

Pyroclastic soils are widespread in the world. In particular, they cover a great part of Campania, a densely populated country of Southern Italy, where some distinct volcanic centers are present. In these soils, precipitations can trigger fast flow-like landslides causing destruction and loss of human lives. The movement style, the high velocity and the long run-out of these landslides are an indication of the occurrence, in the saturated soil mass, of mechanisms of undrained instability due to the inability of soil to sustain the deviator stress related to the slope condition. This paper reports the results of a wide experimental laboratory program carried out on a volcanic ash, which recently has been the seat of a killer landslide, stressing the factors that govern the undrained response of these materials.

Tight-reservoir micropore formation and evolution in sedimentary organic-matter-bearing tuff: a case study from the Permian Tiaohu Formation in the Santanghu Basin, NW China

ABSTRACTRecently, a tuffaceous tight oil reservoir with considerable reserves has been found in the Santanghu Basin, northwestern China. The physical properties of the tuff are characterised by high porosity (5–25%) and low permeability (0.01–0.50 mD). This reservoir has the peculiar property of being a sedimentary organic-matter-bearing tuff formed by air-fall volcanic ash in a lake. Its discovery offered an excellent opportunity to further our knowledge of the formation and evolution of tuffaceous tight-reservoir micropores. This study integrates analyses of organic geochemistry, thin-sections, scanning electron microscopy, CT scanning, mineral and element compositions, mercury injections and measurements of porosity and permeability to determine the characteristics of the formation and evolution of the tuff micropores. (1) The tuff reservoir comprises vitric, crystal-vitric, pelitic and silicified tuffs, which are characterised by high porosity and big throat, moderate porosity and thin throat, low porosity and thin throat, and very low porosity and moderate throat pore structures, respectively. (2) Of the four types, vitric tuff has the properties best suited to a reservoir, and devitrification of volcanic glasses is the principal mechanism of reservoir micropore formation. (3) The tuff of the Permian Tiaohu Formation contains sedimentary organic matter, and organic acids produced during kerogen maturation facilitate the process of devitrification. (4) Evolution of tuff porosity depends mainly on the original composition and burial depth. The porosity of vitric tuff is higher than crystal-vitric tuff at the same depth, but both their porosities initially decrease and then increase with depth. At depths >3000 m, vitric tuff retains the trend towards higher porosity, whereas the porosity of crystal-vitric tuff remains largely unchanged or even decreases. The results of this investigation could be considered a reference regarding the distribution and prediction of tuffaceous reservoirs in similar situations in other parts of the world.

Detailed clay mineralogy of the Triassic-Jurassic boundary section at Kendlbachgraben (Northern Calcareous Alps, Austria)

AbstractThe Triassic-Jurassic boundary (TJB) is marked by one of the five largest Phanerozoic mass extinctions. To constrain existing models for TJB events, we obtained a stratigraphically highly resolved dataset from a marine section at Kendlbachgraben, Austria.The topmost Triassic Kössen Formation contains low to medium-charged smectite and vermiculite as alteration products of mafic-ultramafic minerals. The clay minerals in the boundary mudstone are kaolinite ⩾ illite + muscovite ⨠ smectite > chlorite. Predominant kaolinite suggests humid climate and abundant terrigenous input. In the lowermost Jurassic, the clay mineral pattern changes to illite + muscovite ⨠ kaolinite ⨠ smectite, which reflects change to less humid and more moderate climate.The topmost Kössen Formation also contains clay spherules. Their composition, shape and size indicate that they are alteration products of airborne volcanic glass droplets solidified in the air, settled in the sea and altered rapidly with negligible transport in terrestrial or marine environments. Our data are consistent with sudden climatic change at the TJB, as a result of large-scale volcanic activity of the Central Atlantic Magmatic Province which produced distal airfall volcanic ash.

Composition and Provenance of Volcanic Glass in Late Eocene Manning Formation, East-Central Texas: ABSTRACT

ABSTRACT Airfall volcanic ash beds in the Late Eocene Manning Formation, Brazos River Valley, Texas, contain large quantities of hydrated, but otherwise unaltered and non-corroded, rhyolitic glass shards, along with mica, sanidine, and quartz crystals. Enclosing deposits contain well preserved siliceous microfossils (diatoms, sponge spicules, phytoliths). Unaltered glass occurs only where volcanic ash was deposited in nonmarine, non-swamp environments: lacustrine, fluvial, and forested soil surface. Composition of glass shards from the upper part and top of the Manning Formation was determined by broad-beam electron microprobe analysis. Glasses from four ash beds analyzed to date are very similar in major and minor element composition. The mean and standard deviation in weight% oxides for 398 shards are: 72.7% SiO2 (Standard deviation 0.7%), 11.6% Al2O3 (0.2),5.7% K20 (0.5),2.4% Na2O (0.3),0.7% FeO (0.2), 0.4% CaO (0.1), and 6.5% H2O (by difference). Na and possibly K have likely been substantially leached from glass shards during hydration. MgO, MnO, TiO2, P2O5, and SO3 are all less than 0.2% (below detection limit). The youngest ash layer (top Manning) contains 0.9% FeO compared to 0.7% FeO for two ash layers in the middle Manning and 0.5% FeO for the oldest ash layer in the lower Manning. A possible source for these glasses would be the Sierra Madre Occidental in west Mexico. Age (34.9 and 34.4 Ma for two of the ash beds) and composition is dissimilar to most rhyolites in the Davis Mountains of west Texas.

Determining pre-eruptive compositions of late Paleozoic magma from kaolinized volcanic ashes: Analysis of glass inclusions in quartz microphenocrysts from tonsteins

Glass inclusions in quartz microphenocrysts were analyzed for major and minor elements by electron microprobe and H, Li, Be, B, Rb, Sr, Y, Nb, Mo, Sn, Cs, Ce, Th, and U by ion microprobe. The phenocrysts and inclusions occur as fresh relicts in about eleven strongly kaolinized, air-fall volcanic ash units (tonsteins) that outcrop in five states located in the central Appalachian basin; the ashes were erupted during the Pennsylvanian. Even though the whole-rock tonstein samples are extremely altered, the glass trapped in quartz microphenocrysts preserves pre-eruptive melt compositions, and, consequently, the inclusions are useful for determining compositions of source magmas and identifying geochemical trends indicative of magmatic evolution. Interpretation of inclusion compositions indicates the strongly altered tonsteins were derived from potassium-enriched, metaluminous to mildly peraluminous magma(s). The tonsteins can be divided into two groups on the basis of trapped melt compositions: older tonsteins that have inclusions with high Sr and normative quartz contents and comparatively low concentrations of U, Th, Rb, Y, Cs, Nb, F, and Cl (±Be) and younger tonsteins whose inclusions contain low Sr and normative quartz and high concentrations of U, Th, Rb, Y, Cs, Nb, F, and Cl (±Be). In general, as concentrations of Sr decreased, the magmatic abundances of Rb, Y, Cs, Nb, U, Th, Cl, and F (±Be) increased. The associated magma or magmas were highly evolved, volatile enriched, and contained Rb, Nb, and Y abundances characteristic of continental within-plate granites; compositions ranged from high-silica rhyolite to topaz rhyolite. Pre-eruptive volatile abundances in the source magma(s) were generally high but also highly variable. Chlorine contents of melt(s) ranged from 0.02–0.23 wt%, and F ranged from 0.01–0.7 wt%. Concentrations of H 2O in melt(s) ranged from 1.6–6.5 wt%. The high pre-eruptive H 2O contents are consistent with large eruptive volumes indicating the precursor rhyolites, which weathered to tonsteins, were a result of plinian eruptions. Even though pre-eruptive water concentrations exhibit no recognizable trends with any elements studied, magmatic evolution appears to have been a strong function of F and H 2O in melt(s); the thermal stabilities of quartz and feldspar were controlled by F and H 2O activities at pressures of approximately 0.5–1 kbar.

An update on British Tonsteins

Abstract Tonsteins are thin mudstone beds composed of kaolinite that formed from the alteration of volcanic ash in coal-forming environments. In the Coal Measures of Britain, tonsteins have been recognized as volcanic in origin based on bedform, volcanic textures, field relationships with other volcanics, distinctive volcanic minerals and an igneous geochemistry. The sharp contacts, together with a contrasting composition with adjacent sediments and lateral continuity, are diagnostic of an altered air-fall volcanic ash. Passage into less altered volcanics and into a K-bentonite have both been shown in Britain. A more direct determination of volcanic origin is possible from the identification of volcanic minerals, whole-rock geochemistry and analyses of silicate melt inclusions in volcanic minerals such as β quartz. Once a volcanic origin is established the stratigraphic value of a tonstein is enhanced. Immobile trace elements in tonsteins also provide information on the tectono-magmatic setting of the parent volcano. Two tonstein groups are identified in the Coal Measures of Britain, one formed from basic (basaltic) ash of local origin and the other from acid (rhyolite or rhyodacite) ash, probably associated with distant Plinean or ultraplinean eruptions at a destructive plate margin to the south or southeast.

Factors affecting the geochemistry of a thick, subbituminous coal bed in the Powder River Basin: volcanic, detrital, and peat-forming processes

The inorganic geochemistry and mineralogy of three cores from the Anderson-Dietz 1 coal bed, a 15.2-m-thick subbituminous coal bed in the Tongue River Member (Paleocene) of the Fort Union Formation, were examined (1) to determine if the cores could be correlated by geochemical composition alone over a total distance of 2 km and (2) to identify the major factors that influenced the geochemistry of the coal bed. Chemical data (46 elements on a coal-ash basis) for 81 coal samples and 4 carbonaceous rock samples, with most samples representing a 0.6-m-thick (2-ft) interval of core, were grouped into compositional clusters by means of cluster analysis. Seven major clusters were produced; two of these clusters can be used to correlate the coal bed throughout the study area. Data from scanning electron and optical microscope analyses indicate that several factors influenced the geochemistry of the Anderson-Dietz 1 coal bed. The majority of mineral grains in the coal bed are interpreted to be detrital (water borne); evidence includes the presence of rounded to subrounded quartz grains having two-phase, aqueous fluid inclusions characteristic of hydrothermal or low-to-moderate grade metamorphic quartz. These quartz grains are found throughout the coal bed but are most abundant in samples from the midpart of the bed, which was influenced by detrital input associated with the deposition of the clastic rocks that form the split between the Anderson and Dietz 1 coal beds 900 m to the east of the study area. In addition to the detrital minerals mentioned above, volcanic ash that was fluvially transported to the sites of peat deposition or possibly deposited as air-fall volcanic ash also affected the geochemistry of the coal bed. For example, crandallite(?), a mineral reported to form as an alteration product of volcanic ash, is found in seven samples from the coal bed. The presence of quartz grains containing silicate-melt inclusions in eight samples from the coal bed.provides further support for a volcanic ash component. Other factors that probably affected the geochemistry of the coal bed include (1) detrital input associated with the deposition of the roof rocks of the coal bed, (2) peat-forming processes and plant material, and (3) epigenetic ground-water flow.

Formative Process of Uphill-facing Scarps Distributed around the Ridge Top of Mt. Waga, Ohu Backbone Range, Northeast Japan

Uphill-facing scarps occur perpendicular or slightly oblique to slopes on eastern flank of Mt. Waga of Mts. Mahiru, Ohu Backbone Range, Northeast Japan (Plate 1, 2; Fig. 2). The scarps of which relative height are ca. 5.5 meters run straight (Fig. 3), being concordant with the dominant strike of the joint system around them (Fig. 4). Toppling of base rock around the scarps was discovered (Plate 3; Fig. 5). Sediments burring the linear depression just below Mt. Waga were dislocated to dip to west, the ridge side, and slightly faulted (Fig. 6). Induced from those deformed features of topography, young sediments and base rocks, the uphill-facing scarps have been caused by relative subsidence of the ridge top subsequent to the toppling of the lower mountain slope. Rock mechanics implies the deformation around the ridge top of Mt. Waga has been triggered by earthquakes with recurrence interval of 3, 000yrs. more or less. The last event formed the scarps was presumably the Riku-u Earthquake of 1896 AD. because the dislocated sediments intercalate a thin layer of air fall volcanic ash named To-a of ca 1, 000yrs. B. P.

An Appalachian isochron: A kaolinized Carboniferous air-fall volcanic-ash deposit (tonstein)

The Fire Clay tonstein is a kaolinized, airfall volcanic ash bed that was deposited in a widespread late Carboniferous peat-forming mire. Eleven samples from Kentucky and West Virginia, spanning a distance of 200 km, and two samples from Tennessee and Virginia indicate a characteristic mineralogical signature, as compared with other Appalachian tonsteins, consisting of well-crystallized kaolinite, beta-quartz crystal paramorphs, sanidine, ilmenite, zircon, and brookite. Detrital illite and quartz are rarely present or are in very small amounts, which indicates rapid deposition in a mire. Several normal graded cycles in this tonstein suggest repeated episodes of pyroclastic activity that produced a composite ash layer. A high-silica alkalic rhyolitic source is suggested by the geochemistry of immobile elements and by electron-probe analyses of glass inclusions in volcanic quartz from the Fire Clay tonstein. The rare-earth-element plots (chondrite normalized) of the tonstein show a pronounced negative Eu anomaly and relatively high concentrations of Zr and Th, which are both indicative of a rhyolitic source. Probe analyses of the Fire Clay glass inclusions from four states indicate a chemically identical high-silica rhyolite with peraluminous affinities. 40 Ar/ 39 Ar sanidine plateau dating indicates an age of 312 ± 1 Ma for the Fire Clay tonstein, which is consistent with previous 40 Ar/ 39 Ar dates for this tonstein. This age is in agreement with a late Westphalian B age in the European Carboniferous chronostratigraphy on the basis of an age of 311 Ma for the Westphalian B/C boundary. A new isopachous map of the Fire Clay ash-fall deposit indicates an area of 37,000 km 2 and a probable source to the present-day southwest. The deposit has a minimum preserved compacted volume of 2.8 km 3 , which corresponds to an original uncompacted volume of about 20 km 3 . This preserved volume indicates an ultraplinian volcanic explosion. Pindell and Dewey (1982) proposed an Andean-type arc in this block during the late Carboniferous, prior to South American-North American plate collision. We hypothesize an associated back-arc caldera system in the Yucatan block to explain the high-silica, potassic rhyolitic ash that gave rise to the Fire Clay tonstein.

Volcanic ash dispersed in the Wyodak-Anderson coal bed, Powder River Basin, Wyoming

Minerals derived from air-fall volcanic ash were found in two zones in the upper Paleocene Wyodak-Anderson coal bed of the Fort Union Formation in the Powder River Basin of Wyoming, and are the first reported evidence of such volcanic material in this thick (> 20 m) coal bed. The volcanic minerals occur in zones that are not visually obvious because they contain little or no clay. These zones were located by geophysical logs of the boreholes and X-ray radiography of the cores. The zones correspond to two of a series of incremental core samples of the coal bed that have anomalous concentrations of Zr, Ba, Nb, Sr, and P 2O 5. Two suites of minerals were found in both of the high-density zones. A primary suite (not authigenic) consists of silt-sized quartz grains, biotite, and minor zircon. A minor suite consists of authigenic minerals, including calcite, pyrite, kaolinite, quartz, anatase, barite, and an alumino-phosphate (crandallite?). The original volcanic ash is inferred to have consisted of silica glass containing phenocrysts of quartz, biotite, zircon, and possibly, associated feldspars, pyroxenes, and amphiboles. The glass, as well as the less stable minerals, probably dissolved relatively quickly and contributed to the minor authigenic mineral suite or was removed from the peat as a result of the prevailing hydrologic conditions present in a raised peat formation. This type of volcanic ash suggests that suggests that volcanic material could have rained on the peat; this fallout may have also had a fertilizing effect on the peat by providing nutrients essential for plant growth thus contributing to the thick accumulations of the Wyodak-Anderson bed. Notwithstanding, the presence of these minerals provides evidence for the contribution by volcanic sources to the mineral content of coal, but not as tonsteins.

A tonstein from the Lower Limestone Group of Arran

Synopsis A 4 cm tonstein, in an oxidised coal seam above the Corrie Limestone on the east coast of Arran, formed by the in situ alteration of air-fall volcanic ash. The tonstein consists predominantly of well-crystallised kaolinite as subparallel grains of altered biotite and more equidimensional vermiform rouleaux formed from pre-existing feldspar or pumice fragments. Accessory minerals include pyrite, apatite, quartz, anatase and illite. SEM photos of some kaolinite grains exhibit cornflake texture described by Keller (1978) as characteristic of smectite in bentonites. Chemical analyses by XRF suggests a secondary quartz content between 10–14% which is supported by powder XRD data. Trace element plots show the precursor ash was basic in composition. Fe 2 O 3 content is near 10%, which is considerably higher than most other tonsteins and probably indicates extensive leaching and oxidation during either Permian or Tertiary time. Pyroclastics of the same age occur on the mainland near the top of the Hosie and Hurlet Limestones and may be the stratigraphic equivalent of the Arran tonstein.

Deep Cenezoic Sediments in Front of the Barbados Ridge Complex, Odp Site 672: Hemipelagites, Turbidites, and Possible Contourites in Western Central Atlantic Ocean

The cenozoic sediments cover of western central Atlantic oceanic floor was cored at about 5 000 m depth, close to the deformation front of the Barbados Accretionary Prism. Good preservation of sedimentary structures and high recovery rate allowed the recognition and study of hemipelagic calcareous or siliceous clays and muds, with or without interbedded airfall volcanic ash beds, and with or without interbedded coarser deposits coming from lateral inputs. The latters are either calcareous fine-grained turbidites coming from local shallower oceanic areas, or terrigenous and calcareous (more hypothetical) contourites (reworking turbidites) related to bottom currents running along the South-America margin. Fluid-escape veinstructures, related to dewatering process - upward, vertical to almost horizontal advection - were identified, especially in relation with the beginning of offscraping stress in early Miocene incipient decollementhorizon. Twenty-four color print photos of splitted cores are presented; the macroscopic features they show are commented and discussed.

Facies Interpretation in Ordovician Carbonates of Southern Ontario Using K-Bentonites: ABSTRACT

The ability to interpret accurately paleogeographic and stratigraphic relationships existing in a sedimentary sequence can be significantly enhanced if one or more chronostratigraphic markers occur within the section under study. K-bentonite beds of the Ordovician Trenton and Black River groups of southern Ontario represent the distal remnants of widespread airfall volcanic ash beds that were deposited in a marine environment on a slowly subsiding carbonate platform. Because deposition occurred essentially instantaneously in a geologic time frame, the beds make ideal chronostratigraphic markers, assuming they can be correlated on a regional basis. Sixty-nine samples representing a number of K-bentonite beds were collected from 19 cores distributed across southern Ontario and analyzed by XRF, ICP/MS, and INAA for a specific set of major, minor, and trace elements. Stepwise discriminant analysis of the analytical results indicates that individual beds can be identified on the basis of chemical composition or fingerprint. Two regional cross sections were constructed utilizing the most consistently identifiable K-bentonite beds. Facies relationships worked out on this basis indicate that Black River carbonates were deposited in water that gradually deepened to the southwest. Conversely, and probably due to tectonic forces associated with Taconic orogenesis, Trenton carbonate deposition occurred in relatively deepermore » water on a ramp dipping into the Appalachian basin.« less

Mineralogy and Petrology of the Cretaceous-Tertiary Boundary Clay Bed and Adjacent Clay-Rich Rocks, Raton Basin, New Mexico and Colorado

ABSTRACT The Cretaceous-Tertiary (K-T) is preserved in a sequence of coal-bearing, fluvial rocks in the lower part of the Raton Formation (Late Cretaceous and Paleocene) at several sites in the east-central part of the Raton Basin. The K-T occurs at the top of a kaolinitic claystone commonly referred to as the boundary clay layer, in an interval of coal and carbonaceous shale. The is defined by the disappearance of certain fossil-pollen taxa. The clay layer also contains shocked quartz grains and abundance anomalies of iridium, chromium, and other elements. Each of these characteristics support the hypothesis of an asteroid impact at the end of the Cretaceous. Small goyazite spheres were also found in the clay bed; however, their origin is ncertain. X-ray powder diffraction (XRD), scanning electron microscopy (SEM), and thin-section petrography have shown that the clay bed exhibits unique mineralogic and petrologic characteristics not possessed by other clay-rich beds in coal-bearing rocks of the Raton Basin. The clay bed is composed primarily of well-crystallized kaolinite but also contains subordinate, randomly interstratified, mixed-layer illite/smectite, both of which appear to have been formed by the alteration of vitreous material in a coal-swamp environment. The bulk mineralogy of the clay bed is similar to that of other kaolinite-rich clay beds (tonsteins) that altered in coal swamps from airfall volcanic ash. The alteration process and diagenetic products of the host material in the cl y bed, however, differed from those of the tonsteins, resulting in a unique petrographic fabric. The clay bed is characterized by an overall imbricate fabric that may reflect an original vitreous material that included shards, bubbles, and spheres. The original fallout material of the clay bed first altered to cabbage-like microspherules, indicating a halloysite or allophane precursor, and then to well-crystallized kaolinite; the well-crystallized kaolinite is the diagenetic stable phase for these rocks at their maximum burial conditions. In comparison, the vitreous components of the tonsteins were altered directly to coarse vermicular and platey kaolinite. Alteration of the original fallout material in the bed was probably rapid, leading to hydrated, disordered, fine-grained clay minerals that formed into curled spherical or tubular shapes. The facto s that produced these unique fabrics in the clay bed existed at all K-T sites in the Raton Basin and were much different from those that formed the coarse platey and vermicular fabrics of ordinary tonsteins. The distinctive character of the clay may be due to the unique nature and chemistry of the fallout material and (or) physical or chemical conditions existing shortly after its deposition.

Leachability of uranium and other elements from freshly erupted volcanic ash

A study of leaching of freshly erupted basaltic and dacitic air-fall ash and bomb fragment samples, unaffected by rain, shows that glass dissolution is the dominant process by which uranium is initially mobilized from air-fall volcanic ash. Si, Li, and V are also preferentially mobilized by glass dissolution. Gaseous transfer followed by fixation of soluble uranium species on volcanic-ash particles is not an important process affecting uranium mobility. Gaseous transfer, however, may be important in forming water-soluble phases, adsorbed to ash surfaces, enriched in the economically and environmentally important elements Zn, Cu, Cd, Pb, B, F, and Ba. Quick removal of these adsorbed elements by the first exposure of freshly erupted ash to rain and surface water may pose short-term hazards to certain forms of aquatic and terrestrial life. Such rapid release of material may also represent the first step in transportation of economically important elements to environments favorable for precipitation into deposits of commercial interest. Ash samples collected from the active Guatemalan volcanoes Fuego and Pacaya (high-Al basalts) and Santiaguito (hornblende-hypersthene dacite); bomb fragments from Augustine volcano (andesite-dacite), Alaska, and Heimaey (basalt), Vestmann Islands, Iceland; and fragments of “rhyolitic” pumice from various historic eruptions were subjected to three successive leaches with a constant water-to-ash weight ratio of 4:1. The volcanic material was successively leached by: (1) distilled-deionized water (pH = 5.0–5.5) at room temperature for 24 h, which removes water-soluble gases and salts adsorbed on ash surfaces during eruption; (2) dilute HCl solution (pH = 3.5–4.0) at room temperature for 24 h, which continues the attack initiated by the water and also attacks acid-soluble sulfides and oxides; (3) a solution 0.05 M in both Na,CO, and NaHCO, (pH = 9.9) at 80°C for one week, which preferentially dissolves volcanic glass. The first two leaches mimic interaction of ash with rain produced in the vicinity of an active eruption. The third leach accelerates the effect of prolonged contact of volcanic ash with alkaline ground water present during ash diagenesis.

Origin of montmorillonite bands in the Middle Jurassic of eastern England

Clay-mineral suites from the Bathonian (Middle Jurassic) of eastern England contain significant amounts of montmorillonite. It is suggested that this mineral is derived from the alteration of air-fall volcanic ash, intimately related to synchronous North Sea vulcanism, and that the varying proportions in which montmorillonite occurs reflect fluctuations in the intensity of volcanism and sedimentation rate in the delta embayment environment operating at that time.