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Pyroclastic material in the form of altered volcanic ash or tephra has been reported and described from one or more stratigraphic units from the Proterozoic to the Tertiary. This altered tephra, variously called bentonite or K-bentonite or tonstein depending on the degree of alteration and chemical composition, is often linked to large explosive volcanic eruptions that have occurred repeatedly in the past. K-bentonite and bentonite layers are the key components of a larger group of altered tephras that are useful for stratigraphic correlation and for interpreting the geodynamic evolution of our planet. Bentonites generally form by diagenetic or hydrothermal alteration under the influence of fluids with high-Mg content and that leach alkali elements. Smectite composition is partly controlled by parent rock chemistry. Studies have shown that K-bentonites often display variations in layer charge and mixed-layer clay ratios and that these correlate with physical properties and diagenetic history. The following is a review of known K-bentonite and related occurrences of altered tephra throughout the timescale from Precambrian to Cenozoic.

Permian-Triassic (P-Tr) altered volcanic ashes (tuffs) are widely distributed within the P-Tr boundary successions in South China. Volcanic altered ashes from terrestrial section-Chahe (CH) and marine section-Shangsi (SS) are selected to further understand the influence of sedimentary environments and volcanic sources on diagenetic alterarion on volcanic tuffs. The zircon 206 Pb/ 238 U ages of the corresponding beds between two sections are almost synchronous. Sedimentary environment of the altered tuffs was characterized by a low pH and did not experience a hydrothermal process. The dominant clay minerals of all the tuff beds are illite-smectite (I-S) minerals, with minor chlorite and kaolinite. I-S minerals of CH (R3) are more ordered than SS (R1), suggesting that CH also shows a higher diagenetic grade and more intensive chemical weathering. Besides, the nature of the volcanism of the tuff beds studied is derived from different magma sources. The clay mineral compositions of tuffs have little relation with the types of source volcanism and the depositional environments. Instead, the degree of the mixed-layer clay minerals and the REE distribution are mainly dependent upon the sedimentary environments. Thus, the mixed-layer clay minerals ratio and their geochemical index can be used as the paleoenvironmental indicator.

Ordovician K-bentonite beds have a long history of investigation all around the world. They have been reported from Gondwana, the Argentine Precordillera, the Yangtze Platform, Laurentia, Baltica, and numerous terrains between Gondwana and Baltica, which now constitute a part of Europe. In recent years several K-bentonite beds have also been discovered in the Upper Ordovician of the Siberian Platform. This discovery is significant not only for their value in local and regional chronostratigraphic correlation but also for global geochronology, paleogeography, paleotectonic and paleoclimatic reconstructions. All in all, eight individual K-bentonite beds have been identified in the Baksian, Dolborian and Burian regional stages, which correspond roughly to the Upper Sandbian-Katian Global Stages. Zircon crystals from the uppermost K-bentonite bed within the Baksian regional stage provide a 206Pb/238U age of 450.58 ± 0.27 Ma. We will present preliminary results of the study of the three lowermost beds from the Baksian Regional Stage and suggest that the Taconic-Enisej (also spelled Yenisei or Yenisey) volcanic arc was continuous along the western margin of Siberia.

Cretaceous strata preserved in Wyoming contain numerous large bentonite deposits formed from the felsic ash of volcanic eruptions, mainly derived from Idaho batholith magmatism. These bentonites preserve a near-continuous 40 m.y. chronology of volcanism and their whole-rock and mineral chemistry has been used to document igneous processes and reconstruct the history of Idaho magmatism as emplacement migrated across the Laurentian margin. Using LA-ICP-MS, we analyzed the U-Pb ages and Hf isotopic compositions of nearly 700 zircon grains from 44 bentonite beds from the Bighorn Basin, Wyoming. Zircon populations contain magmatic autocrysts and antecrysts which can be linked to the main pulses of the Idaho batholith and xenocrysts ranging from approx. 250 Ma to 1.84 Ga from country rocks and basement source terranes. Initial εHf compositions of Phanerozoic zircons are diverse, with compositions ranging from −26 to nearly +12. Based on temporal trends in zircon ages and geochemistry, four distinct periods of plutonic emplacement are recognized during the Mid- to Late Cretaceous that follow plutonic emplacement across the Laurentian suture zone in western Idaho and into western Montana with the onset of Farallon slab shallowing. Our data demonstrate the utility of using zircons in preserved tephra to track the regional-scale evolution of convergent margins related to terrane accretion and the spatial migration of magmatism related to changes in subduction dynamics.

Changes in the global atmospheric budget of platinum reportedly correspond to explosive volcanic eruptions. Using inductively coupled plasma mass spectrometry (ICP-MS) elemental analysis we examined eight widely separated stratified sites to evaluate the geographic extent of three late Holocene high magnitude volcanic events. We found characteristic Pt anomalies across the Western Hemisphere dating to the Laki, Iceland (CE 1783–1784), Kuwae, Vanuatu (CE 1452–1453), and Eldgjá, Iceland (CE 934) explosive volcanic eruptions. Pt anomalies in sediments over a broad geographic area indicate distinctive time-correlative atmospheric deposition rates of platinum-rich volcanic ash. These anomalies provide new chronostratigraphic markers for these late Holocene high magnitude volcanic eruptions, which are especially valuable in the Western Hemisphere in strata with limited chronometric control. Pt anomalies provide an important tracer for the age of these volcanic events and ultimately a new chronostratigraphic marker in archaeological, geological, palynological, and paleontological sediments.

The Hazar-Madeıı Basin sediments were deposited along the southern branch of the Neotethys Ocean margin during Late Maastrichtian-Middle Eocene times. X-ray powder diffraction (XRD), ICP-AES, ICP-MS and scanning electron microscopy (SEM) were performed on samples of the Upper Maastrichtian-Middle Eocene Hazar Group and the Middle Eocene Maden Complex from the Hazar-Maden Basin to investigate the main effects of depositional envi- ronmental parameters in three sections belonging to deeper marine (slope), proximal arc volcanic (Mastarhill and Yukaribag sections) and shallow platform marine (Sebken section) settings. Marine sediments contain clay minerals (smectite, smectite/chlorite, chlorite, illite, interstratified illite/smectite, illite/chlorite, palygorskite), clinoptilolite, quartz, feldspar, calcite, dolomite, opal-CT and hematite. The clays are dominated by iron-rich smectites. La, Zr and Th concentrations are high in the shallow marginal Sebken section where the terrestrial detrital contribution is significant, while Sc and Co are more dominant in the deeper marine (slope) Yukaribag section, which is represented by basic-type volcanism and a higher contribution of hydrothermal phases. In a chondrite-normalized REE diagram, the negative Eu anomaly in samples from Sebken, the section which was deposited in a shallow marine environment, is less significant than that of the other two sections indicating the presence of a high terrestrial contribution in that part of the basin. A decrease in LREE /HREE and La /Yb , La /Sin ratios from Sebken to Mastarhill and the Yukaribag sections indi- cates deepening of the basin and an increasing contribution of volcanism in that direction.

The Dnestr Basin of Podolia, Ukraine, is an epicratonic basin consisting of neritic carbonate and calcareous mudstone facies including a nearly complete Silurian sequence ranging from late Llandovery to late Pridoi; in age. The Silurian section has served as a standard for regional and interregional studies as a consequence of its well-documented macro- and microfaunal assemblages. Approximately 24 mid- to Late Silurian K-bentonites are present in this succession, and their lateral persistence has aided in establishing regional correlations. The K-bentonites range from 1 to 40 cm in thickness and occur in the Bagovitsa (late Wenlock), Malinovtsy (Ludlow) and Skala (Pridoli) Formations. Discrimination diagrams based on immobile trace elements together with rare earth element data suggest the K-bentonites had a volcanic origin in a collision margin setting related to subduction. Thickness and stratigraphic distribution considerations are consistent with a source area in the Rheic Ocean.

At least twenty silicified volcanic ash beds have been identified in the Kuibis and Schwarzrand subgroups of the terminal Proterozoic Nama Group of Namibia. Nineteen of the Nama ash beds are in the Schwarzrand Subgroup in the Witputs subbasin. Two of these are in the siliciclastic-dominated lower part of the subgroup, which consists of the Nudaus Formation and Nasep Member of the Urusis Formation and comprises two depositional sequences. Identification and correlation of these ash beds are very well known based on stratigraphic position. Sixteen ash beds are contained within the carbonate-dominated strata of the Huns, Feldschuhhorn and Spitskop members of the Urusis Formation. These strata comprise four large-scale sequences and eighteen medium-scale sequences. Ash beds have been found in three of the large-scale sequences and seven of the medium-scale sequences. Correlations are proposed for these ash beds that extend over large changes in facies and stratal thickness and across transitions between the seaward margin, depocentre and landward margin of the Huns-Spitskop carbonate shelf. A study of whole rock and in situ phenocryst compositions was conducted to evaluate the feasibility of independently testing sequence stratigraphic correlations by geochemically identifying individual ash beds. Whole rock abundances of Al, Fe, Mg, K and Ti vary inversely with Si, reflecting variations in phenocryst concentration due to air fall and hydrodynamic sorting. These sorting processes did not substantially fractionate whole rock rare earth element abundances (REE), which vary more widely with Si. REE abundances are higher in samples of the Nudaus ash bed than in samples of the Nasep ash bed, independent of position in bed, phenocryst abundance, or grainsize, providing a geochemical means for discriminating between the two beds. Variations in the position of chondrite-normalized whole rock REE plots similarly support suspected correlations of ash beds between widely separated sections of the Spitskop Member. Abundances of Fe, Mg and Mn in apatite plot in distinct clusters for Spitskop ash beds that are known to be different and in clusters that overlap for ash beds suspected of correlating between sections. Abundances of REE in monazites differ for the Nudaus, Nasep and Spitskop ash beds in which these phenocrysts were identified. Multivariate statistical analysis provided a quantitative analysis of the discriminating power of different elements and found that whole rock abundances of Ge, Nb, Cs, Ba and La discriminate among the whole rock compositions of the Nudaus and Nasep ash beds and the Spitskop ash beds that are thought to correlate between sections. Each of the above geochemical signatures, by itself, is not definitive because the differences between beds are comparable to the variability within beds and because some signatures are shared by beds known to be different. Taken together, however, weight-of-evidence arguments based on multiple components and phases can successfully discriminate among Nama ash beds. Results from this study support sequence stratigraphic correlations of Spitskop ash beds that document stratal truncations and gaps in the record related to onlap and erosion.

In 1556, Agricola published the first detailed description of a mudstone posthumously, but it was not until 1747 that one of them (a black shale) was formally defined by William Hooson. A review of the world literature since Hooson's time shows a slow, steady activity in the study of mudstones in the 1800s, a sharp break to more frequent activity in the early 1920s, a decline during World War II, followed by a return to the pre-war level, but then followed in the 1990s by a possible decrease in activity. The trends are similar for both conceptual advances and for new techniques. The break in the 1920s was led by new technologies that expanded observations down to the angstrom level with X-ray diffraction and up to the formation scale with down-hole geophysical logging and seismic reflection. A comparison with a similar compilation for igneous and metamorphic rocks shows the same three phases—early slow phase, intermediate rapid phase punctuated by the Second World War, final slower phase—but in the case of the crystalline rocks, the rapid phase begins much earlier, in the 1880s. We suggest that the common techniques of that time—the pétrographie microscope and wet chemistry—were well suited to the investigation of crystalline rocks, but the breakthrough for mudstones had to wait for new technologies.

Ordovician K-bentonite beds have a long history of investigation all around the world. They have been reported from Gondwana, the Argentine Precordillera, the Yangtze Platform, Laurentia, Baltica, and numerous terrains between Gondwana and Baltica, which now constitute a part of Europe. In recent years several K-bentonite beds have also been discovered in the Upper Ordovician of the Siberian Platform. This discovery is significant not only for their value in local and regional chronostratigraphic correlation but also for global geochronology, paleogeography, paleotectonic and paleoclimatic reconstructions. All in all, eight individual K-bentonite beds have been identified in the Baksian, Dolborian and Burian regional stages, which correspond roughly to the Upper Sandbian–Katian Global Stages. Zircon crystals from the uppermost K-bentonite bed within the Baksian regional stage provide a 206Pb/238U age of 450.58 ± 0.27 Ma. We will present preliminary results of the study of the three lowermost beds from the Baksian Regional Stage and suggest that the Taconic–Enisej (also spelled Yenisei or Yenisey) volcanic arc was continuous along the western margin of Siberia.