Tuff Horizons Research Articles

Petrographic-mineralogical characteristics of newly identified tuff horizons from the Kłodnica Member (Tournaisian, Visean) in the Lublin Basin and the associated volcaniclastic conglomerates and sandstones

Petrographic-mineralogical characteristics of newly identified tuff horizons from the Kłodnica Member (Tournaisian, Visean) in the Lublin Basin and the associated volcaniclastic conglomerates and sandstones

Youngest Toba Tuff deposits in the Gundlakamma River basin, Andhra Pradesh, India and their role in evaluating Late Pleistocene behavioral change in South Asia

AbstractThe eruption of Toba ca. 75 ka was the largest volcanic eruptive event during the Quaternary, and evidence for this eruption is widespread in terrestrial sediment sequences in South Asia as primary and reworked distal ash deposits. Youngest Toba Tuff horizons (YTT) have been widely employed as isochrons to understand and link regional sediment sequences and the evidence for environmental and cultural change in the archaeological records preserved within them. We identify the YTT deposits at Retlapalle, Andhra Pradesh, India, and present the optical ages of the K-feldspar grains recovered from sediments immediately underlying and overlying the tephra horizon. We combine these results with particle size and magnetic susceptibility analyses to establish the depositional conditions of YTT, which indicate that accumulation and reworking ceased by ca. 64 ka. We explore the role of YTT deposits as an isochron for examining the effect of the 75 ka Toba super-eruption, highlighting the need for an independent chronological assessment of YTT before using it as a Late Pleistocene chronological marker in reconstructing South Asian paleo-landscapes and hominin adaptations. Further, our findings support the regional continuity of human occupations within South Asia, spanning the eruption of Toba and the enduring utility of Middle Paleolithic tools.

Lower Cambrian volcanism in the Hawker Group and the Billy Creek Formation, Arrowie Basin, Flinders Ranges, South Australia

Widespread tuffs are known from the lower Cambrian succession of the Arrowie Basin, Flinders Ranges, mainly from the lower parts of both the Mernmerna Formation and the Billy Creek Formation. Up to 2.5 m of potassium-rich (K2O 6.32–9.60 wt%) rhyolite tuffs are known from the Mernmerna Formation, with the ‘Big Green Tuff’ from Bunyeroo Gorge yielding a TIMS age of 515.4 ± 0.2 Ma (i.e. unnamed Cambrian stage 3). The petrography of the Big Green Tuff, with numerous glass shards, suggests an explosive subaqueous pyroclastic deposit or ash-flow tuff. The presence of several layers of identical green tuff indicates that these were produced by a multi-phase volcanic event. There are up to thirty tuff horizons (5–50 cm thick) within the Warragee Member of the Billy Creek Formation. The tuffs within the Billy Creek Formation occur over a wide area, with an east–west extent of 250 km and a north–south extent of up to 80 km. It is suggested that they are in part ash-flow tuffs. A previous TIMS age for the Billy Creek Formation was 511.87 ± 0.14 Ma (i.e. unnamed Cambrian stage 4). The source of the tuffs, both from the Mernmerna Formation and the Billy Creek Formation is unknown. Detailed study of the whole-rock and trace-element geochemistry is required to fully document them.

Insights into U-Th-Pb mobility during diagenesis from laser ablation U-Pb dating of apatite fossils

We studied the feasibility of using laser ablation inductively coupled plasma mass spectrometry (LA–ICPMS) for U-Pb dating of vertebrate and invertebrate apatite fossils. Two specimens of Carcharodon Carcharias from Pleistocene (2.58–0.01 Ma) sediments in Baja California peninsula, Mexico, give Gelasian ages of 2.0 ± 0.4 Ma and 2.0 ± 0.6 Ma which are consistent with the palaeontologic record reported for this species suggesting that the U-Pb system in the shark teeth remained closed until the present. Dentine data from an Oligocene (33.9–23.0 Ma) shark tooth, Otodus angustidens, from El Cien Formation, Mexico, give an anomalously young age of 19 ± 0.2 Ma with a low 207Pb/206Pb value of 0.65. This age is largely controlled by a sub-population of anomalously radiogenic data with low concentrations of U and Pb suggesting remobilization of Pb from high to low U domains. Regressing relatively non-radiogenic data from high U and Pb domains using a fixed average crustal value for initial common 207Pb/206Pb gives an age of 28.6 ± 0.2 Ma, which agrees with U-Pb zircon ages from tuff horizons in the sequence. Enamel from the same shark tooth is much lower in U and Pb and shows evidence for substantial mixing with radiogenic Pb from the dentine. Apatite shells from a brachiopod specimen, Lingula cobourgensis, and conodont fossils contain high U concentrations and significant proportions of radiogenic Pb. Some conodont specimens show high Th/U ratios combined with evidence of significant excess 206Pb. The amount of excess 206Pb requires a minimum Th partition coefficient of about 16,000 and likely much higher suggesting enrichment by ion exchange rather than recrystallization. In some cases conodonts with low Th/U can be precisely dated. A specimen of I. simulator gives an age of 302 ± 4 Ma which is consistent with its well constrained Gzhelian range (303.7–298.9 Ma). We conclude that processing LA–ICPMS data at the single cycle level allows more precise absolute dating of fossils. In some cases, separate regression of totally reset and partially reset domains can resolve ages of early and late diagenesis.

U-Pb geochronology and cyclostratigraphy of the middle Ediacaran upper Jibalah Group, eastern Arabian Shield

The Ediacaran Period (ca. 635–539 Ma) represents a major transition in Earth history, beginning with the end of the Cryogenian snowball Earth glaciation and ending with the appearance and early diversification of bilaterian animals. Whereas a steady flow of new radioisotopic ages from key stratigraphic successions has resulted in much improved age constraints for the earliest and latter Ediacaran, few ages exist for the middle Ediacaran Period. The Jibalah Group, in the eastern part of the Arabian Shield, comprises minimally deformed volcano-sedimentary successions deposited in discrete basins, previously constrained to date between ca. 620 Ma and 560 Ma. The Antaq basin is one of the largest and best exposed of the Jibalah basins and contains the Muraykhah Formation (upper Jibalah Group), which is of particular interest because it contains putative Ediacaran fossils, textured organic surfaces, multiple tuff horizons, and spectacular 1–10 m-scale siltstone-sandstone cycles. Here we report new high-precision U-Pb zircon ages from the Muraykhah Formation anchoring an astrochronological analysis of these cycles. The radioisotopic ages confirm that two strong peaks identified in the stratigraphic height domain reflect accumulation paced by the long (405 kyr) and short (131–91 kyr) eccentricity cycles, the latter of which corresponds to the m-scale cyclicity that is visually apparent in the stratigraphy. The combination of radioisotopic ages and astrochronology allows us to develop a continuous, tuned record spanning from 599.1 ± 0.4 to 590.4 ± 0.6 Ma. This record complements other astrochronological analyses from slightly younger Ediacaran strata spanning the global Shuram negative carbon isotope excursion. The integrated radioisotopic and astronomical time scale ages also provides new constraints on the Ediacaran tectonic evolution of the eastern Arabian Shield.

40Ar/39Ar geochronology of Burdigalian paleobotanical localities in the central Paratethys (south Slovakia)

The Lipovany and Mučín paleobotanical localities contain important floral associations within the tuff horizons, which were used for determination of subtropical to tropical climatic conditions during the Early Miocene. Based on the combination of results from plagioclase and biotite 40Ar/39Ar dating, the age of the tuff deposition is around 17.3Ma. For the Lipovany locality, single-grain 40Ar/39Ar convergent ages of 17.49±0.54Ma and 17.28±0.06Ma, for plagioclase and biotite were obtained, respectively. The Mučín locality only provide an imprecise convergent age of 16.5±1.4Ma due to the small size of the analyzed plagioclase crystals. The results thus allowed to include the fossil subtropical flora of the studied localities in the late Ottnangian regional stage (upper part of the Burdigalian). Additionally, these age data indicate that deposition of the overlaying Salgótarján Formation starts much later than originally thought (during Ottnangian-Karpatian boundary).

Late Devonian magmatism and clastic deposition in the upper Earn Group (central Yukon, Canada) mark the transition from passive to active margin along western Laurentia

The Earn Group of central Yukon records the transition from a passive to an active margin along western Laurentia in the Late Devonian. Fine-grained clastic rocks and chert of the lower Earn Group contain late Early to Middle Devonian fossils and were deposited in an offshelf environment. The upper Earn Group comprises a mixture of sandstone and conglomerate, fine-grained siliciclastic rocks, and widespread crystal lithic tuff. Zircons from this succession are precisely dated using chemical abrasion isotope dilution thermal ionization mass spectrometry (CA–ID–TIMS) methods on igneous (ca. 363 Ma) and detrital (ca. 378–363 Ma) grains and the ages were confirmed by Frasnian to Famennian fossils. Abrupt, along-strike facies changes within the upper Earn Group of the Glenlyon–Tay River area occur across mapped faults that are inferred to have originated as syndepositional extension faults in the Late Devonian. Occurrences of ca. 363 Ma tuff horizons within all facies of the upper Earn Group provide a temporal correlation across the area. Diorite plutons intrude lower Paleozoic rocks in the area and have U–Pb zircon crystallization dates of ca. 364 Ma. The diorite has calc-alkaline composition consistent with arc magmatism or crustal contamination. The Late Devonian magmatism in the Earn Group is coincident with onset of arc magmatism in the allochthonous Yukon–Tanana terrane, and extension related to rifting and opening of the Slide Mountain ocean in a back-arc setting. Magmatic rocks in the Earn Group of central Yukon thus represent part of a remnant continental arc and back-arc stranded behind the Slide Mountain ocean in the Mississippian.

Sedimentology of the continental end‐Permian extinction event in the Sydney Basin, eastern Australia

AbstractUpper Permian to Lower Triassic coastal plain successions of the Sydney Basin in eastern Australia have been investigated in outcrop and continuous drillcores. The purpose of the investigation is to provide an assessment of palaeoenvironmental change at high southern palaeolatitudes in a continental margin context for the late Permian (Lopingian), across the end‐Permian Extinction interval, and into the Early Triassic. These basins were affected by explosive volcanic eruptions during the late Permian and, to a much lesser extent, during the Early Triassic, allowing high‐resolution age determination on the numerous tuff horizons. Palaeobotanical and radiogenic isotope data indicate that the end‐Permian Extinction occurs at the top of the uppermost coal bed, and the Permo‐Triassic boundary either within an immediately overlying mudrock succession or within a succeeding channel sandstone body, depending on locality due to lateral variation. Late Permian depositional environments were initially (during the Wuchiapingian) shallow marine and deltaic, but coastal plain fluvial environments with extensive coal‐forming mires became progressively established during the early late Permian, reflected in numerous preserved coal seams. The fluvial style of coastal plain channel deposits varies geographically. However, apart from the loss of peat‐forming mires, no significant long‐term change in depositional style (grain size, sediment‐body architecture, or sediment dispersal direction) was noted across the end‐Permian Extinction (pinpointed by turnover of the palaeoflora). There is no evidence for immediate aridification across the boundary despite a loss of coal from these successions. Rather, the end‐Permian Extinction marks the base of a long‐term, progressive trend towards better‐drained alluvial conditions into the Early Triassic. Indeed, the floral turnover was immediately followed by a flooding event in basinal depocentres, following which fluvial systems similar to those active prior to the end‐Permian Extinction were re‐established. The age of the floral extinction is constrained to 252.54 ± 0.08 to 252.10 ± 0.06 Ma by a suite of new Chemical Abrasion Isotope Dilution Thermal Ionization Mass Spectrometry U‐Pb ages on zircon grains. Another new age indicates that the return to fluvial sedimentation similar to that before the end‐Permian Extinction occurred in the basal Triassic (prior to 251.51 ± 0.14 Ma). The character of the surface separating coal‐bearing pre‐end‐Permian Extinction from coal‐barren post‐end‐Permian Extinction strata varies across the basins. In basin‐central locations, the contact varies from disconformable, where a fluvial channel body has cut down to the level of the top coal, to conformable where the top coal is overlain by mudrocks and interbedded sandstone–siltstone facies. In basin‐marginal locations, however, the contact is a pronounced erosional disconformity with coarse‐grained alluvial facies overlying older Permian rocks. There is no evidence that the contact is everywhere a disconformity or unconformity.

40AR/39AR GEOCHRONOLOGY USING HIGH SENSITIVITY MASS SPECTROMETRY: EXAMPLES FROM MIDDLE MIOCENE HORIZONS OF THE CENTRAL PARATETHYS

40Ar/39Ar radio-isotopic dating of volcanic tuffs intercalated in sediments can provide high accuracy age control on the deposition of sedimentary rocks. State-of-the-art mass spectrometers such as the ARGUS VI+ are able to acquire highly precise ages for relatively small single grains (~90-250 μm for Miocene samples). Single grain measurement can provide insight into the sometimes complex age distributions within volcanic tuffs. The results show that 40Ar/39Ar ages based on multiple grain fusions will not necessarily reflect eruption ages, which can lead to (slight) overestimation of the depositional age. The paper compares multiple and single grain data from different Miocene tuffs in the Central Paratethys, which plays an important role in the establishment of a geological time frame for this area. The examples come from three middle Miocene tuff horizons that span from the Badenian transgression to the Badenian-Sarmatian Extinction Event. The new ages obtained from the Quellgraben section in the Styrian Basin (14.31±0.27 Ma and 14.03±0.04 Ma) are much younger than the previous dating and together with the new data from the Bernhardsthal-4 well, Vienna Basin (15.12±0.19 Ma) indicate, that the Badenian (Langhian) marine flooding did not reach this area before 15.2 Ma. The new weighted mean age of 12.56±0.10 Ma from the Kamenica nad Hronom section in the Danube Basin dates the transition from marine to terrestrial setting, which is possibly connected with a sea level lowstand at the beginning of the Sarmatian.

Structure and Geochemical Features of the Volcanic Rocks in the Tunguska Syneclise (Siberian Trap Province)

To understand the origin of the Siberian trap province, it is necessary to know its internal structure in detail. Only two areas were described, the Noril’sk and Maimecha-Kotuy, while the most important part of the province, the Tunguska syneclise, is still poorly characterized. The main goal of our study is an investigation of the structure and geochemistry of volcanic rocks located in the central part of the province. We have studied basalts and tuffs and compared them with the rocks of the Noril’sk area. All studied rocks belong to three formations: Korvunchansky, Nidymsky and Kochechumsky. The whole thickness of this cross-section is around 1,000 m. Our geochemical data demonstrate that the lower part of the cross-section comprises basalts with TiO2=1 wt% while the upper part consists of the basalts with the average TiO2 content 1.4-1.5wt%. Based on this data, these rocks can be correlated with Morongovsky and Mokulaevsky formations located in the Noril’sk area. Despite many similarities between the volcanic rocks of these two areas, the basalts and tuffs of the Tunguska syneclise have their specific features, i.e. many tuff horizons, small thickness of flows and their poikiloophitic structure that reflect the rock formation near the boundary of the Tunguska syneclise.

THE OLIGOCENE–MIOCENE MENILITE FORMATION IN THE UKRAINIAN CARPATHIANS: A WORLD‐CLASS SOURCE ROCK

This study investigates the hydrocarbon potential of Oligocene–Miocene shales in the Menilite Formation, the main source rock in the Ukrainian Carpathians. The study is based on the analysis of 233 samples collected from outcrops along the Chechva River in western Ukraine in order to analyse bulk parameters (TOC, Rock‐Eval), biomarkers and maceral composition.In Ukraine, the Menilite Formation is conventionally divided into Lower (Lower Oligocene), Middle (Upper Oligocene) and Upper (Lower Miocene) Members. The Early Oligocene and Early Miocene ages of the lower and upper members are confirmed by new nannoplankton data. The Lower Menilite Member is approximately 330 m thick in the study area and contains numerous chert beds and turbidite sandstones in its lower part together with organic‐rich black shales. The shales have a high content of silica which was probably derived from siliceous micro‐organisms. The TOC content of the shales frequently exceeds 20 wt.% and averages 9.76 wt.%. HI values range between 600 and 300 mgHC/gTOC (max. 800 mgHC/gTOC). The Middle Member contains thin black shale intervals but was not studied in detail. The Upper Member is about 1300 m thick in the study area and is composed mainly of organic‐rich shales. Chert layers are present near the base of the Member, and a prominent tuff horizon in the upper part represents a volcanic phase during shale deposition. The member grades into overlying molasse sediments. The average TOC content of the Upper Menilite succession is 5.17 wt.% but exceeds 20 wt.% near its base.Low Tmax and vitrinite reflectance measurements for the Lower (419°C and 0.24–0.34 %Rr, respectively) and Upper (425°C and 0.26–0.32 %Rr, respectively) Menilite Member successions indicate thermal immaturity. Biomarker and maceral data suggest a dominantly marine (Type II) organic matter input mixed with varying amounts of land‐plant derived material, and indicate varying redox and salinity conditions during deposition.Determination of the Source Potential Index (SPI) shows that the Menilite Formation in the study area has the potential to generate up to 74.5 tons of hydrocarbons per m2. The Chechva River outcrops therefore appear to have a significantly higher generation potential than other source rocks in the Paratethys realm. These very high SPI values for the Menilite Formation may explain why a relatively small area in Ukraine hosts about 70% of the known hydrocarbon reserves in the northern and eastern Carpathian fold‐thrust belt.

Petrogenesis of a Large-Scale Miocene Zeolite Tuff in the Eastern Slovak Republic: The Nižný Hrabovec Open-Pit Clinoptilolite Mine

AbstractOver 170,000 metric tonnes of high-grade clinoptilolite tuff were extracted from the open-pit mine in Nižný Hrabovec (eastern Slovak Republic) in 2018, making it one of the world’s major natural clinoptilolite producers. The mine is hosted in a Miocene volcanogenic-sedimentary deposit in the East Slovak basin, with estimated 150 million tonnes of clinoptilolite tuff—the economically most important reserve in the European Union. The 100-m-thick tuff horizon is under- and overlain by SW-dipping shallow-marine Badenian sediments (16.30–15.03 Ma ≅ Langhian stage). The tuff is rhyolitic (74.3–77.6 wt % SiO2, 4.09–5.49 wt % Na2O + K2O) with a generally high-K calc-alkaline affinity (3.07–4.28 wt % K2O), comparable to volcanic rocks in the central segment of the Carpathian-Pannonian region of similar age. Although some parts of the tuff underwent slight geochemical changes during formation, this did not significantly change the whole-rock composition of the strikingly homogeneous raw material present in the mine. Rare earth elements (ΣREE = 117–141 ppm) show a uniform pattern, with enriched light REEs and negative Eu anomalies of 0.42 to 0.6 (when normalized to chondrite), comparable to the upper continental crust composition. Mean 87Sr/86Sr = 0.70880 and 143Nd/144Nd = 0.512463 indicate an enriched magmatic source, with a dominant crustal contribution. X-ray diffraction data and electron probe microanalyses show that clinoptilolite-Ca is the only zeolite phase present in the deposit. High-resolution electron probe microanalytical imaging and measurement techniques reveal that clinoptilolite is present as (1) coarse patches that either form pseudomorphs of volcanic glass shards or grew in voids and (2) ultrafine material making up the matrix of the tuff. Both textural types have Si/Al >4, and their evolution is connected to dissolution-(transport)-precipitation reactions from acidic volcanic ash under alkaline fluid influence and slightly elevated pressure-temperature (44°–84°C) conditions. Authigenic cristobalite, detected as ultrafine-grained matrix crystallites, formed during the zeolitization process from excess Si. The large-scale, remarkably homogeneous and monomineralic natural zeolite deposit formed from a well-equilibrated magmatic source, with no syn- to postsedimentary reworking and with essentially isochemical conditions during the zeolitization process.

Structure and mineralogical composition of the tuff horizon of Petrikov potash deposit

Integrated data of the structure and mineralogical composition of the marking tuff horizon in the Upper-Devonian bed of the North-Shestovich synclinal zone are presented in this paper. Dependence of the tuff bed structure on the volcanic activity stage is revealed. Find out that the prevailing mineral of tuff is the structure-ordered illite 1M. Marked tuff horizon is situated inside the Upper-Devonian salt bed within Petrikov synclinal zone and comes up into the overlaying clay-marl formation of the surrounding geological beds.

Magmatic episodes in the Holy Cross Mountains, Poland – a new contribution from multi-age zircon populations

This contribution reports on new U–Pb zircon age data from magmatic rocks from the Holy Cross Mountains (HCM) of Poland. The analyzed samples were taken from lamprophyre and diabase veins of Podkranów and Janowice-2 as well as from tuff horizon of Kielce Beds (Ludlow). Internal morphologies have been investigated by SEM-BSE and cathodoluminescence images and they have been used as a guide for the selection of genetically various type of grains, e.g. potential auto-, ante- and xenocrysts, that were analyzed by ion microprobe. The U–Pb age of the magmatic events at 414.2 ±6.6 Ma (Kielce tuff), 322 ±12 Ma (Podkranów, lamprophyre), and 300±10 Ma (Janowice-2, diabase) confirmed the time frame of known magmatic activity reported within the HCM, as determined by 40Ar/39Ar geochronology in previous studies. The zircon investigation revealed also multiple populations with record of an earlier pulse of magma system (antecrysts), as well as abundant xenocrysts.

Provenance and structure of the Yancannia Formation, southern Thomson Orogen: implications for the tectono-stratigraphic evolution of the Cambro-Ordovician western Tasmanides

The upper Cambrian Yancannia Formation is a small and isolated basement exposure situated in the southern Thomson Orogen, northwestern New South Wales. Understanding the geology of the Yancannia Formation is important, as it offers a rare glimpse of the composition and structure of the mostly covered basement rocks of the southern Thomson Orogen. It consists of deformed fine-grained, lithic-rich, turbiditic metasediments, suggesting deposition in a proximal, low-energy deep-marine environment. A 497 ± 13 Ma U–Pb detrital zircon date provides its maximum depositional age, the same as previously published for a tuff horizon in a correlative unit. Analysis of sedimentological, geochronological and geophysical data confirms the Yancannia Formation belongs to the Warratta Group. The Warratta Group exhibits many similarities to the Teltawongee Group in the adjacent Delamerian Orogen, including similar provenance, sedimentology and deep-water turbiditic depositional environment. Additionally, there is no sedimentological evidence for deposition of the Warratta Group following the ca 500 Ma Delamerian Orogeny, which suggests that the Warratta Group is syn-Delamerian. However, no geochronological or structural evidence for Delamerian orogenesis was observed in the Warratta Group, suggesting that the group was either unaffected by Delamerian orogenesis, or that no conclusive record remains. The provenance signature of the Warratta Group also bears strong similarities with the upper Cambrian Stawell Zone Saint Arnaud Group in the western Lachlan Orogen. Units east of Yancannia have similar provenance signatures to the Lower Ordovician Girilambone Group of the Lachlan Orogen, suggesting equivalents exist in the southern Thomson Orogen. These are likely to be the Thomson beds, deposited in a deep-marine setting outboard of the Delamerian continental margin. Structural analysis from a ∼10 km, semi-continuous, across-strike section indicates a major, kilometre-scale, upright, shallow northwest-trending, doubly plunging anticline dominates the Yancannia region. This D1 structure was associated with tight-to-isoclinal folding, penetrative cleavage and abundant quartz veining of probable Benambran age. Later dextral transpressional deformation (D2) produced a sporadic, weak cleavage and dextral faulting, possibly of Bindian age. Major south-directed thrusting (D3) on the adjacent Olepoloko Fault occurred in the early Carboniferous and appears to pre-date a later deformation event (D4), which was associated with kink folding.

Contrasting origin of two A-type rhyolite series from the Early Permian Nomgon bimodal volcanic association (Southern Mongolia)

A-type rhyolites of contrasting compositions and eruption characters were revealed among two volcanic series of the Early Permian bimodal association in the Nomgon graben. Rhyolites of the lower volcanic series formed extrusions, lava domes, and tuff horizons. They had low FeOt, Zr, Hf, Nb, Ta, Y, and REE concentrations and also a moderately depleted Nd isotope composition (eNd(Т) = 6.7–7.1). Their formation was related to anatexis of the juvenile continental crust, triggered by the thermal effect of mafic magmas. Rhyolites of the upper volcanic series formed extensive lava flows and dikes. Their composition was characterized by high FeOt, Zr, Hf, Nb, Ta, Y, and REE concentrations, and also depleted Nd isotope characteristics (eNd(Т) = 7.7–9.0). These rhyolite melts formed under long-term crystallizational differentiation of basaltoids in the intracrustal magmatic chambers, with limited participation of crustal contamination. The source of magmas for the upper volcanic series was the sublithospheric mantle.

The age of volcanic tuffs from the Upper Freshwater Molasse (North Alpine Foreland Basin) and their possible use for tephrostratigraphic correlations across Europe for the Middle Miocene

The Middle Miocene Upper Freshwater Molasse sediments represent the last cycle of clastic sedimentation during the evolution of the North Alpine Foreland Basin. They are characterized by small-scale lateral and temporal facies changes that make intra-basin stratigraphic correlations at regional scale difficult. This study provides new U–Pb zircon ages as well as revised 40Ar/39Ar data of volcanic ash horizons in the Upper Freshwater Molasse sediments from southern Germany and Switzerland. In a first and preliminary attempt, we propose their possible correlation to other European tephra deposits. The U–Pb zircon data of one Swiss (Bischofszell) and seven southern German (Zahling, Hachelstuhl, Laimering, Unterneul, Krumbad, Ponholz) tuff horizons indicate eruption ages between roughly 13.0 and 15.5 Ma. The stratigraphic position of the Unterneul and Laimering tuffs, bracketing the ejecta of the Ries impact (Brockhorizon), suggests that the Ries impact occurred between 14.93 and 15.00 Ma, thus assigning the event to the reversed chron C5Bn1r (15.032–14.870 Ma) which is in accordance with paleomagnetic evidence. We combine our data with published ages of tuff horizons from Italy, Switzerland, Bavaria, Styria, Hungary, and Romania to derive a preliminary tephrochronological scheme for the Middle Miocene in Central Europe in the age window from 13.2 to 15.5 Ma. The scheme is based on the current state of knowledge that the Carpathian–Pannonian volcanic field was the only area in the region producing explosive calc-alkaline felsic volcanism. This preliminary scheme will require verification by more high-quality ages complemented by isotopic, geochemical and paleomagnetic data.

A new Paleoproterozoic tectonic history of the eastern Capricorn Orogen, Western Australia, revealed by U–Pb zircon dating of micro-tuffs

The Earaheedy Basin is one of several Paleoproterozoic siliciclastic basins in the eastern part of the Capricorn Orogen in Western Australia. In the absence of an adequate chronostratigraphic framework, multiple conflicting models have been proposed for the tectonic history of the basin. We present in situ SHRIMP U–Pb zircon dates of 1971±11Ma, 1954±5Ma and 1949±10Ma from three thin tuff horizons (“micro-tuffs”) in the Yelma Formation at the base of the basin succession. These micro-tuffs are only slightly younger than the 1990±6Ma Imbin porphyry, which has been interpreted as a basement inlier. Our new dates, combined with field mapping, suggest that the Imbin porphyry is a volcanic–subvolcanic complex within the basin. Micro-tuffs in the Yelma Formation are more than 50m.y. older than c. 1890Ma micro-tuffs within the overlying Frere Formation. Deposition of the Yelma Formation was coeval with continent–continent collision farther west in the Orogen marked by the 2005–1950Ma Glenburgh Orogeny, and the formation is possibly correlative with peripheral foreland deposits of the Padbury Group. The Frere Formation was deposited in shallow marine conditions within an intracratonic basin about 60m.y. after the Glenburgh Orogeny. The Frere Formation is unconformably or disconformably overlain by the Miningarra Group, the youngest unit of which is the Mulgarra Sandstone. This sandstone was deposited after c. 1865Ma, and derived either from exposed Frere Formation or from a now-buried source within the Capricorn Orogen that was uplifted by inception of the 1820–1770Ma Capricorn Orogeny. Therefore, the Earaheedy Basin comprises at least three distinct, unconformity-bound successions deposited in contrasting tectonic settings.

Permian scorpions from the Petrified Forest of Chemnitz, Germany.

BackgroundPaleozoic scorpions (Arachnida: Scorpiones) have been widely documented from the Carboniferous Period; which hosts a remarkable assemblage of more than sixty species including both putative stem- and crown-group fossils. By contrast the succeeding Permian Period is almost completely devoid of records, which are currently restricted to a trace fossil from the early Permian of New Mexico, USA and some limb fragments from the late Permian of the Vologda Region, Russia.Results?Opsieobuthus tungeri sp. nov. from the Petrified Forest of Chemnitz, Germany represents the first complete body fossils of scorpions from the Permian. Explosive volcanism preserved these remarkable specimens in situ as part of the palaeosol horizon and bedrock of the Petrified Forest, immediately beneath the Zeisigwald tuff horizon. This dates to the early Permian (Sakmarian) or ca. 291 Ma. Intriguingly, the specimens were obtained from a palaeosol horizon with a compacted network of different-sized woody roots and thus have been preserved in situ in their likely life position, even within their original burrows. Differences in the structure of the comb-like pectines in the two fossils offer evidence for sexual dimorphism, and permit further inferences about the ecology and perhaps even the reproductive biology of these animals.ConclusionsAs putative members of a Coal Measures genus, these fossils suggest that at least some Carboniferous scorpion lineages extended their range further into the Permian. This contributes towards a picture of scorpion evolution in which both basal and derived (orthostern) forms coexisted for quite some time; probably from the end of the Carboniferous through to at least the mid Triassic.

Rangal Supermodel 2015

More than 6,000 boreholes were compiled to develop a consistent regional scale stratigraphic framework for the Permian Rangal, Baralaba and Bandana coal measures (CMs) within the Bowen Basin. Coal beds and tuff horizons were used as stratigraphic markers, supported by chemostratigraphy and age dating. Results corroborate the general subdivisions of these different coal measures relative to basin location, but increase resolution on migrating depocentres in response to foreland loading and subsidence on coal thickness and splitting patterns. In the north, the Rangal CMs comprise two main seams, correlated as Leichhardt and Vermont. The Yarrabee Tuff is consistently present and splits the Vermont seam. The main Leichhardt seam exhibits relatively simple offset stacking relationships with the underlying Vermont and overlying Phillips seams. In the southwest, the Bandana CMs comprise two to three significant seams—the Aries-Castor, Pollux (Leichhardt equivalent) and Orion—along with the Pisces containing the Yarrabee Tuff. Seams exhibit complex Z splitting and vertical interburden stacking. Locally super-thick seams (crabs) form from convergence of thinner split seams in areas of relative stability over basement highs. In the Taroom Trough, the Baralaba CMs show the greatest response to loading, as seams thin and split along the eastern margin. The variability in the splitting patterns, coupled with the coal measures total thickness, corroborate the extension of the final basin depocentre northward, which was not preserved.