Tuff Beds Research Articles

Fallout tuffs in the Eocene Duchesne River Formation, northeastern Utah—ages, compositions, and likely source

Thin fallout tuffs are common in the terrestrial deposits of the Eocene Duchesne River Formation on the flanks of the Uinta Mountains of eastern Utah. Their ages and compositions provide new insight into the tectonic events and magmatic history of the western Cordillera and provide important constraints on the Cenozoic land mammal chronology. Whole-rock compositions of the volcanic ash show that they underwent post-emplacement argillic alteration, typical of a wetland/floodplain depositional setting. However, immobile element ratios and abundances, such as Zr/Ti, La/Nb, and Y are typical of rhyolites formed in a subduction-related setting. Glass shards preserved in one sample all had SiO2 values >75%, typical of high-silica rhyolite. Preserved phenocrysts in the ash beds include quartz, sanidine, plagioclase, and biotite with variable amounts of accessory zircon, apatite, titanite, and allanite. Biotite compositions have Fe/(Fe+Mg) ratios typical of calc-alkaline igneous rocks and clusters of chemical compositions suggest a genetic relationship to three or four separate eruptions. Sanidine compositions from five samples range from Or73 and Or79. Only one sample had preserved plagioclase with compositions ranging between An22 – An49. Allanite from the ash beds has lower total rare earth elements (REE) concentrations than allanite from other well-studied rhyolites. Titanite in one sample has lower concentrations of REE, Fe, and Al than expected of rhyolites and is probably detrital.Plagioclase and sanidine from two different tuff beds near the middle of the Duchesne River Formation yielded analytically indistinguishable 40Ar/39Ar ages of 39.47 ± 0.16 Ma and 39.36 ± 0.15 Ma, respectively. These dates, along with the compositional data seem to limit the eruptive source for these fallout tuffs to the northeast Nevada volcanic field, one of the few volcanically active regions of western North America at the time. These new radiometric ages, along with stratigraphic relations and previously published ages for tuffs in the Bishop Conglomerate (which unconformably overlies the Duchesne River Formation), constrain the timing of late Laramide uplift in the region from 39 to about 37 Ma and post-Laramide epeirogenic uplift from 34 Ma to 30 Ma. Finally, the ages also provide additional evidence that the Duchesnean North American Land Mammal Age ended in the Eocene, which was originally named and defined from the Duchesne River Formation.

Geochronology and geochemistry of the fossil-flora-bearing Wuda Tuff in North China Craton and its tectonic implications

The Wuda Tuff Flora in the North China Craton (NCC) is a “vegetational Pompeii” and one of the most completely preserved marsh ecosystems of the late Paleozoic Northern Hemisphere. However, the precise age, geochemistry, and petrogenesis of the thick tuff containing this peat-forming flora have not been established. We conducted a comprehensive petrographic, mineralogical, whole-rock geochemical, and zircon UPb and HfO isotopic investigation of five samples from a vertical section through the tuff bed. Our results confirm that the material that buried the flora is a volcanic tuff bed that has undergone intensive post-deposition alteration. Whole-rock Al2O3/TiO2 ratios (65.8–103.5) and negative Eu/Eu* values (0.27–0.38) further suggest a felsic volcanic origin. Secondary ion mass spectrometry zircon UPb dating of the altered tuff bed yielded a weighted mean 206Pb/238U age of 295.9 ± 1.4 Ma (1σ, MSWD = 0.94, n = 39), constraining the age of the Wuda Tuff Flora to the earliest Permian. Depletions of Nb and Ta in both the whole-rock and zircon compositions indicate an arc-related setting for the volcanism. The positive εHf(t) values (0.0 to +5.0) and δ18O values (5.05‰–6.17‰) of zircon grains from the Wuda Tuff suggest a juvenile crust origin of the volcanic magma. Geochemical comparisons with contemporaneous igneous rocks suggest that the volcanic eruption that led to the formation of the Wuda Tuff and associated vegetational Pompeii was most probably caused by arc volcanism along the western margin of the NCC related to subduction of Paleo-Asian Ocean oceanic crust, implying that the Alxa Terrane had not yet amalgamated with the NCC during the early Permian.

Eocene-Oligocene succession at Kıyıköy (Midye) on the Black Sea coast in Thrace

A belt of Upper Eocene-Lower Oligocene marine sedimentary rocks extends from Kiyikoy on the Black Sea coast to Pinarhisar in the Thrace Basin, suggesting a marine connection between the Black Sea and the Thrace Basin during this period. The Cenozoic succession of this marine corridor was studied in the vicinity of Kiyikoy along two measured stratigraphic sections. The sequence lies unconformably over metamorphic basement rocks and consists of ~75 m of bioclastic limestone and sandstone of the Sogucak Formation, overlain by ~40 m of limestone, marl, mudstone, sandstone, and acidic tuff, which are assigned to the newly defined Servez Formation. Larger benthic foraminifera indicate that the lower part of the succession is Late Eocene in age, and nannoplankton from the upper part of the succession suggest an Early Oligocene age; these age determinations are also supported by the Sr-isotope data. A U-Pb age from zircons from a tuff bed is 33.9 ± 0.4 Ma, which falls on the Eocene-Oligocene boundary. The Kiyikoy Upper Eocene- Lower Oligocene sequence was deposited in shallow marine conditions below 50-m water depth. The depositional setting, as well as the relatively reduced thickness of the sequence, shows that any marine connection between the Black Sea and the Thrace Basin along the Kiyikoy-Pinarhisar corridor was not significant. The Late Eocene-Early Oligocene marine connection between the Black Sea and the Thrace Basin occurred along the Catalca gap southeast of Kiyikoy. In the Catalca gap the Upper Eocene-Lower Oligocene sequence is much thicker (350 m) and was deposited at much greater water depth.

The “Lower Kaimur Porcellanite” (Vindhyan Supergroup) is of Sedimentary Origin and not Tuff

Abstract The ‘Lower Kaimur Porcellanite’ from the Proterozoic Vindhyan Supergroup (~1700-900? Ma) is not only a chronostratigraphic marker but also an indicator of the tectonic setting of the basin. A few other silicified shaly units (porcellanites) from the upper strata have been thought to be tuff. New petrographic (optical microscopic; SEM-BSE), chemical, and U-Pb zircon geochronological studies of the lowermost of these suspected tuff units, however, do not support an igneous origin for these beds. The rocks do not contain phenocrysts or glass shards, but contain remains of mineralized microbial spheres, mudclasts, and other detrital grains that include one datable zircon grain (~1715 Ma). Their chemical compositions are not diagnostic of tuff. Despite this result, investigations of other porcellanites from Upper Vindhyan strata is recommended, because they have the potential of identifying crucially important tuff beds.

Zircon U-Pb age of a lapilli tuff bed in the lower Miocene Yoka Formation, northern Hyogo, SW Japan

Zircon U-Pb age of a lapilli tuff bed in the lower Miocene Yoka Formation, northern Hyogo, SW Japan

A pan-latitudinal Rodinia in the Tonian true polar wander frame

A new paleomagnetic and geochronological study was conducted on the Tonian Chéngjiāng Formation from Yúnnán Province, South China, to examine the recent reported true polar wander event (TPW) during 825–750 Ma and clarify the position of South China in the Rodinia supercontinent. Detailed thermal demagnetization revealed two remanent magnetic components. A low-temperature component separated below 300°C is interpreted as a recent viscous remanence. Additionally, a high-temperature component (H1) with unblocking temperature up to 690°C is revealed. Passing both the fold test and reversal test, and recording at least two and half magnetochrons, H1 is suggested to be a primary remanence. The paleomagnetic pole (CJH1, 33.4°N, 56.1°E, dp/dm=7.3°/8.9°) calculated from component H1 has no similarities to the Phanerozoic apparent polar wander path of South China. SHRIMP II U-Pb zircon data from a tuff bed near the paleomagnetic sampling sites suggest that the H1 pole has an age of ca. 800 Ma. Combining the reported Tonian paleomagnetic results from South China and Svalbard, we suggest a ∼63° TPW event during 825–790 Ma. Under this TPW paleomagnetic frame and using the reliable Tonian paleomagnetic results from other continents, we reconstruct the main part of Rodinia. This new configuration of Rodinia reconstructs South China and India at its northern periphery, and within the polar or high latitude zone during ca. 900–750 Ma. The polar location of those cratons, together with the mid-high latitudinal distribution of Australia, Congo and Baltica, suggests that Rodinia was a pan-latitudinal rather than an equatorially distributed supercontinent. The reconstruction also suggests that the breakup of the Rodinia should have been after 750 Ma but before 720 Ma. Despite similarities of their assembly and configuration, Rodinia and Pangea affected the Earth system differently.

Age of the N7/N8 (M4/M5) planktonic foraminifera zone boundary: constraints from the zircon geochronology and magnetostratigraphy of early Miocene sediments in Ichishi, Japan

The age assigned to the Miocene N7/N8 (M4/M5) planktonic foraminifera zone boundary is markedly different from the astronomically tuned Neogene time scales published in 2004 (ATNTS2004) and 2012 (ATNTS2012). This difference is primarily because of the lack of reliable assignment of the zone boundary to magnetostratigraphy, which needs to be resolved for the better refinement of the geologic time scale. Our previous biostratigraphic results from an onland sedimentary sequence of the upper Ichishi Group in central Japan allow location of the stratigraphic position of the lowest occurrence datum (LOD) of Praeorbulina sicana, a key planktonic foraminifera species for defining the N7/N8 (M4/M5) zone boundary. In this study, we performed laser ablation-multiple collector-inductively coupled plasma mass spectrometry U–Pb dating on zircons from two felsic tuff beds in the Ichishi sequence. The age of the zone boundary is approximated by a U–Pb age of 17.03±0.11 Ma for a tuff bed ∼2m above the boundary. We also conducted a magnetostratigraphic analysis in which site-mean characteristic remanent magnetization directions were determined for 19 sites and showed a reverse–normal–reverse (R–N–R) stratigraphic change in polarity. The N7/N8 (M4/M5) zone boundary is within the upper reverse polarity zone. The R–N–R sequence can be correlated with a stratigraphic portion corresponding to chrons C5Dr–C5Dn–C5Cr; hence, the age of the zone boundary is within Chron C5Cr. Our results are thus compatible with the age (16.97 Ma) assigned to the boundary in ATNTS2004 rather than ATNTS2012 and should be considered when the Neogene time scale is updated. A normal polarity subzone found in the upper reverse polarity zone can also be dated at ∼17.0 Ma and is probably correlated with a short normal polarity episode in Chron C5Cr. In terms of tectonic rotation, the site-mean magnetization directions determined in this study suggest an ∼35° clockwise rotation in Ichishi with respect to the North China Block in the Asian continent since ∼17.2 Ma (the upper boundary age of Chron C5Dn). Clockwise rotation of Southwest Japan, which occurred in association with backarc opening of the Japan Sea, was probably in progress during deposition of the upper Ichishi Group sediments.

Geochronological and palaeomagnetic investigation of the Madiyi Formation, lower Banxi Group, South China: Implications for Rodinia reconstruction

New geochronological and palaeomagnetic data are presented for the Madiyi Formation of the lower Banxi Group in the South China Block (SCB). Zircon U–Pb dating of two tuff beds within the sampled section yielded consistent ages of 801.9 ± 6.3 Ma and 804.6 ± 9.6 Ma. A total of 137 palaeomagnetic samples were collected and thermally stepwise demagnetized. Three components were isolated. Component A was identified in 30 samples below 300 °C. It has a mean direction resembling the recent geomagnetic field, and is interpreted of recent viscous remanent magnetization. Component B, typically isolated below ~660 °C, yields a pole in situ close to the Cretaceous poles of the SCB and is interpreted as a Cretaceous overprint. Component C was identified in 90 samples, determined in the high-temperature steps between 660 and 690 °C. It yielded a mean direction of Ds = 293.1°, Is = 69.9° (k = 22.3, α95 = 3.2°) after tilt correction, corresponding to a palaeopole of 35.3°N, 67.9°E (dp = 5.5°, dm = 4.7°) and palaeolatitude of 53.8 +4.9/−4.7°N for the study area. Inclination correction using an empirical flattening factor f = 0.6 yielded a corrected mean direction of Ds* = 293.0°, Is* = 77.3° (k = 51.4, α95 = 2.1°), corresponding to a palaeopole of 34.3°N, 82.4°E (dp = 3.9°, dm = 3.7°) and palaeolatitude of 65.7 +3.7/−3.6°. Component C records five polarity zones and passes a reversal test. The pole diverges from all younger poles of the SCB, and is interpreted to be of primary remanent magnetization. Our results reveal a high-latitude position of the SCB at ~800 Ma. Comparing with other two poles from the Xiaofeng dykes (~821 Ma) and Yanbian dykes (~824 Ma) reveals no significant latitudinal difference for the SCB between ~820 and 800 Ma. However, coeval poles from the global palaeomagnetic database, along with geological evidence, indicate that East Svalbard, Australia and Laurentia were located around the equator at ~800 Ma. The striking difference in palaeolatitude suggests that the SCB was unlikely to be located between southeastern Australia and western Laurentia in the Rodinia supercontinent at ~800 Ma.

Initiation and evolution of forearc basins in the Central Myanmar Depression

AbstractThe forearc basin in Myanmar is significant in understanding the development of continental forearc basins. We present stratigraphic, sandstone petrographic, and U-Pb detrital data from Upper Cretaceous–Eocene strata of Chindwin and Minbu sub-basins in the Central Myanmar Depression. The Upper Cretaceous lower Kabaw Formation consists of turbiditic conglomerate, sandstone, and mudstone in the Minbu sub-basin. The composition of conglomerates are mainly schist and subordinate quartz. Prominent detrital zircon age probability peaks are between 260 and 223 Ma, similar with that of Upper Triassic Pane Chaung turbidites and Kanpetlet schist on the West Burma plate. In the upper Kabaw Formation, turbiditic volcanic-rich sandstones have major age populations ranging from 103 to 70 Ma in both Minbu and Chindwin sub-basins. The Paleocene slope environment Paunggyi Formation, which overlies the Kabaw Formation, mainly consists of conglomerate, sandstone, mudstone, and tuff beds in the Minbu sub-basin. In contrast, the Paunggyi Formation in the Chindwin sub-basin is composed of sandstone and mudstone; major detrital zircon age populations from the Paunggyi Formation are between 100 and 60 Ma. Eocene strata in both basins are composed mainly of shallow marine to delta sandstone and mudstone. Major detrital zircon age populations are 100–36 Ma and 600–500 Ma. The Late Cretaceous–Eocene ages from Upper Cretaceous–Eocene strata overlap with igneous crystallization ages from the Western Myanmar Arc. We propose that the Chindwin and Minbu sub-basins developed as parts of a forearc basin along the west flank of Western Myanmar Arc (present coordinate). The forearc basin initiated in Albian time atop the continental West Burma plate due to the formation of a structural high along the western margin of West Burma plate.

Stratigraphic evolution of the northern part of the Cretaceous Neungju Basin, South Korea

Several nonmarine basins were formed in SW Korea under an extensional or transtensional tectonic regime during the Cretaceous. The Neungju Basin is one of these basins, filled by alluvial to lacustrine deposits interbedded with thick, amalgamated tuff beds. Facies analysis in the northern part of the basin shows that the basin fill is composed of four facies associations, representing (1) alluvial fan, (2) alluvial plain, (3) sandflat, and (4) marginal playa lake environments. The basin fill can be subdivided into four stratigraphic units by interbedded thick amalgamated tuff beds. The lowermost unit (unit I) is composed of alluvial plain deposits. The lack of any vertical depositional trend indicates a balance between basin subsidence and sediment supply. The middle unit (units II and III) show regional variations. The extensive lake formed in the central to northern parts of the basin, which are later partly filled by proximal sediments from surrounding areas. In contrast, only alluvial plain to sandflat environments occurred in the southern part. The uppermost unit (unit IV) shows a transition from alluvial fan to sandflat, comprising a fining-upward trend that reflects a gradual decrease in sediment supply and slope-gradient during denudation of uplifted source areas. The variations in depositional patterns were possibly caused by the relationship between volcanism-driven sediment supply and basin subsidence. Basin subsidence may have been sufficient to compensate the volcanism-driven sediment supply prior to the deposition of units II and III, resulting in the conformable deposition of lake sediments over thick amalgamated tuff beds. In contrast, volcanism-driven sediment supply may have reduced the generation of accommodation prior to the deposition of unit IV, resulting in the deposition of conglomerates over thick tuff beds. The traditional depositional model composed of syn-eruption aggradation and inter-eruption degradation may not sufficiently explain the development of sedimentary succession, in which the balance between volcanism-driven sediment supply versus basin subsidence coeval to volcanism is variable.

Pyroxene 40Ar/39Ar Dating of Basalt and Applications to Large Igneous Provinces and Precambrian Stratigraphic Correlations

AbstractCorrelations within and between Precambrian basins are heavily reliant on precise dating of volcanic units (i.e., tuff beds and lava flows) in the absence of biostratigraphy. However, felsic tuffs and lavas are rare or absent in many basins, and direct age determinations of Precambrian basaltic lavas have proven to be challenging. In this paper, we report the first successful application of 40Ar/39Ar dating to pyroxene from a Neoproterozoic basalt unit, the Keene Basalt in the Officer Basin of central Australia. 40Ar/39Ar analyses of igneous pyroxene crystals yielded an age of 752 ± 4 Ma (mean squared weighted deviation = 0.69, p = 72%), which is underpinned by 40Ar/39Ar plagioclase age (753.04 ± 0.84 Ma) from the basalt. This age is significant because the Keene Basalt is one of the very few extrusive igneous rocks identified within the Neoproterozoic successions of central Australia and is potentially an important time marker for correlating the Neoproterozoic stratigraphy within, and beyond, the central Australian basins. Our geochronological and geochemical data show that the Keene Basalt, which is characterized by enriched elemental and Nd‐Pb isotopic signatures, is strikingly similar to, and coeval with, the 755 ± 3 Ma Mundine Well Dolerite in northwestern Australia. Here we suggest that both are part of the same large igneous province (~6.5 × 105 km2) related to breakup of the supercontinent Rodinia. This study demonstrates the potential of pyroxene 40Ar/39Ar geochronology to date ancient flood basalts and to provide pivotal time constraints for stratigraphic correlations of Precambrian basins.

An extensive K-bentonite as an indicator of a super-eruption in northern Iberia 477 My ago

Zircon and monazite ID-TIMS U-Pb dating of four Lower Ordovician altered ash-fall tuff beds (K-Bentonites) in NW Iberia provided coetaneous ages of 477.5±1, 477±1.3 Ma, 477.2±1.1 Ma and 477.3±1 Ma, with a pooled concordia age of 477.2±0.74 Ma. A conservative estimation of the volume and mass of the studied K-bentonite beds (using data from the Cantabrian Zone) returns a minimum volume for the preserved deposits of ca. 37.5 km3 (Volcanic Explosivity Index - VEI = 6, Colossal). When considering other putative equivalent beds in other parts of Iberia and neighbouring realms the volume of ejecta associated to this event would make it reach the Supervolcanic-Apocalyptic status (VEI=8, >1000 km3). Contrary to most cases of this kind of gargantuan eruption events, the studied magmatic event took place in relation to continental margin extension and thinning and not to plate convergence. We speculate that a geochronologically coincident large caldera event observed in the geological record of NW Iberia could be ground zero of this super-eruption.

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.

Lithic-rich and lithic-poor ignimbrites and their basal deposits: Sovana and Sorano formations (Latera caldera, Italy)

Conceptual models explaining the characteristics of ignimbrites and their basal deposits have mainly focused on variations in particle concentration and speed of the parent pyroclastic currents. Here, we focus on the effects of relative proportions of clasts that are well coupled to the carrier gas phase and those that are poorly coupled, i.e., the proportions of fine and/or low-density clasts compared to coarse and/or dense clasts. We document facies and macroscopic clast populations of two ignimbrite-producing eruptive sequences of similar volume and composition, both erupted from Latera caldera (central Italy). The Sovana ignimbrite has 1–15 vol% dense lithic clasts and locally up to 70 vol% in lithic-rich domains. It preserves evidence of emplacement of multiple overlapping flow units with local internal channeling. The ignimbrite is underlain by two basal deposits: (1) a radially distributed, 20–80-cm-thick vitric tuff referred to as BUS (basal unit Sovana) that thins gradually with distance and shows evidence of emplacement by lateral currents; and (2) a fines-depleted, lenticular, massive lapilli tuff, up to 20 cm thick, with abundant lithic clasts. The Sorano ignimbrite, in contrast, has < 1 vol% dense lithic clasts. It contains multiple reverse coarse-tail graded emplacement units, but we observed no internal channeling. In most places, its basal deposits are 20–100 cm of dominantly planar-parallel, 5–10 cm-thick, reverse coarse-tail graded tuff beds. However, in local depressions near the tops of paleocanyons that were filled by Sorano ignimbrite, the basal deposits are up to 2 m thick and are clearly cross stratified. We interpret the different deposit characteristics in terms of recent modeling and experimental results. The Sovana eruption involved major caldera collapse that introduced abundant dense lithic clasts into the erupting mixtures. BUS may have partly resulted from expulsion of a dilute, fine-grained mixture at the site where the pyroclastic fountain(s) impacted the ground, which then propagated outward as a dilute pyroclastic current ahead of the main ignimbrite-producing currents; the expulsion process is promoted by collapsing mixtures rich in poorly coupled clasts. The fines-depleted layer overlying BUS was deposited by rapid sedimentation of abundant dense clasts from the head of the ash-cloud surge, which accompanied the concentrated underflow that deposited the ignimbrite. Spatial and temporal variations in dense-clast abundance, and likely other parameters, during eruption resulted in multiple flow units. The Sorano eruption lacked poorly coupled clasts and did not produce an expulsion-driven current. Its ash-cloud head deposited ash and small pumice lapilli that were able to move in traction carpets prior to being overridden by a concentrated underflow. The overriding ash-cloud bodies locally deposited cross-stratified horizons in depressions high on the canyon walls, before they were inundated as the concentrated underflows filled the canyons with ignimbrite. To summarize, the differences between the ignimbrites and their basal deposits can be largely attributed to the differences in proportions of poorly coupled relative to well-coupled particles in the collapsing eruptive mixtures; the resulting conceptual model simplifies the previous range of interpretations for basal deposits, and is consistent with recent modeling and experimental results.

Nature and origin of tuff beds in Jurassic strata of the Surat Basin, Australia: Implications on the evolution of the eastern margin of Gondwana during the Mesozoic

Volcanogenic rocks in the Great Australian Superbasin provide one of the principal records of contemporaneous volcanism in eastern Australia during the Mesozoic. However, the paucity of primary Jurassic to Early Cretaceous-age extrusive or intrusive igneous bodies on the Australian continent makes it particularly challenging to deduce their source and character. This, in turn, makes it difficult to ascertain how the eastern margin of Gondwana evolved during this timeframe. Despite some studies of this enigmatic volcanism, there have been little or no detailed analyses of these Jurassic to Early Cretaceous-aged sediments. Based on the multidisciplinary analyses of age-constrained air-fall tuffs (168 to 148 Ma) from the Jurassic Walloon Coal Measures of the Surat Basin, we suggest from zircon grains of a similar age that the tuffs were erupted from volcanoes fed by intermediate to felsic magmas supported by their quartz and feldspars-rich composition, and from zircon grains with low to moderate Nb values (0.5 to 100 ppm) and high U and Th values (30 to 1000 ppm). The mapping of tuff isopachs and a mean zircon crystal size of 170 μm supports the source being from volcanoes approximately 280 to 1000 km from the palaeoeast-southeast with a volcanic explosivity index (VEI) of 8. Our results indicate the tuffs originated from a continental arc setting associated with the Whitsunday Igneous Association, and the long-lived (late Palaeozoic to Cretaceous) westward subduction of the palaeo-Pacific oceanic crust beneath eastern Australia. Such a tectono-magmatic environment would help constrain the timing of the transition of eastern Gondwana from a convergent to a divergent margin.

Revisions to the chronostratigraphic framework of the Upper Jurassic Walloon Coal Measures of the Surat Basin, Australia

The Upper Jurassic Walloon Coal Measures (WCM) in the Surat Basin host the largest coal seam gas (CSG) resource in Australia. Despite this, a poorly defined lithostratigraphic framework hinders the development of reservoir models and groundwater flow simulations. Correlations in the WCM are challenging, owing to the complex arrangement of facies over short distances and the absence of a reliable regional stratigraphic datum. To better correlate the strata, 26 tuff beds were dated using the U–Pb chemical abrasion thermal ionisation mass spectrometry methodology across the Surat Basin CSG fairway. This initially suggested that coal-bearing strata in the basin were diachronous. However, the acquisition of a new date from the Surat Basin has identified a five million year time gap between dated tuffs ~20 m apart. This suggests the presence of an unconformity and that there were two independent episodes of coal accumulation in the basin. Above the unconformity, there are incised valleys with a sedimentary infill that transitions from fluvial- to tidal-influenced facies, as indicated by dinoflagellate cysts and tidal sedimentary structures, including double mud drapes. The cause of the unconformity is likely to be tectonic, as eustatic sea-level was rising during the Kimmeridigian. The marine incursion into the basin is the consequence of a highstand of sea-level during the early Tithonian. The application of the new chronostratigraphic framework should elucidate the evolution of fluviolacustrine systems in the basin and aid in resource prediction. Further dating of tuffs in the basin could refine the stratigraphic framework.

Erratum for ‘The provenance of the Devonian Old Red Sandstone of the Dingle Peninsula, SW Ireland; the earliest record of Laurentian and peri-Gondwanan sediment mixing in Ireland,’ Journal of the Geological Society, London, 175, 411–424

Samples in this paper have been assigned formations based on the Geological Survey of Ireland shapefile released prior to the commencement of the study. However, the authors were not aware that, since obtaining the samples, an updated shapefile had been released. This update affects three of the four apatite samples assigned to the Lower Devonian Ballymore Formation. The location of samples Mb-1, Mb-4 and Mb-5 now places them well within the undifferentiated, Upper Devonian Slieve Mish Group. As outlined in our paper, the apatite ages were originally produced concurrently with apatite fission track analysis and were later used in our study to provide additional provenance information in support of the detrital zircon geochronological data. In the second paragraph of the discussion section we say the following: et al. (1999) obtained an age of 411 Ma for the Cooscrawn Tuff Bed in the Ballymore Formation, which is older than 22 of the 70 detrital apatites analysed in this formation. The reassignment of the three samples to the Upper Devonian Slieve Mish Group nullifies the above statement. However, our interpretation that the depositional age of the Ballymore Formation is younger than the 411 Ma age given by Williams et al. (1999) is predominantly based upon the evidence given by the six youngest detrital zircons from the formation which underlies the Ballymore Formation (i.e. the Slea Head Formation). These zircons give a concordia age of 405 ± 4 Ma. This suggests that the Ballymore Formation was more than likely deposited after 409 Ma. We do not believe that the reassignment of the three samples has any major impact on our provenance interpretations. The ∼420 Ma age of the majority of the apatites in samples Mb-1, Mb-4 and Mb-5 actually fits with the range of Palaeozoic detrital zircons in sample AK-17 which was taken from the Slieve Mish Group, thereby supporting minor input of rocks affected by end-Scandian metamorphism.

The geochemistry, tectonic and palaeogeographic setting of the Karing Volcanic Complex and the Dusunbaru pluton, an early Permian volcanic-plutonic centre in Sumatra, Indonesia

The Karing Volcanic Complex and the Dusunbaru Pluton comprise an Early Permian volcanic-plutonic centre in the Volcanic-Plutonic Arc on the western margin of the Kluet-Kuantan basin of the West Sumatra Block in Sumatra, Indonesia. The Karing Volcanic Complex is mostly composed of ‘Monotonous Intermediate Type’ tuffs which interfinger with the volcaniclastics and coastal and marine sediments of the Mengkarang Formation. Eight lithological Intervals each commencing with tuffs overlain by volcaniclastics and sediments, have been mapped in the Merangin river section. The climax of the volcanicity occurred in Interval IV. In the waning stages of volcanism basalt flows were extruded and the volcanic pile was eroded by lahars and debris flows. Zircon CA-IDTMS dates from tuff beds at the base and top of the section bracket the volcanic activity as having taken place over a period of 630,000 years within the Asselian Stage of the Permian. The Dusunbaru pluton is composed of Gabbroids, Granitoids with subordinate Metabasalts and Metatuffs occurring as xenoliths. These lithologies are present as lithic clasts within the Karing Volcanic Complex tuffs, and it is considered that the Dusunbaru Pluton represents a former magma chamber from which the tuffs were vented. The Mildly-alkaline to Sub-alkaline Metabasalts and Gabbroids, were derived from the N-MORB sector of the lithospheric ‘Mantle Array’ while the compositions of the Granitoids (including gabbroid-granitoid hybrid rocks) were influenced by the reaction of basic magmas with the lower crust. The Sub-alkaline and High–K Late Basalt Lava flows were derived from the melting of basalt below the crust-mantle interface. The rock suites were variably hydrated as a result of alterations. Magmatic alteration has affected the Metabasalts, Gabbroids, Granitoids, their xenoliths and the Late Basalts. Hydrothermal alteration has weakly affected the aubaerial tuffs and strongly changed the mineralogy of some of the Late Basalt flows, in particular the entablatures. The chemistry of the Dusunbaru Pluton is of Cordilleran-type, typical of continent margin volcanic-plutonic arcs. The Dusunbaru Pluton was likely originally emplaced at the interface between the Kuantan Formation (Visean to Permian) and a Proterozoic gneissic basement. A late hydrothermal event/tectonic event during which the Dusunbaru Pluton was faulted against the Karing Volcanic Complex, is attributed to the collision of the Woyla oceanic terrains with Sumatra in the late-mid Cretaceous. The stratigraphy of the West Sumatra Volcanic Arc and the Kluet-Kuantan Basin resembles that of the Sukhothai Volcanic Arc and associated basin sediments in the Indochina Block. A palaeogeographical reconstruction suggests that the West Sumatra Block originally was an appendage to the Indochina Block at a palaeolatitude between 5 and 15° S.

New U-Pb geochronology evidence for 2.3 Ga detrital zircon grains in the youngest Huronian Supergroup formations, Canada

Detrital zircon U-Pb ages from one sandstone and one claystone unit in the upper Huronian Supergroup, near Flack Lake, Ontario, Canada, provide new evidence for the maximum depositional age of the two youngest formations and reinterpretation of the depositional history of the upper Huronian Supergroup. The average age of the youngest zircon grains constrains the age of deposition to sometime after 2315 ± 5 Ma, but prior to intrusion of gabbro (Nipissing) dykes approximately 95 m.y. later. Clusters of late Archean age zircon are prominent in both samples, but a considerable addition from post-Archean sources is also present. The sample from the Bar River Formation contains a greater proportion of post-Archean material, which may be a result of grain-size bias. Our results are similar to U-Pb zircon ages determined from stratigraphically lower purported tuff beds in the Gordon Lake Formation, however our field and Scanning Electron Microscope (SEM) analyses demonstrate that the samples in the present study were not part of tuff units.

Solving a tuff problem: Defining a chronostratigraphic framework for Middle to Upper Jurassic nonmarine strata in eastern Australia using uranium–lead chemical abrasion–thermal ionization mass spectrometry zircon dates

To better predict the architecture of reservoirs and the location of undiscovered resources in fluvial-dominated strata, a sound chronostratigraphic framework is needed. This study reassesses the stratigraphic framework of petroleum-bearing Jurassic fluviolacustrine successions in the Eromanga, Surat, and Clarence–Moreton Basins of eastern Australia. Correlation of the strata is challenging because of the heterolithic facies, the absence of conventional stratigraphic marker beds, and the longevity of palynostratigraphic zones. The abundance of laterally discontinuous volcanic air–fall tuffs and volcanogenic sandstones across the Jurassic of eastern Australia allows for the construction of a new, regional chronostratigraphic framework. High-precision U-Pb zircon chemical abrasion–thermal ionization mass spectrometry (CA-TIMS) dates ranging from 168.07 ± 0.07 Ma to 149.08 ± 0.06 Ma were obtained from 31 samples from 13 wells across 3 basins. Five chronostratigraphic datums were defined and extrapolated to 677 wells within a time interval of 420 ka or less over hundreds of kilometers across eastern Australia. The new chronostratigraphic framework reveals inaccuracies in picking lithostratigraphic units based on lithology and wire-line log characteristics and shows coal-bearing facies of the Walloon Coal Measures to be two episodes of coal (peat) accumulation separated by an unconformity. The study also demonstrates the feasibility of extending chronostratigraphic datums to neighboring basins without tuff beds by dating the youngest zircon in volcanogenic sandstones by U-Pb CA-TIMS following laser ablation–inductively coupled plasma mass spectrometry analysis. The dates provide a substantial revision to the Middle to Upper Jurassic stratigraphy of eastern Australia. The use of precise U-Pb CA-TIMS dates should help elucidate the lithofacies architecture of nonmarine successions in other basins and assist in petroleum development.