Regional Stratigraphic Marker Research Articles

Stratigraphy and sequence stratigraphy of the Neoproterozoic (Cryogenian–Ediacaran) Stuart Shelf, South Australia

The Stuart Shelf is part of the Adelaide Superbasin overlying the Gawler Craton and Cariewerloo Basin in South Australia. The basin is of interest for sediment-hosted copper mineralisation known to be hosted in numerous stratigraphic intervals across the region. Therefore, facies analysis and understanding of spatial distribution of host units are essential for exploration targeting and mineral systems research. Our study presents improved and new definitions of Cryogenian and Ediacaran Stuart Shelf stratigraphy, and a detailed, regional-scale sequence stratigraphic analysis. The Cryogenian non-glacial interlude was of particular interest, as it includes the Tapley Hill Formation, a known host for copper mineralisation. The succession of Tapley Hill Formation (including Sturtian cap carbonates), Brighton Limestone and Angepena Formation represents a third-order depositional cycle, an equivalent to the lowermost cycle of the Cryogenian non-glacial interlude in the adjacent Adelaide Rift Complex. The Ediacaran post-glacial succession includes the Nuccaleena Formation and Tent Hill Formation and is interpreted as a second-order depositional cycle. The Sturtian and Marinoan cap carbonates form regional stratigraphic marker horizons and were deposited in a transgressive systems tract. Variations in facies and thickness can be linked to development of localised depocentres and topographic highs, such as the Pernatty High. Modelling of 3D surfaces reveals a shift in the basin orientation between the Cryogenian and Ediacaran sedimentation from an NNW–SSE axis towards a N–S axis. The Stuart Shelf sequence stratigraphic framework provides new insights in the basin sedimentology and evolution, which aids sediment-hosted mineral systems analysis and improves the understanding of Cryogenian and Ediacaran basin evolution in Australia.

Multiphase cryogenesis and incipient paleosol development during MIS 2 in North-Western Siberia: Detailed chronostratigraphy, paleoenvironmental significance, and comparison with the global and European records

New widely accepted scenario of glacier-free landscape evolution in the north of West Siberia during MIS 2 (Sartanian cryochron) raises a challenge to find new regional stratigraphic markers in the absence of reliable continuous sedimentary units. The cryogenic and pedogenic structures have been long applied for the European Late Pleistocene stratigraphy in the extra-glacial regions. We argue that such structures might also provide important markers for the detailed stratigraphic scheme in the north of West Siberia. Two types of indicators were suggested. Firstly, morphological features and ranges of radiocarbon dates in epigenetic ice wedge pseudomorphs of higher terraces are sensitive to events of abrupt climate amelioration. Regional analysis of radiocarbon dates of ice wedge pseudomorphs indicated four warmer climatic events in the Late Pleistocene – the Early Holocene (ca. 18000–16500 cal BP, ca. 13800–12500 cal BP, ca. 11700–11100 cal BP, and ca. 10400–9000 cal BP). They correlate to climatic events of the Greenland ice core record with minor temporal lag. Secondly, more detailed cryogenic-paleosol-sedimentary sequences formed at lower terraces where sedimentation continued during MIS 2 are another type of suggested indicator. Their morphological and paleobotanical analyses and radiocarbon dating provided a reconstruction of local paleolandscape evolution with events of cryohydromorphic soil development (ca. 15700–15300 cal BP, ca. 11200–9000 cal BP). Similar late-glacial paleosols are encountered in the European loess sequences. We propose that the cryopedogenic horizons, including ice wedge pseudomorphs, cryoturbation structures, and cryohydromorphic paleosols, are reliable regional stratigraphic markers in the north of West Siberia for the Sartanian cryochron.

Deciphering a cryptic unconformity in the Surat Basin using high frequency detrital zircon U-Pb geochronology: Insights into basin dynamics on the north-east margin of Gondwana

The Surat Basin is one of a series of shallow intracratonic basins that comprise the Great Australian Superbasin and developed in response to the initiation of Gondwanan breakup. Within the Surat Basin, the Walloon Coal Measures (WCM) are a significant target for Coal Seam Gas (CSG) production and the identification of the boundary with the overlying Springbok Sandstone (SST) is critical to the stratigraphic framework of the basin, and identification of groundwater flow units it delineates. Identification of the contact on a local (tenement) and regional (basin) scale between the SST and WCM is challenged in CSG producing areas by (i) overall lithological similarity of the units (macro- and microscopically, whole rock geochemistry, wireline signature), (ii) lateral heterogeneity of lithologies on a regional scale due to lateral facies and depositional environment variation in the uppermost WCM and lowermost SST and (iii) lack of consistent regional stratigraphic markers like erosion surfaces or correlated tuff beds. Sediment provenance analysis via detrital zircon (DZ) U-Pb geochronology allows constraint of spatial and temporal changes in sediment provenance by sampling across stratigraphic units along a lateral transect. The resulting large-scale stratigraphic correlations facilitate identification of basin dynamics within a tectonically quiescent basin. Here we present the results of DZ distributions from 26 samples that were sampled from four wells along a 300 km transect from the eastern Eromanga Basin to easternmost extent of the Surat Basin. Our data show a brief pulse of basement derived material at the base of the SST which interrupts the predominant volcaniclastic sediment provenance in the SST and WCM. This brief change in zircon provenance is interpreted as a precursor to higher energy depositional environments in the SST and can be utilised as a regional marker across the Surat and Eromanga basins, indicating a change in source provenance and basin dynamics associated with the SST – WCM boundary.

Eruptive and depositional processes of a low-aspect-ratio ignimbrite (the Southern Kusandong Tuff, South Korea) inferred from magnetic susceptibility variability

Low-aspect-ratio ignimbrites, produced by the most lethal volcanic eruptions, are thin but laterally extensive and can serve as good stratigraphic markers in many sedimentary basins. They are commonly structureless, and require diverse analytical tools to reveal the related geological processes. This study aims to understand better the eruptive and depositional processes of the Cretaceous Kusandong Tuff, which is a several meter-thick and laterally extensive silicic ignimbrite in Korea, through rock-magnetic analyses of magnetic susceptibility (k) and isothermal remanent magnetization (IRM) together with optical and electron microscopic observations. Massive parts of the tuff show systematic vertical variation in k within each locality, and the vertical variation patterns vary from locality to locality, being classified broadly into three types: a single cycle of upward-decreasing k (type A), repetition of two or more upward-decreasing k (type B), and consistently low k (type C). A linear relationship between k and IRM indicates predominant contribution of ferri- and antiferro-magnetic minerals to k in the majority of the tuff. Integrated results of microscope images, temperature variations of k, and IRM unmixing analyses indicate that the major magnetic minerals are coarse magnetite (with occasionally partial maghemitization) of both primary and secondary origins, very fine secondary magnetite, and secondary hematite and goethite. Quantitative comparison of the unmixed magnetic mineral components reveals that those vertical k variations are related to some combination of magnetite destruction and subsequent precipitation of antiferromagnetic minerals by post-depositional alterations at different stages (types A, B, and C) and magnetite supply by deposition of additional cooling unit (type B). Despite the adverse post-depositional modifications of k, this suggests that the massive part of this tuff is a composite unit formed by accumulation of multiple cooling units from multiple pyroclastic current pulses probably associated with fluctuations in eruption rate at one or multiple source vent(s). Comparison by a k-based proxy between localities implies independence of primary magnetic mineral content on the distance from vent(s), potentially implying convection-dominated behavior in the pyroclastic current dynamics. This possible flow dynamics is inferred to be generated by sustained laterally-directed blasts from shallow-level explosion and could, in turn, lead to long-distance travel. This study exemplifies how the methodological approach using magnetic susceptibility can contribute to characterize the nature of low-aspect-ratio ignimbrite, and will help make more use of the Kusandong Tuff as a key regional stratigraphic marker as well.

Alginite-Rich Layers in the Bazhenov Deposits of Western Siberia

In this study, we identified the luminescent layers containing a significant amount of alginite in the Upper Jurassic–Lower Cretaceous Bazhenov Formation named “the alginite-rich layers”. Lithological and geochemical methods were used to determine distinctive features of these layers and to evaluate their impact on the total petroleum generation potential of the Bazhenov Formation. We have shown that the composition of the alginite-rich layers differs significantly from the organic-rich siliceous Bazhenov rocks. Rock-Eval pyrolysis, bulk kinetics of thermal decomposition, elemental analysis, and the composition of pyrolysis products indicate type I kerogen to be the predominant component of the organic matter (OM). Isotope composition of carbon, nitrogen, and sulfur was used to provide insights into their origin and formation pathways. The luminescent alginite-rich layers proved to be good regional stratigraphic markers of the Bazhenov Formation due to widespread distribution over the central part of Western Siberia. They can also be applied for maturity evaluation of the deposits from immature to middle of the oil window, since the luminescence of the layers changes the color and intensity during maturation.

Patterns of geochemical variability across weakly developed paleosol profiles and their role as regional stratigraphic markers (Upper Pleistocene, Po Plain)

Weakly developed paleosols from two distinct interfluve surfaces of Late Pleistocene age provide excellent keys to high-resolution stratigraphic correlation and may serve to trace large-scale genetic packages (systems tract equivalents) across the continental portion of the Po Basin. Twenty-four paleosol profiles from 17 sediment cores were identified and characterized for bulk-geochemical analysis. X-ray fluorescence data were used to trace the degree of weathering. Paleosols, 0.5–1.5 m thick, are pedogenically altered floodplain deposits, developed over time spans of a few thousand of years and mostly partitioned into A-Bk horizons. The most notable paleosol features are dark, organic-matter-rich and carbonate-free mineral surface horizons (A) that overlie bright calcic horizons (Bk) typified by the accumulation of secondary carbonates in the form of pedogenic nodules.Paleosol profiles exhibit a homogeneous geochemical signature that fingerprints a moderate degree of weathering, with little strike- and dip-oriented variability across the different study localities. Plots of Al-normalized calcification and base loss indices against depth reveal systematic increasing values from intensely altered A horizons to underlying Bk horizons. These trends reflect consistent patterns of Ca translocation from surface horizons deeper into the profile, with significant to almost complete Ca removal from A horizons through leaching and accumulation in Bk horizons. Selected trace element ratios (Ba/Sr, Rb/Sr), redox-sensitive trace elements and Zr contents display opposite, up-profile increasing trends that reflect Sr loss in A horizons, with selective Zr concentration in residual minerals.Vertical trends in element ratios are laterally extensive and consistent on a regional basis and represent key pedochemical/stratigraphic markers that can be traced over great distances (tens of kms) throughout the inland portion of the basin. Through quantitative assessment of the degree of weathering, geochemical profiling provides high potential for robust subsurface paleosol correlation that might not be captured by visual core descriptions alone.

Machine learning classifiers for attributing tephra to source volcanoes: an evaluation of methods for Alaska tephras

ABSTRACTGlass composition‐based correlations of volcanic ash (tephra) traditionally rely on extensive manual plotting. Many previous statistical methods for testing correlations are limited by using geochemical means, masking diagnostic variability. We suggest that machine learning classifiers can expedite correlation, quickly narrowing the list of likely candidates using well‐trained models. Eruptives from Alaska's Aleutian Arc‐Alaska Peninsula and Wrangell volcanic field were used as a test environment for 11 supervised classification algorithms, trained on nearly 2000 electron probe microanalysis measurements of glass major oxides, representing 10 volcanic sources. Artificial neural networks and random forests were consistently among the top‐performing learners (accuracy and kappa > 0.96). Their combination as an average ensemble effectively improves their performance. Using this combined model on tephras from Eklutna Lake, south‐central Alaska, showed that predictions match traditional methods and can speed correlation. Although classifiers are useful tools, they should aid expert analysis, not replace it. The Eklutna Lake tephras are mostly from Redoubt Volcano. Besides tephras from known Holocene‐active sources, Holocene tephra geochemically consistent with Pleistocene Emmons Lake Volcanic Center (Dawson tephra), but from a yet unknown source, is evident. These tephras are mostly anchored by a highly resolved varved chronology and represent new important regional stratigraphic markers.

Refining the age, emplacement and alteration scenarios of the olivine-rich unit in the Nili Fossae region, Mars

The Nili Fossae region of Mars exhibits from remote sensing spectral data the largest exposures of olivine-rich materials on the planet. However, it is not clearly constrained how and when these terrains formed. Some of the proposed scenarios favor a mode of formation closely related to Isidis impact basin: either under intense effusive volcanism following the impact, from cooling of an immediate impact melt sheet or silicate impact vapor condensate. These deposits might also be pyroclastic products ejected from now eroded or buried vents. Recent studies also proposed that lag deposits could be responsible for enrichment in olivine after deposition. In this contribution, we mapped the olivine-rich unaltered and altered bedrock exposures using near infrared and thermal inertia data, investigated the geometry and key contacts of the olivine-rich unit, and determined its surface age using crater counts and stratigraphical relationships. We find that the olivine-rich bedrock extends over at least ~18,000 km2 in the Nili Fossae region, with a large part of it being unaltered. Olivine-rich material that overlaps the northern rim of Jezero crater corresponds to a primary deposit (i.e. rather than reworked material). Since this crater is younger than Isidis, we favor the hypothesis of a post-Isidis origin, rather than the impact melt (consistently with Bramble et al., 2017) and impact condensate origin. Based on our observations and crater counts, we estimate an emplacement age of 3.82 ± 0.07 Ga (Mid to Late Noachian). We discuss the origin of the unit, with the most likely scenarios being ash falls and/or pyroclastic surges. To explain the circum-Isidis distribution of these deposits, we favor the hypothesis of a thinned and weakened crust in the region subsequently to the giant impact of Isidis, as suggested by Tornabene et al. (2008). Finally, the distribution of altered bedrock conflicts with the contact metamorphism scenario. As the olivine-rich unit exposed at Jezero crater, future home of the Mars 2020 rover, is a regional stratigraphic marker, return samples for precise dating of this unit should be made one of the major mission targets.

Evidence for distal transport of reworked Andean tephra: Extending the cryptotephra framework from the Austral volcanic zone

Abstract Cryptotephra deposits (non-visible volcanic ash beds) may extend thousands of kilometres and provide valuable chronological isochrons. Here, we present a Lateglacial-early Holocene (c. 16,500 cal yr BP-6000 cal yr BP) tephrostratigraphy from Hooker's Point, East Falkland, South Atlantic. This period spans the last glacial termination across the southern mid-latitudes, a time period during which the palaeoenvironmental record is poorly resolved in southern South America and the South Atlantic. The development of a regional tephrostratigraphy will provide chronological constraint for palaeoenvironmental records from this period. Two cryptotephra deposits from Hooker's Point are linked with Mt. Burney, including the early-Holocene MB1 tephra, while a third is likely to be derived from the R1 eruption of Reclus volcano. The high shard abundance of these cryptotephra deposits suggests they extend further into the Southern Ocean, and may act as regional stratigraphic markers during the Lateglacial. Further peaks in shard abundance are composed of detrital glass (tephra not derived from primary air fall events), with mixed shard morphologies and geochemically heterogeneous glass populations. This detrital glass is likely to have been repeatedly reworked by wind action in the Patagonian Steppe before final deposition in the Falkland Islands. The high abundance of detrital glass in the Hooker's Point sequence suggests long distance transport of reworked tephra is common in this region, and highlights the need to carefully analyse cryptotephra deposits in order to avoid incorrectly describing reworked tephra as new isochrons. A temporal pattern of shard abundance is apparent in the Hooker's Point sequence with a reduction/absence of shards between 14,300–10,500 cal yr BP.

The dinoflagellate cyst Molassedinium bicornutum gen. et sp. nov. from the Oligocene of the North Alpine Foreland Basin, Austria

The organic-walled dinoflagellate cyst Molassedinium bicornutum gen. et sp. nov. described in this paper is recorded in the Oligocene (Rupelian) organic-rich sediments of the Eggerding Formation from the North Alpine Foreland Basin (Molasse Basin), Austria. It was discovered within a diverse and well-preserved dinoflagellate cyst assemblage. It is a gonyaulacacean (leptodinioidean) holocavate cyst with distinctive apical and antapical ectophragmal horns and an apical archaeopyle. The current stratigraphic range of the taxon is restricted to the Rupelian. It may constitute a potentially important regional stratigraphic marker for high-resolution age control. It is associated with a dinoflagellate cyst assemblage indicating inner–outer neritic and nutrient-rich environments.

Late Middle Miocene volcanism in Northwest Borneo, Southeast Asia: Implications for tectonics, paleoclimate and stratigraphic marker

Explosive volcanic events often produce pyroclastic materials that can be recognized from the geological record. These discrete pyroclastics form regional marker beds. Here we report the occurrence of a tephra layer interbedded within very thick coal beds near Mukah, Sarawak, Borneo. Traceable for tens of kilometers in the Mukah area of Sarawak, this tephra layer can be considered as regional stratigraphic marker with precise chronostratigraphic control. Systematic sedimentological, mineralogical, geochemical and zircon UPb geochronological studies have revealed a major effusive volcanic event during the latest Middle Miocene, presumably contemporaneous and/or related to a magmatic event of an earlier phase of the Mt. Kinabalu pluton or magmatism in West Sarawak or East Sabah. The volcanic event had promoted catastrophic flooding of coastal swamps and fall-out from the ash clouds that formed a regionally monotonous tephra layer across the Serravallian- Tortonian boundary. In conjunction with the regional occurrences of trap rocks, structural trends and known tectonic events, we constrained the regional depositional environments, and climate. The tephra layer was deposited in a coastal plain-swamp,- seasonal, shallow, high-moderate energy, fluvial channel-lacustrine environmental setting, wherein atmospheric fallout and eroded material from regoliths formed over older basement and volcanic rocks of the hinterland which were mixed to produce the tephra layer. This tephra layer is sandwiched between the very thick coal beds. A pre-existent volcanic chamber that was active for a long time, also experienced periodic explosive activity from probably the same magma chamber and conduit and including a major explosive activity that recycled early-formed crystals and felsic magma (rhyolite-dacite) during the major effusive event are also recognized. Our findings provide robust evidence for the prevalence of intensive chemical weathering under a wet-humid climate, and relative tectonic quiescence before the major effusive event, and the existence of vast, monotonously gently-sloping coastal plains and luxuriant vegetation akin to the present.

New zircon (U-Th)/He and U/Pb eruption age for the Rockland tephra, western USA

Eruption ages of a number of prominent Quaternary volcanic deposits remain inaccurately and/or imprecisely constrained, despite their importance as regional stratigraphic markers in paleo-environment reconstruction and as evidence of climate-altering eruptions. Accurately dating volcanic deposits presents challenging analytical considerations, including poor radiogenic yield, scarcity of datable minerals, and contamination of crystal populations by magma, eruption, and transport processes. One prominent example is the Rockland tephra, which erupted from the Lassen Volcanic Center in the southern Cascade arc. Despite a range in published eruption ages from 0.40 to 0.63 Ma, the Rockland tephra is extensively used as a marker bed across the western United States. To more accurately and precisely constrain the age of the Rockland tephra-producing eruption, we report U/Pb crystallization dates from the outermost ∼2 μm of zircon crystal faces (surfaces) using secondary ion mass spectrometry (SIMS). Our new weighted mean 238U/206Pb age for Rockland tephra zircon surfaces is 0.598 ± 0.013 Ma (2σ) and MSWD = 1.11 (mean square weighted deviation). As an independent test of the accuracy of this age, we obtained new (U-Th)/He dates from individual zircon grains from the Rockland tephra, which yielded a weighted mean age of 0.599 ± 0.012 Ma (2σ, MSWD = 5.13). We also obtained a (U-Th)/He age of 0.628 ± 0.014 Ma (MSWD = 1.19) for the Lava Creek Tuff member B, which was analyzed as a secondary standard to test the accuracy of the (U-Th)/He technique for Quaternary tephras, and to evaluate assumptions made in the model-age calculation. Concordance of new U/Pb and (U-Th)/He zircon ages reinforces the accuracy of our preferred Rockland tephra eruption age, and confirms that zircon surface dates sample zircon growth up to the time of eruption. We demonstrate the broad applicability of coupled U/Pb zircon-surface and single-grain zircon (U-Th)/He geochronology to accurate dating of Quaternary tephra, and highlight the challenges and opportunities of this technique.

Elemental geochemistry and strontium-isotope stratigraphy of Cenomanian to Santonian neritic carbonates in the Zagros Basin, Iran

A Neo-Tethyan upper Cenomanian–Santonian neritic carbonate ramp succession (Sarvak and Ilam formations), drilled in the Zagros Basin in southwest Iran, was investigated via detailed sedimentology, microfacies analysis, elemental geochemistry and Sr-isotope stratigraphy (SIS). The succession contains two exposure surfaces, which are known as the CT-ES and mT-ES (Cenomanian–Turonian and middle Turonian, respectively), and associated prominent negative carbon-isotope excursions that represent important regional stratigraphic marker horizons. Precise knowledge about the onset of platform exposure and the duration of the exposure-related hiatus, however, is currently lacking due to a rather low-resolved shallow-water biostratigraphic framework and a bulk carbonate carbon-isotope pattern that clearly differs from global Late Cretaceous reference curves. Therefore, the existing bio-chemostratigraphic framework was complemented by bulk carbonate strontium-isotope stratigraphy (SIS). As bulk carbonate material is in particular prone to diagenetic alteration, a careful selection of least altered samples has been carried out by means of elemental geochemistry and petrography. In contrast to what could be expected, the meteoric alteration of limestones beneath both exposure surfaces is not clearly expressed by increasing iron and manganese and coeval decreasing strontium contents. On the contrary, the impact of meteoric diagenesis is well illustrated via pronounced increases in Rb concentrations and concomitant prominent positive shifts to radiogenic strontium-isotope values, an observation that clearly reflects the decay of continentally derived 87Rb into 87Sr. Rubidium corrected strontium-isotope values place the CT-ES around the Cenomanian–Turonian boundary and point to an exposure duration of less than 0.4Myr. This rather short-term CT-ES related hiatus is supported by petrographic evidence, which indicates a youth karstification stage of strata beneath the CT-ES. Following SIS, the Ilam–Sarvak transition at the top of Nezzazatinella–Dicyclina interval zone coincides with the mT-ES. Carbonates placing above this transition (Ilam Formation) are ascribed to the earliest to latest early Santonian, while carbonates immediately beneath the mT-ES (Sarvak Formation) are dated as late Turonian. SIS thus indicates a long-lasting hiatus of 4.5Myr associated with the mT-ES. Emergence represented by the CT-ES is here proposed as harbinger of the mT-ES in the Zagros Basin, which most likely resulted from stepwise peripheral bulging due to ophiolite obduction in combination with a small-scale global sea level fall around 94Ma.

Base of brackish-water mud as key regional stratigraphic marker of mid-Holocene marine flooding of the Baltic Sea Basin

Many modern epicontinental seas were dry land before their marine flooding by the mid-Holocene glacioeustatic sea-level rise, whereas the Baltic Sea Basin was covered by a huge postglacial lake. This change from a postglacial lake to the present-day semi-enclosed brackish-water sea is studied here in sediment cores and acoustic profiles from the Baltic Sea major sub-basins, based on novel datasets combined with information extracted from earlier publications. In shallow areas ( 50m water depth), where it may be locally less clearly expressed due to reworking and bioturbation. Both in the shallow and deep areas, the brackish-water mud is strongly enriched in organic matter compared to underlying sediments. Bioturbation type changes at the flooding surface in response to the increased sedimentary organic content, but no firm-ground ichnofacies were developed because of low erosion. It is concluded that the base of the brackish-water mud is a robust allostratigraphic bounding surface that is identifiable by the lithologic examination of cores over the Baltic Sea. The surface is a distinct reflector in seismic-acoustic profiles, which facilitates mapping and basin-wide stratigraphic subdivision. Detailed geochronologic studies are required to confirm if sediments immediately overlying the erosional flooding surface in shallow areas are younger than the basal part of the brackish-water mud in deep areas that is predicted to be time-equivalent to the erosion.

The Sokoliska Limestone, a new regional marker horizon of coccolith laminites in the Oligocene of the Outer Carpathians: diagnostic features and stratigraphic position

Isochronous horizons of pelagic coccolith limestones are important regional stratigraphic markers in the Oligocene sequences of the Outer Carpathians. Three widely recognized limestone horizons are the Tylawa Limestones, the Jaslo Limestone and the Zagorz Limestone horizons. Another regional marker horizon is described here, the Sokoliska Limestone horizon, situated between the Jaslo Limestone and Zagorz Limestone horizons in the nannoplankton NP24 Zone. Its lithologic characteristics allow it to be distinguished from the other limestones, especially under the optical microscope. Its dark laminae are greatly enriched in non-calcareous material. The limestone contains tests of planktonic foraminifers, but these are much less abundant than in the Jaslo Limestone. The boundaries with the marly shales above and below are often more gradational than is the case with the other limestone horizons. The Sokoliska Limestone has been recognized over a distance of ca. 550 km in the Skole (Tarcau) and Silesian units of the eastern and northern parts of the Outer Carpathians. Four sections of the Sokoliska Limestone horizon in the Polish and Romanian Carpathians are described; the exposure in the Sokoliska cliff of the Solinka River at Bukowiec was selected as the reference locality.

The Old Fort Point Formation: Redefinition and formal subdivision of a distinctive stratigraphic marker in the Neoproterozoic Windermere Supergroup, southern Canadian Cordillera

Abstract The southern Canadian Cordillera strata of the Neoproterozoic Windermere Supergroup form an areally extensive outcrop belt of deep-marine sedimentary rocks. Within this generally monotonous pile of siliciclastic and minor carbonate rocks, the Old Fort Point Formation forms a lithologically and geochemically distinctive unit that serves as a key regional stratigraphic marker. New sedimentologic and stratigraphic work demonstrates that it is lithologically distinctive, mappable and correlatable on a regional scale and deserves formal recognition. The Old Fort Point Formation comprises three lithostratigraphic members that have a consistent stratigraphic relationship across the basin and can easily be distinguished from lithofacies in the enveloping strata of the Windermere Supergroup. The basal Temple Lake Member is composed primarily of siltstone to mudstone that grades upward into rhythmically interstratified limestone-siltstone. The middle Geikie Siding Member is a thin, organic-rich mudstone-pelite. The Whitehorn Mountain Member is the uppermost unit and varies locally and regionally in thickness and lithology, including diamictite, breccia to conglomerate, mudstone to siltstone, subarkose, quartzarenite, calcareous arenite, arenaceous limestone, and limestone. This unique lithostratigraphic unit is here formally named the Old Fort Point Formation and other site-specific names should be discontinued. The use of the name Old Fort Point Formation is an attempt to simplify part of a complicated and informally defined stratigraphic nomenclature currently in use for rocks in the Neoproterozoic Windermere Supergroup, southern Canadian Cordillera.

SHRIMP U–Pb zircon age of tephra identified in the Yahe section of Liushan Basin and its significance as a regional stratigraphic marker

Three tephra layers in the middle part of the Yahe section have been discovered in Liushan Basin in Nanzhao County, Henan Province. This paper presents zircon U–Pb ages measured on the sensitive high-resolution ion microprobe (SHRIMP)-II from zircons separated from one of these tephra layers. Thirteen analyses yielded a 206Pb/238U weighted mean age of 237 ± 2.5 Ma (95 % confidence, Mean Square of Weighted Deviates = 0.72). This age suggests that the Yahe section strata were deposited as early as the Middle Triassic. The new SHRIMP U–Pb data on the tephra from the Yahe section provides strong age constraints on the Triassic stratigraphic subdivision in Liushan Basin. By comparing the reported ages for similar tephra exposed around Liushan Basin, we infer that the tephra layers may be the sedimentary response to the first episode of the Indosinian tectonism and can be used as a marker bed for regional stratigraphic correlation.

Sedimentological constraints on the evolution of the Cordilleran arc: New insights from the Sonsela Member, Upper Triassic Chinle Formation, Petrified Forest National Park (Arizona, USA)

The Sonsela Member of the Upper Triassic Chinle Formation in Petrified Forest National Park, Arizona, forms a distinctive part of an extensive fluviolacustrine complex that blanketed the Colorado Plateau region and surrounding areas. Upper Triassic sediments were deposited within a continental backarc setting, synchronous with the development of the Cordilleran magmatic arc. Although the Chinle Formation was derived from a broad variety of source terranes, petrographic and geochemical data clearly fingerprint the Cordilleran arc as a region important to provenance. The Sonsela Member exhibits a stratigraphically abrupt change in depositional style and sedimentary architecture that likely reflects the dynamics of the evolving backarc fluvial system and syndepositional arc magmatism. The stratigraphic boundary between the two disparate depositional styles is marked by a laterally persistent red silcrete horizon that is also associated with a biostratigraphically significant faunal turnover. The lower Sonsela Member—a coarse-grained assemblage dominated by persistent sheet sandstone and conglomerate bodies—reflects a coarse-grained progradation of sediment that suggests low rates of dynamic subsidence and high rates of sediment input. The upper Sonsela Member—characterized by an overall finer-grained assemblage and ribbon sand bodies with an increased proportion of encasing mudstone—reflects an increasing rate of basin subsidence that likely persisted throughout the deposition of the overlying Petrified Forest Member. These packages reflect a sharp change from lower- to higher-sinuosity river systems that closely coincides with the increased rate of subsidence across the regional stratigraphic marker. Previous studies have suggested the importance of the silcrete horizon as a key biostratigraphic marker associated with a significant faunal turnover. This study supports the hypothesis that this horizon may represent a regional unconformity as opposed to local channeling, perhaps the product of an abrupt change in basin geometry followed by an increase in basin subsidence. Because the Sonsela Member is the stratigraphically lowest unit of the Chinle Formation dominated by volcanic clastic detritus on the Colorado Plateau, architectural analysis of the Sonsela Member serves as the most reliable indicator of short-term fluctuations in arc dynamics of the poorly preserved and disjointed Mesozoic portion of the Cordilleran magmatic arc.

The Campanian Ignimbrite/Y5 tephra layer – A regional stratigraphic marker for Isotope Stage 3 deposits in the Lower Danube region, Romania

A number of volcanic ash layers embedded in Pleistocene loess and fluvial deposits of the lower Jiu and Olt valleys in south-western Romania have been analyzed for glass chemical compositions and general chronological relations. The WDS-EPMA chemical analyses document remarkably similar major and minor element compositions at the four investigated sites, and total alkali–silica diagrams ascribe these volcanic ashes a phonolite to trachyte affinity. A comparison with the Quaternary Carpathian volcanism chemical data refutes this volcanic field as the primary source, even though earlier works have tentatively taken some of these volcanic ashes as evidence of volcanic activity in this particular field. Instead, the chemical data suggest an origin in the Campi Flegrei volcanic province in Italy, and a comparison with chemical data from proximal and distal outcrops of the Campanian Ignimbrite/Y5 tephra confirm that the ash layers presented here are distal occurrences of this widespread tephra layer. The Campanian Ignimbrite/Y5 tephra is the product of one of the largest late Quaternary explosive events in the Mediterranean, and is consistently dated elsewhere to about 39–40 ka. It is one of the most important chronologic/stratigraphic markers of western Eurasia that provide an independent basis for establishing age–depth relationships for the embedding deposits. Optical luminescence age estimates obtained at two of the investigated sites further confirm that these volcanic ash occurrences represent a regionally synchronous depositional event, around 40 ka. These results highlight the important role that the Campanian Ignimbrite/Y5 tephra could play as marker horizon at sub-continental scale, providing that more potential tephra locations such as these from the Pleistocene sedimentary deposits in the Lower Danube region are chemically and chronologically constrained.

A review and analysis of varve thickness records from glacial Lake Ojibway (Ontario and Quebec, Canada)

Varve thickness records from glacial Lake Ojibway provide a detailed picture of ice margin dynamics and paleohydrology for the Laurentide Ice Sheet during the early Holocene. The demise of Lake Ojibway occurred when an ice dam of the shrinking Laurentide Ice Sheet failed, perhaps releasing enough freshwater to impede Atlantic meridional overturning circulation and precipitate the 8200 cal BP climatic cooling event. Previously published records and new information from lacustrine sediment cores are combined to reconstruct a varve record over 2100 years long. Between varves numbered 1 to 1500, average rates of ice margin retreat increased from 140 m/y to as high as 400 m/y. Beginning prior to varve year 1800, and lasting to around year 2000, ice readvanced along a front at least 300 km wide during an episode called the Cochrane. The total distance of the advance was at least 75 km, and averaged around 200 m/y. During the advance, a series of ∼50 anomalously thick varves were deposited, the cause of which is uncertain. A widespread unconformity occurred during the end of the Cochrane advance, which was followed at around varve year 2060 by the return of varves, which thicken abruptly. Unlike previous varves, these varves include ice-rafted pellets of Lake Ojibway sediment. These varves may have resulted from rapid melting and calving, rather than a second advance, and may relate to basin re-filling following a lowstand. There are no direct dates on the Lake Ojibway varves, but a sudden increase in varve thickness at year 1528 is a regional stratigraphic marker that may have resulted from glacial sediment and meltwater (including Lake Agassiz overflow), diverted from routes to the Lake Superior basin, into the Lake Ojibway basin. Dates from the Lake Superior basin suggest that the diversion of this water occurred at around 9040 cal BP. Applying this date to varve 1528 constrains the age of the Lake Ojibway varves numbered 1 to 2100 to between 10,570 and 8470 (±200) cal BP.