Chronostratigraphic Control Research Articles

Triassic-Early Jurassic evaporites of the Saharan Platform, Algeria: Astronomical and geodynamic constraints on stratigraphy and sedimentation

The sequence stratigraphy of the Late Triassic - Early Jurassic evaporites of the Berkine Basin is described. Disconformities occur between all the major evaporitic units but lack of biostratigraphy (or other) chronostratigraphic control precludes their precise dating. The S4 and S3 Halites are predominantly non-marine halites deposited in low-lying salinas with a barrier to the north. The top of the S4 depositional sequence is marked by the D2, usually regarded as the Tr-J boundary in the Berkine Basin. Both the S3 and S4 salt deposits thin rapidly to the south-east and are thicker in the basin centre coincident with a subcrop of Carnian? aged volcanics. A combination of thermal cooling, rifting and reactivation of N–S lineaments parallel to the Hassi Messaoud-El Biod Arch controlled the (∼1500 m thick) depocenter.Time series analysis shows that astronomical forcing played a key role in the deposition of the S3 and S4 bedded halites. The prevailing climate was monsoonal with major replenishment of the basin indicated by long eccentricity cycles (405 kyr). Sedimentation rates were estimated using eCOCO analysis with average rates of 15 cm/kyr. Well to well comparisons shows that in marginal areas thinner sedimentary sequences relate to slower accumulation rates and periods of non-deposition or deflation. The pattern is similar in the S4 and S3 halite, but the reduced mud content and amalgamation of halite beds suggests a more arid climate in S3 times.The S1+S2 unit marks the first widespread deposition of sulphate in the basin. The lowermost anhydrite beds of the S1 + S2 rest unconformably on the underlying S3 and overstep the basin margins in the south-east; the sequence is capped by the B Horizon a basin-wide carbonate shelf deposit about 25 m thick indicating increased marine influence. Above there is a rapid return to thinly bedded mudstone-halite dominated sedimentation (Lias Salifère) which is overlain by the Lias Anhydritique an alternating sequence of halite and anhydrite deposits. Astronomical parameters of the whole sequence indicate an average sedimentation rate of ∼10 cm/kyr in this marine-influenced section, slower than the halite units. Although the time series analysis cannot provide precision dating of the evaporitic sequences the results indicate that there are important breaks in the depositional record. The combined S4 and S3 halites account for 4.75 Ma and the rest of the Liassic 9.4 Ma. It seems clear that much of the depositional record is missing.These Saharan Platform basins bear much in common with other western Mediterranean evaporite basins. Many show the same overall pattern of sedimentation with increased sulphate deposition above the Tr-J boundary. In late Triassic time they formed a contiguous low-lying zone flanked by cratonic highlands. This zone spanned the Gondwana-Laurussia boundary immediately prior to its break-up and Greater Adria formed a barrier between these basins and the developing Neotethys to the east. The major changes seen in the Saharan Platform are mirrored by the break-up of Adria and the separation of Gondwana and Laurasia and the ultimate connection of the western Mediterranean and the central Atlantic.Although the cyclostratigraphy can tell us much about the absolute duration of sedimentation, the lack of significant anchor points forbids the construction of a detailed chronostratigraphic framework. In particular, the precise location of the Tr-J boundary remains unclear. Although it is usually placed at the base of the D2 Horizon, biostratigraphical evidence from Southern Tunisia places the S3 in the Norian. The sharp contrast between the S4 and S3 halites with the later evaporite sequence suggests that positioning the boundary between the S3 halite and the S1 + S2 unconformity should be given consideration.

Application of elemental chemostratigraphy to refine the stratigraphy of the Adavale Basin, Queensland

A prerequisite to understanding the evolution and resource potential of a basin is to establish a reliable stratigraphic framework that enables the correlation of rock units across multiple depocentres. Establishing a stratigraphic model for the Adavale Basin is challenging due to its structurally complexity, lack of well penetration and its lateral changes in facies. Biostratigraphy appears broad-scale, and despite providing chronostratigraphic control for the Lower Devonian Gumbardo Formation when combined with U/Pb zircon geochronology, the rest of the Devonian succession is hampered by a lack of microfossil assemblages and their poor preservation. The aim of this study is to establish an independent chemostratigraphic correlation across the Adavale Basin using whole rock inorganic geochemistry. Within this study, a total of 1489 cuttings samples from 10 study wells were analysed by Inductively Coupled Plasma – Optical Emission Spectrometry and Inductively Coupled Plasma – Mass Spectrometry for whole rock geochemistry, in order to establish an independent chemostratigraphic zonation scheme. Based on key elemental ratios selected to reflect changes in feldspars, clay minerals and provenance, the Devonian-aged stratigraphy is characterised into four chemostratigraphic mega-sequences that encompass the Gumbardo Formation (Mega-sequence 1); the Eastwood Formation, the Log Creek Formation and the Lissoy Sandstone (Mega-sequence 2); the Bury Limestone and the Boree Salt formations (Mega-sequence 3); and the Etonvale and the Buckabie formations (Mega-sequence 4). These mega-sequences have been further subdivided into a series of chemostratigraphic sequences that can be correlated across the study wells, establishing a regional correlation framework.

Ice sheet and palaeoclimate controls on drainage network evolution: an example from Dogger Bank, North Sea

Abstract. Submerged landscapes on continental shelves archive drainage networks formed during periods of sea-level lowstand. The evolution of these postglacial drainage networks also reveals how past climate changes affected the landscape. Ice-marginal and paraglacial drainage networks on low-relief topography are susceptible to reorganisation of water supply, forced by ice-marginal rearrangement, precipitation and temperature variations, and marine inundation. A rare geological archive of climate-driven landscape evolution during the transition from ice-marginal (ca. 23 ka) to a fully submerged marine environment (ca. 8 ka) is preserved at Dogger Bank, in the southern North Sea. In this study, our analysis of high-resolution seismic reflection and cone penetration test data reveal a channel network over a 1330 km2 area that incised glacial and proglacial lake-fill sediments. The channel network sits below coastal and shallow marine sediments and is therefore interpreted to represent a terrestrial drainage network. When mapped out, the channel form morphology reveals two distinct sets. The first set comprises two low-sinuosity, wide (>400 m) channels that contain macroforms of braid and side bars. These channels are interpreted to have originated as proglacial rivers, which drained the ice-sheet margin to the north. The second set of channels (75–200 m wide, with one larger, ∼400 m wide) has higher sinuosity and forms a subdendritic network of tributaries to the proglacial channels. The timing of channel formation lacks chronostratigraphic control. However, the proglacial rivers must have formed as the ice sheet was still on Dogger Bank, before 23 ka, to supply meltwater to the rivers. Ice-sheet retreat from Dogger Bank led to reorganisation of meltwater drainage and abandonment of the proglacial rivers. Palaeoclimate simulations show a cold and dry period at Dogger Bank between 23 and 17 ka. After 17 ka, precipitation increased, and drainage of precipitation formed the second set of channels. The second set of rivers remained active until marine transgression of Dogger Bank at ca. 8.5–8 ka. Overall, this study provides a detailed insight into the evolution of river networks across Dogger Bank and highlights the interplay between external (climate) and internal (local) forcings in drainage network evolution.

A geochemical and biostratigraphic approach to investigating regional changes in sandstone composition through time; an example from Paleocene–Eocene strata, Taranaki Basin, New Zealand

AbstractA geochemical and biostratigraphic approach has been applied to investigate the spatial and stratigraphic variability of Palaeogene sandstones from key wells in Taranaki Basin, New Zealand. Chronostratigraphic control is predominantly based on miospore zonation, while differences in the composition of Paleocene and Eocene sandstones are supported by geochemical evidence. Stratigraphic changes are manifested by a significant decrease in Na2O across the New Zealand miospore PM3b/MH1 early Eocene zonal boundary, at approximately 53.5 Ma. The change in Na2O is associated with a decrease in baseline concentrations of many other major (MnO, CaO, TiO2) and trace elements, and is interpreted to reflect a significant change in sandstone maturity. Paleocene sandstones are characterized by abundant plagioclase (albite and locally Na–Ca plagioclase), significant biotite and a range of heavy minerals, while Eocene sandstones are typically quartzose, with K-feldspar dominant over plagioclase, low mica contents and rare heavy minerals comprising a resistant suite. This change could reflect a change in provenance from local plutonic basement during the Paleocene Epoch to relatively quartz- and K-feldspar-rich granitic sources during Eocene time. However, significant quartz enrichment of Eocene sediment was also likely due to transportation reworking/winnowing along the palaeoshoreface and enhanced chemical weathering, driven in part by long-term global warming associated with the Early Eocene Climatic Optimum. The broad-ranging changes in major-element composition overprint local variations in sediment provenance, which are only detectable from the immobile trace-element geochemistry.

Chronostratigraphy of the Baringo-Tugen Hills-Barsemoi (HSPDP-BTB13-1A) core – 40Ar/39Ar dating, magnetostratigraphy, tephrostratigraphy, sequence stratigraphy and Bayesian age modeling

The Baringo-Tugen Hills-Barsemoi 2013 drillcore (BTB13), acquired as part of the Hominin Sites and Paleolakes Drilling Project, recovered 228 m of fluvio-lacustrine sedimentary rocks and tuffs spanning a ~3.29–2.56 Ma interval of the highly fossiliferous and hominin-bearing Chemeron Formation, Tugen Hills, Kenya. Here we present a Bayesian stratigraphic age model for the core employing chronostratigraphic control points derived from 40Ar/39Ar dating of tuffs from core and outcrop, 40Ar/39Ar age calibration of related outcrop diatomaceous units, and core magnetostratigraphy. The age model reveals three main intervals with distinct sediment accumulation rates: an early rapid phase from 3.2–2.9 Ma; a relatively slow phase from 2.9–2.7 Ma; and the highest rate of accumulation from 2.7–2.6 Ma. The intervals of rapid accumulation correspond to periods of high Earth orbital eccentricity, whereas the slow accumulation interval corresponds to low eccentricity at 2.9–2.7 Ma, suggesting that astronomically mediated climate processes may be responsible for the observed changes in sediment accumulation rate. Lacustrine transgression-regression events, as delineated using sequence stratigraphy, dominantly operate on precession scale, particularly within the high eccentricity periods. A set of erosively based fluvial conglomerates correspond to the 2.9–2.7 Ma interval, which could be related to either the depositional response to low eccentricity or to the development of unconformities due to local tectonic activity. Age calibration of core magnetic susceptibility and gamma density logs indicates a close temporal correspondence between a shift from high- to low-frequency signal variability at ~3 Ma, approximately coincident the end of the mid-Piacenzian Warm Period, and the beginning of the cooling of world climate leading to the initiation of Northern Hemispheric glaciation c. 2.7 Ma. BTB13 and the Baringo Basin records may thus provide evidence of a connection between high-latitude glaciation and equatorial terrestrial climate toward the end of the Pliocene.

WITHDRAWN: Chronostratigraphy of the Baringo-Tugen-Barsemoi (HSPDP-BTB13-1A) core – 40Ar/39Ar dating, magnetostratigraphy, tephrostratigraphy, sequence stratigraphy and Bayesian age modeling

The Baringo-Tugen-Barsemoi 2013 drillcore (BTB13), acquired as part of the Hominin Sites and Paleolakes Drilling Project, recovered 228 m of fluviolacustrine sedimentary rocks and tuffs spanning a ~3.29–2.56 Ma interval of the highly fossiliferous and hominin-bearing Chemeron Formation, Tugen Hills, Kenya. Here we present a Bayesian stratigraphic age model for the core employing chronostratigraphic control points derived from 40Ar/39Ar dating of tuffs from core and outcrop, 40Ar/39Ar age calibration of related outcrop diatomaceous units, and core magnetostratigraphy. The age model reveals three main intervals with distinct sediment accumulation rates: an early rapid phase from 3.2 to 2.9 Ma; a relatively slow phase from 2.9 to 2.7 Ma; and the highest rate of accumulation from 2.7 to 2.6 Ma. The intervals of rapid accumulation correspond to periods of high Earth orbital eccentricity, whereas the slow accumulation interval corresponds to low eccentricity at 2.9–2.7 Ma, suggesting that astronomically mediated climate processes may be responsible for the observed changes in sediment accumulation rate. Lacustrine transgression-regression events, as delineated using sequence stratigraphy, dominantly operate on precession scale, particularly within the high eccentricity periods. A set of erosively based fluvial conglomerates correspond to the 2.9–2.7 Ma interval, which could be related to either the depositional response to low eccentricity or to the development of unconformities due to local tectonic activity. Age calibration of core magnetic susceptibility and gamma density logs indicates a close temporal correspondence between a shift from high- to low-frequency signal variability at ~3 Ma, approximately coincident the end of the mid-Piacenzian Warm Period, and the beginning of the cooling of world climate leading to the initiation of Northern Hemispheric glaciation c. 2.7 Ma. BTB13 and the Baringo Basin records may thus provide evidence of a connection between high-latitude glaciation and equatorial terrestrial climate toward the end of the Pliocene.

Chronostratigraphic model of a high-resolution drill core record of the past million years from the Koora Basin, south Kenya Rift: Overcoming the difficulties of variable sedimentation rate and hiatuses

The Olorgesailie Drilling Project and the related Hominin Sites and Paleolakes Drilling Project in East Africa were initiated to test hypotheses and models linking environmental change to hominin evolution by drilling lake basin sediments adjacent to important archeological and paleoanthropological sites. Drill core OLO12-1A recovered 139 m of sedimentary and volcaniclastic strata from the Koora paleolake basin, southern Kenya Rift, providing the opportunity to compare paleoenvironmental influences over the past million years with the parallel record exposed at the nearby Olorgesailie archeological site. To refine our ability to link core-to-outcrop paleoenvironmental records, we institute here a methodological framework for deriving a robust age model for the complex lithostratigraphy of OLO12-1A. Firstly, chronostratigraphic control points for the core were established based on 40Ar/39Ar ages from intercalated tephra deposits and a basal trachyte flow, as well as the stratigraphic position of the Brunhes-Matuyama geomagnetic reversal. This dataset was combined with the position and duration of paleosols, and analyzed using a new Bayesian algorithm for high-resolution age-depth modeling of hiatus-bearing stratigraphic sections. This model addresses three important aspects relevant to highly dynamic, non-linear depositional environments: 1) correcting for variable rates of deposition, 2) accommodating hiatuses, and 3) quantifying realistic age uncertainty with centimetric resolution. Our method is applicable to typical depositional systems in extensional rifts as well as to drill cores from other dynamic terrestrial or aquatic environments. We use the core age model and lithostratigraphy to examine the interconnectivity of the Koora Basin to adjacent areas and sources of volcanism.

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.

Detecting eustatic and tectonic signals with carbon isotopes in deep-marine strata, Eocene Ainsa Basin, Spanish Pyrenees

We explore the potential of high-resolution carbon isotope stratigraphy to provide an independent record of global sea-level changes in a 1600-m-thick succession representing ∼5 m.y. of slope sedimentation in the Eocene Ainsa Basin of Spain. The restricted physiographic setting of the basin results in a bulk δ 13 C carb signal that accurately correlates with the coeval eustatic curve from the New Jersey (USA) passive margin. We show that much of the deep-water sediment gravity flow (SGF) deposits are emplaced during eustatic lowstands and fine-grained marly intervals between SGFs correlate with rising and highstand sea levels. However, we also detect a substantial interval of SGF deposition during a sea-level highstand, confirming the nonuniqueness of the controls on clastic deposition. This approach provides a new way to assess the origin of depositional sequences and improve stratigraphic predictions in basins with limited chronostratigraphic control.

Palaeoenvironmental drivers of vertebrate community composition in the Belly River Group (Campanian) of Alberta, Canada, with implications for dinosaur biogeography.

BackgroundThe Belly River Group of southern Alberta is one of the best-sampled Late Cretaceous terrestrial faunal assemblages in the world. This system provides a high-resolution biostratigraphic record of terrestrial vertebrate diversity and faunal turnover, and it has considerable potential to be a model system for testing hypotheses of dinosaur palaeoecological dynamics, including important aspects of palaeoecommunity structure, trophic interactions, and responses to environmental change. Vertebrate fossil microsites (assemblages of small bones and teeth concentrated together over a relatively short time and thought to be representative of community composition) offer an unparalleled dataset to better test these hypotheses by ameliorating problems of sample size, geography, and chronostratigraphic control that hamper other palaeoecological analyses. Here, we assembled a comprehensive relative abundance dataset of microsites sampled from the entire Belly River Group and performed a series of analyses to test the influence of environmental factors on site and taxon clustering, and assess the stability of faunal assemblages both temporally and spatially. We also test the long-held idea that populations of large dinosaur taxa were particularly sensitive to small-scale environmental gradients, such as the paralic (coastal) to alluvial (inland) regimes present within the time-equivalent depositional basin of the upper Oldman and lower Dinosaur Park Formations.ResultsPalaeoenvironment (i.e. reconstructed environmental conditions, related to relative amount of alluvial, fluvial, and coastal influence in associated sedimentary strata) was found to be strongly associated with clustering of sites by relative-abundance faunal assemblages, particularly in relation to changes in faunal assemblage composition and marine-terrestrial environmental transitions. Palaeogeography/palaeolandscape were moderately associated to site relative abundance assemblage clustering, with depositional setting and time (i.e. vertical position within stratigraphic unit) more weakly associated. Interestingly, while vertebrate relative abundance assemblages as a whole were strongly correlated with these marine-terrestrial transitions, the dinosaur fauna does not appear to be particularly sensitive to them.ConclusionsThis analysis confirms that depositional setting (i.e. the sediment type/sorting and associated characteristics) has little effect on faunal assemblage composition, in contrast to the effect of changes in the broader palaeoenvironment (e.g. upper vs. lower coastal plain, etc.), with marine-terrestrial transitions driving temporal faunal dynamics within the Belly River Group. The similarity of the dinosaur faunal assemblages between the time-equivalent portions of the Dinosaur Park Formation and Oldman Formation suggests that either these palaeoenvironments are more similar than characterized in the literature, or that the dinosaurs are less sensitive to variation in palaeoenvironment than has often been suggested. A lack of sensitivity to subtle environmental gradients casts doubt on these forces acting as a driver of putative endemism of dinosaur populations in the Late Cretaceous of North America.Electronic supplementary materialThe online version of this article (doi:10.1186/s12898-016-0106-8) contains supplementary material, which is available to authorized users.

The Miocene mammal record of the Vallès-Penedès Basin (Catalonia)

The land mammal record of the Vallès-Penedès Basin (Catalonia, NE Spain) ranges from the early Miocene (Ramblian) to the late Miocene (Turolian), that is from about 20 to 7Ma. Here we present an updated review of the mammal succession focusing on biochronology as well as on environmental and faunal changes. Based on faunal similarities with central Europe, we interpret this basin as a transitional zone between the forested environments of northern regions and the more arid landscapes of the inner Iberian Peninsula. The quality of the Vallès-Penedès record and its chronostratigraphic control is clearly better for the late Aragonian and the Vallesian (between 12.6–9.0Ma), especially for small mammals. Therefore, we analyze small mammal diversity dynamics during this interval. Contrary to previous analyses, which found an abrupt extinction event coinciding with the early/late Vallesian boundary (the Vallesian Crisis), our results show that this pattern is due to uneven sampling. Instead, taxonomic richness slowly decreased since the late Vallesian as a result of a series of extinctions that mostly affected forest-dwelling taxa.

Sedimentology of the Lower Cretaceous reservoirs of the Sea Lion Field, North Falkland Basin

The Sea Lion Field in the North Falkland Basin was discovered in 2010 by Well 14/10-2, which tested oil from Lower Cretaceous sands of the Sea Lion fan complex. Appraisal of the field involved the drilling of a further eight wells from which an extensive wireline-log data set and 455 m of core were acquired. This unique data set has allowed detailed study of the reservoir sedimentology and has significantly benefited reservoir characterization. The reservoirs are interpreted to be deposited in a deep-water lacustrine basin as base-of-slope to basin-floor fans fed by canyons or channel-feeder systems from the east. Deposition is considered to have been affected both by turbidity currents and by liquefied sediment gravity flows, such that reservoir facies are dominated by those associated with high- and low-density turbidites, mass-flow deposits, and, to a lesser extent, hybrid-event beds. In the absence of significant chronostratigraphic control, the description of the first-order reservoir architecture is primarily driven by seismic interpretation, whilst the core affords significant control of the internal architecture of individual fans. Several lines of evidence point towards the Sea Lion sands being derived from a coexisting, shallow-water system on the eastern margin of the basin. Palaeogeographical reconstruction links this shoreface system to a large, contemporaneous, southerly-prograding delta via wave reworking and longshore drift of delta-front sands. Evolution of this delta has played a significant role in the timing, positioning and emplacement of fans along the eastern margin of the North Falkland Basin. The overall impact of the core data set has been to improve confidence in the reservoir characterization and spatial distribution of facies within the reservoir model, and to narrow significantly the range of uncertainty, which has direct implications for field development planning.

Landscape distribution and ecology of Plio-Pleistocene avifaunal communities from Lowermost Bed II, Olduvai Gorge, Tanzania

Plio-Pleistocene avifaunal communities are used to reconstruct Lowermost Bed II landscapes at the early hominin site of Olduvai Gorge, Tanzania. These deposits are laterally extensive, have strong chronostratigraphic control, and were excavated using a landscape archaeological approach. Such factors allow for horizontal spatial-correlation of avian communities across the paleolandscape over a geologically short time frame (approximately 65,000 years). Lowermost Bed II avifaunal communities point to an extensive freshwater wetland system across the extent of paleo-Lake Olduvai's eastern margin.

U-Pb Geochronology of the Blake River Group, Abitibi Greenstone Belt, Quebec, and Implications for Base Metal Exploration,

The 2704 to 2695 Ma Blake River Group in the southern Abitibi greenstone belt comprises a well preserved submarine volcanic sequence that hosts a large number of VMS and important Au-rich VMS deposits, including the world-class Horne and LaRonde-Penna deposits. Establishing precise chronostratigraphic control on the VMS deposits within the Blake River Group is critical because numerous distinct events took place within a period of 9 m.y. Nineteen new high-precision U-Pb ages temporally constrain the host rocks of many polymetallic VMS deposits and associated synvolcanic intrusions, demonstrating that these VMS deposits formed throughout the protracted volcanic evolution of the entire group. Ages on host rocks of the Horne (2702.2 ± 0.9 Ma), Quemont (2702.0 ± 0.8 Ma), and Fabie (2701.9 ± 0.9 Ma) deposits reveal that they are among the oldest VMS deposits in the Blake River Group. The giant Horne Au-rich VMS deposit had already formed when the Cu-Zn deposits of the Noranda mine sequence, including Millenbach and Amulet, were generated at ~2698 Ma and is thus unrelated, consistent with its different volcanological setting and metal content. Large Au-rich VMS deposits of the Bousquet Formation, including LaRonde Penna and Bousquet 2-Dumagami, were formed at 2698 to 2697 Ma and are distinctly younger than the Horne and Quemont deposits. There were, therefore, two major time-stratigraphic intervals within the Blake River Group that were favorable for the formation of Au-rich VMS deposits. Rhyolite hosting the large Bouchard-Hebert VMS deposits yielded an age of 2695.8 ± 0.8 Ma. Important mineralizing events in the Blake River Group occur at ca. 2-m.y. intervals apart and are associated with major magmatic episodes. Recognition of specific time-stratigraphic intervals for different styles of mineralization and geologic settings is essential to improve exploration models within the Blake River Group and for similar volcanic assemblages elsewhere in the Archean.

Paleomagnetic and chronostratigraphic constraints on the Middle to Late Miocene evolution of the Transylvanian Basin (Romania): Implications for Central Paratethys stratigraphy and emplacement of the Tisza–Dacia plate

From the Oligocene onwards, the complex tectonic evolution of the Africa–Eurasia collision zone led to paleogeographic and biogeographic differentiation of the Mediterranean and Paratethys, two almost land-locked seas, in the area formerly occupied by the western Tethys Ocean. Episodic isolation of the basins triggered strong faunal endemism leading to the introduction of regional stratigraphic stages for the Paratethys. Chronostratigraphic control on the Paratethys stages remains rudimentary compared to the cyclostratigraphically constrained Mediterranean stages. This lack of chronostratigraphic control restricts the insight in the timing of geodynamic, climatic, and paleobiogeographic events and thereby hinders the identification of their causes and effects. In this paper, we here derive better age constraints on the Badenian, Sarmatian and Pannonian Central Paratethys regional stages through integrated 40Ar/39Ar, magnetostratigraphic, and biostratigraphic research in the Transylvanian Basin. The obtained results help to clarify the regions Middle Miocene geodynamic and paleobiogeographic evolution. Six new 40Ar/39Ar ages were determined for tuffs intercalating with the generally deep marine basin infill. Together with data from previous studies, there is now a total of 9 radio-isotopically dated horizons in the basin. These were traced along seismic lines into a synthetic seismic stratigraphic column in the basin center and serve as first order tie-points to the astronomically tuned Neogene timescale (ATNTS). Paleomagnetically investigated sections were treated similarly and their polarity in general corroborates the 40Ar/39Ar results. The integrated radio-isotopic and magnetostratigraphic results provide an improved high-resolution time-frame for the sedimentary infill of the Transylvanian Basin. Early Badenian deep water sedimentation is characterized by accumulation of the Dej Tuff Complex in response to a period of intensive volcanism, the onset of which is constrained between the first occurrence (FO) of Orbulina suturalis at 14.56Ma and 14.38±0.06Ma. During the subsequent Badenian Salinity Crisis (BSC) up to 300m of salt accumulate in the basin center. The faunal turnover that marks the Badenian–Sarmatian Boundary is dated at 12.80±0.05Ma. A second phase of intense volcanism occurs at 12.4Ma and leads to deposition of the middle Sarmatian tuff complex (Ghiriş, Hădăreni, Turda and Câmpia Turzii tuffs). Rates of sediment accumulation strongly diminish in the basin center at the onset of the Pannonian stage coincident with an approximately 20° CW tectonic rotation of the Tisza–Dacia plate. Concurrent enhanced uplift in the Eastern a'nd Southern Carpathians leads to the isolation of the Central Paratethys and triggers the transition from marine to freshwater conditions. An additional Pannonian to post-Pannonian 6° of CW rotation is related to the creation of antiform geometries in the Eastern Carpathians which are notably larger in the north than in the south. An 8.4Ma age is determined for the uppermost Pannonian sediments preserved in the central part of the Transylvanian Basin. Two sections belonging to middle Pannonian Zone D, and the lower part of Zone E (Subzone E1) are found to cover the 10.6–9.9Ma time-interval.

A medial to distal volcaniclastic record of an andesite stratovolcano: detailed stratigraphy of the ring-plain succession of south-west Taranaki, New Zealand

The >25 ka volcaniclastic ring-plain succession in south-west Taranaki has been remapped to establish a much more detailed understanding of the older stratigraphic record of Mt. Taranaki. Coastal cliff exposures show a range of volcaniclastic lithofacies, including debris-avalanche and lahar deposits, and allow a detailed chronological reconstruction of past volcanic and sedimentary events. Five new debris-avalanche deposits were identified, and their distribution in coastal cross-sections mapped. In addition, four previously described units were renamed and their stratigraphic position and lateral extent redefined. Chronostratigraphic control of the younger (<50 ka) sequence was obtained by radiocarbon dating of wood found within, or peat interbedded with, the deposits. Emplacement ages of the older units were estimated from their stratigraphic position and underlying marine wave-cut surfaces. Overall, at least 14 widespread debris-avalanche deposits occur within the <200 ka ring-plain record of Mt. Taranaki, suggesting one major edifice failure on average every 14,000 years, with an increase in frequency since 40 ka. The stratigraphic reconstruction of the ring-plain succession showed that the same pattern of deposition was repeatedly produced throughout the existence of Mt. Taranaki. Depending on their sedimentological characteristics, the different volcanic and sedimentary lithofacies can be related to phases of edifice-construction or collapse events. Based on the identified cyclic sedimentation pattern, we present a new episodic stratigraphy that integrates existing and new lithostratigraphic units into a coherent chronostratigraphic framework that can be applied to the entire volcanic and volcaniclastic succession at Mt. Taranaki. This model takes into account the complex geological processes that have taken place on the volcano and provides a more uniform stratigraphic terminology that could be applied to repeatedly collapsing stratovolcanoes elsewhere.

Devonian sea-level change in Bolivia: A high palaeolatitude biostratigraphical calibration of the global sea-level curve

During the Devonian Period the cool water Malvinokaffric Realm was located at high palaeolatitudes in the southern hemisphere. The Realm is defined by its highly endemic marine benthic fauna, which makes extra-Malvinokaffric correlations problematic. Investigation of the Devonian palynomorphs from an extensive regional sub-surface dataset (24 wells) in the Chaco Basin, Bolivia, reveals the presence of a number of stratigraphically restricted and regionally correlative epiboles (peak abundances) of the distinctive palynomorphs Bimerga bensonii, Crucidia camirense, Evittia sommeri, Petrovina connata and Ramochitina magnifica. These palynomorph epiboles were then located at outcrop by a high-resolution palynological investigation (225 samples) of two Bolivian outcrop localities in the Chaco Basin: Bermejo, Santa Cruz Department and Campo Redondo, Chiquisaca Department. The important Evittia sommeri epibole in the basal Los Monos Formation is related to a marine transgression that is present in both these outcrop sections. Additional chronostratigraphic control on this marine transgression comes from the occurrence of rare goniatites in the base of the Huamampampa Formation at Campo Redondo, which are at least early Eifelian in age (post Chotĕc Event). Based on the goniatite together with spore data, the marine transgression with Evittia sommeri in the basal Los Monos Formation can be tentatively assigned to the Mid Devonian (mid-late Eifelian) Kačák Event. The presence of a datable Malvinokaffric goniatite has shown that key spore taxa which are used zonally in both Laurussia and Gondwana do not all have coincident first occurrences in both areas. Furthermore, despite the appearance of some cosmopolitan elements in the microflora, Mid and Late Devonian spore assemblages in Bolivia are distinct from other regions in being relatively impoverished in both progymnosperm and lycopod spores. This floral difference is attributable to the Malvinokaffric Realm continuing to retain a distinctive cool climatic signature throughout this interval. However, the sporadic occurrences of extra-Malvinokaffric macrofauna in restricted stratigraphic intervals of the Middle Devonian in South America and South Africa are significant. The oldest known occurrence of the extra-Malvinokaffric brachiopod Tropidoleptus in Bolivia is coincident with the late Eifelian basal Los Monos Formation transgression. Hence there was a relationship between influxes of this fauna and marine transgressions, i.e. temporary periods of reduced climatic gradient. The migration of Devonian spores, and particularly heterosporous spores, between Gondwana and Laurussia clearly occurs during the Mid and Late Devonian. This is at variance with models claiming a wide Rheic Ocean during much of the Devonian.

Testing the limits of Paleozoic chronostratigraphic correlation via high-resolution (&lt;500 k.y.) integrated conodont, graptolite, and carbon isotope ( 13Ccarb) biochemostratigraphy across the Llandovery-Wenlock (Silurian) boundary: Is a unified Phanerozoic time scale achievable?

The resolution and fidelity of global chronostratigraphic correlation are direct functions of the time period under consideration. By virtue of deep-ocean cores and astrochronology, the Cenozoic and Mesozoic time scales carry error bars of a few thousand years (k.y.) to a few hundred k. y. In contrast, most of the Paleozoic time scale carries error bars of plus or minus a few million years (m. y.), and chronostratigraphic control better than +/- 1 m. y. is considered high resolution. The general lack of Paleozoic abyssal sediments and paucity of orbitally tuned Paleozoic data series combined with the relative incompleteness of the Paleozoic stratigraphic record have proven historically to be such an obstacle to intercontinental chronostratigraphic correlation that resolving the Paleozoic time scale to the level achieved during the Mesozoic and Cenozoic was viewed as impractical, impossible, or both. Here, we utilize integrated graptolite, conodont, and carbonate carbon isotope (delta C-13 carb) data from three paleocontinents (Baltica, Avalonia, and Laurentia) to demonstrate chronostratigraphic control for upper Llandovery through middle Wenlock (TelychianSheinwoodian, similar to 436-426 Ma) strata with a resolution of a few hundred k.y. The interval surrounding the base of the Wenlock Series can now be correlated globally with precision approaching 100 k.y., but some intervals (e. g., uppermost Telychian and upper Shein-woodian) are either yet to be studied in sufficient detail or do not show sufficient biologic speciation and/or extinction or carbon isotopic features to delineate such small time slices. Although producing such resolution during the Paleozoic presents an array of challenges unique to the era, we have begun to demonstrate that erecting a Paleozoic time scale comparable to that of younger eras is achievable. (Less)

Diatom-based correlation of early to mid-Pliocene drillcores from the southwestern Ross Sea, Antarctica

The ANDRILL AND-1B drillcore in southern Victoria Land Basin, Antarctica recovered an upper Miocene to early Pleistocene continental shelf stratigraphic section, including a spectacular Pliocene and Pleistocene sequence of alternating diamictite and diatom-rich sediments. This punctuated sequence reflects variation between glacial sediments deposited by ice advance over the site and open-marine diatom productivity and sedimentation. An early Pliocene age for the base of diatom-rich sediment is constrained through integrated diatom biostratigraphy and magnetostratigraphy. The presence of marine diatom Shionodiscus tetraoestrupii from the top of the core to 583.64mbsf indicates the lowest diatom-bearing sediments are younger than 5Ma. This drillcore provides important new chronostratigraphic control for paleoenvironmental changes also recorded in three drillcores from the western coast of the McMurdo Sound area, DVDP-10 and -11 in Taylor Valley and CIROS-2 from the seaward edge of Ferrar Fjord. The refined continental shelf biostratigraphy developed from AND-1B provides a framework for regional correlation to understand better the timing and character of large paleoenvironmental changes in the western Ross Sea that involve the history of the West Antarctic Ice Sheet (WAIS). Multivariate analysis, along with traditional biostratigraphic approaches, enables the correlation and comparison of coeval intervals in these drillcores. A composite stratigraphic sequence from these four cores suggests the early and mid-Pliocene Ross Sea experienced extended intervals of open-marine conditions with minimal sea-ice cover and high diatom production and sedimentation. This new information provides important constraints on Antarctic paleoclimate and ice sheet history during an important interval when global climate was warmer than today. The history preserved in these four drillcores will be an important tool to guide and test future ice sheet and climate models.

Early to mid-Pliocene environmentally constrained diatom assemblages from the AND-1B drillcore, McMurdo Sound, Antarctica

In extended lower to mid-Pliocene stratigraphic record of near-shore environmental variation in the southwestern Ross Sea, Antarctica is preserved in ANDRILL’s McMurdo Ice Shelf Project AND-1B drillcore.A94meter-thick sequence of three diatomite and diatomaceous mudstone intervals represents the highest latitude record (78° S) of persistent open-marine conditions on the Antarctic continental shelf, in response to a protracted period of Pliocene warmth. This core provides new chronostratigraphic control for Antarctic paleoclimatic events, and describes a sequence of distinct diatom assemblages comprised of both extant and extinct species that are unknown on the Antarctic shelf today. Common taxa include those with modern distributions close to the Polar Frontal Zone (e.g. Shionodiscus tetraoestrupii, Stellarima stellaris, Thalassionema nitzschioides), suggesting marine temperatures in the southwestern Ross Sea warmer than at present. Representatives of the modern sea-ice assemblage (e.g. Fragilariopsis curta, Stellarima microtrias) and cold open-marine species associated with the retreating sea-ice front (e.g. Eucampia antarctica var. recta) are also present, but in much lower abundance than in the Ross Sea today as suggested by core top and surface sample data. The co-occurrence of species from these three distinct modern environments in Pliocene sediments, in close proximity to the continent, suggests that modern ecological zonal concepts cannot be applied directly in Pliocene paleoenvironmental analyses. Species present in the 4 m-thick lowest diatomite unit (~4.5 Ma) indicate a cold openmarine environment,with the period of deposition likely comprising the length of one interglacial period. The middle diatomite unit (~4.1-4.3 Ma; 14m) records warmer and perhaps seasonally mixed conditions with often abundant Thalassionema nitzschioides and Shionodiscus tetraoestrupii and little evidence of sea-ice associated species. Documentation of changes in diatom assemblages through the upper 75m-thick diatomite interval (~3.3-3.6Ma) offers the opportunity to resolve environmental limits of Pliocene diatom species thatwill serve as a guide to diatom-based paleoenvironmental interpretations for other Pliocene deposits. This upper unit records deposition during several glacial/interglacial cycles indicating an extended interval with open marine productivity during the mid-Pliocene. In addition, environmental associations for extinct species are inferred by their co-occurrence with extant taxa. Persistent intervals of pure diatomite suggest high productivity in an open-marine setting and minimal to no influence of sea ice. Commonly occurring species can be placed in five ecological cateories; Neritic/Stratified, Warm, Cold/Sea Ice, Heavy/Wind Mixed and Mixed. Comparison of this long diatom-bearing interval with coeval Antarctic deposits presents a regional and continental picture of Pliocene warmth, suggesting East Antarctic Ice Sheet was smaller with sub-polar marginal conditions.