High Paleolatitudes Research Articles

Calcareous nannoplankton fluctuation within the Albian-Cenomanian Boundary Event of the Tethyan Himalaya

A hemipelagic succession 29m thick, situated in South Tibet within the Tethyan Himalaya tectonic unit, has been investigated for its calcareous nannofossil content. A total of 17 samples were subject to qualitative and semi-quantitative analysis. The studied interval belongs to the upper Albian-lowermost Cenomanian and extends into the UC0 nannofossil zone; based on the last occurrence of Hayesites albiensis, the UC0a and UC0b-c subzones were recognized. The most abundant nannofossil of the Youxia section is Watznaueria barnesiae. Other common taxa are Eiffellithus turriseiffelii, Eprolithus floralis, Rhagodiscus spp., and Zeugrhabdotus spp. In the lowermost part of the studied section, below the beginning of the Albian-Cenomanian Boundary Event (ACBE), i.e., prior to the δ13C positive excursion related to OAE1d, the nannofossils confined to high paleolatitudes, namely Repagulum parvidentatum, Seribiscutum primitivum, and Sollasites horticus, are present with a low abundance. This occurrence is believed to be evidence of a short episode of cooler surface waters linked to a transgressive event. The nannofossil abundance and diversity, along with the fluctuation patterns of the nutrient and temperature indices throughout the section, reflects a primary signal of mesotrophic to eutrophic conditions from the base of the succession up to the two oldest δ13C peaks of ACBE, both late Albian in age and within the OAE1d. By contrast, the dominance of Watznaueria barnesiae, representing more than 80% of the total assemblages, along with the significant drop in abundance and diversity shown by nannofossils within late phases of ACBE, are interpreted as a diagenetic signal. Mesotrophic to eutrophic conditions returned towards the top of the studied succession, where Biscutum constans and Zeugrhabdotus erectus again show a higher abundance.

Docodontans from the Lower Cretaceous of Yakutia, Russia: New insights into diversity, morphology, and phylogeny of Docodonta

Docodonta are the most abundant and taxonomically diverse group of mammaliaforms in the high paleolatitude Lower Cretaceous Teete locality in Yakutia, Russia. Docodontans are represented by the tegotheriids Khorotherium yakutense and Ergetiis ichchi gen. et sp. nov., and possibly two more taxa known from fragmentary specimens (Docodonta indet.). Ergetiis ichchi gen. et sp. nov. shows a reduction and posterior shift of the angular process of the dentary which could indicate a reduction of the mandibular middle ear potentially in a way analogous to the transitional mammalian middle ear of Eutriconodonta. A revised phylogenetic analysis of Docodonta suggests a stemward position of Docodontidae and a sister group relationship for Simpsonodontidae and Tegotheriidae.

High‐Latitude Geomagnetic Secular Variation at the End of the Cretaceous Normal Superchron Recorded by Volcanic Flows From the Okhotsk‐Chukotka Volcanic Belt

AbstractThe Cretaceous Normal Superchron (CNS, 84–121 Ma) is a singular period of the geodynamo's history, identified by a prolonged absence of polarity reversals. To better characterize the paleosecular variation (PSV) of the geomagnetic field at the end of this interval, we sampled seven continuous sequences of lava flows from the Okhotsk‐Chukotka Volcanic Belt, emplaced 84–89 Ma in the vicinity of the Kupol ore deposit (NE Russia). From a collection of 1,024 paleomagnetic cores out of 82 investigated lava flows, we successfully determined the paleodirections of 78 lava flows, which led to 57 directional groups after removing the serial correlations. The resulting paleomagnetic pole is located at 170.0°E, 76.8°N (A95 = 5.2°, N = 57), in good agreement with previous estimates for north‐eastern Eurasia. Aiming at quantifying PSV at a reconstructed paleolatitude (λ) of ∼80°N, we obtained a virtual geomagnetic pole (VGP) scatter , the value of which was corrected for within‐site dispersion and is little dependent on the choice of the selection criteria. Compared to previous paleodirectional data sets characterizing PSV at various paleolatitudes during the CNS, our Sb estimate confirms a relative latitudinal increase Sb(λ = 90°)/Sb(λ = 0°) on the order of 2–2.5. Focusing on PSV at high paleolatitude within the 70°–90° range, we show that Sb was ∼15% lower at the end of the CNS than during the past 10 Myr, confirming that the singular polarity regime of the geodynamo observed during the CNS is likely accompanied with reduced PSV.

Paleosols of the Maastrichtian dinosaur-bearing Chorrillo Formation (southern Patagonia, Argentina): Paleoenvironmental and paleoclimate implications

The Maastrichtian dinosaur-bearing Chorrillo Formation in southern Patagonia (⁓50° S, Austral-Magallanes Basin, Argentina) is a pedogenically modified fluvial succession, which records sediment deposition at mid–high paleolatitudes in the Southern Hemisphere. In order to reconstruct the paleoenvironment and paleoclimates for the Chorrillo Formation, we performed a paleopedological study (abiotic components) of the unit within a well-defined sedimentological–paleontological context, and considering new paleobotanical data of the unit. Using detailed macro and micromorphological features and clay mineralogy of the paleosols, we show that the Chorrillo Formation paleosols are overall smectite-rich soils with vertic and redoximorphic features (i.e., moderately developed hydromorphic Vertisol-, calcic Vertisol-, poorly developed hydromorphic Vertisol-, Histosol-, and argillic Vertisol-like paleosols). The small-scale or high-frequency stacking of such paleosols indicates that they developed under different hydrologic conditions, and subtle differences in grain-size (parent material) and topographic relief on a distal floodplain. Conversely, the large-scale or small-frequency vertical stacking of different paleosols is linked to avulsion processes. Paleobotanical remains through the Chorrillo succession demonstrates different ecological requirements for the inhabited part of the fluvial floodplain. Abiotic and biotic climate proxies suggest that these paleosols formed under a broadly temperate–warm and seasonally humid climate. Overall, these combined data record environmental and climatic conditions during the uppermost Cretaceous, and preserve a record of Maastrichtian terrestrial conditions in the Southern Hemisphere.

Using Constrained Optimization (CONOP) to examine Ordovician graptolite distribution and richness from the Central Andean Basin and their comparison with additional data from North America and Baltoscandia

Ordovician graptolite fossils represent one of the first abundant, well-documented records of marine zooplankton. An accurate assessment of their stratigraphic ranges, biogeography, and taxon richness is crucial to studies on the Great Ordovician Biodiversity Event. Unfortunately, our understanding of graptolite richness from high paleo-latitudes is poor and recent work has demonstrated that many high paleo-latitude taxa are often misidentified endemic species. Hence, high paleo-latitude localities likely contain a hidden graptolite richness.This work uses CONOP9 to construct a range chart and correlation model from the graptolite occurrences at thirty-eight Ordovician stratigraphic sections from the Central Andean Basin of South America, and parts of Laurentia and Baltoscandia. Thirteen graptolite biozones spanning the early Tremadocian (Tr1)–middle Dapingian interval (Dp2) are identified. Some taxa in the Central Andean Basin composite exhibit unusual ranges and occur in biostratigraphically unexpected positions in the composite range chart. In order to examine the possible causes of these unusual ranges, sections from Baltica and Laurentia were added to the analyses. These additional data facilitated results that were more consistent with the generally accepted positions of certain taxon.The first raw taxon richness curve of graptolites from the Central Andean Basin obtained with CONOP9 shows a diversification phase in the early Floian that continues to a peak in the late early Floian. A second peak occurs at the beginning of the middle Floian. A decreasing trend is observed from the top of the Didymograptellus bifidus Biozone to the “Isograptus victoriae” Biozone. These results generally agree with the graptolite richness curves published for the same interval in Baltica, Australasia, and the Yangtze region of SW China, but differs from the patterns observed for the Jiagnan and Yiyang regions of SE China. The rarified regional taxon richness curve, obtained with CONOP64, is also in agreement with global curves previously proposed by different authors.

Astrochronology and carbon-isotope stratigraphy of the Fengcheng Formation, Junggar Basin: Terrestrial evidence for the Carboniferous-Permian Boundary

The Carboniferous–Permian Boundary (CPB) is a major stratigraphic boundary and a marker for global stratigraphic correlation. However, the delimitation and global correlation of the CPB is hindered by the erosion during the Late Paleozoic Ice Age. Several carbonate platforms show a negative carbon isotope excursion (NCIE) in response to the CPB, which has great potential for the delimitation of this boundary. However, the response of terrestrial profiles to the CPB is unclear, and the global synchroneity of this NCIE is controversial. We studied a high paleolatitude section to evaluating the feasibility of the NCIE for delimitating CPB. We used astrochronological analysis of the 340 m Gamma Ray log of the Fengcheng Formation in Mahu Sag, Junggar Basin, northwest China, where the CPB may have been recorded. We constructed a high-resolution chronostratigraphic framework for the Fengcheng Formation, which precisely defines the chronostratigraphic attribution of the Fengcheng Formation as the Gzhelian–Asselian, and confirms that the CPB is recorded in the Fengcheng Formation. In addition, a global correlation of carbon isotope profiles revealed a negative excursion in terrestrial carbon isotope profiles across the CPB, which is consistent with the characteristics of NCIEs in several marine profiles. The similarity of the NCIE records supports their global synchrony and confirms that they can be used to delimit the CPB in terrestrial sections. Combining the astronomical time scale and the carbon isotope profile, we constrain the position of the CPB to the depth range of 4694.84–4703.56 m, with 4698.9 m as a preliminary estimate of the CPB. This new astronomical timescale and the stratigraphic position of the CPB enable the revision of the chronostratigraphic attribution of the terrestrial strata in the Junggar Basin. Overall, our results contribute to studies of the Carboniferous–Permian global carbon cycle, high-latitude glaciation, eustatic fluctuations, global volcanism, and their potential relationships.

A new typothoracine aetosaur (Archosauria, Pseudosuchia) from the Upper Triassic of India with insights on biostratigraphy, diversification, and paleobiogeography

ABSTRACT A new typothoracine aetosaur is described based on multiple isolated and articulated left paramedian and lateral osteoderms recovered from the Upper Triassic lower Dharmaram Formation of India. The partial carapace of the new taxon is reconstructed as strongly discoidal based on the curvature of the paramedian osteoderms with the widest one positioned dorsal to the mid-dorsal trunk vertebra. Asymmetric lateral osteoderms with acute flexion are considered as precaudals with the angle of flexion decreasing posteriorly. Phylogenetic analysis recovered the new taxon as deeply nested within the clades Typothoracinae and Paratypothoracini, and a sister taxon to Kocurypelta silvestris. The autapomorphic characters involving paramedian osteoderms dorsal to the trunk vertebrae include dorsal surface ornamented by large, irregular pits surrounding the dorsal eminence and radiating ridges in other areas, straight anterior margin of the anteromedial corner of the anterior bar in dorsal view, and raised or ridged and ornamented posteromedial corner. The current study highlights the significance of the new aetosaur, and the age of the lower Dharamaram Formation is modified here as mid-Norian to Rhaetian based on global correlation with other coeval horizons. The recovery of this taxon marks the first record of Paratypothoracini from high paleolatitudes of the Gondwanan region. The study corroborates the earlier findings of a strong Laurasian faunal influx in India during the Late Triassic suggesting possible land bridges and/or conducive environmental conditions for faunal dispersal. http://zoobank.org/urn:lsid:zoobank.org:pub:FCC37C0E-107F-4B15-9460-4E802B465865

LITHOLOGY CONTROLS AMMONOID SIZE DISTRIBUTIONS

ABSTRACT Body-size distributions of organisms across environments in space and time are a powerful source of information on ecological and evolutionary processes. However, most studies only focus on selected parameters of size distributions (e.g., central tendency or extremes) and rarely take into account entire distributions and how they are affected by the collection style and facies. Here we analyze the impact of facies, region, taxonomy, and collection style over size distributions using diameter as a proxy of Late Devonian ammonoids in their entirety using non-metric multidimensional scaling and PERMANOVA based on Kolmogorov distance. The effects are then compared with effects on mean sizes. In all analyses, lithology was the dominant effect, with sizes greater by 59% in marls and by 33% in limestones, as compared to black shales. The effect of complete sampling style was a decrease in size by 11%. Kurtosis was an important parameter differentiating size distributions, with platykurtic distributions in marls and leptokurtic distributions in limestones, suggesting that this parameter may reflect different degrees of time averaging. Most size distributions were positively skewed, but most strongly in marls. Complete sampling led to skewness values close to zero (symmetrical distributions) and high kurtosis. Samples from higher paleolatitudes were on average smaller, but contained outliers with the largest sizes, highlighting the need to analyze entire distributions. Lithology and collection differences need to be accounted for when evaluating size differences across space (polar gigantism) and time (Lilliput effect). Similarly, differences in facies may affect species determination.

Paleomagnetic Constraints From 925 Ma Mafic Dykes in North China and Brazil: Implications for the Paleogeography of Rodinia

AbstractCoeval Neoproterozoic mafic dykes with a well‐constrained isotopic age of 925 Ma are distributed in the North China Craton (NCC) of China and São Francisco Craton (SFC) of Brazil. Several recent studies favor the hypothesis that these two cratons were connected during 925 Ma and constituted building cratons for the supercontinent Rodinia. However, the paleo‐positions of the two cratons in Rodinia have not yet been paleomagnetically resolved. This paper presents the paleomagnetic results of these dykes in both NCC and SFC. Detailed thermal and alternating‐field demagnetization revealed that these dykes record stable components of magnetization with unblocking temperatures below the Curie temperature of magnetite. Rock magnetic results and scanning electron microscope analysis further confirm that the major magnetic minerals in dyke samples are pseudo‐single domain titanomagnetites. The mean directions of the characteristic remanent magnetization (ChRM) of the dykes are distinct from those of rocks of different ages in the sampling localities, and are supported by positive baked contact tests, indicating primary remanent magnetization. The ChRM of the SFC dykes also exhibits dual polarity. Our results suggest that the NCC and SFC were in the moderate to high paleolatitudes of the Southern Hemisphere during approximately 925 Ma, suggesting a possible paleogeographic connection between the two cratons. A new paleogeographic reconstruction of both cratons during 925 Ma is proposed, which is supported by other geological evidence.

New dalmanitid trilobite taxa from the Lower Devonian of Argentina: comments on spinosity in endemic taxa from southern high paleolatitudes

Three new Lower Devonian dalmanitid trilobite taxa are recognized from the Talacasto Formation in the Precordillera Basin, Argentina, which includes two monospecific genera (Ivanites leonorae n. gen., n. sp. and Aguaditaspis mediaspina n. gen., n. sp.), and one additional new genus and species left in open nomenclature. These taxa combine characters of the subfamilies Dalmanitinae Vogdes, 1890 and Synphoriinae Delo, 1935, challenging their taxonomic distinction. Forty percent of the dalmanitids from high paleolatitudinal basins from southwestern Gondwana (Malvinokaffric) exhibit non-homologous dorsal spinosity. Aguaditaspis mediaspina n. gen., n. sp. shares similar spines with Trypaulites calypso (Hall, 1861). The unnamed new genus resembles the spinosity of endemic Malvinokaffric dalmanitids, like Dalmanitoides Delo, 1935 and Fenestraspis Braniša & Vaněk, 1973. Considered defensive, the recorded spinosity along with putative sublethal-attack marks, suggest higher predation pressure than previously thought at high paleolatitudes. This evidence is in accordance with some hypothesis on rising predation pressure in the mid-Paleozoic marine ecosystems.

U–Pb age constraints on the Jurassic succession and paleoflora of Mount Flora, Antarctic Peninsula

The spectacular fossil plant assemblage preserved in the non-marine Mount Flora Formation of the northern Antarctic Peninsula represents the diverse Jurassic flora that once covered the Gondwanan continents at high paleolatitudes. The depositional facies of the formation plays a key role in the tectonic interpretations and basin evolution models that attempt to reconcile large igneous province magmatism, continental break up, and magmatic arc development throughout the Jurassic and Cretaceous. Limited U–Pb in situ geochronology reported from the Mount Flora Formation and adjacent units lack the necessary resolution to overcome ambiguous correlations and biostratigraphic discrepancies. We present two high-precision U–Pb zircon ages (CA-ID-TIMS method) from a distinct tuffaceous interval of the Mount Flora Formation at Hope Bay, which document a terminal Middle Jurassic age (Callovian Stage) for the formation and its paleoflora. In excess of 1400 new collected fossil plant specimens exhibit a highly diverse Jurassic plant association that dominated the Antarctic Peninsula nearly 17 million years after its disappearance from northern Patagonia. This suggests similar paleoecological conditions were established diachronously throughout basins of southern Gondwana, possibly facilitating floral migrations in response to local climate change. The depositional facies of the Mount Flora Formation, its age proximity to the marine Nordenskjöld Formation in the Antarctic Larsen Basin and its coincidence with a regional unconformity in the northern Patagonia point out to a complex interplay among magmatic arc development, tectonic extension and continental break up that dominated the geologic and paleoenvironmental evolution of southern Gondwana near the end of the Middle Jurassic.

A new bauriamorph therocephalian adds a novel component to the Lower Triassic tetrapod assemblage of the Fremouw Formation (Transantarctic Basin) of Antarctica

ABSTRACT The Fremouw Formation (Transantarctic Basin) of Antarctica preserves an exceptional fossil record of tetrapods from the beginning of the Mesozoic at high paleolatitude. Many of these fossils can be identified to species that also occur in the much richer Lystrosaurus declivis Assemblage Zone of South Africa’s Karoo Basin, but endemic species also occur. A third category of Antarctic fossils includes those that can only be broadly classified or lack sufficient diagnostic features for a synapomorphy-based identification. Therocephalians are a prime example of the last category, with Akidognathidae indet., Baurioidea indet., and cf. Ericiolacerta parva, currently recognized from the lower Fremouw Formation. Here we describe the most complete skull for an Antarctic therocephalian discovered to date. Notictoides absens, gen. et sp. nov., preserves a unique dental formula, a large intranarial portion of the septomaxilla, two bony excrescences along the rostral margin of the lacrimal, and a complete secondary palate formed by the maxilla. These features, in combination with a sagittal crest with a slit-like parietal foramen, a long, low dentary, and reduced upper and lower antecanine tooth count, suggest a relatively derived position for N. absens within baurioids. Indeed, the results of an updated cladistic analysis unite N. absens with the clade including Nothogomphodon, Ordosiodon, and Bauriidae. More broadly, the discovery of a third endemic species in the Lower Triassic rocks of the Fremouw Formation suggests that diversification in the wake of the Permian–Triassic mass extinction was likely more geographically complex than currently appreciated.

North China block underwent simultaneous true polar wander and tectonic convergence in late Jurassic: New paleomagnetic constraints

Jurassic witnessed two synchronous events on the Earth, named true polar wander (TPW) and closure of the Mongol–Okhotsk ocean (MOO) between Siberia and eastern Asian blocks. The paleogeographic change of the north China block (NCB) should thus include both TPW and plate tectonic components in Jurassic. We performed a new combined paleomagnetic and U–Pb geochronological study for Middle and Late Jurassic volcanic rocks from the northern NCB and obtained three new well-dated paleopoles: 82.7°N, 282.6°E, A95=3.2° (ca. 160 Ma); 84.1°N, 228.9°E, A95=6.6° (ca. 165 Ma); and 89.3°N, 216.8°E, dp/dm = 5.4°/7.1° (ca. 170 Ma). A positive untilting test indicates a pre-folding origin interpretation for the ca. 160 Ma pole. Quality of these poles is enforced by three positive conglomerate tests, the presence of magnetic reversals and secular variation analyses. Comparison of the apparent polar wander paths of Eurasia and the NCB suggests that the NCB accreted to Eurasia around 140 Ma. The NCB and its then connected blocks did not undergo any significant southward paleogeographic shift during Jurassic. In context of TPW, we estimate a net northward plate moving velocity at 0.50–0.75°/myr for the NCB in Jurassic. The long-term accumulation of MOO subducted slabs at the high paleolatitudes may have critically contributed to, or perhaps led to, the Jurassic TPW.

Changes in fluvial architecture induced by discharge variability, Jaicós Formation (Silurian-Devonian), Parnaíba Basin, Brazil

Discharge variability-based facies models are key to understanding depositional dynamics and controlling mechanisms of fluvial accumulation. The well-exposed Silurian-Devonian Jaicós Formation, lower Parnaíba Basin, NE Brazil, allows for detailed fluvial architectural analyses in terms of variations in discharge patterns. This field-based study comprises a collection of high-resolution sedimentological, stratigraphic and paleocurrent data from 120 m of integrated logs. Eight architectural elements were recognized and fit into two distinct fluvial styles, termed fluvial units 1 and 2. Fluvial unit 1 is dominated by deposits of straight- and sinuous-crested bedforms, as constituents of downstream accretionary bars and subcritical, aggradational dunes. These elements indicate deposition in a low discharge variability perennial system, where water flow was perennial but bedforms migrated during peak-discharge events, related to a humid climate. Fluvial unit 2 abruptly overlies unit 1 and is dominated by small- to large-scale humpback dunes and antidunes deposited by unstable, transcritical and supercritical flows during non-periodic, peak-flood events. These deposits characterize an ephemeral system, in a high discharge variability regime linked to a semi-arid climate. The Jaicós fluvial system was deposited in a low subsidence basin, in low accommodation settings. Minimally affected by tectonic activity and latitudinal displacements, the Jaicós Formation displays a climate-controlled change in fluvial architecture that reflects shifts in discharge regime. These climate changes were probably triggered by global oscillations in the carbon cycle that were common during the Silurian, but most evidence is from low paleolatitude marine deposits. Thus, the abrupt fluvial architecture change in the Jaicós Formation comprises an example of climate-induced change in a hinterland fluvial system situated at moderate to high paleolatitudes in western Gondwana.

Tectonically conditioned record of continental interior paleoclimate during the Carnian Pluvial Episode: The Upper Triassic Los Rastros Formation, Argentina

Abstract Discerning paleoclimate parameters in depositional systems of the continental interior is challenging because the system response and stratigraphic record of climate are controlled by tectonic processes and are mediated through landscape and hydrological evolution of fluvial lacustrine systems. Climate and tectonic signals cannot be deconvolved from stratigraphic patterns alone but require additional information or data sets that directly record climate or tectonic influence. The Carnian Los Rastros Formation in northwest Argentina provides an excellent case study that integrates an appropriate range of information in a system with strong climate and tectonic signals, being deposited in part during the Carnian Pluvial Episode and spanning the active rift phase of the Ischigualasto–Villa Unión Basin. We examined the stratigraphic and spatial patterns of carbon (C) and oxygen (O) stable isotopes in lacustrine carbonates from the Los Rastros Formation in multiple parts of the basin to constrain paleohydrological conditions and paleotemperatures. Practically all C and O isotope values are characterized by negative values: δ18Ocarb –11.6‰ and –15.7‰ (χ average –13.1‰; 1σ = 1.6) and δ13Ccarb –2.6‰ to –8.0‰ (χ average –5.1‰; 1σ = 2.1), reflecting the latitude, altitude, and continentality of the lake system and its vegetated and humid catchment area. Stratigraphic patterns of stable isotope data from two different localities (Cerro Bola North and Cerro Bola South) show a change from short water-residence time to long residence time and back to short residence time. This contrasts with sedimentologic, organic geochemical, and small-scale stratigraphic patterns that indicate an overfilled lake basin, which is expected to contain a completely open-hydrology isotopic signature. Paleotemperatures calculated from marginal lacustrine carbonates show a warm and quite variable paleothermal range consonant with their continental interior position and with Global Climate Models estimates for high paleolatitudes. Warmer paleotemperatures (linked to aridity, probably smaller lake size, and less thermal mass) precede the Carnian Pluvial Episode, whereas relatively cooler paleotemperatures coincide with the Carnian Pluvial Episode (linked to humidity, probably larger lake size, and more thermal mass). Carbon and oxygen stable isotope signatures integrated with sedimentologic and physiographic information allow us to propose that tectonics, specifically, half-graben tilting during the active synrift phase, dominated over climate effects as the cause of hydrological fluctuations of this system, even during the Carnian Pluvial Episode. Without appropriate stratigraphic-tectonic context, single-proxy reconstructions of continental-interior paleoclimate can be misleading. A robust interpretation of climate effects requires characterization of tectonic effects, geomorphology, paleohydrology, and sedimentary system responses.

Calcareous nannofossils from the Great Australian Bight (IODP Site U1512) as a record of the peak and early decline of the mid-Cretaceous super greenhouse

Abstract International Ocean Discovery Program drilling in the Great Australian Bight (Site U1512) provides a record of Turonian to Santonian hemipelagic sedimentation deposited in an elongate epicontinental basin that opened westward to the Southern Ocean during the peak and the early decline of the Cretaceous hot greenhouse. Despite its relatively high paleolatitude (at about 60°S), the biostratigraphy of calcareous nannofossils from this succession is directly correlative to low-latitude zonation schemes throughout the Turonian to Santonian. These calcareous nannofossil assemblages are somewhat restricted in species composition, lacking some elements generally found in Late Cretaceous assemblages (e.g., holococcoliths). In addition, two biostratigraphically-important high latitude taxa (Thiersteinia ecclesiastica, and Zeugrhabdotus kerguelenesis) have biostratigraphic ranges notably shorter than elsewhere, indicating range truncation through local extirpation. Quantitative paleoecological analysis of the calcareous nannofossil assemblages indicates that the major change in the paleoenvironment was the evolution from conditions that were quite warm and had relatively elevated surface water fertility in the Turonian to cooler and possibly more oligotrophic conditions in the Coniacian–Santonian. The first cooling began at about 92 Ma, accelerated at about 91.2 Ma and then continued to gradually cool during the Coniacian–Santonian. This paleotemperature scenario is consistent with those documented from other localities globally.

Eocene to Oligocene high paleolatitude neritic record of Oi-1 glaciation in the Otway Basin southeast Australia

Multiple stable isotope investigations from upper Eocene to lower Oligocene deep-water marine sequences record the transition from global greenhouse to the icehouse conditions (Oi-1 glacial). While Southern Ocean high latitude deep sea records of this transition are well known, their shallow marine equivalents are rare and have the potential to record the eustatic and oceanic consequences of Paleogene glacial variability. The well-known high paleolatitude (~55°S) neritic carbonate sequence at Browns Creek and Castle Cove in the Otway Basin in southeast Australia spans the Eocene-Oligocene boundary. During this time the area lay on the northeastern margin of the Australo-Antarctic Gulf facing the evolving Southern Ocean. The importance of this record has been hampered by a lack of a consistent stratigraphy and contradictory microfossil interpretations. To reconcile these issues we combine new bio-, chemo- and lithostratigraphic analyses of the outcrops and a new core (Colac-2) with pre-existing data to revise the stratigraphy. This confirms the middle/upper Eocene boundary is near the base of the section. The overlying upper Eocene siliciclastic strata are truncated by an unconformity (of ~0.8 Ma in duration) and overlain by glauconitic sand (the Notrostrea greensand) deposited after ~35.9 Ma. Subsequently deepening to middle to outer neritic depths deposited cyclic carbonates. Shallowing after ~35 Ma deposited laterally variable calcareous siliciclastic facies. These strata were tilted and eroded prior to 34 Ma leading to shallow water facies that may have been subaerially exposed during uplift. Brachiopod strontium isotope dates and an 0.5‰ carbon isotope excursion above this unconformity suggests the top of the Browns Creek and the base of the Castle Cove section correlate to Eocene-Oligocene transition (EOT-1) at ~34 Ma. The subsequent persistence of positive C/O isotope values above this level records the transition to the Oi-1 glaciation at ~33.7 Ma. Strong cyclicity in the inner shelf Castle Cove limestone is interpreted to record the commencement of obliquity dominated glacio-eustacy during the Oi-1 glacial phase. The shallowing from outer to inner shelf palaeodepths from the late Eocene to the early Oligocene is likely related to the onset of cryosphere expansion, however, palaeodepth estimates are complicated by the onset of regional compressional tectonism at the Eocene/Oligocene boundary that caused localized tilting and an unconformity with possible antisiphoning effects in this near-field site.

The sedimentological evolution and petroleum potential of a very thick Upper Cretaceous marine mudstone succession from the southern high latitudes—a case study from the Bight Basin, Australia

During IODP Expedition 369, a 690 m thick succession of silty claystone spanning the early Turonian to the late Santonian was encountered at Site U1512 in the Bight Basin, offshore southern Australia. Stacking patterns, sedimentary facies and palynological assemblages reveal that the succession was rapidly deposited with hyperpycnal and hypopycnal flows in a marine prodelta setting, which was subject to basin restriction. The dominance of clay-rich facies and phytoclasts in the succession was likely the result of a major river system delivering a high sediment load into the Bight Basin when a warm, wet climate prevailed. A combination of high sedimentation rates (19–272 m/Myr) and accelerated subsidence prevented the delta from rapidly prograding into more distal regions of the basin. The complete Turonian to Santonian mudstone succession yields low total organic carbon (~1 wt%) and Type IV kerogens. However, palynofacies assemblages become progressively marine in character and total organic carbon values vary between 1 and 1.5 wt% with depth. This may indicate that the base of the hole at Site U1512 was close to potential organic-rich black shales associated with Ocean Anoxic Event 2. Low amplitude and irregular reflections on seismic data and disparities between biostratigraphic zonations suggest the upper 350 m of the Turonian to Santonian succession may represent a mass movement that happened during the Pleistocene. This study reveals that Site U1512 material likely represents a near-stratigraphically complete marine mudstone succession from high paleolatitudes, as well as the only depositional record that was fully cored from the Bight Basin.

Revision of the Cambro-Ordovician acritarch genus Vulcanisphaera Deunff, 1961

After its first description in Cambrian–Ordovician boundary strata from Gondwana, the acritarch genus Vulcanisphaera Deunff, 1961 has been found subsequently on most other paleocontinents, with a total of 32 species assigned to this genus. A comprehensive revision of the published literature indicates that the genus contains only three species that are easily differentiable: (1) V. africana Deunff, 1961; (2) V. mougnoana Martin, 1973; and (3) V. simplex Jardiné et al., 1974. The other species are either synonymous with these three or must be excluded from the genus. The generic and specific diagnoses of Vulcanisphaera are emended herein. The new classification scheme is tested and validated by using quantitative multivariate analyses based on morphometric studies on abundant material from England, Algeria and Iran. The species show large morphological variability, and transitional forms between V. africana and V. mougnoana are observed. The stratigraphic ranges of the species have been determined. V. mougnoana occurs from the Miaolingian to the Early Ordovician, V. africana is found with certainty from the Furongian to the Floian and V. simplex is recorded in the latest Furongian and throughout the Tremadocian. The genus Vulcanisphaera shows a cosmopolitan paleogeographic distribution with most occurrences recorded in higher paleolatitudes.

Diagenetic and environmental control of the clay mineralogy, organic matter and stable isotopes (C, O) of Jurassic (Pliensbachian-lowermost Toarcian) sediments of the Rodiles section (Asturian Basin, Northern Spain)

Abstract Clay mineralogical and geochemical analyses (δ18Ocarb, and paired carbonate and organic matter carbon isotope), completed by the characterization of organic matter, have been performed on the Pliensbachian sediments exposed on the Rodiles section (Asturian Basin, Northern Spain). The objectives were to precise the Pliensbachian climate evolution at a relatively low paleolatitude of the Tethyan domain by comparison with previously published high-resolution clay mineralogical and geochemical data from the NW Tethyan domain (e.g. Paris Basin and Cardigan Bay Basin, West Wales). The clay mineral assemblages are dominantly composed of illite and illite-smectite mixed-layers (I–S) associated with minor proportions of chlorite and kaolinite. Smectites are not identified what may result either to their absence at the time of sedimentation or to their progressive illitization due to the burial diagenesis. Rock-Eval data show that organic matter reaching more than 5% in black-shale horizons is thermally mature with Tmax comprised between 444 and 454 °C indicating that the section is in the oil window and that the burial temperature reached or exceeded 80–90 °C. The low δ18Obulk values comprised between −3 and −6‰ clearly results from a diagenetic influence (thermal effect due to burial) but their fluctuations compared with δ18O measured on well-preserved belemnites guards show that the overall trend is however preserved. By comparison with higher latitude sedimentary basins, the most striking feature is the very small proportions of kaolinite (generally less than 5% while the proportions of this mineral reach 40% at higher paleolatitudes), except at the Pliensbachian/Toarcian transition, where the proportion of kaolinite increases significantly. Clay minerals likely originated from the erosion of the Iberian Massif located to the South of the Asturian Basin in a semi-arid climatic belt. This may explain the scarcity of kaolinite by comparison with sedimentary basins located at higher latitude influenced by more humid climate. The increasing proportions of kaolinite by the end of the Pliensbachian suggest southward expansion of the humid belt as a consequence of cooler temperatures and the establishment of steeper latitudinal gradients of temperature likely resulting from the formation of restricted polar ice cap. The main δ13C (on both 13Cinorg and 13Corg) Pliensbachian excursions including the negative excursion of the Sinemurian/Pliensbachian Boundary Event (SPBE) and the Late Pliensbachian positive excursion are well recognized.