Displays

Over very long timescales, mountain building or orogenesis is associated with increased weathering, the drawdown of atmospheric CO₂, and global cooling. Considering the Phanerozoic glaciation in particular, a multimillion‐year delay appears to exist between peaks in low‐latitude mountain uplift and the maximum extent of glaciation, implying a complex causal relationship between them. We show, using a combination of physical climate/circulation modelling and geochemical modelling approaches, that global silicate weathering can be modulated by orogeny in three distinct phases. High, young mountain ranges experience preferential precipitation and the highest erosion. As mountain ranges denude, precipitation decreases, but runoff temperature rises, sharply increasing chemical weathering potential and CO₂ drawdown. In the final phase, erosion and weathering are throttled by flatter topography. We hypothesise that orogeny acts as a capacitor in the climate system, granting the potential for intense transient CO₂ drawdown when mountain ranges are denuded. Intriguingly, depending on the future evolution of the Tibetan Plateau, the mechanism suggests such a scenario potentially happening 10–50 × 10⁶ years in the future.

How to cite: Joshi, M. and Mills, B.: A mechanism to explain the timing of glaciations related to orogenic episodes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11136, https://doi.org/10.5194/egusphere-egu2020-11136, 2020

The Late Paleozoic Ice Age (LPIA) is the longest-lived and most extreme glacial period (from ca 360 to 260 Ma) of the Phanerozoic. Over this time span, ice masses are thought to have covered most of Gondwana, from South America to Australia. In southern Africa, the sedimentary, stratigraphic and geomorphic evidence of this glaciation is recorded in the Karoo Supergroup. The Kaokoland region of northern Namibia is characterized by a dense network of deep (200-700 m), large (5-15 km) and U-shaped incised valleys formed during the LPIA (Martin, 1981). A recent reappraisal of the morphology and sedimentary infill of these outstanding geomorphic features attests of their glacial origin. Valley flanks are spectacularly striated and scratched while valley floors are characterized by extensive whalebacks and roches moutonnées. Moreover, the sedimentary infill at the base of these valleys is mainly composed of coarse deposits (conglomerates, diamictites, erratics, striated clasts) interpreted as glaciogenic in origin. Of particular interest, however, is the presence of coarse (ranging from sand to boulders) glaciogenic sediments plastered on the sub-vertical and striated valley sides. Vitally, the elevation of these deposits in the valleys appears to correspond to a linear bench-like level, which may reflect a marginal moraine allowing for the maximum thickness of the LPIA glaciers to be derived, an unprecedented advance. For the first time in the characterization of a pre-Pleistocene glacial epoch, an ice thickness has been inferred. Collectively, these features prove that the valleys were carved and occupied by ice masses during the LPIA from which ice volume, and in turn their contribution to global eustasy, can directly be inferred. In addition, postglacial sedimentary succession abutting on valley flanks and showcasing marine, deltaic and estuarine affinities clearly indicate that these glacial valleys formed fjords in the immediate aftermath of the LPIA, after the retreat of the ice margins. Sealed by the Karoo Supergroup sediments through Carboniferous to early Cretaceous times, these major glaciogenic morphologic features have subsequently been exhumed during the Cenozoic. Thus, some desertic landscapes of northern Namibia correspond to a glacial relief inherited from the LPIA at ca ~ 300 Myr ago.

Martin, H., 1981, The Late Paleozoic Dwyka Group of the South Kalahari Basin in Namibia and Botswana and the subglacial valleys of the Kaokoveld in Namibia, in Hambrey, M.J., and Harland, W.B. (eds.) Earth’s Pre-Pleistocene Glacial Record: New York, Cambridge University Press, 61–66

How to cite: Dietrich, P., Griffis, N., Kettler, C., Le Heron, D., and Montañez, I.: Exhumed fjords of Namibia: A glimpse of the Late Paleozoic Ice Age in the Karoo Supergroup of the Kaokoland basin, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-14059, https://doi.org/10.5194/egusphere-egu2020-14059, 2020

Late Carboniferous to Permian French intracontinental basins have been studied during the 20^th century, mainly for industrial applications due to their carbonaceous resources. This period is also of great interest for its palaeogeographical, palaeoenvironmental and palaeoclimatic evolution associated with large geodynamic modifications, such as the transition from the Variscan orogeny to the breakup of Pangaea, and with the change from an icehouse to a greenhouse climate.

The end-Carboniferous (i.e. late Pennsylvanian) to Permian Autun and Lucenay-lès-Aix basins, studied here, are located south of the Paris Basin (France). The Autun Basin crops out in the Morvan area and lies on a Devonian and Carboniferous magmatic substratum. The Lucenay-lès-Aix area is a subsurface basin, under a ca. 250 m-thick Meso-Cenozoic cover, located in the southern termination of the outcropping Decize-La Machine Basin.

The study is based on subsurface data, using cored boreholes, well-log and seismic profiles. Our investigations have been achieved through a sedimentological approach including facies associations analysis and sequence stratigraphy, mineralogy and petrography, supplemented by a geochemistry approach to characterise the deposition and preservation mechanisms of organic matter (OM).

In the Autun Basin, three cores encompass a part of the lower Autunian, near the Gzelian/Asselian boundary. Sedimentological observations indicate lacustrine-dominated environments, with black shales and intercalated turbidites, interrupted by more proximal facies (microbial deposits and coarse river-mouth sediments). Palynofacies and Rock-Eval analyses show a dominant Type I OM, i.e. lacustrine algae, in the laminated fine-grained deposits. The geochemical results indicate periods of both high primary productivity and sedimentary OM storage, reflected in high total organic carbon and total nitrogen contents in sediments (TOC and TN, up to 21.5 wt.% and 0.76 wt.%, respectively), and very low δ¹³C_org values (down to -29.1‰, σ = 0.07‰).

In the Lucenay-lès-Aix Basin, the sedimentation dated from the late Gzhelian to the late Sakmarian is mostly characterised by alluvial, deltaic, lacustrine and floodplain deposits (coal), with a great volcaniclastic component. The geochemical proxies in coal deposits show high TOC and TN contents (up to 71 wt.% and 1.6 wt.%, respectively), with δ¹³C_org values averaging -23.7‰ (σ = 0.03‰), indicating a Type III OM, as already reported for these deposits at that time.

In both basins, the main clay assemblage, slightly affected by burial diagenesis and mostly detrital, is dominated by kaolinite, illite, R1 type illite/smectite and chlorite/vermiculite mixed-layers. Kaolinite/illite ratio (K/I) is used as a proxy of the continental runoff, considering that kaolinite was formed in soils under more humid climate, although reworked kaolinite from previous deposits under high runoff conditions is possible. Moderate K/I values in black shales indicate low continental runoff conditions, while a substantial runoff indicated by high K/I ratio is associated with the more proximal coarse detrital sediments (river-mouth/delta). These interpretations are consistent with the mixture of both lacustrine and terrestrial OM (i.e. C3 vascular plants, Type III).

How to cite: Mercuzot, M., Bourquin, S., Pellenard, P., Thomazo, C., Beccaletto, L., Schnyder, J., Baudin, F., Ducassou, C. D., and Pierson-Wickmann, A.-C.: Contribution of sedimentology, organic geochemistry and clay mineralogy to reconstruct the palaeoenvironments of late Carboniferous to Permian of the northeastern Massif central, France., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-14346, https://doi.org/10.5194/egusphere-egu2020-14346, 2020

The Late Carboniferous - early Permian was a time-interval of major geological and climatological changes, mostly due to the transition to greenhouse conditions from the maximum glacial coverage (Late Palaeozoic Ice Age or LPIA). This climatic change produced an increase of the extinction rates on land plants and a variation on the constitution and distribution of palaeofloras during this time. The restructuring of ecosystems during Late Pennsylvanian is interpreted as the "collapse of the rainforests". A replacement of hygrophytic (“Stephanian flora”) by mesophytic and meso-xerophytic flora (“Autunian flora”) that tolerate seasonally dry climate is described.

In the Euramerican Province, the continental vegetation during the Pennsylvanian was a hygrophilous flora comprising pteridosperms, marattialean ferns, lycopsids, Calamites and Cordaites trees. At the Late Pennsylvanian-early Permian, the “Carboniferous hygrophilous flora” proliferated in the wet depressions (lowlands) and the mesophilic or even meso-xerophytic flora, grew on the heights (uplands) in the dewatered habitats. Later, this xerophytic flora will be dominant in the landscapes during the middle and late Permian.

There are detailed palynostratigraphic studies that allow precise palynological datings for the Carboniferous period in the Euramerica Province. However, few palynological works have been published relative to the early Permian sedimentary record in this province. In a broad sense, these latter studies only differentiate the early Permian flora (“Autunian flora”) due to the presence of sporomorph taxa as Potonieisporites novicus and Vittatina costabilis, and the middle-late Permian (“Thuringian flora”) mainly characterised by Lueckisporites virkkiae and Nuskoisporites dulhuntyi. This lack of precision was probably due to the different sedimentation rates in the intramontane basins and the “border effect” (as a phytogeographic barrier) caused by the Variscan Belt.

The number of works and the wrong use of non-chronostratigraphic terms like “Autunian” and “Thuringian”, making it necessary to re-calibrate the palynological assemblages in the Euramerica Province. A detailed biostratigraphic study allows us to show here, for the first time, a new palynostratigraphic chart derived from palynological studies from some of the best known low-latitude basins radiometrically dated (Pyrenees, Autun, Lodève, Collio and Tregiovo basins) and from basins with well-known internal lithostratigraphic correlation (e.g., the Cantabrian Mountains and the Iberian Ranges).

Based on the results obtained here, the microflora evolution in the early-middle Permian has been described at low latitudes of the Euramerican Province. Furthermore, this study provides a solid base for stabilising the palynozones for the Permian in the southern domain of the Variscan Belt.

How to cite: Juncal, M. A., Diez, J. B., Lloret, J., de la Horra, R., Gretter, N., Ronchi, A., Barrenechea, J. F., Borruel-Abadía, V., and López-Gómez, J.: Re-calibrating the Late Palaeozoic palynostratigraphy in the basins of the southern domain of the Variscan Belt (southwestern Europe)., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19788, https://doi.org/10.5194/egusphere-egu2020-19788, 2020

Developed on the North China Craton, the intra-cratonic Ordos basin contains a complete Paleozoic to Cenozoic sediment record allowing long-term paleo-environmental and climate change investigation. During the Carboniferous-the early Permian period, convergence between the North China block and the paleo-Yangtze plate to the south lead to a general marine regression characterised by a series of second-order transgression/regression cycles diachronous along the eastern margin of the Ordos. However, the detailed mechanisms that induced these cycles, as well as the associated paleoecological changes, are still unknown. In this study, we integrated the description of numerous core samples with electric-log data and 2-D seismic data to reconstruct the sediment facies associations across the first-order regression from the Carboniferous tidal flat depositional system to the early Permian prograding fluvial delta system. δ¹⁸O, δ¹³C and clay content (w(Illite + Kaolinite)/w(smectite) ratio) stratigraphic variations were then used to reconstruct the paleo-sea level from the late Carboniferous to the early Permian. We conclude that the direction of second-order transgression/regression mainly stroke to the east during the late Carboniferous and switched clockwise towards the north during the early Permian. We suggest that the variability of the second-order cycles, diachronous in space and time was mainly linked to local variations in sediment supply and regional uplift.  Using detrital zircon U-Pb data, major and trace elements content and heavy minerals assemblages (HMA), we estimated the sediment provenance area. The sediment volumes deposited in the basin through time were obtained using 3Dseismic data. During the Carboniferous, the coarse-grained sediments deposited in the eastern Ordos were derived from the uplifting Helan Mountain. By the early Permian, the detrital material became multi-sourced issuing from both the Yinshan range to the north and the Qinling range to the south. During the first stage, regression was controlled by regional uplift, while the sediment supply controlled the second stage. Indeed, based on sediment dispersal volume calculation, we can infer that the sediment supply during the early Permian was more extensive than during the Late Carboniferous – early Permian. We correlate this observation to a more humid climate during the early Permian: multi- paleoecological indexes, including the sporopollenin content and microsomal type assemblage, suggest that glaciation prevailed during the Late Carboniferous – early Permian shallow-marine stage. In contrast, the early Permian alluvial and deltaic series were deposited under a warmer, interglacial climate (Sakmarian). Finally, the typical interglacial coal accumulation pattern occurs earlier than the Pennsylvanian–Permian transition it characterises around the world (Artinskian).

How to cite: Fu, C., Yu, X., Jolivet, M., Li, S., Peng, Z., and Shi, S.: Mechanism of Carboniferous-Permian transgression/regression in the east Ordos basin and associated paleoecological variability: Insight from detrital geochronology and palaeontology data, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2573, https://doi.org/10.5194/egusphere-egu2020-2573, 2020

This study documents the sedimentary and structural response of continental crust in relatively hot lithosphere that is subjected to extension. We focus on the Permian rift system in the Western Pyrenees, where the narrow, post-orogenic intracontinental extensional Bidarray Basin is in contact with late Variscan granulites of the Ursuya massif. The western margin of the N-S trending Bidarray Basin preserves alluvial fans dominated by hyperconcentrated flows and interdigitating eastward into a N-S trending fluvial system. Structural analysis of the Ursuya granulites shows that they underwent orogen-parallel mid-crustal flow and were exhumed owing to strain localization during retrogressive metamorphism within an extensional shear zone flanking an E-W elongated domal structure. We show that the Bidarray Basin formed during Permian time on the hanging wall of a south-vergent detachment system that developed in response to the formation of an immature “a-type” metamorphic core complex (the Ursuya massif) under regional E-W extension, resulting in homogeneous thinning of the hot crust. This core complex was later exposed by denudation during Cenomanian time. The preservation of the Permian and Triassic paleogeography and structure indicates that there has been no lateral motion between Iberia and Europe in the study area. The Cretaceous Pamplona transfer zone, responsible for the shift of the Mesozoic rift axis, reactivated a N-S trending Permian crustal heterogeneity.

How to cite: Saspiturry, N., Cochelin, B., Razin, P., Leleu, S., Issautier, B., Serrano, O., Baudin, T., and Allanic, C.: Permian inheritance : post-orogenic extension and metamorphic core complex formation (Western Pyrenees), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21820, https://doi.org/10.5194/egusphere-egu2020-21820, 2020

Sea-level fluctuation is an important parameter controlling the sedimentation in deep-marine environments and influenced also the expansion of oxygen-depleted conditions in neritic settings during oceanic anoxic events (OAEs). Despite this fundamental role, sea-level fluctuation remains on a short timescale (<1 Myr) one of the least constrained parameters for numerous OAEs. Here we refine the sequence stratigraphic framework for the uppermost Pliensbachian–Toarcian with a special focus on the Toarcian OAE interval. This study is based on sedimentological and total organic carbon isotope data used to correlate 16 sections located in the central High Atlas (Morocco). Palinspastically, those sections formed a 50-kilometer proximal–distal transect along the northern Gondwana continental shelf, which allow reconstructing the shoreline migration through time and space. Our sequence stratigraphic interpretation is then compared to the geochemical signals (e.g. detrital index, chemical index of alteration) measured on samples collected in deep-environment settings from numerous basins distributed worldwide. Our study shows that the relative sea-level changes recorded in Morocco can be correlated over large distances across those basins, indicating that the relative sea-level changes were driven by eustatic fluctuations. This study gives insights into the relationship between relative sea-level fluctuations and the geochemical record.

How to cite: Krencker, F.-N., Fantasia, A., El Ouali, M., Kabiri, L., and Bodin, S.: Latest Pliensbachian–Toarcian eustatic calibration using shallow-marine sedimentological record coupled with basinal geochemical analyzes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8499, https://doi.org/10.5194/egusphere-egu2020-8499, 2020

Over the last decades, studies on Jurassic palaeoenvironments have been mostly focussed on the early Toarcian as this latter was marked by the Toarcian Oceanic Anoxic Event (T-OAE; ca. 183 Ma), which was one of the most extreme hyperthermal of the Phanerozoic. Hence, little is know about palaeoclimatic and palaeoenvironmental changes during the Aalenian time interval, although it is likely marked by an abrupt cooling in the aftermath of the Toarcian warm mode. Available palaeontological and geochemical datasets suggest that the Aalenian is also characterized by faunal turnovers and potential carbon-cycle perturbations. Despite those evidence, there is still no consensus about the modality of Aalenian palaeoenvironmental and palaeoclimatic changes as well as their potential triggering mechanisms. In addition, data from outside Europe are absent, leading to large uncertainties whether the observed changes are of gobal significance. In this study, we focus on the upper Toarcian–lower Bajocian interval of two marl/limestone alternation successions, namely Le Brusquet (Vocontian Basin, SE France) and El Peñon (Andean Basin, N Chile). Palaeoenvironmental and palaeoclimatic conditions are inferred based on high-resolution mineralogical (whole-rock and clay fraction) and geochemical (carbon isotopes, Rock-Eval pyrolysis, phosphorus, mercury) analyses. Additionaly, we provide a cyclostratigraphic framework for the Aalenian based on high-resolution magnetic susceptibility spectral analysis. The carbon isotope composition of bulk organic matter reveals evident correlatable fluctuations between sites from both hemispheres, providing the first evidence that the carbon cycle was globally and repeatedely disturbed during the Aalenian. The Toarcian–Aalenian transition is associated with a decrease in detrital and nutrient input (phosphorus), which is likely related to the shift towards the Aalenian cool mode. Interestingly, the middle–upper Aalenian transition is characterized by a sharp increase in terrigenous and nutrient influxes suggesting a more humid and warmer episode. The concomitance between strongly expressed precession cycles and palaeoenvironmental changes suggests moreover the influence of orbital parameters on the Aalenian sedimentary record.

How to cite: Fantasia, A., Adatte, T., Spangenberg, J. E., Thibault, N., Mattioli, E., Bernárdez, E., Krencker, F.-N., and Bodin, S.: Evidence for repeated and global palaeoenvironmental perturbations during the Aalenian , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9527, https://doi.org/10.5194/egusphere-egu2020-9527, 2020

Mesozoic sea-level fluctuations have been a matter of debate for several decades, especially the extend and origin of sea-level cycles that have a periodicity of about 1 Myr or less. The debate lies in the main driving mechanism for sequence development (global sea-level or sediment flux variations) as well as the reason behind water exchanges between the continents and the oceans (glacio- or aquifer-eustatism). In this study, we focus on the carbonate-dominated sedimentary record of the Bajocian (Middle Jurassic) in the Central High Atlas Basin of Morocco. Several aspects make this basin an appropriate location for discussing Middle Jurassic sea-level changes. Firstly, the outstanding exposures of the High Atlas Mountains, with continuous exposures for 10s of kilometres, allow to describe and track sedimentary packages and their bounding surfaces from proximal to distal settings. Moreover, a combination of ammonite and brachiopod biostratigraphy with carbon-isotopes chemostratigraphy allows to temporarily constrain their development, which permits to correlate and compare the Central High Atlas sedimentary record to other basins. Finally, due to high-subsidence rates, thick Bajocian sedimentary sequences have accumulated, minimizing condensation and hiatus that might prevail in other basins due to a lack of accommodation space creation. Two Bajocian long-term transgressive-regressive (T-R) packages are observed throughout the basin. They are modulated by several medium-term T-R packages, that have each an approximate duration of 1 Myr. These sequences can also be correlated on a basinwide scale. Combined with sedimentological and facies analyses, architectural evidence along proximal-to-distal transect illustrates that several of the medium-term sequences are characterized by the presence of a falling stage and lowstand systems tracts, demonstrating that medium-term T-R stacking patterns are not solely linked to fluctuation in sediment supply, but also to episodes of relative sea-level fall of at least 30m of amplitude. This is confirmed by backstripping analysis performed in a composite section from the center of the Basin. Comparison with Bajocian deposits from France and Scotland, where good biostratigraphic dating is also available, shows that similar contemporaneous sea-level fall can be observed, highlighting their potential global character. The two long-term Bajocian sequences are more difficult to correlate on a global scale, suggesting that they are rather primarily linked to fluctuation in regional sediment supply or dynamic topography processes. The exact cause of the Bajocian medium-term sea-level falls is currently unknown, but it is here interesting to note that a relatively cool globate climate has been postulated for the Middle Jurassic, suggesting that glacio-eustasy was their most likely driver.

How to cite: Bodin, S., Danisch, J., Mau, M., Krencker, F.-N., Nutz, A., and Kabiri, L.: Ample and recurrent sea-level fluctuations during the Bajocian: A hint towards middle Jurassic glacio-eustatism, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13124, https://doi.org/10.5194/egusphere-egu2020-13124, 2020

Isolating the eustatic signal from the sedimentary record remains challenging, yet much progress is being made toward understanding the timing, magnitude, and rate of eustasy on both long-term (10⁷-10⁸ yr) and short-term (10⁵-10⁶ yr) scales throughout the Phanerozoic. Knowledge of timing, magnitude, and rate, in turn, provides insights into driving mechanisms (tectono-eustasy vs. climatically mediated eustasy; e.g., glacio- or aquifer-eustasy). As an example, we review the current state of knowledge of Cretaceous eustasy. A literature-based review of sea-level change estimates has been conducted, and the results were evaluated against the different driving mechanisms. A further evaluation of driving mechanisms has been derived from a global geodynamic and associated paleoclimate model.

An analysis of short-term sea-level cycles reveals four broad episodes of magnitude change. Three of these episodes reflect trends of increasing sea-level change magnitudes from the Berriasian to early Hauterivian, late Hauterivian to Aptian, and Santonian to Maastrichtian. The fourth episode reflects a decreasing magnitude trend from the Albian to Coniacian. In addition, the maximum magnitude of sea-level change, at an approximate stage level, has been identified and categorised as slight (less than 10 m), modest (10 to 40 m), or significant (41 to 65 m). Significant magnitudes are inferred for the Valanginian, Aptian, Albian, and Maastrichtian; exclusively slight magnitudes are restricted to the Berriasian.

Because climatically driven eustasy is the likely cause of short-term sea-level change, an assessment of the characteristic maximum magnitude limits of the principal climatic drivers (thermo‑, aquifer-, and glacio-eustasy) has been made. Such a comparison argues for glacio-eustasy as the driver of significant short-term sea-level change and is supported by climate proxy data demonstrating that the Valanginian, Aptian, Albian, and Maastrichtian are intervals of cooling.

While the mechanisms, frequency, and magnitude short-term sea-level cycles linked to thermo- and glacio-eustasy are understood, the likely contribution of aquifer-eustasy remains enigmatic and, for the most part, untested. To better understand the role of aquifer-eustasy, paleoclimate simulations aimed at assessing the spatio-temporal pattern of aridity and humidity under differing CO₂ forcing have been undertaken during time slices considered reflective of the differing Cretaceous climates and paleogeographic configurations (Valanginian, Turonian, and Maastrichtian). Only modest changes in the spatial extent of arid and humid zones are observed in response to large changes in CO₂ forcing. The simulations also demonstrate that the greatest aquifer charge is more likely during lower CO₂/cooler intervals, indicating that aquifer-eustasy works in phase with both glacio- and thermo-eustasy in contrast to the aquifer-eustasy paradigm. Additionally, using information on modern water table depths, we estimate that aquifer eustasy would be unable to contribute significantly to Cretaceous sea-level change. Indeed, even in the most optimistic case, the largest possible total aquifer-eustasy response remains smaller than 7 m. Our results indicate that glacio-eustasy is the most likely driver of Cretaceous short-term eustatic cycles because aquifer-eustasy is unable to account for the estimated Cretaceous magnitudes.

How to cite: Davies, A., Simmons, M., Ray, D., Gréselle, B., van Buchem, F., and Robson, C.: Timing, Magnitude, Rate, and Drivers of Eustasy: A Review of the Cretaceous Period, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3088, https://doi.org/10.5194/egusphere-egu2020-3088, 2020

During the greenhouse climate of the mid-Cretaceous, the Western Interior Seaway (WIS) experienced semi-restricted conditions with poor water-column ventilation, leading to the accumulation of black organic-rich shales. In the Maverick Basin, the southernmost extent of the WIS, the main phase of organic-matter deposition occurred in the early to late Cenomanian, before Oceanic Anoxic Event 2 (OAE 2). A sea-level rise prior to the event may have caused the basin to become better ventilated during the Cenomanian–Turonian transition, and ocean circulation likely played a major role on productivity and the preservation of organic matter. Widely different regimes of ocean circulation are suggested to have operated, with alternating incursions of water masses from both the north and the south. Foraminiferal assemblages suggest that during the early phase of OAE 2, Tethyan waters were drawn northward into the WIS (Elderbak & Leckie, 2016), whereas dinocyst occurrences indicate an influx of boreal surface waters into the Maverick Basin at that time (Eldrett et al., 2014; 2017). This cooler episode correlates with the so-called Plenus Cold Event, recognized in northern Europe by southward invasion of boreal faunas.

Here we present neodymium-isotope records (ε_Nd) of fish teeth and detrital fractions from the Eagle Ford Formation that record the presence of distinct water masses at depth and allow testing of suggested mechanisms of ocean circulation. Mid- to late Cenomanian values of ε_Nd around -3 (this study) are unusually radiogenic compared to coeval open ocean ε_Nd records from the North Atlantic, where values typically lie between -4 and -10 (Martin et al., 2012, Robinson & Vance, 2012) and may reflect a strong influence of regional volcanism close to the WIS and/or weathering of mafic volcanic rocks in the water-mass source area. An excursion to positive ε_Nd values in the WIS during OAE 2 may reflect changes in local weathering, or alternatively, the incursion of water masses carrying a signature of volcanic activity. The coeval emplacement of several Large Igneous Provinces (LIP), including the High Arctic LIP (Estrada et al., 2015) and the Caribbean LIP, may have influenced the seawater chemistry of the WIS, as reflected in Os and Cr concentrations and isotope ratios from the USGS Portland core (Du Vivier et al., 2014; Holmden et al., 2016).  Comparison of seawater and detrital ε_Nd signatures with records north and south of the Maverick Basin will elucidate the direction and degree of deep-water exchange in the southern WIS.

References:

Du Vivier, A.D.C. et al., 2014, EPSL, 389, 23-33

Elderbak, K. & Leckie, R.M., 2016. Cret. Res., 60, pp.52-77.

Eldrett, J.S., et al., 2014. Geology, 42(7), pp.567-570.

Eldrett, J.S., et al., 2017. Climate of the Past (13), pp.855–878.

Estrada, 2015. Int. J. Earth Sci. (104), pp.1981–2005.

Holmden et al., 2016. Geochim. Cosmochim. Acta 186 (2016) 277–295

Martin, E.E., et al., 2012. EPSL, 327, pp.111-120.

Robinson, S.A. & Vance, D., 2012. Paleoceanography, 27(1).

How to cite: Batenburg, S., Jenkyns, H., Bryant, R., Leckie, M., Dickson, A., Robinson, S., Ruhl, M., and Eldrett, J.: Oceanography of the Western Interior Seaway during OAE 2 using Nd isotopes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19580, https://doi.org/10.5194/egusphere-egu2020-19580, 2020

Belemnite calcite has been used extensively for Jurassic and Cretaceous stable oxygen isotope temperature reconstructions since the 1950s. However, with the advent of clumped isotope thermometry, a consistent offset between reconstructed δ¹⁸O temperatures vs Δ₄₇ temperatures from the same belemnites has been observed. We investigate the causes of this offset by analyzing samples from the aragonitic phragmacone and calcitic rostrum from the same Cylindroteuthis belemnites, along with other aragonitic benthos, from the Callovian-aged Christian Malford Lagerstätte, U.K. Our new clumped isotope data suggest that the water-calcite ¹⁸O-fractionation factor in belemnite calcite was larger than that of the commonly used δ¹⁸O thermometry equations (e.g. Kim and O’Neil, 1997), and which is currently observed in other marine calcifiers. Our reconstructions suggest that the oxygen isotope fractionation is compatible with that observed in slow-forming abiotic calcites (e.g. Coplen, 2007) and in rapidly precipitating Travertines (Kele et al. 2015). The application of more established δ¹⁸O thermometry equations (Kim and O’Neil, 1997) to belemnite calcite for temperature reconstructions has resulted in a consistent underestimation of belemnite calcification temperatures, which has led to erroneous conclusions about belemnite life habits, and underestimation of global temperatures during these greenhouse times. We therefore advocate the use of calcite equations based on low precipitation rate experiments (e.g. Coplen, 2007; Kele et al., 2015) for belemnite rostra temperature reconstructions.

How to cite: Vickers, M., Bernasconi, S., Ullmann, C., Hesselbo, S., Price, G., and Korte, C.: Belemnites, clumped isotopes and oxygen fractionation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-260, https://doi.org/10.5194/egusphere-egu2020-260, 2019

The closure of Late Miocene Mediterranean-Atlantic gateways and the restriction of Mediterranean Outflow Water (MOW) led to the Messinian Salinity Crisis (MSC), the dynamics of which is not well understood. However, restriction of the Mediterranean-Atlantic exchange and the Mediterranean Outflow Water (MOW) is one of the prerequisites to generate hypersaline conditions for evaporitic deposition. During the Late Miocene, MOW circulation was active through a Mediterranean-Atlantic exchange of the Betic, Riffian, and possibly Gibraltar gateways. This connection is thought to have ceased or reduced with the onset of the MSC, before re-establishing through the Gibraltar gateway since the Pliocene to the present. In this study, we define the sedimentary evolution of the Neogene Basins of the Gulf of Cádiz to investigate MOW evolution during the latest Miocene. Seismic interpretation shows an Upper Messinian sedimentary unit of transparent seismic facies. It could also be found in the lower Guadalquivir and Gharb basins, and towards the West Portuguese margin. Biostratigraphic dating indicate an onset of deposition predating the MSC. Distribution of this transparent unit implicates the dominant deposition of hemipelagic/pelagic deposits during a period of quiescence in the Atlantic margins, subsequent to MOW disconnection. This suggests that weakening or cut-off of the intermediate bottom currents of the Mediterranean-Atlantic exchange through the Betic-Gibraltar-Riffian paleo-gateways precedes the onset of MSC evaporites. This work is crucial for the understanding of sedimentary, paleoceanographic and climatic implications of the Latest Miocene Mediterranean isolation in the Atlantic margins.

How to cite: Ng, Z. L., Hernández-Molina, F. J., Duarte, D., Sierro, F. J., Ledesma, S., Llave, E., Roque, C., and Rogerson, M.: Latest Miocene Mediterranean-Atlantic gateway restriction: The Atlantic's side of the story, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-82, https://doi.org/10.5194/egusphere-egu2020-82, 2019

Cenozoic (past ~66 Myr) sea-level history reflects temperature changes and cryospheric evolution of the Earth from essentially ice-free conditions in the Early Eocene to bipolar ice sheets in the Quaternary. We derived a global mean sea level (GMSL) estimate for the Cenozoic using a new astronomically calibrated Pacific benthic foraminiferal δ¹⁸O splice from published records and a 2 Myr-smoothed Pacific bottom water temperature record based on published benthic foraminiferal Mg/Ca data. Our GMSL estimates are similar to sea-level estimates derived from “backstripping” (progressively accounting for compaction, loading and thermal subsidence) of cores from the mid-Atlantic U.S. continental margin. Peak global warmth, elevated GMSL, high CO₂, and largely ice-free conditions occurred during the Early Eocene “Hothouse.” During the Middle-Late Eocene “Cool Greenhouse,” small ice sheets associated with lower atmospheric CO₂ drove sea-level changes. Continental-scale ice sheets began in the Oligocene “Icehouse” (ca. 34 Ma), a permanent East Antarctic ice sheet began in the middle Middle Miocene (ca. 12.8 Ma), and full, bipolar glaciation began in the Quaternary (ca. 2.55 Ma). The Last Glacial Maximum (20-27 ka) was the largest lowering of GMSL (~130 m) of the Mesozoic-Cenozoic and GMSL rise during last deglaciation (ca. 19-10 ka) exceeded 40-45 mm/yr.  Sea-level rise progressively slowed from 10 ka to 2 ka and was then at stillstand until late 19^th to early 20^th century when rates began to rise. Despite large uncertainties in proxies, our study reaffirms that throughout the Cenozoic, high long-term (10⁷-year scale) CO₂ was associated with warm climates and high sea levels.  However, sea level-change was dominated by periodic, astronomically controlled (10’s kyr-Myr scale) Milankovitch variations superimposed upon longer-term changes driven by CO₂.

How to cite: Miller, K., Browning, J., Schmelz, W. J., Kopp, R., Mountain, G., and Wright, J.: Cenozoic sea-level and cryospheric evolution from deep-sea geochemical and continental margin records, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10017, https://doi.org/10.5194/egusphere-egu2020-10017, 2020

The Ediacaran–Cambrian (E–C) transition witnessed profound biological and oceanic changes, among which the appearance of explosive radiation of skeletonized animals is considered as the most remarkable one. Although the prominent rise of oxygen concentration in atmosphere and ocean is believed to be the major trigger of the “Cambrian Explosion”, it is still an open question about the persistence of anoxic/ferruginous deep waters. In this study, the δ¹⁵N_bulk, δ¹³C_org values, TOC and TN contents, as well as trace elements concentrations of Well ZK4411 fresh core samples are analyzed to explore redox conditions of deep ocean, the nitrogen cycle and their relationships with biological evolution. The Mo concentrations, Mo–U covariations and Th/U ratios of studied samples indicate the oxic water condition of the mid-depth ocean during the late Ediacaran and Cambrian Fortunian stage, widespread anoxic/euxinic water conditions with temporarily sulfidic water condition during Cambrian Stage 2 and early Stage 3, and oxic water condition during the middle and late Stage 3. Based on a combination of δ¹⁵N_bulk values from this study and other published data, the low δ¹⁵N_bulk values (4⁺ assimilation in the photic zone, and the extinction of Ediacaran-type and small shelly-type animals during widespread anoxic event of Cambrian Stage 2 and early Stage 3. However, the high δ¹⁵N_bulk values, close to those of modern ocean sediments, imply that an established large NO₃⁻ reservoir and the existence of the well-oxygenated mid-depth ocean during the middle to late Stage 3, which is coincident with the appearance of larger, diversified skeletonized animals as exemplified by the Chengjiang Biota. All these results indicate a stepwise oxygenation of the early Cambrian deep ocean and an increasing supply of nitrogen nutrient, which leads to the Cambrian diversification and ecological radiation.

How to cite: Tian, H., Gai, H., and Wu, Y.: Nitrogen isotope evidence for the stepwise oxygenation of the mid-deep ocean across the Yangtze Block during the Ediacaran-Cambrian transition in South China , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13097, https://doi.org/10.5194/egusphere-egu2020-13097, 2020

The Ordovician was an important interval in Earth’s history, characterized by major sea level fluctuations, carbon cycle and climatic perturbations, as well as a profound increase in marine biodiversity – the Great Ordovician Biodiversification Event (GOBE) (1-4){Sepkoski, 1981 #6}. Recently, direct links between Ordovician climatic evolution and biotic turnover have been proposed, based on geochemical data obtained from the East Baltic (5). However, the potential impact of late diagenetic modification of the geochemical compositions remains to be evaluated. Based on calcitic fossil brachiopods and bulk rock carbonates, this study documents the Early (Floian) to Late Ordovician (Sandbian) carbon (C) and oxygen (O) isotope evolution on the Swedish island of Öland, which was situated in the central part of the Ordovician Palaeobasin on the palaeocontinent of Baltica. The near-primary nature of the carbon and oxygen isotopic trends and its potential palaeoenvironmental significance is evaluated using optical, chemical and statistical methods. The results suggest that diagenetic alteration may have shifted both C and O isotope compositions to higher values, in contrast to classical interpretations. Nevertheless, both long-term and shorter-duration C and O isotope trends of palaeoenvironmental significance are discernible. Carbon isotope compositions suggests that despite the influence of late diagenesis, prominent C isotope perturbations are robust enough to be recorded in both bulk carbonates and calcitic brachiopods. Our Baltic oxygen isotope record reveals a long-term increase in carbonate oxygen isotopic composition during the Ordovician – consistent with the general O isotope Phanerozoic trend (1, 2, 6), which is most pronounced in the Darriwilian (Middle Ordovician). Therefore, the Ordovician brachiopod fossils, although partially altered, preserve a record of Middle Ordovician climate amelioration; supporting recent suggestions of Middle Ordovician climatic cooling (5, 7).

REFERENCES

1. J. Veizer et al., ⁸⁷Sr/⁸⁶Sr, δ¹³C and δ¹⁸O evolution of Phanerozoic seawater. Chemical geology 161, 59-88 (1999).
2. H. Qing, J. Veizer, Oxygen and carbon isotopic composition of Ordovician brachiopods: Implications for coeval seawater. Geochimica et Cosmochimica Acta 58, 4429-4442 (1994).
3. A. Munnecke, M. Calner, D. A. Harper, T. Servais, Ordovician and Silurian sea–water chemistry, sea level, and climate: a synopsis. Palaeogeography, Palaeoclimatology, Palaeoecology 296, 389-413 (2010).
4. J. J. Sepkoski, Jr., A factor analytic description of the Phanerozoic marine fossil record. Paleobiology 7, 36-53 (1981).
5. C. M. Ø. Rasmussen et al., Onset of main Phanerozoic marine radiation sparked by emerging Mid Ordovician icehouse. Scientific Reports 6, (2016).
6. J. Veizer et al., Oxygen isotope evolution of Phanerozoic seawater. Palaeogeography, Palaeoclimatology, Palaeoecology 132, 159-172 (1997).
7. S. Stouge, G. Bagnoli, J. A. Rasmussen, Late Cambrian (Furongian) to mid-Ordovician euconodont events on Baltica: Invasions and immigrations. Palaeogeography, Palaeoclimatology, Palaeoecology, (2019).

How to cite: Edward, O., Korte, C., Ullmann, C. V., and Rasmussen, C. M. Ø.: Late diagenetic versus near-primary isotopic compositions in Ordovician carbonate rocks and fossils: A Baltoscandian example, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-14482, https://doi.org/10.5194/egusphere-egu2020-14482, 2020

Metazoan reef builders receded globally during the Carboniferous, after the Late Devonian extinction event, with only few exceptions of coral-bearing bioconstructions reported worldwide. Among the latter, two exceptional extended coral reefs, dated as Late Viséan–Serpukhovian and Kasimovian-Gzhelian in age, respectively, were recently reported from southern China. The scarcity of coral buildups worldwide suggests global unfavorable conditions, with specific settings considered to represent refugia. To constrain these environmental conditions, seawater composition is reconstructed using carbon and oxygen isotopes originating from five measured sections located in southern China.

The resulting δ¹³C data reveals several environmental changes throughout the Carboniferous attributed to climate changes, ocean current variations, and proliferation of terrestrial plants. During the Late Viséan-earliest Serpukhovian, the high δ¹³C values (_˜3‰) are interpreted as recording short-lived glacial events, with the expansion of ice-sheets in South America and eastern Australia. The scarcity of coral reef growth suggests that the cooling acted as an inhibiting factor during this period. Conversely, the development of the exceptional coral reefs in southern China could be explained by the persistence of warm oceanic currents in the epicontinental sea, located in equatorial position. During the Kasimovian-Gzhelian, the gradual δ¹³C positive shift from -0.7 to +4.7‰ coincides with a short-lived warming, which should be suitable for the recovery of coral communities. However, in spite of the mild climate, the scarcity of Pennsylvanian coral reef leads to consider other inhibiting factors (e.g. biological competition and aragonite seas). Interestingly, the disappearance of coral reefs in southern China correlates with negative δ¹³C shifts (e.g. Mid-Viséan, Late Gzhelian), interpreted as related to intensified upwellings.

The reconstitution of the Carboniferous environmental conditions documents several factors contributing to the metazoan reef demise and recovery subsequent to the Late Devonian extinction events, and adds to our current knowledge of the longest reef recovery in the Phanerozoic.

How to cite: Maillet, M. and Samankassou, E.: Oceanic δ13C, environmental changes and Carboniferous coral reef development: Records from Tianlin and Ziyun (southern China), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5054, https://doi.org/10.5194/egusphere-egu2020-5054, 2020

Large-scale studies of Carboniferous–Permian trilobites by Russian paleontologists were completed about 80 years ago. As a result, the systematic and stratigraphic distributions of the previously described taxa require revision in accordance with modern data. In addition, in recent decades, a fairly large number of new, not yet described fossil materials have accumulated.

The author conducted a revision of all localities of the trilobites of the Carboniferous and Permian of the former USSR. Most of localities received accurate geographic location and age in accordance with modern data, which allowed us to review the stratigraphic distribution of many species and some genera. So, e. g., the endemic genus Anujaspis (North-East of Russia), widely known in the literature, which was long considered as Artinskian, is actually not younger than the Bashkirian.

In the process of research, the author established 6 new species: Brachymetopus (Conimetopus) alekseevi Mychko, 2019 (Asselian of the Urals), Ditomopyge (Carniphillipsia) mosquensis Mychko et Alekseev, 2018 (Gzhelian of the Moscow Region), Pseudophillipsia (Pseudophillipsia) darvazica (Lower Permian of the Darwaz; unpublished), D. (D.) arctica (Kasimovian of the Novaya Zemlya; unpublished), D. (D.) zhirnovskiensis (Kasimovian of the Volgograd Region; unpublished), Paraphillipsia urushtensis (Upper Permian of the Northern Caucasus; unpublished).

The other interesting group is Cyclida – enigmatic the Late Paleozoic–Mesozoic arthropods. Their taxonomic position is still open. Now 55 species and 17 genera have been described. Cyclids on the territory of Russia are extremely rare: until now only 6–7 forms from Carboniferous and Permian were known. Since 2016, the author has been searching for new material and revising all previously known cyclids of this territory.

Based on the new material, the author (with co-authors) established 2 new genera (Skuinocyclus and Prolatcyclus) and 2 new species: Skuinocyclus juliae Mychko & Alekseev, 2018 and Prolatcyclus kindzadza Mychko et. al., 2019. According to the results of the revision of previously known forms, it was found that Cyclus spinosus and C. tuberosus are synonyms and represent a new undescribed genus; C. miloradovitchi is a typical species for the new genus Uralocyclus Mychko & Alekseev, 2018. A new species of cyclids from the Carboniferous of Urals is currently under description.

Some researchers include cyclids in the subclass Branchiura along with modern parasitic carp lice. However, in the structure of cyclids there are features that contradict this: Skuinocyclus juliae has small lugs on the ventral side of the carapaces, which significantly complicated the ectoparasitic lifestyle. New taxa expand our knowledge of the biodiversity of this extinct animal group.

The study was funded by Russian Foundation for Basic Research (RFBR), project No. 18-35-00165.

How to cite: Mychko, E.: New rare Arthropods (Trilobites and Cyclids) from Carboniferous and Permian of Russia, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-105, https://doi.org/10.5194/egusphere-egu2020-105, 2019

Several characteristics and properties of Jurassic carbonates have received great attention as they have economic significance and was attractive for several researchers. Micro-fauna was studied for various purposes such as biostratigraphy and paleo-environment. Stable isotope composition analysis of ancient successions of carbonates is one of the strongest and powerful tools for the purpose of reconstructing paleoclimate and paleo-environment. This study aims to refine the stratigraphic signatures on the Carbon and Oxygen isotope composition through picking different timeline surfaces during middle Jurassic Tuwaiq Limestone Formation and related sequences. Field observations were carried out and samples were collected. Several laboratory analyses were performed including thin section petrography, X-Ray Diffraction (XRD) and Scanning Electron Microscope (SEM). Powder samples were prepared to analyze isotopic composition.

Tuwaiq Mountains Formation consists of skeletal, oolitic, intraclasts, fossiliferous planar laminated limestones and trough crossbedded limestone in the most upper part of the studied section. The cherty and bioturbated limestone is predominated also as well as large reef bioherm within the thick massive beds. The studied section consists of several types of foraminiferal assemblage species including Trocholina elongata, Redmondoides lugeoni, Kurnubia palastiniensis, Cladocoropsis, and minor amounts of Lenticulina and Nautiloculina oolithica. Other non-foraminiferal species also exist including noncostate gastropods, brachiopods fragments, bivalve, echinoderm, and coral fragments. Based on previous studies and the results of fossil associations that are found in the studied section, the  Tuwaiq MountainsLimestone Formation varies from fore-bank into lagoonal and intertidal sub-environments. Due to the absence of sponge spicules species, deep marine intra-shelf basin was excluded. Tuwaiq Mountain Limestone Formation is composed of shallowing upward outer lagoon (T1) graded upward into a back reef (T2) and reef (T3) Members.   This is reflected in the vertical signatures of both Oxygen and Carbon isotopes. Both of them are enriched upward because of the change  to more shallow marine lithofacies (low temperature and higher organic productivity). Two breakthroughs were observed in the upper part of Tuwaiq Formation where strong depletion for both Carbon and Oxygen isotopes is found, this is thought to be a result of a nearby continental source of negative lighter carbon and oxygen isotopes (δ12C and δ16O). Chert nodules and stratified chert might have an impact on isotope curves and thought to be associated with the overlying intensive occurrences of sponge spicules that are rich in silica in the lower parts Hanifa Formation. The overall enrichment in Oxygen isotopes is probably associated with the predominant Callovian-Oxfordian polar glaciation.

How to cite: Abdlmutalib, A. and Abdullatif, O.: Integration of lithofacies, biofacies, and stable isotope analysis of Middle Callovian Tuwaiq Formation: Implications for paleoenvironments and paleoclimate, Central Saudi Arabia., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1481, https://doi.org/10.5194/egusphere-egu2020-1481, 2019

During the Albian, the open marine carbonate production underwent a profound revolution with the onset of the dominance of planktonic production in the total carbonate budget. This led to the deposition of vast amounts of chalk across the world’s Oceans as a result of the accumulation of large amounts of nannoplankton. The worldwide Upper Cretaceous white chalks are however not the first true chalks (i.e. deposits dominated by calcareous nannofossils) to be recorded in Earth’s History. Already during the Barremian, chalks were deposited in the North Sea Central Graben. These chalks did not extend until the Albian, since a ‘nannoconid crisis’ occurred at the onset of the early Aptian OAE-1a, with the deposition of an organic-rich marlstone layer named the Fischschiefer. To better understand if climatic changes have governed the occurrence of the Barremian true chalks and the switch to organic-rich marlstones during OAE-1a, we have reconstructed the evolution of climate in the North Sea Basin based on clay mineral assemblages. Clay mineral composition and distribution are proven indicators of paleo­climate and evolution of a basin as the formation of clay minerals in soils depends on the climate under which it develops. Hence, based on high-resolution clay mineral data from various cores from the North Sea, a paleoclimatic reconstruction of the late Hauterivian to early Aptian stratigraphic interval is proposed. Based on a long-term decrease of kaolinite content, a trend toward aridification is observed during the late Barremian, concordant with the development of the first true chalks. A sharp increase in kaolinite content is recorded at the onset of OAE-1a, with its highest peak occurring towards the end of the event. This suggest that a significant increase in humidity accompanies the unfolding of OAE-1a in the North Sea Basin. Further investigation is needed to confirm the hypothesis that paleoclimatic changes in the Boreal Realm are responsible for the onset of chalk deposition and the change in clay mineral assemblages.

How to cite: Blok, C., Fantasia, A., Anderskouv, K., Ineson, J., Edvardsen, N., Adatte, T., and Bodin, S.: New insight on the paleoclimatic evolution in the Boreal Realm at the onset of the Early Cretaceous chalk sea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9192, https://doi.org/10.5194/egusphere-egu2020-9192, 2020

The Cenomanian–Turonian boundary interval (~94 Ma) was marked by a period of climatic turbulence, and featured the widespread expansion of strongly oxygen-depleted conditions across a large part of the global ocean; collectively these environmental degradations are referred to as an oceanic anoxic event or OAE (specifically OAE 2 for this time interval). Extremely high sea-surface temperatures are documented for several regions during OAE 2, likely beginning at the onset of the event, but a shift towards colder conditions during the early stages of the OAE (the Plenus Cold Event or PCE) is also recorded in several locales, before a return to a very warm climate during the latter part of the crisis. The overarching high temperatures are thought to have resulted from major volcanic activity during the emplacement of one or more oceanic plateaus, as evidenced by a globally documented shift in osmium-isotope ratios to very unradiogenic values just below the base of OAE strata that indicates a very large flux of mantle-like osmium to the open ocean at that time. Intriguingly, the PCE cooling has been shown as likely non-synchronous globally, suggesting a local control in addition to/instead of global forcing; whilst the high temperatures associated with OAE 2 appear to have continued long after the OAE itself ceased.

This study presents new osmium-isotope data from the New Jersey shelf of the proto-North Atlantic (ODP Leg 174AX: Bass River), correlating the results with a previously generated sea-surface temperature dataset from the same site. These results are then compared with other temperature archives and osmium records of oceanic-plateau activity for OAE 2. The new data indicate intense oceanic-plateau activity prior to and in the earliest stages of the OAE, with a decline in mantle-osmium output before the end of the event, consistent with previous findings. However, when the osmium data are directly correlated with temperature records, both at Bass River and other sites, they clearly show that not only were high sea-surface temperatures maintained after the OAE, but also after oceanic-plateau activity (and presumably associated volcanism and CO₂ emissions) fell. Thus, a reduction in mantle carbon output did not manifestly result in an immediate reduction of atmospheric CO₂. Moreover, the beginning of the osmium recovery broadly correlates with the end of the PCE cooling at all locations where both osmium and temperature trends have been studied. Consequently, although the PCE cooling was not globally synchronous and its precise timing at individual locations was likely controlled by local processes, some feature of the oceanic-plateau development allowed the cooling spells to occur when plateau activity was most intense, before a reduction in that intensity stymied the spread of cold conditions and resulted in the restoration of high temperatures in the latter stages of the OAE and beyond. These data highlight the need for further work to understand the complexity of and nuances in the relationships between large-scale volcanism and major climate/environment perturbations, both for OAE 2 and for other events.

How to cite: Percival, L., van Helmond, N., Selby, D., Goderis, S., and Claeys, P.: Links between temperature changes and oceanic-plateau emplacement during the Cenomanian–Turonian Oceanic Anoxic Event (OAE 2), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13304, https://doi.org/10.5194/egusphere-egu2020-13304, 2020

The Cenomanian-Turonian (C-T) mass extinction occurred during a peak global greenhouse interval, with eustatic sea-level elevated nearly 300 m above present stand. The time period spanning the Cenomanian–Turonian boundary was characterized by profound paleoenvironmental changes at global scale. World-wide consequences of these events have been the focus of new reports and their influence on local to regional depositional systems. A faunal turnover is recorded in the uppermost Cenomanian, marked by the disappearance of most of the Cenomanian taxa. The macrofossil contents of two Upper Cretaceous sections from Wadi Qena, central Eastern Desert, Egypt have been collected and studied in detail. These fossils, ranging in age from Late Cenomanian to middle Turonian. The Cenomanian- Turonian sequence of centeral Eastern Desert is represented by the fossiliferous Galala formation (about 90 meter) at base and Umm Omeiyid formation (about 40 meter) at top. The Galala Formation is characterized by shale, fossiliferous marl, marly limestone, sandstone and siltstone interbeds. The Umm Omeiyid Formation overlies the Galala Formation unconformably. It consists of unfossiliferous siltstone with fine, medium-grained hummocky cross-stratified sandstone intercalations with a few fossiliferous limestone beds and marl intercalations. The Cenomanian-Turonian boundary cuts within the upper part of the Galala Formation at the last occurrence (LO) of Vascoceras cauvini and the first occurrence (FO) of Vascoceras proprium. There is rapid faunal change across the Cenomanian-Turonian boundary in all the studied sections. Most benthic fauna become extinct at the Upper Cenomanian and new taxa appeared at the Lower Turonian. All Upper Cenomanian cephalopod taxa become extinct at the same level and new taxa appeared at the Lower Turonian. The faunal diversity decreased from the Upper Cenomanian to the Lower Turonian. The ammonite ranges are used for a biostratigraphic zonation of the Cenomanian- Turonian succession in the northern and central parts of Wadi Qena. five ammonite zones have been distinguished (Neolobites vibrayeanus Zone , Vascoceras cauvini Zone , Vascoceras proprium Zone , Choffaticeras (Choffaticeras) segne Zone  and Coilopoceras requienianum Zone).

How to cite: Hefny, A., El Qot, G. M., El Araby, A. E. M., and Aly, M. F.: The Cenomanian–Turonian boundary mass extinction (Late Cretaceous): Ammonoid biodiversity in the Eastern Desert, Egypt, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8319, https://doi.org/10.5194/egusphere-egu2020-8319, 2020

The Aptian-Albian transition is marked by the unfolding of the Oceanic Anoxic Event (OAE) 1b, a protracted environmental perturbation characterized by occurrence of several sub-events out of which the Kilian and Paquier events are the most well-known ones. So far, the conditions leading to the unfolding of the OAE 1b cluster and its sub-events, as well as their consequences, remain elusive as most of the studies have focussed on the Paquier level, thereby precluding a broader perspective on this event. In this study, we focus on an extended stratigraphic interval from the Brier section (Vocontian Basin, SE France) spaning the Kilian to Paquier levels interval. Our goal is to better understand the processes having led to organic matter (OM) accumulation across this stratigraphic interval as well as to constrain the exogenic carbon cycle framework in which these changes are inscribed. For this purpose, we have performed high-resolution bulk-rock pyrolysis analyses, paired stable carbon isotope measurements on both bulk carbonate and organic matter, and handheld XRF analyses.

Measured total organic contents (TOC) average 1.5% with peaks reaching 3% in the Paquier level. Apart for the Kilian, Paquier and HN 12 levels, which are characterized by the dominance of marine organic matter, the remainder of the studied interval is characterized by the accumulation of continental organic matter. Moreover, there is a good correlation between changes in the long-term TOC content and detrital input as inferred from changes in element concentration such as aluminium and thorium. A preservation model therefore best explains the long-term OM accumulation across the studied interval. Sporadic episodes of enhanced marine OM productivity account only for the deposition of the Kilian, Paquier and HN 12 levels.

Carbon isotope analyses shows that the Kilian and Paquier levels are both associated with a 0.5 – 1‰ negative excursion in the bulk carbonate record. In the bulk OM record, the C-isotope signal is however different. The Kilian level is hence characterized by a 3‰ negative excursion whereas the Paquier level is characterized by a 4‰ positive excursion. This discrepancy is due to the fact that the bulk OM C-isotope record is strongly influenced by the mixing of different types of organic matter. By applying a correction factor tacking into account the type of organic matter, as characterized by the pyrolysis analyses, both OM and carbonate C-isotope records can be reconciled.

Importantly, our paired C-isotope record shows that in between the Kilian and Paquier levels, two others episodes of similar negative C-isotope excursion occur, with an abrupt onset and a total amplitude of 1‰. These episodes likely correspond to the Monte Nerone level observed in Italy.  The unfolding of OAE 1b cluster is thus thightly tied to a very dynamic exogenic carbon cycling, characterized by repeated injections into the oceans-atmosphere of light isotopic carbon, potentially similar to the Early Eocene scenario.

How to cite: Charpentier, M., Ullmann, C. V., Rudra, A., Sanei, H., and Bodin, S.: Changes in carbon cycling across the OAE 1b cluster (Aptian-Albian transition): New insights from the Vocontian Basin, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13036, https://doi.org/10.5194/egusphere-egu2020-13036, 2020

The evolution of planktonic foraminifera in Cretaceous is forced by the paleo-oceanographic changes associated with Oceanic Anoxic Events (OAEs). The paleo-oceanographic conditions including global sea level rise, loss of ocean stratifications and subsequent loss of niche partitioning, ocean acidification and warming were common during early-middle Cretaceous. The evolution of planktonic foraminifera is associated globally with such changes in oceans. The global warming of early to middle Cretaceous was replaced by a period of cooling during early Maastrichtian. The stratification in global oceans resumed and the diversity of planktonic foraminifera increased. This study deals with signals of paleo-oceanographic conditions of Maastrichtian time preserved in Mughal Kot Section, Eastern Tethys.

A thick succession of hemipelagic sediments of Maastrichtian age is exposed in Mughal Kot Formation, Indus Basin, Pakistan. The studied section revealed abundant taxa of Globotruncana, Globotruncanita and Heterohelix. Based on these taxa a local assemblage biozone of Maastrichtian (~76-~69 Ma) age is erected. The overall species richness in the studied section is very low. This significantly low richness is thought to be associated with high sedimentation rate as 1100 meter thick strata is deposited during Maastrichtian. Presence of turbidite beds supports such high sedimentation rates. However, a consistent decrease in species richness from base (early Maastrichtian) to the top (late Maastrichtian) is recorded in the section. This decrease is associated with the dominance of opportunistic taxa of Heterohelix. The proliferation of opportunistic taxa at the expense of specialized taxa during late Maastrichtian gives a strong clue that the global cooling of early Maastrichtian was replaced by warming during late Maastrichtian. Such warming may have resulted in the melting of polar ice, uniformity in the physical properties of water masses in global oceans and hence destruction of habitat for the dwelling of specialized foraminifera. 

How to cite: Wadood, B., Khan, S., Li, H., and Liu, Y.: Paleoceanographic reconstruction of Maastrichtian from the hemipelagic sediments of Pakistan, Eastern Tethys, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2258, https://doi.org/10.5194/egusphere-egu2020-2258, 2020

The rifting and the amalgamation of earth landmass is a continuous process. The assembly of the Gondwana lasted from ~730 Ma to 500 Ma, and most of the mass is covered by glaciation at the southern hemisphere. Afterthought experienced multiple episodes of rifting and collision of small ribbon shape microcontinents. The extra-peninsular Gondwana sequence is discontinuous in the Himalayan orogenic belt while peninsular Gondwana sequence is broadly distributed in numerous intracratonic basins of peninsular India. The detrital zircon U-Pb ages from Permo-Carboniferous sequence peak at ~1164 with a subordinate peak at ~1305 Ma. This result emphasised that the sediments were mainly sourced from the Stenian magmatism in Albany-Fraser orogeny or the East Africa-Nibua and eastern coast of India, and southwest Australia. Also, the unit also contains sporadic volcanic unit (Baraha Volcanics). The Saptakoshi Formation, uncomfortably overlain the Khokha Diamictite, yield the peaks at ~522 Ma and 941 Ma with a younger peak at ~113 Ma with some older peaks at ~1811 and 1917 Ma. This younger detritus possibly sourced from the Rajhmahal basalt ~~115-120 Ma) while the remaining grains show a similar trend to the underlying Diamictite and overlying Tamrang Formation. Additionally, the Tamrang Formation have peaks at ~976 Ma, and 1716 Ma, identically identical to the Greater Himalayan sequence. The U-Pb age distribution of these three units coincide with the Tethys Himalaya further brings the possibility that either they share the same provenance or recycled from the Tethys Himalaya till Permian and onwards there was input from the Lhasa terrane, South Qiangtang terrane, and Indo China blocks.

How to cite: Baral, U. and Lin, D.: The break-up of the Indian subcontinent from Gondwana: constrain by detrital zircon U-Pb dating of mid- Paleozoic-early Cenozoic strata in eastern Nepal, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-66, https://doi.org/10.5194/egusphere-egu2020-66, 2019

The late Carboniferous to early Permian interval was a time of major global changes on Earth. It encompassed the final consolidation of the Pangaea supercontinent and the subsequent development and infill of continental basins. This interval also witnessed a climate transition towards warmer conditions, including the substantial retreat of the extensive continental ice sheets developed during the Late Palaeozoic Ice Age (LPIA). The end of this climate transition represented an evolution form an icehouse to a greenhouse period at the early to middle Cisuralian, which in turn was a time of important tectonic changes in Pangaea. However, our understanding of this whole chain of processes still has many gaps and many of its features still generate debate, especially those concerning the origin and the end of this transition period. This study focuses the late Carboniferous to early Permian interval by means of a multidisciplinary study in the Pyrenean (P) and Basque-Cantabrian (BC) basins, that constitute a near equatorial complex extensional structure located in the middle of the late-Variscan fold belt. The present-day Pyrenean Ranges, extending E-W along almost 1000 Km, from the Mediterranean coast to the Basque-Cantabrian Cordillera, is the result of intense inversion of previous Permian to Cretaceous extensional to transtensional rift basins. In those basins, the time-interval has been traditionally studied in two separated structural units: Basque-Cantabrian Pyrenees and Aragonese-Catalan Pyrenees. However, this multidisciplinary study, proposes the existence of a lateral tectono-sedimentary connection for the entire basin.  Nine late Carboniferous - early Permian sub-basins have been studied in the Basque-Cantabrian and Pyrenean domain. From west to east they are: Frieres, Sotres, Carmona, Anayet, Aragorn-Bearn, Castejón-Sas, Erill Castell, Gramós and Camprodón. Three lithological units, separated by unconformities and dated by means of pollen associations and radiometric data, represent the Permian record of these basins. They constitute three cycles of sedimentation of similar age (Gzhelian-Asselian, Asselian-Sakmarian, and Sakmarian-Kungurian) related to the same post-Variscan geodynamic stages of evolution affecting the whole Pyrenean and Basque-Cantabrian basins. The two younger cycles broadly coincide with other similar cycles defined in SE France and Sardinia, thus pointing to a common tectono-sedimentary evolution for the westernmost continental Peri-Tethys domain during this time-interval. In addition, the middle cycle (Asselian-Sakmarian) mostly represents a calc-alkaline volcano-sedimentary event in most of the sub-basins, related to a post-Variscan extensional phase. Detailed studies of paleosols, sedimentology, and mineralogy indicate a progressive evolution towards warmer conditions that were accelerated during the Artinskian, probably related to the end of the global icehouse period associated to the LPIA in the equatorial late-Variscan fold belt domain.

How to cite: Barrenechea, J. F., Lloret, J., de la Horra, R., Gretter, N., Borruel-Abadía, V., Ronchi, A., López-Gómez, J., Juncal, M., and Díez, J. B.: Late Carboniferous-early Permian tectono-sedimentary cycles and climate proxies in the Pyrenean and Basque-Cantabrian basins, N Spain., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3629, https://doi.org/10.5194/egusphere-egu2020-3629, 2020

The Phanerozoic Parnaíba Basin occupies 600.000km² in northeast Brazil, covering cratons and Neoproterozoic belts. Its Central-West region is mostly represented by the Jurassic-Cretaceous Sequence (Mosquito, Corda Grajaú, Codó and Itapecuru formations) recording magmatic events from the Central Atlantic Magmatic Province, with depocenters migrations and shifts on depositional environments related to Pangea breakup.  This work discusses the Jurassic-Cretaceous siliciclastic units testing possible sedimentary source areas with U-Pb and combined Lu-Hf data on detrital zircons, using LA-ICP-MS. The basalts from Mosquito Formation are dated at +/- 198Ma and the Codó Formation present accurate Aptian fossil data. This formation records a hypersaline lake system, succeeded by a transgression that represents pioneer marine ingression within an intracontinental rift. The other units (Corda, Grajaú and Itapecuru) are constituted by siliciclastic sediments involved in intracontinental sub-environments. The Corda Formation consists of aeolian system, sand sheets and wadis deposited in a desertic setting. The contact between the subsequent Grajaú Formation is abrupt, represented, at the base, by thick coarse braided river facies grading laterally and upwards to ephemeral channels in association with low amplitude Aeolian dunes, evidencing still arid conditions. Interlayered beds of fluvial and aeolian sandstones within lacustrine deposits, indicates that Codó and Grajaú formations consists the same seasonal fluvial-lacustrine system. The last Itapecuru Formation, is represented by a thick red sandstone succession deposited in a deltaic system. Paleocurrents measurements below Codó Formation (i.e. Corda and lower Grajaú) points a W-NW sense of direction, whereas paleocurrents above Codó Formation (i.e. upper Grajaú and Itapecuru) presents a regional sense to E-NE. Detrital zircons geochronology analysis helped to identify the source area of sediments through the comparison of the main ages of possible uplifted tectonic terranes. The preliminary results revealed that sandstones below Codó Formation shows a major Neoproterozoic population (56, 41% to 40%) with age peaks at 583 and 628 Ma; and also Paleoproterozoic (43, 48% to 35,05%); Archean (4,35%) and Paleozoic (2,61%) populations. Sandstones above Codó Formation, also show a Neoproterozoic major detrital zircon population (40% to 37,12%) with 625, 665 and 783 Ma age peaks. Two other populations are present: Paleoproterozoic (22.68% to 20%) with peaks at 1749 and 1881 Ma, and Archean (24,45% to 15,47%). This last source has a greater contribution than in the formations below the Codó maker. We envisaged that the shift from W-NW to E-NE sandstones paleocurrent is coherent with the rise on Archean contribution, possibly related to the Amazon Craton to the West. In addition, the youngest Phanerozoic detrital zircons obtained in all samples are minor (6,66% to 6,18%). The integration of field stratigraphic analysis, paleocurrents and detrital zircon provenance studies corroborate to the hypothesis that Codó Formation must represent a Cretaceous stratigraphic datum for the transition of a rift and post-rift phase, thus the change of source areas is consistent.

The authors acknowledge support from Shell Brasil Petroleo Ltda. and ANP (Brazil’s National Oil, Natural Gas and Biofuels Agency) through the R&D levy regulation (Technichal Cooperation #20.219-2).

How to cite: de Assis, A. P., da Cruz, K. A. C., da Silvia Schmitt, R., and de Medeiros, S. R.: Jurassic-Cretaceous paleogeography of Central-West Parnaiba Basin, NE Brazil - stratigraphy and sedimentary provenance of detrital zircons, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22505, https://doi.org/10.5194/egusphere-egu2020-22505, 2020

The Espírito Santo Basin is part of the Brazilian rift system basins, formed by the break-up of the Supercontinent Gondwana and subsequent opening of the South Atlantic Ocean. The Aptian sedimentary succession of the basin is named Mucuri Member, lower unit of Mariricu Formation, and is contemporary to the pre-salt main carbonate reservoirs. Proximal deposition consists mostly of clastic sediments, interpreted as fluvial and coastal systems in the margins of a wide lake, which synchronously accumulated carbonates to the distal portions. The economic interest for oil is centered on the distal carbonates, thus the proximal sections lack detailed studies. The main objective of this study is the tectono-stratigraphic analysis of the marginal Mucuri Member. Leading methodology is seismic stratigraphy based on 220 2D lines and 1 3D volume, coupled with petrophysical and lithological analysis of 103 well log data and 5 cored wells. The combined analysis of seismic and lithological parameters resulted in the individualization of four seismic facies, which correspond to the predominance of one or two amongst five facies associations interpreted in cored wells. Seismic facies (SF) 1 is predominantly composed of offshore and lower shoreface successions; SF 2 is characterized by a dominance of poorly confined fluvial channels facies association; SF 3 records the interaction between fluvial and coastal successions composing mound-like structures, and is interpreted as wave-dominated deltaic facies association; SF 4 corresponds to sandspits structures and is restricted to regions where the coastal waves interact with basement highs. Four seismostratigraphic units were identified (named, from the base upwards: MUC1, MUC2, MUC3 and MUC4), delimited by three subparallel horizons. Unit-bounding reflectors coincide with gamma ray maxima or minima representing shales or anhydrites, respectively, deposited in deep water environments. The Mucuri Member records an enlargement in depositional area from the base upwards witnessing an overall lacustrine base level rise during deposition. The geometry of the depositional area during MUC1 and MUC2 was conditioned by the paleorelief of the preceding rift basins. MUC3 and MUC4 seismic units record a decrease in thickness as remnant topography was gradually filled; both units transcended and draped the half-grabens. The Early Cretaceous Mucuri Member composes the beginning of the post-rift sequence of Espírito Santo Basin, marked by the onset of thermal subsidence and cessation of mechanical subsidence.

How to cite: Amarante, F. B. D., Schilling, A. B., Kuchle, J., Iacopini, D., Scherer, C. M. D. S., Alvarenga, R. D. S., and Ene, P. L.: Tectono-stratigraphic evolution of the Aptian Pre-Salt of the onshore Espírito Santo Basin, SE Brazil, an example of proximal passive margin sag basin, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3702, https://doi.org/10.5194/egusphere-egu2020-3702, 2020

The Late Cretaceous Nise Formation is located on the Mid-Norwegian Continental Shelf. On the Halten Terrace, the formation is generally described as a mudstone interbedded with thin sandstones and carbonate stringers. The spatial distribution, connectivity, sediment source and depositional processes of the sandstones are uncertain. 

Six cores and associated thin sections of the Nise Formation on the Mid-Norwegian Continental Shelf have been described. Three of the cores are from the Halten Terrace area, and one core each are from the Vestfjorden Basin, Nyk High and the Møre/Vøring Basin transition respectively. The main focus has been on the Halten Terrace while the cores from the other areas (tens to hundreds of kilometres away from Halten) are included to compare facies, depositional environment, source of sediment, grade of bioturbation and other characteristics with those of the Halten Terrace.

An isochore map of the Nise Formation in the central part of the Halten Terrace has been constructed based on interpretation of wireline logs from approximately 280 well penetrations. The map reveals a distinct thickness increase in the central parts of the Halten Terrace, suggesting the infilling of a former sub-basin. Additionally, the map supports an eastern and/or northern sediment source, which might exclude the Greenlandic landmasses as a provenance area. The cores from the Halten Terrace, supported by additional wireline well logs, indicate that the upper parts of the formation contain a greater amount of sandstone facies compared to the lower parts. These sandstones are interpreted as densely bioturbated distal turbidites. The comparison of facies development in the cores from the different areas indicates variability in depositional systems between the areas at the time of deposition.

Different depositional scenarios of the Nise Formation on the Mid-Norwegian Continental Shelf are being developed in order to increase the understanding of the Nise Formation on the Halten Terrace. The proposed scenarios include various sediment input models and alternative depositional environments in the different areas. The scenarios mainly display marine, isolated systems. 

How to cite: Hope Blå, M., Felix, M., Watt, J., and Næss, A.: Proposed depositional scenarios of the Nise Formation on the Mid-Norwegian Continental Shelf, with focus on the Halten Terrace., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20037, https://doi.org/10.5194/egusphere-egu2020-20037, 2020

Sequence stratigraphy of the mixed siliciclastic-carbonate successions is considered to be more complex than their counterparts of pure siliciclastic or carbonate systems. This is due to the higher number of controlling factors in the depositional environment. Consequently, the reservoir and source rock characterization within these systems is relatively more difficult in term of the spatial distribution of the lithologic unit and its petrophysical properties heterogeneity.

Following Scharland et al., 2001, the Miocene Dam Formation is regarded as a third-order sequence which consists of both siliciclastic and carbonate sediments, with the dominance of carbonates. Three fourth-order sequences are well exposed in the Lidam area, Eastern Saudi Arabia. Sequence boundaries are delineated using the existing Rhizolith and desiccation cracks in the most upper part of the high stand systems tract, where it is dominated by shallow marine carbonate deposits. The early low stand systems tract is of dolomitic mudstone interbedded with dissolved chicken-wire anhydrite. Fine sandstone of estuarine origin, in addition to intertidal mudstone-sandstone succession, represent the late low stand systems tract. Transgressive surfaces are either sharp erosional surfaces or ravinement surfaces with intraformational lag deposits. Subtidal quartz skeletal wackestone with pieces of evidence of storm events represent the transgressive systems tract. Going toward the end of the vertical succession of the Dam Formation, shallow marine siliciclastics dominate. A set of incised channels filled with intraformational boulders of calcareous sandstone lies over the shallow marine siliciclastics succession, indicating a great drop in the sea level. Finally, a fluvial system of high energy braided stream origin, composed of medium to coarse ferruginous sandstone within a fining upward succession and high abundance of plant remains, represent the Hufouf Formation base and the start of a new third-order sequence. The heterogeneity of the lithofacies is intense, but the sequence stratigraphic framework helps in their rearrangement vertically into packages, which helps in the prediction of their spatial distribution.   

How to cite: Bashri, M., Abdullatif, O., Salih, M., Kaminski, M., Babalola, L., and Adam, A.: Mixed siliciclastic-carbonate sequence stratigraphy, Miocene Dam Formation, Saudi Arabia, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21218, https://doi.org/10.5194/egusphere-egu2020-21218, 2020

Heterogeneities within clinoforms, which can be in the form of variations in internal geometry and/or change in sediment distribution, may act as fluid flow barriers or conduits for hydrocarbon or freshwater reservoirs. These heterogeneities can lead to considerable uncertainty in estimating pore-fluid recovery factors by up to 35%. And yet, variations in sediment petrophysical characteristics within clinoforms have been poorly documented. Understanding the rock physics of clinoform heterogeneities along continental margins is a key to reducing the uncertainties in predicting the dynamics and the volume of recoverable pore fluids within these structures.

The Miocene sedimentary record of the New Jersey continental margin is a prime candidate for studying continental-margin clinoform structures and the variation in their petrophysical properties. The margin has experienced a stable tectonic history, smooth and gradual thermal subsidence, and continuous sediment loading in the Miocene, resulting in deposition of well-developed siliciclastic clinoformal sequences. We use data from three IODP Expedition 313 boreholes and an encompassing high-resolution 3D multichannel seismic volume, collected in 2015 by the R/V M.G. Langseth on the New Jersey continental shelf, to predict shallow-marine sedimentological properties of the Miocene clinoforms at a significantly higher resolution (~5 m laterally) than previously achieved (~100s of m). We identify 76 system tracts and 22 sequences spanning ~8 m.y. of the Miocene. The results of our 3D stratigraphic analysis provide a detailed structural framework for analyses of the Miocene deposits to: 1) define the sedimentary structure in terms of stratal packages and 2) estimate the internal clinoform heterogeneity associated with phases of known mid-Cenozoic sea-level change.

Our statistical analysis of the estimated elastic properties, including P-wave velocity (Vp), density and clay volume, reveals repeating spatial patterns in the internal rock physics properties of the Miocene clinoforms. We show that diagenesis and sediment compaction within the dipping parts of clinoforms cause a continuous increase of Vp in the seaward direction, with a magnitude that decreases from top to bottom. Our results also suggest that lithofacies change in clinoforms imposes a stronger influence on density, as lateral changes in lithofacies are more pronounced in sediment density than in Vp. In the Miocene sedimentary record, the transgressive system tracts show a seaward coarsening trend in grain size and a 3%-5% increase in density from clinoform topsets to bottomsets. Highstand and lowstand system tracts show a fining trend basinward, with a ~8% and 5% reduction in density, respectively. We further demonstrate that the identified trends can provide a standard model allowing incorporation of clinoforms in reservoir characterization techniques, such as model-based seismic inversion, and enable setting of guidelines on how the petrophysical properties change regionally in shallow-marine siliciclastic environments of continental margins.

How to cite: Aali, M., Nedimovic, M. R., Fulthorpe, C. S., Mountain, G. S., Richards, B., and Austin, J. A.: Rock physics modelling of siliciclastic shallow-marine clinoforms on the New Jersey continental shelf using 3D seismic data and well logs , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12238, https://doi.org/10.5194/egusphere-egu2020-12238, 2020

The climatic, biotic, and tectonic events of the Permian are amongst the most profound in Earth history. Global orogeny leading to Pangaean assembly culminated by middle Permian time, and included multiple orogenic belts in the equatorial Central Pangaean Mountains, from the Variscan-Hercynian system (east) to the Ancestral Rocky Mountains (west). Earth’s penultimate global icehouse peaked in early Permian time, transitioning to full greenhouse conditions by late Permian time, thus archiving the only example of icehouse collapse on a fully vegetated Earth. The Late Paleozoic Icehouse was the longest and most intense glaciation of the Phanerozoic, with hypothesized low-elevation glaciation posited for both eastern and western tropical Pangaea during early Permian time. Reconstructions of atmospheric composition record the lowest CO₂ and highest O₂ levels of the Phanerozoic, with average CO₂ levels comparable to the Quaternary, rapidly warming climate. Fundamental shifts occurred in atmospheric circulation: a global megamonsoon developed and the tropics became anomalously arid with time. Extreme environments are well documented in the form of voluminous dust deposits, acid-saline lakes and groundwaters, extreme continental temperatures and aridity, and major extinctions/extirpations, ultimately culminating at the Permo-Triassic boundary with the largest extinction of Earth history.

We seek to elucidate paleoclimatic conditions and forcings through the Permian at temporal scales ranging from the millennial to the Milankovitch  and beyond by acquiring continuous core in continental lowlands known to harbor stratigraphically complete records dominated by loess and lacustrine strata. We have identified sites in the western U.S. and Europe as the key sites globally to achieve our objectives, as these represent the western and eastern limits, respectively of the Pangaean tropics. Identified sites harbor arguably the most complete continental Permian sections in the paleoequatorial region, with adjacent paleo-uplands hypothesized to have hosted glaciation. We will also address the nature and character of the modern and fossil microbial biosphere, Mars-analog conditions, and exhumation histories of source regions.

How to cite: Soreghan, G., Beccaletto, L., Benison, K., Bourquin, S., Hamamura, N., Hamilton, M., Heavens, N., Hinnov, L., Looy, C., Pfeifer, L., and Pochat, S.: DeepDust - A Proposed Drilling Project to Probe Continental Climate of the Late Paleozoic Icehouse-Greenhouse Transition, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11830, https://doi.org/10.5194/egusphere-egu2020-11830, 2020

The expansion of ice masses across southern Africa during the Late Palaeozoic Ice Age (LPIA) at about 300 Ma has been recognised in the literature for over a century, including the distribution of upland areas in controlling the configuration of ice masses (Davis, 1908). In Namibia, increasing attention has focussed on long and deep palaeovalley networks in the north, but comparatively little work has been attempted in the topographically subdued plains of the south. The desert terrain of the Mariental area exposes diamictites of the Dwyka Formation discontinuously over about 300 km, extending further south to the Noordoewer area at the Namibian-South African border along the Orange River. Whilst examined at a stratigraphic level, the nature of the contact between the Dwyka glacial rocks and underlying lithologies has not been systematically investigated. This paper presents some preliminary results from fieldwork in austral winter 2019, in which we describe a highly varying basal contact that records the processes of growth, flow and expansion of ice masses across this part of Gondwana. Subglacially-produced unconformities may exhibit classic glacially-striated pavements at basin margins, which substitute for soft-sediment striated surfaces in comparatively more “basinal” areas. Where these features are absent, additional criteria may be sought. In Mariental, spectacular soft-sediment shear zones exhibit a combination of brittle and ductile end products are recognised, overprinted by shear bands. This type of subglacial unconformity developed over well differentiated, unconsolidated, siliciclastic materials. Where ice advanced over more poorly sorted material or cannibalised pre-existing diamictites, “boulder-pavements” formed in which in single clast-thick boulder-dominated intervals were facetted and striated in situ by overriding ice. By integrating measurements of striation orientations, fold vergence and palaeocurrent information, former ice flow pathways can potentially be reconstructed over a wide area, which is suspected to have been dominated by Piedmont glaciers.

How to cite: Le Heron, D. P., Kettler, C., Griffis, N., Dietrich, P., and Montañez, I.: Complex development of a 300-million-year old subglacial unconformity in southern Namibia, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8856, https://doi.org/10.5194/egusphere-egu2020-8856, 2020

The organic-rich lacustrine beds of the Autun Basin (France) were deposited from the late Gzhelian (late Carboniferous) to the Sakmarian (early Permian), encompassing the Carboniferous-Permian boundary (∼299 Ma). Those deposits reach up to 1500 m thick, and correspond to a tropical, intra-mountainous late-orogenic basin infilling associated with the Variscan orogeny (Marteau, 1983; Schneider et al., 2006). Organic-rich and laminated facies are attributed to distal lacustrine environments which sometimes alternate with silty to sandy rich deltaic depositional environments (Mercuzot et al., 2019). The four successive formations (respectively the Igornay, Muse, Surmoulin and Millery fms) yield series of oil-shale beds (successively the Moloy, Igornay, Lally Muse, Surmoulin and Les Télots beds) (Marteau, 1983; Garel et al., 2017). The oil-shale beds are at least several m thick in the basin, except for the “Margenne” boghead bed which is only 0.3 m thick (Marteau, 1983; Garel et al., 2017). Recently, accurate U-Pb ages obtained on zircons from volcanic layers of the Autun Basin have placed the Carboniferous-Permian boundary within the Lally oil-shale beds (Pellenard et al., 2017).

In this work, we present a detailed study of the 364-m thick Chevrey 1 core, based on a Rock-Eval pyrolysis survey. The Chevrey 1 core encompasses the successive Igornay and Muse fms., including the Lally oil shale bed and the C/P boundary. TOC varies from 0.2 to 21 wt%, whereas HI values range from 22 to 421 mgHC/gTOC. The Lally oil-shale bed seems to correspond to a 2.5-m thick interval of maximum organic preservation between -145.02 m and -142.55 m, with TOC peaks reaching 12-21 wt%. However, the broad organic-rich interval seems much larger, with TOC around 6.1 wt% on average and HI values of 282 mgHC/gTOC on average between -157.3 m and -126.1 m. Moreover, a long-term progressive increase of TOC accumulation, highlighted by several organic pulses is obvious, starting at -282.4 m and pre-dating the Lally oil shale bed occurrence. We thus evidence for the first time that the Lally oil shale bed corresponds to the short-lived apex of a long-term lacustrine organic rich sequence of increasing paleo-productivity and/or paleo-anoxia that is ∼ 200m in thickness and therefore, is not only limited to a thin, (pluri)-meter-thick organic rich interval associated with short-lived anoxia and/or primary productivity pulse, as previously admitted. These findings rise the question of the paleoenvironmental mechanism(s) behind the occurrence of oil-shale intervals within the Autun Basin. Although further works are needed to fully understand those mechanisms, a preliminary cyclostratigraphy study using the Chevrey 1 TOC record suggests that the organic accumulation was likely controlled by climatic cycles in the Milankovitch frequency bands, and that the ∼ 200 m long-term organic trend may be linked to ~2 Myrs eccentricity.

How to cite: Schnyder, J., Martinez, M., Baudin, F., Mercuzot, M., Pellenard, P., Thomazo, C., Bourquin, S., and Beccaletto, L.: Long-term lacustrine paleo-productivity and/or paleo-anoxia trends controlled by eccentricity cycles in the continental Autun Basin (France) at the Carboniferous/Permian boundary, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19962, https://doi.org/10.5194/egusphere-egu2020-19962, 2020