Jean-François Deconinck

Depositional conditions and organic matter preservation pathways in an epicontinental environment: the Upper Jurassic Kashpir Oil Shales (Volga Basin, Russia)

Abstract The Middle Volgian Kashpir Oil Shales Formation, located on the Russian Platform, is a lateral equivalent of the North Sea and West Siberian petroleum source rocks. In the Volga Basin, this formation is 6 m thick and shows alternations of marls and black shales. The organic carbon content is often higher than 1%, although bioturbation and benthos are abundant, even in the black shales. In the marls, highly degraded organic matter (OM) dominates, while aliphatic, sulphur-rich OM is dominant in the black shales. The combination of sedimentological and geochemical studies allowed to determine that the redox conditions of the sediment regularly fluctuated from oxic to anoxic, under relatively productive waters, while climatic conditions were increasingly arid. It is proposed that OM deposition occurred in relation to the increasing aridity, through the combination of recurrent disruption of salinity stratification and aeolian supply of iron, which both promoted phytoplankton productivity.

A record of glacial/interglacial alternations in Pleistocene sediments off New Jersey expressed by clay mineral, grain-size and magnetic susceptibility data

Abstract Clay mineral, grain-size and magnetic susceptibility measurements have been performed on a high resolution sampling of Pleistocene clayey silts and silty clays drilled at ODP Site 902 (Hole D) located on the upper slope off New Jersey (NW Atlantic). The combination of data from different techniques, and their comparison with δ 18 O and Gamma-Ray Attenuation Porosity Evaluator (GRAPE) data, allow us to clearly distinguish between sediments deposited during glacial and interglacial cycles. Glacial sediments are poorly-sorted, rich in either clay or sand, and characterized by the abundance of chlorite and high magnetic susceptibility values. In contrast, interglacial sediments are well-sorted, silt-dominated and poor in chlorite. The Hudson River and its tributaries draining the Appalachian Highlands appear to constitute the perennial sources of the detrital input including quartz, feldspars, amphibole and a clay fraction dominantly composed of illite, random mixed-layers and kaolinite. During glacial stages, the terrigenous sedimentation was enhanced by additional input of chlorite from glacial erosion in Northeastern America. The variations in chlorite amounts supplied to the ocean were probably controlled by the ice-sheet advances and retreats. These variations are quantified by the kaolinite/chlorite ratio (K/C) which constitutes a reliable marker of glacial and interglacial influences on the terrigenous sedimentation.

Contrôle géodynamique de la sédimentation argileuse du Callovien-Oxfordien moyen dans l'Est du bassin de Paris: influence eustatique et volcanique

The Middle Callovian to Middle Oxfordian clay deposits drilled in the eastern Paris Basin (borehole HTM 102, ANDRA) show a strong mineralogical change occurring in the lowermost Oxfordian (Scarburgense subzone, Mariae zone). Such a change probably results from the initial development of connections between the young Atlantic Ocean and the Paris Basin. A bentonitic layer identified close to the boundary between Lower and Middle Oxfordian and also recognized in the subalpine Basin (South-East of France) at the same age, reflects a volcanic activity probably resulting from an extensional regime located in the North Sea.

Multi-proxy orbital chronology in the aftermath of the Aptian Oceanic Anoxic Event 1a: Palaeoceanographic implications (Serre Chaitieu section, Vocontian Basin, SE France)

In the early Aptian, the Oceanic Anoxic Event (OAE) 1a is well defined by a negative ?13C excursion followed by a positive δ13C excursion, spanning the Deshayesites deshayesi and Dufrenoya furcata ammonite biozones. A cyclostratigraphic approach is performed in the Vocontian Basin, France, to estimate the time required for the carbon cycle recovery following the major disturbance associated to OAE1a and to provide durations of ammonite and foraminifer biozones. The Serre Chaitieu section, which consists of hemipelagic blue-grey marls with occasional marker limestone horizons and encompassing the Deshayesites deshayesi Zone to the end of the Epicheloniceras martini Zone, was used as a reference section in the Vocontian Basin. Using field Spectral Gamma Ray (SGR), 450 measurements were performed throughout the section, and a sample of each measured sediment was collected to further perform calcimetry, clay mineralogy, and magnetic susceptibility (MS) measurements. Detrital clay mineral assemblages consist of illite, illite/smectite mixed-layers (I-S), kaolinite and chlorite. Fluctuations of clay minerals are mainly driven by climate change, progradation/drowning of peri-vocontian platforms and sea-level changes. The proportions of illite and kaolinite covary and fluctuate in opposition with I-S. Cyclic fluctuations of relative proportions of clay minerals are particularly well recorded by the kaolinite/chlorite ratio (K/C). Spectral analyses, using the multi-taper and the amplitude spectrogram methods, were performed on SGR, MS, CaCO3 and K/C signals to detect sedimentary cycles related to an orbital forcing throughout the series. The geochronometer 405-kyr eccentricity cycle well expressed and significant (up to 99% confidence level) is used to provide a robust temporal framework. More than five 405-kyr eccentricity cycles are recognised, providing a total duration of at least 2.49 myr for the whole sedimentary succession. The minimum duration of the D. furcata Zone is assessed at 0.42 myr, and the duration of the E. martini Zone at 1.52 myr. Amplitude spectrograms show a strengthened signal of obliquity during the D. furcata Zone, which is coherent with the global cooling that has been depicted for this interval, and which could have favored the development of lowextension polar ice and thus the lowering of the sea level. Durations of C-isotope zones, worldwide correlated, are also calculated. From these results, the duration of the return to equilibrium in the carbon cycle in the aftermath of OAE1a could be calculated at 1.35 myr.

The Purbeckian facies of the Boulonnais: a tsunami deposit hypothesis (Jurassic-Cretaceous boundary, northern France)

Abstract We reconsider the origin of the Purbeckian deposits (Uppermost Jurassic–Lowermost Cretaceous) of the Boulonnais. Made up of an erosional conglomerate overlain by wood fragments and clays containing continental and marine fossils, they are attributed to a tsunami triggered by either an earthquake or the impact of a bolide in the palaeo-Barents Sea.

Biodiversity across the Guadalupian-Lopingian Boundary: first results on the ostracod (Crustacea) fauna, Chaotian section (Sichuan Province, South China)

ABSTRACT The Middle Permian-Late Permian boundary (Guadalupian-Lopingian boundary, GLB) interval is characterised by important faunal assemblage changes. This extinction-turnover episode is considered by some authors to be the first step of the end-Permian biodiversity drop. The forty-five meters thickness of sediment encompassing the GLB in Chaotian section (Sichuan Province, South China) was sampled and processed for ostracod study. This study presents the first analysis of ostracod faunas in the GLB interval. A total of 154 species belonging to 29 genera are identified. Three species are described as new: Bairdia chaotianensis Zazzali, n. sp., Microcheilinella wujiapingensis Zazzali, n. sp., Microcheilinella pagodaensis Zazzali, n. sp. All the ostracods discovered in the section belong to shallow marine taxa. So these results are not consistent with previous interpretations (lagoonal environment or deep water setting) based on other evidences. Abundance and diversity present a rapid and noticeable decline in the Early Capitanian. Recovery is then recorded about three meters above the GLB. At specific level, a 93% extinction rate and a 96% turnover rate are recorded at the GLB. Moreover, Palaeocopida, straight dorsal border ostracods known to progressively disappear from the Late Permian to the basal Middle-Triassic, are here less abundant and diversified after the GLB. This could reflect the first step of their disappearance at the end of the Palaeozoic.

Orbital Chronology of the Lower–Middle Aptian: Palaeoenvironmental Implications (Serre Chaitieu Section, Vocontian Basin)

A detailed cyclostratigraphic study conducted on several proxies including spectral gamma ray, magnetic susceptibility, clay minerals, and carbonate content suggests that the return to equilibrium in the carbon cycle after the major disturbance linked with Oceanic Anoxic Event 1a occurred in about 2 Myr. The minimum duration of the D. furcata zone is estimated at 0.46 Myr and that of the E. subnodosocostatum zone at 1.45 Myr. Strengthening of the obliquity record in the furcata zone confirms the cooling that characterizes this period.

Astrochronology of the Valanginian Stage from GSSP Candidates and Hypostratotype

The Valanginian Stage currently displays no radiometric age, which severely hampers palaeoceanographic reconstructions for this time interval. An astrochronology of the Valanginian Stage using the stable 405-kyr eccentricity cycle was performed on biostratigraphically well-calibrated standard sections from the Vocontian Basin (southeastern France). High-resolution gamma-ray spectrometry signals were obtained from orbitally driven marl–limestone alternations from five sections in the basin, and they display the same long-term trends. The spectral analyses present the pervasive record of the 405-kyr eccentricity cycle together with precession, obliquity, and 2.4-Myr eccentricity. Based on the identification of the 405-kyr eccentricity cycle, the duration of the Valanginian Stage is assessed at 5.08 Myr. Since the Weissert Event appears to be ~3 Myr older than the onset of the Parana–Etendeka Large Igneous Province activity, a link between these events is unlikely. We therefore propose, following Grocke et al. (2005) and Westermann et al. (2010), that continental organic carbon storage and carbonate platform demise are responsible for the onset of the δ13C positive excursion. In addition, a stronger obliquity control appears in the O. (O.) nicklesi and C. furcillata subzones. This may be linked to the limited production of polar ice suggested for this time interval.