Johann Schnyder

Characterization of organic matter in the fine-grained siliciclastic sediments of the Shemshak Group (Upper Triassic–Middle Jurassic) in the Alborz Range, Northern Iran

Abstract Bulk organic geochemical and microscopic studies (Rock-Eval pyrolysis, light transmitted–uv microscope) were carried out on the shales of the Upper Triassic–Middle Jurassic Shemshak Group in the northern, central and southern Alborz Range of northern Iran. Total organic carbon (TOC) values range from 0 to 29.4 wt% (1.2 wt% on average) indicating a generally poor–moderate organic carbon content. Upper Triassic shales in the lower part of the Shemshak Group have been mainly deposited in marine/lake settings under dysoxic–anoxic conditions, with TOC=0.7 wt% on average. Toarcian–Aalenian shales in the upper part of the Shemshak Group were deposited under comparatively deeper marine oxic–dysoxic conditions with the lowest TOC contents recorded (0.3 wt% on average). Carbonaceous shales at different stratigraphic levels of the Shemshak Group show the highest TOC contents (14.2 wt% on average). Tmax values range from 439 to 599 °C (average 500 °C), indicating that the organic matter has experienced high temperatures during deep burial and active post-sedimentary tectonics. The hydrogen index (HI)–Tmax diagram shows the presence of Type IV kerogen of altered organic matter with a very low mean HI value. The palynofacies is characterized by the dominance of amorphous organic matter probably predominately derived from degradation of marine–non-marine phytoplankton. The Upper Shemshak Group has low potential to produce petroleum, whereas the Lower Shemshak Group is an important effective petroleum source rock in the Alborz Range. The latter may have generated a considerable amount of petroleum at some localities (e.g. Tazareh and Paland) in the geological past.

Subtropical climate conditions and mangrove growth in Arctic Siberia during the early Eocene

The early Eocene (ca. 56–47.8 Ma) was an interval of exceptional warmth with reducednpole-to-equator temperature gradients. Climate proxies indicate mean annual air temperaturesn(MATs) and sea-surface temperatures (SSTs) exceeding 8–18 °C and frost-free, mild wintersnin polar areas, features that have proven difficult to reproduce with the most elaborate climatenmodels. A full appraisal of the early Eocene polar climate has been, however, limited by possiblenseasonal biases associated with geochemical proxies and the lack of data from the vast EurasiannArctic. Here we present multiproxy data from lower-middle Eocene coastal plain sediments ofnthe New Siberian Islands (Russia) showing that taxodioid Cupressaceae, palms, and the mangroven Avicennia grew in Arctic Siberia above 72°N under air temperatures averaging 16–21 °Cnannually and 5.5–14 °C in winter. Kaolinite contents are exceptionally high (up to 60% of claynassemblages) and comparable to those found in present-day subtropical soils formed undernhigh mean annual precipitation (MAP >1000 mm) and warm (MAT >15 °C) conditions. Then Avicennia pollen records the northernmost mangrove growth ever documented and indicatesnearly Eocene SSTs exceeding 13 °C in winter and 18 °C in summer. Considering the high MAPnestimated for Arctic Siberia and other pan-Arctic landmasses, we propose that the heat fromnwarm river waters draining into the Arctic might have amplified early Eocene polar warmth.nOur results provide the first climate constraints for the early Eocene of Arctic Siberia and supportnthe view that most climate models underestimate polar warming in greenhouse conditions.

The brittle and ductile components of displacement along fault zones

Abstract The total offset across a fault zone may include offsets by discontinuous faulting as well as continuous deformation, including fault-related folding. This study investigates the relationships between these two components during fault growth. We established conceptual models for the distributions of displacement due to faulting (i.e. brittle component or near-field displacement), to folding (i.e. ductile component) and to the sum of both (i.e. far-field displacement) for different mechanisms of fault-related folding. We then compared these theoretical displacement profiles with those measured along mesoscale normal faults cutting carbonate-rich sequences in the Southeast Mesozoic sedimentary basin of France. The near-field and far-field displacement profiles follow either a flat-topped or a triangular shape. Several fold mechanisms were recognized, sometimes occurring together along the same fault and represent either fault-propagation folds, shear folds or coherent drag folds. In the last case, local deficit in the fault slip is balanced by folding so that the brittle and ductile components compose together a coherent fault zone. Common characteristics of these faults are a high folding component that can reach up to 75% of the total fault throw, a high displacement gradient (up to 0.5) and a strong fault sinuosity.

Palaeobiogeography of the genus Latochara (fossil Charophyta) in the Upper Jurassic of Southern Europe. New data from Oléron Island (France).

A charophyte assemblage including Latochara tenuicostata (PECK 1937) and Mesochara gr. voluta (PECK 1937) was found in the Lower Tithonian bone-bed 148 of the stratigraphic section at the Pointe de Chassiron, Oleron Island (Charente-Maritime, Southwestern France). This bed, which is well known for its rich and diverse fauna of tetrapods, was deposited in a margino-littoral environment of Purbeckian facies in the Late Jurassic Charente palaeogulf. The occurrence of Latochara at Oleron Island is the most southern location known in the distribution of this genus during the Late Jurassic. This suggests that the boundary between the charophyte Boreal and Tethyan provinces, defined on the basis of this distribution, is not solely determined by latitudinal (i.e. thermal) factors. Ecological factors such as salinity and competition with other charophyte families are hypothesized as alternative explanations.

Late Jurassic dinosaur footprints from Chassiron–La Morelière (Oléron Island, western France)

A new dinosaur tracksite was recently discovered in the Upper Jurassic deposits from the Oléron Island (Atlantic Ocean, western France). Tracks are located along the coastline in the tidal area of La Morelière near “Pointe de Chassiron”. They were observed in situ in an evaporitic interval from the “Purbeck beds” Unit. Sediments consist of calcite recrystallised after gypsum dissolution within limestone layers which are composed of thin laminites. The track-bearing surface shows abundant mud cracks and some ripple marks. Ichnofossils consist of medium-sized tridactyl footprints of theropods forming a narrow trackway. In order to obtain 3D reconstructions of the track-bearing surface, the trackway was scanned with a resolution of 0.5xa0mm using an Artec Eva 3D white-light scanner. As indicated by the track dimensions, the probable trackmaker is assigned to a medium-sized non-coelurosaur tetanuran, such as a megalosauroid or an allosauroid theropod. The palaeoenvironmental context is interpreted as a tidal flat in an evaporitic basin. In addition to the tridactyl tracks, we report sauropod- and thyreophoran-like tracks on two other surfaces from Chassiron–La Morelière.

Integrated stratigraphy of a continental Pliensbachian–Toarcian Boundary (Lower Jurassic) section at Taskomirsay, Leontiev Graben, southwest Kazakhstan

Abstract During the Early Jurassic, major palaeoclimatic changes, associated with large carbon-cycle perturbations, occurred at the Pliensbachian–Toarcian boundary. Although the detailed marine palaeoclimatic record of this time interval and its impact on the marine biota is well recorded, much less is known about the continental realm. The current study documents new palynological and high-resolution carbon isotope data measured on bulk organic matter from the continental Lower Jurassic section of Taskomirsay in Kazakhstan, Central Asia. Both datasets allow a transition zone between the Pliensbachian and Toarcian to be identified. In addition, the spore and pollen distribution suggests a warming trend from the Pliensbachian to the Toarcian, most probably associated with a shift in floristic associations from the Siberian to the Sino-European palaeofloristic provinces, as recorded elsewhere in Central Asia during the Early Jurassic. The Taskomirsay section is thus of primary interest for palaeoclimatic studies as it is one of the very few well-dated continental section worldwide that records the Pliensbachian–Toarcian palaeoclimatic changes. Supplementary material: Table of δ13Corg data, Taskomirsay section, Karatau Graben, Kazakhstan is available at https://doi.org/10.6084/m9.figshare.c.3495645

An unexpected record of the PETM in terrestrial and organic sediments of Avesnois, between the Paris and Belgian Basins, NW Europe

The stratigraphy of the Late Paleocene-Early Eocene has been revised in Avesnois thanks to drillings supporting a geological mapping project. Detailed sampling and analyses were performed along selected drillings, among which sedimentological, mineralogical, chemostratigraphic (δ13Corg), biostratigraphic and palynological data have been obtained. New Mbs/Fms are defined around the Paleocene-Eocene boundary (PEb) and compared to the lithostratigraphic nomenclatures of the Belgian and Paris Basins (Steurbaut, 1998; Aubry et al., 2005). Biostratigraphy based on the study of foraminifera and mammal fauna, palynological content and chemostratigraphy enable correlation to be established between adjacent basins as well as sea level and landscape evolutions to be refined during this critical interval. Upper Thanetian marine units In the AVE 007-drilling (Mormal Forest, Locquignol, between Englefontaine, Landrecies and Maroilles villages), the Coniacian chalk is overlain by a flint conglomerate, whose sandy-clayey matrix contains Thanetian pollen and dinocysts. Widespread in Avesnois and containing often reworked Upper Cretaceous foraminifera and occasionally glauconite, this unit constitutes the Vervins Mb (cf. Argile a silex de Vervins interpreted as a marine conglomerate by Gosselet, 1879). It is often overlain by the Mondrepuis-Bettrechies Sand Mb (cf. Sable verts de lAvesnois), a thin sandy glauconiferous unit, more clayey and silty westward (cf. Tuffeau de Valenciennes Mb). The three Mbs constitute the Hainaut-Valenciennois Fm and can be correlated to the Belgian Grandglise Mb (upper part of the Hannut Fm) and Bois-Gilles Fm, and to the Paris Basin Châlons-sur-Vesles and Bracheux Fms, all Thanetian in age. Uppermost Paleocene to lowermost Eocene terrestrial units In AVE 007 the Vervins Mb is unconformably overlain by flint gravels (Mormal Mb), then a pyritic sand (Le Quesnoy Sand Mb, cf. Sables et Gres du Quesnoy of Gosselet, 1890), and a lignitic clay (Locquignol Mb), silty and ochreous at the top. All but the top ochreous paleosol contain spores and pollen grains and are devoid of dinocysts, they were thus probably deposited in terrestrial environments. The Carbon Isotope Excursion (CIE) of the PEb begins below the top of the sandy unit and continues in the lignitic clay. The palynological study confirms the Locquignol Mbs earliest Eocene age and allows correlation with the base of the Tienen Fm. To the East of AVE 007 along the Belgium-France border, fluvial fossiliferous gravels and sands have been studied in past sand quarries (Erquelinnes Sand Mb). Above a sharp unconformity, they i) overly Upper Thanetian marine sands (Bois-Gilles Fm, NP9a, then Grandglise Mb, NP8), ii) contain a mammal fauna referred as MP7 in the Mammalian biochronological scale for the European Paleogene (Missiaen et al., 2013) and iii) record the first part of the CIE onset in the basal fossiliferous gravel and marl beds. Three km west of Erquelinnes and toward AVE 007 a few drillings have recognized a sandy and pyritic unit containing resinite fragments, lignite and peat beds (Vieux Reng Mb), overlying a thin basal flint gravel bed (Mormal Mb). These five Mormal, Le Quesnoy, Locquignol, Vieux-Reng and Erquelinnes Sand Mbs form the Sambre Fm. Such fluvial gravels and sands, more or less lignitic, clayey or marly are widespread in northern France and southeastern Belgium. They belong to the so-called Upper Landenian and are correlated to the Tienen Fm. In AVE 007, they record the CIE onset marking the PEb and 2.5 m of Upper Paleocene fluvial units, in contrast with Belgian drillings such as Doel and Kallo where a hiatus is present at the PEb. Lower Ypresian hiatus and marine units In AVE 007, a laminated silty unit overlies the Locquignol Mb, contains dinocysts Wetzeliella sp. and agglutinated foraminifera similar to those of the base of the Orchies Clay Mb. Widespread in Avesnois-Valenciennois that marine unit is named Avesnois Mb, it is locally richer in very fine sand, and correlated to the base of the Kortrijk Clay Fm (Ieper Gp). In Avesnois, Sparnacian lagoonal units are absent upon the terrestrial ones described above, again marking a reverse trend when compared with the Belgian and adjacent Basins. This may result from restricted terrestrial environments coinciding with Variscan accidents and structural highs between the Artois anticline and Ardenne Masssif, or to their deposition followed by subsequent erosion prior to the Lower Ypresian marine transgression (cf. sea level drop 2 of Dupuis et al, 2011). Correlations and PETM impact on land All terrestrial units here described fill fluvial channels incised in marine Thanetian and older units such as the Cretaceous or the Variscan basement in Avesnois and Belgium, as already reported elsewhere in northern France (cf. sea level drop 1 of Dupuis et al., 2011). They exhibit locally cross stratification, lignite, flora or vertebrate fauna, for example at Leval, Hoegaarden and Dormaal, and/or are intensively silicified with scarce root or leaf casts. Similar fluvial sediments and silcretes are recognized in the first Sparnacian units and paleogeography of the Paris, Dieppe-Hampshire and London Basins, where the PEb is also present, the CIE being recorded on thicknesses between 3 m (as in AVE 007) to >20 m (as in Sinceny). These features may be regarded now as constant in those Sparnacian landscapes recording the PETM, with very rapid lateral and vertical facies shift, particular paleosols, a few fossiliferous fluvial to lacustrine units and variable deposition rates/sediments preservation. Terrestrial sediments, flora and fauna have been described there before the CIE onset as well, for example at Cobham, Vasterival or Rivecourt (e.g. Smith et al., 2014). Lihons, an intermediate outlier situated between AVE 007 and Sparnacian sites of those basins has further been investigated: marine Thanetian sands are overlain by thin fluvial, lacustrine then lagoonal sediments, the two latter recording the CIE of the PETM on an interval 4 m thick. Here again the basal terrestrial sediments have been deposited before the PEb. In AVE 007, depositional environments are fluvial for the Mormal and Le Quesnoy Mbs, and evolve toward flood plain then palustrine ones for the Locquignol Mb, probably here after the river bed migration, but still recording sporo-pollen fluvial inputs from the hinterland, the whole indicating a rather humid subtropical climate. Compared to the Locquignol Mb, the sporopollinic assemblage of the Mormal and Le Quesnoy Mbs (below the CIE onset) is different, particularly regarding the Normapolles distribution, the Tricolpates, Tricolporates and Monocolpates relative pollen abundances and the pollen ratio of Plicapollis pseudoexcelsus/Juglandaceae. In the three Mbs, rapid vertical variations characterize pollen and fern spores assemblages, with peaks of particular taxa reaching 25% of the total counts, such as for P. pseudoexcelsus, T. platycaryoides, C. dorogensis, P. mcgregorii, T. robustus. They suggest rapid floral changes in the catchment and/or sources changes in the surrounding landscapes compartments for the rivers supplying the fluvial sediments. Resinite particles are irregularly abundant as well as microcharcoals are occasionally abundant, notably slightly before the CIE onset and 1.5 m above it, still within the CIE. The clay mineral assemblage is homogeneous along marine Thanetian units and strongly dominated by Illite-Smectite (IS) mixed layers, whatever the facies, while it is more variable in the marine Ypresian silts (abundant IS, followed by Kaolinite, Illite, Chlorite and Vermiculite). In contrast terrestrial units are dominated by IS followed by Kaolinite, Illite and very rare Chlorite, and no variation is observed across the PEb. Those data would suggest that major environmental changes have regionally begun before the PETM, at the very end of the Paleocene, when terrestrial realm has settled, in relation with a regional uplift. Clay minerals reflect constant reworking of preexisting weathering profiles established upon Cretaceous strata and Variscan basement, while vegetation modifications are subtle and more likely attributable to changes of depositional environments

Carbon and nitrogen dynamics in decaying wood: paleoenvironmental implications

The effect of early diagenesis on carbon and, especially, nitrogen isotopes (δ13C and δ15N) of organic matter is not well understood and is of interest for accurate paleoenvironmental reconstructions. Wood samples were incubated in distilled water and river water to assess the effects of early diagenesis on carbon and nitrogen dynamics. Elemental content and isotopic composition of carbon and nitrogen as well as mass loss of wood pieces were determined. Mass loss in river water was three times greater than in distilled water. This difference was attributed to the development of two different types of fungi characterised by various degradation rates. Carbon dynamics of wood samples showed similar patterns in both type of water: (i) a sharp increase in carbon content, possibly related to carbohydrate degradation, before it slowly returned towards initial values, and (ii) no significant changes in δ13C values. In contrast, nitrogen dynamics of samples showed complex patterns: (i) N release associated with 15N depletion in distilled water, attributed to uptake of 15N-enriched pool (i.e. proteins) by fungi, and (ii) N accumulation associated with 15N enrichment in river water. The latter pattern was attributed predominantly to microbially mediated importation of 15N-enriched nitrate from river water. Although challenging, the present results suggest that early diagenesis may average an environmental signal by integrating individual signals (woods, fungi, water) and microbial processes. Considering the non-linear behaviour of early diagenesis, this integration is probably almost instantaneous on the geological time scale, which may not preclude paleoenvironmental reconstructions.