Triassic Buntsandstein Research Articles

The Triassic Buntsandstein sedimentary basins of Sardinia (Italy) and their transition to the Brianzonese Ridge (W Alps): crossroads through the W-Tethys areas

Sardinia represented a key area during the Western Tethys opening in late Early-early Middle Triassic times since it was located at the nodal point of several paleogeographic domains. New and revised sedimentological data and petrographical investigations evidence that the Sardinian Buntsandstein Group formational units differ from each other and were deposited in different alluvial to transitional depositional systems related to Alpine rifting basins. Petrographic analysis revealed directional NS and EW compositional differences: they hint at morphotectonic rises separating the diverse depositional areas through lasting barriers related to paleoeuropean ridges in progressive dismantling. The data suggest that 1) the dominating environmental and (micro)climate conditions were diverse from area to area, 2) the sedimentary basins evolved differently through time, and 3) were diachronous in timing. Moreover, the sources of debris were variable, both in morphology and substrate composition, with reliefs from articulated to smoothed. A stratigraphic and sedimentological framework for the Sardinian Buntsandstein is suggested. Comparison analysis with close coeval successions of the Western Tethydian area, particularly regarding the Upper Permian? - Lower Middle Triassic deposits of the Brianconnais ridge have been attempted, as well as their palaeogeographic localization, to frame the Sardinian Buntsandstein Group with the coeval units of SW Europe.

Improved reservoir quality assessment by evaluating illite grain coatings, quartz cementation, and compaction – Case study from the Buntsandstein, Upper Rhine Graben, Germany

Reservoir quality (RQ) in the Buntsandstein of the Upper Rhine Graben is in the center of attention as it is a possible target formation for geothermal energy production and hydrocarbon exploration surrounding existing fields. An understanding of properties affecting reservoir quality of the target lithology is still fairly poor and the success of accurately targeting high RQ intervals is limited. This is due to the fact, that the effect of compaction and the interplay between enhanced chemical compaction (i.e. pressure dissolution of quartz grains), illitic contact coatings and quartz cementation in the lithology has been underestimated. The understanding and quantification of controlling factors on reservoir qualities in fluvio-eolian sedimentary rocks, deposited in a (semi-)arid climate has been improved in recent years and this case study highlights the benefits of detailed petrographic analyses in understanding diagenetic and compactional processes in this lithology. This is especially relevant in the observation of grain coating clay minerals, whose effect depends on their specific location, i.e. either at grain contacts between quartz grains (GTG coating) or at the interface between detrital quartz grains and the intergranular volume (GTI coating).Illitic GTI coatings affect syntaxial quartz overgrowth precipitation, as precipitation sites are locally blocked. The negative correlation between the grain coating coverage and quartz cement volumes support these findings across multiple sample sets, and they may locally preserve intergranular porosity. Illitic GTG coatings on the other hand enhance chemical compaction (i.e. pressure dissolution) and will reduce the IGV. The negative correlation between these two properties again underlines the negative effect of this process on reservoir properties. Studied samples from a deep Buntsandstein well in the central URG show low reservoir quality due to either intense quartz cementation (0.7–31.7%) or a high degree of mechanical and chemical compaction (IGV: 2.3–38.0%). Higher illitic GTG and GTI coating coverages play a substantial role in controlling reservoir quality development, as demonstrated by comparing data from other fluvio-eolian lithologies (Triassic Buntsandstein and Permian Rotliegendes) to results of this study. In relation to their respective burial histories, higher illitic GTI coating coverages always correlate with smaller syntaxial quartz cement volumes. Similarly, higher illitic GTG coating coverages always correlate with lower IGV values in the three compared sample series.As both, the precipitation of quartz cements and compaction, are a function of the burial history, i.e. effective stresses and experienced temperatures, understanding the interaction of both these processes may enable the prediction of reservoir properties in undrilled areas.

Quantitative facies analysis of a fluvio-aeolian system: Lower Triassic Buntsandstein Group, eastern France

Understanding the factors that control the temporal and spatial evolution of the Lower Triassic Buntsandstein Group, eastern France, is important not just for palaeoenvironmental reconstruction but also because it is an important reservoir for the lithium-rich geothermal brines in the Upper Rhine Graben region. The interval of interest in this study, the Lower Grès Vosgien Formation (LGV), is made up of c. 200 m of clastic deposits of mixed fluvial and aeolian origin. Given the proximity between the outcrops and the areas of mineral and heat exploration, this region offers a unique opportunity for quantitative analysis and correlation between outcrops and reservoirs. The LGV is the thickest Formation in the Buntsandstein Group. However, hitherto, no detailed architectural analysis has been published to reveal the controlling factors of its depositional elements. In this study, high-resolution facies analysis was applied to thirteen outcrops, and a core, to quantify fluvial and aeolian depositional architectures, and to comprehend palaeoenvironmental conditions during their deposition. Two facies associations were identified. The fluvial channel facies association, composed mainly of sandstones deposited in poorly confined, occasionally ephemeral channels, corresponds to 93 % of the total thickness. The analysis of >800 sets of strata characterised by lower-, transitional-, and upper-flow regime sedimentary structures, distributed within different architectural elements, enabled the distinction of temporal and spatial variabilities, and an estimate of fluvial palaeohydraulic characteristics. The wind- and water-laid facies association, which accounts for 7 % of the total thickness, records an architecture controlled by a fluctuating water-table level, and ephemeral floods. Results revealed that aeolian dunes and sand sheets accumulated in distinct temporal moments. The vertical trends, recorded in the core, indicate an upward increase in the frequency of intercalations between the two facies associations, and a decrease in the occurrences of thicker cross-bedded sandstone sets of fluvial origin, which, associated with the overall retrogradational stacking pattern of the LGV, is interpreted as the preserved record of a large distributive fluvial system (DFS). The occurrence of laterally extensive aeolian deposits, overlying fluvial deposits, is often associated by authors with periods of relatively more arid conditions at a regional scale. However, evidence indicates channel belt avulsion as an alternative interpretation for the establishment of the two facies associations. The findings enhance the knowledge about the depositional controlling factors in braided fluvial and aeolian systems, and improve predictive models that account for reservoir heterogeneity in accumulated successions of this type.

Osteology and phylogenetic relationships of a new archosauriform reptile from the Middle Triassic (Anisian) of Germany

ABSTRACT Skeletal remains of archosauriform reptiles from the Lower and lower Middle Triassic Buntsandstein Group in the Central European Basin are rare. This paper reports on a partial, almost completely disarticulated skeleton of a previously unknown archosauriform reptile from the lower Middle Triassic (Anisian) Röt Formation of Rotfelden in Baden-Württemberg (Germany). Marcianosuchus angustifrons gen. et sp. nov. is distinguished by the following combination of features: posterodorsal process of premaxilla slender, slightly inclined posterodorsally, and with rounded apex; posterodorsal surface of frontal covered by slightly diverging, fine longitudinal grooves; squamosal with distinct lateral ridge extending for entire length of element; teeth with proportionately small, weakly recurved crowns; humerus considerably shorter than femur; distal shaft of ischium rod-like, not plate-like; pubis with flat distal portion forming ‘pubic apron’: neural spines of posterior cervical and anterior dorsal vertebrae with transversely greatly expanded apices that are Y-shaped in anterior or posterior view; dorsal vertebrae with centra taller dorsoventrally than long anteroposteriorly; dorsal osteoderms more or less rectangular in outline, longer anteroposteriorly than wide mediolaterally, with slightly rounded anterior and concave posterior margins and bearing dorsal ridge or eminence; and dorsal surfaces of osteoderms with unsculptured anterior region and posterior region bearing keel or eminence and covered by sculpturing of radially arranged grooves and pits. Marcianosuchus angustifrons represents the first definitive record of a non-archosaurian archosauriform from the Buntsandstein Group of Germany. Its body plan most closely resembles that of Euparkeria capensis among well-known non-archosaurian archosauriform reptiles, but the two taxa are clearly distinct.

Diagenesis and controls on reservoir quality of Lower Triassic red bed sandstones (Buntsandstein) from a marginal basin facies, southwest Germany

After decades of hydrocarbon exploration and production in the Lower Triassic Buntsandstein from the Central European Basin center, the marginal southern German Buntsandstein has gained interest as a potential lithology for geothermal applications and lithium brine extractions. For this purpose and in order to better understand reservoir quality, porosity and permeability in combination with point-counted petrographic data of sandstones from SW Germany covering the Lower, Middle, and Upper Buntsandstein are studied. Textural, mineralogical, and petrophysical data show that the eolian and fluvial sandstones represent a heterogeneous reservoir system. Permeabilities vary over six orders of magnitude from 0.001 to 515 mD, while porosities are in the range of 7.9–21.5%. Different controls on reservoir quality exist. Large grain sizes ≥0.31 mm together with total quartz contents are the major controls on reservoir quality. Additional controls on reservoir quality are exerted by authigenic phases. Since porosity loss is dominated by compaction, the influence of authigenic clay phases on reservoir quality is minor. However, illite coatings on detrital quartz efficiently inhibited syntaxial quartz overgrowth. Where illite coatings lack, syntaxial quartz overgrowths locally stabilized the granular framework and, thus, preserved primary porosity. These authigenic phases, however, represent a local control, which cannot be applied on a regional scale. Similarly, enhanced secondary porosity due to feldspar dissolution is locally restricted and minor. The comparison with other studies on the Buntsandstein shows that its reservoir system is highly heterogeneous over small and large scales and within the same stratigraphic levels. In this study, quartz-rich and authigenic clay-poor sandstones with relative large grain sizes interpreted as eolian deposits reveal the best reservoir qualities.

The significance of outcrop analog data for reservoir quality assessment: A comparative case study of Lower Triassic Buntsandstein sandstones in the Upper Rhine Graben

Subsurface reservoir quality assessments are among the most important workflows in reservoir geology. The application has been standard practice in the hydrocarbon industry and is also adopted for geothermal reservoir systems hosted in clastic rocks. With the emergence of fluid mining for metals, such as lithium for the energy transition, yet another field may adopt the standard practice. However, as drill cores and hard geological data from the subsurface are expensive commodities they may be substituted by outcrop analog studies to fill in data gaps in the understanding of the deep subsurface. However, reservoir quality controls, such as pore-filling cements or natural fractures may be altered or overprinted during uplift to the surface and exposure to undersaturated meteoric fluids and associated stress release. A complete understanding of geochemical and geomechanical processes affecting the rocks since their deposition is needed to account for that, otherwise data may not be representative of the subsurface. In this case study using petrographic (point-counting) and petrophysical (porosity and permeability) analyses we highlight the possibilities and limitations of outcrop analog data used for the evaluation of subsurface reservoir quality. The focus is on the Triassic Buntsandstein within the Upper Rhine Graben in southwestern Germany and eastern France and exposed outcrop analogs along the western margin of the graben. Here, we show that some data (detrital composition, quartz cement, K-feldspar cement, and clay mineral cement volumes) may directly be applicable to subsurface reservoir quality assessment. However, other data (porosity, permeability, IGV, carbonate cement, and iron hydroxide cement contents) cannot be used analogously to subsurface samples and we outline possible approaches to mitigate some of the mismatches in the frame of a reservoir quality study. In particular, the correction for uplift diagenetic dissolution of carbonate cemented nodules and goethite precipitation will be the key to gain more accurate results in the presented area.

Lithium-rich geothermal brines in Europe: An up-date about geochemical characteristics and implications for potential Li resources

Lithium (Li) is a strategic metal - especially for batteries in electric vehicles - for which worldwide demand is constantly increasing. Presently, several investigations are examining if a part of lithium could be extracted from deep European geothermal fluids. Among the data from geothermal and hydrocarbon wells found in the literature review carried out by BRGM and EIFER, only six areas stand out with deep fluids containing high Li concentrations from 125 to 480 mg/l in Italy, Germany, France and the United-Kingdom. Except the UK fluid, which has a relatively low salinity (TDS = 19 g/l) and reservoir temperature (around 52 °C), these deep Li-rich fluids are Na-Cl brines (TDS ≥ 56 g/l) with high concentrations of Na (> 18 g/l) and Cl (> 25 g/l) and high temperatures (≥ 120 °C and up to 380 °C in Italy). If high TDS and temperature values seem to be key factors for triggering high Li concentrations in such fluids, these two factors alone cannot be sufficient. Indeed, our study confirms that Li concentrations not only depend on temperature and fluid salinity, but also on the type of reservoir rock and its mineralogical constituents, as demonstrated by the good results obtained using two different Na-Li thermometric relationships existing in the literature. One fits well with the brines from low to high temperature (120–250 °C) reservoirs in deep tectonic sedimentary basins over a crystalline basement (France, Germany) and the other with the fluids from ultra-high temperature (≥ 300 °C) reservoirs in volcano-sedimentary environment (Italy). If the high chloride concentrations values in these brines mainly depend on the fluid origin (evaporated seawater or freshwater, halite dissolution, primary neutralization fluids or parent-geothermal fluids in high-temperature and -pressure volcanic environments, water mixing, etc.), the other aqueous major species are mostly controlled by hydrothermal water-rock interaction processes. At these temperatures (≥ 120 °C), the fluid-rock interaction processes are generally dominated by plagioclase and K-feldspar dissolution, followed by albitization of these minerals, dissolution of white micas and biotite, precipitation of illite, and chloritization. According to the two Na-Li thermometric relationships, existing mineralogical and isotopic data, this study suggests that the main sources of Li are white micas and biotite dissolution. Among the European areas, it shows that the Upper Rhine Graben (URG) along the French/German border is probably the most promising zone. For the URG geothermal brines, the main source and control of lithium, at about 225 °C, at reservoir depth, is probably the up to 450-m-thick micaceous continental sandstone of Triassic Buntsandstein. A minor contribution from the granite basement can also not be excluded. Though the Li concentration values of these brines (≥ 150 mg/l) seem to be favourable for geothermal Li exploitation, it is essential to make an as accurate as possible estimate of the Li resource of these brines, design the Li extraction process, and examine the economic conditions of its exploitation.

Controls on compactional behavior and reservoir quality in a Triassic Buntsandstein reservoir, Upper Rhine Graben, SW Germany

The main reservoir quality controls in Triassic Buntsandstein deposits in the Upper Rhine Graben and surrounding outcrops in Germany and France are discussed to be authigenic mineral precipitates and the vicinity to structural elements (faults and fractures). To better understand reservoir quality controls in this lithology, a detailed diagenetic study based on petrographic methods (optical and SEM) in combination with petrophysical measurements (porosity and permeability) on core material (1133–1591 m TVD) was initiated. This case study highlights that large detrital grain sizes and low compactional porosity loss in association with blocky authigenic cements such as quartz and dolomite are beneficial to reservoir quality, whereas variable illite textures have a dual, but opposite effect on reservoir qualities. Grain coating illite (tangential and radial) may preserve intergranular porosity by inhibition of syntaxial quartz precipitation, while at the same time, if present at grain contacts, tangential illite enhances chemical compaction and reduces the available intergranular volume. The presence of early diagenetic carbonate cemented nodules is also beneficial for reservoir quality in stabilizing the grain framework against compactive porosity loss. Overall, detrital grain size and the intergranular volume, as a result of compaction and cementation, are derived as the main reservoir quality controls. The formation of hydrothermally mineralized veins prior to hydrocarbon charging does not show a distinct impact on porosity and permeability in the sandstone matrix. Fractures and interstices between vein minerals such as barite, galena, tetrahedrite, and siderite are equally covered in hydrocarbon residue, as are all present detrital and authigenic minerals in the sandstones indicating that vein precipitates, if not pervasive, do not alter the overall reservoir quality. This study may indicate additional potential to identify high reservoir quality intervals in the studied succession.

On the origin of bleaching phenomena in red bed sediments of Triassic Buntsandstein deposits in Central Germany

The Lower Triassic Buntsandstein in Central Germany comprises one of the major clastic, lithified hydrocarbon reservoirs, it is a major groundwater aquifer and it is considered as a potential underground gas storage lithology. Thus, sandstones of the Buntsandstein from Thuringia and NE Hesse were used to analyse their geochemical composition combined with mineralogical data in order to investigate the origin of several bleaching phenomena in primary red beds, since these give a hint for mineral alteration, mineral formation and mineral reactivity in the course of fluid-rock-interactions. This is relevant for e.g. recent Carbon Capture and Storage (CCS) efforts. The origin of the observed bleaching phenomena is diverse and it is linked to certain stages of diagenesis and different episodes of fluid flow: (1) eodiagenetic bleaching coupled with palaeosoil formation and iron mobilisation, (2) mesodiagenetic bleaching as a result of hydrocarbon migration, (3) Tertiary bleaching due to CO2 degassing from Miocene volcanism, (4) Tertiary telodiagenetic bleaching due to weathering under humid climate conditions, and (5) weathering processes related to recent telodiagenesis/pedogenesis. This study emphasizes the multiple nature of fluids, which can induce iron mobilisation and bleaching of red beds.

Formation of hydrothermal fluorite-hematite veins by mixing of continental basement brine and redbed-derived fluid: Schwarzwald mining district, SW-Germany

Hydrothermal fluorite-hematite veins represent a relatively rare vein type, which is sometimes associated with more common, fluorite-barite-quartz‑carbonate veins containing base metal mineralization. The origin and significance of the fluorite-hematite veins, and how they relate to the more common expression of mineralization, is essentially unknown. Here, we present new data illuminating the formation conditions of these “sulfide-barren” fluorite-hematite veins in a mining district known for hydrothermal Pb + Zn + Ag ores for which formation conditions are well understood: the Schwarzwald district in SW Germany. The exemplary Ödsbach-Hesselbach vein was chosen to study the fluids involved in fluorite-hematite precipitation through the first, direct investigation of fluid inclusions hosted in hydrothermal hematite using combined infra-red light microscopy, microthermometry and LA-ICPMS. The results indicate that the hematite-precipitating fluids are nearly identical to those observed in the associated fluorite, showing consistent salinities of 24.6–25.0 wt% (NaCl+CaCl2) and homogenization temperatures of 150 to 155 °C. Moreover, the trace element abundances determined by the LA-ICPMS analyses show a composition similar to other inclusions observed in the common mineralization in the mining district, including elevated Pb (>5000 μg/g) and Zn (>2000 μg/g) concentrations at low Cl/Br mass ratios, and sulfur contents mostly below the limits of detection. The results suggest fluid mixing between a continental basement brine and a redbed-derived fluid from the Triassic Buntsandstein. Therefore, the formation of the rare fluorite-hematite veins was probably triggered by a change of the sedimentary fluid endmember representing a shift from the Middle Triassic Muschelkalk fluid, which drove the more common Pb + Zn + Ag mineralization, towards fluids derived from the Buntsandstein. Furthermore, the absence of sulfur is interpreted to have precluded base metal sulfide precipitation, and the formation of hematite points to unusually oxidized conditions and hence towards a lack of a reducing agent, which would also have been required to generate a mineralization of base metal sulfides.

Lower Triassic marine Buntsandstein deposits in the Central European Basin

Lower Triassic marine Buntsandstein deposits in the Central European Basin

Triaxial testing and hydraulic\u2013mechanical modeling of sandstone reservoir rock in the Upper Rhine Graben

The Upper Rhine Graben (URG) is a highly favorable location for deep geothermal energy utilization. One of the main reservoir horizons is the lower Triassic Buntsandstein formation. While quantification of thermal and hydraulic parameters and their coupling is common, geomechanical factors are often neglected due to high costs and the expense in gathering data. Equivalent experiments with analog rocks, like triaxial tests, can help to improve this knowledge. Triaxial tests have been performed and evaluated on two different Buntsandstein rock examples as analogs to quantify the mechanical behavior under stress conditions found in the Buntsandstein of the URG. Both samples, Tennenbach (TenSst) and Pfinztal (PfSst) Sandstone, show high Young’s moduli under axial loading. With confining pressures up to 90 MPa, the axial failure strength is up to 448 (TenSst) and 561 (PfSst) MPa. The results demonstrate the usability of linear elasticity for the expected stress range in the Buntsandstein reservoir of the URG. The deformation behavior is described and a linear failure criterion is derived, predicting failure strength with a high accuracy. These experimental results can be further employed as calibration parameters for a hydro-mechanical evaluation of the deformation processes.

Zircon size-age sorting and source-area effect: The German Triassic Buntsandstein Group

This study stresses how zircon-grain length versus age correlation significantly can improve provenance interpretations. Hence, taking sedimentary sorting effects into account, we present a detrital-zircon provenance study for continental deposits of the Central European, Lower Triassic Buntsandstein Group in Germany. The Buntsandstein Group was deposited in an extended intracontinental sag basin of Pangea under arid climate conditions. It included several intrabasinal tectonic swells, some of which have been assumed topographic highs acting as detrital source areas. This study of the Lower and Middle Buntsandstein subgroups in Central Germany indicates that particularly the Massif Central and the Bohemian Massif contributed to the basin fill of the central part of the Central European Basin. The Rhenish Massif, an assumed highland, was submerged. This interpretation is based on >1000 zircon grains from 12 sandstone beds. The unbroken grains in seven of the zircon separates have mean lengths of 190–220μm and five are composed 100–140μm long grains in average. All have a main population of 340–320Ma Carboniferous ages, representative of granitic, probably zircon-rich, rocks from the Variscan Orogen of the Massif Central and the Bohemian Massif. This first-cycle detritus was mixed with recycled, abraded zircon grains from Ediacaran to Palaeozoic metasedimentary units feeding the basin with 610–460Ma (Ediacaran to Ordovician) and 2Ga age grains, as is typical for Gondwana-derived material. The Ediacaran to Ordovician age population is common only in five samples with mean grain lengths of 100–140μm. Grains of such ages mainly are <130μm in the east, but equally common in all grain sizes in western sandstone beds. A western group was fed from the Massif Central and an eastern group derived from the Bohemian Massif. However, abrasion, age-population variations for pre-Variscan grains, and dominating wind directions indicate that two different fluvial systems delivered the Bohemian material and that this was combined with aeolian and alongshore lacustrine transport from the south and west. We conclude that interpretation of detrital zircon ages preferably are separately for different size classes, because zircon studies performed on broad grain-size intervals without considering possible size-age correlations may disguise sorting effects that potentially can lead to a misleading source-area interpretation.

Ichnological evidence of a horseshoe crab hot-spot in the Early Triassic Buntsandstein continental deposits from the Catalan Pyrenees (NE Iberian Peninsula)

Xiphosurids (horseshoe crabs) are aquatic chelicerate arthropods commonly related to marine environments. Although today only four marine species exist, in the geological record they were much more diverse (especially during Carboniferous and Triassic periods), and even inhabited non-marine settings. Here we analyze an exceptional xiphosurid trace-fossil site preserved in the lowermost Buntsandstein red-beds from the Early Triassic of the Catalan Pyrenees. Ichnological and sedimentological analyses were mainly conducted during fieldwork. Fossiliferous surfaces were cleaned and mapped in order to recognize distribution and orientation patterns. Two main ichnotaxa were identified: (1) Selenichnites isp. (>400 trace fossils), consisting of a lunate, anteriorly convex lobe (prosoma imprint) followed by a rounded to triangular impression (opisthosoma imprint) and a posterior straight thin groove (telson imprint); and (2) Kouphichnium isp. (about 70 trace fossils), consisting of long (up to 2 m), often curved, complex series of parallel rows of morphologically variable tracks and imprints. Selenichnites isp., interpreted as resting and/or feeding traces (cubichnia/fodinichnia), were mostly oriented against the current, preventing tracemakers being overturned. Kouphichnium isp. correspond to locomotion traces (repichnia). Trace fossils are found at the uppermost succession of fining- and energy decreasing- upwards fluvial red beds with no marine influence. These ichnofossils are the earliest Triassic evidence of life from the Catalan Pyrenees, and thus shed light on ecosystems composition at the dawn of the Mesozoic era, when biota recovered from the greatest mass extinction in Earth history.

Provenance Signatures Recorded In Transgressive Sandstones of the Upper Cretaceous Iberian Seaway

Abstract: Maximum Mesozoic transgressions flooded many continental areas worldwide, forming a wide and shallow seaway on the Iberian Microplate during the Late Cretaceous. Several siliciclastic and mixed siliciclastic–carbonate units developed along the Iberian Massif (cratonic area) while, at the same time, epeiric carbonate platforms spanned Iberia. These siliciclastic deposits unconformably cover the Variscan basement, as well as the Permian and Triassic deposits. This study focuses on sandstone provenance in central Spain, in a transgressive, relatively quiet tectonic setting with a wide rock variety in the potential source areas. Results shown in this paper can be applied in analogue systems; transgressive sandstones often form important hydrocarbon reservoirs due to their high primary porosity. The Cretaceous siliciclastic deposits are included in four formations ranging in age from the Cenomanian to the early Coniacian. The deposits were initially of fluvial origin, followed by a coastal-tidal setting. Quantitative petrographic analysis of sandstones from these formations reveals compositional variations related to provenance and weathering degree. The oldest formation (Utrillas Fm.) was deposited on a paleorelief surface. Sandstone composition changes from arkosic sandstones in the depressed area (Petrofacies P1a) to younger quartzarenites when sediments were deposited on top of the paleotopographic high (Petrofacies P1b). Quartz grains in both sub-petrofacies are angular to subangular, and the framework is poorly sorted. Intense weathering conditions are deduced for Petrofacies P1b, manifested by grain infiltration, clay coatings, and the absence of plagioclase grains and alteration features on K-feldspars. All these observations have been related to wetter conditions during P1b deposition. Main source areas of the Utrillas Fm were Ordovician metasediments and orthogneisses, Permian volcanic rocks, and Triassic Buntsandstein facies. Petrofacies 2 encompasses the three coastal-tidal formations, constituted mainly by subarkoses generally showing a subangular and poorly sorted framework. The deduced source areas are mainly associated with Variscan granitoids, pre-Ordovician schists and paragneisses, and Ordovician metasediments. Glauconite and carbonate grains support intrabasinal inputs. The low degree of sorting of this petrofacies related to a low-energy sedimentary environment produced lower values of original porosity compared to transgressive sandstone hydrocarbon reservoirs. Also, intrabasinal inputs promoted carbonate cementation and contribute to almost occlude the porosity.

Geothermal implications for fracture-filling hydrothermal precipitation

In geothermal reservoirs, fluid circulation is greatly dependent on the geometry, density, and hydraulic properties of fractures. The Soultz-sous-Forêts geothermal site located in the Upper Rhine Graben in Alsace, France, consists of a granitic basement overlain by a 1.4km-thick sedimentary succession. Core analysis and borehole wall imagery collected from reconnaissance well EPS1, drilled vertically to a depth of 2230m, revealed an extensive fracture network throughout the granite and overlying sediments, including both open fractures and fractures sealed through mineral precipitation (primarily quartz, illite, chlorite, calcite, dolomite, barite, pyrite and galena). Here we present a combined experimental and modelling study that aims to provide insights into the permeability anisotropy in the Triassic Buntsandstein sandstone (1–1.4km depth) and the impact of mineral precipitation. We targeted borehole samples that best represented the variability of fractures within the Buntsandstein. Forty cylindrical samples (40mm in length and 20mm in diameter) were prepared from the chosen borehole samples such that they contained sealed or partially-sealed fractures either parallel or perpendicular to their axis. We also prepared samples of the intact host rock. These samples were then subject to porosity and permeability measurements, and thin sections were made for Scanning Electron Microscopy (SEM) to characterise the nature of the fractures and the precipitated minerals. Permeability measurements of the Buntsandstein host rock yielded values ranging from 10−15m2 to less than 10−18m2. SEM and X-ray powder diffraction analyses suggest that prevalent pore-filling illitic clays can explain the low permeability of the sandstone host rock. Additionally, we found that the permeability of fractures depends on the nature of the filling and the extent of sealing, with barite providing the most effective precipitate. Taking into account the geothermal fluid composition at Soultz-sous-Forêts, we employ a kinetic model for the barite crystal growth rate with temperature to provide an estimate for the time scale over which open fractures can seal through barite precipitation (from months to days depending on temperature). The rate increases dramatically as the temperature of the geothermal brine decreases, highlighting the risk of mineral precipitation at geothermal sites, where fluid temperature fluctuates due to circulation through the reservoir rock and fluid mixing around the injection well. An improved knowledge of the time dependency of fracture permeability will provide insights into the permeability anisotropy in the Buntsandstein and may have repercussions for the geothermal exploitation and for the ongoing fluid flow modelling of the Soultz-sous-Forêts geothermal reservoir.

Constraining the Permian/Triassic transition in continental environments: Stratigraphic and paleontological record from the Catalan Pyrenees (NE Iberian Peninsula)

The continental Permian–Triassic transition in southern Europe presents little paleontological evidence of the Permian mass extinction and the subsequent faunal recovery during the early stages of the Triassic. New stratigraphic, sedimentological and paleontological analyses from Middle–Upper Permian to Lower–Middle Triassic deposits of the Catalan Pyrenees (NE Iberian Peninsula) allow to better constrain the Permian–Triassic succession in the Western Tethys basins, and provide new (bio-) chronologic data. For the first time, a large vertebra attributed to a caseid synapsid from the ?Middle Permian is reported from the Iberian Peninsula—one of the few reported from western Europe. Osteological and ichnological records from the Triassic Buntsandstein facies reveal a great tetrapod ichnodiversity, dominated by small to medium archosauromorphs and lepidosauromorphs (Rhynchosauroides cf. schochardti, R. isp. 1 and 2, Prorotodactylus–Rotodactylus), an undetermined Morphotype A and to a lesser degree large archosaurians (chirotheriids), overall suggesting a late Early Triassic–early Middle Triassic age. This is in agreement with recent palynological analyses in the Buntsandstein basal beds that identify different lycopod spores and other bisaccate and taeniate pollen types of late Olenekian age (Early Triassic). The Permian caseid vertebra was found in a playa-lake setting with a low influence of fluvial water channels and related to the distal parts of alluvial fans. In contrast, the Triassic Buntsandstein facies correspond to complex alluvial fan systems, dominated by high-energy channels and crevasse splay deposits, hence a faunal and environmental turnover is observed. The Pyrenean biostratigraphical data show similarities with those of the nearby Western Tethys basins, and can be tentatively correlated with North African and European basins. The Triassic Pyrenean fossil remains might rank among the oldest continental records of the Western Tethys, providing new keys to decipher the Triassic faunal biogeography and recovery.

Selected Properties and Systematic Position of Soils Developed from Red Sandstones and Clays of the Lower Triassic Buntsandstein in the Nw Part of the Holy Cross Mountains (Poland) / Niektóre właściwości i pozycja systematyczna gleb wytworzonych z czerwonych piaskowców i iłów dolnego triasu w NW obrzeżeniu Gór Świętokrzyskich

AbstractThe aim of the study was to compare the properties of soils developed from the Lower Triassic Buntsandstein sediments in the north-western part of the Holy Cross Mountains (Poland). These are deposits of continental genesis and unique features such as red beds. Two representative soil pedons - the Bartków profile (pBK) developed from clay and the Góra Czerwona profile (pGC) developed from sandstone were selected for detailed analyzes. The morphology of profiles, their micromorphological features, mineralogical composition, and physico-chemical properties were examined. Most of the properties of the soils are a consequence of the original parent rock lithology. A specific feature of the morphology of the soils are the presence of red color (about 10R by the Munsell color scale) related to the presence of hematite. As shown by the XRD data, hematite is not the product of the current soil-forming processes, but it is a lithogenic component, which was inherited from the parent rock. Under the influence of climatic factors primary rock structure has been transformed into a new pedogenic one. Soils developed from clays have a characteristic angular blocky structure. Micromorphological analysis showed that an important role in the formation of soil structure involves geogenic susceptibility of Triassic clays to specific cuboid disintegration. This is indicated by the pore system of planes as an orthogonal nets visible in thin section. The soils developed from sandstone have a weak (unstable) subangular blocky structure. The main reason is the insufficient dispersion of the clay-ferruginous fraction from the sandstone matrix. Microscopic observations indicate that fine factions occurs as loose microaggregates, which results in a feature that smaller rock fragments and individual quartz grains are not bonded into soil aggregates. The studied soils are characterized by specific physical-chemical properties. Some of them strongly depend on the mineralogical properties of the soil substrate. Strong acidity (pH 3-4) and a very low content of base cations (below 1.0 cmol(+)kg-1) are due to a lack of carbonate minerals in sandstones and weak weathering of aluminium silicate. High exchangeable Al content in clay (16.5 cmol(+)kg-1) should be related to the geochemical properties of the red bed-type rocks such as the Lower Triassic Buntsandstein deposits. Soils developed from the Lower Triassic Buntsandstein clays have a sequence of genetic horizons: Ap, Bw, Bw/C, C and fulfill the criteria assigned to dystrophic typical brown soils (BDt) in the Polish Soil Classification (PSC 2011), whereas in the WRB they were classified as Endoeutric Chromic Cambisols (Loamic). Soils developed from the Lower Triassic Buntsandstein red sandstone can be classified as dystrophic humus brown soil (BDpr) in the PSC (2011). Within the WRB classification that soil can be assigned to Epidystric Chromic Endoleptic Cambisols.

Late Permian paleomagnetic results from the Lodève, Le Luc, and Bas-Argens Basins (southern France): Magnetostratigraphy and geomagnetic field morphology

Paleomagnetic results are presented from 271 stratigraphically-ordered horizons at four locations in southern France. Our focus is mainly on the Late Permian (258 horizons), but results from 13 horizons in the Triassic Buntsandstein are also reported. We argue that the Permian results extend magnetostratigraphic coverage up to the upper Capitanian Stage, some 6 million years after the end of the Permo-Carboniferous Reversed Superchron defined by the Illawarra Reversal in the Wordian Stage. When combined with published data, an overall mean paleopole at 49°N, 161°E (A95=4°, N=9) is obtained. This is virtually identical to the upper Permian pole obtained by Bazhenov and Shatsillo (2010) using the intersecting great-circle method. Agreement between the two procedures, which are based on entirely independent data, supports the geocentric axial dipole (GAD) model.

Floodplain habitats of braided river systems: depositional environment, flora and fauna of the Solling Formation (Buntsandstein, Lower Triassic) from Bremke and Fürstenberg (Germany)

The Solling Formation is the most distinctive unit of the Early Triassic Buntsandstein of the epicontinental Central European Basin. The Solling Formation of Bremke and Furstenberg has yielded one of the richest and most diversified plant collections of the Middle Buntsandstein to date, one of the oldest floras in Europe after the end-Permian mass extinction. Based on the plant fossils, the Middle Buntsandstein ecosystem from Bremke and Furstenberg represents not only one of the earliest floras in Europe after the end-Permian extinction but also one of the earliest Triassic occurrences of insect herbivory from any documented flora worldwide and thus provides a rare glimpse into the third pulse of herbivore expansion. Integrated palaeobotanical, palaeontological and sedimentological studies have enabled reconstruction of two different floodplain environmental settings of the Solling Formation, including their vegetation, the plant–insect interactions and revealing how important taphonomy and environmental settings were for the preservation of Middle Buntsandstein plants. At Bremke a levee-crevasse splay complex is reconstructed that tributed into perennial backwsamps and at Furstenberg unconfined subaerial flows formed a sandy aggradational floodplain with ephemeral ponds. A rich plant community was established and became preserved in backswamps and ponds. This suggests that the scarcity of Buntsandstein floras is clearly related to taphonomical processes and not to extreme environmental conditions under arid or semi-arid climates.