Late Jurassic Research Articles

Multiple phase rifting and subsequent inversion in the West Netherlands Basin: implications for geothermal reservoir characterization

Abstract. Aiming to contribute to the energy transition, this study provides an integrated picture of the geothermal system hosted in the West Netherlands Basin and shows how the reconstruction of the basin's geological history can contribute to the correct exploration and exploitation of its geothermal resources. In the West Netherlands Basin, the main geothermal targets are found in the Cretaceous and Jurassic strata that were deposited during the rifting and post-rifting stages and were deformed during the subsequent basin inversion. Despite multiple studies on the tectonic setting, the timing and tectono-stratigraphic architecture of the rift system and its overall control on the development and evolution of geothermal systems are still to be fully deciphered. In this study, a detailed seismo-stratigraphic interpretation of the syn- and post-rift intervals in the West Netherlands Basin will be given within the framework of geothermal exploration. A recently released and reprocessed 3D seismic cube is used, covering a large portion of the onshore section of the basin. We identified two major Jurassic rifting episodes and a Late Cretaceous inversion event. During the Jurassic rifting phases, the compartmentalization of the basin and the creation of accommodation space led to the deposition of the Late Jurassic Nieuwerkerk Formation, which is the main regional geothermal producing target. Within this formation, we individuate growth synclines located in the central portions of the Jurassic half-grabens as sites that show good potential for geothermal exploration.

Comment on ‘The Generation of Eocene Mafic Dike Swarms During the Exhumation of a Core Complex, Biarjmand Area, NE Iran’ by Azizi et al. (2023), Journal of Petrology, 64, 1–18

Abstract Azizi, H., Daneshvar, N., Asahara, Y., Minami, M. & Anma, R. (2023). The generation of Eocene mafic dike swarms during the exhumation of a Core complex, Biarjmand area, NE Iran. Journal of Petrology64, 1–18 have attributed the E–W-oriented mafic dike swarm in the Biarjmand metamorphic core complex to an Eocene extensional event which is much younger than a previously suggested Late Jurassic–Early Cretaceous age. They proposed that the emplacement of these dikes occurred in a rapid extensional regime coeval with the exhumation of the core complex after gravitational instability in the Central Iran/Eurasia collision zone. I appreciate the opportunity this article provides to shed light on specific aspects of the Late Mesozoic–Early Cenozoic continental extension within the Eurasian sector of the Neotethys subduction system. However, I here bring to attention certain discrepancies within Azizi et al.'s publication. Specifically, the assignment of an Eocene age to the emplacement of the E–W-oriented dike swarm, even though purportedly supported by U–Pb zircon dating, appears to be at odds with field observations and previously published geochronological data. Furthermore, the article contains internal contradictions in its presentation of the core complex model for the study area. It is important to note that Malekpour-Alamdari, A., Axen, G. J., Heizler, M. T. & Hassanzadeh, J. (2017). Large-magnitude continental extension in the northeastern Iranian plateau: insight from K-feldspar 40Ar/39Ar thermochronology from the Shotor Kuh-Biarjmand metamorphic core complex. Geosphere13, 1207–1233 previously documented the geochronological-based metamorphic core complex model of the area. Regrettably, despite its direct relevance, these earlier works have not been acknowledged in Azizi et al.'s paper. In this comment, I outline the problems with the structural and regional geology, the zircon U–Pb age of the dike samples, the age of the dike swarm, and the geodynamic interpretations in Azizi et al.'s work.

Remagnetization of the Upper Permian–Lower Triassic limestones in the western Lhasa Terrane and its tectonic implications

SUMMARY The drift history of the Lhasa terrane plays an essential role in understanding the tectonic evolution of the Bangong-Nujiang Tethyan Ocean and the Neo-Tethyan Ocean, as well as the evolutionary history of the Tibetan Plateau. Here, a combined rock magnetic, petrographic, and palaeomagnetic study is performed on the Upper Permian–Lower Triassic limestones (∼259–251 Ma) in the western Lhasa terrane. The site-mean direction for the 28 sites is Dg = 32.1°, Ig = 50.3°, kg = 47.9 and α95 = 4.0° in situ and Ds = 342.9°, Is = 32.7°, ks = 43.2 and α95 = 4.2° after tilt-correction, yielding a palaeopole at 68.9°N, 314.4°E with A95 = 4.3°, corresponding to a palaeolatitude of 18.0° ± 4.3°N. The fold tests are not significant because the sampling section shows monoclinic features with minor variations in their bedding attitudes. The palaeopoles for the directions before and after tilt-correction are compared with reliable Late Permian–Palaeogene palaeopoles obtained from the Lhasa terrane. Based on these comparisons, the studied limestones were remagnetized prior to tilting and this remagnetization most likely occurred during the Early Cretaceous. The depositional environment of the limestones may have changed from anoxic to suboxic and oxic during the Early Cretaceous, leading to the oxidation of iron sulphide to authigenic magnetite. Meanwhile, the Late Jurassic–Early Cretaceous convergence between the western Lhasa and Qiangtang terranes may have resulted in tectonic fluid migration and the formation of calcite veins and stylolites in the limestones. This is supported by the presence of small calcite veins and stylolites in some samples, as well as the fact that the framboidal oxides were formerly sulphides (mostly pyrite), implying that the majority of the iron oxides observed in the limestones were authigenic. These processes indicate that chemical remanent magnetization caused by the growth of magnetic minerals related to tectonic fluid migration was most likely the mechanism for the limestone remagnetization.

Gravity flow deposits in Mesozoic sediments of Chukotka microplate (North‐East Russia)

AbstractIn the Mesozoic succession of the Anyui–Chukotka fold system (North‐East Russia), five stratigraphic intervals were recognised that have an abundance of gravity flow deposits. These are the Olenekian (Lower Triassic), Upper Carnian, Upper Norian, Oxfordian–Kimmeridgian and Valanginian. The Triassic gravity flow deposits formed on the south‐facing, passive margin of the Chukotka microplate and consist of greywackes and lithic arenites. Palaeocurrent data indicate that the flows were directed towards the south‐east. The Olenekian gravity flow units consist of clast‐rich sandstone deposited on the continental slope, and clast‐poor sandstone deposited at the base of the slope. Upper Carnian mud‐poor sandstones were deposited at the base of the slope and the Norian thin‐bedded turbidites were upper to mid‐slope deposits. The continental margin was affected by tectonism and was uplifted in the latest Triassic–earliest Jurassic, possibly due to the initiation of the southward translation of the Arctic Alaska–Chukotka microplate. Following an Early–Middle Jurassic uplift of the area, sedimentation resumed in the Late Jurassic and earliest Cretaceous. Several syn‐orogenic depressions (Rauchua, Pegtymel, Pevek, Myrgovaam and Kytepveem) developed on the south‐western margin of the Chukotka microplate, and deposition in these basins included gravity flow deposits at various times. In both the Oxfordian–Kimmeridgian and Valanginian successions, gravity flow deposits included arkosic and subarkosic sandstones with a northern source area of granitoid complexes and deformed Triassic strata. The intervening Tithonian–Berriasian gravity flow deposits consisted mainly of thin‐bedded turbidites. These sediments had a southern source, which included a volcanic arc that had accreted to the southern margin of the Chukotka microplate.

Fossil-calibrated phylogenies of Southern cave wētā show dispersal and extinction confound biogeographic signal.

The biota of continents and islands are commonly considered to have a source-sink relationship, but small islands can harbour distinctive taxa. The distribution of four monotypic genera of Orthoptera on young subantarctic islands indicates a role for long-distance dispersal and extinction. Phylogenetic relationships were inferred from whole mtDNA genomes and nuclear sequences (45S cassette; four histones). We used a fossil and one palaeogeographic event to calibrate molecular clock analysis. We confirm that neither the Australian nor Aotearoa-New Zealand Rhaphidophoridae faunas are monophyletic. The radiation of Macropathinae may have begun in the late Jurassic, but trans-oceanic dispersal is required to explain the current distribution of some lineages within this subfamily. Dating the most recent common ancestor of seven island endemic species with their nearest mainland relative suggests that each existed long before their island home was available. Time estimates from our fossil-calibrated molecular clock analysis suggest several lineages have not been detected on mainland New Zealand, Australia, or elsewhere most probably due to their extinction, providing evidence that patterns of extinction, which are not consistently linked to range size or lineage age, confound biogeographic signal.

Origin of bedding-parallel calcite “beef” layers in the Upper Jurassic Haynesville shale, northwestern Louisiana

Origin of bedding-parallel calcite “beef” layers in the Upper Jurassic Haynesville shale, northwestern Louisiana

Stratigraphy and Depositional History of the Middle–Upper Jurassic Platform Carbonate Succession in Kıraç Mountain (Western Central Taurides, S Turkey)

Stratigraphy and Depositional History of the Middle–Upper Jurassic Platform Carbonate Succession in Kıraç Mountain (Western Central Taurides, S Turkey)

Geochemistry and Reservoir Properties of the Upper Jurassic Formations, in Selected Oilfield Central Iraq

Geochemical analysis of subsurface organic matter in the most productive source rocks, the Sargelu and Naokelekan formations, has provided insight into the potential of hydrocarbon production in the Jurassic strata of the selected oil field in central Iraq.The aim of this study is to conclude if the source rock can provide reservoir properties such as porosity and permeability. This attempt is made to support petroleum production from both reservoir and source rocks. The cause of this idea was adopted due to some of very good source rock were kept the produced hydrocarbons within the thin impermeable layers.Rock Eval pyrolysis returned high total organic carbon ranging from 0.55 to 34.82 wt%, Type II kerogen with a hydrogen index of up to 368 mg HC/gTOC, and a rock potential of 0.58 to 50.9 kghc/ton rock. Mature organic material was found in both formations, with a Tmax between 434 and 450 °C. This study of source rock properties shows that the rocks have good properties that need to be considered as the Sargelu and Naokelekan formations are the main sources of hydrocarbons in the studied area.The multi-story source-reservoir rocks are represented in this secession. The reservoir properties of the Naokelekan Formation are impressive in that it can draw crude oil from the underlying Sargelu Formation, which produces oil and releases it into the local petroleum system. The effective porosity in the Naokelekan Formation reached 14% in the main reservoir area, while permeability values mostly reached 4 mD. This indication potentially underscores the objective of this study to upgrade the oil-producing source rock into reservoir intervals.

Late Mesozoic Exhumation of the Huangling Massif: Constraints on the Evolution of the Middle Yangtze River

AbstractPlate subduction leads to complex exhumation processes on continents. The Huangling Massif lies at the northern margin of the South China Block. Whether the Huangling Massif was exhumed as a watershed of the middle reaches of the Paleo‐Yangtze River during the Mesozoic remains under debate. We examined the exhumation history of the Huangling Massif based on six granite bedrock samples, using apatite fission track (AFT) and apatite and zircon (U‐Th)/He (AHe and ZHe) thermochronology. These samples yielded ages of 157–132 Ma (ZHe), 119–106 Ma (AFT), and 114–72 Ma (AHe), respectively. Thermal modeling revealed that three phases of rapid cooling occurred during the Late Jurassic–Early Cretaceous, late Early Cretaceous, and Late Cretaceous. These exhumation processes led to the high topographic relief responsible for the emergence of the Huangling Massif. The integrated of our new data with published sedimentological records suggests that the Huangling Massif might have been the watershed of the middle reaches of the Paleo‐Yangtze River since the Cretaceous. At that time, the rivers flowed westward into the Sichuan Basin and eastward into the Jianghan Basin. The subduction of the Pacific Plate beneath the Asian continent in the Mesozoic deeply influenced the geomorphic evolution of the South China Block.

Extensional tectonics of the Indian passive continental margin in the Middle and Late Jurassic: Constraints from detrital zircon ages in the eastern Tethyan Himalaya

Knowledge the initial extension in the Indian continent during the Jurassic is important for understanding the dynamics of its breakup from Eastern Gondwana. The absence of Jurassic magmatic activity in the eastern Tethyan Himalaya hinders the understanding of this process. We report a provenance and tectonic setting study on the Middle Jurassic Zhela and Late Jurassic Weimei Formations sandstone in Gyangze, eastern Tethyan Himalaya. Detrital zircons of Jurassic sediments indicate four major age peaks: ∼500 Ma, ∼820 Ma, ∼950 Ma, and ∼2450 Ma, which reflect the affinity between the Tethyan Himalaya and India. The differences between the crystallization age and depositional age of isolated detrital zircons from the Middle Jurassic Zhela and Late Jurassic Weimei Formations, indicate that they were formed in an extensional continental margin and deposited on the passive continental margin of India. Combined with regional geological information, our results show that extensional tectonics was dominant on the northern margin of the Indian continent during the Jurassic. The source-sink sedimentary system and the topography remained stable at this time. After the extension event in the Middle and Late Jurassic, the Kerguelen mantle plume commenced activity, indicating the transition from the lithospheric thinning process to active rifting. The mantle plume activity in the Early Cretaceous led to large-scale uplift in the southeast part of the Tethyan Himalaya, that ultimately led to the breakup of India from Eastern Gondwana.

Ikaite versus seep-related carbonate precipitation in the Late Jurassic–Early Cretaceous of West Spitsbergen: evidence for cold versus warm climates?

Ikaite versus seep-related carbonate precipitation in the Late Jurassic–Early Cretaceous of West Spitsbergen: evidence for cold versus warm climates?

Mid-Cretaceous Accelerated Cooling of the Beishan Orogen, NW China: Evidence from Apatite Fission Track Thermochronology

Abstract The Beishan orogen, a significant component of the southern Altaids, presents an opportunity for investigating the intracontinental deformation and exhumation history of the Altaids during the Mesozoic era. Although previous studies indicated that the Beishan orogen has experienced multiple reactivation since the late Mesozoic, the precise extent of these events remains poorly constrained. Here, we provide a comprehensive synthesis of field observations and apatite fission track (AFT) thermochronological dating throughout the Beishan orogen. Detailed field observations confirmed four major E-W trending thrusts in our study area. Based on the youngest truncated strata associated with the thrusts and previous dating results from neighboring regions, we propose that these thrust sheets likely developed in the late Middle Jurassic. AFT dating results from seven pre-Mesozoic granitoid samples and associated with thermal history modeling demonstrate that the Beishan orogen experienced a rapid basement cooling during the mid-Cretaceous (~115–80 Ma). Moreover, a compilation of previously published and newly gained AFT data reveals a comparable mid-Cretaceous cooling event in other parts of Central Asia, such as Qilian Shan, Eastern Tianshan, and Altai-Sayan. This observation suggests that the mid-Cretaceous cooling event is more likely to be regional rather than localized. This mid-Cretaceous cooling pulse is interpreted as a tectonic exhumation controlled by boundary faults and related to the rotation of the Junggar and Tarim basins. These processes are linked to distant plate-margin events along the Eurasian continent.

Bacteria-Driven Fossil Ecosystems as Paleoindicators of Active Continental Margins and the Role of Carbonate Sediment-Hosted Vents in Geodynamic Reconstructions

Continental rifting of the Tisza microplate started during the Late Jurassic and resulted in phreatic eruptions, peperite, and the construction of a volcanic edifice in the Early Cretaceous in the Mecsek Mountains (South Hungary). In the SE direction from the volcanic edifice at Zengővárkony, a shallow marine (depth 100–200 m) carbonate sediment hosted a vent environment, and iron ore deposition occurred at the end of the Valanginian to early Hauterivian, hosting a diverse, endemic fauna of approximately 60 species. The detailed mineralogical analysis of the transport conduits included Fe oxides (ferrihydrite, goethite, hematite, and magnetite), quartz, mixed carbonate, pyrite, feldspar, Fe-bearing clay minerals, apatite, sulfates (barite, gypsum, and jarosite), and native sulfur. Filamentous, microbially mediated microtextures with inner sequented, necklace-like spheric forms (diameter of 1 μm) and bacterial laminae are also observed inside decapod crustacean coprolites (Palaxius tetraochetarius) and in the rock matrix. This complex ecological and mineralogical analysis provided direct evidence for the presence of bacteria in fossil sediment-hosted vent (SHV) environments on the one hand and for the intimate connection between bacteria and decapod crustaceans in hydrothermal environments 135 Ma before. This observation completes the fossil food chain of chemosynthesis-based ecosystems, from primary producers to the top carnivores reported for the first time from this locality.

New data on the shell anatomy of Selenemys lusitanica, the oldest known pleurosternid turtle in Europe

One of the most diverse turtle lineages in the Upper Jurassic and Lower Cretaceous record of Europe is that of the stem turtles Pleurosternidae (Paracryptodira), also distributed in North America. The oldest European representative of this lineage is the Portuguese Selenemys lusitanica, known from upper Kimmeridgian to lower Tithonian levels (Late Jurassic). The information on the shell of this taxon is so far very limited, being exclusively restricted to that provided in the publication in which the taxon was defined. New specimens from several upper Kimmeridgian to Tithonian localities in the Consolação Sub-basin of the Portuguese Lusitanian Basin, both from the Leiria District and the Lisbon District, are studied here. They include the most complete shell of the species found so far, as well as several isolated plates that allow us to improve knowledge about it. Thus, new information about the shell anatomy of Selenemys lusitanica is provided, but also about its intraspecific variability.

Upper Jurassic–lowermost Cretaceous hybrid build-ups of the Western Tethys Realm: Cement-rich microencruster-microbialite-calcified sponge framework

This study summarizes and provides new data concerning the composition, fabrics, depositional environment and palaeogeographic distribution of Upper Jurassic–lowermost Cretaceous (Kimmeridgian–Berriasian) upper slope build-ups with complex microencruster-microbialite-calcified sponge frameworks associated with large amount of early marine cement crusts. The focus is on reef carbonates from the Štramberk-type limestones from the Carpathians (Getic Carbonate Platform) and Apuseni Mountains in Romania, with additional data from the Plassen Carbonate Platform (Northern Calcareous Alps, Austria) and the Štramberk Carbonate Platform (Western Carpathians, Czech Republic and Poland). The microencrusters, often of problematic biological affinity, are mainly Crescentiella morronensis (Crescenti 1969), Labes atramentosa Eliášová 1986 and Radiomura cautica Senowbari-Daryan & Schäfer 1979. Specialized encrusting calcified sponges are also common, with Perturbatacrusta leini Schlagintweit & Gawlick 2011 as the most abundant and characteristic form. Light-dependent microencrusters (e.g., “Lithocodium-Bacinella”) that are abundant in coeval overlying coral-dominated reefs, are rare, as are corals (microsolenids adapted to low-light level). Three dominant types of fabric-based framework varieties are considered for the studied build-ups. The main differences between these types reflect a bathymetric zonation in their development. The abundance of Crescentiella, microbialites, massive radiaxial fibrous cement crusts, and poorly diversified microencruster/sponge assemblages, are all suggesting that Type I framework variety is characteristic for build-ups formed in the deepest parts of an upper slope environment. Type II most probably developed at slightly lower depths than Type I, as revealed by its main features (clusters of calcified sponges and microencrusters associated with less extensive cement crusts) and distribution between the other two types in the sedimentary sequences. Type III, the most common, characterizes build-ups formed at the shallowest depths of the upper slope environment and is composed of alternating layers of diverse microencrusters (including several light-dependent species), calcified sponges, rare microsolenid corals, microbialites and early marine cement crusts. The distribution patterns of these framework varieties (Type I to Type III) together with their associated facies, strengthen this depositional interpretation. From a process-based perspective, these upper slope build-ups exemplify triple hybrid carbonates which are intimate combinations of microbial and skeletal components (microencrusters/microbialites and calcified sponges), associated with abiotic precipitates (early marine cement crusts). Their palaeogeographic importance is reflected in their formation on the slopes of rimmed carbonate platform systems facing open oceanic domains, predominantly within the central Western Tethys Realm. Such hybrid build-ups appear to have been absent on the southern European shelf where other reef frameworks commonly developed on carbonate ramps and ramp-type carbonate platforms.

Late Jurassic Tethyan igneous records in North Sumatra: Geochronological and geochemical constraints

The Jurassic igneous rocks on Sumatra Island are important for investigating the Mesotethyan evolution and regional correlation in Southeast Asia. This paper presents new laser ablation−multicollector−inductively coupled plasma−mass spectrometry zircon U-Pb-Hf geochronological and isotopic, whole-rock elemental, and Sr-Nd isotopic data for the newly identified Glebruk dolerite and andesite in North Sumatra. New zircon dating results suggest that these mafic−intermediate volcanic rocks, with ages of 150−146 Ma, were formed during the Late Jurassic. The Glebruk dolerite and andesite belong to the calc-alkaline series, are enriched in the light rare earth elements, and exhibit depletion in Nb, Ta, and Ti. Thus, they resemble arc-like volcanic rocks. They have low (87Sr/86Sr)i ratios of 0.7038−0.7048 and strong positive εNd(t) and εHf(t) values of +6.4 to +7.5 and +4.8 to +18.6, respectively. Their geochemical signatures suggest that these Late Jurassic volcanic rocks were derived from a depleted mantle wedge that was metasomatized by the slab-derived melts. The Glebruk volcanic rocks were formed in a continental arc setting in response to the Late Jurassic northward subduction of the Woyla Ocean beneath West Sumatra. Our results and regional geological observations suggest that the Mesotethyan subduction-related igneous rocks could extend from South Qiangtang through West Yunnan to North Sumatra. The Woyla Ocean could represent the southern extension of the Mesotethyan Ocean.

Groundwater Exploration in Carbonate Reservoirs Using Borehole Investigations: A Case Study from South Dobrogea, Romania

The Late Jurassic–Early Cretaceous (J3–K1) transboundary aquifer is the most important groundwater body in southern–southeastern Romania, shared with Bulgaria and hosted in karstic–fractured carbonates. We conducted an integrated evaluation of this aquifer by analyzing three 700 m deep groundwater exploration–exploitation boreholes, which intercepted it in the Cernavodă area (South Dobrogea region). The evaluation was based on geophysical wireline logging, drilling information, and borehole production tests. A K-means clustering of the logging data was performed for lithology typing, formation boundaries identification, and the delineation of probable producing intervals associated with secondary porosity development. Petrophysical interpretation was carried out via depth-constrained (zonal) inversion, using multimineral models, the estimated formation boundaries, and variable uncertainties for the main input logs. The optimal interpretation models were correlated with borehole testing results to gain insight into the hydrogeological properties of the aquifer complex. The fractured–vuggy interval with the highest water-producing potential was identified in the lower section of the J3-age Rasova Formation (639–700 m depth), comprising mainly undolomitized limestones. A southeast-to-northwest trend of increasing productivity of the boreholes, correlated with an increasing lateral dolomitization intensity within the Rasova Formation, suggests a highly heterogeneous character of the aquifer. The differences in productivity are due not only to local porosity variations but also to various degrees of pore space connectivity that are related to the amount of fracturing or karstification. The novel findings of this study have important practical implications for the optimal placement, design, and drilling program of future groundwater exploitation boreholes in the Cernavodă area and neighboring sectors.

Docodontans (Mammaliaformes) from the Late Jurassic of Germany

ABSTRACT Docodon hercynicus sp. nov. from the Upper Jurassic (upper Kimmeridgian) Süntel Formation of the Langenberg Quarry in Lower Saxony is the first docodontan recorded from Central Europe. The two lower molars available are characterised by vertical enamel ridges at the distal flank of cusp a, which are typical for Docodon. This is only the second record of Docodon from Europe with the genus otherwise restricted to the Kimmeridgian-Tithonian of the Morrison Formation in western North America. A distal fragment of a much smaller lower molar from the same strata exhibits similar vertical enamel ridges and indicates the possible presence of a second Docodon species in the Langenberg Quarry. Docodon hercynicus n. sp. is further evidence for the close terrestrial faunal interrelationships between Europe and North America. It represents only the third record of a docodontan from Late Jurassic-earliest Cretaceous strata of Europe and supports a relic occurrence and late survival of mammaliaforms on a palaeo-island in the Lower Saxony Basin within the European Archipelago. Zoobank: urn:lsid:zoobank.org:pub:90E8BCB2-B4DC-49D4-B10C-115ECF60E1F3

Substrate control in track registration and preservation: insights across the Triassic–Jurassic boundary in southern Africa

Tracks registration is influenced by the dynamic interplay between the pedal anatomy of the trackmaker, its behaviour, and the substrate conditions it interacts with. Differences in substrate conditions, especially those linked to grain size and moisture content, often result in the most dramatic variations in track morphology. In the upper Stormberg Group, main Karoo Basin of southern Africa, diverse trace fossils, primarily comprising Late Triassic–Early Jurassic dinosaur tracks, are preserved. Numerous studies have extensively documented individual ichnosites, investigating variations between sites over time, with recent studies suggesting that track abundance and anatomical fidelity increase up-stratigraphy. Despite the well-established link between substrate and track morphology, past studies have not specifically focused on substrate conditions, often emphasizing macro-sedimentary features instead. Here, we examine the micro-sedimentary features of track-bearing units in the upper Stormberg Group using petrographic techniques to better understand the palaeosubstrate and its effect on fossil track registration and preservation. The analysis revealed that very fine-grained sandstones and substrates modified by microbial activity tend to preserve tracks with greater abundance and/or higher anatomical fidelity. Furthermore, the prevalence of very fine-grained and microbially modified strata, and their associated track trends increases in younger stratigraphic units. Across the Triassic – Jurassic boundary in southern Africa, a boom in dinosaur track abundances is observed and credited to the proliferation of dinosaur populations during the Early Jurassic. Our findings, however, suggest that the observed local increase in track abundance (and anatomical fidelity) up-stratigraphy may be linked to substrate composition differences, which were ultimately controlled by large-scale changes in the palaeoenvironment from high-energy meandering fluvial to lower-energy aeolian-lacustrine settings in the Late Triassic and Early Jurassic, respectively. These findings have implications for global macroevolutionary patterns, palaeo-geographical reconstructions, and biostratigraphic correlations in the early Mesozoic.

Variations of sedimentary environment under cyclical aridification and impacts on eodiagenesis of tight sandstones from the late Middle Jurassic Shaximiao Formation in Central Sichuan Basin

Understanding eodiagenesis is essential to decoding the diagenetic pathways of tight sandstones that act as excellent unconventional oil & gas reservoirs. Great paleoclimate change is capable of influencing eodiagenetic processes of tight sandstones through variations of sedimentary environment. However, it is less noted how climate gradients between the greenhouse and the hothouse conditions impact the eodiagenesis of tight sandstones. We examined eodiagenetic processes that has been operated in the Shaximiao Formation sandstones using petrographic observation, scanning electron microscopy, geochemistry, and XRD analysis to reveal impacts of the transitional climate changes on the differential eodiagenesis and implications for the diagenesis-porosity evolution. Based on sequence stratigraphy, the Shaximiao Formation is divided into four sub-members SXM1, SXM2, SXM3, and SXM4, respectively. Dark sandstones and mudstones mainly occurred in the SXM1 and the SXM2. Grey-green clastic rocks are dominant in the SXM3, whereas red mudstones frequently appear in the SXM4. Paleoclimate indices denote that a cyclical aridification from the warm-humid to hot-semiarid conditions took place from the SXM1 to the SXM4. It could have been caused by the megamonsoon effect and the paleogeographic shift along with the breakup of the Pangaea supercontinent. Combined with the migration of depocenters, the paleoclimate change resulted in transformation of sediment provenances from mafic igneous rocks to quartzose sedimentary rock along with the decreasing textural maturity. Therefore, sedimentary environments varied from the high-saline to low-saline and from low-oxygen to high-oxygen conditions respectively, which had a crucial impact on eodiagenetic cements which were formed in the Shaximiao Formation. Chlorite and laumontite cement precipitation was promoted by high-saline alkaline fluids. Chlorite proportions show an arched trend from the SXM1 to the SXM4, compatible with those of primary and secondary porosities. In contrast, laumontite proportions exhibit a decreasing variation from the SXM1 to the SXM4. High percentages of early cements are favorable to improving resistance to the compaction and preservation of primary pores. However, high chlorite (>5%) and laumontite (>10%) proportions are destructive for the reservoir quality. Large quantities of laumontite cements occupy primary pores and impede diagenetic fluids flowing and are not favorable to the dissolution. By contrast, an excess of chlorite cements can be dissolved to produce secondary pores during the organic acid release. Thus, the SXM2 is potential as a reservoir for oil & gas. Therefore, those eodiagenetic cements can control the late diagenetic evolution and the reservoir quality. Observations made here have implications for understanding tight sandstone reservoirs elsewhere in the world.