Jurassic Research Articles

Jurassic sediments geochemical constraints on provenance, weathering process, and palaeoclimate variation of the north margin of Qaidam Basin, north‐eastern Tibetan Plateau

The Jurassic sequence in the northern Qaidam Basin can be represented by the lower coal‐bearing swamp‐lacustrine sediments unit and the upper reddish clastic deposits suite, which recorded dramatic transition of deposit environment changes. Whole‐rock geochemical data of this succession in Dameigou profile were used to reconstruct the provenance and chemical weathering history. Based on the major element discrimination diagrams, trace element ratios and chondritenormalized REE patterns, most sediments were derived from a similar source area with felsic‐intermediate rocks as dominant contributors. Multiple geochemical parameters and index displayed good correlation, via vertical variations sequence, Jurassic chemical weathering history of northern Qaidam Basin was established, and four stages were recognized from the profile. Phases I and III experienced intense chemical weathering, while Phases II and IV are characterized by mild to moderate chemical weathering. Based on this, long‐term Jurassic climatic evolution of the Qaidam Basin was reconstructed. Early‐middle period of Early Jurassic and middle period of Middle Jurassic developed warm and humid climate. On the contrary, late Early Jurassic‐early Middle Jurassic and late Middle Jurassic‐Late Jurassic was deposited under relatively semiarid to arid climatic conditions.

Migmatites record multiple episodes of crustal anatexis and geochemical differentiation in the Sulu ultrahigh‐pressure metamorphic zone, eastern China

AbstractPartial melting of ultrahigh‐pressure (UHP) metamorphic rocks is common during collisional orogenesis and post‐collisional reworking, indicating that determining the timing and processes involved in this partial melting can provide insights into the tectonic evolution of collisional orogens. This study presents the results of a combined whole‐rock geochemical and zirconological study of migmatites from the Sulu orogen in eastern China. These data provide evidence of multiple episodes of crustal anatexis and geochemical differentiation within the UHP metamorphic rocks. The leucosomes contain higher concentrations of Ba and K and lower concentrations of the rare earth elements (REE), Th and Y, than associated melanosomes and granitic gneisses. The leucosomes also have homogenous Sr–Nd–O isotopic compositions that are similar to proximal (i.e. within the same outcrop) melanosomes, suggesting that the anatectic melts were generated by the partial melting of source rocks that are located within individual outcrops. The migmatites contain zircons with six different types of domains that can be categorized using differences in structures, trace element compositions, and U–Pb ages. Group I domains are relict magmatic zircons that yield middle Neoproterozoic U–Pb ages and contain high REE concentrations. Group II domains represent newly grown metamorphic zircons that formed at 230 ± 1 Ma during the collisional orogenesis. Groups III, IV, V, and VI zircons are newly grown anatectic zircons that formed at 222 ± 2 Ma, 215 ± 1 Ma, 177 ± 2 Ma, and 152 ± 2 Ma, respectively. The metamorphic zircons have higher Th/U and lower (Yb/Gd)N values, flat heavy REE (HREE) patterns with no significantly negative Eu anomalies relative to the anatectic zircons, which are characterized by low Th/U ratios, steep HREE patterns, and negative Eu anomalies. The first two episodes of crustal anatexis occurred during the Late Triassic at c. 222 Ma and c. 215 Ma as a result of phengite breakdown. The other two episodes of anatexis occurred during the Jurassic period at c. 177 Ma and c. 152 Ma and were associated with extensional collapse of the collision‐thickened orogen. The majority of Triassic anatectic zircons and all of the Jurassic zircons are located within the leucosomes, whereas the melanosomes are dominated by Triassic metamorphic zircons, suggesting that the leucosomes within the migmatites record more episodes of crustal anatexis. Both metamorphic and anatectic zircons have elevated εHf(t) values compared with relict magmatic zircon cores, suggesting that these zircons contain non‐zircon Hf derived from material with more radiogenic Hf isotope compositions. Therefore, the Sulu and Dabie orogens experienced different episodes of reworking during the exhumation and post‐collisional stages.

Early evidence of molariform hypsodonty in a Triassic stem-mammal

Hypsodonty, the occurrence of high-crowned teeth, is widespread among mammals with diets rich in abrasive material, such as plants or soil, because it increases the durability of dentitions against wear. Hypsodont postcanine teeth evolved independently in multiple mammalian lineages and in the closely related mammaliaforms since the Jurassic period. Here, we report the oldest record, to our knowledge, of hypsodont postcanines in the non-mammaliaform stem-mammal, Menadon besairiei, from the early Late Triassic. The postcanines are long and columnar, with open roots. They were not replaced in older individuals and remained functional after the total wear of the crown enamel. Dental histology suggests that, convergently to hypsodont mammals, wear was compensated by the prolonged growth of each postcanine, resulting in dentine hypsodont teeth most similar to extant xenarthran mammals. These findings highlight the constraints imposed by limited tooth replacement and tooth wear in the evolutionary trajectories of herbivorous mammals and stem-mammals.

A new calmanostracan crustacean species from the Cretaceous Yixian Formation and a simple approach for differentiating fossil tadpole shrimps and their relatives

BackgroundCalmanostraca is a group of branchiopod eucrustaceans, with Triops cancriformis and Lepidurus apus as most prominent representatives. Both are regularly addressed with the inaccurate tag “living fossil”, suggesting that the morphology has remained stable for several millions of years. Yet, T. cancriformis and L. apus represent only a fraction of the morphological diversity occurring in Calmanostraca, comprising the two groups Notostraca and Kazacharthra. Notostracans, commonly called tadpole shrimps, comprise the two groups Lepidurus and Triops with their elongated and rather narrow (in dorsal view) head shields. Kazacharthrans are exclusively fossil calmanostracans with broad and rather short shields, known from the Jurassic and Triassic period. One formation where fossil calmanostracans have been found is the Yixian Formation of northeastern China (Lower Cretaceous, 125–121 million years). It is part of the Jehol Group, an ecosystem known for its exceptionally well-preserved fossils, including vertebrates and plants, but also diverse arthropods. Two calmanostracan species have to date been described from the Yixian Formation, Jeholops hongi and Chenops yixianensis.ResultsWe describe here a new calmanostracan crustacean from the Yixian Formation, Notostraca oleseni, and additionally a simple tool using a morphospace analysis to delineate different species. Measurements characterising the shield and trunk proportions of different calmanostracan species were performed, data were size-corrected, and used for this morphospace analysis to compare the different morphologies. As sclerotised body parts are more likely to be preserved in fossils than soft tissue, shields and parts of the trunk are in many cases the only morphological structures available for study. Therefore, the present analysis represents a simple tool for distinguishing between different species, as well as allowing the inclusion of specimens that are only preserved fragmentarily. Additionally, it provides a tool to demarcate the kazacharthran-like specimen described, but not formally named, by Wagner et al. (Paleontol Res. 22:57–63, 2018). Hence, we amended the description and name the species Calmanostraca hassbergella.ConclusionOur results indicate a large diversity in shield and trunk morphology in calmanostracans, in contrast to their often claimed highly conserved and uniform morphology. Especially extinct forms such as Notostraca oleseni add up to this result and point to the species richness and morphological diversity within Calmanostraca.

Petrology and 40Ar-39Ar dating of paragneisses from the Devrekani Massif (Central Pontides, Northern Turkey): Implications for the Jurassic high-T metamorphism in an extensional tectonic environment

The Devrekani Massif in the northern part of the Central Pontides (Northern Turkey) provides important clues to the regional tectonics and geodynamic processes associated with Jurassic high grade metamorphic conditions. This study reports new paragenetic assemblages, mineral compositions, whole-rock geochemistry and 40Ar-39Ar geochronological data from the paragneisses in the massif, and, discusses the P–T conditions and geodynamic implications of the Jurassic metamorphism during continental extension in the Central Pontides. Upper amphibolite to lower granulite facies paragneisses form one of the main lithological units in the massif. Within these, there are five different mineral parageneses with diagnostic mineral assemblages of: quartz, K-feldspar (An0-1Ab4-26Or73-96), plagioclase (An18-35), biotite [XPhl: 0.28–0.57; Mg/(Mg + Fe2+): 0.33–0.61], sillimanite, cordierite [Mg/(Mg + Fe2+): 0.48–0.71] and garnet (Alm43-80Grs0-18Prp5-23And0-4Sps10-33) with minor hercynite. Based on Na-in-Crd thermometry and GASP barometry results, the peak metamorphic conditions are 775 ± 25 °C and 6 ± 1 kbar in the massif. The field relations, petrography and bulk chemical data suggest that the paragneisses, derived from shale-wackestone and pelitic sedimentary protoliths, are typical rock lithologies of an active continental margin. They display enrichments in LILE over HFSE, coupled with negative Nb and Ti anomalies, which are geochemical signatures of subduction-related sources. It is likely that the peak metamorphism took place during the Middle–Upper Jurassic period (ca. 174–156 Ma), suggesting that the metamorphic rocks cooled to 300–350 °C at ca. 156 Ma. The mineral assemblages reveal that the prograde history passed from sillimanite zone conditions up to the cordierite-garnet-K-feldspar zone. The petrological and geochronological data indicate that the protoliths are related to multiple sources such as volcano-sedimentary successions. We conclude that the Devrekani Massif represents the products of pre-Jurassic sedimentation, and Permo-Carboniferous continental arc magmatism, overprinted by Jurassic metamorphism.

Late Triassic - Late Jurassic subsidence analysis in Neuquén Basin central area

This paper presents a detailed study of the tectonic subsidence evolution during the Late Triassic - Late Jurassic period in the central area of the Neuquén Basin. The methodology consisted of the integration of outcrop and subsurface data, in order to obtain tectonic subsidence curves for the analyzed period in the study area. The origin of the basin was linked to a lithospheric mechanical extension episode in the Late Triassic - Early Jurassic that, once completed, gave rise to a post-rift stage. The post-rift stage, comprised between Early Jurassic and Early Cretaceous, was classically assumed as dominated by thermal subsidence. Results obtained in this paper show that there is a change in the geometry of the tectonic subsidence curves, which depart considerably from the expected behavior for a basin dominated by thermal subsidence. Therefore, the post-rift stage of the Neuquén Basin for the study area would have had a more complex tectonic evolution than that determined exclusively by the thermal subsidence, and that superposition of mechanisms must be considered. From the available data, a shift from thermal to dynamic subsidence from the Middle Callovian onwards seems to be the most plausible explanation for the subsidence evolution observed in the study area.

Structural and chemical features of coal-similar wood organic substances of the Devonian and Jurassic periods

The results of investigation of organic matter chemical structure of the coal-similar wood of Devonian and Jurassic geological periods by the methods of IR-, EPR-, chromate-masses-spectrometry and element analysis are presented. It is shown that the investigated samples are the products of the metamorphism of natural lignins, which confirms hypothesis about the important role of lignins in the processes of coal formation.

Structural and Chemical Features of Organic Matter in Carbonized Wood of the Devonian and Jurassic Periods

The results of studies of the chemical structure of organic matter in Devonian and Jurassic carbonized wood using IR, ESR, and GC–MS techniques along with elemental analysis are presented. It is shown that the examined samples consist of metamorphism products of natural lignin, which confirms the hypothesis of the key role of lignin in coal formation.

Characteristics and exploration potential of Jurassic oil and gas reservoirs in Mahu sag of the Junggar Basin

Characteristics and exploration potential of Jurassic oil and gas reservoirs in Mahu sag of the Junggar Basin

A new Jurassic scansoriopterygid and the loss of membranous wings in theropod dinosaurs.

Powered flight evolved independently in vertebrates in the pterosaurs, birds and bats, each of which has a different configuration of the bony elements and epidermal structures that form the wings1,2. Whereas the early fossil records of pterosaurs and bats are sparse, mounting evidence (primarily from China) of feathered non-avian dinosaurs and stemward avians that derive primarily from the Middle-Upper Jurassic and Lower Cretaceous periods has enabled the slow piecing together of the origins of avian flight3,4. These fossils demonstrate that, close to the origin of flight, dinosaurs closely related to birds were experimenting with a diversity of wing structures3,5. One of the most surprising of these is that of the scansoriopterygid (Theropoda, Maniraptora) Yi qi, which has membranous wings-a flight apparatus that was previously unknown among theropods but that is used by both the pterosaur and bat lineages6. This observation was not universally accepted7. Here we describe a newly identified scansoriopterygid-which we name Ambopteryx longibrachium, gen. et sp. nov.-from the Upper Jurassic period. This specimen provides support for the widespread existence of membranous wings and the styliform element in the Scansoriopterygidae, as well as evidence for the diet of this enigmatic theropod clade. Our analyses show that marked changes in wing architecture evolved near the split between the Scansoriopterygidae and the avian lineage, as the two clades travelled along very different paths to becoming volant. The membranous wings supported by elongate forelimbs that are present in scansoriopterygids probably represent a short-lived experimentation with volant behaviour, and feathered wings were ultimately favoured during the later evolution of Paraves.

A Jurassic leap for flowering plants.

Comprehensive phylogenomics and molecular dating support a Triassic origin for flowering plants that is substantially earlier than their appearance in the fossil record, leaving a puzzling gap spanning the entire Jurassic period.

Upper Triassic\u2012Middle Jurassic resedimented toe-of-slope and hemipelagic basin deposits in the Dinaridic Ophiolite Belt, Zlatar Mountain, SW Serbia

In the Dinaridic Ophiolite Belt, small (m-scale) to large (km-scale) blocks of Middle to Upper Triassic, and Middle to Upper Jurassic, more or less silicified bedded limestone are widely present, both as parts of para-autochthonous successions and as redeposited blocks in ophiolitic mélanges. The studied, approximately 230-m-thick succession in the wider area of Zlatar Mountain, is one of the most important para-autochthonous sections in the Serbian part of the Dinaridic Ophiolite Belt. The succession is made up of an alternation of redeposited carbonate toe-of-slope deposits, sand to clay-sized siliciclastic rocks, hemipelagic mudstones and radiolarite–spongiolite basin facies. In the lower part of the sequence, the components of the siliciclastic beds were derived mostly from low- and medium-grade metamorphic rocks. Similar components, together with sand-sized fragments of ophiolitic rocks, were encountered in small amounts in some redeposited carbonate beds. The chronostratigraphic assignment of the succession is based mostly on foraminifers, but age-diagnostic radiolarians and other microfossil groups were also considered. In the lower part of the probably continuous succession, a Norian–Rhaetian assemblage was recognized; a Sinemurian–Pliensbachian assemblage was encountered up-section, whereas the upper part of the succession could be assigned to the Bajocian–Bathonian. Considering the paleogeographic reconstructions and the analogies of age-equivalent sections, the succession records the depositional history of the Bosnian Basin during the Late Triassic to Middle Jurassic period and may contribute to the understanding of the evolution of the Adriatic margin of the Neotethys Ocean in the transition interval from passive to active margin stages.

Oil-bearing heterogeneity and threshold of tight sandstone reservoirs: A case study on Triassic Chang7 member, Ordos Basin

The Ordos Basin is the foremost tight-oil reservoir in China with respect to location and production, where tight-oil reservoirs are characterized by low porosity, low permeability, low pressure, and high heterogeneity. Until now, the cause of the oil-bearing heterogeneity of such reservoirs has not been reported. In this study, the cause of the oil-bearing heterogeneity and threshold of tight sandstone reservoirs in the seventh member of the Triassic Yanchang Formation (Chang7 Member), Ordos Basin, were investigated by core and thin-section observations using a fluorescence microscope, cathodoluminescence microscope, isotope ratio mass spectrometer, and thermal stage, among other devices. The results revealed that calcitic cementation is the major genetic mechanism for the oil-bearing heterogeneity of tight sandstone reservoirs in the study area, and that it is mainly in Phase 1. The clumped isotope revealed that the calcitic cements in Phase I were formed at 18–42 °C during the Middle–Late Triassic to Middle Jurassic periods; thus, they are the diagenetic products, and may be related to the early small-scale tectonic movements in the basin. The synchronous brine inclusions associated with the hydrocarbon inclusions correspond to a homogenization temperature of 90–120 °C. In conjunction with the basin simulation, this suggests that the hydrocarbons were charged in one phase in the Early Cretaceous period (100–130 Ma). Moreover, the modified cements in the Chang7 oil-free tight sandstone were formed earlier than the period in which the oil was charged in the oil-bearing sandstone. The tight sandstone reservoirs mainly contain nanoscale throats (diameter < 1 μm). With respect to the residual oil saturation, average size of mesopores, and the correlation between the oil saturation and porosity, it was concluded that tight oil may be present above the threshold characterized by a 15 mm mesopore average size and 2% porosity. It is first specified that the formation of oil-free calcitic-cemented sandstone is the diagenetic response of the early tectonic movement, which provides a basis for the determination of the influence of calcitic-cemented sandstone on hydrocarbon accumulation. Moreover, the results of the study on the oil-bearing threshold can be helpful in selection of the exploration zone and the resource estimation of unconventional tight-oil reservoirs.

Lake evolution and its influence on the formation of oil shales in the Middle Jurassic Shimengou Formation in the Tuanyushan area, Qaidam Basin, NW China

The non-marine Qaidam Basin is a petroliferous basin in northwest China. The Tuanyushan area is located in the Saishiteng Depression in the northern Qaidam Basin. Coal and oil shales are widely developed in the Middle Jurassic Dameigou and Shimengou formations in this area where the sedimentary sequence and controls on coal accumulation have already been well documented. However, the geochemical characteristics of lacustrine fine-grained sediments, including lacustrine oil shales, in the shale member of the Shimengou Formation and the main controlling factors of the formation of the oil shales are ambiguous. This paper aims to reconstruct the lake evolution history during the Middle Jurassic period and reveal its influence on the formation of the oil shales in this area. Oil shales and fine-grained sediments were systematically sampled to determine their mineralogical and geochemical characteristics (major, trace and rare earth elements; stable carbon and oxygen isotopes). Based on lithological variations and total organic carbon (TOC) contents, a complete third-order sequence is identified and can be further divided into four system tracts (lowstand system tract, LST; transgressive system tract, TST; highstand system tract, HST and regressive system tract, RST) that correspond to four lake evolution stages (A–D). Changes in the lake level show an initial shallow lake, followed by a continuous upward deepening trend, followed by a shallowing trend. Shallow lake facies developed in the LST, TST and RST, whereas semi-deep to deep lake facies developed in the HST. Stable carbon and oxygen isotopes indicate that the Shimengou Lake was semi-closed to closed in the Middle Jurassic. Therefore, the water properties responded strongly to climate changes. According to elemental and mineralogical analyses, a moist climate prevailed, except during the early stages of the TST (stage B1) and HST (stage C1) when there was a semiarid climate. The semiarid climate influenced the water properties and detrital input and was the major controlling factor for the formation of the higher quality oil shales within a saline water environment. In comparison, under the moist climatic condition in the HST, the stable semi-deep to deep-water environment was the major controlling factor for the formation of lower quality oil shales within a fresh water environment.

THE LOWER BAJOCIAN GAETANI LEVEL: LITHOSTRATIGRAPHIC MARKER OF A POTENTIAL OCEANIC ANOXIC EVENT

In this paper we document in detail the transition from the Rosso Ammonitico Lombardo to the Radiolarites outcropping at Alpe Turati (Albavilla, Como) in the Lombardy Basin of the Southern Alps. At this location, the Jurassic succession was deposited on a flank of the Corni di Canzo paleohigh: as found on other structural shallow areas, the upper Pliensbachian–Toarcian–Aalenian interval is represented by pseudonodular to nodular marly limestones of the Morbio Limestone and Rosso Ammonitico Lombardo units. In the early Bajocian deposition of biosiliceous sediments is ubiquitously testified by the Radiolarites (Bajocian–Callovian). At Alpe Turati, the interval immediately below the basal green member of the latter unit, consists of a 10 cm thick black shale, pointing to oxygen-depleted bottom waters. Dysoxia persisted during the Bajocian–Callovian as evidenced by dark green stratified cherts. Calcareous nannofossil biostratigraphy indicate an early Bajocian age for the black shale that correlates with the core of a distinct C isotopic negative anomaly also known in various basins outside the Southern Alps. Very dark grey to black lithologies of early Bajocian age have been documented in a few sections from Poland, Corsica and Morocco. We name Gaetani Level the black shale interval in recognition of Maurizio Gaetani’s pioneering and extensive work on Jurassic sedimentary successions of the Lombardy Basin. The association of the Gaetani black shale with a C isotopic anomaly suggests that it could be the sedimentary record of an Oceanic Anoxic Event whose regional to global extension must be ascertained.

Early–Middle Jurassic magmatism and skarn–porphyry mineralization in NE China: Geochronological and geochemical constraints from the Sankuanggou skarn Fe–Cu–(Mo) deposit, and tectonic implications

The Early–Middle Jurassic tectonic setting of NE China is poorly defined. Mineralization of this period, especially skarn–porphyry mineralization and the ore-related magmatism, can provide well constraints on the geodynamic setting. An analysis of the spatio-temporal distribution of skarn–porphyry deposits in NE China, with the Sankuanggou skarn Fe–Cu–(Mo) deposit chosen as a typical example, was conducted in an attempt to elucidate the tectonic setting of the region. The Sankuanggou deposit is located in the Great Xing'an Range, in the eastern segment of the Central Asian Orogenic Belt. Re-Os isochron dating of molybdenites indicates the ages of the skarn- and veinlets–disseminated-type mineralization are 172.9 ± 3.3 and 173.4 ± 6.3 Ma, respectively. This suggests that the mineralization was contemporaneous with emplacement of granodiorite (175.9 ± 1.6 Ma), and related to Early Jurassic magmatism. Zircons from the Sankuanggou granodiorite have εHf(t) values of +8.7 to +11.2. Sulfur isotopic compositions (δ34S) of the sulfides range from −3.59‰ to +0.92‰, indicating that the ore-forming materials were mainly derived from magma. The Re content of molybdenite (11.3–63.3 ppm) suggests that the ore-forming materials mainly originated from a mixed crust–mantle source. Geochemical characteristics and Sr–Nd–Pb–Hf isotopic compositions suggest that the ore-forming granodiorite was formed in a continental arc setting, melts of which largely originated from partial melting of a mantle wedge induced by the subducted slab. Based on the spatio-temporal distribution of the skarn–porphyry deposits in NE China, we propose that the Early–Middle Jurassic period (200–160 Ma) is likely to have been one of the most important metallogenic epochs in NE China. These deposits formed in a continental arc setting, which was closely related to the westward subduction of the paleo-Pacific Plate during this period.

Structure styles and tectonic evolution of the Abrolhos Sub-Basin, Perth Basin, Western Australia

Based on examination and re-interpretation of 2D seismic and well data, three groups of faults were recognized in the Abrolhos Sub-basin: a) major normal faults transecting the Permian, Triassic and Jurassic, b) small normal faults developed in the Early Permian, and c) small normal faults developed in the Triassic and Jurassic.

On the Middle Jurassic – Early Cretaceous Megatrigoniinae (Bivalvia, Trigoniida): their biogeography, evolution and classification

On the Middle Jurassic – Early Cretaceous Megatrigoniinae (Bivalvia, Trigoniida): their biogeography, evolution and classification

New insights into multiple provenances evolution of the Jurassic from heavy minerals characteristics in southern Junggar Basin, NW China

Abstract The method of random forest was used to classify the heavy mineral assemblages of 2 418 Jurassic samples in the southern Junggar Basin, and determine the distribution of the heavy mineral assemblages from the same provenance systems. Based on the analysis of heavy minerals assemblages, ZTR index, sedimentary characteristics, U-Pb zircon ages, whole-rock geochemical and paleocurrent direction analysis, the study reveals that five important provenances were providing sediments to the southern Junggar Basin in the Jurassic period: The North Tianshan (NTS), Central Tianshan (CTS), Bogda Mountains, Zhayier Mountains and Kalamaili Mountains. During the Early Jurassic, NTS-CTS, Kalamaili Mountains and Zhayier Mountains are primary provenances, Bogda Mountains started to uplift and supply clastic materials in the Middle Jurassic. There are three sedimentary area in the Jurassic of southern Junggar Basin: the western part, the central part and the eastern part. In the western part, the clastic materials of the Early Jurassic was mainly from NTS blocks and Zhayier Mountains, and the sediments were dominantly derived from the Zhayier Mountains during the Middle–Late Jurassic. In the central part, the main provenance of the Early Jurassic switched from NTS to CTS. In the Xishanyao Formation, the main source went back to NTS again. The NTS was the primary provenance during the sedimentary periods of Toutunhe Formation and Qigu Formation. In the eastern part, the contribution of CTS and Kalamaili Mountains were considered as major provenances in the Early Jurassic−Xishanyao Formation, small proportion of sediments were from NTS. The Bogda mountains uplifted and started to provide sediments to the Junggar Basin in the sedimentary period of Xishanyao Formation, and became the major source during the Toutunhe Formation period, with small amount of sediments from CTS. The provenance from CTS was hindered during the sedimentary period of Qigu Formation owing to the uplifting of the Bogda mountains, and the sediments were mainly from the Bogda mountains and NTS.

Sequence stratigraphic analysis of the Lower Jurassic Precipice Sandstone and Evergreen Formation in the Surat Basin, Australia: Implications for the architecture of reservoirs and seals for CO2 storage

Abstract The Lower Jurassic Precipice Sandstone and Evergreen Formation in the Surat Basin, Australia are a highly prospective interval for carbon capture and storage (CCS). However, their stratigraphic nomenclature is inconsistently applied, and a basin-wide stratigraphic framework that is ubiquitously agreed upon remains elusive. In particular, a sandy transitional sandstone interval that occurs above the main Precipice Sandstone reservoir unit has caused confusion about how the two units should be subdivided. To address this issue, we integrate core observations with seismic interpretation and wireline log correlation to construct a sequence stratigraphic framework for the Lower Jurassic System that goes beyond lithostratigraphy. The main findings of our work include: (1) the identification of three 3rd-order sequences, SQ1-SQ3, from base to top. Two main sandstone intervals, the Precipice Sandstone and the Boxvale Sandstone Member (Evergreen Formation), represent the lowstand systems tracts (LSTs) of SQ1 and SQ3, respectively. The Westgrove Ironstone Member in the upper Evergreen Formation – a distinctive oolitic ironstone unit – represents the transgressive system tract (TST) of SQ3; (2) the transitional interval above the main Precipice Sandstone reservoir corresponds to the lower TST in SQ1 (lower Evergreen Formation). It is manifest as an upward-fining succession with a retrogradational facies stacking pattern, and is significantly different from the main Precipice Sandstone reservoir which shows an aggradational stacking pattern; (3) the Precipice Sandstone and lower Evergreen Formation progressively onlap the western and the eastern margins of the Surat Basin, and this confines the Precipice Sandstone to the central portion of the basin; (4) the three sequences (SQ1-SQ3) approximately coincide with global sea-level cycles documented from the early Jurassic. Two episodes of accelerated tectonic subsidence were responsible for the thinning and onlap of SQ1 and SQ3 onto the basin margins. In contrast, weaker tectonic subsidence was responsible for the overall finer grained sediment, the uniformity in thickness, and widespread distribution of SQ2. The results of our stratigraphic analysis indicate that the architecture of the reservoir and seal significantly differs from previous thinking, such that: (1) the Precipice Sandstone, the main reservoir unit, has a distribution that is limited to the basin centre; (2) the transitional interval above the main Precipice Sandstone reservoir forms poorly-connected, localized reservoirs that onlap and backstep towards the basin margins; and, (3) SQ2 and SQ3 are sealing units because they are very widespread and mud-dominated. Viewing the Precipice Sandstone and Evergreen Formation from a sequence stratigraphic standpoint provides important information about how to subdivide them and helps clarify the stratal relationships that were previously unclear from a mixture of terminology. Furthermore, this study gives insights into the spatial-temporal evolution of this reservoir and sealing interval that is an important CCS target in Australia.