Muddy Limestone Research Articles

Mechanisms by Which an Evaporated Lagoon Sedimentation System Controls Source–Reservoir Preservation in Lei32 Sub-Member Unconventional Oil and Gas

The muddy limestone in the Lei32 sub-member of the Middle Triassic Leikoupo Formation in the well Chongtan 1 (CT1) of the Sichuan Basin has yielded promising industrial oil and gas production. This discovery has the potential to become a significant strategic reservoir in the future for oil and gas exploration in the Sichuan Basin. However, the understanding of hydrocarbon accumulation in the muddy limestone of the Lei32 sub-member remains insufficient, which poses limitations on further exploration selection and deployment strategies. This study focuses on the analysis of core samples and laboratory data in the wells CT1 and Jianyang 1 (JY1), aiming to investigate the source rock and reservoir characteristics of the muddy limestone in the Lei32 sub-member, as well as the primary controlling factors influencing the development of the source rock and reservoir. The results show that the Lei32 sub-member in the Central Sichuan Basin is an evaporated lagoon deposition; the tight argillaceous limestone and limy mudstone exhibit the characteristic of source–reservoir integration, belonging to a new type of unconventional oil and gas reservoir. The reservoir space of the argillaceous limestone and limy mudstone in the Lei32 sub-member primarily consists of inorganic and organic micro–nanopores and microfractures. The average porosity and permeability are 2.7% and 0.19 mD, indicating a low-porosity and low-permeability unconventional reservoir. The clay minerals and gypsum content are the important factors influencing reservoir porosity, and the fractures are key factors influencing permeability. This study will elucidate the specific features of hydrocarbon accumulation in the muddy limestone reservoirs of the Lei32 sub-member and provide insights into its exploration potential.

Factors controlling tight oil and gas reservoir development in the Jurassic siliciclastic-carbonate rocks in Sichuan Basin, China

The prediction of “sweet spot” of multi-lithology composite tight oil and gas reservoirs within mixed siliciclastic-carbonate sequences is a hot topic in oil and gas exploration. There are mixed lacustrine carbonate-siliciclastic rocks in the Da'anzhai Member of Ziliujing Formation in the Eastern Slope of western Sichuan Depression. The reservoir lithology is complex, and the main factors controlling the development of high-quality reservoirs are yet to be known. Based on a large number of drilling, core and thin section observation, well log and seismic data, this paper systematically studies the characteristics and factors controlling the development of the mixed siliciclastic-carbonate reservoir. The results show that the reservoir is composed primarily of coquina, sandstone, breccia and shale in lithology, and its development is mainly controlled by sedimentary microfacies, diagenesis, tectonism (fracturing) and paleogeomorphology during the sedimentary period. The effective fractures in reservoirs of the low-energy shell shoal facies are well developed with relatively good physical properties. With the change of sedimentary microfacies from low-energy shoal to high-energy shoal to arenaceous shoal, the hydrocarbon production capacity worsens step by step. Diagenesis, such as dissolution and fracturing, has a certain effect on reservoir physical properties. The palaeogeomorphic highs and slopes had well-developed fractures during the sedimentary period which are conducive to constructive dissolution, and thus they are favorable zones for reservoir development. The development of structural fractures further amplifies the influence of dissolution; thus the fracture zones are also favorable for the development of high-quality reservoirs. The favorable sedimentary facies zone and fracture development degree serve as the core factors for the formation of high-quality mixed reservoirs. The muddy limestone with certain dissolved pores and structural fractures deposited under the low-energy shell shoal setting is of relatively high-quality reservoir in the Da'anzhai Member.

Objective-Driven Solid-Surface-Roughness Characterization for Enhanced Nuclear-Magnetic-Resonance Petrophysics

Summary Typically, smooth solid surfaces of reservoir rocks are assumed in formation evaluation, such as nuclear-magnetic-resonance (NMR) petrophysics and reservoir-wettability characterization through contact-angle measurements. Measuring the degree of surface roughness (R), or smoothness, and evaluating its effects on formation evaluation are topics of much research. In this paper, we primarily focus on details in characterizing solid-surface roughness and its applications in NMR pore-size analysis. R can be measured by contact techniques and noncontact techniques, such as stylus profilometer, atomic-force microscopy, and different kinds of optical measurements. Each technique has different sensitivities, measurement artifacts, resolutions, and field of view (FOV). Intuitively, although a finer resolution measurement provides the closest account of all surface details, the correspondingly small FOV might compromise the representativeness of the measurement, which is particularly challenging for charactering heterogeneous samples such as carbonates. To balance the FOV and measurement representativeness, and to minimize artifacts, laser scanner confocal microscopy (LSCM) is selected in this study. Results for the more than 27 rock samples tested indicate that rocks of similar rock types have similar R-values. Grainy limestones have relatively higher R-values compared with dolostones, consistent with the dolostone’s crystallization surface features. Muddy limestones have smoother surfaces, resulting in the lowest R-values among the rocks studied. For sandstones, R varies with clay types and content. For rocks containing two distinct minerals, two R-values are observed from the R profiles, which for these rock types justifies the use of two NMR surface relaxivity (ρ2) parameters for determining the pore-size distribution (PSD) from the NMR T2 distribution. The novelty here is the integration of LSCM and NMR to obtain an NMR PSD relevant for permeability, capillary pressure, and other petrophysical parameters. Typically, ρ2 is calibrated using the total surface area from Brunauer-Emmett-Teller (BET; Brunauer et al. 1938) gas adsorption, but this underestimates the NMR pore size because of surface-roughness effects. In our novel approach, we use R measured from LSCM to correct ρ2 for surface-roughness effects, and thereby obtain the NMR pore size more relevant for permeability and other petrophysical parameters. We then compare the roughness-corrected NMR PSD against pore size from microcomputed tomography (micro-CT) scanning (which is roughness independent). The good agreement between roughness-corrected NMR and micro-CT pore sizes in the micropore region validates our new technique, and highlights the importance of surface-roughness characterization in NMR petrophysics.

The depositional mechanism of organic-rich siliceous shales in Upper Yangtze area: Response to the Kwangsian Orogeny in South China

The organic-rich siliceous shale in Upper Ordovician is the major source rock for shale gas in the South China, while the deposition mechanism of these rocks is still under debated. Here we present systematic geochemical and petrographic characteristics of the Upper Ordovician sediments to evaluate the redox conditions, paleo-productivity, terrestrial input, and to investigate the depositional mechanisms of these organic rich siliceous shales in the Wufeng Formation. In shallow water areas, the Linxiang Formation is dominantly characterized by grey thick Limestone. The Upper Daduhe Formation is composed of calcareous mudstone intercalated with argillaceous limestone, while the Lower Daduhe Formation consists of limestone interbedded with mudstone. At deep water area, the Linxiang Formation is dominated by limestone and muddy limestone. The lower and upper Wufeng Formation are mainly characterized by grey argillaceous shale and organic-rich siliceous shale respectively. It is suggested that the lithological and environmental changes in the Yangtze area were the response to the development of the Kwangsian Orogeny. During the late Ordovician, the Yangtze basement constantly descended and the sea level rose relatively due to the collision between the Yangtze and Cathaysia blocks. Thus, the oxygen concentration decreased with the rising sea level according to the UEF values and Ce/Ce* ratios, which resulted in anoxic water condition and promoted the preservation and enrichment of organic matter in marine sediments. Meanwhile, uplifts (e.g., Jiangnan - Xuefeng, Qianzhong and Chuanzhong) constantly expanded around the Sichuan Basin, leading to high terrigenous input that is reflected in geochemical and Nd isotopic compositions. Additionally, primary productivity gradually increased, because the Si/Al ratios exhibit an increasing trend in both shallow and deep water areas, and it may be related to the high clastic influx which provided adequate nutrients for siliceous organisms. Overall, anoxic water condition and high biogenic Si influx caused by Kwangsian Orogeny are considered as the most important factors controlling the organic-rich siliceous shale in the Wufeng Formation.

Depositional model for mixed carbonate-clastic sediments in the Middle Cambrian Lower Zhangxia Formation, Xiaweidian, North China

In order to make accurate decisions in interpreting depositional environments of sedimentary rocks, a multi-proxy approach is best employed. In the Middle Cambrian Lower Zhangxia Formation exposed at Xiaweidian in the Northern China, lack of this form of approach puts doubts on the various models (e.g. carbonate ramp and isolated platform) proposed by previous workers. In this study, we integrated field outcrop investigation with laboratory examinations of thin sections with light and electron microscopies to further understand sedimentary environments of the Lower Zhangxia Formation. Dominant rock types of this formation include oolitic limestone, muddy limestone, flat-pebble limestone and calcareous mudstone. Evidence from ooid features and lime-mud content of oolitic limestones suggests their deposition in an environment with intermittent high energy level. The muddy limestones were formed on a restricted platform with lower wave energy, which is supported by the existence of pyrites in a reducing environment and the input of terrestrial clays from neighboring clastic environments. The flat-pebble limestones were formed by storm reworking of early deposits on restricted platform below a fair-weather wave base, due to their composition and clast features. Mudstones with occurrences of terrestrial silts could be associated with clastic shallow marine adjacent to the restricted platform. A mixed carbonate-clastic depositional model is suggested for this formation and can be used as model for other researchers working in the North China. Cited as : Zhang, X., Pang, X., Jin, Z., Hu, T., Toyin, A., Wang, K. Depositional model for mixed carbonate-clastic sediments in the Middle Cambrian Lower Zhangxia Formation, Xiaweidian, North China. Advances in Geo-Energy Research, 2020, 4(1): 29-42, doi: 10.26804/ager.2020.01.04

A novel multi-technique approach used in the petrophysical characterization of the Maquoketa Group (Ordovician) in the southeastern portion of the Illinois Basin: Implications for seal efficiency for the geologic sequestration of CO2

Quantifying the petrophysical properties of low-permeability sedimentary units helps to determine the possibility of upward flow of supercritical carbon dioxide when evaluating a site for safe confinement of geologic carbon storage. This research examines fine-scale pore characteristics that affect the sealing capacity of the Upper Ordovician Maquoketa interval, a thick and heterogeneous sequence of carbonates, siltstones, and clay-rich rock units in the Illinois Basin. This unit has been previously identified as a regional caprock that would likely isolate and effectively store any CO2 injected into underlying reservoirs. We applied a multi-technique approach to quantify pore-size distribution, pore surface area, porosity, permeability, and capillary entry pressure. These laboratory-based techniques include mercury porosimetry, gas adsorption, portable X-ray fluorescence, X-ray diffraction, total organic carbon analyses, and, to a lesser extent, scanning electron microscopy and petrography. In addition, we developed a lithofacies model that interpreted the combined wireline responses from multiple well locations. This model confirms that the Maquoketa Group is dominated by muddy limestone, dolomitic/calcitic shale, and silty shale. The results of these evaluations indicate that these sequences have low porosity (0.4–3.1 %) and low permeability (0.04–7.1 mD) values and capillary entry pressures adequate to inhibit invasion of supercritical CO2 driven by buoyancy forces. Laboratory results also indicate that portions of the Maquoketa Group may also function as a low-volume reservoir for CO2. That is, should supercritical CO2 migrate upward and percolate into this unit, most of the CO2 will likely be securely trapped by means of capillary mechanisms.

Paleogeographic and Age Findings on Planktonic Foraminiferal Assemblages of Yuksekova Complex in the Northeast of Elazig (Eastern Turkey)

The Yüksekova Complex is one of the parts of oceanic crust representing the Bitlis-Zagros suture zone in the eastern Turkey. The volcanosedimentary rocks of the unit, which is widely exposed around the Elaziğ (E-NE Turkey) province are located in the Seki-Hacihasan district (E Elazığ) and Asker Mountains (NE of Elazığ). In this area, the planktonic foraminifera bearing pink-red micritic limestones and related volcanogenic rocks were examined and aged. The volcanogenic rocks related to the micritic limestones are mainly spilitic and pillow shaped basalts. The volcanogenic rocks contain mainly of plagioclase ± pyroxene ± partly olivine set in amygdaloidal, intersertal, hyalo-microlitic porphyric textures. The sediments consist mainly of thin- to- medium-bedded muddy limestones. The pelagic red limestones are represented mostly by biomicrite and contain a large quantity of planktonic foraminifera and slight amount of thin shelled bivalve fragments, radiolaria, and echinoderm fragments. The planktonic foraminifera are composed mainly of Globotruncana arca, G. gagnebini, G. lapparenti, G. cf. falsostuarti, Globotruncanita stuarti, Gt. stuartiformis, Gt. conica, Globotruncanella citae and Rosita fornicata scattered into a micritic matrix. Small amounts of silt and sandsized quartz grains are also observed in some thin sections. Accordingly, based on planktonic foraminifera bearing micritic limestones, the age of the Yüksekova Complex is proposed Late Campanian-Late Maastrichtian. Thus, the Late Campanian-Late Maastrichtian aged red pelagic limestones of the Yüksekova Complex were deposited in the deeper parts of the Late Cretaceous Ocean, probably a deep shelf edge.

Depositional process and astronomical forcing model of lacustrine fine-grained sedimentary rocks: A case study of the early Paleogene in the Dongying Sag, Bohai Bay Basin

Abstract The Paleogene strata of the Bohai Bay Basin include multiple sets of lacustrine fine-grained sediments, which provide a detailed record of the climate changes that occurred in the lake basin. Early Paleogene fine-grained sedimentary rocks are the main target layers of continental shale oil exploration in eastern China. Therefore, studies of the geological responses to astronomical factors have important theoretical and practical significance for understanding the depositional process of lacustrine fine-grained sedimentary rocks. In this paper, we focus on the lower Es3 (Es3l)-upper Es4 (Es4u) units in four shale oil wells in the Dongying Sag of the Bohai Bay Basin. Based on the previous establishment of a high-resolution astronomical time scale in this area, we have further studied the petrological characteristics and lithofacies of the fine-grained sedimentary rocks. Then the geological responses to astronomical cycles on different scales are investigated, and the models for the depositional process of fine-grained sedimentary rocks under the control of astronomical factors are revealed. The results indicate that the cyclic changes in mineral content and lithofacies in fine-grained sedimentary rocks in the Dongying Sag are controlled by short eccentricity and precession cycles. Periods near the maximum short eccentricity range had large precession amplitudes, humid climates, and the development of organic-rich laminated calcareous mudstone lithofacies. In contrast, periods near the minimum short eccentricity range had small precession amplitudes, a dry climates, and the development of organic-containing layered muddy limestone lithofacies. This study explores the depositional process of fine-grained sedimentary rocks from the perspective of astronomical cycles, which is conducive to the comprehensive prediction of favorable shale oil layers.

Application of Strong Sealing High-performance Water-based Drilling Fluid in the Horizontal Well of the Missan Oilfield

In the Missan oilfield, because of its reservoir being easy leakage in the up and low zones, and wellbore being collapse in the middle zone with narrow density window, borehole instability, lost circulation, borehole collapse, sticking and other downhole complicated conditions were easily induced. A strong sealing and high performance water-based drilling fluid was mainly used to solve the problems of borehole instabilities in mud shale and muddy limestone in the horizontal wells of Missan Oilfield, for solving the problem of sidewall instability, aphysical/chemical plugging was used for strengthening the plugging of drilling fluid, improving the loading ability in formation, reducing the risk of lost circulation. The principle of regulation, permeable pressure were used to improve the semi-permeable membrane characteristics of mudstone and shale, reduce the activity of drilling fluid, inhibit the expansion ability of mudstone hydration and expansion in formation, on the basis of the static drilling fluid, permeable pressure was added to the wellbore for further improvement of borehole stability. The system in use reflects the stable rheological properties, it can be very good for plugging micro fractures, greatly improving the load ability in formation, the delaying time of borehole stability, guaranteeing the borehole stability in drilling, it provides support for the smooth drilling of the first horizontal well and consequent horizontal well drilling in Missan Oilfield.

Characteristics of Changchengian rifts in southern margin of North China Craton and its hydrocarbon geological conditions

The Meso-Neoproterozoic strata are wildly distributed in North China Craton (NCC), of which Changchengian strata are most widely developed. Taking Changchengian strata in south margin of NCC as the study object, and combined with comprehensive analysis of isotopic chronology and petrology, it can be concluded that the Xiong'er rift is a plume rift which responds to breakup of Columbia supercontinent. Seismic data shows that Changchengian rifts are developed in the Qinshui Basin and the southern part of Ordos Basin covered by Phanerozoic strata, respectively are large-scale graben rifts and half-graben rifts. Aero magnetic data indicates that a NE-trending rift is developed in the west of the Xiong'er rift, and the Qinshui Basin rift is the extension of the north branch of the Xiong'er rift. The filling process of Changchengian rifts can be divided into four stages: the early rifting stage developing thick andesitic volcanic rocks, the late rifting stage developing large suite of coarse clastic sedimentary rocks, the depression stage developing fine clastic rocks, and the epeiric sea stage developing carbonate rocks. The dark argillaceous rocks are developed in Cuizhuang Formation and Chenjiajian Formation during the depression stage, and the black shale in Cuizhuang Formation is the effective source rocks. The bitume is filled in fractures of dolomite in Luoyukou Formation, as well as dissolution pores and large caves in Longjiayuan Formation. The argillaceous sandstone and muddy limestone of Lower Cambrian is the effective cap rocks, which can form an potential accumulation assemblage of Changchengian strata with underlying source rocks of Cuizhuang Formation and reservoirs of Luoyu Group, and this assemblage may be still effective at present.

Small microbialites from the basal Triassic mudstone (Tieshikou, Jiangxi, South China): Geobiologic features, biogenicity, and paleoenvironmental implications

We report small microbialites from the calcareous mudstone slightly above the Permian–Triassic boundary (PTB) in the Tieshikou section, southern Jiangxi Province, South China. The newly found microbialite is a bowl-like structure, which contrasts with the surrounding calcareous mudstone. The small microbialite is composed of columnar forms and fan-shaped structures. Mini-columnar structures resemble mini stromatolites. In plane view, most branches are patchy or strip-shaped, with clotted structures, resembling that of a thrombolite. Fan-shaped cement precipitates are comprised of multiple crystal fans that have a radiating texture and that show distinct growth laminae. The radiating fabrics are represented by rod-shaped filaments under SEM, which are interpreted as calcified trichomes/filaments. Prominent micropores occur within the rods, and they may be the plane views of multiple sheaths of the false branching zone of trichomes. Thus, both the columnar forms and radiating fans are probably microbial in origin. Moreover, three types of nano-sized structures: intraparticle micropores, fibrous biofilms, and filamentous sheaths are also pronounced in both the columnar structures and cement fans of the Tieshikou microbialite, all suggestive of biogenicity. These well-preserved fibrous biofilms probably represent calcified extracellular polymeric substances (EPS), and linear sheaths are interpreted as the traces of bacterial activities during the precipitation of dolomite crystals. The Tieshikou microbialite therefore shares similar biogenetic mechanisms with other PTB microbialites. The combination of the absence of pyrite framboids, slightly negative carbon isotope values, and an association with abundant ammonoids, bivalves, and gastropods indicates oxic conditions during the growth of the Tieshikou microbialite. Moreover, the nodular preservational state resembles widely distributed nodular mudstones or muddy limestones of the Lower Triassic successions worldwide. This implies that nodular mudstone or argillaceous limestones may also have been deposited in microbe-rich environments, in which terrigenous supply was abundant.

Permian–Triassic boundary microbialites at Zuodeng Section, Guangxi Province, South China: Geobiology and palaeoceanographic implications

A previously unknown microbialite bed in the Permian–Triassic (P–Tr) boundary beds of Zuodeng section, Tiandong County, Guangxi, South China comprises a thin (5cm maximum thickness) stromatolite in the lower part and the remaining 6m is thrombolite. The Zuodeng microbialite has a pronounced irregular contact between the latest Permian bioclastic limestone and microbialite, as in other sites in the region. The stromatolite comprises low-relief columnar and broad domal geometries, containing faint laminations. The thrombolite displays an irregular mixture of sparitic dark coloured altered microbial fabric and light coloured interstitial sediment in polished blocks. Abundant microproblematic calcimicrobe structures identified here as Gakhumella are preserved in dark coloured laminated areas of the stromatolite and sparitic areas in thrombolites (i.e. the calcimicrobial part, not the interstitial sediment) and are orientated perpendicular to stromatolitic laminae. Each Gakhumella individual has densely arranged segments, which form a column- to fan-shaped structure. Single segments are arch-shaped and form a thin chamber between segments. Gakhumella individuals in the stromatolite and thrombolite are slightly different from each other, but are readily distinguished from the Gakhumella- and Renalcis-like fossils reported from other P–Tr boundary microbialites in having a smaller size, unbranching columns and densely arranged, arch-shaped segments. Renalcids usually possess a larger body size and branching, lobate outlines. Filament sheath aggregates are also observed in the stromatolite and they are all orientated in one direction. Both Gakhumella and filament sheath aggregates may be photosynthetic algae, which may have played an important role in constructing the Zuodeng microbialites. Other calcimicrobes in the Zuodeng microbialite are spheroids, of which a total of five morphological types are recognized from both stromatolite and thrombolite: (1) sparry calcite spheroid without outer sheaths, (2) a large sparry calcite nucleus coated with a thin sparry calcite sheath, (3) a large nucleus of micrite framboid aggregates rimmed by a thin sparry calcite sheath (bacterial clump-like spheroids), (4) a large nucleus of micrite framboid aggregates coated with a thin micritic sheath, and (5) a small sparry nuclei rimmed by coarse-grained, radiated euhedral rays. The irregular contact beneath the Zuodeng microbialites is interpreted as a subaerial exposure surface due to regional regression in South China. The demise of the Zuodeng microbialites may have been due to rapid rise in sea-level because they grew in relatively shallow marine conditions and are overlain by muddy limestones containing pelagic conodonts. Also siliciclastic content increases above the microbialite, suggesting a possible climate-related increase in weathering as the transgression progressed.

New data on pre-Eocene karst in the Tatra Mountains, Central Carpathians, Poland

Sparry limestone, ferruginous muddy limestone and limestone breccia have been found in the Western Tatra Mts. They occur within Jurassic rocks of the Choc Nappe just below red conglomerates of probable Eocene age. The deposits found bear strong resemblance to the infill of subterranean karst forms. They differ significantly from Quaternary karst deposits of the Tatra Mts. The δ 18 O values of spelean carbonates suggest crystallisation at relatively high temperatures (over 20°C) whereas their relatively negative δ 13 C values imply the presence of soil-derived CO 2 linked with vegetation dominated by C3 pathway plants. The karst forms and their infill were formed before the Eocene transgression, which shows unequivocally that the Tatra Mts. were subjected to karstification at that time.

Sequence stratigraphy of the late middle Jurassic open shelf platform of the Tuwaiq Mountain Limestone Formation, central Saudi Arabia

Middle Jurassic (Callovian) strata of Tuwaiq Mountain Limestone Formation, central Saudi Arabia are composed of thin- to thick-bedded, grainy and muddy limestones and dolostones with various skeletal and nonskeletal components. Facies analysis documents low- to high-energy environments, including lagoon, back reef, reef framework, fore-reef and intra-shelf basin lithofacies associations. Because of the narrow lateral distribution of facies and the presence of distinctive fossils, the depositional system likely represents an open marine shelf. Two third-order depositional sequences can be distinguished in the stratigraphic measured section. Transgressive sequences show deepening-upward trends, in which shallow water (lagoonal and back reef) facies are overlain by deeper-water (fore-reef and intra-shelf basin) facies. Regressive sequences show shallowing-upward trends in which deep-water facies are overlain by shallow-water facies. Most sequence boundaries in the study area are of the nonerosional (SB2) type. Correlation of depositional sequences in the studied section reveals that relatively shallow marine conditions predominated in the area.

Fluctuations of redox conditions across the Permian–Triassic boundary—New evidence from the GSSP section in Meishan of South China

Oceanic anoxia has long been considered as one of the main causes for the end-Permian mass extinction. However, the results obtained by different researchers are rather divergent from different sections, or even on the same section using the same redox proxy. This study aims to examine the causes for some of these divergent results using high-resolution pyrite framboid sampling at the Meishan GSSP section in South China. Detailed microfacies analysis shows that the uppermost Late Permian strata comprises two significantly different sedimentary facies: one characterized by silicious muddy limestone and recognized as representing autochthonous background sediments; the other distinguished by bioclastic grainstone, interpreted to be allochthonous in origin and have been transported from the nearby platform margin. These two different sedimentary facies represent two distinctly different redox conditions. Together with the facies analysis, a statistical analysis of pyrite framboids was carried out to evaluate the redox evolution across the Permian–Triassic boundary. Abundant framboids with average diameters of about 6μm are found in background sediments beneath the extinction boundary, indicating generally anoxic bottom water conditions. But this condition was punctuated by transient intervals of rapid oxygenation interpreted to have been caused by intrusion of intermittent turbidity flows. Our study also showed that anoxic conditions persisted into the immediate aftermath of the mass extinction, thereafter it was quickly followed by a relatively long period of oxic conditions (with rare framboids). However, the redox conditions returned to anoxia (with abundant pyrite framboids averaging about 5μm in diameter), accompanied by a rapid global transgression. The oxygenation manifested near the Permian–Triassic boundary coincides with the negative excursion of carbon isotope. This would imply that, contrary to previous interpretations, this great δ13C negative excursion was probably not caused by the upwelling of anoxic deep ocean waters.

Dynamic processes from plate subduction to intracontinental deformation: Insights from the tectono-sedimentary evolution of the Zhaosu–Tekesi Depression in the southwestern Chinese Tianshan

The Zhaosu–Tekesi Depression, located to the northern margin of the South Tianshan orogenic belt (STB), is an ideal region for investigating Paleozoic orogenic processes and later modifications of the southwestern Chinese Tianshan. In this study, we employ 2D seismic and borehole data revealing the geological structure and tectonic evolution of the Zhaosu–Tekesi basin, and we report new zircon U–Pb geochronological and Hf isotopic data and whole-rock elemental and Sr–Nd isotopic analytical results of the Late Silurian magmatic rocks, as well as the whole-rock trace element data of the Early Carboniferous carbonate rocks. Borehole data (wells ZC1 and ZC2) and seismic imaging suggest that the Zhaosu–Tekesi Depression is composed of Silurian, Lower Carboniferous, Permian, Jurassic, Upper Cretaceous and Paleogene strata, with six unconformity surfaces (C1a/S3, P1/C1a, P2t/P1, J/P2t, K2d/J and E/K2d). The Silurian strata exhibit southward thrust structures and consist of basalt, andesite, dacite, tuff, minor muddy limestone and diorite intrusions. Zircon SHRIMP U–Pb dating for the well ZC1 diorites yield a crystallization age of ∼422Ma, with positive εHf(t) values (3.3–10.6). The diorites and andesites are calc-alkaline and show enrichment in LILEs (such as Th and U) and depletion in Nb, Ta and Ti, relatively high initial 87Sr/86Sr ratios (0.703376–0.706737) and low εNd(t) values (−0.5 to 1.4), and high Th/Yb (0.29–0.35) and Ta/Yb (1.31–1.58) ratios, indicating their formation in an active continental margin setting. The Lower Carboniferous strata mainly comprise oolitic limestone, bioclastic micritic limestone and interbedded calcareous siltstone. High Sr/Ba (1.31–2.33), V/Sc (10.4–18.6) and low Th/U (0.01–0.23) ratios suggest that they were potentially deposited in the mixing near-shore marine environment. These results, in combination with previous data, indicate that the southwestern Chinese Tianshan is controlled by northward subduction of the South Tianshan Ocean during the Late Silurian to Early Carboniferous. The presence of the Lower Permian terrigenous strata and rift basin structure constrains the timing of collision between the CTB–KYB and the Tarim Craton as the late Carboniferous. Thrusting reactivation and syn-tectonic growth strata in the basin suggest that the southwestern Chinese Tianshan experienced Late Cretaceous compression and Neogene modifications after the Late Permian post-faulting thermal subsidence, Triassic peneplanation and Jurassic regional subsidence. Intracontinental subduction due to distant effect is the main tectonic control on the Miocene rapid uplift of the Tianshan. The Zhaosu–Tekesi basin evolution sheds light on dynamic processes of the southwestern Chinese Tianshan from plate subduction to intracontinental deformation.

Potential Cretaceous-Paleogene boundary tsunami deposit in the intra-Tethyan Adriatic carbonate platform section of Hvar (Croatia)

An exceptional 47-m-thick succession of Maastrichtian to Paleocene inner-platform carbonates is exposed on the Dalmatian island of Hvar (Adriatic Sea, Croatia) in a seaside locality called Majerovica. The middle part of this succession is an ~5-m-thick intraformational massive deposit, which is underlain by well-bedded peritidal inner-platform limestones containing latest Maastrichtian rudists and shallow-water benthic foraminifera. This deposit includes a polygenic, matrix-supported carbonate breccia characterized by ripped-up platform limestone lithoclasts, up to boulder sized, and polygenic microbreccia in a muddy matrix. The microbreccia contains rare small intraclasts of pelagic mudstone containing terminal Maastrichtian planktonic foraminifera. The deposit is overlain in turn by mudstone containing a planktonic foraminiferal association belonging to the P0 and Pα zones of the basal Paleogene, and by shallow-water muddy limestones containing planktonic foraminifera belonging to the P1 zone. While facies suggest that the deposit was emplaced over the inner platform by a single large tsunami, the biostratigraphic assessment of this section and the presence of enhanced concentrations of platinum group elements, such as iridium, in the topmost part of the massive deposit lend support to the hypothesis that this tsunamite is related to the Chicxulub impact in Yucatan. This is potentially the first case of a tropical carbonate platform sedimentary succession recording the Cretaceous-Paleogene boundary event, which provides a new constraint for modeling both the western Tethyan paleogeography and the catastrophic aftermaths of the Chicxulub impact at the end of the Mesozoic Era.

Lower Devonian trace fossils and their paleoenvironmental significance from the western Yangtze Plate, South China

Abundant and diverse trace fossils occur in the littoral-neritic sediments recording an Early Devonian transgression in South China. The well-exposed Ganxi section is located in the Longmen Mountain 100 km northwest of Chendu, Sichuan. The Lower Devonian strata are dominated by sandstones, siltstones, muddy shales, limestones, bioclastic limestones, and muddy limestones. About thirteen ichnogenera were systematically described, including Arenicolites, ?Balanoglossites, Chondrites, Cylindrichnus, Diplocraterion, Planolites, Palaeophycus, Phycodes, Rhizocorallium, Rusophycus, Skolithos, Thalassinoides, and Zoophycos. Six trace fossil associations under different sedimentary environments have been recognized as follows: the Skolithos and Diplocraterion-Skolithos associations mainly consist of domichnia derived from a high-energy zone of the foreshore to upper shoreface; the Rusophycus-Phycodes association is characterized by fodinichnia and cubichnia generated in a lower-energy zone of the upper shoreface; the Chondrites-Palaeophycus association mainly consists of domichnia, chemichnia, and fodinichnia as indicators of the lower-energy and suboxic conditions of a coastal lagoon; the Rhizocorallium association includes feeding and dwelling burrows formed in a lower-energy zone of the upper offshore enriched with food; and the Zoophycos association is characterized by fodinichnia and pascichnia as indicators of the lower offshore frequently influenced by storm waves. The entire sequence shows several transgressive-regressive cycles from the Lochkovian to Pragian and much storm influence in the Emsian.

A late Cretaceous elasmosaurid of the Tethys Sea margins (southern Negev, Israel), and its palaeogeographic reconstruction

AbstractRecent research on the late Cretaceous (Santonian), Menuha Formation of the southern Negev, Israel, has revealed several unconformities in its exposures, spatial changes in its lithofacies, agglomerations of its carbonate concretions and nodules at a variety of localities. At Menuha Ridge Site 20, portions of a new elasmosaurid skeleton were found within deposits of laminated bio-micritic muddy limestone with thin phosphatic layers. The sediments are rich in microfossils – foraminifera and ostracods preserved in the carbonate mud. Planktic foraminifera species (e.g.Dicarinella asymetrica,D.concavata,Sigalia decoratissima carpatica) appear as well as species indicative of opportunistic life strategies typical of a forming upwelling system in the region. Marine ostracods (e.g.Brachycythere angulata,Cythereis rosenfeldi evoluta) and many echinoid spines suggest an open marine environment. Using a multidisciplinary approach, we offer here a reconstruction of the micro-regional palaeogeography along a segment of the ancient shoreline of the Tethys Sea during the Santonian, and explain the environmental conditions under which the various fauna lived. This new elasmosaurid is examined in light of the above and compared with evidence from the adjacent areas along the margins of the southern Tethys Sea.

Análise estratigráfica da Formação Brejo Santo, Bacia do Araripe, Nordeste do Brasil: implicações paleogeográficas

The Araripe Basin has been the subject of several publications in the recent years, notably with respect to the rich paleontological collection of Brejo Santo, Crato and Romualdo Formations. However, papers detailing stratigraphic aspects of Brejo Santo Formation are scarce. The formation (Mesozoic era - Dom Joao Stage) is represented by a predominantly pelithic succession (up to 450 m thick) outcropping in eastern portion of the basin. The Brejo Santo Formation unconformably overlies the Cariri Formation, presumably of Paleozoic age, and makes gradational contact with the overlying Missao Velha Formation. It consists of pelithes, such as massive to laminated reddish-brown chalks and calciferous shales with decimetric intercalations of white to greenish-grey siltstones and centimetric layers of reddish mottled, speckled and striped shales, with calcareous nodules, and subordinately, muddy limestones and highly fossiliferous calciferous sandstones and fine sandstones with planar cross-stratifications. The measured paleocurrents have a consistent dispersion pattern for SE, SW and S, suggesting the installation of an open wide basin toward the south. The facies and facies associations described were interpreted as generated by (i) lake systems in which periodically the level of the lakes and/or ponds suffered sudden variations, causing changes in coloration related to seasonal fluctuations in the level of the lake (periods of subaerial exposure) and (ii) ephemeral river systems which fed these lakes. The widespread occurrence of fossil organisms, such as non-marine ostracods and conchostracheans suggest that the sediments of Brejo Santo Formation were possibly deposited in lacustrine-fluviatile systems favorable to the formation of red beds layers, under hot climatic conditions with well-defined dry seasons, corroborating the depositional system interpretation.