Cratonic Basins Research Articles

Molecular markers of Neoproterozoic-Lower Paleozoic petroleum systems and their geological significance: A case study of the cratonic basins in western China

In this article, we report the investigation of the Ediacaran and Lower Paleozoic petroleum systems using their molecular markers. The cratonic basins located in western China were selected as a case study, whereas worldwide published samples where compiled for comparison. The study aimed in characterizing specifically and generally the molecular markers, and providing their geological significance as well. Gas chromatography-mass spectrometry experiments (GC-MS) were applied on Ediacaran reservoir solid bitumen extracts (Dengying Formation, member-Z2dn2) and Cambrian oil/rock extracts samples respectively from Sichuan Basin and Tarim Basin. The results show a predominance of n-alkanes over isoprenoids (e.g. Pr/nC17, Ph/nC18), a noticeable absence of 25-norhopane and monoaromatic steroids among all the samples. The values of pristane to phytane ratio (Pr/Ph) fall below 0.8 for all the solid bitumen samples, whereas they are equal to 0.9, on average, for the Lower-Middle Cambrian oils and rock extracts. Relatively high values of gammacerane to C30 17α (H)-hopane ratio (G/C30H), low values of C30 17α (H)-diahopane/C30H (average = 0.09), and a V-shaped distribution pattern for the following C29–C28–C27 regular steranes with C27 > C29 > C28 and C28 < C27 < C29 respectively for Ediacaran and Cambrian samples, have been noticed as well. Moreover, the values of the following ratios: ββ/(ββ+αα) and 20S/(20S + 20R) C29 steranes, tricyclic terpanes/(tricyclic terpanes +17α-hopanes), and Ts/(Ts + Tm) fall below their respective equilibrium endpoints, notwithstanding the calculated equivalent vitrinite reflectance values range within 1.69%–2.20%. The data shows that the source rocks of all the Ediacaran and Lower-Middle Cambrian samples of the western China were deposited in an anoxic and stratified environment, with the initial organic matter showing a prevalence of type II kerogen (marine algal). The organic matter have experienced none or at least a very slight biodegradation rate, in spite that the maturity rate corresponds to post-maturity level.

Differences in east-west thermo-rheological structure and lithospheric deformation in the Qaidam Basin, Northern Tibetan Plateau, China

ABSTRACT To investigate the differences in east-west lithospheric deformation in the Qaidam Basin, we present thermo-rheological models of two profiles across the western Qaidam Basin (WQB) and eastern Qaidam Basin (EQB). The differences in east-west geodynamic deformation styles are also described, involving GPS motions, focal mechanisms (P axes), seismic anisotropy (SKS-wave splitting), and low-velocity zones (LVZs). The WQB is characterized by a warm destabilized cratonic basin with a weak lower crust, and the rheological structure changes from a rather weak crème brûlée-1 regime into a strong jelly sandwich-1 regime, from the Altyn-Qaidam boundary to the northeastern corner of the WQB. Combined with the limited distribution of the LVZs and the strong crust-mantle decoupling revealed by the unmatched pattern between P axes (N20ºE) and SKS-wave splitting (N110°E), the crust-mantle mixing related to the under-thrusting of the Tarim Basin along the Altyn Tagh Fault is suggested as the primary tectonic dynamic inducement of the destabilized WQB craton, which weakens the lithospheric strength greatly and contributes to the shallow brittle deformation. However, the EQB performs as a typical cold and rigid cratonic basin characterized by a jelly sandwich-2 rheological regime, which is sufficiently strong to maintain crust-mantle coupling. The LVZs beneath the EQB revealed by recent wide-angle seismic profiles have only a small effect on the lithospheric strength drop. The EQB could be regarded as a solid basin, anchored in the NE Tibetan Plateau, which shows a strong resistance to NE extrusion of the weak plateau material, leading to clockwise rotation.

Contrasted Chemical Weathering Rates in Cratonic Basins: The Ogooué and Mbei Rivers, Western Central Africa

Despite the absence of tectonic activity, cratonic environments are characterized by strongly variable, and in places significant, rock weathering rates. This is shown here through an exploration of the weathering rates in two inter-tropical river basins from the Atlantic Central Africa: the Ogooué and Mbei River basins, Gabon. We analyzed the elemental and strontium isotope composition of 24 water samples collected throughout these basins. Based on the determination of the major element sources we estimate that the Ogooué and Mbei rivers total dissolved solids (TDS) mainly derive from silicate chemical weathering. The chemical composition of the dissolved load and the area-normalized solute fluxes at the outlet of the Ogooué are similar to those of other West African rivers (e.g., Niger, Nyong, or Congo). However, chemical weathering rates (TZsil+rate expressed as the release rate of the sum of cations by silicate chemical weathering) span the entire range of chemical weathering intensities hitherto recorded in worldwide cratonic environments. In the Ogooué-Mbei systems, three regions can be distinguished: (i) the Eastern sub-basins draining the Plateaux Batéké underlain by quartz-rich sandstones exhibit the lowestTZsil+rates, (ii) the Northern sub-basins and the Mbei sub-basins, which drain the southern edge of the tectonically quiescent South Cameroon Plateau, show intermediateTZsil+rates and (iii) the Southern sub-basins characterized by steeper slopes record the highestTZsil+rates. In region (ii), higher DOC concentrations are associated with enrichment of elements expected to form insoluble hydrolysates in natural waters (e.g., Fe, Al, Th, REEs) suggesting enhanced transport of these elements in the colloidal phase. In region (iii), we suggest that a combination of mantle-induced dynamic uplift and lithospheric destabilization affecting the rim of the Congo Cuvette induces slow base level lowering thereby enhancing soil erosion, exhumation of fresh primary minerals, and thus weathering rates. The study points out that erosion of lateritic covers in cratonic areas can significantly enhance chemical weathering rates by bringing fresh minerals in contact with meteoric water. The heterogeneity of weathering rates amongst cratonic regions thus need to be considered for reconstructing the global, long-term carbon cycle and its control on Earth climate.

Relations between earthquake distributions, geological history, tectonics and rheology on the continents.

This paper is concerned with the distribution of earthquakes, particularly their depths, with the temperature of the material in which they occur, and with the significance of both for the rheology and deformation of the continental lithosphere. Earthquakes on faults are generated by the sudden release of elastic energy that accumulates during slow plate motions. The nonlinear high-temperature creep that localizes such energy accumulation is, in principle, well understood and can be described by rheological models. But the same is not true of seismogenic brittle failure, the main focus of this paper, and severely limits the insights that can be obtained by simulations derived from geodynamical modelling of lithosphere deformation. Through advances in seismic tomography, we can now make increasingly detailed maps of lithosphere thickness on the continents. The lateral variations are dramatic, with some places up to 300 km thick, and clearly relate to the geological history of the continents as well as their present-day deformation. Where the lithosphere thickness is about 120 km or less, continental earthquakes are generally confined to upper crustal material that is colder than about 350°C. Within thick lithosphere, and especially on its edges, the entire crust may be seismogenic, with earthquakes sometimes extending into the uppermost mantle if the Moho is colder than 600°C, but the continental mantle is generally aseismic. Earthquakes in the continental lower crust at 400-600°C require the crust to be anhydrous and so are a useful guide or proxy to both composition and strength. These patterns and correlations have important implications for the geological evolution of the continents. They can be seen to have influenced features as diverse as the location of post-collisional rifting; cratonic basin formation; the location, origin and timing of granulite-facies metamorphism; and the formation, longevity and strength of cratons. In addition, they have important consequences for earthquake hazard assessment in the slowly deforming edges and interiors of continental shields or platforms, where the large seismogenic thickness can host very large earthquakes. This article is part of a discussion meeting issue 'Understanding earthquakes using the geological record'.

Geological conditions, reservoir evolution and favorable exploration directions of marine ultra-deep oil and gas in China

By analyzing the structural background, petroleum geological conditions, and typical regional (paleo) oil and gas reservoirs in marine ultra-deep oil and gas regions in China, this paper reveals the evolution processes of the marine ultra-deep oil and gas reservoirs and the key controlling factors of accumulation. The marine ultra-deep oil and gas resources in China are buried at depth of greater than 6000 m, and are mainly distributed in the Precambrian and Lower Paleozoic strata in the Sichuan, Tarim and Ordos cratonic basins. The development of marine ultra-deep source rocks in China is controlled by cratonic rifts and cratonic depressions with the background of global supercontinent breakup-convergence cycles. The source rocks in Sichuan Basin have the most developed strata, followed by Tarim Basin, and the development strata and scale of Ordos Basin needs to be further confirmed. The marine ultra-deep reservoir in China is dominated by carbonate rocks, and the reservoir performance is controlled by high-energy sedimentary environment in the early stage, superimposed corrosion and fracture in the later stage. The regional caprocks are dominated by gypsum salt rocks, shale, and tight carbonate rock. The ultra-deep oil and gas fields in China have generally experienced two stages of oil-reservoir forming, cracking (or partial cracking) of paleo-oil reservoirs, and late finalization of cracked gas (or highly mature to over mature oil and gas). The oil and gas accumulation is controlled by static and dynamic geological elements jointly. Major hydrocarbon generation center, high quality and large-scale reservoir resulted from karstification of high energy facies belt, thick gypsum rock or shale caprock, and stable trapping and preservation conditions are the key factors for accumulation of ultra-deep oil and gas. We propose three favorable exploration directions, i.e. the areas around intracratonic rift and intracratonic depression, and craton margin.

Geological characteristics, ore-forming fluids, and genetic models of uranium mineralization in superimposed basin and craton basin: a study on uranium-bearing basins in Xingmeng area, North China

Geological characteristics, ore-forming fluids, and genetic models of uranium mineralization in superimposed basin and craton basin: a study on uranium-bearing basins in Xingmeng area, North China

The late Paleoproterozoic to Mesoproterozoic rift system in the Ordos Basin and its tectonic implications: Insight from analyses of Bouguer gravity anomalies

The Ordos Basin is the largest cratonic basin in the North China Craton (NCC) with minimal intra-basin deformation in the Phanerozoic, making itself a good proxy to study the Precambrian evolution of the NCC. The distribution of the Precambrian strata and associated structural deformation in the Ordos Basin, however, remain an enigma due to the thick covering of the Phanerozoic strata. Herein, we depicted the residual thicknesses of the oldest sedimentary strata above the basement in the Ordos Basin, the late Paleoproterozoic to Mesoproterozoic Changcheng and Jixian systems, by combined forward and inverse modeling of the Bouguer Gravity Anomaly (BGA) data. The distribution of associated basement-involved faults was extracted by applying the signal-processing methods (e.g., the small sub-domain filtering and the total horizontal derivative) to the BGA data. The results were further calibrated by available geological constraints from limited borehole loggings, outcrops and seismic reflection data. They reveal an E- to NE-striking rift system with the development of plenty of syn-depositional normal faults and several uplifts and sags in the late Paleoproterozoic to Mesoproterozoic period in the Ordos Basin. The rift deposits reach ~8000 m in thickness in the southwestern Ordos Basin but thin toward north-northeast. The preexisting basement tectonics played an important role in the development of this rift system. Combined with regional tectonics, our findings imply that the late Paleoproterozoic to Mesoproterozoic extension in the Ordos Basin may not be related to the mantle plume centered in the Xiong’er Rift to the southeast, but instead likely results from the oceanic subduction beneath the southern margin of the NCC. This supports that the southern margin of the NCC may not connect with any other continental blocks in the reconstruction of the Paleoproterozoic Columbia Supercontinent.

Exploration status of the deep Sinian strata in the Sichuan Basin: Formation conditions of old giant carbonate oil/gas fields

To construct a giant Sichuan gas province, it is in an urgent need of exploring new replacement areas with abundant natural gas resources and great exploration potential. In this paper, the formation conditions (e.g. source rock, reservoir and play) and the distribution laws of the deep-seated old giant marine carbonate oil/gas fields in China's Craton Basins were investigated systematically. Then, the accumulation and enrichment conditions of deep Sinian natural gas in the Sichuan Basin were analyzed. Finally, potential and favorable target zones of natural gas exploration in the Dengying Formation of Upper Sinian were evaluated. And the following research results were obtained. Firstly, the effectiveness and scale of source kitchens, reservoirs and reservoir–caprock assemblages and the proximity are the necessary conditions for the formation of deep-seated giant carbonate oil/gas fields, and paleo-uplifts, paleo-slopes and paleo-fault zones are the favorable areas of searching deep-seated giant carbonate oil/gas fields. Secondly, in the Sichuan Basin, three sets of high-quality source rocks are developed in the Neoproterozoic–Cambrian System, and their maturity of organic matter is still in the optimal window for cracking gas, so the gas generation scale is large. Thirdly, after the microbial carbonate rocks of Dengying Formation in the Sichuan Basin were reconstructed by constructive diagenesis, effective reservoirs are formed and distributed widely. Fourthly, owing to the integrated source rocks and cap rocks of Dengying Formation, the marginal platform and the intra platform have favorable conditions for proximal hydrocarbon accumulation. In conclusion, the deep Sinian in the Sichuan Basin has good conditions of hydrocarbon accumulation and it is an important replacement area of natural gas exploration. Paleo-uplifts and slopes in the central Sichuan Basin have been the favorable locations of natural gas accumulation for a long period. Large-scale exploration shall focus on four favorable areas, including the marginal platform zone in the fourth member of Dengying Formation, the marginal platform zone in the second member of Dengying Formation, the bioherm shoalbody of Dengying Formation intra platform in the paleo-uplifts and slopes of the central Sichuan Basin, and the bioherm shoalbody of Dengying Formation in the eastern Sichuan Basin.

Discovery of Sinian source rocks in proterozoic in the Tarim Basin, NW China

As one of three cratonic basins in China, the Tarim Basin has the most abundant oil and gas resource, and its Precambrian petroleum system is a new region for hydrocarbon exploration. After conducting numerous field investigations in the Tarim Basin, we discovered the Sinian source rocks with 150m of thickness in Kuruktag area. These source rocks have total organic carbon (TOC) contents between 0.6% and 1.6%, with equivalent vitrinite reflectance (%Ro) between 1.23% and 1.42%. In addition, dark mudstone and a large amount of reservoir bitumen is present in the Upper Sinian outcrops in Aksu region. Biomarkers analysis indicates that reservoir bitumens in the Sinian and Cambrian strata were derived from the Sinian dark mudstones. Meanwhile, biomarkers of the reservoir bitumens from well ST1 in the Lower Cambrian Xiaoerbulake Formation indicate that the bitumen was originated from the Sinian source rocks and subsequently experienced biodegradation influence. Furthermore, a special hydrocarbon charging event with homogenization temperature of 50–60 °C has been elucidated from coeval aqueous phase of oil inclusions from in well ZS5, indicating that the oil and gas charging event in the Sinian source rocks occurred throughout the basin. As a result, the Sinian petroleum system in the Tarim Basin may contain high-quality source rocks, which contribute to the massive amount of Lower Paleozoic oil and gas resources in the basin. This study has important practical significance for oil and gas exploration of the Precambrian petroleum system in the Tarim Basin.

Joint Inversion of High-Frequency Receiver Functions and Surface-Wave Dispersion: Case Study in the Parnaíba Basin of Northeast Brazil

ABSTRACTWe assess the performance of the joint inversion of receiver functions (RF) and surface-wave dispersion in the characterization of the sedimentary package comprising the Parnaíba basin. This procedure is routinely utilized in passive-source crustal studies to retrieve S-wave velocity variations with depth, and has seldom been used with higher-frequency datasets to investigate fine sedimentary structure. The Parnaíba basin is a Paleozoic cratonic basin composed of five supersequences, accumulating ∼3.5 km of sedimentary rocks interbedded by Late Cretaceous diabase sills. The dataset used for this research was acquired between 2015 and 2017 through deployment of 10 short-period and one broadband seismic stations distributed along an approximately 100-kilometer-long linear array in the center of the basin. The deployment was carried out under the Parnaíba Basin Analysis Project, a multi-institutional and multidisciplinary effort funded by BP Energy do Brasil. High-frequency RFs (f&amp;lt;4.8 Hz) were calculated from deconvolution of teleseismic P waveforms (30°&amp;lt;Δ&amp;lt;90°) after rotation into the great-circle path, whereas high-frequency dispersion curves (0.25–2 Hz) were obtained through multiple filter analysis of empirical Green’s functions developed from cross-correlation (ZZ component) and stacking (six months) of time–frequency-normalized ambient seismic noise recordings. S-wave velocity–depth profiles down to ∼5 km depth were developed through an iterative, linearized joint inversion approach. Comparison to independent active-source seismic profiles overlapping with our passive-source seismic line reveals the inverted velocity models successfully retrieve sedimentary thickness (top of the Cambrian), sedimentary velocity structure, and depth to the Cenozoic sedimentary sequence. In addition, high-velocity zones at depths ranging from 1.5 to 2.5 km are observed in the inverted velocity–depth profiles, which are interpreted as due to the Late Cretaceous sills interbedding the basin’s sedimentary rocks. The relative low cost of our approach makes it ideal for basic characterization of relatively unknown sedimentary basins.

Differential enrichment of shale gas in upper Ordovician and lower Silurian controlled by the plate tectonics of the Middle-Upper Yangtze, south China

Abstract The Upper Ordovician Wufeng Formation and Lower Silurian Longmaxi Formation marine shale are the major shale gas exploration and development target in the Middle-Upper Yangtze region. Although three national shale gas demonstration areas have been built and several high-yield shale gas wells have been discovered in the Wufeng Formation and Longmaxi Formation, shale gas in these areas has not been fully commercialized because the enrichment degree of shale gas varies greatly in different tectonic settings. By analyzing the formation processes and distribution characteristics of the organic-rich shale, reconstructing the burial history and hydrocarbon generation history, and comparing the shale gas enrichment and preservation conditions in different tectonic settings in South China, this paper points out that (1) The craton depression and foreland basin on the cratonic margin are favorable zones for the development of organic-rich shale. Because the Yangtze plate is small, the foreland basin is greatly affected by the input of terrigenous clastic and the thickness and area of organic-rich shale in the craton basin is larger than that in the foreland basin. The organic-rich shale is mainly distributed in south Sichuan deep-water shelf and west Hubei-east Chongqing deep-water shelf, and the enrichment interval is the Dillcellograptus complexus-Cystograptus Vesicolus graptolite zone from the Wufeng Formation to the bottom of Longmaxi Formation are favorable intervals for shale gas enrichment. (2) Since the Late Jurassic, significant differential intracontinental tectonic deformation, uplift and denudation occurred in the Middle-Upper Yangtze region. Moderate compression and uplift were beneficial to the development of micro-fractures and bedding fractures in shale, resulting in good preservation conditions of shale gas reservoirs with a high-pressure coefficient. In contrast, strongly deformed areas, where faults and high-angle large fractures were developed, are not conducive to the preservation of shale gas reservoirs. (3) The craton depression of the Middle-Upper Yangtze region, where organic-rich shale was deposited, is favorable to shale gas enrichment because it provides a good material basis, and also a relatively stable and compression-resistant tectonic environment for later tectonic deformation and uplift-derived destruction. Based on the analysis and evaluation of tectonic construction–transformation conditions under the macro-control of plate tectonics, it can well explain the existing commercial shale gas discoveries and failure cases and can guide the optimization of future exploration targets.

Source and distribution of mercury in natural gas of major petroliferous basins in China

Based on the analysis of the natural gas samples of 146 wells from four major petroliferous basins of China, the source and distribution of mercury in natural gas in major petroliferous basins of China were identified. Studies have shown that the mercury concentration of natural gas in petroliferous basins of China varies widely, ranging from 0.01 to 4050 μg/m3. The gas well with the highest mercury concentration is in the Xushen gas field in the Songliao Basin. The mercury concentration in the craton basin is relatively low, and the secondary gas reservoir in the Tarim Basin has the lowest mercury concentration. In the rift basin, due to the presence of deep faults which cut the basement and mixing effects of deep fluids, the mercury concentration in natural gas may be abnormally high, e.g. the Xushen gas field in the Songliao Basin. In relatively stable basins where deep and large faults do not develop, mercury is primarily of organic origin. In rift basins where deep fluids are mixed with gas reservoirs, mercury in natural gas may mainly come from the deep fluids and is characterized by abnormally high mercury concentration and R/Ra values.

Geochronology, paleomagnetic signature and tectonic models of cratonic basins of India in the backdrop of Supercontinent amalgamation and fragmentation

Geochronology, paleomagnetic signature and tectonic models of cratonic basins of India in the backdrop of Supercontinent amalgamation and fragmentation

Development of a Dense Cratonic Keel Prior to the Destruction of the North China Craton: Constraints From Sedimentary Records and Numerical Simulation

AbstractIt remains unclear why some cratonic lithospheres like the North China Craton (NCC) have undergone significant thinning and catastrophic destruction. Here we provide evidence for a dense Phanerozoic cratonic keel beneath the NCC that provides a reason for the craton destruction during the late Mesozoic. We observed two long‐lasting (&gt;100 Myr) episodes of surface subsidence that formed the saucer‐shaped basin topography on the NCC from the Paleozoic to early Mesozoic, which was not associated with lithosphere stretching or a high sea level. Based on a detailed geological and geochemical analysis and 2‐D numerical experiments, we show that this subsidence most likely reflects the secular cooling and eclogitization of iron‐rich materials infiltrated into the cratonic mantle since the early Phanerozoic, a process that is the most prominent over the eastern NCC. This result implies that the lithosphere beneath the eastern NCC became gravitationally unstable prior to its Mesozoic destruction as its keel became progressively denser during the Paleozoic. An early Mesozoic partial removal of this dense keel possibly led to widespread surface uplift of the NCC, as indicated by regional basin inversion. We conclude that the long‐term subsidence of a cratonic basin reflects a gradual increase of the lithospheric density and that this type of subsidence should not be considered a characteristic of stable cratons as previously assumed.

The electrical conductivity structure of the Tarim basin in NW China as revealed by three-dimensional magnetotelluric inversion

The Tarim basin, located in northwestern China, is the largest inland basin with oil, gas, and mineral resources in China. To study the deep structure of the Tarim basin, we conducted a magnetotelluric (MT) survey of the entire basin using 756 datasets to build a three-dimensional (3-D) electrical conductivity model. Based on the initial 3-D model, which was derived from a two-dimensional (2-D) minimum support gradient (MSG) inversion, the electrical conductivity structure of the Tarim basin was imaged by a 3-D MT regularized inversion using the contracting integral equation method (CIE) and quasi-Newton optimization. Based on simulation results, a thick resistive anomaly exists near the center of the Tarim basin, which suggests that the basin has the properties of a cratonic basin. The basin is also surrounded by resistive lithosphere faults, which indicates that it has the properties of a fault basin. The crystalline basement of the Tarim basin is divided into a resistive craton basement, high resistance basalt basement, and conductive crevice water metamorphic basement based on the shallow resistivity structure. In addition, the lithospheric thickness was estimated using the deeper resistive structure.

Stratigraphy, sedimentology and paleogeography of a Paleozoic succession, Ghadames and Jefarah basin, Libya and Tunisia

The sedimentology and tectonics and their relationship of the Paleozoic series that fill the Ghadames basin and its northern extension consisting of the Jefarah basin in Libya and Tunisia was studied. These formations belong to the Gondwana cycle and are typically interpreted as being deposited in a cratonic basin truncated by the Hercynian unconformity. This study is based on wells. The stratigraphic correlation between the wells is based on the definition of second order cycles, electrofacies and electrosequences notion. This allowed to produce isopach maps of facies distributions and to reconstruct the paleogeography of the different stratigraphic units. These data allow to address the nature of the deformation and also clarifies the behavior of active high regional areas during the Palaeozoic. The Paleozoic succession in the Ghadames and Jefarah basins can be divided into five first order sequences, bounded by major tectonic unconformities with sequence durations of 40–70 Ma. Within these five sequences 18 s order sequences (10–40 Ma) were differentiated, describing Sequence Boundaries (SB), Maximum Flooding Surfaces (MFS) and sedimentological characteristics.

Terrestrial heat flow and crustal thermal structure in the northern slope of Tazhong uplift in Tarim Basin

Considerable breakthroughs have been obtained recently in deep and ultra-deep carbonate strata of the northern slope of Tazhong uplift, which is characterized by a burial depth of more than 6300 m and is characterized by high burial temperature. However, the lack of systematic temperature research and effective temperature prediction for the ultra-deep strata in northern slope of the Tazhong has seriously restricted the petroleum exploration in the future. The continuous steady-state temperature logs and well test temperature were obtained, and the thermal conductivity were measured to analyze the current thermal state and deep thermal structure characteristics of the northern slope of Tazhong uplift. The geothermal gradient is generally between 19.4 and 28.5 ℃/km, and the geothermal heat flow is 34.3–59.2 mW/m2 with an average of 45.4 mW/m2, which is typical of low heat flow background of the Precambrian craton basin. In addition, the spatial distributions of the estimated formation temperature at the depth of 6˜8 km are also similar and are mainly controlled by the basement structural pattern. The temperature at the depth of 6000 m is 118˜172 ℃, the temperature at 7000 m is 136˜198 ℃, and the temperature at 8000 m is 190˜224 ℃. The temperature gradually increases from Shunbei to Shunnan area, but the temperature near SN4 fault zone is about 10˜20 °C higher than the surrounding rock. This thermal disturbances is possibly caused by hydrothermal fluids passing through the fracture system from the deep strata to the Ordovician strata, with hot alkaline brine from the Lower Cambrian providing heat source, and fault activities providing power and acting as channels for fluids upwelling. Furthermore, the 1-D theoretical crustal thermal structures of the Shunbei and Shunnan area were constructed by solving steady-state heat conduction equation. The results show that the thickness of the “thermal” lithosphere and the Moho temperature in Shunbei area are 177 km and 580 °C, respectively. However, the thickness of the “thermal” lithosphere in Shunnan area is relatively thin, with an average of 149 km. The heat flow distribution on the northern slope of Tazhong indicates the crustal heat flow contributes more to the surface heat flow and has the typical thermal structure characteristics of “hot crust and cold mantle” model.

The Late Neoproterozoic sedimentary sequences in the Yutang section southwest Tarim Basin and their tectonic implications and hydrocarbon perspective: Insight from basinology

The Late Neoproterozoic sedimentary sequences in the Tarim Basin are pivotal for understanding basin evolution and related tectonic setting, and evaluating the hydrocarbon perspective. In this study, tectonostratigraphic investigation has been conducted on the Late Neoproterozoic succession in the Yutang area, including strata of the Kexili, Yutang, Kuerkake and Kezisuhumu formations, along the Xinjiang-Tibet Highway ~90 km southwest of the Yecheng at the southwest Tarim Basin. The results indicate that the sediments represent a fluvial/alluvial-lacustrine-shallow marine tectonostratigraphic sequence complex upward the section, which is comparable with the deposition of sedimentary basin obeying evolution path from syn-rifting, late-rifting, transitional to post-rifting stage. These results lead to an interpretation of evolution from rift basin to passive margin or cratonic basin for the southwest Tarim Basin during the Late Neoproterozoic. Of these strata, the lacustrine dark shales of the late-rifting stage and the dolomites of the post-rifting stage can server as high-quality source rocks and reservoirs, respectively.

133,000 Years of Sedimentary Record in a Contourite Drift in the Western Alboran Sea: Sediment Sources and Paleocurrent Reconstruction

The Djibouti Ville Drift is part of a contourite depositional system located on the southern side of the Djibouti Ville Seamount in the Alboran Sea (Western Mediterranean). The sedimentary record of a core located in the drift deposits has been characterized to achieve the possible sediment sources for the Saharan dust supply and the paleocurrent variability related to Mediterranean intermediate waters for the last 133 kyr. Three end-member grain-size distributions characterize the sediment record transported by the bottom current to address the different aeolian populations, i.e., coarse EM1, silty EM2, and fine EM3. For these particles, the most likely source areas are the Saharan sedimentary basins and deserts, as well as the cratonic basins of the Sahara-Sahel Dust Corridor. The prevalence of these main source areas is shown in the core record, where a noticeable change occurs during the MIS 5 to MIS 4 transition. Some punctual sediment inputs from the seamount have been recognized during sea-level lowstand, but there is no evidence of fluvial supply in the drift deposits. The paleocurrent reconstruction allows the characterizing of the stadial and cold periods by large increases in the mean sortable silt fraction and UP10, which point to an enhanced bottom current strength related to intermediate water masses. Conversely, interglacial periods are characterized by weaker bottom current activity, which is associated with denser deep water masses. These proxies also recorded the intensified Saharan wind transport that occurred during interstadial/stadial transitions. All these results point to the importance of combining sediment source areas with major climatic oscillations and paleocurrent variability in palaeoceanographic sedimentary archives, which may help to develop future climate prediction models.

The interplay of dynamic topography and eustasy on continental flooding in the late Paleozoic

Global sea level change can be inferred from sequence stratigraphic and continental flooding data. These methods reconstruct sea level from peri-cratonic and cratonic basins that are assumed to be tectonically stable and sometimes called reference districts, and from spatio-temporal correlations across basins. However, it has been understood that long-wavelength (typically hundreds of km) and low-amplitude (<2 km) vertical displacements of the Earth's surface due to mantle flow, namely dynamic topography, can occur in the absence of crustal deformation. Dynamic topography can drive marine inundation or regional emergence of continents and must be taken into consideration for eustasy estimates. Our analysis indicates that the long-term trend in global-scale maximum flooding over the late Paleozoic generally correlates with global sea level curves. The first-order flooding history of North America correlates with some estimates of eustasy. The Paleozoic inundation of South America does not follow long-term sea level variations. The flooding lows during the Early Carboniferous and high during the Late Carboniferous are at odds with estimates of eustasy and can be explained by dynamic uplift and subsidence, respectively. Our dynamic topography models indicate that the Yangtze Platform of South China experienced significant dynamic subsidence during the transition from Permian to Triassic largely due to proto-Pacific subduction and its northward motion to collide with North China. The reference districts – Western New York, Oklahoma and Kansas, and West Texas in North America – were to some degree affected by dynamic uplift and subsidence associated with long-lived Panthalassa subduction zones, closure of the Rheic Ocean and large-scale upwelling above the African deep-mantle structure during late Paleozoic times. This indicates that some published global sea level curves may include non-eustatic signals such as dynamic uplift or subsidence. The interpretation of stratigraphic data gathered from these reference districts should be treated with caution to estimate global sea level variations.