Marine Rocks Research Articles

Onshore–offshore structure and hydrocarbon potential of the South Yellow Sea

Recent high-resolution airborne gravity data taken over the South Yellow Sea and its western onshore–offshore transition zone, combined with ground gravity data taken over the onshore area (Subei Basin), China, show that the South Basin of the South Yellow Sea and the Subei Basin correspond to the same gravity low anomaly. Magnetic data also support our interpretations. Both areas have similar strata, structures and hydrocarbon potential, and form a large Cenozoic terrestrial sedimentary basin controlled by the Tanlu Fault. Cenozoic terrestrial strata are well developed in the South Basin of the South Yellow Sea, and thick Meso–Paleozoic marine strata are preserved in the Central uplift area. Future hydrocarbon exploration in the South Yellow Sea should focus on the Cenozoic continental sequence, especially the Paleogene in the South Basin, as well as the Meso–Paleozoic marine rocks in the Central uplift area. The western part of the middle depression and middle and western parts of the north depression in the South Basin of the South Yellow Sea have the greatest potential for hydrocarbon accumulation.

Ichnofauna from the Harbans Bed of the Badhaura Formation (Sterlitmakian), Rajasthan, India

In the first ever systematic study of trace fossils from the Badhaura Formation, the authors described a nesting burrow, which they ascribed to a stomatopod. The purpose of this paper is two-fold: primarily, to document ichnofauna from (post-glacial marine late Palaeozoic rocks of peninsular India) the Badhaura Formation (Sterlitmakian) representing marine rocks deposited following the Late Palaeozoic glaciation and secondly to contribute to the data on post-glacial ichnofauna from constituent continents of the Gondwanaland. Trace fossils described here are from the Harbans Bed, the topmost lithounit of the Badhaura Formation. The ichnofauna includes Arenicolites tenuis, Beaconites isp., Curvolithus isp., Cylindrichnus concentricus, Didymaulichnus lyelli, Ophiomorpha isp., Palaeophycus tubularis, Planolites beverleyensis, P. montanus, Rosselia chonoides, R. socialis, Skolithos linearis, Taenidium cameronensis, Thalassinoides paradoxicus, Thalassinoides isp. and a flask-shaped brood chamber assigned to a stomatopod crustacean. This mixed assemblage is assigned to distal Skolithos ichnofacies and is suggestive of a period of relatively quiet, shallow water conditions of deposition. The ichnofauna, when viewed in context of peri-gondwanic ichnofaunas, mainly consisting of simple tracks and trails, from late Palaeozoic post-glacial deposits of other Gondwanan continents, is interesting due to dominance of domichnia. Profusion of brood chambers along with Thalassinoides in the Badhaura Formation validates the concept of pre-Mesozoic Thalassinoides being non-decapod in origin and suggestive of adaptive convergence.

Trace element content of sedimentary pyrite as a new proxy for deep-time ocean–atmosphere evolution

Sedimentary pyrite formed in the water column, or during diagenesis in organic muds, provides an accessible proxy for seawater chemistry in the marine rock record. Except for Mo, U, Ni and Cr, surprisingly little is known about trace element trends in the deep time oceans, even though they are critical to developing better models for the evolution of the Earthʼs atmosphere and evolutionary pathways of life. Here we introduce a novel approach to simultaneously quantify a suite of trace elements in sedimentary pyrite from marine black shales. These trace element concentrations, at least in a first-order sense, track the primary elemental abundances in coeval seawater. In general, the trace element patterns show significant variation of several orders of magnitude in the Archaean and Phanerozoic, but less variation on longer wavelengths in the Proterozoic. Certain trace elements (e.g., Ni, Co, As, Cr) have generally decreased in the oceans through the Precambrian, other elements (e.g., Mo, Zn, Mn) have generally increased, and a further group initially increased and then decreased (e.g., Se and U). These changes appear to be controlled by many factors, in particular: 1) oxygenation cycles of the Earthʼs ocean–atmosphere system, 2) the composition of exposed crustal rocks, 3) long term rates of continental erosion, and 4) cycles of ocean anoxia. We show that Ni and Co content of seawater is affected by global Large Igneous Province events, whereas redox sensitive trace elements such as Se and Mo are affected by atmosphere oxygenation. Positive jumps in Mo and Se concentrations prior to the Great Oxidation Event (GOE1, c. 2500 Ma) suggest pulses of oxygenation may have occurred as early as 2950 Ma. A flat to declining pattern of many biologically important nutrient elements through the mid to late Proterozoic may relate to declining atmosphere O2, and supports previous models of nutrient deficiency inhibiting marine evolution during this period. These trace elements (Mo, Se, U, Cu and Ni) reach a minimum in the mid Cryogenian and rise abruptly toward the end of the Cryogenian marking the position of a second Great Oxidation Event (GOE2).

Contribution of the Magnetic Susceptibility to the Characterization of the Babouri-Figuil Cretaceous Basin

Magnetic susceptibility data from marine rocks can be used for global correlation due to synchronous variations in global erosion. This work, aims to show that, the magnetic susceptibility signature which exists in the cretaceous sediments of the Babouri-Figuil basin, resides mainly in the detrital constituents. Variations of amplitudes are observed, which can be useful for regional correlation.This amplitude variation results in the transgressive and regressive movements that give way to a fluctuation of detrital deposits because of erosion. Considering the sedimentological study, carried out, different microfacies have been identified in the basin, namely, fluvial facies and lake facies that sometimes contain fossils. The combination of detailed sedimentological analysis and sequence stratigraphy is based on the modeling of vertical and lateral variations within the basin. Finally, the interpretation of the magnetic susceptibility signal suggests that in the detrital domain, this signal is mainly controlled by environmental parameters such as the agitation of the water and the rate of sedimentation, as well as diagenesis.

BIOLOGICAL EFFECTS ON THE SOURCE OF GEONEUTRINOS

Detection of antineutrinos from U and Th series decay within the Earth (geoneutrinos) constrains the absolute abundance of these elements. Marine detectors will measure the ratio over the mantle beneath the site and provide spatial averaging. The measured mantle Th/U may well be significantly below its bulk earth value of ~4; Pb isotope measurements on mantle-derived rocks yield low Th/U values, effectively averaged over geological time. The physics of the modern biological process is complicated, but the net effect is that much of the U in the mantle comes from subducted marine sediments and subducted upper oceanic crust. That is, U subducts preferentially relative to Th. Oxygen ultimately from photosynthesis oxidizes U(IV) to U(VI), which is soluble during weathering and sediment transport. Dissolved U(VI) reacts with FeO in the oceanic crust and organic carbon within sediments to become immobile U(IV). These deep marine rocks are preferentially subducted relative to Th(IV)-bearing continental margin rocks. Ferric iron from anoxygenic photosynthesis and oxygen in local oases likely mobilized some U during the Archean Era when there was very little O2in the air. Conversely, these elements behave similarly in the absence of life, where the elements occur as U(IV) and Th(IV), which do not significantly fractionate during igneous processes. Neither do they fractionate during weathering, as they are essentially insoluble in water in surface environments. Th(IV) and U(IV) remain in solid clay-sized material. Overall, geoneutrino data constrain the masses of mantle chemical and isotopic domains recognized by studies of mantle-derived rocks and show the extent of recycling into the mantle over geological time.

Lithostratigraphy, sedimentology, and cyclicity of the Duwi Formation (late Cretaceous) at Abu Tartur plateau, Western Desert of Egypt: evidences for reworking and redeposition

The Duwi Formation (late Cretaceous) represents the first onset of fully marine conditions and comprises sediments that were confined by a preexisting depression enclosed by the Dakhla uplift in the west and Kharga uplift in the east. The Duwi Formation overlies unconformably the Qusseir Formation and underlies conformably the Dakhla Formation. The Duwi Formation could be subdivided vertically into three remarkable informal lithostratigraphic units. The Duwi Formation comprises a heterogeneous suite of shallow marine rocks (phosphorite, shale, mudstone, marlstone, glauconite, and dolomite). A model has been presented for the mode of phosphorite accumulation. This model includes aspects from both the traditional upwelling phosphogenic model and key elements from the non-upwelling model. The role played by high-energy events (e.g., internal waves) in phosphorite accumulation was achieved by the interplay between the physicochemical and hydrodynamical processes. Internal waves are thought to be the triggering mechanism for the density flows. Internal waves and induced bottom currents are sporadic but strong enough to bring sediments into suspension. Accumulation of phosphorites by high-energy waves is evidenced by the predominance of many types of bedform sequences. Such type of bedforms are represented by fining upward, firm grounds, erosive pockets and pot holes, planar lags, imbrication, sorting, and fragmentation. The Duwi Formation has been divided into several meter-scale cycles of three types (gradual, non-gradual, and truncated). This represents the higher frequency sea-level fluctuations forming shallowing–upward cycles that were arranged into the larger regressive megasequence during a third-order sea level change.

Unraveling species concepts for the Helicoprion tooth whorl

The genus Helicoprion (Chondricthyes, Euchondrocephali) is preserved primarily by a continuous spiral root that forms the base for more than 130 teeth. Helicoprion is found globally in Lower Permian marine rocks and at least 100 specimens exist in public collections worldwide. Ten species of the genus are reviewed in the context of new morphometric analyses conducted on dozens of specimens. Helicoprion whorls share a common inner spiral geometry that results in exponential growth of tooth and root dimensions. Novel growth equations permit calculation of whorl diameter, volution number and tooth count from incomplete specimens. The results of meristic and geometric analyses identify taxobases that distinguish three emended species concepts. Differentiation of form is evident among specimens only after the second volution or roughly the 85th tooth. Helicoprion davisii has widely spaced, stout teeth with tall cutting surfaces and is distinguished from H. bessonowi, which has narrow, closely spaced teeth with short cutting surfaces. Helicoprion ergassaminon is an intermediate form, having narrow, closely spaced teeth with tall cutting surfaces. Several large specimens in the study are too dissimilar to place in the new emended species concepts.

New Paleogene paleoclimate analysis of western Washington using physiognomic characteristics from fossil leaves

The Chuckanut and Manastash formations were deposited in a lowland fluvial environment in western Washington during the Paleocene–Eocene. We provide a thorough paleotemperature analysis using the physiognomic characteristics from the fossil dicotyledonous leaves from these formations using two techniques, Leaf Margin Analysis (LMA) and Climate Leaf Analysis Multivariate Program (CLAMP). This work differs from previous analyses by 1) analyzing more localities and including the assessment of the never-before described Taneum Ridge Body and Manastash Main Body within the Manastash Formation and the Maple Falls Member of the Chuckanut Formation, and 2) using two different techniques to decipher mean annual temperatures through time. In this study, mean annual temperatures (MAT) were determined from 125 distinct morphotypes and are several degrees higher at each locality than previously reported. MAT values derived from LMA ranged from 17.0° to 28.8°C for the lowermost and 13.4° to 19.5°C for the uppermost Chuckanut Formation. The Manastash Formation had MATs ranging from 17.1°C to 29.1°C. Using CLAMP, paleotemperatures ranged from 15.3° to 19.4°C for the lowermost and 12.3° to 15.6°C for the uppermost Chuckanut Formation and 15.8 to 21.3°C for the Manastash Formation. These data are compared to other Eocene–Paleocene formations in the western US; the Chuckanut and Manastash formations record some of the warmest paleotemperatures in the Paleogene in the western US because of their low-elevation locations. These paleotemperatures may reflect the terrestrial signal of paleoclimate optima defined in marine rocks during the Paleocene–Eocene.

Organic Geochemical Characteristics and Depositional Environments of the Eocene Deposits in the Eastern Black Sea Region, NE Turkey

Type of organic material, thermal maturity, depositional environment, and source rock potential of Eocene aged clastic rocks of the Eastern Black Sea Region were investigated. Based on total organic carbon values, clastic rocks range from weak-good source rock. Samples contain a mixture of Type I–Type III kerogen. Based on T max values, Eocene aged clastic rocks were thermally mature. Moretane/hopane ratio of samples <0.15, 22S/(22S + 22R) homohopane (C32), and 20S/(20S + 20R) sterane (C29) maturity parameters were equal or very close to equilibrium value of oil generation. The deposition environment was transitional marine environment and clastic rocks do not have the potential to form hydrocarbon.

Geochemical Characterization of the Calacoa Geothermal Zone

The Calacoa geothermal zone (CGZ) is located in the western Cordillera of the Andes in southern Peru, approximately 10km on the western slope of Ticsani Volcano. In Calacoa there are mainly volcanic rocks that have been deposited over a sedimentary Cretaceous basement. The geochemical interpretation of the results shows that thermal waters are an alkaline-chloride-sulphate water type. The Cl-SO4-HCO3 diagram shows that most of the geothermal waters plot close to the mature water portion, which is typical of geothermal deep fluids. High B concentrations lead to a relatively high B/Cl ratios as shown on a B-Cl binary diagram. This can be used to elucidate the reactions of waters with sedimentary marine rocks at deep depths. The δ18O vs δD plot indicates that the geothermal water originates by a mixing of meteoric water with magmatic water. The results for chemical geothermometry allowed us to estimate temperatures between 170 and 230°C at the depth of the geothermal resources.

An acid-free method of microfossil extraction from clay-rich lithologies using the surfactant Rewoquat

Marine rocks characterized by high clay content provide excellent conditions for fossil preservation, particularly for organic-walled microfossils such as retiolitid graptolites and chitinozoans. Nevertheless, the phyllosilicate minerals, which constitute the clay component, make microfossil extractions difficult. The problem results from the tendency of phyllosilicates to form aggregates in low pH values, as standard methods of microfossil extraction employ acids for rock digestion. Consequently, the use of acids for clay-rich rocks is often inefficient and time-consuming. We propose a method of rock disintegration using the surfactant Rewoquat and compare it with two commonly applied approaches: digestion in buffered acetic acid and in HCl-HF. Using examples from the Mulde Brick-clay Member from the Silurian of Gotland and the Daleje Shale from the Devonian of the Prague Basin we observed that disintegration in Rewoquat was faster (days) than digestion in acid (months), and allowed to recover calcareous in addition to organic-walled fossils. The yield and preservation was comparably good, except for conodonts, which were strongly etched after using HCl-HF. Retiolitid graptolites recovered using Rewoquat were preserved in 3D and showed a lower degree of fragmentation. The fossil content of the residue obtained using Rewoquat was higher due to dispersion of clay aggregates. For observation of delicate fossils we recommend to coat the sample with the surfactant. Application of Rewoquat can reveal the most delicate forms and growth stages, and thus provide a better insight into the ontogeny, autecology, and body size distributions of a number of fossil groups.

Leaf structural adaptations of two Limonium miller (Plumbaginales, Plumbaginaceae) taxa

Limonium gmelinii (Willd.) O. Kuntze 1891 subsp. hungaricum (Klokov) So? is Pannonian endemic subspecies that inhabits continental halobiomes, while Limonium anfractum (Salmon) Salmon 1924 is one of the indicators of halophyte vegetation of marine rocks and its distribution is restricted to the southern parts of Mediterranean Sea coast. In this work, micromorphological and anatomical characters of leaves of these two Limonium taxa were analyzed, in order to examine their adaptations to specific environmental conditions on saline habitats. The results showed that both taxa exhibited strong xeromorphic adaptations that reflected in flat cell walls of epidermal cells, thick cuticle, high palisade/spongy tissue ratio, high index of palisade cells, the presence of sclereid idioblasts in leaf mesophyll and mechanical tissue by phloem and xylem. Both taxa are crynohalophytes and have salt glands on adaxial and abaxial epidermis for excretion of surplus salt. Relatively high dimensions of mesophyll cells, absence of non-glandular hairs and unprotected stomata slightly increased above the level of epidermal cells, are also adaptations to increased salinity.

A New Fossil Amiid from the Eocene of Senegal and the Persistence of Extinct Marine Amiids after the Cretaceous–Paleogene Boundary

We report a new fossil amiid from Eocene rocks of West Africa representing the first record of this clade from Senegal. The new specimen has a maxilla that is very similar in size to that of Amia calva. It is distinctly smaller than reported remains of another West African Eocene taxon, †Maliamia gigas. We tentatively refer the Senegal specimen to Vidalamiini because it has the large postmaxillary process diagnostic of this clade; however, it also exhibits anatomical features not previously described in extinct amiids. We recovered the specimen in rocks of the Lam-Lam Formation in Central-Western Senegal that we interpret to have been a shallow marine depositional environment. The occurrence of an Eocene marine amiid contradicts existing hypotheses that marine amiids were generally absent after the Cretaceous–Paleogene boundary having been replaced by freshwater taxa. Research completed since the initial discovery of †Maliamia gigas indicates that this Eocene taxon was also found in shallow marine rocks.Nous avons découvert un poisson fossile dans les sédiments éocènes de l'Afrique de l'Ouest, le premier représentant du clade des Amiidae au Sénégal. Le nouveau fossile a presque la même taille que l'espèce actuelle, Amia calva. Le fossile sénégalais a une longue processus postmaxillaire diagnostique de Vidalamiini. Le fossile montre aussi plusieurs charactéristiques anatomiques qui n'ont jamais été décrites chez les Amiidés fossiles. Nous avons trouvé le fossile dans les sédiments de la Formation du Lam-Lam au Centre-Ouest du Sénégal, et nous les interprétons comme des sédiments marins. La découverte d'un amiidé dans les gisements éocènes marins contredit l'hypothèse selon laquelle on ne trouve plus d'amiidés marins au-dessus de la limite Crétacé–Paléogène et qu'ils ont été remplacés par les amiidés d'eau douce. Les travaux stratigraphiques menés depuis la découverte de †Maliamia gigas ont montré que cette espèce était aussi présente dans les gisements marins.

Beyond black shales: The sedimentary and stable isotope records of oceanic anoxic events in a dominantly oxic basin (Silurian; Appalachian Basin, USA)

Episodes of instability in the global climato-oceanic system have become the hallmark of the Silurian Period. These events are marked by six large positive carbon isotope excursions associated with major extinctions. The widespread environmental consequences of these perturbations of the global carbon cycle remain poorly documented. A high-resolution chronostratigraphic reconstruction of the Appalachian foreland basin (AFB) provides a framework to evaluate environmental signatures of two of these carbon isotope excursions (Valgu and Ireviken events; Telychian–Sheinwoodian) at a regional scale. Integration of published biostratigraphic range data, mapping of sequence stratigraphic surfaces and facies, and generation of high-resolution carbon isotope profiles for several transects provides precise correlation of the basin stratigraphy yielding a high-resolution temporal framework. This chronostratigraphic reconstruction of the AFB highlights the importance of time-specific facies (TSF) during these intervals. For instance, packages of red, green, and gray marine rocks separately occupied discrete time intervals—cutting across “local” facies belts indicative of shallow to deep marine environments. Similarly, widespread iron mineralization occurred during discreet time slices as “bathtub rings” rimming the margins of the basin. Changes from one time-specific facies to the next are coincident with changes in carbon isotope values, indicating a genetic link between fractionation of the global carbon reservoir and redox changes from generally oxic to anoxic oceanic conditions in parts of the AFB.The basal part of the study interval was deposited during the Valgu Event. This first event begins with an unconformity marking global lowstand. This fall in sea level was coincident with rising carbon isotope values as recorded in adjacent basins. A slight sea level rise and flooding of the subaerial unconformity occurred near the peak in carbon isotope values. Oxidized iron minerals were deposited in a zone that extended from the lower shoreface into the basin center—pinching out into shallower parts of the basin. Thin pyrite–phosphorite–glauconite layers, associated with black shales in the basin center, were deposited over these ferruginous strata. As the event ended and carbon isotope values shifted negatively to a more stable baseline, temperatures warmed, sea level reached a Silurian high, and oxic conditions associated with very low rates of organic carbon (OC) burial were established. These conditions were relatively stable for nearly four million years. This long period of stasis was punctuated by the Ireviken Event, which displays a strikingly similar progression of TSF patterns, yet more intense and longer lived than the preceding Valgu Event.With the first positive shift in carbon isotopes at the onset of the Ireviken Event, the basin redox system changed from largely oxidizing to reducing, coincident with a rapid drop in sea level and an abrupt reappearance of iron mineralization on the basin margins. The sea level fall associated with the shift to peak values of the carbon isotope excursion is marked by the progradation of a significant body of sand well out into the basin. During this sea level drop, conditions in the basin center became highly reducing and periodically sulfidic. Widespread ironstones and ankeritic carbonates were deposited isochronously, rimming the shallow basin margins where they alternated with light gray pyritic shales, indicating fluctuating redox conditions. A large negative carbon isotope excursion of ~3‰ is coincident with a shift from regression to transgression, suggesting glacial melting via methane release. Continued deposition of ironstones through this time interval suggests a primary link to shallow marine redox processes rather than large-scale transgression or regression of the shoreline. The ironstones are overlain by gray to black pyritic shales deposited across all environments, revealing a final stage of redox progression into sustained sulfidic conditions as peak values of the carbon isotope excursion returned to a high stable baseline near +4‰.The similar expression of TSFs within the AFB of these two global events demonstrates that fractionation of the global atmospheric and oceanic carbon reservoirs had a predictable, cascading impact on shallow sea floor environments. The associated redox processes reveal the dynamic regional interactions of a shallow oxic epicontinental water mass with a broader anoxic oceanic water mass, providing valuable insight into the origin of some of the largest extinctions in the Silurian Period.

Origins of natural gases from marine strata in Northeastern Sichuan Basin (China) from carbon molecular moieties and isotopic data

To determine the origin, maturity, formation mechanism and secondary process of marine natural gases in Northeastern Sichuan area, molecular moieties and carbon isotopic data of the Carboniferous and Triassic gases have been analyzed. Typical samples of marine gas precursors including low-maturity kerogen, dispersed liquid hydrocarbons (DLHs) in source rocks, residual kerogen and oil have been examined in a closed system, and several published geochemical diagrams of gas origins have been calibrated by using laboratory data. Results show that both Carboniferous and Triassic gases in the study area have a thermogenic origin. Migration leads to stronger compositional and weak isotopic fractionation, and is path dependent. Carboniferous gases and low-H2S gases are mainly formed by secondary cracking of oil, whereas high-H2S gases are clearly related to the TSR (Thermal Sulfate Reduction) process. Gases in NE Sichuan show a mixture of heavy (13C-enriched) methane in comparison to the lower maturated ethane of Triassic gas samples, suggesting a similar source and maturity for ethane and propane of Carboniferous gases, and a mixture of heavy ethane to the propane for Triassic gases. Based on the data plotted in the diagram of Chung et al. (1988), the residual kerogen from Silurian marine shale and palaeo oil reservoirs are the main source for Carboniferous gases, and that the residual kerogen from Silurian and Permian marine rocks and Permian paleao oil reservoirs constitute the principal source of Triassic gases.

Late Cretaceous mosasauroids (Reptilia, Squamata) of the Big Bend region in Texas, USA

ABSTRACTUpper Cretaceous marine rocks of the Big Bend Region of trans-Pecos Texas preserve a number of marine-adapted mosasauroids. At least three unnamed taxa of basal mosasauroids are represented by remains from shaly limestones in the middle Turonian portion of the Boquillas Formation. These occur along with remains of larger derived mosasaurs referable to Russellosaurina and an undescribed tylosaurine. Derived mosasaurs from the middle to late Coniacian include the first report of Tylosaurus kansasensis outside of Kansas, T. nepaeolicus, Platecarpus planifrons, and Platecarpus aff. P. planifrons. Clidastes liodontus is found in the latest Coniacian or early Santonian part of the Pen Formation. An undescribed species of Ectenosaurus, Clidastes sp. and an indeterminate plioplatecarpine occurs in the middle Santonian to early Campanian interval of the Pen Formation. The mosasaur fauna from the Big Bend region is quite similar to that from the Smoky Hill Chalk of Kansas, a thousand kilometres to the north. We refine the position of the Cenomanian–Turonian boundary within the Ernst Member of the Boquillas Formation, based on ammonite faunas. We also corroborate previous interpretations describing the time-transgressive nature of the onset of deposition of the Pen Formation based on a west-to-east descending level of the Inoceramus (Cremnoceramus) undulatoplicatus FAD.

Slowed by sulphide

The Triassic–Jurassic period extinction marked a rapid turnover in the marine realm. Biomarkers in marine rocks suggest that the development of sulphidic conditions in the early Jurassic delayed marine recovery.

Integrated potential field study on the subsurface structural characterization of the area North Bahariya Oasis, Western Desert, Egypt

The present study aims mainly to delineate and outline the regional subsurface structural and tectonic framework of the buried basement rocks of Abu El Gharadig Basin, Northern Western Desert, Egypt. The potential field data (Bouguer gravity and total intensity aeromagnetic maps) carried out in the Abu El Gharadig Basin had been analyzed together with other geophysical and geological studies. The execution of this study is initiated by transformation of the total intensity aeromagnetic data to the reduced to pole (RTP) magnetic map. This is followed by applying several transformation techniques and various filtering processes through qualitative and quantitative analyses on both of the gravity and magnetic data. These techniques include the qualitative interpretation of gravity, total intensity magnetic and RTP magnetic maps. Regional–residual separation is carried out using the power spectrum. Also, the analytic signal and second vertical derivative techniques are applied to delineate the hidden anomalies. Aeromagnetic anomalies in the area reflect significant features on the basement tectonics, on the deep-seated structures and on the shallow-seated ones. Major faults and intrusions in the area are indicated to be mainly along the NE–SW, NW–SE, ENE–WSW and E–W directions. The Bouguer gravity map indicates major basement fracturing, as well as variations in the sedimentary basins and ridges and subsequent tectonic disturbances. The most obvious anomalous trends on the gravity map, based on their frequencies and amplitudes, are along the NE–SW, ENE–WSW, E–W and NW–SE trends. The main of Abu EL Gharadig Basin depositional center does not show sharp variations, because of the homogeneity of the marine rocks and the great basement depths.

Allochthonous Mesozoic marine sequences of northeastern Asia and western North America: Correlation of the stratigraphic levels and geodynamic depositional settings

The problem of correlating the isolated outcrops of allochthonous Middle Mesozoic deposits developed in northeastern Asia and western North America includes several aspects: (1) the stratigraphic subdivision of the sequences using the radiolarian assemblages and taking into account the complex nappe-thrust structure of the region; (2) the analysis of their facies composition with reconstructing the geodynamic depositional settings; (3) the search for features in common (with respect to the previous two aspects) in all these localities. The first widely applied radiolarian analysis revealed that the Middle Mesozoic marine rocks from separate nappes and slices of tectono-stratigraphic sections in East Asia enclose 20 different-age (Norian to Hauterivian) radiolarian assemblages. The correlation of these assemblages revealed the wide lateral distribution of their host Middle Mesozoic marine sequences in the regions under consideration. The significance of the presented materials is determined by the following facts: (1) using the radiolarian analysis, the Middle Mesozoic marine sequences of northeastern Asia are first subdivided into stage and substage units with defining their analogs in western North America; (2) despite the discrete distribution of Mesozoic allochthons, it is shown that the defined stratigraphic units are widespread in northeastern Asia; (3) it is established that these allochthons consist of rocks formed in different geodynamic depositional settings: ocean floor, island arc, fore-arc, and marginal-sea domains. This spectrum of heterogeneous rocks is traceable practically through the entire northeastern Asia region.

Accumulation and Reformation of Silurian Reservoir in the Northern Tarim Basin

Abstract:Silurian sandstone in Tarim Basin has good reservoir properties and active oil and gas shows, especially thick widely‐distributed bituminous sandstone. Currently, the Silurian was found containing both bitumen and conventional reservoirs, with petroleum originating from terrestrial and marine source rocks. The diversity of their distribution was the result of “three sources, three stages” accumulation and adjustment processes. “Three sources” refers to two sets of marine rocks in Cambrian and Middle‐Upper Ordovician, and a set of terrestrial rock formed in Triassic in the Kuqa depression. “Three stages” represents three stages of accumulation, adjustment and reformation occurring in Late Caledonian, Late Hercynian and Late Himalayan, respectively. The study suggests that the Silurian bitumen is remnants of oil generated from Cambrian and Ordovician source rocks and accumulated in the sandstone reservoir during Late Caledonian‐Early Hercynian and Late Hercynian stages, and then damaged by the subsequent two stages of tectonic uplift movements in Early Hercynian and Pre‐Triassic. The authors presumed that the primary paleo‐reservoirs formed during these two stages might be preserved in the Silurian in the southern deep part of the Tabei area. Except for the Yingmaili area where the Triassic terrestrial oil was from the Kuqa Depression during Late Himalayan Stage, all movable oil reservoirs originated from marine sources. They were secondary accumulations from underlying Ordovician after structure reverse during the Yanshan‐Himalayan stage. Oil/gas shows mixed‐source characteristics, and was mainly from Middle‐Upper Ordovician. The complexity and diversity of the Silurian marine primary properties were just defined by these three stages of oil‐gas charging and tectonic movements in the Tabei area.