Geological Journal

Triassic–Jurassic sedimentary successions (Baluti and Sarki formations) in northern Iraq record a variety of environmental changes that may be related to global Triassic–Jurassic (Tr/J) boundary events. The diversity of some benthic fauna decreases through the transitional boundary beds. The coastal marine environment of the lower part of the Baluti Formation is followed by shallower tidal flat and supratidal marginal marine environments at the transitional boundary with the Jurassic‐age Sarki Formation. The alternating calcareous mudrocks and dolomitic limestones of the transitional succession are overlain by a succession of calcareous mudrocks and dolomicrites that form a dolocrete bed in the latest Triassic. The early Jurassic carbonates (lower part of Sarki Formation) were deposited in a shallow‐marine to lagoonal environment. Geochemical evidence supports this interpretation. TOC% increases towards the Tr/J boundary and the lower part of the Sarki Formation. This increase can be interpreted as resulting from the primary precipitation of dolocrete as palaeosol horizons. The variations in the oxygen isotope ratios mainly reflect the facies and diagenetic effects. Th/K ratio is generally constant and shows an increase in the calcareous mudrock beds of the upper part of the Baluti Formation, possibly related to the degradation of K‐bearing clay minerals. Low Th/U ratios are due to the depletion in thorium, typical of many marine carbonates rather than to an increase in authigenic uranium. This explanation is also corroborated by the presence of abundant fossils in some of the studied carbonates. Copyright © 2013 John Wiley & Sons, Ltd.

The Mangxiang Formation black shales are the most important hydrocarbon source rocks in the Wuyu Basin. Trace and rare earth element (REEs) of the black shales from the Wuyu Basin were studied in order to understand their depositional environments and palaeoclimate. Thirty one black shale samples from the Wuyu Basin were analysed by inductively coupled plasma–mass spectrometry and X‐ray fluorescence. The black shales are characterized by moderate SiO₂ (52.55–58.73%) contents and medium K₂O/Na₂O (2.21–4.83) ratio values but relatively high MgO + Fe₂O₃ (5.8–6.91%) and Al₂O₃ (15.1–17.5%) contents. The Chemical Weathering Index of Alternation (CIA) ranges from 57 to 70, together with medium Th/U (2.66–4.97) ratio values, reflecting a weak to moderate degree of chemical weathering of the source area. The palaeoredox condition of black shale was slightly oxic (or dysoxic) during black shale deposition as evidenced by slightly Ce anomalies (0.92–0.99) and Mn enrichment (EF = 1.6). The moderate palaeosalinity values (9.32‰–17.75‰), together with medium B/Ga (3.90–5.57) ratio values, indicate a brackish water environment. The palaeoclimate index ∑(Fe + Mn + Cr + Ni + V + Co)/∑(Ca + Mg + Sr + Ba + K + Na) ranges from 0.28 to 0.50 and low Sr/Ba (0.15–1.22) ratio values, indicating a semiarid to semimoist climatic condition during the sedimentation of the black shale. The high ω(La)_N/ω(Yb)_N ratio values (1.02–1.09) indicate a fast sedimentary rate during black shale deposition. In this study, a preservation model of the Mangxiang Formation black shale was established. The model indicates that excellent preservation may be the major controlling factor for the accumulation of organic matter. Copyright © 2015 John Wiley & Sons, Ltd.

The Xianshuihe Fault Belt (XSF), along which the syntectonic Zheduoshan batholith was emplaced, has great significance for the reconstruction of the tectonic framework in the eastern margin of the Tibetan Plateau. In this contribution, formation process and evolution of the XSF are discussed based on the structural deformation in the field and the geochronology of Zheduoshan batholith. The results show that the XSF current arc‐shaped protrusion to the north‐east probably was formed by a fracture of the clockwise rotation compression that extended northward to the periphery with the eastern Himalayan tectonic syntaxis as the centre. It is a complex fault belt formed by the superposition of multi‐stage structures. In the early‐stage formation and evolution of the XSF, the Oligocene‐Miocene migmatite zone and Miocene granites of the Zheduoshan batholith were emplaced. Among them, the lower limit of the XSF's initial activity time was not less than 47 Ma that was limited by the Zircon U–Pb geochronology of migmatite zone formed under the compression system. During the emplacement of Miocene granites, the XSF underwent a process from compression to sinistral strike‐slip, and the geochronology indicates that the onset of the XSF sinistral strike slip should not be less than 14 Ma. After syntectic magmatism, the XSF also experienced the shear deformation (from ductile to brittle) with sinistral kinematics.⁴⁰Ar‐³⁹Argeochronology results show that the ductile shear deformation mainly occurred around 5.5–3.2 Ma and accompanied a staged and differential uplift from north to south. It extended to the south along the weak crustal zone of Anninghe, Daliangshan, Xiaojiang, and other faults, forming the Xianshuihe–Anninghe–Xiaojiang sinistral strike‐slip fault system on the eastern margin of the Tibetan Plateau, and large‐scale sinistral strike slip began around 5 Ma. Our new insights lay a foundation for understanding and dissecting the formation and evolution of the Tibetan Plateau eastern margin.

The Ordovician genus‐diversity trajectories of trilobites and the radiation patterns previously proposed in South China are reviewed on the basis of an extensive and intensive study of 32 representative collections assigned respectively to thesuecicus,hirundoandclavusgraptolite‐zone intervals and made along shelf environment gradients from 15 re‐measured representative early‐Middle‐Ordovician sections. Analyses of new data indicate that components of the Whiterock Fauna (Paleozoic Evolutionary Fauna) spread across the whole of the environmental spectrum and in general increased steadily through the Ordovician. The major faunal innovations appeared first in the inner shelf during the Early Ordovician and the novelties then expanded seawards into the outer shelf. The Whiterock Fauna first exhibited a higher representation in shallow outer‐shelf faunas and made its early appearance in deep outer‐shelf habitats during the early Middle Ordovician. However, it was not until theclavusinterval (late early Middle Ordovician) that the onset of the major Ordovician radiation or the radiation of the Whiterock Fauna as a whole commenced in deep outer‐shelf environments. From the Darriwilian onwards, members of the Whiterock Fauna had further developed and dominated all environments, but it was not until the Hirnantian that they comprised the whole fauna. The beginning of the radiation in South China was dominated by the expansion of reedocalymenine calymenids, trinucleids and cyclopygids. All of these three trilobite clades had their early diversification centred in high latitude zones of Gondwana. In terms of the macroevolution of trilobites, these specialized trilobites were only diagnostic of a regional Ordovician radiation. All of them represented a subset of the Paleozoic Evolutionary Fauna, which became extinct before or during the latest Ordovician mass extinction and bore no relationships to the Silurian Fauna. Therefore, the Whiterock Fauna of South China only played a minor role in the global macroevolution of the Paleozoic Evolutionary Fauna. As revealed by close investigations, the Ordovician radiation in South China may have been directly triggered by two major transgressive‐regressive cycles during the Tremadoc/Arenig and Lower/Middle Ordovician. The increase of oxygen levels in oceans during the early Middle Ordovician may have provided an essential condition for the radiating fauna to eventually inhabit the new ecological niches of the deep outer shelf. Copyright © 2007 John Wiley & Sons, Ltd.

The high rate of species extinction in recent decades is seen by many ecologists as heralding an extinction of catastrophic magnitude in the near future. The ecological consequences of such a biodiversity crisis are hard to predict, but some indication of likely effects can be gained from the knowledge of mass extinctions in the past. The Late Ordovician extinction was one of the five great extinctions in the geological record. It occurred in two phases about 0.5–1million years (Ma) apart and resulted from climatic and related environmental changes associated with the rapid growth and decay of the large Gondwanan ice cap. Overall, an estimated 86% of species became extinct, 61% of genera and 12–24% of families, but few or no orders or higher taxa. The extinction severely affected both marine benthos and plankton. Using brachiopod data as a measure of ecological change, it can be seen that the number of within‐habitat species (alpha diversity) was severely reduced and the number and distinctness of benthic communities (beta diversity) on marine shelves also declined sharply. Concurrently the number of palaeogeographic provinces fell from ten to five, possibly as a result of a loss of endemic species and preferential survival of cosmopolitan species. At the peak of extinction, following the second extinction phase, the ecological structure of both benthic and planktonic ecosystems had been severely disrupted and downgraded in complexity as a wide variety of niches were ‘vacated’. In spite of the profound biodiversity and ecological crisis within this ‘survival’ interval, communities returned to their pre‐extinction levels of alpha and beta diversity during the subsequent ‘recovery’ interval. In spite of the large amount of vacant ecospace to be filled there was very little innovation in terms of adaptive strategy, so that the structure of the emerging Silurian communities was similar to that of the Ordovician. In these terms the ecological recovery was remarkable, but it was also prolonged over about 4–5 Ma. On a geological time scale the biosphere returned to ‘normal’, but on a human time scale the mass extinction produced a very severely degraded biosphere. Copyright © 2001 John Wiley & Sons, Ltd.

In the southern flank of the Menderes Massif, western Turkey, an E–W‐trending quartz–mica vein was recognized in the augen gneisses of the so‐called ‘core’ series. Rb‐Sr determinations of two undeformed or slightly deformed mica books from the vein yielded Palaeocene–Early Eocene ages (c. 62–43 Ma) for the vein formation. The new data are interpreted to closely post‐date the time of major Barrovian‐type High Temperature/Medium Pressure (HT/MP) main Menderes metamorphism (MMM) and coeval crustal‐scale top‐to‐the‐N‐NNE Tertiary deformation that affected the whole Massif. Subsequent cooling occurred in the Middle Eocene (36 ± 2 Ma Ar‐Ar mica ages).

Top‐to‐the‐N‐NNE structures were later overprinted by top‐to‐the‐S‐SSW fabrics within normal‐sense ductile shear zone, located between the core augen gneisses in the footwall and the structurally overlying cover schists in the hanging wall. The Massif was at the surface by the Early Miocene. Field relations clearly demonstrate that the age of top‐to‐the‐S‐SSW deformation is well constrained between the age of the MMM and the Early Miocene, and that exhumation was mainly tectonically controlled. Copyright © 2000 John Wiley & Sons, Ltd.

Petrographical, mineralogical, and geochemical characteristics of a blanket‐type lateritic bauxite deposit, bordering the Deccan basalt in Kachchh, are investigated to interpret the parent rock, lateritization processes, and palaeogeographical position of the basin. The presence of partly weathered vesicular basalt at the base of the succession along with partly preserved plagioclase, augite, and hornblende in petrographical observations suggests the in situ nature of these bauxites. X‐ray diffraction analysis reveals gibbsite as the major bauxitic mineral associated with kaolinite, goethite, titanium‐rich anatase, calcite, haematite, and quartz as subconstituents. The presence of gibbsite, kaolinite, and goethite in the majority suggests that warm and humid climatic conditions prevailed during the lateritic bauxite formation. SEM‐EDS analysis shows that most of the gibbsites are mesocrystalline with particle sizes 50–200 μm that coexists with microcrystalline goethite, kaolinite, rutile, and haematite. These deposits are classified as bauxite, ferruginous bauxite, and laterite, based on chemical composition. The high CIA values (86.6–98.1%) suggest their formation under high run‐off and water‐logged conditions similar to that which prevails in the equatorial region. The Zr–Cr–Ga ternary plot testifies to the mafic composition of the parent rock. The major oxides and chondrite‐normalized trace and REEs behaviours, (La/Sm)_N ratio (~5.77), and Eu anomaly of these lateritic bauxites suggest that they (bauxites) were formed in situ by the intensive leaching and alteration of the basaltic parent rock, and the process of bauxitization followed the path of deferruginization and destruction of kaolinite. The development of a lateritic regolith over the Deccan basalt was possible during Palaeocene time when the Indian Plate was in complete isolation and the Kachchh region was positioned near the equator in the Southern Hemisphere.

To make a better understanding of the stratigraphic correlation and ocean processes during the transition from Series 2 to Series 3 in the Cambrian, this study investigated the characteristics and origin of the palaeokarst in the uppermost part of the Longwangmiao Formation (Toyonian) and established the carbon isotopic curves of the shallow‐water succession. Macroscopically, the palaeokarst features have been identified, including the exposed surface, karstic trough, discolouration, and breccia in the upper part of the Longwangmiao Formation. The exposed surface is divided into the bedrock, weakly weathering zone, intensely weathering zone, dissolved‐collapsed zone, and weathering clay layer at the Dingzhai section. The reddish aluminous oxide and the stratiform‐dissolved pore are recognized in the Gaodongmiao section. The clast‐support breccia and matrix‐support breccia develop in the palaeokarst zone. Microscopically, three dissolution‐filling zones in grainstone (the matrix zone, half‐dissociated zone, and mixed‐filling zone) dissolved fibrous cement in the early diagenetic stage, microspar cement in a bright homogeneous luminescence, and medium‐crystal calcite showing a zonation luminescence are identified indicating the facies‐controlled attributes. The δ¹³C values are characterized by a lower range (4.4‰ to −2.1‰) in the karst zone. The REE pattern shows enriched middle REE, negative Eu anomalies, and low Y/Ho ratio, which is different from the nature of seawater. It is proposed that the palaeokarst in the upper part of the Longwangmiao Formation is the eogenetic karst controlled by high‐frequency sea‐level fluctuations. Two negative δ¹³C excursions (values −4.4‰ in D2 and −3.6‰ in D1) are respectively recognized, the top boundary and middle part of the Longwangmiao Formation. Based on the regional and global correlations, the excursion of D2 is the diagenetic signal in the shallow‐water setting, which correlated to ROECE event (Redlichiid‐Olenellid Extinction Carbon isotope Excursion) that occurred around the boundary between Series 2 and Series 3 in the Cambrian. The local signal of D2 is attributed to the sea‐level falling.

Penecontemporaneous dissolution has been considered as the dominant origin of reservoir formation, whereas epigenetic dissolution has also played certain roles in the improvement of reservoir capacity in the Upper Cretaceous Mishrif Formation in the Middle East. Here, we report a possible new reservoir origin based on a case study in the Halfaya oilfield of Iraq, that is, facies‐controlled eogenetic karstification. The most representative evidence is that early selective dissolution is not only present in relative high‐energy categories of rocks but also in low‐energy ones in the cored intervals. Meanwhile, the occurrence frequency of karstification varies among different environments and associated lithologies. In grainstones and packstones, spongy‐like dissolution pores and irregular karst channels are widely developed, with fillings and massive plastic breccias. In contrast, in wackestones, lots of high‐angle karst channels are developed based on biological burrows, and the karst is more frequently observed in the shoal environment than in the low‐energy environments. These characteristics are indication of eogenetic karstification. The model of this karstification can be attributed to a multiple superimposition of short‐term exposure during the penecontemporaneous stage and the medium‐term exposure after the shallow burial stage. The karst has impact on reservoir capacity, according to which 3 areas are divided in the increasing order, namely, the tight bedrock area, spongy‐like dissolution pores area, and karst channels and vugs‐filled area, among which the karst channels and vugs‐filled area is the most favourable for oil accumulation. This understanding might be general to the Mishrif reservoirs in the entire Middle East.