Geological Journal最新文献

This response addresses the comments made by Saraswati et al. on our recent publication in Geological Journal. In our study, we used a multi-analytical screening approach, including visual, XRD and trace element analysis, to evaluate diagenetic alteration of carbonate shell samples. Contrary to the claims made by Saraswati et al. our findings are based on well-preserved samples with minimal alteration. The Sr-isotope data from the Khari Nadi Formation (23.07–18.09 Ma) are largely consistent with existing biostratigraphic constraints, but suggest a possible extension of the upper age limit into the Burdigalian. Our Sr-isotope ages for the Chhasra Formation (15.11–12.29 Ma) are younger than those suggested previously, based on larger benthic foraminifers, but largely corroborate recent nannoplankton data. As a result of the new age data from Kutch, we suggest that the sedimentation rates, derived from multiple samples, increased significantly from the Oligocene to Miocene. We assert that ‘established’ ages are also subject to refinements as the rock units become amenable to new dating techniques. We welcome any criticism that is constructive, and remain open to any further data that could refine or challenge our interpretations.

Troctolite is relatively rare compared with other ultramafic–mafic rocks, but its origins and rheological deformation are significant for understanding melt–rock interactions and fractional crystallization of mafic magmas. The Keketoukeleke ultramafic–mafic complex in the South Altyn Tagh, northwest China, consists mainly of dunite, coarse- and fine-grained troctolite. Based on petrographic observations, major and trace element variations of the dunites and troctolites, and low olivine Zr/Y and Ti/Y ratios, we propose that the troctolites were formed by fractional crystallization and late localized water-bearing melt injection. In addition, the olivine in the dunites has A- and E-type fabrics, whereas the olivine in the troctolites has a weak fabric and plagioclase has a pronounced fabric with the [010] axes aligned subnormal to the foliation and [100] axes subparallel to the lineation. The results suggest that the troctolite rheological deformation was concentrated mainly in plagioclase, and the olivine only underwent grain rotation or grain boundary slip. Furthermore, the dating of baddeleyite in troctolite suggests they crystallized at 378.6 ± 2.3 Ma, suggesting the Keketoukeleke ultramafic–mafic complex emplaced the continental crust extension after deep subduction–exhumation process of South Altyn.

The southern margin of the Junggar Basin (SMJB) represents the typical intra-continental basin–orogen coupling structure of the Central Asian Orogenic Belt and is a key area to study the deformation mechanisms and the geodynamic evolution processes of the North Tianshan Orogen. Herein, we compiled data from boreholes, gravity and magnetism, seismicity- and magnetotellurics-derived geological profiles and field data to recover the sedimentary history of the SMJB and discuss the intra-continental deformation driven by the basin–orogen coupling mechanism. The results show that the sedimentary and tectonic evolution of the SMJB were both profoundly controlled by the intra-continental orogeny of the North Tianshan Orogen and its coupling with the Junggar Basin during the Meso-Cenozoic period. Consequently, the SMJB is dominated by thick-skinned thrust nappes accompanied with strike-slip faulting and thrusting. The foreland thrust belt of the SMJB is characterised by three structural belts, from south to north, including the basement-involved thrust belt, the cover-detached foreland thrust-fold belt and the thrusted foreland basement uplifts, respectively. Meanwhile, as indicated by the geometry of the basement thrusts, the thickness of décollements and the structure of the foreland basement, the stress field in the SMJB is obviously stronger in the west and weaker in the east. The sedimentation and deformation migrated northwards into the basin area in a stepwise process, that corresponded to the pace of the overthrusting of the North Tianshan Orogen onto the Junggar Block. Intense regional compression induced the rapid uplifting of the North Tianshan and the significant crustal shortening of the Junggar Block, driving the three structural belts to form accordingly during the Late Jurassic to the Neogene. There are at least one or two décollements within the SMJB, representing one of the main features of basin–orogen coupling structure in most cases. The décollement of some layers represents the decoupling of the sedimentary cover with the basement, which helps to accommodate the lateral crustal shortening of the SMJB by a translation into vertical uplift. As a result, the detached foreland thrust-fold belts in the shallow level of the upper crust overthrusted upon the basement-involved nappes of the mountain's side, forming the opposite thrust system. Coevally, the basement of the basin, in the deep level of the upper crust keeps underthrusting beneath the North Tianshan Orogen, forming a typical crocodile mouth-like structure. In general, both the shallow and deep deformation in the SMJB have been formed by the intense intra-continental compression during the Meso-Cenozoic, which were driven by the basin–orogen coupling mechanism.

This is a comment on the paper of Singh et al. (Geological Journal, 2024:1–20) on Strontium isotope stratigraphy of marine Oligocene–Miocene sedimentary successions of Kutch Basin, western India. Kutch hosts stratotypes of some lithostratigraphic and chronostratigraphic units of India. In this study, the Sr-isotope estimated ages of the studied formations deviate significantly from their known ages based on biostratigraphy. The authors have not validated the interpreted ages with biostratigraphy. We believe a scrupulous screening of samples and validation of Sr-isotope data with biostratigraphy are two essential requirements of Sr-isotope stratigraphy. Unfortunately, Singh et al. made a new contribution to Kutch stratigraphy that falls short of meeting both conditions, leading to incorrect ages of the regional chronostratigraphic units of India.

This research focuses on the field observations, petrography, mineral chemistry and geochemistry of the serpentinised peridotite of Al-Barramiya ophiolitic sequence to place constraints on their magmatic history and their geodynamic evolution. Al-Barramiya ophiolitic rocks are a dismembered ophiolite which was strongly deformed and metamorphosed under greenschist to lower amphibolite facies. They comprise a mantle section dominated by highly serpentinised peridotite with less metapyroxenite and chromitite, as well as a crustal portion represented by metagabbros. Along shear zones, the ophiolite sequence was affected by several types of alteration. Extensive carbonate alteration is common in the ultramafic section, resulted in talc carbonates, listvenites and magnesite, while rodingitisation is common in the metagabbro resulted in rodingite. Despite the extensive serpentinisation, some fresh relics of primary mantle minerals such as Cr-spinel, olivine and pyroxenes are preserved sporadically in the serpentinised peridotite. Few Cr-spinel crystals are sometimes surrounded by subhedral flakes of Cr-chlorite (kämmererite) that was formed due to replacement of Cr-spinel during later alteration or regional metamorphism. The serpentinite samples are depleted in the total REE (0.56–1.19 ppm) with slightly negative to slightly positive Eu anomalies (0.89–1.28). The fresh cores of Cr-spinel have Cr# mostly > 60, and the relics of pyroxenes and olivine are Mg-rich suggesting that the Al-Barramiya serpentinites are residual to high degrees of melt extraction. The estimated degrees of partial melting range between 18.2% and 20.7%. All the mineralogical and geochemical characteristics of the ultramafic section of the Al-Barramiya ophiolites are most consistent with its formation in a fore-arc setting.

The Permian shales in the Kaijiang-Liangping Trough within the Sichuan Basin represent a promising frontier for marine shale gas exploration, whereas there has been limited systematic research on their sedimentary environment and organic matter (OM) enrichment mechanisms. Therefore, we present total organic carbon (TOC) analysis, major/trace element analyses and scanning electron microscope experiments for the Permian marine shales from the trough to determine their paleoenvironmental conditions and influencing factors of OM enrichment. The results show that the paleoclimate changed from dry climate to humid and warm climate (P₃w¹ [the first member of the Wujiaping Formation]) and semi-humid to semi-arid climate (P₃w² [the second member of the Wujiaping Formation]) and P₃d-I (Dalong Formation-I) and then to arid climate again during the shale deposition period from the P₂g (Gufeng Formation) to the P₃d-II. The shales with the highest TOC contents (TOC > 3%, P₃d-I and P₂g), lower TOC contents (TOC < 1%, P₃w¹ and P₃w²) and higher TOC contents (1% < TOC < 2%, P₃d-II) were formed under the control of anoxic environment and high paleoproductivity, oxic-suboxic environment and high paleoproductivity, anoxic-euxinic environment and lower productivity, respectively. Only appropriate sedimentation rates promote OM enrichment. Terrestrial input, paleoclimate, volcanic activity and hydrothermal upwelling mainly indirectly affect OM accumulation by influencing paleoproductivity. The degree of redox conditions is the primary factor affecting OM enrichment, followed by paleoproductivity. Nonetheless, anoxic to euxinic environments are most appropriate for OM preservation. Weak volcanic activity can boost paleoproductivity, but severe volcanic activity might introduce excessive harmful compounds that limit organism survival, resulting in a fall in paleoproductivity. Additionally, element P brought by volcanic ashes doesn't contribute to OM accumulation.