Geosystems and Geoenvironment最新文献

The Babouri-Figuil and Mayo Oulo-Lere basins located in Northern Cameroon are small elongated sedimentary asymmetric half-grabens associated with the development of the West and Central African Rift System (WCARS). The evolution of these basins began during the Precambrian pre-rift period and continued through the Late Jurassic - Early Cretaceous periods, although details of their development remain unclear. The aim of this study is to understand the evolution and development of the border faults and their control on syn-rift geometry, accommodation space, and the resulting stratigraphic architecture. Using a combination of multiples surface and subsurface well logs data, we demonstrate the relationship and evolution of various stages of rifting and the associated sedimentary architecture. The basin is composed of an alluvial to lacustrine succession that developed during its tectonic evolution. A 3-stage tectonic model is proposed for the evolution of the basin that uses fault propagation analysis to describe the sedimentary architecture and evolution of basin geometry. The three stages of the model are the early rift, rift climax, and late rift; here, the early rifting phase is marked by the reactivation of faults in the Lower Cretaceous. The rift climax phase resulted in the interconnection of small segmented border faults leading to the high rate of activation of major border faults and to the development of the half graben structure of the basins. The late rift stage is marked by a decrease in border fault activity and accommodation. Sediment distribution and the facies architecture are controlled by tectonic subsidence created throughout the rifting phases. The stratigraphic signature of the Babouri-Figuil and Mayo Oulo-Lere basins are punctuated by the lacustrine water transgression-regression cycles, and controlled by the balance between the rate of sediment supply and tectonic accommodation. The progressive change between overfilled to starved sedimentary architecture is illustrated by this stratigraphic succession. Additional age assessment and sub-surface data in future work could help confirm the structures identified on the surface.

The growing need of rare earth elements in different sectors of the modern-day world has led to their increasing demand, which is in accordance with their limited supply. In such a scenario, investigations related to search for potential zones of rare earths have become the need of the hour. In one such investigation, undertaken by Geological Survey of India, the authors report profuse intrusion of the apatite veins into the syenite-pyroxenite suite in the eastern part of the Eastern Ghats Mobile Belt (EGMB) in Nayagarh area of eastern Odisha. These apatite veins are anomalously rich in REEs, particularly LREEs, which on average ranges between 37,000 to 53,000 ppm. Rare earths and apatite are generally directly proportional, where apatites tend to carry sufficient REE budget, depending on the phosphorous fugacity and other ambient conditions inducing their genesis. The major-oxide chemistry of apatite veins indicates that apatites in the study area can be divided into two types based on SiO₂ wt.% and their inverse relationship with P₂O₅ and CaO wt.%. Mineral chemistry data shows overall SiO₂ depletion, with dominance of CaO, followed by P₂O₅. Apatite veins, as per mineral chemistry, contain numerous veinlets (identified microscopically), where one type is REE-bearing and another type is REE-free. The REE-bearing phases are rich in Ce, Nd, La and Pr and are represented by allanite, monazite, and bastnaesite. Thorite and zircon are prevalent. Earlier instances of apatite intrusions into granulite have been reported from parts of south India. On the contrary, in this area, the intrusion is typically restricted to the younger intrusive units of the EGMB, represented by syenite-pyroxenite suite. The intrusives also show moderately high values of REEs. In this paper, an attempt has been made to characterize the apatite veins and the syenite-pyroxenite suite, they have intruded, in respect to their mineralogy and tectonics and to elucidate the potentiality of the area in terms of future REE exploration.

Accessible silicate reservoirs on Earth (i.e., the depleted mantle and continental crust) are depleted in Nb relative to chondrites, as expressed by their subchondritic Nb/Ta ratios. Mixtures of continental crust and depleted mantle cannot produce chondritic Nb/Ta ratios; therefore, another geochemical reservoir with a superchondritic Nb/Ta ratio is necessary to balance the low Nb contents of the continental crust. We investigated alkaline igneous rocks with superchondritic Nb/Ta ratios (20.4–24.8). Their high Nb/Ta ratios were inherited from their lithospheric mantle source rather than being produced by magmatic differentiation. Geochemical data show that an enriched mantle domain with superchondritic Nb/Ta ratios can be produced by the interaction of subducted carbonate-rich sediment-derived melts with the lithospheric mantle, and the enriched lithospheric mantle is a potential superchondritic reservoir.

The Bundelkhand Craton (BuC) represents a significant Archean terrane within the northern Indian Craton and yet its tectono-metamorphic evolutionary history remains relatively understudied. Our investigation involves detailed petrography, geochemistry, and bulk composition modeling of both garnet-bearing and garnet-absent amphibolites with a two-fold objective: (i) to constrain the protolithic nature and tectonic settings involved in the genesis of these rocks, and (ii) to propose a tectono-metamorphic evolutionary history for the BuC. The BuC amphibolites originate from basalt and andesitic-basalt protoliths. Their trace element compositions reveal negative anomalies in Nb, Ta, and Ti, while their rare earth element (REE) normalized patterns indicate enrichment in light REEs over heavy REEs. The basaltic protolith is interpreted to have formed during orogeny in a compressional tectonic regime at the active margins of island arcs in a subduction-related setting. This interpretation is supported by various discrimination plots for amphibolites, such as Nb/Th vs Zr/Nb, Zr vs Zr/Y, and Th/Nb vs Ce/Nb, as well as high Th/Yb and low Nb/Yb contents— all of which suggest an island arc setting influenced by subduction. These amphibolites have undergone three distinct phases of metamorphism, as evidenced by petrography, mineral chemistry, and bulk composition modeling. This interpretation is further supported by geochemical discrimination diagrams which indicate that a subduction tectonic setting was active during peak metamorphism. During the pre-peak phase, the garnet-bearing amphibolites experienced pressure and temperature conditions ranging from 6.25 to 6.5 kbar and 580 to 590ºC, while the garnet-absent amphibolites underwent conditions from 5.0 to 5.8 kbar and 400 to 450ºC. Peak metamorphism was observed at pressures ranging from 6.8 to 7.4 kbar and temperatures from 760 to 805ºC for the garnet-bearing amphibolites, and at pressures from 7.0 to 7.4 kbar and temperatures from 785 to 810ºC for the garnet-absent amphibolites. The metamorphic retrograde conditions for the garnet-bearing amphibolites are defined by P-T conditions ranging from 4.45 to 4.75 kbar and 585 to 615ºC, while for the garnet-absent amphibolites, it ranges from 3.1 to 4.0 kbar and 620 to 710ºC. The mineral assemblages and P-T conditions delineate a clockwise P-T path for both, garnet-bearing and garnet-absent amphibolites from the Babina and Mauranipur regions. This suggests that the rocks underwent a burial process amid subduction tectonic settings in an arc-related environment, followed by a decompression stage that brought them to the surface.

The question of which specific tectonic regimes played an essential role in shaping the Neoarchean evolution of the North China Craton (NCC) has been a contentious and controversial topic. Mafic rocks, containing valuable geochemical information from the mantle and deep crust, serve as an important source of data to provide essential constraints on above issue. This study presents bulk-rock geochemistry, zircon U-Pb geochronology and Hf-Nd isotopes for the Neoarchean mafic rocks in the core area of the Longgang Block of the NCC. Petrographically, these mafic rocks are composed of the amphibolite and metadiabase. Zircon U-Pb dating results revealed that they were synchronously emplaced at ca. 2.5 Ga. Geochemically, the amphibolites belong to subalkaline tholeiite series, exhibiting enrichment in light rare earth elements (LREEs), intensely negative Nb, Ta and Ti anomalies, and slightly enriched Nd and relatively variable zircon Hf isotopic compositions. These geochemical and isotopic features show large affinity to arc-like magmatism, implying that they were derived from subduction-related metasomatized lithospheric mantle. In contrast, the metadiabases exhibit high-Ti alkaline basalt affinities, OIB-like REE, and trace element patterns. They also show clearly positive Nb, Ta and Ti anomalies, indicating that they originated from a mantle plume-related tectonic environment. The new geochemical and isotopic data reveal that these Neoarchean mafic rocks could have resulted from coexisting mantle plume and subduction processes. Combined with available regional structural, geochemical, and metamorphic data, it is likely that during the Neoarchean, both a mantle plume and subduction jointly controlled the crustal growth and tectonic evolution of the Longgang Block in the eastern NCC.

Rainfall is a significant triggering factor for landslides, after tectonics and structurally vulnerable lithology— particularly in Himalayan Range. Globally, extensive efforts have been undertaken to determine the specific rainfall threshold conditions that lead to the initiation of a slide on a regional scale. Rainfall-induced landslides disrupt life and cause extensive damage to properties in the Himalayan region. This study has a two-fold objective; to determine the relationship between the occurrence of landslides and a significant triggering factor, namely rainfall, based on intensity-duration (I-D) and antecedent rainfall methods, and also to determine the best fit distribution for rainfall data based on goodness of fit tests within the four western-most districts of the Garhwal Himalaya, in Dehradun, Rudraprayag, Tehri Garhwal, and Uttarkashi. The rainfall patterns of these four districts conform to the log-logistic (3P) distribution, and the rainfall threshold has been fitted over a power-law equation with the lower boundary demarcated by a quantile regression that is presented as a threshold with an established relationship of $y\left(I\right)=1.38D^{-0.126}$ (I=rainfall intensity, D=duration). The results suggest that a rainfall intensity of 0.45-0.50 mm per hour over short durations (48 h) has the potential to trigger landslides in this region. Antecedent rainfall of around 80 mm in the 15 days prior to a landslide event significantly raises the landslide risk. Further, lithologies like mud and sandstones are highly susceptible to landslides and can be triggered by rainfall of 10–20 mm occurring, consecutively, over a 5-day period.

This paper reviewed the application of airborne geophysical and remote sensing datasets in the mapping of orogenic gold deposits in different geologic settings around the world by examining more than forty publications in peer-reviewed journals. The paper indicates the role of aeromagnetic, aeroradiometric datasets (airborne geophysical) and LandSat and ASTER datasets (remote sensing) in mapping epithermal orogenic gold deposits. The paper further highlighted the importance of understanding the geologic settings of epithermal gold mineralization in terms of the mineral system before mineral mapping can be done. Case studies drawn from fourteen (14) publications were presented to show the successful mapping of epithermal gold deposits using airborne geophysical and remote sensing datasets. However, the challenges of the methods of mapping as presented in the paper indicated the limitations of the methods in terms of ambiguity in interpretation, especially when a single method is used. Also, the cost of data acquisition and the inability of the exploration methods to estimate the tonnage and grade of the epithermal gold deposits pose a limitation to the use of airborne geophysical and remote sensing datasets in epithermal gold mapping. The paper was able to justify the use of the methods solely for mapping, which essentially is to focus exploration on certain areas, thereby, saving time and money. Further analyses on tonnage estimation can be done by wildcat drilling and geochemical analysis in mapped areas obtained from airborne geophysical and remote sensing datasets. In addition, the paper presented new technologies that are less expensive than conventional airborne geophysical methods that are capable of probing deep into the subsurface with higher resolution and the use of integrated techniques of airborne geophysical and remote sensing datasets to cater for the ambiguity associated with exploration data interpretation. The paper finally highlights ongoing research in the mapping of epithermal gold deposits in the Ife–Ilesa schist belt involving the development of machine learning algorithms to process voluminous datasets to produce a reliable predictive mineral potential map.

Precambrian sedimentary successions exposed worldwide record microbial deposits within both marine and continental strata. The Camaquã Basin records the transition from the Ediacaran to Cambrian periods in strata that comprise marine to lacustrine environments. The Camaquã Basin fossils comprise microbial mats and organic-walled microfossils in very close association. Carbonaceous laminae are common within the siltstone and mudstone layers and represent remarkably well-preserved Ediacaran microbial mats. These mats occurred along shallow marine through deep and shallow lake settings. The excellent preservation of these structures seems to be the product of calm hydrodynamic conditions and floodings. These hydrodynamic undisturbed environments allowed microbial mat growth. Besides, episodic floodings brought clay particles, which deposits guaranteed isolation from organic decomposition of mats. In general, the siltstones and mudstones record microbially induced sedimentary structures (MISS) that are typical from Ediacaran strata, such as wrinkle marks and elephant skin structures. Our research revealed that the occurrence of these structures is not limited to marine settings, as common in records from this period, but also present in lacustrine environments. Field studies were combined with optical and scanning electron microscopy plus energy-dispersive X-ray spectroscopy (SEM-EDS) analyses to demonstrate the vast occurrence of microbial mats along the deposits that registered the Camaquã Basin development. Geochemical data (whole-rock and total organic carbon (TOC) analyses) contribute to improving sedimentological descriptions, bringing light regarding the depositional environment where these microbial communities have developed. The excellent preservation state of organic matter on these marine and lacustrine siliciclastic sequences represents an impressive record in a Proto-Gondwana basin.

Conventional sequence stratigraphy is based, explicitly or implicitly, on the hypothesis that steady external forcing results in a steady stratigraphic configuration (equilibrium response), so that an unsteady stratigraphic configuration is usually believed to result from unsteady external forcing. Recent advances in autostratigraphy, on the other hand, have led to a significantly different notion that steady external forcing generally results in an unsteady stratigraphic configuration (non-equilibrium response). To advance this debate, it is necessary to clarify what exactly is meant by a steady stratigraphic configuration. Here, we propose a quantitative criterion for defining the latter concept in terms of the straightness of the shoreline trajectory, and specifically a straight shoreline trajectory or the shoreline being held still as a sign to express steady stratigraphic configurations. In such a definition, a steady stratigraphic configuration means that the ratio of the rate of aggradation and the rate of progradation is constant, or one of these two rates is zero. Based on this criterion, a total of 7 types of steady stratigraphic configurations can be clarified, most of which require unsteady external forcing and are thus realized by non-equilibrium response, although special cases exist. The reason that non-equilibrium responses dominate the stacking of strata is that it is common for a growing basin-margin depositional system to change its surface area. The size-changing system will easily change the stacking pattern (unsteady stratigraphic configuration) if the external forcing is steady, or, if the steady stratigraphic configuration is maintained, the rate of external forcing must change in a particular pattern (unsteady external forcing). Equilibrium responses can occur, but in very special cases. Conventional sequence stratigraphy should take into account the importance of non-equilibrium response.

A new biostratigraphic biozonation for the Paleocene to Late Oligocene succession of the onshore Niger Delta has been developed based on the identification of significant stratigraphic dinoflagellate cyst events. This study is aimed at applying rarely used dinoflagellate cysts events in dating, correlating and inferring the paleoenvironment of penetrated sedimentary successions in the studied area. Palynological processing and analysis were carried out on two hundred and twenty ditch cutting samples recovered from two exploration wells (BN-1 and FJ-1) located in the Greater Ughelli and Northern Depobelts of the onshore Niger Delta respectively. Abundant and diverse dinoflagellate cysts recovered included marker Peridinales species Apectodinium sp., Palaeocystodinium australinum, and Gonyaulacales such as Areosphaeridium arcuatum, Cordosphaeridium gracile, Tuberculodinium vancampoae, Polysphaeridium zoharyi and Hystrichokolpoma rigaudiae. Three dinoflagellate cysts Palynological Zones - DPZ 100 (Early to Late Paleocene), DPZ 200 (Early to Late Eocene) and DPZ 300 (Late Eocene to Late Oligocene) are proposed herein. The biozones were subdivided into ten subzones based on the Top occurrence, Quantitative Tops and Quantitative base as well as ACME events of dinoflagellate cysts marker taxa which are used to date the rock sequence to be Paleocene to Late Oligocene. Abundant and diverse shallow outer neritic to inner neritic dinoflagellate cysts are identified from thick grey soft shales at the lower section of the studied wells. The upper section show significant reduction in dinocysts assemblage dominated by inner (coastal) neritic taxa. This dinocysts occurrence and distribution suggests inner (coastal) neritic paleoenvironment at the upper sections which deepened into shallow outer neritic conditions at the lower intervals of the studied wells. The thick brown to grey shales at the lower section and the medium to fine grained, moderately well sorted sandstone interbeds at the middle section of the studied wells are interpreted to be potential source and reservoir rocks respectively. These datasets are useful for correlating possible hydrocarbon source rocks and potential reservoir carrier beds in the Gulf of Guinea.