Rapid Subsidence Research Articles

Initiation and early evolution of a forearc basin: Georgia Basin, Canada

AbstractThe lower Nanaimo Group was deposited in the (forearc) Georgia Basin, Canada and records the basin's initiation and early depositional evolution. Nanaimo Group strata are subdivided into 11 lithostratigraphic units, which are identified based on lithology, paleontology, texture and position relative to both the basal nonconformity and to each other. Significant topography on the basal nonconformity, however, has resulted in assignment of lithostratigraphic units that are not time correlative, and hence, cannot reliably be used to accurately reconstruct basin evolution. Herein, we present a sequence stratigraphic framework for lower Nanaimo Group strata in the Comox Sub‐Basin (northern Georgia Basin) that integrates both facies analysis and maximum depositional ages (MDAs) derived from detrital zircon. This stratigraphic framework is used to define significant sub‐basin‐wide surfaces that bound depositional units and record the evolution of the basin during its early stages of development. Seven distinct depositional phases are identified in the lower 700 m of the lower Nanaimo Group. Depositional phases are separated by marine flooding surfaces, regressive surfaces, or disconformities. The overall stratigraphy reflects net transgression manifested as an upwards transition from braided fluvial conglomerates to marine mudstones. Transgression was interrupted by periods of shoreline progradation, and both facies analysis and MDAs reveal a disconformity in the lowermost part of the Nanaimo Group in the Comox Sub‐Basin. Stratigraphic reconstruction of the Comox Sub‐Basin reveals two dominant depocenters (along depositional strike) for coarse clastics (sandstones and conglomerates) during early development of the Georgia Basin. The development and position of these depocenters is attributed to subduction/tectonism driving both subsidence in the north‐northwest and uplift in the central Comox Sub‐Basin. Our work confirms that in its earliest stages of development, the Georgia Basin evolved from an underfilled, ridged forearc basin that experienced slow and stepwise drowning to a shoal‐water ridged forearc basin that experienced rapid subsidence. We also propose that the Georgia Basin is a reasonable analogue for ridged forearc basins globally, as many ridged forearcs record similar depositional histories during their early evolution.

Regional 1D hydrocarbon maturation modelling of the Cenomanian–Turonian Lokpanta Shale, southern Benue Trough, Nigeria: Implications for the origin of Niger Delta deep sea oils

Basin modelling, constrained by geochemical data from eight wells, was carried out, across the late Cretaceous and Early Palaeogene sediment packages of south-eastern Nigeria sedimentary basins. The study was aimed at establishing the sediment burial history, thermal maturation of the source rock and timing of hydrocarbon generation. The Late Cenomanian to Early Turonian Lokpanta Shale, which is the basal unit of the Eze-Aku Formation in the Eze-Aku Group, consists of alternating dark grey to black shales, marl and siltstones. The upper sections are mainly alternating sandstones, shales and limestones. This unit is the key petroleum source rock for the basin. Numerical 1D basin model of the study area revealed that the Cenomanian to Turonian times was the main phase of rifting in the Benue Trough evidenced by rapid subsidence. Subsidence rates varied widely in all the wells studied, ranging from 29–61 m/Mya (million years ago) and averaging 44 m/Mya. Subsidence rates also varied widely through geological time from the Cretaceous to the Palaeogene. The Cenomanian–Turonian and the Maastrichtian ages recorded the highest subsidence rates (169.75 and 168.28 m/Mya, respectively). These phases of rapid subsidence correspond to the main phase of rifting (Cenomanian–Turonian) and periods of increased sediment supply (Campanian and Maastrichtian), due to rapid erosion and unroofing of the structurally inverted Benue Trough, post-Santonian. Vitrinite reflectance values (1.87–4.78% Ro) indicated that the Lokpanta source rock (Late Cenomanian–Early Turonian) is mature to overmature. The vitrinite maturation profiles and the geochemical data suggested the generation of hydrocarbon before the Santonian compressional uplift of the Abakaliki–Benue Trough with its resultant sediment folding, which displaced the depo-centre from the Abakaliki Basin to the Anambra and Afikpo platforms. The subsequent erosion and non-deposition in the Abakaliki Basin raised the Lokpanta Shale above the oil generative window. The renewed sedimentation in the Campanian resulted in sagging due to sediment load creating the Anambra Basin. The lack of an effective trapping mechanism in the pre-Santonian may imply that hydrocarbon generated before the uplift migrated away, probably to upper and/or lower horizons (observed as oil shows in the region); some of which may be contributing to the Niger Delta crude. This is evidenced by the correlation of the Niger Delta deep sea samples to the Cretaceous (Lokpanta Shale) source rock and occurrence of biomarkers of Cretaceous origin (ab-hopanes and oleananes in the Opuama Channel complex, northern depobelt, Niger Delta).

Holocene Sedimentary Record and Coastal Evolution in the Makran Subduction Zone (Iran)

The western Makran coast displays evidence of surface uplift since at least the Late Pleistocene, but it remains uncertain whether this displacement is accommodated by creep on the subduction interface, or in a series of large earthquakes. Here, we address this problem by looking at the short-term (Holocene) history of continental vertical displacements recorded in the geomorphology and sedimentary succession of the Makran beaches. In the region of Chabahar (Southern Iran), we study two bay-beaches through the description, measurement and dating of 13 sedimentary sections with a combination of radiocarbon and Optically Stimulated Luminescence (OSL) dating. Our results show that lagoonal settings dominate the early Holocene of both studied beach sections. A flooding surface associated with the Holocene maximum transgression is followed by a prograding sequence of tidal and beach deposits. Coastal progradation is evidenced in Pozm Bay, where we observe a rapid buildup of the beach ridge succession (3.5 m/years lateral propagation over the last 1950 years). Dating of Beris Beach revealed high rates of uplift, comparable to the rates obtained from the nearby Late Pleistocene marine terraces. A 3150-year-old flooding surface within the sedimentary succession of Chabahar Bay was possibly caused by rapid subsidence during an earthquake. If true, this might indicate that the Western Makran does produce large earthquakes, similar to those that have occurred further east in the Pakistani Makran.

Convergence of the Pamir and the South Tian Shan in the late Cenozoic: Insights from provenance analysis in the Wuheshalu section at the convergence area

AbstractIn response to collision and convergence between India and Asia during the Cenozoic, convergence took place between the Pamir and South Tian Shan. Here we present new detrital zircon U-Pb ages coupled with conglomerate clast counting and sedimentary data from the late Cenozoic Wuheshalu section in the convergence zone, to shed light on the convergence process of the Pamir and South Tian Shan. Large Triassic zircon U-Pb age populations in all seven samples suggest that Triassic igneous rocks from the North Pamir were the major source area for the late Cenozoic Wuheshalu section. In the Miocene, large populations of the North Pamir component supports rapid exhumation in the North Pamir and suggest that topography already existed there since the early Miocene. Exhumation of the South Tian Shan was relatively less important in the Miocene and its detritus could only reach a limited area in the foreland area. Gradually increasing sediment loading and convergence of the Pamir and South Tian Shan caused rapid subsidence in the convergence area. Since ca. 6–5.3 Ma, the combination of a major North Pamir component and a minor South Tian Shan component at the Wuheshalu section is consistent with active deformation of the South Tian Shan and the North Pamir. During deposition of the upper Atushi Formation, a larger proportion of North Pamir–derived sediments was deposited in the Wuheshalu section, maybe because faulting and northward propagation of the North Pamir caused northward displacement of the depocenter to north of the Wuheshalu section.

Sedimentary Evolution and Transgressions of the Western Subei Basin in Eastern China since the Late Pliocene

AbstractSedimentary sequences in the Subei Basin are complex and have been affected by interactions between the ocean and rivers since the Late Pliocene, including the Yellow River, Huaihe River, and the Yangtze River. This sedimentary evolution, in particular the timing of Pleistocene transgressions, has long been a matter of controversy owing to the lack of precise chronological evidence. The aim of this study is to explore the evolution of the sedimentary environment throughout the past 3.00 Ma in this region on the basis of a comprehensive analysis of particle size and foraminifera and ostracods collected in the TZK9 core from the Subei Basin combined with geochronological studies of magnetostratigraphy, AMS14C and optically stimulated luminescence (OSL). The results show that fluvial facies in the sedimentary environment from 3.00 to 1.01 Ma. There were fluvial facies and reflects six sea‐level high stands from 1.01 to 0.25 Ma. The study area was affected by four large‐scale transgressions since 0.25 Ma. The four marine sedimentary layers known as DU7 (buried at 48–52 m), DU5 (buried at 35–41 m), DU3 (buried at 16–23 m), and DU1 (buried at 2–4 m) are recorded in the MIS7 (210–250 ka), MIS5, MIS3, and Holocene, respectively. The magnitude of the DU5 transgression was identical to that of the DU3 transgression, both were larger than the DU7 transgression, and the DU1 transgression was the weakest. The variation of transgression strength reflects the influence of global changes in sea level, tectonic subsidence, shell ridges, and sand dams. In the TZK9 core, we found evidence of seven sea‐level high stands from the Early–Middle Pleistocene, and the first one caused by regional rapid subsidence and could be traced back to 0.83‐0.84 Ma. The sea‐level high stands and the age of the first one recorded above was different from other cores in eastern China, this was caused by the lack of absolute age control and the differences in paleotopography during this period. This study reconstructs sedimentary evolution, determines the transgression and its age, establishes the chronology since the Late Pliocene, and provides a scientific framework for further paleoenvironmental and tectonic studies. The results of this study highlight the important role that local tectonics and global sea level play in the sedimentary evolution and transgressions that have occurred in the western Subei Basin.

Basin Evolution and Massive Sulfide Deposition at Rammelsberg (Germany): Updating the Subsidence Analysis

The Rammelsberg sulfide deposit is classically considered as a SEDEX-type deposit. The origin of SEDEX-type massive sulfides links with the evolution of their hosting basins. They frequently constitute the source for the metal-enriched basinal brines transported afterwards as mineralizing hydrothermal fluids. This study revisits previous data concerning the analysis of the basin that hosts the Rammelsberg deposit, the Goslar basin, updating its subsidence analysis and providing new tectonic and total subsidence curves from two different paleogeographic locations: the depocenter and the basin margin. The basin evolution is defined by five stages depicting different subsidence intensity and mechanisms for each of these locations. The stratigraphic position of Rammelsberg coincides with a drastic change in the basin evolution. A rapid tectonic subsidence event is proposed as a trigger mechanism for hydrothermal activity. The paleogeographic location and the relation between supply of mineralizing fluids and sedimentation rate were critical for the concentration or dissemination of sulfides.

Geological setting and Ediacaran–Palaeozoic evolution of the western slope of the East European Craton and adjacent regions

A set of geological maps and geological cross-sections was prepared to document the geological setting of sedimentary basins developed on the western slope of the EEC and adjacent areas to the west. On the basis of these data and literature on the subject, the evolution of the sedimentary basins in the study area was reviewed, with special emphasis on the Ediacaran–Lower Palaeozoic basin. The basin originated during late Ediacaran rifting, related to the latest stages of breakup of the Precambrian super-continent Rodinia/Pannotia, associated with large-scale igneous activity. The rifting ultimately led to the formation of the Tornquist Ocean and subsequently, during the latest Ediacaran to Middle Ordovician, the SW margin of the newly formed Baltica became a passive continental margin. The upper Cambrian depocentre in the Bilgoraj-Narol Zone and the Łysogory Block tentatively is interpreted as a small, narrow foredeep, related to the docking of the Malopolska Block to the western margin of Baltica. From the Late Ordovician through the Silurian, a gradual change to a collisional tectonic setting is observed across the entire SW margin of Baltica, as well as in the zones adjacent to it from the west, which together became the site of development of the extensive Caledonian foredeep basin, related to the convergence and collision of Avalonia and Baltica. The oblique character of the collision resulted in a prominent diachronism in the development of the foredeep basin. This refers to the initiation of basin subsidence, the starved basin phase, the main phase of rapid subsidence and supply of detritus from the west, and the termination of basin development. The Early Mississippian (Bretonian) phase of uplift and erosion and, to a lesser degree, also the Late Pennsylvanian one significantly affected the structure of the western EEC. During the Mississippian, extensive magmatic activity took place at the SW margin of East European Craton, in the region referred to here as the Baltic-Lublin Igneous Province.

Environmental Impact Assessment of Deep Pit Project in floodplain near Yangtze

The construction of deep pit has a great impact on the surrounding environment, which is mainly reflected in the subsidence of surrounding soil. Because of the particularity in geological and hydrogeological conditions of riverside floodplain area, environmental impact assessment is needed. According to the monitoring data of practical engineering, the deep pit in the floodplain area near Yangtze River may cause a large subsidence of the surrounding soil with a rapid subsidence rate and a wide range of subsidence effects. The subsidence of the surrounding soil increases with the depth and time of excavation, and the post-construction subsidence effect is not obvious. The environmental effects of the deep foundation pit project in this project have provided valuable experience for guiding the implementation of the deep foundation pit near the floodplain in the future.

Zircon U-Pb ages and REE composition constraints on the provenance of the continental slope-parallel submarine fan, western Qiongdongnan Basin, northern margin of the South China Sea

Abstract The Ledong Submarine Fan in the western Qiongdongnan Basin runs parallel to the continental slope, with an area of over 10,000 km2 and maximum thickness of greater than 2000 m. Using bulk rare earth element (REE) compositions of the sediments and detrital zircon U-Pb age data, we analysed the provenance of the Ledong Submarine Fan and its regional tectonic implications. The zircon grains in the Ledong Submarine Fan show a wide range of shapes, from prismatic crystals to oval grains with mostly rounded corners, indicating the long transport distance of these zircon grains prior to deposition. The zircon ages have a wide range from 27 to 3006 Ma, with five major age peaks at ca. 27 Ma, 88 Ma, 256 Ma, 415 Ma and 785 Ma, along with a number of subordinate age peaks at ca. 1024 Ma, 1376 Ma, 1879 Ma and 2374 Ma. These age distribution patterns are similar to those from the Red River at Yen Bai. The total REE content range is between 87.72 and 210.38 mg/g in the Ledong Submarine Fan. The chondrite-normalized REE patterns show light REE (LREE) enrichment and weakly negative Eu anomalies, which are similar to those in the Red River at Hanoi. The evidence from zircon U-Pb ages and REE geochemistry indicates that the provenance of the Ledong Submarine Fan is the Red River System. The development of the Ledong Submarine Fan indicates that a rapid subsidence occurred in the western Qiongdongnan Basin during the late Miocene, and the strike slip reversal of the Red River Fault from sinistral to dextral motion was the key factor that induced gravity flow to form the Ledong Submarine Fan.

Evolution of sinkholes over Wink, Texas, observed by high-resolution optical and SAR imagery

Two caprock and cover collapse sinkholes, Wink Sinks #1 and #2, in Winkler County, Texas, collapsed in 1980 and 2002, respectively. They are located in a region underlain by the Salado Formation on the eastern edge of the Delaware Basin. The sinkhole collapses were associated with intense hydrocarbon drilling and production activities in the Hendrick oilfield. Production records and prior investigations imply the sinkholes developed after salt dissolution due to freshwater intrusion, cavity formation, roof failure, and successive upward cavity migration. High-resolution (1 m) aerial photography shows the ensuing expansion of Wink Sink #2 from 2002 through 2010 and relative stability of the ground surface after 2010. Very high-resolution (25 cm) staring spotlight mode TerraSAR-X reveals the spatiotemporal details of Wink sinkhole deformations. The vicinities around the two existing sinkholes (Wink Sinks #1, #2) deformed at a rate of 5 cm/yr in line-of-sight (LOS) direction from October 2015 to March 2016, and the deformation was possibly caused by the gradual deposit of the debris from overlying unconsolidated Alluvium and Santa Rosa sandstones infilling the cavity after the sinkhole collapse. Specifically, the largest deformation of 53 cm/yr in LOS direction was found 1 km east of Wink Sink #2. Our study suggests that severe drought in 2011 widened and created fractures as well as ground fissures in the rocks overlying and underlying the salt formation due to the increased effective stress. Fractures, ground fissures, and the abandoned unplugged boreholes or improperly completed wells opened one or more vertical pathways of freshwater from groundwater flow and precipitation toward the salt bed, finally causing the rapid salt dissolution and the opening of new cavities. To explain the onset of a rapid subsidence near the Wink sinkholes, our study considers the combined effect of natural perturbation (i.e., drought, rock fractures, ground fissures, subsurface drainage, precipitation) and anthropogenic effects (i.e., improper well control). Our method to explore high-resolution imagery can be applied to characterize the sinkhole deformation and mitigate sinkhole-related geohazards.

Potential and favorable areas of petroleum exploration of ultra-deep marine strata more than 8000 m deep in the Tarim Basin, Northwest China

At present, deep and ultra-deep oil and gas have become the research and exploration hotspots. With little drilling data available, the geological understanding on deep formations is still in the exploratory stage, but most petroleum geologists have universally acknowledged that the deeply-buried strata of Chinese marine craton basins are equipped with the conditions for abundant oil and gas resources. A series of discoveries have been made in the Ordovician and Cambrian strata. For Precambrian strata, besides breakthroughs in the Sichuan Basin, a lot of research or drilling work has been initiated in other basins. In the Tarim Basin, the current burial depth of the Lower Paleozoic and the Precambrian mainly ranges from 5000 to 12000 m. This is attributed to rapid subsidence and deposition since the Neogene. A large number of exploration discoveries have been made, and commercial productivity projects have been constructed at the depth less than 8000 m. Strata more than 8000 m deep will be a significant major prospecting exploration field in the future. The analyses of hydrocarbon accumulation conditions of the ultra-deep layers of the Tarim Basin provide theoretical guidance for ultra-deep oil and gas exploration. This research is based on source rocks in the Cambrian, Sinian, and Nanhua System, formation and preservation of ultra-deep carbonate reservoirs, phase of petroleum and accumulation assemblages in ultra-deep strata, the exploration fields deeper than 8000 m were also evaluated to sort out the most favorable areas and, future exploration focuses on the Ordovician, Cambrian, and Sinian strata are pointed out.

Source rock evaluation and thermal maturity modeling of Jake oil fields, Fula sub-basin, Muglad Basin, Sudan

Comprehensive source rock evaluation, burial history modeling, and thermal maturity of oil generation have been investigated in petroleum system of Jake oil fields. The consideration of geological, geochemical, and palynofacies data was used for source rock and thermal maturity. Basin Mod® software was applied to construct the burial and thermal histories and to determine the timing of hydrocarbon generation and expulsion. The modeling was achieved through transient heat flow and collaborated with measured %Ro. The Lower Cretaceous Abu Gabra Formation was believed to be organic-rich shale and characterized by source rock of excellent abundance and potential that possesses total organic carbon (TOC) ranging from 0.37 to 6.27 wt% and production yield 1.31–49.23 mg HC/g rock. Kerogens can be classified as oil-prone types I–II. Vitrinite reflectance (Ro) and Tmax values suggested that most samples were of early maturation for oil generation. Palynofacies and geochemical combination indicated that amorphous phytoclasts of plant debris were dominated by high TOC and hydrogen index (HI), whereas assemblage kerogens confirmed the presence of oil source rock. Geohistory models showed the highest rapid subsidence rates during the Early-Late Cretaceous. The results demonstrated that petroleum could be generated in the upper (1950–2500 m) and the lower intervals (2550 total depth) of Jake South field. Maturity and hydrocarbon generation modeling indicated that the source rocks reached the early maturation stage in the Early Cretaceous (127 Ma) at a temperature of 90 °C. The middle maturation stage began in the Late Cretaceous (64 Ma) with a temperature of 110 °C. Essential oil generation started approximately 63 Ma ago and continues to the present day.

Heterogeneity of free and occluded bitumen in a natural maturity sequence from Oligocene Lake Enspel

Sedimentation in Oligocene Lake Enspel was rapidly terminated by a basaltic lava flow. This introduced a preservational barrier while imparting a ‘natural flash pyrolysis’, during which the organic matter in underlying stratigraphic units was subjected to rapid thermal maturation resulting in hydrocarbon generation. Samples from these strata exhibit a steep maturity gradient (0.25–1.07% optical vitrinite reflectance, or RO) over uniform organofacies. This offers the opportunity to investigate bitumen generation during rapid thermal maturation mechanistically, in particular the nature of Bitumen 2—occluded bitumen, which is only recoverable after the digestion of the mineral matrix and was frequently dismissed as an artifact of incomplete extraction. Elaborate sequential extraction of the contact metamorphic sequence of oil shales at Enspel revealed systematic changes in bitumen composition. These trend progressively towards those of occluded bitumen, which exhibits a systematically elevated thermal maturity, a higher degree of catalytic biomarker-rearrangement and the conspicuous absence of molecular signatures from vascular plants that are present in the free bitumen. One plausible explanation involves a contribution of allochthonous clay-adsorbed organic matter to Bitumen 2. This could represent a mixture of older reworked bitumen and an early-diagenetic snapshot of clay adsorbed organic matter. Alternatively, a close association of early-generated bitumen with clay minerals may have led to enhanced isomerization and catalytically influenced ‘uniformization’ of alkane signatures. Deviations from the expected relationships between various thermal maturity parameters suggest variable dependence on the time-pressure-temperature pathway (i.e. metamorphic facies). The maturation of organic matter likely behaves differently under a contact metamorphic regime or during rapid subsidence and exhumation, as compared to slow maturation during regional subsidence. Our data also suggest that geologically brief shallow intrusive or extrusive magmatism might not be as destructive to the sedimentary hydrocarbon inventory as hitherto thought. This study draws attention to the small-scale compositional heterogeneity of bitumen that can be studied using sequential extraction methods. More importantly, it suggests that occluded bitumen could potentially harbor information on organic matter that pre-dates in situ primary productivity and may be derived from allochthonous biomass and detrital input.

Monitoring of large-scale deformation in mining areas using sub-band InSAR and the probability integral fusion method

ABSTRACTThe large-scale and rapid land subsidence that occurs in mining areas often leads to problems, such as densely spaced interference fringes and the temporal decorrelation of interferometric synthetic aperture radar (InSAR) interferograms. To solve these problems, sub-band InSAR is applied to monitor the large-scale deformation that occurs in mining areas. First of all, four different bandwidth images with three sub-band bandwidth parameters are used to extract simulated mining-induced subsidence with seven different deformation magnitudes. The results of the simulation experiment suggest the following conclusions. In monitoring subsidence with different deformation magnitudes using images with different bandwidths, an optimal monitoring value exists; wider image bandwidths lead to smaller optimal monitoring values and higher monitoring accuracies. Therefore, an appropriate sub-band bandwidth should be selected that depends upon the image bandwidth and the subsidence level to achieve optimal monitoring. The optimal sub-band bandwidth for monitoring subsidence of different magnitudes in mining areas is determined through simulation experiments, and these conclusions can provide a technical basis for selecting the appropriate sub-band bandwidth for the monitoring of subsidence in mining areas. Although sub-band InSAR can reduce the number of interference fringes and the difficulty of unwrapping, the simultaneous introduction of large amounts of noise leads to reduced monitoring precision, and the application of the probability integral method in the prediction of mine subsidence is more mature. Therefore, the combined use of sub-band InSAR and the probability integral fusion method to monitor mining-induced deformation is proposed in this paper. The probability integral method is used to perform noise peeling on the interferometric phases of the sub-bands to improve the monitoring accuracy of sub-band interferometry. Then, according to the results of the simulation experiment, the fusion method with the appropriate sub-band bandwidth parameters is applied to monitor the surface deformation associated with working face 52,304 from 2 December 2012 to 13 December 2012. Finally, the monitoring results are compared with the results of monitoring using conventional differential interferometric synthetic aperture radar (D-InSAR) and global positioning system (GPS) field survey data. The results show that the reliability and accuracy of the fusion method are much better than those of conventional D-InSAR in monitoring the large-scale deformation that occurs at the edges of subsidence basins.

The nature of the heat source of mafic magmatism during the formation of the Vilyui rift based on the ages of dike swarms and results of numerical modeling

Possible mechanisms of rifting and the thermal regime of the lithosphere beneath the rift zone of the Vilyui sedimentary basin are considered based on the available isotopic ages of dike swarms, rates of sedimentation, and results of numerical modeling. Temporal correlations between the intrusion of mafic magma and a sharp increase in the rate of subsidence and sedimentation in the rift basin prove the contribution of both plate-tectonic and magmatic factors to the formation of the Vilyui rift. The results show a relationship between the rapid extension of the lithosphere and the formation of mafic dike swarms in the Yakutsk-Vilyui Large Igneous Province of the Siberian Platform at the Frasnian-Famennian boundary, with a peak at ~374.1 Ma, and at the end of the Late Devonian, with a peak at ~363.4 Ma. There were two pulses of dike formation during rapid subsidence of the basin basement in the period 380-360 Ma, with a sedimentation rate of 100-130 m/Myr, at a background rate of 10-20 m/Myr. Analysis of numerical thermomechanical models revealed that the best-fit model is that combining the mechanisms of intraplate extension (passive rifting) and the ascent of a mantle magmatic diapir (active rifting). A conclusion about the nature of the heat source of trap magmatism has been drawn: The plume-driven regime of the lithosphere can better explain the dynamics of extension during rifting than the decompression melting mechanism.

Diagenesis, burial history, and hydrocarbon potential of Cambrian sandstone in the northern continental margin of Gondwana: A case study of the Lalun Formation of central Iran

Undeformed Cambrian–Ordovician sandstones on the northern margin of Gondwana are in places important hydrocarbon reservoirs. The factors that favoured diagenetic preservation of porosity in such sandstones are reviewed and compared to the Lower Cambrian Lalun Formation of central Iran, in order to assess what aspects of geological history have controlled the cementation and porosity of the sandstones. The Lalun Formation is up to 640 m thick and comprises braided-fluvial sandstones with minor shales that are overlain by marine shales, sandstones and carbonates. In four sections along a ∼650 km traverse, the sandstones are predominantly quartzarenites, locally arkosic and lithic, and contain minimal detrital matrix. Provenance analysis and paleoflow suggest derivation from the Arabian Shield. Diagenesis was dominated by quartz overgrowths and abundant intra-grain microfractures healed by silica cement, with modest amounts of poikilotopic and mosaic calcite, authigenic clay, and iron oxide as early grain rims and late-stage fracture fills. Minor phases include dolomite, ankerite, siderite, magnetite and barite. Marine strata contain more carbonate cement, with silica cement prominent in topmost nearshore sandstones. Burial-history analysis suggests relatively rapid burial in a cratonic setting to more than 4 km by the end of the Silurian and a maximum temperature of ∼150 °C during burial diagenesis, sufficient for hydrocarbon generation by the end of the Paleozoic. Dissolution of K-feldspar, ferromagnesian grains and, locally, calcite cement generated secondary porosity, but porosity and permeability values are low, averaging 1.5% and <0.1 mD, respectively. Rapid subsidence of thick Cambrian fluvial successions at the northeastern Gondwanan margin may have limited early dissolution and promoted occlusion of primary porosity by quartz overgrowths, limiting permeability pathways for fluids that might have generated secondary porosity later in the basinal history. In contrast with Cambrian and Ordovician sandstone reservoirs across North Africa and the Middle East, the existence of Paleozoic source rocks may be problematic in central Iran.

Effects of trench‐perpendicular ridge subduction on accretionary wedge deformation: Clues from analogue modelling

Basement highs (e.g., seamounts and ridges) often exist on subducted oceanic plates. However, their effects on the deformation of accretionary wedges have not been well understood, in particular the sequential cross‐sectional evolutionary processes caused by ridge subduction. To evaluate the effects of ridge subduction on accretionary wedges, analogue models were run to observe the deformation processes in both plan and cross‐sectional views. The results show that ridge subduction induces an inverted‐U‐shaped uplifted area, while seamount subduction always causes a circular uplifted area in plan view. Both ridge and seamount subduction will result in a radial fan‐shaped strike‐slip fault system in plan view, with opposite slip directions of the faults in the two wings. During ridge subduction, the faults in the left wing of the ridge axis are sinistral, and those in the right are dextral, which is reverse to that of seamount subduction. This may result from the backward flow in the overlying sediments during the rapid subsidence in the wake of the seamount, which would reverse the initial movement along the strike‐slip faults. The subducted ridge induces migration of wedge material from the frontal margin to the distal part, expressed by changes in structural geometry and kinematics (e.g., reduction in wedge length and taper angle and increase in wedge height). The accretionary prism adapted to the variation of taper angle by the development of back‐thrust faults and out‐of‐sequence thrust fault. Our model results shed lights on understanding the interior structural deformation pattern and mechanism caused by natural cases of ridge subduction, such as Gagua Ridge and North d'Entrecasteaux Ridge.

Basinal setting and origin of thick (1·8 km) mass‐flow dominated Grand Conglomérat diamictites, Kamoa, Democratic Republic of Congo: Resolving climate and tectonic controls during Neoproterozoic glaciations

AbstractThe Kamoa sub‐basin, in the south‐eastern part of the Democratic Republic of Congo, is a rift basin that hosts a world‐class stratiform copper deposit at the base of a very thick (1·8 km) succession of matrix‐supported conglomerates (diamictite) (Grand Conglomérat Formation) that has been interpreted by some as the product of deposition in the aftermath of a planet‐wide glaciation. Newly available subsurface data consisting of more than 300 km of drill core throws new light on the origin of diamictite and associated facies types, and their tectonic, basinal and palaeoclimatic setting. Initiation of rifting is recorded by a lowermost subaqueous succession of fault‐related mass flow conglomerates and breccias (the ‘Poudingue’) with interdigitating coeval and succeeding sandstone turbidites (Mwashya Subgroup). Overlying diamictites of the Grand Conglomérat were deposited as subaqueous debrites produced by mixing and homogenization of antecedent breccias and gravel from the Poudingue and Mwashya sediments with basinal muds. Failure of over‐steepened basin margins and debris flow was likely to be triggered by faulting and seismic activity, and was accompanied by syn‐depositional subaqueous basaltic magmatism recorded by peperites and pillow lavas within diamictites. The thickness of diamictites reflects recurring phases of faulting, volcanism and rapid subsidence allowing continued accommodation of rapidly deposited resedimented facies well below wave base. A distal or indirect, glacial influence in the form of rare dropstones and striated clasts is evident, but tectonically‐driven mass flow destroyed any primary record of glacial climate originally present in basin margin sediments. Such basin margin settings were common during Rodinia rifting and their stratigraphy and facies record a dominant tectonic, rather than climatic, control on sedimentation. Deposition occurred on tectonic timescales inconsistent with a Snowball Earth model for Neoproterozoic diamictites involving a direct glacial contribution to deposition.

Pyrolysis S2-peak characteristics of Raniganj shales (India) reflect complex combinations of kerogen kinetics and other processes related to different levels of thermal maturity

The Permian shales of the Raniganj basin, India, have experienced a dramatic burial history of rapid subsidence (Jurassic-Early Cretaceous) followed by igneous intrusion (Cretaceous) and rapid uplift and erosion (Late Cretaceous-Tertiary). This has left these shales with mixed kerogen macerals with a characteristic thermal signature, ranging from early mature to post mature, which is reflected in the characteristics of their pyrolysis S2 peak. Many of these shales are today at peak thermal maturity but reached that condition more than 100 million years ago. They have significant potential to be exploited as a shale gas resource and their S2 peak characteristics should help to identify sweet spots for such exploitation. Here we analyze single-rate and multi-rate heating ramp Rock-Eval data from a suite of these shales at varying stages of thermal maturity. The S2 peak shapes provide significant insight to the kerogen kinetics involved in their thermal evolution. However, detailed fitting of the peak shapes with kerogen-kinetic mixing models indicate that factors other than static first-order reaction kinetics are also involved in their S2-peak-shape characteristics. Such factors likely include the catalytic effects, influenced by sulfur and charcoal, on some kerogen components, and kerogen pore-size distribution changes during their complex burial and thermal maturation histories. It is likely that the peak-mature shales contain significant, already generated, gas trapped within some of the kerogen nanopores that may be released during the S2 pyrolysis heating ramp rather than during the S1 heating ramp. This causes the S2 peak to broaden in the mature shales, a characteristic that could be used as an exploration marker for zones best suited to shale gas exploitation. Cited as : Wood, D.A., Hazra, B. Pyrolysis S2-peak characteristics of raniganj shales (India) reflect complex combinations of kerogen kinetics and other processes related to different levels of thermal maturity. Advances in Geo-Energy Research, 2018, 2(4): 343-368, doi: 10.26804/ager.2018.04.01

Geochemical characteristics and hydrocarbon generation potential of the first member of Shahejie Formation (E2s1) source rocks in the Dongpu Depression, East China

The Dongpu Depression, containing huge amounts of hydrocarbons, is a typical petroliferous province in the Bohai Bay Basin, east China. As one set of the main source rocks, the first member of Shahejie Formation (E2s1) shows huge exploration potential with approximately 0.83 × 108 t resources remaining to be discovered. However, the geochemical characteristics and thermal evolution history of E2s1 source rocks have still not been determined. In this study, the sedimentary features were firstly analysed based on the geological data. Then the organic matter abundance, kerogen type, and thermal maturity of E2s1 source rocks in the present day were evaluated according to the Rock‐Eval pyrolysis, micro‐components of kerogen, total organic carbon (TOC), and vitrinite reflectance analysis. Furthermore, the evolution of thermal history and hydrocarbon generation was determined using the PetroMod procedure. The results show that dark mudstones and salt rocks were well developed in E2s1, which are propitious for organic matter to grow and preserve. E2s1 source rocks contain abundant organic matter and moderate to good generative source rock potential. E2s1 source rocks mainly consist of Type II kerogen and have entered early to maturation stage for hydrocarbon generation. Due to the limited buried depth, E2s1 source rocks only attained lower thermal maturity and generated finite hydrocarbons. The initial rapid subsidence and the second rift after uplift erosion resulted in the higher geo‐temperature at the end of E3d and the present day. Corresponding to the geo‐temperature evolution, there are two stages of hydrocarbon generation for E2s1 source rocks: The first stage was in the late E3d period, and the late stage was from the late Neogene to the present day. Moreover, the hydrocarbon generation rate and quantity of the late stage are much higher than the first stage, indicating the major hydrocarbon generation stage. Due to the limited hydrocarbon generation capacity, oils generated from E2s1 source rocks are low maturity; thus, it cannot migrate for a long distance and only accumulated around the sag belts.