Basin Margin Faults Research Articles

Evolution of Neoproterozoic siliciclastic Kerur Formation in the light of sequence stratigraphic framework: Badami Basin, Karnataka, India

Field-based sedimentology, state of the art facies analysis and sequence stratigraphic framework analysis have revealed the controls of local and global tectonics, basin-marginal slope, climate and changes in relative sea level (RSL) over the sedimentation pattern and evolution of a Neoproterozoic Kerur Formation within the Badami Group of Kaladgi Supergroup in India. The entire succession shows three major cycles of deposition. Facies study and fluvial architectural elemental analysis suggest considerable variations in depositional environments as well as palaeogeography. A transition from basin-margin alluvial cone deposits to braided system, initially with fluctuating ephemeral flows then to a steadier semi-perennial nature, is discernible within the 1st cycle, in response to decreasing depositional slope with rising water table. The initial alluvial cone and braided ephemeral streams of high slope areas is designated as a product of low accommodation systems tract (LAST), while the semi-perennial system with steadier flows, representing the axial river of the initial rift valley, appears to be a product of high – accommodation systems tract (HAST). The 2nd cycle begins with a perennial and steady braided river system and grades upward to a shallow marine succession, comprising wave-dominated, well-sorted sandstone, with a granular transgressive lag at the base. Thus, the bottommost fluvial interval of the 2nd cycle constitutes the lowstand systems tract (LST). The marine succession represents deposits of outer shelf offshore to foreshore-beach settings and is composed of an initially deepening and fining upward transgressive systems tract (TST), followed by a coarsening and shallowing upward highstand systems tract (HST) with a maximum marine flooding surface (MFS) in between, demarcated by a shale-rich condensed zone. The 3rd cycle, with its prograding alluvial fan and aggrading braided fluvial deposits and restricted occurrence, represents only the low accommodation systems tract (LAST) with a subaerial unconformity at the base. The basin evidently initiated in the western sector, followed by its eastward expansion during the first major rejuvenation of the basin margin faults, after the deposition of the 1st cycle. After the basin-wide deposition of the 2nd cycle, restricted development of the 3rd cycle took place in the western sector only, following the second major rejuvenation of the fault system. The proposed sedimentological model, supported by established geochronological constraints, suggests that the sedimentation in the 1st cycle begins with scree cones, alluvial fans and braided ephemeral channel networks, originated from faulted basin margins within a riftogenic setting possibly related to the global-scale extensional tectonics of Rodinia breakup. After the expansion of the basin, the marine inundation has been correlated to the transgression that possibly took place during the post-rift maturation stages.

Recent Holocene activity and regional tectonic significance of the northern segment of the red river fault zone

The Red River Fault Zone is a large-scale right-lateral strike-slip fault zone with relatively strong activity during the Quaternary Period. This fault, located on the southeastern margin of the Qinghai‒Tibetan Plateau, plays a key role in the extrusion, rotation and escape of the continental blocks constituting the Qinghai‒Tibetan Plateau. Furthermore, this fault represents the southwestern boundary of the Sichuan–Yunnan Block, which has experienced strong deformation and frequent seismic activity. The northern segment of the Red River Fault Zone is the most active part of the whole fault. However, surface erosion and vegetation coverage have obscured the activity of the northern segment; therefore, the study of its activity has obviously been insufficient. There is still controversy over whether all the secondary faults on the northern segment are active Holocene faults. Studying the activity characteristics of the northern segment, which is densely populated, is particularly important for seismic risk prevention in this area. Based on remote sensing interpretations and field geological surveys, this paper describes the latest activity characteristics of the Cangshan Piedmont Fault, Fengyi–Dingxiling Fault and Midu Basin Margin Fault, including their spatial distributions and kinematic characteristics. According to the ages of the offset strata in profiles, the above three secondary faults were all active in the late Holocene. The latest active age of the Cangshan Piedmont Fault was later than 543-494 cal BP, and two palaeoseismic events that occurred in this section during the Holocene occurred at 2700 and 473 cal BP. The latest active age of the Fengyi–Dingxiling Fault was later than 2760–2700 cal BP; in this section, one Holocene palaeoseismic event occurred between 1777 cal BP and 2730 cal BP, another occurred between 2730 cal BP and 5664 cal BP, and the third occurred between 6449 cal BP and 8360 cal BP. The latest active age of the Midu Basin Margin Fault was later than 558-510 cal BP, and two Holocene palaeoseismic events occurred later than 2318-2114 cal BP and 558-510 cal BP. Based on the results of this paper and previous studies, we believe that the Fengyi–Dingxiling Fault on the northern segment of the Red River Fault Zone is at risk for future strong earthquakes. Additionally, abundant geological and geomorphologic evidence suggests that the northern segment is dominated by normal faults, reflecting the local strain response to secondary clockwise rotation of the Sichuan–Yunnan Block along the boundary fault. This finding is in line with the eastwards extrusion and escape of materials on the Qinghai‒Tibetan Plateau caused by the northwards and northeastwards pushing of the Indian Plate. To a certain extent, these observations reflect the tectonic deformation coordination between block rotation and boundary fault slip in the Sichuan–Yunnan Block in the context of continental block extrusion on the Qinghai–Tibetan Plateau.

Eocene extensional tectonics in the Amakusa region, northern Ryukyu arc

AbstractPaleogene surface tectonics in Japan is not well understood because of the paucity of onshore Paleogene stratigraphic records except for those from accretionary complexes. Paralic Paleogene formations remaining in SW Japan are usually so thin that it is difficult to decipher the tectonics from them. However, the Eocene paralic sedimentary package with a thickness of kilometers indicates syn‐depositional tectonic subsidence by a few kilometers in the Amakusa archipelago, west of Kyushu Island. Thus, we made a detailed geological map of the Eocene formations in an area of ~50 square kilometers in the northwestern part of the archipelago. We identified NE‐SW and NW‐SE trending normal faults, most of which were recognized by previous researchers, and also discovered low‐angle faults. NW‐SE trending ones are known to be of the Miocene. NE‐SW trending and low‐angle normal faults are the oldest map‐scale structures in the Eocene ones. It is not obvious within the above‐mentioned area whether those normal faults are accompanied by growth strata. However, the significant southeastward thickening of the Eocene formations across the Amakusa archipelago suggests that they filled a large half graben with the basin margin fault along the eastern side of the archipelago. This basin model is consistent with the N‐S to NW‐SE transport directions of the low‐angle and NE‐SW trending normal faults. Since many NE‐SW to EW trending Eocene grabens were formed in the offshore regions west of Kyushu Island and in the East China Sea, the Amakusa region was probably a northeastern branch of the rift system. The geologic structures and depositional ages of the Eocene formations indicate that the Eocene extensional tectonics removed the overlying strata to some extent for the high‐P/T Takahama Metamorphic Rocks which crops out to the south of our study area.

Responses of Stream Geomorphic Indices to Piedmont Fault Activity in the Northern Segment of the Red River Fault Zone

Based on a digital elevation model (DEM) and GIS technology, we extracted and analyzed stream geomorphic indices—such as the slope, relief degree of the land surface (RDLS), hypsometric integral (HI) and channel steepness index (ksn)—of the Cangshan Piedmont Fault, Fengyi–Dingxiling Fault and Midu Basin Margin Fault in the northern segment of the Red River Fault Zone. This work indicates that all the stream geomorphic indices show higher values, with the highest values along the Cangshan Piedmont Fault, followed by the Fengyi–Dingxiling Fault, and the lowest values along the Midu Basin Margin Fault, forming a decreasing trend from north to south. Based on lithology, climate and tectonics, we infer that neotectonic activity is the main factor controlling the development of the drainage geomorphology. The results show that the northern segment of the Red River Fault Zone is highly active and that the activity level shows a decreasing trend from north to south. The results of this study are consistent with previous conclusions that the overall activity of the Red River Fault Zone weakens from north to south, and the activity in the northern segment has been the most intense since the Late Pleistocene.

Trapped in a graben: deposition of Huronian gold-bearing conglomerates in a fault-influenced, valley-confined, fluvial system in the southern Cobalt Basin, Ontario, Canada

Isopachs of Huronian strata of the Elliot and Hough Lake groups in the southern part of the Cobalt Basin can be used to define the geometry of a 4 km wide valley system that directly influenced the location of gravel-bed rivers bearing detrital gold and auriferous pyrite in the Mississagi Formation. Distribution and thickness of these and underlying formations can be directly linked to initial valley formation parallel to existing north-south-oriented faults in the Archean basement. Thickness distributions were directly influenced by active subsidence associated with transverse, east-south-east (ESE)-oriented, normal faults, related to extension along the Huronian transform-rift margin further south. Strata underlying the Mississagi Formation were largely removed by erosion in the northern part of the paleovalley system, but thickened and then thinned south of the ESE faults. Pyrite and detrital gold in the Mississagi Formation may have been concentrated from reworking of coarse clastic rocks of the Matinenda and Ramsay Lake formations, along with significant contributions from erosion of proximal Archean basement within 3–5 km of the preserved basin margins. There is strong evidence to suggest that stream flow was initially concentrated in three main structurally influenced valley systems in the north, with one lateral tributary in the south-eastern part of the basin. The fluvial systems merged, and thickened, south of the Tee Lake fault, possibly reflecting trans-tensional influences on the basin margin faults.

Basement cross‐strike Bomdila Fault beneath Arunachal Himalaya: Deformation along curved thrust traces, seismicity, and implications in hydrocarbon prospect of the Gondwana sediments

Structural mapping and fieldwork in the Lesser and Higher Himalayan sequences in the western Arunachal Himalaya reveal crucial deformation fabrics in the Main Central Thrust (MCT) and Dirang Thrust (DT) zones. The top‐to‐SW ductile shear in the MCT and DT zones is correlated with the swing in the trend of MCT and DT from NE to NNW. The curved MCT and DT as traced by previous authors on the regional map of Arunachal Himalaya are studied. It is found that at places where these faults swing, shear senses developed at the meso‐scale. These shear senses are studied in meso‐ and micro‐scales. Seismicity in the western Arunachal Himalaya is influenced by basement cross‐strike crustal‐scale NW‐trending buried Bomdila Fault (BF). Landslides occur frequently along the Bhalukpong‐Bomdila–Sela traverse are also linked with the transverse BF. In its southern part, the BF coincides with the Dhansiri Lineament and is the basin margin fault in the upper Assam shelf. The Gondwana sediments extended further south below the Brahmaputra alluvium along the fault is to be explored for hydrocarbon exploration.

The Coles Hill Uranium Deposit, Virginia, USA: Geology, Geochemistry, Geochronology, and Genetic Model

Abstract The Coles Hill uranium deposit, with an indicated resource of about 130 Mlb of U3O8, is the largest unmined uranium deposit in the United States. The deposit is hosted in the Taconian (approx. 480–450 Ma) Martinsville igneous complex, which consists of the Ordovician Leatherwood Granite (granodiorite) and the Silurian Rich Acres Formation (diorite). The host rock was metamorphosed to orthogneiss during the Alleghanian orogeny (approx. 325–260 Ma), when it also underwent dextral strike-slip movement along the Brookneal shear zone. During the Triassic, extensional tectonics led to the development of the Dan River Basin that lies east of Coles Hill. The mineralized zone is hosted in brittle structures in the footwall of the Triassic Chatham fault that forms the western edge of the basin. Within brittle fracture zones, uranium silicate and uranium-bearing fluorapatite with traces of brannerite form veins and breccia-fill with chlorite, quartz, titanium oxide, pyrite, and calcite. Uranium silicates also coat and replace primary titanite, zircon, ilmenite, and sulfides. Sodium metasomatism preceded and accompanied uranium mineralization, pervasively altering host rock and forming albite from primary feldspar, depositing limpid albite rims on igneous feldspar, altering titanite to titanium oxide and calcite, and forming riebeckite. Various geothermometers indicate temperatures of less than ~200°C during mineralization. In situ U-Pb analyses of titanite, Ti-oxide, and apatite, along with Rb/Sr and U/Pb isotope systematics of whole-rock samples, resolve the timing of geologic processes affecting Coles Hill. The host Leatherwood Granite containing primary euhedral titanite is dated at 450 to 445 Ma, in agreement with previously obtained ages from zircon in the Martinsville igneous complex. A regional metamorphic event at 330 to 310 Ma formed anhedral titanite and some apatite, reequilibrated whole-rock Rb/Sr and U-Pb isotopes, and is interpreted to have coincided with movement along the Brookneal shear zone. During shearing and metamorphism, primary refractory uranium-bearing minerals including titanite, zircon, and uranothorite were recrystallized, and uranium was liberated and mixed locally with hematite, clay, and other fine-grained minerals. Uranium mineralization was accompanied by a metasomatic episode between 250 and 200 Ma that reset the Rb-Sr and U-Pb isotope systems and formed titanium oxide and apatite that are associated and, in places, intimately intergrown with uranium silicate dating mineralization. This event coincides with rifting that formed the Dan River Basin and was a precursor to the breakup of Pangea. The orientation of late-stage tectonic stylolites is compatible with their formation during Late Triassic to Early Jurassic basin inversion, postdating the main stage of uranium mineralization and effectively dating mineralization as Mesozoic. Based on the close spatial and temporal association of uranium with apatite, we propose that uranium was carried as a uranyl-phosphate complex. Uranium was locally reduced by coupled redox reactions with ferrous iron and sulfide minerals in the host rock, forming uranium silicates. The release of calcium during sodium metasomatic alteration of primary calcic feldspar and titanite in the host rock initiated successive reactions in which uranium and phosphate in mineralizing fluids combined with calcium to form U-enriched fluorapatite. Based on the deposit mineralogy, oxygen isotope geochemistry, and trace element characteristics of uranium silicate and gangue minerals, the primary mineralizing fluids likely included connate and/or meteoric water sourced from the adjacent Dan River Basin. High heat flow related to Mesozoic rifting may have driven these (P-Na-F-rich) fluids through local aquifers and into basin margin faults, transporting uranium from the basin or mobilizing uranium from previously formed U minerals in the Brookneal shear zone, or from U-enriched older basement rock.

Structural Characteristics of Luonan Basin Based on Gravity Data

The Luonan Basin is a typical intermountain basin in the Qinling orogenic belt. The tectonic characteristics of the basin and its adjacent areas are very complex. Previous studies on Luonan Basin mainly focus on paleontology and sedimentation, and lack of reports on basin studies using geophysical data. In order to study the tectonic characteristics of Luonan Basin, this paper collected 1:500000 gravity data in the study area, and used the sliding average filtering method to separate the remaining gravity anomalies from the Bouguer gravity anomalies, and analyzed the causes of gravity anomalies. In this paper, the normalized total horizontal derivative vertical derivative, total horizontal gradient mode and vertical second derivative are used to infer the faults in the study area by referring to the remaining Bouguer gravity anomalies and geological data. According to the inferred results and geological data, the boundary of the basin is determined. Three gravity profiles across the basin were fitted by a 2.5D human-computer interaction inversion method. The results show that the high gravity is caused by the basement uplift, and the low gravity is the reflection of the Meso-Cenozoic and low-density intrusive rocks. The faults in the study area are mainly in NE, NW and nearly EW directions. The development of the basin is controlled by the basin margin faults. The east-west length of the basin is 85km, the maximum north-south width is 7km, and the basin area is 420km². Cretaceous, Paleogene and Neogene are mainly developed in the basin. The basement depth of the basin is "deeper in the middle than in the west, and deeper in the west than in the east", with the maximum depth of about 1130m.

Metallogenic characteristics and tectonic setting of the Jiaodong gold deposit, China

Jiaodong is the largest gold deposit concentration area in China. In recent years, great breakthroughs have been made in deep prospecting, and it has become the third largest concentration area of gold deposits in the world. A series of prospecting discoveries in the Jiaodong area provided the basic conditions for summarizing the regional metallogenic law, constructing the deposit model, carrying out research on the genesis of the deposit and innovating metallogenic theory. The Jiaodong gold deposits are mainly distributed in Northwest Jiaodong, Qi-Peng-Fu and Mu-Ru metallogenic regions, and mainly occur in Precambrian metamorphic rock series, Jurassic Linglong-type granite, Early Cretaceous Guojialing-type granite and Laiyang group. The orebodies are controlled by Sanshandao, Jiaojia, Zhaoping, Xilin-Douya, Jinniushan and other major ore-controlling fault zones. The main mineralization types include altered rock-type, quartz vein-type, stockwork type, sulfide quartz vein-type, interlayer detachment zone-type, altered conglomerate-type, basin margin fault breccia-type and pyrite carbonate vein-type and so on. The important progress of deep prospecting is summarized: two super giant deposits have been evaluated in the Sanshandao and Jiaojia areas; the amount of altered rock-type gold resources identified by deep prospecting has exceeded that of quartz vein-type gold deposits in the Linglong gold deposit field; the Hushan large scale gold deposit has been discovered in Qixia gold deposit field; and the Liaoshang large-size gold deposit is a new discovery of deep prospecting in Pengjiakuang gold deposit field. The ore-controlling faults of gold deposits have several changing steps with dip angle varying from steep to gentle along the dip angle. The gold ore bodies are mainly enriched in sections along the steep, gentle turning points and relatively gentle parts of the fault dip angle, forming a stepped distribution pattern. In the Early Cretaceous, the subduction and rollback of the Paleo-Pacific plate induced crust mantle interaction, resulting in large-scale magmatic and fluid activities. The crust tension and magma uplift form the dome extension structure of granite, which provides a channel for the migration of ore-forming fluid and also provides favorable space for the deposition of ore-forming fluid.

Petro-mineralogical and geochemical characteristics of Shahaba Limestone from Gogi-Kanchankayi sector, Bhima Basin, Karnataka with reference to Uranium mineralisation

The Shahabad Limestone Formation of Bhima Basin from Gogi-Kanchankayi area occurs in heterogeneous forms like massive/blocky limestone, argillaceous/ siliceous limestone and laminated/ flaggy limestone. These limestones are primarily composed of micrite, which often alters into sparry calcite on diagenesis with associated impurities of quartz, feldspar, barite, chlorite, glauconite, sulphides and carbonaceous matter. Geochemically, these limestones comprises of variable CaO with low MgO and P2O5 content. Trace elements concentration shows elevated Ba, Rb and depleted Sr. The current study classified these limestones as non-dolomitic and non-phosphatic types deposited in shallow marine carbonate platform setting with low energy conditions. Post-sedimentation, basin tectonics has resulted in reactivation of the basin margin fault causing intense fracturing of limestone. Subsequent hydrothermal movement along those fractures has resulted in re-mobilisation and re-precipitation of sulphides and carbonaceous matter, and along with alteration has facilitated the precipitation of the uranium bearing minerals.

The Hera orebody: A complex distal (Au–Zn–Pb–Ag–Cu) skarn in the Cobar Basin of central New South Wales, Australia

AbstractThe Hera Au–Pb–Zn–Ag deposit in the southeastern Cobar Basin of central New South Wales preserves calc‐silicate veins and remnant sandstone/carbonate‐hosted skarn within a reduced anchizonal Siluro‐Devonian turbidite sequence. The skarn orebody distribution is controlled by a long‐lived, basin margin fault system, that has intersected a sedimentary horizon dominated by siliciclastic turbidite, with lesser gritstone and thick sandstone intervals, and rare carbonate‐bearing stratigraphy. Foliation (S1) envelopes the orebody and is crosscut by a series of late‐stage east–west and north–south trending faults. Skarn at Hera displays mineralogical zonation along strike, from southern spessartine–grossular–biotite–actinolite‐rich associations, to central diopside‐rich–zoisite–actinolite/tremolite–grossular‐bearing associations, through to the northern most tremolite–anorthite‐rich (garnet‐absent) association in remnant carbonate‐bearing lithologies and sandstone horizons; the northern lodes also display zonation down dip to garnet present associations. High‐T, prograde skarn assemblages rich in pyroxene and garnet are pervasively replaced by actinolite/tremolite–biotite‐rich retrograde skarn which coincides with the main pulse of sulfide mineralization. The dominant sulfides are high‐Fe–Mn sphalerite–galena–non‐magnetic high‐Fe pyrrhotite–chalcopyrite; pyrite, arsenopyrite; scheelite (low Mo) is locally abundant. The distribution of metals in part mimics the changing gangue mineralogy, with Au concentrated in the southern and lower northern lode systems and broadly inverse concentrations for Ag–Pb–Zn. Stable isotope data (O–H–S) from skarn amphiboles and associated sulfides are consistent with magmatic (or metamorphic) water and sulfur input during the retrograde skarn phase, while hydrosilicates and sulfides from the wall rocks display comparatively elevated δD and mixed δ34S consistent with progressive mixing or dilution of original magmatic (or metamorphic) waters within the Hera deposit by unexchanged waters typical of low latitude (tropical) meteoritic waters. High precision titanite (U–Pb) and biotite (Ar–Ar) geochronology reveals a manifold orebody commencing with high‐T skarn and retrograde Pb–Zn‐rich skarn formation at ≥403 Ma, Au–low‐Fe sphalerite mineralization at 403.4 ± 1.1 Ma, foliation development remobilization or new mineralization at 390 ± 0.2 Ma followed by thrusting, orebody dismemberment at 384.8 ± 1.1 Ma and remobilization or new mineralization at 381.0 ± 2.2 Ma. The polymetallic nature of the Hera orebody is a result of multiple mineralization events during extension and compression and involving both magmatic and likely formational metal sources.

Lithological variations and drainage evolution in a marginal region: A case study from Chandrapur block of the Pranhita-Godavari Basin, Central India.

Field mapping and interpretation of sedimentary structures in Gojoli region of Chandrapur block along the eastern margin of the Pranhita-Godavari valley suggests westward flowing channels during the Neoproterozoic. Present channels in this part of Chandrapur block are south/ southeast flowing and possess clear imprints of the basin-margin faults and lineaments. Drainage parameters in its Neoproterozoic stratigraphic sequence suggest impervious substrates probably due to high degree of consolidation and filling of rock-fractures by hydrothermal solution activities. However, the same for the Gondwana’s friable sandstone (towards western region) and Archaean gneisses (in the eastern part) has been observed consistent probably due to its porous nature and development of thick weathering profile. The affiliation of drainage with tectonism is therefore, from Neoproterozoic to Cenozoic era within contrasting lithology, was out of synchronization intermittently.

An insight to the initiation of Cretaceous sedimentation in Northeast Indian Craton

The time transgressive Cretaceous sedimentation in northeast Indian craton is represented by the deposition of the Upper Cretaceous Mahadek sediments in Meghalaya. The Lower unit of these sediments is comprised of alternate sandstone and conglomerate horizons, and the Upper unit is composed of arkosic to sub-arkosic sandstones. These sediments are overlain by the calcareous litho-units of Langpar Formation of Cretaceous–Paleocene age. Lithofacies identified from various exposures and studied lithologs reflect a variation from marginal marine to marine depositional set-up in the Mahadek sediments. The Lower Mahadek facies is dominated by large-scale trough beds of medium to coarse-grained sediments which initiates with a conglomerate horizon succeeded by thin beds of shale and fine-grained sandstone with parallel laminations, which are followed by successive deposition of gritty sand and pebbly beds with coarsening upward sequence, suggesting product of delta plain in a marginal marine setting. Moreover, the bioturbated coaly fine-grained sandstone is further representative of tidal flat-tidal channel swamp formed in reducing condition and the trace fossil assemblages depict the channel fills. The Upper Mahadek facies is dominated by glauconitic-gritty fossiliferous sandstone showing fining upward sequence and hummocks, typical of marine environment. This fining upward sequence along with the vertical and lateral facies variation implies superimposition of retrograding shorelines on an earlier progradational alluvial fan sand complex. These different-scale cycles are interpreted as the sedimentary response to movements of the basin margin faults at variable frequencies, related to the contemporary faults. This depositional model of the Cretaceous sediments may further help in paleogeographic reconstruction of the northeast Indian craton during the Cretaceous period.

Evolution of Damodar Fan Delta in the Western Bengal Basin, West Bengal

ABSTRACT Fan delta is formed when an alluvial fan prograde directly into the standing body of water from the adjacent highland. An ancient fan delta was developed 6-10m above from the present sea level at the lower course of Damodar river in the western part of Bengal basin. The present study attempted to represent the evolutionary history of Damodar fan delta in the context of many associative features favor to fan delta formation such as adjacent highland, basin margin faults, accommodation space and standing water body. These elements have been taken into consideration to identify the location, time and processes for the fan delta formation. Data related to tectonic and depositional history, sea level fluctuation and palaeodrainage network have been collected from various sources. These reveal that the western part of Bengal basin is characterized by numbers of north-south trending normal faults, shifting river courses and repeated marine transgression and regression phases. After collecting a huge amount of sediment from Chhotanagpur plateau, Damodar river deposited its sediment in the eastern side of these north-south trending faults in the subaerial and subaqueous environment throughout the geological period. In this region Chhotanagpur plateau, fault-dominated stable shelf and transgressed Bay of Bengal play the role of adjacent highland, accommodation space and standing water body respectively to develop Damodar fan delta. Sedimentation process started in late Cretaceous and still it is continuing in a subaerial environment. These fan forming processes led Damodar river to shift its course and make this region flood prone.

Sequence stratigraphic architectures and responses to syndepositional tectonic evolution in the Paleogene Lingshui Sag, Qiongdongnan Basin, northwestern South China Sea

ABSTRACT The syndepositional tectonic evolution and sequence stratigraphic architecture of the Paleogene Lingshui Sag, Qiongdongnan Basin, South China Sea, are investigated with seismic profiles, boreholes and well logs. First, according to the recognized sequence boundaries, the Paleogene sequence stratigraphic framework is established. Then, based on analyzing the subsidence history and paleogeomorphology evolution, the Paleogene Lingshui Sag is clarified to present episodic tectonic evolution, showing tectonic stage-I, stage-II and stage-III. From stage-I to stage-III, the major basin-marginal faults gradually lost control over the subsidence center, which moved away from the initial basin margin to the late central basin. Structural slope break belts, such as fault slope break belts and gentle slope break belts, formed through the above three tectonic stages and controlled the formation of four types of relevant distinct sequence stratigraphic architectures, namely, Types A1, A2, B1 and B2. Finally, the responses of sequence stratigraphic architectures to tectonic evolution are described, and the favorable hydrocarbon reservoirs in the different architectures are discussed, which constitute stratigraphic traps, fault traps, lenticular lithologic traps and updip wedge-out traps. This information is beneficial in prospecting for subtle traps in basins located in deepwater areas.

Paleoenvironment and age constraints of Paleoproterozoic alluvial fans in the Sao Francisco Craton, Brazil

The Ouricuri do Ouro Formation represents sedimentary deposits of the Statherian period in São Francisco Craton. Previous works have focused just on the Formation mapping, interpreting the depositional system only in general aspects. However, this study focuses on detailed facies analysis and geochronological investigations of the Ouricuri do Ouro Formation in the central area of São Francisco Craton, Brazil. Four facies associations are recognized: i) Proximal deposits dominated by non-cohesive debris flow; ii) Proximal sheetfloods; iii) Intermediate sheetfloods; and iv) Sandy plains of distal floods. Paleocurrent directions derived from cross-bedding indicate a radial pattern to NE-N. The vertical stacking of distal deposits overlying the proximal facies association suggests a retrogradation of the alluvial fan system, indicating a change in the dynamic of the depositional system. Retrogradation might have been caused by a decrease in the regional gradient declining of the tectonic activity or the retreat of the basin's margin faults system. Furthermore, the presence of higher frequency fining-upward cycles also suggests a climatic control in those deposits. These cycles could be results of the progressive decrease in flow capacity and competence, associated with a decrease of superficial runoff over time. Allied with the dominant paleocurrent, the geochronological data of four tuffaceous sandstones reveal the contribution of Rhyacian to Mesoarchean zircons, suggesting that they came with sediments transported from SW and S of the studied area during the Statherian. The transported sediments came from the southern portion of Gavião Paleoplate. The contribution of volcanic zircons recognized in the tuffaceous sandstones is related to syndepositional magmatism of Statherian Rift of Espinhaço Supergroup. The tuff layer at the base of drill hole (Sample FLV-61 A) contains a zircon age with dominant fraction represented by Statherian zircons defining the syn-sedimentary age of 1722 ± 13 Ma at the basal section of the Ouricuri do Ouro Formation. The present results and available geochronological data constraints the depositional interval of Lower Espinhaço Basin or Megasequence (in sense of Chemale et al., 2012; Guadagnin et al., 2015a, b) between <1.80 and >1.68 Ga in the Chapada Diamantina Region.

NEOTECTONIC AND MORPHOTECTONIC CHARACTERISTICS OF THE ELMALI BASIN AND NEAR VICINITIES

Elmalı Basin, which consists of the Elmalı county and some villages, approximately 120 km far from Antalya city, is one of the recent depositional areas in the extensional neotectonic region of southwestern Turkey. The basin provides signifi cant data for the understanding of the geological evolution to record from the basement to recent depositional units. For this reason, these kinds of structures are used to determine the deformations phases for the evolutionary history of basin formation. In the content of the study, following applications have been investigated: (a) mapping of the area and structures in 1/25.000 scale, (b) information about deformation and tectonic activity along the basin margin faults, (c) the distribution of peak ground acceleration (PGA) in case of an earthquake with 6.5 magnitude and (d) morphometric analyzes of the basin to understand the tectonic uplift by using the digital elevation model. The new results combined with existing data imply that the Elmalı Basin has a number of deformation structures and active faults on both western and eastern sites of the basin. Depending on the morphometric indices, western side of the basin created higher uplift ratio compared with the eastern side of the basin.

Estimation of palaeo-slope and sediment volume of a lacustrine rift basin: A semi-quantitative study on the southern steep slope of the Shijiutuo Uplift, Bohai Offshore Basin, China

The sequence architecture and depositional systems of the Palaeogene lacustrine rift succession in the southern steep slope of the Shijiutuo Uplift (SU) in Bohai Offshore Basin (BOB) were investigated using combined 3-D seismic, well log and core data. Four second-order/composite sequences and seven third-order sequences were identified. A detailed analysis revealed fan delta, braid delta and lacustrine depositional systems in the third-order sequences 7 (SQ7). Eleven seismic facies were chosen in the SQ7 to calculate the area and sediment budget. We also calculated the palaeo-slope parameters in the southern steep slope of the SU, including the gradient of basin margin fault slope belt, the shape, average width, average height, width/height ratio and cross sectional area of palaeo-valley. The gradient of the fault slope break belt was inversely proportional to the area and volume of sediments. The higher gradient corresponded to the smaller area and volume of sediments. The width and cross sectional area of palaeo-valleys dominated the volume and facies of sediments. A larger cross sectional area of a palaeo-valley involved much more sediments supply and more favorable conditions for the origin of large-scale deltas. Because of favorable sediment transportation path and existence of large-scale braid delta, the eastern part of the SU formed better quality reservoir than that in the west.

Characteristics of hydrothermal sedimentation process in the Yanchang Formation, south Ordos Basin, China: Evidence from element geochemistry

Hydrothermal sedimentation occurred in the Triassic Yanchang Formation, Ordos Basin, China. However, their macroscopic features at the scale of the stratum and hydrothermal sources still lack correlational research. This paper performed element geochemical study on a large number of core samples collected from the Yanchang Formation of a new drilling well located in the south Ordos Basin. The SiO2/(K2O+Na2O) vs. MnO/TiO2 crossplot and Fe vs. Mn vs. (Cu+Co+Ni)×10 ternary diagram demonstrate that the Yanchang stratum in the study area has, in general, hydrothermal components. The Al/(Al+Fe+Mn) and (Fe+Mn)/Ti ratios of the core samples range from 0.34 to 0.84 and 4.81 to 50.54, averaging 0.66 and 10.67, respectively, indicating that the stratum is a set of atypical hydrothermal sedimentation with much terrigenous input. Data analysis shows that the hydrothermal source in the study area was from the deep North Qinling Orogen around the south margin of the basin, where some active tectonic and volcanic activities took place, rather than from the relatively stable internal basin. Early Indosinian movement and volcanic activities activated basement faults around the southern margin of the basin, providing vents for the deep hydrothermal fluid upwelling. The hydrothermal indicators suggest that the study area experienced 4 episodes of relatively stronger hydrothermal activity, namely during the Chang 10, Chang 9–1, Chang 7–3 and Chang 6–2 periods. We also propose a new hydrothermal sedimentation model of hydrothermal fluids overflowing from basin margin faults, for the Yanchang Formation, which is reported here for the first time.

Arc-oblique fault systems: their role in the Cenozoic structural evolution and metallogenesis of the Andes of central Chile

The evolution of the Main Cordillera of Central Chile is characterized by the formation and subsequent inversion of an intra-arc volcano-tectonic basin. The world’s largest porphyry Cu-Mo deposits were emplaced during basin inversion. Statistically, the area is dominated by NE- and NW-striking faults, oblique to the N-striking inverted basin-margin faults and to the axis of Cenozoic magmatism. This structural pattern is interpreted to reflect the architecture of the pre-Andean basement. Stratigraphic correlations, syn-extensional deposits and kinematic criteria on fault surfaces show several arc-oblique structures were active as normal faults at different stages of basin evolution. The geometry of syn-tectonic hydrothermal mineral fibers, in turn, demonstrates that most of these structures were reactivated as strike-slip ± reverse faults during the middle Miocene – early Pliocene. Fault reactivation age is constrained by 40Ar/39Ar dating of hydrothermal minerals deposited during fault slip. The abundance and distribution of these minerals indicates fault-controlled hydrothermal fluid flow was widespread during basin inversion. Fault reactivation occurred under a transpressive regime with E- to ENE-directed shortening, and was concentrated around major plutons and hydrothermal centers. At the margins of the former intra-arc basin, deformation was largely accommodated by reverse faulting, whereas in its central part strike-slip faulting was predominant.