scholarly journals Nature and significance of rift-related, near-surface fissure-fill networks in fractured carbonates below regional unconformities

2020 ◽  
Vol 177 (6) ◽  
pp. 1168-1185 ◽  
Author(s):  
Kit Hardman ◽  
Robert E. Holdsworth ◽  
Edward Dempsey ◽  
Ken McCaffrey
Keyword(s):  
Wall Rock ◽  
Surface Phenomenon ◽  
Low Permeability ◽  
Early Permian ◽  
Near Surface ◽  
Content Type ◽  
Basement Rocks ◽  
Supplementary Material ◽  
Fissure Fill ◽  
Migration Pathways

Fissure-fill networks are a widely recognized, but relatively little described, near-surface phenomenon (<1–2 km) hosted in carbonate and crystalline basement rocks below regional unconformities. Faults and fractures in otherwise tight Devonian carbonate basement rocks of the Tor Bay region, Devon, SW England are associated with the development of millimetre- to decametre-wide fissures containing red-coloured early Permian sedimentary material, vuggy calcite mineralization and wall rock collapse breccia. These features preserve evidence about the style and history of fault deformation and reactivation in near-surface settings and on fluid-related processes, such as elutriation and/or mineralization. Field observations, palaeostress analysis and fracture topology analyses show that the rift-related faults and fractures created a network of long-lived open cavities during the development of the Portland–Wight Basin in the early Permian. Once formed, they were subjected to episodic, probably seismically induced, fluid fluxing events and local karstification. The large, well-connected networks of naturally propped fractures were (and possibly still are) important fluid migration pathways within otherwise low-permeability host rocks. These structures are probably equivalent to those observed in many other rift-related, near-surface tectonic settings and suggest that the Tor Bay outcrops can be used as a global analogue for sub-unconformity open fissure systems hosted in low-permeability basement rocks.Supplementary material: Appendix A is available at https://doi.org/10.6084/m9.figshare.c.5023103

Jurassic–Cretaceous arc magmatism along the Shyok–Bangong Suture from NW Himalaya: Formation of the peri-Gondwana basement to the Ladakh Arc

2021 ◽  
pp. jgs2021-035
Author(s):  
Wanchese M. Saktura ◽  
Solomon Buckman ◽  
Allen P. Nutman ◽  
Renjie Zhou
Keyword(s):  
Late Cretaceous ◽  
Nw Himalaya ◽  
Content Type ◽  
Magmatic Arc ◽  
Basement Rocks ◽  
Ladakh Himalaya ◽  
Accreted Terranes ◽  
Volcaniclastic Rocks ◽  
Supplementary Material ◽  
Depositional Age

The Jurassic–Cretaceous Tsoltak Formation from the eastern borderlands of Ladakh Himalaya consists of conglomerates, sandstones and shales, and is intruded by norite sills. It is the oldest sequence of continent-derived sedimentary rocks within the Shyok Suture. It also represents a rare outcrop of the basement rocks to the voluminous Late Cretaceous–Eocene Ladakh Batholith. The Shyok Formation is a younger sequence of volcaniclastic rocks that overlie the Tsoltak Formation and record the Late Cretaceous closure of the Mesotethys Ocean. The petrogenesis of these formations, ophiolite-related harzburgites and norite sill is investigated through petrography, whole-rock geochemistry and U–Pb zircon geochronology. The youngest detrital zircon grains from the Tsoltak Formation indicate Early Cretaceous maximum depositional age and distinctly Gondwanan, Lhasa microcontinent-related provenance with no Eurasian input. The Shyok Formation has Late Cretaceous maximum depositional age and displays a distinct change in provenance to igneous detritus characteristic of the Jurassic–Cretaceous magmatic arc along the southern margin of Eurasia. This is interpreted as a sign of collision of the Lhasa microcontinent and the Shyok ophiolite with Eurasia along the once continuous Shyok–Bangong Suture. The accreted terranes became the new southernmost margin of Eurasia and the basement to the Trans-Himalayan Batholith associated with the India-Eurasia convergence.Supplementary material:https://doi.org/10.6084/m9.figshare.c.5633162

Formation of wide-blocky calcite veins by extreme growth competition

2020 ◽  
pp. jgs2020-104
Author(s):  
Liene Spruženiece ◽  
Michael Späth ◽  
Janos L. Urai ◽  
Estibalitz Ukar ◽  
Michael Selzer ◽  
...  
Keyword(s):  
High Resolution ◽  
Growth Rates ◽  
Polarized Light ◽  
Wall Rock ◽  
Scattered Electron ◽  
Thin Sections ◽  
Content Type ◽  
Calcite Veins ◽  
Supplementary Material ◽  
Electron Back Scattered Diffraction

Liassic limestones on the coast of Somerset in the UK contain dense arrays of calcite microveins with a common, but poorly understood microstructure, characterized by laterally wide crystals that form bridges across the vein. We investigated the mechanisms of formation and evolution of these ‘wide-blocky’ vein microstructures using a combination of high-resolution analytical methods, including virtual petrography, optical cathodoluminescence and scanning electron microscopy techniques (e.g. energy-dispersive X-ray spectrometry, back-scattered electron imaging, cathodoluminescence and electron back-scattered diffraction), laboratory experiments and multiphase field modelling. Our results indicate that the studied veins formed in open, fluid-filled fractures, each in a single opening and sealing episode. As shown by the optical and electron back-scattered diffraction images, the vein crystals grew epitaxially on grains of the wall rock and we hypothesize that their growth rates differed depending on whether the crystals were on a wall rock grain substrate that fractured intergranularly (slow growth rates) or transgranularly (rapid growth rates). Our multiphase field models support this hypothesis, showing that wide, blocky crystals only form where there are significant differences in the growth rate and are dependent on the type of seed grain. These results provide strong evidence for extreme growth competition, a process that we propose controls vein-filling in many micritic carbonate reservoirs, as well as demonstrate that the characteristics of the fracture wall can affect the filling processes in syntaxial veins.Supplementary material: The description and images of the studied thin sections are available at https://doi.org/10.6084/m9.figshare.c.5172371. High-resolution optical microscopy mosaics (under plane-polarized- and crossed polarized light) of the thin section collection in PetroScan file format are available on request from the authors.

Fluids sources in basement-hosted unconformity-uranium ore systems: tourmaline chemistry and boron isotopes from the Patterson Lake corridor deposits, Canada

2021 ◽  
pp. geochem2021-037
Author(s):  
E.G. Potter ◽  
C.J. Kelly ◽  
W.J. Davis ◽  
G. Chi ◽  
S-Y. Jiang ◽  
...  
Keyword(s):  
Shear Zones ◽  
Isotopic Signature ◽  
Strong Interactions ◽  
Content Type ◽  
B Values ◽  
Isotopic Signatures ◽  
Link Type ◽  
Basement Rocks ◽  
Supplementary Material ◽  
Over Time

The Patterson Lake corridor is a new uranium district located on the southwestern margin of the Athabasca Basin. Known resources extend almost one kilometer below the unconformity in graphite- and sulfide-bearing shear zones within highly altered metamorphic rocks. Despite different host rocks and greater depths below the unconformity, alteration assemblages (chlorite, illite, kaolinite, tourmaline and hematite), ore grades and textures are typical of unconformity-related deposits. This alteration includes at least three generations of Mg-rich tourmaline (magnesio-foitite). The boron isotopic composition of magnesio-foitite varies with generation: the earliest generation only observed in shallow samples from the Triple R deposit (Tur 1) contain the heaviest isotopic signature (δ11B ≈ +26 to +19 ‰), whereas subsequent generations (Tur 2, Tur 3) yield lighter and more homogeneous isotopic signatures (δ11B ≈ +17.5 to +19.9 ‰). These results are consistent with precipitation from low temperature, NaCl- and CaCl2-rich brine(s) derived from an isotopically heavy boron source (e.g. evaporated seawater) that interacted with tourmaline and silicates in the basement rocks and/or fluids derived from depth (with low δ11B values). The lower δ11B values in paragenetically later magnesio-foitite reflect greater contributions of basement-derived boron over time whereas minor compositional variations reflect local metal sources (e.g. Cr, V, Ti) and evolving fluid chemistry (decreasing Na and Ca, increasing Mg) over time. The δ11B and chemical variation in magnesio-foitite over time reinforce the strong interactions with basement rocks in these systems while supporting incursion of basinal brines well below the unconformity contact.Thematic collection: This article is part of the Uranium Fluid Pathways collection available at: https://www.lyellcollection.org/cc/uranium-fluid-pathwaysSupplementary material:https://doi.org/10.6084/m9.figshare.c.5727555

scholarly journals Basement reservoir plumbing: fracture aperture, length and topology analysis of the Lewisian Complex, NW Scotland

2020 ◽  
Vol 177 (6) ◽  
pp. 1281-1293 ◽  
Author(s):  
K. J. W. McCaffrey ◽  
R. E. Holdsworth ◽  
J. Pless ◽  
B. S. G. Franklin ◽  
K. Hardman
Keyword(s):  
Oil And Gas ◽  
Fractured Reservoirs ◽  
Crystalline Basement ◽  
Fracture Aperture ◽  
Power Law Distribution ◽  
Content Type ◽  
Scaling Properties ◽  
Link Type ◽  
Basement Rocks ◽  
Supplementary Material

Upfaulted ridges of Neoarchean crystalline basement rocks formed in the Faeroe-Shetland basin as a consequence of Mesozoic rift processes and are an active target for oil exploration. We carried out a comprehensive fault and fracture attribute study on the extensive exposures of geologically equivalent crystalline basement rocks onshore in NW Scotland (Lewisian Gneiss Complex) as an analogue for the offshore oil and gas reservoirs of the uplifted Rona Ridge basement high. Our analysis shows a power-law distribution for fracture sizes (aperture and length), with random to clustered spacing and high connectivity indices. Regional variations between the Scottish mainland and the Outer Hebrides are recognized that compare directly with variations observed along the Rona Ridge in the Faeroe-Shetland basin. Here we develop a model for the scaling properties of the fracture systems in which variations in the aperture attributes are a function of the depth of erosion beneath the top basement unconformity. More generally, the combination of size, spatial and connectivity attributes we found in these basement highs demonstrates that they can form highly effective, well-plumbed reservoir systems in their own right.Supplementary material: Additional methods and results are available at: https://doi.org/10.6084/m9.figshare.c.5017139Thematic collection: This article is part of the The Geology of Fractured Reservoirs collection available at: https://www.lyellcollection.org/cc/the-geology-of-fractured-reservoirs

Age and geochemistry of the Paleoproterozoic Bhatwari Gneiss of Garhwal Lesser Himalaya, NW India: implications for the pre-Himalayan magmatic history of the Lesser Himalayan basement rocks

2018 ◽  
Vol 481 (1) ◽  
pp. 319-339 ◽  
Author(s):  
Aranya Sen ◽  
Koushik Sen ◽  
Hari B. Srivastava ◽  
Saurabh Singhal ◽  
Purbajyoti Phukon
Keyword(s):  
Crystalline Rock ◽  
Arc Magmatism ◽  
Content Type ◽  
Basement Rocks ◽  
Petrographic Analyses ◽  
History Of ◽  
Supplementary Material ◽  
Arc Setting ◽  
Magmatic History ◽  
Log Ratio

AbstractThe Bhatwari Gneiss of Bhagirathi Valley in the Garhwal Himalaya is a Paleoproterozoic crystalline rock from the Inner Lesser Himalayan Sequence. On the basis of field and petrographic analyses, we have classified the Bhatwari Gneiss into two parts: the Lower Bhatwari Gneiss (LBG) and the Upper Bhatwari Gneiss (UBG). The geochemical signatures of these rocks suggest a monzonitic protolith for the LBG and a granitic protolith for the UBG. The UBG has a calc-alkaline S-type granitoid protolith, whereas the LBG has an alkaline I-type granitoid protolith; the UBG is more fractionated. The trace element concentrations suggest a volcanic arc setting for the LBG and a within-plate setting for the UBG. The U–Pb geochronology of one sample from the LBG gives an upper intercept age of 1988 ± 12 Ma (n = 10, MSWD = 2.5). One sample from the UBG gives an upper intercept age of 1895 ± 22 Ma (n = 15, MSWD = 0.82), whereas another sample does not give any upper intercept age, but indicates magmatism from c. 1940 to 1840 Ma. Based on these ages, we infer that the Bhatwari Gneiss has evolved due to arc magmatism and related back-arc rifting over a time period of c. 100 Ma during the Proterozoic. This arc magmatism is related to the formation of the Columbia supercontinent.Supplementary material: LA–ICP-MS calibration and adjusted major element data used for computing the isometric log-ratio transformations are available at https://doi.org/10.6084/m9.figshare.c.4272158

scholarly journals The nature and age of basement host rocks and fissure fills in the Lancaster field fractured reservoir, West of Shetland

2019 ◽  
Vol 177 (5) ◽  
pp. 1057-1073 ◽  
Author(s):  
R. E. Holdsworth ◽  
R. Trice ◽  
K. Hardman ◽  
K. J. W. McCaffrey ◽  
A. Morton ◽  
...  
Keyword(s):  
Fractured Reservoirs ◽  
Source Rocks ◽  
Fractured Reservoir ◽  
Fracture Permeability ◽  
Near Surface ◽  
Thin Sections ◽  
Content Type ◽  
Link Type ◽  
Fracture Systems ◽  
Supplementary Material

Hosting up to 3.3 billion barrels of oil in place, the upfaulted Precambrian crystalline rocks of the Lancaster field, offshore west of Shetland, give key insights into how fractured hydrocarbon reservoirs can form in such old rocks. The Neoarchean (c. 2700–2740 Ma) charnockitic basement is cut by deeply penetrating oil-, mineral- and sediment-filled fissure systems seen in geophysical and production logs and thin sections of core. Mineral textures and fluid inclusion geothermometry suggest that a low-temperature (<200°C) near-surface hydrothermal system is associated with these fissures. The fills help to permanently prop open fissures in the basement, permitting the ingress of hydrocarbons into extensive well-connected oil-saturated fracture networks. U–Pb dating of calcite mineral fills constrains the onset of mineralization and contemporaneous oil charge to the mid-Cretaceous and later from Jurassic source rocks flanking the upfaulted ridge. Late Cretaceous subsidence and deposition of mudstones sealed the ridge, and was followed by buoyancy-driven migration of oil into the pre-existing propped fracture systems. These new observations provide an explanation for the preservation of intra-reservoir fractures (‘joints’) with effective apertures of 2 m or more, thereby highlighting a new mechanism for generating and preserving fracture permeability in sub-unconformity fractured basement reservoirs worldwide.Supplementary material: Analytical methods and isotopic compositions and ages are available at https://doi.org/10.6084/m9.figshare.c.4763237Thematic Collection: This article is part of the Geology of Fractured Reservoirs collection available at: https://www.lyellcollection.org/cc/the-geology-of-fractured-reservoirs

Anatomy of two Permian greenschist- to blueschist-facies tectonic mélanges in the Solonker Suture Zone (Inner Mongolia, northeastern China): evidence for divergent double subduction and soft collision

2020 ◽  
Vol 177 (5) ◽  
pp. 981-996 ◽  
Author(s):  
Huichuan Liu ◽  
Jichun Wang ◽  
Chun-Kit Lai ◽  
Yinglei Li ◽  
Jingfan Wang ◽  
...  
Keyword(s):  
Inner Mongolia ◽  
Northeastern China ◽  
Controversial Issues ◽  
Early Permian ◽  
Content Type ◽  
The Matrix ◽  
Ophiolitic Rocks ◽  
Supplementary Material ◽  
Tectonic Mélange ◽  
East West

The timing and processes of the Paleo-Asian Ocean (PAO) closure are still controversial issues due to ambiguous interpretation of the regional geological records. We describe the structural and kinematic features of two Permian greenschist-/blueschist-facies tectonic mélanges at Zhurihe and Erdaojing in Inner Mongolia (northeastern China), separated by the Solonker Suture. The tectonic mélanges have block-in-matrix fabrics. The Zhurihe tectonic mélange in the south is dominated by NE-trending folds, and east–west- to NE-trending reverse/strike-slip faults, indicating a NW–SE compressive direction. Blocks in the Zhurihe mélange include ophiolitic fragments (c. 292 Ma) and ocean island sequences (c. 274 Ma), which show E-MORB and OIB geochemical affinities, respectively. The matrix includes blueschist, greenschist, quartz schist and clastics, of which the blueschist yielded a zircon U–Pb age of c. 255 Ma, and is geochemically E-MORB-like. The Erdaojing mélange north of the Solonker Suture is dominated by east–west-trending folds, and NE- and NW-trending reverse faults, indicating a north–south compressive orientation. The Erdaojing mélange is composed of early Permian (c. 281 Ma) ophiolite blocks, and middle Permian (c. 266 Ma) actinolite schist and clastics as the matrix. The Erdaojing ophiolitic rocks are geochemically N-MORB tholeiitic. Our results define two parallel Permian MORB-type ophiolitic belts separated by the central Solonker Suture. These observations are evidence for the Permian divergent double subduction and Early Triassic soft-collision model for the PAO along the Solonker Suture.Supplementary material: Detailed analytical methods and results are available at: https://doi.org/10.6084/m9.figshare.c.5008910

The largest arthropod in Earth history: insights from newly discovered Arthropleura remains (Serpukhovian Stainmore Formation, Northumberland, England)

2021 ◽  
pp. jgs2021-115
Author(s):  
Neil S. Davies ◽  
Russell J. Garwood ◽  
William J. McMahon ◽  
Joerg W. Schneider ◽  
Anthony P. Shillito
Keyword(s):  
Atmospheric Oxygen ◽  
Early Carboniferous ◽  
Late Carboniferous ◽  
Three Dimensions ◽  
Early Permian ◽  
Content Type ◽  
Delta Plain ◽  
3D Mesh Model ◽  
Supplementary Material ◽  
Body Fossil

Arthropleura is a genus of giant myriapods that ranged from the early Carboniferous to Early Permian, with some individuals attaining lengths >2 m. Although most of the known fossils of the genus are disarticulated and occur primarily in late Carboniferous (Pennsylvanian) strata, we report here partially articulated Arthropleura remains from the early Carboniferous Stainmore Formation (Serpukhovian; Pendleian) in the Northumberland Basin of northern England. This 76 × 36 cm specimen represents part of an exuvium and is notable because only two comparably articulated giant Arthropleura fossils are previously known. It represents one of the largest known arthropod fossils and the largest arthropleurid recovered to date, the earliest (Mississippian) body fossil evidence for gigantism in Arthropleura, and the first instance of a giant arthropleurid body fossil within the same regional sedimentary succession as the large arthropod trackway Diplichnites cuithensis. The remains represent 12–14 anterior Arthropleura tergites in the form of a partially sand-filled dorsal exoskeleton. The original organism is estimated to have been 55 cm in width and up to 2.63 m in length, weighing c. 50 kg. The specimen is preserved partially in three dimensions within fine sandstone and has been moderately deformed by synsedimentary tectonics. Despite imperfect preservation, the specimen corroborates the hypothesis that Arthropleura had a tough, sclerotized exoskeleton. Sedimentological evidence for a lower delta plain depositional environment supports the contention that Arthropleura preferentially occupied open woody habitats, rather than swampy environments, and that it shared such habitats with tetrapods. When viewed in the context of all the other global evidence for Arthropleura, the specimen contributes to a dataset that shows the genus had an equatorially restricted palaeogeographical range, achieved gigantism prior to late Paleozoic peaks in atmospheric oxygen, and was relatively unaffected by climatic events in the late Carboniferous, prior to its extinction in the early Permian.Supplementary material: Images of 3D mesh model of Arthropleura are available at https://doi.org/10.6084/m9.figshare.c.5715450

Volcano-sedimentary processes at Las Derrumbadas rhyolitic twin domes, Serdán-Oriental basin, Eastern Trans-Mexican Volcanic Belt

2021 ◽  
pp. SP520-2021-144
Author(s):  
Marie-Noëlle Guilbaud ◽  
Corentin Chédeville ◽  
Ángel Nahir Molina-Guadarrama ◽  
Julio Cesar Pineda-Serrano ◽  
Claus Siebe
Keyword(s):  
Volcanic Belt ◽  
Debris Avalanche ◽  
Density Currents ◽  
Content Type ◽  
Mexican Volcanic Belt ◽  
Wide Range ◽  
Eruptive Episode ◽  
Basin Floor ◽  
Supplementary Material ◽  
Trans Mexican Volcanic Belt

AbstractThe eruption of the ∼10 km3 rhyolitic Las Derrumbadas twin domes about 2000 yrs ago has generated a wide range of volcano-sedimentary deposits in the Serdán-Oriental lacustrine basin, Trans-Mexican Volcanic Belt. Some of these deposits have been quarried, creating excellent exposures. In this paper we describe the domes and related products and interpret their mode of formation, reconstructing the main phases of the eruption as well as syn-and-post eruptive erosional processes. After an initial phreatomagmatic phase that built a tuff ring, the domes grew as an upheaved plug lifting a thick sedimentary pile from the basin floor. During uplift, the domes collapsed repeatedly to form a first-generation of hetero-lithologic hummocky debris avalanche deposits. Subsequent dome growth produced a thick talus and pyroclastic density currents. Later, the hydrothermally-altered over-steepened dome peaks fell to generate 2nd generation, mono-lithologic avalanches. Subsequently, small domes grew in the collapse scars. From the end of the main eruptive episode onwards, heavy rains remobilized parts of the dome carapaces and talus, depositing lahar aprons. Las Derrumbadas domes are still an important source of sediments in the basin, and ongoing mass-wasting processes are associated with hazards that should be assessed, given their potential impact on nearby populations.Supplementary material at https://doi.org/10.6084/m9.figshare.c.5752296

Geochemical evaluation of the in-situ regolith of Madengi hills of Dodoma, Tanzania. Implication on bedrock mapping and delineating gold mineralization target

2021 ◽  
pp. geochem2021-074
Author(s):  
Godson Godfray
Keyword(s):  
Gold Mineralization ◽  
Early Stage ◽  
Shear Zones ◽  
Soil Geochemistry ◽  
Residual Soils ◽  
Content Type ◽  
Geochemical Evaluation ◽  
Supplementary Material ◽  
Pathfinder Elements

Successful gold exploration projects depend on a piece of clear information on the association between gold, trace elements, and mineralization controlling factors. The use of soil geochemistry has been an important tool in pinpointing exploration targets during the early stage of exploration. This study aimed to establish the gold distribution, the elemental association between gold and its pathfinder elements such as Cu, Zn, Ag, Ni, Co, Mn, Fe, Cd, V, Cr, Ti, Sc, In, and Se and identify lithologies contributing to the overlying residual soils. From cluster analysis, a high similarity level of 53.93% has been shown with Ag, Cd, and Se at a distance level of 0.92. Au and Se have a similarity level of 65.87% and a distance level of 0.68, hence is proposed to be the most promising pathfinder element. PCA, FA, and the Pearson's correlation matrix of transformed data of V, Cu, Ni, Fe, Mn, Cr, and Co and a stronger correlation between Pb and U, Th, Na, K, Sn, Y, Ta and Be shows that source gold mineralization might be associated with both hornblende gneisses interlayered with quartzite, tonalite, and tonalitic orthogneiss. From the contour map and gridded map of Au and its pathfinder elements, it has been noted that their anomalies and target generated are localized in the Northern part of the area. The targets trend ESE to WNW nearly parallel to the shear zones as a controlling factor of Au mineralization emplacement.Supplementary material:https://doi.org/10.6084/m9.figshare.c.5721965

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