scholarly journals Geochemistry of macrofossil, bulk rock, and secondary calcite in the Early Jurassic strata of the Llanbedr (Mochras Farm) drill core, Cardigan Bay Basin, Wales, UK.

2021 ◽  
pp. jgs2021-018
Author(s):  
Clemens V. Ullmann ◽  
Dominika Szűcs ◽  
Mengjie Jiang ◽  
Alexander J.L. Hudson ◽  
Stephen P. Hesselbo
Keyword(s):  
Sea Level ◽  
Carbon Isotope ◽  
Analytical Data ◽  
Early Jurassic ◽  
Sediment Provenance ◽  
Carbonate Content ◽  
Level Control ◽  
Content Type ◽  
The Core ◽  
Supplementary Material

The Llanbedr (Mochras Farm) core (Wales, United Kingdom) yielded a > 1,300 m long mudrock sequence which has excellent potential for establishing an integrated stratigraphic scheme for the entire Early Jurassic Epoch. Lithological variations in the core are predominantly driven by hierarchical changes in carbonate content which also dominate – or may impact upon – many geochemical and physical properties of the core. The bulk carbonate carbon isotope record displays systematic fluctuations the largest of which correspond to previously identified phases of environmental perturbation. The magnitudes of negative carbon isotope excursions in carbonate are inflated compared to equivalents previously described elsewhere due to diagenesis and concomitant loss of primary carbonate.The marine macrofossil record of Mochras reveals biological and isotopic patterns that are generally comparable to other UK basins. Potentially significant differences between Cleveland and Cardigan Bay Basin are observed in the Pliensbachian and Toarcian fossils. This different expression may be related to different habitat structure or palaeoceanographic and water depth differences between these basins. Minima in macrofossil δ18O values generally coincide with peaks in macrofossil wood abundance and sea-level lowstands inferred from sequence stratigraphic interpretation of other UK sections. This relationship points to a possible relative sea-level control on observed oxygen isotope records and sediment provenance.Supplementary material: Analytical data for the Mochras core pertaining to this contribution is available online at https://doi.org/10.6084/m9.figshare.c.5463508

Rapid cooling during late-stage orogenesis and implications for the collapse of the Scandian retrowedge, northern Scotland

2020 ◽  
Vol 178 (1) ◽  
pp. jgs2020-022
Author(s):  
B.M. Spencer ◽  
J.R. Thigpen ◽  
R.D. Law ◽  
C.A. Mako ◽  
C.S. McDonald ◽  
...  
Keyword(s):  
Analytical Data ◽  
Accelerated Cooling ◽  
High Grade ◽  
Supplementary Data ◽  
Cooling Rates ◽  
Thrust Sheet ◽  
Content Type ◽  
Data Table ◽  
Supplementary Material ◽  
Supplementary Data Table

New 40Ar/39Ar thermochronological and deformation temperature analyses in the Scandian (c. 435–420 Ma) orogenic retrowedge of northern Scotland demonstrate accelerated cooling during late syn- to post-orogenic exhumation of the high-grade orogenic core. Initial cooling rates of 10–30°C myr−1 immediately following peak orogenesis transitioned to rapid rates of 45–90°C myr−1 during final exhumation of the Naver thrust sheet in the orogenic core. The flanking ductile thrust sheets exhibit a similar, albeit less pronounced, acceleration of cooling, with rates increasing by c. 150–300% following peak orogenesis. Closer to the foreland, the Moine thrust sheet did not experience increased cooling rates. Calculated unroofing rates of 3.75 mm a−1 in the high-grade Naver thrust sheet suggest increasing, rapid exhumation in the orogenic core during a presumed collapse phase of orogenesis. This is contrary to the expectation of decreasing erosional efficiency as topography is diminished and is interpreted to suggest that unroofing of the Scottish Caledonides may have been partially enhanced by upper crustal extensional deformation during ductile flow of the infrastructure of the orogenic core. Similar processes have been interpreted in the East Greenland Caledonides, which form the northern extension of the Scandian retrowedge.Supplementary material:40Ar/39Ar analytical data for muscovite (Supplementary Data Table 1), 40Ar/39Ar analytical data for amphibole (Supplementary Data Table 2), and electron microprobe analytical data for amphibole samples (Supplementary Data Table 3) is available at: https://doi.org/10.6084/m9.figshare.c.5087057

Terreneuvian bio- and chemostratigraphy of the South Sichuan Region (South China)

2021 ◽  
pp. jgs2020-167
Author(s):  
Ben Yang ◽  
Michael Steiner
Keyword(s):  
Carbon Isotope ◽  
South China ◽  
Cambrian Explosion ◽  
The South ◽  
Yangtze Craton ◽  
Content Type ◽  
Link Type ◽  
Small Shelly Fossils ◽  
Assemblage Zone ◽  
Supplementary Material

Classical sections, such as the Maidiping and Daqiao Mine sections of South Sichuan (China), expose early Cambrian deposits that are crucial for understanding the biological and environmental evolution of Yangtze Craton. These sequences are rich in Terreneuvian small shelly fossils, which can be assigned to assemblages I and III from South China. The Anabarites trisulcatus– Protohertzina anabarica Assemblage Zone (Assemblage I) is recognized at the lower Maidiping Formation. The second assemblage (Paragloborilus subglobosus – Purella squamulosa Assemblage Zone) cannot be verified in South Sichuan, although previous reports claimed its existence based on the occurrence of Paragloborilus subglobosus. The third assemblage (Watsonella crosbyi Assemblage Zone) is confirmed in the upper Maidiping Formation. The abundant bioclasts in this interval indicate abrasions and bioerosions by winnowing or starved sedimentation. Carbon isotope values from the Maidiping section present no negative excursion at the presumed Ediacaran–Cambrian transition. A positive carbon isotope excursion is observed in the upper Maidiping Formation (Assemblage III) which is correlated to the ZHUCE excursion in the Dahai Member of eastern Yunnan. The shallow water deposits of South Sichuan can be correlated with the South China, western Mongolia and Siberia successions based on biozonations and carbon isotope trends.Thematic collection: This article is part of the Advances in the Cambrian Explosion collection available at: https://www.lyellcollection.org/cc/advances-cambrian-explosionSupplementary material:https://doi.org/10.6084/m9.figshare.c.5326834

Hardground, gap and thin black shale: spatial heterogeneity of arrested carbonate sedimentation during the Jenkyns Event (T-OAE) in a Tethyan pelagic basin (Gerecse Mts, Hungary)

2021 ◽  
pp. SP514-2020-266
Author(s):  
Tamás Müller ◽  
Gregory D. Price ◽  
Emanuela Mattioli ◽  
Máté Zs. Leskó ◽  
Ferenc Kristály ◽  
...  
Keyword(s):  
Carbon Isotope ◽  
Black Shale ◽  
Carbonate Production ◽  
Content Type ◽  
Oceanic Anoxic Event ◽  
Carbonate Sedimentation ◽  
Environmental Perturbations ◽  
Carbon Isotope Excursion ◽  
Supplementary Material ◽  
Tethys Ocean

AbstractThe Jenkyns Event or Toarcian Oceanic Anoxic Event (T-OAE) was an episode of severe environmental perturbations reflected in carbon isotope and other geochemical anomalies. Although well studied in the epicontinental basins in NW Europe, its effects are less understood in open marine environments. Here we present new geochemical (carbon isotope, CaCO3, [Mn]) and nannofossil biostratigraphic data from the Tölgyhát and Kisgerecse sections in the Gerecse Hills (Hungary). These sections record pelagic carbonate sedimentation near the margin of the Tethys Ocean. A negative carbon isotope excursion of ∼6‰ is observed in the Tölgyhát section, in a condensed clay and black shale layer where the CaCO3 content drops in association with the Jenkyns Event. At Kisgerecse, bio- and chemostratigraphic data suggest a gap in the lower Toarcian. The presence of an uppermost Pliensbachian hardground, absence of the lowermost Toarcian Tenuicostatum ammonite zone, and the condensed record of the Jenkyns Event at Tölgyhát, together with a condensed Tenuicostatum Zone and the missing negative carbon isotope anomaly at Kisgerecse implies arrested carbonate sedimentation. A calcification crisis and sea-level rise together led to a decrease in carbonate production and terrigenous input, suggesting that volcanogenic CO2-driven global warming may have been their common cause.Supplementary material at https://doi.org/10.6084/m9.figshare.c.5355342

Primary versus carbonate production in the Toarcian, a case study from the Llanbedr borehole (Mochras Farm, Wales)

2021 ◽  
pp. SP514-2021-19
Author(s):  
Alessandro Menini ◽  
Emanuela Mattioli ◽  
Stephen P. Hesselbo ◽  
Micha Ruhl ◽  
Guillaume Suan
Keyword(s):  
Sea Water ◽  
Initial Perturbation ◽  
Carbonate Production ◽  
Nutrient Delivery ◽  
Content Type ◽  
High Productivity ◽  
Oceanic Anoxic Event ◽  
The Core ◽  
C Isotopes ◽  
Supplementary Material

AbstractThe leading hypothesis for the Toarcian oceanic anoxic event (T-OAE; ∼183Ma) and the associated negative C-isotope excursion is the massive release of 12C favouring greenhouse and continental weathering. The nutrient delivery to shallow-basins supported productivity and, because of O2-consumption by organic-matter respiration, anoxia development. However, several works showed that calcareous nannoplankton experienced a decrease during the T-OAE. Nannofossil fluxes measured in the Llanbedr borehole (Mochras Farm; Wales, UK) were the highest prior to the negative C-isotope excursion, along with high amounts of taxa indicative of nutrient-rich environments (Biscutaceae). Such conditions attest to high productivity. Fluxes show the lowest values in the core of the event, along with a size decrease of Schizosphaerella and a peak in Calyculaceae. The recovery of nannofossil fluxes and Schizosphaerella size occurred concomitant with the return of C-isotopes to more positive values. Concomitantly, deep-dwellers (Crepidolithus crassus) dominated, indicating a recovery of the photic-zone productivity. These observations demonstrate that the cascade of environmental responses to the initial perturbation was more complex than previously considered. In spite of elevated nutrient delivery to epicontinental basins in the early Toarcian, carbonate and primary productions of nannoplankton were depressed in the core the T-OAE likely because of prolonged thermohaline sea-water stratification.Supplementary material at https://doi.org/10.6084/m9.figshare.c.5541440

U–Pb detrital zircon constraints on active margin magmatism and sedimentation after the Grampian Orogeny in western Ireland

2020 ◽  
pp. jgs2020-093
Author(s):  
Peter D. Clift ◽  
Amy L. Luther ◽  
Madison E. Avery ◽  
Paul B. O'Sullivan
Keyword(s):  
Detrital Zircon ◽  
Analytical Data ◽  
Sedimentary Rocks ◽  
Detrital Zircons ◽  
Upper Ordovician ◽  
Content Type ◽  
Igneous Intrusions ◽  
Supplementary Material ◽  
Western Ireland ◽  
Harbour Area

Early Ordovician collision of the Lough Nafooey Arc (part of the Baie Verte Oceanic Tract) with the passive continental margin of Laurentia peaked at c. 475 Ma in Scotland and Ireland and was followed by subduction polarity reversal. We examined Upper Ordovician–Silurian sedimentary rocks from western Ireland to see whether collision was followed by renewed arc magmatism. Despite the scarcity of dated igneous intrusions between the Grampian (c. 470 Ma) and Acadian (c. 420 Ma) orogenies in Ireland, detrital zircons show a continuity of activity peaking at 480–440 Ma, implying no hiatus in regional magmatism. Differences in zircon U–Pb age spectra highlight the isolation of basins in the southern Killary Harbour area from those north of the South Mayo Trough. These latter rocks were largely derived by erosion from Moine and Upper Dalradian sources. By contrast, the Killary Harbour Basin shows a decreasing influence from the Dalradian after c. 436 Ma and an increasing influence of contemporaneous magmatic zircons. These were transported from sources along-strike from the present NE, probably at the southern end of the Scandian Mountains in SE Greenland. The western Irish basins formed as pull-apart basins in a forearc setting and are analogous to Cenozoic pull-apart basins in Sumatra.Supplementary material: U-Pb zircon analytical data is available at a decreasing influence from the Dalradian after c. 436 Ma and an increasing influence of contemporaneous https://doi.org/10.6084/m9.figshare.c.5209849

Did an earthquake located off Jersey trigger a mudflow preserving the only loess outcrop known under the seas?

2020 ◽  
Vol 177 (5) ◽  
pp. 911-922
Author(s):  
Jean-Pierre Lefort ◽  
Paul Chambers ◽  
Guzel Danukalova ◽  
Jean-Laurent Monnier ◽  
Eugenia Osipova ◽  
...  
Keyword(s):  
Slope Failure ◽  
Core Sample ◽  
Seismic Zone ◽  
Content Type ◽  
The Core ◽  
Loess Deposit ◽  
Mollusc Species ◽  
Supplementary Material ◽  
Rule Out

A 5 m long core sample, retrieved from between Jersey and the Cotentin (France) has been studied using different techniques. Between the lower part and the top, the core is a yellowish loess, a black mud and a grey shelly formation. The basal formation, which displays a typical loess granulometry, is the only loess found anywhere in situ on the subtidal seabed worldwide. Elsewhere, this fragile sediment has always been washed out by the successive Quaternary transgressions. The overlying mud formation, which displays characteristics of a landslide, sealed the loess deposit. Because this core has been sampled on the rim of a submarine valley located in front of a major onshore geological disruption, clearly associated with an active seismic zone, it is possible that an earthquake triggered a slope failure causing a gravity collapse of materials that protected the loess deposit from subsequent erosion, although we cannot completely rule out the effects of a large flood.Supplementary material: Tables showing the results of the pollen analysis and distribution of mollusc shells and other fossils in borehole O-VC15, and a figure showing the distribution of mollusc species in core O-VC15 and a possible reconstruction of the palaeoenvironment during sediment deposition are available at https://doi.org/10.6084/m9.figshare.c.4991753

scholarly journals Thin-section detrital zircon geochronology mitigates bias in provenance investigations

2021 ◽  
pp. jgs2021-070
Author(s):  
Isabel C. Zutterkirch ◽  
Christopher L. Kirkland ◽  
Milo Barham ◽  
Chris Elders
Keyword(s):  
Detrital Zircon ◽  
Sample Selection ◽  
Sediment Provenance ◽  
Thin Sections ◽  
Zircon Age ◽  
Content Type ◽  
The North ◽  
Supplementary Material ◽  
Northern Carnarvon Basin ◽  
Depositional Age

Detrital zircon U-Pb geochronology has enabled advances in the understanding of sediment provenance, transportation pathways, and the depositional age of sedimentary packages. However, sample selection and processing can result in biasing of detrital zircon age spectra. This paper presents a novel approach using in-situ detrital zircon U-Pb measurements on thin-sections to provide greater confidence in maximum depositional ages and provenance interpretations. New U-Pb age data of 310 detrital zircon grains from 16 thin-sections of the Triassic Mungaroo Formation from two wells in the Northern Carnarvon Basin, Australia, are presented. Whilst detrital zircon age modes are consistent with previous work, there are some differences in the relative proportions of age modes, which is partly attributed to a lack of small grains in hand-picked grain mounts. The relative sample bias is quantified via grain size comparison of dated zircon (in thin-sections or hand-picked mounts) relative to all zircons identified in bulk-mounts and thin-sections. The youngest age mode (∼320 – 195 Ma) is consistent with an active margin to the north, likely South West Borneo and/or Lhasa terrane. The dated zircons reveal a maximum depositional age of 197 Ma for the upper part of Mungaroo Formation, suggesting deposition continued into the Early Jurassic.Supplementary material:https://doi.org/10.6084/m9.figshare.c.5628911

Fluid flow and syntectonic veining in an Ediacaran-Cambrian foreland fold–thrust zone, western margin of the São Francisco Craton, Brazil

2021 ◽  
pp. jgs2020-061
Author(s):  
Melina C. B. Esteves ◽  
Frederico M. Faleiros
Keyword(s):  
Fluid Flow ◽  
Fluid Inclusion ◽  
Analytical Data ◽  
Microstructural Analysis ◽  
Fluid Pressure ◽  
Content Type ◽  
Western Margin ◽  
Metasedimentary Rocks ◽  
Supplementary Material ◽  
Fault Valve

The western margin of the São Francisco Craton, central Brazil presents a 1300 km long foreland fold–thrust belt where Ediacaran-Cambrian (560–520 Ma) metasedimentary rocks from the Bambuí Group were subsequently deformed during post-collisional stages (520–495 Ma) related to Gondwana assembly. This scenario provides an opportunity to quantify fluid flow regimes and fault-related processes that were active in exhumed foreland fold–thrust zones, which were estimated based on structural, microstructural and fluid inclusion studies of syntectonic veins and host rocks. Kaolinite-bearing synkinematic mineral assemblages from metasedimentary rocks, thermodynamic models and grain-scale deformation accommodated by dissolution–precipitation creep and intracrystalline deformation indicate metamorphic and deformational conditions of 250–270°C. Subhorizontal extensional veins formed under subhorizontal shortening and subvertical extension, supporting vein development under a fold–thrust regime that formed regional NW–SE-trending thrust fault zones and megafolds with NW–SE-trending axes. Orientation and growth microstructures indicate that NW–SE-trending subvertical cleavage-parallel veins formed under subhorizontal NE–SW extension, compatible with those inferred to produce mapped kilometre-scale gentle folds with NE–SW-trending traces. Two primary aqueous fluid inclusion assemblages (FIA) are distinguished by salinity variation: 2–21 wt% NaCleq. in subhorizontal veins and 6–0 wt% NaCleq. in cleavage-parallel subvertical veins. Fluid inclusion thermometry and microstructural analysis suggest that veins crystallized between 250 and 270°C under fluid pressure fluctuating within a range of 50–500 MPa (subhorizontal veins) and 80–320 MPa (cleavage-parallel subvertical veins), evidencing fault-valve behaviour. Trends of coupled decreases in salinity and homogenization temperatures in both FIA indicate downward mixing of meteoric fluids, which was more effective in subvertical veins and was in both cases enhanced by fault-valve behaviour. Dominance of moderate salinity and absence of CO2 and CH4 indicate that the fluids are dominated by formation waters. The salinity signature is similar to those of formation waters and metamorphic fluids derived from rocks of shallow marine environments worldwide.Supplementary material: Details of samples and analytical data are available at https://doi.org/10.6084/m9.figshare.c.5275031

Timing of slip across the South Tibetan detachment system and Yadong–Gulu graben, Eastern Himalaya

2020 ◽  
Vol 178 (1) ◽  
pp. jgs2019-197
Author(s):  
Hanwen Dong ◽  
Kyle P. Larson ◽  
Dawn A. Kellett ◽  
Zhiqin Xu ◽  
Guangwei Li ◽  
...  
Keyword(s):  
Late Miocene ◽  
Analytical Data ◽  
Eastern Himalaya ◽  
Model Parameters ◽  
The South ◽  
Southern Tibet ◽  
Content Type ◽  
The North ◽  
Extensional Strain ◽  
Supplementary Material

The Yadong–Gulu graben preserves the kinematic and temporal relationships between east–west-directed extension in southern Tibet and north–south extensional strain in the Himalaya. In the Yadong region, distinct outer and inner top-down-to-the-north segments of the South Tibetan detachment system (STDS) are recognized. Herein, we combine high- to medium-T (U–Pb, 40Ar/39Ar) and low-T (apatite fission-track, apatite (U–Th)/He and zircon (U–Th)/He) thermochronometry to investigate the timing of slip across the STDS and Yadong–Gulu structures. These data demonstrate that the cessation of the Yadong shear zone, the structurally outer ductile segment of the STDS, occurred c. 20 Ma and that motion along the inner brittle–ductile Zherger La detachment continued after c. 16.6 Ma, ending by 11 Ma. The cooling history in the immediate STDS footwall is characterized by two main episodes of relatively rapid cooling and exhumation. The first occurred in the middle Miocene (c. 15–11 Ma), and is common along-strike of the innermost STDS footwall, related to cooling of the STDS. The second occurred in the late Miocene–Pliocene (c. 7–3 Ma), and is local to the Yadong–Gulu graben footwall in NW Bhutan, indicating that late Miocene–Pliocene slip along the graben system contributed to exhumation of the STDS east of the graben rift.Supplementary material: Tables of analytical data, dating results, and input data and model parameters of HeFTy are available at https://doi.org/10.6084/m9.figshare.c.5132941

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