Formation Of Red Beds Research Articles

Persistent cooling in the Ordovician (Darriwilian–Sandbian) revealed by conodont δ18O records in the Tarim Basin, NW China: Climatic and sedimentary implications

The Great Ordovician Biodiversification Event (GOBE) was considered to be primarily triggered by climate cooling. However, the precise timing of the greenhouse-icehouse transition during the Ordovician Period remains largely unconstrained. In order to reconstruct palaeotemperatures during the Middle−Late Ordovician transition in the Tarim Basin, northwestern China, a high-resolution SIMS-based oxygen isotope (δ18Oapatite) profile was obtained from 65 conodont elements from the Nanyigou section. The oxygen isotope record based on mono-generic samples reveals an increase in δ18Oapatite by 1.6‰ (VSMOW) during the late Darriwilian to Sandbian, translating to tropical sea-surface water cooling of ∼7 °C, probably marking the initiation of polar ice sheet growth and the prelude of the Hirnantian maximum. This δ18Oapatite shift agrees well with the time-equivalent δ18Oapatite records from other continents, thus suggestiing a global signal. Furthermore, the positive shift of δ18Oapatite overall coincides with inorganic carbon isotope (δ13Ccarb) positive excursion, indicating that enhanced organic carbon burial may have led to the atmospheric pCO2 drawdown. Additionally, the temporal coincidence between the δ18Oapatite positive excursion and development of the purplish-red nodular limestone of the Tumuxiuke Formation suggests that global cooling may have contributed to the formation of coeval marine red beds.

The Mechanism of Mineral Dissolution on the Development of Red-Bed Landslides in the Wudongde Reservoir Region

The Wudongde reservoir region exhibits a notable prevalence of landslides within the red-bed reservoir stratum. The red bed is a clastic sedimentary rock layer dominated by red continental deposits. It is mainly composed of sandstone, mudstone, and siltstone. The lithology is diverse and uneven. In this study, we delve into the impact of mineral dissolution on the development of red-bed landslides in the reservoir area by utilizing the Xiaochatou landslide as a representative case study. Considering the inherent susceptibility of red-bed formations to erosion, collapse, and softening when exposed to water, an investigation was conducted to examine the consequences of mineral dissolution on landslides occurring in these areas. We conducted a mineral analysis and an identification of rock samples from the Xiaochatou landslide site, revealing alternating layers of sandstone and mudstone. Sandstone and conglomerate specimens were immersed in deionized water, and advanced techniques such as scanning electron microscopy (SEM), ion chromatography (IC), and inductively coupled plasma (ICP) analysis were used to examine the effects of water immersion. We also employed the hydrogeochemical simulation software PHREEQC to understand the dissolution mechanism of gypsum during soaking. Our findings reveal that sandstone and conglomerates harbor a notable quantity of gypsum, which readily dissolves in water. Prolonged immersion leads to erosion cavities within the sandstone, thereby augmenting its permeability. The concentration of SO42− ions in the soaking solution emerges as the highest, followed by Ca2+ and Na+. The notable significance is the dissolution of gypsum, whose intricate mechanism is contingent upon diverse environmental conditions. Variations in ion concentration profoundly influence the saturation index (SI) value, with the pH value playing a crucial role in shifting the reaction equilibrium. Regarding the deformation mode of the landslide, it manifests as a combination of sliding compression and tension cracking. The fracture surface of the landslide assumes a step-like configuration. As the deformation progresses, the mudstone layer takes control over the sliding process, causing the sandstone to develop internal narrow-top and wide-bottom cracks, which propagate upward until the stability of the slope rock mass is compromised, resulting in its rupture. In this manuscript, we delve into the dissolution traits of red-bed soft rock in the Wudongde reservoir area, using a landslide case as a reference. We simulate this rock’s dissolution under environmental water influences, examining its interaction with diverse water types through rigorous experiments and simulations. This study’s importance lies in its potential to shed light on the crucial engineering characteristics of red-bed soft rock.

Palaeomagnetism of the mid-Cretaceous red beds from the Tethyan Himalaya: direction discrepancy and tectonic implications

Understanding the northern extension of Greater India is vital for modelling the India–Asia collision process and the formation of the Tibetan Plateau. We present new palaeomagnetic data from the mid-Cretaceous ( c. 106 Ma) Gyabula Formation red beds in the Tethyan Himalaya. Well-defined high laboratory unblocking temperature component magnetizations were isolated from 19 sites and pass the fold tests, indicating that they are pre-folding magnetizations. The tilt-corrected site-mean direction is D s = 222.9°, I s = +39.4° with α 95 = 4.2°. The site-mean inclination increases from 39.4° to 45.8° after anisotropy-based inclination shallowing correction. The declination and inclination differ considerably from those of neighbouring sections. This directional discrepancy of the red beds may be attributed to the fact that the sampled section (sites ZB1–11, 40–52) is overturned and to local vertical-axis rotation. The combination of our new and previously published palaeomagnetic data shows that the Tethyan Himalaya was located at 27.5° ± 2.4°S for the sampled area. Our new results, together with the reliable Cretaceous palaeomagnetic data obtained from the Tethyan Himalaya as well as coeval palaeolatitudes expected from the Indian craton, support a smaller Greater India and that the Tethyan Himalaya did not rift from the Indian craton during the mid-Cretaceous. Supplementary material: Detrital zircon U–Pb ages data, hysteresis parameters data, palaeomagnetic datasets for the Gyabula Formation, anisotropy of isothermal remanent magnetization data, available latest Jurassic–Early Cretaceous palaeomagnetic results from the Tethyan Himalaya and supplementary figures are available at https://doi.org/10.6084/m9.figshare.c.6795683 Thematic collection: This article is part of the Mesozoic and Cenozoic tectonics, landscape and climate change collection available at: https://www.lyellcollection.org/topic/collections/mesozoic-and-cenozoic-tectonics-landscape-and-climate-change

Fluid Flow, Alteration, and Timing of Cu-Ag Mineralization at the White Pine Sediment-Hosted Copper Deposit, Michigan, USA

Abstract White Pine, located in Michigan’s Upper Peninsula, is an archetypal sediment-hosted stratiform copper deposit. The Midcontinent rift system is one of only seven basins globally that host a giant sediment-hosted stratiform copper deposit. Despite many similarities with other deposits of this type, White Pine displays some important differences, including the late Mesoproterozoic age, a thick basalt sequence, an apparent lack of evaporites, and a lacustrine depositional setting. This study analyzes paleofluid flow related to the formation of White Pine and places a particular emphasis on structural and diagenetic fluid pathways. Most of the ore is located in a 30-m-wide zone spanning the Copper Harbor Formation red beds and the overlying Nonesuch Formation shales. Sedimentation of these units was accompanied by subtle synsedimentary faulting. Premineralization phases include calcite concretions and nodules, illite and hematite grain coatings, isopachous chlorite rims, emplacement of liquid petroleum (now pyrobitumen), and bleaching. Mineralization introduced native copper into the footwall sandstones and a zoned suite of native copper and sulfur-poor copper sulfide minerals across a migrating redox front in the overlying shales where copper minerals nucleated on authigenic and detrital chlorite grains. Postmineralization phases include quartz cement, calcite cement, and calcite veins that partially overlapped inversion of synsedimentary faults. Contrary to previous studies, we identified evidence for only one phase of mineralization. An Re-Os chalcocite age of 1067 ± 11 Ma places mineralization 11 to 17 m.y. after host-rock deposition. Sulfide δ34S values of –14.0 to 29.9‰ suggest an important contribution from sour gas and thermochemical sulfate reduction of seawater. Carbon (δ13C) and oxygen (δ18O) isotope compositions of five calcite generations range from –15.1 to –1.3‰ and 10.4 to 41.3‰, respectively, and record early meteoric pore water displaced by later seawater. White Pine is both a sediment-hosted stratiform copper deposit and a paleo-oil field. Synsedimentary faults controlled the sedimentological character of the upper Copper Harbor Formation, and together these imparted a strong control on fluid flow and later diagenetic processes. Early oxidized meteoric fluids were displaced by liquid petroleum and sour gas, which were in turn succeeded by metal-rich but sulfate-poor oxidized seawater. Burial compaction during deposition of the overlying Freda Formation drove fluids through White Pine due to its situation on a paleotopographic high near the basin margin. Mineralization occurred at ~125°C at depths of ~2.0 km and spanned incipient basin inversion related to the distal effects of Grenvillian orogenesis. The hightenor copper mineral assemblage is the product of an abundant supply of metal from basaltic volcanic detritus in the Copper Harbor Formation and low seawater sulfate concentrations in late Mesoproterozoic oceans. This demonstrates that viable sediment-hosted stratiform copper systems can form when a readily leachable metal source rock is present, even if hypersaline and sulfate-rich brines are not.

Neotocite textures as a clue to the exploration of Red Bed type sediment-hosted stratabound copper (SSC) deposits – evidence from Ravar–Tabas–Eshghabad copper belt, Central Iran

ABSTRACT Red Bed type sediment-hosted stratabound copper (SSC) deposits consist of replaced, disseminated to veinlets copper sulphides, hosted by thick, haematite-stained sequences of sandstone, conglomerate, and siltstone. More than 120 sediment-hosted stratabound copper (SSC) deposits and occurrences have been reported in Iran. Ravar–Tabas–Eshghabad is the major Red Bed type SSC belt in Iran, with a cumulative tonnage of more than 400 Mt ore at ~1–2% Cu. These deposits are hosted in Garedu Red Bed Formation (including reddish conglomerates and micro-conglomerates, red siltstones and sandstones), and copper sulphides (chalcocite, with minor chalcopyrite and bornite) are concentrated in local reducing conditions that are achieved around plant fossil accumulations, within grey paleochannels in the upper part of the Garedu Red Bed Formation. These deposits are usually explored using geochemical, geophysical, and remote sensing methods. However, the host layers usually extend laterally up to tens of kilometres, and the critical point is to find the paleochannels in the host horizons where organic matter and plant fossils accumulate, especially in underground mining and boreholes. The lateral extension of ore-bearing lenses is usually 50–150 m, so it is not easy to find them within the reduced horizons. Recognizing neotocite textures (including diffusion, radial DLA (diffusion-limited aggregation), cat footprint, and spotty), and distribution in the host rocks of the Red Bed type SSC deposits can be used as a clue to find the ore lenses, organic matter, and plant fossil accumulations.

Physical and geochemical reconstruction of a 2.35–2.1 Ga volcanic arc (Toumodi Greenstone Belt, Ivory Coast, West Africa)

This study reconstructs the evolution of one of the oldest Paleoproterozoic arcs, which formed shortly after a period of momentous change when the Earth transitioned out of the Archean. We present new stratigraphic, geochemical and geochronological data of the Toumodi Greenstone Belt (Ivory Coast, West Africa) integrated with local and regional data to better constrain its construction and evolution. The belt consists of four main stratigraphic events: [1] A ca. 2345 Ma tholeiitic sequence formed on the sea floor far from any landmass. This juvenile crust is the oldest Birimian supracrustal sequence known and represents either a dismembered oceanic plateau or ridge basalt; [2] Nascent arc formation (ca. 2220–2160 Ma) developed initially in a subaqueous environment and transitions into emergent volcanic centres. Near Toumodi, most deposits consist of volcaniclastic debris emplaced in subaerial to shallow-water environments, but also include minor pyroclastic vent-proximal deposits. Magmas for this event are mainly basaltic andesite to andesite and are enriched in LILE and have negative Nb, Ta and Ti signatures, all of which are consistent with water-fluxed melting typical of arc magmatism. Inherited zircons and mafic xenocrysts/xenoliths together reflect the older history and nature of underlying mid- to lower-crustal rocks, [3] An intra-arc rifting-event marked by a unconformity between events 2 and 4 the correlation with pillowed tholeiitic basalts; and [4] Construction of a mature arc (ca. 2150–2100 Ma) that consists of emergent dacitic volcanic centres and more distal volcaniclastic sediments, and overlain by post-magmatic sedimentation and red-bed formation (ca. 2100–2050 Ma). Magmas for this time period are dominantly dacitic and also display geochemical characteristics of arc magmatism, but also contain high-K, indicative of melting in a thickened volcanic arc.

Location and shape of the Lhasa terrane prior to India-Asia collision

The precollisional location and shape of the Lhasa terrane are crucial for constraining the closure of the Neo-Tethys Ocean and the ensuing India-Asia collision; however, estimation of these features of the Lhasa terrane remains highly controversial. Here, we carried out a new paleomagnetic investigation on the Lower Cretaceous Duoni Formation red beds in the central-eastern Lhasa terrane. The tilt-corrected site-mean direction is declination (Ds) = 339.0°, inclination (Is) = 26.8°, ks = 78.4, and α95 = 2.3° (k—precision parameter; α95—the radius that the mean direction lies within 95% confidence; s—stratigraphic coordinates) (N = 50), corresponding to a paleopole at 64.2°N, 324.2°E, with A95 = 1.9° (A95—the radius that the mean pole lies within 95% confidence). These new paleomagnetic data pass a positive fold test and indicate that the studied area was located at 14.3 ± 1.9°N during the Early Cretaceous. No significant inclination shallowing is present in the Lower Cretaceous Duoni Formation red beds. Our new results, combined with previously published reliable Cretaceous paleomagnetic results, show that the Lhasa terrane was located at a paleolatitude of ∼22.9°N to 10.1°N from west to east and was oriented at ∼298°−296° prior to India-Asia collision.

Temporal and spatial distribution of Precambrian red beds and their formation mechanisms

Red beds are irrefutable evidence of important geological events; understanding their formation mechanism and sedimentary environment provide insights about Earth's history and the evolution of life. In this paper, Precambrian red beds from over 70 outcrops in nearly 30 regions in different countries are statistically assessed using their lithology, sedimentary environment, and spatio-temporal distribution. Twelve sets of red beds are identified, five of which are distributed globally; the other sets are regional or local. The depositional environments for red beds are varied. They include terrestrial (aeolian, alluvial, and fluvial) and shallow marine environments (dominantly, tidal and glacial marine), as well as the transition areas from shallow to deep-water environments. While the former two are dominant environments, the latter has fewer occurrences – most of which are concentrated in strata of the Ediacaran Period. Red beds are lithologically diverse; terrestrial red beds are mainly composed of conglomerate and sandstone, while shallow marine red beds primarily consist of siltstone, mudstone, and carbonate rocks. The formation of red beds is directly controlled by the concentration of dissolved Fe and O2 content, and penecontemporaneous geological events. Here, they are sorted into two groups based on their chronological coupling with glaciations, the assembly and breakup of supercontinents, eustatic sea-level changes, the flourishing of photoautotrophs like cyanobacteria, as well as δ13C and 87Sr/86Sr isotope anomalies. The formation of the first group of red beds is closely related to the Paleoproterozoic and Neoproterozoic glaciations, the Great Oxidation Event, and the Neoproterozoic Oxygenation Event. Much of the dissolved Fe scavenged from seawater during oxygenation events precipitated into banded iron formations (BIF). The transmittance of light through water reduced during glaciations. It prevented the production of O2 and limited Fe2+ oxidation – which worked as a protector for oceanic Fe re-accumulation. The melting of glaciers improved the availability of light which restarted the production of photosynthetic O2 that aided the formation of red beds. The formation of the second group of red beds was aided by supercontinent fragmentation that created many shallow margin habitats for photoautotrophs, facilitating O2 production. The appearance of this group of red beds coincides with and is geochemically supported by pulses of O2 increase recorded by Cr and C isotopes, as well as by iron speciation data. Future research should examine how Precambrian red beds can be used to systematically decode paleoenvironment changes.

Location of the Lhasa terrane in the Late Cretaceous and its implications for crustal deformation

The Cenozoic crustal deformation of the Asian continent is constrained by the location of its southern margin in the Late Cretaceous, which is, however, affected by potential remagnetization and inclination shallowing of redbeds in the Linzhou Basin. Our paleomagnetic results from the Shexing Formation redbeds in the Late Cretaceous yield a mean paleo-inclination of ~21.5°, consistent with the recalculation of previous data. Rock-magnetic, paleomagnetic and petrographic analyses indicate that the redbeds have recorded the original paleomagnetic field and have not suffered from significant inclination shallowing. The paleopole from 70 redbeds sites is at 71.8°N, 281.5°E with A 95 = 2.6°, yielding a paleolatitude of ~12.0 ± 2.6°N at ~70–91 Ma for the Linzhou Basin. This result indicates a latitudinal convergence of 1264 ± 477 km north of the sampling area within Asia since the Late Cretaceous. Combined with previous data, our results show that the southern margin of Asia was at ~11–15°N in the Late Cretaceous, aligning in a WNW-ESE direction, which is similar to its present-day orientation. A review of available data shows that the sharp bending of the southern margin of Asia around the eastern Himalayan syntaxis (EHS) is probably due to different crustal movement patterns around the EHS. The shape of the southern margin of Asia determines the arc-shaped orientation of Himalayan orogen. • The Shexing Formation redbeds have recorded the prefold magnetizatioin. • The southern margin of Asia was oriented in a WNW-ESE direction at ~12°N. • Different movement around the eastern Himalayan syntaxis make a sharp turn. • The southern margin of Asian plays a primary role in shaping the Himalayan orogen.

Unconformity-controlled bleaching of Jurassic-Triassic sandstones in the Ordos Basin, China

Bleaching of red beds, a type of hydrocarbon-induced alteration, is generally attributed to redox reactions between ferric iron minerals and hydrocarbon-bearing solutions. Herein, we report sandstone bleaching occurs interbedded with the coal- and dark mudstone-bearing strata at shallow depths below two unconformity surfaces separating sandstone formations of Triassic-Jurassic age in the Ordos Basin, China. Field observations, petrography, and geochemistry suggest that uplift events controlled the formation of red beds via supergene alteration and bleaching via hydrocarbon circulation. The color of sandstones below the unconformities grade from red, yellow, and white colors at shallow depths (few meters to tens of meters) to dark yellow, gray-green and gray colors at deeper depths. Organic matter (carbonaceous plant debris) and pyrite in the unaltered sandstone gave rise to the gray color. The red/yellow sandstones are characterized by the presence of extensive iron oxide/hydroxide grain coatings, exhibit intense dissolution and extensive kaolinization of detrital feldspar and biotite and lithics and are mainly composed of detrital quartz. The white, bleached sandstone presents similar petrographic characteristics as the unbleached sandstone except for a lack of iron oxide/hydroxide cements. δ18OVSMOW (9.8‰ to 15.8‰) and δDVSMOW (−103‰ to −119‰) values of kaolinite, and chemical indices of alteration of sandstones indicate a weathering origin for the kaolinite and the dissolution of labile minerals in the red and yellow sandstones. The original color of the bleached sandstone was gray during very early diagenesis and shifted to red/yellow due to the oxidation of pyrite and ferromagnesian silicate minerals (e.g., biotite) into hematite or goethite cements by the meteoric water circulation during regional uplift following the deposition of each formation. Supergene alteration associated with unconformities also created significant secondary porosity, and allowed later hydrocarbons to flow along the unconformities. The lithological properties of the weathered rocks below unconformities are highly heterogeneous both vertically and laterally and have a significant influence on fluid flow. This study provides direct evidence for hydrocarbon migration along unconformities and improves understanding of fluid-rock interaction in subsurface reservoirs.

Paleomagnetic and geochronological results of the Risong Formation in the western Lhasa Terrane: Insights into the Lhasa-Qiangtang collision and stratal age

To constrain the Lhasa-Qiangtang collisional age, a combined paleomagnetic and geochronologic investigation has been performed on the Risong Formation redbeds and volcanic rocks in the Wuma area of the western Lhasa terrane (LT). A U-Pb dating result from the volcanic interlayer at the bottom of the Risong Formation redbeds yields the youngest weighted mean 206 Pb/ 238 U age of 110.6 ± 1.3 Ma, and the two detrital zircon U-Pb chronological results from the Risong Formation redbeds provide the youngest weighted mean 206 Pb/ 238 U age of 106.9 ± 3.3 Ma (the top strata of section A) and 120.5 ± 3.0 Ma (the top strata of section D), respectively. Our new geochronological results suggest that the studied Risong Formation redbeds formed during the Early Cretaceous, rather than during the Late Jurassic as assigned by 1:250,000 scale Wuma regional geological survey report. Characteristic remanent magnetization (ChRM) directions from two different limbs of folds concentrate in two different groups, and both site-mean directions pass positive fold tests. One group including 32 sites yields a tilt-corrected site-mean direction that is D = 356.1°, I = 33.1° with α 95 = 2.3°, corresponding to a paleopole at 75.2°N, 278.1°E with dp/dm = 1.5°/2.6° and a paleolatitude of 18.1° ± 1.5°N for the study area (32.41°N, 83.39°E). The other group including 24 sites provides a tilt-corrected site-mean direction that is D = 60.5°, I = 48.5° with α 9 5 = 2.0°, corresponding to a paleopole at 38.7°N, 159.6°E with dp/dm = 1.7°/2.6° and a paleolatitude of 29.5° ± 1.7°N for the study area (32.41°N, 83.39°E). After syntectonic-sedimentation-correction, the Fisherian site-mean inclination of 41.7° ± 1.6° and corresponding paleolatitude of 24.0° ± 1.2°N obtained from 56 sites of two different limbs are consistent with the inclination-only mean of 39.6 ± 3.1° and corresponding paleolatitude of 22.5° ± 2.2°N deduced from the ChRM directions of all the 56 paleomagnetic sites, suggesting that the observed inclination discrepancy of the Risong Formation redbeds may result from the syntectonic sedimentation. Comparing the Early Cretaceous paleolatitudes observed from the western LT with those from the western Qiangtang terrane shows insignificant paleolatitude difference, supporting that the Lhasa-Qiangtang collision in the western part occurred at or before the Early Cretaceous. • New dating shows the sampled Risong Formation redbeds formed during the Early Cretaceous rather than the Late Jurassic. • The Wuma sampling area was located at ~22.5° ± 2.2°N during ~120.5–106.9 Ma. • The Lhasa-Qiangtang collision occurred before ~120 Ma in the western part.

Mineral paragenesis and stratigraphic setting of the Neoproterozoic volcano-sedimentary succession of Samran Group, Jabal Farasan Area, West central Arabian Shield, Saudi Arabia

Jabal Farasan is located in the west central part of the Arabian Shield. The present study aims to reveal the stratigraphic setting and mineral paragenesis of the exposed volcano-sedimentary succession of the Samran Group in Jabal Farasan area. The succession of Samran Group is composed of three successive facies: the lower interbedded andesitic tuffs and tuffaceous andesites and trachyte’s (F1), the middle-altered quartz diorite/andesite (F2) and, an upper interbedded andesitic tuff, tuffaceous andesites and trachytes (F3). The vertical variations in the petrographic lithotypes depend mainly on the position of the depositional sites relative to the volcanic centers (proximal and distal). Both the volcanic rocks and the associated units are subjected to syn-and post-depositional diagenetic/metamorphic processes. These processes include the formation of the Fe2+-silicates: (i.e., chlorite instead of the volcanic ashes and the formation of plagioclase minerals). Post diagenetic recrystallization of the tuffaceous mudstones and the formation of microcrystalline quartz. The diagenetic oxidation of the Fe2+-silicates led to the formation of volcaniclastic red beds in certain stratigraphic horizons.

Formation of Late Ordovician marine red beds: A case study of Sandbian deposits in the Tarim Basin, Northwest China

The late Ordovician was an important transition interval between the mid-Ordovician biological radiation and end-Ordovician extinction. Marine red beds (MRB) of Late Ordovician are distributed worldwide, including in the Americas, South Africa, England, and China. Among them, the conditions that contributed to the widespread Katian MRB was a combination of terrigenous input, cool-water settings, sediment oxidation, volcanic material input, and seasonal aridity. In contrast, Hirnantian MRB have a more limited distribution, and their occurrence might be related to regional tectonic settings and icehouse conditions. Sandbian MRB are distributed worldwide, but are relatively poorly studied. Even though Sandbian MRB are very similar in some characteristics to the Katian MRB, we don't know which factors were important to their formation and how these were related to marine environmental change and tectonic evolution. The Dawangou section near Kalping in the Tarim Basin of northwest China is an important global reference section for the auxiliary GSSP of the Middle–Late Ordovician boundary and for Sandbian through early Katian biostratigraphy and chronostratigraphy. Diffuse reflection spectroscopy, major and trace element analysis, rare earth element analysis, and sedimentology were employed to study the interval of Sandbian MRB in the Kanling Formation (within the N. gracilis graptolite zone) relative to underlying and overlying deposits. The red-colored strata have elevated total Fe and hematite contents, whereas the grey and grey-green samples have much a lower total Fe and a significant lack of hematite and goethite. The total Fe content for strata within the Sandbian MRB varies with the clay content. The AlFe correlation within the Sandbian MRB is 0.9, whereas it is 0.6 for the non-red sediments. The Th/U ratio is much higher in the red-colored beds (averaging 7.0) relative to the underlying and overlying sediments (averaging 4.4). However, a redox proxy of Ni/Co ratios does not exhibit a significant difference between the underlying and overlying sediments (averages of 2.6 and 2.8) and the reddish Sandbian MRB interval (average of 2.6). The results indicated that the Sandbian marine red beds of the Tarim were formed during conditions of marine oxygenation that helped form and preserve the iron oxide. However, the regional distribution and aspects of the sediment geochemistry imply that another essential process for the formation of these Sandbian MRB was the influx of terrestrial iron-rich sediments from the exposed uplifts into the mixed shelf regions; and that this influx was partly governed by the regional patterns of southward subduction of the South Tianshan ocean and of northward subduction of the Altyn to the margins of the Tarim block.

Geochemical record of the subsurface redox gradient in marine red beds: A case study from the Devonian Prague Basin, Czechia

AbstractMarine red beds are usually interpreted as indicating water column oligotrophy, good bottom‐water oxygenation and redox conditions. Lower Devonian successions of the Prague Basin, Czechia, exhibit a distinct centimetre to metre‐scale alternation of layers of marine red beds, grey carbonates, marls and black shales. In order to understand why the redox potential fluctuated so rapidly, reflectance spectroscopy, microscopy, elemental geochemistry data and stable isotopes of Mo have been analysed in this paper. Whilst the grey and black facies only contain goethite, the marine red beds are enriched with synsedimentary and early diagenetic, submicronic hematite, which is present in micrite, skeletal interiors, microstromatolites and oncoids. It was formed by microbially mediated precipitation, the replacement of detrital Fe phyllosilicates, and/or by the oxidation of microbially precipitated Fe‐bearing aluminosilicate precursors. The marine red beds are frequently enriched in Fe, depleted in U, V, Mo and Cu, and show negative δ98Mo values indicating oxic conditions. Peloidal micrite, microbial coatings and cements with the marine red beds exhibit positive (up to 9) Ce/Ce* anomalies. The non‐red facies show opposite patterns. This geochemical variability is probably related to Mn oxyhydroxide cycling and organic matter remineralization along the sediment subsurface redox gradient, particularly by reactions between pore water and various elemental pools. These patterns, combined with the centimetre‐scale colour alternation of the sediments, may reflect redox zonation that has been preserved beneath the ancient seafloor. Four zones are recognized: (i) the oxic zone of Fe‐oxide precipitation (marine red beds); suboxic zones of (ii) Fe enrichment, and (iii) U‐Mo enrichment; and (iv) suboxic–anoxic zone of Cu, V (± Mo) enrichment. The presented model of a marine red bed origin from redox reactions in the sediment subsurface contradicts models of the formation of marine red beds through iron enrichment from Fe2+ supersaturated ocean waters following periods of ocean anoxia.

Reply to Comment by Zhao et al. on “Paleomagnetism of the Late Cretaceous Red Beds From the Far Western Lhasa Terrane: Inclination Discrepancy and Tectonic Implications”

AbstractIn this manuscript, we checked all the pieces of evidence separately and conclude that the evidence Zhao et al. (2021, https://doi.org/10.1029/2020TC006520) provided cannot be regarded as arguments supporting their points. Our observations show that (i) sampling units and depositional environment support that the Jingzhushan Formation red beds could be affected by the syntectonic sedimentation, (ii) characteristic remanent magnetization directions observed from the Jingzhushan Formation red beds represent primary magnetization acquired during the syntectonic sedimentation, not secondary remagnetization due to strain reorientation of remanence, and (iii) inclination discrepancy of the Jingzhushan Formation red beds could be attributed to syntectonic sedimentation. Therefore, the paleomagnetic results obtained from the Jingzhushan Formation red beds are reliable for paleogeographic reconstructions.

The Upper Jurassic Garedu Red Bed Formation of the northern Tabas Block: elucidating Late Cimmerian tectonics in east-Central Iran

The Garedu Red Bed Formation (GRBF) of the northern Tabas Block (Central-East Iranian Microcontinent, CEIM) is a lithologically variable, up to 500-m-thick, predominantly continental unit. It rests gradually or unconformably on marine limestones of the Esfandiar Subgroup (Callovian–Oxfordian) and is assigned to the Kimmeridgian–Tithonian. In the lower part, it consists of pebble- to boulder-sized conglomerates/breccias composed of limestone clasts intercalated with calcareous sandstones, litho-/bioclastic rudstones and lacustrine carbonates. Up-section, sharp-based pebbly sandstones and red silt-/fine-grained sandstones of braided river origin predominate. Palaeocurrent data suggest a principal sediment transport from west to east and a lateral interfingering of the GRBF with marine greenish marls of the Korond Formation at the eastern margin of the Tabas Block. Westwards, the GRBF grades into the playa deposits of the Magu Gypsum Formation. Red colours and common calcretes suggest arid to semi-arid climatic conditions. The onset of Garedu Red Bed deposition indicates a major geodynamic change with the onset of compressive tectonics of the Late Cimmerian Tectonic Event (LCTE), being strongest at the eastern margin of the northern Tabas Block. When traced southwards, the same tectonic event is expressed by extension, indicating a shift in tectonic style along the boundary fault between the Tabas and Lut blocks. The complex Upper Jurassic facies distribution as well as the spatio-temporal changes in tectonic regime along the block-bounding faults are explained by the onset of counterclockwise vertical-axis rotation of the CEIM in the Kimmeridgian. The block boundaries accommodated the rotation by right-lateral strike slip, transpressional in today’s northern and transtensional in today’s southern segments of the block-bounding faults. Rotation occurred within bracketing transcurrent faults and continued into the Early Cretaceous, finally resulting in the opening of narrow oceanic basins encircling the CEIM. Palaeogeographically, the GRBF is part of a suite of red bed formations not only present on the CEIM, but also along the Sanandaj-Sirjan Zone (NW Iran), in northeastern Iran and beyond, indicating inter-regional tectonic instability, uplift and erosion under (semi-)arid climatic conditions across the Jurassic–Cretaceous boundary. Thus, even if our geodynamic model successfully explains Late Jurassic tectonic rotations, fault motions and facies distribution for the CEIM, the basic cause of the LCTE still remains enigmatic.

Late Cretaceous-Cenozoic tectonic-sedimentary evolution and U-enrichment in the southern Songliao Basin

Late Cretaceous-Cenozoic tectonic-sedimentary evolution is fundamental to U (Uranium) migration and U-enrichment in the southern Songliao Basin, northeast China. To establish the genetic connection between tectonic-sedimentary evolution and U-enrichment, this paper presents a mineralogical study, apatite fission-track (AFT) analysis, and SEM-CL analysis of quartz grains. Two stages of mineralization were proposed, including the U pre-enrichment stage during the Late Cretaceous, and the ore-forming stage during the Cenozoic. Based on the density plots of the AFT ages, paleoclimate studies, and SEM-CL analysis of quartz grains, it is inferred that the U pre-enrichment was closely related to the rapid uplift of the Great Xing'an Range and the Zhangguangcai Range, and the O2-rich atmosphere during 97–90 Ma in the southern Songliao Basin. The O2-rich atmosphere increased U mobility and led to the formation of red beds during the Late Cretaceous, which widely developed U pre-enrichment in the basins of Northeast China. The mineralogical study reveals that the Qianjiadian uranium deposit developed in two stages. The diagenetic process experienced the transition from early alkaline to late acidic conditions, to promote the Stage I mineralization (U pre-enrichment). The dominant occurrence spaces for U were the dissolution pores of quartz, tightly packed with calcite cement. U-enrichment during alkaline ambient conditions was mainly via adsorption. With the increase of the buried depth, the early alkaline diagenesis returned to acidic conditions due to the abundant organic material. U(VI) was reduced to pitchblende due to the reducing capacity of the organic matter (OM). Stage II (ore-forming stage) mineralization was strictly related to the hydrocarbon re-concentration. According to the thermal history study of the CC1 and Q2 boreholes, the southern Songliao Basin experienced four episodes of rapid uplift during the Cenozoic at 83–80 Ma, 68–52 Ma, 33–23 Ma, and −8 Ma, respectively. The Cenozoic uplifts led to hydrocarbon migration at 65 Ma, 23 Ma, and present. The re-concentration of hydrocarbon prompted the U mineralization in the Qianjiadian uranium deposit in the southern Songliao Basin during the stable period of tectonic movements. The reducing conditions caused by hydrocarbons promoted U mineralization. The dominant spaces of coffinite were intergranular dissolved pores, formed during the transformation from the feldspar to the kaolinite under acidic conditions. The post-rift phase and structural inversion phase directly controlled the U pre-enrichment stage and the ore-forming stage, respectively. Overall, the tectonic-sedimentary evolution played an important role in controlling the U leaching from granites, the dominant occurrence spaces, and U recycling.

The formation of marine red beds and iron cycling on the Mesoproterozoic North China Platform

Abstract Marine red beds (MRBs) are common in sedimentary records, but their genesis and environmental implications remain controversial. Genetic models proposed for MRBs variably invoke diagenetic or primary enrichments of iron, with vastly different implications for the redox state of the contemporaneous water column. The Xiamaling Formation (ca. 1.4 Ga) in the North China Platform hosts MRBs that offer insights into the iron cycling and redox conditions during the Mesoproterozoic Era. In the Xiamaling MRBs, well-preserved, nanometer-sized flaky hematite particles are randomly dispersed in the clay (illite) matrix, within the pressure shadow of rigid detrital grains. The presence of hematite flake aggregates with multiple face-to-edge (“cardhouse”) contacts indicates that the hematite particles were deposited as loosely bound, primary iron oxyhydroxide flocs. No greenalite or other ferrous iron precursor minerals have been identified in the MRBs. Early diagenetic ankerite concretions hosted in the MRBs show non-zero I/(Ca+Mg) values and positive Ce anomalies (>1.3), suggesting active redox cycling of iodine and manganese and therefore the presence of molecular oxygen in the porewater and likely in the water column during their formation. These observations support the hypothesis that iron oxyhydroxide precipitation occurred in moderately oxygenated marine waters above storm wave base (likely <100 m). Continentally sourced iron reactivated through microbial dissimilatory iron reduction, and distal hydrothermal fluids may have supplied Fe(II) for the iron oxyhydroxide precipitation. The accumulation of the Xiamaling MRBs may imply a slight increase of seawater oxygenation and the existence of long-lasting adjacent ferruginous water mass.

Marinoan-aged red beds at Shennongjia, South China: Evidence against global-scale glaciation during the Cryogenian

The continuous existence of the Marinoan Ice Age, a late Cryogenian panglaciation (Snowball Earth), is challenged by the occurrences of widespread glacial marine and nonglacial deposits consisting of stratified diamictite, laminated siltstone and claystone, and even carbonates, which indicate an episode of partial deglaciation. Red beds that commonly accompany these glacial marine and nonglacial deposits have not received enough attention to date. The middle Nantuo Formation of the Cryogenian Marinoan epoch in the Sanshengtai section, Shennongjia area, is composed of 6 lithofacies, i.e., Green Massive Diamictite, Green Laminated Lithic Feldspar Wacke, Green Laminated Siltstone and Claystone, Red Laminated Lithic Feldspar Wacke, Red Crudely Stratified Diamictite, and Red Laminated Siltstone. Among them, the last three constitute the red beds and are interpreted as glacial lacustrine and nonglacial deposits, which can be found and roughly correlated across several continents. High Fe2O3 and low Ce/Ce* values in the red beds suggest a probable association between the formation of red beds and the Neoproterozoic oxygenation events. Coincidences between the occurrences of red beds and high values of Al/Na, Ti/Na, and chemical index of alteration (CIA) indicate the favorability of warm intervals with high weathering intensities for the formation of red beds, whose presence indicates partial deglaciation during the Marinoan Ice Age and provides a new clue to contradict the worldwide Snowball Earth hypothesis.

Paleomagnetism of the Late Cretaceous Red Beds From the Far Western Lhasa Terrane: Inclination Discrepancy and Tectonic Implications

AbstractThe precollisional locations and geometries of the Lhasa terrane (LT) are critical to constrain the India‐Asia collision. However, the inclinations of the Cretaceous paleomagnetic data obtained from the northern limb of folds are obviously lower than those obtained from the southern limb, which cause large discrepant paleolatitudes of the LT prior to India‐Asia collision. Here, we carried out a new paleomagnetic investigation on the Late Cretaceous Jingzhushan Formation red beds in the far western LT. The tilt‐corrected site mean direction yielded a palaeopole at 74.4°N, 226.0°E with A95 = 3.8° (N = 54). This paleomagnetic data set passes fold tests and indicates that the studied area was located at 19.6° ± 3.8°N during the Late Cretaceous. However, the mean inclination calculated from the northern limb of folds (Is = 19.0°) is significantly lower than that of the southern limb of folds (Is = 51.8°). This inclination discrepancy of the Jingzhushan Formation red beds may be attributed to the syntectonic sedimentation. Nevertheless, the site mean direction obtained from both limbs of folds is generally consistent with the site mean direction after syntectonic‐sedimentation correction. Our new paleomagnetic results, combined with the reliable Cretaceous paleomagnetic results from the LT showed that the southern margin of Asia had a present‐day relatively east‐west alignment prior to India‐Asia collision.