Ancient Analogues Research Articles

Mapping Ocean Gateways from 3D Seismic Data Using Color Processing and Analogues from Seismic Data

Mapping sedimentary features to delineate ancient ocean gateways from 3D seismic data has long been a challenge due to their limited vertical resolution. We present the technique of color processing of seismic data which improves the vertical resolution to very few seismic samples corresponding to 5–10 m. The result is a representation of the seismic impedance contrast correlation in Red-Green-Blue (RGB) colors, which creates images similar to satellite images that assist the geological interpretation because the RGB colors can be associated with mud-rich and sand-rich sediments. Using this approach, we have extracted six different sedimentary bedforms from different bottom flow settings and correlated these with the bedform-velocity matrix by Stow et al. (2009). Then we studied bedform assemblies in their regional context for the area covered by the 3D seismic data set. We conclude with comparing a modern and an ancient analogue – both from 3D seismic data – and infer paleo flow direction and velocity using oceanographic measurements available for the modern analogue. The method provides insights into ocean bottom currents and their impact on the formation of contourite depositional systems in modern and ancient ocean gateways which contribute to the reconstruction of the paleoclimate as an input to present day climate change models.

About this title - Oceanic Gateways: Modern and Ancient Analogues and their Conceptual and Economic Implications

About this title - Oceanic Gateways: Modern and Ancient Analogues and their Conceptual and Economic Implications

Elevated sea surface temperature and enhanced primary productivity during Ocean Anoxic Event 1d in the eastern Tethys: Calcareous nannofossil evidence from southern Tibet, China

Mid-Cretaceous oceanic anoxic events (OAEs) are characterized by major disturbances to the global carbon cycle and are considered ancient analogs for anthropogenic global warming. Among these events, OAE 1d, occurring around the Albian-Cenomanian boundary, remains understudied in the eastern Tethys region, leaving a critical palaeogeographic gap that hampers a global understanding of its impacts. In this paper, we present a detailed record of calcareous nannofossil assemblages from the lower to middle part of the Lengqingre Formation in the Chaqiela section of Gamba, southern Tibet, China. The study interval contains abundant, moderately to well preserved calcareous nannofossil assemblages, with Watznaueria barnesiae, Discorhabdus ignotus, Biscutum constans, and Zeugrhabdotus erectus as the dominant species constituting >50% of the assemblages. These nannofossil assemblages allow the study interval to be constrained to Upper Cretaceous (UC) Biozones UC0 through UC2, with the Albian-Cenomanian boundary located in the lower part of the Lengqingre Formation, and the middle part assigned to Cenomanian in the study area. As such, the position of OAE 1d in this section is accurately determined. Calculated nutrient and temperature indices based on nannofossil species with preferred ecologies reveal significant palaeoceanographic changes across the OAE 1d. These changes show increased abundance of warm-water taxa (e.g., Rhagodiscus asper and Zeugrhabdotus diplogrammus) and high-productivity taxa (e.g., D. ignotus, B. constans, and Z. erectus), suggesting increased sea surface temperature and enhanced productivity during OAE 1d in the southwestern shelf sea of the eastern Tethys Ocean.

Revisiting Elevated δ13C Values of Sediment on Modern Carbonate Platforms

AbstractThe measured carbon isotopic compositions of carbonate sediments (δ13Ccarb) on modern platforms are commonly 13C‐enriched compared to predicted values for minerals forming in isotopic equilibrium with the dissolved inorganic carbon (DIC) of modern seawater. This offset undermines the assumption that δ13Ccarb values of analogous facies in the rock record are an accurate archive of information about Earth's global carbon cycle. We present a new data set of the diurnal variation in carbonate chemistry and seawater δ13CDIC values on a modern carbonate platform. These data demonstrate that δ13Ccarb values on modern platforms are broadly representative of seawater, but only after accounting for the recent decrease in the δ13C value of atmospheric CO2 and shallow seawater DIC due to anthropogenic carbon release, a phenomenon commonly referred to as the 13C Suess effect. These findings highlight an important, yet overlooked, aspect of some modern carbonate systems, which must inform their use as ancient analogs.

Firmgrounds and hardgrounds in the Coniacian carbonate platform of the Iberian basin: Origin and model for development of omission surfaces in tidal environments

Distinctive erosional and omission surfaces occur at several stratigraphic levels in the tidal carbonates of the 3rd-order Coniacian sequence (Iberian Basin). They are ancient analogs of omission surfaces developed on lithified carbonates in subaerial and coastal settings. Omission surfaces consist of (i) firmground Glossifungites ichnofacies (Balanoglossites-Thalassinoides); and (ii) hardground Trypanites ichnofacies (scalloped and planar surfaces and Gastrochaenolites-Entobia surfaces). Firmgrounds are also related to erosion or ferruginous crusts. Hardground surfaces are related to bioerosion, dissolution and physical erosion. Grain size and textural features in Balanoglossites and Thalassinoides firmground surfaces are essentially the same, suggesting that even bathymetry could be similar. Several stages in hardgrounds consist of different, scalloped or planar surfaces related to bioerosion, dissolution and physical erosion. Gastrochaenolites-Entobia borers represent a major change in the trace fossil associations and imply different processes in their origin, being originated at slightly different depths with Gastrochaenolites representing shallower environments. The studied field sections display a cyclicity on the scale of meters that tentatively reflects the presence of 4th-order parasequence sets. Two kinds of sedimentary discontinuities have been used for correlation: omission surfaces and ferruginous crusts representing regional sea level falls and rises. Part of the described surfaces does not appear to have been previously recognized in older carbonate deposits. Their common presence of similar surfaces along modern coasts and in karst terrains, as well as their abundance in the Coniacian sequence, suggests that they might also be abundant in the geologic record in other sedimentary basins for defining palaeoshorelines.

In situ sulfur isotope analysis of pyrite from the Ezuri Kuroko‐type volcanogenic massive sulfide deposit, northeastern Japan: Contribution of microbial sulfate reduction to initial sulfide mineralization

AbstractVolcanogenic massive sulfide (VMS) deposits are ancient analogues of seafloor massive sulfide (SMS) deposits. The importance and contribution of microbial activity during initial mineralization has recently been recognized in both VMS and SMS deposits. Here, we report in situ sulfur isotope compositions (δ34S) of pyrite from the Ezuri Kuroko‐type VMS deposit in northeastern Japan as determined by secondary ion mass spectrometry. During the evolutionary process of sulfide mineralization, pyrite textures changed from framboidal to colloform to euhedral. Initial framboidal pyrite had highly negative δ34S values down to −31.8‰ (average ± 1SD = −18.8‰ ± 13.0‰; n = 21), banded colloform pyrite exhibited medium δ34S values (−11.7‰ ± 10.4‰; n = 8), and euhedral pyrite displayed the highest average δ34S value of +2.7‰ ± 1.6‰ (n = 5); thus, δ34S varied as different textures of pyrite were produced during mineralization. The maximum isotopic fractionation between framboidal pyrite and past seawater sulfate (δ34S ca. +20‰) is −52‰; such values can be produced only by microbial sulfate reduction (MSR) in an open system. Framboidal pyrite with a low δ34S value is observed at the centers of sulfide‐rich areas within polished sections and has often been replaced by later sulfide minerals (sphalerite, galena, and chalcopyrite); thus, our S isotope data from the Ezuri pyrite reveal that sulfur derived from MSR induced and acted as a nucleation point for later sulfide mineral growth. Combined with previously reported data, our results endorse the importance and universality of MSR‐derived sulfur during the initial mineralization stage of both VMS and SMS deposits.

Paleoenvironmental changes during the Middle-Late Ordovician transition, Northwestern Tarim Basin, NW China and implications for the great Ordovician Biodiversification Event

• Black shales of Saergan Formation had a mainly felsic source region likely in an active continental margin. • The paleoenvironment exhibited a semiarid climate with high salinity and anoxic marine conditions during the Middle–Late Ordovician in the Awati Sag. • The Middle–Late Ordovician was a transition period which the paleoenvironment changed greatly. • The GOBE may be triggered by supergreenhouse climate. The changes in the geological environment during the Middle–Late Ordovician transition are well documented by geochemical evidence. We study the paleoenvironmental changes during the Middle–Late Ordovician and their effects on the Great Ordovician Biodiversification Event (GOBE) through the analysis and testing of the major elements, trace elements and rare earth elements (REEs) present in 21 samples from the Middle–Upper Ordovician Saergan Formation in the Awati Sag, Northwestern Tarim Basin, NW China. Compared with post-Archaean Australian Shale, the Saergan Formation is enriched in Ca, P, U and REEs. The total abundance of REEs ranges from 107.3 to 674.8 ppm, with obvious enrichment of light REEs and negative Eu anomalies, as well as Ce shows a weak negative anomaly. Furthermore, the parent rock of Saergan Formation is mainly felsic. Combined with the provenance attributes and tectonic background, the source area is considered to be mainly in an active continental margin tectonic environment. The black shales underwent moderate weathering and simple cyclical sorting, with an immature composition. Multiple geochemical indicators show that the Middle–Late Ordovician is a transition period and the paleoenvironment had semiarid climate and hypersaline anoxic marine conditions. The supergreenhouse climate may accelerate chemical weathering and increase nutrient inputs. In addition, the warm climate melting of the polar ice sheet which leads to the relative shallowness of the sea surface temperature gradient, sea level rise and expansion of marine habitats. The resultant environmental changes could have accelerated the diversification of benthos, which provides an ancient analogue for anthropogenic climate change.

Radiaxial fibrous calcite forms via early marine‐diagenetic alteration of micritic magnesium calcite

ABSTRACTMarine radiaxial fibrous calcites are common pore‐filling carbonate fabrics in Palaeozoic and Mesozoic carbonates, yet recent analogues are scarce. Although ancient marine radiaxial fibrous calcites are often considered a cement phase and used for palaeoenvironmental interpretations, their origin (primary precipitation versus diagenetic alteration of a pre‐existing fabric) and their specific formation processes remain ambiguous. This may be related to insufficient information from a few recent analogues and limitations of traditional methods in investigating ancient radiaxial fibrous calcites. This study documents a Holocene occurrence of marine pore‐filling radiaxial fibrous Mg calcites and focuses on their origin, formation processes and implications for ancient analogues. The radiaxial fibrous Mg calcites examined in this study share morphological and mineralogical similarities with numerous ancient marine radiaxial fibrous calcites with a Mg calcite precursor. Based on petrological and geochemical data, the radiaxial fibrous Mg calcites are interpreted as secondary porewater products formed during early burial alteration of micritic Mg calcite precursor cement. These findings provide implications for, at least in part, ancient marine radiaxial fibrous calcites in the context of the formation process and mechanism and are relevant for palaeoceanographic and palaeoenvironmental interpretations based on the presence of this carbonate phase.

Unraveling an alkaline lake and a climate change in Northeastern Brazil during the Late Aptian

The Codó Formation is an Aptian marine-influenced lacustrine depositional system in the Parnaíba Basin (Brazil). This unit is an important sedimentary succession because of the high organic matter content and for being an ancient analog for the depositional systems of the Brazilian Pre-salt interval. However, mineralogical and lithogeochemical studies have not been sufficiently carried out on the Codó Formation, which hinders detailed sedimentological knowledge. We report new mineralogical and lithogeochemical data that allow a better understanding of the environmental, climatic, and provenance evolution of the Codó deposits. Mineralogical and lithogeochemical studies were performed on 50 lutite samples collected from the 1-UN-32-PI core and 13 outcrop lutite samples obtained from sites close to the Grajaú and Imperatriz regions. A magnesium-sulfate-rich shallow alkaline hypersaline lake was identified in the lower Codó Formation based on the identification of a clay mineral assemblage composed of authigenic saponite and detrital illite. Hydrothermal processes were interpreted as the magnesium source of this alkaline lake. A salinity-alteration event was identified at the upper limit of the alkaline lake and was interpreted as the first marine ingression in the Codó Formation. This event changed the lake's pH from alkaline to acid, inhibiting the formation of Mg-smectites, and changing its hydrochemical conditions, originating a marine-influenced sulfate-rich acid saline/hypersaline sabkha. This facies succession coincides with a clay mineral assemblage composed of detrital montmorillonite, illite and kaolinite. A regional paleoclimate humidification process was identified from the sabkha to a lagoon facies succession in the upper Codó Formation. This last succession reveals the marine-dominated interval of the Codó Formation and coincides with a clay mineral assemblage composed of detrital kaolinite, montmorillonite and illite. Based on lithogeochemical data, a common post-Archean intermediate source area was also interpreted for these three environments.

Response of calcareous nannoplankton to the Paleocene–Eocene Thermal Maximum in the Paratethys Seaway (Tarim Basin, West China)

The Paleocene-Eocene Thermal Maximum (PETM) was a rapid global warming occurred 56 million years ago and has been widely viewed as an ancient analogue to the ongoing warming driven by anthropogenic CO 2 emissions. The complete and continuous Paleogene shallow marine strata well preserved and outcropped in the Tarim Basin, northwestern China are ideal to study the paleoenvironmental change of the Paratethys Seaway during the PETM. To date, no high-resolution calcareous nannofossil biostratigraphy has been performed for the PETM interval in the Tarim Basin. Outcrop samples taken from the Qimugen Formation in the Kuzigongsu section contain abundant, moderately well preserved calcareous nannofossils, allowing for the establishment of a high-resolution biostratigraphic framework. Overall, 73 species of calcareous nannofossils from 33 genera were observed, with the dominant species including Coccolithus pelagicus , various Toweius species, Pontosphaera exilis , and Micrantholithus flos. The five calcareous nannofossil datums allow for the recognization of nannofossil Zone NP6 through Zone NP10. The common occurrence of shallow-water taxa ( Micrantholithus ) throughout the section suggests a middle to outer neritic setting as the depositional environment of the Kuzigongsu section. The stratigraphic distribution of “excursion taxa” ( Coccolithus bownii, Discoaster araneus , D. acutus, Rhomboaster spp.) is consistent with the range of negative excursions in δ 13 C carb and δ 18 O carb , indicating that these “excursion taxa” are micropaleontological markers for identifying the presence of the PETM in the Paratethys Seaway. During the PETM, the deteriorated preservation and extremely low abundance of nannofossils and near-zero wt% CaCO 3 values suggest that ocean acidification occurred in the shallow water of the Paratethys Seaway. In addition, a significant increase in the species Neochiastozygus junctus , which is a high productivity indicator suggests increased surface ocean productivity. Higher primary productivity may be triggered by enhanced continental weathering delivering increased nutrient through river runoff. • A new, shallow marine Paleocene–Eocene Thermal Maximum (PETM) record was discovered in the eastern Tethys. • High-resolution calcareous nannofossil biostratigraphy across the PETM was established. • The PETM “excursion taxa” are marker species for identifying PETM record in the eastern Tethys. • Low pre- and syn -PETM carbonate contents were attributed to ocean acidification and terrestrial dilution. • Marine productivity increased during the PETM due to elevated nutrient input from continental weathering.

Extensive Early Marine Seafloor Cementation in a Modern Epeiric Sea Induced by Seawater Properties and a Shallow Redox Boundary Below the Seafloor

Extensive Early Marine Seafloor Cementation in a Modern Epeiric Sea Induced by Seawater Properties and a Shallow Redox Boundary Below the Seafloor

Recurrent ancient geomagnetic field anomalies shed light on future evolution of the South Atlantic Anomaly

The strength of the geomagnetic field has decreased rapidly over the past two centuries, coinciding with an increasing field asymmetry due to the growth of the South Atlantic Anomaly. The underlying processes causing the decrease are debated, which has led to speculation that the field is about to reverse. Here, we present a geomagnetic field model based on indirect observations over the past 9,000 y and identify potential ancient analogs. The model is constructed using a probabilistic approach that addresses problems with age uncertainties and smoothing of sedimentary data that have hampered previous attempts. We find evidence for recurrent hemispherical field asymmetries, related to quasiperiodic millennial-scale variations in the dipole moment. Our reconstruction indicates that minima in the dipole moment tend to coincide with geomagnetic field anomalies, similar to the South Atlantic Anomaly. We propose that the period around 600 BCE, characterized by a strongly asymmetric field, could provide an analog to the present-day field. The analogy implies that the South Atlantic Anomaly will likely disappear in next few hundred years, accompanied by a return to a more symmetric field configuration and possibly, a strengthening of the axial dipole field.

Stratigraphic evolution and karstification of a Cretaceous Mid‐Pacific atoll (Resolution Guyot) resolved from core‐log‐seismic integration and comparison with modern and ancient analogues

AbstractAtolls are faithful recorders helping us understand eustatic variations, the evolution of carbonate production through time, and changes in magmatic hotspots activity. Several early Cretaceous Mid‐Pacific atolls were previously investigated through ocean drilling, but due to the low quality of vintage seismic data available, few spatial constraints exist on their stratigraphic evolution and large‐scale diagenesis. Here, we present results from an integrated core‐log‐seismic study at Resolution Guyot and comparison with modern and ancient analogues. We identify six seismic‐stratigraphic units: (1) platform initiation with aggradation and backstepping through the Hauterivian which ended by platform emersion; (2) reflooding of the platform with progradation and aggradation through the Barremian till the early‐Aptian when ocean anoxic event 1a resulted in incipient drowning; (3) platform backstepping till the mid‐Aptian when the platform shifted to progradation and aggradation till the mid‐Albian; (4) platform emersion; (5) reflooding with backstepping ending at the latest‐Albian by platform emersion; and (6) final drowning. The stratigraphic surfaces bounding these units are coeval with some of the Cretaceous eustatic events, which suggest an eustatic control on the evolution of this atoll and confirm that several previously reported sea‐level variations in the early Cretaceous are driven by eustasy. Changes in subsidence and carbonate production rates and suspected later magmatism have also impacted the stratigraphic evolution. The suspected later magmatism could lead to environmental perturbations and potentially platform demise. Contrary to previous studies, we identify two emersion events during the mid‐ and late‐Albian which resulted in intensive meteoric dissolution and karstification. The platform margin syndepositional fractures interacted with the subaerial exposure events by focusing the dissolution which formed vertically stacked flank‐margin fracture‐cave system. The study gives a unique insight into the interplay between eustasy, subsidence, and volcanic activity(ies) on long‐term evolution of early Cretaceous shallow‐marine carbonates. It also documents the impact and distribution of hypogenic and epigenic fluid‐flow in atolls serving as an analogue for isolated carbonate platforms.

Mid-latitude alluvial and hydroclimatic changes during the Paleocene–Eocene Thermal Maximum as recorded in the Tremp-Graus Basin, Spain

A short episode (~170 kyr) of extremely high global temperatures that occurred ~56 Ma, known as the Paleocene–Eocene Thermal Maximum, is widely considered an ancient analogue of the ongoing anthropogenic warming. This ancient hyperthermal event consisted of three phases, onset, core and recovery, which are respectively represented in the mid-latitude south Pyrenean Tremp-Graus Basin by three successive terrestrial units, the Claret Conglomerate, the Yellowish Soils and the Gypsum-rich Unit, each of them recording a different sedimentary response to evolving climatic and hydrological regimes. The Claret Conglomerate is mainly made up of calcareous conglomerates and is acknowledged to record an abrupt hydrological change during the onset phase. This unit most likely formed in an alluvial megafan, an interpretation here reinforced with new architectural information and by comparison with naturally occurring small-scale fan-like accumulations. Assuming the similarity between the onset of the Paleocene–Eocene Thermal Maximum and the anthropogenic warming, recent hydrological and meteorological data from mid-latitude Spain were compiled for comparison purposes. This comparison concurs with published modelling results about hydroclimate changes in the Pyrenees during the hyperthermal event, postulating an enhancement of seasonal contrast with augmented frequency and intensity of floods but no significant change in total rain. Furthermore, the comparison suggests that the Claret Conglomerate accumulated during extreme rainfall episodes that mainly occurred in autumn. The Yellowish Soils are mostly composed of silty marls with ubiquitous small-sized carbonate nodules and intercalated calcarenites. The marls were deposited in floodplains and the calcarenites in point bars and crevasse splays of meandering rivers. The scarcity of conglomerates entails a near absence of strong currents, and the carbonate nodules perennial or seasonal arid conditions. These nodules occur within cumulate palaeosols, the development of which required regular sedimentary increments during inundations on the floodplains. Sedimentation rates of siliciclastic material on the floodplains increased significantly during the core of the hyperthermal event, as well as at river outlets in coastal settings, which shows that erosion was accelerated and rules out the occurrence of protective, dense vegetation. Combined, these characteristics suggest persistent very dry summers, but a smooth wet season without intense precipitation events. Lastly, the profusion of gypsum in the youngest unit is clear proof of an arid climate during the recovery of the hyperthermal event. In essence, the studied deposits provide a unique window into the sequence of hydroclimatic change during the rise, peak and decline of an ancient global warming event in a mid-latitude terrestrial setting, against which ongoing climate-change data and future projections can be compared.

The macroscopic Paleo-pedogenic features and their significance in deducing the continuity of sedimentation and the climatic changes during the Eocene/Oligocene transition, Dur At Talah sequence, central Libya

The studied sequence is 150 m thick of siliciclastic strata, well-exposed as a prominent escarpment that extends along 25°:45ʺN at the southern flank of Sirt Basin and dated as of Late Eocene age. It is built up from the base to top by three sedimentary units: (i) very fine sandstones and mudstones called New Idam Unit (NIU), (ii) medium-grained sandstones called Lower Sarir Unit (LSU) and (iii) very coarse to microconglomeratic sandstones called Upper Sarir Unit (USU). Pedogenic modifications which are differentially recurring from the base to the top of this sequence have not been paid sufficient attention in the previous studies. This study benefited the widespread macroscopic pedogenic features, introduced them and discussed their usefulness for estimating the continuity of the sedimentation and thus inferring the paleoclimate responsible for their formation.The exposed pedogenic features characteristic of each unit are described and then discussed in light of the documented modern and ancient analogues and case studies in order to comprehend the depositional continuity through time. The discussion is passed via analysing the prevailed climate because it is the fundamental factor controlling the formation of these features. Ultimately, this research allowed, for the first time, the recognition of a major (end Eocene) unconformity embedded within the sequence. This is certainly of interest for the hydrocarbon exploration in and around the basin.

An ancient analogue of carbonate beach complex: the Upper Triassic limestones of Tran Formation, southwestern Bulgaria

An ancient analogue of carbonate beach complex: the Upper Triassic limestones of Tran Formation, southwestern Bulgaria

Transient Slow Slip Characteristics of Frictional‐Viscous Subduction Megathrust Shear Zones

AbstractThe deep roots of subduction megathrusts exhibit aseismic slow slip events, commonly accompanied by tectonic tremor. Observations from exhumed rocks suggest this region of the subduction interface is a shear zone with frictional lenses embedded in a viscous matrix. Here, we use numerical models to explore the transient slip characteristics of finite‐width frictional‐viscous megathrust shear zones. Our model utilizes an invariant, continuum‐based, regularized form of rate‐ and state‐dependent friction (RSF) and simulates earthquakes along spontaneously evolving faults embedded in a 2D heterogeneous continuum. The setup includes two elastic plates bounding a viscoelastoplastic shear zone (subduction interface melange) with inclusions (clasts) of varying distributions and viscosity contrasts with respect to the surrounding weaker matrix. The entire shear zone exhibits the same velocity‐weakening RSF parameters, but the lower viscosity matrix has the capacity to switch between RSF and viscous creep as a function of local stress state. Results demonstrate a mechanism in which stress heterogeneity in these shear zones both (a) sets the “speed limit” for earthquake ruptures that nucleate in clasts such that they propagate at slow velocities; and (b) permits the transmission of slow slip from clast to clast, allowing slow ruptures to propagate substantial distances over the model domain. For reasonable input parameters, modeled events have moment‐duration statistics, stress drops, and rupture propagation rates that overlap with some natural slow slip events. These results provide new insights into how geologic observations from ancient analogs of the slow slip source may scale up to match geophysical constraints on modern slow slip phenomena.

Tectonics and drainage development in central Amazonia: The Juruá River

Understanding the factors that controlled the development of large rivers lacks information from modern and ancient analogues. The Amazonian basin contains several of the largest rivers on Earth, with many having tectonic control. The study of these rivers can contribute to test the claim that large rivers have a tectonic forcing. Addressing the evolution of the Juruá River is relevant, since this is the longest Amazonian river that parallels the Solimões Megashear Zone, a major NE-striking structure with a long history of reactivation of faults and folds even in the Cenozoic. This work discusses the geological evolution of the Juruá River along the Pleistocene-Holocene based on digital elevation models, integrated with subsurface data (well log and seismic reflection). The goal was to characterize some ancient large river valleys in forested areas of Amazonia and to analyze whether they might be related to previous paths of the Juruá River. A physical connection was found between this river and three elongated belts (i.e., PV1, PV2, and PV3) with low topographic and dissection values and surfaces marked by sinuous to meandering strings or curved lines. These characteristics led to relate the belts to ancient Juruá River valleys, an interpretation also suggested by the correspondence between paleovalley morphologies and subsurface Pleistocene-Holocene deposits with concave-up erosional bases. A tectonic hypothesis could explain the development of these paleovalleys around drainage basins with an annular pattern associated with a large convex-up relief, which we relate to rock upwarping by folding. Reconstructing the geological evolution of the Juruá River can introduce new data in discussions focused on the origin of large tropical rivers and on the neotectonic history of the South American intraplate.

Modern brackish bryostromatolites (\u201cbryoliths\u201d) from Zeeland (Netherlands)

Bryostromatolites are found in stressed environments from the Paleozoic to the Recent. They are formed by alternating layers of bryozoans and microbes. This study investigates recent bryostromatolites in brackish ponds in the Netherlands to better understand ancient analogues and the environments which hosted them. They formed a fringing reef at the site Ronde Weel and a barrier reef at Kaaskenswater. The ponds had low biodiversity with only one bivalve species, two gastropod species, one ostracod species, and three diatom species comprising most of the easily fossilizable taxa; one isopod species, one decapod species, and two polychaete species were also present. Observations of microbial layers and cementation practices indicate that an alternation of bryozoan-favouring conditions and microbe-favouring conditions is essential to forming bryostromatolites. The collected bryostromatolites only had tiny living bryozoan patches. Water tests confirmed a brackish environment but with enriched arsenic and titanium concentrations and periodic euxinia. The extreme environment explains the lack of biodiversity and may provide information about the environments in which past bryostromatolites formed.

A late Permian–Triassic trench‐slope basin in the Central Qiangtang metamorphic belt, Northern Tibet: Stratigraphy, sedimentology, syndepositional deformation and tectonic implications

AbstractRecords of sedimentation and deformation in trench‐slope basins contain valuable tectonic information about the associated oceanic subduction zone. Here, we present a multidisciplinary study on newly discovered late Permian–Triassic sedimentary successions in the >500‐km‐long Central Qiangtang metamorphic belt (CQMB) to better understand the type of basin and the concomitant tectonism. The Mayer Kangri succession contains lithofacies associations of submarine fan siliciclastic rocks, slope‐environment limestone, deep marine chert and minor olistostromes from the forearc basin. The conodont assemblages and sandstone and andesite interlayers yield continuous stratigraphic ages from the Lopingian to middle Norian. The clastic sediments had two provenances, including the epicontinental arc in the North Qiangtang block (NQB) and synchronous volcanism in the accretionary wedge. Moreover, a large suite of the Anisian–early Carnian radiolarian cherts (>30 m thick) was discovered in the Lanling area. Regionally, the CQMB shows evident spatiotemporal variations in late Permian–Triassic sedimentation, with a general depositional trend of southward deepening and getting younger. The three identified subzones include a bathyal setting, a carbonate platform setting and a deep marine setting from north to south. These observations indicate that the late Permian–Triassic sedimentary successions in the CQMB were deposited in a trench‐slope basin environment during the northward subduction of the Longmu Co–Shuanghu Tethys Ocean beneath the NQB. Generally, the CQMB and the concomitant trench‐slope basin is among the well‐preserved ancient analogs characterized by extensional tectonism. The syndepositional horst‐graben‐like structure, forearc basin‐derived olistostromes, abyssal radiolarian cherts and synchronous volcanism provide new implications for the geological evolution of the trench‐slope basin.