Marker Beds Research Articles

Preservation of probable MIS 7 deglacial and nonglacial deposits near the edge of the Hudson Bay Lowland in Manitoba, Canada

The stratigraphic principle of superposition assumes that sediments at surface are youngest. And yet, patches of older preserved landscapes continue to be identified in the regions formerly covered by the Laurentide Ice Sheet. Two gravel pits, at the edge of the western Hudson Bay Lowland near the geographic centre of the North American Ice Sheet Complex, reveal additional patches where older glacial and nonglacial sediment is preserved. These sites expose 1–4 m of a darker, highly overconsolidated, clayier till, and 0–2 m of a lighter, less consolidated, sandier till over glaciofluvial and post-glacial gravels and sands; the waning stages of deposition occurred at 214 ± 22 ka (1σ minimum age model quartz grain optical age estimates, n = 2). This was during the latter end of the cool Marine Isotope Stage (MIS) 7-d (within 1σ range) or near the end of MIS-8 (within 2σ range). Next followed a warmer period similar to, or warmer than, present day with higher precipitation; inferred from pollen and macrofossils deposited in nonglacial low-energy floodplain or pond sediments. Our study highlights the need to accurately date Quaternary sediments, given that the shallowest nonglacial sediments at both gravel pits were deposited during an old (MIS 7) interglacial; there is likely no record of the youngest interglacial (MIS 5). Preservation of older sediment also means that in Quaternary stratigraphy, disjoint regional nonglacial “organic marker beds” should not automatically be considered correlative. Identification of similar “old” patches, together with till stratigraphy and composition, is essential to accurately model glacial sediment transport over time.

Correlation of the Coniacian and Santonian stages of the Upper Cretaceous in the Anglo-Paris Basin

The correlation of the Coniacian and Santonian chalks of the Anglo-Paris Basin is described on the basis of detailed lithological logs and extensive records of macrofossils and microcrinoids. In the almost complete absence of ammonites, inoceramid bivalves afford the highest resolution correlation of these stages in chalks, but their value here is limited by the absence of key genera and species, most notably in the Upper Coniacian and middle and Upper Santonian. Echinoids and other macrofossils (brachiopods, stalked crinoids, belemnites) have proved useful, but many are long-ranging or uncommon. Some marker beds, including flints and marl seams, provide useful correlations across the basin, but are locally absent. For the Upper Santonian, the stemless benthonic crinoids Uintacrinus and Marsupites provide high-resolution correlation, both within the basin and to other regions. The successions on the basin margins, in the far north of France (Nord, Pas de Calais) and the southwest (Touraine) are condensed and yield ammonites in association with important inoceramid species. The controls on sedimentation caused by sea-level changes are evaluated on a basinal and global scale, most especially for the Upper Santonian.

Application of 9-component S-wave 3D seismic data to study sedimentary facies and reservoirs in a biogas-bearing area: A case study on the Pleistocene Qigequan Formation in Taidong area, Sanhu Depression, Qaidam Basin, NW China

To solve the problems in restoring sedimentary facies and predicting reservoirs in loose gas-bearing sediment, based on seismic sedimentologic analysis of the first 9-component S-wave 3D seismic dataset of China, a fourth-order isochronous stratigraphic framework was set up and then sedimentary facies and reservoirs in the Pleistocene Qigequan Formation in Taidong area of Qaidam Basin were studied by seismic geomorphology and seismic lithology. The study method and thought are as following. Firstly, techniques of phase rotation, frequency decomposition and fusion, and stratal slicing were applied to the 9-component S-wave seismic data to restore sedimentary facies of major marker beds based on sedimentary models reflected by satellite images. Then, techniques of seismic attribute extraction, principal component analysis, and random fitting were applied to calculate the reservoir thickness and physical parameters of a key sandbody, and the results are satisfactory and confirmed by blind testing wells. Study results reveal that the dominant sedimentary facies in the Qigequan Formation within the study area are delta front and shallow lake. The RGB fused slices indicate that there are two cycles with three sets of underwater distributary channel systems in one period. Among them, sandstones in the distributary channels of middle-low Qigequan Formation are thick and broad with superior physical properties, which are favorable reservoirs. The reservoir permeability is also affected by diagenesis. Distributary channel sandstone reservoirs extend further to the west of Sebei-1 gas field, which provides a basis to expand exploration to the western peripheral area.

High-resolution digital outcrop model of the faults, fractures, and stratigraphy of the Agardhfjellet Formation cap rock shales at Konusdalen West, central Spitsbergen

Abstract. Structure-from-motion (SfM) photogrammetry has become an important tool for the digitalisation of outcrops as digital outcrop models (DOMs). DOMs facilitate the mapping of stratigraphy and discontinuous structures like folds, faults, and fractures from the centimetre to kilometre scale. With pristine, treeless exposures, the outcropping strata in Svalbard, Arctic Norway, hold exceptional potential for analogue studies and are ideally suited for the acquisition of high-resolution DOMs. Here, we present the acquisition, processing, and integration of the Konusdalen West digital model data set, comprising both DOM and derived digital terrain model (DTM) data. Drone-based image acquisition took place over 2 weeks in July and August 2020. The Konusdalen West DOM and DTM cover a 0.12 km2 area and span a 170 m elevation difference. The DOM covers the upper two-thirds of the mudstone-dominated Late Jurassic–Early Cretaceous Agardhfjellet Formation. The Agardhfjellet Formation and its time equivalents are regional cap rocks for CO2 sequestration and petroleum accumulations on the Norwegian Continental Shelf. A total of 15 differential GNSS control points were used to georeference and quality assure the digital data assets, 5 of which function as reference checkpoints. SfM processing of 5512 acquired images resulted in high-confidence, centimetre-scale resolution point clouds, textured mesh (DOM), tiled model, orthomosaics, and a DTM. The confidence-filtered dense cloud features an average inter-point distance of 1.57 cm and has an average point density of 3824.9 points per metre. The five checkpoints feature root mean square errors of 2.0 cm in X, 1.3 cm in Y, 5.2 cm in Z, and 5.7 cm in XYZ. Increased confidences and densities are present along the western flank of the Konusdalen West outcrop, where a fault fracture network in mudstone-dominated strata is best exposed and photographed most extensively. Top and side view orthomosaics feature maximum resolutions of 8 mm per pixel, enabling the mapping of faults, formation members, marker beds, fractures, and other sub-centimetre features. Additional structural measurements and observations were taken in June 2021 to place the data in the geological context. Data described in this paper can be accessed at Norstore under https://doi.org/10.11582/2022.00027 (Betlem, 2022b).

The Cretaceous deposits of the Paris Basin

Abstract Since the creation of the Cretaceous system by Jean-Baptiste d'Omalius d'Halloy, numerous analyses have provided details about the Cretaceous sedimentary successions of the Paris Basin, where many boreholes have been drilled. We present a synthesis of the Cretaceous deposits using data from recent drilling carried out in the central and eastern parts of the basin which, during the early Cretaceous, corresponded to a continental environment represented by Wealden facies. However, from the Berriasian to the Barremian, the southeastern part of the basin was occasionally invaded by a shallow sea during transgressions originating from the Tethys Ocean. The sedimentary successions of this age presented in this article have been studied thanks to drilling carried out by Andra (French Agency for Nuclear Waste Disposal). Several new results are presented here: a discussion on the age of the Argiles ostréennes, the identification of an equivalent of oceanic anoxic event (OAE)1a, and the construction of a continuous section of the Albian clays in their type area. Regarding the Upper Cretaceous chalk, the Cenomanian to Maastrichtian stages are represented over the entire basin and correlations are proposed based on biostratigraphic data and various marker beds including marly levels, bentonites, hardgrounds and eco-events.

A far-traveled basalt lava flow in north-central Oregon, USA

Widely separated basalt lava-flow outcrops in north-central Oregon, USA, expose products of a single eruptive episode. A Pliocene lava flow, here informally termed the Tetherow basalt, issued from vents near Redmond, in the Deschutes basin of Oregon, as a plains-forming basalt now exposed in continuous outcrops northward for 60 km. A similar basalt crops out 47 km farther north, near Maupin, within what was then a slightly incised ancestral Deschutes River canyon. The northernmost outcrops of this lava flow lie on Fulton Ridge, in the Dalles basin, near the confluence of the Deschutes and Columbia Rivers. Complementary lines of evidence confirm these rocks are all from the same volcanic eruption. Outcrops in the Deschutes and Dalles basins are chemically similar high-titanium basalts, petrographically similar to each other and distinct from other lava flows in the area. Paleomagnetic directions from 11 scattered sites are similar and indistinguishable by various tests for a common mean. Three new 40Ar/39Ar ages indicate the Tetherow basalt eruption occurred between 5.5 Ma and 5.0 Ma, likely at ca. 5.2 Ma. The widely separated outcrops of this lava flow span 160−180 km along the ancestral Deschutes River and downstream Columbia River. The lava flow’s length and erupted volume of 15−20 km3 are extraordinarily large in a non-flood-basalt setting. This lava flow provides a datum with which to describe regional physiographic history, assess incision rates, and infer tectonic history. Spanning different depositional basins, the Tetherow basalt is a useful chronologic and stratigraphic marker bed.

Comments on « Environmental and climatic controls of the clay mineralogy of Albian deposits in the Paris and Vocontian basins (France) » [Cretaceous Res. 108 (2020) 104342] and « Astronomical calibration of the OAE1b from the Col de Pré-Guittard section (Aptian-Albian) Vocontian Basin » [Cretaceous Res. 150 (2023) 105618

We wish to point out an obvious error made by one of us (J.F. Deconinck), and kindly reported by J.G. Bréhéret, the reason why we have jointly decided to publish this correction. The error affects at varying degrees the scientific quality and partly invalidates the conclusions of the following articles: Corentin et al. (2020) « Environmental and climatic controls of the clay mineralogy of Albian deposits in the Paris and Vocontian basins (France) » (Cretaceous Research, 108). and Ait-Itto et al. (2023) « Astronomical calibration of the OAE1b from the Col de Pré-Guittard section (Aptian–Albian) Vocontian Basin » (Cretaceous Research, 150).Indeed, for the Vocontian basin, the analytical work, whatever its quality, was carried out on a stratigraphic interval which does not correspond to the presentation made. The two articles in question did not take into account the detailed lithostratigraphy and biostratigraphy earlier published, notably by Bréhéret (1997) and Bréhéret & Brumsack (2000) mentioning known marker beds as presented in these works. Thus, for one of the two sections, the displayed time window assumed to be the lower Albian, actually corresponds to a small part of the middle Albian and a part of the upper Albian. This invalidates many of the proposed results and conclusions.

High-resolution X-ray fluorescence-based provenance mapping of Eocene fluvial distributary fans that fed ancient Gosiute Lake, Wyoming, USA

The Green River Formation of Wyoming, USA, is host to the world’s largest known lacustrine sodium carbonate deposits, which accumulated in a closed basin during the early Eocene greenhouse. Alkaline brines are hypothesized to have been delivered to ancient Gosiute Lake by the Aspen paleoriver that flowed from the Colorado Mineral Belt. To precisely trace fluvial provenance in the resulting deposits, we conducted X-ray fluorescence analyses and petrographic studies across a suite of well-dated sandstone marker beds of the Wilkins Peak Member of the Green River Formation. Principal component analysis reveals strong correlation among elemental abundances, grain composition, and sedimentary lithofacies. To isolate a detrital signal, elements least affected by authigenic minerals, weathering, and other processes were included in a principal component analysis, the results of which are consistent with petrographic sandstone modes and detrital zircon chronofacies of the basin. Sandstone marker beds formed during eccentricity-paced lacustrine lowstands and record the migration of fluvial distributary channel networks from multiple catchments around a migrating depocenter, including two major paleorivers. The depositional topography of these convergent fluvial fans would have inversely defined bathymetric lows during subsequent phases of lacustrine inundation, locations where trona could accumulate below a thermocline. Provenance mapping verifies fluvial connectivity to the Aspen paleoriver and to sources of alkalinity in the Colorado Mineral Belt across Wilkins Peak Member deposition, and shows that the greatest volumes of sediment were delivered from the Aspen paleoriver during deposition of marker beds A, B, D, and I, each of which were deposited coincident with prominent “hyperthermal” isotopic excursions documented in oceanic cores.

Newly discovered Botryococcus-rich source rocks in the western Qaidam Basin, NW China and their implications for shale oil exploration in saline lacustrine basins

Owing to the high hydrocarbon content and photosynthetic efficiency of phytoplankton, planktonic microalgae account for the main source of petroleum hydrocarbons in marine and continental petroliferous basins. Therefore, the presence of algal residues in such basins is considered an important indicator of high-quality hydrocarbon source rocks. The southwestern region of the Qaidam Basin, characterized by lacustrine deposits from the Paleogene, has substantial petroleum resources. However, studies on the development of source rocks and the mechanisms involved in their organic matter enrichment are insufficient, thus hampering ongoing shale oil exploration efforts. Herein, abundant Botryococcus fossils were discovered in the main source rocks of the upper member of Xiaganchaigou Formation of Paleogene in Southwest Qaidam, accounting for >50 % of the total organic matter content. The palynofacies assemblages reflected relatively distal and oxygen-rich sedimentary environments. The algae-rich rocks usually exhibited distinctive laminated structures alternately composed of quartz-feldspathic, clay and carbonate laminae, indicating periodic climate fluctuations. The Botryococcus, which was mainly preserved in the coarse-grain quartz-feldspathic laminae, probably reflected heavy precipitation conditions and subsequently high nutrient inputs. Finally, the oxygen-rich, low salinity and eutrophic water was likely taking form which benefit the growth of these hydrobiontic algae. These algal-rich shales exhibited typically high hydrogen index (IH) and total organic carbon (TOC) values, indicating their high hydrocarbon-generation potential. Thus, they are important marker beds for high-quality source rocks in petroliferous basins. Concurrently, the coarse-grained detrital mineral laminae displayed excellent reservoir physical properties, probably providing sufficient reservoir space for planktonic algae-derived liquid hydrocarbon. Therefore, these Botryococcus-rich source rocks might represent important targets for ongoing shale oil exploration.

A tsunami deposit in the Stonewall Formation (Upper Ordovician), northeastern margin of the Williston Basin, Canada, and its tectonic and stratigraphic implications

The dolomudstone of the Stonewall Formation was deposited on the present-day northeastern side of the low-relief, relatively shallow, tropical Williston Basin of central Laurentia during the latest Ordovician. This unit hosts a geographically widespread carbonate conglomerate, up to 0.5 m thick and consisting of rudstone to floatstone, that thins and eventually disappears westward and southward. The bed contains dolomudstone intraclasts in a variably argillaceous dolomudstone matrix and varies in composition laterally and vertically. Intraclasts range from millimetres to centimetres in size, are spheroidal to tabular to irregular in shape, and are angular to rounded. They record incipient microcrystalline calcite precipitation in localised domains in lime mud that began just below the sediment–water interface, forming lightly cemented nodules in an unconsolidated matrix, and indicate exhumation and deposition by a single, short-duration, high-energy but variable event that fragmented and abraded many of the nodules and washed in clay and lime mud. The composite nature of the bed in terms of varying proportion of intraclasts versus mud, and range of intraclast size with variable rounding and sorting, suggest that the event fluctuated in intensity and current direction. We interpret this bed as having been formed by a succession of tsunami waves and backwash that scoured the sea bottom and reworked the nodules, and brought in suspended illite from shallow water and the adjacent land surface. Despite drainage from tsunami-inundated coastal areas, the absence of allochthonous quartz sand and silt suggests that there was no sediment of this size stored there. Backwash transport may also have played a role in the formation of argillaceous marker beds which have been attributed previously to sea level fall and subaerial exposure. Other anomalous beds that punctuate shallow epeiric sea successions may also be tsunami deposits, especially if the ambient paleoclimate and bathymetry were not conducive to generating or leaving a signal of major storms, and sea level fluctuation can be discounted.

Geochronology of reworked ash and its implications for accommodation space variations in distal foreland basins, McMurray Formation, Alberta, Canada

A bentonite bed consisting of reworked volcanic ash from the Barremian to Aptian lower McMurray Formation is dated using chemical abrasion thermal ionization mass spectrometry (CA-TIMS) on zircon. The bentonite bed is from a cored interval in the McKay Paleovalley in the northern Athabasca Oil Sands Region of Alberta, Canada. Deposition of the lower McMurray Formation took place in the distal portion of the Western Canada Sedimentary Basin early in the Cretaceous-to-Paleocene phase of widespread contractional deformation in the Canadian Cordillera. A 30 g sample of volcanic ash yielded >10,000 very small (<50 µm) zircon grains with the vast majority (~99.7%) being highly abraded and clearly detrital. Twenty-six sharply faceted grains were sufficiently large to be mounted for laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) analysis, which yielded seven Lower Cretaceous dates. These Lower Cretaceous grains were further analyzed using CA-TIMS. The five youngest CA-TIMS dates agree with a weighted mean of 121.51 ± 0.11 Ma which is conservatively interpreted as a maximum depositional age (MDA). The good agreement between the five zircon dates indicates the MDA is probably the depositional age. This study demonstrates that with careful analysis of zircon populations, high-confidence MDAs can be derived even from reworked ash beds that are dominated overwhelmingly by detrital zircon. The date from this study overlaps a previously published MDA of 121.39 ± 0.27 Ma for the top of the lower McMurray Formation in Firebag Tributary in the northeastern Athabasca Oil Sands Area. The overlapping dates establish a widespread chronostratigraphic surface within the northern Athabasca Oil Sands Area. Both ages are from ash beds preserved in coal seams, suggesting that the coal-bearing interval is a regional marker bed and that the ash beds may form an intraformational datum. Further, the occurrence of the ash beds in the McMurray Formation in close proximity to regional flooding surfaces suggests that tectonic activity in the emerging Canadian Cordillera influenced accommodation space variations in the distal portions of the adjacent foreland basin.

The Potash Identification (PID) Plot: A Rapid Screening Crossplot for Discrimination of Commercial Potash

Potash minerals are the primary source of potassium (K), which is used for the manufacture of gunpowder, fertilizer, and as a sodium-seasoning substitute. Commercial potash minerals are all evaporites. Because potassium-40 (40K) is radioactive (decaying to argon-40 (40Ar) and releasing a gamma ray (GR) in the process), commercial potash mineralization is often discovered when GR (γ ray) logs in petroleum wells drilled through evaporite sequences “go off scale.” However, not all potash minerals may be commercial sources of potassium via underground mining techniques, and potassium is not the only radioactive element. For example, the mineralogy of the McNutt “potash” Member of the Salado Formation in southeast (SE) New Mexico is extremely complex, consisting of multiple thin (i.e., less than 10 ft thick) beds of six low-grade (radioactive) potash minerals, only two of which are commercial for underground mining. There are also four nonradioactive evaporite minerals, one of which may interfere with potash milling chemistry and numerous claystones and marker beds (shales and/or volcanics), with GR count rates comparable to the low-grade potash mineralization in this sequence. Because of this complexity, traditional borehole wireline (WL) and logging-while-drilling (LWD) potash assay techniques, such as GR log-to-core assay transforms, may not be sufficient to identify potentially commercial potash mineralization for underground mining (Teufel, 2008) in SE New Mexico. Crain and Anderson (1966) and Hill (2019) developed linear programming and multimineral analyses, respectively, to estimate potash mineralogy and grades from multiple borehole geophysical measurements. However, both of these approaches require large sets of multiple log measurements. In SE New Mexico, petroleum wells are drilled through the Salado Formation evaporite (including the McNutt “potash” Member) with air, then cased and cemented in place without running WL measurements. Then, the wells are drilled out to total depth (TD) in the underlying sediments with water-based mud. Complete log suites are run from TD to the casing shoe, with only the GR and neutron logs recorded through the cased evaporite sequence for stratigraphic and structural correlation. As a result, essentially all recent oil and gas wells in SE New Mexico have casedhole gamma ray and neutron logs through the Salado evaporite. Hill and Crain (2020) developed a simple crossplot involving only GR and neutron log data, which could discriminate between anhydrous and hydrated potassium evaporite minerals. Logs from these wells could provide a rapid potash screening database if used properly. This technique can be used with both openhole and casedhole petroleum well logs, as well as corehole WL logs, and provides discrimination of commercial potash mineralization from noncommercial (potash and non-potash) radioactive mineralization. Case histories of the use of PID crossplots in evaporite basins of Michigan, Nova Scotia, Saskatchewan, and SE New Mexico are described. This technique may also be useful in screening potential potash deposits elsewhere in the world.

Lagoonal Microfacies, Lithostratigraphy, Correlation and Shale Migration of the Basal Middle Eocene Seeb Formation (Rusayl Embayment, Sultanate of Oman)

The study improves the understanding of the basal part of the Eocene Seeb Formation of Oman, informally known as “Unit 1”, in terms of microfacies, lithostratigraphy and shale migration within the context of regional tectonics. We logged four sections bed-by-bed over a distance of 8.3 km, collected samples and analyzed thin-sections as well as XRD samples. For the first time, the microfacies and stratigraphic correlation of the lowermost part of the limestone-dominated Seeb Formation were studied in detail. In the analyzed area, Unit 1 is ~20 to 40 m thick, with the thickness increasing to the SE. In the upper part of Unit 1 is a laterally continuous shale horizon. The limestones of Unit 1 contain mostly packstones and grainstones. The dominant standard microfacies types are SMF 18-FOR and SMF 16. The former is dominated by benthic foraminifera, and the latter by peloids. Both SMFs indicate restricted lagoonal conditions. Foraminifera are common in Unit 1 and indicate a middle Eocene age. Considering the abundance of encountered foraminiferal bioclasts, it appears probable that the lagoon barrier was mainly composed of foraminiferal tests. Gutter casts, slumps and debrites indicate an active, partly unstable syndepositional slope, which was likely initially created by uplift of the Saih Hatat Dome and Jabal Nakhl Subdome. Differential regional uplift due to a more pronounced overall doming in the NW (Jabal Nakhl Subdome) than in the SE (Saih Hatat Dome) explains more accommodation space and greater thickness towards the SE. For the first time, we report visco-plastic shale migration/intrusion within the Seeb Formation, related to a shale horizon of Unit 1. This shale locally migrated as indicated by (1) local thickness variations, (2) detached limestone boulders floating in the shale, (3) limestone beds that have been cut-off by the shale and (4) dragged by the shale (5) an upward shale intrusion/injection which then spread parallelly to bedding similar to a salt tongue and (6) tilting overlying limestones. We suggest that shale migration is related to post-“mid”-Eocene E-W convergence between Arabia and India and to faulting or to the second, late Paleogene/early Neogene, faulting interval of the Frontal Range Fault. The shale horizon in the upper part of Unit 1 is a marker bed, which can be correlated across the study area.

Evidence of postdepositional remobilization of redox-sensitive metals across sapropel boundaries: New insights from LA-ICP-MS and EDX mapping analyses

Deoxygenation in marine systems is globally increasing due to anthropogenic pressures and climate change impacts, leading to significant ecological and environmental consequences. Understanding past deoxygenation events is crucial for assessing the response of ocean stressors to climate variability and for predicting and managing the impacts of current and future changes. Deep-marine basins with organic-rich deposits serve as valuable palaeo-archives, preserving evidence of profound palaeoceanographic changes involving deoxygenation events. Such events have been broadly studied and reconstructed using redox-sensitive trace metals (RSTMs) as proxies for oxygenation conditions. Assessing these proxies is therefore crucial for further understanding deoxygenation events. To do so, this study focuses on RSTMs distribution across the most recent sapropel, S1, which is a well-preserved organic-rich sediment layer that represents the most recent large-scale deoxygenation event in the Eastern Mediterranean. High-resolution geochemical microanalyses, including Laser Ablation ICP-MS and SEM-EDX, were conducted to investigate sub-mm scale geochemical signals and post-depositional remobilization of RSTMs within S1 and surrounding sediments. Statistical analyses have also helped to identify the geochemical fractions within S1, including detritic, carbonate, and authigenic, enriched in RSTMs and organic matter. The study revealed distinct specific geochemical intervals, such as the marker bed, oxidized interval, unoxidized interval, and synsapropel interval. These intervals provided insights into the diagenetic processes and post-depositional reactions occurring within and across sapropel S1 boundaries. The enrichment of RSTMs within pyrite aggregates in the synsapropel interval was primarily attributed to post-depositional processes rather than the initial syndepositional conditions of S1. Large pyrite surfaces below S1 and the precipitation of Mn-oxyhydroxides over S1 played a significant role in the fixation and absorption of dissolved RSTMs derived from postdepositional oxidation. Furthermore, the sub-mm scale analysis revealed spatial heterogeneities in mineralogical composition and elemental distribution, providing valuable information on the complexity of post-depositional reactions and processes. The findings highlight the importance of microscale geochemical analyses for understanding the dynamics of RSTM fixation and enrichment processes in marine sediments. By differentiating between syndepositional and post-depositional geochemical signals, this study contributes to the interpretation of palaeoenvironmental conditions and enhances our understanding of the spatial complexity of deoxygenation processes. Such insights are crucial for accurately assessing the impacts of deoxygenation on biogeochemical cycling and ecosystem functioning in marine environments.

Ibex Hollow Tuff from ca. 12 Ma supereruption, southern Idaho, identified across North America, eastern Pacific Ocean, and Gulf of Mexico

Abstract The Ibex Hollow Tuff, 12.08 ± 0.03 Ma (40Ar/39Ar), is a widespread tephra layer erupted from the Bruneau-Jarbidge volcanic field of southern Idaho. Tephra from this eruption was deposited across much of western and central North America and adjacent ocean areas. We identified the Ibex Hollow Tuff at Trapper Creek, Idaho, near its eruption site, and at 15 distal sites, from the Pacific Ocean to the Gulf of Mexico, by the chemical composition of its glass shards, using electron-microprobe analysis, instrumental neutron activation analysis, and laser-ablation–inductively coupled plasma–mass spectrometry. By these methods, we distinguished the Ibex Hollow Tuff from overlying and underlying tephra layers near its source and at distal sites. Fluvially reworked Ibex Hollow Tuff ash was transported by the ancestral Mississippi River drainage from the interior of the North American continent to the Gulf of Mexico, where it is present within an ~50-m-thick deposit in marine sediments in the subsurface. The minimum fallout area covered by the ash is ~2.7 million km2, with a minimum volume of ~800 km3, and potential dispersal farther to the north and northeast. The areal distribution for the Ibex Hollow Tuff is similar to that of the Lava Creek B (0.63 Ma) supereruption. The Ibex Hollow Tuff represents a unique chronostratigraphic marker allowing a synoptic view of paleoenvironments at a virtual moment in time across a large terrestrial and marine region. The Ibex Hollow Tuff is also an important marker bed for North American Land Mammal Ages, and it coincides with climatic cooling in the middle to late Miocene documented in marine cores.

Sedimentary Processes and the Temporal Resolution of Sedimentary Strata

AbstractAn appealing strategy for reconstructing the timing and tempo of paleoenvironmental change from sedimentary strata is to linearly interpolate between marker beds of known age. This method requires significant assumptions, but more advanced age modeling methods are usually not feasible. We used experiments to explore how changes in sedimentary processes invalidate these assumptions and affect estimates of time from the strata. When sedimentary processes changed to favor widespread deposition, we found that measuring time linearly systematically overestimated time duration from the resulting strata (time dilation) and misestimated the beginning and end of geologic intervals (phase shifting). When simple age models must be used for sedimentary strata, geologic evidence for transient changes in spatial sediment dispersal may help identify sections of dilated and shifted time, and better resolve time in sedimentary strata.

Clay mineral and Nd, Pb, and Sr isotope provenance of a MIS 4-3 sediment record from the Lomonosov Ridge, central Arctic Ocean

Modern techniques for detrital mineral provenance were applied to sediment core 96/12-1pc from the Lomonosov Ridge in the central Arctic Ocean. The techniques include quantitative clay mineralogy analysis combined with determination of Nd, Pb, and Sr isotopes from clay fraction. The clay mineral assemblage and the isotope signatures depict distinct changes during the Marine Isotope Stage (MIS) 4-3 transition corresponding to the Middle Weichselian deglaciation. This transition is characterised by a homogenous, 48 cm thick, dark grey, silty clay layer with a distinctive IRD concentration, forming a prominent marker bed for the central Arctic Ocean sediments. The elevated smectite and kaolinite contents in the transitional interval are possible weathering products of the Siberian basaltic rocks, such as the Putorana Plateau, feeding the shelves of the Kara Sea and the western Laptev Sea. The Nd and Sr isotope values are compatible with input from the basaltic rocks and fall within the isotopic range of sediments from these shelves. The abrupt changes in the Nd, Pb and Sr isotopic data from the distinct grey layer attributed to the MIS 4-3 transition likely mark a pronounced deglaciation event. An increase in coarse debris in the grey layer indicates a change in the sedimentation regime with a strong iceberg rafting component. This change may also be related to a sudden release of meltwater from a large ice-dammed lake in the northern Siberia.

Characteristics and Geological Significance of High-Frequency Cycles in Salinized Lake Basins: The Paleogene Kumugeliemu Group in the Xinhe Area, Northern Tarim Basin

Salinized lake basins have distinctive sedimentary response characteristics, similar to marine shallow-water carbonate platforms. High-frequency cycles can also be used to reveal more sedimentological information, such as relative lake-level fluctuations, lithofacies sequence combinations, and paleogeographic evolution. In this article, a comprehensive study on the stratigraphic shelf delineation and high-frequency cycles of the Paleozoic Kumugeliemu Group in Xinhe area, northern Tarim Basin, was performed using drilling cores, logging curves, and seismic analyses. As a result of the study, the following data were obtained: the three sets of marker beds in the Kumugeliemu Group in the study area could be divided into a bottom sandstone component (E1-2 km1), a lower gypsum mudstone component (E1-2 km2), a salt rock component (E1-2 km3), and an upper gypsum mudstone component (E1-2 km4) by petrology vertical overlay combination and isochronous tracking correlation, which constituted two third-order cycles (ESQ1, ESQ2). They were further divided into seven fourth-order cycles (Esq1–Esq7). Due to the droughty and saline lacustrine depositional system background, the internal rock fabric changed frequently and showed a periodic vertical overlay pattern. Stratified gypsum salt, gypsum mud (sand) rock, and gypsum rock were used as the cycle interface. A single cycle was mainly characterized by an upward shallower depositional sequence of rapid lake transgression followed by a slow lake regression, composed of massive sandstone–lamellar mudstone–lime dolomite–gypsum rock, massive sandstone–lamellar mudstone–gypsum rock (gypsum salt), massive sandstone–massive gypsum mud (sand) rock–gypsum rock, and other cycle structure types. The complete sedimentary cycle was superposed by a single cycle and compared by the inter-well thickness difference, indicating that the study area had a paleogeomorphology pattern of “West-Low–East-High”. The thickness of the cycles decreased gradually from bottom to top vertically, and five sedimentary stages were determined, i.e., freshwater, brackish, brackish water, salt lake, and semi-saltwater, reflecting the evolutionary process of increasing salinity, lake basin filling, and gradual salinization and shrinkage.

Gzhelian cyclothem development in the western North China cratonic basin and its glacioeustatic, tectonic, climatic and autogenic implications

The Gzhelian Ximing Sandstone–to–No. 8 Coal succession of the Taiyuan Formation in the Linxing gas field and adjacent areas records the complex nested architecture of the Palougou Cyclothem and component cyclothems 2–10 developed in the western North China underfilled cratonic basin. Integrated facies and sequence stratigraphic analysis of core, well log, outcrop, and seismic data reveals architectural variation of the Palougou Cyclothem in the study area from southwestward retrograding incised-valley estuary and lagoon (TST) through upward and northeastward prograding coastal plain to epeiric shelf (HST) into downward and eastward prograding open-coast tidal flat and epeiric shelf with aggrading distributive fluvial system (FSST). The component cyclothems vary in the internal architecture through the Palougou Cyclothem from mixed retrogradational–progradational (R–P; R/P > 1) cyclothems 2–4 in the early TST to retrogradation-dominated (R/P ≫1) cyclothems 5–7 in the late TST, progradation-dominated (R/P ≪1) Cyclothem 8 in the HST, and fluvial aggradation-dominated cyclothems 9 and 10 in the FSST. The Palougou Cyclothem and component cyclothems constrained by conodont-bearing limestone marker beds and radiometric dating have been observed to be substantially synchronized with the Gzhelian interglacial–glacial cyclothem and nested 400-kyr cyclothems in the North American Midcontinent and Donets Basin. The cyclic change in accommodation at the scale of the Palougou Cyclothem and component cyclothems has been ascribed to the third-order interglacial–glacial cycle and fourth-order, long eccentricity (ca. 400 kyr)-driven glacioeustatic fluctuations coupled with the tectonically driven source–to–sink reorganization. The progressively decreasing siliciclastic flux and increasing vegetation through both the Palougou Cyclothem and individual component cyclothems have been ascribed to the third- and fourth-order equatorial seasonally dry–wet climatic cycles coupled with the tectonically driven source–to–sink reorganization, in concert with the Gzhelian third- and fourth-order interglacial–glacial eustatic cycles. Both the transgressive fluvial degradation and the falling-stage fluvial aggradation have been ascribed to a higher-gradient fluvial profile relative to the gradient of the receiving North China epeiric shelf. The autogenic shoreline retreat, advance and channel migration modified the depositional response to the third- and fourth-order sea level rise through time. The termination of the falling-stage fluvial channel belts in the lateral and distal floodplain mires has been ascribed to the sustained fluvial aggradation and autodetachment of the fluvial fan from the syn-depositional shoreline in response to steady fall of relative sea level. This study contributes to a proper understanding of the Gzhelian North China intracratonic cyclothem development in response to the coupled glacioeustatic, tectonic, climatic, and autogenic processes at the third- and fourth-order scales, but also provides a context within which to interpret the symbiotic relationship, and predict the distribution, of superimposed coal–sandstone–shale gas reservoirs, and to investigate the impact of depositional heterogeneities on hydrocarbon generation and migration pathways.

Middle–Upper Pleistocene tephras in the Papua New Guinea highlands

Eighteen Middle–Upper Pleistocene (ca 200 ka) tephra units in the highlands of Papua New Guinea (PNG) are mapped and described. The study area is about 24 000 km2, extending from Tari in the west to Kainantu and the Kassam Pass in the east. It is estimated that at least 75 000 km2 of highland PNG received ≥50 cm tephra from various highland sources, and the total volume would have been well over 300 km3. Total tephra thicknesses near sources are >20 m, with thicknesses of individual units near source ranging from 1–2 to >4 m. Several units were deposited with coarse ash and lapilli basal layers that have been weathered to fine ash and clay sizes. Hagen, Giluwe and Yelia volcanoes are the main sources, with smaller centres near Giluwe and north of Hagen also producing mappable tephra units. One unit (Birip) was erupted about 40 ka, but most of the tephra units are 200 ka and older. Volumes of tephra suggest that many of the eruptions had volcanic explosivity indices of 4–6. On stable bedrock, tephras are present on slopes up to 35°, demonstrating the high stability of the tephra materials. Gaps in the tephra cover coincide with bedrock that weathers to unstable regolith where tephras did not accumulate; the tephra cover thus provides an indicator of bedrock stability. Mapped tephra units provide the potential for correlation of other Quaternary materials and can be used as widespread marker beds. The tephras are also important as the parent material for the dominant agricultural soils in the highlands.