Lacustrine Environments Research Articles

Chapter 8. Albertine Rift Fluvial Systems and Terrestrial Rift Valley Environments

Whilst Albertine Rift marginal lacustrine and deltaic depositional environments each produce characteristic lithofacies that together can form a complex stratal architecture, ≥80% of the onshore area of these early continental rift basins is actually dominated by fluvial systems and their associated rift valley terrestrial environments. Albertine Rift rivers today can be classified as flank fan drainage, flank drainage rivers or long-axial systems. Dependent upon slope gradient and sinuosity, and reflecting the bed or suspended load transported, alluvial channels can cycle through four main stages before entering a delta distributary system at the lake shoreline. Higher energy flows closer to rift margins are dominated by gravel bed loads, transitioning to mixed and then suspended loads as gradients decrease out across the rift valley floor. Typical fluvial geomorphological features such as riffle-pool sequences, channel and side bars, meanders, point bars and ox-bow lakes are accompanied by development of characteristic ichno- and rhizo- fabrics in riparian sands and interfluve silts and clays. The four different fluvial stages can be recognized in onshore Pleistocene - Holocene sedimentary successions of both Lake Edward and Lake Albert, with well-sorted fluvial sand bodies, containing termite nests, correlating with petroleum reservoir intervals in the subsurface.

Sedimentary Environment Reconstruction and Organic Matter Enrichment Mechanisms in Various Lithofacies of the Lacustrine Shale: A Case Study of the Da’anzhai Member, Central Sichuan Basin, China

Jurassic lacustrine shale in the Sichuan Basin is a focal exploration area in China, while the pronounced heterogeneity presents challenges, necessitating detailed research as a prerequisite. This study aims at the Jurassic Ziliujing Formation Da’anzhai shale in the central Sichuan Basin, systematically characterizing its geological features. Employing geochemical methods, we reconstructed the paleo-sedimentary environments and explored the mechanisms behind the organic matter enrichment. The findings reveal that the Da’anzhai shale exhibits three lithofacies: organic-rich argillaceous shale (ORA), organic-poor argillaceous shale (OPA), and organic-rich mixed shale (ORM). The sedimentary period was marked by a warm and humid climate, predominantly depositing in anoxic environments with freshwater to brackish conditions. The watershed areas that are provenance sources for the shale clastics are experiencing strong weathering. Significant differences in the sedimentary environments of various lithofacies’ shale were observed. Redox conditions and paleoclimate were identified as the primary factors controlling organic matter enrichment in the Da’anzhai shale of the study area. Salinity also played a role in organic matter enrichment, while terrigenous debris influx and paleo-productivity did not exert a significant controlling effect on organic matter enrichment. Utilizing the reconstructed ancient sedimentary environments, we developed sedimentary models for different lithofacies’ shale, contributing to a deeper understanding of lithofacies’ diversity and the mechanisms governing organic matter enrichment in lacustrine environments. This study provides new information for further understanding the response mechanism of lacustrine shales to the Toarcian Oceanic Anoxic Event in the Early Jurassic from the perspective of elemental geochemistry.

Full-Body Impressions: A category of trace fossils unique to exceptionally preserved bedding planes and indicators of true substrates

Certain biogenic sedimentary structures can be considered to be reliable indicators of true substrates. Full-body impressions, or FBIs, are trace fossils that are shallow, highly-detailed impressions of the ventral axial anatomy of low body mass animals made while living . Their mode of preservation makes FBIs distinct from undertracks, body moulds, or passive tool marks. FBIs are made at the air- or water-substrate interface during sedimentary stasis. Associated trace fossils, like those that exhibit excavations with preserved waste mounds and very near-surface burrow roofs, located on the same bedding plane, can support the interpretation of true substrate preservation. Integration of the biogenic and sedimentological data from both Carboniferous and Pleistocene sites with well-preserved bedding-plane trace fossil assemblages has confirmed the conditions under which true substrates are preserved. These include periods (as brief as days to weeks) of sedimentary stasis, followed by gentle deposition without erosion, and without the need to invoke biofilms or biomats as consolidants. The potential for these taphonomic conditions exist in both alluvial and lacustrine environments, as well as intertidal-estuarine valleys, as described by other published examples of FBI preservation. Future FBI discoveries will prove useful in identifying true substrates across a range of time periods and environments.

Understanding an exceptional Afropollis‐dominated flora in the middle Cretaceous palaeotropics of the southern Tethys

AbstractAfropollis is a common element in Cretaceous low‐latitude palynofloras ranging from the Barremian to the Cenomanian. Despite an abundant fossil record, its affinities are still elusive. Nine Afropollis‐rich subsurface samples from the Cenomanian Bahariya Formation in the northern part of the Western Desert of Egypt were palynologically analysed, and the palynoflora was compared with previously published material. The geographical distribution of Afropollis‐yielding strata was compared with known macrofloras. The Bahariya material yielded a distinct palynomorph assemblage dominated by an unusually high frequency of Afropollis jardinus, reaching up to 60.7% of the total palynomorph count. The dominance of A. jardinus may be ecologically related to a near‐coastal lacustrine or lagoonal environment. Its association with two kinds of macrofloras suggests that the plants producing A. jardinus may have been part of a tropical azonal vegetation type, either aquatic or mangrove‐like.

Organic Sulfur Markers as Proxies of Depositional Paleoeenvironments Related to Recôncavo and Amazon Basins, Brazil.

This study employed organic sulfur markers (S-markers) associated with geochemistry parameters to evaluate the paleoenvironment of different depositional settings in 24 samples collected in vertical sections of outcrops of the Candeias and Barreirinha Formations in Recôncavo and Amazon basins, respectively. A total of twenty-one S-markers from benzothiophene (BT), dibenzothiophene (DBT), and benzonaphtothiophenes (BNT) classes were optimized and quantified by gas chromatography-triple quadrupole mass spectrometry (GC-MS/MS). S-markers efficiently evaluated and differentiated the depositional paleoenvironment in the source rocks based on the individual compound, in cross-validation with saturated biomarkers, and associated with parameters such as total organic carbon (TOC) and Rock-Eval pyrolysis. Samples from the lacustrine environment presented low concentrations of BT, DBT, and BNT, and samples from the marine environment showed high BT, DBT, and BNT concentrations. The variations in ∑DBT and TOC indicated that the quantity and/or the type of organic matter exert some control over the distribution of DBTs. Although the formations are from different paleoenvironments, the organic matter input was similar, as indicated by high proportions of 1,2-BNT and 2,1-BNT relative to 2,3-BNT, thus characterizing the algal input with a microbial contribution for both sites. The sum of the BNTs was directly related to the amounts of amorphous organic matter (AOM) in the vertical distribution of outcrops. These results are in accordance with the finding that BNTs may originate from the microbial activity. The DBT/Phen vs pristane/phytane (Pr/Ph) relationship attested to differences in the redox conditions of the depositional paleoenvironments of the formations under study. The 4,6-DMDBT/2,4,6-TMDBT and 2,4,6-TMDBT/(2,4,7 + 2,4,8)-TMDBT ratios indicated immaturity for hydrocarbon generation.

Transport and depositional processes of mud in lacustrine environments：From sediment plumes to hyperpycnal flows

Lacustrine mudstones are usually considered as deposited dominantly through vertical suspension fallout in low-energy environments influenced by only occasional fine-grained turbulent flows, while the deposition of their marine counterpart have been increasingly documented to be subject to more complex and dynamic processes. To provide insights into the dynamic processes that transport and deposit mud in lacustrine settings, this study presents an integrated petrographic, sedimentological, geochemical, and sequence-stratigraphic analysis of an ancient lacustrine mudstone-dominated succession using outcrops of the Lucaogou Formation in the Junggar Basin. Detailed facies characterization has revealed seven facies associations, which show spatial and temporal variations within a stratigraphic framework and stack to form eight repetitive parasequence styles. Process-based sedimentological analysis allows the identification of multiple transport mechanisms in different depositional settings, which strongly suggests that mud in lake systems is commonly subject to multiple dynamic transport processes (i.e., sediment plumes, turbidity flows, hyperpycnal flows, and wind-driven bottom currents), rather than suspension fallout under overall quiet conditions. The differences in the expression of parasequence styles can be attributed to the combination of allogenic and autogenic processes, which systematically change with distances from the shoreline. It is important to note that river-floods can discharge large amounts of mud into the deep lake setting. The strong correlation between geochemical data and the distribution of facies generated by riverine processes indicates that in saltwater lakes terrigenous mud is mainly transported to the deep water by sediment plumes and turbidity flows, whereas in freshwater lakes it is transported primarily by hyperpycnal flows. This study demonstrates that the transport and depositional processes of lake mud are diverse and vary systematically in response to lake conditions and environmental changes. High-resolution petrographic and sedimentologic analysis is essential to identify these dynamic processes, which allows more critical and accurate interpretations of depositional conditions and long-term depositional trends in lake systems.

Sedimentary factors controlling the organic matter enrichment in oil shale, Seyitömer Lacustrine Basin, Western Anatolia: New implications from organic and inorganic geochemistry

AbstractWestern Anatolia, an important segment of the Alp‐Himalayan Belt, hosts numerous Neogene lacustrine basins with potential oil shale reserves surpassing 1.6 billion tonnes. Among these basins, the Seyitömer Basin (Kütahya) stands out, containing oil shales that are intercalated with claystone, marl, limestone and coal layers. This study presents a comprehensive dataset of organic and inorganic chemistry to gain insight into the palaeoclimate and palaeoenvironmental factors that control the enrichment of organic matter. The studied samples exhibit high total organic carbon contents, averaging 12.85% (ranging from 2.22 to 36.21%), high hydrogen indices (486–812 mgHC/g rock) and low oxygen indices (33–70 mgCO2/g total organic carbon), indicating their substantial hydrocarbon‐source potential. These characteristics indicate predominantly ‘excellent’ to occasionally ‘very good’ source rock qualities and very high oil potential. Their pyrolysis, gas chromatography and gas chromatography–mass spectrometer parameters indicate an immature‐early mature characteristic for Seyitömer oil shale samples. They comprise dominantly Type‐I kerogen with minor Type‐II kerogen and lacustrine algal organic matter. Their sedimentological characteristics, along with various geochemical values, such as total organic carbon versus S, B versus Ga and dibenzothiophene/phenanthrene, reveal a moderately deep fresh/brackish to saline lacustrine environment. The Ga/Rb, K/Al and Sr/Cu ratios suggest dominantly humid and warm climate conditions, occasionally interrupted by periods of less humidity. The prevalence of warm and humid climate conditions leads to intense chemical weathering processes, supported by high Chemical Index of Alteration and low Rb/Sr ratios in the associated oil shale samples. Intense chemical weathering and high runoff resulted in dissolved nutrient enrichment, promoting ecological dynamics favourable to increased productivity. Their low Pr/Ph and Mo/total organic carbon ratios, high Ni/Co ratio and, relatively low MoEF/UEF ratio, along with well‐developed lamination of the oil shales, indicate the presence of anoxic conditions in the bottom water. These anoxic conditions would have facilitated the preservation potential of organic matter in the samples. Thus, the palaeoclimate conditions integrated with sedimentary factors have an important role in the ecological dynamic and physical–chemical environmental conditions, ultimately contributing to the organic matter enrichment of the studied samples. This work provides a case study to better understand the sedimentary factors controlling organic matter enrichment in the lacustrine basins of the Alp‐orogenic belt.

Site characterization of the Endurance CO 2 store, Southern North Sea, UK

The Endurance saline aquifer CO 2 store in the UK North Sea is the planned storage site for CO 2 emissions that will be captured from the East Coast Cluster. The site has been characterized to assess its suitability for injectivity, containment and capacity. The injection target is the Early Triassic Bunter Sandstone Formation, consisting of fine-grained sandstones deposited in fluvial, lacustrine and playa lake environments that were subject to repeated aeolian and fluvial reworking. The sandstones have excellent reservoir quality and are likely to be well connected due to the lack of preserved heterolithic layers. The trap is a large, four-way dip-closed anticline, sealed by mudstones and evaporites of the Middle Triassic Dowsing Formation. The top seal sediments are laterally extensive and geomechanically strong, and faulting observed in the overburden does not appear to connect to the reservoir, providing confidence that CO 2 containment will be secure. The risk of leakage through existing legacy wells is assessed as low. The maximum effective CO 2 storage capacity is c . 450 Mt: Phase 1 development of 100 Mt is equivalent to a storage efficiency factor of 3%, which is achievable without the need for brine production. First injection is planned from 2027.

Reconstructing early sedimentation patterns of the lower Codó Formation (Early Cretaceous, NE Brazil): Evidence of marine influence and the onset of an alkaline hypersaline lake

The upper Aptian lower Codó Formation is an attractive stratigraphic unit within the Parnaíba and São Luís basins, northeastern Brazil, formed during the break-up of Western Gondwana. Located in a large sag basin, the formation records a dynamic depositional history of deltaic, lacustrine, and sabkha environments. This sedimentary evolution is reflected in a diverse lithological succession characterized by intermittent terrestrial input, episodic marine incursions, and restricted lacustrine phases culminating in sulfate evaporite precipitation. While the later stages of the formation have been extensively studied, the initial sedimentary processes and environmental conditions remain poorly understood. This research aims to reconstruct the depositional paleoenvironments during the initial stages of Codó sedimentation, elucidating the onset of the alkaline hypersaline lake environment and the earlier marine incursions in the study region. To achieve this, it was conducted a high-resolution facies analysis using core descriptions, trace fossil analysis, petrography, various types of X-ray fluorescence, scanning electron microscopy, X-ray diffractometry, and isotope analyses (δ1³C, δ1⁸O, and ⁸⁷Sr/⁸⁶Sr) on a ∼63-m-thick succession from borehole 2-TV-1-MA. Thirteen facies were recognized and grouped into brackish lake, prodelta, delta front, and alkaline hypersaline lake facies successions. The lower Codó Formation exhibits cyclic sedimentation, indicating fluctuating water dynamics associated with shifts between open and closed drainage systems controlled by paleoclimate. Tectonic subsidence drove the basin's evolution from a brackish lacustrine-deltaic environment with an incipient marine influence (balanced-fill basin stage) to a restricted system with the establishment of an alkaline hypersaline lake (underfilled basin stage).

Vertebrate taphonomy and taphofacies of the Capianga Member, Aliança Formation (Jurassic), Jatobá Basin, Brazil

In addition to the study of sedimentary facies, the reconstruction of paleoenvironments depends on a detailed understanding of the taphonomy of a fossil accumulation. The Capianga Member of the Aliança Formation (Middle to Late Jurassic of the Jatobá Basin, northeastern Brazil) reveals vertebrate fossil accumulations composed predominantly of disarticulated bone and osteoderms, scales, spines and isolated teeth from a lacustrine paleoenvironment. However, the taphonomic history of the accumulation has not yet been studied extensively. The present work aims to interpret taphonomic facies based on lithological data and the systematic collection of fossils from the Capianga Member of the Aliança Formation, located in the northeastern portion of the Jatobá Basin, mainly in the municipality of Ibimirim. Six lithofacies were identified: Fm – massive claystones, Fl – Shale laminated Lt – calcilutites, Lc – calcarenites, Gf – fibrous gypsum and Scl – calciferous sandstones. Two primary taphonomic classes were recognized, consisting essentially of disarticulated elements, including (1) incomplete, semi-complete, or rarely complete bioclasts (such as osteoderms and fish scales) and (2) small-sized, occasionally complete bioclasts (osteoderms and scales) that sometimes compose bonebeds. According to the vertebrate taphonomy characteristics, three taphofacies were identified, with the Taphofacies A and B occurring in calcarenites and the Taphofacies C in calcilutites. The interpretation of these taphofacies reveals a multi-episodic history of the lacustrine environment. The integrated model suggests that the genesis of the identified taphofacies is linked to the action of lowering the water level of the paleolake, as well as the agents responsible for transport and disarticulation. These agents are related to pre-burial sub-aerial exposure, the action of unidirectional turbidity currents in shallow waters, and the likely influence of storms.

Redox conditions influence the chemical composition of iron-associated organic carbon in boreal lake sediments: A synchrotron-based NEXAFS study

The global carbon and iron cycles are intimately linked as redox-sensitive iron oxides readily bind organic carbon in a variety of environmental settings, including marine and lacustrine sediments. While these iron-organic carbon complexes sequester vast quantities of organic carbon, the composition of the organic matter within them remains unknown for lacustrine environments. Here we present C K1s and Fe L3,2 edge Near Edge X-ray Absorption Fine Structure (NEXAFS) spectra of surface sediments and authigenic iron complexes from adjacent basins of a pristine boreal lake located in Québec, Canada, with contrasting oxygen exposure regimes. We demonstrate differences in organic carbon speciation in sediments from both basins, as well as co-localization of organic carbon and iron on a sub-micron scale in 100 nm thick samples. Differences in redox cycling across these two basins allow for a direct comparison of the effect of oscillating redox conditions on the composition of organic carbon sequestered by iron. Our results suggest that reactive organic molecules, which may be polysaccharides, were found preferentially associated with iron in the perennially oxic sediments compared to more phenol rich organics in the seasonally anoxic sediments, highlighting the importance of iron oxides in the protection and preservation of labile organic compounds. Traces of aliphatic carbon were observed in sediments from the anoxic basin, alongside carboxyl and aromatic functionalities. This carboxyl-rich aliphatic material could possibly interact with the sediment mineral matrix either through a ligand exchange mechanism between the mineral phases and the carboxyl functionalities, or via non-specific hydrophobic interactions involving the aliphatic moieties. Finally, our work also shows that OC:Fe ratios should be used with caution when inferring a binding mechanism between OC and iron oxides.

Aliphatic and polycyclic aromatic compounds in coal and coal-based solid wastes: Relationship with coal-forming paleoenvironment and implications for environmental pollution

In this study, coal and coal-based solid wastes (coal gangue, fly ash, bottom ash, desulfurized gypsum and tar residue) were collected from major coal mines, power plants and coking plants in Lianghuai mining area (LH), China, and were analyzed for 76 polycyclic aromatic compounds (PACs), 27 n-alkanes and 2 isoprenoids (phytane and pristane). The total n-alkanes concentrations and ∑76 PACs in raw coals (640 ± 600 and 180 ± 87 μg/g) were higher than those in coal-based solid wastes (47 ± 40 and 24 ± 25 μg/g), but were lower than those in tar residue (3700 and 63,000 μg/g). It was discovered that the depositional paleoenvironment in LH was mostly a lacustrine and freshwater environment with oxidizing conditions and mixed organic matter input, but the Huainan coalfield had stronger oxidizing conditions and more input of terrestrial organic matter than that of the Huaibei coalfield. Alkylated PACs made up 56 ± 12 % of the ∑76PACs in raw coals, whereas solid wastes mainly consisted of 16 EPA PAHs (66 ± 16 %). Coal combustion and gangue weathering altered the structural properties of n-alkanes and PACs, resulting in a significant loss of n-alkanes and PACs, a higher proportion of parent PACs, and an increased abundance of short n-alkanes in the products (No apparent change of n-alkanes composition was observed through gangue weathering). The toxicity of PACs in raw coal and its solid wastes in LH from high to low was tar residue, raw coal, coal gangue, and coal-fired products. This investigation further confirmed that traditional diagnostic ratios may distort source information, and that they should not be used to assess PACs sources from raw coal particles or coal gangues, but rather to identify combustion sources near the point source. In addition, Retene/(Retene + Chrysene) < 0.03 may indicate direct contamination of raw coal particles.

Evaluating evaporites as potential archives of lake and seawater lithium isotopic ratios: An experimental study using various natural saline fluids

Reconstructing the lithium isotope composition (δ7Li) of ancient seawater and lacustrine environment is crucial for tracing silicate weathering processes that regulate Earth's carbon cycle and climate. This study examines the reliability of evaporite minerals in preserving the Li isotope signature of the original brines from which they precipitate. We conducted evaporation experiments on modern natural waters from Qinghai Lake, the Yellow Sea, and Chaka Salt Lake, each with unique salinity and chemical composition, to examine the behavior of lithium isotope ratios in the evolving brine and precipitated evaporite minerals.Our results show that initial hydrochemical conditions of the lake and ocean are the primary control on the evolution of major and trace elements and the mineral precipitation sequences during evaporative concentration. Aragonite, calcite, gypsum, and/or nesquehonite precipitated prior to halite. Li isotope fractionation of these non-halite precipitates varied significantly between 4‰ and 17‰. However, the δ7Li values of halite closely mirror those of the initial lake and seawater, indicating negligible Li isotope fractionation (<1‰) during halite precipitation. The δ7Li values of the evolving brines remain unchanged throughout the evaporation experiment. Solid phases precipitating before halite constitute <1–2% of the total moles of salt precipitated from the complete evaporation of the initial waters, thus, having little to no influence on the δ7Li values of evolving brines. Although halite comprises >77% of the total moles of salt, it does not fractionate Li isotopes. Therefore, halite more accurately reflects the δ7Li values of the co-evolving brine as well as those of the initial, unevaporated lake and seawater. These results underscore the potential of late stage evaporites, particularly halite, to serve as high-fidelity archives of ancient lacustrine and seawater δ7Li values. Such records are instrumental in reconstructing past silicate weathering and climatic conditions.

Outcrop spectral gamma-ray signals of distal arid continental basins: Facies identification and controls

Spectral gamma ray (SGR) studies can help link outcrop exposures to downhole data and provide useful case studies to reduce uncertainty in interpreting geophysical logs where core and other geological data is unavailable. However, this technique is underused in continental outcrop studies due to difficulties in determining facies comprising similarly sourced sediment, despite the fact that these deposits form prolific reservoirs for hydrocarbon and geothermal exploration, as well as carbon capture and hydrogen and storage. This work demonstrates the usefulness of spectral gamma ray measurements as an outcrop logging technique in determining quantifiable facies from the dryland continental sediments of the Cedar Mesa Sandstone Formation, Utah, USA. Complex sedimentary interactions between the aeolian, fluvial and lacustrine depositional environments are present, resulting in shared similar sedimentary characteristics within the preserved strata, such as mineralogy, grain size, sorting and provenance signals. Consequently, the geophysical signatures observed within the spectral gamma ray logs, have been suppressed, making it difficult to distinguish sedimentary facies from solely the spectral gamma ray responses. However, by combining K:Th cross-plot analysis and gamma ray log motif interpretation, this work demonstrates that individual facies within arid continental settings can be distinguished based on SGR outcrop logging, and unique spectral trends and values can be quantified and used to correlate facies across the depositional basin.

Holocene paleolimnological changes in rundvågshetta lakes in the Soya Coast region and their paleoenvironmental significance with glacial isostatic adjustment in East Antarctica

This study investigated Holocene paleolimnological changes inferred from a multi-proxy dataset of C, N, and S elements, biomarkers, and microscopic observations of microalgae and cyanobacteria in sediment cores from two Rundvågshetta lakes (Maruwanminami-ike and Maruwan-oike) in the Soya Coast region of Antarctica, along with sedimentary facies and accelerator mass spectrometry 14C dating. We discuss biological production and sources, the transition from marine to brackish and lacustrine environments in the lakes, paleoenvironmental changes, transition ages, and relative sea level (RSL) falls (or subsidence) from glacial isostatic adjustment (GIA) in the Soya Coast region. The ages of the Maruwanminami-ike (MwS4C-01, length 147 cm) and Maruwan-oike sediment cores (Mw4C-01, length 226 cm) were 1.4 (5.8 cm core depth)–4.9 and 2.3–5.7 cal ker BP, respectively. The local reservoir effects of the Rundvågshetta lakes were very high, possibly because of the influence of dead carbon in the glacially eroded marine sediments and bed rock. The coastal marine environments of Lakes Maruwanminami-ike (4.9–2.6 cal ker BP) and Maruwan-oike (5.7–3.3 cal ker BP) were characterized by low biological production. A transition period marked by a stratified brackish lake environment in Lakes Maruwanminami-ike (2.6–2.4 cal ker BP) and Maruwan-oike (3.3–2.9 cal ker BP) was characterized by stratified conditions with an anoxic layer at the bottom of the photic zone. The freshwater lake environment of Lake Maruwanminami-ike [(65.6–0 cm, 2.4–1.4 (5.8 cm core depth) cal kyr BP)] was characterized by high biological (cyanobacteria and green algae) production, while that of Lake Maruwan-oike (2.9–2.3 cal ker BP) was characterized by low biological production due to the influence of glacial clay. Around 7.2–3.0 cal ka BP in the Holocene was probably a warm period, and progressive glacial retreat with RSL fall occurred during the warm periods in the Soya Coast region. The RSL falls of all raised beaches and isolated basins in the Soya Coast region ranged from 0.36 ± 0.03–4.4 ± 0.5 mm/yr with an average 2.1 ± 0.2 mm/yr (standard deviation, SD). They were similar to GNSS–based GIA–induced uplift with an average of 2.4 ± 1.5 mm/yr (SD) at Soya Coast region.

Chronology of Sedimentation and Landscape Evolution in the Okavango Rift Zone, a Developing Young Rift in Southern Africa

AbstractThe Kalahari Basin in southern Africa, shaped by subsidence and epeirogeny, features the Okavango Rift Zone (ORZ) as a significant structural element characterized by diffused extensional deformation forming a prominent depocenter. This study elucidates the Pleistocene landscape evolution of the ORZ by examining the chronology of sediment formation and filling this incipient rift and its surroundings. Modeling of cosmogenic nuclide concentrations in surficial eolian sand from distinct structural blocks around the ORZ provides insights into sand's residence time on the surface. Sand formation occurred from ∼2.2 to 1.1 Ma, coinciding with regional tectonic events. Notably, provenance analyses of sand within ORZ's lowermost block where large alluvial fans are found indicate different source rocks and depositional environments than those of the eolian sands found at a higher elevation. This suggests that the major phase of rift subsidence and the following incision of alluvial systems into the rift occurred after eolian dune formation. Luminescence dating reveals that deposition in alluvial fan settings in the incised landscape began not later than ∼250 ka, and that a lacustrine environment existed since at least ∼140 ka. The established chronological framework constrains the geomorphological effects of the different tectono‐climatic forces that shaped this nascent rifting area. It highlights two pronounced stages of landscape development, with the most recent major deformation event in the evolving rift probably occurring during the middle Pleistocene transition (1.2–0.75 Ma). This event is reflected as a striking change in the depositional environments due to the configurational changes accompanying rift progression.

Facies Analysis and Paleoenvironmental Reconstruction of Ghar Formation in Al-Muthanna Governorate, Southern Iraq

The Oligocene-Miocene succession in Al-Muthanna Governorate of southern Iraq shows significant mixed siliciclastic-carbonate formation. Fourteen samples are collected from two wells and four outcrops in Al-Muthanna area for studying the facies and paleoenvironments of the Ghar Formation. It is mainly composed of sandstone, calcareous sandstone, and carbonate rocks. Six facies associations are identified in the mixed siliciclastic-carbonate deposit: the carbonate type, including sabkha, lagoon, intertidal-subtidal, and lacustrine; the mixed type, including shoreface; and the siliciclastic type, including distributary channels. The major minerals in this formation are quartz, calcite, and dolomite, whereas feldspar and gypsum are the minor minerals. Clay minerals such as Palygorskite and Montmorillonite were also identified. The carbonate rocks of the Ghar Formation reflected deposition in the evaporitic platform (Sabkha), shallow restricted (lagoon), tidal and lacustrine environments. Ghar formation is considered transitional in conditions between a very restricted shallow marine and near-shore environment, whereas the clastic facies in Ghar Formation indicated deposition in the delta front channel.

Mid to Late-Holocene environmental dynamics recorded in Lake Pup Lagoon, East Antarctica: Insights from environmental magnetism and biogeochemical proxies

Paleoenvironmental archives in East Antarctica have revealed significant changes during the Holocene, marked by ice sheet retreat leading to the isolation of submarine basins. These basins offer valuable insights into past climate, glaciology, and oceanography shifts that impact sedimentary processes. In this study, environmental magnetism and biogeochemical proxies to investigate the Mid-to-Late-Holocene transitions in Pup Lagoon, a coastal isolation basin is presented. Our findings reveal distinct stratigraphic zones reflecting shifts from marine to lacustrine environments. The results reveal predominant mechanical weathering in the Stornes region, producing coarse-grained “soft” ferrimagnetic minerals. Notably, a period of warm oceanographic conditions between 6000 and 4722 cal. yr BP was characterized by mixed magnetic grain sizes and ultrafine superparamagnetic grains, indicating relatively oxic open waters in the basin. Subsequent shifts to reducing conditions coincide with persistent marine sea ice cover from 4722 to 2634 cal. yr BP, favoring the retention of coarse-grained ferrimagnets. Finer magnetic grain sizes between 2634 and 2185 cal. yr BP was attributed to the increased freshwater inputs associated with the Mid-Holocene Hypsithermal. Further, diagenetic changes under persistent sea ice cover between 2185 and 1970 cal. yr BP led to the selective dissolution of fine-grained ferrimagnets. Transitioning to freshwater isolated basin conditions between 1970 and 588 cal. yr BP, fine ferrimagnet precipitation indicate oxic to suboxic conditions alongside drier conditions. Biogenic productivity increased post-isolation, which was reflected in increased (Total Organic Carbon) TOC and (Total Nitrogen) TN percentages. Additionally, the presence of greigite in the isolated phase sediment indicates reducing conditions owing to organic matter decomposition. Notably, χfd% exhibits an inverse trend to sea ice concentration, potentially indicating anoxic-dysoxic conditions due to the presence of sea ice. These observations align with broader regional sea ice concentration changes, emphasizing the interconnected behavior of local and regional factors shaping Antarctic coastal environments.

Swimming, confusion, and plenty of brews: Negotiating ambivalence within Windermere's fragile waters

Windermere is a complex and contested freshwater site which encounters fluctuating social and environmental pressures. Swimmers at Windermere regularly practice across all four seasons while negotiating social concerns such as access, conflicting user groups, public health communications, and swim safety, alongside environmental complications including extreme weather, wastewater, run-off, plastic pollution, algal blooms, biosecurity, and climate change. Simultaneously, these entangled pressures generate ongoing adaptation, ambivalence, and avoidance within the swim communities. Furthermore, they disrupt individualised and inwardly focused understandings of ‘healthy’ outdoor swimming practices. In contribution to the special issue (on outdoor swimming), this article reflects on how outdoor swimming researchers may methodologically attend to these social and environmental complexities within contested lacustrine environments through an immersive 12-month wet ethnographic approach, combining ‘lake-hangouts’ and ‘swim-along interviews’ with different swimmers at Windermere. The article discusses how these relational in-situ approaches can continue to broaden inwardly focused understandings of ‘healthy’ outdoor swimming practices towards the wider social and environmental relations for both the participants and researcher. The article also highlights senses of ambivalence and ethical tension while negotiating conflicting concerns of ill-health, in and out of Windermere's fragile waters.

Hydrocarbon-generating potential of the Middle Permian Lucaogou source rock in the eastern Junggar Basin, China

The Middle Permian Lucaogou Formation is the most significant source rock in the eastern Junggar Basin. Previous studies have confirmed its excellent hydrocarbon-generating potential in the Jimsar Sag. However, its potential in other areas of the eastern Junggar Basin remains uncertain. Based on total organic carbon and pyrolysis, organic petrology, hydrocarbon simulation experiments, basin simulation, and combined well-seismic coupling interpretation, this study systematically compares the hydrocarbon-generating potential of the Lucaogou source rock in the Jimsar Sag with other areas of the eastern Junggar Basin. It discusses the sedimentary environment of high-quality source rocks and depicts the distribution of practical source kitchens. The Lucaogou source rocks in the eastern Junggar Basin are oil-prone, dominated by type I–II kerogen, and generally classified as good to excellent source rocks. Nowadays, the area of the Lucaogou source rocks that have entered the main oil-generating window is approximately 11 × 103 km2. Except for the bulge area, the Lucaogou source rocks in the eastern Junggar Basin successively entered the hydrocarbon-generating threshold during the Jurassic and the main oil-generating window in the Cretaceous. The Lucaogou source rocks in the Jimsar Sag and other parts of the eastern Junggar Basin share similar biomarker fingerprints, characterized by relatively low ratios of Pr/Ph, Pr/n-C17, Tm/C30 hopane, C19/C20 tricyclic terpene, and C24 tetracyclic terpene/C26 tricyclic terpene, and high β-carotene content, gammacerane index, and Ts/Tm ratios. These characteristics reflect deposition in a strongly reducing brackish lacustrine environment with parental sources dominated by lower organisms such as algae and bacteria. Generally, the Lucaogou source rocks in the eastern Junggar Basin have an oil-generating intensity of more than 3 × 106 t/km2. Several oil-generating centers with an intensity of more than 5 × 106 t/km2 have developed in the front of the Bogda Mountain, Jimsar Sag, Dongdaohaizi Sag, Wucaiwan Sag, and Shazhang Fault Zone, covering a total area of approximately 12,500 km2. These characteristics of the Lucaogou source rocks promise favorable potential for forming large and medium oil fields. The results further consolidated the oil and gas resources in the eastern Junggar Basin and provided valuable references for exploring future oil and gas fields.