Shallow Lacustrine Research Articles

Early Pleistocene charophyte flora from Dursunlu (Ilgın Basin, Turkey): palaeoecological implications

Here, a charophyte assemblage from lower Pleistocene deposits at the coal mine locality of Dursunlu (Ilgın Basin, Turkey) is described and illustrated for the first time. The assemblage is composed of 11 species recovered from 50 samples taken from an 18 m-thick sedimentary sequence. The base of the sequence is dominated by lignites and organic-rich claystones/siltstones with root marks attributed to palustrine facies. The charophyte assemblage in this palustrine interval is dominated by Chara vulgaris and C. globularis. The upper part of the section is constituted by light grey-yellowish marls, siltstones and a limestone bed related to shallow lacustrine deposits. Two charophyte species dominate this lacustrine interval, Chara hispida and Nitellopsis obtusa, which occurred in intervals in association with Chara pappii, Chara molassica var. notata, Chara sp.1, Chara sp.2, Sphaerochara prolifera, Sphaerochara intricata and Lychnothamnus barbatus var. antiquus. The palaeoenvironmental characteristics and evolution of the Ilgın palaeolake during the Early Pleistocene (Günz glaciation) are inferred from a facies analysis and the ecological requirements of the charophyte assemblages. The results suggest that during this glacial period, the Ilgın Basin was occupied by a very shallow eutrophic lake with a dense palustrine vegetation belt, which evolved to a shallow (2–15 m deep), stable, oligotrophic, alkaline and oligohaline lake. The increase in water availability in the palaeolake can be related to climatic oscillation during the Günz glaciation. Finally, the lake depocentre migrated towards the NW due to tectonism, and these deposits were later subjected to intense pedogenic alteration.

Hydrogeochemical controls and recharge-flow system of groundwater in the lowland of the Fukui-Ohno Basin, central Japan, based on carbon isotopic signatures

• River water mixing and CO 2 dissolution in groundwater was modeled with δ 13 C tracers. • Decomposition of both C3 and lacustrine organic matter was related to CO 2 dissolution. • Modeling identified similar amounts of vertical and horizontal groundwater recharge. Carbon stable isotopic compositions of dissolved inorganic carbon (δ 13 C DIC ) and chemical compositions of groundwater were used to investigate the processes of supply of DIC to groundwater in the lowland area of the Fukui-Ohno Basin and to examine constraints on the groundwater recharge-flow system. The groundwater in the basin is dominantly of Ca-HCO 3 type with relatively high concentrations of Ca 2+ and HCO 3 − and lower concentrations of Mg 2+ ; this groundwater chemistry can be attributed to weathering within the aquifer of mainly anorthite and some pyroxene at partial pressures of CO 2 that are typical of soils. The δ 13 C values of the CO 2 gas initially equilibrated with DIC in the groundwater were calculated to be −28.6‰ to −22.8‰, indicating that the groundwater in the lowland basin has two main sources of carbon: (1) C3 organic matter (δ 13 C ≈ −29‰), distributed mainly in mountains surrounding much of the basin and in aquicludes within the basin; and (2) organic matter in shallow lacustrine deposits within the lowland basin (δ 13 C ≈ −20‰). The results of modeling accounting for these two sources demonstrated that roughly equal proportions (50 ± 20%) of DIC in the groundwater are sourced from lacustrine deposits and C3 organic matter. The distributions of these types of organic matters in and around the lowland basin suggest that there are two major and roughly equal components of groundwater recharge: a vertical component of downward percolation of precipitation in the lowland area and a mainly horizontal component of groundwater flow originating in the mountains around the study area and flowing through aquifers into the lowland basin.

Improving paleoenvironment in North China aided Triassic biotic recovery on land following the end-Permian mass extinction

The driver of the Early–Middle Triassic biotic recovery on land following the end-Permian crisis is puzzling. Here, we show the biotic recovery was gradual and spanned up to 8 Myr after the end-Permian mass extinction, based on continuous, well-dated sections over large areas in the northeastern Ordos Basin, North China. Initial recovery began in the Olenekian, marked by the disappearance of microbially induced sedimentary structures and reappearance of bioturbation, and continued in the Anisian, with a bloom of plants and tetrapods and intense bioturbation. Sedimentary environments changed from Induan braided-eolian conditions to Olenekian–Anisian shallow lacustrine and meandering river systems, marking an overall deepening lacustrine system. Carbonate δ13C and geochemical proxies of weathering intensity, salinity and clayiness reveal an overall warm and semi-humid paleoclimate in the Olenekian–Anisian. This improved stable paleoenvironment of warm and semi-humid conditions likely contributed to the biotic recovery following the Permian-Triassic hyperthermal-related crisis.

Unraveling the ecological mechanisms of bacterial succession in epiphytic biofilms on Vallisneria natans and Hydrilla verticillata during bioremediation of phenanthrene and pyrene polluted wetland

In wetland ecosystem, the microbial succession in epiphytic biofilms of submerged macrophytes remains to be fully elucidated, especially submerged macrophytes used to remediate organic pollutants contaminated sediment. Herein, 16 S rRNA gene sequencing was used to investigate the bacterial dynamics and ecological processes in the biofilms of two typical submerged macrophytes (Vallisneria natans and Hydrilla verticillata) settled in sediment polluted by polycyclic aromatic hydrocarbons (PAHs) at two growth periods. The results presented that the variations of bacterial community in the biofilms were influenced by attached surfaces (explanation ratio: 17.30%), incubation time (32.30%) and environmental factors (39.10%). Bacterial community assembly was mainly driven by dispersal limitation which triggered more positive co-occurrence associations in microbial networks, maintaining ecological stability in the process of bioremediation of PAHs. Additionally, the functional redundancy strength of bacterial community was more affected by attached surface than incubation time. The structural equation model illustrated that community assembly drove β-diversity and explained a part of ecological functions. Environmental factors, community assembly, and β-diversity jointly affected microbial networks. Overall, our study offers new insights into the microbial ecology in biofilms attached on the submerged macrophytes settled in PAH-polluted sediment, providing important information for deeply understanding submerged macrophyte-biofilm complex and promoting sustainable phytoremediation in shallow lacustrine and marshy ecosystems.

The Jurassic – Cretaceous transition in the West Cameros Basin (Tera Group, Burgos, Spain): Sedimentological and palynostratigraphical insights

The Jurassic–Cretaceous boundary transition is widely distributed within the West Cameros Basin. This horizon is constituted by the Tera Group, consisting of two depositional sequences (DS1 and DS2) comprising four geological formations spanning the Tithonian to the Berriasian. Here, a combined sedimentological and palynostratigraphic study was made covering the Tera Group. The sedimentological results for the DS 1 differentiate three facies associations (FA 1-1, 1-2 and 1-3) corresponding to floodplains and braided channels of a distributive fluvial system shifting to shallow lacustrine facies in a semi-arid climate. For the DS 2, three FA were also differentiated (FA 2-1, 2-2, and 2-3), corresponding to tidal-influenced meandering channels and floodplains in a distal fluvial to a delta-plain system that shifts to shallow lacustrine facies. The palynological results were only productive in samples from the Jaramillo Formation. The palynological record shows an assemblage dominated by spores including Concavissimisporites, Leptolepidites and Taurocusporites, as well as Classopollis pollen. According to the stratigraphic ranges of some selected taxa, the suggested chronostratigraphic assignment is lower Berriasian for the Jaramillo Fm. A comparison of the microfloras from the Tithonian–Berriasian recorded in the Iberian Peninsula was carried out. The following observations can be concluded: 1) the change in the plant communities, already observed since the Late Jurassic onwards, became more pronounced between the Tithonian–Berriasian boundary and the late Berriasian; and 2) a noteworthy occurrence of some biostratigraphically important taxa, specifically for the Tithonian–Berriasian interval.

Perhydrous organic carbon logging evaluation method in shore and shallow lacustrine shale formations: A case study of the Mesozoic lacustrine shale in the western Sichuan Basin, China

The types of organic carbon in shore and shallow lacustrine shales are more diverse than those in marine formations. It is of great importance to establish log interpretation models of the separate types of organic carbon content for hydrocarbon generation potential estimation. First, we summarize the main rock types and their logging characteristics in the shore and shallow lacustrine shale reservoirs and analyze the types of organic carbon in shore and shallow lacustrine shales using core pyrolysis analysis and kerogen maceral microscopic identification. Second, we establish the calculation method of the total organic carbon (TOC) content in the lacustrine shale formation by regression analysis. Third, we divide the organic matter in shore and shallow lacustrine shales into two parts: perhydrous organic carbon (perhydrous macerals) with a higher hydrogen index, higher H/C atomic ratio, and stronger hydrocarbon generation potential and subhydrous organic carbon (subhydrous macerals) with a lower hydrogen index, lower H/C atomic ratio, and poorer hydrocarbon generation potential. Furthermore, we introduce the concept of the mixed skeleton, combined with the formation component triangular chart, and establish calculation methods for perhydrous and subhydrous organic carbon contents. Finally, we apply this method to the Mesozoic shore and lacustrine shale in the western Sichuan Basin, China. The application indicates that the kerogen types of the Mesozoic shore and shallow lacustrine formations in the western Sichuan Basin are mainly type III and type IIB kerogen with a low content of perhydrous macerals. The perhydrous organic carbon content calculated by the logging evaluation model is positively correlated with the perhydrous maceral content obtained by kerogen maceral microscopic identification and the oil generation potential S2 obtained by core pyrolysis analysis. Logging analysts should use the perhydrous organic carbon content to estimate the hydrocarbon generation potential of shore and shallow lacustrine shale formations.

Geochemical characteristics and depositional paleoenvironment of source rocks from the Lower Cretaceous Chijinbao Formation in Jiuxi Basin, China

The Lower Cretaceous Chijinbao Formation is one of the primary source rocks in the Jiuxi Basin in Northwest China. However, it is poorly studied due to the lack of a complete stratigraphic sequence exposed by drilling. In this research, an integrated analysis of palynofacies, organic geochemistry, and element geochemistry was systematically conducted on samples with longitudinal continuity collected from the Chijinbao Formation in the Hanxia Section of the Jiuxi Basin to determine the distribution of source rocks and the source and type of organic matter and to reconstruct the depositional paleoenvironment of the Chijinbao Formation. The lower part of the sequence, which is governed by poor source rocks with Type III organic matter, is characterized by palynofacies A (woody tissue–charcoal assemblage) grown in a proximal alluvial fan–shallow lacustrine environment. The excellent source rocks with mostly Type II 1 organic matter are considerable in the middle part of the sequence, which is marked by palynofacies B (amorphous organic matter assemblage), and occur in a distal semi-deep lacustrine–deep lacustrine environment. The upper part of the sequence is largely composed of poor and fair source rocks, represented by palynofacies C (amorphous organic matter–woody tissue assemblage), and a small amount of sporadically distributed good and excellent source rocks (approximately 1/3), which may be deposited in a proximal fan delta and a local semi-deep lacustrine–deep lacustrine environment. A semiarid–semi-humid climate and a freshwater–brackish water environment prevailed during the deposition of the Chijinbao Formation. The lower part of the sequence is characterized by the highest terrestrial detritus influx, strong weathering, humid paleoclimate, oxic water column conditions, and the lowest paleoproductivity. The middle part of the sequence has the lowest terrestrial detritus influx, relatively weak weathering, relatively arid paleoclimate, predominantly anoxic water environment, and the highest paleoproductivity. The climate became humid again during the deposition of the upper part, which is typified by an intermediate terrestrial detritus influx, strong weathering, fluctuations of oxic–anoxic bottom-water conditions, and moderate paleoproductivity. The correlation analysis of element proxies suggests that the increase in salinity during the deposition of the middle part is associated with an arid climate. Amorphous organic matter, which is derived mostly from aquatic organisms and degraded terrestrial higher plants, is the major hydrocarbon generation component in the source rocks of the Chijinbao Formation. It increases in abundance with an improvement in water reducibility and productivity, favoring the formation and preservation of excellent source rocks. • Excellent source rocks distribute in middle part of Chijinbao Formation. • AOM dominate organic components in excellent source rocks of Chijinbao Formation. • Excellent source rocks were deposited under anoxic and relative arid conditions. • High salinity correlates with arid climate during the deposition of middle part.

Distribution and Controlling Growth Factors of Ooids in Qinghai Lake, Northern Tibet Plateau, China

Ooids are coated carbonate grains, which exist in shallow water marine and lacustrine environments. There is an ongoing debate about whether the origin of ooids is inorganic or organic. Qinghai Lake is the largest inland lake in China, and ooids are seen on the lake shore. This paper focuses on whether environmental energy has an impact on the growth and size of ooids. Through hydrochemical analysis, thin section observation, and scanning electron microscope, the carbonate coats of beach sands from Qinghai Lake were studied. The research shows that the carbonate-coated grain content from the different shores of the lake present variations. The hydrodynamics and particularly the waves seem to control the distribution of carbonate coats in the lake shore, not the hydrochemical condition. In addition, the integrity and thickness of carbonate coats from the shores with a strong hydrodynamic force are high and thick, respectively. The carbonate coats are often observed on medium-grained sands, and the maximum carbonate-coated grain occurred under the strongest waves, indicating that ooids can be produced only when hydrodynamic force and particle size are well matched. Bacteria or extracellular polymeric substances are not observed within the ooid cortices by scanning electron microscopy. So, bacteria may not be a major factor in the formation and growth of ooids, but hydrodynamic forces appear to play a great role in carbonate grain coat distribution, integrity, thickness, and ooid grain size.

Sedimentary Characteristics Analysis and Sedimentary Facies Prediction of Jurassic Strata in the Northwest Margin of Junggar Basin—Covering the W105 Well Region in the Wuerhe Area

As the northwestern area of the Junggar Basin is a key area for oil and gas exploration, the sedimentary facies of the Jurassic formations in the Wuerhe area has long been a focus of research. The target strata are Jurassic strata, including five formations: the Lower Jurassic Badaowan and Sangonghe, the Middle Jurassic Xishanyao and Toutunhe and the Upper Jurassic Qigu. Disputes over the are sedimentary facies division exist in this area. Considering the W105 well region in this area as an example, the overall sedimentary facies of single-well logging facies is analyzed and then expanded to two cross-sections and characterized. Based on previous studies, a detailed overview of the regional stratigraphy is obtained by well logs and other data. Then, two cross-sections are selected and analyzed. The single-well and continuous-well facies of 10 wells in the sections are analyzed to grasp the sand bodies’ spatial distribution. Finally, a planar contour map of the net to gross ratio is mapped to analyze the sources and the distribution of the sand bodies in each period. The sedimentary facies map is also mapped to predict the sedimentary evolution. The results show that the sedimentary facies of the Badaowan Formation in the study area was an underwater distributary channel of the fan-delta front, and the sand body spread continuously from northwest to southeast. The Sangonghe Formation entered a lake transgression period with a rising water level, at which time shore–shallow lacustrine deposits were widespread throughout the region. The period of the Xishanyao Formation entered a regression period, the northwest region was tectonically uplifted, and the central and southeastern regions facies were dominated by the fan-delta front and shallow lacustrine. During the Toutunhe Formation period, the northwest region continued to uplift and was dominated by delta plain facies. During the period of the Qigu Formation, the thickness of stratigraphic erosion reached its maximum, and the non-erosion area of the study area was mainly deposited by the fan-delta plain. Overall, the Jurassic system in the W105 well area is a fan delta–lacustrine–fan delta sedimentary system.

The Guinardera quarry (Sant Martí de Tous, Barcelona): A new chert exploitation location during historical times

In 2014, an anthropic accumulation of chert material was discovered in La Guinardera area, at the southwest of the Sant Martí de Tous town (Barcelona, NE Iberian Peninsula). In 2018 a first archaeological intervention was carried out in two locations: La Guinardera and La Guinardera Nord. After the fieldworks, these two accumulations were interpreted as chert workshops. These workshops are in the St. Genís Formation, included within La Noguera lacustrine system and dated to the Priabonian age (upper Eocene). The St. Martí de Tous area presents shallow lacustrine conditions typical of sabkha environments, in which layers of gypsums and sandy lutites are interspersed with tabular red sandstone levels, yielding different varieties of chert.
 The Guinardera chert is characterized by a fairly homogeneous matrix, presenting a fine texture, with a microcrystalline and spherulitic length-slow chalcedony matrix, and a combination of grey colours, in general of dark hues, with an opaque diaphaneity but translucent at the edges.
 The archaeological assemblage from La Guinardera Nord site allows us to identify a chert workshop for the production of gunflints. The heterogeneity of the assemblage at La Guinardera site precludes assigning it to any single chrono-cultural period or function.
 The technological characterization of La Guinardera Nord site reveals distinctive attributes of a gunflint workshop that can be differentiated from prehistoric workshops. The presence of square and thick preforms, oxide traces on butts and ventral faces, marked bulbs and thick platforms, together with fresh edges on flakes and blades and the near-absence of patinated materials, corroborate it.
 The presence of these two deposits within the above-mentioned formation shows us a repeated landscape exploitation pattern for raw material extraction, since references chert use range from the Middle Palaeolithic (e.g., Abric Romaní) to historical times (e.g., La Guinardera Nord).

Geological Characteristics and Exploration Prospect of Black Shale in the Dongyuemiao Member of Lower Jurassic, the Eastern Sichuan Basin, China

China has yielded huge commercial production from the marine organic-rich shale but shows a slow exploration process in the lacustrine organic-rich shale. Multiple lacustrine shales in the Lower Jurassic of the eastern Sichuan Basin, rich in organic matters, are potential targets for shale hydrocarbon exploration and development. An investigation of the Dongyuemiao member, Lower Jurassic, was firstly conducted utilizing the macroscopic and microscopic analyses on outcrops and drilling cores to reveal the characteristics of sedimentary subfacies, mineral compositions, organic matter content and types, thermal maturity level, and reservoir quality. The dark shales in the Dongyuemiao member can be grouped into four general categories: shore, shallow, semi-deep, and deep lacustrine shales. The semi-deep and deep lacustrine shales generally have higher values in thickness (>20 m), average total organic carbon (TOC) content (>1.5 wt.%), and average porosity (>2%) relative to shore and shallow lacustrine shales. All four categories of shales primarily consist of type II kerogen and have thermal maturity levels exceeding the vitrinite reflectance value of 0.9–1.0% (or the Tmax of ∼440°C). Thermally powered pore generation generally promoted the pore system as indicated by the positive correlation between porosity and Tmax. Notably, the semi-deep lacustrine shale in the vicinity of the Qiyueshan Fault Zone shows abnormally high porosity and low oil saturation index (OSI) at Tmax>∼465°C potentially due to the promoted hydrocarbon expulsion through multiscale fractures. Except for the vicinity of the Qiyueshan Fault Zone, the semi-deep and deep lacustrine shales generally show the better exploration prospect relative to the shore and shallow lacustrine shales. Additionally, the high content of clay minerals (>40 wt%) reduced the brittleness of the semi-deep and deep lacustrine shales which may challenge the artificial hydraulic fracturing.

Fish swim traces from the Jindong Formation (Cretaceous) Korea: Implications for lake basin ichnofacies and paleoecology

The track rich Cretaceous Jindong Formation is famous for abundant tetrapod track assemblages, which occur at multiple, closely-spaced stratigraphic levels in the Gyeongsang Basin lake deposits of South Korea. However, until now no fish traces or fish body fossils had been reported. Thus, the discovery of three Undichna ichnospecies (U. simplicitas, U. bina, and U. britannica) provide the first direct evidence of fish in the basin. Paired in-phase traces vary in width and indicate a range of fish sizes among the tracemakers. Undichna assemblages represent a shallow lacustrine Undichna ichnocoenosis. Bird tracks (Ignotornis) registered on the same surface represent the superposition of a shorebird ichnocoenosis. Dinosaur tracks (Brontopodus and Caririchnium) associated with surfaces only ∼1 m below the bird and fish traces, represent additional large tetrapod ichnocoenoses, and demonstrate multiple, superimposed ichnocoenoses, interpenetrating localized sedimentary successions which are little separated in space or time. The Undichna and shorebird, ichnocoenoses are considered, in schema followed here, as representative of higher-level ‘archetypal’ Charachichnos and Grallator ichnofacies respectively. However, it is not clear whether a third ichnocoenosis and ichnofacies is represented by the Brontopodus and Caririchnium assemblages or the extent to which the overprinting of different ichnofaunas in localized areas always allows for the unambiguous application of ichnocoenois and ichnofacies schema.

Sedimentary Environments and Coal Accumulation of the Middle Xishanyao Formation, Jurassic, in the Western Dananhu Coalfield, Turpan-Hami Basin

The Dananhu coalfield, at the southern margin of Turpan-Hami Basin, Xinjiang, has good coal-forming geological conditions. Due to the low research level on the depositional environment and coal accumulation, the comprehensive coal mining is restricted. Based on drilling and sectioning outcrop data, the depositional characteristics, lithofacies, paleogeography, and coal accumulation of the coal-bearing formations in western Dananhu coalfield are analyzed. The middle member of the Xishanyao Formation is the major coal-bearing strata of the Dananhu coalfield, composed of conglomerate, sandstone, siltstone, mudstone, and combustible organic rock, which can be further subdivided into 14 lithofacies according to substance composition, sediment texture, and structure. During the early Middle Jurassic coal-accumulating period, alluvial fan, fluvial plain, fluvial delta, and shallow lacustrine deposits were developed from northeast to southwest in the western Dananhu mining area. Coal accumulation was most developed in the center regions of the study area extending from northeast to southwest, which were primarily concentrated in margin fans, floodplains, interdeltaic bays, and lakeshore environments, especially the transitional zone between the fluvial plain and the delta plain in the west of No. 1 mining area. This coal accumulation was mainly controlled by synsedimentary tectonic subsidence and clastic sediments supply or sedimentary environment, where tectonic subsidence was the primary controlling factor and the source supply was the secondary controlling factor for coal accumulation, which provided theoretical support for the coal prediction and geological exploration in western Dananhu coalfield.

Geochemical characteristics and possible sources of crude oils in the Baiyun deep-water area of the Pearl River Mouth Basin, South China Sea

Two classification methods, the distribution of biomarkers and the alternating least squares (ALS) chemometric analysis were used to investigate the genetic characteristics of oils collected from the Baiyun deep-water area of the Pearl River Mouth Basin in the South China Sea. Results of the two methods were generally consistent that most of the oils were originated from the shallow lacustrine source rocks, but oils from the West Sub-Sag (WSS) and the South Sub-Sag (SSS) have a distinct geochemical characteristic with low abundances of bicadinanes and oleanane, and relatively lighter δ13C value (<−28.0‰). The ALS chemometric analysis can quantitatively determine the sources of the oils. It shows that mixing has commonly occurred between the oils, and three end-member oils were identified (i.e., EM1, EM2, and EM3), which were derived from the semideep–deep Wenchang Formation source rocks that are characterized by a predominance of C27 steranes and low concentrations of bicadinanes and oleanane, the shallow lacustrine Enping Formation source rocks in the Main Sub-Sag (MSS) that are rich in bicadinanes but depleted in oleanane, and the shallow lacustrine Wenchang and/or Enping formations source rocks in the East Sub-Sag (ESS) that have a relatively high abundance of bicadinanes, a moderate content of oleanane, and a predominance of C29 steranes, respectively. The oils from the Panyu Low Uplift (PLU) are dominated by >83% of EM2 oil. Oils from the SSS and WSS are dominated by EM1 oil (68.0%–93.1%). Oils from the ESS and eastern part of the MSS (E-MSS) are mixed oils consisting mainly of EM2 and EM3 oil, with an increasing proportion of EM3 moving from the E-MSS to central ESS. The maturity of the oils was assessed based on diamondoid indices, and oils from the PLU have the highest maturities (1.43%–1.59% Rc), whereas oils from the SSS have the lowest maturities (1.23%–1.25% Rc), corresponding to the different maturities of their source rocks. The semideep–deep lacustrine oils discovered in the WSS and SSS highlight the hydrocarbon generation potential of these types of source rocks in the Wenchang Formation, where further oil exploration should be focused.

Shallow lacustrine versus open ocean coastal clastic deposits: Morphosedimentary diagnostic indicators and interpretation

Lacustrine coasts, especially those associated with shallow highly fluctuating lakes, do not evolve in exactly the same way as marine coasts. Climate variability can significantly modify the morphology of the water body, affecting the lake surface area, the available fetch, and the water depth, resulting in possible changes to wave generation and propagation. To look more closely at the nature of coastal sediments generated under such specific environments, we selected examples of both shallow lacustrine and open ocean beach deposits. The first location was Gallocanta Lake, a shallow body of water in the Iberian Chain in central Spain; and the second was Mazagón Beach, in the Gulf of Cádiz, a mesotidal sandy beach exposed to Atlantic waves and tides. Both deposits are formed by beach and ridge and runnel systems. The sedimentary structures of the two deposits were compared, together with a genetic scheme for these. In both cases, applying wave propagation models facilitated the interpretation of the sedimentary processes responsible for their genesis. Although the general morphosedimentary characteristics of the two deposits are roughly similar, a more detailed analysis revealed significant differences, some of which could be used as diagnostic indicators of shallow lacustrine environments in geoarchives. Effective fetch seemed not to be a determining factor for explaining the textural differences between the two deposits. Local factors such as wind strength and direction, and approaching wave refraction patterns are important in controlling the final energy reaching both types of coasts, and, hence, the resulting geometry of the clastic coastal deposits. The type and maturity of the available sediment also influence the final textural properties of the deposits. Open ocean beaches receive sporadic high energetic storms; these partially erode the sedimentary record and reduce the preservation potential of coastal deposits compared with those from shallow lake beaches. Comparing lacustrine deposits with marine analogues and identifying similarities and differences between the two coastal environments may help to infer the main factors underlying the formation and development of coastal deposits in non-tidal, fetch-limited environments.

Syn-rift sedimentary evolution and hydrocarbon reservoir models in a graben rift sag, Songliao Basin, Northeast China

Identifying hydrocarbon reservoirs in lacustrine rift basins is a challenging task. This study uses seismic, well, and core data to analyse the sequence stratigraphy and describe the depositional systems and hydrocarbon reservoir models in the syn–rifted successions of Sujiatun sag in order to locate prospects in the Late Jurassic–Early Cretaceous continental rift basin. SQ2 is composed mainly of fan–delta and shallow lacustrine depositional systems. SQ3 and SQ4 consist of shallow lacustrine and semi–deep lacustrine depositional systems. SQ5 and SQ6 are composed of braid-delta and shallow lacustrine depositional systems, forming favourable reservoir sand bodies. Data obtained from thin section observation and physical property analysis show that the physical properties of braid-delta reservoirs are better than those of fan-delta reservoirs in terms of hydrocarbon accumulation. Geochemical analysis of the mudstones in SQ3 and SQ4 shows that they are excellent source rocks. Analysis of the conditions and characteristics under which the hydrocarbon reservoirs formed, indicate that Sujiatun sag models can be divided into six categories: a tilted fault–block hydrocarbon reservoir model, Syn trending fault–block hydrocarbon reservoir model, reverse trending fault hydrocarbon reservoir model, fault-lithologic hydrocarbon reservoir model, an updip pinch-out hydrocarbon reservoir model, and a lenticular lithologic hydrocarbon reservoir model. Research indicates that the distinctive depositional systems reflect three episodic syn–rifted stages. In particular, the rift climax influences the development of source rocks. Well-sorted sedimentary conditions and a large number of dissolution pores are key to improving a reservoir. The types and distribution of hydrocarbon reservoirs vary greatly over different structural stages and structural zones. Fault activity and a combination of different faults types, reservoirs, and source rocks jointly determine the enrichment degree of the hydrocarbon accumulation in different reservoirs. This study analyses the sedimentary evolution and hydrocarbon reservoirs within the syn–rifted successi to provide useful insight for oil and gas exploration in syn–rifted basins and may also be effective for use in areas in which wells or outcrop data are lacking.

Unusual intraclast conglomerates in a stormy, hot‐house lake: The Early Triassic North China Basin

AbstractEarly Triassic temperatures were some of the hottest of the Phanerozoic, sea‐surface temperatures approached 40°C, with profound consequences for both the sedimentology and faunal distributions in the oceans. However, the impact of these temperatures in terrestrial settings is unclear. This study examines shallow lacustrine sediments from the Lower Triassic succession of North China. These consist of diverse fluvial to shallow lacustrine sandstones and also spectacular, coarse conglomerates composed of diverse, intraformational clasts reworked from the interbedded sediments. The conglomerate beds can show inverse grading and high angle, flat‐pebble imbrication in their lower part and vertically orientated flat pebbles in their upper part. The cobbles include cemented and reworked conglomerate intraclasts and sandstone concentrically‐laminated concretions that record multi‐step histories of growth and reworking, pointing to rapid cementation of the sandy lake bed (likely facilitated by high temperatures). The conglomerates record frequent, high‐energy events that were capable of brecciating a lithified lake bed and transporting cobbles in wave‐influenced sediment‐gravity flows. Initially, powerful oscillatory flows brecciated and deflated the lake bed and subsequently helped to sustain turbulence during short‐distance lateral flow. It is possible that hurricanes, originating from the adjacent hyper‐warm, Palaeo‐Tethyan Ocean travelled into the major lakes of the North China continent during the Early Triassic.

Tectono-climatic controls of the early rift alluvial succession: Plio-Pleistocene Corinth Rift (Greece)

Proximal alluvial sediments represent a useful sedimentary archive to reconstruct the tectono-climatic history of continental rift basins. However, poor dating of coarse fluvial successions usually prevents high-resolution distinction of tectonic and climatic processes, and thus good determination of process rates. This paper presents a dating study of Plio-Pleistocene Kalavryta river system during the early history of the Corinth Rift (northern Peloponnese, Greece) based on magnetostratigraphy and palynology. This river system developed across several active normal fault blocks that are now uplifted along the southern rift margin. The detailed sedimentary record constrains alluvial architectures from the proximal basin to the river outlet where small deltas built into a shallow lake. In four magnetostratigraphy sections the correlation to the reference scale relies on the identification of the Gauss/Matuyama magnetic reversal and biostratigraphic elements. The river system developed between about 3.6 to 1.8 Ma, with sediment accumulation rates (SARs) ranging from 0.40 to 0.75 mm yr−1. SAR is lower in the alluvial fans than in the deltaic system, and higher at the centre of the normal fault depocentres than at the fault tip. By comparison with worldwide Cenozoic SARs, our values are higher but lie in the same range as those determined in coarse alluvial foreland basins. Moreover, in the context of overfilled intra-mountainous rift basins, these rates are minimum values and can be used as a proxy for accommodation rate. Therefore, early rift stratal wedges and growth synclines attest high sedimentation rates and also high rates of tectonic processes. Finally, in the distal river system, floral compositions and changes of vegetation deduced from palynological data are coherent with alternating fluvio-deltaic and shallow lacustrine deposits, which are linked to relative base level variations. Dry/cool climate is preferentially recorded during periods of low lake level, while the warm/moist climate is mainly recorded in prodelta deposits during periods of high lake level. This correlation suggests that, despite the dominant control of active faulting, climate is a key control of syn-rift stratigraphic architectures.

Dropstone deposition: Results of numerical process modeling of deformation structures, and implications for the reconstruction of the water depth in shallow lacustrine and marine successions

ABSTRACTDropstones in lacustrine and marine sediments show a wide range of sizes: from less than a millimeter to many meters. Their size and shape determine the velocity and the acceleration when they settle through the water column, and this, in turn, determines in principle the imprint that they make in the bottom sediment. Although these parameters are crucial for dropstone deposition, the unknown material (sediment) properties (like strength, porosity, pore-water content, viscosity, etc.) of the bottom sediment play a just as important role in this process as the water depth, which can physically be understood as the length of the pathway traveled vertically through a dense medium before the impact. Reconstruction of the principal environmental conditions at the time of dropstone fall and deposition consequently requires considering the variety of factors affecting the final imprint depth of a dropstone, the combination of several numerical methods.Here, we show the results of numerical modeling of dropstones with different sizes that settle through water columns with different depths. Our results show how environmental factors control the deformation structures formed at the sedimentary surface during the impact of a dropstone, and how deep the imprint caused by the settling dropstone will be.

Paleoclimatic variability in the southern Tethys, Egypt: Insights from the mineralogy and geochemistry of Upper Cretaceous lacustrine organic-rich deposits

The effect of paleoclimate during global warming periods on Upper Cretaceous lacustrine deposits is not fully understood. In this study, comprehensive sedimentological, mineralogical, and geochemical data were used to unravel the past climatic changes and their influence on the accumulation of Campanian lacustrine organic-rich deposits in Egypt. Three sedimentary lithofacies associations are recorded within the uppermost part of the Quseir Formation. The laminated clay-rich mudstone lithofacies has the highest organic matter (TOC up to 1.2 wt.%). Sedimentological observations indicate that the bulk lithologic assemblage was originated in a shallow lacustrine setting. The studied samples consist of clay minerals, quartz, pyrite, carbonate, fluorapatite, and halite. The siliciclastic content reaches up to 78wt.%, whilst the carbonate content ranges from 3 to 22 wt.%. Compared to Post-Archean Australian shales (PAAS), the studied samples are significantly rich in Mg, Ca, P, Al, V, Ga, and Cr contents. The mudstone samples are characterized by high ratios (C-value, CIA, CIW, Mg/Ca, Fe/Mn, Ga/Rb) and low values (K2O/Al2O3), reflecting relatively warm and humid conditions during deposition of these sediments. The prevalence of smectite, as well as the sedimentary features, support that the lacustrine organic-rich mudstones are mainly deposited under warm seasonal climate (arid/humid). Additionally, the presence of dolomite and halite along with the Sr/Ba ratios (1.30–2.13), strongly suggest a saline water setting. The changes between aridity and humidity periods are probably the main reason for the significant variation in water chemistry, especially salinity. Therefore, the paleoclimatic variations largely controlled the depositional process during the formation of the Upper Cretaceous lacustrine organic-rich mudstones in the southern Tethys.