Shallow Lacustrine Environments Research Articles

The lower Barra Velha formation (Aptian) in the Atapu field, Santos basin: Geological model for a pre-salt succession

The characterization of facies, microfacies and facies successions allowed to develop a conceptual geological model for the lower succession of the Barra Velha Formation in the Atapu field, Aptian of the Santos Basin. The obtained data and interpretations were based on the detailed description (scale 1:10) of ca. 90 m of two almost continuous well-cores and petrographic analysis. The facies were defined based on their primary depositional attributes, which are well recognized despite diagenetic modifications. Lutites, microbialites, tufa-rocks, calcirudites constitute the main recognized facies. The clastic lutites recorded transport and deposition of terrigenous and intraformational clasts by hydrodynamic and gravitational processes in low energy and shallow lacustrine environment. Microbialites and tufa-rocks record in situ carbonate deposition in alkaline saturated waters. Injectites, diastasis cracks, loop bedding, slump folds, many of the normal and listric faults and fractures are syn-depositional structures interpreted as primarily induced by tectonics. Facies changes allow to divide the succession in three stratigraphic intervals. Lutites predominates in intervals 1 and 3 whereas tufa-rocks constitute the interval 2. In the lower part of interval 1 labile fresh siliciclastic components and tractive and gravitational structures are abundant, indicating dry climate but with sporadic rainfall during a period of relative high water level of the lake. The upward decreasing of siliciclastic fragments, the appearance of coarse microbialite and a fish mortality level in the upper part of interval 1, mark the transition to the tufa-rocks of interval 2. These latter features denote a relative dryer climate and higher alkalinity waters, and a low water level of the lake. The interval 3 lutites cover abruptly the tufa-rocks recording the drowning of the succession. This may be related to renewed rifting or to the beginning of the basin thermal subsidence.

Morphological Typology and Petrography of Plio-Quaternary Continental Carbonates of Aït Chaïb, Skoura Basin, Middle Atlas, Morocco

The Plio-Quaternary Ait Chaib basin occupies the northwestern part of the Skoura Syncline, which is part of the northern Middle Atlas Folded Zone. This intracontinental basin was formed probably in the Pliocene after the Tortono-Messinian regression sea. The Plio-Quaternary deposits are bound by fluvial conglomerates and continental carbonates. They consist of barrage travertines; travertine limestones and oncholitic travertine silts and sands of the upstream and downstream deposits. These travertines laterally pass to lacustrine limestones at the bottom of the basin. The genesis of these continental carbonates, correlative to phases of karstification, has affected the Meso-Cenozoic carbonate deposits of the folded Middle Atlas, and is linked to favorable paleo-environmental conditions of geological, geomorphological, hydrogeological, hydrological and bioclimatic order. The petrographic study of these lacustrine limestones shows laminar structure allowing to assimilate into continental stromatolites. They were formed by cyanobacterial mats in a shallow lacustrine environment in which water were was non-turbid and supersaturated in limestone solution, coming from karstic springs staking out the Middle Atlasic North Accident. The important diagenesis evolution is linked to the surface water table contain, which has as a floor gypsiferous lagoonal marl of the Upper Miocene age.

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.

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.

Putative arthropod trace fossils from the Orcadian Basin at Achanarras Quarry (Middle Devonian of Scotland)

Achanarras Quarry, Caithness, Scotland displays a diverse fossil fish fauna that is presumed to have inhabited shallow-lacustrine environments present in the Orcadian Basin during the Early to Middle Devonian. Although Achanarras Quarry itself exposes deep-lacustrine facies, the ecology of their depositional environment remains unknown, in stark contrast to the detailed environmental reconstructions available for the lake margin. I report putative arthropod trace fossils from Achanarras Quarry that are tentatively interpreted as having been formed in a deep-lake environment. Transport of doomed pioneers from the thriving shallow-water ecosystems by turbidite flows is discussed as a possible scenario for their formation. The infrequent and ephemeral intrusions of animals into the deep waters of Lake Orcadie fit the broader narrative of the colonization of deep-lake ecosystems after the Devonian. These interpretations of deep-lacustrine trace fossils from Achanarras, along with their place within the narrative of lake ecosystem evolution, are made cautiously, however, given the paucity of the specimens and the uncertainty surrounding their sedimentary setting.

The sedimentary architecture of hyperpycnites produced by transient turbulent flows in a shallow lacustrine environment

Hyperpycnal flows are river-derived extrabasinal turbidity currents transporting both sand and clay to lacustrine, coastal, shelf and deepwater sedimentary environments. Experimental research in the past twenty years has shown that the presence of clay in sediment-laden flows promotes a transitional behavior between fully turbulent flow and a quasi-laminar plug flow regime. However, to date, most work concerning the fluid dynamic interpretation of lacustrine hyperpycnal flows has been founded on concepts based on fully turbulent flows, and relatively little is known about the influence of clay on the rheological and turbulence characteristics of these flows. With the help of 3D seismic volume and wire-line logging data, the present study undertakes a description and interpretation of subsurface mixed sandstone–mudstone bedforms observed in cores from the Upper Cretaceous Heidimiao Sandstone, Nenjiang Formation, Songliao Basin, NE China. Six lithofacies and five lithofacies associations are recognized from the cores and interpreted as formed under both turbulent and transitional flows. Typical bedforms of transitional flows include large current ripples and low-amplitude bedwaves. The delta-fed shallow lacustrine hyperpycnal flow deposits are characterized by proximal sinuous channels (extending for c. 20 km) and lobe deposits (extending for >20 km), showing a fan-like geometry associated with distributary channel extension in the down-dip direction. Basal erosion in the proximal channels is produced by the initial turbulent and turbulence-enhanced transitional flows, with channel infill dominated by upper transitional plug flows or quasi-laminar plug flows. The lobe deposits possess a fan-like geometry with large aspect ratio, and were formed from turbulence-enhanced and lower transitional plug flows. The downdip transformation of sediment-laden flows in the lacustrine basin varies from an initial turbulent flow, via quasi laminar plug flows to upper and lower transitional plug flows, to turbulence enhanced transitional flows. These transitional flows are interpreted to have experienced gradual dilution and deceleration and eventually transformed to turbulent flows in their distal regions. A new model for delta-fed shallow lacustrine hyperpycnal flow deposits is presented incorporating decelerated transitional flows and flow transformations. This new model can aid understanding of the depositional processes of lacustrine transitional flows and the facies distribution of hyperpynal flows in both modern and ancient sediments.

Evolution of the early Mesozoic Cordilleran arc: The detrital zircon record of back-arc basin deposits, Triassic Buckskin Formation, western Arizona and southeastern California, USA

Abstract A shift in the depositional systems and tectonic regime along the western margin of Laurentia marked the end of the Paleozoic Era. The record of this transition and the inception and tectonic development of the Permo-Triassic Cordilleran magmatic arc is preserved in plutonic rocks in southwestern North America, in successions in the distal back-arc region on the Colorado Plateau, and in the more proximal back-arc region in the rocks of the Buckskin Formation of southeastern California and west-central Arizona (southwestern North America). The Buckskin Formation is correlated to the Lower–Middle Triassic Moenkopi and Upper Triassic Chinle Formations of the Colorado Plateau based on stratigraphic facies and position and new detrital zircon data. Calcareous, fine- to medium-grained and locally gypsiferous quartzites (quartz siltstone) of the lower and quartzite members of the Buckskin Formation were deposited in a marginal-marine environment between ca. 250 and 245 Ma, based on detrital zircon U-Pb data analysis, matching a detrital-zircon maximum depositional age of 250 Ma from the Holbrook Member of the Moenkopi Formation. An unconformity that separates the quartzite and phyllite members is inferred to be the Tr-3 unconformity that is documented across the Colorado Plateau, and marks a transition in depositional environments. Rocks of the phyllite and upper members were deposited in wholly continental depositional environments beginning at ca. 220 Ma. Lenticular bodies of pebble to cobble (meta) conglomerate and medium- to coarse-grained phyllite (subfeldspathic or quartz wacke) in the phyllite member indicate deposition in fluvial systems, whereas the fine- to medium-grained beds of quartzite (quartz arenite) in the upper member indicate deposition in fluvial and shallow-lacustrine environments. The lower and phyllite members show very strong age and Th/U overlap with grains derived from Cordilleran arc plutons. A normalized-distribution plot of Triassic ages across southwestern North America shows peak magmatism at ca. 260–250 Ma and 230–210 Ma, with relatively less activity at ca. 240 Ma, when a land bridge between the arc and the continent was established. Ages and facies of the Buckskin Formation provide insight into the tectono-magmatic evolution of early Mesozoic southwestern North America. During deposition of the lower and quartzite members, the Cordilleran arc was offshore and likely dominantly marine. Sedimentation patterns were most strongly influenced by the Sonoma orogeny in northern Nevada and Utah (USA). The Tr-3 unconformity corresponds to both a lull in magmatism and the “shoaling” of the arc. The phyllite and upper members were deposited in a sedimentary system that was still influenced by a strong contribution of detritus from headwaters far to the southeast, but more locally by a developing arc that had a far stronger effect on sedimentation than the initial phases of magmatism during deposition of the basal members.

Intensifying aeolian activity following the end‐Permian mass extinction: Evidence from the Late Permian–Early Triassic terrestrial sedimentary record of the Ordos Basin, North China

AbstractSedimentary successions provide direct evidence of climate and tectonics, and these give clues about the causes of the mass extinction around the Permian–Triassic boundary. Terrestrial Permian–Triassic boundary strata in the eastern Ordos Basin, North China, include the Late Permian Sunjiagou, Early Triassic Liujiagou and late Early Triassic Heshanggou formations in ascending order. The Sunjiagou Formation comprises cross‐bedded sandstones overlaid by mudstones, indicating meandering rivers with channel, point bar and floodplain deposits. The Liujiagou Formation was formed in braided rivers of arid sand bars interacting with some aeolian dune deposits, distinguished by abundant sandstones where diverse trough and planar cross‐bedding and aeolian structures (for example, inverse climbing‐ripple, translatent‐ripple lamination, grainfall and grainflow laminations) interchange vertically and laterally. The Heshanggou Formation is a rhythmic succession of mudstones interbedded with thin medium‐grained sandstones mainly deposited in a shallow lacustrine environment. Overall, the sharp meandering to braided to shallow lake sedimentary transition documents palaeoenvironmental changes from semi‐arid to arid and then to semi‐humid conditions across the Permian–Triassic boundary. The die‐off of tetrapods and plants, decreased bioturbation levels in the uppermost Sunjiagou Formation, and the bloom of microbially‐induced sedimentary structures in the Liujiagou Formation marks the mass extinction around the Permian–Triassic boundary. The disappearance of microbially‐induced sedimentary structures, increasingly intense bioturbation from bottom to top and the reoccurrence of reptile footprints in the Heshanggou Formation reveal gradual recovery of the ecosystem after the Permian–Triassic boundary extinction. This study is the first to identify the intensification of aeolian activity following the end‐Permian mass extinction in North China. Moreover, while northern North China continued to be uplifted tectonically from the Late Palaeozoic to Late Mesozoic, the switch of sedimentary patterns across the Permian–Triassic boundary in Shanxi is largely linked to the development of an arid and subsequently semi‐humid climate condition, which probably directly affected the collapse and delayed recovery in palaeoecosystems.

The largest species of <italic>Asianopodus</italic> footprints from Junggar Basin, Xinjiang, China

A discovery has been made where thirteen footprints, two invertebrate traces and symmetrical ripple marks were found in a greyish green calcareous fine sandstone bed. The discovery was found in the Lower Cretaceous Shengjinkou Formation of Urho area, northwestern Junggar Basin, Xinjiang and it formed three trackways and four isolated tracks. Classic methods were used to study the dinosaur footprints in the Urho area, which included measuring the stratigraphic sections and the footprint parameters in the field. Similar standards of measurements were used for the measurement of footprint length and width, digit length and width, the divarication between digits, the length and width of anterior triangle, pace and stride lengths. In addition, photos of each footprint were taken and the orientation and distance between the neighboured tracks were measured. Distributions and outlines of the footprints were drawn in the computer by using CorelDRAW. On the other hand, silicone rubber demould was used for the well preserved specimens for a detailed research and from the detailed study of the footprints and trackways, two different types of theropod footprints were identified. The large three-toed footprints were a new species, named Asianopodus niui ichnosp. nov., which are large sized tridactyl (47.0–56.0 cm long and 31.0–42.0 cm wide, width/length between 0.66–0.75), the divarication between digits II and IV is 49.00°–55.34°, subsymmetrical, digit III is slightly longer than digit II, but more longer than digit IV, the length/width of anterior triangle between 0.34–0.37, V-shape tracks with a distinct bulbous heel impression, without manus and tail impressions, the phalangeal formula is 3-3-2, the pace length is between 164–180 cm, the stride length is 328–336 cm. It is significant to note that this is the largest Asianopodus footprints that have been discovered. The medium-sized tracks belong to Asianopodus pulvinicalx , which are middle sized tridactyl (21.0–27.0 cm long and 18.0–20.0 cm wide, width/length between 0.72–0.86), the divarication between digits II and IV is 46.63°–51.40°, subsymmetrical, digit III is more longer than digit II and digit IV, the length/width of anterior triangle between 0.44–0.54, V-shape tracks with a distinct bulbous heel impression and without manus and tail impressions, the phalangeal formula is 3-3-3, the pace length is between 76.5–83.2 cm, the stride length is 160 cm. According to the footprint length, pace and stride length, relative stride length of the tracks, we calculated that the body length of A. niui ichnosp. nov. is 5.89 m and the speed is 8.14 km/h, while the body length of A. pulvinicalx is 2.52 m and speed is 6.48 km/h. Based on the symmetric ripple marks and invertebrate traces on the surface of the footprints layer, we infered that both the trackmakers of the Urho dinosaur footprints walked in the shore and shallow lacustrine environment.

Depositional model of early Cretaceous lacustrine carbonate reservoirs of the Coqueiros formation - Northern Campos Basin, southeastern Brazil

Abstract Recent discoveries of giant hydrocarbon accumulations in coquinas and other lacustrine carbonate rocks of Rift and Post-rift (pre-salt) super-sequences of the Campos and Santos Basins have motivated a series of studies focused on understanding the genesis of these unique reservoirs. An integrated petrologic-sedimentologic-stratigraphic-seismic study of the Coqueiros Formation delineated the depositional model and evolution of reworked bioclastic rocks and other associated rocks deposited on the external high in the northern Campos Basin during the transition between late-rift and sag stages. Bioclastic rudstones and grainstones with moderate to high shell reworking represent 65% of the samples described. The following pieces of evidence point to a predominantly shallow lacustrine depositional environment, dominated by storm currents: (i) presence of high-angle (>14°) longshore cross-bedded rudstones; (ii) limited occurrence of muddy sediments interbedded with thick packages of amalgamated bioclastic rudstones; (iii) great lateral continuity of rudstone deposits devoid of matrix on the structural high, consistent with processes of storm action on large expanses of shallow lakes; (iv) deposits of massive rudstones, with disorganized shells, similar to the modern coastal bioclastic deposits of California and Shark Bay. Three sets of facies association are proposed: (i) high-energy facies, composed of bioclastic rudstones and grainstones devoid of matrix deposited above the fair weather wave base (FWB); (ii) low energy facies, composed of hybrid rocks with muddy or peloidal matrix and other fine grained rocks deposited below the FWB or in protected shallow environments; (iii) an alkaline set, composed of mainly magnesian clay ooids deposited in a more alkaline composition of lake water. The evolution of the facies associations over time demonstrates an overall fall of the relative lake level. High proportion of low energy facies associated with a deeper lacustrine environment occurs at the Coqueiros Fm. base, while at the top predominate high energy facies associated with a shallow lacustrine environment, in addition to magnesian clay-rich facies associated to restricted and alkaline water bodies. Paleocurrents suggest the activity of two main wind direction (northwest and southeast) over the external high that result in clean bioclastic facies with good reservoir potential on both flanks. The knowledge of wind directions during the late Barremian and early Aptian in the northern Campos Basin is extremely important for targeting the best depositional facies for hydrocarbon accumulation in the Coqueiros Formation.

Sedimentary characteristics of hyperpycnites in a shallow lacustrine environment: A case study from the Lower Cretaceous Xiguayuan Formation, Luanping Basin, Northeast China

Hyperpycnites are extrabasinal turbidites related to hyperpycnal flows which form during river floods. Compared to intrabasinal turbidite systems in deep marine or lacustrine settings, hyperpycnites in shallow lakes settings are not understood. In this work, we studied the typical sedimentary characteristics of shallow lacustrine hyperpycnal flow deposits in outcrops of the Luanping Basin, Northeast China. Based on the sedimentary architecture and analysis of the facies associations in these outcrops, the hyperpycnites can be classified into the following three types: (a) bedload‐dominated erosional channel deposits, (b) bedload‐ and suspended‐load‐dominated channel‐to‐lobe transitional deposits, and (c) suspended‐load‐dominated lobe deposits. The erosional channels in proximal locations are filled by mud clasts, parallel‐laminated sandstones, massive sandstones, cross‐bedded sandstones, massive siltstones, and mudstones. The channel‐to‐lobe transitional deposits in a middle location are composed of parallel‐laminated sandstones, cross‐bedded sandstones, mud clasts near internal erosional surfaces, ripple cross‐bedded sandstones, massive siltstones, and mudstones. The lobe deposits in distal locations are composed of ripple cross‐bedded sandstones, massive siltstones, and mudstones. The sedimentary process of the shallow lacustrine hyperpycnal flows shows obvious response to periodic flood energy fluctuations. Gentle slope, small capacity of flood discharges and semiarid paleoclimate may be the possible reason for the observed sedimentary architecture of the shallow lacustrine hyperpycnal flow deposits. Therefore, this study offers new insights into the sedimentology of shallow lacustrine deposits in rift basins.

Geochemistry and origins of petroleum in the Neogene reservoirs of the Baiyun Sag, Pearl River Mouth Basin

A total of 25 light oils/condensates (5 condensates from 2 new exploratory wells) were collected from the Baiyun Sag in the Pearl River Mouth Basin (PRMB) in the South China Sea and investigated to establish their origins and sources. Three oil families were identified among samples following analysis of various geochemical parameters using biomarkers, stable carbon isotope compositions, and Hierarchical Cluster Analysis (HCA). Family-I oils, distributed in the northern structural belts, are characterized by significantly high pristane to phytane (Pr/Ph) ratios (4.77–7.93), moderate oleanane to C30 hopane (OL/C30H) ratios (0.18–0.97) and raised bicadinane-T/C30H (T/C30H) ratios (1.67–7.67). Family-II oils are distributed along the eastern flank of the Main Sub Sag towards the northeastern tectonic zones, and, have relatively high Pr/Ph ratios (3.14–4.68), elevated OL/C30H values (0.73–2.29) and moderate T/C30H values (1.64–4.52). The geochemical characteristics of the oils suggest that oils of both Family-I and Family-II are of terrestrial origin, but from two entirely independent sources. Oils of Family-III (a newly discovered oil group) have relatively lower Pr/Ph ratios (0.89–1.99), moderate OL/C30H values (0.13–0.82) and lower T/C30H values (0.34–1.61), indicating a reduced contribution of higher plant organic matter in related source rocks. Family-III oils are consistent in geochemical characteristics with shallow lacustrine source rocks and their related oils discovered in the Zhu I and III depressions of the PRMB, suggesting that they are likely to be derived from source rocks deposited in a shallow lacustrine environment during the Eocene Wenchang-Enping period. The oils of Family-III mainly occur in the southwest flank of the Baiyun Sag, which implies that this is the preferred direction for migration of lacustrine oils. The discovery of lacustrine oil demonstrates that the Baiyun Sag possesses enormous exploration potential in lacustrine oils and that this may become the next favored petroleum exploration target.

Lithofacies and pore characterization in an argillaceous-siliceous-calcareous shale system: A case study of the Shahejie Formation in Nanpu Sag, Bohai Bay Basin, China

Shale oil has gradually become the predominant target for unconventional hydrocarbon exploration in recent years. Unlike marine shales that have been extensively studied, lacustrine shales are not adequately characterized in the literature. In this study, we examined Shahejie Formation shales in the Nanpu Sag in the Bohai Bay Basin of China, and performed organic geochemistry, mineralogy, scanning electron microscopy (SEM), and N2 and CO2 gas adsorption analyses. The results show that the Shahejie shale can be divided into four lithofacies: siliceous shale, calcareous shale, argillaceous shale, and mixed shale. Calcareous shale was deposited in deep lacustrine environment, and usually has the highest total organic carbon (TOC) content with type I kerogen. Mixed shale, which was deposited in the transitional zone, has moderate TOC values. Argillaceous shale and siliceous shale were deposited in shallow lacustrine environment or near shore, and always have the lowest TOC values. Inorganic mineral pores are the dominant type of pore in all four shale groups. Argillaceous shale has a large amount of intraparticle pores with low average pore size and poor connectivity compared with calcareous shale. Limited organic matter pores are present in low matured lacustrine shales, whereas the porosity of some lacustrine shales may increase due to the occurrence of large number of organic pores leading to significant increase of total porosity. Micro-fissures within calcareous shale greatly improve reservoir quality. Overall, siliceous shale and argillaceous shale usually have low TOC, low residual hydrocarbon, relatively strong adsorption capacity, and poor connectivity of pores, which contain little free oil and are unfavorable for shale oil exploration. Calcareous shale has high TOC, high residual hydrocarbon, large pore size, and excellent fracturing nature, and should be the best target for lacustrine shale oil exploration.

A late Miocene ostracod record from the northeastern Tibetan Plateau

Lake Qinghai, the largest inland brackish lake in China, is located on the northeastern margin of the Tibetan Plateau and is affected by both the Asian Monsoon and Westerlies climate systems. It is very sensitive to climate and environmental changes. Moreover, its sediments accumulated over the Cenozoic Era and contain abundant information about past tectonics and environmental changes. Here we present a record of late Miocene ostracods from Lake Qinghai sediment cores. A taxonomic analysis of the ostracods yielded 7 families, 14 genera, and 31 species. Six distinct sediment facies were recognized and their corresponding paleolimnological and paleoenvironmental changes were inferred from the ostracod assemblages. The first appearance of ostracod shells at about 7.60 Ma reflects a switch from an eolian sedimentation facies (ca. 8.10–7.60 Ma) to a shallow lacustrine environment (ca. 7.60–5.85 Ma). Ostracod shells were absent from ca. 5.85 to 4.64 Ma in eolian sediments. A transition to a deep lacustrine facies began at ca. 4.64 Ma, and ended at ca. 3.58 Ma. Thus, Lake Qinghai existed in the late Miocene and became larger and deeper in the early Pliocene (ca. 4.64 Ma). During the time interval ca. 3.58–0.70 Ma, littoral lacustrine sedimentation occurred at the core sites. After ca. 0.70 Ma, there were alternating shallow lacustrine and lakeshore environments at the sites, until ca. 0.12 Ma. Changes in ostracod assemblages and their abundance during the intervals ca. 4.64–3.56 Ma and ca. 0.70–0.12 Ma indicate that the sedimentary environment fluctuated over short time scales. Given Lake Qinghai’s sensitivity to environmental and tectonic changes, related to its special location, we concluded that changes in the relative and absolute abundances of ostracods reflect the evolution of the Asian monsoon-arid environment and Plateau growth, as well as global climate changes.

Western Gondwana non-marine ostracods from Early Cretaceous low-latitude ephemeral lake, Northeastern Brazil

In the beginning of Gondwana’s break up, small rift lakes were formed in the area that is nowadays located in Northeastern Brazil. These lakes captured the drainage systems and were subjected to hot and arid climatic conditions in a low-latitude area. These conditions are evidenced by the Lower Cretaceous fluvio-lacustrine deposits that filled the Sousa Basin, which integrates a complex of basins along the Rio do Peixe (State of Paraíba). The Sousa Formation is the most representative unit in the Sousa Basin, being predominantly composed of siltstones and shales deposited in shallow lacustrine environments with fluvial influence. The sedimentation occurred under a semi-arid climate with alternating rainy and dry seasons, indicating the presence of ephemeral lakes. This is also supported by the lithology, the sedimentary structures (mud cracks) and the evaporites (gypsum). The lacustrine environment is favorable to the proliferation of non-marine ostracods, which are microcrustaceans of great importance for the study of current and paleolake deposits. Non-marine ostracod assemblages recovered from the Sousa Formation, sampled from core 2-FC-1-PB, revealed undescribed species. In this work, these ostracods were studied taxonomically, with the new species Cypridea paraibensis sp. nov., Cypridea vianai sp. nov. and Alicenula sousaensis sp. nov., as well as the already described Alicenula leguminella (Forbes, 1855) Martens, Rossetti and Horne 2003, Brasacypris ovum Krömmelbein 1965, Cypridea ambigua Krömmelbein 1962, and Reconcavona swaini Krömmelbein 1962 being identified. Another set of taxa remained in open nomenclature, since the morphological descriptions did not fit existing species. Thus, the descriptions referring to the described groups take into consideration criteria such as similarity and variability of morphological factors (Alicenula ex gr. leguminella and Alicenula sp. 1), degree of preservation, and quantity of recovered carapaces.

The hydrocarbon trap distribution patterns of the simple dip area of a continental basin: A case study from the western slope of the Songliao basin, China

Simple dip areas of continental sedimentary basins feature stratal overlap, gentle changes in slope, uniform dip angles, and minimal fault development. Locating hydrocarbon traps in these environments therefore presents a difficult challenge. This paper analyzes reservoir sand body and hydrocarbon trap development and their distributions within the gently dipping western area of the Songliao basin and discusses the determinants influencing the distribution of reservoir sand bodies and oil and gas bearing zones. Sand bodies within the simple east-dipping western basin margin exhibit thin singular layering, extensive planar distribution, and frequent interbedding with mudstone layers. In the homoclinal slope areas that lack significant fault development, sand bodies served as important hydrocarbon migration pathways. Spatial shifts in the paleosedimentary system caused by basin subsidence and late stage tectonism can help constrain the search for hydrocarbon traps. The spatial patterns of the depositional system influenced the distribution of different types of traps. Southerly prograding delta lobes were subsequently tilted to the east, creating sand bodies that form up-dip, wedge-out lithologic–stratigraphic traps along their western edge. Some of the delta lobes are folded or cut by faults along their peripheral edges, making structural traps and combination traps, the primary types in this region. The delta lobes in the western provenance system, however, dip in a direction parallel to that of sediment supply. Most subaqueous distributary channel sand bodies are continuous into the western source area and lack seal conditions. Lenticular lithologic traps are therefore found only along the outer edges of delta lobes and the adjacent shallow lacustrine environments.

Upper Jurassic travertine at El Macanudo, Argentine Patagonia: a fossil geothermal field modified by hydrothermal silicification and acid overprinting

AbstractThe Deseado Massif hosts numerous Late Jurassic (150 Ma) fossil geothermal systems related to an extensive volcanic event developed in a diffuse extensional back-arc setting. Detailed mapping, petrography and mineralogical observations of El Macanudo outcrops verify that it represents a hot-spring-related travertine partially replaced by silica and delineated by six sedimentary facies. These are large concentric cones (F1), laminated vertical columnar structures (F2), porous layers (F3), shrubby and irregular lamination (F4), low-amplitude wavy bedding (F5) and mounds and breccias (F6). The Macanudo Norte Outcrop rocks constitute a silica-replaced travertine sequence, with development of large conical stromatolites in a deep pool or geothermally influenced shallow lacustrine environment, surrounded by a subaerial travertine apron terrace; whereas, the Macanudo Sur Outcrop is a subaerial travertine mound sequence. Structurally controlled vent areas occur in both northern (F1) and southern (F6) outcrops, mainly located along regional NNE- and ENE-trending faults. The other sedimentary units display a concentric distribution of travertine facies with respect to the interpreted vent areas. The El Macanudo palaeo-hot spring deposit is situated in an eroded Jurassic volcanic centre, and records a complex evolutionary-fluid history. The sediments archived three different Jurassic events, when large and long-lasting hydrothermal systems were active across the region. This relative temporal sequence was formed by: (1) travertine precipitation; (2) development of a silica cap, where early silicification was responsible for exceptional preservation of some stromatolitic fabrics; and (3) acid alteration, recorded by dissolution textures and clay formation, and caused by a palaeo-phreatic water-level drop.

Ostracoda (Crustacea) from the Pozo D-129 Formation (upper Barremian?–Aptian), Golfo San Jorge basin, Patagonia, Argentina: Taxonomic descriptions, palaeoenvironments and palaeogeographical implications

In this study, we present complete descriptions and illustrations of nine ostracod species recovered from the Pozo D-129 Formation (Golfo San Jorge basin), Patagonia Argentina, discussing their depositional palaeoenvironment and palaeobiogeography. The species described in this study are Musacchiocythere sarugnata (Musacchio and Palamarczuk) and Metacypris herreriensis (Musacchio) (Family Limnocytheridae), Alicenula cf. leguminella (Forbes) (Family Darwinulidae), Neuquenocypris (Protoneuquenocypris) antiqua Musacchio and Simenoni, and Rhinocypris diademae Musacchio and Simenoni (Family Ilyocyprididae), Pattersoncypris cf. angulata (Krömmelbein and Weber) and Zonocypris sp. (Family Cyprididae), Damonella ultima (Krömmelbein and Weber) and Candona sp. (Family Candonidae). The Pozo D-129 Formation is the main source rock of hydrocarbons of the Golfo San Jorge basin, and its deposits represent an extensive lacustrine system. The samples studied come from the Cerro Chenques in the Sierra Silva anticline of the San Bernardo fold belt, Chubut Province, and from the well AAB.x-1002 (Anticlinal Aguada Bandera), Santa Cruz Province. The outcrop succession consists of several shallowing and coarsening upward sedimentary cycles 5–8 m thick, evolving from deep lacustrine, sublittoral and shallow lacustrine environments. The micropalaeontological association represents a moderate energy thanatocoenosis, and thus is a good indicator of the depositional palaeoenvironment. Taphonomic attributes and taxonomic composition of the association indicate deep and sublittoral environments, consistent with previous facies models for the outcrop succession, and suggest stressed water bodies. Previous studies of palynomorphs suggested a latest Barremian? to early Aptian Age for the study section, whilst the ostracod species Damonella ultima, and the charophytes Clavator harrisi Peck and Porochara ultima (Peck) are part of the Flabellochara [= Clavator] harrisi zone of Aptian Age. The ostracod fauna shows affinities to contemporaneous faunas of Brazil and West Africa, providing evidence of potential faunistic interchange between distant regions.

Sedimentology of the Essaouira Basin (Meskala Field) in context of regional sediment distribution patterns during upper Triassic pluvial events

Upper Triassic continental clastics (TAGI: Trias Argilo-Greseux Inferieur) in the Essaouira Basin are largely restricted to the subsurface, which has limited analysis of the depositional environments and led to speculation on potential provenance of the fluvial systems. Facies analysis of core from the Meskala Field onshore Essaouira Basin is compared with tentatively time-equivalent deposits exposed in extensive outcrops in the Argana Valley, to propose a process orientated model for local versus regional sediment distribution patterns in the continuously evolving Moroccan Atlantic rift during Carnian to Norian times. The study aims to unravel the climatic overprint and improve the understanding of paleo-climatic variations along the Moroccan Atlantic margin to previously recognised Upper Triassic pluvial events.In the Essaouira Basin, four facies associations representing a progressive evolution from proximal to distal facies belts in a continental rift were established. Early ephemeral braided river systems are succeeded by a wet aeolian sandflat environment with a strong arid climatic overprint (FA1). This is followed by the onset of perennial fluvial deposits with extensive floodplain fines (FA2), accompanied by a distinct shift in fluvial style, suggesting increase in discharge and related humidity, either locally or in the catchment area. The fluvial facies transitions to a shallow lacustrine or playa lake delta environment (FA3), which exhibits cyclical abandonment. The delta is progressively overlain by a terminal playa with extensive, mottled mudstones (FA4), interpreted to present a return from cyclical humid-arid conditions to prevailing aridity in the basin.In terms of regional distribution and sediment source provenance, paleocurrent data from Carnian to Norian deposits (T5 to T8 member) in the Argana Valley suggest paleoflow focused towards the S and SW, not directed towards the Meskala area in the NW as previously suggested. A major depo-centre for fluvial sediments is instead located in the southern Argana Valley, possibly the Souss Basin. To effectively source the reservoir sandstones found in the Meskala Field, a more local provenance area has hence to be envisaged. Despite this, the direct comparison of the genetic evolution of sedimentary sequences in the Argana Valley and Essaouira Basin shows a similar progression from dominantly arid ephemeral depositional environments to humid perennial sedimentation, returning to prominent arid conditions. This suggests climatic control in both regions, where an enhanced humid signal drives perennial fluvial flow in otherwise arid dominated sequences. On a regional scale, this is suggested to record the impact of strong Triassic pluvial events previously recognised in other basins along the Central Atlantic margin during the Carnian to Norian periods.