Sedimentary Differentiation Research Articles

Sedimentary differentiation caused by glaciation and orogeny during the Ordovician–Silurian transition in the southern Sichuan Basin, China

The Wufeng–Longmaxi shales, which were deposited during the Ordovician–Silurian transition in the southern Sichuan Basin, exhibit distinct lithofacies transitions. The shift from organic-rich siliceous shale to organic-poor mixed shale and argillaceous shale is attributed to the influences of the Hirnantian glaciation and the Kwangsian Orogeny. Nevertheless, debates persist regarding the mechanisms underlying these significant lithofacies transformations. This study employs sequence stratigraphy, petrology, and geochemical analysis of key wells to unveil crucial insights. The findings indicate: 1) the Wufeng Formation to the Long-1 Member could be divided into two third-order sequences; 2) the waterbody underwent a transition from anoxic and strongly restricted conditions to oxic and moderately restricted conditions; and 3) the trend in formation thickness changed from an E–W orientation to a NE–SW orientation. The results suggest that considerable sediment condensation and anoxia resulting from glacier melting in the early Rhuddanian played a pivotal role in the formation of organic-rich siliceous shale. During the Middle Rhuddanian–Aeronian, the accelerated uplift caused by the Kwangsian Orogeny offset the transgression induced by glacier melting, leading to shallow water conditions and increased terrigenous influx. This diluted the organic matter content in the sediment, and subsequent turbidity currents altered sediment composition, driving the lithofacies transformation into organic-poor mixed shale and argillaceous shale. The study posits that the sedimentary differentiation in the southern Sichuan Basin during the Ordovician–Silurian transition is linked to the initiation and progression of the far-field effects of the Kwangsian Orogeny.

Sedimentary differentiation triggered by the Toarcian Oceanic Anoxic Event and formation of lacustrine shale oil reservoirs: Organic matter accumulation and pore system evolution of the Early Jurassic sedimentary succession, Sichuan Basin, China

Sedimentary differentiation triggered by the Toarcian Oceanic Anoxic Event and formation of lacustrine shale oil reservoirs: Organic matter accumulation and pore system evolution of the Early Jurassic sedimentary succession, Sichuan Basin, China

Complexity and Heterogeneity Evaluation of Pore Structures in the Deep Marine Shale Reservoirs of the Longmaxi Formation, China

The structural evolution and sedimentary differentiation of the Sichuan Basin in China are complex, with intricate reservoir pore structures that significantly impact shale gas production. This study examines the complexity and heterogeneity of the microscopic pore structures in the deep marine shale reservoir in the Longmaxi Formation. Pore structure characterization techniques are used to compare deep and shallow–medium marine shales, and siliceous and silty shales. The results reveal the factors influencing pore structure and their impact on exploration and development. The key points are as follows: (1) The pore structure of deep siliceous shale is the most complex due to its diverse range of pore development patterns, pore types, and sizes. (2) The box dimension of full pore size is about 1.52 for deep marine shale and 1.46 for shallow–medium shale. Organic matter (OM) content, the degree of pore development, and inorganic mineral content all correlate positively with the complexity of the pore structure in deep marine shale, which affects the formation of high-quality reservoirs. (3) Lateral heterogeneity of pore structures shows strong regional variations in the study area. Heterogeneity is more pronounced in the deep marine shale than in the medium and shallow shale formations. OM mesopores significantly influence the overall heterogeneity of the shale pore system. The deep marine shale reservoir is situated in an area with strong regional variations. The pore structure of high-quality reservoirs is more complex than those of shallow–medium marine shales, displaying notable heterogeneity. Pore structures with fractal dimension values close to that of the shallow–medium formations (box dimensions within 1.5) offer promising targets for the exploration and development of deep marine shale gas.

Paleo-trade wind directions over the Yangtze Carbonate Platform during the Cambrian–Ordovician, Southern China

AbstractThe Sichuan Basin was a part of the Yangtze Carbonate Platform (YCP) during the Cambrian–Ordovician, and marine carbonates were deposited in the basin during this interval. Although previous studies have evaluated the paleogeography, paleoclimate and paleoecology of this basin, they have primarily focused on the paleoecology and biological evolution in the basin; however, analysis of paleogeography and paleoclimate is lacking. This study integrated outcrop sedimentological and magnetic fabric data to document sedimentary differentiation and anisotropy of magnetic susceptibility (AMS) within the YCP. The aims of this study were to infer paleowind directions during each epoch of the Cambrian–Ordovician and to constrain the paleogeographic location of the YCP. The northwestern, central and southeastern sides of the YCP were characterized by high-energy deposition (e.g. sub-angular to rounded intraclasts), medium-energy deposition (e.g. sub-angular to sub-rounded intraclasts) and low-energy deposition (e.g. angular to sub-angular intraclasts), respectively. The centroid D-Kmaxvalues for the Early, Middle and Late Cambrian were 116° ± 52°, 145° ± 57° and 159° ± 62° from the present north, respectively; corresponding values for the Early, Middle and Late Ordovician were 169° ± 70°, 139° ± 73° and 91° ± 68° from the present north, respectively. Sedimentary differentiation and AMS results indicated that the prevailing wind directions during the Early Cambrian, Middle Cambrian, Late Cambrian, Early Ordovician, Middle Ordovician and Late Ordovician were 296° ± 52°, 325° ± 57°, 339° ± 62°, 349° ± 70°, 319° ± 73° and 271° ± 68° from the present north, respectively. The present study provides evidence for the location of the YCP during the Cambrian–Ordovician via the correspondence between the paleowind directions over the YCP and the trade winds in the Northern and Southern hemispheres. The novelty of this study lies in the following aspects: (1) it integrates microfacies and AMS analyses to establish paleowind patterns; (2) it constrains the paleo-hemispheric location of the YCP during the Cambrian–Ordovician; and (3) it provides a reference for further studies of the paleoclimate and paleogeography of the YCP during the Cambrian–Ordovician.

Post-phosphogenesis processes and the natural beneficiation of phosphates: Geochemical evidence from the Moroccan High Atlas phosphate-rich sediments

The Upper Cretaceous-Paleogene sedimentary series of the High Atlas hosts underexplored phosphate-rich sediments and presents an excellent example of how complex interactions between various geological processes control the accumulation and distribution of phosphates. Genetically-linked phosphatic lithofacies from this series were investigated for their mineralogical and geochemical compositions with the purpose of assessing the impact of post-phosphogenesis sedimentary processes on the geochemical behavior of genetically-linked phosphatic lithofacies and understanding the distribution of chemical elements. Sampled phosphatic horizons from five representative sections along the Marrakesh High Atlas borders were investigated using optical and scanning electron microscopies (OM and SEM), X-ray diffraction (XRD), inductively coupled plasma optical emission spectroscopy (ICP-OES), and inductively coupled plasma mass spectrometry (ICP-MS) techniques. Results indicated that the studied lithofacies are composed of a mixture of phosphatic and non-phosphatic particles in variable proportions depending on the facies type. However, the phosphatic fraction is dominated by carbonate fluorapatite (CFA), showing an average CO32− content of 8.40 ± 0.37 wt%. Geochemical results revealed variations in the chemical composition of the different phosphate facies. Low-grade pristine phosphatic sediment (P2O5 < 12%wt.) can be turned by the effects of storms and bottom currents into high-grade granular phosphate beds containing up to 24 wt% P2O5. This natural enrichment is mirrored in the bulk rock by increasing P2O5 concentrations against decreasing detrital phase-associated chemical elements (K, Al, Si, and Ti). Among the investigated lithofacies, karst-filling phosphate show relatively lower uranium and cadmium contents as it experienced a polyphase evolution, including winnowing, submarine reworking, transport, and subaerial weathering, traced using some redox-sensitive proxies such as cerium and uranium. Despite this polyphase sedimentary differentiation, the apatite resisted chemical changes and preserved its original geochemical signature, specifically the rare earth elements signal that reflects oxic seawater conditions. However, the differentiation processes modified the bulk rock composition of phosphates through the preferential leaching of gangue phases (e.g., carbonates). Furthermore, these processes triggered the oxidation of organic matter and sulfides, removing some associated elements of environmental concern, such as uranium and cadmium. Conclusively, the combination of mechanical winnowing/reworking and subaerial chemical weathering is the most effective way of the natural beneficiation of phosphate, both economically and environmentally.

Geological characteristics of the southern segment of the Late Sinian—Early Cambrian Deyang—Anyue rift trough in Sichuan Basin, SW China

Based on the latest drilling, seismic and field outcrop data, the geological characteristics (e.g. strata, development and sedimentary evolution) of the southern segment of the Late Sinian—Early Cambrian Deyang—Anyue rift trough in the Sichuan Basin are analyzed. First, the strata in the southern segment are complete. The first to second members of Dengying Formation (Deng 1 + Deng 2) are found with relatively stable thickness (400–550 m), and the third to fourth members (Deng 3 + Deng 4) show great thickness difference between the marginal trough and the inner trough, which is up to 250 m. The Cambrian Maidiping Formation and Qiongzhusi Formation in southern Sichuan Basin are relatively thin, with the thickness changing greatly and frequently. Second, the Deyang—Anyue rift trough extended southward during the Deng 4 period, affecting southern Sichuan Basin. Compared to the middle and northern segments of the rift trough, the southern segment is generally wide, gentle and shallow, with multiple steps, and alternating uplifts and sags, which are distributed in finger shape. Third, the Deng 1 + Deng 2 in southern Sichuan Basin records the dominance of carbonate platform and unobvious sedimentary differentiation, and the Deng 4 exhibits obvious sedimentary differentiation, namely, basin—slope—secondary slope—slope—secondary slope—platform margin—restricted platform, from the inner trough to the marginal trough. Fourth, the rift trough in southern Sichuan Basin has evolved in four stages: stabilization of Deng 1—Deng 2, initialization of Deng 3—Deng 4, filling of Maidiping—Qiongzhusi, and extinction of Canglangpu Formation.

Reconstruction of paleowind directions during the Cambrian-Ordovician in the Tarim Basin, Northwestern China

The Tarim Basin was in the lower latitudes during the Cambrian-Ordovician, during which it experienced marine carbonate platform deposition. Although the platform is in the Trade Winds belt, the specific prevailing quantitative paleowind direction remains uncertain. This study integrated drill core and outcrop sedimentological data with outcrop magnetic fabric data to document sedimentary differentiation and anisotropy of magnetic susceptibility (AMS) within the Tarim Carbonate Platform (TCP). The aims of this study were to infer paleowind directions during each epoch of the Cambrian-Ordovician and to constrain the paleogeographic location of the TCP. The characteristics of the south, central, and northern sides of the platform were indicative of high-energy deposition (e.g., sub-angular to sub-rounded intraclasts with sparite cements), medium-energy deposition (e.g., angular to sub-rounded intraclasts with microspar and sparite cements), and low-energy deposition (e.g., angular to sub-angular intraclasts with microspar cements), respectively. AMS measurements yielded mean paleocurrent directions for the Early, Middle, and Late Cambrian of 226° ± 42°, 207° ± 43°, and 193° ± 36° from present north, respectively, whereas those for the Early, Middle, and Late Ordovician were 178° ± 39°, 165° ± 43°, and 155° ± 52° from present north, respectively. The results of sedimentary differentiation and AMS indicated that the prevailing wind directions of the Early Cambrian, Middle Cambrian, Late Cambrian, Early Ordovician, Middle Ordovician, and Late Ordovician were 226° ± 42°, 207° ± 43°, 193° ± 36°, 178° ± 39°, 165° ± 43°, and 155° ± 52° from present north, respectively. Since the TCP has rotated clockwise by ∼90° since the Ordovician, the paleowind directions of the Early, Middle, and Late Cambrian were 146° ± 42°, 117° ± 43°, and 103° ± 36° from paleo-north, respectively. The paleowind directions of the Early, Middle, and Late Ordovician were 88° ± 39°, 75° ± 43°, and 65° ± 52° from paleo-north, respectively. The TCP shifted from Southern Hemisphere to the Northern Hemisphere from the Early Cambrian to the Late Ordovician. The TCP was close to the Equator during the Early Ordovician. The novelty of this study related to: (1) integrating microfacies and AMS analyses to establish paleowind patterns; (2) constraining the paleo-hemispheric location of the TCP during the Cambrian-Ordovician; (3) providing references for the further study of paleoclimate and paleogeography of the TCP during the Cambrian-Ordovician.

The composition of heavy minerals of the sandy lands, Northeast China and their implications for tracing detrital sources.

Comprehending heavy mineral composition of the sandy land in Northeast China (NESL) is of great significance for interpreting generation, pathways, source and geochemistry of sediments in this area. To this end, the fine-grained (<63 μm) aeolian-fluvial sediments and loess deposits, which were taken from the Onqin Daga Sandy Land, the Horqin Sandy Land, the Hulun Buir Sandy Land and the Songnen Sandy Land, and from the downwind loess section (L1), respectively, were analyzed to construct the heavy mineral data set of NESL source and sink and to evaluate feasibility of the heavy mineral method in tracing the source of aeolian dust in Northeast China. Additionally, the <63 μm, 63–125 μm and 125–250 μm fractions of the fluvial sands from the different Balan River reaches having a same source, were analyzed to valuate the impact of the river transport-sedimentation process on the heavy mineral composition. The results show that the NESL shows moderate similarities in the heavy mineral composition, with ilmenite, epidote, zircon and amphibole as the primary minerals. In the source-to-sink system in the NESL, limited by sedimentary differentiation, post-deposition alteration and similar source material composition, the heavy mineral composition of the loess and that of sandy-land sources does not well correlate, indicating single heavy mineral method is incapable of unequivocally detecting loess sources when not considering the physical geographical conditions. The sediments in the different Balan River reaches show clear diversities and grain-size dependency in heavy minerals composition, indicating the river transport-deposition processes exert a clear control on the heavy-mineral composition in the sediment downstream. Both a wide grain-size window and more numbers of samples are needed to obtain a complete heavy-mineral picture in the source area.

Sedimentary filling characteristics and controlling factors of lacustrine microbial carbonates sequence in the Santos Basin, Brazil

Based on comprehensive analysis of seismic, logging, core, thin section data, and stable isotopic compositions of carbon and oxygen, the sedimentary filling characteristics of the Lower Cretaceous Barra Velha Formation sequence in H oil field, Santos Basin, are studied, and the high-frequency sequence stratigraphic framework is established, and the spatial distribution of reef-shoal bodies are predicted and the controlling factors are discussed. During the depositional period of the Barra Velha Formation, the study area is a slope-isolated platform-slope sedimentary pattern from southwest to northeast and the change of climate background from rift to depression periods has resulted in the variation of sedimentary characteristics from the lower third-order sequence SQ1 (BVE 300 Member) of low-energy deep water to the upper third-order sequence SQ2 (BVE 200 and 100 members) of high-energy shallow water in the Barra Velha Formation. The activities of extensional faults and strike-slip faults in rift period and the sedimentary differentiation from platform margin to intra-platform in depression period made the sedimentary paleogeomorphology in these two periods show features of “three ridges and two depressions”. The reef-shoal bodies mainly developed in the SQ2-LHST period, with vertical development positions restricted by the periodic oscillation of the lake level, and developed on the top of each high-frequency sequence stratigraphic unit in SQ2-LHST in the platform. The strike-slip fault activity controlled the distribution of the reef-shoal bodies on the plane by changing the sedimentary paleogeomorphology. The positive flower-shaped strike-slip faults made the formation of local highlands at the margins of and inside the shallow water platforms and which became high-energy sedimentary zones, creating conditions for the development of reef-shoal bodies.

Control of paleogeographic pattern on sedimentary differentiation of evaporite-carbonate symbiotic system: A case study of the sixth sub-member of Ordovician Majiagou Formation M5 Member in central-eastern Ordos Basin, NW China

The relationship between paleogeographic pattern and sedimentary differentiation of evaporite-carbonate symbiotic system is examined based on logging, core and thin section data, by taking the sixth sub-member of fifth member of Ordovician Majiagou Formation (M56) in the central-eastern Ordos Basin as an example. (1) Seven sub-geomorphic units (Taolimiao west low, Taolimiao underwater high, Taolimiao east low, Hengshan high, East salt low, North slope and Southwest slope) developed in the study area. (2) The “three lows” from west to east developed dolomitic restricted lagoon, evaporite evaporative lagoon and salt evaporative lagoon sedimentary facies respectively, the “two highs” developed high-energy grain beach and microbial mound, and the north and south slopes developed dolomitic flats around land. (3) The paleogeographic pattern caused natural differentiation of replenishment seawater from the northwest Qilian sea, leading to the eccentric sedimentary differentiation of dolomite, evaporite and salt rock symbiotic system from west to east, which is different from the classic “bull's eye” and “tear drop” distribution patterns. (4) As the Middle Qilian block subducted and collided into the North China Plate, the far-end compression stress transferred, giving rise to the alternate highland and lowland in near north to south direction during the sedimentary period of M56 sub-member. (5) Taolimiao underwater high and Hengshan high developed favorable zones of microbial mounds and grain shoals in south to north strike in M56 sub-member, making them favorable exploration areas with great exploration potential in the future.

Genetic types of mudstone in a closed-lacustrine to open-marine transition and their organic matter accumulation patterns: A case study of the paleocene source rocks in the east China sea basin

Organic mudstone can be a high-quality source rock in petroleum basins. The accumulation patterns of various types of mudstone can also serve as a useful record of paleogeography. Due to the impact of tectonic movements, the East China Sea Basin (ECSB) saw its deposition setting transform from closed-lacustrine to open marine settings. This provides an opportunity to explore organic matter accumulation patterns within such a transformation setting. This study classified the constituent mudstones according to their differences in composition and sedimentary structure as follows: coaly mudstone or carbonaceous mudstone, calcareous mudstone, clayey mudstone, and siliceous mudstone. Considering the different sedimentary structures, siliceous mudstone can be subdivided into laminated, wavy, and slumped mudstones. Using integrated multiple indices of the paleo-environment, which includes clay mineral content, trace mineral content, and micro-paleontology assemblages, we reconstructed the paleography setting during the Paleocene and inferred that during this time, the ECSB was characterized by sedimentary differentiation. We discerned two different organic mudstone accumulation patterns by combining the results from rock-eval pyrolysis and paleo-environmental indices: 1) A closed-lacustrine mudstone accumulation pattern characterized by abundant sediment supply and dominated by allogenous mudstone formation by the flocculation process. The fresh to slightly salty water environment provided adequate nutrition to microorganisms and an appropriate temperature led to the increased accumulation of type II 1 and II 2 kerogen. 2) An open-marine mudstone accumulation pattern which revealed a sediment-starved setting. The flocculation process and fine-graded turbidities were the main processes leading to mudstone accumulation. More plants were concentrated in shallow water with declining salinity, leading to peat and mass carbon mudstone accumulation. • Classified the mudstone into six types according to its genesis difference. • Build the relationship between the paleo-environment and the abundance/types of organic matters. • Restore the mudstone distribution and the sedimentary facies association evolution process. • Put out the organic matter accumulation pattern in the transition setting.

Geochemical characteristics of organic-rich shale, Upper Yangtze Basin: Implications for the Late Ordovician–Early Silurian orogeny in South China

The tectonic–sedimentary environment of the Upper Yangtze Basin in China underwent significant changes from the Late Ordovician to Early Silurian. The widely distributed shale formed during this interval has recorded key information about paleo-ocean conditions. Analysis of major, trace and rare earth elements was conducted on 135 shale samples collected from five wells in the study area. The covariation of the MoU enrichment factors indicates that the Upper Yangtze Basin in the Wufeng stage was characterized by sedimentary differentiation. The southern basin was dominated by a reducing bottom water environment under euxinic conditions. The northern and central basins were dominated by a suboxic–anoxic environment. Up to the early Longmaxi stage, the Upper Yangtze Basin formed a relatively uniform sedimentary environment with gradually increasing oxygen content in the bottom water. The paleoproductivity index indicates that marine organisms flourished in the Wufeng to early Longmaxi stages, making an important contribution to the siliceous content in organic-rich shale; subsequently, terrestrial siliciclastics gradually dominated. Analysis of paleogeography, rare earth elements, and weathering indicates that Chuanzhong and Qianzhong uplifts in the Wufeng stage were the provenance area of the Upper Yangtze Basin, with a relatively high weathering degree. Weathering decreased in the beginning of the Longmaxi stage and then gradually increased, with the Xuefeng Uplift becoming the main provenance areas after the early Longmaxi stage. Controlled by stress transfer from the continuous syn–Caledonian Kwangsian Orogeny in eastern South China, the Upper Yangtze Basin and Xiangzhong Basin were squeezed and pushed up successively. The uplift separated the open shallow sea and formed a relatively limited deep-water shelf environment, resulting in a relatively euxinic bottom water environment, which was favorable for organic matter accumulation. The tectonic–sedimentary evolution of the Upper Yangtze Basin in the Late Ordovician to Early Silurian proposed in this study indicates that the formation of organic-enriched shale was not only related to glacio-eustatic change but also strongly controlled by tectonic activities in South China.

Sedimentary pattern and exploration significance of Permian reefs and shoals around the Wusheng-Shizhu intra-platform depression, eastern Sichuan Basin, China

There exists the Wusheng-Shizhu and Chongqing-Qijiang intra-platform depressions in the eastern Sichuan Basin during the Late Permian to the Early Triassic. The clear understanding of the sedimentary pattern and evolution of reef-shoal around the depression-margin has a practical significance for guiding the exploration and development of the Permian–Triassic reef and shoal gas fields. Based on observation, the systematic sampling and laboratory analysis of typical outcrops of the Changxing Formation (i.e. Juandong, Juhuaba, Lengshuixi, and Taiyuan) are located in the eastern Sichuan Basin. Combined with drilling data and previous studies, the study area is dominated by a ramp sedimentary environment, except for the Kaijiang-Liangping Trough in the Early Changxingian. The main sediments are micrite-containing chert nodules and micrite. With the regression that took place during the Middle-Late Changxingian, there is a distinct sedimentary differentiation that began to appear. There is a “two depressions–one platform–one trough” sedimentary pattern from south to north in the study area in the Changxingian. In essence, there exists the Chongqing-Qijiang and the Wusheng-Shizhu intra-platform depressions (two depressions), platform-flat (one platform) between the Wusheng-Shizhu intra-platform depression and the Kaijiang-Liangping Trough, and the Kaijiang-Liangping Trough (one trough). The reef-shoal of the northern depression-margin distributes between the 2nd Member and lower part of the 3rd Member. By comparison, the reef-shoal of the southern depression-margin distributes in the upper part of the 3rd Member; this indicates that the deposition of reef-shoal shows “earlier in the north and later in the south.” It is considered that the sedimentary palaeogeomorphology of the Changxingian is characterized by “higher in the north-lower in the south.” The clear understanding of the sedimentary pattern and evolution of reef-shoal around depression-margin reveal a new field for exploration and development of the Permian Changxing reef-shoal gas fields in the eastern Sichuan Basin.

Comparison of detrital mineral compositions between stream sediments of the Yangtze River (Changjiang) and the Yellow River (Huanghe) and their provenance implication

ABSTRACT A comparative comparative study on the detrital mineral composition of stream sediments of the Yangtze River (Changjiang) and Yellow River (Huanghe) shows that, light minerals of the Yangtze River basin were mainly quartz, feldspar, and detritus, the compositional characteristics of light minerals differed among tributaries, the main stream had a generally higher maturity index than tributaries; heavy mineral content tended to decrease progressively from the upper stream to lower stream of the Yangtze River, the primary assemblage was magnetite-hornblende-augite-garnet-epidote, and diagnostic minerals of different river basins were capable of indicating the nature and distribution of the source rock. Detrital mineral assemblages in sediments of tributaries and the main stream of the Yellow River were basically similar, Primary heavy mineral assemblage was opaque mineral-garnet-epidote-carbonate mineral and alteration mineral. Variations in the contents of garnet, opaque mineral, and hornblende mainly reflected the degree of sedimentary differentiation in suspended sediment and the hydrodynamic intensity of a drainage system. The heavy mineral differentiation index F revealed sedimentary differentiation of diagnostic detrital mineral composition due to changes in regional hydrodynamic intensity and can serve as an indicator for studying the dynamic sedimentary environment of a single-provenance river and the degree of sedimentary differentiation of its detrital minerals. Changes in detrital mineral content of the Yellow River was not completely controlled by provenance but reflected gravity sorting of the detrital mineral due to variations in the ephemeral river hydrodynamic intensity and sedimentary environment, however the index changing of Yangtze River were mainly influenced by the complex sediment sources. Therefore caution must be exercised in using the detrital mineral composition of marginal sea to determine the contribution of the Yangtze River and Yellow River. © 2019 China Geology Editorial Office.

Middle–Late Triassic sedimentation in the Helanshan tectonic belt: Constrain on the tectono-sedimentary evolution of the Ordos Basin, North China

The Helanshan tectonic belt is located to the west of the Ordos Basin, and separates the Alxa (or Yinshan) Massif to the west from the Ordos block to the east. Triassic sedimentation in the Helanshan tectonic belt records important information about tectono-sedimentary process between the Alxa Massif and the Ordos block. Detailed geological mapping and investigation on the lithological package, sedimentary facies and paleocurrent orientation have been conducted on the Middle to Upper Triassic clastic rocks in the Helanshan tectonic belt. The succession is characterized by upward-fining sequence and comprises coarse grained alluvial-fluvial facies in the lower part as well as deltaic-lacustrine facies in the upper part. Based on detailed study and comparisons on the sedimentary sequence along various sections, the Middle to Upper Triassic strata have been revealed that show clear southeastward-deepening sedimentary differentiation and transgression from southwest to northeast, which are consistent with the southeastward flowing paleocurrent. These features indicate a southeastward-dipping paleogeography in the Helanshan tectonic belt, which was original western part of southeastward orientated fluvial-lacustrine system in the northwestern proto-Ordos Basin. Further to the east, the Triassic succession in the Ordos Basin displays gradually thickening and alluvial-fluvial system flowed from southeast to northwest, showing a huge thick sedimentary wedge in the western basin margin. Together with the Late Permian–Early Triassic closure of the Paleo-Asian Ocean to the north, the Late Triassic extensional structures and diabase dykes in the Helanshan tectonic belt, all the above sedimentary features could be mostly interpreted as records of an extensional basin correlated to post-collisional collapse of the Central Asian Orogenic Belt.

Paleogeographic evolution of the Lower Yangtze region and the break of the “platform-slope-basin” pattern during the Late Ordovician

Wide distribution of the black shales and diversification of the graptolite fauna in South China during the Late Ordovician resulted from its unique paleogeographic pattern, which was significantly affected by the paleogeographic evolution of the Lower Yangtze region. In the study, 120 Upper Ordovician sections from the Lower Yangtze region were collected, and a unified biostratigraphic framework has been applied to these sections to establish a reliable stratigraphic subdivision and correlation. Under the unified time framework, we delineate the distribution area of each lithostratigraphic unit, outline the boundary between the sea and land, and reconstruct the paleogeographic pattern for each graptolite zone. The result indicates that, with the uplift and expansion of the ‘Jiangnan Oldland’ in the beginning of the late Katian, the oldland extended into the Yangtze Sea gradually from south to north, which finally separate the Jiangnan Slope and the Yangtze Platform. Consequently, the longstanding paleogeographic pattern of “platform-slope-basin” in South China was broken. The paleogeographic change led to sedimentary differentiation among the two sides of the ‘Jiangnan Oldland’ during the Late Ordovician. This event also led to the closure of the eastern exit of the Upper Yangtze Sea, and formed a semi-closed, limited and stagnant environment for the development of the organic-rich black shales during the Late Ordovician. The major controlling factors of these paleogeographic changes in the Lower Yangtze region were not consistent from the Katian to the Hirnantian. In the late Katian, the sedimentary differentiation between the east and west sides mostly resulted from regional tectonic movement - the Kwangsian Orogeny. However, during the Hirnantian, the whole Yangtze region became shallower, which was mostly influenced by the concentration of the Gondwana ice sheet and the consequent global sea level drop.

PALEOGEOGRAPHIC FEATURES OF THE FORMATION OF SHALE METACONGLOMERATES OF NOVOKRIVOROZHSKAYA SUITE OF KRYVYI RIH STRUCTURE

The paper considers the conditions of formation of novokrivorozhskaya suite that meets the basis of the section of Kryvyi Rih structure. Paleogeographic reconstructions novokrivorozhskaya suite was based on these methods: comparison of intensity factors of weathering and sedimentary differentiation of A.A. Predovsky, the attitude indicator elements and depositional facies conditions – the conditions of sedimentation. Using techniques of V.K. Golovenok, and by comparing the chemical composition of rocks novokrivorozhskaya suite with an average composition of rock-forming oxides clarke in magmatic rocks there was the attempt to reconstruct the climatic conditions of sedimentation suite. The formation analysis usage allowed to determine the likely sources of clastic material and ways of transportation.

Sedimentary differentiation of aeolian grains at the White Sands National Monument, New Mexico, USA

Abstract Gypsum (CaSO4·2H2O) has been identified as a major component of part of Olympia Undae in the northern polar region of Mars, along with the mafic minerals more typical of Martian dune fields. The source and age of the gypsum is disputed, with the proposed explanations having vastly different implications for Mars’ geological history. Furthermore, the transport of low density gypsum grains relative to and concurrently with denser grains has yet to be investigated in an aeolian setting. To address this knowledge gap, we performed a field study at White Sands National Monument (WSNM) in New Mexico, USA. Although gypsum dominates the bulk of the dune field, a dolomite-rich [CaMg(CO3)2] transport pathway along the northern border of WSNM provides a suitable analog site to study the transport of gypsum grains relative to the somewhat harder and denser carbonate grains. We collected samples along the stoss slope of a dune and on two coarse-grained ripples at the upwind margin of the dune field where minerals other than gypsum were most common. For comparison, additional samples were taken along the stoss slope of a dune outside the dolomite transport pathway, in the center of the dune field. Visible and near-infrared (VNIR), X-ray powder diffraction (XRD), and Raman analyses of different sample size fractions reveal that dolomite is only prevalent in grains larger than ∼1 mm. Other minerals, most notably calcite, are also present in smaller quantities among the coarse grains. The abundance of these coarse grains, relative to gypsum grains of the same size, drops off sharply at the upwind margin of the dune field. In contrast, gypsum dominated the finer fraction (

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Properties of five soils developed on different positions of a Legon hill catena in the Accra Plains, Ghana, were studied. Nyigbenya, Toje, Adenta, Haatso and Alajo series were located on the higher upperslope, lower upperslope, middle slope, lower slope and bottom slope positions, respectively. All the soils had bulk density between 1.00 and 1.40 Mg m-3 and pH (KCl) > 5.1. Alajo series showed relatively higher pH (H2O) (6.6 to 7.6), than all the other soils. Clay fraction increased from 263 g kg-1 at the upper to 721 g kg-1 at the bottom slope. All the soils except Alajo series showed very low levels of exchangeable bases and cation exchange capacity (CEC). Alajo series contained close to 10-fold levels of exchangeable bases and CEC compared to those of the other soils. Phosphorus retention was low (< 35%) in all the soils except the last horizon of Alajo series which showed a very high amount (96.2%) and had stronger correlation with clay content of the upper 30 cm of the soils. Alajo series showed the highest levels of organic C (1.8 to 9.5 g kg-1). This study shows that: (1) The properties of Nyigbenya, Toje, Adenta and Haatso series suggest that they were formed from stratified parent materials; (2) The within and across pedon textural differences along the catena were caused by sedimentary differentiation, illuviation and pedogenic formation of clay in the subsoil; (3) The levels of phosphate retention, CEC and exchangeable bases in the soils could be attributed mainly to the type and amount of clay they contained. Nyigbenya and Toje series were classified as Rhodic Kandiustalf according to Soil Taxonomy. Adenta series was classified as Typic Kandiustalf and Haatso series as Kandic Haplustalf. Alajo series, on the other hand was classified as Typic Natraquert. According the WRB classification system, Nyigbenya and Toje series were Nitisols whereas Adenta and Haatso series were Lixisols. Alajo series was classified as Natric Vertisol. Key words: Alfisols, catena, pedogenesis, soil taxonomy, World reference base.

Multi-proxy palaeosol evidence for late Quaternary (MIS 4) environmental and climate shifts on the coasts of South Africa

Palaeosols are common along the coastline of southern South Africa as stacked aeolian dune deposits but have rarely been studied. We selected two late Quaternary palaeosols exposed in a marine cliff-face at Koeberg and coastal barrier dune at Goukamma, South Africa in order to improve our understanding of their pedogenesis and palaeoclimate dynamics. Palaeosol-based proxies explored include: elemental geochemistry by X-ray fluorescence spectrophotometry, δ13C and δ18O isotopes, micromorphology and clay mineralogy by x-ray diffraction. Selected physico-chemical soil properties were analyzed by routine laboratory procedures. The palaeosols comprise predominantly loamy sand to sandy clay loam textures, have a high pH (>6.5), and very low electrical conductivity (<0.89 mS cm−1). SiO2 and CaO are the most abundant of all the elements in the cambic and calcic horizons respectively. The low levels of Al in the parent materials most likely invalidated the applicability of chemical weathering indices (CIA) to assess weathering intensity. In the case of chemical index of weathering (CIW), the age and sedimentary settings of the palaeosols overruled the possibility for K metasomatism and illitization by metamorphism. The indices WI-1 and W1-2 developed by Darmody et al. (2005) appear more consistent with depth. The palaeo MAT computed from palaeosol carbonate oxygen isotope is 14 and 11 °C for Koeberg and Goukamma respectively, while the maximum MAP obtained from the cambic horizon (Bw) of the Goukamma coastal barrier is 653 mm y−1. The layering seen in the thin section of the calcic layer at Goukamma indicates deposition, possibly by sedimentary differentiation across a palaeo-slope. Clastic calcite and muscovite mica are the dominant minerals in these palaeosols indicating impeded chemical weathering. Similar to many other parts of the world, the coastlines of South Africa has experienced environmental and climate oscillations in the Quaternary. We conclude that along the southern South Africa coasts, a palaeosol based approach to palaeoenvironmental and palaeoclimate reconstruction in combination with other proxies such as pollen and marine based isotopes can provide insights into the environmental oscillations of the late Quaternary.