Changes In Sedimentary Environment Research Articles

Paleogeography and shale development characteristics of the Late Permian Longtan Formation in southeastern Sichuan Basin, China

Abstract The Late Permian Longtan Formation is extensively developed within the southeastern Sichuan basin which lies on the western margin of the Yangtze Plate in southwestern China. The stratigraphy and sequence boundaries of the Longtan Formation were interpreted from a combined study of field data, well log data and various data analysis in order to determine the extent and thickness of shale sequences. This interpretation defined the broad sequence stratigraphic framework and separated the Longtan Formation into three 3rd-order sequences. In addition, the formation was divided into five sedimentary facies using sedimentary, paleontological, geophysical and other data. The facies identified were fluvial, low energy shore, tidal flat, melanged accumulated shelf and platform basin. Three paleogeographic maps were created based on the 3rd-order sequences: SQ1, SQ2 and SQ3. The study shows that SQ1- SQ3 sedimentary environment of the study area, which extends from the southwest to northeast, was developed followed by fluvial, shore, tide, shallow water melanged accumulation shelf, deep water melanged accumulation shelf and platform basin. The mud shale rock types of the Longtan Formation that were identified are carbonaceous mud shale, silty mudstone and lime mudstone. According to the divided standard of mud shale effective thickness, there are three mud shale assemblages present in the sequences SQ1, SQ2, SQ3, namely, mud shale with sandstone, mud shale with limestone, and mud shale. SQ1 has the thickest mud shale identified on well logs. Mud shale with sandstone was well developed in the southwest of the study area and was interpreted to have been deposited in a shore or tidal flat sedimentary environment. This rock assemblage has a large effective thickness and extensive lateral distribution. In contrast, the assemblage of mud shale with limestone was mainly developed in the northeast of the study area and was interpreted to have formed in a melanged accumulation shelf sedimentary environment. The effective thickness of the mud shale with limestone is small and the distribution is limited. There is developed mud shale in above two type's sedimentary environment, but the distribution is also limited. The Longtan Formation's mud shale reached stage B diagenesis and is principally composed of quartz and clay minerals. The average quartz content is 43.40% and the average clay mineral content, which is mainly composed of illite, mixed illite/smectite, chlorite and kaolinite, is 39.58%. The sedimentary environment has controlled the distribution of minerals within the sequences. The clay mineral content gradually decreases as the sedimentary environment changes from shore-tidal flat in the southwest to deep water melanged accumulation shelf in the northeast. Therefore this comprehensive analysis shows that the southwest of the study area is the key area for shale gas exploration in the Longtan Formation.

Biomarkers stratigraphy of Irati Formation (Lower Permian) in the southern portion of Paraná Basin (Brazil)

This study covers the application of biomarkers as stratigraphic markers in a Lower Permian marine section from the Paraná Basin, southern Brazil. The biomarker variations reveal changes in sedimentary environment that were not detected by sedimentological and paleontological methods and will contribute to high resolution stratigraphy. The Irati Formation contains two lithostratigraphic units: (1) the lower unit (Taquaral Member) is composed of shales, deposited in a marine epicontinental to restricted environment and (2) the upper unit (Assistência Member) which consists of carbonates interbedded with shales and organic-rich shale intervals deposited in a restricted marine environment. The biomarker differences observed in the Taquaral Member can be explained by two different types of organic matter. The lower and upper parts are characterized by a high relative abundance of high molecular weight n-alkanes, C27 (17α) trisnorhopane, C31 αβR homohopane, high hopanes/steranes ratio, and low hydrogen index values. This facies contains greater relative amount of terrestrial organic matter. The low hopanes/steranes ratio, together with the high hydrogen index values and amorphous organic matter relative proportion observed in the middle part of the Taquaral Member indicate a more marine organic matter contribution. In the Assistência Member based on gammacerane/C30 αβ hopane ratio, C28ααα (20R) methylcholestane/C29ααα (20R) methylcholestane ratio, total organic carbon, hydrogen index values, palynofacies and sedimentary structures it was possible to distinguish changes in salinity and redox conditions. The bituminous shales intervals were deposited in an anoxic environment (high total organic carbon, high hydrogen index values, high preserved amorphous organic matter content) with lower salinity (lower gammacerane/C30 αβ hopane and C28ααα (20R) methylcholestane/C29ααα (20R) ethylcholestane ratios) on the other hand, the limestones intervals were deposited in an aerobic environment (low total organic carbon, low hydrogen index values, high oxidized organic matter content) with higher salinity (higher gammacerane/C30 αβ hopane and C28ααα (20R) methylcholestane/C29ααα (20R) methylcholestane ratios).

인공구조물 건설 후 안면도 가경주 간석지의 퇴적환경 및 미지형변화 분석

인공구조물 건설 후 안면도 가경주 간석지의 퇴적환경 및 미지형변화 분석

Characteristics of depositional environment and evolution of Upper Cretaceous Mishrif Formation, Halfaya Oil field, Iraq based on sedimentary microfacies analysis

As one of the most important carbonate targets in the Middle East, Upper Cretaceous Mishrif Formation has been highlighted for a long time. Although consensus has been reached on the overall sedimentary background, disputes still exist in understanding the sedimentary environment changes among sub-regions due to relatively limited research, rare outcrop, and incomplete drilled core, which hinders the analysis on sedimentary environment and thus the horizontal and vertical correlation. In this study, taking the Halfaya Oil Field as an example, the sedimentary microfacies analysis method was introduced to comprehensively characterize the cored interval of Mishrif Formation, including Single Layers MC1-1 to MA2. A total of 11 sedimentary microfacies are identified through system identification of sedimentary microfacies and environmental analysis, with reference to the standard microfacies classification in the rimmed carbonate platform. Then three kinds of environments are identified through microfacies assemblage analysis, namely restricted platform, open platform, and platform margin. Systematic analyses indicate that the deposits are mainly developed in the open platform and platform margin. Meanwhile, rock-electricity interpretation model is established according to the electricity response to cored intervals, and is then employed to interpret the uncored intervals, which finally helps build the sedimentary evolution pattern through horizontal and vertical correlation. It is proposed that the Single Layers MC1-1 to MB2-3 were deposited in the open platform featured by low water level, including sub-environments of low-energy shoal within platform and inter-shoal sea; Single Layers MB2-2 to MB1-2B were deposited in the open platform and platform margin, including sub-environments of high-energy shoal on the platform margin, low-energy shoal within platform, inter-shoal sea, and open sea; and Single Layers MB1-2A to MA2 were again deposited in the open platform with high water level, and the circumstance of open sea was dominant. The deposition of Single Layers MC1-1 to MA2 actually corresponded to a retrogradation-progradation process. Results of this study will not only provide significant guidance to the exploration and development of Mishrif Formation, Halfaya Oil Field, but also support that the theory of sedimentary environment correlation with adjacent areas is reliable.

A century of change in sediment accumulation and trophic status in Lake Fuxian, a deep plateau lake of Southwestern China

Lake sediments are often regarded as archives enabling the reconstruction of environmental change. However, little is known about the modern sedimentary environmental change in lakes in vulnerable elevated plateau regions. This study was conducted to investigate a century change of sediment accumulation and trophic status in Lake Fuxian, a deep plateau lake of Southwestern China. Seven sediment cores were collected during an axial transect of Lake Fuxian. The sediment cores were sectioned, oven-dried, ground, and weighed. Sediment ages and sediment accumulation rates (SARs) were determined from the activity of radionuclides (210Pb, 214Pb, and 137Cs). Grain sizes in the cores were determined using an automated laser optical particle-size analyzer. The particulate concentrations of total organic carbon (TOC), total nitrogen (TN), and total phosphorus (TP) were determined using the concentrated sulfuric acid-potassium dichromate digestion method followed by quantification with a UV-3600 spectrophotometer. SARs were similar over ~ 100 years, with an average of 0.096 g cm−2 a−1 (1.6 mm a−1), which is lower than that of other shallow lakes in the plateau area. Prior to the 1910s, SARs were relatively constant, suggesting little human influence, whereas post-1910s, significant fluctuations occurred coincident with an increase in human activities. Since the 1980s, the SARs recovered due to improvements in land management and strategies for reducing soil loss from the catchment area. The grain size indicated that the hydrodynamic process near the outlet of the lake was higher than that in other regions. The temporal distributions of sedimentary nutrients and C/N ratios in the lake indicated that the onset of nutrient enrichment in different parts of the Lake Fuxian was not synchronous. The maximum values of nutrient and C/N ratios were focused on the central-northern and southern part of the lake. Changes in human activities are important drivers influencing environmental impacts on Fuxian Lake over the past century. More attention should be paid to the control of nutrient inputs from sub-catchments to minimize further environmental pollution of this plateau lake. The results are of significance to the management of plateau lakes elsewhere, such as those in Tibet.

Paleomagnetism and rock magnetism from sediments along a continental shelf-to-slope transect in the NW Barents Sea: Implications for geomagnetic and depositional changes during the past 15 thousand years

Abstract Paleomagnetic and rock magnetic data were measured on glaciomarine silty-clay successions along an E-W sediment-core transect across the continental shelf and slope of the Kveithola paleo-ice stream system (south of Svalbard, north-western Barents Sea), representing a stratigraphic interval spanning the last deglaciation and the Holocene. The records indicate that magnetite is the main magnetic mineral and that magnetic minerals are distinctly less abundant on the shelf than at the continental slope. The paleomagnetic properties allow for the reconstruction of a well-defined characteristic remanent magnetization (ChRM) throughout the sedimentary successions. The stratigraphic trends of rock magnetic and paleomagnetic parameters are used for a shelf-slope core correlation and sediment facies analysis is applied for depositional processes reconstruction. The new paleomagnetic records compare to the PSV and RPI variation predicted for the core sites by a simulation using the global geomagnetic field variation models SHA.DIF.14k and CALS7K.2 and closest PSV and RPI regional stack curves. The elaborated dataset, corroborated by available 14C ages, provides a fundamental chronological framework to constrain the coupling of shelf-slope sedimentary processes and environmental changes in the NW Barents Sea region during and after deglaciation.

PALEOENVIRONMENTAL DISTRIBUTION OF ORDOVICIAN CALCIMICROBIAL ASSOCIATIONS IN THE TARIM BASIN, NORTHWEST CHINA

The stratigraphic and facies distribution of 20 calcimicrobial genera (including calcified cyanobacteria and associated problematic calcified microfossils) are reported for the entire Ordovician succession in the Tarim Basin in northwestern China based on examination of drill cores and 8500 thin sections from 64 wells from the Tabei, Bachu, Tazhong, and Tadong uplifts. A total of ten calcimicrobial associations are recognized in the Lower to Upper Ordovician based on taxonomic composition and distribution within four paleoenvironment types: reef (marginal and patch reef), open platform/bank (marginal and patch bank), lagoon, and tidal flat. The temporal distribution of the calcimicrobial genera closely follows changes in sedimentary environments; an extensive literature survey reveals similar relationships in much of the Paleozoic and Mesozoic. Based on their paleoenvironmental preferences, calcimicrobes can be classified into five paleohabitat types: (1) reef-adapted (Acuasiphonoria, Razumovskia, Phacelophyton , Gomphosiphon, Epiphyton, Renalcis, and Izhella); (2) open platform/bank-adapted (Subtifloria and Bevocastria); (3) both reef and open platform/bank-adapted (Bija, Apophoretella, Rothpletzella, and Wetheredella); (4) lagoon-adapted (Hedstromia, Cayeuxia, Zonotrichites, Ortonella, and Garwoodia), and (5) not only reef and open platform/bank-adapted but also tolerant of tidal flat conditions (Girvanella and Proaulopora). The occurrences of these calcimicrobes in strata not only can indicate ancient sedimentary facies but also can reveal paleoecological parameters of ancient seas, such as nutrient levels (e.g., N and P), predation pressure, and sea level, especially in strata absence of other well-studied facies fossils.

Assessment for the Coastal Sedimentary Environment in the Pearl River Delta (PRD) Region, China by Using 14C Dating

Since late Quaternary, the intense change of coastal sedimentary environment in the Pearl River Delta (PRD) region has resulted in accumulating the thick sedimentary layers with the different causes and properties which were influenced by the fluctuation of sea levels, the change of ancient climate and human activities. Based on the interpretation of high resolution of these sediments, such as the period, sedimentary facies and discontinuous faces, we can infer the evolution of the ancient climate of regional Quaternary environment in the background of global change to know more details about the formation of the PRD. Taking the Xilin Hill fault as the example, we explained the reason of the PRD formation, inferred its interactions with local/regional climate and sea-level changes, and analyzed the evolution of the PRD formation and its controlling factors of the coastal sedimentary environment by using 14C dating technology. It is noted that the neo-tectonic movement and the fault activities played a critical role in controlling coastal geological environment. This is helpful to further investigate the evolution of the PRD coastal sedimentary environment, the history of inversion region, and the sedimentary facies model of the interaction between land and sea. More investigations will be conducted in the near future.

メコンデルタ堆積物におけるベリリウム-7,鉛-210及び放射性セシウム同位体の分布と堆積環境の季節変化

メコンデルタ堆積物におけるベリリウム-7,鉛-210及び放射性セシウム同位体の分布と堆積環境の季節変化

Chapter 6 Geological framework of the Andaman–Nicobar Islands

Abstract The Andaman–Nicobar archipelago that forms the western margin of the Andaman Sea is a sediment-dominated accretionary wedge (outer-arc island) associated with a convergent margin tectonic setting. The Andaman accretionary ridge consists of two stratigraphically and structurally distinct terranes, juxtaposed and telescoped into a north–south-trending high-relief fold-thrust belt formed along the obliquely subducting eastern margin of the Indo-Australian oceanic lithospheric plate. The geology and structure of the ridge reflect the complexity of the evolving tectonics and stratigraphy of an accretionary wedge. Pre-Cretaceous meta-sedimentary rocks, Upper Cretaceous ophiolites and Palaeogene–Neogene sedimentary formations indicate rapid, spatial and temporal changes in lithology, sedimentology, sedimentary and tectonic environments, and palaeogeographic setting. This chapter outlines the current geodynamic setting, evidence for the history of accretion and regional geology and introduces the regional stratigraphic framework.

Human-induced changes in sedimentary facies and depositional environments, Sarum area, Red Sea coast, Saudi Arabia

This paper aims to study the environmental impact of the human inference on the coastal zone of Sarum area, Red Sea coast, Saudi Arabia. In the past, the study area was occupied by coastal lagoons that were connected with the sea with several inlets. Change detection technique based on multi-date satellite images in the last 44 years indicated the occurrence of environmental changes accompanied the construction of the main asphaltic road of the Red Sea that separated some coastal lagoons at the studied area from the Red Sea. Field investigation and litho- and bio-facies distribution indicated the existence of six lateral facies types: gypsum, microbial mats, halite, Quaternary coral reef terrace, bioclasts, and siliciclastic sand sheets. The lateral distribution of these facies is related to the following changes: (1) disconnection between the coastal lagoons and the sea, (2) extensive contraction of the coastal lagoons and formation of saline ponds, (3) exposure of the submerged Quaternary coral reef terrace, (4) increasing of the salinity of the ponds up to 270‰, and (5) deposition of the evaporitic facies types (gypsum and halite) over intertidal and subtidal marine carbonates. Based on the above-mentioned criteria, the human activity leads to environmental changes in the Sarum area from coastal lagoons dominated with carbonate deposits into hypersaline ponds dominated with evaporitic deposits.

Sedimentary Environment and Relative Sea Level Changes of the Asmari Formation Deposited in Lurestan Basin, Southwest of Zagros

Asmari Formation is deposited in the Zagros Foreland basin during Oligo-Miocene time. In the presented research, Sheykh Makan (north of Kabir-Kuh Anticline) section located at Lurestan Basin is studied to achieve an understanding of facies changes, paleoenvironment and also sequence stratigraphic framework (relative sea level changes) of the Asmari Formation. Results of this study revealed that 15 microfacies grouped to four facies belts named as tidal flat, lagoon, barrier and open marine are the main constituents. A homoclinal ramp is determined as the depositional setting of the Asmari Formation. In Sheykh Makan Section, studied interval contains three third-order sequences aging oligmiocene. It overlies gradually shaly Pabdeh Formation and is overlain by evaporitic Gachsaran Formatin.

Sedimentary environmental change induced from late Quaternary sea-level change in the Bonaparte Gulf, northwestern Australia

Low-latitude continental shelves, mixed siliciclastic–carbonate sedimentary systems, provide an understanding of sedimentary environments driven by paleoclimatological processes. The Bonaparte Gulf, northwestern Australian continental shelf, is among the widest in the world, ranging to 500 km, with shallow carbonate terraces and platforms that were exposed during periods of lower sea level. The dominant sediments type switches between carbonate and siliciclastic over a sea-level cycle. However, the mechanism of sedimentary environmental change in the Bonaparte Gulf is not clearly understood. Here, we present a record of sedimentary environmental change from ca. 24 to 35 ka that is related to sea-level variability and exposure of carbonate terraces and platforms. Multi-proxy data from a marine sediment core show a sea-level change induced switch in sedimentary environment from siliciclastic to carbonate-dominated sedimentation during the last glaciation. Radiocarbon ages constrain the timing of this switch to ca. 26 ka, associated with a local sea-level fall of −90 m.

Carbon isotope composition and comparison of Lower Triassic marine carbonate rocks from Southern Longmenxia section in Guang’an, Sichuan Basin

The Early Triassic was a period of ecological restoration for the earth’s system after the end-Permian mass extinction. Geochemical records linked to the variations in marine ecosystems during this period have attracted the interest of geologists for many years. Based on petrographic analysis of samples and evaluations of their reliability as proxies for original seawater, this study investigated the carbon and oxygen isotopic compositions of 350 carbonate rock samples from the Lower Triassic (and adjacent strata) in the southern Longmenxia section of Guang’an, eastern Sichuan Basin. The results indicate that the Triassic carbonate rocks from the southern Longmenxia section favorably preserved the original seawater’s δ13C signal. Furthermore, carbon and oxygen isotopic compositions are found to be poorly correlated, with a determination coefficient as low as 0.0205 and only 44 rock samples show results of Mn/Sr>2 and/or δ18O -6.5‰. This curve presents favorable comparability on a global scale, specifically in the δ13C minima near the Permian-Triassic boundary, at the top of the Jia1 and within the the Jia3, as well as in the δ13C maxima at the tops of the Ye1 and Ye4, at the base of the Jia2 and at the top of the Ye3. The peaks and troughs corresponding to these maxima and minima all reflect global signals. By comparing these results to previous research on coeval carbon isotopic curves established within the chronostratigraphic framework, the ages of these critical evolution points are determined. The results show that the Ye1 roughly corresponds to the Griesbachian substage; the Ye2 through Ye4 correspond to the Dienerian substage; Jia1 corresponds to the Smithian substage; from the Jia2 to the lower part of the Jia4 correspond to the Spathian substage; and the middle and upper parts of the Jia4 belongs to the Aegean Substage of the Middle Triassic. Around the boundary between the Jia1 and Jia2 (which represents the Smithian-Spathian boundary (SSB)), the value of δ13C increases rapidly from -0.911‰ to 3.679‰. The span during which the seawater’s carbon isotope experiences this drastic change may be less than 36 kyr. The oxygen isotope, which is more sensitive to sedimentary environmental changes, exhibits changes prior to the carbon isotope near the SSB, indicating a significant increase in the salinity of the seawater before a sharp rise in the carbon isotope; this event leads to the formation of evaporites and dolomites.

Sedimentary mode and reservoir distribution of the Cambrian carbonate & evaporate paragenesis system in the Sichuan Basin

The Cambrian carbonate & evaporite paragenesis system in the Sichuan Basin is made up of the Longwangmiao, Gaotai and Xixiangchi Fms. So far, great breakthrough has been made only in the Longwangmiao Fm instead of the latter two, and the Anyue Gasfield was discovered in the center of this basin. In this paper, therefore, the Cambrian carbonate & evaporite paragenesis system in the Sichuan Basin was analyzed in terms of its structural–sedimentary setting, sequence stratigraphic framework, sedimentary facies and the distribution of evaporites by using various geologic, logging and seismic data. Then, the geological model of sedimentary facies was established and the distribution range of favorable reservoirs was predicted. Based on these studies, the following results are obtained. Firstly, the palaeotectonic framework is characterized by the style of “one depression between two uplifts” in the setting of a large SE dipping slope, and the stratigraphic filling is in the structure of “onlapping at the bottom and truncation at the top” which is thin in the west and thick in the east. Secondly, three third-order sequence cycles which, on the whole, become shallow upward are developed from bottom to top, and gypsum-salt rocks are mainly located at the high system tract (HST) of third-order sequences and concentrated in the Wanzhou–Yibin sag. Thirdly, the geological model of sedimentary facies is composed of three major sedimentary structural layers from bottom to top, namely the evaporative carbonate ramp, the evaporative diamictic restricted platform and the evaporative restricted platform. The sedimentary environment changes from the open to the closed and the penesaline for a long time, and then back to the open. The distribution of shoals changes from the pattern of “dual banks” in a large area to more scattered shoals and banded shoals, while the evaporative lagoon and tidal flat shrink. Fourthly, the reservoir distribution is complicated and mainly controlled by the sedimentary pattern of palaeotectonics, the grain shoals and the exposed surfaces of sequences with different orders. The favorable areas for reservoir development are located above the paleo-uplift and its surroundings, at the slope-break belt in the platform and in the periphery of platform margin.

Early–middle Miocene topographic growth of the northern Tibetan Plateau: Stable isotope and sedimentation evidence from the southwestern Qaidam basin

This study presents facies analysis and carbonate stable isotopic records of a 4435-m-thick Oligocene–middle Miocene (~30 to ~11Ma) stratigraphic section from the southwestern Qaidam basin, which place constraints on the interplay between topographic growth and climatic/hydrologic changes in the northern Tibetan Plateau. Three types of carbonate, including marl/limestone, pedogenic carbonate and carbonate cement, were collected and went through detailed screening for diagenesis. Carbonate cements are primary early diagenetic carbonates formed in shallow ground water setting. The majority of marls/limestones and pedogenic carbonates are primary micritic carbonates, whereas a few of them showing alterations were sub-sampled to avoid diagenetic phases. A negative shift in the δ18O values of marls/limestones and depositional environment change from palustrine to shallow lacustrine associated with paleohydrological change at ~20Ma are likely caused by tectonic activity in adjacent ranges. Around 15–14Ma, a decrease of ~1.5‰ in the most negative δ18O values of carbonate cements is interpreted to reflect the topographic growth of surrounding mountain ranges (i.e., Altun Shan, East Kunlun Range). This inference is supported by the sedimentary facies change from marginal lacustrine to fluvial that was accompanied by an abrupt increase in sedimentation rate. Finally, a ~1.5‰ positive shift in the most negative δ18O values of carbonate cements and pedogenic carbonates, enriched δ13C values of pedogenic carbonates, as well as sedimentary environment change from fluvial to braided river/alluvial fan at 13–12Ma, most likely reflect a regional aridification event observed over a large area of the Central Asia. This aridification event may have been associated with continued topographic growth of the northern Tibetan Plateau to a critical elevation to block moisture (e.g., Westerlies) from penetrating the interior of Central Asia.

Distribution and Environmental Significances of Radionuclides in the Sediment of the Changyi Coastal Wetland

The radionuclides contents in the surface sediments and sediment cores were determined by gamma spectrometry method. The distribution of radionuclides and its indication to material sources, human activities and other sedimentary environmental evolution events were discussed and the sedimentation rate was estimated. The results showed that the average activity concentrations of 238U,232Th, 226Ra and40K in the surface sediment were (54.4±11.7), (57.9±9.7), (28.6±4.3) and (542±21) Bq·kg-1, respectively. Regional differences of radioactivity level reflected the material sources and influences of human activities on the sediment characteristics. According to the vertical distribution of excess 210Pb, the sedimentation rate of high tidal areas was estimated at 0.23 cm·a-1. In the sediment core, the variable coefficients of 238U,232Th and 226Ra contents were high. According to the vertical profiles of 238U,226Ra and 238U/226Ra ratio, the process of sedimentation could be divided into four periods: period of slow deposition, period of dramatic change in sedimentary environment caused by great migration of the Yellow River channels, period of stable deposition after the Yellow River ran into one single channel, and period of alternate degradation and growth under the influence of human activities. The vertical variation of radionuclides in the sediment core could serve as an effective environmental indicator since it could record the environmental evolution processes of the coastal wetland.

Middle to late-Holocene decreased fluvial aggradation and widespread peat initiation in the Ishikari lowland (northern Japan)

This study investigated the influence of sea-level and climate changes on the decreased fluvial aggradation and subsequent widespread peat initiation in the middle to late-Holocene in the Ishikari lowland, which is a coastal floodplain formed in response to the postglacial sea-level change. By introducing a new approach to separately evaluate the rates of organic and clastic sediment input, we demonstrated that the peat began to form when the fluvial sedimentation rate was significantly decreased (less than 0.6 mm/yr), while plant macrofossil analysis suggested that lowering of water level is also important to the peat initiation. Such changes in sedimentary environment may be associated with the abrupt abandonment of crevasse splays. The concentrated ages of the peat initiation around 5600–5000, 4600–4300, and 4100–3600 cal. BP suggest that an allogenic control promoted the abandonment of crevasse splays, and different onset ages can be explained by different fluvial responses of the Ishikari River and its tributaries. The abandonment of crevasse splays could result from sea-level fall or decreased precipitation. While submillennial sea-level fluctuations coincident with the peat initiation have not been reported in coastal lowlands of Japan, the close comparison of the onset ages and decreased precipitation recorded in a stalagmite from China, which represents the strength of the East Asian summer monsoon (EASM), suggests that decrease in precipitation led to the abandonment of crevasse splays. Our results may indicate that similar fluvial responses might be common in other coastal floodplains affected by the EASM.

Tephrostratigraphy and sedimentary environment changes since the last 300ka recoded in a deep-sea core off coast of Akita

Tephrostratigraphy and sedimentary environment changes since the last 300ka recoded in a deep-sea core off coast of Akita

Mineralogical and geochemical characteristics of Miocene pelitic sedimentary rocks from the south-western part of the Pannonian Basin System (Croatia): Implications for provenance studies

Fifty-two samples of Miocene pelitic sedimentary rock from outcrops on Medvednica, Moslavačka Gora and Psunj Mts., and boreholes in the Sava Depression and the Požega Sub-depression were investigated. These sediments formed in different marine (with normal and reduced salinity), brackish, and freshwater environments, depending on the development stage of the Pannonian Basin System. Carbonate minerals, clay minerals and quartz are the main constituents of all pelitic sedimentary rocks, except in those from Moslavačka Gora Mt in which carbonate minerals are not present. Feldspars, pyrite, opal-CT, and hematite are present as minor constituents in some rocks. Besides calcite, dependent on the sedimentary environment and diagenetic changes, high-magnesium calcite, aragonite, dolomite and ankerite/Ca-dolomite are also present. Smectite or illite-smectite is the main clay minerals in the samples. Minor constituents, present in almost all samples, are detrital illite and kaolinite. In some samples chlorite is also present in a low amount. Major elements, trace elements and rare earth elements patterns used in provenance analysis show that all analysed samples have a composition similar to the values of the upper continental crust (UCC). The contents of major and trace elements as well as SiO2/Al2O3, K2O/Al2O3, Na2O/K2O, Eu/Eu*, La/Sc, Th/Sc, La/Co Th/Co, Th/Cr, Ce/Ce* and LREE/HREE ratios, show that the analysed pelitic sedimentary rocks were formed by weathering of different types of mostly acidic (silicic), i.e. felsic rocks.