Evolution Of Sedimentary Environments Research Articles

Redox conditions and nitrogen cycling in the Late Ordovician Yangtze Sea (South China)

The late Ordovician was a critical interval in Earth's history, during which changes in the biosphere, climate and environment occurred. In this study, we analyse the nitrogen isotope composition and multiple geochemical indices of Ordovician-Silurian (OS) shales in the Yangtze Sea (South China) to elucidate the evolution of nitrogen cycling and sedimentary environments. The nitrogen isotope values of Wufeng shale samples (Upper Ordovician) range from −0.4‰ to +1.6‰, and those of Longmaxi shale samples (Lower Silurian) range from +0.6‰ to +3.1‰. According to the longitudinal distribution of nitrogen isotope, there was a positive shift from the Wufeng Formation to Longmaxi Formation. Moreover, the redox-sensitive element ratios and nitrogen isotopes were strongly related. These relationships imply that the pre- and post-Hirnantian intervals of nitrogen cycle perturbation were directly related to changes in the oceanic redox conditions in the Yangtze Sea region. During the depositional stage of the Wufeng Formation (pre-Hirnantian) the water column was deep, causing anaerobic reduction in the sedimentary environment. The nitrogen cycle was dominated by anaerobic nitrogen fixation with little isotopic fractionation; thus, the δ15N values are light. In contrast, during the depositional stage of the Longmaxi Formation (post-Hirnantian), global cooling and tectonic uplift led to sea-level change, and sedimentation was increasingly affected by the supply of terrigenous clastic materials. The oxygen content of the water column increased, and redox stratification was maintained dynamically. Denitrification and anaerobic ammonium oxidation (anammox) played important roles in the Longmaxi Formation nitrogen cycle, leading to strong isotopic fractionation. Nitrogen isotopes are a reliable indicator of palaeo-environmental changes and have great value for the restoration of palaeo-depositional conditions and the comparative analysis of global Ordovician-Silurian geological events.

New Discovery and Significance of Microbialites of the Middle Ordovician Majiagou Formation in Liujiang Basin, Hebei, China

The concept, classification, genesis, and significance of microbialites have been constantly improving for over a century, but they are still hot spots in the field of geology because of the strong attractions and many unsolved mysteries such as the mechanism of the microbial deposition process, the microbial induced sedimentary structures, and so on. At the same time doubt and confusion arises among students and beginners because of the mysteries. Generally, any findings of various stromatolites, oncolites, and thrombolites are all classified under microbialites. Moreover, microbialites have also been discovered from the middle Ordovician Majiagou formation in Liujiang Basin, Hebei, China. As an important geological practice base for thousands of students and beginners every year, in order to enhance the interest in geosciences the further research of the microbialites found in Liujiang Basin is necessary. In this study, the classification scheme of microbialites has been summarized. On this basis, the study shows that the main microbialites in the study area are microbial carbonate rocks, which include wavy, columnar, conical-shaped stromatolite carbonate rocks and oncolite carbonate rocks. The variety, macroscopic characteristics, and clear internal features of microbialite carbonate rocks are all have considerable significance in judging hydrodynamic conditions, sedimentary environment, paleogeographic characteristics, and biological evolution. And the evolutionary process of different kinds of microbialites has been discussed. However, the roles of microbialites in the formation mechanism and ecological significance are still need further study. With the new discovery of microbialites from the middle Ordovician Majiagou formation in Liujiang Basin, a new window of study microbialites has been opened for everyone. More findings and detailed characteristics of microbialites in this area are urgently needed during geological practice.

Review of the syn-rift to early post-rift depositional systems of the Cretaceous Mauléon rift: sedimentary record of continental crust hyperextension and mantle denudation (Western Pyrenees)

The Mauléon Basin, in the northwestern Pyrenean Belt, is related to Early Cretaceous rifting and mantle denudation. Here we review the evolution of depositional systems in the Mauléon Rift Basin during Albian and Cenomanian time. This review includes the lithostratigraphy, regional distribution, boundaries, age and facies sedimentology of the basin’s syn-rift formations and their members. We construct paleogeographic maps to elucidate (1) the 3D distribution of sedimentary facies and depositional systems during the Albian and Cenomanian from the Iberian proximal margin to the hyperextended domain and (2) the link between major extensional structures and sedimentation during rifting and mantle denudation. The Mauléon Rift was supplied during most of the Albian by sediments from the Iberian proximal margin. The southern margin had a steep and abrupt topographic boundary related to a northward crustal rollover along the south-dipping Saint-Palais detachment. This feature controlled the deposition of coarse-grained turbidites at the base of the margin that abruptly gave way to low-density turbidites, then deep-basin deposits in the hyperextended domain. During uppermost Albian to Early Cenomanian time, mantle denudation occurred in the eastern Mauléon Basin and the vergence of the detachment systems reversed. Minor debris-flow deposits formed at the foot of fault scarps associated with the newly formed north-dipping detachments. Elsewhere, sediment from deltaic systems to the west in the Saint-Jean-de-Luz area deposited low-density turbidites in the hyperextended domain. During the post-rift stage, the flux of coarse sediment from the detachment footwall gradually declined as deformation waned, and low-density turbidites expanded onto the hyperextended domain from the European Upper Cretaceous carbonate platform. These paleogeographic reconstructions, in addition to offering a synthetic view of the evolution of sedimentary environments during rifting, offer new insight into the post-rifting exhumation of the lower crust and mantle.

Sedimentary Environment Evolution of DLK01, a Borehole in the Holocene on the Southern Coast of Changxing Island, Liaodong Peninsula

Ma, H.; Fang, J.; Ni, J.; Zhang, L.; Liu, Y., and Li, J., 2020. Sedimentary environment evolution of DLK01, a borehole in the Holocene on the southern coast of Changxing Island, Liaodong Peninsula. In: Liu, X. and Zhao, L. (eds.), Today's Modern Coastal Society: Technical and Sociological Aspects of Coastal Research. Journal of Coastal Research, Special Issue No. 111, pp. 78–83. Coconut Creek (Florida), ISSN 0749-0208.Clayey water electric conductivity, foraminifera analysis, and grain-size analysis of the DLK01 borehole in a shrimp pond in the south of Changxing Island, Liaodong Peninsula, were conducted. Combined with accelerator mass spectrometry 14C, the DLK01 core was divided into six sedimentary units from bottom to top: unit I, continental flooding deposit; unit II, storm surge or high tide deposit, formed earlier than 8352 cal BP; unit III, intertidal zone deposit, formed at 8352 cal BP and representing the Holocene transgression that changed from continental facies to marine–continental transitional facies; unit IV, shallow sea deposit, formed in 7738 cal BP and representing the rise of the sea surface in the Holocene; unit V, intertidal zone deposit, unconformably contacted with unit IV, indicating that the marine–continental transitional facies deposit was formed by the drop of seawater; and unit VI, salt marsh deposit to intertidal zone deposit, formed as the sea level dropped farther since 1340 cal BP.

Sedimentary environment evolution and provenance analysis of northwestern Liaodong Bay from the Middle Pleistocene

The environmental evolution and provenance of the sediments since 1.2 Ma penetrated by the borehole of JXC-1 in Northwestern Liaodong Bay are studied in this paper using the data from accelerator mass spectrometry 14C dating, optically-stimulated luminescence dating, paleomagnetic dating, qualitative and quantitative analysis of benthic foraminifera, and the geochemistry analysis. Based on regional stratigraphic correlation, the evolution of sedimentary environment and provenance are discussed. The study suggests that large-scale transgression began by the end of Middle Pleistocene, and invaded into this region for three times from Late Middle Pleistocene to Holocene, in MIS 7, MIS5, and MIS1, that is in consistent with the global pattern of sea level changes. The results of this study are different from previous researches in the Bohai Sea, which found three times of transgressions in MIS5, MIS3, and MIS1 respectively. There are two possibilities, either no transgression in the Bohai Sea, or the transgression did not enter Liaodong Bay during the time of MIS3. Provenance analysis shows that the northwest of Liaodong Bay is a mixed depositional area. Sediments are mainly sourced from the Yellow River, Liaohe River (formerly Shuangtaizi River) and Daliao River. During the three marine phases, the sediments of the JXC-1 borehole came mainly from the Yellow River characterized by high content of CaO, and deposited under the control of Bohai Sea circulation and coastal currents. The sediments in depths of 36.00–46.00 m and 58.20–70.30 m of the borehole are high water level lacustrine sediments, mainly came from the Yellow River. However, the sediments in depths of 46.00–58.20 m are mainly brought in by the Liaohe and Daliao rivers. All these studies suggest that the sediments coming from the Yellow River may be distributed northward up to 40.5°N. As a major conclusion different from previous understandings, this paper provides an important reference for the evolution of the sedimentary environment and the provenance of the Late Quaternary deposits of the Bohai Sea.

The Cretaceous stratigraphy, Songliao Basin, Northeast China: Constrains from drillings and geophysics

Abstract The Cretaceous nonmarine sedimentary strata are widespread in Songliao Basin, Northeast China. As the largest oil producer in China, an abundance of data has been generated by the petroleum industry, including sedimentology, seismology, geochemistry, and geochronology in the Songliao Basin. This article reviews the achievements in China Cretaceous Continental Scientific Drilling SK and presents the new results of the China Continental Geothermal Drilling SR1 in stratigraphy and geophysics. The results allow us to establish the relationships among SK-1, SK-2, and SR1, provide some constrains on the tectonic setting of Songliao Basin, and interpret the sedimentary facies and environmental evolution. After stratigraphic correlation of SK and SR boreholes, the result indicates that the geological boundary between the late Cretaceous Mingshui formation and the Paleogene Yi’an formation is at the depth of 115 m. The magnetotelluric sounding anomaly areas are corresponding with the unconformities, which are equivalent to seismic horizon T03, T1, T2, and T3, respectively. In addition, the typical “steer’s-head” geometry in the magnetotellurics indicates that the SLB has experienced five evolution stages.

Differential enrichment mechanism of organic matters in the marine-continental transitional shale in northeastern Ordos Basin, China: Control of sedimentary environments

The organic-rich shales deposited in marine-continental transitional environments are well developed in China. Previous studies mainly focused on the petrology and geochemical characteristics of the transitional shale. However, the factor controlling organic matter (OM) enrichment in the shale has not been studied adequately, limiting the optimal reservoir selection in shale gas exploration. Based on laboratory analyses and field investigations, this paper documents the geochemical features of the transitional shales in Upper Paleozoic (the Benxi, Taiyuan and Shanxi Formations) of the northeastern Ordos Basin, China. The sedimentary environment evolution and the relevant influence on OM enrichment are discussed to reveal the factor controlling the interlayer difference of the OM enrichment in the shale. The results show that the shale in the Taiyuan Formation hosts the highest OM content in the Upper Paleozoic. The sedimentary setting of the study area transited from barrier-flats to fluvial deltas, accompanied by gradually arid climates, decreasing degree of reduction and salinity of waters, and increasing clastic influx. During the middle stage of the marine-continental transition (the depositional stage of the Taiyuan Formation), the warm and humid climate was beneficial to the OM production, and the anoxic condition was conducive to the OM preservation. In addition, the increasing clastic influx ensured the OM input, and the moderate depositional rate favored the OM preservation. All the factors above commonly established an excellent environment for the OM enrichment. Our results reveal the OM enrichment mechanism in transitional shales, which is helpful to the optimal reservoir selection in the shale gas exploration in the Ordos Basin as well as the similar basins.

A compositional approach to the reconstruction of geochemical processes involved in the evolution of Holocene marine flooded coastal karst basins (Mljet Island, Croatia)

Geochemical patterns in the environment are always the result of certain processes. Therefore, it is essential to decipher a process to properly evaluate the environmental role and potential of chemical elements/compounds. This allows the distinction between natural and anthropogenic influence on elemental concentrations. However, if the compositional nature of geochemical data is neglected, erroneous or misleading conclusions regarding the processes involved are probable. In this study the reconstruction of depositional environments and processes through the Holocene in two sediment cores obtained from submerged sinkholes located on the island of Mljet, Croatia, was performed by taking into account the compositional nature of geochemical, mineralogical and grainsize data. Problems involving compositional data are always multivariate; for example, the concentration of a single element does not carry any interpretative information, as only the ratios between elements do. This has led to the discovery of a large number of geochemical proxies based on elemental ratios, which describe certain environmental conditions and processes involved. Nevertheless, some proxies have been found to be restricted to only some specific environments, thus preventing them from being used in general; therefore, some kind of relation between different proxies is necessary to obtain final conclusions. However, when using simple elemental ratios, those correlations cannot be obtained due to the nature of compositional data. With a sequential binary partition of a compositional vector, orthonormal log ratio (olr) coordinates (proxies) can be constructed. When based on expert knowledge, those proxies fully acknowledge the geochemical properties of the chosen elements with one major difference - that the correlation between newly obtained variables is mathematically well grounded. As a result, the final conclusion is more accurate. In this research, geochemical proxies obtained as a representation in olr coordinates of the elements that are enriched compared to the local soil were used to perform principal component analyses. It helped to unravel the evolution of sedimentary environments. Mineralogical (XRD and heavy mineral data) and grain size analyses supported the conclusions obtained based solely on geochemical data. Furthermore, data analysis suggests that the proxies for redox conditions described in the literature should be used with caution, as their use is somewhat limited.

Glacier Relics of the Last Glacial and Holocene Periods Discovered in the Middle‐low Mountains of Eastern China: Sedimentary Sequences and Environmental Evolution of Mengshan Mountain in Shandong Province since 80 ka

AbstractThe goal of this research study is to describe academic issues which have been debated in the field of Chinese geosciences for a century. In 1922, Jonquei S. Lee (Li Siguang) discovered Quaternary glacial relics at Taihang Mountainin eastern China. In 1947, he published his research findings in the magazine Mount Lushan in Glacial Age. The research results had established three Ice Ages: Poyang (Gonzi), Dagu (Minde), and Lushan (Lisi). However, at that time, no Wurm glacial relics of the last Ice Age had been found in Lushan Mountain. Since then, the research team represented by Shi Yafeng, who is considered to be “the father of glaciers in China”, questioned Jonquei S. Lee's research results and concluded that “Professor Jonquei S. Lee's Quaternary glacier research in Lushan Mountain having misread the debris flow”. In 2005, the “middle‐low mountains” in eastern China were finally defined as follows: “We clearly and unambiguously believe that there were no glacial activities in the middle‐low mountainous areas of eastern China (east of 102° to 104°E; below 3,000 and 2,500 m) during the Quaternary Period”. Currently, the long‐standing academic debate appears to have come to a conclusion.As of 2015, the author and others began to investigate and study the Quaternary glacial relics in Mengshan Mountain (1,156m above sea level), Shandong Province, one of the “middle‐low mountains” of eastern China. The relics have been observed to posses the systematic features of glacial erosion, trough and valley striations, and moraine deposits. The applied dating method shave confirmed that there were not only glacial relics of the last Ice Age (Wurm), but also Holocene glacial relics in the Mengshan Mountain area. Therefore, in order to further establish the corresponding relationship between the glacier, loess, stream sediment series, and MIS in the Mengshan Mountain area, a large number of chronological studies have been carried out regarding the various types of sediments in the area, and 24 dating datahave been obtained using OSL, CRN, and 14Cmethods. On this basis, the corresponding relationship between the sedimentary sequences and the MIS was established for the first time in eastern China, which in dicates the environmental changes which had occurred in eastern China since 80ka. These discoveries s and chronological study results confirm the existence of the Last Ice Age, as well as Holocene glacial relics at Mengshan Mountain, there by confirming that Quaternary glaciation had occurred in the middle‐low mountain areas of eastern China.

Evolution Characteristics and Stratigraphic Division of Quaternary Sedimentary in the South Wing of Yangtze River Delta, East China

This paper discusses the sporo pollen assemblage, evolution of sedimentary environment, sedimentary facies and the lithotope characteristics revealed by boreholes since pliocene epoch by carrying out 14C dating, sporo pollen and micro-paleontological analysis of sediments in borehole BK01 (depth 237.80 m) in the southern flank of the Yangtze River Delta Plain. According to the results of this study, there are 17 species of foraminifera that have been identified as 11 genera, including 16 benthic foraminifera and 1 planktonic foraminifera；8 species of Ostracoda that have been identified as 8 genera; sporo pollen analysis shows that there are 37 types, including 18 species of woody plant pollen, 11 species of herbaceous plant pollen, 8 species of fern spores, of which, in the sporo pollen assemblage, woody plants have the highest content (about 85.1%), ferns spores have about 8.7%, and herbaceous plants have only 6.3%. Sedimentary environment records can be divided into 18 sporo pollen assemblage zones according to borehole lithology, sporo pollen assemblage and micropaleontology analysis. The results show that the BK01 bore strata from bottom to top can be divided into carbonaceous mudstone (Ech), Jiaxing Formation (N-Qp1j), Qiangang Formation (Qp2q) Dongpu Formation (Qp3d), Ningbo Formation (Qp3n) and Zhenhai Formation (Qhzh).

Sedimentology and sequence stratigraphy of the middle and upper Eocene succession from Jebel Kabbara (central Tunisia)

During the Maastrichtian-Middle Eocene interval, a resistant area (Kasserine Island) occurred in central Tunisia. The emerged area narrowed especially on its northern edge because of the eustatic rise and associated compressive tectonics having marked central Tunisia during the Paleocene-Eocene transition. Located at the NE boundary of the Kasserine Island, the study area saved the shoreline variations and therefore the sedimentary environment evolution. In this area, sedimentological and sequence stratigraphy analysis of middle and upper Eocene deposit are scarce in previous works. In this context, this paper aims to study the depositional facies, early diagenesis and sequence stratigraphy of the Cherahil Formation (Middle and Late Eocene) in Jebel Kabbara (central Tunisia) in order to refine the deposit facies and environment and paleographic variations in the NE boundary of the Kasserine Island during Middle and Late Eocene.The studied succession (Cherahil Formation) outcrops at Jebel Kabbara, SW of Jebel Cherahil and is located 42 km from Kairouan city. This Formation is subdivided into lower and upper lithostratigraphic members separated by Siouf member. The field examination and microscope observation of thin sections and XRD clays mineralogy set apart five carbonate facies. The depositional facies occurred broadly within a shallow-water carbonate platform with alternatively episodes of open marine conditions and episodes of restricted lagoonal conditions.The analysis of the depositional facies stacking and the field examination of the unconformity surfaces allow subdividing the Cherahil Formation into eight third-order depositional sequences, including only transgressive and high-stand systems tracts. The sequence boundaries generally consist of erosive surfaces corresponding to superimposed subaerial exposure surfaces and transgressive ones. According to the available biostratigraphic data, the depositional sequences of Cherahil Formation may be broadly correlated with the Haq's et al. TA3 2nd order global eustatic cycles.

Differential Characteristics of the Upper Ordovician‐Lower Silurian Wufeng‐Longmaxi Shale Reservoir and its Implications for Exploration and Development of Shale Gas in/around the Sichuan Basin

The Upper Ordovician Wufeng‐Lower Silurian Longmaxi shale is widely distributed in the Sichuan Basin and its periphery, which is the key stratum for marine shale gas exploration and development (E&D) in China. Based on sedimentary environment, material basis, storage space, fracability and reservoir evolution data, the reservoir characteristics of the Wufeng‐Longmaxi shale and their significance for shale gas EE&D are systematically compared and analyzed in this paper. The results show that (1) the depocenter of the Wufeng (WF)‐Longmaxi (LM) shale gradually migrates from east to west. The high‐quality shale reservoirs in the eastern Sichuan Basin are mainly siliceous shales, which are primarily distributed in the graptolite shale interval of WF2‐LM5. The high‐quality reservoirs in the southern Sichuan Basin are mainly calcareous‐siliceous and organic‐rich argillaceous shales, which are distributed in the graptolite shale interval of WF2‐LM7. (2) Deep shale gas (the burial depth >3500 m) in the Sichuan Basin has high‐ultrahigh pressure and superior physical properties. The organic‐rich siliceous, calcareous‐siliceous and organic‐rich argillaceous shales have suitable reservoir properties. The marginal area of the Sichuan Basin has a higher degree of pressure relief, which leads to the argillaceous and silty shales evolving into direct cap rocks with poor reservoir/good sealing capacity. (3) Combining shale gas exploration practices and impacts of lithofacies, depth, pressure coefficient and brittle‐ductile transition on the reservoir properties, it is concluded that the favorable depth interval of the Wufeng‐Longmaxi shale gas is 2200∼4000 m under current technical conditions. (4) Aiming at the differential reservoir properties of the Wufeng‐Longmaxi shale in the Sichuan Basin and its periphery, several suggestions for future research directions and EE&D of shale gas are formulated.

The spatial discrepancy of Miocene transgression and its corresponding channel transformation pattern: A case study of the carabobo region in the northeast Venezuela basin

The Eastern Venezuela Basin serves as a world-class petroliferous basin, with the sedimentary environment controlled by interactions between the Caribbean and South American plates. This complex tectonic movement could have an important impact on the unsynchronized sedimentary evolution process and heterogeneous marine transgression of different regions within the basin. Thus, it is important to clarify the impact of interactions between the fluviatic- and marine-deposits on the development of the sedimentary environment, the transition of different delta types, and sand-body distribution within this complicated tectonic movement. Based on interpretation of 3D seismic data, electrical well-logging facies and analysis of the sedimentary characteristics of the core, we were able to identify the different sedimentary association types and understand the evolutionary process of the sedimentary system within the study area. We found that the differential sedimentary process in the eastern and western Carabobo block in the Eastern Venezuela Basin was caused by differential uplift and the corresponding spatial discrepancy transgression process in the Middle and Early Miocene foreland basins in this region. The flat geomorphology in the eastern Carabobo block contributed to the obvious sea level transgression process from North to South, but the uplift in the center of the western Carabobo block continuously blocked the sea level transgression. The different transgression processes led to the different sedimentary evolution processes in the eastern and western Carabobo blocks. In the eastern Carabobo block, braided river deltas (23.3–20.43 Ma) gradually changed into meandering river deltas (20.43–15.97 Ma) and finally into tide-influenced estuaries (15.97 Ma). However, in the western subsiding Carabobo block, impacted by the uplift in the center of this area, braided river deltas (23.3–15.97 Ma) in front of the uplift (adjacent to the sea) quickly changed into tide-influenced estuaries (15.97 Ma) with the generation of meandering distributary channels. However, the relatively weak transgression impacting the area behind the uplift resulted in braided channels being retained. Along with the evolution of the sedimentary environment, the tide-related deltaic distributary channel began to change from a braided channel to a meandering one. After rebuilding the paleo-hydrodynamic field, ancient geomorphology, and combining the hydrodynamic equations, we were able to quantitatively explain why sausage-shaped mouth bars occurred in the West and isolated mouth bars occurred in the East. Differential sedimentary environment evolution in eastern and western Carabobo is related to the spatial discrepancy of transgression, which resulted in the rate of change in hydrodynamic intensity, the direction of water flow confluence, and paleo-geomorphology.

Sedimentary Environment Evolution in East China's Coastal Tidal Flats: The North Jiangsu Radial Sand Ridges

Xu, M.; Meng, K.; Zhao, Y., and Zhao, L., 2019. Sedimentary environment evolution in East China's coastal tidal flats: The North Jiangsu radial sand ridges. Journal of Coastal Research, 35(3), 524–533. Coconut Creek (Florida), ISSN 0749-0208.The North Jiangsu radial sand ridges (NJRSR) are a unique sedimentary system comprising the largest silt and mud coastal tidal flat in China. Many factors influence its tidal geomorphology sensitivity when responding to the marine environment and human activities. In this study, three typical core samples were collected from sampling points near Xinyang Gang (XYG), Chuandong Gang (CDG), and Xiaoyangkou Gang (XYK). 210Pb and 137Cs radionuclide dating was used to estimate the sedimentation rate of the tidal flat. On the basis of combined sediment grain size and reclamation history over the last 50 years, the evolution of the NJRSR coastal tidal flat sedimentary environment under the impact of human activities and the changing coastal environment was examine. The result indicated that the predominant type of sedimentary deposition is sandy silt; however, sediment composition varies because of coastal reclamation activities extending into the sea and with changes in sediment source and depositional forces. Average sedimentation rates of the three core samples were 1.27, 1.85, and 2.43–3.3 cm/y, respectively, reflecting increasing sedimentation rates from north to south. However, sedimentation rates over the past century have gradually declined in core XYG because of a reduced supply of materials from northern sources. Moreover, sand content has increased in the north since the 1980s, and grain size has become coarser. In the middle sample, a sudden change in sediment composition occurred in the 1950s–60s, exhibited by a significant reduction in sand content but an increase in silt content and finer grain sizes. In the south, grain size coarsened initially before becoming finer, likely because of a reduction of eroded material from the Yellow River in the north, increased coastal reclamation activities, and changes in sedimentary dynamics.

Distribution of biogenic silica in seafloor sediments on the East China Sea inner shelf: Seasonal variations and typhoon impact

The content and distribution of biogenic silica (BSi) in sediments, as effective proxies for marine paleo-productivity and sedimentary environment evolution studies, has significant seasonal and event-based differences. Based on measurements of BSi content in the seafloor sediments acquired during three surveys in winter and before and after typhoons in summer in the Fujian-Zhejiang coastal mud deposition center (MDC-ZFC), respectively, the seasonal and event-based (typhoon process) differences of BSi in seafloor sediments are discussed by combining with data of the sediments grain sizes and the abundances of dominant diatom species. Conceptual models of the modern sedimentary environment (including BSi) are summarized. The study results show that the BSi in the seafloor sediments in the study region is relatively low, generally less than 1%. Generally, the BSi in the winter seafloor sediments is lower than that under normal summer sea conditions and the BSi in the sediments after the summer typhoons is higher than that under normal summer sea conditions. The differences in the origin of biogenic siliceous materials (seasonal growth of siliceous organisms) and the marine hydrodynamic environment (such as seafloor sediment resuspension and elutriation caused by strong marine hydrodynamic processes) are the major factors for different relative BSi content. The seasonal and event-based variations of BSi have important significance in paleo-oceanography and event sedimentology studies.

Interrelationship Between Facies Association, Diagenetic Alteration and Reservoir Properties Evolution in the Middle Miocene Carbonate Build Up, Central Luconia, Offshore Sarawak, Malaysia

Carbonate build-ups are proven prolific hydrocarbon reservoirs located around the globe. In Southeast Asia, the carbonates are usually associated with grain-rich facies types. Understanding the carbonate reservoirs in terms of their internal characteristics, depositional environment, diagenesis and their impact on petrophysical properties are essential parts of successful exploration and production strategy. The present study provides a detailed work on carbonate reservoir facies, diagenetic history and reservoir quality including qualitative and quantitative analysis of thin sections of the Miocene carbonate platform, Central Luconia. The Central Luconia carbonates are subdivided into eight lithofacies based on specimens collected from five wells. Detailed lithofacies analysis and correlation of carbonate rocks from Well A and Well B allowed us to reconstruct the evolution of sedimentary environments and relative sea-level changes in a platform during the Early to Middle Miocene of Cycle IV and V. Five lithofacies are identified based on texture, sedimentary structure, grain composition, and fossil contents. These include (1) coated grain to packstone (av. O = 6%, av. Kh = 1 mD), (2) coral (m) lime grainstone (av. O = 14.7%, av. Kh = 6 mD), (3) oncolite lime grain to packstone (av. O = 10%, av. Kh = 4 mD), (4) skeletal lime packstone (av. O = 15%, av. Kh = 4.6 mD), and (5) coral (p) lime mud to packstone (av. O = 4%, av. Kh = 0.5 mD), respectively. Micritization, cementation, fractures, compaction, and dissolution are the dominant diagenetic parameters which are identified. The diagenetic features include dolomitization and dissolution (i.e., leaching) making Well A and Well B unique location to study facies distribution and its implication on reservoir potential. By integrating these parameters, facies 2, 3 and 4 are interpreted as a relatively good quality reservoir, whereas facies 1 and 5 are considered as poor reservoir quality. Among all the diagenetic features, dissolution phenomenon contributes to porosity enhancement and reservoir quality, whereas micritization, compaction and cementation have a negative impact on reservoir quality.

Sedimentary environment and depositional evolution of the Mesoproterozoic Bingmagou Formation on the southern margin of the North China Craton

The Precambrian sedimentary strata on the southern margin of the North China Craton are well developed and widely exposed, making the region ideal for the study of depositional processes. However, because of the length of the depositional history and the lack of biogenic criteria, interpretations of the sedimentary environments of the Precambrian strata are often based on the tectonic background, geographical environment, rock type and sedimentary structures, resulting in controversies in the literature. In this study of the Bingmagou Formation in the Ruyang Group on the southern margin of the North China Craton, analysis of petrologic features, palaeocurrents and sedimentary facies is combined with regional correlation of relevant strata and the reconstruction of ancient landforms to explain the depositional environments and environmental transitions. Dominated by marine deposits on the southern margin of the North China Craton, the sedimentary strata of the Ruyang Group unconformably overlie the Archean crystalline basement or Proterozoic Xionger Group. As the lowermost unit of the Ruyang Group, the Bingmagou Formation, which was depositionally controlled by topography and faults and received abundant detrital material, is a highly distinctive set of sedimentary strata and represents an environmental transition from alluvial fan to sandy coast.

Effect of aqueous Si/Mg ratio and pH on the nucleation and growth of sepiolite at 25 °C

Sepiolite [Mg4Si6O15(OH)2·6H2O] is a trioctahedral 2:1 Mg-silicate that has been often used to reconstruct the evolution of sedimentary environments and facies in the geological record. To date, however, the reaction paths underlying sepiolite formation are poorly constrained and most of the existing models are based on empirical observations. In order to shed light on the mechanisms controlling the formation of this mineral phase, in the present study, sepiolite was precipitated at 25 ± 1 °C from modified seawater and MgCl2 solutions undersaturated with respect to brucite and amorphous silica. Although a suite of hydrous Mg-silicates, such as kerolite, saponite, stevensite and talc, were oversaturated in the solutions at a higher level relative to sepiolite at any time of reaction, poorly crystallized, aluminous sepiolite was the only precipitate after 91 days. The precipitated sepiolite [Mg3.4-3.8Al0.1-0.4)∑3.8-3.9(Si5.9-6.0Al0-0.1)O15(OH)2·nH2O] shares a number of structural and chemical similarities with natural sepiolite, such as a fibrous crystal shape and an atomic Si/(Si + Mg+Al) ratio of ∼0.61.The proposed reaction path for the formation of sepiolite is based on the temporal evolution of the chemical compositions of the experimental solution and solids: (i) Nucleation and growth of Al-sepiolite occurred during the first 8 days of the experimental runs via condensation and polymerization of SiOH tetrahedra onto Mg–Al–O–OH template sheets at a precipitation rate of ∼2.19 ± 0.01 × 10−10 mol s−1. (ii) At decreasing pH and in the absence of [Al]aq this intermediate phase transformed into aluminous sepiolite at a slower crystal growth rate of ∼1.08 ± 0.02 × 10−12 mol s−1. This finding explains the high abundances of sepiolite in highly alkaline, evaporitic, lacustrine and soil environments, where the growth rates of sepiolite are considered faster (10−11 to 10−10 mol s−1, Brady, 1992). We propose that (i) low rates of Mg2+ ion dehydration and silica condensation and polymerization at the surface of the initial precipitate, (ii) the formation of MgS040 aquo-complexes and (iii) the reduced sorption rates of [Si]aq and [Mg]aq at the active growth sites on sepiolite surfaces at pH ≤ 8.3 retard the precipitation of sepiolite in marine-diagenetic environments.

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.

Porosity and reservoir potentiality of the Cherahil Formation limestone (middle-upper Eocene) in the Gulf of Gabes (Tunisia)

During the middle and upper Eocene, the deposits in the Gulf of Gabes correspond to the Cherahil Formation, which is sub-divided into three units, which are as follows from base to top: the Lower Cherahil A, the Siouf and the Upper Cherahil B members. The Siouf member has a lateral equivalent in the Souar Formation named Reineche member.The Cherahil Formation has never been considered by oil companies as a particular drilling target in the Gulf of Gabes (offshore east Tunisia) despite the presence of hydrocarbon at the bottom of Cherahil Formation in Sidi Behara and Sidi Litayem oil fields in Sfax Area (onshore east Tunisia) and in its equivalent carbonate beds in Jebel Trozza (Central Tunisia). Therefore, the evaluation of porosity in the carbonate levels of Cherahil Formation in 20 drilling wells were performed on well logging by applying Wyllie method. The obtained results show that the studied carbonates are characterized by an economically important total porosity average ranging between 5% and 55%, and both vertical and lateral variations. The vertical porosity variation was controlled by the sea-level fluctuation that, in turn, controlled the evolution of carbonate sedimentary environments and relative facies. The lateral porosity variation followed the Tunisian middle-upper Eocene paleogeography changes controlled by NW-SE synsedimentary tectonic trends. Considering the important features of the Cherahil Formation and the coexistence of components of an oil system in the Gulf of Gabes, this formation can be an important potential reservoir and subsequently a new petroleum exploration target in the Gulf of Gabes.