Delta Mouth Research Articles

Human‐initiated autocyclic delta failures

ABSTRACTRiver regulations have resulted in changes in the hydrology and particle budgets of fluvial systems. Since the 19th century, many rivers have been significantly modified to control flood hazards, to gain land from swamp areas for agricultural purposes, and to stabilize river‐levels and lake‐levels to facilitate navigation. These dramatic changes of the river courses have impacted the sediment budgets and grain‐size dissemination along them as well as the sediment distribution at the delta mouths in the downstream lakes, which could lead to slope instabilities. Deposits of such catastrophic lacustrine mass movements caused by delta collapses have been, for instance, observed in Lake Brienz (Switzerland), where relatively thick (0.5 to 1.3 m) and voluminous (>1 million m3) megaturbidites are stacked in the deep basin witnessing these processes. This study uses sediment cores and seismic data to reconstruct the megaturbidites' history in Lake Brienz. Data reveal that mass‐movement deposits, originating from the Aare Delta, one of the two main inflows, have mean ages of 1853, 1905, 1942 and 1996 ce and that they were unprecedented in, at least, half a millennium. The fact that the numbers of floods and earthquakes have not changed radically over this time period implies that human impact is the most likely explanation for these failure events. Therefore, the recurrent delta collapses are attributed to the focused sediment accumulation at the front of the channelized inflow in the proximal delta region, caused by the modification of the Aare River through its straightening and channelization during the late 19th century. These findings indicate that river regulation can affect delta sedimentation, leading to autocyclic delta collapses. Those collapses, in turn, can potentially generate tsunami waves, representing an additional natural hazard for shoreline communities.

EstuarIndex: an eco-geomorphological index to assess the conservation state of estuaries

The main objective of this work is to present a novel methodology to assess the environmental, ecological, and conservation status of mid-latitude estuaries by means of an index-based method, EstuarIndex. The EstuarIndex is an integration of four sub-indexes, assessing the status of the main subsystems present on estuaries: sandy environments, dunes, tidal flats, and the drainage network. Each sub-index includes variables combining three types of factors: morphosedimentary and oceanographic factors, ecological factors, and management and protection factors. The environmental status is finally evaluated in five possible classes: (1) Very low, (2) Low, (3) Medium, (4) High, and (5) Very High, according to EU requirements for classifying the conservation status of habitats. Three pilot zones along the Spanish coast have been chosen for testing the method: San Vicente de la Barquera estuary (N Spain), Guadiana estuary (SW Spain), and Ebro River delta mouth (E Spain). They present different oceanographic and geomorphological conditions. The results obtained have proven that EstuarIndex is a suitable methodology for the application to other estuaries. The systematic application of EstuarIndex on broad time scales would allow evaluating the system trends, what seems key for implementation of more realistic restoration strategies, and it has a great potential as a tool for environmental management in natural protected areas. Furthermore, it may help in the detection of the most relevant site-specific vulnerabilities for long-term sustainability in response to both natural and artificial drivers.

Lateral variations in the signature of earthquake‐generated deposits in Lake Iznik,NWTurkey

AbstractUsing lake‐sediment cores to document past seismicity requires a comprehensive understanding of possible lateral variations in depositional processes. This study aims to reveal the lateral variations in earthquake‐induced event deposits throughout Lake Iznik, a large lake located on the middle strand of the North Anatolian Fault. Based on stratigraphic, sedimentological and geochemical analyses of 14 sediment cores from two subbasins across the lake, five different types of event deposits (T1–T5) were identified and characterised. One event deposit type (T5) is restricted to a delta mouth, characterised by the occurrence of authigenic Fe‐Mn carbonates and interpreted to result from flood events. The four other types of event deposits are characterised by their synchronicity between cores and their age consistency with historical earthquakes and are interpreted to be likely generated by earthquakes. The locally prominent 1065 CE historical earthquake that ruptured the sub‐lacustrine Iznik Fault produced at least three different types of event deposits. One deposit type (T2) is only observed for this very local earthquake, implying that the type of event deposit might also depend on ground‐motion parameters. At the lake scale, the occurrence of various event deposits depends on the flow distance from the source of sediment destabilisations to the coring site.

Historical morphological changes (1956-2017) and future trends at the mouth of the Ebro River delta (NE Spain)

The present work focuses on the recent morphological changes recorded in the Ebro River delta, by using aerial photographs in the last six decades and by analyzing changes in the main constitutive features through geomorphological maps. Geomorphological maps of the years 1956 and 2017 are here presented. The results obtained give very valuable clues about the recent trends of the river delta mouth, which can be used to predict future coastal changes to be expected in the following decades. In this sense, by analysing changes in the surface by means of the geomorphological maps together with a shoreline analysis, a differential behavior has been described at both sides of the river mouth: the left side, El Garxal wetland, shows an accretionary trend with rates reaching +40 m/year, while the right side, San Antonio Island, shows erosive trends of more than -20 m/year. This last side also presents a surface reduction of more than 50 ha in the last 60 years. These results suggest that, approximately by the year 2050, the emerged San Antonio Island may disappear if shoreline retreat trends are maintained, making El Garxal exposed to easterly storms, the main erosive dynamic processes in this zone and, therefore changing the entire configuration of the Ebro River delta mouth in the upcoming years. Despite possible solutions have been described in recent works, they may not contribute to a total recovery of the most natural part of the Ebro Delta. Urgent management plans are required to attempting to slow or reverse these trends, otherwise one of the most valuable ecosystems of the delta could disappear.

Bangladesh's vulnerability to cyclonic coastal flooding

Abstract. In the Ganges–Brahmaputra–Meghna delta, covering most of Bangladesh, more than 165 million people live in low-lying coasts facing major extreme climatic events, such as cyclones. This article reviews the current scientific literature publications (2007–2020) in order to define vulnerability in the context of coastal Bangladesh facing cyclonic flooding. Based on this review, a new metric, called the socio-spatial vulnerability index (SSVI), is defined as function of both the probability of the cyclonic flood hazard and the sensitivity of delta inhabitants. The main result shows that the districts of Shariatpur, Chandpur and Barisal situated in the tidal floodplain of the Ganges–Brahmaputra–Meghna delta are in the fourth quartile, i.e., highest category, the most vulnerable areas. These districts are very densely populated (from 870 up to 1400 inhabitants per square kilometer) and exposed to inundation hazards with a large number of vulnerability factors. Finally, the delta's mouth was identified as a very vulnerable area to cyclonic flooding as well.

Storm Surge and Tidal Dissipation in Deltaic Wetlands Bordering a Main Channel

AbstractDeltas are complex systems where tidal and riverine signals interact with each other. In river‐dominated deltas, where river discharge is sufficiently high, tidal amplitude is attenuated and distorted. Here we use wavelet analysis to examine the water level signal in stations located in wetlands adjacent to the main river outlet in the Wax Lake system, part of the Mississippi River Delta in Louisiana, USA. We study the signal attenuation in lateral wetlands at different frequency bands, following the propagation of the water level signal from the Gulf of Mexico to the innermost stations. During high river discharge, the astronomical tide measured inside the wetlands is reduced of 90%–98% in comparison to the tide in Atchafalaya Bay. Storm surge events, largely occurring at lower temporal frequencies, propagate conserving their energy once the signal enters the delta mouth. The river discharge signal, mostly present at frequencies lower than 1.59 μHz, is felt depending on river discharge conditions and the station position within the wetlands. Our results suggest that lateral wetlands in the Wax Lake system act as a low pass filter, attenuating tidal components but not the low frequency components of storm surges.

Multi-decadal dynamics of the Saloum River delta mouth in climate change context

The coast of Senegal is subject to climate variability that affects coastal dynamics. The Sangomar spit bounding the Saloum River delta provides a striking example. The spit dynamics has been characterized by complex changes involving both erosion and accumulation. Using a methodological approach combining remote sensing and data analysis in a Geographic Information System (GIS) we calculated a mean annual erosion rate of −3.55 m of the spit shoreline from 1954 to 1987. In 1987 the spit was breached by storm in its most eroded sector (- 4.59 m/year). This breach widened significantly to attain 5.25 km in 2018 and has now become the Saloum River mouth. The breach has resulted in exacerbated erosion of adjacent shorelines, especially at the down-drift coast, where the retreat attains a peak of −20.16 m/year, the average erosion rate remaining at −3.56 m/year for the entire coast. Extension of the spit tip has favoured progradation of the Sangomar Pointe which has lengthened by 5.74 km between 1954 and 2018, diverting gradually the Saloum River former mouth towards the South, and corresponding to a 5,185,009 m3 sedimentation. Extrapolation of shoreline mobility reveals high erosion rates due to sea-level rise by 2050 and 2100.

Melhorias na proposta do sistema de codificação de bacias hidrográfica de Pfafstetter

ABSTRACT The coding basin system proposed by Pfafstetter (1989) is an important reference point and adopted by the principal water resources management system in the world. This adoption is due to the method’s simplicity and the topological relationship between the river stretches built-into the codes. Otherwise, the Pfafstetter basin coding system can only be applied in a hydrographic vector represented by an anti-arborescence binary graph. This type of representation causes loss of hydrographic information due to the simplification of regions where there are anabranching, braided or delta areas that implies multiple confluences, cycles or disruptions of the network. This paper proposes improvements to the Pfafstetter basin coding system, maintaining it simple, while keeping the topological relationship between the stretches and including the possibility to represent the hydrography with multiple confluences, cycles, delta mouths, sinks, water masses and disruptions to drainage.

Multidecadal biogeomorphic dynamics of a deltaic mangrove forest in Costa Rica

Térraba-Sierpe National Wetland (TSNW) is the largest wetland in Costa Rica located in the southeastern part of the country. The protected area comprises the Térraba-Sierpe delta covered with dense mangrove vegetation. We aim to analyze the geomorphological evolution of the coastal bars and the land use/cover change between 1948 and 2012 using aerial photographs and satellite images interpretation as well as field corroboration. We determined the geomorphological dynamics of three coastal bars (supratidal, intertidal, and subtidal), ten land uses/covers, and deforestation rates of six delta mouths for 64 years. In addition, we estimated a net periodic loss by deforestation between 1948 and 2012 exceeding 2562 ha, with an annual net loss of 40 ha per year. Moreover, these outputs quantify the anthropic impact over 40 years and give good evidence from an example of environmental policies implementation since the 1990s to protect these fragile tropical ecosystems.

Clay mineralogy and lithogeochemistry of lutites from the Lower Cretaceous Crato Member, Araripe Basin, NE Brazil: Implications for paleoenvironmental, paleoclimatic and provenance reconstructions

The Aptian Crato Member of the Araripe Basin is a 50-m-thick succession of laminated limestones alternated with siliciclastic rocks. This unit has been receiving attention worldwide mainly because it is one of the most famous fossil Konservat-Lagerstätten, where were identified well-preserved paleofauna and paleoflora. Clay mineralogy and lithogeochemical data have been widely used in paleoenvironmental, paleoclimatic and provenance reconstructions. However, these techniques were not systematically used to study the Crato Member rocks. This study reports the mineralogical and lithogeochemical characterization of the Crato Member lutites looking for paleoenvironmental, paleoclimatic, and provenance reconstructions. Facies analysis, X-ray diffractometry, lithogeochemical, and scanning electron microscopy studies were carried out on thirteen Crato Member lutite samples in the Três Irmãos quarry, near Nova Olinda County. Regional arid paleoclimate conditions were identified using clay mineral assemblages and two paleoenvironmental models with subtle paleoprecipitation and provenance differences were interpreted. In the lower Três Irmãos quarry, a tidal-dominated delta mouth bar facies succession coincident with a clay mineral association with lesser kaolinite contents was identified, which indicates drier conditions. Lithogeochemical data suggests provenance from Archean felsic rocks. In the upper Três Irmãos quarry, a marginal paleolagoon facies succession coincident with a clay mineral association with higher kaolinite contents was recognized, which indicates wetter conditions. Lithogeochemical data indicate provenance from mixed Post-Archean to Archean felsic-mafic rocks. The paleolagoon water was interpreted as brackish and sudden paleosalinity increases indicated seawater inputs, which were also responsible for raising the paleolagoon water table in the marginal paleolagoon facies succession, which presented paleoenvironmental proxies suggesting a more expressive marine character.

Stratigraphic architecture of climate influenced hyperpycnal mouth bars

AbstractInvestigation of the process sedimentology of mouth bar deposition demonstrates a link to the seasonality associated with abrupt climate warming events during the early Eocene. While a relationship between fluvial sedimentology and climate events has been established previously, such documentation within mouth bar deposits is missing. Deltaic mouth bars of the Douglas Creek Member of the Green River Formation in the Uinta Basin record a fluvial–lacustrine response to abrupt warming events associated with the Early Eocene Climatic Optimum. This study ties the observed sedimentology of these mouth bars and their deposition via hyperpycnal currents to these climatic events. Detailed measured sections and analysis of stratigraphic architecture of outcrops in the eastern Uinta Basin document the sedimentology and stratigraphy of delta mouth bar complexes. Internal structures include climbing ripple cross‐lamination, convolute bedding, and those associated with transitional to upper‐flow regime bedforms such as planar laminations with normal grading and convex‐upward stratifications. The dominance of upper‐flow regime bedforms as well as interpreted long run out distances suggests that density currents deposited mouth bars tens of kilometres from the shoreline under Froude supercritical flow conditions. The highly seasonal climate regime of the early Eocene, capable of producing high‐sediment yield flooding events, likely provided the mechanism for hyperpycnal current formation within a relatively freshwater lake and controlled the timing of mouth bar deposition. Discrete lacustrine carbonate‐dominated packages separate periods of episodic fluvial discharge, recording reduced fluvial deposition between warming events. The established link between the occurrence of upper‐flow regime bedforms and the timing of climate warming events suggests that these climate conditions may be important for their formation and preservation. The study has implications for expanding existing mouth bar depositional models to include climatically controlled upper‐flow regime bedforms.

Estuarine Mapping and Eco-Geomorphological Characterization for Potential Application in Conservation and Management: Three Study Cases along the Iberian Coast

Geomorphological changes in recent decades in three estuaries along the Iberian coast were analysed using aerial orthophotographs. A hierarchical classification scheme, based on a literature review representing 26 estuarine eco-geomorphological features relevant to estuarine dynamics and functioning, is described. The estuaries selected were San Vicente de la Barquera (N Spain), Guadiana River (SW border between Spain and Portugal) and the Ebro River Delta mouth (NE Spain). For these systems, a 60-year time series of high-resolution maps was developed, analysing the changes in feature surfaces. The main subsystems analysed were beach, dunes, saltmarshes and the drainage network. The results of the cartographies showed general behaviour common to all transitional systems, relationships among main subsystems and processes inherent to each one. This work illustrates how beaches and dunes serve as a protective barrier for the tidal flats, acting as a sediment buffer for the entire system. The subsystems are connected by the drainage network responsible for the exchange of matter and energy between them. Furthermore, an accuracy assessment was performed in one of the study zones to identify the limitations of mapping with aerial photographs. The results explain the changes with time but also the processes and relationships between the estuarine features at a long-term scale. This work adds an important perspective towards a general understanding of their dependence on intrinsic and boundary conditions.

Seeking enlightenment of fluvial sediment pathways by optically stimulated luminescence signal bleaching of river sediments and deltaic deposits

Abstract. Reconstructing sediment pathways in fluvial and deltaic systems beyond instrumental records is challenging due to a lack of suitable methods. Here we explore the potential of luminescence methods for such purposes, focusing on bleaching of the optically stimulated luminescence (OSL) signal of quartz sediments in a large fluviodeltaic system across time and space. We approach this by comparing residual doses of sand and silt from the modern Mississippi River channel with estimated residual doses of sand isolated from Late Holocene Mississippi Delta mouth bar and overbank deposits. Further insight is obtained from a comparison of burial ages of paired quartz sand and silt of Mississippi Delta overbank deposits. In contrast to some previous investigations, we find that the bleaching of the OSL signal is at least as likely for finer sediment as for coarser sediment of the meandering Mississippi River and its delta. We attribute this to the differences in light exposure related to transport mode (bedload vs. suspended load). In addition, we find an unexpected spatiotemporal pattern in OSL bleaching of mouth bar sand deposits. We suggest this may be caused by changes in upstream pathways of the meandering channel belt(s) within the alluvial valley or by distributary channel and coastal dynamics within the delta. Our study demonstrates that the degree of OSL signal bleaching of sand in a large delta can be highly time- and/or space-dependent. Silt is shown to be generally sufficiently bleached in both the modern Mississippi River and associated paleo-deposits regardless of age, and silt may therefore provide a viable option for obtaining OSL chronologies in megadeltas. Our work contributes to initiatives to use luminescence signals to fingerprint sediment pathways within river channel networks and their deltas and also helps inform luminescence dating approaches in fluviodeltaic environments.

Sediment Transport and Morphological Changes at the Mouth Delta of Shirakawa River

Intertidal mudflats are important for land conservation, flooding risk alleviation and biological environment. Mudflat profiles have been measuring monthly along some cross-shore lines exceeding 1,000 m in the vicinity of a river mouth since December 2000. For the mud profile evolution, the annual accretion rate was 4.2 cm/year and 1.6 cm/year on the right and left lines, the seasonal variation in the range -5 cm and 5 cm, and the maximum episodic variation 12.1 cm and 22.0 cm. The wind waves could play many contributions for deposition and erosion of sediment. Sediment budgets based on the monthly bed level and net sediment flux monitoring were estimated to examine the relative contributions of tides and river discharge to sediment transport. The estimated sediment budgets are interpreted using the water mass balance equation and the horizontal tidal current pattern. The intertidal flat accreted during normal discharge conditions are primarily attributable to the alongshore sediment flux toward the river mouth. However, the flat was eroded when the large offshore suspended sediment transport occurred on the flat during the large river discharge. The net alongshore tidal current causes alongshore sediment fluxes toward the river mouth on this intertidal flat adjacent to the river mouth. In observation periods, there were two times of a significant flood occurred on July 12, 2012 with the discharge water rates of 2,300 m3/s and on June 21, 2016 with 1,600 m3/s. Also two earthquakes scaled of 7 on the Japanese scale struck Kumamoto Prefecture on April 14 and 16, 2016, and then about the 0.4 m ground subside was occurred at the present field site. A large amount of sediment from Shirakawa River was discharged into the delta due to the flood. The clinoform along the water route approximately propagated 150 m in the offshore direction and the bed level increased by 1.5 m. The estimated sediment discharge was 1.1x105 m3/yr./km2 during 2014 to 2016, corresponds four times during 1978 to 1997.

Coastal habitat and biological community response to dam removal on the Elwha River

AbstractHabitat diversity and heterogeneity play a fundamental role in structuring ecological communities. Dam emplacement and removal can fundamentally alter habitat characteristics, which, in turn, can affect associated biological communities. Beginning in the early 1900s, the Elwha and Glines Canyon dams in Washington, USA, withheld an estimated 30 million Mg of sediment from river, coastal, and nearshore habitats. During the staged removal of these dams, the largest dam removal project in history, over 14 million Mg of sediment were released from the former reservoirs. Our interdisciplinary study in coastal habitats, the first of its kind, shows how the physical changes to the river delta and estuary habitats during dam removal were linked to responses in biological communities. Sediment released during dam removal resulted in over a meter of sedimentation in the estuary and over 400 m of expansion of the river mouth delta landform. These changes increased the amount of supratidal and intertidal habitat, but also reduced the influx of seawater into the pre‐removal estuary complex. The effects of these geomorphic and hydrologic changes cascaded to biological systems, reducing the abundance of macroinvertebrates and fish in the estuary and shifting community composition from brackish to freshwater‐dominated species. Vegetation did not significantly change on the delta, but pioneer vegetation increased during dam removal, coinciding with the addition of newly available habitat. Understanding how coastal habitats respond to large‐scale human stressors, and in some cases the removal of those stressors, is increasingly important as human uses and restoration activities increase in these habitats.

Impact of depositional environment and diagenesis on the Upper Triassic Xujiahe tight-sand reservoir in Guang’an area, Central Sichuan Basin, SW China

The Upper Triassic Xujiahe tight-sand reservoir is characterized by low porosity and ultra-low permeability as well as intense heterogeneity. Investigation on depositional facies and their impact on reservoir quality has been proven to be a powerful tool to predict high-quality reservoirs. This study attempted to explain impacts of depositional environment and diagenesis on Xujiahe tight-sand reservoir quality in Guang’an area by: (1) identifying petrofacies and wireline logs patterns and analyzing the spatial variation of sedimentary facies, (2) determining diagenesis through detrital components, rock properties and their spatially variation as well. The results suggest that (1) T3x2, T3x4 and T3x6 were mainly deposited in delta system, including braided river in delta plain, delta distributary channels, underwater distributary channel, and mouth bar, etc., (2) the Upper Triassic Xujiahe Formation in the Guang’an area is typical tight-sand reservoir with porosity of 0.68–15.94% and permeability of 0.001–274 mD. Poor reservoir quality is attributed to intense compaction and dissolution during middle diagenetic stage A2, which are dominated by feldspathic litharenite and lithic sandstone. (3) Potential high-quality reservoirs in T3x2, T3x4 and T3x6 are associated with delta plain distributary channel, delta front underwater distributary channel and mouth bar, since high-energy delta channel microfacies typically resulted in a large number of primary pores in sand body. Considerable secondary pores were resulted from organic acid derived from source rocks during middle diagenetic stage, improving reservoir quality significantly.

Record of continental to marine transition from the Mesoproterozoic Ampani basin, Central India: An exercise of process-based sedimentology in a structurally deformed basin

The Mesoproterozoic Ampani Group of rocks, a structurally deformed sedimentary package hosted within the Bastar Craton in central India, was studied for process-based facies and paleoenvironmental analyses. Outcrop mapping on 1:1500 scale, deconvolution of deformation pattern, and process-based facies analyses have led to the identification of fifteen facies types, clubbed under four facies associations. A range of paleoenvironmental settings varying from continental fluvial to distal marine shelf is inferred. Deductive paleohydrology revealed poorly-efficient 'dirty river' character for the Ampani River system with low water discharge. However, at times of catastrophic sheet floods release of sediments trapped at the river mouth in form of hyperpycnal underflows triggered formation of river mouth delta. Reworking of delta front sediment in wave-dominated coastline resulted development of beach-foreshore and shoreface (proximal to distal). Variation in the relative proportion of bar and interbar products within the shoreface successions exposed at different studied sections is interpreted as signature of relative bathymetric variation. The pro-deltaic Ampani shelf was storm infested.Tectonic perturbance in the basin hinterland in course of Ampani sedimentation is inferred from occurrence of a disparately thick lobate high-density flow deposit towards the top of shoreface succession and increase in feldspar content upward within the shoreface succession. Addition of detritus from a ∼1600Ma Mesoproterozoic provenance in upper part of shoreface also strengthen the contention. Deconvolution of deformation pattern and delineation of environmental products ranging between continental and deep marine allowed us to infer the Ampani sediment package as fining-upward in character evolved in a transgressive mode.

Deltaic coastline of the Siwalik (Neogene) foreland basin: evidences from the Gish River section, Darjeeling Himalaya

This paper presents the sedimentological analysis of about 1.3 km succession of Neogene Siwalik Group exposed in the Gish River section, Darjeeling foothills. Contrary to the existing alluvial fan‐braided stream depositional model, the facies analysis shows that the succession accumulated in a shallow marine deltaic setting. Nine facies identified represent deposition from various processes related to unidirectional and oscillatory current, suspension settlement in quiet water and also to processes related to soil formation. Abundant trace fossils of marine affinity occur throughout the succession and include Cylindrichnus, Rosselia, Teichichnus, Rhizocorallium, Chondrites and Zoophycos. The studied succession has been subdivided into seven facies associations that can be interpreted in terms of the different sub‐environments of a river‐dominated delta. The lower ~550 m of the succession comprises sand–mud alternation of delta front, delta mouth and delta plain deposits (FA3, 4, 5 and 6), organized in stacked, 10–30 m coarsening‐upward units, and is inferred to represent progradation of the delta lobes. The overlying 300 m mud‐dominated interval is dominated by prodelta to open marine bay‐fill succession (FA1 and 2); the pebbly sandstones and conglomerates of the uppermost ~500 m represent a braidplain delta environment. A kilometre‐scale coarsening‐upward trend of the Siwalik deposits of this section is attributable to southward propagation of the thrust front related to the activity of the Main Boundary Thrust (MBT). The sedimentological analysis shows that a marine embayment existed in the Eastern Himalaya during the Siwalik time and the deltaic succession reported here provides the link between the upland transverse drainages recorded from Western Himalaya and the deep marine Bengal Fan succession. Copyright © 2017 John Wiley & Sons, Ltd.

Features and genesis of Paleogene high-quality reservoirs in lacustrine mixed siliciclastic\u2013carbonate sediments, central Bohai Sea, China

The characteristics and formation mechanisms of the mixed siliciclastic–carbonate reservoirs of the Paleogene Shahejie Formation in the central Bohai Sea were examined based on polarized light microscopy and scanning electron microscopy observations, X-ray diffractometry, carbon and oxygen stable isotope geochemistry, and integrated fluid inclusion analysis. High-quality reservoirs are mainly distributed in Type I and Type II mixed siliciclastic–carbonate sediments, and the dominant pore types include residual primary intergranular pores and intrafossil pores, feldspar dissolution pores mainly developed in Type II sediments. Type I mixed sediments are characterized by precipitation of early pore-lining dolomite, relatively weak mechanical compaction during deep burial, and the occurrence of abundant oil inclusions in high-quality reservoirs. Microfacies played a critical role in the formation of the mixed reservoirs, and high-quality reservoirs are commonly found in high-energy environments, such as fan delta underwater distributary channels, mouth bars, and submarine uplift beach bars. Abundant intrafossil pores were formed by bioclastic decay, and secondary pores due to feldspar dissolution further enhance reservoir porosity. Mechanical compaction was inhibited by the precipitation of pore-lining dolomite formed during early stage, and oil emplacement has further led to the preservation of good reservoir quality.

Conceptualizing delta forms and processes in Arctic coastal environments

AbstractClimate warming in the Arctic directly causes two opposite changes in Arctic coastal systems: increased melt‐water discharge through rivers induces extra influx of sediments and extended open water season increases wave impact which reworks and erodes the shores. A shoreline change analysis along the southern coast of Disko Island in western Greenland was conducted with aerial photographs and satellite images from 1964, 1985, and 2012. The decadal morphologic evolution of this 85 km section showed that large parts of the coast had undergone very limited changes. However, two deltas were highly dynamic and popped up as hotspots.The Tuapaat delta and Skansen delta showed large progradation rates (1.5 and 7 m/yr) and migration of the adjacent barriers and spits. The dynamic behavior at the delta mouths was mainly caused by classic delta channel lobe switching at one delta (Tuapaat), and by a breach of the fringing spit at the other delta (Skansen). The longshore and cross‐shore transports are responsible for reworking the sediment with a result of migrating delta mouths and adjacent subaqueous mouth bars. Seaward progradation of the deltas is limited due to the steep nature of the bathymetry in Disko Bay.Finally, a schematic conceptual overview of processes and associated morphological responses for deltas in Arctic environments is presented, including the climate drivers affecting delta evolution. Copyright © 2016 John Wiley & Sons, Ltd.