Depositional Events Research Articles

Understanding stream bank erosion and deposition in Iowa, USA: A seven year study along streams in different regions with different riparian land-uses

Agricultural activities such as row-cropping and grazing, have accelerated stream bank erosion. Accelerated stream bank erosion increases nonpoint source pollutants in aquatic ecosystems, significantly degrading them. Mitigating stream bank erosion is a priority worldwide, especially in agricultural watersheds. The objective of this study was to analyze the impacts of riparian land-use management on stream bank erosion and deposition, along with analyzing its temporal and spatial patterns. The study was conducted in three regions of Iowa (central, northeast and southeast) along 30 stream reaches adjacent to seven different riparian land-uses. The riparian land-uses were riparian forest buffers, grass filters, pastures with the cattle excluded from the stream, intensive rotational grazing, rotational grazing, continuous grazing and row crop fields. Seasonal erosion and deposition data (Spring, Summer and Autumn) were collected along these reaches for 5 years and yearly for the following two years. To analyze the data, conventional statistical methods (ANOVA and Tukey's test) along with innovative techniques (percentile plots, cumulative erosion curves and bubble charts) were utilized. Based on the analysis, of this extensive in time (seven years) and large in size (1500 pins measured 17 times in three regions) field dataset, major results were obtained in regard to stream bank erosion in Iowa, USA. Stream banks exhibited high year-to-year variation in erosion and deposition showcasing the need for long-term datasets to better understand stream bank erosion and deposition. Seasonal erosion, also had high variability with Spring recording the most erosion followed by Summer and Autumn. Certain seasons exhibited high stream bank erosion indicating that managers need to focus on these seasons, to reduce erosion effectively. In addition, seasonal measurements can highlight depositional events that might be masked with annual measurements. Riparian land-uses significantly impacted stream bank erosion. Riparian forest buffers and grass filters significantly mitigated stream bank erosion while traditional agricultural practices like continuous grazing and row-crop agriculture had accelerated stream bank erosion. Finally, the percentile plots, cumulative erosion curves and bubble charts captured some stream bank responses that would have been unnoticed using conventional statistical methods, allowing decision makers, stakeholders and the general public, to support and approve measures to mitigate this environmental problem. Nature-based solutions utilizing riparian perennial vegetation can sustainably mitigate stream bank erosion.

Grain-Size Analysis of Hurricane-Induced Event Beds in a New England Salt Marsh, Massachusetts, USA

Castagno, K.A.; Donnelly, J.P., and Woodruff, J.D., 2021. Grain-size analysis of hurricane-induced event beds in a New England salt marsh, Massachusetts, USA. Journal of Coastal Research, 37(2), 326–335. Coconut Creek (Florida), ISSN 0749-0208.Tropical cyclones pose a growing threat to coastal infrastructure and livelihood. Because instrumental and historic records are too short to help us understand interactions between tropical cyclones and climate on a longer scale, proxy records are the only means for reconstructing millennia of tropical cyclone impacts. This study determines grain-size trends in storm-induced overwash deposits along a transect of sediment cores from a salt marsh in Mattapoisett, Massachusetts, to characterize sorting trends and compare deposits associated with individual storms. The overwash deposits preserved within the high-marsh peat provide a record spanning the last two millennia. Building on a 2010 study, a different approach was used to accurately determine the grain-size distribution of overwash deposits from cores in a transect running perpendicular to the adjacent sandy/gravely barrier. Although maximum grain-size values are expected to decrease as distance from the barrier increases, not all event deposits that were studied follow this trend within uncertainty. Analysis of the storm event beds reveal a significant difference in settling trends between historic and prehistoric deposits, with historic deposits largely displaying landward-fining trends and prehistoric deposits largely displaying landward-coarsening trends. This suggests changes in the hydrodynamic or that geomorphic regime may have altered the way in which storm beds were deposited at this site. This new in-depth, transect-based approach has utility for improving the accuracy of future storm reconstructions, particularly for events for which no historic record exists.

Debris-Flow and Debris-Flood Susceptibility Mapping for Geohazard Risk Prioritization

ABSTRACT Regional-scale assessments for debris-flow and debris-flood propagation and avulsion on fans can be challenging. Geomorphological mapping based on aerial or satellite imagery requires substantial field verification effort. Surface evidence of past events may be obfuscated by development or obscured by repeat erosion or debris inundation, and trenching may be required to record the sedimentary architecture and date past events. This paper evaluates a methodology for debris-flow and debris-flood susceptibility mapping at regional scale based on a combination of digital elevation model (DEM) metrics to identify potential debris source zones and flow propagation modeling using the Flow-R code that is calibrated through comparison to mapped alluvial fans. The DEM metrics enable semi-automated identification and preliminary, process-based classification of streams prone to debris flow and debris flood. Flow-R is a susceptibility mapping tool that models potential flow inundation based on a combination of spreading and runout algorithms considering DEM topography and empirical propagation parameters. The methodology is first evaluated at locations where debris-flow and debris-flood hazards have been previously assessed based on field mapping and detailed numerical modeling. It is then applied over a 125,000 km2 area in southern British Columbia, Canada. The motivation for the application of this methodology is that it represents an objective and repeatable approach to susceptibility mapping, which can be integrated in a debris-flow and debris-flood risk prioritization framework at regional scale to support risk management decisions.

Combining 137Cs, 210Pb and dendrochronology for improved reconstruction of erosion–sedimentation events in a loess gully system (southern Poland)

AbstractDetailed knowledge on the past events of erosion and redeposition is necessary to assess the frequency and magnitude of soil degradation and to provide more complete data, among all, to calculate rainfall thresholds and to develop strategies for controlling erosion. Therefore, in this study, we aimed to obtain a more comprehensive and accurate reconstruction of erosion–sedimentation events in a gully system by combining analyses of 137Cs and 210Pb with dendrochronological dating in the same sediment profiles. We aimed to precisely determine the number and the age of erosion–sedimentation events, which led to the development of study deposits and to determine the type of erosion providing material for deposition in particular events. To this purpose, 136 soil samples and 62 roots and tree stems samples were collected in eight research profiles at the gully bottom. We compared the results obtained with the two methods. Isotope analysis allowed us to estimate more general age of sediments and the type of erosion that provided material for individual layers. Dendrochronology provided a more precise reconstruction of the age of soil degradation events. By combining both techniques, we determined rainfall events responsible for erosion and sedimentation in the studied gully since 1980s and a rainfall threshold of 60 mm day−1 for soil erosion and redeposition in the study area.

A tsunamigenic delta collapse and its associated tsunami deposits in and around Lake Sils, Switzerland

Large lacustrine mass movements and delta collapses are increasingly being considered as potential tsunamigenic sources and therefore hazardous for the population and infrastructure along lakeshores. Although historical reports document tsunami events in several lakes in Switzerland, and although the propagation of lake tsunamis has been studied by numerical wave modeling, only little is known about on- and offshore lacustrine tsunami deposits. In Lake Sils, Switzerland, a large prehistoric mass-movement deposit originating from the Isola Delta with a minimum estimated volume of 6.5 × 106 m3 and a basinal thickness of > 6 m in the seismic record has been identified by previous studies and radiocarbon dated to around 700 Common Era. Here, we combine (i) comprehensive sedimentological investigation of sediment cores recovered from the on- and offshore settings, (ii) mineralogical fingerprinting of the inflows from key catchments to characterize sediment provenance, and (iii) numerical tsunami modeling, to test the hypothesis of a tsunamigenic delta collapse in Lake Sils. We observe a clastic event deposit consisting of coarse-grained, fining-upward sand overlying an organic-rich peat deposit in the shallow water. This layer thins and fines landward on the coastal plain. Toward the deeper water (20–40 m), the deposit transforms into a thicker and more heterogeneous sediment package with multiple sequences of fining-upward sand and a well-pronounced clay cap at the top. Radiocarbon dating of the peat underlying the event deposit yields a maximum age of 225–419 calibrated Common Era. The tsunami models, which indicate wave heights reaching up to 5 m, simulate areas of inundation that coincide with the location of event deposits. Based on our results, we propose that the historically undocumented Isola Delta collapse generated a basin-wide tsunami that inundated the lakeshore, transporting large amounts of unconsolidated sediment along the lakeshore toward the coastal plain and into the deeper lake basin.

Providing context to the Homo naledi fossils: Constraints from flowstones on the age of sediment deposits in Rising Star Cave, South Africa

Rising Star Cave in the Cradle of Humankind, South Africa, contains one of the richest hominin-bearing deposits in the world, and is the type locality for the Homo naledi fossils. This paper provides a stratigraphic and geochronological framework, within which published and future fossil finds from Rising Star Cave can be placed. Detailed mapping of flowstone horizons combined with new age constraints based on both U-Th disequilibrium and 234U/238U dates and one new OSL date help define seven periods of flowstone formation that punctuate episodes of clastic sedimentation and erosion. Clastic sediments entered the cave through an opening in the roof of the Postbox Chamber from about 600 ka onward, until the opening was choked by coarse breccia blocks, probably sometime after 180 ka. Depositional and erosional events changed the internal morphology of the cave chambers over time, and thereby changed the access route into the Dinaledi Chamber where the bulk of the H. naledi fossils were found.Periods of pervasive flowstone formation at all levels of the cave occurred at >600 ka, ~500–400 ka, ~225–190 ka and ~110-90 ka. Additional periods of localised flowstone growth restricted to individual chambers (or parts thereof) occurred at ~300 ka, ~160 ka, ~70 ka, ~50 ka, ~30 ka, and ~10 ka. Flowstone horizons bracket sedimentary units that include a variety of sediment types that changed with time. The oldest flowstones overlie lithified mud clast breccias (LMCB), which were partly eroded before they were covered by externally derived laminated orange sands (LOS) and 500–400 ka flowstones. These flowstones and sands were removed between 290 ka and 225 ka, with sediment transported to deeper parts of the cave via erosion channels characterised by massive orange sands (MOS). In this period of time, the access route into the Dinaledi Chamber may have further changed due to the collapse of the Dragon's Back block. Deposition of laminated orange-red mud (LORM) from suspension occurred between 225 and 190 ka and temporally overlaps with widespread flowstone formation around ~225 ka and ~200 ka. The LORM deposits were largely removed from the upper chambers by ~110 ka, before the youngest group of flowstones formed in the cave; some of which are still growing today.The U-Th ages from Rising Star Cave, combined with other dating constraints reveal age clusters of flowstone formation, which coincided with warmer interglacial or interstadial periods. The patterns recognised in Rising Star Cave overlap with periods of flowstone formation recognised in nearby caves in the Cradle of Humankind, thereby confirming a regional climatic control on flowstone growth in caves during the past 500 ky.The new ages further constrain the minimum age of H. naledi to ~241 ka. Thus, H. naledi entered the cave between 241 ka and 335 ka, during a glacial period, at which time clastic sediments inside the cave were undergoing erosion. H. naledi would probably have entered the cave through an access point in the roof of the Postbox Chamber and made its way along a SW trending fracture towards the Dragon's Back and Dinaledi Chambers.

U–Pb geochronology and Hf isotope systematics of detrital zircon from the late Ediacaran Kalyus Beds (East European Platform): palaeogeographic evolution of southwestern Baltica and constraints on the Ediacaran biota

A maximum depositional age (MDA) of 551.2 ± 4.2 Ma for the black argillites of the Kalyus Beds, Nagoryany Formation (East European Platform, northeastern Republic of Moldova) is reported based on the youngest cluster of detrital zircons dated by LA-ICP-MS. Their source can be linked to an arc-related volcanism that fed the tuffite layers interbedded in the basal part of the Kalyus Beds and reasonably support our inference that the reported MDA is consistently compatible with the true depositional age (TDA) of the Kalyus Beds. The volcanic event that led to ash deposition for the tuffites may be related to the active margin processes which developed adjacent to the Baltica palaeocontinent. Hf isotope values of two detrital zircons from the youngest cluster suggest mantle input, and a third detrital zircon was derived from felsic rocks that originated from re-melting of the older crustal rocks. The older detrital zircon clusters correlate well with known published ages of the different lithotectonic units from the East European Platform. The precise geochronological U–Pb ages, including the Kalyus detrital zircon age, offer outstanding data to calibrate the sedimentary and volcanic events, and the evolution of the Ediacaran biota in the Moldova-Podillya Basin, specifically the Miaohe biota biozone, for which a few newly identified taxa significantly increase its taxonomic diversity. The anoxic conditions together with the associated phosphorites that controlled the deposition of the Kalyus argillites represent a major depositional event in this area of the Baltica palaeocontinent, and can be correlated with other worldwide anoxic events during the late Ediacaran.

Plateau lake ecological response to environmental change during the last 60 years: a case study from freshwater Lake Yangzong, SW China

The effects of anthropogenic activity and climate change on lake ecosystems for nine large plateau lakes in Yunnan Province have recently received much attention; however, most studies have focused on the deepest lake sediments and ignored the lake sediments adjacent to the populated area. To evaluate the sedimentation rates and ecological changes, four 210Pb- and 137Cs-dated sediment cores were collected along the long axis of Yangzong Lake, a fault-controlled north-south strike lake located in Yunnan Plateau in July 2006 and July 2016, respectively. We combined a biological indicator (diatom assemblage) with geochemical indicators (total organic carbon (TOC), total nitrogen (TN), and total phosphorus (TP)) for core S0 in the southern part of the lake to examine the ecological changes in response to anthropogenic disturbances within the lake over the last 60 years. The results showed that the sedimentation rate in the southern part of the lake (S1) was significantly higher than that in the central (S2) and northern (S3) parts of the lake. Redundancy analysis (RDA) and Pearson correlation analysis identified significant correlations between TP and Aulacoseira granulata, Cyclotella ocellata, and Cyclotella rohomboideo-elliptica (correlation coefficients: − 0.66, − 0.63, and 0.81, respectively, p < 0.01). There was a clear shift in the diatom community from oligotrophic to eutrophic species becoming dominant after the 1990s, coinciding with the intensifying human activities around the lake. Furthermore, there were two periods of diatom absence in core S0, which was associated with two periods of rapid depositional events. These are likely associated with the intensification of industry, mining, road construction, and agriculture within the lake catchment. These findings highlight the sensitivity of freshwater ecosystems in the shallow-water area of a lake to human activities, with implications for future water quality in fault-controlled lakes.

Lineal Energy of Proton in Silicon by a Microdosimetry Simulation

Single event upset, or Single Event Effect (SEE) is increasingly important as semiconductor devices are entering into nano-meter scale. The Linear Energy Transfer (LET) concept is commonly used to estimate the rate of SEE. The SEE, however, should be related to energy deposition of each stochastic event, but not LET which is a non-stochastic quantity. Instead, microdosimetry, which uses a lineal calculation of energy lost per step for each specific track, should be used to replace LET to predict microelectronic failure from SEEs. Monte Carlo simulation is used for the demonstration, and there are several parameters needed to optimise for SEE simulation, such as the target size, physical models and scoring techniques. We also show the thickness of the sensitive volume, which also correspond to the size of a device, will change the spectra of lineal energy. With a more comprehensive Monte Carlo simulation performed in this work, we also show and explain the differences in our results and the reported results such as those from Hiemstra et al. which are commonly used in semiconductor industry for the prediction of SEE in devices.

Effects of a large-scale, natural sediment deposition event on plant cover in a Massachusetts salt marsh.

In mid-winter 2018, an unprecedented sediment deposition event occurred throughout portions of the Great Marsh in Massachusetts. Evaluation of this event in distinct marsh areas spanning three towns (Essex, Ipswich, and Newbury) revealed deposition covering 29.2 hectares with an average thickness of 30.1±2.1 mm measured shortly after deposition. While sediment deposition helps marshes survive sea level rise by building elevation, effects of such a large-scale deposition on New England marshes are unknown. This natural event provided an opportunity to study effects of large-scale sediment addition on plant cover and soil chemistry, with implications for marsh resilience. Sediment thickness did not differ significantly between winter and summer, indicating sediment is not eroding or compacting. The deposited sediment at each site had similar characteristics to that of the adjacent mudflat (e.g., texture, bivalve shells), suggesting that deposited materials resulted from ice rafting from adjacent flats, a natural phenomenon noted by other authors. Vegetative cover was significantly lower in plots with rafted sediment (75.6±2.3%) than sediment-free controls (93.1±1.6%) after one growing season. When sorted by sediment thickness categories, the low thickness level (1–19 mm) had significantly greater percent cover than medium (20–39 mm) and high (40–90 mm) categories. Given that sediment accretion in the Great Marsh was found to average 2.7 mm per year, the sediment thickness documented herein represents ~11 years of sediment accretion with only a 25% reduction in plant cover, suggesting this natural sediment event will likely increase long-term marsh resilience to sea level rise.

Late Quaternary episodes of clastic sediment deposition in the Tarimba Cave, Central Brazil

The study presents three sedimentary profiles from Tarimba cave in central Brazil. The data allow constraining the late Quaternary sedimentary evolution of cave systems in the region. A multi-techniques approach was used to characterize sediment texture (grain size and circularity index), mineralogical composition (X-Ray Diffraction), chemical composition (ICP-OES) and deposition ages (Optically Stimulated Luminescence and radiocarbon). Eight sedimentary facies were identified, including a facies formed by autochthonous sediments (Guano) and seven facies formed by allochthonous siliciclastic sediments. The siliciclastic facies range from clayey to gravelly deposits that correspond to high-density gravity flows (Diamicton) and water laid traction process of varied deposition energy (Channel, Backswamp, and Slackwater). The studied deposits reflect an intense cave filing from the Middle to Late Pleistocene in the last 200 Ka. Subsequent depositional events formed three sedimentation phases: the first before 200 Ka corresponding to a larger granulometric interval, clay to gravel, identified through erosive surfaces; the second, after 197 Ka, is composed of sandy sediments and oscillations in the water flow energy. The third phase between 87 and 52 Ka (which covered the other stages) was responsible for the last fluvial deposition.

An eco-parametric method to derive sedimentation rates for coastal saltmarshes

The saltmarsh plant Spartina alterniflora was introduced to the Jiangsu coasts, China and serves as an ecological engineer to reduce near-bed shear stress, trap fine-grained sediments and protect the coast from wave-induced erosion. The saltmarshes thus could record the Spartina colonization-driven changes within the sedimentary layers. Based on these ecological and sedimentological changes in sediments, we present a new eco-parametric method to estimate the sedimentation rate for the newly-formed wetlands in the Yancheng Wetland Nature Reserve for Rare Birds, Jiangsu. Sediment cores and satellite imagery were used to identify the thickness of accumulated sediment layers and the time since the Spartina colonization. We defined the original ground on which Spartina alterniflora initially colonized using pigment concentrations, grain size and stable carbon isotopic compositions of organic matter (δ13C) in sediments. We also determined the time mark of the Spartina colonization by examining the Landsat images over 1982–2018 to discriminate the Spartina alterniflora from other native plants and geomorphological features. These two datasets yielded a sedimentation rate of 3.3 cm/yr for Core A and of 9.6 cm/yr for Core B, the latter evidenced by an increase of ~ 0.51 m in the bed level from 2008 to 2014. Combining the 210Pb dating method, we further estimated the sedimentation rate for the layers beneath the original ground, which was comparable to that of the bare flats in the Jiangsu coast. Even though this new method is only applicable to newly-formed saltmarshes, it helps identify the recent sedimentation events as well as reveal the environmental changes and the evolution of saltmarsh-bare flat systems due to the interplay between vegetation, hydrodynamics and sediment dynamics. It thus could be an efficient and cost-effective tool for an improved understanding of the response of coastal wetlands to a changing climate/environment.

Great earthquake at 7.3 ka inferred from tsunami deposits in the Sukumo Bay area, Southwestern Japan

AbstractTsunami deposits in Kyushu Island, Southwestern Japan, have been attributed to the 7.3 ka Kikai caldera eruption, but their origin has not been confirmed. We analyzed an 83‐cm‐thick Holocene event deposit in the SKM core, obtained from incised valley fill in the coastal lowlands near Sukumo Bay, Southwestern Shikoku Island. We confirmed that the event deposit contains K‐Ah volcanic ash from the 7.3 ka eruption. The base of the event deposit erodes the underlying inner‐bay mud, and the deposit contains material from outside the local terrestrial and marine environment, including angular quartz porphyry from a small inland exposure, oyster shell debris, and a coral fragment. Benthic foraminifers and ostracods in the deposit indicate various habitats, some of which are outside Sukumo Bay. The sand matrix contains low‐silica volcanic glass from the late stage of the Kikai caldera eruption. We also documented the same glass in an event deposit in the MIK1 core, from the incised Oyodo River valley in the Miyazaki Plain on Southeastern Kyushu. These two 7.3 ka tsunami deposits join other documented examples that are widely distributed in Southwestern Japan including the Bungo Channel and Beppu Bay in Eastern Kyushu, Tachibana Bay in Western Kyushu, and Zasa Pond on the Kii Peninsula as well as around the caldera itself. The tsunami deposits near the caldera have been divided into older and younger 7.3 ka tsunami deposits, the younger ones matching the set of widespread deposits. We attribute the younger 7.3 ka tsunami deposits to a large tsunami generated by a great interplate earthquake in the Northern part of the Ryukyu Trench and (or) the Western Nankai Trough just after the late stage of the Kikai caldera eruption and the older 7.3 ka tsunami deposits to a small tsunami generated by an interplate earthquake or Kikai caldera eruption.

Quantitative and semi–quantitative analyses using a portable energy dispersive X–ray fluorescence spectrometer: Geochemical applications in fault rocks, lake sediments, and event deposits

Quantitative and semi–quantitative analyses using a portable energy dispersive X–ray fluorescence spectrometer: Geochemical applications in fault rocks, lake sediments, and event deposits

Geocognitive Technologies Improve Basin- and Petroleum-System Analyses

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 197610, “Application of Geocognitive Technologies to Basin- and Petroleum-System Analyses,” by Paolo Ruffo, Marco Piantanida, SPE, and Floriana Bergero, Eni, et al., prepared for the 2019 Abu Dhabi International Petroleum Exhibition and Conference, Abu Dhabi, 11-14 November. The paper has not been peer reviewed. Eni and IBM developed a cognitive engine exploiting a deep-learning approach to scan documents searching for basin geology concepts, extracting information about petroleum system elements (e.g., formation name; geological age; and lithology of source rocks, reservoirs, and seals), and enabling basin geologists to perform automated queries to collect all information related to a basin of interest. The cognitive engine succeeded in identifying the correct formations, lithologies, and geological ages of the petroleum systems with an accuracy in the range of 75 to 90%. Introduction While commercial databases often provide summary information about basins that can be extracted easily with queries or even interactive tools, the explorationist needs to integrate such information with more up-to-date and in-depth descriptions of structural and sedimentary events occurring in the basin, descriptions that can be found only in unstructured documents. Key information about basins can be scattered across paragraphs, tables, and image captions of hundreds of technical articles, or can be embedded within pictures. Even when exploiting a traditional search engine with the name of the desired basin, the results can be unsatisfactory: first, not all the results might be relevant; second, many different variants of the basin name are often used within publications. In the optimistic hypothesis that the subset of relevant documents is found by the search engine, all key concepts related to a basin need to be understood by the geologist by careful examination of the paper text and images. Moreover, even if the published information (structured and unstructured) on a basin is found, there are different opinions expressed by different authors, in addition to the uncertainty of the data itself (such as the age of a formation or the details of the geological evolution of the basin), so that multiple conceptual models of the basin can be drawn from the uncertain and scarce information available. Some of these conceptual models are more probable while others are less probable, but sometimes the latter happen to be economically more valuable. Therefore, recovery of all relevant information about a basin is crucial, but also important is the preservation of differing opinions about the data - what might be termed minority reports.

Late Cretaceous-Paleogene Formation of the Bekrit Syncline, Middle Atlas, Morocco: Sedimentology, Geochemistry, Palynology and Paleoenvironments

Globally, the Paleogene period experienced several events marking the Paleocene-Eocene transition which is especially to one of the hottest periods that the Earth has ever known (Paleocene-Eocene Thermal Maximum, PETM), followed by the biological crisis that affected benthic foraminifera. The Bekrit Syncline, subject of this study, shows three Formations of range from the Cretaceous to the Eocene ages (the El Koubbat, Irbzer and Bekrit-Timahdite Formations). Sedimentological, palynological and geochemical analyses of collected samples, in the Bekrit syncline, were studied to determine the paleogeographic and paleoenvironmental conditions of the Middle Atlas. The carbonate rocks were analyzed by microscopic study and geochemical analysis by X-ray Fluorescence (XF) and Inductively Coupled Plasma Spectrometry (ICP). The results obtained highlight the sedimentary events that took place during the Paleogene. Thus, the Maastrichtian is characterized by a closed euxinic marine environment shared by a more open and oxidizing environment. On the other hand, the Paleocene was a restricted marine environment with evaporitic sedimentation, while the Eocene sees the installation of a carbonate platform with a bioclastic Shoal.

Active Subaquatic Fault Segments in Lake Iznik Along the Middle Strand of the North Anatolian Fault, NW Turkey

AbstractThe seismic activity of the middle strand of the North Anatolian Fault (MNAF), Northwestern Turkey, is debated because of its quiescence during the instrumental period, in contrast to a significant historical activity documented by several chronicles over the last two millennia. Here, we focus on Lake Iznik, bordered by the MNAF, to get a new insight into its long‐term seismicity and its tectonic setting. The study of lacustrine sediment cores reveals 14 earthquake‐induced turbidites since their ages correspond to seismic events during the past two millennia. Bathymetry and high‐resolution seismic reflection data allow to describe two hitherto unknown subaquatic active fault structures (the South Boyalica and Iznik faults) that belong to the MNAF system. Sediment cores sampled on both sides of the Iznik Fault document an event deposit and a sedimentary unit vertically offset of ~40 cm interpreted as the last rupture during the 1065 CE destructive earthquake. Older events are supposed on this fault more than thousand years ago. Further studies will help to estimate the horizontal co‐seismic offset of this oblique‐slip fault and the calendar of older ruptures. The current seismic gap of thousand years on this segment greatly increases the seismic hazard in this region and must be considered in the seismic risk assessment of the NAF system.

Revisiting the Paleo Elbe Valley: Reconstruction of the Holocene, Sedimentary Development on Basis of High-Resolution Grain Size Data and Shallow Seismics

The Paleo Elbe Valley is the most prominent subsurface structure in the southern North Sea. During the Weichselian (marine isotope stage (MIS) 2), the valley traversed the exposed sea floor and drained the southern margin of the Scandinavian ice sheet. Today the valley is filled with up to 16 m thick sediments, but the responsible processes and drivers remain unknown. To unravel these processes and describe the valley’s evolution with Holocene transgression, we use shallow seismic data and vertical high-resolution grain-size core data. At the base of the western shore, supralittoral fine sands are overlain by a thin layer of clay dated to 9.8 cal. ka BP. The major sediment package consists of marine silt with internal seismic reflectors inclined in a northeastern direction, indicating a sediment transport from the southwest. The valley infill started when the western shore was flooded around 9.6 cal. ka BP and can be divided into two phases. During the first one (9.6–8.1 cal. ka BP) the sedimentation rate was highly driven by wind and waves. The second phase (8.1–5.0 cal. ka BP) was mainly tidal dominated but shows also storm event deposits in the north. Around 5.0 cal. ka BP the valley was almost filled.

Shales of Palaeo-Mesoproterozoic Vindhyan Basin, central India: insight into sedimentation dynamics of Proterozoic shelf

AbstractThe Vindhyan Supergroup represents the largest Proterozoic sedimentary basin fill in the Indian shield. In addition to some significant palaeobiological discoveries, the sedimentary sequence of the Vindhyan, particularly its argillaceous intervals, holds crucial information for our understanding of sedimentation dynamics in Proterozoic clastic shelves. Here we attempt an extensive, although not exhaustive, review of the physical characteristics of six argillaceous (shale) intervals (Arangi, Koldaha, Rampur, Bijaygarh, Rewa and Sirbu shale) from the Son valley sector, Vindhyan Basin, augmented with new observations to unravel the status of current understanding in terms of palaeo-flow dynamics, shelf sedimentation processes and dispersal pattern, depositional cyclicity and basinal tectonics. The sedimentary attributes of Vindhyan shales reveal their deposition largely in relative bathymetry fluctuating from distal shoreface or inner shelf (near to fair-weather wave base) to distal shelf below storm wave base. More often than not, the Vindhyan shelf was storm-infested and the operation of both storm-generated return flow and Coriolis-force-guided geostrophic currents are documented from different stratigraphic intervals of argillaceous successions. The thick arenaceous intervals interrupting the deposits of the Koldaha, Rewa and Sirbu shales at multiple stratigraphic levels indicate the presence of a fan delta and braided fluvial system during intermittent regressive stands of sea level or event deposition during a sea-level highstand, respectively. Based on facies pattern and flow vectors, a rift-related half-graben model is inferred for Arangi and Koldaha shale and a low-gradient stable-shelf model with well-defined energy gradient is proposed for successions from Rampur shale onwards.

Characteristics and distribution of the event deposits induced by the 2011 Tohoku-oki earthquake and tsunami offshore of Sanriku and Sendai, Japan

Deep-sea turbidite deposits are useful tools for submarine paleoseismology. It is fundamental to understand what kind of disturbance occurred owing to the earthquake and which event-induced deposits were formed in a particular area. The 2011 Tohoku-oki earthquake and tsunami were the most destructive geohazards in Japanese history. To understand their influence in the seafloor environment and to characterize the related event deposits, undisturbed sediment cores were collected using a multiple corer along two transects off Sanriku and Sendai, Japan, during July–August 2011. The deposits from the 2011 Tohoku-oki earthquake and tsunami were widely observed along the seafloor, from the outer shelf to the lower trench slope off Sanriku. Their composition indicated multiple sources for the deposited material, reflecting that the seafloor disturbance occurred across a wide area. Two types of sedimentary structures (Types 1 and 2) have been recognized in the event deposits. The Type 1 event deposit is further divided into two subtypes, 1-1 and 1-2. Subtype 1-1 is characterized by a fine-grained turbidite sequence with a sharp basal contact and a parallel and/or cross-laminated basal coarse-grained bed that is a few cm thick, suggesting that sediment particles were transported over relatively long distances by turbidity currents. Relatively short-distance transport by sediment resuspension and settling is hypothesized for subtype 1-2 deposits, which have a thin basal coarse-grained bed without internal structure, and for Type 2, which are composed of homogeneous muddy sediments above a sharp surface but without any basal coarse-grained bed. However, organic geochemical characteristics of the event deposits suggest that long-distance transport was limited even for subtype 1-1 deposits. The wide spatial distribution of the event deposits, which also are evident across a wide range of water depths, reflects the magnitude of the earthquake and size of the tsunami it triggered.