Recent Sedimentary Deposits Research Articles

Is the current stress state in the Central Amazonia caused by surface water loading?

We present new fault data for the region of the Manaus, Central Amazonia, Brazil. Field measurements concentrate on the Miocene–Holocene sedimentary deposits exposed on the Amazonas River Basin, in order to investigate the development of this region in this time-interval. Two faulting events are distinguished since the Miocene. The oldest one is related to NW–SE extension during Miocene times and associated with paleoseismicity, while the younger is associated with NE–SW extension direction and seems to persist today. These two deformational events may be thereby considered Neotectonic. Moreover, the second extensional pulse with NE–SW orientation can be explained by the surface hydrological loading, which induces the Central Amazonia flexural subsidence and may promote extensional stresses in the upper crust.

Outer-rise normal fault development and influence on near-trench décollement propagation along the Japan Trench, off Tohoku

Multichannel seismic reflection lines image the subducting Pacific Plate to approximately 75 km seaward of the Japan Trench and document the incoming plate sediment, faults, and deformation front near the 2011 Tohoku earthquake epicenter. Sediment thickness of the incoming plate varies from <50 to >600 m with evidence of slumping near normal faults. We find recent sediment deposits in normal fault footwalls and topographic lows. We studied the development of two different classes of normal faults: faults that offset the igneous basement and faults restricted to the sediment section. Faults that cut the basement seaward of the Japan Trench also offset the seafloor and are therefore able to be well characterized from multiple bathymetric surveys. Images of 199 basement-cutting faults reveal an average throw of approximately 120 m and average fault spacing of approximately 2 km. Faults within the sediment column are poorly documented and exhibit offsets of approximately 20 m, with densely spaced populations near the trench axis. Regional seismic lines show lateral variations in location of the Japan Trench deformation front throughout the region, documenting the incoming plate’s influence on the deformation front’s location. Where horst blocks are carried into the trench, seaward propagation of the deformation front is diminished compared to areas where a graben has entered the trench. We propose that the décollement’s propagation into the trench graben may be influenced by local stress changes or displacements due to subduction of active normal faults. The location and geometry of the up-dip décollement at the Japan Trench is potentially controlled by the incoming outer-rise faults.

A simple approach to define Holocene sequence stratigraphy using borehole and cone penetration test data

AbstractCone penetration testing has been widely used since the 1950s for determining the subsurface geotechnical conditions of unconsolidated sediments. This paper highlights the potential value of cone penetration testing as an aid to define the stratigraphic structure of Holocene sedimentary deposits. By calibrating cone penetration test logs with adjacent borehole logs and by utilizing all the available information produced during geotechnical surveying, stratigraphic models that accurately describe the vertical and lateral boundaries, as well as the stacking pattern, of Late Quaternary systems can be constructed. The widespread application and technical simplicity of cone penetration testing, combined with simple data interpretation via correlation with adjacent borehole logs, yield a useful and inexpensive tool for sedimentological investigations. This methodology is illustrated using data from 36 cone penetration tests and 11 boreholes on the Holocene deltaic plain of the Aliakmon River, Greece. Sedimentological and stratigraphic information from core log correlations, the spatial distribution of cone penetration test parameters, sediment grain size and per cent concentration of organic matter are utilized. The results suggest, that in sequence stratigraphic terms, the delta is divided into a lowstand systems tract composed by fluvial gravels and sands (U0) of Late Pleistocene age, as well as from red oxidized clays, and a transgressive systems tract represented by fluvial channel sands (U1), overlain by a thin transgressive sand sheet of coastal origin (U2), characterized by fining upward trends. The highstand systems tract is constituted by a variety of stratigraphic units (U3 to U7) and depositional environments, characterized by coarsening upward sequences, representing both aggradational and progradational facies, and dominated by the presence of three prograding wedges. Detailed definition of the thickness, vertical boundaries and stacking pattern of the resolved stratigraphic units, presented as a two‐dimensional stratigraphic model, demonstrates the applicability of the proposed method.

Provenance of Pliocene and recent sedimentary deposits in western Amazônia, Brazil: Consequences for the paleodrainage of the Solimões-Amazonas River

Integrated data on paleocurrents, the morphology of detrital minerals and zircon grains, chemical compositions and U–Pb geochronology, reveal that the flow of the modern Solimões-Amazonas River has changed from west to east since the Plio–Pleistocene. This finding is supported by several lines of evidence, including paleocurrent directions and detrital mineral assemblages in the Içá Formation and in recent sediments. The Içá Formation, which was most likely deposited during the Pliocene, has NE and SE paleocurrents, a high proportion of stable detrital mineral assemblages and U–Pb zircon ages that we interpreted as being derived from the Amazonian craton (e.g., the Rondonian-San Ignácio and Sunsas-Grenvillian geochronologic provinces) and neighboring provinces, including the Neoproterozoic to Cambrian Brazilian Pampean mobile belts. A small proportion is derived from the Cambrian to Silurian Famatinian continental arch. Another source is the Precambrian and Paleozoic basement from the Andes cordillera, which includes several metamorphic inliers in Colombia, Peru and Bolivia. The overlying recent deposits have different provenances and are characterized by a more variable detrital assemblage with zircon grains that are enriched in trace elements and depleted in Si and have Mesoproterozoic ages. In our interpretation, the erosion of the Iquitos Arch after deposition of the Içá Formation allowed the westward expansion of the Solimões-Amazonas system in the Plio-Pleistocene.

Recent sediment transport and deposition in the Cap-Ferret Canyon, South-East margin of Bay of Biscay

The Cap-Ferret Canyon (CFC), a major morphologic feature of the eastern margin of the Bay of Biscay, occupies a deep structural depression that opens about 60km southwest of the Gironde Estuary. Detailed depth profiles of the particle-reactive radionuclides 234Th and 210Pb in interface sediments were used to characterise the present sedimentation (bioturbation, sediment mass accumulation, and focusing) in the CFC region. Two bathymetric transects were sampled along the CFC axis and the southern adjacent margin. Particle fluxes were recorded from the nearby Landes Plateau by means of sediment traps in 2006 and 2007. This dataset provides a new and comprehensive view of particulate matter transfer in the Cap-Ferret Canyon region, through a direct comparison of the canyon with the adjacent southern margin. Radionuclide profiles (234Th and 210Pb) and mass fluxes demonstrate that significant particle dynamics occur on the SE Aquitanian margin in comparison with nearby margins. The results also suggest show three distinct areas in terms of sedimentary activity. In the upper canyon (<500m), there is little net sediment accumulation, suggesting a by-pass area. Sediment focusing is apparent at the middle canyon (500–1500m), that therefore acts as a depocenter for particles from the shelf and the upper canyon. The lower canyon (>2000m) can be considered inactive at annual or decadal scales. In contrast with the slow and continuous accumulation of relatively fresh material that characterises the middle canyon, the lower canyon receives pulses of sediment via gravity flows on longer time scales. At decadal scale, the CFC can be considered as a relatively quiescent canyon. The disconnection of the CFC from major sources of sediment delivery seems to limit its efficiency in particle transfer from coastal areas to the adjacent ocean basin.

Introduction to the special issue ‘Tracer Applications in Sediment Research’

Introduction to the special issue ‘Tracer Applications in Sediment Research’

Using Chernobyl‐derived 137Cs to document recent sediment deposition rates on the River Plava floodplain (Central European Russia)

AbstractFloodplain sedimentation is one of the most dynamic geomorphic processes within plain and lowland landscapes. There is generally a good understanding of longer term floodplain evolution, but quantitative information on overbank deposition rates for recent shorter timescales is lacking. This paper describes the application of Chernobyl‐derived 137Cs to quantify floodplain aggradation rates for the River Plava (a small river draining a severely contaminated part of the upland region of Central European Russia), based on detailed sampling of four representative floodplain study sites. Two approaches have been employed for estimating post‐Chernobyl (1986–2009) floodplain accumulation rates. The first was based simply on locating the Chernobyl fallout‐associated 137Cs peak in overbank sediment sections. The second involved quantification of the increase in the total 137Cs inventory at individual sampling points associated with the post‐Chernobyl deposition of contaminated suspended sediment. It has been shown that considerable local‐scale variability of overbank deposition rates exists, with aggradation rates on the low level floodplain (6 ± 1.2–14 ± 2.8 mm year−1) exceeding by 1.5–3 times the values for the middle level floodplain (2 ± 0.4–9 ± 1.8 mm year−1) and by 3–6 times the values for the upper level floodplain (1 ± 0.2–5 ± 1.0 mm year−1) floodplain levels. Combining these estimates with information on the areas occupied by different floodplain levels within the 54 km long valley section, derived from detailed geomorphic surveys of the selected reaches, it has been estimated that about 9700 ± 1950 t of sediment have been stored on floodplain during the 1986–2009 period. The role of floodplain storage in the overall basin sediment budget and conveyance losses within the main channel system have been evaluated. Copyright © 2012 John Wiley &amp; Sons, Ltd.

New geophysical and geological evidence for the present day southernmost active deformational front of the Rif thrust-and-fold belt and the oceanic accretionary prism of Cadiz: the Dhar Doum–Lalla Zahra fault, Northwestern Atlantic Coastal Morocco

Combining the results from field observations, high resolution 2D-electric resistivity and high resolution seismic refraction tomography, we have found new evidence for active faulting in the Dhar Doum area, northwestern Atlantic coast of Morocco. Qualitative interpretation of the inverted resistivity and seismic refraction data reveals the presence of two thrust faults that cut the topographic surface and displace the most recent sedimentary deposits. Furthermore, one of these seismically imaged faults strongly correlates with the Dhar Doum–Lalla Zahra fault. The steepening at the surface and the flattening with depth of this fault are likely to indicate an important strike-slip component associated with a major reverse component. If extended offshore to the west, the Dhar Doum fault could be a continuation of the southernmost deformational front of the Cadiz oceanic accretionary prism. Therefore, this structure is very likely to represent the actual active southern deformational front of the oceanic accretionary prism, as well as that of the Rif fold-and-thrust belt. The southward thickening of the growth strata composing the uppermost dune system and the occurrence of Plio-Quaternary conglomerates north of the Dhar Doum–Lalla Zahra fault suggest that the neotectonic activity may have started by the end of the Pliocene and have continued to the present as indicated by the folding-and-faulting of the most recent sedimentary deposits and the presence of high scarps. The identification of active faulting in the study area that may have a profound influence on seismic hazard analysis of the whole Rif foredeep basin.

Present deep-submarine canyons activity in the Bay of Biscay (NE Atlantic)

Present sedimentation in three canyons of the Bay of Biscay (Audierne, Blackmud and Capbreton) is studied by the combined analysis of cores and current meter data collected over a 7month period. At the current meter mooring locations, interface cores were collected to characterize the recent sedimentation processes. In the two canyons located in the Northern part of the Bay of Biscay (Audierne and Blackmud), there is no evidence of recent sedimentary deposits. Canyons are by-passing or erosive areas. In the southern part of the Bay of Biscay (Capbreton), recent turbidite are deposited. In the three canyons, current meters recorded energetic currents with velocities showing alternating upslope and downslope motions, and a period corresponding to the semi-diurnal component M2. These currents are supposed to be related to deep internal tides. The high speed of the current (1m/s) in Audierne and Blackmud is consistent with the lack of preservation of recent sediments on the canyon floors. In Capbreton Canyon, the magnitude of currents is less and recent turbidites are preserved. In addition to periodical current motion, small magnitude gravity event corresponding to a low-concentration turbulent surge or a high-concentration nepheloid layer initiated during a storm was recorded during the mooring period. These results suggest that deep-sea canyons in the Bay of Biscay have behaviour at present varying between by passing or erosion areas and sediment trapping. These examples suggest that sediments are moved up and down by low-energy, tide-initiated hydrodynamic events during most of the time. During higher magnitude, short-duration gravity events, the sediments are transferred down canyon towards the deep sea.

Recent sediment transport and deposition in the Lisbon–Setúbal and Cascais submarine canyons, Portuguese continental margin

Recent sediment transport and deposition in the Lisbon–Setúbal and Cascais submarine canyons, Portuguese continental margin, were investigated on the basis of water column profiles of suspended particulate matter, records of near-bottom currents and settling fluxes of particulate matter obtained with benthic landers, and analysis of surface sediments. The results show that fine-grained predominantly lithogenic sediment derived from adjacent shelf areas accumulates in the upper reaches of the canyons. Sediment deposited further down in the middle and lower reaches of the canyons is essentially similar to the hemipelagic sediment found on the adjacent continental slope, indicating that down-canyon transport of sediment from the upper to the lower canyon is limited. Tidal currents measured at various depths in the Lisbon–Setúbal Canyon appear sufficiently strong to resuspend and transport sediment, but net up-canyon flow of the bottom water may retain shelf-derived lithogenic sediment in the upper canyon. Sediment gravity flows, which in the nearby Nazaré Canyon are an effective mechanism for down-canyon sediment transport, appear rare in the Lisbon–Setúbal Canyon and probably also in the Cascais Canyon. Turbidity current events recorded in the sedimentary record of the canyons may correspond to seismic events of 1969 and 1755 AD.

Extended survival of several organisms and amino acids under simulated martian surface conditions

Recent orbital and landed missions have provided substantial evidence for ancient liquid water on the martian surface as well as evidence of more recent sedimentary deposits formed by water and/or ice. These observations raise serious questions regarding an independent origin and evolution of life on Mars. Future missions seek to identify signs of extinct martian biota in the form of biomarkers or morphological characteristics, but the inherent danger of spacecraft-borne terrestrial life makes the possibility of forward contamination a serious threat not only to the life detection experiments, but also to any extant martian ecosystem. A variety of cold and desiccation-tolerant organisms were exposed to 40 days of simulated martian surface conditions while embedded within several centimeters of regolith simulant in order to ascertain the plausibility of such organisms’ survival as a function of environmental parameters and burial depth. Relevant amino acid biomarkers associated with terrestrial life were also analyzed in order to understand the feasibility of detecting chemical evidence for previous biological activity. Results indicate that stresses due to desiccation and oxidation were the primary deterrent to organism survival, and that the effects of UV-associated damage, diurnal temperature variations, and reactive atmospheric species were minimal. Organisms with resistance to desiccation and radiation environments showed increased levels of survival after the experiment compared to organisms characterized as psychrotolerant. Amino acid analysis indicated the presence of an oxidation mechanism that migrated downward through the samples during the course of the experiment and likely represents the formation of various oxidizing species at mineral surfaces as water vapor diffused through the regolith. Current sterilization protocols may specifically select for organisms best adapted to survival at the martian surface, namely species that show tolerance to radical-induced oxidative damage and low water activity environments. Additionally, any hypothetical martian ecosystems may have evolved similar physiological traits that allow sporadic metabolism during periods of increased water activity.

Luminescence dating of Holocene sedimentary deposits on Rømø, a barrier island in the Wadden Sea, Denmark

This study presents an Optically Stimulated Luminescence (OSL) based chronology for the evolution of the barrier island Rømø and related back-barrier lagoon deposits in the Danish Wadden Sea. The chronology is constructed using 78 quartz OSL ages obtained using a Single Aliquot Regenerative dose (SAR) protocol; samples were retrieved from seven sediment cores, each up to 25 m long, taken from locations on the island and within the back-barrier lagoon. The reliability of the OSL results is assessed using preheat plateaus, thermal transfer tests, and dose recovery measurements, and by comparison with independent age control. The OSL ages compare favourably with three 14C age estimates, and range between 220±20 and 15 800±1200 years. Rømø appears to have developed as a progradational barrier system, and the island appeared for the first time approximately 8000 years ago. It then gradually expanded towards the west, north and south as a result of an abundant sediment supply. Luminescence dating is concluded to be well-suited for establishing precise and accurate chronologies for barrier island deposits, and the technique has provided an excellent basis for the reconstruction of barrier island evolution.

Hindcasting cyanobacterial communities in Lake Okaro with germination experiments and genetic analyses

Cyanobacterial blooms are becoming increasingly prevalent worldwide. Sparse historic phytoplankton records often result in uncertainty as to whether bloom-forming species have always been present and are proliferating in response to eutrophication or climate change, or if there has been a succession of new arrivals through recent history. This study evaluated the relative efficacies of germination experiments and automated rRNA intergenic spacer analysis (ARISA) assays in identifying cyanobacteria in a sediment core and thus reconstructing the historical composition of cyanobacterial communities. A core (360 mm in depth) was taken in the central, undisturbed basin of Lake Okaro, New Zealand, a lake with a rapid advance of eutrophication and increasing cyanobacteria populations. The core incorporated a tephra from an 1886 volcanic eruption that served to delineate recent sediment deposition. ARISA and germination experiments successfully detected akinete-forming nostocaleans in sediment dating 120 bp and showed little change in Nostocales species structure over this time scale. Species that had not previously been documented in the lake were identified including Aphanizomenon issatschenkoi, a potent anatoxin-a producer. The historic composition of Chrococcales and Oscillatoriales was more difficult to reconstruct, potentially due to the relatively rapid degradation of vegetative cells within sediment.

Comparative effects of mercury contamination and wastewater effluent input on Gram-negative merA gene abundance in mudflats of an anthropized estuary (Seine, France): a microcosm approach

The macrotidal Seine estuary (France) is one of the most man-altered and mercury-contaminated European estuaries. Molecular quantification by competitive PCR has shown that the highest quantities of Gram-negative merA genes in intertidal freshwater mudflat sediments are located in recent sediment deposits independently of mercury concentrations, suggesting that particle-attached allochtonous mercury-resistant merA bacteria are deposited on mudflat surfaces. To investigate this hypothesis, a microcosm experiment was carried out to evaluate the respective contributions of (i) the input of allochtonous merA bacteria supplied by WWTP-treated effluents and (ii) merA gene abundance corresponding to a response of the sediment's autochthonous bacterial community to mercury contamination. Gram-negative merA gene quantification and T-RFLP analysis of both 16S rDNA and merA genes demonstrated that deposited allochtonous bacteria did not develop in estuarine sediments, whereas mercury contamination (10 μg g −1 wet sediment) selected an autochthonous mercury-resistant merA bacterial community. Thus, in mudflats of highly anthropized macrotidal estuaries, i.e. those subjected to intense hydrosedimentary processes and continuously contaminated by mercury and fecal bacteria, inputs of allochtonous merA bacteria are largely responsible for the high quantities of merA genes on the surface of mudflat sediments.

Late Bronze Age paleogeography along the ancient Ways of Horus in Northwest Sinai, Egypt

AbstractThe northwest Sinai contained the eastern frontier of New Kingdom Egypt during the Late Bronze Age. The ancient Pelusaic branch of the Nile Delta influenced the environmental setting of this region at that time. Fortresses were built along the coastal byway through the study area known as the Ways of Horus to protect Egyptian‐held territory from immigrants and intruders from Canaan and the Mediterranean Sea. Building on previous geomorphic studies in the region, this paper presents the results of field investigations of Holocene sedimentary deposits, aided by satellite photography, used to create a paleogeographic map that places archaeological sites in their proper environmental context. CORONA satellite photographs from the late 1960s reveal surface features that have been obscured by more recent agricultural development in the region. Canals dug for an agricultural project provided easy access to the shallow subsurface for mapping the extent of Holocene sediments representing barrier coast, lagoon, estuarine, fluvial, and marsh depositional environments. © 2008 Wiley Periodicals, Inc.

Recent sediment transport and deposition in the Nazaré Canyon, Portuguese continental margin

Processes, pathways and fluxes of sediment transport and deposition in the Nazaré submarine canyon, Portuguese continental margin, were investigated by water column profiling of suspended particulate matter, recording of near-bottom currents and suspended particulate matter fluxes with benthic landers, and by analysis of surface sediment cores. The results show that fine-grained suspended sediment from the adjacent shelf is captured in the upper canyon and actively transported through the canyon to the abyssal plain under influence of tidal currents and intermittent sediment gravity flows. Tidal currents are actively resuspending and transporting sediment in the upper to middle canyon, as reflected by high concentration of suspended particles in bottom waters, high horizontal and vertical sediment fluxes in the bottom water layer and high sediment accumulation rate on the seabed. Diminishing strength of tidal currents leads to focussed sediment deposition in the middle canyon. Further transport of sediments toward the lower canyon takes place predominantly by sediment gravity flows occurring on yearly or longer timescale. Turbidity currents strong enough to transport sand-sized material to the lower canyon occur only on centennial or longer timescales.

Miocene to Holocene landscape evolution of the western Snake River Plain region, Idaho: Using the SHRIMP detrital zircon provenance record to track eastward migration of the Yellowstone hotspot

We report new U-Pb detrital zircon sensitive high-resolution ion microprobe (SHRIMP) age data (702 grains) from 13 samples collected from Miocene to Holocene sedimentary deposits in the western Snake River Plain region. These samples effectively show that modern stream sediments of the Snake River system reliably and repeatedly record the detrital zircon age populations that are present as sources in their drainage basins across the Cordilleran thrust belt and Basin and Range Province. We use this framework and the provenance of Neogene sedimentary rocks in the region to test the effect of the migrating Yellowstone hotspot on regional drainage patterns in southern Idaho since the middle Miocene. Our results indicate that Neogene paleodrainages were fi rst directed radially away from the tumescent Yellowstone highland, then subsequently reversed their fl ow toward the subsiding Snake River Plain basin. This occurred in east-progressing time-constrained intervals starting at 16 Ma. In northern Nevada, the drainage divide is represented by a northeast-trending, southeast-migrating crest of high topography. Specifi cally, middle to late Miocene (16‐ 10 Ma) sedimentary deposits of the western Snake River Plain and Oregon-Idaho graben contain early to middle Eocene (52‐42 Ma) detrital zircon populations sourced in Challis magmatic rocks north of the Snake River Plain. Middle Jurassic (160 Ma) and middle to late Eocene (42‐35 Ma) detrital zircons, sourced from rocks in northern Nevada, are not present. Late Eocene detrital zircons from Nevada are present in two younger than 7 Ma sedimentary units of the Idaho Group along the Oregon-Idaho border. This indicates that by the late Miocene, southeastward headward erosion of the paleo‐Owyhee River into the Owyhee Plateau had captured drainage from north-central Nevada and directed it northwestward toward the subsiding western Snake River Plain. The modern Owyhee Plateau is still a topographic high, in contrast to the modern Snake River Plain, suggesting that lowering of the regional Snake River Plain base level, rather than crustal subsidence, drove stream capture. By the late Pliocene (3 Ma), Middle Jurassic detrital zircons are recorded in the Glenns Ferry Formation and Tuana Gravel of the central Snake River Plain, suggesting that surface subsidence reversed the fl ow direction of paleo‐Salmon Falls Creek from southward into Nevada to northward toward Idaho. Miocene strata of the western Snake River Plain lack recycled Proterozoic detrital zircons that are ubiquitous in sedimentary rocks of the central and southeast Idaho thrust belts. Such detrital zircons appear on the central and western Snake River Plain in early Pliocene to Holocene (4‐0 Ma) deposits. This records capture of drainage from the eastern Snake River Plain. The Yellowstone hotspot controlled the east-migrating continental divide, in the wake of which formed the western-draining, and progressively eastward-collecting, Snake River system.

Rapid development and infilling of a buried gully under cropland, central Belgium

Analysis of contemporary and past gully erosion and infilling processes allowed to reconstruct the long-term evolution of a permanent gully system under cropland. An active and a buried gully under cropland were investigated. The recent sediment deposits within the active gully, adjacent to the buried gully, showed that the recent gully was filling in at a mean rate of 6.4 cm a − 1 . In the buried gully, several erosion and deposition phases could be identified and the type of deposited sediments revealed a complex infilling history. Charcoal, pottery and brick fragments of different sizes were found at all depths of the gully infilling. Their age indicates that the first gully incised after the midst of the 17th century, most probably in the second half of the 18th century or the early 19th century. Gully morphology and analogy with the processes in the recent gully indicate that the buried gully filled in rapidly. Overall, five cycles of cut and fill occurred in 350 years or less and four cycles even within little less than a few decades, indicating that gully development and infilling under cropland can be very rapid processes.

Combined thin-skinned and thick-skinned deformation in the central Andean foreland of northwestern Argentina

The Choromoro Basin, an intramontane structural depression in the foreland fold-and-thrust belt of the central Andes of northwestern Argentina, is underlain by Proterozoic–Recent sedimentary deposits, with Cretaceous–Paleogene and older rocks exposed along the margins and Neogene and Quaternary deposits exposed within it. The Cretaceous–Paleogene deposits represent part of an extensive rift system. Late Cenozoic Andean compression followed rifting and initiated the foreland basin stage of sedimentation. The Choromoro Basin developed in response to the fragmentation of the Andean foreland during the latest Cenozoic Andean orogeny. Regionally, the Choromoro Basin is located in an area dominated by a thick-skinned structural style of deformation, which is marked by high-angle, eastward-verging reverse faults rooted in a mid-crustal detachment that uplifted the basement blocks that bound the basin along its western and eastern margins. Faulting is associated with eastward-verging folds that involve the basement rocks and overlying Cretaceous–Neogene sedimentary strata. In contrast, Neogene beds in the Choromoro Basin show a thin-skinned structural style of deformation with an eastward-verging, imbricated thrust system and associated folds related to a shallow detachment located at approximately 2.5–3 km depth in the Cenozoic succession. The superposition in space and time of thick- and thin-skinned structural styles in the Choromoro Basin gives rise to a combined style of deformation. Several orogens, including the Andes, show the coeval development of thick- and thin-skinned structural styles, but they are spatially distributed in the orogen and foreland realms, respectively. In the Choromoro Basin, the thin-skinned deformation developed in an area dominated by thick-skinned deformation.

Particulate N and P characterizing the fate of nutrients along the estuarine gradient of the River Neva (Baltic Sea)

The estuary of the River Neva is eutrophied due to the high nutrient load from St. Petersburg. Previous studies have suggested that a large part of the N and P load is efficiently retained in the estuary. However, the fate of particulate N and P in the estuarial sea area has remained uncertain. Variations in the C, N and P sedimentation rate and in sediment organic matter (loss on ignition, LOI), total nitrogen (TN) and phosphorus (TP) were therefore studied in the recent sediment deposits along the estuarine gradient of the River Neva. The high gross sedimentation rate of N and P indicated efficient retention of both N and P in the inner estuary. An increase in the concentrations of settling particulate C and N along the estuarine gradient was consistent with the increase in the concentrations and burial of organic matter and TN in sediments. However, although the concentration of P in settling matter also increased along the estuarine gradient, a decrease was noted in the sediment TP concentration. This decrease correlated significantly with an increase in the concentration of sediment organic matter, but not with water depth along the estuarine gradient. The result indicates that an increase in organic matter and changes in hydrodynamic conditions along the estuarine gradient can reduce the capacity of the top 0–10 cm sediment layer to retain P. The average concentration of N in settling particulate matter was 23.3 mg g −1 DW and in the sediment surface 6.5 mg g −1 DW. Corresponding values for P were much closer to each other, that is, 3.3 and 2.8 mg g −1 DW, respectively. This suggests that, in the surface sediments (0–1 cm), settled P was more efficiently retained and/or accumulated than N. In contrast, in the 0–10 cm sediment layer N seemed to be more efficiently retained than P.