Overbank Research Articles

Architectural and facies organisation of slope channel fills: Upper Cretaceous Rosario Formation, Baja California, Mexico

Abstract Slope channel systems, well-known for their significance in hydrocarbon exploration and sediment transport and deposition in deep-water settings, are inherently complicated in the rock record. To unravel the complexity, a better understanding of their internal organisation is needed. Here an integrated qualitative and quantitative examination of internal architecture and vertical facies successions is reported from the San Fernando slope channel system of the Rosario Formation (Upper Maastrichtian), Baja California, Mexico. The San Fernando system (ca. 7–9 km wide, 400 m thick) comprises four channel complex sets (CCSs), of which this study focuses on the best exposed-CCS-B. Geologic mapping and photomosaic interpretation indicate that CCS-B features an erosion-surface-bounded and coarse-grained lower succession that records braid-like channel fills at the channel belt axis. The overlying upper succession is characterised by fine-grained overbank or passive channel fills intercalated with conglomeratic sinuous channel fills at the channel belt axis. By contrast, fine-grained overbank deposits and a capping hemipelagic interval (abandonment) dominate the upper succession at the channel belt margin. Visual inspection of the measured sections from CCS-B suggests multiple fining-upward packages, which are partly confirmed by stochastic analysis of vertical facies transitions using the Markov chain technique. This technique also quantitatively demonstrates that distinct preferred vertical facies transitions characterise different stratigraphic levels (lower succession vs. upper succession), and varying depositional environments (axis vs. margin) within the channel belt of CCS-B. The results of this study merit close consideration as analogues to submarine channel systems characterised by a lower braided pattern and/or an upper meandering pattern, and should find wide applicability in subsurface prediction of reservoirs, in other outcrop-based studies, and in constraining transition probability-based stochastic geological modeling.

Tracing provenance and chemical weathering changes in Ankara Stream sediments, central Turkey: Geochemical and Sr–Nd–Pb–O isotopic evidence

This study aims to analyze the changes in sediment provenance through the geochemical and Sr–Nd–Pb–O isotopic composition of the Ankara Stream bed and overbank sediments (ASBS and ASOS) and to examine the weathering and sediment transport processes in response to step climate variability. Major, trace, and rare earth elements, as well as Sr–Nd–Pb–O isotopic composition, exhibit generally similar variations for the investigated samples. These sediments have relatively high εNd(0) values and low 87Sr/86Sr ratios. Similar Pb isotopic ratios also characterize these sediments but have slightly different δ18O isotope values with relatively higher δ18O isotope values in the ASBS and lower δ18O isotope values in the ASOS samples. Chemical indices indicate low–moderate chemical weathering in the step-dominated climate. The low average Zr/Sc and Zr/Nb ratios indicate that such geochemical change was dominated by the composition of the source rocks but not sedimentary recycling with zircon enrichment, reflecting first-cycle deposit. Low radiogenic 87Sr/86Sr ratios and negative εNd(0) values indicate that the investigated sediments are relatively derived from young magmatic provinces, which are intermediate sources. Finally, the findings show that the geochemical compositions of the ASBS and ASOS were dominated by the composition of the source materials that originate from Galatia province and Late Cretaceous- Early Tertiary volcanic rocks in central Anatolia. Reasonable semiarid to arid climatic conditions are observed in the region. Active tectonism, increased erosion, and the investigated sediments are also transported rapidly to the stream channel without chemical alteration, and the sediments are the first-cycle products.

The modern Kaoping transient fan offshore SW Taiwan: Morphotectonics and development

Using bathymetry and seismic reflection profiles, this study examined and determined the transient nature of the Kaoping Fan located in the topographically complex slope offshore southwest Taiwan. Kaoping Fan is located west of the lower reach of the Kaoping Canyon at the lower Kaoping Slope, ranging from 2,200 to 3,000 m water depth, and has a relatively small areal extent restricted in the topographic lows confined by structural highs due to mud diapiric uplifting and thrust faulting. Kaoping Fan shows an asymmetrical triangular fan-shaped bathymetric feature elongated in an NW-SE direction but with a strong skew toward the east. The fan deposits consist of three main seismic facies: layered high-amplitude reflections in the upper section and stratified, parallel to sub-parallel low-amplitude reflections with variable continuity and channel fill facies in the lower section. In the absence of ground-truthing from core data, the seismic patterns suggest that the Kaoping Fan recorded the onset of channelized and over-bank deposits in the lower part and layered turbidite facies in the upper part subsequently. The development of the Kaoping Fan can be divided into three stages in terms of canyon activities and fan-feeding processes. Initially, Kaoping Fan was mainly fed by a point sediment source at the apex of the fan. Secondly, Kaoping Fan was maintained as a slope fan, mainly fed laterally by over-spilled sediments from the canyon. Finally, the Kaoping Canyon completely passes through the Kaoping Fan and supplies over-spilled sediments laterally, forming a transient fan with canyon incision and sediment by-passing. The accumulation of sediments and the growth of Kaoping Fan are primarily controlled by inherited complex paleo-topography and the evolution of Kaoping Canyon. The sediment delivery system of Kaoping Fan is characterized by lateral supply of over-spilling sediment flows and sediments bypassing to and beyond the base of slope. The Kaoping Fan together with the ponded Fangliao Fan in the topographically complex Kaoping Slope can be used as a type model for evaluating the topographic effects on the development of submarine fans on complex slopes in general.

Stratigraphy, sedimentology and inferred flow dynamics from the July 2015 block-and-ash flow deposits at Volcán de Colima, Mexico

The July 2015 block-and-ash flow (BAF) events represent the first documented series of large-volume and long-runout BAFs generated from sustained dome collapses at Volcán de Colima. This eruption is particularly exceptional at this volcano due to (1) the large volume of BAF material emplaced (0.0077±0.001km3), (2) the long runout reached by the associated BAFs (max. ~10km), and (3) the short period (~18h) over which two main long-sustained dome collapse events occurred (on 10 and 11 July, respectively). Stratigraphy and sedimentology of the 2015 BAF deposits exposed in the southern flank of the volcano based on lithofacies description, grain size measurements and clast componentry allowed the recognition of three main deposit facies (i.e., valley-confined, overbank and ash-cloud surge deposits). Correlations and lithofacies variations inside three main flow units from both the valley-confined and overbank deposits left from the emplacement of the second series of BAFs on 11 July provide detailed information about: (1) the distribution, volumes and sedimentological characteristics of the different units; (2) flow parameters (i.e., velocity and dynamic pressure) and mobility metrics as inferred from associated deposits; and (3) changes in the dynamics of the different flows and their material during emplacement. These data were coupled with geomorphic analyses to assess the role of the topography in controlling the behaviour and impacts of the successive BAF pulses on the volcano flanks. Finally, these findings are used to propose a conceptual model for transport and deposition mechanisms of the July 2015 BAFs at Volcán de Colima. In this model, deposition occurs by rapid stepwise aggradation of successive BAF pulses. Flow confinement in a narrow and sinuous channel enhance the mobility and runout of individual channelized BAF pulses. When these conditions occur, the progressive valley infilling from successive sustained dome-collapse events promote the overspill and lateral spreading of the upper and marginal regions of the main flow body, generating highly mobile overbank flows that travel outside of the main valley. Volume- and distance-dependent critical channel capacities for the generation of overbank flows are used to better estimate the inundation area of these hazardous unconfined pyroclastic flows. These results highlight the importance of including and correctly assessing the hazards posed by large volume and long runout BAFs associated with frequent, small VEI, sustained dome-collapse eruptions.

Evaluation of potentially toxic element contents and Pb isotopic compositions in Ankara Stream sediments within an urban catchment in central Turkey

This study aimed to interpret the spatial distribution, enrichment factor (EF), and geo-accumulation of potentially toxic elements in the bed sediments (ASBS) and Pleistocene-aged overbank sediments (ASOS) of the Ankara Stream in Turkey. The potential toxic pollution of the investigated potentially toxic elements was defined by evaluating the EF, geo-accumulation index (I geo), and Pb anthropogenic contribution. The distributions of As, Ni, and Co in the analyzed sediments were uniform in the ASBS and ASOS along the length of the stream. The EF and I geo values indicated that ASBS and ASOS were significantly polluted by As and Ni and moderately polluted by Co, Zn, and Cu. ASBS was locally strongly polluted by Pb and Cd. Pb anthropogenic contribution was estimated at an average of > 100% in the ASBS and ASOS. The potentially toxic element levels and distribution patterns revealed that industrial activities, gasoline combustion, vehicular emission, phosphate fertilizers and natural sources were probable causes of metal pollution in Ankara Stream sediments. These potentially toxic elements were transported by atmospheric conditions and stream water. The potentially toxic elements in Ankara Stream increased with each passing day and were important in evaluating the potential toxicity risks to living organisms. Thus, suitable measures should be taken to ensure appropriate treatment of wastewater before toxic residues are disposed into the nearby stream. Chimney filter, treatment plants, and appropriate technology should be used in the organized industrial zone to prevent atmospheric pollution.

Conceptual models for short‐eccentricity‐scale climate control on peat formation in a lower Palaeocene fluvial system, north‐eastern Montana (USA)

AbstractFluvial systems in which peat formation occurs are typified by autogenic processes such as river meandering, crevasse splaying and channel avulsion. Nevertheless, autogenic processes cannot satisfactorily explain the repetitive nature and lateral continuity of many coal seams (compacted peats). The fluvial lower Palaeocene Tullock Member of the Fort Union Formation (Western Interior Williston Basin; Montana, USA) contains lignite rank coal seams that are traceable over distances of several kilometres. This sequence is used to test the hypothesis that peat formation in the fluvial system was controlled by orbitally forced climate change interacting with autogenic processes. Major successions are documented with an average thickness of 6·8 m consisting of ca 6 m thick intervals of channel and overbank deposits overlain by ca 1 m thick coal seam units. These major coal seams locally split and merge. Time‐stratigraphic correlation, using a Cretaceous–Palaeogene boundary event horizon, several distinctive volcanic ash‐fall layers, and the C29r/C29n magnetic polarity reversal, shows consistent lateral recurrence of seven successive major successions along a 10 km wide fence panel perpendicular to east/south‐east palaeo‐flow. The stratigraphic pattern, complemented by stratigraphic age control and cyclostratigraphic tests, suggests that the major peat‐forming phases, resulting in major coal seams, were driven by 100 kyr eccentricity‐related climate cycles. Two distinct conceptual models were developed, both based on the hypothesis that the major peat‐forming phases ended when enhanced seasonal contrast, at times of minimum precession during increasing eccentricity, intensified mire degradation and flooding. In model 1, orbitally forced climate change controls the timing of peat compaction, leading to enhancement of autogenic channel avulsions. In model 2, orbitally forced climate change controls upstream sediment supply and clastic influx determining the persistence of peat‐forming conditions. At the scale of the major successions, model 2 is supported because interfingering channel sandstones do not interrupt lateral continuity of major coal seams.

Vertical accretion sand proxies of gaged floods along the upper Little Tennessee River, Blue Ridge Mountains, USA

Understanding environmental hazards presented by river flooding has been enhanced by paleoflood analysis, which uses sedimentary records to document floods beyond historical records. Bottomland overbank deposits (e.g., natural levees, floodbasins, meander scars, low terraces) have the potential as continuous paleoflood archives of flood frequency and magnitude, but they have been under-utilized because of uncertainty about their ability to derive flood magnitude estimates. The purpose of this paper is to provide a case study that illuminates tremendous potential of bottomland overbank sediments as reliable proxies of both flood frequency and magnitude. Methods involve correlation of particle-size measurements of the coarse tail of overbank deposits (>0.25mm sand) from three separate sites with historical flood discharge records for the upper Little Tennessee River in the Blue Ridge Mountains of the southeastern United States. Results show that essentially all floods larger than a 20% probability event can be detected by the coarse tail of particle-size distributions, especially if the temporal resolution of sampling is annual or sub-annual. Coarser temporal resolution (1.0 to 2.5year sample intervals) provides an adequate record of large floods, but is unable to discriminate individual floods separated by only one to three years. Measurements of >0.25mm sand that are normalized against a smoothed trend line through the down-column data produce highly significant correlations (R2 values of 0.50 to 0.60 with p-values of 0.004 to <0.001) between sand peak values and flood peak discharges, indicating that flood magnitude can be reliably estimated. In summary, bottomland overbank deposits can provide excellent continuous records of paleofloods when the following conditions are met: 1) Stable depositional sites should be chosen; 2) Analysis should concentrate on the coarse tails of particle-size distributions; 3) Sampling of sediment intervals should achieve annual or better resolution; 4) Time-series data of particle-size should be detrended to minimize variation from dynamic aspects of fluvial sedimentation that are not related to flood magnitude; and 5) Multiple sites should be chosen to allow for replication of findings.

Strong sea forcing and warmer winter during solar minima ∼2765 yr B.P. recorded in the growth bands of Crassostrea sp. from the confluence of river Ganges, Eastern India

Long term variation of solar activity plays a key role in controlling climatic oscillations during glacial-interglacial cycles. The records of such climatic shifts can be retrieved from sedimentary archives in overbank deposits found in the estuary regions of major rivers in the tropics which are fed by glaciers. In this study we have shown the effect of solar variability on regional climate by altering the river discharge and incursion of warm water pool into the region adjoining Bay of Bengal during seasonal dry period. The incremental growth bands present in the modern day Meretrix sp and Late Holocene Crassostrea sp. were examined for reconstruction of temperature and water composition at the head bay region of the river Ganges. The conventional C14 techniques on fossil oyster yielded age of 2765 ± 130 yr B·P., which coincides with a solar minima. Analysis of clumped isotope thermometry on the growth bands provided temperature estimates for the growth of shells. The temperature estimates for the modern shell, suggesting range of values showed a range between 13° and 42 °C, close to the observed temperatures recorded in the climatological data while the fossil shell had a range of values between 22° and 38 °C. The δ18O measured in the aragonite together with the estimated temperature were used to deduce the water composition during growth at equilibrium condition. The water δ18O varied between −4.8‰ and 1.2‰ for the modern sample, close to the observed values of water measured near this locality, while the range in water composition inferred for the paleo samples was from −2.37‰ to 0.82‰, suggesting a stronger influence of sea water throughout the year. The results are consistent with the argument of infiltration of water from neighbouring warm water pool into the estuary. A similar approach can be extended to evaluate the effects of climate variability due to differential action of river discharge into the sea at seasonal time scales based on available mollusc shells in the sedimentary successions from the region.

O-153 砂質海底扇状地システムにおけるオーバーバンク堆積物の発達過程 : 房総半島鮮新統清澄層を例として(16. 堆積相と堆積システム・シーケンス,口頭発表,一般発表)

O-153 砂質海底扇状地システムにおけるオーバーバンク堆積物の発達過程 : 房総半島鮮新統清澄層を例として(16. 堆積相と堆積システム・シーケンス,口頭発表,一般発表)

Holocene evolution of the southern Tauranga Harbour

ABSTRACTTauranga Harbour is a major economic centre and home to New Zealand’s largest port. Despite the importance of the harbour basin, and especially of the shipping channel, the subsurface stratigraphy of this area has not been studied in detail. We use sediment cores retrieved in support of a recent dredging campaign to characterise the subsurface stratigraphy and interpret the sediments in terms of paleoenvironment. Four main facies (volcanic sand, lithic silt, coarse sand and pumiceous sand) were identified using textural, mineralogical and geochemical characteristics. They represent distinct depositional environments at different stages of the Holocene marine transgression (HMT). The oldest facies, deposited prior to the HMT (volcanic sand and lithic silt facies), represent the channel and overbank deposits of an aggrading river system with rapidly migrating channels. During the deposition of this facies, a change in sediment source is evident from geochemical data. This preceded a transition to a predominantly offshore sediment source (coarse sand facies), likely supplied by longshore drift from the Kaituna River, which was associated with the development of Mt Maunganui tombolo and Matakana barrier island. During the Holocene stillstand, the barrier island and tombolo restricted sediment input from offshore, resulting in the deposition of a predominantly locally derived unit (pumiceous sand facies), which still dominates sedimentation in the harbour today.

Discovery of Paleogene Deposits of the Central High Plains Aquifer In the Western Great Plains, U.S.A.

Abstract: Recent drilling in southwestern Kansas has produced intact cores to investigate the subsurface lithostratigraphy, sedimentary provenance, and chronostratigraphy of strata constituting the central High Plains Aquifer (HPA). In large portions of the HPA, groundwater withdrawals greatly exceed rates of recharge, leading to dramatic water-level declines and growing concerns for long-term sustainability. Two cores, HP1A (98 m) and CMC1 (50 m), are the first and deepest intact cores of the HPA ever collected. The cores show decameter-scale intercalations between suspended-load fluvial deposits composed of fine-grained sands with pedogenically modified overbank deposits, and very coarse-grained sands and gravels consistent with high-energy, bedload-dominated fluvial systems. Six intervals in HP1A and one from CMC1 were analyzed for detrital-zircon U-Pb ages by LA-ICP-MS. The HP1A samples show maximum depositional ages (MDAs) ranging from ∼ 27.5 to 36.4 Ma with depth. A MDA of ∼ 30 Ma was measured in the CMC1 core. These Paleogene zircons likely originated from explosive volcanism associated with the mid-Cenozoic ignimbrite flare-up (44–18 Ma), which blanketed much of western North America with high-volume air-fall tuffs. We propose that a large evaporite-dissolution basin in southwestern Kansas provided the accommodation space to preserve a record of Paleogene strata. The lack of younger middle–late Miocene zircons from cores in southwestern Kansas is striking given that such grains, likely derived from the Snake River Plain–Yellowstone hotspot volcanic provinces (16.1–0.6 Ma), are readily identified in the Ogallala Formation in north-central Kansas and Nebraska. The MDAs suggest Eocene to Oligocene age deposits that are time-equivalent to the Arikaree and White River groups in Nebraska are also present in Kansas.

The environmental and geomorphological impacts of historical gold mining in the Ohinemuri and Waihou river catchments, Coromandel, New Zealand

Between 1875 and 1955 approximately 250,000Mgyr−1 of mercury-, arsenic-, and cyanide-contaminated mine tailings were discharged directly into the Ohinemuri River and its tributaries, in the Coromandel Region, North Island, New Zealand. A devastating flood on 14 January 1907 deposited large amounts of mine waste across the floodplain of the Ohinemuri and Waihou rivers in the vicinity of the township of Paeroa. The 1907 mine-waste flood deposit was located as a dirty yellow silt in cores and floodplain profiles, with a thickness ranging from 0.15–0.50m. Geochemical analysis of the mine waste shows elevated concentrations of Pb (~200–570mgkg−1) and As (~30–80mgkg−1), compared to early Holocene background concentrations (Pb<30mgkg−1; As<17mgkg−1). Bulk sediment samples recovered from the river channel and overbank deposits also show elevated concentrations of Pb (~110mgkg−1), Zn (~140–320mgkg−1), Ag (~3mgkg−1), and Hg (~0.4mgkg−1). Using the mine-waste deposit as a chronological marker shows that sedimentation rates increased from ~0.2mmyr−1 in the early Holocene, to 5.5–26.8mmyr−1 following the 1907 flood. Downstream trends in the thickness of the flood deposit show that local-scale geomorphic factors are a significant influence on the deposition of mine waste in such events. Storage of mine waste is greatest in the upstream reaches of the floodplain. The volume of mine waste estimated to be stored in the Ohinemuri floodplain is ~1.13Mm3, an order of magnitude larger than recent well-publicised tailings-dam failures, such as the 1996 South America Porco, 2000 Romanian Baia Mare and Baia Borsa accidents, and constituted, and was recognised at the time, a significant geomorphological and environmental event. The mine-waste material remains in the floodplain today, representing a sizable legacy store of contaminant metals and metalloids that pose a long-term risk to the Ohinemuri and Waihou ecosystems.

Natural levee evolution in the Rhine-Meuse delta, the Netherlands, during the first millennium CE

This paper presents reconstructions on natural levee development in the Rhine-Meuse delta, the Netherlands, during the first millennium CE, covering the full delta plain. It is the first study that performs this on a delta scale, which allows seeing the delta-wide trends on levee-forming controls and their feedbacks. We mapped the levee morphology and elevation by combining LiDAR imagery, lithological borehole data, soil mapping, radiocarbon dates, archaeological data, and GIS-reconstruction techniques. From the detailed levee reconstructions we quantified natural levee dimensions and evaluated the temporal changes therein. The dimensions and the changes therein were then linked to external forcings (increasing suspended sediment load, variable flooding intensity) and to natural preconditions (e.g., delta plain width, flood basin configuration).We show that natural preconditions are an important control on levee shape. This is demonstrated for the upper delta where the relatively narrow delta plain combined with strong compartmentation (i.e., the occurrence of many alluvial ridges and enclosed flood basins) caused the flood levels to be amplified allowing the natural levees to grow relatively high. Compartmentation also seems to have stimulated trapping of coarse-grained overbank sediments, explaining the clear downstream trend in levee width. This effect was probably further aided by the clearance of the riparian forests, mainly in the upstream and central delta, which caused the coarser fraction of the suspended load to be further dispersed into the flood basin leading to wider levees. In the first millennium CE several new river courses formed that avoided the areas of natural levee relief of abandoned alluvial ridges. On these fossil alluvial ridges, the topographical expression gradually reduced because of widespread flood basin trapping of overbank sediment, which led to topographic levelling. The natural levees that formed during this period along the new courses appear to be relatively high compared to precursor generations in the upper and central delta. This is most likely related to the increased suspended sediment supply and intense flooding regime during their formation. The hypotheses generated with this new delta-wide overview help to better understand the controls in the development of levees, which are important elements in river landscapes and in fluvial sedimentary records.

New data towards the development of a comprehensive taphonomic framework for the Late Jurassic Cleveland-Lloyd Dinosaur Quarry, Central Utah.

The Cleveland-Lloyd Dinosaur Quarry (CLDQ) is the densest deposit of Jurassic theropod dinosaurs discovered to date. Unlike typical Jurassic bone deposits, it is dominated by the presence of Allosaurus fragilis. Since excavation began in the 1920s, numerous hypotheses have been put forward to explain the taphonomy of CLDQ, including a predator trap, a drought assemblage, and a poison spring. In an effort to reconcile the various interpretations of the quarry and reach a consensus on the depositional history of CLDQ, new data is required to develop a robust taphonomic framework congruent with all available data. Here we present two new data sets that aid in the development of such a robust taphonomic framework for CLDQ. First, x-ray fluorescence of CLDQ sediments indicate elevated barite and sulfide minerals relative to other sediments from the Morrison Formation in the region, suggesting an ephemeral environment dominated by periods of hypereutrophic conditions during bone accumulation. Second, the degree of abrasion and hydraulic equivalency of small bone fragments dispersed throughout the matrix were analyzed from CLDQ. Results of these analyses suggest that bone fragments are autochthonous or parautochthonous and are derived from bones deposited in the assemblage rather than transported. The variability in abrasion exhibited by the fragments is most parsimoniously explained by local periodic re-working and re-deposition during seasonal fluctuations throughout the duration of the quarry assemblage. Collectively, these data support previous interpretations that the CLDQ represents an attritional assemblage in a poorly-drained overbank deposit where vertebrate remains were introduced post-mortem to an ephemeral pond during flood conditions. Furthermore, while the elevated heavy metals detected at the Cleveland-Lloyd Dinosaur Quarry are not likely the primary driver for the accumulation of carcasses, they are likely the result of multiple sources; some metals may be derived from post-depositional and diagenetic processes, and others are potentially produced from an abundance of decomposing vertebrate carcasses. These new data help to support the inferred depositional environment of the quarry as an ephemeral pond, and represent a significant step in understanding the taphonomy of the bonebed and Late Jurassic paleoecology in this region.

Late Miocene fluvial distributary system in the northern Danube Basin (Pannonian Basin System): depositional processes, stratigraphic architecture and controlling factors of the Piešťany Member (Volkovce Formation)

Aim of the study was to investigate the detailed stratigraphic architecture of the alluvial Volkovce Formation, which was deposited in the northern Danube Basin during the interval between 10 and 6 Ma. Two type locality exposures were subject to facies analysis and interpretation of depositional processes and >500 boreholes served for correlation of spatial variability in the character of the formation. The study was focused on the northern Blatne and western Risňovce depressions, which are partly separated by the Považský Inovec Mountains horst. It was revealed, that a mostly gravelly fluvial distributary system of the Piesťany Member was deposited in the northern Blatne depression, partly confined by the exposed horst and by incised palaeotopography. The sequence is composed of amalgamated braided river channel bodies with no overbank strata, in response to high sediment supply and low accommodation. The palaeo-stream then crossed the horst in its southern part towards the Risňovce depression, where there was four times more accommodation. The submerged horst acted as a boundary, where the depositional processes changed to a meandering river and significant overbank deposits started to be preserved. Only small streams were present in the northern Risňovce depression, which led to very low sediment supply compared to the high accommodation there. The depositional system was affected mainly by the incised palaeotopography, by the location of sediment input together with spatial differences in accommodation rate caused by the activity of the Ripňany Fault on the southeastern side of the Považský Inovec Mts.

Fluvial architecture in actively deforming salt basins: Chinle Formation, Paradox Basin, Utah

AbstractDetermining the response of fluvial systems to syn‐sedimentary halokinesis is important for reconstructing the palaeogeography of salt basins, determining the history of salt movement and predicting development and architecture of sandstone bodies for subsurface fluid extraction. To assess both the influence of salt movement on fluvial system development and the use of lithostratigraphic correlation schemes in salt basins we have analysed the Triassic Chinle Formation in the Paradox Basin, Utah. Results indicate that sandstone body development proximal to salt bodies should be considered at two scales: intra‐ (local) and inter‐ (regional) mini‐basin scale. At the intra‐mini basin or local scale, conformable packages of up to 12 m deep meandering fluvial channel deposits and associated overbank deposits are developed, which may thin, pinch‐out or become truncated towards salt highs. When traced down the axis of a mini‐basin, individual stories extend for a few hundred metres, and form part of amalgamated channel‐belt packages up to 60 m thick that can be traced for at least 25 km parallel to palaeoflow. Where salt movement outpaces sediment accumulation, progressive low angle unconformities are developed along the flanks of salt highs. Significantly, in mini‐basins with high sand supply, sandstone bodies are present across salt highs where they show increased amalgamation, decrease in thickness due to truncation and no change in internal sandstone body character. At inter mini‐basin or regional scale, spatial and temporal variations in accommodation space generated by differential salt movement strongly influence facies distributions and facies correlation lengths. Broad lithostratigraphic packages (5–50 m thick) can be correlated within mini‐basins, but correlation of these units between adjacent mini‐basins is problematic. Knowledge of fluvial system development at a regional scale is critical as, fluvial sediment distribution is focussed by topography generated by growing salt bodies, such that adjacent mini‐basins can have significant differences in sandstone body thickness, distribution and lateral extent. The observations from the Chinle Formation indicate that lithostratigraphic‐based correlation schemes can only be applied within mini‐basins and cannot be used to correlate between adjacent mini‐basins or across a salt mini‐basin province. The key to predicting sandstone body development is an understanding of the timing of salt movement and reconstructing fluvial drainage system development.

Evolution of C4 plants and controlling factors: Insight from n-alkane isotopic values of NW Indian Siwalik paleosols

The δ13C values of long chain n-alkanes from paleosol samples (n=74) of four NW Indian Siwalik sections have been used to reconstruct the relative abundance of C3-C4 plants, after isotopic characterization of modern plants from the Ganges floodplain considered equivalent to the past Siwalik floodplain. The values from the Naladkhad (11.6–8.9Ma) and Ranital (11.3–7.4Ma) sections of the Kangra sub-basin indicate the presence of ca. 20% C4 plants at ca. 11Ma. The existence of C4 plants at ca.11Ma indicate an early appearance of C4 plants, compared with the published data from the Siwalik regions. The Ranital section shows a patchy occurrence of C4 plants between 11.3Ma and 7.4Ma and the Jabbarkhad section shows (5.2–3.0Ma) a gradual increase in C4 plant abundance. The C4 plant abundance fluctuated between 83% and 0% (5.8–1.3Ma) in the Haripur Khol section of the Subathu sub-basin. Comparison of C4 plant abundance from the four sections shows that each section was characterised by a distinct evolutionary pattern. The δD values of the same n-alkanes from the Kangra and Subathu sub-basins indicate two episodes of Indian summer monsoon intensification at ca. 9Ma and ca. 4Ma. The correlation between δDC29 and δ13CC29 values varied in different sections. It was also observed that the C4 plant abundance variation could be linked to the variable channel/overbank ratio of the studied sections. This observation suggests that Indian summer monsoonal rainfall, type of overbank sediments and moisture content controlled the abundance of C4 plants in the Siwalik floodplain.

Lost in dating – Problems with the absolute chronologies and sedimentation rates of Late Glacial and Early Holocene oxbow lake deposits in Central Europe

Abstract This article presents potential errors related to the construction of absolute chronologies for Late Glacial and Early Holocene sediments accumulated in oxbow lakes in Central Europe and their influence on palaeoecological reconstruction. Our study is based on two sediment cores: (i) from the San River (Stubno-Naklo site in south-eastern Poland; S-N) and (ii) from the Dniester River (Luka site in western Ukraine) valleys. Accelerator mass spectrometry (AMS) 14C dating was performed on terrestrial plant macrofossils from each site to produce Bayesian age–depth models and to calculate sedimentation rates (SRs). The SR in both sites was characterised by high variability reflecting the developmental stage of oxbow lake. The highest SR values (1.18–1.23 cm yr-1) were identified at the S-N site at the beginning of the Holocene (ca. 11,550–11,470 cal. BP) and related to enhanced overbank deposition. The lowest SR values (0.01–0.02 cm yr-1) were identified in the transitional layer between the Younger Dryas and the Preboreal chronozone (ca. 11,810–11,160 cal. BP) in the Luka profile; this may have been related to the sedimentary gap(s) caused by falls in the water table and/or temporary desiccation of the water body. Moreover, our study revealed the presence of three distinctly aged AMS 14C dates in the Younger Dryas section of the Luka profile. These radiocarbon inversions were completely undetected by pollen analysis, which indicated the domination of Pinus and Betula without any significant fluctuations throughout the entire subsection. The presence of such ‘cryptic’ insertions of older material could lead to erroneous palaeoenvironmental reconstruction. Similarly the S-N site revealed anomalously ‘aged’ dates which were also difficult to detect by pollen analysis.

Uniqueness Deposit of Sediment on Floodplain Resulting From Lateral Accretion on Tropical Area

Kampar rivers has a length of 413 km with average depth of 7.7 m and width of 143 m. Sixty percent of this rivers are meandering fluvial system which transport and deposit a mixture of suspended and bed-load (mixed load) along low energy. River channel that moving sideways by erosion is undergoing lateral migration and the top of the point bar becomes the edge of the floodplain and the fining-upward succession of the point bar will be capped by overbank deposits of Kampar River. Along the Kampar Rivers, there are more than 60% of floodplain sediments and almost all of the floodplain formed by bend migration on the suspended-load channels of Kampar watershed. This formation consist of succession of fine to medium sand and silt/mud, with root traces, that form as drapes on the prograding bank. These beds dip mostly channel wards and quickly wedge out as they grade up and onto the floodplain. The depositional model is presented showing how lateral accretion can make a significant contribution to the preservation of fine-grained within channel deposits in contemporary floodplains. The examples presented here demonstrate that analogues to ancient point-bar deposits containing alternating sandstone and shale sequences are common in the low-energy fluvial environments of Riau rivers especially Kampar rivers.

Late Holocene lowland fluvial archives and geoarchaeology: Utrecht's case study of Rhine river abandonment under Roman and Medieval settlement

Fluvial lowlands have become attractive human settling areas all around the world over the last few millennia. Because rivers kept changing their course and networks due to avulsion, the sedimentary sequences in these areas are archives of both fluvial geomorphological and archaeological development. We integrated geological and archaeological datasets to demonstrate the concurrence of the gradual abandonment of a major Rhine channel (Utrecht, The Netherlands), the development of human habitation in the area, and the interactions between them.The Utrecht case study highlights the stage-wise abandonment of a natural river channel, due to avulsion, coincident with intensifying human occupation in Roman and Early Medieval times (1st millennium AD). The analyses make maximum use of very rich data sets available for the study area and the tight age control that the geo-archaeological dataset facilitates, offering extra means of time-control to document the pacing of the abandonment process. This allows us to quantify change in river dimensions and meander style and to provide discharge estimates for successive stages of the abandonment phase over a 1000-year period of abandonment succession, from mature river to eventual Late Medieval overbuilt canal when the Rhine branch had lost even more discharge.Continued geomorphic development during this period - which includes the 'Dark Ages' (450-1000 AD) - appears to have been crucial in the development of Utrecht from Roman army fortress to Medieval ecclesial centre. The settlement dynamics in and around the city of Utrecht changed during the various phases of abandonment. In the bifurcating network of river branches forming the Rhine-Meuse delta, the main Rhine branch hosted the Roman limes military border and transport route. The Rhine- Vecht bifurcation at Utrecht provided an excellent location to raise a Roman fort. Continued geomorphic activity during abandonment in Early Medieval times was characterised by enhanced overbank sedimentation and shifts in the position of bifurcations. River flooding became more incidental in this stage, and alluvial-ridge occupancy became sensitive to flooding events for several centuries. We conclude by demonstrating that similar human-river interactions during Roman times occurred in several other deltas within the former Roman empire, with differences depending on the position of a settlement within the delta, the overall hydrological situation, and the ability of societies to control the changing environment.