Saw-tooth bars are shore-oblique sand bars that are found along most ebb-tidal deltas of the Frisian Wadden Islands. Although they might significantly affect sediment transport pathways and volumes on the deltas, their general characteristics and dynamics are largely unknown. The main aim of this paper is to determine the typical saw-tooth bar heights, wave lengths, widths, orientations, migration rates and depths of occurrence. To this end, we analysed bathymetries from the Dutch and German Frisian Wadden Islands between 1970 and 2015. Bar heights range between 0.5 and 2 m, and wave lengths range between 300 and 900 m, with an average of 670 m, and the bar crests have a down contour orientation of approximately 25°. The bars are between 800 and 2200 m wide. Saw-tooth bars are located at depths from 3 to 12 m, depending on the slope of the area. Migration speeds of up to 112 m/year were found, using a spatial correlation method. Bar height and migration speed are positively linearly correlated, as well as bar wave length and bar width, bar height and the orientation of the depth contours and migration speed and the orientation of the area. The derived characteristics are compared to those of other bar types to evaluate hypotheses regarding the formation mechanism of the bars.
Ocean Dynamics - Ebb-tidal deltas are shallow features seaward of tidal inlets, acting as a wave filter for the nearby barrier island and a source of sediment for the landward tidal basin. On many... 相似文献
The problem of insufficient age-control limits the utilisation of the 8.2 ka BP event for modelling freshwater forcing in climate change studies. High-resolution radiocarbon dates, magnetic susceptibility and lithostratigraphic evidence from a lake sediment core from Nedre Hervavatnet located at Sygnefjell in western Norway provide a record of the early Holocene. We use the method of radiocarbon wiggle-match dating of the lake sediments using the non-linear relationship between the 14C calibration curve and the consecutive accumulation order of the sample series in order to build a high-resolution age-model. The timing and duration of Holocene environmental changes is estimated using 38 AMS radiocarbon dates on terrestrial macrofossils, insects and chironomids covering the time period from 9750 to 1180 cal BP. Chironomids, Salix and Betula leaves produce the most consistent results. Sedimentological and physical properties of the core suggest that three meltwater events with high sedimentation rates are superimposed on a long-term trend with glacier retreat between 9750 and 8000 cal BP. The lake sediment sequence of Nedre Hervavatnet demonstrates the following: only a reliable high-resolution geochronology based on carefully selected terrestrial macrofossils allows the reconstruction of a more refined and complex environmental change history before and during the 8.2 ka event. 相似文献
Turbidites embedded in lacustrine sediment sequences are commonly used to reconstruct regional flood or earthquake histories. A critical step for this method to be successful is that turbidites and their trigger mechanisms are determined unambiguously. The latter is particularly challenging for prehistoric proglacial lake records in high-seismicity settings where both earthquake-generated and flood-generated turbidites interrupt the background varved sedimentation. This calls for a new method to allow efficient and objective identification and classification of turbidites. This study examined turbidites in five long (9 to 17 m) sediment cores from Eklutna Lake, a proglacial lake in south-central Alaska, using standard core logging and grain-size data. A novel statistical approach is presented, in which varve-thickness distributions were first analyzed to objectively identify the thickest turbidites and distinguish them from background sedimentation. For each turbidite, a selection of variables were then measured, including: basal grain-size, thickness, magnetic susceptibility and spectrophotometric variables. Triggering mechanisms were discriminated by a combination of principal component analysis and clustering, and by calibration with historical events. Using this approach, a 2250 year long lake-wide event stratigraphy was constructed, with 94 prehistoric events, including 24 earthquake and 70 flood events. Basal grain-size and thickness variables turn out to be the most effective proxies for discrimination. This statistical approach is a powerful and new method to identify turbidites and their triggering mechanisms in long prehistoric sediment records. It opens up new prospects for palaeoseismological, palaeohydrological and palaeoclimate studies in proglacial lakes worldwide. 相似文献
Amalgamation of a number of continental fragments during the Late Neoproterozoic resulted in a united Gondwana continent.
The time period in question, at the end of the Precambrian, spans about 250 million years between ∼800 and 550 Ma. Geological
activity focused along orogenic belts in Africa during that time period is generally referred to as “Pan African.” We identify
three age-related classes of tectonic terranes within these orogenic belts, differentiated on the basis of the formation-age
of their crust: juvenile (e.g. mantle derived at or near the time of the orogenesis, ∼0.5–0.8 Ga), Paleoproterozoic (∼1.8–2.5 Ga),
Archean (>2.5 Ga). We combine African mineral deposits data of these terranes on a new Neoproterozoic tectonic map of Africa.
The spatial correlation between geological terranes in the belts and mineral occurrences are determined in order to define
the metallogenic character of each terrane, which we refer to as their “metallogenic fingerprint.” We use these fingerprints
to evaluate the effectiveness of mobilization (“recycling”) of mineral deposits within old crustal fragments during Pan African
orogenesis. This analysis involves normalization factors derived from the average metallogenic fingerprints of pristine older
crust (e.g. Palaeoproterozoic shields and Archean cratons not affected by Pan African orogenesis) and of juvenile Pan African
crust (e.g. the Nubian Shield). We find that mineral deposit patterns are distinctly different in older crust that has been
remobilized in the Pan African belts compared to those in juvenile crust of Neoproterozoic age, and that the concentration
of deposits in remobilized older crust is in all cases significantly depleted relative to that in their pristine age-equivalents.
Lower crustal sections (granulite domains) within the Pan African belts are also strongly depleted in mineral deposits relative
to the upper crustal sections of juvenile Neoproterozoic terranes. A depletion factor for all terranes in Pan African orogens
is derived with which to evaluate the role of mineral deposit recycling during orogenesis. We conclude that recycling of old
mineral deposits in younger orogenic belts contributes, on average, to secular decrease of the total mineral endowment of
continental crust. This could be of value when formulating exploration strategies. 相似文献
Deltas are important coastal sediment accumulation zones in both marine and lacustrine settings. However, currents derived from tides, waves or rivers can transfer that sediment into distal, deep environments, connecting terrestrial and deep marine depozones. The sediment transfer system of the Rhone River in Lake Geneva is composed of a sublacustrine delta, a deeply incised canyon and a distal lobe, which resembles, at a smaller scale, deep‐sea fan systems fed by high discharge rivers. From the comparison of two bathymetric datasets, collected in 1891 and 2014, a sediment budget was calculated for eastern Lake Geneva, based on which sediment distribution patterns were defined. During the past 125 years, sediment deposition occurred mostly in three high sedimentation rate areas: the proximal delta front, the canyon‐levée system and the distal lobe. Mean sedimentation rates in these areas vary from 0·0246 m year?1 (distal lobe) to 0·0737 m year?1 (delta front). Although the delta front–levées–distal lobe complex only comprises 17·0% of the analysed area, it stored 74·9% of the total deposited sediment. Results show that 52·5% of the total sediment stored in this complex was transported toward distal locations through the sublacustrine canyon. Namely, the canyon–levée complex stored 15·9% of the total sediment, while 36·6% was deposited in the distal lobe. The results thus show that in deltaic systems where density currents can occur regularly, a significant proportion of riverine sediment input may be transferred to the canyon‐lobe systems leading to important distal sediment accumulation zones. 相似文献