1,721,039 research outputs found
Sezione 309010, Foglio 309 Montepulciano, Carta Geologica Regionale, Scala 1:10.000, Regione Toscana
No full textMetadati Carta Geologica Regione Toscana scala 1:10.000. http://159.213.57.103/geoweb/listmet/lista_metadati_10k.ht
Climatic and tectonic signature of a Late Pliocene valley (Siena Basin, Northern Apennines, Italy)
No full textIncised valleys entirely filled with fluvial deposits are rarely described in the literature, and even rarer are accounts of incised valleys whose filling is not driven by relative sea-level changes. Valleys of this type have not been widely recognized in the stratigraphic record, because they are likely to occur in areas undergoing intense erosion or to be encased within alluvial deposits with no strong lithologic contrast. In this paper, an upper Pliocene fluvial valley fill, encased within shallow marine middle Pliocene deposits and pre-Neogene bedrock (eastern Siena Basin, Northern Apennines, Italy) and accumulated in response to tectonics and climate changes, is described. The recognition of this fluvial body (up to 40 m thick) in an incisedvalley setting is based on its overall geometry, lithofacies characteristics, gravel composition, and fossil content. Upvalley fill is represented by amalgamated gravels emplaced by heavily sediment-laden flows, whereas downvalley fill shows a more organized depositional style and consists of two fining-upward successions. Notable is the presence at the top of the lower succession of decameter-thick floodplain fines, which extend from one valley wall to another and have no fine-grained correlatives in the upvalley fill. Valley incision resulted from a drop in relative sea level, arising from late middle Pliocene regional uplift. Stratigraphic and paleontological data constrain valley filling to the late Pliocene–early Pleistocene time span. Such a filling stemmed from an increase in sediment supply, which resulted from the interplay between uplift of the Chianti Ridge and a climatic change toward humid conditions, as recorded in the coeval strata of the adjacent Valdarno Basin. Beyond the regional significance, an important implication of the case study is that the incision and filling factors need not be genetically related. Moreover, the stratigraphic architecture of the studied fluvial fill is discussed in terms of the relations between sediment supply and sediment storage en route from the source area to the depositional site and their association with tectonic movements. Specifically, the valley-wide floodplain fines are thought to record the response of the fluvial system to a tectonic rejuvenation of catchments, which modified the valley gradient and promoted upstream gravel storage, whereas the overlying gravels manifest the reestablishment of a new equilibrium river profile
Influence of extreme and annual floods on point-bar sedimentation: Inferences from Powder River, Montana, USA
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Morpho-sedimentary evolution of a microtidal meandering channel driven by 130-years of natural and anthropogenic modifications of the Venice Lagoon (Italy)
Full text linkAbstract Tidal channels form the pathways for tidal currents to propagate and distribute clastic sediments and nutrients, thus providing a primary control on tidal-landscape ecomorphodynamics. Most tidal channels in both estuarine and lagoonal environments have a tendency to meander, yet very few studies exist that investigate the full spectrum of processes controlling tidal meander morpho-sedimentary evolution. The Venice Lagoon (Italy) offers a unique opportunity to shed light on this topic, because a long record of morphological and sedimentary data is available, which allows one to relate tidal channel evolution to the hydrodynamic and morphological changes undergone by the lagoon. In particular, during the last 130 years, feedback between rising relative sea levels and anthropogenic interventions have caused severe modifications of the Lagoon hydro- and morpho-dynamics. Here we investigate how these modifications fed back into the morpho-sedimentary evolution of a meandering tidal channel located in the northern Lagoon. Combining extensive datasets of aerial photographs, topographic and bathymetric surveys, geophysical investigations, sedimentary core analysis, and numerical modeling, we show that enhanced local tidal ranges and water discharges determine adjustments of channel cross-sectional geometries proportional to increasing tidal prisms, while changes in local tidal asymmetries caused modifications of the local sediment transport regime, resulting in the development of bar-pool patterns according to the dominant tidal phase. Such bar-pool patterns eventually determine channel migration through a bar-push mechanism controlled by a fluvial-like, quasi-linear relationship between local channel curvature and lateral migration rates. Critical differences in sediment transport regime are however highlighted between fluvial and tidal meanders, the latter being potentially characterized by high concentrations of suspended sediment during periods of slack waters when wind-driven sediment transport processes are not negligible. This could hamper the formation of high-relief bedforms, with profound implications for the sedimentology of tidal point-bar deposits
Planform-asymmetry and backwater effects on river-cutoff kinematics and clustering
No full textRiver bends occasionally meander to the point of cutoff, whereby a river shortcuts itself and isolates a portion of its course. This fundamental process fingerprints a river's long-term planform geometry, its stratigraphic record, and biogeochemical fluxes in the floodplain. Although meander cutoffs are common in fast-migrating channels, timelapse imagery of the Earth surface typically does not offer a long enough baseline for statistically robust analyses of these processes. We seek to bridge this gap by quantifying cutoff kinematics along the Humboldt River (Nevada) – a stream that, from 1994 to 2019, hosted an exceptionally high number of cutoffs (specifically, 174 of the chute type and 53 of the neck type). A coincidence between major floods and cutoff incidence is first suggestive of hydrographic modulation. Moreover, not just higher sinuosity but also upstream planform skewness is associated with higher cutoff incidence and channel widening for a sub-population of chute cutoffs. We propose a conceptual model to explain our results in terms of channel-flow structure and then examine the distances between adjacent cutoffs to understand the mechanisms governing their clustering. We find that both local and nonlocal perturbations together trigger the clustering of new cutoffs, over distances capped by the backwater length and over yearly to decadal timescales. Our research suggests that planform geometry and backwater controls might sway the occurrence of cutoff clusters – both local and nonlocal – thereby offering new testable hypotheses to explore the evolution of meandering-river landscapes that have significant implications for river engineering and stratigraphic modelling. © 2020 John Wiley & Sons, Ltd
Sezione 321100, Foglio 321 Abbadia S. Salvatore, Carta Geologica Regionale, Scala 1:10.000, Regione Toscana
No full textMetadati Carta Geologica Regione Toscana scala 1:10.000. http://159.213.57.103/geoweb/listmet/lista_metadati_10k.ht
Tidal Flow Asymmetry and Discharge of Lateral Tributaries Drive the Evolution of a Microtidal Meander in the Venice Lagoon (Italy)
No full textTidal landscapes are extensively characterized by the presence of meandering channels, the latter being important for the ecomorphodynamic evolution of these environments. It remains unclear whether changes in the relative strength of maximum flood and ebb currents (i.e., tidal flow asymmetries), together with the widespread presence of lateral tributaries, cause tidal meanders to evolve differently from their fluvial relatives. Here, we investigate the evolution of a meandering channel in the Venice Lagoon (Italy) that receives water from two major tributaries along its outer bank. Using a 2-D numerical model, we first analyze the changes in local tidal flow asymmetries, both natural and anthropogenically induced, which occurred during the last 120 years. The effects of these modifications on the meander morphodynamics are then investigated by means of a 3-D numerical model, and results are compared to modern and historical field data spanning more than one century. We show that under asymmetric tidal flows, tidal meanders develop depositional patterns according to the dominant flow direction, similar to that of fluvial meanders. In addition, the morphological effects of the nondominant tidal flow become increasingly negligible as tidal flow asymmetry increases. We also show that enhanced sediment and water fluxes from major lateral tributaries, sourced from wind-wave exposed tidal flats, can critically influence the development of erosional and depositional patterns within tidal meanders otherwise sheltered from wind action by the presence of salt marshes
Reading tidal processes where their signature is cryptic: The Maastrichtian meandering channel deposits of the Tremp Formation (Southern Pyrenees, Spain)
No full textTidal currents can propagate tens of kilometres landward from the shoreline, forming dense networks of meandering channels, which drain vegetated areas and range in width from tens to hundreds of metres. Sedimentary products of these inland tidal channels are poorly documented in the stratigraphic record, as most studies on tidal processes occurring in inland areas are focused on fluvio-tidal interaction. The present study contributes to filling this gap, investigating Late Maastrichtian meandering-channel deposits of the Tremp-Graus Basin (Southern Pyrenees, Spain). These deposits accumulated in a growth syncline, which allowed the development of a 20 to 30 km wide, 80 to 100 km long, tidal embayment. Sporadic occurrence of poorly-developed rhythmites and bidirectional currents confirms the occurrence of tidal processes, whereas their local dominance is inferred from several pieces of evidence, including: (i) disproportion between depth of the larger channels and related catchment area; (ii) landward-accretion of point-bar bodies; (iii) landward fining of point-bar bodies, and (iv) reconstruction of palaeo-flow patterns at the meander bend scale. This work shows that three-dimensional architectural modelling and reconstruction of bar planform transformation styles can provide a critical contribution to the understanding of tidal control on sedimentation in inland areas
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