532 research outputs found
The role of discharge variability in the formation and preservation of alluvial sediment bodies
Extant, planform-based facies models for alluvial deposits are not fully fit for purpose, because they over-emphasise plan form whereas there is little in the alluvial rock record that is distinctive of any particular planform, and because the planform of individual rivers vary in both time and space. Accordingly, existing facies models have limited predictive capability. In this paper, we explore the role of inter-annual peak discharge variability as a possible control on the character of the preserved alluvial record. Data from a suite of modern rivers, for which long-term gauging records are available, and for which there are published descriptions of subsurface sedimentary architecture, are analysed. The selected rivers are categorized according to their variance in peak discharge or the coefficient of variation (CVQp = standard deviation of the annual peak flood discharge over the mean annual peak flood discharge). This parameter ranges over the rivers studied between 0.18 and 1.22, allowing classification of rivers as having very low ( 0.90) annual peak discharge variance. Deposits of rivers with very low and low peak discharge variability are dominated by cross-bedding on various scales and preserve macroform bedding structure, allowing the interpretation of bar construction processes. Rivers with moderate values preserve mostly cross-bedding, but records of macroform processes are in places muted and considerably modified by reworking. Rivers with high and very high values of annual peak discharge variability show a wide range of bedding structures commonly including critical and supercritical flow structures, abundant in situ trees and transported large, woody debris, and their deposits contain pedogenically modified mud partings and generally lack macroform structure. Such a facies assemblage is distinctively different from the conventional fluvial style recorded in published facies models but is widely developed both in modern and ancient alluvial deposits. This high-peak-variance style is also distinctive of rivers that are undergoing contraction in discharge over time because of the gradual annexation of the channel belt by the establishment of woody vegetation. We propose that discharge variability, both inter-annual peak variation and âflashinessâ may be a more reliable basis for classifying the alluvial rock record than planform, and we provide some examples of three classes of alluvial sediment bodies (representing low, intermediate, and high/very high discharge variability) from the rock record that illustrate this point
A meta-study of relationships between fluvial channel-body stacking pattern and aggradation rate: implications for sequence stratigraphy
A quantitative comparison of 20 literature case studies of fluvial sedimentary successions tests common assumptions made in published models of alluvial architecture concerning (1) inverse proportionality between channel-deposit density and floodplain aggradation rates, and (2) resulting characteristics of channel-body geometries and connectedness. Our results do not support the relationships predicted by established stratigraphy models: the data suggest that channel-body density, geometry, and stacking pattern are not reliable diagnostic indicators of rates of accommodation creation. Hence, these architectural characteristics alone do not permit the definition of accommodation-based âsystems tractsâ and âsettingsâ, and this calls into question current sequence stratigraphic practice in application to fluvial successions
Glaciolacustrine deposits formed in an ice-dammed tributary valley in the south-central Pyrenees: new evidence for late Pleistocene climate
Combined geomorphic features, stratigraphic characteristics and sedimentologic interpretation, coupled with optically stimulated luminescence (OSL) dates, of a glacio-fluvio-lacustrine sequence (LinĂĄs de Broto, northern Spain) provide new information to understand the palaeoenvironmental significance of dynamics of glacier systems in the south-central Pyrenees during the Last Glacial Cycle (â130âŻka to 14âŻka). The LinĂĄs de Broto depositional system consisted of a proglacial lake fed primarily by meltwater streams emanating from the small Sorrosal glacier and dammed by a lateral moraine of the Ara trunk glacier. The resulting glacio-fluvio-lacustrine sequence, around 55âŻm thick, is divided into five lithological units consisting of braided fluvial (gravel deposits), lake margin (gravel and sand deltaic deposits) and distal lake (silt and clay laminites) facies associations. Evolution of the depositional environment reflects three phases of progradation of a high-energy braided fluvial system separated by two phases of rapid expansion of the lake. Fluvial progradation occurred during short periods of ice melting. Lake expansion concurred with ice-dam growth of the trunk glacier. The first lake expansion occurred over a time range between 55âŻÂ±âŻ9âŻka and 49âŻÂ±âŻ11âŻka, and is consistent with the age of the Viu lateral moraine (49âŻÂ±âŻ8âŻka), which marks the maximum areal extent of the Ara glacier during the Last Glacial Cycle. These dates confirm that the maximum areal extent of the glacier occurred during Marine Isotope Stages 4 and 3 in the south-central Pyrenees, thus before the Last Glacial Maximum. The evolution of the LinĂĄs de Broto depositional system during this maximum glacier extent was modulated by climate oscillations in the northern Iberian Peninsula, probably related to latitudinal shifts of the atmospheric circulation in the southern North-Atlantic Ocean, and variations in summer insolation intensity
Scales and causes of heterogeneity in bars in a large multi-channel river: RĂo ParanĂĄ, Argentina
To date, published studies of alluvial bar architecture in large rivers have been restricted mostly to case studies of individual bars and single locations. Relatively little is known about how the depositional processes and sedimentary architecture of kilometreâscale bars vary within a multiâkilometre reach or over several hundreds of kilometres downstream. This study presents Ground Penetrating Radar and core data from 11, kilometreâscale bars from the RĂo ParanĂĄ, Argentina. The investigated bars are located between 30 km upstream and 540 km downstream of the RĂo ParanĂĄ â RĂo Paraguay confluence, where a significant volume of fineâgrained suspended sediment is introduced into the network. Barâscale crossâstratified sets, with lengths and widths up to 600 m and thicknesses up to 12 m, enable the distinction of large river deposits from stacked deposits of smaller rivers, but are only present in half the surface area of the bars. Up to 90% of barâscale sets are found on top of finerâgrained rippleâlaminated barâtrough deposits. Barâscale sets make up as much as 58% of the volume of the deposits in small, incipient midâchannel bars, but this proportion decreases significantly with increasing age and size of the bars. Contrary to what might be expected, a significant proportion of the sedimentary structures found in the RĂo ParanĂĄ is similar in scale to those found in much smaller rivers. In other words, large river deposits are not always characterized by big structures that allow a simple interpretation of river scale. However, the large scale of the depositional units in big rivers causes smallâscale structures, such as ripple sets, to be grouped into thicker cosets, which indicate river scale even when no obvious largeâscale sets are present. The results also show that the composition of bars differs between the studied reaches upstream and downstream of the confluence with the RĂo Paraguay. Relative to other controls on downstream fining, the tributary input of fineâgrained suspended material from the RĂo Paraguay causes a marked change in the composition of the bar deposits. Compared to the upstream reaches, the sedimentary architecture of the downstream reaches in the top ca 5 m of midâchannel bars shows: (i) an increase in the abundance and thickness (up to metreâscale) of laterally extensive (hundreds of metres) fineâgrained layers; (ii) an increase in the percentage of deposits comprised of ripple sets (to > 40% in the upper bar deposits); and (iii) an increase in barâtrough deposits and a corresponding decrease in barâscale crossâstrata (< 10%). The thalweg deposits of the RĂo ParanĂĄ are composed of dune sets, even directly downstream from the RĂo Paraguay where the upper channel deposits are dominantly fineâgrained. Thus, the change in sedimentary facies due to a tributary pointâsource of fineâgrained sediment is primarily expressed in the composition of the upper bar deposits
Multi-scale classification of fluvial architecture: An example from the Palaeocene-Eocene Bighorn Basin, Wyoming
Sudden changes in fluvial style across the Permian / Triassic boundary in the eastern Iberian Ranges, Spain: Analysis of possible causes
The sedimentary record of the Late Permian and Early Triassic of the eastern Iberian Ranges shows four major, sudden,
or very rapid, vertical changes in fluvial style. The Late Permian sedimentary cycle starts with the Boniches Formation, of
alluvial fan-braided fluvial origin, which grades vertically over within a few metres into the Alcotas Formation, deposited by
low to high sinuosity, avulsion-prone rivers with extensive floodplains. The Alcotas Formation contains calcimorphic soils,
plant remains and pollen and spore assemblages. However, the upper third of the unit is devoid of all organic remains and
soils and is characterized by a dominant red colour, the sandstone levels were deposited by high-sinuosity, meandering rivers.
This major change took place during the Late Permian and is probably coeval with the emplacement of the Emeishan basaltic
Large Igneous Province (LIP) in SE China. Rocks of the Boniches and Alcotas Formations are separated by an angular
unconformity from the overlying strata, which consist of the Late Permian conglomeratic Hoz del Gallo Formation, of
alluvial fanâgravel braided fluvial origin and the sandy Cañizar Formation, of low-sinuosity sandy river origin. The Permianâ
Triassic boundary lies, probably between the upper part of the Hoz del Gallo Formation and the first metres of the Cañizar
Formation. Late Permian pollen and spore assemblages have been found in the Hoz del Gallo Formation but the Cañizar
Formation is barren, with the exception of an Anisian (Middle Triassic) assemblage at the top. Tectonic extensional pulses in
the Iberian Basin caused the changes observed between the lower and upper parts of the Boniches Formation, at the base of
the Hoz del Gallo Formation and between the lower and upper part of this Formation. The changes observed in the
uppermost part of the Alcotas Formation are not easily explained by tectonic causes, nor those in the passage from the Hoz
del Gallo Formation to the Cañizar Formation. Similar sedimentary characteristics of the sandy Cañizar Formation such as
amalgamated sandstone bodies, erosion and reactivation surfaces, dominant trough cross-stratification, tabular geometry,
absence of plant remains and pollen and spores, and absence of silts and clays to those of coeval formations in places as far
away as Australia, South Africa and Brazil suggest a global rather than local cause for these abrupt changes in fluvial style.
This global cause was probably die-off of plant cover over extensive areas of the catchment, related to the end of the Permian mass extinction and possibly related to the emplacement of the West Siberian basaltic Large Igneous Province (LIP),
responsible for drastic atmospheric and marine changes
Sustained fluvial deposition recorded in Marsâ Noachian stratigraphic record
Orbital observation has revealed a rich record of fluvial landforms on Mars, with much of this record dating 3.6â3.0 Ga. Despite widespread geomorphic evidence, few analyses of Marsâ alluvial sedimentary-stratigraphic record exist, with detailed studies of alluvium largely limited to smaller sand-bodies amenable to study in-situ by rovers. These typically metre-scale outcrop dimensions have prevented interpretation of larger scale channel-morphology and long-term basin evolution, vital for understanding the past Martian climate. Here we give an interpretation of a large sedimentary succession at Izola mensa within the NW Hellas Basin rim. The succession comprises channel and barform packages which together demonstrate that river deposition was already well established >3.7 Ga. The deposits mirror terrestrial analogues subject to low-peak discharge variation, implying that river deposition at Izola was subject to sustained, potentially perennial, fluvial flow. Such conditions would require an environment capable of maintaining large volumes of water for extensive time-periods, necessitating a precipitation-driven hydrological cycle
Scales and causes of heterogeneity in bars in a large multi-channel river: RĂo ParanĂĄ, Argentina
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