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What is a crevasse splay? inizia ad imparare
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A sedimentary fluvial deposit when a stream breaks it's levees and deposits onto a floodplain
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What are alluvial rivers? What other river category is there? inizia ad imparare
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Those flowing across their own deposits. The other category is incised rivers, which flow within eroded valleys
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subdivision of the stratigraphic record into mappable rock bodies on the bases of it's bounding discontinuities
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Describe the graded river profile inizia ad imparare
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A graded longitudinal profile of a river is the natural shape it assumes due to the dynamic balance between water and sediment - stepper at the source, flattening to a fraction of a degree at the mouth
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A curve of deepest points along the channel
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A narrow channel cutting off a point bar from the bank
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A lake created by an abandoned meander
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What are allogeneic processes? And autogenic? inizia ad imparare
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For a given depositional system, allogeneic processes are those which act outside of the system and affect the stratigraphic sequence, whereas autogenic are the internal ones
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It is a crescent-shaped depositional feature made of well sorted alluvial deposit accumulated at the inside bend of a river.
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What is a microscopic heterogeneity in a fluvial deposit? inizia ad imparare
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A porosity-permeability variation
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What is a mesoscopic heterogeneity in a fluvial deposit? inizia ad imparare
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Variation between bedding units and sedimentary structures
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What is a macroscopic heterogeneity in a fluvial deposit? inizia ad imparare
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Variability associated with the deposition of channels and bars
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What is a megascopic heterogeneity in a fluvial deposit? inizia ad imparare
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Variations across major sedimentary units and entire basins
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Because of turbulence, internal shear forces and bank-bed friction
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How is a bottom of a straight channel shaped and why inizia ad imparare
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There is a sinuous thalweg with alternate bars on the insides of it's bends, due to the same factors that cause point bars in meandering to rivers
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What factors lead to a braided river? inizia ad imparare
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Increased discharge, high discharge variations, higher slope, presence of coarse non-cohesive sediment which the river is unable to carry (may be brought suddenly due to a major flood or volcanic eruption)
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How is sinuousity defined? inizia ad imparare
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A ratio of the curvilinear and straight distance between two points
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Which characteristics of single-thread and anastomosed rivers stand out in comparison with meandering and braided? inizia ad imparare
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Low sinuousity, narrow channels (stable stream position), typically found on broad low-slope plains
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Where do anastomosed rivers form? inizia ad imparare
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In areas of rapid aggradation, such as confined, rapidly subsiding basins or where rapid base-level rise is matched by an abundant sediment supply.
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How does bank vegetation influence channel evolution? inizia ad imparare
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It stabilizes the bank, inhibits braiding and prevents flooding
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How can a dam affect the river? inizia ad imparare
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It reduces discharge variability, often leading to a development of a meandering style
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How are mid-channel braid bars formed? inizia ad imparare
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Outside of gentle bends in the thalweg, by a similar process as in point bar formation
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What is the definition of a bedload? inizia ad imparare
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Larger grains which are moved by sliding or rolling along the bed
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How does a debris flow deposit look like? inizia ad imparare
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Poorly sorted, contains large pebbles, cobbles or even boulders mixed together, embedded in a sand-silt-clay matrix. The matrix may show subtle grading, while the clasts may show a preferred orientation imposed by internal shear in the last flow moments
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What is river competence and what does it depend on? inizia ad imparare
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It is the maximum grain size that can be transported, depends on velocity and depth of the flow
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What is river capacity and what does it depend on? inizia ad imparare
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The total volume of sediment that can be moved, it depends on the magnitude of discharge
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What is a crevasse of a river? inizia ad imparare
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What is the difference between a current ripple and wave ripple? inizia ad imparare
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The former is asymmetrical (stepper on the lee side), the latter symmetrical
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Characterize trough cross-stratification inizia ad imparare
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In sections parallel to the flow we have curved planar erosional surfaces separating sets of foreset laminae. In perpendicular sections bowl-shaped trough surfaces separate concordant concave laminae. Formed by migration of ripples with irregular crests.
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Characterize planar cross-stratification inizia ad imparare
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In a section parallel to the flow we have flat erosional surfaces separating foreset laminae. In a perpendicular section laminae are also flat. Formed by migration of ripples with straight crests.
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What is the difference between ripples and dunes/megaripples? inizia ad imparare
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They are respectively smaller and larger than 5 cm in height. Dunes have larger wavelengths, may be covered by smaller ripples and they correspond to higher Froude numbers
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v/√(gl), where v - flow velocity, g - gravitational acceleration, l - flow depth
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What are upper flow regime bedforms and what stratigraphic record do they leave? inizia ad imparare
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Froude number close to 1: upper plane bed, leaving horizontal laminae. Fr>1: antidunes, small upstream migrating bedforms that do not get preserved in the stratigraphic record
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How to identify the growth of an active point bar? inizia ad imparare
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Succession of vegetation - the oldest part will have trees, younger grass, youngest will be bare
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What are floodplain deposits like? inizia ad imparare
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Mainly fine grained clastic units
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What is the process of avulsion? inizia ad imparare
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Permanent diversion of a channel through a crevasse, if it has built up a n alluvial ridge and the diversion results in a slope advantage for the channel
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What is a perennial river? inizia ad imparare
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A river which flows all year round
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How do outcrops in ephemeral arid environment braidplains look like? inizia ad imparare
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Tabular sandstone bodies several meters thick, consisting of plane-laminated sandstone. Or flood sheets comprising thinning and fining upward assemblages of cross-bedding and ripples
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How could an outcrop of a braided river in an arid gravel-dominated environment look like? inizia ad imparare
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Graded bedding of large grains due to size sorting during transportation. Or poorly sorted deposits from violent debris flows.
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What depositional sequence is typical in alluvial settings and why? inizia ad imparare
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Fining-upward. Aggradation of a channel results in decreasing flow depth and velocity, and consequently in a decrease in the competency and capacity of the flow. Development of point bars also tends to follow a fining-upward trend
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What are thick sandstone sheets in alluvial settings like and what could they be attributed to? inizia ad imparare
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4-16 m thick, possibly extending for many kilometers. They may reflect allogeneic causes such as gentle basin tilting or base level change
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How does sediment deposit look like in ephemeral streams? inizia ad imparare
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They are accumulated in flash floods, forming successions of stacked fining-upward sandstone sheets
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What does a coarsening-upward sequence in proximal alluvial deposits indicate? How is it called and how big can it be? inizia ad imparare
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It is a record of increasing source-area relief and depositional slope during tectonism (allogenic factor). It is referred to as tectonic cyclotherm and can be up to hundreds of meters thick and basin-wide
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What is the concept of accommodation in stratigraphy? inizia ad imparare
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The space available for sedimentation and how this volume changes in response to allogenic forces
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What is an eustatic sea-level? inizia ad imparare
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Distance from center of the Earth to the sea surface
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What happens to the river when the base level drops? inizia ad imparare
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If the exposed slope is stepper than river's graded profile, the river will erode its bed developing an incised valley. If the slope is more gentle, the river will increase sinuousity. If the river carries lots of sediment it may prograde and not incise.
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What happens to a river valley during a stable sea-level period? inizia ad imparare
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The valley will widen, which can be preserved in the form of terrace remnants along the valley walls
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What does a sequence boundary represent in non-marine systems? inizia ad imparare
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The final position of the subaerial erosion surface immediately prior to the commencement of a new phase of base-level rise
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What happens to incised valleys during base-level rise? inizia ad imparare
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How does base-level rise affect alluvial deposition? inizia ad imparare
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A decrease in slope in the lower course of the river leads to a reduction of competency and, consequently, in the grain size of the sediment transported and deposited
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What is an alloformation sequence related to base-level changes? inizia ad imparare
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FSST (falling-stage systems tract) -> SB (sequence boundary) -> LST (lowstand systems tract) -> TST (transgressive systems tract) -> MFS (maximum flooding surface) -> HST (highstand systems tract) -> SB
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How is the maximum flooding surface reflected in stratigraphy? inizia ad imparare
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Marine influence on typically fluvial deposits. Possible tidal influence (tidal bedding, reversing cross-bedding, sigmoidal bedding...)
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Why is there no erosion surface within coastal fluvial systems which would correspond to lowstand erosion? inizia ad imparare
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Because such surfaces are continually modified by channel scour, even during transgression
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What happens to a channel during a transition from cold to warm phase? How about the other way? inizia ad imparare
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Incision, because discharge increases while sediment yield is low. Rivers of anastomosing and meandering style tend to develop. The other way as well, except the discharge increase is not due to melting snow but reduced evapotranspiration.
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Which river style develops in glacial and interglacial periods, respectively? inizia ad imparare
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Glacial - braided. Interglacial - meandering
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How are sedimentary rocks formed? inizia ad imparare
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Weathering of source rock, transportation (water, wind, mudflow, glacier etc.), deposition and lithification (cementing, compacting), or direct precipitation (e g evaporites, reefs)
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How do mudstone-sandstone-carbonates proportions differ between the outcrop statistics and bulk chemistry of the Earth's crust? Why? inizia ad imparare
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Mudstone constitutes ~50% of the outcrops but it should be ~80% based on crust chemistry based estimations (sandstones/carbonate make up the rest in ~3/2 ratio). Reason: mudstones may preferentially land on oceanic floors.
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What are terrigenous clastic rocks? inizia ad imparare
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Sedimentary rocks formed from clasts of particles (blocks/boulders/cobbles/pebbles/granules/sand/silt/clay) with a fragmental texture (discrete grains in tangential contact with each other)
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What are rudites? What other categories are there? inizia ad imparare
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Rudites are sedimentary rocks with gravel-sized (>~2 mm) grains. Other categories are arenite (sand grains) and lutite (clay grains)
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What are the two main rudite types and what are their characteristics? inizia ad imparare
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Breccias (angular clasts) and conglomerates (more rounded grains)
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A fine-grained sedimentary rock characterized by fissibility, which is the tendency to break into thin slabs along it's laminations
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Above 256 mm: boulders, 64-256 mm: cobbles, 4-16 mm: pebbles, 2-4 mm: granules; 1/16 - 2 mm: sand, 1/256 - 1/16 mm: silt; below 1/256 mm: clay
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Φ = -log_2(S), where S is grain size in millimeters
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What is a measure of grain sorting? inizia ad imparare
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Standard deviation of the grain size distribution
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Which environments is well rounded and angular sand typical for, respectively? inizia ad imparare
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Well rounded: eolian, nearshore (surf zone). Angular: glacial, turbidity currents
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What is the difference between detrital and authigenic minerals? What are the examples of each category? inizia ad imparare
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Detrital (quartz, feldspar) survive weathering and are transported in sediment grains. Authigenic (gypsum, halite) form in-situ in the depositional site in response to geochemical processes
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Definition and examples of penecontemporaneous sedimentary structures inizia ad imparare
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Created not during, but shortly after deposition. Examples: load casts, mud cracks
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Definition and examples of primary sedimentary structures inizia ad imparare
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Created during deposition. Examples: graded bedding, cross-bedding, wave ripples
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What's the difference between strata and laminae? inizia ad imparare
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Thickness, the boundary is usually set at 1 cm
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What are sole markings and what processes are responsible for their formation? inizia ad imparare
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Structures formed in the bottom of a bed by: 1) uneven weight distribution upon a softer (mud) layer (load casts), 2) current action reworking the mud surface, 3) activities of living organisms on that surface
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Genesis-based subcategories of rudites inizia ad imparare
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Epiclastic (physical disintegration or weathering of preexisting rocks), pyroclastic (explosive volcanic activity), cataclastic (local Earth movements or solution phenomena), meteoritic (extraterrestrial)
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Volume % of sub-2 mm particles in conglomerates inizia ad imparare
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Clast-supported: <15%, matrix-supported: 15-80%
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