Comparative Sedimentology

What is a crevasse splay?

A sedimentary fluvial deposit when a stream breaks it's levees and deposits onto a floodplain

What are alluvial rivers? What other river category is there?

Those flowing across their own deposits. The other category is incised rivers, which flow within eroded valleys

Define allostratigraphy

subdivision of the stratigraphic record into mappable rock bodies on the bases of it's bounding discontinuities

Describe the graded river profile

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

Define a thalweg

A curve of deepest points along the channel

What is a chute channel?

A narrow channel cutting off a point bar from the bank

What is an oxbow lake?

A lake created by an abandoned meander

What are allogeneic processes? And autogenic?

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

What is a point bar?

It is a crescent-shaped depositional feature made of well sorted alluvial deposit accumulated at the inside bend of a river.

What is a microscopic heterogeneity in a fluvial deposit?

A porosity-permeability variation

What is a mesoscopic heterogeneity in a fluvial deposit?

Variation between bedding units and sedimentary structures

What is a macroscopic heterogeneity in a fluvial deposit?

Variability associated with the deposition of channels and bars

What is a megascopic heterogeneity in a fluvial deposit?

Variations across major sedimentary units and entire basins

Why do rivers meander?

Because of turbulence, internal shear forces and bank-bed friction

How is a bottom of a straight channel shaped and why

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

What factors lead to a braided river?

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)

How is sinuousity defined?

A ratio of the curvilinear and straight distance between two points

Which characteristics of single-thread and anastomosed rivers stand out in comparison with meandering and braided?

Low sinuousity, narrow channels (stable stream position), typically found on broad low-slope plains

Where do anastomosed rivers form?

In areas of rapid aggradation, such as confined, rapidly subsiding basins or where rapid base-level rise is matched by an abundant sediment supply.

How does bank vegetation influence channel evolution?

It stabilizes the bank, inhibits braiding and prevents flooding

How can a dam affect the river?

It reduces discharge variability, often leading to a development of a meandering style

How are mid-channel braid bars formed?

Outside of gentle bends in the thalweg, by a similar process as in point bar formation

What is the definition of a bedload?

Larger grains which are moved by sliding or rolling along the bed

How does a debris flow deposit look like?

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

What is river competence and what does it depend on?

It is the maximum grain size that can be transported, depends on velocity and depth of the flow

What is river capacity and what does it depend on?

The total volume of sediment that can be moved, it depends on the magnitude of discharge

What is a crevasse of a river?

A break in the bank

What is pedogenesis?

Soil formation

What is the difference between a current ripple and wave ripple?

The former is asymmetrical (stepper on the lee side), the latter symmetrical

Characterize trough cross-stratification

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.

Characterize planar cross-stratification

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.

What is the difference between ripples and dunes/megaripples?

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

Froude number definition

v/√(gl), where v - flow velocity, g - gravitational acceleration, l - flow depth

What are upper flow regime bedforms and what stratigraphic record do they leave?

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

How to identify the growth of an active point bar?

Succession of vegetation - the oldest part will have trees, younger grass, youngest will be bare

What are floodplain deposits like?

Mainly fine grained clastic units

What is the process of avulsion?

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

What is a perennial river?

A river which flows all year round

How do outcrops in ephemeral arid environment braidplains look like?

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

How could an outcrop of a braided river in an arid gravel-dominated environment look like?

Graded bedding of large grains due to size sorting during transportation. Or poorly sorted deposits from violent debris flows.

What depositional sequence is typical in alluvial settings and why?

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

What are thick sandstone sheets in alluvial settings like and what could they be attributed to?

4-16 m thick, possibly extending for many kilometers. They may reflect allogeneic causes such as gentle basin tilting or base level change

How does sediment deposit look like in ephemeral streams?

They are accumulated in flash floods, forming successions of stacked fining-upward sandstone sheets

What does a coarsening-upward sequence in proximal alluvial deposits indicate? How is it called and how big can it be?

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

What is the concept of accommodation in stratigraphy?

The space available for sedimentation and how this volume changes in response to allogenic forces

What is an eustatic sea-level?

Distance from center of the Earth to the sea surface

What happens to the river when the base level drops?

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.

What happens to a river valley during a stable sea-level period?

The valley will widen, which can be preserved in the form of terrace remnants along the valley walls

What does a sequence boundary represent in non-marine systems?

The final position of the subaerial erosion surface immediately prior to the commencement of a new phase of base-level rise

What happens to incised valleys during base-level rise?

They become estuaries

How does base-level rise affect alluvial deposition?

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

What is an alloformation sequence related to base-level changes?

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

How is the maximum flooding surface reflected in stratigraphy?

Marine influence on typically fluvial deposits. Possible tidal influence (tidal bedding, reversing cross-bedding, sigmoidal bedding...)

Why is there no erosion surface within coastal fluvial systems which would correspond to lowstand erosion?

Because such surfaces are continually modified by channel scour, even during transgression

What happens to a channel during a transition from cold to warm phase? How about the other way?

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.

Which river style develops in glacial and interglacial periods, respectively?

Glacial - braided. Interglacial - meandering