Unit 6 - Depositional Environments

Question 1

What does facies analysis mean?

Answer 1

• the method used to interpret the environment of deposition of a sedimentary succession

Question 2

What does facies mean?

Answer 2

• it refers to a unit that is identified by having certain specific characteristics.

Question 3

Depositional facies is defined by?

Answer 3

• lithology (physical characteristics) of the sediment

this includes:

• sedimentary structures
• colour
• diagenetic character
• fossil content.

Question 4

What is a depositional system?

Answer 4

• three-dimensional association of facies, which are genetically linked by sedimentary processes and environments of deposition

Question 5

Interpretation of facies tell us what?

Answer 5

the responsible processes for the depostion of the facies but not about the environment of the deposition → to identify that, we need to consider the vertical and lateral association of facies

Question 6

What is a genetic unit?

Answer 6

• a group of facies associations that were formed by closely related processes in related depositional environments

Example: a delta lobe would form a genetic unit, containing several sandstone facies (channel deposits) as well as mudstone, siltstone and coaly facies deposited in inter-channel areas.

Question 7

the arrangement of multiple facies is highly dependent on ?

Answer 7

the depostional environment

Question 8

Where do aeolian sediments mostly occur?

Answer 8

Desert environments

Question 9

How thick are aeolian dune sets normally?

Answer 9

meters to 10s of meters → barely happens subaqueous → therefore large set size is often taken as aeolian origin

Question 10

Periods of wetting and drying in aeolian successions can be caused by ?

Answer 10

• rise in sea or lake level or increase in rainfall

Question 11

What happens to aeolian dunes during wetting periods?

Answer 11

• They are less active and might get eroded

Question 12

The four types of channels of river forms (fluvial environments)?

Answer 12

Question 13

What are the two most common types of river systems?

Answer 13

• meandering
• braided

Question 14

An old meander loop often turns to an ?

Answer 14

ox-box lake

Question 15

What is the floodplain?

Answer 15

• the surrounding low-lying land of a river, which may be flooded during periods of flooding

Question 16

What happens, when a river breaks through its banks?

Answer 16

They form temporary crevasse channels

Question 17

If river go over their banks during a flood, how will they distribute their sediments?

Answer 17

• They will deposit their coarser sediments closer to the river → they produce thin beds, that become finer and thinner, the further away from the river

Question 18

What is a levee?

Answer 18

• if meandering rivers repeatedly produce floods over many years → elevated ridges of sediment, known as levees, close to the channel will start to form

Question 19

What is the difference between a meandering river and a braided river?

Answer 19

• Meandering rivers only have one active channel at a time
• braided rivers have number of active channels, separated by sandy or gravelly bars

Question 20

Where do braided rivers form?

Answer 20

• slightly steeper slopes

→ with a high proportion of sandy or gravelly sediment

Question 21

In which direction do individual channels of braided river systems migrate?

Answer 21

• laterally, shifting their course frequently
• the intervening bars migrate both downstream and across the streams

→ Braided rivers therefore produce compound sandbodies consisting of a number of mutually-erosive channel bodies

Question 22

Anastomosing rivers create which kind of sand bodies?

Answer 22

• relatively narrow but thick multi-storey sandbodies

Question 23

Meandering Rivers produce classically what kind of sandbodies?

Answer 23

erosive-based, upward-fining sandbodies

Question 24

What do rivers form when they leave the confines of a valley ?

Answer 24

• they usually form cones of sediment known as alluvial fans

Question 25

Describe alluvial fans

Answer 25

• sediment becomes finer away from fan apex
• migration of fan produces an upwards coarsing trend

Question 26

When the relative sealevel falls, what happens to the sediment passing through an incised valley?

Answer 26

• it will bypass it

Question 27

When the relative sealevel rises, what happens to the sediment passing through an incised valley?

Answer 27

Sedimentation will start in the incised valley, which will continue into the highstand period

Question 28

Beaches form the boundary between …?

Answer 28

Shallow marine and terrestrial environments

Question 29

Why does a beach produce an upward-coarsening facies sequence, as it progrades offshore?

Answer 29

Because in general, the energy is greatest, and the sediment coarsest, on the higher parts of the beach profile

Question 30

Why are waters in a lagoon brackish?

Answer 30

Because of the mixing of fresh water and of sea water → brackish means intermediate salinity

Question 31

How is a delta formed , generally?

Answer 31

major river reaches a standing body of water, such as a lake or the sea.

→ river supplies the sediment faster than the basinal processes in body of water can rework it, then the shoreline will project locally into the basin, forming a delta.

Question 32

What will a river do on the subaerial part of the delta plain?

Answer 32

Split itself up into two or more little channels causing sediment to be supplied to many points along the delta front

Question 33

What are reasons for the different delta forms?

Answer 33

climate, tectonic setting, sediment supply, energy of receiving basin

Question 34

What will fluvial dominated deltas do?

Answer 34

• supply more sediment to coastline than can be reworked by basinal processes → resultant delta will therefore form a pronounced protuberance of the shoreline

Question 35

What will tidal dominated deltas do?

Answer 35

the tidal processes will most likely produce a radial pattern of distributary channels which become broader towards the basin

Question 36

What will wave-dominated deltas do ?

Answer 36

• a big amount of the sediment supplied by river mouth will be reworked into beach ridges on either side. The resulting delta will, therefore, often cause only a slight deflection of the coastline

Question 37

What happens generally at the mouths of channels on fluvial-dominated deltas, where sediment laden river waters interact with the basinal water?

Answer 37

• fluvial flow loses power and starts to deposit sediment → sediment fines away from the apex
• Progradation oft delta mouthbar therefore produces an upward-coarsening sequence dominated by current-generated structures

Question 38

What will the vertical facies of wave dominated deltas look like?

Answer 38

• Facies sequences will be very similar to those produced by prograding beaches → differing mainly by the presence of associated distributary channels

Question 39

What is a big difference between the physical processes like tides and waves of coastal and shallow marine environments and deep marine clastic environments ?

Answer 39

tides and waves, which dominate coastal and shallow marine environments are generally absent or ineffective in the deep marine environment

Question 40

What kind of environments are deep oceans most of the time?

Answer 40

• low-energy environments in which fine grained carbonate or clastic muds are able to accumulate
• coarse-grained sediment is transported into these environments by several infrequent and shortlived processes, of which the sediment gravity flow processes (slumps, debris flows, turbidity currents etc.) are dominant.

Question 41

What were the earliest deep marine sandstones?

Answer 41

Turbidites

Question 42

Facies models for turbidites, debris flow deposits (debrites) and slump deposits - it is just a graphik, to check out the succesions!

Answer 42

Question 43

In deep marine clastic environments, examination of some turbidite successions showed that facies sequences, in which the thickness and/or mean grain size of the sandstone beds increased or decreased upwards what is the explanation for that?

Answer 43

• these trends were attributed to the deposition on different parts of a submarine fan

Question 44

What is a submarine fan?

Answer 44

• occur offshore from major river systems or off the continental shelf
• they are sourced from a single point (like deltas) ( or multiple sourced) and contain a system of distributary channels supplying lobes
• upward-thickening trends in turbidite sequences are attributed to the progradation of fan lobes
• upward-thinning trends to deposition within a channel.

Question 45

Multiple source submarine fan (Graphic)

Answer 45

Question 46

What are the “natural” sediments in marine environments?

Answer 46

Carbonates

Question 47

What is a big difference between clastic grains and carbonate grains?

Answer 47

• clastic grains have their origin most of the time further away from their site of deposition
• carbonate grains are formed at or very close to their eventual site of deposition

Question 48

What are bioclasts?

Answer 48

• majority of carbonate grains are precipitated by organisms

or by biochemical processes

• clasts derived from skeletal material are known as bioclasts → most of the time broken pieces of skeletons

Question 49

What are pellets or pelloids?

Answer 49

• ovoid shapes composed of micrite (lime mud) and microscopic shells

Question 50

What are ooids?

Answer 50

• small (less than 2mm) near-spherical carbonate grains
• generally form in moderate to high energy shallow marine environments

Question 51

What are intraclasts?

Answer 51

• common for carbonates to be partially lithified at, or close to, the sediment surface

→ If erosion then occurs, sediment may break into groups of grains lightly cemented together →called intraclasts

Question 52

Where are the majority of carbonates deposited?

Answer 52

• marine or lacustrine environments