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[Meer zien]Voorbeeld 4 van de 32 pagina's
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Voorbeeld van de inhoud
SEDIMENTAIRE SYSTEMENCHAPTER 23: SEQUENCE STRATIGRAPHY AND SEA-
LEVEL CHANGES
Change is base level (usually relative sea level) results in a change in patterns of sedimentation in
almost all depositional environments. Possible causes for (relative) sea-level changes: tectonic
activity, change in water volume (eustatic sea-level change, eustasy) and sedimentation.
Transgression: relative sea-level rise and land moves inward. Regression: shoreline moves seawards
as a result of sedimentation occurring at the coast. Forced regression: shoreline moves seawards
due to a relative sea level drop. See pages 352 & 353. A forced regression may be distinguished from
a simple regression by evidence of erosion in the coastal and shallowest marine deposits. Sediments
will be deposited in places where there is space available to accumulate material: concept of
accommodation. This is determined by changes in relative sea-level. Higher relative sea-level
space filled up with sediment till equilibrium profile is reached. The equilibrium profile is a notional
surface of deposition relative to sea-level and sedimentation occurs on any point in the shallow
marine environment until this surface is reached. Patterns of sedimentation and sea-level and
shoreline trajectory (pg. 355):
1. If the rate of sediment supply is very low then the shoreline will move landward without
deposition occurring and with the possibility of erosion (transgression)
2. With moderate sedimentation rates, but high rates of sea-level rise, deposition will occur as the
shoreline moves landward (transgression)
3. If it is a higher sedimentation rate, then as fast as the sea level rises the space is filled up with
sediment and the shoreline stays in the same place. (shoreline remains fixed)
4. At high sedimentation rates, the shoreline will still move seawards, even though the sea level is
rising. (regression)
5. During periods when the sea level is static the addition of sediment causes the shoreline to shift
seawards. (regression)
6. At low rates of sea-level fall and/or high rates of sediment supply deposition occurs as the
shoreline moves seawards (forced regression)
7. If the rate of sea-level fall is relatively high and the rate of sedimentation is low, there is no
sedimentation, and there may be erosion (forced regression)
8. A coast undergoing rapid erosion during sea-level fall could theoretically fall into this category.
Low angle onshore slope will allow the shoreline to move further landward during sea-level rise. A
gently sloping seafloor will result in the shoreline shifting further seaward during sea-level fall. These
are aspects of physiography. For a fixed shoreline the sediments will build up without variation,
aggradation. The pattern of shallowing up of the facies through a succession is called progradation,
by product of regression (4,5,6). When the pattern is one of deeper deposits progressively through
the succession, we call this retrogradation, characteristic to transgression. To preserve a cycle of
sedimentation a condition of net accommodation creation through time is required. Two main types
of continental margin are recognised, each resulting in different stratigraphic patterns when there
are sea-level fluctuations: (a) shelfbreak margins, have a shallow shelf area bordered by a steeper
slope down to the deeper basin floor (b) ramp margins, do not have distinct change in slope at the
shelf edge.
A sequence boundary marks the end of the end of the previous depositional sequence and the start
of a new one. When there is no unconformity in the basin floor a correlative conformity will mark
the sequence boundary. This is a surface that is laterally equivalent to the unconformity that forms
the sequence boundary on the shelf. The point of the furthest landward extent of the shoreline is
,called the maximum flooding surface (MFS), this does not represent the highest sea-level in the
cycle = due to sediment starvation. The fluvial deposition is no longer confined to incised valleys,
resulting in deposition in rivers and on overbank areas over wide areas of the coastal plain.
A parasequence is defined as a genetically related succession of bedsets that is bounded by marine
flooding surfaces (or their correlative surfaces) on top and at the bottom. They form because the
actual variation in sea level is not smooth but occurs in series of shorter stages (pg. 362). Expanded
succession: facies units become thicker. Foreshortened succession: facies become thinner as
erosion occurs. Although an individual parasequence shows an internal progradational bed pattern,
they may be arranged into parasequence sets that overall show progradational, aggradational or
retrogradational patterns (image pg. 365.) With a high sedimentation rate or slow sea-level fall, the
individual parasequences are stacked against each other in an attached falling stage systems tract,
whereas faster sea-level fall and/or lower sediment supply results in a detached falling stage
systems tract. Variables that control sequence development
1. the magnitude of relative sea-level change
2. the rate of relative sea-level change
3. the supply of sediment.
4. the physiography of the margin
Consider: Rapid changes in relative sea-level can lead to the complete omission of systems tracts.
The physiography of the margin and the magnitude of the relative sea-level changes determine the
proportion of the deposition that occurs in shallow marine environments. Sedimentation rates are
important and may disrupt the affects of sea-level.
Supply of material in carbonate settings is governed mainly by the factors that control biogenic
productivity, such a water temperature, salinity, nutrient supply, Suspended sediment content,
water depth and the area of the shelf that is available for production. These last two are dependent
of the relative sea level fluctuation, thus there is a direct link between relative sea level and
sediment supply in carbonate system. An important difference with clastic systems tracts is that the
carbonate productivity varies because most carbonate material is biogenic and forms in shallow
water. During high stand and transgressive systems tracts wide areas of shallow water allow more
sediment to be formed, whereas at falling stage and lowstand production of carbonate sediment is
much lower. Parasequences in carbonate depositional systems normally show a shallowing-up
character. Lower subtidal zone comprised of wackestones that coarsen up into packstones and
grainstones are deposited in the higher energy wave-reworked zone of shallower water.
Climate directly controls the volume of water in a lake. These fluctuations can be of greater
magnitude than global eustasy, accommodation is also determined by tectonic subsidence.
Looking a sequence stratigraphy, a progradational trend is indicated if the higher parasequences
indicate generally shallower water than the lower ones, while retrogradational trend has a
characteristic deepening up trend and if all parasequences have the same range of facies the trend is
aggradational. The recognition of key surfaces such as sequence boundaries (erosion surface) and
maximum flooding surfaces (condensed facies) is an important step in the analysis of the
sedimentary succession. For petrophysical logging gamma ray logs are very useful, they are used to
assess the relative proportions of sand and mud within a succession. In general sandier deposits are
characteristic to shallower marine deposition than mud. A trend of decreasing value upwards can be
related to increase in sand content and thus a shallowing upwards trend. A flooding surface,
fluvial/estuarine facies or sequence boundary will have an increasing value upwards. Maximum
flooding surfaces may also be picked up on spectral gamma logs as they contain more potassium do
to the abundance of glauconite.
,Causes of sea-level change:
Tectonic forces and related thermal effects acting on the margins of continents result in the land
mass being raised or lowered relative to the sea level. Eustasy is a global phenomenon involving
changes in the volume of water in the world’s oceans, so every shoreline will experience the same
amount of sea-level rise or fall at the same time. Melting of continental ice caps at the poles can
release large quantities of water tot the oceans that can potentially raise the sea levels around the
world. The connection between climate change, glacial accretion/wastage and global sea-level
changes is established as a glacio-eustatic mechanism. Thermo-tectonic causes of sea-level change:
new oceanic crust takes up more space than older oceanic crust due to the fact this crust is hot and
buoyant, this will cause higher sea-levels. During supercontinent formation the total length of
spreading ridges will be reduced and result in more capacity in the ocean and cause the sea-level to
drop. High spreading rates = rise, low spreading rates = drop. Thermal expansion: as water warms
up, the volume increases. Thus, when global temperatures rise the sea-level will rise as well, the
effect is quite minimal though compared to other causes. Milankovitch cycles: the eccentricity of
the earth’s orbit of the sun, changes on the obliquity of the axis of rotation of the earth and the
precession of the axis of rotation may result in global climatic cycles on the scale of tens of
thousands of years.
, CHAPTER 4: PROCESSES OF TRANSPORT AND
SEDIMENTARY STRUCTURES
The simplest mechanism of sediment transport is due to gravity, which causes rock falls and
accumulations called scree, which build up as talus cones. The angle of rest varies for the clast size
and varies from 30 to 36 degrees, with larger clast sizes being able to rest at a bigger angle.
Transport by water is by far the most significant and can transport over large distances. Wind
blowing over land can pick up dust and sand and carry it large distances. Over longer time periods
ice also functions as a transport mechanism. When there is a very high concentration of sediment in
water the mixture forms a debris flow and is gravity driven.
There are two types of fluid flow: laminar flows, all molecules within the fluid move parallel to each
other in the direction of transport; turbulent flows, molecules in the fluid move in all directions but
with a net movement in the transport direction. The Reynold number is obtained by relating the
velocity of flow (v), the ratio between the density of the fluid and the viscosity of the fluid (V) and
the depth of flow (l). Re= (v*l)/V, this flow will be laminar if Re<500 and turbulent when Re>2000.
There’re three mechanisms of particles moving: rolling, saltation and suspension. Particles being
caries by rolling or saltation are called bedload, and the material in suspension is called suspended
load. Higher energy is needed rolling suspension. Rolling grains are moved by frictional drag
between the flow and the clasts. The fluid velocity at which a particle becomes entrained in the flow
can be referred to as critical velocity. The Hjülstrom diagram shows the relationship between water
flow velocity and grain size, it demonstrates important features of sediment movement in currents.
The lower line shows the velocity to keep a rock in motion while the lower line shows the velocity
required to start the motion from rest. The cohesive properties of clay particles mean that fine-
grained sediments require high velocities to re-erode them once they are deposited, especially once
they are compacted.
A deaccelerating flow will form a normal graded bed (fining upwards), while a flow with an
increasing velocity through time may result in a reverse graded bed (coarsening upwards). The
settling velocity of particles in a fluid is given by stokes law where V=gD2(ρs-ρf)/18μ. Stokes law only
accurately predicts for small grains, because of the turbulence created by the drag of larger grains
falling through the fluid, same is true for plate-like clasts and micas, which will settle lasts because of
the greater drag. Higher viscosity fluids can transport bigger sediments and rocks.
A bedform is a morphological feature formed by the interaction between flow and cohesionless
sediments on a bed. We will focus on ripple marks and sand dunes, which leave distinctive layering
in the preserved strata. A fluid flowing over a surface can be divided into a free stream, which is the
portion of the flow unaffected by the boundary effect, a boundary layer, within which the velocity
starts to decrease due to friction with the bed and a viscous layer, a region of reduced turbulence
that is typically less than a millimetre thick. If all particles are contained within the viscous layer it is
hydraulically smooth, if parts stick out it is hydraulically rough. “steps” are formed by grains being
clustered by turbulent sweeps. Expansion of flow over the step result in an increase in pressure and
the sediment transport rate is reduced, resulting in deposition on the lee side of the step, which
forms a series of layers at the angle of the slope. These thin inclined layers of sand are called cross-
laminae and build the sedimentary structure called cross-lamination. When viewed from above the
ripples might be straight/sinuous ripples or linguoid ripples, which are unconnected arcuate forms.
Straight ripples tend to develop into linguoid ripples over time and at high velocities. A perfectly
straight ripple would generate planar cross-lamination. Linguoid ripples develop a pattern of trough
cross-lamination (see image pg. 55). Climbing ripples are indicators of rapid sedimentation as their
formation depends on the addition of sand to the flow at a rate equal to or greater than the rate of
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