Download - Systems Tracts
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Systems Tracts
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Systems tract- idealized type-1 sequence
shown is representative of a shelf-break
margin.
Deposition in a basin is not uniform and
continuous but occurred in a series of
discrete packets bounded by seismic
reflection terminations.
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These packages are known
as systems tract. The term was first used by Brown and Fisher
(1977) to represent contemporaneous
depositional systems. Depositional systemsare a three- dimensional assemblage oflithofacies, genetically linked by active(modern) or inferred (ancient) processes andenvironments.
A system tract is therefore a three-dimensional unit of deposition, and theboundaries of a system tract are depositionalboundaries of onlap, downlap, etc.
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Systems Tract
Within one relative sea-level cycle,
three main systems tracts are
frequently developed.
The system tract represents the
fundamental mapping unit that contains
depositional systems for which apaleogeographic map can be drawn.
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Lowstand system tract
The basal system tract in a type 1depositional sequence.
It is deposited during an interval ofrelative sea-level fall at the offlapbreak, and subsequent slow relativesea-level rise.
Falling relative sea-level at the offlapbreak of a shelf-break margin will havean extreme effect on the river system.
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Prior to relative sea-level fall the river
have a more or less a graded riverprofile with erosional upper portion and
a depositional lower portion.
With relative sea-level fall the river will
have to adjust to the lowered baselevel.
The river incises into the previous
deposited topsets. These rewroked sediments, and the
fluvial load from the land, are delivered
directly on to the previous highstand
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Because the river is not free to avulse,
the sediments are focused towards the
same point in the slope.
Because instability the sedimentation
processes are dominated by large-
scale slope failure resulting in bypassof the slope and deposition of
submarine fans in the basin.
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At the river sea-level low point the riverprofile stabilizes again, and a
prograding topset-clinoform system can
then be stablish.
The first topset of this system will onlapbelow the level of the previous offlap
break. This is known as a downward
shift in coastal onlap below the levelof the offlap break.
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Relationship
betweensea-level,
topset
accomodatio
n volume,
and
systems
tracts, in a
simple
numerical
model
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Low stand submarine fans Two distinctive fan units can be recognized
within the lowstand submarine fan; an initial
basin floor fan unit, detached from the foot ofthe slope, and a subsequent slope fan unit,abutting the slope, occasionally referred toas slope front fill.
Submarine fan deposits on the lower slope orbasin.
Associated with erosion of canyons into theslope.
Turbidites and debris flow.
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Lowstand prograding wedge.
Topset clinoform system deposited
during accelerating relative sea-level
rise.
It is separated from the overlying
transgressive system tract by a
maximun prograding surface.
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(TST)
It is the middle systems tract of both type 1
and type 2 sequences. It is deposited during the part of the relativesea-level rise cycle when topsetaccomodation volume is increasing fasterthan the rate of sediment supply.
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Transgressive Systems Tract
It contain mostly topsets, with fewassociated clinoforms, and is entirely
retrogradational.
The active depositional systems aretopset systems:
alluvial,
paralic (coal deposits formed along the margin of the
sea), coastal plains
shelfal.
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Transgressive Systems Tract
Widw shelf area are characteristic of
transgressive systems tract.
The Transgressive Systems Tract
passes distantlly into a condensedsection characterized by extremely low
rates of deposition and the
development of condensed facies such
as glauconitic, organic reach and/or
phosphatic shales, or pelagic
carbonates.
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Transgressive Systems Tract
The maximum rate of rise of sea-leveloccurs some time within the
transgressive sytems tract, and the end
of the systems tract occurs when the
rate of topset accomodation volume
decreases to a point where it just
matches sediment supply, and
progradation begins again. This point is known as Maximum
Flooding Surface.
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Transgressive Systems Tract
Topsets of the TST tend to have a
lower sand percentage than those of
other systems tracts, because little of
the mud-grade sediment bypasses thetopsets.
TST can therefore often hast sealing
horizons to topset reservoirs, and alsosource beds.
Present-day depositional systems over
much of the glove form a TST.
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(HST)
The HST is the youngest sytems tract ineither a type 1 or a type 2 sequence.
It represent the progradational topset-clinoform system deposited after maximumtransgression, and before a sequenceboundary, when the rate of creation ofaccommodation is less than the rate of
sediment supply.
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g s an ys ems rac
(HST)
Deposits are similar initially to those of TST, but theinfill of shelf areas by progradation, and thedecrease in the rate of sea-level rise, may lead to adecrease in tidal influence and adecrease in theamount of coal, and of overbanks, laggonal and
lacustrine shales.
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Type 2 sequence boundary and
the shelf-margin systems tract Relative sea-level may fall over the
proximal area of the highstant topsets,without falling at the offlap break.
A sequence boundary results, but not onecharacterized by fluvial incision orsubmarine fan deposition.
The sequence boundary is recognized in
the seismic lines by a downward shift incoastal onlap to a position landward of theofflap break, where topset reflections canbe seen onlapping an older topset.
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Type 2 sequence boundary
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Type 2 sequence boundary
The sequence boundary is overlain by
a shelf-margin system tract of topsets
with a predominantly aggradationalstacking pattern.
The rate of sea-level fall at the
shoreline is equal to , or less than, thesubsidence.
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The second part of the wedge is
characterized by a slow relative rise in sea-level, the infilling of incised valleys, and
continued shoreline progradation.