Systems Tracts - UPRMgeology.uprm.edu/Classes/GEOL4046/systemstracts.pdf · Systems Tract •Within one relative sea-level cycle, three main systems tracts are frequently developed.

Systems Tracts

• Systems tract- idealized type-1 sequenceshown is representative of a shelf-breakmargin.

• Deposition in a basin is not uniform andcontinuous but occurred in a series ofdiscrete packets bounded by seismicreflection terminations.

These packages are knownas systems tract.

• The term was first used by Brown and Fisher(1977) to represent contemporaneousdepositional 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.

Systems Tract

• Within one relative sea-level cycle,three main systems tracts arefrequently developed.

• The system tract represents thefundamental mapping unit that containsdepositional systems for which apaleogeographic map can be drawn.

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.

• Prior to relative sea-level fall the riverhave a more or less a graded riverprofile with erosional upper portion anda depositional lower portion.

• With relative sea-level fall the river willhave to adjust to the lowered baselevel.

• The river incises into the previousdeposited topsets.

• These rewroked sediments, and thefluvial load from the land, are delivereddirectly on to the previous highstandclinoform.

• Because the river is not free to avulse,the sediments are focused towards thesame point in the slope.

• Because instability the sedimentationprocesses are dominated by large-scale slope failure resulting in bypassof the slope and deposition ofsubmarine fans in the basin.

• At the river sea-level low point the riverprofile stabilizes again, and aprograding topset-clinoform system canthen be stablish.

• The first topset of this system will onlapbelow the level of the previous offlapbreak. This is known as a downwardshift in coastal onlap below the levelof the offlap break.

• Relationshipbetweensea-level,topsetaccomodation volume,andsystemstracts, in asimplenumericalmodel

Low stand submarine fans

• Two distinctive fan units can be recognizedwithin the lowstand submarine fan; an initialbasin 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.

Lowstand prograding wedge.

• Topset clinoform system depositedduring accelerating relative sea-levelrise.

• It is separated from the overlyingtransgressive system tract by amaximun prograding surface.

Transgressive Systems Tract(TST)

• It is the middle systems tract of both type 1and 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.

Transgressive Systems Tract• It contain mostly topsets, with few

associated clinoforms, and is entirelyretrogradational.

• The active depositional systems aretopset systems:– alluvial,– paralic (coal deposits formed along the margin of the

sea),– coastal plains– shelfal.

Transgressive Systems Tract

• Widw shelf area are characteristic oftransgressive systems tract.

• The Transgressive Systems Tractpasses distantlly into a condensedsection characterized by extremely lowrates of deposition and thedevelopment of condensed facies suchas glauconitic, organic reach and/orphosphatic shales, or pelagiccarbonates.

Transgressive Systems Tract• The maximum rate of rise of sea-

level occurs some time within thetransgressive sytems tract, and the endof the systems tract occurs when therate of topset accomodation volumedecreases to a point where it justmatches sediment supply, andprogradation begins again.

• This point is known as MaximumFlooding Surface.

Transgressive Systems Tract• Topsets of the TST tend to have a

lower sand percentage than those ofother systems tracts, because little ofthe mud-grade sediment bypasses thetopsets.

• TST can therefore often hast sealinghorizons to topset reservoirs, and alsosource beds.

• Present-day depositional systems overmuch of the glove form a TST.

Highstant Systems Tract(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 ofsediment supply.

Highstant Systems Tract(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 andlacustrine shales.

Type 2 sequence boundary andthe shelf-margin systems tract

• Relative sea-level may fall over theproximal 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 inthe seismic lines by a downward shift incoastal onlap to a position landward of theofflap break, where topset reflections canbe seen onlapping an older topset.

Type 2 sequence boundary

Type 2 sequence boundary

• The sequence boundary is overlain bya shelf-margin system tract of topsetswith a predominantly aggradationalstacking pattern.

• The rate of sea-level fall at theshoreline is equal to , or less than, thesubsidence.

Low stand systems tracts on a rampmargin

• Previous descrived systems aredeveloped on a shelf-break margin.

• On a ramp margin, the lowstandsystems tract consis of a relatively thinlowstand wedge that may contain twoparts:– The first part is characterized by stream

incision and sediment bypass of thecoastal plain. This occurs during a relativefall in sea-level when the shoreline stepsrapidlly basinward.

• The second part of the wedge ischaracterized by a slow relative rise in sea-level, the infilling of incised valleys, andcontinued shoreline progradation.