Systems Tracts Are Inter

8/3/2019 Systems Tracts Are Inter

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Systems tract (Brown and Fisher, 1977): a linkage of contemporaneous depositional systems,

forming the subdivision of a sequence.A systems tract includes all strata accumulated across the basin during a particular stage

of shoreline shifts.

Systems tracts are interpreted based on stratal stacking patterns, position within the

sequence, and types of bounding surfaces. The timing of systems tracts is inferred

relative to a curve that describes the base-level fluctuations at the shoreline.

Systems tracts are inter-preted based on stratal stacking patterns, position

within the sequence and types of bounding surfaces,and are assigned particular positions along an inferred

curve of base-level changes at the shoreline.

Each systems tract is defined by a specific type

of stratal stacking pattern, closely associated with a type

of shoreline shift (i.e., forced regression, normal regres-sion, or transgression), and represents a

specific sedi-mentary response to the interaction between sediment

flux, physiography, environmental energy, and changes

in accommodation (Posamentier and Allen, 1999).

The lowstand and the shelf-margin systems tracts

are similar concepts, as being both related to the same

portion of the reference sea-level curve (the stage of

fallearly rise), so they were used interchangeably as

part of a depositional sequence (Vail, 1987; Posamentier

and Vail, 1988; Vail et al., 1991). A sequence composed

of lowstand, transgressive and highstand systems

tracts was defined as a type 1 sequence, whereas a

combination of shelf-margin, transgressive and high-stand systems tracts was said to have

formed a type 2

sequence (Posamentier and Vail, 1988).

The lowstand systems tract, as defined by Posamentier

et al. (1988), includes a lowstand fan, accumulated


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during falling sea level, and a lowstand wedge, repre-senting deposition during sea-level

lowstand and early

rise (depositional sequence II in Fig. 1.7). The lowstand

fan systems tract consists of autochthonous (shelf-perched deposits, offlapping slope wedges)

and allo-chthonous gravity-flow (slope and basin-floor fans)

facies, whereas the lowstand wedge systems tract

includes part of the aggradational fill of incised valleys,

and a progradational wedge which may downlap onto

the basin-floor fan (Posamentier and Vail, 1988).

The lecture seen in the movie involves a geologic model that makes the followingassumptions::

y Sea level position variedy Subsidence was constanty Sediment supply was constant

The sequence is divided by surfaces system tracts. Each systems tract is representedby a collection of the sediments of the associated sedimentary depositional systemsthat were active during the different phases of base level change. Thus system tractsediments can be considered as sedimentary units that were deposited synchronouslyand can be mapped as being enclosed by continuous surfaces that extend from sub-aerial and to sub-aqueous settings.

The systems tracts defined in order of deposition to form the ideal sequence are:

y Early Phase Lowstand System Tracty Late Phase Lowstand Systems Tracty Transgressive Systems Tracty Highstand Systems Tract

Early Phase Lowstand System Tract is associated with:

y Falling stage of relative sea level induced by eustasy falling rapidly and/or

tectonic uplift outpacing the rate of change in sea level positiony Fluvial incision up dip with formation of an unconformity or sequence boundary

and the focus of sediment input at the shoreliney Forced regressions induced by the lack of accommodation producing stacking

patterns of downward stepping prograding clinoforms over the condensedsection formed during the previous transgressive and highstand systems tracts

y Slope instability caused by the rapid deposition of sediment from the fluvialsystems


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y Basin floor fans formed from sediment transported from the shelf margin whenthis fails under the weight of the rapid sediment accumulation associated with theforced regression

y Shelf margin and slope fans form when rates of sedimentation slows and slopeinstability is reduced so sediment is not displaced so far downslope

y

Onlap of sediments onto the prograding clinoforms below the shelf breaky The lower bounding surfaces of the Early Phase Lowstand System Tract are the

updip unconformity and the top of the downdip condensed section. Thesesurfaces form by different mechanisms and have different time significance

y The top of the downdip condensed section immediately underlies thedownlapping prograding clinoforms of the forced regression

y The top of the Early Phase Lowstand System Tract in theory is marked by aninitial onlap onto the often eroded surface of the prograding clinoforms of theforced regression

Late Phase Lowstand Systems Tract is associated with:

y A slow relative sea level rise is induced when eustasy begins to rise slowlyand/or tectonic uplift slows

y Sediment is now outpaced by an increase in accommodation and in response thesediment begins to onlap onto the basin margin

y River profiles stabilizey Valleys backfilly Prograding lowstand clinoforms form and are capped bytopset layers that onlap,

aggrade, become thicker upward and landward

Transgressive Systems Tract is associated with:

y A rapid relative sea level rise above the shelf margin occurs when eustasybegins to rise rapidly, exceeding the effects of any tectonic uplift

y Condensed sequences are often composed of sediment layers rich in the tests offauna that are no longer masked by sediment accumulation becausesedimentation rates are very slow in response to the greater area of sea floorexposed to sedimentation

y Ravinement erosion surface formed when the transgressing sea reworks eitherthe prior sequence boundary or the sediments that may have collected during theforced regression that may have followed the formation of that sequenceboundary.

y Maximum flooding surface forms when the last fine-grained widespreadtransgressive sediment collects before the High Stand builds out over it.

Highstand Systems Tract is associated with:

y Slow rise of relative sea level followed by a slow fall; essentially a still stand ofbase level when the slower rate eustatic change balances that of tectonic motion

y Sediment outpacing loss of accommodation


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y River Profiles stabilizey River valleys are dispersed laterally in a position landward of the shelf margin.y Prograding highstand clinoforms develop capped by aggrading topsets that

become thinner upward.y

The regressive deposits that form when sediment accumulation rates exceed therate of relative sea-level rise and increase in accommodation constitutes the uppersystems tract in either a type 1 or type 2 sequence.

The base of this systems tract is formed by the maximum flooding surface (mfs) overwhich the Highstand Systems Tract sediments prograde and agrade. The top of thissystems tract is formed by the eroded unconformity surface that develops when asea level fall initiates erosion of the now subaerial Highstand system sedimentsurface and the start of the Falling Stage Systems Tract.

This systems tract is commonly widespread on the shelf and may be characterizedby one of more aggradation to progradational parasequence sets with progradingclinoform geometry. They onlap the sequence boundary in a landward direction anddownlap the top of the Transgressive and/or Lowstand Systems Tracts in abasinward direction

LST

Throughout this web site it is proposed that the Lowstand Systems Tract is bounded bythe Falling Stage Systems Tract and the Transgressive Systems Tract (Plint andNummedal, 2000; Coe et al, 2002). This system tract is represented by the sedimentary

accumulation that straddes the lowest position of the relative sea level curve. It often forms aprograding wedge at the base of a shelf margin with its lower boundary onlapping ontothe prograding clinoforms (see animated gif) and/or downlapping onto a downslope fan.Stacking patterns of parasequences of the Lowstand Systems Tract exhibitbackstepping onlapping retrogradational aggrading clinoforms that thicken updipcapturing the effect of the rate of rise in relative sea level is greater than the ratesediment accumulation. This change in sediment geometry can be explained asoccuring when accommodation starts to expand in response to a relative rise in sealevel that occurs when a rise eustasy exceeds the rate of subsidence.

The upper boundary of the Lowstand Systems Tract is marked by the development of

the Transgressive Surface that steps up onto the shelf margin (see animated gif). Thecharacterization of the boundary between Falling Stage Systems Tract (Plint andNummendal, 2000) , or the Early Lowstand Systems Tract (Posamentier and Allen,1999), and the overlying sediments becomes "fuzzy" if the onlapping wedge of theLowstand Systems Tract fills incised valleys and becomes subaerial. The depositionalsetting of this onlapping Lowstand Systems Tract wedge occurs below the shelf marginbreak, the depth of water in the adjacent basin determines whether the sediments aresubaerial, and/or submarine. This subaerial onlap might be equated by some with the


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Transgressive Systems Tract. On this web site these latter onlapping strata are onlyreferred to as a part of the Transgressive Systems Tract when they rise above the shelfbreak. It is recognized that in outcrop and wells it may be nearly impossible todetermine if this onlapping occurs below or above the shelf margin break. This onlap offluvial and coastal plain deposits has been defined as "coastal" onlap by Mitchum

(1977).

Posamentier and Allen (1999) refer to the Lowstand SystemTract defined above as the Late Lowstand Systems Tract. Theyrefer to the Falling Stage Systems Tract as the Early LowstandSystems Tract. Traditionally the sediments of the LowstandSystems Tract, as defined by Posamentier and Allen (1999),included the deposits that accumulated after the onset of relativesea-level fall directly on the sequence boundary over the

Highstand Systems Tract as a basin-floor fan, slope fan, andlowstand wedge. As can be recognized on this web site, this "oldLowstand Systems Tract" is now divided into the Falling StageSystems Tract with its basin-floor fans, and slope fans while, asindicated above, the Lowstand Systems Tract sediments nowform lowstand wedges. These latter often filling incised valleysthat cut down into the Highstand Systems Tract. This systemstract is equated with development of limited accomodation

associated with a small rise relative rise sea level during anessetnially lowstand of the sea.

.


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Maximum Flooding Surface (MFS)

A surface of deposition at the time the shoreline is at its maximum landward position(i.e. the time of maximum transgression) (Posamentier & Allen, 1999)

The surface marks the time of maximum flooding or transgression of the shelf and itseparates the Transgressive and Highstand Systems Tract. Seismically, it is oftenexpressed as a downlap surface. Marine shelf and basinal sediments associated withthis surface are the result of slow rates of deposition by pelagic-hemipelagic sedimentsand they are usually thin and fine grained. These fine sediments make up thecondensed section (Mitchum, 1977).

An mfs is often characterized by the presence of radioactive and often organic richshales, glauconite, and hardgrounds. There are commonly widespread thin beddedconcentrations of fauna (condensed sections) with high abundance and diversity. Anmfs can often be the only portion of a sedimentary cycle which is rich in fauna. Often ina landward direction the maximum flooding surface may match the underlyingtrangressive surface formed during or just after the inital transgressive phase thatimmediately follow sea level lowstands. In this case Glossifungites burrows may occurwithin this surface. The mfs is not commonly burrowed or bored. Any burrowing or


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boring are likely connected to the preceding transgressive surface before the waterdeepens and conditions become inimical for colonization but favour preservation.

The mfs often mark the bounding surface between coarsening and/or fining upwardcycles and are used to relate these cycles to deepening and shallowing in the

geological section. It is synomous with the maximum transgressive surface (Helland-Hansen and Martinsen, 1996); final transgressive surface (Nummedal et al., 1993) atthe top of retrogradational strata, downlapped by highstand normal regressive strata(Catuneanu, 2006).

Transgressive Surface

This is a marine-flooding surface that forms the first significant flooding surface in asequence. The TS, in most siliciclastic and some carbonate successions, marks theonset of the period when the rate of creation of accommodation space is greater thatthe rate of sediment supply. It forms the base of the retrogradational parasequence

stacking patterns of the Transgressive Systems Tract. In areas of high sediment supply,e.g. on rimmed carbonate platforms, the rate of sediment supply may keep pace withthe rate of relative sea-level rise and thus the TS will mark a change from aprogradational to an aggradational parasequence stacking patterns. The TS oftenmarks the base of the most prominent onlap.

If no lowstand or falling stage systems tract facies are preserved above the sequenceboundary, the TS may coincide with this boundary. A TS is often characterized by thepresence of a surface marked by consolidated muds of firmgrounds or hardgrounds thatare cemented by carbonates. Both surfaces are often penetrated by either burrowing orboring organisms. For instance Glossifungites burrows are found penetrating the firm

grounds and are often filled by an overlying widespread winnowed, sorted and oftenconglomeratic ravinement sediment, or lag. Cemented surfaces may be colonized andbored by a Trypanites ichnofacies and infilled by the sediments associated with thebase of the transgressive system tract and are often wave winnowed.

If the rate of sediment supply is low over the transgressive surface this may mergelandward with the maximum flooding surface. When a TS extends over LST valley fill,the response on the resistivity log curve may show a small local increase resistivityfollowed by a low. This increase in resitivity is in response to the carbonate cementationof the hardground, while the low is associated with deposition of trangressive shales.

maximum flooding surface


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A widespread marine flooding surface that separates the underlying transgressivesystems tract from

the overlying highstand systems tract. The surface also marks the deepest water facies within a

sequence. The maximum flooding surface represents a change from retrogradational to progradational

parasequence stacking patterns. It commonly displays evidence of condensation or slow deposition,

such as abundant burrowing, hardgrounds, mineralization and fossil accumulations. On wireline logs, the

shales that immediately overlie the maximum flooding surface commonly have different characteristics

than other shales and can often be recognized on the basis ofresistivity, gamma ray, neutron and

density logs. These shales can also be recognized by electrofacies analysis when the analysis is designed

to do so.

condensed section

In sequence stratigraphy, a section offine-grained sedimentary rocks that accumulated slowly, thereby

representing a considerable span of time by only a thin layer. In condensed sections, fossils and organic,

phosphatic and glauconitic material tend to be concentrated compared with rapidly deposited sections

that contain few fossils. Condensed sections are most commonly deposited during transgressions. In

such cases they are associated with "maximum flooding surfaces" and form important sequence

stratigraphic markers.

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Systems Tracts Are Inter