Relative Sea-level,Relative Sea-level,tectonics, and eustasytectonics, and eustasy

Definition of sea-levelDefinition of sea-level

In order to understand the controls onIn order to understand the controls onsequence development, it is firstsequence development, it is firstnecessary to define what is meant by:necessary to define what is meant by: EustasyEustasy

Relative sea-level and water depthRelative sea-level and water depth

Global EustasyGlobal Eustasy

Eustasy- the world-wide sea-level regimeEustasy- the world-wide sea-level regimeand its fluctuations, caused by absoluteand its fluctuations, caused by absolutechanges in the quantity of seawater, e.g.changes in the quantity of seawater, e.g.by continental icecap fluctuations.by continental icecap fluctuations.

See also; glacio- eustatism, sedimento-See also; glacio- eustatism, sedimento-eustatism.eustatism.

Global EustasyGlobal Eustasy

Eustasy is measured between the sea-Eustasy is measured between the sea-surface and a fixed datum, usually thesurface and a fixed datum, usually thecenter of the Earth.center of the Earth.

It can vary by changing ocean-basinIt can vary by changing ocean-basinvolume; (e.g. by varying ocean-ridgevolume; (e.g. by varying ocean-ridgevolume);volume);

by varying ocean-water volume (e.g. byby varying ocean-water volume (e.g. byglacio-eustasy).glacio-eustasy).

Eustatic vs. relative sea-levelEustatic vs. relative sea-level

Relative sea-levelRelative sea-level

Relative sea-level is measured between the sea-Relative sea-level is measured between the sea-surface and a local moving datum, such assurface and a local moving datum, such asbasement or a surface within the sediment pile.basement or a surface within the sediment pile.

Tectonic subsidence or uplift of a basementTectonic subsidence or uplift of a basementdatum, sediment compaction involvingdatum, sediment compaction involvingsubsidence of a datum within the sediment pile,subsidence of a datum within the sediment pile,and vertical eustatic movements of the sea-and vertical eustatic movements of the sea-surface all contribute to relative sea-levelsurface all contribute to relative sea-levelchanges.changes.

Relative sea-levelRelative sea-level

¬¬ ⇑⇑ relative sea-level relative sea-level ““risesrises”” are due to: are due to:•• SubsidenceSubsidence

•• Eustatic sea-level riseEustatic sea-level rise

¬¬ ⇓⇓ relative sea-level relative sea-level ““fallsfalls”” are due to: are due to:•• Tectonic upliftTectonic uplift

•• Eustatic sea-level fallEustatic sea-level fall

AccommodationAccommodation

Eustasy and subsidence rate together controlEustasy and subsidence rate together controlthe amount of space available for sedimentthe amount of space available for sedimentaccumulation termed accumulation termed accommodation spaceaccommodation space..

It is defined as the space available forIt is defined as the space available forsediment to accumulate at any point in time.sediment to accumulate at any point in time.

Accommodation is controlled by Accommodation is controlled by base levelbase levelbecause In order for sediment to accumulate,because In order for sediment to accumulate,there must be space available below basethere must be space available below baselevel.level.

Base levelBase level

The base level datumThe base level datumvaries according tovaries according todepositional setting.depositional setting.

In alluvialIn alluvialenvironments baseenvironments baselevel is controlled bylevel is controlled bythe graded streamthe graded streamprofile, which isprofile, which isgraded to sea-level orgraded to sea-level orlake-level at its distallake-level at its distalend.end.

Base levelBase level

In deltaic and shoreline systems baseIn deltaic and shoreline systems baselevel is effectively equivalent to sea-level.level is effectively equivalent to sea-level.

In shallow marine base level is sea-level,In shallow marine base level is sea-level,although fair-weather wave base can formalthough fair-weather wave base can forma temporary base level in the form of aa temporary base level in the form of agraded shelf profile.graded shelf profile.

∆∆ accommodation= accommodation=∆∆ eustasy + eustasy + ∆∆ subsidence + subsidence + ∆∆ compaction compaction

Sediment supply fills the accommodationSediment supply fills the accommodationcreated and controls water depth.created and controls water depth.

Sediment supply fills availableSediment supply fills availableaccommodation.accommodation.

If the rate of sediment supply exceeds theIf the rate of sediment supply exceeds therate of creation of accommodation at arate of creation of accommodation at agiven point, water depth will decrease:given point, water depth will decrease:

∆∆ water depth= water depth= ∆∆ eustasy + eustasy + ∆∆ subsidence + subsidence + ∆∆compaction - sediment depositedcompaction - sediment deposited

Accommodation through timeAccommodation through time

To understand how accommodation variesTo understand how accommodation variesthrough time it is useful to consider howthrough time it is useful to consider howdifferent rates of tectonic subsidence and ,different rates of tectonic subsidence and ,in this , case, same sinusoidal eustaticin this , case, same sinusoidal eustaticsea-level curve combine to give differentsea-level curve combine to give differentrates of addition and destruction ofrates of addition and destruction ofaccommodationaccommodation

Orders of cyclicity and globalOrders of cyclicity and globalcorrelationcorrelation

A depositional sequence represents a completeA depositional sequence represents a completecycle of deposition bounded above and below bycycle of deposition bounded above and below byerosional unconformities.erosional unconformities.

The continental encroachment cycles is definedThe continental encroachment cycles is definedby the very largest scale (> 50 my) cycles ofby the very largest scale (> 50 my) cycles ofsedimentary onlap and offlap of the supersedimentary onlap and offlap of the supercontinents. Two of these cycles in thecontinents. Two of these cycles in thePhanerosioc.Phanerosioc.

They are called first order cycle and areThey are called first order cycle and areconsidered to be the results of tectono-eustatic,considered to be the results of tectono-eustatic,I.e. changes in ocean basin related to tectonicI.e. changes in ocean basin related to tectoniccycles.cycles.

Second-order (3- 50 my) cycles are the buildingSecond-order (3- 50 my) cycles are the buildingblocks of the first-order sequences.blocks of the first-order sequences.

May be caused by changes in the rate ofMay be caused by changes in the rate oftectonic subsidence or uplift in the sedimenttectonic subsidence or uplift in the sedimentsource terrane.source terrane.

Third-order (0.5- 3 my) sequence cycle are theThird-order (0.5- 3 my) sequence cycle are thefoundation of sequence strat. Because they arefoundation of sequence strat. Because they arethe scale of seismic data.the scale of seismic data.

They are identified by recognition of individualThey are identified by recognition of individualcycles of accommodation creation andcycles of accommodation creation anddestruction.destruction.

May be controlled by glacio-eustatic.May be controlled by glacio-eustatic. Fourth-order cycles (parasequences) individualFourth-order cycles (parasequences) individual

shallowing upward facies cycles.shallowing upward facies cycles. May be related to autocyclic processes.May be related to autocyclic processes.