SEQUENCE STRATIGRAPHY(Part-I)
Sequence Stratigraphy involves integration of log, core and seismic data to interpret deposition and architecture of sediments.
Sequence
Sequence is a relatively conformable succession of genetically related strata bounded at its top and base by unconformities and their correlative conformities. It is composed of a succession of systems tracts and is interpreted to be deposited between eustatic-fall inflection points." Note that a sequence is terminated by a fall in sea level.
Growth of a sequence
Sequence stratigraphy is necessarily an understanding of geologic processes. By going through the development of a sequence process-by-process and component by component (process-response modeling), we gain an understanding of the basic model and some insight into the variations possible.
Requirements for the Growth of a Sequence
“Accommodation”
It is a sequence stratigraphic term that stands for the amount of space available for sediment accumulation. Factors affecting the amount of accommodation, or accommodation space, include subsidence and eustasy.
Accommodation Space Equation
Over long time scales (105-108years), sediment accumulation is strongly controlled by changes in eustatic sea level, tectonic subsidence rates, and climatic effects on the production of sedimentary grains. Several of these factors are linked to one another through the accommodation space equation. This balance of terms is most easily explained for marine sediments, but can be easily modified to include terrestrial sedimentation. A number of processes can cause the surface of the oceans to move up and down relative to the center of the earth. “The distance from the sea surface to the center of the earth is eustatic sea level”. In addition, the lithosphere can also move up and down relative to the center of the earth, and changes in the distance from some arbitrarily chosen reference horizon and the center of the earth are called uplift or subsidence. The distance between this reference horizon and the sea surface is called relative sea level or accommodation space.”
Filling of Accommodation Space
Accommodation space can be filled with sediments or water. The distance between the sediment surface/top and the ocean surface is known as water depth. The accommodation space not filled with water is filled with sediment. The rates of change of tectonic subsidence, eustatic sea level, sediment thickness and water depth are linked to one another through the accommodation space equation:
T + E = S + W
Where T is the rate of tectonic subsidence,
E is the rate of eustatic sea-level rise,
S is the rate of sedimentation, and
W is the rate of water depth increase (or deepening).
These four variables are defined such that positive values correspond to tectonic subsidence and eustatic sea-level rise (factors that increase accommodation space) and sediment accumulation and water depth increase (factors that reflect filling of accommodation space). Reversing the signs of these variables accommodates tectonic uplift, eustatic sea-level fall, erosion, and shallowing of water depth, respectively.
The accommodation space equation represents a simple volume balance, with the terms on the left controlling the amount of space that can be occupied by sediments and water and the terms on the right describing how much water or sediment fills the accommodation space. As written, the equation is an approximation. In reality, sediment thickness and water depth must be corrected for compaction of sediments and for the isostatic effects of newly deposited sediment.
Through section measurement, changes in sediment thickness can be known, and through facies analysis, changes in water depth can be known or approximated. However, without outside information, the rates of eustatic sea-level change and tectonic subsidence cannot be isolated, nor can their effects be distinguished from one another for a single outcrop. In other words, there is no unique solution to this equation as it has two unknowns. Thus, it is impossible in most cases to ascribe water depth or sedimentation changes to eustasy or tectonics without having regional control or outside information.
Eustatic Sea Level
“The distance from the sea surface to the center of the earth is eustatic sea level”.
Eustasy affects positions of shorelines and processes of sedimentation, so interpretation of eustasy is an important aspect of sequence stratigraphy.
Causes of Eustatic Sea-Level Change
Changes in sea level can result from movement of tectonic plates altering the volume of ocean basins, or when changes in climate affect the volume of water stored in glaciers and in polar icecaps.
Explanation
The volume of ocean basins is controlled primarily by the rate of seafloor spreading and secondarily by sedimentation in ocean basins. Because hot and young oceanic lithosphere is relatively buoyant, it floats higher on the asthenosphere and displaces oceanic waters upwards and onto continents. Older and colder oceanic lithosphere is denser; floats lower on the asthenosphere, and allows oceanic waters to stay within ocean basins. Long-term (102 k.y. - 105 k.y.) changes in the global rate of seafloor spreading can change the global average age and density of oceanic lithosphere, resulting in tens to a couple hundred meters of eustatic change. Filling of ocean basins with sediments derived from continental weathering is a relatively slow and minor way of changing ocean basin volumes and is capable of meters to tens of meters of eustatic change over tens to hundreds of millions of years.
CONTROLS ON THE VOLUME OF SEAWATER
The three most important controls on the volume of seawater are, Glaciation, Ocean temperature, and The volume of groundwater.
a) Glaciation
Continental and mountain glaciation is perhaps the most efficient and rapid means of storing and releasing ocean water. Due to Archimede's principle, ice caps over polar oceans do not affect eustatic sea level, so frozen seawater must be placed on a landmass to lower eustatic sea-level. Continental glaciation is capable of driving high amplitude (10 - 100 m) and high frequency (1 - 100 k.y.) eustatic changes.
b) Seawater Temperature
Because water expands at temperatures higher and lower than 4 degrees C, and because the depths of the oceans average around 5 km, small changes in the temperature of seawater can lead to significant changes in ocean water volume. Changes in water temperature can drive a few meters of eustatic change over short time scales (0.1 - 10 k.y.).
c) The Volume of Ground Water
Ocean water is continuously being recycled through continents as groundwater and surface water, such as rivers and lakes. Over relatively short time scales (0.1 - 100 k.y.), changes in the amount of water sequestered on the continents can cause up to a few meters of eustatic change.
Tectonic Subsidence
“The lithosphere can move up and down relative to the center of the earth, and changes in the distance from some arbitrarily chosen reference horizon and the center of the earth are called uplift or subsidence”.
Causes of Tectonic Subsidence
“Tectonic subsidence is also called driving subsidence and is distinguished from the isostatic effects of sediment and water loads. Tectonic subsidence, as its name implies, is driven by tectonic forces that affect how continental lithosphere floats on the asthenosphere.”
MECHANISMS
Three main mechanisms that affect this isostatic balance and therefore drive tectonic subsidence include stretching, cooling, and loading.
a) Stretching
Stretching of continental lithosphere in most situations results in the replacement of relatively light continental lithosphere with denser asthenosphere. The resulting stretched and thinned lithosphere sinks, causing tectonic subsidence. Stretching occurs in several types of sedimentary basins including rifts, aulacogens, back arc basins, and cratonic basins.
b) Cooling
Cooling commonly goes hand-in-hand with stretching. During stretching, continental lithosphere is heated, becomes less dense, and tends to uplift from its decreased density (the net effect in a stretched and heated basin may result either in uplift or in subsidence).
As continental lithosphere cools, it becomes denser and subsides. Cooling subsidence decreases exponentially with time yet can cause a significant amount of subsidence hundreds of millions of years following initial cooling. Cooling subsidence is especially important on passive margins and in cratonic basins.
c) Tectonic Loading
Tectonic loading can also produce subsidence. The additional weight of tectonic loads such as accretionary wedges or folds and thrust belts causes continental lithosphere to sink, leading to tectonic subsidence. Because the lithosphere responds flexurally, the subsidence occurs not only immediately underneath the load, but in broad region surrounding the load. Tectonic loading is particularly important in orogenic regions such as foreland basins.
Transgression
It is the migration of shoreline out of a basin and onto land (backstepping) during retrogradation. A transgression can result in sediments characteristic of shallow water being overlain by deeper water sediments.
Regression
The migration of shoreline into a basin during progradation due to a fall in relative sea level. Deposition during a regression can juxtapose shallow-water sediments atop of deep-water sediments.
Forced Regression
Forced regression is a seaward movement of the shoreline in response to relative sea-level lowering. Falling sea level & high sedimentation cause forced regression and prograding clinoforms. These clinoforms overlie the sequence boundary and underlie the transgressive surface (enveloped by the sequence boundary and transgressive surface).
Progradation
Progradation is the lateral outbuilding of strata in a sea-ward direction. It can occur as a result of a sea-level rise accompanied by a high sediment flux (causing a regression). Hence the beds are deposited successively basinward because sediment supply exceeds accommodation. Thus, the position of the shoreline migrates into the basin during episodes of progradation, a process called regression. Regression usually occurs during the late stages of the development of a Highstand Systems Tract and/or a Falling Stage Systems Tract.
Progradation Sediments build out into the basin in areas of progradation.
Retrogradation
Retrogradation is the landward movement of coast-line, in response to a transgression. This can occur during a sea-level rise with low sediment flux. Hence the beds are deposited successively landward because sediment supply is limited and cannot fill the available accommodation. Thus, the position of the shoreline migrates backward onto land, a process called transgression, during episodes of retrogradation.
Retrogradational stacking patterns of parasequences refer to patterns in which facies become progressively more distal when traced upward vertically
Aggradation
Vertical build up of a sedimentary sequence. Usually occurs when there is a relative rise in sea level produced by subsidence and/or eustatic sea-level rise, and the rate of sediment influx is sufficient to maintain the depositional surface at or near sea level (i.e. carbonate keep-up in a HST [highstand Systems Tract] or clastic HST). Occurs when sediment flux = rate of sea-level rise. It produces aggradational stacking patterns in parasequences when the patterns of facies at the top of each parasequence are essentially the same.
Fig. Aggradation. Stratigraphic sequences build upward in areas of aggradation.
Toplap
Toplap is the termination of strata against an overlying surface mainly as a result of nondposition (sedimentary bypassing) with perhaps only minor erosion.
Baselap
Baselap is a term describing termination of strata along the lower boundary of a depositional sequence, used only where discrimination between onlap and downlap is difficult or impossible
Onlap
It is the termination of shallowly dipping, younger strata against more steeply dipping, older strata. Onlap is a particular pattern of reflections in seismic data that occurs during periods of transgression. Rising of sea level & high sedimentation produce onlapping and aggrading clinoform enveloped by 1st onlapping parasequence and transgressive surface.
Downlap
The termination of more steeply dipping overlying strata against a surface or underlying strata that have lower apparent dips; a term used to describe base-discordant relation in which initially inclined strata terminate downdip against an initially horizontal or less inclined surface.
Truncation
Termination of strata or seismic reflections is interpreted as strata along an unconformity surface due to post-depositional erosional or structural effects
Topset
Topset is a horizontal deltaic deposit composed of coarse alluvial sediment. It represents current or past surface of the delta.
Parasequences
Relatively conformable depositional units bounded by marine flooding surfaces, surfaces that separate older strata from younger and show an increase in water depth in successively younger strata. In addition to these defining characteristics, most parasequences are asymmetrical shallowing-upward sedimentary cycles. Note that a parasequence is terminated by a rise in sea level. Parasequences are usually too thin to discern on seismic data, but when added together, they form sets called parasequence sets that are visible on seismic data.
Parasequence Boundary
A marine flooding surface or its correlative surface is called parasequence boundary.
DEPOSITIONAL SETTING
By genetically related, it is meant that all facies within a parasequence were deposited in lateral continuity to one another, that is, Walther's Law holds true within a parasequence.
a) For Siliciclastic
For a typical siliciclastic wave-dominated shoreline, a particular suite of facies should occur in a predictable order. A parasequence that spanned all of these facies would begin with bioturbated offshore mudstones, pass through the storm beds of the transition zone or lower shoreface, continue through the trough cross bedding of the shoreface, pass upwards into the seaward inclined laminae of the foreshore, and be capped by a backshore or coastal plain coal bed. In reality, a single parasequence at a single outcrop rarely passes through all of these facies, but instead includes only a portion of this facies succession; however, all of the facies that do occur appear in the correct order as predicted by Walther's Law. For example, a typical sandy wave-dominated parasequence in an outcrop might include only offshore and transition zone facies, or only shoreface, foreshore, and coastal plain facies, but offshore facies would not be overlain by coastal plain facies within a single parasequence.
Subscribe to:
Post Comments (Atom)
0 comments:
Post a Comment