View source for Parasequences (fourth- and fifth-order sequences)

{{publication 
 | image   = exploring-for-oil-and-gas-traps.png
 | width   = 120px
 | series  = Treatise in Petroleum Geology
 | title   = Exploring for Oil and Gas Traps
 | part    = Predicting the occurrence of oil and gas traps
 | chapter = Exploring for stratigraphic traps
 | frompg  = 21-1
 | topg    = 21-68
 | author  = John C. Dolson, Mike S. Bahorich, Rick C. Tobin, Edward A. Beaumont, Louis J. Terlikoski, Michael L. Hendricks
 | link    = http://archives.datapages.com/data/specpubs/beaumont/ch21/ch21.htm
 | pdf     = 
 | store   = http://store.aapg.org/detail.aspx?id=545
 | isbn    = 0-89181-602-X
}}
Parasequences are deposited during fourth- or fifth-order eustatic cycles. They are generally [[Depocenter#Sediment_supply_rate_and_facies_patterns|progradational]] and have a shoaling-upward association of facies.<ref name=ch21r47>Van Wagoner, J., C., Mitchum, R., M., Campion, K., M., Rahmanian, V., D., 1990, Siliciclastic Sequence stratigraphy in Well Logs, Cores and Outcrops: Concepts for High-Resolution Correlation of Time and Facies: [http://store.aapg.org/detail.aspx?id=1196 AAPG Methods in Exploration Series No. 7], 55 p.</ref> In siliciclastic parasequences, [[grain size]] can either fine or coarsen upward, reflecting an upward decrease in water depth. Some workers<ref name=ch21r45>Van Wagoner, J., C., 1995, [http://archives.datapages.com/data/specpubs/memoir64/front/000i.htm Overview of sequence stratigraphic foreland basin deposits: terminology, summary of papers, and glossary of sequence stratigraphy], in Van Wagoner, J., C., Bertram, G., T., eds., Sequence Stratigraphy of Foreland Basin Deposits: Outcrop and Subsurface Examples from the Cretaceous of North America: [http://store.aapg.org/detail.aspx?id=146 AAPG Memoir 64], p. ix–xxi.</ref> consider fourth-order sequences (deposited during cycles 100,000–200,000 years in duration) to be the building blocks of most reservoir or field studies.

==Periodic vs. episodic parasequences==
[[file:exploring-for-stratigraphic-traps_fig21-12.png|300px|thumb|{{figure number|1}}Characteristics of an upward-coarsening parasequence formed in a deltaic environment. From Van Wagoner et al.;<ref name=ch21r47 /> courtesy AAPG.]]

A parasequence can be either periodic or episodic.<ref name=ch21r48>Weber, L., J., Sarg, J., F., Wright, F., M., 1995, [[Sequence stratigraphy]] and reservoir delineation of the middle Pennsylvanian (Desmoinesian), Paradox basin and Aneth field, southwestern U., S., A., in Read, J., F., Weber, L., J., Sarg, J., F., Wright, F., M., eds., Milankovitch Sea-Level Changes, Cycles, and Reservoirs on Carbonate Platforms in Greenhouse and Ice-House Worlds: SEPM Short Course No. 35, 79 p.</ref> A '''periodic parasequence''' has regional continuity and forms in response to deposition during a global sea level cycle. An '''episodic parasequence''' has limited lateral extent and forms in response to (for example) [[tidal flat]] migration or [[Deltaic environments|delta]] lobe shifts. Episodic parasequences are of very short duration—generally less than 10,000 years. Periodic parasequences have average durations of 100,000 years.

==Parasequence sets==
A parasequence set is a succession of genetically related parasequences that forms a distinctive stacking pattern. A parasequence set is generally bounded by a [[Definitions_of_depositional_system_elements#Maximum_flooding_surface|marine flooding surface]].<ref name=ch21r45 />

[[file:exploring-for-stratigraphic-traps_fig21-13.png|300px|thumb|{{figure number|2}}Characteristics of two upward-fining parasequences formed in a tidal flat to subtidal environment. From Van Wagoner et al.;<ref name=ch21r47 /> courtesy AAPG.]]

==Upward-coarsening parasequence==
[[:file:exploring-for-stratigraphic-traps_fig21-12.png|Figure 1]] shows the characteristics of an upward-coarsening parasequence formed in a deltaic environment.

==Upward-fining parasequence==
[[:file:exploring-for-stratigraphic-traps_fig21-13.png|Figure 2]] shows the characteristics of two upward-fining parasequences formed in a tidal flat to subtidal environment.

==See also==
* [[Sequence stratigraphy]]
* [[Hierarchy of sequences]]
* [[Third-order sequences]]
* [[Traps in a framework of sequence stratigraphy]]

==References==
{{reflist}}

==External links==
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* [http://archives.datapages.com/data/specpubs/beaumont/ch21/ch21.htm Original content in Datapages]
* [http://store.aapg.org/detail.aspx?id=545 Find the book in the AAPG Store]

[[Category:Predicting the occurrence of oil and gas traps]] 
[[Category:Exploring for stratigraphic traps]]