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Biofacies and changing sea level (view source)
Revision as of 19:32, 30 January 2014
, 19:32, 30 January 2014Initial import
{{publication
| image = exploring-for-oil-and-gas-traps.png
| width = 120px
| series = Treatise in Petroleum Geology
| title = Exploring for Oil and Gas Traps
| part = Critical elements of the petroleum system
| chapter = Sedimentary basin analysis
| frompg = 4-1
| topg = 4-123
| author = John M. Armentrout
| link = http://archives.datapages.com/data/specpubs/beaumont/ch04/ch04.htm
| pdf =
| store = http://store.aapg.org/detail.aspx?id=545
| isbn = 0-89181-602-X
}}
Biofacies are identified by an assemblage of fossils and are interpreted to reflect a specific environment. The mapped distribution of the biofacies assemblage reflects the distribution of the interpreted environment. Biofacies are especially useful in mudstone-dominated facies such as the GOM basin Cenozoic strata.
==Traditional biofacies model==
The traditional biofacies model is based on the modern distribution of organisms.<ref name=ch04r44>Hedgpeth, J., W., 1957, Classification of marine environments: Geological Society of America Memoir 67, p. 17–27.</ref> This is a good model for a relative highstand of sea level (see figure below), in which neritic biofacies occur mostly on the shelf and bathyal biofacies occur mostly on the slope.
==Biofacies distribution during lowstand==
The lowering of sea level moves the water mass- and substrate-linked biofacies assemblages seaward—possibly far enough to place the inner neritic biofacies at the physiographic shelf/slope break. This movement causes the middle to outer neritic biofacies to shift basinward onto the upper slope of the clinoform (Figure 4-30B). The magnitude of relative sea level fluctuation, as well as the angle of the basin slope, controls how far the biofacies move across the physiographic profile. This pattern of low sea level biofacies distribution is confusing because the commonly used biofacies nomenclature is based on high sea level patterns where, by convention, the neritic biofacies are on the shelf (Figure 4-30A). During a lowstand, neritic biofacies may occur in situ on the physiographic slope.
[[file:sedimentary-basin-analysis_fig4-30.png|thumb|{{figure number|4-30}}After Armentrout.<ref name=ch04r7>Armentrout, J., M., 1991, Paleontological constraints on depositional [[modeling]]: examples of integration of biostratigraphy and seismic stratigraphy, Pliocene–Pleistocene, Gulf of Mexico, in Weimer, P., Link, M., H., eds., Seismic Facies and Sedimentary Processes of Submarine Fans and Turbidite Systems: New York, Springer-Verlag, p. 137–170.</ref><ref name=ch04r9>Armentrout, J., M., 1996, High-resolution sequence biostratigraphy: examples from the Gulf of Mexico Plio–Pleistocene, in Howell, J., Aiken, J., eds., High Resolution [[Sequence stratigraphy]]: Innovations and Applications: The Geological Society of London Special Publication 104, p. 65–86.</ref> Copyright: Springer-Verlag, Geological Society of London.]]
==Holocene GOM basin example==
On the Gulf of Mexico shelf, elements of the foraminiferal fauna also move in the seaward direction due to the modification of the environment by the Mississippi River.<ref name=ch04r77>Poag, C., W., 1981, Ecologic Atlas of Benthic Foraminifera of the Gulf of Mexico: Woods Hole Marine Science Institute, 174 p.</ref> High rates of deltaic sedimentation with coarser sediment grains, abundant terrigenous organic matter, and modified salinity and temperature greatly affect the local environment. Biofacies distribution responds to these environmental modifications. [See Pflum and Freichs<ref name=ch04r73>Pflum, C., E., Freichs, W., E., 1976, Gulf of Mexico deep water foraminifers: Cushman Foundation Foraminiferal Research Special Publication 14, 125 p.</ref> for a discussion of the delta-depressed fauna.]
==Lowstand fluvial influence on biofacies==
At times of low sea level, when the river systems discharge their sediment load directly on the upper slope, the inclined depositional surface may help sustain downslope transport of the terrigenous material and associated fluids. The modification of the local slope environment near the sediment input point could result in seaward excursions in ecological patterns similar to those caused on the shelf by the modern Mississippi River.<ref name=ch04r73 /><ref name=ch04r77 /> These seaward ecological excursions could extend to bathyal depths where downslope transport is sustained by the inclined surface and gravity-flow processes (see Figure 4-29).
==Biofacies mixing==
The downslope transport of shallow-water faunas by sediment gravity-flow processes may result in the mixing of biofacies assemblages from different environments.<ref name=ch04r119>Woodbury, H., O., Murray, I., B., Pickford, P., J., Akers, W., H., 1973, Pliocene and Pleistocene depocenters, outer continental shelf, Louisiana and Texas: AAPG Bulletin, vol. 57, p. 2428–2439.</ref> The further mixing of stratigraphically separate assemblages by rotary drilling complicates the identification of mixed assemblages. Such problems can be overcome in three ways:
* Careful sample analysis, specifically looking for mixed assemblages
* Use of closely spaced sidewall cores that may sample unmixed in situ assemblages occurring in beds interbedded with displaced assemblages
* Evaluation of the mapped pattern of age-equivalent interpretations from a large number of wells
Armentrout<ref name=ch04r7 /> <ref name=ch04r9 />) carefully reexamines rapid changes of biofacies and patterns of rapid biofacies variations within age-equivalent intervals between wells. Once these local patterns were reevaluated and accepted as reliable, the interpretations were mapped for each depositional sequence. Figure 4-29 is the results of this analysis and further supports the biofacies models of Figure 4-30.
==See also==
* [[Sea level cycle phase]]
* [[Determining sea level cycle order]]
* [[Sea level cycle phase and systems tracts]]
* [[Identifying systems tracts]]
* [[Systems tracts and trap types]]
* [[Identifying sea level cycle phase with biostratigraphy]]
* [[Constructing age model charts]]
* [[Superimposed sea level cycles]]
==References==
{{reflist}}
==External links==
{{search}}
* [http://archives.datapages.com/data/specpubs/beaumont/ch04/ch04.htm Original content in Datapages]
* [http://store.aapg.org/detail.aspx?id=545 Find the book in the AAPG Store]
[[Category:Critical elements of the petroleum system]]
[[Category:Sedimentary basin analysis]]
| image = exploring-for-oil-and-gas-traps.png
| width = 120px
| series = Treatise in Petroleum Geology
| title = Exploring for Oil and Gas Traps
| part = Critical elements of the petroleum system
| chapter = Sedimentary basin analysis
| frompg = 4-1
| topg = 4-123
| author = John M. Armentrout
| link = http://archives.datapages.com/data/specpubs/beaumont/ch04/ch04.htm
| pdf =
| store = http://store.aapg.org/detail.aspx?id=545
| isbn = 0-89181-602-X
}}
Biofacies are identified by an assemblage of fossils and are interpreted to reflect a specific environment. The mapped distribution of the biofacies assemblage reflects the distribution of the interpreted environment. Biofacies are especially useful in mudstone-dominated facies such as the GOM basin Cenozoic strata.
==Traditional biofacies model==
The traditional biofacies model is based on the modern distribution of organisms.<ref name=ch04r44>Hedgpeth, J., W., 1957, Classification of marine environments: Geological Society of America Memoir 67, p. 17–27.</ref> This is a good model for a relative highstand of sea level (see figure below), in which neritic biofacies occur mostly on the shelf and bathyal biofacies occur mostly on the slope.
==Biofacies distribution during lowstand==
The lowering of sea level moves the water mass- and substrate-linked biofacies assemblages seaward—possibly far enough to place the inner neritic biofacies at the physiographic shelf/slope break. This movement causes the middle to outer neritic biofacies to shift basinward onto the upper slope of the clinoform (Figure 4-30B). The magnitude of relative sea level fluctuation, as well as the angle of the basin slope, controls how far the biofacies move across the physiographic profile. This pattern of low sea level biofacies distribution is confusing because the commonly used biofacies nomenclature is based on high sea level patterns where, by convention, the neritic biofacies are on the shelf (Figure 4-30A). During a lowstand, neritic biofacies may occur in situ on the physiographic slope.
[[file:sedimentary-basin-analysis_fig4-30.png|thumb|{{figure number|4-30}}After Armentrout.<ref name=ch04r7>Armentrout, J., M., 1991, Paleontological constraints on depositional [[modeling]]: examples of integration of biostratigraphy and seismic stratigraphy, Pliocene–Pleistocene, Gulf of Mexico, in Weimer, P., Link, M., H., eds., Seismic Facies and Sedimentary Processes of Submarine Fans and Turbidite Systems: New York, Springer-Verlag, p. 137–170.</ref><ref name=ch04r9>Armentrout, J., M., 1996, High-resolution sequence biostratigraphy: examples from the Gulf of Mexico Plio–Pleistocene, in Howell, J., Aiken, J., eds., High Resolution [[Sequence stratigraphy]]: Innovations and Applications: The Geological Society of London Special Publication 104, p. 65–86.</ref> Copyright: Springer-Verlag, Geological Society of London.]]
==Holocene GOM basin example==
On the Gulf of Mexico shelf, elements of the foraminiferal fauna also move in the seaward direction due to the modification of the environment by the Mississippi River.<ref name=ch04r77>Poag, C., W., 1981, Ecologic Atlas of Benthic Foraminifera of the Gulf of Mexico: Woods Hole Marine Science Institute, 174 p.</ref> High rates of deltaic sedimentation with coarser sediment grains, abundant terrigenous organic matter, and modified salinity and temperature greatly affect the local environment. Biofacies distribution responds to these environmental modifications. [See Pflum and Freichs<ref name=ch04r73>Pflum, C., E., Freichs, W., E., 1976, Gulf of Mexico deep water foraminifers: Cushman Foundation Foraminiferal Research Special Publication 14, 125 p.</ref> for a discussion of the delta-depressed fauna.]
==Lowstand fluvial influence on biofacies==
At times of low sea level, when the river systems discharge their sediment load directly on the upper slope, the inclined depositional surface may help sustain downslope transport of the terrigenous material and associated fluids. The modification of the local slope environment near the sediment input point could result in seaward excursions in ecological patterns similar to those caused on the shelf by the modern Mississippi River.<ref name=ch04r73 /><ref name=ch04r77 /> These seaward ecological excursions could extend to bathyal depths where downslope transport is sustained by the inclined surface and gravity-flow processes (see Figure 4-29).
==Biofacies mixing==
The downslope transport of shallow-water faunas by sediment gravity-flow processes may result in the mixing of biofacies assemblages from different environments.<ref name=ch04r119>Woodbury, H., O., Murray, I., B., Pickford, P., J., Akers, W., H., 1973, Pliocene and Pleistocene depocenters, outer continental shelf, Louisiana and Texas: AAPG Bulletin, vol. 57, p. 2428–2439.</ref> The further mixing of stratigraphically separate assemblages by rotary drilling complicates the identification of mixed assemblages. Such problems can be overcome in three ways:
* Careful sample analysis, specifically looking for mixed assemblages
* Use of closely spaced sidewall cores that may sample unmixed in situ assemblages occurring in beds interbedded with displaced assemblages
* Evaluation of the mapped pattern of age-equivalent interpretations from a large number of wells
Armentrout<ref name=ch04r7 /> <ref name=ch04r9 />) carefully reexamines rapid changes of biofacies and patterns of rapid biofacies variations within age-equivalent intervals between wells. Once these local patterns were reevaluated and accepted as reliable, the interpretations were mapped for each depositional sequence. Figure 4-29 is the results of this analysis and further supports the biofacies models of Figure 4-30.
==See also==
* [[Sea level cycle phase]]
* [[Determining sea level cycle order]]
* [[Sea level cycle phase and systems tracts]]
* [[Identifying systems tracts]]
* [[Systems tracts and trap types]]
* [[Identifying sea level cycle phase with biostratigraphy]]
* [[Constructing age model charts]]
* [[Superimposed sea level cycles]]
==References==
{{reflist}}
==External links==
{{search}}
* [http://archives.datapages.com/data/specpubs/beaumont/ch04/ch04.htm Original content in Datapages]
* [http://store.aapg.org/detail.aspx?id=545 Find the book in the AAPG Store]
[[Category:Critical elements of the petroleum system]]
[[Category:Sedimentary basin analysis]]