Biofacies and changing sea level

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Exploring for Oil and Gas Traps
Series Treatise in Petroleum Geology
Part Critical elements of the petroleum system
Chapter Sedimentary basin analysis
Author John M. Armentrout
Link Web page
Store AAPG Store

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.[1] 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

Figure 1 Traditional biofacies model is based on the modern distribution of organisms. After Armentrout.[2][3] Copyright: Springer-Verlag, Geological Society of London.

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 1B). 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 1A). During a lowstand, neritic biofacies may occur in situ on the physiographic slope.

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.[4] 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[5] for a discussion of the delta-depressed fauna.]

Lowstand fluvial influence on biofacies

Figure 2 Biofacies map of the study area in the Gulf of Mexico. After Armentrout;[2][3] courtesy Springer-Verlag.

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.[5][4] These seaward ecological excursions could extend to bathyal depths where downslope transport is sustained by the inclined surface and gravity-flow processes (see Figure 2).

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.[6] 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[2][3] 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 2 is the results of this analysis and further supports the biofacies models of Figure 1.

See also

References

  1. Hedgpeth, J. W., 1957, Classification of marine environments: Geological Society of America Memoir 67, p. 17–27.
  2. 2.0 2.1 2.2 Armentrout, J. M., 1991, Paleontological constraints on depositional modeling: examples of integration of biostratigraphy and seismic stratigraphy, Pliocene–Pleistocene, Gulf of Mexico, in P. Weimer, and M. H. Link, eds., Seismic Facies and Sedimentary Processes of Submarine Fans and Turbidite Systems: New York, Springer-Verlag, p. 137–170.
  3. 3.0 3.1 3.2 Armentrout, J. M., 1996, High-resolution sequence biostratigraphy: examples from the Gulf of Mexico Plio–Pleistocene, in J. Howell, and J. Aiken, eds., High Resolution sequence stratigraphy: Innovations and Applications: The Geological Society of London Special Publication 104, p. 65–86.
  4. 4.0 4.1 Poag, C. W., 1981, Ecologic Atlas of Benthic Foraminifera of the Gulf of Mexico: Woods Hole Marine Science Institute, 174 p.
  5. 5.0 5.1 Pflum, C. E., and W. E. Freichs, 1976, Gulf of Mexico deep water foraminifers: Cushman Foundation Foraminiferal Research Special Publication 14, 125 p.
  6. Woodbury, H. O., I. B. Murray, P. J. Pickford, and W. H. Akers, 1973, Pliocene and Pleistocene depocenters, outer continental shelf, Louisiana and Texas: AAPG Bulletin, vol. 57, p. 2428–2439.

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