Changes

==Interpreting the maps==

==Interpreting the maps==

[[file:sedimentary-basin-analysis_fig4-29.png|300px|thumb|{{figure number|2}}.]]

[[file:sedimentary-basin-analysis_fig4-29.png|300px|thumb|{{figure number|2}}.]]

Biofacies map patterns are defined by distribution of benthic foraminiferal biofacies.<ref name=ch04r7 /><ref name=ch04r9 /> [[:file:sedimentary-basin-analysis_fig4-29.png|Figure 2]] shows the biofacies distribution below, within, and above the ''Glob alt'' sandstone interval. In upward stratigraphic order, these intervals are interpreted as the sediment accumulated during (1) falling, (2) low, and (3) rising phases of sea level, respectively. A scenario to explain biofacies and sediment patterns in the example is as follows.

Biofacies map patterns are defined by distribution of benthic foraminiferal biofacies.<ref name=ch04r7 /><ref name=ch04r9 /> [[:file:sedimentary-basin-analysis_fig4-29.png|Figure 2]] shows the biofacies distribution below, within, and above the ''Glob alt'' sandstone interval. In upward stratigraphic order, these intervals are interpreted as the sediment accumulated during (1) falling, (2) low, and (3) rising phases of sea level, respectively. A scenario to explain biofacies and sediment patterns in the example is as follows.

* During the lowering of sea level, the biofacies distributions and sites of maximum sediment accumulation move seaward (Figure 4-29A) where they are deposited on top of the preceding condensed section and associated maximum flooding surface (Figure 4-18). Within the initial lowering phase, the rate of slope and intraslope basin sediment accumulation increases, with fine-grained deposits above the underlying condensed section. As lowering progresses, the river systems bypass sediment across the shelf, depositing it directly on the upper slope. Remobilized sand and sand supplied directly from rivers during floods may be transported downslope by gravity-flow processes, depositing potential reservoirs.<ref name=ch04r81>Prior, D., B., Bornhold, B., D., Wiseman, W., J. Jr., Lowe, D., R., 1987, Turbidity current activity in a British Columbia fjord: Science, vol. 237, p. 1330–1333., 10., 1126/science., 237., 4820., 1330</ref> These sands accumulate at changes in the depositional gradient as slope fan and basin-floor fan deposits within the intraslope basins (minibasins).<ref name=ch04r22>Bouma, A., H., 1982, [http://archives.datapages.com/data/specpubs/history2/data/a110/a110/0001/0550/0567.htm Intraslope basins in northwest Gulf of Mexico: a key to ancient submarine canyons and fans], in Watkins, J. S., and C. L. Drake, eds., Studies in Continental Margin Geology: AAPG Memoir 34, p. 567–581.</ref>

* During the lowering of sea level, the biofacies distributions and sites of maximum sediment accumulation move seaward ([[:file:sedimentary-basin-analysis_fig4-29.png|Figure 2A]]) where they are deposited on top of the preceding condensed section and associated maximum flooding surface ([[:file:sedimentary-basin-analysis_fig4-18.png|Figure 3]]). Within the initial lowering phase, the rate of slope and intraslope basin sediment accumulation increases, with fine-grained deposits above the underlying condensed section. As lowering progresses, the river systems bypass sediment across the shelf, depositing it directly on the upper slope. Remobilized sand and sand supplied directly from rivers during floods may be transported downslope by gravity-flow processes, depositing potential reservoirs.<ref name=ch04r81>Prior, D., B., Bornhold, B., D., Wiseman, W., J. Jr., Lowe, D., R., 1987, Turbidity current activity in a British Columbia fjord: Science, vol. 237, p. 1330–1333., 10., 1126/science., 237., 4820., 1330</ref> These sands accumulate at changes in the depositional gradient as slope fan and basin-floor fan deposits within the intraslope basins (minibasins).<ref name=ch04r22>Bouma, A., H., 1982, [http://archives.datapages.com/data/specpubs/history2/data/a110/a110/0001/0550/0567.htm Intraslope basins in northwest Gulf of Mexico: a key to ancient submarine canyons and fans], in Watkins, J. S., and C. L. Drake, eds., Studies in Continental Margin Geology: AAPG Memoir 34, p. 567–581.</ref>

* The biofacies associated with this lowstand depositional phase, the sandstone interval (Figure 4-29B), show similar basinward excursion in outer neritic and upper bathyal biofacies as deposited during the presandstone phase. The inner and middle neritic shallow-water biofacies of the sandstone interval show a seaward shift, relative to the seismic stratigraphically defined shelf/slope break—interpreted as an indication of coastal progradation associated with lowered sea level.

* The biofacies associated with this lowstand depositional phase, the sandstone interval ([[:file:sedimentary-basin-analysis_fig4-29.png|Figure 2B]]), show similar basinward excursion in outer neritic and upper bathyal biofacies as deposited during the presandstone phase. The inner and middle neritic shallow-water biofacies of the sandstone interval show a seaward shift, relative to the seismic stratigraphically defined shelf/slope break—interpreted as an indication of coastal progradation associated with lowered sea level.

* Once sea level begins to rise, the supply of sand-rich sediment to the basinal slope area is rapidly cut off. Mud accumulates during this postsandstone interval, culminating with the overlying upper condensed section. These mudstones and those of the pre-sandstone interval provide a seal for the interbedded lowstand sandstones. The biofacies pattern for this postsandstone interval shows a northward shift toward the basin margin as the coastline regresses across the shelf during transgression (Figure 4-29C). The occurrences of neritic biofacies of the postsandstone interval do not shift northward as far as their position during the presandstone interval. This is interpreted as sediment accumulation rate exceeding the rate of accommodation space formation by sea level rise plus basin subsidence. The consequence is coastal progradation, as suggested by comparing the mapped patterns of presandstone and postsandstone biofacies in the High Island–East Addition area just west of the seaward extension of the Texas/Louisiana border.

* Once sea level begins to rise, the supply of sand-rich sediment to the basinal slope area is rapidly cut off. Mud accumulates during this postsandstone interval, culminating with the overlying upper condensed section. These mudstones and those of the pre-sandstone interval provide a seal for the interbedded lowstand sandstones. The biofacies pattern for this postsandstone interval shows a northward shift toward the basin margin as the coastline regresses across the shelf during transgression ([[:file:sedimentary-basin-analysis_fig4-29.png|Figure 2C]]). The occurrences of neritic biofacies of the postsandstone interval do not shift northward as far as their position during the presandstone interval. This is interpreted as sediment accumulation rate exceeding the rate of accommodation space formation by sea level rise plus basin subsidence. The consequence is coastal progradation, as suggested by comparing the mapped patterns of presandstone and postsandstone biofacies in the High Island–East Addition area just west of the seaward extension of the Texas/Louisiana border.

==See also==

==See also==