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==Stratigraphic cross sections==

==Stratigraphic cross sections==

[[file:geological-cross-sections_fig1.png|left|thumb|{{figure number|1}}(a) Stratigraphic and (b) structural cross sections of the Ranger Formation in the Long Beach unit of the Wilmington field, California. Sections are projected onto a north-south plane. (From Slatt et al.<ref name=pt06r122>Slatt, R. M., Phillips, S., Boak, J. M., Lagoe, M. B., 1993, Scales of geological heterogeneity of a deep-water sand giant oil field, Long Beach unit, Wilmington field, California, in Rhodes, E. G., Moslow, T. F., eds., Marine Clastic Reservoirs—Examples and Analogs: New York, Springer-Verlag.</ref>)]]

[[file:geological-cross-sections_fig1.png|thumb|{{figure number|1}}(a) Stratigraphic and (b) structural cross sections of the Ranger Formation in the Long Beach unit of the Wilmington field, California. Sections are projected onto a north-south plane. (From Slatt et al.<ref name=pt06r122>Slatt, R. M., Phillips, S., Boak, J. M., Lagoe, M. B., 1993, Scales of geological heterogeneity of a deep-water sand giant oil field, Long Beach unit, Wilmington field, California, in Rhodes, E. G., Moslow, T. F., eds., Marine Clastic Reservoirs—Examples and Analogs: New York, Springer-Verlag.</ref>)]]

Stratigraphic cross sections show characteristics of correlatable stratigraphic units, such as reservoir sandstones or sealing shales. They may also be vital in understanding the timing of deformation by showing the drape of sediment over developing folds or the thickening of the section across growth faults. The following elements of cross section design are presented as if they were a sequence. In practice, however, each choice affects and is affected by the others.

Stratigraphic cross sections show characteristics of correlatable stratigraphic units, such as reservoir sandstones or sealing shales. They may also be vital in understanding the timing of deformation by showing the drape of sediment over developing folds or the thickening of the section across growth faults. The following elements of cross section design are presented as if they were a sequence. In practice, however, each choice affects and is affected by the others.

The ''datum'' is the level or reference horizon from which elevations and depths are measured in the cross section. In a stratigraphic cross section, the geologist takes advantage of the principle of original horizontality to produce an interpretation of what the chosen slice of the earth might have looked like at some time in the past. By “hanging” all the available vertical information on a stratigraphic horizon or datum that can be correlated along the full length of the cross section, the data are transformed to reflect a different horizontal plane, one that existed at an earlier time (see [[:file:geological-cross-sections_fig1.png|Figure 1a]]). The assumption that this surface was horizontal when deposited assumes no original depositional slope.

The ''datum'' is the level or reference horizon from which elevations and depths are measured in the cross section. In a stratigraphic cross section, the geologist takes advantage of the principle of original horizontality to produce an interpretation of what the chosen slice of the earth might have looked like at some time in the past. By “hanging” all the available vertical information on a stratigraphic horizon or datum that can be correlated along the full length of the cross section, the data are transformed to reflect a different horizontal plane, one that existed at an earlier time (see [[:file:geological-cross-sections_fig1.png|Figure 1a]]). The assumption that this surface was horizontal when deposited assumes no original depositional slope.

The purpose of the cross section is to determine which horizon can serve as the datum. Because it is shown as horizontal, the thickness variations of the units directly above and below the datum are most simply interpretable on the cross section. The cross section in [[:file:geological-cross-sections_fig1.png|Figure lb]] uses the horizon labeled F as a datum because this has been interpreted as the top of a chronostratigraphic sequence.<ref name=pt06r122 />

The purpose of the cross section is to determine which horizon can serve as the datum. Because it is shown as horizontal, the thickness variations of the units directly above and below the datum are most simply interpretable on the cross section. The cross section in [[:file:geological-cross-sections_fig1.png|Figure 1b]] uses the horizon labeled F as a datum because this has been interpreted as the top of a chronostratigraphic sequence.<ref name=pt06r122 />

An unconformity is commonly used as a datum. In many circumstances, unconformities represent relatively uniform and geologically important time horizons and are therefore useful features on which to hang cross sections. However, caution must be used since the sedimentary layers may reflect paleotopographic relief.

An unconformity is commonly used as a datum. In many circumstances, unconformities represent relatively uniform and geologically important time horizons and are therefore useful features on which to hang cross sections. However, caution must be used since the sedimentary layers may reflect paleotopographic relief.