6.4 SEDIMENTARY MODELS, INCREMENTS, AND CYCLES
281
pach maps of genetic sequences of strata define larger morphologic units of shelf, hinge
line, and basin.
It is apparent though, from the data in this chapter, that increments of sedimentation
are generated by many sedimentary models. Therefore, incremental mapping is actually
feasible with many sedimentary facies. The concept of incremental mapping, though
seldom the formal terminology, is widely applied in facies analysis. It forms an integral
part of environmental and paleogeographic analyses to locate favorable hydrocarbon
reservoir facies.
6.4.3 Sequence Stratigraphy
The depositional environments of ancient sedimentary rocks is normally carried out
simultaneously with a stratigraphic analysis of the formations studied. It is necessary,
therefore, to consider the relationship between facies analysis and stratigraphy. For over
a century fossils provided a vital link between these two pursuits, paleoecology aiding
environmental interpretation, and biostratigraphy establishing the chronology. Now,
however, paleontology is joined by seismic surveying as a linking tool. The advent of
modern high-quality seismic data has lead to a new discipline, known by its advocates
as seismic sequence stratigraphy (Payton, 1977; Berg and Wolverton, 1985; Wilgus et al.,
1988; Steele et al., 1995; Emery and Myers, 1996; Miall, 1997).
Sequences of sedimentary rocks may be differentiated into units, and the units correlated from section to section in many different ways. These include facies, lithostratigraphy, and chronostratigraphy. Facies are defined by their geometry, lithology, sedimentary structures, paleocurrent pattern, and paleontology. In the subsurface facies
may also be characterized by their geophysical properties, such as log profile and seismic character (Selley, 1996). Facies may be subdivided into subfacies, and genetic increments and sequences may be recognized.
The lithostratigraphy delineates mappable rock units arranged in a hierarchy of
groups, formations, and members. The chronostratigraphy is commonly based on biostratigraphy, and can be used to attribute the rocks to geological systems, stages, or series. These units can be divided in to upper, middle, and lower, as appropriate.
The foregoing are essentially observational activities. They form the basis for subsequent interpretations. The facies may be studied to interpret their environments of
deposition. The chronostratigraphy may be interpreted to establish geochronology.
Geochronologic units are the intervals of time during which a particular chronostratigraphic unit was deposited. They are arranged in the hierarchy of periods, epochs, or
ages. Geochronologic units may be subdivided into early, middle, and late, as appropriate. A useful way of distinguishing between chronostratigraphic and geochronologic
units is to consider an hourglass, or its miniaturized version, the egg timer. The sand
that flows through the hourglass is the chronostratigraphic unit. The time taken is the
geochronologic unit.
Sequence stratigraphy is basically "a geologic approach to the stratigraphic interpretation of seismic data" (Vail et al., 1977, p. 51). Of fundamental importance is the
seismic delineation of depositional sequences. A depositional sequence is defined as a
"stratigraphic unit composed of a relatively conformable succession of genetically related strata, bounded at top and bottom by unconformities or their correlative conformities" (Vail et al., 1977, p. 53).
281
pach maps of genetic sequences of strata define larger morphologic units of shelf, hinge
line, and basin.
It is apparent though, from the data in this chapter, that increments of sedimentation
are generated by many sedimentary models. Therefore, incremental mapping is actually
feasible with many sedimentary facies. The concept of incremental mapping, though
seldom the formal terminology, is widely applied in facies analysis. It forms an integral
part of environmental and paleogeographic analyses to locate favorable hydrocarbon
reservoir facies.
6.4.3 Sequence Stratigraphy
The depositional environments of ancient sedimentary rocks is normally carried out
simultaneously with a stratigraphic analysis of the formations studied. It is necessary,
therefore, to consider the relationship between facies analysis and stratigraphy. For over
a century fossils provided a vital link between these two pursuits, paleoecology aiding
environmental interpretation, and biostratigraphy establishing the chronology. Now,
however, paleontology is joined by seismic surveying as a linking tool. The advent of
modern high-quality seismic data has lead to a new discipline, known by its advocates
as seismic sequence stratigraphy (Payton, 1977; Berg and Wolverton, 1985; Wilgus et al.,
1988; Steele et al., 1995; Emery and Myers, 1996; Miall, 1997).
Sequences of sedimentary rocks may be differentiated into units, and the units correlated from section to section in many different ways. These include facies, lithostratigraphy, and chronostratigraphy. Facies are defined by their geometry, lithology, sedimentary structures, paleocurrent pattern, and paleontology. In the subsurface facies
may also be characterized by their geophysical properties, such as log profile and seismic character (Selley, 1996). Facies may be subdivided into subfacies, and genetic increments and sequences may be recognized.
The lithostratigraphy delineates mappable rock units arranged in a hierarchy of
groups, formations, and members. The chronostratigraphy is commonly based on biostratigraphy, and can be used to attribute the rocks to geological systems, stages, or series. These units can be divided in to upper, middle, and lower, as appropriate.
The foregoing are essentially observational activities. They form the basis for subsequent interpretations. The facies may be studied to interpret their environments of
deposition. The chronostratigraphy may be interpreted to establish geochronology.
Geochronologic units are the intervals of time during which a particular chronostratigraphic unit was deposited. They are arranged in the hierarchy of periods, epochs, or
ages. Geochronologic units may be subdivided into early, middle, and late, as appropriate. A useful way of distinguishing between chronostratigraphic and geochronologic
units is to consider an hourglass, or its miniaturized version, the egg timer. The sand
that flows through the hourglass is the chronostratigraphic unit. The time taken is the
geochronologic unit.
Sequence stratigraphy is basically "a geologic approach to the stratigraphic interpretation of seismic data" (Vail et al., 1977, p. 51). Of fundamental importance is the
seismic delineation of depositional sequences. A depositional sequence is defined as a
"stratigraphic unit composed of a relatively conformable succession of genetically related strata, bounded at top and bottom by unconformities or their correlative conformities" (Vail et al., 1977, p. 53).