Seismic sequence boundaries

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Problem 10.12a

Interpret the section shown in Figure 10.12a. Assume that out-of-the-plane data are not important. Pick events that involve angularities between primary reflections in order to identify unconformities and/or seismic sequence boundaries. Note the thinning/thickening of different units.

Figure 10.12a.  Seismic section (from Emery and Myers, 1996).

Background

Seismic sequence analysis is based on the concept that changes in sea level produce more-or-less systematic patterns in marine sediments. Sequence stratigraphy assumes that reflections parallel time surfaces (surfaces that at some past time were the tops of the solid earth) and that stratigraphic patterns result from a combination of (1) absolute sea level (eustasy). (2) uplift or subsidence, (3) supply of sediments, and (4) climatic conditions. Sediments deposited when sea level is lower than it was before and afterward yield lowstand tracts; those deposited when sea level is rising beyond its previous highest value produce transgressive tracts; and those produced when sea level is higher than it was before and afterward yield highstand tracts.

A fall of sea level generally produces an unconformity (see problem 10.13) somewhere, where aerial and sometimes marine exposures are subject to erosion. A sequence consists of the sediments deposited beginning with a sea-level fall and extending to the next sea-level fall. Sequence boundaries are usually evident on seismic lines that are long enough and where the data quality is good and the resolution is sufficient. Changes of sea level are apt to occur at the same time over a large area.

Interpretation begins by noting angularities between reflections (shown in Figure 10.6b and discussed in problem 10.6), which are then used to identify unconformities and other features. Onlap angularities are produced by rising sea level, erosional unconformities by a sea-level fall. During a rise of sea level following a fall, the coastline moves landward (transgresses) unless sedimentation is rapid enough to maintain the coastal position. The interpretation procedure is to first mark the unconformities to define sequences, then map them in three-dimensions, noting changes in the thickness of sequences and the relationship to other sequences, and finally attributing significance to specific sequences.

A fall of sea level, which marks the beginning of a sequence, causes the coastline to move seaward, changing the kinds of sediments deposited, sediments generally becoming coarser as the coastline comes closer. If the sea-level fall moves the coastline below the shelf edge, we expect increased lowstand deposition. Sediments that lie on the slope at the angle of repose will fail more often because storm waves can disturb them more easily, and these sediments will be redeposited farther seaward. During the latter parts of the low-stand as sea level is rising but still below the shelf edge, we expect lowstand progradation resulting in reflections that are more regular and continuous. As sea level rises above its previous level, we expect transgression (landward movement of the coastline) and then less sediment will be available for deposition at seaward locations; this condition will extend into the next highstand. We expect a retrogradational pattern (successive units not reaching as far seaward) during a rapid transgression, then an aggradational one as the transgression comes to an end, and finally progradational as the sequence comes to an end with the next sea-level fall. Thus we expect that cyclical changes of sea level will produce cycles in the depositional patterns. Of course we should not expect the same pattern of seismic facies to be repeated exactly, because conditions will be different during each cycle.

Figure 10.12b.  Interpretation of Figure 10.12a; arrows indicate reflection terminations.

Solution

The following are observations about this single section and tentative suggestions as to their possible meanings. One cannot expect firm conclusions based on a single section.

The major reflection events have been picked in Figure 10.12b and identified by letters. The seafloor reflection is labeled . We see a single peak with about a cycle of forerunner; we interpret this as an embedded wavelet that is nearly zero-phase with standard polarity, although the larger trough following the major peak suggests that the embedded wavelet is not completely zero phase.

The unit is thicker over the right-hand 40% of the section. is a rough surface with a channel cut in it between 2.0 and 4.5 km. We interpret the -to- units as fluvial sedimentation. -to- appears to be progradational. Both and are cut by channels and it appears that the still deeper channel in probably fixed the locations of the shallower channels as far up as ; it is not unusual for channels to localize subsequent channels. The -to- unit thickens to the right with some suggestion of progradation. The source of these sediments appears to be to the right.

At about 9 km there is a listric fault (problem 10.6) either at or at least before . There is a suggestion of onlap onto , and the sediments immediately above it thicken to the right. appears to be out-of-phase with most of the reflections, and there are suggestions of both downlap and onlap onto and of toplap below . , , and indicate progradation. Both and have mounds at the base of their steeper slopes and in places we see both onlap and downlap onto them. , which has been picked discontinuously, may not represent the same horizon. The source of sediments for units below is to the left of the section, in contrast to those above .

Figure 10.12c.  Seismic section in Gulf of Mexico.

and appear to be the best candidates for significant sequence boundaries. Other strong reflections such as , , , and may also be minor sequence boundaries.

The progradation from to may be deltaic. marks a relative sea-level fall that eroded the tops of the -to- sediments. The -to- sediments are mainly lowstand.

Problem 10.12b

Interpret the Gulf of Mexico section shown in Figure 10.12c, making the same assumptions as with Figure 10.12a.

Solution

There are rather clear evidences of faulting, which must be resolved before stratigraphic interpretation can begin. The reflections are all more-or-less parallel and obvious angularities are few. A number of changes in seismic character (seismic facies) are evident, and these greatly assist in identifying the same horizons across the faults. The most obvious of these are very weak reflection zones underlain by fairly strong reflections that are probably sequence boundaries. These weak-reflection zones are probably predominately shale.

An interpretation of this section is shown in Figure 10.12d with some of the stronger reflections or more obvious seismic facies separations indicated by letters. The general increase in dip to the right below event and the general thickening to the right indicate that the right-hand side of the section was subsiding more rapidly than the left-hand side while the sediments were being deposited. The down-to-the-right synthetic faults are growth faults; the throw increases with time, resulting in thicker units on the downthrown (hanging-wall) sides. Because of the increase of velocity with depth, the throws increase with depth more rapidly than they appear to. The dipping event may be the deep continuation of a listric fault that is soling out.

Figure 10.12d.  Interpretation of Figure 10.12c.

There appear to be two antithetic faults (faults opposite in sense to the more important growth faults), which are associated with the extension involved with rollover anticlines that locally reverse the dip.

The sediments in the Gulf of Mexico are predominently clastics, and this area (in about 200 m of water) was probably marine for a very long time, the coastline being further away during highstands than lowstands. Lowstands generally result in larger volumes of sediments being available for deposition than highstands because more surface is exposed to areal erosion then. Hence, we expect most of this section to be lowstand deposition, and highstands to be thinner with changes more concentrated than in lowstand sediments. Thus sharp changes in physical properties and distinctive reflections will be more probable during the highstand and especially at its end of the highstand. We generally associate the strongest and most continuous reflections with the sequence boundaries; these are indicated by solid lines. The reflections indicated by long dashes are interpreted as the tops of slope-fan units, and those identified by dots as the tops of lowstand prograding sediments. Many of the lettered reflections are probably sequence boundaries, and there are probably more sequence boundaries than have been picked.

The unit between and thins to the right, presumably because the coastline is a long way off to the left and, hence, fewer sediments are available for deposition as one goes to the right. Units often thicken on the downthrown side of growth faults. At times this area was presumably undergoing rapid subsidence because of movement of underlying salt, and some of the thickening is probably the consequence of this.

Often well logs can be interpreted in sequence-stratigraphic terms. A log in a well shown in Figure 10.12d has been interpreted as showing a number of sequence boundaries indicated by the letters , , , , and (the initials of distinctive paleontological species). These generally correspond to the facies changes indicated by the solid lines in Figure 10.12d. Two other sequence boundaries between and are identified in this well, but the resolution of the seismic section does not permit their identification. Event H is probably also a sequence boundary although it was not identified in the well.

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