Seismic Stratigraphy

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Sediments deposited between two picked reflections during salt uplift

Erosional Channels

Some tasks of the interpreter are now apparent:

To pick the sequence boundaries (which are unconformities or their correlative conformities).
To classify the internal configuration of reflections within the sequence (conformable, divergent, sigmoid, oblique, hummocky, chaotic, and so on).
To recognize the depositional and erosional setting of the sequence from its context in the section (open basin, shoreline, prograding shelf, continental, local sediment source, etcetera).
To infer what type of rocks would be expected within the sequence.

It’s important to understand the different depositional sequence types that can occur at the top and base of boundaries. Figure 1 shows the reflection relationships which can occur at the base of a sequence.

Reflection relationships at the base of a sequence, seismic section, Seismic Stratigraphy, seismic, seismic interpretation, sedimentary layers — **Figure 1**: Reflection relationships at the base of a sequence

Figure 2 provides the same relationship for those at the top of a sequence.

Concordance, toplap, and erosional truncation, seismic section, Seismic Stratigraphy, seismic, seismic interpretation, sedimentary layers — **Figure 2**: Concordance, toplap, and erosional truncation

We can illustrate the interpretation approach with some examples of erosional channels. These may be cut by rivers, glaciers, or by marine currents.

Channel Examples

Figure 3 illustrates a channel cut by a river. We see how the flanking reflections are breached by the channel, and how the channel has become filled with sediment of quite different genesis. In fact, the sediment in this case is of glacial origin, and higher in velocity than the surrounding rocks because we notice the velocity pull-up in the reflections below the channel.

Erosional channel cut by a river, seismic section, Seismic Stratigraphy, seismic, seismic interpretation, sedimentary layers — **Figure 3**: Erosional channel cut by a river

Figure 4 shows us that marine currents, even in a deep open basin, can cause dramatic erosion. In this case, the channel is full of water, of a velocity lower than the surrounding rocks, which we can see from the velocity pull-down in the reflections below.

Deep basin marine channel cut, seismic section, Seismic Stratigraphy, seismic, seismic interpretation, sedimentary layers — **Figure 4**: Deep basin marine channel cut

Figure 5 illustrates a similar channel cut in past geologic time. Channels cut by rivers often have rough, steep sides. They may contain chaotic fill of terrestrial origin or, if they have been inundated by the sea, marine sediments onlapping the channel walls. Channels cut by marine currents tend to be smoother. In the case of Figure 5, the filling sediments onlap the channel walls very clearly.

Marine channel cut in past geologic time, seismic section, Seismic Stratigraphy, seismic, seismic interpretation, sedimentary layers — **Figure 5**: Marine channel cut in past geologic time

Figure 6 illustrates that marine channels may remain static (as at the left) or they may prograde. The right channel is continually eroding away at its left bank and depositing sediments on its right bank, much as a meandering river erodes its outer bank and deposits a point bar on its inner bank.

Static marine channel (left) and prograding marine channel (right), seismic section, Seismic Stratigraphy, seismic, seismic interpretation, sedimentary layers — **Figure 6**: Static marine channel (left) and prograding marine channel (right)

A counterpart to Figure 6, but in past time, is seen in Figure 7. The channel fill has two components: chaotic sediments on the right, deposited by the channel itself as it prograded leftward, and orderly marine sediments which onlap the final right wall of the channel and drape significantly into the channel.

Chaotic marine sediments (right) and orderly, onlap marine sediments (left), seismic section, Seismic Stratigraphy, seismic, seismic interpretation, sedimentary layers — **Figure 7**: Chaotic marine sediments (right) and orderly, onlap marine sediments (left)

Drape is a useful tool as it allows us to characterize rocks by their resistance to compaction. The value of this is that clays and shales are notably more compactible than other rock types. In Figure 7, the marine fill thickens where the channel is deepest; compaction soon after deposition has allowed more deposition. The interpretation is that the layers showing the thickening are rich in shales. In contrast, in Figure 5, there is no drape into the channel; the fill is less compactible, and we expect less shale content.