Timing of Diapiric Movement

Interpretation Example: The Timing of Diapiric Movement

Figure1 is a marine section crossing two salt pillows.

Figure 1

The section represents 27 km (17 miles) of line; we note how the vertical exaggeration (about 3:1) makes it easy to see the thickening and thinning of different intervals. An interpreter has picked the section for us, at several levels. On the basis of correlations between the section and wells drilled in the area, he has identified the intervals between the picks as Lower Tertiary (TL), Upper Cretaceous (KU), Lower Jurassic (JL), Upper and Lower Triassic (THU, TRL), and the Zechstein salt (Ze). He has also suggested several faults, in a qualitative manner; we see that, as usual, the deep faults are absorbed in the thick and mobile salt section.

The Upper and Lower Triassic maintain substantially constant thickness over and between the salt pillows. The same parallelism seems to be maintained in the lowest part of the Lower Jurassic. We can say, therefore, that there was no significant movement of salt until some time in the Lower Jurassic, at the earliest. At this time, the section would have appeared as in Figure4 (f); the whole depositional system had remained remarkably stable, with the rate of subsidence generally well matched to the rate of sediment deposition.

Figure 4

Let us look first at the evolution of the left pillow, which may have been triggered by rejuvenated faulting in the deep zone below the salt. We do not know (from this one line) what thickness of Jurassic and Lower Cretaceous sediments were deposited, for any Upper Jurassic and Lower Cretaceous sediments which were present have been eroded. However, the thinning of the remaining Lower Jurassic sediments over the left pillow tells us that the salt had moved, and the left pillow had started to form, before this erosion was complete. Further, the parallelism between the erosional surface and the lowest reflections in the Upper Cretaceous tells us that the erosional surface was substantially horizontal, and the salt had ceased moving, by the early Upper Cretaceous. This situation is shown in Figure4 (e).

The parallelism of reflections continues through the Upper Cretaceous and into the Lower Tertiary; there was no movement of the left pillow during this time ( Figure3 (d)).

Figure 3

Then, some time in the Lower Tertiary, the big uplift started. Looking at the Tertiary reflections just to the right of the left uplift, we see thinning toward the high, then a zone of parallelism, and then renewed thinning; we infer that the salt started to move, stopped again, and then surged upwards ( Figure3 (c)). Subsequent erosion of the Tertiary sediments led to the situation of Figure 2 (b).

Figure 2

No movement of the left pillow has occurred since that time, and sedimentation is proceeding uneventfully at present ( Figure2 (a)).

We turn now to the right pillow. Several interpretations are possible; we discuss only the simplest. The right pillow did not start to form at the same time as the left pillow ( Figure4 (e)). Everything was stable through the period of Jurassic and Lower Cretaceous deposition and erosion; at the end of the erosion a topographic high remained at the right of the line. The salt started to move quite early in the Upper Cretaceous, and continued until about three-quarters of the Upper Cretaceous was in place; then it stopped to rest. We can see the clear pattern of onlap reflections in the Upper Cretaceous during the period of uplift. The uplift to the right pillow, in distinction from the left pillow, caused some drop-down faulting over the crest.

Everything then remained stable until early in the Lower Tertiary ( Figure3 (d)). The right pillow waited while the left pillow made its false start, and then the final movement started on both. During this period the withdrawal of salt from between the pillows caused significant subsidence in the middle of the line ( Figure3 (c)).

During the final erosion, the collapse faulting over the right pillow hastened the erosion; a valley developed, and broadened to the left ( Figure2 (b)). Since then the valley has been filled, though differential compaction of these young sediments has caused some draping into the valley ( Figure2 (a)).

We turn now to Figure5 .

Figure 5

This shows a salt wall, some 3-4 km in width, which once broke to the surface. This dramatic example gives us a delightful record of the salt movement.

Figure6 illustrates complete withdrawal of the salt from the area between two pillows.

Figure 6

Extensional subsidence in this area has ruptured the Triassic rocks, and salt has filled the space between them. The right pillow has engendered a fault of very considerable throw.

These examples are dramatic ones, brought about by the extreme mobility and low density of salt. In some areas, similar effects can be caused by mobile shale. However, the techniques which we learn from these dramatic examples are of general utility in establishing the order of events in geologic time. And the importance of this to the interpreter is major; we see from Figure7 (a sedimentary series with the potential for petroleum accumulation),

Figure 7

Figure8 (the importance of timing: migration before trap formation)

Figure 8

and Figure9 (the importance of timing: migration after trap formation) that we have no interest in traps which formed after the migration of hydrocarbons was complete.

Figure 9

Thus it may emerge, as an area is developed, that some structures contain oil or gas, and that others do not; the interpretation task, then, is to relate the drilling success to the timing of structure, and to propose drilling locations only on those structures which are of sufficient age. This is a very important skill.