Interpretation of salt uplift

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Problem 10.7a

Figure 10.7a shows a salt uplift at a shelf edge. How could one tell that this feature is not caused by reef growth, an igneous intrusion, or shale flowage instead of salt uplift?

Background

Under sufficiently high pressure and over long periods of time, salt and shale and some other rocks become plastic and flow; salt, which at depth is lighter than surrounding rocks, is the most common diapiric material. Salt flow may producc pillows, domes, anticlines, salt walls, and other types of features, and the flowing often results in arching of the overlying beds as the salt tries to rise because of its buoyancy, or rises to maintain its depth as the entire section subsides (sinks). In some cases the salt pierces the overlying beds and may even reach the surface. The pierced beds are usually “dragged” upwards, resulting in steep dips adjacent to the sides of the salt structure; this may also be caused by the sediments subsiding while the salt remains at roughly the same depth. Reflections from within the salt are rarely observed. As the salt moves out, the surrounding region from which the salt comes often becomes a rim syncline as sediments subside.

Shale diapirs generally result from overpressuring (see problem 5.9) because interstitial water, laid down as the shale is deposited, cannot escape; as a result, the shale loses shear strength and becomes somewhat fluid (diapiric). Shale diapirs “freeze” into place once the water escapes. Shale diapirs often have a somewhat similar appearance to salt structures.

Figure 10.7a.  Migrated section across a salt dome (courtesy of Grant Geophysical).

Reefs are discussed in problem 10.11, unconformities in problem 10.13. An igneous intrusion would probably have a magnetic signature that could be detected by magnetic measurements.

Solution

If there should be a clear reflection from underneath the feature, its stacking velocity might provide important evidence as to a characteristic velocity. Salt should have an interval velocity around 4.5 km/s whereas the velocity of shale would be much lower and igneous rocks probably higher; a limestone reef should have velocities of about 4.5 km/s or higher. If the regional history were known from other sources, it might indicate the most likely solution. If gravity and/or magnetic data were available, they would help in resolving the difficulty (see problem 10.8).

Since all we have to go on is the single seismic section, we pick the top of the diapir feature in Figure 10.7a at about 3.1 s. Primary reflections appear to extend to at least 4.5 or 5.0 s, and they are bent upward adjacent to the poor reflection zone where the uplift lies. They also suggest a rim syncline, especially prominent to the right of the feature; rim synclines commonly lie above the region of salt withdrawal as salt moved into the uplift, thus reinforcing the interpretation that this is a salt dome. Shale diapirs also occasionally show withdrawal synclines. We note that the vertical extent exceeds the height of most reefs. The pull-up of reflections on both sides of the feature may represent sediments uplifted with the salt. Such pull-up is not usually associated with reefs (although they may show minor pull-up and onlap because of differential compaction), and there is usually little reflection pull-up surrounding igneous intrusions. Shale diapirs ordinarily are not associated with as much pull-up of flank reflections as we see here. Thus we conclude that the feature is probably a salt uplift.

Problem 10.7b

Does the relief above the unconformity U indicate post-unconformity salt movement, down-drop along faulting at the shelf edge, or differential compaction because of the weight of the postunconformity section?

Solution

There is some minor thinning of the sections below and above , especially just to the left of the salt dome, suggesting some residual upward movement of the salt both before and after . The strong reflection above (at about 1.5 s at the left edge) appears to be undisturbed to the left of the uplift, suggesting that there was no further salt uplift after the time of its deposition, but this and other reflections drop down to the right of the uplift, suggesting that the salt continued to move from the right after . There are indications of a pair of faults cutting suggesting a graben above the salt that does not extend much higher than ; this is consistent with the extension that the downdropping would have produced.

Some parts of the section below show thickness variations which indicate that salt withdrawal underneath them was occurring at the time of their deposition. Several parts of the section below to the left of the uplift suggest progradation.

Regional dip is presumably to the right but reflections below 2.5 s at the right end of the line show counter-regional dip, possibly indicating a growth fault and a rollover anticline just beyond the end of the line.

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