Salt Tectonics Short Course

3- Growth of Salt Domes (cont.)

3.11.5- Allochthonous salt and Extension

The sketch illustrated in fig. 133, shows one of the possible mechanisms of emplacement of allochthonous salt. In this hypothesis several steps are considered:

a) Extrusive Salt Wall.

b) Growth Faulting:

- The extension created by the fault heave of a growth-fault is balanced by shrinking of the stem of the salt wall.

- The shortening due to reduction of the stem induces the salt to flow up-dip and down-dip forming a salt tongue.

- As the shortening of the stem balances the lengthening, due to the fault heave, there is no extension.

c) Salt Welding

-With the continuation of the growth-fault displacement the shrinking of the salt wall stem is complete. The salt tongue will be disconnected from the mother salt layer and the stem will become a salt weld.

Fig. 133- In this hypothesis, the emplacement of an allochthonous salt sheet is explained by interaction between the extension created by the growth-fault, the shrinking of the extrusive salt wall. Subsequently, there is formation of a salt tongue and a secondary salt weld. On the bottom of the salt tongue, the horizontal planes represent periods of lateral flow and the vertical steps represent periods of vertical flowing.

When the extension induced by the growth-fault displacement cannot be balanced by shrinking of the stem, there is a local sedimentary lengthening.

3.11.6- Salt Intrusion Mechanisms

The following figures (fig. 134 & 135) illustrate the principal mechanics invoked to explain salt intrusions.

Fig.134- Salt can be intruded by (i) Injection, (ii) Reverse faulting, (iii) Thrusting and (iv) Shearing. Each of these mechanisms have been proposed to explain certain structures observed on seismic lines requires particular geological conditions.

Fig. 135- In the ground or in a time slice, it is often possible to observe the frontal morphology of the salt intrusions proposed by Pollard for the igneous intrusions as illustrated above.

3.11.7- Basinward Evolution of Allochthonous Salt

Allochthonous salt is found in the majority of the salt basins such as Gulf of Mexico, offshore Angola, Offshore Brazil, etc. In these basins, as illustrated in fig. 136, the allochthonous salt seems to evolve basinward. Indeed, seaward, the predominant salt structures seem to be:

(i) Salt stocks

(ii) Salt canopies

(iii) Salt nappes or Salt sheets.

Fig. 136- In the Gulf of Mexico, as in other salt basins, very often (see fig. 121) a typical evolution of salt structures can be observed seaward. Salt stocks become canopies, before coalesce into large salt sheets (salt sheets are also named nappes).

3.11.8- Allochthonous Salt in the Gulf of Mexico

Petroleum explorationists working in Gulf of Mexico know how important is concept of allochthonous salt for petroleum exploration. Two perpendicular regional seismic lines, located in fig. 137, showing large reactivated allochthonous salt sheets are illustrated fig. 138 and 139.

Fig. 137- The perpendicular seismic lines illustrated in fig. 138 and 139 are located seaward of the present time Mississippi mouth.

Fig. 138- A large reactivated allochthonous salt layers is here illustrated by a dip regional line regional lines. The down-dip flowage of the allochthonous salt created a local compensatory subsidence, which is the responsible for the growth-faults and the associated depocenters. A small salt window is likely in the center of the line.

Fig. 139- On a strike line (see intersection with the line of fig. 138), the transverse geological section of the reactivated salt tongue is clearly depicted. The down-dip salt flowage, within the salt tongue, created a large depocenter, which strongly contrasts with the “normal” stratigraphy recognized outside of the tongue on the ends of the line.

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