Universidade Fernando Pessoa
8- Salt Reduction
Contents: 8.1- Depocenter and Apparent Mound 8.2- Inversion, Salt Roller, Relict Roller and Fault Weld 8.3- Apparent Downlaps
8.1- Depocenter and Apparent Mound
As illustrated in fig. 216, apparent mounds are often developed in association with salt welds.
Fig. 216- In this sketch, interdomal synforms become apparent mounds by complete salt evacuation, that is to say, by total evacuation of the salt. Depocenters become apparent mounds. They are inverted structures, in which the high structural points become low structural points
Fig. 217- In apparent mounds, as illustrated on this seismic line from the North Sea, the proposed hydrocarbon traps must be critically tested. Indeed, the age of the tectonic inversion must predate the age of the hydrocarbon migration, which is not always the case.
8.2- Inversion, Salt Rollers, Relict Rollers & Fault Welds
The geological sketch, illustrated in fig. 218, allow us to understand:
(i) How salt flowage can invert the fault movement of an associated growth-fault.
(ii) How a salt layer become a down-dip succession of a salt roller.
(iii) How a total salt reduction creates a salt weld and changes salt roller into relic salt rollers.
Fig. 218- An apparent inversion along the growth-fault planes and relic roller can be explained by salt tectonics, that is to say, by extension and halokinesis.
The geological evolution depicted on the sketch illustrated in fig. 218, can be summarized as follows:
a) Deposition of an isopachous prekinematic interval (in green) above the allochthonous salt layer.
b) Slight seaward tilting of the basin initiates salt flowage. Flowage creates small basinward growth-fault.
d) Synkinematic deposition of the blue and yellow intervals.
e) The thickness of the synkinematic intervals increases toward the growth-fault planes, where accumulation (space available, or subsidence if eustacy is stillstand).
f) The continuation of salt flowage creates more space available for sediments in the down-thrown faulted block increasing the normal down-dip movement of the normal faults.
g) At a given degree of salt reduction, the previous continuous salt layer becomes a discontinuous layer composed of a succession of elongated prisms of salt, localized at the down-dip toes of the fault blocks.
h) Since the landward flanks of the salt rollers shrink, by withdrawal and salt flowage, the overburden collapses.
i) The collapse of the up-thrown sediments creates an inversion of the fault displacement. The fault plays with a similar movement of those of the reverse faults. The fault movement and geometry of the faulted blocks should not be taken as an evidence of a compressional tectonic regime.
j) The inverse movement of the fault plane creates space and obliges the sediments of the downthrown block to extend creating normal accommodation faults.
k) When salt reduction becomes almost total, the shrinkage of the salt roller creates relic rollers, which can be laterally disconnected, as illustrated in fig. 219.
Fig. 219- On this line, the shrinkage of the salt rollers is not enough to invert the geometry of the upper stratigraphic intervals of the overburden along the fault planes.
In the next theoretical geological sketch (fig. 220), it is illustrated the normal slip and the reverse dip created by salt reduction, when a salt dome becomes successively a salt roller and then a relic roller. Taking a look at the unconformity (in red) it is easy to recognize:
(i) At the stage of the salt diapir, the movement of the normal-faults is real normal.
(ii) Then the diapir shrinks and becomes a salt roller.
(iii) The unconformity shows a reverse position, that is to say, its position is lower on the up-thrown faulted block than on the downthrown block.
Fig. 220- Due to the withdrawal of a salt roller, the geometry of an unconformity displaced by a normal fault can change into apparent reverse fault geometry. The fault geometry is not the consequence of a compressional tectonic regime.
(iv) Finally, if the salt reduction is extreme, the salt roller becomes a relict roller and the inverse displacement is maximum.
(v) The evolution “salt diapir / relict roller” is accompanied by the formation of a fault weld, as illustrated in fig. 221.
Fig. 221- In spite of a late reactivation of the fracture zones (see fig. 215), the shrinkage of the western flanks of the diapirs reverses the fault geometry.
8.3- Apparent Downlaps
Salt reduction changes the original reflection terminations and very often induces apparent downlap geometric relationships. The mechanism of such a changes is illustrated on the figure below (fig. 222).
Fig. 222- On this evolution, the geometrical relationships are apparent downlaps. Onlaps are progressively tilted, toward the tectonic disharmony, due to the salt flowage. If the salt evacuation is complete, the tectonic disharmony becomes a primary salt weld, as illustrated in fig. 223.
Fig. 224- Even outside of the regional geological setting, the presence of a salt roller, on the right part of the line, strongly suggests that the seismic surface is not a downlap surface, but a tilted onlap surface, induced by salt flow, which creates a wide fault weld.
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Last modification: Março 19, 2006