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Originally presented at the 1998 Hedberg (AAPG) Research Conference at Galveston, TX
Raft features created by gravity gliding on a salt layer dominate the shelf area offshore West Africa. In the deepwater area downdip of these rafts, a series of compressional features absorb this extension. These salt features have a major impact on sedimentation. Major minibasins form due to thin-skinned extension and linked compression, compared to other salt basins worldwide where minibasins form due to differential loading. These minibasins trap thick sections of basin-floor fans on the upper slope. Continued salt movement allows many depositional lows to later become highs, either as turtle structures or compressional over-printed structures. This yields attractive exploration targets with thick reservoir sections being preserved in structural highs.
Applying the salt tectonic models that are present in West Africa to the subsalt province of the deepwater Gulf of Mexico could yield a dynamic exploration model that will lower risk and uncertainty.
Description of Salt Structural Style
Gravity gliding (raft tectonics) is a well-recognized mechanism of salt movement in offshore West Africa, offshore Brazil, and in the eastern Mediterranean. Salt served as a decollement surface that allowed thin-skinned extensional features to form above the Aptian salt layer in West Africa (Figure 1). Gravity gliding is the translation of fault blocks down a gentle slope. Post breakup of the South Atlantic during the late Aptian, the outer continental margin gradually subsided into the newly formed oceanic basin, and Albian carbonates were deposited on the shelf. This subsidence and sedimentary loading allowed the gravity gliding phenomena to start. Rafts of the newly deposited sediment formed as the margin overextended. The rafts translated downdip and young sediments infilled the expanding depocenters that grew with continued sedimentation. Salt rollers underlie the rafts, and as the salt was squeezed out downdip, listric faults soled out onto the evacuating salt layer, and salt and fault welds formed.
In addition, reactive diapirs formed in the void left behind as the rafts moved downdip. Reactive diapirs form during regional extension, and collapse grabens form over the crest of laterally stretched diapirs. Reactive diapirs use the additional space created by the extension as a weakness zone to rise through sediments. As extension continues, the reactive diapirs collapse, leaving behind inverted basins that are "inverted turtles".
This extension is accommodated downdip by the formation of a downdip fold and thrust belt. Locally, there is also displacement of salt into allochthonous sheets. This linked system forms varying types of salt structures that segment out into provinces:
Factors influencing the distribution of salt features
Regional mapping shows that there are several main factors that control the location of the rafts, compressional toe features, diapiric salt features and major mini-basins. These are, in order of importance:
Similar linked systems are seen along the West Africa margin, and many structural analogs can be found offshore Congo and Angola in the Lower Congo Basin. The area is also very analogous to the offshore Campos Basin in Brazil. The linked nature of the rafts and compressional features look alike, and their impact on Tertiary sedimentation appears similar.
The interaction of all of these factors could have an impact on models related to salt sheets in the Gulf of Mexico. The study area is most analogous to an area offshore Louisiana in the Mississippi Canyon deepwater region during the early Miocene. The seaward features resemble the Mississippi Fan Fold Belt. The toe thrusts and salt walls could be an example of early salt feeders for the large salt sheets. Detailed analysis of the features offshore Gabon could yield important clues to the subsalt minibasins in the Gulf of Mexico. Also, after initial emplacement of salt sheets, gravity gliding could be the major force that allows secondary features, such as toe thrusts and colliding minibasins, to form.
Other points of the analogy are important to examine as exploration in the deepwater continues:
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