Kenya2

Tanzania Offshore

On this West-East geological cross-section of the southern Tanzania through the Mandawa onshore geographic basin and the adjacent offshore several major fracture zones are, easily, recognized : (i) Mandawa Fracture zone, (ii) Seagap Fracture Zone ; (iii) Davie ridge and the Davie Fracture Zone. The Mandawa fracture zone limits, roughly, the onshore from the offshore. It is, probably, the main responsible for the abrupt water depth change between the onshore and offshore environments. In fact, presently, the basin has not shelf, i.e., the shoreline corresponds, practically, to the shelf break. The Seagap fracture zone seems to have a compressional strike-slip movement. Not only the sedimentary packages are, laterally, displaced but those of the eastern faulted are, slightly, shortened as well. The Davie Ridge is, probably, the northward continuation of the subduction zone hypothesized on Mozambique offshore (see Page 35 and 36B). It marks the eastern limit of the Karoo sediments, that is to say, non-marine sediments (shales, red beds, siltstones) and great quantities of volcanics, extending from the Carboniferous and Permian periods to the Late Triassic Epoch. The Davie fracture zone, which is eastern fracture zone of the area, seems to have played with an extensional strike-slip movement, mainly, during the Atlantic-type divergent margin (Cretaceous time). Stratigraphically, the pre-breakup rift-type basins are, easily, recognized. They have a Karroo and Early Jurassic age. The Atlantic-type divergent margin is quite well developed with an important thermal subsidence during the Jurassic and Cretaceous, while during the Tertiary the sedimentary packages are roughly isopachous. In other words, during the Jurassic and Cretaceous the major factor in the creation of the space available for sediments was the subsidence, while during the Tertiary, eustasy was paramount. Notice, in the onshore, the presence a salt deposits at the base of the Atlantic-type margin (Kizimbani-1 well, recognized 750 m of evaporites , which according certain geoscientists seem to emplaced between Bathonian and Late Aptian strata (Balduzzi et al., 1992).

On this tentative geological interpretation of a manual autotrace of an old unmigrated seismic line of the Tanzania offshore, the Davie Ridge is, easily, recognized, eastward of the Karimbas Graben (just the eastern part is recognized on this autotrace). The eastern structural can correspond to the Davie Fracture Zone, which is, often, considered as the northward continuation of the Comores alignment (see Page 43). In spite of the presence of a lot of multiples and diffractions on the original seismic line, at the stratigraphic point of view, it is possible to advance that the two upper sedimentary packages correspond to Tertiary sediments and the green packages correspond, probably, to Cretaceous sediments. However, the proposed breakup unconformity, as well as, the Jurassic sedimentary package are quite conjectural.