Stratigraphy

The subsurface geology of coastal Tanzania is known from a limited number of exploration wells and several deep stratigraphic boreholes. The post-Karroo, entirely marine, sedimentary section is characterized by unconformity-bounded megasequences with major unconformities recognized at:
• Base Pliocene
• Base Miocene
• Base Middle Eocene
• Base Paleocene
• Base Upper Cretaceous
• Base Middle Jurassic (Break-Up Unconformity)

Below the Break-Up Unconformity are Younger (Jurassic rift system) and Older Karroo sediments of restricted marine and continental facies respectively.

There are no wells in the MDOB, but using vintage shallow-water seismic data and coastal zone well control it has been possible to tie directly sown to Base Upper Cretaceous unconformity level on the new Western Geophysical survey. Additional control for the deep-water area is provided by DSDP sites 241 and 242. These data points are, however, located off Kenya and Mozambique, 400 and 500 km from the MDOB survey area respectively. Only Site 241 is tied by multi-channel seismic data and calibrates with certainty the Base Miocene reflector. The bottom hole formation is dated as Late Senonian (Upper Cretaceous), but the hole does not penetrate to the next deepest significant reflector (Coffin & Rabinowitz, 1988). Site 242 proves sediments possibly as old as Turonian.

For the deep-water area a seismostratigraphic scheme has been established in which seven major layers are recognized. By analogy with the Coastal Basin the layers have been interpreted to be unconformity-bounded megasequences reflecting the response of the Tanzanian margin to episodic continental doming and rifting and oceanic spreading events. The seismic characteristics of the defined layers, with preliminary stratigraphic assignments, are:

•  Layer 1 - Karroo (or Basement): a general lack of coherent reflections interpreted as Older Karroo or Pre-Cambrian terrain. In places high-angle dipping reflectors are seen which are interpreted as preserved remnants of Younger Karroo section.

•  Layer 2 - Middle Jurassic to Lower Cretaceous: essentially parallel, sub-horizontal, high frequency, continuous reflectors showing an onlap relationship to the top of Layer 1. The sequence is separated from Layer 3 by a significant high amplitude reflector.

•  Layer 3 - Upper Cretaceous: similar in character to Layer 2, displaying an overall higher frequency, but with less amplitude contrast

• Layer 4 - Paleogene: a zone of high frequency reflectors which exhibits a stacked, mounded geometry displaying internal clinoforms and external onlaps. These features are interpreted as successive channel cut and fill sequences and associated erosional remnants. This layer can be subdivided into several sequences, each characterized by a prominent zone of high frequency, very high amplitude reflectors at their bases.

• Layer 5 - Lower Miocene: displays a deeply erosive unconformable base with associated high amplitude reflectors. The upper part of the layer shows generally very high frequency, parallel, continuous reflections with a reduced amplitude character.

• Layer 6 - Middle and Upper Miocene: displays very high frequency, generally continuous reflection character. Strong amplitude packets indicative of channel geometries occur throughout the layer. The base of the layer is unconformable towards the shelf.

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FIGURE 4. SEISMOSTRATIGRAPHIC INTERPRETATION

FIGURE 5. STRATIGRAPHIC FRAMEWORK FOR TANZANIA COASTAL AND MAFIA DEEP OFFSHORE BASINS

• Layer 7 - Pliocene to Recent: similar to Layer 6, but with less continuous character. It also shows an unconformable base in parts.

Combining data from the Coastal Basin wells with the seismostratigraphic interpretation of the Western geophysical data it is possible to establish a composite chronostratigraphic and seismostratigraphic scheme for the Tanzania deep offshore area to act as a framework for the assessment of the petroleum potential of the area (Fig. 5).

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