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The Takutu basin is an intracratonic graben 280 km (174 mi) long and 40 km (25 mi) wide in northern Brazil and adjoining Guyana, lying entirely within the center of the early Precambrian Guyana shield. Acidic metavolcanic rocks and thick Proterozoic quartzite lie to the north of the basin. Granulite, gneiss, and granite border the graben to the south and east. High mountains arise along the south-bounding fault whereas more subdued topography flanks the north side of the basin. Triassic basalt forms a wide band of outcrop along the southern and eastern margins of the rift. There are very poor and sparse outcrops of the basin fill. The graben is filled with up to 4,000 m (13,123 ft) of Cretaceous and Jurassic sedimentary rock underlain by 1,500 m (4,921 ft) of mafic volcan cs of Triassic age and possibly older (i.e., Proterozoic).
The geologic history of the Takutu graben is interpreted to extend back into Precambrian time because it occupies an ancient suture zone in the Guyana shield. Renewed rifting and major subsidence occurred in Mesozoic time resulting in the deposition of thick nonmarine clastics, evaporites, and carbonates. A basal(?) Jurassic clastic-carbonate sequence overlies the eroded basalt. It is overlain by thick Cretaceous Aptian salt and interbedded shales that were deposited over most of the basin and contain good oil source rocks. The only indication of a marine environment is found within the subsurface post-salt clastics in Brazil. Lacustrine an deltaic depositional processes were dominant as indicated from well and seismic data.
Two main structural styles, namely pre-salt and post-salt, occur in the basin. The former is characterized by block faulting and horst and graben development. Non-piercement halokinetic forms swells and ridges in the post-evaporite Takutu Formation. Wrenching and salt solution are interpreted on seismic records. A large, cross-basin arch is present in Guyana where at least six undrilled prospects have been mapped.
Three widely spaced exploratory wells have been drilled down to the mafic volcanics. The wells are located on structures near the rift margin or in areas of thinner basin fill. Two of the tests were dry and abandoned while Home Karanambo #1 was classed as a noncommercial oil discovery in fractured basalt. Several clastic depocenters have been interpreted and delineated from the seismic and drilling results. They lie near the southern and eastern unexplored basin margins, distant from the wells drilled to date.
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