A Palaeomagnetic Study of the Bukoban System, Tanzania

Summary Results are reported from five members of the Precambrian Bukoban system which outcrops between Lake Victoria and Lake Tanganyika in central Africa. Of 32 sites in the Gagwe Amygdaloidal Lavas, 28 are significant after magnetic cleaning and give a stable mean direction of D = 239°, I =−25° (α 95 = 13°). One lava sample gave a K‐Ar age of 813±30 My. Sixteen of 27 sites in sills intruding the Bukoba Sandstone are significant after cleaning and 13 are magnetized with opposite polarity to the lavas. They yield a mean direction of D = 77°, I =+ 32° (α 95 = 19°) and their axis of magnetization is not significantly different from that of the Gagwe Amydgaloidal lavas. Two samples of sills have given K‐Ar ages of 806±30 and 803±30 My. A Mesozoic dyke has been found in the area; it has a direction of magnetization of D = 349°, I = 44° and has given a K‐Ar age of 190±7 My. Reconnaissance studies have been made on four red bed members of the system. The first three collections proved to be stable, and directions before and after thermal demagnetization are similar. The Bukoba Sandstone gave a direction of D = 132°, I =−19° (10 samples) and the Malagarasi Sandstone gave a direction of magnetization of D = 263°, I =−16° (10 samples). Four sites (20 samples) in the Kigonero Flags yielded a mean direction of D = 280°, I =−47° (α 95 = 27°). The bulk of a collection made from the Manyovu Red Beds behaved in an unstable fashion on thermal demagnetization and the mean direction ( D = 246°, I =−2°) has a very large associated scatter. The formations studied were deposited between about 800 and 1000 My and the results provide the first dated poles for this interval from Africa. They imply a polar shift or movement of the Tanganyika craton through approximately 80° of arc during this interval.