Radiometric ages and paleomagnetism of the Sigura-gura formation, upper part of the “Toba Tuffs” in Sumatra, Indonesia

Abstract

A radiometric age determination by means of KAr and fission-track method was carried out on the Sigura-gura Formation, upper part of the “Toba Tuffs” in Sumatra, Indonesia. The results are as follows: (1) KAr age: 0.96±0.15 Ma. (2) Fission-track ages by re-etched method of external crystal surface are 1.11±0.12 and 1.11±0.14 Ma. (3) Fission-track ages by external detector method of crystal surface are 0.87±0.09 and 0.86±0.10 Ma. (4) The Paleomagnetic polarity of the Sigura-gura Formation is reversed. So this reversed polarity surely belongs to the Matuyama Reversed Epoch though it was previously correlated with the Blake Event in the Brunhes Normal Epoch. (5) The ash-flow deposit, which was the uppermost part of the Sigura-gura Formation, shows the youngest fission-track age, 0.03±0.01 Ma in previous and this works. So, this must be a different unit, called the Porsea Formation, which overlies the Sigura-gura Formation. (6) The Porsea Formation may be a product of the same volcanic activity which erupted the Toba ash layer identified in the deep sea sediments at about 0.075 Ma ago. (7) In addition, six fission-track age values were obtained from the Younger Toba Tuffs of “Toba Volcanic Complex”. They are dated at 0.62-0.1 Ma.