Friday, November 20, 2015

Stratigraphy Of Bali : Related Volcanic Deposits

The basic Stratigraphy Of Bali and adjacent islands as previously understood was largely established by Purbo-Hadiwidjojo (1971) (Figure 1). The oldest rocks exposed in Bali are the Ulakan Formation of Lower Miocene age, as volcanic rocks comprising lavas, breccias and tuffs with intercalations of calcareous sandstones, but Kadar (1972) determined the oldest rock units in Bali are calcareous sandstones of Late Miocene age.

As described by Wheller and Varne (1986), the oldest Bali volcanic rocks are pillow basalts of Late Pliocene age. The southern part of Bali and Nusa Penida Islands are formed of uplifted coral reefs of Pliocene to Pleistocene age (Kadar, 1977). The basement rocks of the mapped area consist of pillow basalts and pumiceous tephras, which are exposed in the southern part of the caldera in the vicinity of Bangli.

Purbo-Hadiwidjojo (1971) described that both rock units are included in the Ulakan Formation. Most of the island is composed of subaerial volcanic deposits which were erupted from the extinct Quaternary Bratan, Batukau, and Seraya Volcanoes, and the two active volcanoes Batur and Agung (Purbo-Hadiwidjojo, 1971).

Geologic Map Of Bali Island
Geologic map of Bali Island (after Purbo-Hadiwidjojo, 1971).

The Bratan and Batur volcanic products of several kinds make up a large proportion of the rock over the island of Bali. There is no separation between Bratan and Batur caldera products. In this case, the author attempted to separate these volcanic products by petrographical and chemical analyses, and material behavior in the field.

The rocks mainly consisting of ash-flow deposits ( ignimbrite ) with some are welded, overlies the sedimentary and older volcanic rocks. Some of the volcanic rocks erupted from Mount Pohen, Sengayang and Lesong, occupy the area in the centre of the island, particularly basaltic andesitic lavas and tephras. Volcanologists who have examined the Batur Caldera and areas elsewhere, largely the ignimbrites, have questioned aspects of the locally established sequence.

Marinelli and Tazieff (1968) believed that the paroxysm phase of the Batur Caldera formation occurred 22,000 years ago, producing rhyodacitic ignimbrite. Moreover the Caldera II culminated in the major ignimbrite eruption about 23,700 years ago were followed by periodic strombolian eruptions of the basaltic magma.

Reinvestigation in 2006 of the Batur Caldera area, resulted that major revisions are required in the previously used stratigraphy. Corroboratory evidence has come from 14 C dating of partially and non-welded ignimbrites. Charcoal found within the partially welded ignimbrite (Gunungkawi Ignimbrite) around the temple of Gunung Kawi, Tampaksiring, or about 15 km southward from the source, has given an age of 20,150 years B.P. Another charcoal was found within the partially welded ignimbrite (Ubud Ignimbrite) in Tukad Wos, Ubud, about 25 km southwestward from the Batur Caldera.

The age of this charcoal is 29,300 years B.P. Both of these ages were dated from the small twigs of charcoals scattered in the units in nearly random orientations. The third location of charcoal was found in Tukad Blingkang, within non-welded ignimbrite, about 2 km from Lake Batur inside caldera I. In this place, there was a big log, occuring in the lower part of Blingkang Ignimbrite that had given an age of about 5,500 years B.P.

References:
Kadar, D., 1977. Upper Pliocene planktonic foraminiferal zonation of Ambengan drill hole, southern part of Bali island. Geological Research Development Centre (Indonesia) Special Publication,1, p.137-158.
Marinelli, G. and Tazieff, H., 1968. L'Ignimbrite et la caldera de Batur (Bali, Indonesie). Bulletin Volcanologique,32, p.89-120.
Purbo-Hadiwidjojo, M., 1972. The geologic map of Bali, Indonesia, scale 1:250.000. Geological Survey of Indonesia.
Sutawidjaja, Igans., 2009. Ignimbrite Analyses of Batur Caldera, Bali, based on 14 C Dating. Jurnal Geologi Indonesia, Vol. 4 No. 3 September 2009: 189-202.
Wheller, G.E. and Varne, R., 1986. Genesis of dacitic magmatism at Batur volcano, Bali, Indonesia: Implications for the origin of stratovolcano calderas. Journal of Volcanology and Geothermal Research,28, p. 363-378.
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