Welded rhyolitic tuffs or “Ignimbrites” in the Pasoemah region, West Palembang, South Sumatra

The Pasoemah region S of the Goemai Mts. in W. Palembang is largely occupied by Quaternary volcanics, which form a sharply dissected plateaulike country, whose surface gradually slopes downward in an ENE direction from ± 1000 m to ± 300 m above sea-level, conformably to the courses of the Selangis and Lematang Rivers above their point of confluence. Where the Lematang River unites with the Moelak River, the acid welded tuffs of the Pasoemah highland, to which attention will be drawn in this paper, are cut off by a steep bluff, formed undoubtedly by retrogressive erosion, which was substantially facilitated by the presence of vertical cleavage planes in the rhyolitic tuff series. In the Goemai Mts., described elsewhere in detail by K.A.F.R. Musper (1937) and also dealt with by the present writer in a previous paper (J. Westerveld, 1941), a core of strongly folded lower-Cretaceous sediments, cut by various intrusiva, is covered unconformably by a steeply tilted series of Eogene or old-Miocene andesitic tuffs and breccias, the Lower Kikim tuffs, which again are covered with slight unconformity by the old-Miocene Upper Kikim tuffs or basal section of the Batoeradja-Telisa series. The base of the Pasoemah volcanics is generally formed by the S-ward dipping Telisa beds or upper part of the latter series; a monotonous sequence of Globigerina marls and shales with intercalated andesitic tuffs and breccias, layers of glauconitic sandstone, platy or concretionnary limestones, and occasional horizons with plant remains. Below the Quaternary tuff mantles this series unquestionably merges S-ward into the late-Miocene Lower Palembang beds, which only seem to be exposed quite locally at the bottom of the Selangis gorge NE of Pageralam (Musper, 1937, p. 41). The lower and thickest portion of the flat-lying, post-Tertiary, volcanic sequence is formed by welded rhyolitic tuffs, and the upper part by andesitic tuffs and agglomerates from the andesitic volcanoes, which border the Pasoemah highland on the W (G. Dempo), the S (the volcanoes of the Semendoh highland) and the E (the G. Isau-isau). Of these eruption points the Dempo volcano and the Semendoh volcanoes lie outside the map region.