Indonesian Journal on Geoscience

10

Authors: S. Poedjoprajitno

Landform evolusion in Manado region has been strongly influenced by tectonic activities beside marine and volcanic ones. Based on surface and subsurface (shallow drilling) geological data, landform evolution of the studied areas can be grouped into four morphological environment units consisting of nine environment sets of morphology, these are: ( a). flood basin, ( b). tidal beach, (c).swamp, (d). river channel, (e).volcanic eruption, (f). flood basin influenced by water, (g).coastal/shore, (h). near shore, and (i). offshore. Collumnar analyses on nine drilling data show that the landform evolution history in the Manado region has undergone more than one tectonic event and three eruptions, proved by the presence of discontinuities on morphological environment units and composition, vertically. Keywords: evolution, morphology, landform, facies, tectonic, volcanism, marine

13

Authors: S. Bronto, S. Mulyaningsih, G. Hartono, and B. Astuti

The Semilir Formation was typically originated from products of a very explosive volcanic activity, i.e. breccias, lapillistones, and tuffs containing abundant pumice. It has a light grey to white colour and high silica andesite to dacite in composition, mainly rich in volcanic glass and quartz. Sedimentary structures of these volcanic rocks are massive, grading, planar bedding, and cross-bedding to antidunes, with grain size varies from ash (≤ 2 mm) to lapilli (2 – 64 mm) to bomb and block (> 64 mm). The formation is widely distributed from the west side (Pleret and Piyungan areas, Bantul Regency, Special Province of Yogyakarta) until Eromoko area in the east (Wonogiri Regency, Jawa Tengah Province). Stratigraphically, the Semilir Formation underlies the Nglanggeran Formation, and overlies the Mandalika Formation in the eastern part and Kebo-Butak Formation in the western part. Geomorphological- and lithological analyses of the Semilir Formation in areas of Parangjoho and Songputri Dams, Eromoko Sub-regency, Wonogiri Regency indicate that the two depressions were alternatively volcanic sources of the Semilir Formation in the Eromoko area. This is proved by the presence of co-ignimbrite breccias(co-ignimbrite lag fall deposits), that descriptively they are polymict breccias. This rock is characterized by a mixing of pumice and various hard rock fragments that primarily are juvenile materials (volcanic blocks, bombs), accessory-, and accidental rock fragments set in pumice-rich volcanic ash and lapilli sizes. The accessory materials came from older volcanic rocks, whereas the accidental ones were originated from basement rocks. During a caldera forming event or a destruction period of an older composite volcanic cone(s), all older rocks resting above the magma chamber were ejected to the surface by a very high magmatic pressure. Since they were heavier than the juvenile material, most accessory and accidental rock fragments were left (lag fall) in caldera rim behind the ash and pumice flow. In the dam areas of Parangjoho and Songputri, the lag fall fragments consisting of andesite, pyroxene andesite, dacite, and pumice, being 10 – 150 cm in diameter are set in pumice-rich lapilli tuffs. Some of the rock fragments are volcanic blocks and bombs, while the older rocks are angular to very angular shape, having prismatic jointing or jigsaw-crack structures. The eruptions in the Parangjoho and Songputri craters were controlled by north-south trending fractures, and they resemble to the Katmaian caldera explosion type. Keywords: Parangjoho, Songputri, Dams, Semilir Formation, co-ignimbrite breccia, volcanic block and bom, pumice

14

Authors: Iskandar Zulkarnain

Five samples consisting of two Permian-Triassic basalts, two Triassic-Jurassic granitic rocks, and a Miocene andesite were collected from the Madina Regency area in North Sumatra that is regionally situated on the West Sumatra Block. Previous authors have proposed three different scenarios for the geological setting of West Sumatra Permian Plutonic-Volcanic Belt, namely an island-arc, subduction related continental margin arc, and continental break-up. Petrographic analysis of the Mesozoic basaltic samples indicates that they are island-arcs in origin; however their trace element spider diagram patterns (Rock/MORB ratio) also show the character of back-arc marginal basin, besides the island-arc. Furthermore, their REE spider diagram patterns (Rock/Chondrite ratio) clearly reveal that they were actually generated in a back-arc marginal basin tectonic setting. Meanwhile, the two Mesozoic granitic rocks and the Miocene andesite reflect the character of an active continental margin. Their spider diagram patterns show a significant enrichment on incompatible elements, usually derived from fluids of the subducted slab beneath the subduction zone. The high enrichment on Th makes their plots on Ta/Yb versus Th/Yb diagram are shifted to outside the active continental margin field. Although the volcanic-plutonic products represent different ages, their La/Ce ratio leads to a probability that they have been derived from the same magma sources. This study offers another different scenario for the geological setting of West Sumatra Permian Plutonic-Volcanic Belt, where the magmatic activities started in a back-arc marginal basin tectonic setting during the Permian-Triassic time and changed to an active continental margin during Triassic to Miocene. The data are collected through petrographic and chemical analyses for major, trace, and REE including literature studies. Keywords: Mesozoic, volcanic-granitic rocks, back-arc marginal basin, active continental margin, magma sources