Volcanostratigraphic Sequences of Kebo-Butak Formation at Bayat Geological Field Complex, Central Java Province and Yogyakarta Special Province, Indonesia

Abstract

-Bayat Complex is usually used as a work field for students of geology and other geosciences. The study area is located in the southern part of the Bayat Complex. Administratively, it belongs to Central Java Province and Yogyakarta Special Province. The lithology of Bayat is very complex, composed of various kinds of igneous, sedimentary, metamorphic, and volcanic rocks. Most of previous researchers interpreted Bayat as a melange complex constructed within a subduction zone. Kebo-Butak is one of formations that forms the Bayat field complex. The formation is composed of basalt, layers of pumice, tuff, shale, and carbonaceous tuff. Most of them are known as volcanic rocks. These imply that volcanic activities are more probable to construct the geology of Bayat rather than the subducted melange complex. The geological mapping, supported by geomorphology, petrology, stratigraphy, and geological structures, had been conducted in a comprehensive manner using the deduction-induction method. The research encounters basalt, black pumice, tuff with basaltic glasses fragments, zeolite, argilic clay, as well as feldspathicand pumice tuff. Petrographically, the basalt is composed of labradorite, olivine, clinopyroxene, and volcanic glass. Black pumice and tuff contain prismatic clinopyroxene, granular olivine, and volcanic glasses. Feldspathic tuff and pumice tuff are crystal vitric tuff due to more abundant feldspar, quartz, and amphibole than volcanic glass. Zeolite comprises chlorite and altered glasses as deep sea altered volcanic rocks. The geologic structure is very complex, the major structures are normal faults with pyrite in it. There were two deep submarine paleovolcanoes namely Tegalrejo and Baturagung. The first paleovolcano erupted effusively producing basaltic sequence, while the second one erupted explosively ejecting feldspathic-rich pyroclastic material. The two paleovolcanoes erupted simultaneously and repeatedly.

Volcanism in The Pre-Semilir Formation at Giriloyo Region; Allegedly as Source of Kebo-Butak Formation in the Western Southern Mountains

“…Parallel Nikol of lithic tuffs exposed at Cengkehan; from the upper right counterclockwise are C01A, C01B, C02, and C03. It shows lithic fragments with irregular shapes but some featuring circle shape, vuggy and cementing with silicic materials Based on the regional geology by the previous studies, reported Kebo-Butak Formation aged Late Oligocene to Early Miocene, was exposed at Tegalrejo-Gedangsari (Gunungkidul; Mulyaningsih, 2016) and Gunung Kebo and Butak (Bayat, Klaten;Surono et al, 1992). It consisted of submarine volcaniclastic deposits associated with calcareous sediments.…”

“…It consisted of submarine volcaniclastic deposits associated with calcareous sediments. The distinguished volcanic rocks were basalt (as sill, dykes and lava), black colour of rich-glass crystal tuffs, layered yellowish to creamy crystal tuff, and laminated light grey dacitic tuff (Mulyaningsih, 2016). That s why, Surono (2009) interpreted Kebo-Butak Formation as deep-sea turbiditic sequence affected by volcanic origin.…”

“…According to Surono et al (1992), Surono (2008), Kebo-Butak Formation from the bottom to the top are composed of basalt lava with pillow structures, intersecting of marls, calcareous claystones and calcareous sandstones, glass-and crystals-rich sandstones. Mulyaningsih (2016) determined the Kebo-Butak Formation as deep-to-neritic volcanic rocks of construction phase volcanic origin, shown by the intersecting volcanic deposits and calcareous marine sediments. Semilir Formation lies on Kebo-Butak Formation in an unconformity manner, consisted of basal layer with basalt fragments which was locally exposed at Watuadeg (Bronto & Mulyaningsih, 2001).…”

“…Rahardjo et al (1995) and Surono et al (1992) determine the Southern Mountain stratigraphy; from the bottom to the top are Kebo-Butak Formation, Semilir Formation, Nglanggeran Formation, Sambipitu Formation, Oyo Formation and Wonosari Formation; the three oldest formations were volcanic origin, while the others were calcareous sedimentary rocks. The Kebo-Butak Formation was volcanic origin deposited in submarine, so that often intersecting with calcareous sedimentary rocks in the frontier facies, while the nearest vents were lava with pillow structures and thick beds of tuff and lapillistone (Mulyaningsih, 2016). The distinguished volcanic origin of the Semilir Formation was composed by very widely distributed volcanic sequences of dacitic-ryolitic tuff and lapillistone by very explossive Plinian-Mega-Plinian volcanic eruptions (Mulyaningsih & Sanyoto, 2012;Mulyaningsih et al, 2011;Bronto et al, 2008).…”

Petrogenesis Lava Bantal Nampurejo di Dusun Kalinampu Dan Sekitarnya, Desa Jarum, Kecamatan Bayat, Kabupaten Klaten, Provinsi Jawa Tengah

“…Penelitian terfokus pada Satuan Lava Bantal yang terdapat di Desa Jarum, Kecamatan Bayat, Kabupaten Klaten, Provinsi Jawa Tengah. Lokasi ini berada di sebelah utara Gunung Kidul, Provinsi Daerah Istimewa Yogyakarta Lava Bantal Nampurejo memiliki struktur bantal dan memiliki komposisi basalt (Surono, 2008) dan menurut Mulyaningsih (2016) terjadi dua proses vulkanisme di lokasi penelitian yang mengakibatkan terendapkannya lava bantal dan endapan vulkanik feldspar. Penelitian ini bertujuan untuk mengetahui linkungan tektonik lava bantal.…”