LITOSTRATIGRAPHIC AND SEDIMENTOLOGICAL SIGNIFICANTS OF DEEPENING MARINE SEDIMENTS OF THE SAMBIPITU FORMATION GUNUNG KIDUL RESIDENCE, YOGYAKARTA

Abstract

Sambipitu Formation in the Southern Mountains plays an important role due to its stratigraphic position, between syn-volcanism and post- volcanism periods. The formation widely distributes along the southern slope of the Baturagung Mountains, Gunung Kidul Residence, Yogyakarta Province. Stratigraphically, the Sambipitu Formation is conformably underlain by dominated unit of volcanic breccias of the Nglanggran Formation, and conformably overlain by dominated unit of marl of the Oyo Formation. Based on detail section along the river of Ngalang, the Sambipitu Formation can be divided into Lower and Upper Members. The Lower Member is dominated by sandstone and siltstone, which is alternated by breccias. The Upper Member is dominated by siltstone and mudstone, which is intercalated by sandstone, marl and conglomerate. The Lower Member was deposited on an environment influenced by tidal current, which was highly affected by gravity flows of volcanic material. This deposition environment was getting deeper to be an inner shelf, where the Upper Member was deposited. Furthermore, based on Rock-eval pyrolysis, TOC value of the Sambipitu Formation ranges from 0.08% to 0.43%, whilst the PY (potential yield) value less than 0.15 mg HC/g rock. Thus, on the basis of those two parameters, the Sambipitu Formation is included into oil prone source rock potential of poor category. Moreover, Tmax value of the Sambipitu Formation ranges from 226°C - 335°C, with the HI (hydrogen Index) value varies from 0 – 12. It indicates that this formation contains kerogen Type III. Therefore, the organic thermal maturation based on plotting of Tmax vs HI, this formation falls into an immature category.

Keywords

• Lithostratigraphy
• volcanic material
• tidal flat
• inner shelf
• Sambipitu Formation