Abstract
The diagenetic process is critical issue to understand the possibility of petroleum systems. Diagenetic mechanism to help make a deep understanding of tectonic and lithology perspective that accelerates it. This paper attempts to find out about the anomaly of tuff diagenesis in Sumberan, Gunung Kidul (near the east borderline of Yogyakarta province), and the main factor affecting it. The research area is belonging to Kebo–Butak formation (Southern Mountain zone) of late Oligocene to the early Miocene age that deforms to creating a complex geological structure. It indicates the diagenetic factors of the rocks in the shallow surface are caused by overburden pressure and it could create shear failure. Overburden pressure causes physical and chemical character changes of rock. Its physical characteristics can be seen from the change of rock compaction. Meanwhile, the chemical characteristics shown by mineral composition is changed. Sumberan has three units of lithology, they are tuffaceous siltstone, tuffaceous sandstone and sandy tuff. Uniquely, the topside of outcrop experiences further diagenesis than the bottomside proven by transformation of tuff to montmorillonite and zeolite. Joints and faults become main structural control that commonly NE-SW direction and sinistral characteristic with massive thrust and normal faulting. The method used in this research is geological structure measurement, lithology identification, and sample laboratory report. Structural measurement is purposed to figure out tectonic orientation and mechanism, meanwhile, laboratory identification consists of petrology, petrography, lithostratigraphy, and XRD. The result of the study shows that overburden pressure does not fully control anomaly of tuff diagenetic. The anomaly is controlled by mineralogy variation of every single bed and dynamic paleostress of the research area.
Published Version
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