ANALYSIS OF PALEOBIODIVERSITY FOR DETERMINING CYCLE AND RECONSTRUCTING DEPOSITIONAL MODEL IN THE RAJAMANDALA FORMATION LIMESTONES, CIKAMUNING AREA, WEST JAVA

• Main Author: Asri Riswanty, Maisi
• Format: Theses
• Language: Indonesia
• Subjects: Geologi, hidrologi & meteorologi
• Online Access: https://digilib.itb.ac.id/gdl/view/41661
• Institution: Institut Teknologi Bandung

Rajamandala Formation has been studied a lot before because it was considered to become analog model of subsuface Oligo-Miocene carbonate reservoir in Kujung Formation, Jawa Timur. However, the study about the association of species diversity with other aspects (depositional cycle, porosity, fracture, and diagenesis) in Rajamandala Formation of Cikamuning Ridge has not ever been done before. The objectives of the study are to know the number of depositional cycle, the association of depositional cycle with paleobiodiversity in carbonate facies and the controlling factors that affect the deposition cycle. The methods are observation of outcrop and core data, thin section analysis, qualitative and quantitative analysis of biota and modeling of facies and biodiversity. Analysis for specific study has been conducted to an outcrop measuring section and a core section with 49 thin sections in the platform-1 and platform-2 of Rajamandala Formation, Cikamuning Area. Based on facies distribution vertically and laterally, the study area consists of thirteen facies asaociation with five phases of depositional environment including reef margin, fore slope, proximal turbidite, distal turbidite, and deep water with debris flow mechanism. The third order sequence stratigraphy in the study area has four depositional cycles based on biostratigraphy and bedding correlation analysis e.g. platform-1, platform-2, turbidite carbonate cycle, dan deep water carbonate breccia. Analysis of paleobiodiversity that controlled by third order sequence indicates the existing of eight depositional cycles in the area. Meanwhile, the high frequency carbonate depositional cycles at sixth order comprise a normal cycle and ?40 iv abnormal cyclicities. Most of the depositional cycle is abnormal paleobiodiversity cycle because it only has two reef development stages in a cycle such as diversification and domination stage whereas the normal stage has complete of stabilization, colonization, diversification, and domination stage orderly. The stage of stabilization and colonization in this study more represent the rework due to allogenic control in the progradation keep up phase so it can be assumed as the upper of domination stage. The abnormality of paleobiodiversity cycle can be caused by autogenic and allogenic factor. At the high frequency cycle, autogenic factor become the main factor of abnormality, such as ecological ability, periodical disturbance manifested as hardground (lithified carbonate as basal rock) due to sea level fluctuation associated with climate in the Milankovitch cycle range of precession. At the third order carbonate cycle in Cikamuning area, allogenic factor is more influential, such as the forming of stacking pattern, hardground and surface exposure, eustacy or sea level fluctuation associated with Milankovitch cycle of 400-kyr eccentricity, sedimentation-accomodation rate, outer influx, and subsidence associated with tectonic. The integration result of facies distribution analysis vertically and laterally, sequence stratigraphy analysis based on biostratigraphy and bedding correlation, and cyclicity analysis based on paleobiodversity shows the study are regionally comprise four depositional cycles at the third order sequence with the overall thickness ±300-350 m. Each cycle is bounded by the sequence boundary. Meanwhile, at the high frequency cycle (fourth or fifth order) spesifically, there are thirteen depositional cycle based on the analysis integration in Cikamuning area. The fuction of mechanical layer from carbonate and paleobiodiversity could become ones of the controlling factors of secondary porosity especially indicated by grain contact from specific type of biota. Several depositional cycle that show the existance of specific secondary porosity by fracture and dissolution from grain, matrix, cement, and stylolite, also indicate the specific characteristic of facies and biota.