THE ICHNOFOSSIL STUDY TO RECONSTRUCT THE MODEL OF MIOCENE SEDIMENTARY DEPOSITIONAL SYSTEM OF KUTAI BASIN, IN THE AREA OF SAMARINDA, EAST KALIMANTAN

• Main Author: Arifullah, Ery
• Format: Dissertations
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/39203
• Institution: Institut Teknologi Bandung

As the largest and one of the most prolific basin in Indonesia, geological study, especially in sedimentology, has been running intensively for understanding the depositional system in the Kutai Basin. The interaction of fluvial, tides and wave processes are distinguished and has been interpreted being a deltaic depositional system. However, that interpretation generates the problem and remains potentially debated, because not only delta can be produced from the interaction of fluvial, tides and wave processes but non deltaic depositional system is generated potentially in transition zone. Because of the paleoecological significance of ichnofossil, ichnofossil was a potential tool for decoding the depositional system interpretation in the Kutai Basin. Yet, an ichnological study in such depositional system is lacking, despite the variation and bioturbation intensity in the outcrops and cores in the Kutai Basin were prevailed. The aim of this study is the determination of ichnofossil association and reconstruction of the depositional system in the Samarinda Area of Serravallian and Tortonian intervals. Since ichnofossil is insitu and the part of sedimentation event and a part of a sedimentary structure, then ichnofossil association and ichnofabric variables can be utilized for depositional modeling in space and time. How the depositional system can be inferred from ichnofossil association and ichnofabric variables are the problem should be solved. Six hundred and forty ichnofabric units were observed at the twenty outcrops that situated at Serravallian-Tortonian intervals in Samarinda Area. Identifying ichnotaxa and ascertaining the bioturbation index (BI), ichnodiversity (ID), number of behavior (NB), penetration depth (PD) and ichnofossil diameter (Dm) variabels were conducted in each ichnofabric units. That ichnological features are integrated with facies and paleocurrent analyses. Thirty-four ichnotaxon were identified, they are Arenicolites, Bergaueria, Chondrites, Conichnus, Cylindrichnus, Diplocraterion, Fugichnia, Gyrolithes, Heimdallia, Helminthoidichnites, Macanopsis, Macaronichnus, Monocraterion, Ophiomorpha, Paleophycus, Phycodes, Phycosiphon, Planolites, Platicytes, Polykladichnus, Psilonichnus, Rhizocorallium, Rosellia, Trackway, Schaubcylindrichnus, Scolicia, Siphonichnus, Skolithos, Taenadium, Teichichnus, Thalassinoides and Zoophycus. Twenty-one of them are dominant in the ichnofabric unit and used it as the basis for naming ichnofossil association. However only six as the principal ichnofossil associations, such as Ophiomorpha, Skolithos, Paleophycus, Thalassinoides, Planolites and Chondrites associations. They perform as monospecific variant commonly. In addition, the average of BI, ID, NB, PD and Dm variables from 600 ichnofabric units respectively are 2,49; 1,69; 1,28; 2,28 and 2,18. That values were categorized low grade. These findings are an indicator of shallow marine and brackish environment. Ichnodisparity and space using are the new components generated from principal component analysis. Ichnodisparity is a function of BI, ID and NB and space using is a function of PD and Dm. The other important result was scores of each ichnofabric units that have intrinsic paleoecological value. That score is dispersed at PC-1 and PC-2 axes that could be clustered based on sedimentary processes criteria. In the Serravallian, zone A is dominated by Skolithos and Paleophycus, zone B is dominated by Skolithos and Ophiomorpha and zone C is dominated by Ophiomorpha, Planolites, and Skolithos. In the Tortonian interval, zone D is dominated by Ophiomorpha and zone E were dominated by Skolithos. Overall, Serravallian ichnofossil association more diverse than the Tortonian one. PC-1 was justified as ichnodisparity that implied of food supply and hydrodynamic energy. PC-2 was good reason for space using that implied of paleosalinity and paleotemperature fluctuation. The orientation of distribution of the PC-1 and PC-2 scores was correlated to the paleosedimentation that west-east trend in Kutai Basin during Miocene. The food supply and hydrodynamic energy may be controlled by the sediment supply influx from the continent as well as from the sea. The paleosalinity and paleotemperature fluctuation may be dictated by increasing intensity of spring tides or tidal range. Integration of the PC-1 and PC-2 maps and the result of facies analysis can generate the paleogeographic maps and paleoecological models. The paleogeographic of Serravallian indicate the three depositional systems such as wave-dominant coasts (zone A), fluvial-dominated delta (zone B) and mixed wave-tides coasts (zone C). Ichnodisparity elevates from zone A to zone C. The unique ichnofossil zonations are indicated. Wave-dominant coasts system is indicated by Paleophycus and mixed wave-tides coasts is indicated by Planolites. The paleogeographic of Tortonian suggest two depositional systems such as fluvial-dominated delta (zone D) that represented by Skolithos and estuary (zone E) that marked by Ophiomorpha. Space using increase from zone E to zone D. window, paleocommunity structures, and strategies can be accessed. In the Tortonian, the tendency of increase of paleosalinity and paleotemperature fluctuation is accompanied by raising of Ophiomorpha. Stratigraphically, Skolithos association is increased upward in the Serravallian, but decreased upward in the Tortonian. Thus, Serravallian and Tortonian depositional systems were controlled by the fluvial influx and spring tides/tidal range respectively.