Geology and tectonics of pre-Tertiary rocks in the Meratus Mountains, South East Kalimantan, Indonesia

Abstract

Between 1981 and 1982 the Banjarmasin Quadrangle in South-East Kalimantan was mapped by the Geological Research and Development Centre, Bandung at the scale of 1 : 250,000. This thesis reports the results of a follow-up study to the mapping programme, which was directed towards determining the age, origin and tectonic evolution of the Pre-Tertiary rocks which form the Meratus Mountains in the eastern part of the Banjarmasin Sheet. The study consists of detailed field-mapping of Pre-Tertiary rocks in well exposed river sections at the scale of 1 :10,000.Measured sections of sedimentary units were made and all structural features were recorded. A comprehensive collection of rock samples was made for laboratory studies. Thin sections were used to determine the composition and origin of sedimentary and igneous rocks. Macro-and microfossils have been examined to determine the depositional environments and the ages of the sedimentary units. From these studies the Pre-Tertiary rocks are divided into a number of tectonostratigraphic units, whose age, origin, structural and tectonic evolution has been determined as far as possible. This information has been used to compile a synthesis of the tectonic development of the Meratus Mountains in the context of plate tectonics and the development of the western Indonesian region. Isotopic and palaeontological dating has shown that the units exposed in the Meratus Mountains range in age from Early Cretaceous to Early Palaeocene. The oldest unit is the Paniungan Formation of Berriasian to Barremian age. It grades upward into the Upper Barremian to Lower Aptian Batununggal Formation. These formations are interpreted as shelf to slope sediments. It is suggested that shortly after deposition, most parts of the shelf to slope sediments were juxtaposed by strike-slip faulting with oceanic crust now represented by the Meratus Ophiolite Complex. Subduction generated a calcalkaline volcanic arc which then collided with the Sunda continent in the Cenomanian time. This collision zone was disrupted and sliced by strike-slip faults, forming a pull-apart basin within it. The absence of Palaeocene to Lower Eocene deposits reflects uplift, subsequently followed by rifting, regional subsidence and deposition of an Eocene-Miocene transgressive sequence. The present configuration of the Meratus Mountains resulted from late Middle Miocene and Plio-Pleistocene tectonic events.