Sedimentology of the Semirara Formation in Semirara Island: implications for the Miocene sedimentation and tectonics of south Philippines

Abstract

This paper describes the sedimentology of the Semirara Formation in the type locality, Semirara Island, which lies south of Mindoro on the western side of the Philippines volcanic arc. The history of sedimentation is set within the evolution of a series of rift basins which were compressed and inverted during collision with the arc. The Semirara Formation is mostly composed of continental clastic deposits of Upper Miocene age. The outcrop is dominated by fluvial channel sandstones interbedded with thick coals and estuarine and floodplain siltstones. Palaeocurrent analysis indicates that the rivers flowed towards the North and the petrography of the sandstones indicates that they were derived from a continental landmass rather than the volcanic arc. It is suggested that the Palawan/Mindoro microcontinent was being uplifted and eroded during the late Miocene and shedding sediments into a series of rapidly subsiding basins which developed under an extensional regime within the Palawan block, prior to collision with the Philippines volcanic arc. At the present day the basins are partially inverted following collision with the arc, beginning in late Miocene times. Inversion occurred in a regional sinistral strike slip regime and spread southward away from the northern edge of the indenting microcontinent. Coal in the Semirara Formation is currently being worked in an open cast coal mine, associated sandstones and organic rich mudstones form potential reservoir and source rocks for petroleum exploration in the three basins described. The sandstones are mostly fluvial channel sandstones deposited by a variety of rivers which can be summarised as four types: 1) Mixed load meandering rivers, 2) Sand-bed meandering rivers, 3) Large sand-bed braided rivers and 4) Shallow braided streams. The fluvial sandstones are interbedded with thick coal seams, up to 20 m thick, and siltstones which are interpreted as freshwater swamp and floodplain deposits. Occasional marine incursions are also recognised indicating an overall depositional setting on a coastal plain.