The Eocene unconformity on Southeast and East Sundaland

Abstract

The Early Palaeogene landmass of Sundaland extended as far southeast as West Sulawesi. The cratonic nature of the Malay Peninsula and China did not extend into or beyond the Borneo region of Sundaland, where the pre-Eocene outcrops were dominated by Cretaceous rocks. Deep water sediments, melange and ophiolite terrain were characteristic of this non-cratonic southeastern peninsula of Sundaland. By 45 Ma (magnetic anomaly 19), India was in flush collision with Eurasia, seafloor spreading ceased at the NW Wharton Basin, and a new fracture system was initiated, separating Australia from Antarctica, and extending WNW along the SE India Ocean Ridge towards the Red Sea. This event was felt throughout the whole of Southeast Asia, and is recorded as the major Eocene Unconformity, both onland and offshore Sundaland. The NNE directed push of cratonic India into the continental margin of Eurasia resulted in a general clockwise rotation of the Sundaland Peninsula, made possible by major right-lateral shear along the pre-existing structural grain (extrusion tectonics). The Sunda arc-trench system was also initiated at 45 Ma. The oroclinal bending resulted in crustal pull-apart in some places and compression in others. Anticlockwise rotation and left-lateral shear were imparted to eastern Sundaland by the collision of Australia to the east. Rejuvenated rivers carried an increased load from the uplifted areas, active faulting created river capture and lakes, many of which date back to the Eocene in most places, but were locally delayed to the Oligocene. Marine platform carbonates became widespread and characteristic of E aiid SE Sundaland from Late Oligocene through Miocene, except in the Rajang Group Basin and some rift-related river/delta systems. Late Miocene to Pliocene uplift was important in non-cratonic Sundaland, causing internal subdivision offormerly more extensive Cenozoic basins.