Abstract

The extensive field program undertaken by Mobil Oil Indonesia in the Lengguru Thrust-Fold Belt of Irian Jaya accessed a large number of previously isolated localities in the rugged interior of the region. Accurate positioning of geologic data on GPS controlled traverses has enabled the compilation of stratigraphic sections through the Tertiary New Guinea Limestone Group and delineation of the shelf edge in the underlying Late Cretaceous clastics.

The Late Cretaceous rocks of West Lengguru range from inner and mid-shelf sandstones of the Ekmai Sandstone in the north, to bathyal shales and pelagic limestones of the Simora Formation (new name) in the south. A reversal of paleo-slope direction during the Paleocene resulted in the onset of shallow water carbonate deposition across much of Lengguru and bathyal sedimentation (Imskin Formation) to the northeast where previously, shelf sandstones of the Ekmai had accumulated.

Examination of over 3000 samples of the New Guinea Limestone has enabled the authors to refine earlier lithostratigraphic subdivisions and recognise six biozones of the East Indian Letter Classification. Facies analysis shows depositional environments consistent with an extensive, isolated carbonate platform. This platform had a remarkably stable history throughout most of the Tertiary with the exception of a regressive event in the Early Oligocene. In the Mid- to Late Miocene, carbonate accumulation was abruptly halted by a transgression. A distinct standstill period is apparent in the final stages of platform sedimentation. This was followed by rapid deepening in the latest Miocene and ultimately, inundation of the region with the molasse sediments of the Pliocene-Pleistocene Buru Formation. Subsidence is postulated to have occurred as the result of collision between the Australian Plate and Melanesian Arc and subsequent crustal flexuring.