THE STRUCTURE OF FOOTHILLS SOUTH OF THE KUBOR RANGE, PAPUA NEW GUINEA

Abstract

The Kubor Range, which trends northwest-southeast and has some peaks of over 4000 m, is composed predominantly of Permian granodiorite, intruding Palaeozoic metamorphic rocks. The part of the Papuan basin to the south is aligned in the same direction; in it there are up to 7000 m of Mesozoic sediments and 2000 m of Tertiary sediments. The Mesozoic sequence contains two incompetent units, the Jurassic Mari! Shale and the Upper Cretaceous Chim Formation; these are separated by the more rigid Kondaku Tuff in the north and the Kerabi Formation further south. The lower part of the Tertiary sequence grades from shelf facies Darai Limestone in the south to the terrigenous clastics of the basinal Aure Group in the north; both are overlain by terrigenous clastics of the Orubadi Formation.

Most of the structural features were caused by gravity sliding from the north down the basinward slope of the Kubor Granodiorite during the Pliocene. This sliding occurred within the incompetent units in the Mesozoic sequence, namely in the Maril Shale and Chim Formation. The latter is present only in the north. Movement occurred there within both units and synclinal klippen of Tertiary rocks survive. These lie discordantly on underlying structures caused by movement within the Maril Shale. To the south, sliding within the Marti Shale has given rise to imbricate structures at the surface; these are replaced further south by asymmetrical anticlines separated by thrust planes.