THE GEOLOGY OF WESTERN PAPUA

Abstract

Two principal phases of sedimentary deposition are known in Western Papua, the Mesozoic and Tertiary. Knowledge of older sediments is limited to a small outcrop of Permian on the western end of the Kubor Range beneath which are granites and metamorphic rocks forming basement.

The Mesozoic rocks outcrop in the central ranges of New Guinea and have been penetrated in 20 wells in the southern coastal area. Elucidation of the detailed Mesozoic structure and stratigraphy is incomplete but the outline of the history of sedimentation is emerging and appears to be similar in broad structural pattern to that of the overlying Tertiary sequence.

Folding emergence and erosion occurred at the end of the Mesozoic in response to epeirogenic uplift of the northeastern margin of the Australian continent; with the subsequent Tertiary transgression being represented for the most part by Eocene and Oligocene limestones of limited thickness. On the basis of facies characteristics and sediment thickness it appears that during the Miocene western Papua was the site of an arcuate orthogeosynclinal complex. In the sector north of tilte Papuan Gulf the subdivision into a eugeosynclinal and miogeosynclinal trough is clearly represented by the development, in Kereruan times, of the Aure Trough and the Omati Basin. During the Lower and Middle Miocene huge thicknesses of greywackes and shales were deposited in the Aure Trough and westwards and southwestwards of the trough a limestone succession consisting mainly of basinal, fore -reef limestones and shoal reefs was developed over an area some 200 miles wide with its maximum thickness at Ornati. Definition of limestone facies within this huge limestone area is still incomplete. The Upper Miocene was marked by widespread development of argillaceous and arenaceous facies with only minor development of limestone. Movements during the Upper Miocene led to restriction of the Pliocene basin of sedimentation and the formation of shallow water arenaceous rocks with lesser mudstones and coals. The final major phase of folding and uplift probably began in late Pliocene times and has continued to the present day, accompanied by vulcanism and vigorous erosion.