Publications

Apatite Fission-Track (AFT) analysis of 44 surface and drill-hole samples constrain the Cenozoic tectonic history of the western Irian Jaya Fold Belt and the adjacent Arafura Platform (Fig. 1). These samples indicate two recent periods of cooling in response to uplift and erosion, 1) Early Miocene ages consistent with regional metamorphism in Papua New Guinea (PNG), and 2) Late Miocene-Pliocene dates that correlate with arc collision in northern PNG and contemporaneous island-wide deformation driven by changing plate convergence on the northern Australian margin. Fission-track samples were collected during field work in 1995 and supplied by Pertamina, Freeport Indonesia, Amoseas Indonesia, and Conoco Indonesia. Twenty of the samples failed the Chi' test, indicating that the dated grains from those samples represent statistically valid multiple grain-age populations as a result of mixed provenance or differential annealing of apatite grains of varying composr ion. For these samples each grain-age population is described.A total of 11 samples were analyzed from the south flank of the Central Range (Fig. 2) between 136'15'E and 139'25'E. Data from a surface sample from the Late Pennian-Jurassic Tipuma Fm Southwest of the PT Freeport Indonesia Grasberg Cu-Au Mine indicate slight age reductior,, with an AFT age of -180 Ma compared to a 190-245 Ma chronostratigraphic age. The Late Miocene-Recent Bum Fm in the same area contains multiple populations of grain-ages that cluster at 0-5 and 20-35 Ma. Length data from these samples suggest slight to moderate annealing presumably prior to uplift and cooling of the source terranes. Two samples of the Late Cretaceous Ekmai Fm and Cambrian-Ordovician Tuaba Fm collected southeast of Grasberg have experienced severe age reduction, based on poor-moderate apatite yields. Multiple grain-ages in the Ekmai Fm cluster at 8-5 and 40-50 Ma with a small number of -100 Mi grains representative of the provenance age. The Tuaba Fm sample gives an -1 1 Ma AFT age, well below the 439- 5 10 Ma stratigraphic age, indicating exposure to elevated paleotemperatures >110",C since deposition.The Noordwest-1 well (Fig. 1) provided four samples of the Late Miocene-Recent Bum Fm which is repeated by a thrust fault separating the upper and lower two samples. The upper two samples are -38 Ma and -48 Ma, while the deeper samples give slightly older ages of -74 Ma and 75 Ma. The presence of AFT ages in excess of the stratigraphic ages, in conjunction' with mean track lengths in excess of 12.5pm, indicates that the section was cooler in the past and has only recently reached the current moderate temperatures.Sampling in the northern and central fold belt (Fig. 2) resulted in eight AFT ages from outcrop and shallow boreholes. Apatite yields from samples of the Paleocene-Eocene Waripi Fm and Late Permian- Jurassic Tipuma Fm were poor in the vicinity of Grasberg, but indicate severe age reduction. The Waripi Fm sample gives a 6 .Ma AFT age while the Tipuma Fm sample contains grain-age populations of 0-5 Ma and older. Approximately 45 km NNW of Grasberg, in the vicinity of the Derewo Fault and the lrian Jaya Ophiolite, 2 samples contained large amounts of apatite. An Ekmai Fm sample approximately 5 km south of the Derewo Fault gives an AFT age of 12.8 Ma with long (>15pm) track lengths suggesting total annealing of pre-existing tracks. Another Ekmai sample -5 km from the Derewo Fault and due north of Grasberg has experienced high temperatures resulting in the reduction of the AFT age to -32 Ma, approximately half the stratigraphic age. This sample contains multiple grain-age populations of 0-6 and 25-35 Ma, and older grains retaining provenance ages. Thermal modelling suggests that the sample experienced temperatures of -80-1 00°C at 10 Ma. The Middle to Late Miocene AFT data from the northernmost portion of the fold belt may record cooling resulting from uplift at the onset of collision at the northern Australian margin.ln the west, two samples of the Albian Woniwogi Fm from the Southwest shore of Lake Paniai give AFT ages of 5.6 and 10.2 Ma. The younger sample was collected from a shallow borehole which penetrated multiple Miocene intrusives. Although apatite is abundant in the sample, no measured lengths were present and the age is therefore representative of postintrusion cooling. The nearby sample from the Woniwogi Fm however retains two grain-age populations of 0-5 and -15-25 Ma. Shallow boreholes in the vicinity of Etna Bay provided 4 samples of intermediate composition intrusives. AFT ages for single population samples are 6-8 and -35 Ma, while a multiple age population sample contains grain-ages of 0-6 Ma and older grains.Southwest of the Central Ranges, 12 samples were collected in the area of the Waghete 1:250,000 map sheet (Fig. 3) containing moderate to excellent apatite yields. Sampling along the axis of an anticline Southeast of Modio resulted in AFT ages of -5 and -12 Ma from the Tipuma Fm in the core of the fold, and a -58 Ma age from the structurally higher Albian Woniwogi Fm. Two samples of the Permian Aiduna Fm from the north side of the Aiduna Fault (described as both a thrust and a transcurrent fault) give AFT ages of -5 and -7 Ma, but fail the Chi test. Both are composed of clusters of grain-ages of 0-4 Ma, while the younger sample contains an additional population of -15-25 Ma age grains. These Late Miocene- Pliocene surface sample AFT ages, common along the 400km length of the fold belt define both the timing and extent of cooling driven by uplift.Five samples from the South Oeta-1 well (Fig. 1) were collected above a thrust fault at depth resulting in AFT ages of 35-37 Ma for the Late Cretaceous Ekmai and Piniya Fms. Below the thrust, the Paleocene-Eocene Waripi Fm sample is older than its stratigraphic age, while the Albian Woniwogi Fm sample gives an AFT age of 75 Ma indicating moderate age reduction from its -100 Ma stratigraphic age. It is significant that the Waripi Fm sample (below the thrust) has a longer mean track length than adjacent samples suggesting that thrust faulting of the section occurred recently, probably within the last 1-2 Ma. The presence of a 75 Ma AFT age neas TD at current temperatwes of -1 70°C further indicates very recent thrust repetition of the section at depth and that burial by the thick (2+ km) Late Pliocene to Recent Buru Fm was both rapid and recent.In eastern Irian Jaya, AFT sample ages from the E-2- X drill-hole (Fig. 1) range from -185 Ma in the Aptian Woniwogi Fm to -297 Ma in the Late Proterozoic- Cambrian(?) Kariem Formation. The upper sample from the Woniwogi Fm is older than the depositional age, indicating little or no exposure to the elevated paleotemperatures seen in the western Papuan Basin. Thermal modelling suggests that high paleotemperatures resulting in age reduction of the Late Proterozoic-Cambrian rocks are likely the result of the intrusion of a gabbroic unit into the Kariem Fm and not a subsequent Late Cretaceous thermal event recorded in the western Papuan Basin. Similarly, results from the Digul-1 drill-hole suggest that Mesozoic rocks reached maximum temperatures prior to development of the Late Cretaceous unconformity resulting in total (or near total) resetting of the apparent AFT ages.These well data suggest that Late Cretaceous heating associated with the opening of the Coral Sea, was limited to the Papuan Basin east of the Arafura Platform. A major lineament corridor is interpreted at the Irian Jaya-PNG boundary from combined gravity, magnetics, thermochronology and surface data. These long-lived basement structures which segment the fold belt and the adjacent Arafiura and Fly Platforms are significant controls on burial history, source and reservoir rock distribution, and the locations of both depocenters and economic Cu-Au mineral deposits.The main phase of cooling in response to uplift, recorded by AFT ages, ranges from Late Miocene- Pliocene, with Late Pliocene-Recent deformation continuing in the foreland and southernmost fold belt. These data correlate with stratigraphic and paleogeographic data recording the initiation of orogenic compression and development of much of the fold belt in the Middle Miocene-Pliocene, followed by a subsequent period of deformation characterized by shortening and strike-slip faulting. An earlier thermal event is recorded by a significant population of Oligocene-Early Miocene AFT ages, contemporaneous with a regional metamorphic event in north PNG. AFT data suggest the thermal maximum of Mesozoic sediments in eastern Irian Jaya was reached prior to the Late Cretaceous as in the successful Kutubu fields of PNG. Another promising generation-trapping pathway for Eastern Irian Jaya, particularly for gas, is dependent on adequate Late Miocene-Pliocene burial and source rock quality.