Search

International audience; Recent field and analytical work have considerably improved our understanding of the structure of the MullerRange along the frontal trend of the Papuan Fold and Thrust Belt (PFTB) in western Papua New Guinea (PNG).However numerous questions remain unresolved concerning the geological evolution of the region. In particular,the Late Cretaceous to Oligocene history of the region is largely unknown due to the absence of a continuousstratigraphic record. Here, we use both new and existing low-temperature thermochronology data to investigatethe geological history of the Muller Range. Thermal history models based on these data suggest two majorCenozoic cooling episodes. The youngest, and best constrained, is clearly recorded in the stratigraphic recordand relates to Neogene collision at the northern margin of the Australian continent. An older episode of comparableor greater magnitude occurred in the Eocene to Oligocene and may relate to the removal of1500–3000m of Late Cretaceous to Eocene section across the Muller Range prior to the widespread deposition ofthe shelfal Darai Limestone. We suggest that extension along major faults beneath the Muller Range accommodatedsedimentation from the Late Cretaceous to the Eocene, consistent with long-lived extensional structuresobserved in neighbouring regions. In contrast to the Muller Range area, an almost continuous Late Cretaceous toEocene stratigraphic sequence is preserved in the hinterland

The Holocene and pre-Holocene sediments and stratigraphy of the Yarra Delta have been examined using nearly 600 geotechnical bores. The oldest Holocene unit is the Coode Island Silt that has two depocentres, each up to 20.0–25.0 m thick, separated by a NW–SE belt of older pre-Holocene units. The northern depocentre represents estuarine infill to the Yarra and Maribyrnong, a river system, whereas the southern depocentre appears to be an offshore bay facies. The youngest unit is the Port Melbourne Sand, which is largely restricted to the area south of the present Yarra River. It is between 5.0 and 28.0 m thick, and is diachronous with the underlying Coode Island Silt. New 14C shell dates from the Coode Island Silt and Port Melbourne Sand have shown an age range between 8341 and 2760 yrs BP. These sediments infill former swamplands covering low-stand river valleys of the Yarra and Maribyrnong rivers across West Melbourne, Fishermans Bend and South Melbourne. After ca 2760 yrs BP active sedimentation in the delta ceased as base-levels fell, and Yarra and Maribyrnong river sediments bypassed the delta because of falling bay levels. The Yarra and Maribyrnong river courses also shifted progressively westwards behind growing beach barriers of the Port Melbourne Sand. A comparable stratigraphy exists between the Yarra Delta and the adjacent Port Phillip Bay, i.e. marine and lagoonal shelly sediments of the Coode Island Silt and barrier sands of the Port Melbourne Sand infill last-glacial channels cut into the middle Pleistocene Fishermans Bend Silt.

The initial stage of continental extension between Australia and Antarctica was associated with lateral changes in extension direction along the margin that reflects the three-dimensional nature of strain during continental rifting. In the Cretaceous Otway Basin, this change in extension direction was related to substantial rheological differences in the lithosphere across the boundary between two Paleozoic fold belts, the Lachlan and Delamerian, with the net extension direction at a high angle to this boundary. The initial Early Cretaceous rifting preserved within the onshore Otway Basin has two main structural subdomains in the eastern and western Otway Basins distinguished by different structural trends of Early Cretaceous normal faulting. This is not controlled by a variation in preexisting structural weaknesses within the underlying Paleozoic basement because the same geometry of extensional structures also occurs within the basement to the north irrespective of the preexisting structural grain. The eastern Otway Basin is dominated by NE-striking NW-dipping normal faults. In the western Otway Basin, the faults define arrays of predominantly NE-dipping or SW-dipping faults separated by wide accommodation zones defined by folding and variably striking and dipping faults. The partitioning of strain along the boundary between the eastern and western Otway Basins is accommodated by a progressive change in strike of faults and not via a transfer fault. Younger rifting in the Late Cretaceous had a similar extension direction in the western Otway basin, but had a dominant seaward dip, extension appears to have been hindered in the eastern Otway basin by a Proterozoic/Paleozoic basement feature. These factors produced a region of diverging extension along the lithospheric boundary between the Delamerian and Lachlan Fold Belts that lead to failure along this boundary and the formation of a localized sinistral trans-tensional graben, the Shipwreck trough, in the early Late Cretaceous. As a result, the younger rifting stepped south of the eastern Otway Basin leaving Bass Strait (the strait between the Australian mainland and Tasmania) a failed rift. The formation of oceanic crust in the Mid to Late Eocene followed the boundary of Late Cretaceous rifting, which led to the formation of the Tasman Fracture Zone.