Martin S. Norvick

Basement controls on rifting and the associated formation of ocean transform faults—Cretaceous continental extension of the southern margin of Australia

Abstract 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.

Paleogene basalts prove early uplift of Victoria's Eastern Uplands

VandenBerg (2010) has written a very well-supported account of the Eocene–Oligocene paleovalleys and basalts of the southern margins of the Eastern Uplands in Gippsland. He is probably quite correct in suggesting that this part of the Eastern Uplands contains residual topography that has suffered little tectonic disturbance since the cutting of a regional paleoplain prior to the Eocene. However, he fails to recognise that this stable province is very much restricted to the coastal fringes of the highlands, and post-Miocene tectonic disruption begins just north of his study area, as documented by Holdgate et al. (2008). It is only by broadening the focus to cover the whole of the Southeast Australian Highlands from Melbourne to Wollongong that we can fully understand the setting of VandenBerg’s paleovalleys (Figure 1). As shown in Figure 1, the Southeast Australian Highlands is made up of numerous remnants of a once more extensive paleoplain, the so-called ‘High Plains.’ The paleoplain is often capped by Eocene, Oligocene or Lower Miocene basalt, which preserves paleotopography that must to have been cut prior to extrusion. The fragments of paleoplain bear subdued relief (100–200 m, but with monadnocks up to 500 m above valley bottoms). They are separated either by deep, younger river gorges, or by abrupt linear steps that probably represent post-Miocene faults. VandenBerg (2010) mentioned some of these faults in the Omeo-Benambra area. The best-documented faults are in southern NSW, for example at Lake George, where the paleoplain is offset by 200–250 m, of which 130 m is preserved as postMiocene half graben fill (Abell 1991; Singh et al. 1981). The biggest fault throws are probably west of the Kosciuszko Main Range. However, this remote NSWVictorian border area has not been mapped in detail except for localised site investigations for the Snowy Mountain Hydroelectric Scheme in the late 1950s (see Sharp 2004a, b). The surfaces of some paleoplain fragments are relatively flat. However, others show distinct regional tilt, well in excess of any purely erosional dip, particularly the NE-dipping Kosciuszko tilt-block and the NW-dipping plateaus adjacent to the Murray Basin. In general, the paleoplain fragments step up to the highest elevations at Mt Kosciuszko and Mt Bogong, and then step down again towards the Murray Basin. The highest, most disrupted and most faulted paleoplain fragments occur in a broad belt from the Namadji Mountains in the southern ACT to Lake Mountain in Victoria. Within this area, sub-basalt remnants of Tertiary fluviatile and lowland swamp sediments occur at current high elevations (Holdgate et al. 2008), for example at Hotham Heights (now at *1700 m ASL) and Kiandra (now at *1450 m ASL). This tectonised province is flanked to the northwest by the NW-tilted plateaus mentioned above. However, to the east and south of the disrupted belt is a less disturbed province that rises up to the lip of the coastal scarp in southeastern NSW. The largest untectonised paleoplains occur in the Monaro and middle Shoalhaven, where Taylor et al. (1990) and Brown (2006) respectively have described Eocene–Oligocene paleovalley complexes. The relatively undisturbed nature of these large paleoplain fragments may have led these authors to also support a uniform ancient age of uplift for the highlands, whereas a different style of more disrupted paleoplains occur further west in the Kiandra-Adaminaby area (Sharp 2004a). The coastal scarp also continues westwards into Victoria in a much more eroded and fragmented form, where it follows the Gippsland edge of the Eastern Uplands. It is within this relatively untectonised area where VandenBerg’s paleovalleys occur, probably occupying pre-Eocene valleys in the Gippsland coastal scarp, in contrast to areas further to the north which have been significantly disrupted. The distribution of modern earthquakes, which can be downloaded from the Geoscience Australia website (http://webmap.ga.gov. au/imf-natural_hazards/imf.jsp?site1⁄4natural_hazards_ earthquake), also shows a distinct diminution of seismic activity in the less disrupted province. Thus, VandenBerg (2010) has very successfully demonstrated the well-preserved paleovalley system in a limited untectonised area of the Southeast Australian Highlands, where initial uplift may have been Australian Journal of Earth Sciences (2011) 58, (93–94)

Geological evolution of the Holocene Yarra Delta and its relationship with Port Phillip Bay

ABSTRACT 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.