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Folded reservoir rocks (Eumeralla Formation) within the Otway Basin are of significant exploration interest for their geothermal and tight gas reservoir potential. Brittle deformation within the Eumeralla Formation (ca 113–100 Ma), along the Otway coastline of Victoria, reflects a protracted history of extension, contraction, erosion and uplift, since Australian–Antarctic separation in the Early Cretaceous. This paper investigates the relative timing of heterogeneously distributed fracture populations within both folded and unfolded areas along the Otway coastline. Data collection occurred across two stratigraphic units spanning the Early Cretaceous (Eumeralla Formation) to the late Cenozoic (Demons Bluff Group), in order to measure fracture geometries and determine the relative timing of fracture formation. An unmanned aerial vehicle has been used to facilitate the systematic and inexpensive acquisition of high-resolution orthophotographs along coastal platforms, in order to complement traditional field mapping of fracture populations. A NE–SW-orientated fracture set is observed exclusively within the Eumeralla Formation (St George fold hinge), while a NW–SE-orientated fracture set pre-dates hinge-parallel fractures. Significant fracture formation has been linked to a period of mid-Cretaceous uplift within the eastern Otway Basin (ca 95 Ma). The Eumeralla Formation and Demons Bluff Group also host a NNW–SSE-orientated fracture set that is inferred to largely post-date mid-Cretaceous folding. Sinistral displacement of fold hinges within the Eumeralla Formation, combined with thrust faulting within Cenozoic sediments, is interpreted to have occurred during an episode of late Miocene to early Pliocene shortening. This paper provides new insights into the history of brittle deformation in the eastern Otway Basin. [ABSTRACT FROM AUTHOR]

The provenance of the upper Cambrian to Upper Ordovician sedimentary rocks of Tasmania and Waratah Bay in southern Victoria provides information about the complex and dynamic tectonic environment present during their deposition. This paper uses U–Pb detrital zircon data to constrain stratigraphic comparisons and tectonic reconstructions of these rock sequences. Multivariate statistics are used to investigate the similarity between the U–Pb ages and quantify the disparity among different samples from various locations. In western and central Tasmania, the Tyennan region supplied most detrital zircons during the late Cambrian and Early Ordovician. The overlying Middle Ordovician Pioneer Sandstone records a switch in provenance with zircons derived from the Mount Read Volcanics (MRV) mixed with zircons similar to those from continental-derived Paleozoic sedimentary rocks deposited throughout east Gondwana. The Middle to Upper Ordovician Gordon Group in western and central Tasmania lacks detrital zircons younger than 1.2 Ga, which indicates a return to a local provenance from Precambrian rocks. In southern Tasmania, the switch to zircons derived from the MRV and east Gondwana-like sources occurred earlier within the Cambrian Deadmans Bay Formation, which is dominated by the east Gondwana Paleozoic zircon age signature. In the East Tasmania Terrane, Ordovician sedimentary rocks from Lefroy have detrital zircon populations dominated by Neoproterozoic and earliest Paleozoic sources similar to the Ordovician sedimentary rocks in the Lachlan Orogen. In southern Victoria, the Bear Gully Chert from Waratah Bay exhibit both Tyennan and distal Gondwana detrital sources. The switching of detrital zircon sources in the west Tasmanian sedimentary sequences implies the docking of Tasmania with mainland Australia during the Cambrian Tyennan Orogeny. The arrival of the distal zircons into these basins occurred at different times in the different areas, reflecting a complex local topography and paleogeography. Paleozoic sedimentary rocks in Tasmanian exhibit multisource detrital U–Pb age signatures that change over time, implying tectonic activity during their deposition. Paleozoic sedimentary rocks in northeastern Tasmania show Gondwana-wide detrital signatures similar to Lachlan Orogen. The Ordovician Bear Gully Chert at Waratah Bay in southern Victoria shows mixed Tasmanian and distal Gondwana detrital populations. The change in detrital zircon signature in western Tasmania suggests that VanDieland docked with the Australian continent during the Cambrian Tyennan Orogeny. [ABSTRACT FROM AUTHOR]

This paper presents the results of a small number of dates from sand samples collected at building sites in the eastern suburbs of Sydney. OSL ages of well-sorted, quartz sand ranges from 31 to 24 ka from within well-developed podzol soil profiles within a metre of the surface. The source of this sand appears to be from the east off the inner continental shelf during a period just prior to the maximum of the Last Glacial Maximum (LGM, Marine Isotope Stage 2) when sea-level was approximately 70-90 m below present level. During a later phase of the LGM, there is evidence from other coastal sites in central NSW and Gippsland, Victoria, that the dominant wind was from the west consistent with evidence from arid and semiarid Australia. Dune orientations in northern NSW into Queensland and other evidence at this time appear to reflect winds from the southeast quadrant. This study highlights the existence of a potential pivot area in dominant wind direction during the LGM between sustained westerly flow in southern area including Victoria and northeast Tasmania and southeast flow of northern NSW into Queensland. In central NSW, the circulation switched from easterly in Stage 2 to westerly as the glacial stage intensified in the LGM sometime after ca 25 ka. [ABSTRACT FROM AUTHOR]

A substantial database of 40Ar/39Ar ages, collected recently from micas in western and central Victoria, has been used in several recent papers as support for continuous, diachronous deformation across western and central Victoria lasting through much of the Early Palaeozoic. This paper reviews these ages, together with field evidence collected over the last ten years. It provides an alternative interpretation, that mica growth and overgrowth in western Victoria was not continuous but episodic, occurring at ca 455 Ma, 440 Ma and 425 Ma, with little or no mica growth recorded from between these times. These ages have been obtained from mica in regional cleavage, crenulation cleavage and in quartz veins, and from across the entire width of the Stawell and Bendigo structural zones of western Victoria. A sharp change in mica ages occurs at the Mt William Fault, east of which no mica growth older than about 380 Ma is recorded. Several ages used in support for diachronous deformation are not related to deformation: an 40Ar/39Ar age of 417 Ma from Chewton is from the aureole of a Devonian granite, and an age of 410 Ma from the Melbourne Zone is shown to contain a substantial amount of inherited mica. If it is accepted that mica growth can be used to date deformation, then the 40Ar/39Ar ages indicate episodic, not continuous, deformation in western Victoria (Stawell and Bendigo Zones). The sharp decrease in the deformation age in the Melbourne Zone, east of the Mt William Fault, agrees well with field evidence that shows continuous sedimentation in the Melbourne Zone in the period (Ordovician to mid-Early Devonian) during which the Stawell and Bendigo zones were undergoing deformation. Some correlation also exists between the 40Ar/39Ar ages from western Victoria and well-constrained deformational events in the eastern Lachlan Orogen. The pattern of deformation has important... [ABSTRACT FROM AUTHOR]

Presents the discussion and reply on the paper concerning the Llandovery-Ludlow graptolites from central Victoria. Involvement of professor Arthur Vandenberg; Reply from researchers A. C. Sandford and Red Rickards; Explanations on silurian stratigraphy.

This paper reviews the role that geophysical methods have played in mapping mineral systems beneath Murray Basin cover in the Stawell corridor of western Victoria. North of Stawell, the Stawell corridor has been shown to extend more than 100 km to the north-northwest, beneath increasing thicknesses of Murray Basin sediments. Based on a geological model developed at Stawell, geophysical methods have played an essential role in mapping new mineral systems in the belt beneath this cover. Regional geophysical data (principally magnetics and gravity) supplied by GeoScience Victoria have been supplemented by company surveys for more detail in selected areas. In addition, various electrical methods have been tested for direct detection of mineralisation beneath deep conductive cover. The results have led to the detection of several mineralised systems many kilometres from outcrop and with no surface expression. Petrophysical measurements on core samples and geophysical logging of drillholes have been essential to the interpretation of geophysical data, and they have also contributed to further understanding of the mineralising processes. [ABSTRACT FROM AUTHOR]

We demonstrated that the Strathbogie batholith is thin and sheet-like (or pizza-like) but did not specifically address its detailed emplacement dynamics, other than to suggest overlapping or stacked sheets (Phillips I et al., i [12]). However, for the Strathbogie batholith, the overall shape and the absence of bounding faults are features that suggest that, for this batholith, cauldron subsidence would not be a preferred model. We thank Colin Branch for his astute comments regarding cauldron subsidence as a possible mechanism for granite magma emplacement. [Extracted from the article]

Evidence from bay floor channelling, seismic surveys and core dating has been used to suggest that Port Phillip Bay dried out for a period between about 2800 and 1000 cal. yr BP as sandbars blocked it off from the sea. This model is now supported by the examination of radiocarbon ages from archaeological excavations of Aboriginal shell middens on the shoreline of the Mornington Peninsula on Bunurong Country. This shows a near-continuous use of marine resources by Bunurong people over the last 6000 years for areas south of Rosebud, while those to the north are only of more recent date, following the refilling of the bay in the last millennium. This study provides an example of the integration of traditional, archaeological and geoscience evidence and the way in which local environmental changes impact on society. Radiocarbon dates from Aboriginal places support the argument that a sandbar blocked much of Port Phillip Bay from the sea between about 2800 and 1000 cal. yr BP. Dates from Aboriginal places south of the sand bar show continuity since the bay formed following the post-Pleistocene rise in sea-levels. Dates north of the sandbar indicate that Aboriginal people began exploiting marine resources in this area once the bay refilled about 1000 years ago. [ABSTRACT FROM AUTHOR]

Age distributions among the detrital zircon populations in western and central Victorian Paleozoic metasedimentary rocks suggest that most were deposited in Andean-type back-arc environments, with variable proportions of arc and continental sediment sources. Notable exceptions are the upper Cambrian Knowsley East Shale and the Silurian(?) sample of 'Glen Creek Lithic Sandstone', which both contain unimodal zircon populations. In the case of the Knowsley East Shale, this age is very close to the inferred depositional age, implying a possible forearc environment, with sediment derived directly from the contemporary volcanic arc. On the other hand, the implied trench environment for the 'Glen Creek Lithic Sandstone' is most likely inherited from its very local sediment source in the fault-bounded and mainly Cambrian Glen Creek erosional window. The only analysed rock units that may have been deposited in non-arc settings are the middle Silurian Humevale Siltstone and the Lower Devonian Norton Gully Sandstone. The zircon populations in these units carry strong continental signals, and they may have been deposited in an extensional marine basin bounded by continental blocks. The upper Silurian Grampians Group may have been deposited in a similar rift setting, but the zircon age histogram for this unit suggests that it was more proximal to an arc terrane. These assignments of depositional/tectonic environment should be regarded as indicative only, particularly since the method provides only subtle distinctions between some extensional and convergent settings. However, if the assignments are correct, there are some important implications for the geological history of this part of the Lachlan Orogen. For example, contrary to present perceptions, the Stawell Zone rocks may not have been prominently uplifted during the Ordovician, and the basin in the Bendigo Zone may have been receiving sediment input from further west in the Grampians–Stavely Zone. Early Cambrian to Early Devonian sedimentation in western and central Victoria occurred mainly in back-arc marine basins, initially associated with an Andean-type margin. Locally, some Cambrian sediments were formed in trench or forearc environments, with detritus supplied directly from a contemporary volcanic arc. Cambrian to Silurian back-arc sediments were deposited at varying distances from any active or former arc terranes and with varying but always major contributions of continent-derived sediments. Early Devonian sediments in the Melbourne Zone may have been deposited in what was effectively an intracontinental marine rift. [ABSTRACT FROM AUTHOR]

The Dorchap Dyke Swarm hosts the first recorded occurrence of lithium–caesium–tantalum (LCT) pegmatites in Victoria, Australia. Syn-orogenic emplacement of pegmatite dykes occurred along a northwest-trending shear system during the Benambran Orogeny. Pegmatites are derived from fractionated melt associated with the Mount Wills Granite, which is an S-type, peraluminous granite originating from supracrustal melting of Ordovician sedimentary sequences. A distinct, eastward-oriented fractionation trend across the Dorchap Dyke Swarm has highlighted a 20 × 8 km highly fractionated zone in the northeastern Dorchap Range, which includes spodumene- and petalite-bearing pegmatites. A distinct pattern of elemental enrichment (P > Cs > Be > Nb ≥ Ta > Li > Sn) is observed across the strongly fractionated zone of the Dorchap Dyke Swarm. Subsequent metasomatic fluid movements and hydrothermal overprinting have resulted in redistribution of mobile elements in the Dorchap Range, either as hydrothermal alteration species or in some instances as the development of exomorphic halos. Additionally, a regional alteration overprint, likely associated with subsequent metamorphism of pegmatite dykes has resulted in alteration of primary petalite to spodumene plus quartz, and primary spodumene to cookeite. Bulk rock geochemical data from the Dorchap Dyke Swarm suggest a syn-collisional setting for dyke intrusion, consistent with the inferred tectonic setting of the central Lachlan Fold Belt at the time of pegmatite emplacement. Pegmatite dykes locally contain an overprinted structural foliation, which is consistent with the primary structural trend of deformed metasediments and may indicate that dyke emplacement was syngenetic with regional folding and compression of the surrounding Omeo Metamorphic Complex during the Benambran Orogeny. Subsequent hydrothermal alteration of some dykes likely occurred immediately following the Bindian Orogeny. Dorchap Dyke Swarm are the first recorded Li–Cs–Ta pegmatites in Victoria, Australia. Pegmatites were emplaced synchronous with or immediately following the Benambran Orogeny. Dorchap Dyke Swarm pegmatites are geochemically correlated with the Mount Wills Granite. [ABSTRACT FROM AUTHOR]

The magmas that formed each Devonian, S-type, volcanic complex in central Victoria were generated through partial melting of a distinct assemblage of rock types in the crustal source region. The more silicic rhyolites were formed independently of the rhyodacitic to dacitic magmas, rather than as differentiates. Within every complex, each ignimbritic unit is also heterogeneous in its chemical and isotopic characteristics. Thus, in this part of Paleozoic Australia, magma heterogeneity occurred on at least three scales—the individual rock unit, the complex and across the region. This means that the Selwyn Block, the logical source of the ignimbrite magmas, is highly heterogeneous, probably in three dimensions. The statistically significant differences in the ages of the various complexes suggest that the heat sources for partial melting of the deep crust were themselves rather local in extent, and probably reflected numerous asynchronous intrusions of mantle-derived mafic magmas, most likely in relatively small batches. Thus, there should be no thick underplated or intraplated layer of gabbroic rock in the deep crust here. Thinner bodies of hot mafic magmas were probably emplaced in different places and at different levels, inducing local high-grade metamorphic conditions. Published radiometric dates for various volcanic units suggest that the magmatic heating of the crust occurred over a period of at least 17 Myr, although maximum intensity occurred between ca 375 and ca 371 Ma. Thus, a model of the deep crust here should involve numerous, perhaps thin metasedimentary units with highly variable ages and original clay contents, as well as sills of Late Devonian mafic rocks scattered both vertically and horizontally and intruded episodically. The dominantly meta-igneous sources of crustally derived I-type magmas generally lie at higher structural levels, where they did not experience metamorphic temperatures sufficient to cause the higher-temperature melting reactions needed to form I-type volcanic magmas. Each Late Devonian, S-type, volcanic magma in central Victoria formed through melting of a distinct assemblage of crustal source rocks, most likely within the Selwyn Block. The rhyolites formed independently of the rhyodacitic to dacitic magmas, not as differentiates. The mafic magma heat sources for partial melting of the deep crust were emplaced as numerous, relatively small batches, in sheets distributed horizontally and vertically. The dominantly meta-igneous and shallower crustal sources of I-type magmas did not experience the higher temperatures required to form I-type volcanic magmas. [ABSTRACT FROM AUTHOR]

The morphology and development of several submarine canyons offshore southeast South Australia and western Victoria are described. The existence of three of those canyons had been foreshadowed in 1963 by N. Boutakoff, who thought them to be linked to ancient courses of the Glenelg River. These canyons occur on the outer continental shelf where their heads are situated in depths shallower than 1000 m. Sinuous channels are visible within two of the canyon heads, indicating that water and sediment may still travel downslope and cause erosion, and thus are geologically recent. Several other canyons are also documented and named. Two canyons are confined to depths below 3000 m; they may be much older and seem not to be linked to ancient river courses. They are also oblique to the upper canyons. The other characteristic feature of the area is the presence of numerous undersea slides. These occur at three specific depths (∼1200, ∼1500 and ∼1800 m) and are coincident with stratigraphic horizons in which continental groundwater flows have been identified in adjacent exploratory oil and gas wells drilled into the continental shelf. Sapping of groundwater may likely have occurred during very wet periods inland. We suggest that these undersea slides could be the first step in the formation of deep-sea canyons that are not necessarily linked to ancient river courses. We also postulate that the 'sliding' of large piles of sediment down the continental slope has tsunamigenic potential and may have occurred during significant wet climate on land. Several deep-sea canyons and numerous submarine slides are documented along the continental shelf offshore the coastline of western Victoria and southeast South Australia in the Otway Basin. Some of these canyons are linked to previous courses of the Glenelg River, which changed over time in response to regional tectonic changes such as faulting and significant upwarping. The orientation of most of these canyons tends to align with ancient terrestrial lineaments that likely extend offshore. Sinuous channels within the upper canyon reaches are clearly visible and not infilled with sediment; they are likely active and linked to ongoing groundwater seepage. It is proposed that the underwater slides are generated by the submarine discharge of continental groundwater on the continental slope, at three specific water depths with discharge associated with particular lithologies. These slides may still be active. Superimposed slides may engender the formation of new canyons. The volume of some of these undersea slides is of tsunamigenic scale. [ABSTRACT FROM AUTHOR]

The monzogranites and syenogranites of the Early Devonian (Emsian), post-tectonic, high-level, S-type Wilsons Promontory batholith contain magmatic garnet and cordierite, in addition to biotite, as well as biotite–quartz pseudomorphs after early orthopyroxene. The batholith forms the southern tip of the Australian mainland and consists of four penecontemporaneous plutons, emplaced at shallow crustal depth, intruding Ordovician to Lower Devonian lower greenschist-facies, quartz-rich metaflysch rocks in the Bassian tectonic zone. The magmas were initially H2O-undersaturated and at T > 800 °C, having been generated by partial melting of greywacke-dominated, Paleoproterozoic to Mesoproterozoic (ca 1.6 Ga) basement rocks in the Selwyn Block (the northern tip of the VanDieland microcontinent). However, the specific source rocks differ from those of the S-type granitic plutons in the neighbouring Melbourne tectonic zone. This probably reflects compositional variations within the Selwyn Block, which has been proposed to form the basement terrane beneath both the Melbourne and Bassian zones. The compositional zonation in the batholith is due to sequential intrusion of separate magma batches rather than in situ differentiation, and the main mechanism responsible for primary chemical variations in the rocks appears to have been peritectic assemblage entrainment. Some local differentiation also occurred, probably by filter pressing, to produce aplites and tourmaline-bearing pegmatites. Normal granitic rocks (i.e. not aplites or pegmatites) with SiO2 >76 wt% were not produced through differentiation of other granitic magmas in the batholith. At the global scale, this suggests that fractionation models for the origins of high-silica granitic and rhyolitic magmas may need revision. S-type magmas of the Wilsons Promontory batholith were H2O-undersaturated and at T > 800 °C. They were generated by partial melting of greywacke-dominated, Paleoproterozoic to Mesoproterozoic (ca 1.6 Ga) basement. The sources differ from those of the plutons in the Melbourne Zone, suggesting heterogeneity in the Selwyn Block. Most compositional variation is due to intrusion of separate magma batches and peritectic assemblage entrainment, not differentiation. [ABSTRACT FROM AUTHOR]

Rather than being inherited as detrital grains, many of the rounded and/or embayed cores in zircon crystals from granitic rocks are probably antecrysts, formed early in the magmatic systems. Using laser-ablation, sector-field ICP-MS, we obtained internally consistent weighted-mean concordia ages of 375.3 ± 2.5 Ma and 376.9 ± 2.6 for early crystallisation of two samples from the previously undated Mount Disappointment pluton in the Melbourne Zone. Within uncertainty, our date of 376.4 ± 2.4 Ma for the Baringhup pluton of the Harcourt batholith is the same as the published and our new date for the Mount Alexander pluton, in the same batholith. Using the same techniques, we produced an improved date of 370.9 ± 6.5 Ma for the I-type Ercildoun Granite in the neighbouring Bendigo Zone of the Lachlan Fold Belt. We also obtained dates that confirm published ages for the Mount Bute Granite in the Stawell Zone, the Tynong pluton of the Tynong batholith in the Melbourne Zone and the Oberon pluton of the Wilsons Promontory batholith in the Bassian Zone. These dates confirm the Late Devonian (Fransian) age of most of this widespread plutonism, as well as the reported Emsian age of the Wilsons Promontory batholith. However, part of what has been mapped as the Mount Wombat pluton of the Strathbogie batholith could be significantly older than the rest. Many rounded or embayed cores in zircon crystals are antecrysts rather than inherited detrital grains. The Mount Disappointment pluton is confirmed as Late Devonian in age, at 375.2 ± 2.5 Ma. At 378.2 ± 2.0 Ma, the Baringhup pluton of the Harcourt batholith is the same age as the Mount Alexander pluton, in the same batholith. Part of the Mount Wombat pluton in the Strathbogie batholith may be older than the rest of the pluton. [ABSTRACT FROM AUTHOR]

The southern boundary of the high level, discordant, Devonian Strathbogie batholith in central Victoria includes two near-linear sections that strike east–west, and one north–south. Field mapping has delineated the 100 km southern boundary with 2200 waypoints taken where granite and hornfels are within 10 m of one another. There is 790 m of topographic relief along the southern boundary and, where the mapping is overlain on contours, there are intervals that reflect a steep-dipping contact and others that reflect near-horizontal contacts. Of the latter, some reflect the granite roof and others the floor. Complementary evidence from mineralogic and textural data indicates a strong correlation between the interpreted roof of the granite and the abundance of tourmaline, aplite and pegmatite. When combined, the roof and floor intervals indicate a batholith thickness up to 350 m; nowhere along the southern boundary can a thickness of 1 km be demonstrated. Along the southern boundary, at least, the batholith shape is a thin, tabular and extensive composite granite sheet analogous to a pizza rather than a pipe. The roof of the granite is nearly horizontal in both east–west and north–south sections, and the plateaux today are close to the batholith roof. On regional and local scales, granite emplacement was determined by pre-existing anisotropies, such as master joint sets, across which the propagation of magma-filled fractures (granite sheets) was arrested. The emplacement records a fine balance between magma pressure, and the interplay of lower confining (overburden) pressures and pre-existing anisotropies that also facilitated roof uplift. The opportunistic magmas used the slightest variations of those parameters. This interpretation of the Strathbogie batholith matches examples in the Himalaya and Chilean Andes but differs markedly from the portrayal of very large batholiths beneath Cu–Au porphyry systems. The boundary of the Strathbogie batholith can be mapped in moderately outcropping terrain to an accuracy of 10 m or better. Steep intervals of the boundary are interspersed with near-horizontal intervals that delineate the granite floor and roof along the 100 km of the southern boundary. The batholith, at least near its southern boundary, comprises near-horizontal sheets of <500 m aggregate thickness. The boundary determined by geophysics is adequate for regional studies but has been misleading in three-dimensional interpretations. [ABSTRACT FROM AUTHOR]

A fully integrated 3D basin model was built to study the lithospheric structural evolution and the thermal history of the offshore part of the Gippsland Basin to investigate the type, timing and magnitude of its initial opening mechanism and their effects on source rocks. The applied workflow included seismic interpretation of major faults and magmatic features, seismic reinterpretation of existing main stratigraphic boundaries and adjustment of the basement topography. Different scenarios for a McKenzie-type basal heat flow evolution were tested based on a two-step inversion approach, which translates the thickness changes of lithospheric layers into basal heat flow. Finally, a crustal layer model was assigned to the existing 3D numerical basin model and the effects on thermal maturation of petroleum source rocks were investigated. The results of the calibrated inversion model suggest intense lithospheric stretching and mantle upwelling during the Early Cretaceous caused initial breakup and subsidence of the Gippsland Basin. This syn-rift phase was accompanied by increasing basal heat flow from initially ∼50 mW/m2 to a maximum of 80–90 mW/m2. The resulting upwelled Moho with an associated crustal thickness of locally only 8 km agrees with geophysical observations. Elevated paleogeothermal gradients between 50 and 60 °C/km prevailed from ca 120 to 80 Ma. Episodes of compression, uplift, erosion, and thermal subsidence resulted in fast and continuous cooling during the early post-rift and a constant basal heat flow of 45–50 mW/m2 since the Eocene. Two main pulses of petroleum generation were identified in the Paleocene and in the Miocene–Pliocene. This novel two-step crustal layer inversion method allows a detailed reconstruction of the thermal history of rift basins and can provide useful information about the maturation of petroleum source rocks. A temperature history reconstruction of the Gippsland Basin is given based on a large-scale numerical 3D model. Tectonically induced strong variability in paleoheat flows is recognised. Strong maturation and petroleum generation developed during the Paleogene and Neogene. [ABSTRACT FROM AUTHOR]

Paleomagnetic studies in the Lachlan Orogen have indicated rotation of elements within it, but the evidence for rotation is limited and has significant statistical uncertainty. This study presents a detailed paleomagnetic investigation, assisted by X-ray analysis of magnetic mineralogy of middle Cambrian to lower Silurian sedimentary and igneous rocks in Victoria and New South Wales. Four demagnetisation behaviours (types A, B, C and D) have been recognised. Type A is characterised by a single component, resulting in an approximately linear path over the magnetite unblocking temperature range. Type B specimens exhibit slow progressive demagnetisation over a wide temperature range, with clustering of demagnetised directions above the magnetite Curie temperature, indicating hematite. Behaviour C is also characterised by clustering of directions, but this extends over a wider temperature range. Samples with behaviour D show noisy and non-informative demagnetisation, indicating very low magnetic stability. Hysteresis data confirm that samples with behaviour B and C have high coercivities and remain unsaturated above 300 mT. Mineral liberation analysis of selected samples indicates presence of iron oxides, magnetite, titanomagnetite, botryoidal hematite and iron hydroxides. Only three sites yielded specimens with type A primary magnetisation, with a poorly defined pole from the Dookie Volcanics on the central limb of the orocline, which does match a previous result from the same district. Two paleopoles, of type B and C specimens from the rocks in Tabberabbera Zone and the Rockley Volcanics, plot within the confidence limit of poles defining the late Silurian to Early Devonian Australian apparent polar wander path. These poles match other poles representing Devonian overprints around the margins of the Lachlan Orogen that have been interpreted to be the result of alteration mineralisation produced by fluids driven by tectonic compression during events in the Bindian (420–410 Ma) or early Tabberabberan (405–380 Ma) orogenies. Paleomagnetic techniques have been used to test the rotation within the elements of the Lachlan Orocline in southeast Australia. Results suggest that the original remanence in the rocks is obscured owing to post-deformation overprint. This overprint is widespread in the Lachlan Orogen and is associated with the deformation during Bindian/Tabberabberan orogenic events. [ABSTRACT FROM AUTHOR]

Examines the timing relationships between felsic magmatism and mineralization in Victoria. Pulses of post-tectonic magmatism in the area; Geodynamic frameworks of the Victorian Lachlan Fold Belt.

Holocene coastal infill was examined in Anglesea Estuary, an intermittently open barrier-type estuary in Victoria. Based on coring and radiocarbon dating, it was found that accommodation space in the estuary was almost entirely occupied in the early–mid Holocene corresponding to periods of higher sea-level. The pattern of infill is in stark contrast to established models of estuarine sedimentation developed on the east Australian coast. The contemporary estuary is composed of two distinct sedimentary basins. The upper basin is infilled by autochthonous peaty-marsh sediments, whereas the lower basin is infilled by saltmarsh material. The traditional tripartite flood-tide delta–central mud basin–fluvial delta sequence is absent. The contemporary estuary appears to have evolved as two distinct basins, which are connected through hydrodynamic rather than sedimentological processes. This pattern of connected basins occurs from the top of the catchment to the wave base, with the contemporary estuary infilling the two basins coincident with modern sea-level. Low sediment supply from both terrestrial and marine sources and bedrock-limited accommodation space means that Anglesea can be considered to be geologically constrained barrier estuary. Geological-control limits mud basin development. Low allochthonous sediment supply significantly affected estuarine facies in Anglesea. Anglesea Estuary infilled to higher mean sea-level elevations in the mid Holocene. Anglesea Estuary is composed of two distinct sedimentary basins. [ABSTRACT FROM AUTHOR]

Information, mainly from the granitic and silicic volcanic rocks in the Stawell, Bendigo and Melbourne structural zones in the state of Victoria, shows that the sources of both the S- and I-type rocks of the Stawell and Bendigo zones (SBZ) contrast in ages and chemistry with the sources of similar granitic rocks in the Melbourne Zone, consistent with the absence of the mainly Proterozoic Selwyn Block beneath most of the SBZ. Below a mid-crustal décollement in the SBZ, the crust is evidently highly variable and possibly includes thinned Proterozoic crust. There is geochronological evidence for ca 400 and ca 370 Ma granulite-grade metamorphic events here, and, after this double bout of metamorphism, and depletion in the silicic melt component, the constituents of the entire deep crust of the SBZ would have densities similar to those of overlying, much lower-grade Cambrian metabasaltic to boninitic rocks. Thus, granitic magmas may have formed here by partial melting of a variety of rock types, probably with back-arc affinities, with ages that may extend back to the Proterozoic. Therefore, the basement of the SBZ is unlikely to consist solely of thick ocean-floor rocks, as in some current interpretations. The sources of the Devonian granitic rocks of the Stawell and Bendigo zones (SBZ) contrast in ages and chemistry with those of the Melbourne Zone granites. Two Devonian granulite-facies events left the melt-depleted deep SBZ crust with densities similar to those of overlying Cambrian metabasaltic rocks. The SBZ Devonian granitic magmas probably formed by partial melting of heterogeneous Proterozoic to Cambrian arc-related crust, below the mid-crustal décollement. [ABSTRACT FROM AUTHOR]

The Proterozoic to Cambrian VanDieland microcontinent was accreted to mainland Australia at ca 400 Ma, and its northern tip, the Selwyn Block, forms the basement in central Victoria. Here, mainly Late Devonian, silicic magmas were derived from the Selwyn Block and intruded into the shallow crust. We use the phase petrology of Late Devonian, S-type rhyolitic ignimbrites and a xenolith of pelitic migmatite, together with Nd-model ages for the silicic magmatic rocks to constrain the lithological characteristics of the metasedimentary component of the Selwyn Block, to infer minimum depths and temperature conditions here in the Late Devonian, and the likely ages of the source rocks for the S-type magmas. The most abundant source rocks are inferred to be volcaniclastic metagreywackes, with minor metadacites, meta-andesites and metapelites. The metapelitic xenolith cannot have been the source for any of the silicic magmas but constrains the upper amphibolite-facies part of the Selwyn Block to depths around 17 km, where temperatures reached ∼775 °C. The older ignimbrite magma was formed by partial melting at perhaps 770 °C and a depth of at least 33 km, while the younger ignimbrite magma formed at ∼23 km and 900 °C. These depths suggest source rocks in the Paleoproterozoic to Mesoproterozoic lower parts of the Selwyn Block. Nd-model ages of the silicic magmatic rocks confirm a dominance of Mesoproterozoic to Paleoproterozoic sources. If the inferred rock types in the Mesoproterozoic formations were as current correlations suggest, the sources for the Late Devonian silicic magmas would have to lie in the undocumented Paleoproterozoic basement of the Selwyn Block. Rock types here must include andesitic to dacitic volcanic components as well as volcaniclastic greywackes and minor pelites, which suggests a continental arc setting. The Late Devonian magmatism in the region may record the progression from amphibolite- to granulite-facies conditions during post-orogenic extension, with heat advected to the crust by mantle-derived mafic magmas. These processes would have resulted in mafitisation of the deep Selwyn Block. [ABSTRACT FROM AUTHOR]

In Branch ([1]), describing the upper Paleozoic volcanic cauldrons, ring complexes and associated granites of the Georgetown Inlier, North Queensland, I invoke a mechanism of underground cauldron subsidence for the formation of both surface volcanic cauldrons filled with ignimbrite, and sub-surface flat-topped granite batholiths that intrude at or near the bottom of the volcanic pile. ([5]) have shown that the Strathbogie batholith is a relatively thin tabular body of granite intruded at a shallow depth and provide two possible interpretations of its vertical profile preferring a sheet-like form with a central feeder pipe. As the lower magma chambers were depleted, large crustal blocks subsided, allowing up to a kilometre or more of ignimbrite sheets to accumulate in rift valley-like structures at the surface (referred to as cauldrons). [Extracted from the article]

HyLogger hyperspectral data obtained from seven orogenic gold deposits in central Victoria, including Bendigo, Ballarat, Maldon, Fosterville, Costerfield, Castlemaine and Wildwood, are presented. The data demonstrate that fresh diamond drill core displays substantial mineralogical variation that can be attributed to the effects of cryptic hydrothermal alteration that might not otherwise be recognised. The most significant hyperspectral response lies in the white mica compositions, which vary in a systematic manner between high-Al muscovitic zones (Al–OH absorption around 2208 nm) that define a phyllic alteration halo around mineralised structures, and low-Al phengitic–chlorite zones (Al–OH absorption >2014 nm) inferred to represent either more distal alteration or possibly regional metamorphic background. An extensive ferroan dolomite alteration halo overlaps the phyllic and sulfidic alteration zones and extends beyond the sampled core in most instances. This ferroan dolomite halo has previously been defined petrographically, geochemically and using carbonate staining techniques, and is further characterised using thermal infrared hyperspectral data in drill core from the Ballarat goldfield. The mineralogical trends identified by the hyperspectral data are best developed in diamond drill core from the Costerfield, Fosterville and Ballarat goldfields, and are less pronounced at the other deposits. At Bendigo and Castlemaine the reasons for this are not immediately clear, but may be related to the close timing of gold mineralisation relative to peak metamorphism. The Maldon area lies within the contact aureole of the Harcourt Batholith and so has been thermally overprinted leading to the recrystallisation of earlier hydrothermal assemblages. The Wildwood deposit is similar to the Magdala deposit at Stawell and differs from the other goldfields in its geological setting, host rock lithologies and style of hydrothermal alteration, with the development of Fe-rich chlorite closely associated with gold mineralisation. The results demonstrate how hyperspectral data can be used to define large hydrothermal alteration footprints associated with orogenic gold mineralisation in central Victoria that are of direct benefit to mineral explorers, as well as independently characterising lithological variations in drill core. [ABSTRACT FROM AUTHOR]

The molybdenite and scheelite mineralisation in the Native Dog Pluton at Monkey Gully near Yea is hosted within an I-type (post-orogenic) pluton, which shows extensive fractionation and magma mixing, and was emplaced in an extensional environment. The pluton comprises four principal rock types: tonalite, granodiorite, dacite and leucogranite. Emplacement of the pluton was in an extensional northwest to southeast paleostress field. Early extensional quartz veins, related to cooling, are overprinted by both dacitic dykes and late-stage quartz sheeted veins. The late-stage veins host the molybdenum and tungsten mineralisation in the deposit.206Pb/238U zircon ages of 356 ±14 Ma and 375 ± 22 Ma place pluton formation and mineralisation at the onset of the Kanimblan Orogeny and later than other major molybdenum deposits in Victoria. Key factors governing the source for the granite and its associated mineralisation are: (1) the presence of a highly fractionated and sulfur-rich leucogranite; and (2) the pluton's location in a regional jog overlying the Selwyn basement block. Finally, a model is developed to explain the differences between this Melbourne Zone molybdenum and tungsten deposit compared with other metallogenic porphyry deposits. [ABSTRACT FROM AUTHOR]

Presents a discussion on the evolution of deep-lead palaeodrainages and gold exploration at Ballarat, Victoria. Development of the landscape around Ballarat in the Early Cenozoic period; Significance of Permian glaciation.

Hylogger-2 reflectance spectroscopy data from two drill holes at the Leven Star gold deposit, provides an insight into the hydrothermal alteration associated with vein sets and faults that host gold mineralisation. The metasedimentary host rock is a biotite-rich hornfels, which overprints a regional greenschist facies metamorphism. Gold mineralisation is associated with quartz (±carbonate) stockwork-breccia zones. HyLogging short-wave infrared reflectance spectra detected carbonates, chlorites, epidote (propylitic: carbonatisation, chloritisation, epidote), muscovite (phyllic/sericitic) and kaolinite, illite, ammonium alunite, montmorillonite and dickite (intermediate argillic alteration) assemblages that were confirmed using petrographic observations, drill core and geochemical data. The gold occurs within a sulfidation halo with carbonate phases (ankerite–siderite–dolomite) in structurally controlled veins and faults, which also host a late Sb–As–Bi–Cu–W assemblage, attributed to an intrusion-related style of mineralisation superimposed on a hydrothermal orogenic gold event. [ABSTRACT FROM AUTHOR]

Among the Devonian igneous complexes of central and northeastern Victoria, the S-type Tolmie Igneous Complex (TIC) occupies a unique spatial position, astride the Governor Fault and the Mount Wellington Greenstone Belt, a probable terrane boundary, and also bridges the boundary between the Strathbogie and Tabberabbera granite provinces. This stitching position makes it important to determine the nature of the crustal source involved in its genesis. Here we present a new U–Pb zircon age for a sample of the Toombullup Ignimbrite, which forms a large part of the TIC. This age (377 ± 2 Ma) allows us to comment far more reliably on the initial Sr and Nd isotope ratios of the TIC magmas. The corrected data (initial87Sr/86Sr = 0.70804 to 0.71085, εNdt= –0.08 to 0.44) confirm a heterogeneous sediment-dominated protolith, but unlike those of other S-type granitic and felsic volcanic rocks of the region, one composed of materials that had not long previously been extracted from the mantle. This finding has importance in the context of future models for the geological and tectonic development of this part of southeastern Australia. [ABSTRACT FROM PUBLISHER]

Alluvial-channel and overlying eolian deposits exposed in coastal cliffs in the Port Campbell australite (Australian tektite) strewn field, SW Victoria, were studied stratigraphically and dated by optically stimulated luminescence (OSL) for quartz sand grains to illuminate the occurrence of australites in this area. The channel deposits, termed the Dog Trap Bay Sand, yielded OSL ages from 420 ± 40 to 230 ± 15 ka for four sites, and the two eolian units OSL ages of 203 ± 16 and 93 ± 11 ka. The ages support field evidence that, contrary to the conclusions of a previous study by others, the channel deposits are stratigraphically distinct from and much younger than the Pliocene Hanson Plain Sand, which forms a regionally extensive system of marine strandlines to the north and NE of Port Campbell. The OSL ages show some preference for times of negative temperature anomalies marking the last four glacial periods of the Middle and Late Pleistocene as indicated by ice-core data from Antarctica. Eolian and at least some alluvial deposition evidently occurred when sea-level was low and rivers extended across the exposed continental shelf. Others have suggested that the Dog Trap Bay Sand contains rare australite fragmentsin situ. However, these occurrences may have resulted from reworking of sandy material, with the australites possibly derived from overlying younger units known to contain numerous specimens. The OSL ages imply that a discrepancy ofca350 ka or more exists between the apparent stratigraphic occurrence, as distinct from the terrestrial age, of australites in the Port Campbell strewn field and the australite40Ar/39Ar age of 770 ± 20 ka. Stratigraphic evidence in this area for the terrestrial age of australites is inconclusive. [ABSTRACT FROM PUBLISHER]

The western sub-province of the Paleozoic Lachlan Orogen in Victoria is dominated by thick turbidite sequences overlying Cambrian basement volcanics. The region was subjected to multiple Cambrian to Late Devonian regional deformation events, followed by extensive post-tectonic granitic magmatism. The western Lachlan Orogen is considered a typical ‘orogenic’ gold province and hosts a large number of goldfields, including the world-class Bendigo–Ballarat goldfields. A variety of geochronological methods (e.g. U–Pb zircon; Re–Os sulfide; 40Ar/39Ar whole-rock, mica) have been used to constrain the timing of gold mineralisation, and the relationship to metamorphism/deformation/magmatism. Regional granitic magmatism is relatively well constrained from U–Pb zircon dating, with the timing of deformation/metamorphism and gold mineralisation reliant largely on 40Ar/39Ar dating results. Owing to inconsistencies in the available 40Ar/39Ar data and recent revisions to 40Ar/39Ar monitor ages and decay constants, we recalculate and re-evaluate all existing 40Ar/39Ar age results. These revisions confirm that the western Lachlan Orogen is characterised by multiple deformation/metamorphism events, with the Stawell structural zone deformed during the ca 500 Ma Delamarian and ca 445 Ma Benambran orogenies, the Bendigo Zone deformed during the Benambran orogeny (with minor Tabberabberan overprinting), and the Melbourne Zone affected by the ca 380–370 Ma Tabberabberan orogeny. Post-tectonic granitic magmatism occurred in two main time intervals, the Early Devonian (ca 400 Ma) and the Late Devonian (ca 380–370 Ma), with the former limited to the Stawell and northwest Bendigo Zones, and the latter distributed throughout the Bendigo and Melbourne Zones and southeast Stawell Zone. Gold mineralisation occurred in two main episodes at ca 445 Ma and ca 380–370 Ma, with another possible (minor) event at ca 410–400 Ma. The ca 445 Ma event is prevalent across the Stawell and Bendigo Zones, with Late Devonian gold mineralisation restricted to the Melbourne and eastern Bendigo Zones. The timing of the two main events is supported by geological constraints, the reproducibility of 40Ar/39Ar results and, in the case of the Bendigo goldfield, coincidence with Re–Os data. Suggestions of a single Devonian age gold mineralisation event are not supported by the available data. The two main gold mineralisation episodes (ca 445 Ma; ca 380–370 Ma) coincide with the waning stages of the Benambran and Tabberabberan orogenies, respectively. Crustal thickening and consequent metamorphic devolatilisation during the Benambran orogeny may have been the main cause of fluid flow related to gold mineralisation at ca 445 Ma. In contrast, crustal anatexis is considered responsible for metamorphic fluid generation and Early Devonian gold mineralisation. [ABSTRACT FROM PUBLISHER]

The Lake Boga Granite in northern Victoria contains a suite of well-crystallised secondary uranyl phosphates, including torbernite, saléeite, metanatroautunite and ulrichite. The minerals crystallised in miarolitic cavities and on joints, after dissolution of primary uraninite and fluorapatite by oxidising groundwater. A more restricted assemblage occurs in the nearby Wycheproof Granite. The 230Th/U dating method was used to reveal any links between formation ages of these minerals and climatic fluctuations during the Quaternary. Out of 104 analyses, 77 gave apparent ages less than about 500 ka, the upper limit beyond which secular equilibrium in the 230Th/U decay series is closely approached. The age distribution curve shows a broad peak around 400 ka, coinciding with a global interglacial period (Marine Isotope Stage 11). Thereafter, there is a slow tapering off towards younger ages, but with no direct correlation with interglacial peaks and with no ages less than 100 ka. Throughout this time interval (ca 500–100 ka), the southeastern Australian region was undergoing aridification, suggesting that regional rather than global climate was the more significant influence on uranium phosphate crystallisation. While caution needs to be applied when interpreting these results, the overall distribution pattern might be best explained by fluctuating weathering affecting a finite primary uranium source that was progressively depleted from about 400 ka and exhausted by around 100 ka. The study has also revealed that the Lake Boga and Wycheproof granites acted as natural reservoirs for the sequestration of uranium in phosphate-rich assemblages over periods of several hundred thousand years. [ABSTRACT FROM AUTHOR]

The Late Miocene-Pliocene Loxton Sand strandplain extends across the western part of the Murray Basin in southeastern Australia. Gamma logging on groundwater bores surrounding Lake Tutchewop, which lies close to the eastern limit of this strandplain, showed that a heavy mineral layer within the Loxton Sand is displaced across the northern extension of the Leaghur Fault. A crevasse splay sand within the overlying Pliocene Shepparton Formation is also displaced, indicating that tectonic activity along the Leaghur Fault occurred in the Early Pliocene. This coincides with the Kosciusko Uplift, a major phase of Neogene tectonism across southeastern Australia, which also resulted in movement along the Danyo, Hindmarsh, Tyrell and Avoca Faults in northwestern Victoria. [ABSTRACT FROM AUTHOR]

The Alligator Rivers Uranium Field (ARUF) includes the mined and unmined Jabiluka, Ranger, Koongarra and Nabarlek unconformity-related uranium deposits and several small prospects including the newly discovered King River prospect. Uranium mineralisation is hosted by a variety of metamorphosed Nimbuwah Domain lithologies that are unconformably overlain by the Kombolgie Subgroup, a basin package of unmetamorphosed arenites and mafic volcanics. All of the uranium deposits and prospects preserve an identical alteration assemblage that is subdivided into a distal and proximal alteration zone. The distal alteration zone comprises an assemblage of sericite and chlorite that replace albite and amphibole. In some cases, this alteration can be traced >1000 m from the proximal alteration zone that is dominated by uraninite, hematite, chlorite and sericite. Uranium precipitated in the basement as uraninite at 1680 Ma at around 200°C from a fluid having δ18Ofluid values of 3.0±2.8‰ and δDfluid values of -28±13‰ VSMOW reflecting an evolved marine source. These geochemical properties are indistinguishable from those recorded by diagenetic illite and chlorite that were collected from the Kombolgie Subgroup sandstones across the ARUF. The illite and chlorite formed in diagenetic aquifers, and where these aquifers intersected favourable basement rocks, such as those containing graphite or other reductants, U was precipitated as uraninite. Therefore, it is proposed that the Kombolgie Subgroup is the source for fluids that formed the deposits. A post-ore alteration assemblage dominated by chlorite, but also comprising quartz±dolomite±sulfide veins cut the uranium mineralisation at all deposits and has historically been recorded as part of the syn-ore mineralisation event. However, these minerals formed anywhere between 1500 to 630 Ma from fluids that have distinctly lower δ18Ofluid values around 1.5‰ and lower δDfluid values around -45‰ reflecting a meteoric water origin. Despite unconformity-related uranium deposits having a large alteration halo, they remain difficult to find. The subtle alteration of albite to sericite several hundred metres from mineralisation occurs in isolation of any increase in trace elements such as U and radiogenic Pb and can be difficult or impossible to identify in hand specimen. Whole rock geochemical data indicate that Pearce Element Ratio (PER) analysis and General Element Ratio (GER) analysis may vector into this subtle alteration because it does not rely on an increase in trace elements to identify proximity to ore. PER and GER plots, Al/Ti vs (2Ca + Na + K)/Ti, Na/Al vs (Na + K)/Al, K/Al vs (Na + K)/Al and (Fe + Mg)/Al vs (Na + K)/Al provide a visual guide that readily distinguish unaltered from altered samples. A plot of (Na + K)/Al and (Fe + Mg)/Al on the x-axis against the concentration of trace elements on the y-axis reveals that U, Pb, Mo, Cu, B, Br, Ce, Y, Li, Ni, V and Nd are associated with the most intensely altered samples. The lithogeochemical vectors should aid explorers searching for uranium mineralisation in a prospective basin environment, but exploration must first focus on the characteristics of the basin to assess its mineralisation potential. A holistic model that describes the evolution of the Kombolgie Subgroup from deposition through diagenesis to formation of the uranium deposits in the underlying basement rocks is presented and has application to other basins that are considered prospective for unconformity-related uranium deposits. The model outlines that explorers will need to consider the thickness of the sedimentary pile, its lithological composition relative to depositional setting, the depth to which the sediments were buried during diagenesis and the degree of diagenesis achieved, which may be time dependant, before deciding on the prospectivity of the basin. [ABSTRACT FROM AUTHOR]

The passive southern margin of the Australian continent contains a rich record of late Miocene-Pliocene neotectonic deformation and uplift that continues to the present day as witnessed by unusually high levels of seismicity for a so-called 'stable continental region.' To date, however, few studies have sought to estimate the magnitude of exhumation triggered by this deformation and uplift. Here we combine apatite fission track analysis (AFTA), apatite (U-Th)/He dating and vitrinite reflectance (VR) data from the Nerita-1 well in the Torquay sub-basin with seismic reflection data from this basin and the adjoining Otway Ranges to constrain the magnitude and driving mechanisms of exhumation in this part of the southern Australian margin. The Cenozoic succession in this basin has been deformed by folding and reverse faulting and contains a major, low-angle mid-Miocene unconformity that can be traced for distances of ∼1500 km along the margin. Paleothermal data from Nerita-1 show that the sub-mid-Miocene succession has been more deeply buried by ∼1 km of now missing section, and indicate that exhumation began between 10 and 5 Ma, in excellent agreement with stratigraphic constraints. Our estimates of removed section and higher than previous estimates based on extrapolation of seismic reflectors, but are corroborated by AFTA results from nearby wells. Seismic data show that late Miocene-onwards intraplate deformation in the Torquay-sub-basin and Otway Ranges has been accomplished by reverse-reactivation of normal faults within Cretaceous-early Paleogene syn-rift successions, resulting in folding of overlying post-rift late Paleogene-Neogene sediments. The probable cause of this deformation and uplift is increased levels of intraplate stress induced by enhanced coupling of the Indo-Australian and Pacific plates ∼10 Myr ago, and our results thus demonstrate the key role that plate boundary-controlled stress fields play in intraplate uplift and deformation. [ABSTRACT FROM AUTHOR]

Imagery of meandering river-like channel features up to 5 m deep and 100 m wide on the floor of Port Phillip Bay have been revealed by recent multibeam surveys. They are in water depths to -22 m below the present bay water level. This young channelling overlies older infilled Pleistocene-early Holocene channels considered to be cut by the Yarra and Werribee rivers during the last glacial or previous low stand minima >10 000 cal. yr BP and infilled during the Holocene transgression. Vibrocoring data indicate the young channels incise shelly mud. Twelve 14C shell dates in vibrocores from bay floor to 3.5 m are in two units-a lower unit range between 2880 cal. yr BP at 0.7 m to 9491 cal. yr BP at 3.5 m, and an upper unit ∼0.5 to 0.7 m thick <869 cal. yr BP forming the present bay floor. Seismic and vibrocore evidence suggests the post-869 cal. yr BP upper unit also covers the base of the young channels. This evidence implies that Port Phillip dried out to -22 m between ∼2800 and ∼1000 cal. yr BP at a time of stable present day sea-levels. The most likely cause is a sand blockage to the Port Phillip Bay channel entrances, coupled with high evaporation rates. Palynological and foraminiferal evidence suggests variable salinities occurred immediately prior to the sedimentary hiatus, with restoration of marine conditions after. The results have wide implications on Port Phillip's water balance, salinity, pre-European climatic events and Aboriginal occupation. [ABSTRACT FROM AUTHOR]

A ∼400 km long deep crustal reflection seismic survey was acquired in central Victoria, Australia, in 2006. It has provided information on crustal architecture across the western Lachlan Orogen and has greatly added to the understanding of the tectonic evolution. The east-dipping Moyston Fault is confirmed as the suture between the Delamerian and western Lachlan Orogens, and is shown to extend down to the Moho. The Avoca Fault, the boundary between the Stawell and Bendigo Zones, is a west-dipping listric reverse fault that intersects the Moyston Fault at a depth of about 22 km, forming a V-shaped geometry. Both the Stawell and Bendigo Zones can be divided broadly into a lower crustal region of interlayered and imbricated metavolcanic and metasedimentary rocks and an upper crustal region of tightly folded metasedimentary rocks. The Stawell Zone was probably part of a Cambrian accretionary system along the eastern Gondwanaland margin, and mafic rocks may have been partly consumed by Cambrian subduction. Much of the Early Cambrian oceanic crust beneath the Bendigo Zone was not subducted, and is preserved as a crustal-scale imbricate thrust stack. The seismic data have shown that a thin-skinned structural model appears to be valid for much of the Melbourne Zone, whereas the Stawell and Bendigo Zones have a thick-skinned structural style. Internal faults in the Stawell and Bendigo Zones are mostly west-dipping listric faults, which extend from the surface to near the base of the crust. The Heathcote Fault Zone, the boundary between the Bendigo and Melbourne Zones, extends to at least 20 km, and possibly to the Moho. A striking feature in the seismic data is the markedly different seismic character of the mid to lower crust of the Melbourne Zone. The deep seismic reflection data for the Melbourne Zone have revealed a multilayered crustal structure that supports the Selwyn Block model. [ABSTRACT FROM AUTHOR]

Over 2% of total world gold production has come from the Victorian gold fields; however, most of the shallow deposits have now been discovered, and new methods are required for exploration beneath the thick sedimentary cover of the Murray Basin. Consequently, as part of the GeoScience Victoria Gold Undercover Initiative, new magnetotelluric (MT) data have now been obtained to provide a greater understanding of potential gold bearing structures extending to mid-crustal and greater depths. Coinciding with part of the 2006 Central Victorian Seismic Survey, a 170 km long MT transect lies within the Bendigo and Melbourne Zones, two of the fault bounded subdivisions of the western Lachlan Fold Belt. Additional transient electromagnetic (TEM) data have now been obtained at each of the MT sites allowing further processing, including compensation for static shifts that are significant in this area. The revised MT section provides an image consistent with the seismic section. The major fault zones align with several dipping conductive slabs supporting the orientation and depth extent revealed by the seismic profile. There is some evidence in the MT data that near-surface geological structures can be extrapolated to mantle depths. [ABSTRACT FROM AUTHOR]

Profiles of five Paleogene rivers reconstructed from basalts that flowed down them show a complex landscape. All five flow fields are in Victoria's Eastern Uplands, two in the Baw Baw region in the southwest and three in the Nunniong-Deddick region in the east. Three flow fields (Toorojil, Aberthomson, Timbuchan) originate on high plateaus (<1000 m), from which they descend steadily via deeply incised valleys to lowland settings of below 200 m. Local topography at the edge of the upland plateaus shows Paleogene basalt lying between 650 and 900 m below adjacent mountain peaks, proving that the Paleogene streams were deeply incised. Two additional Paleogene (Eocene, ∼38-42 Ma) basalt fields in the Nunniong-Deddick region (Butrindal and Bondick) indicate that the Eocene Snowy River valley's base was overlooked by mountains up to 800-1050 m higher. In the Baw Baw region, the Paleogene northern edge of the Gippsland Plains is marked by an upland front at which Paleogene basalts debouch onto and spread over pre-basaltic alluvial fans. Pliocene alluvial fans overlying the basalt debouch from the same positions, showing that there has been no significant Neogene shift of the upland's edge. The Paleogene landscape contained all major geomorphic elements visible today: high plateaus bordered by steep escarpments that were breached by deep valleys that descended to dissected lowlands, debouching onto alluvial fans of the Gippsland Plain. The Paleogene landscape is best viewed as a snapshot taken during the prolonged evolution of the landscape. The uplift must therefore be considerably older, and is tied to the onset of major sedimentation into the receiving basins of the Southern Rift System at 93 Ma (Cenomanian), also recorded as a regional cooling event in apatite fission-track ages. [ABSTRACT FROM AUTHOR]

Three gravity and magnetic profiles through the central and eastern part of the Melbourne Zone, Lachlan Fold Belt, have been modelled, with tight control from surface geology to investigate the deep crust. The largely non-magnetic upper crust (folded Ordovician to Lower Devonian strata of the Melbourne Zone intruded by Devonian granite plutons) has been modelled using recently acquired density measurements, providing a good match with the gravity profiles. The top of the basement is modelled at ∼10-15 km depth in the centre of the zone, and ∼8-10 km depth in the south of the zone, reflecting a thinning of the folded cover sequence toward the south, consistent with outcrops of basement Cambrian rocks on the south coast of Victoria. Long-wavelength magnetic anomalies in the Melbourne Zone are modelled as highly magnetic tabular bodies lying in the basement below the folded strata of the Melbourne Zone. The only rock type that matches the density and magnetic susceptibility is serpentinised ultramafic rock. Thus, a large ophiolitic body is thought to exist in the upper part of the basement to the Melbourne Zone, at depths of 15-25 km in the central part of the zone, and 10-15 km in the south of the zone. This unit is correlated with the Cambrian mafic-ultramafic complex of Tasmania, agreeing with previous correlations of the Selwyn Block with Tasmania. [ABSTRACT FROM AUTHOR]

Structural and fluid-flow evolution at the Fosterville gold deposit in central Victoria was influenced by three distinct stress regimes that each played a role in localising the sulfide-hosted gold deposits. Major gold mineralisation at Fosterville is younger than at the majority of auriferous quartz-reef-hosted deposits in the Bendigo Zone, which formed at around 440 Ma, during regional east-west shortening of the host Ordovician turbidites. In contrast, wall-rock-hosted disseminated, fine-grained auriferous pyrite-arsenopyrite mineralisation at Fosterville post-dated both the development of chevron-shaped upright folds and fold-related faults and the emplacement of Early to Middle Devonian felsic dykes. Here we show that the primary structural control on the distribution of the auriferous sulfides was related to local reactivation of the west-dipping Fosterville Fault and associated low-displacement splay faults that truncate a south-plunging syncline. Analysis of slickenline lineations on slip surfaces indicates most faults initiated as dip-slip, reverse structures during regional east-west compression, and were subsequently reactivated with sinistral-reverse displacement during later northwest-southeast compression, coincident with gold mineralisation. Kinematic analysis of conjugate sets of quartz-carbonate veins that are temporally and spatially associated with the auriferous sulfides at Fosterville also indicates mineralisation occurred during northwest-southeast shortening. A third stress regime, characterised by north-south shortening, is inferred to have been active during the late stages of mineralisation at Fosterville. Late stibnite mineralisation associated with bedding-discordant faults at Fosterville is similar to that associated with many Devonian-age gold deposits in the Melbourne Zone to the east. The absence of quartz reefs, the scarcity of visible free gold in primary ore and the wall-rock-hosted disseminated character of the sulfide ores at Fosterville are atypical of gold deposits in the Bendigo Zone. However, due to the important role of fold and fault relationships in localising mineralisation the geometry and structural setting of the Fosterville deposit is similar to many other goldfields of the Bendigo Zone. [ABSTRACT FROM AUTHOR]

The Otway, Gippsland and central coast basins (including the Torquay Basin, Sorrento Graben and Port Phillip Basin) preserve one of the most complete records of Mesozoic and Cenozoic sedimentation in southern Australia. However, robust age constraints on sedimentation are scarce. Strontium isotope analysis of calcitic bioclasts from the Jan Juc Marl of the Torquay Group, Torquay Basin, gives a range of possible ages between 24.2 and 27.9 Ma for the Jan Juc Marl, and an age of 24.24 Ma (+1.3 Ma, -1.2 Ma) for the base of the Point Addis Limestone. These data demonstrate that the Jan Juc Marl and the lower Point Addis Limestone are age-equivalent facies representing deep- and shallow-water sedimentation, respectively. 40Ar/39Ar whole-rock analysis of the underlying Angahook Formation basalt of the Demons Bluff Group gives an eruption age of 28.7 ± 0.2 Ma. The upper boundary of the Angahook Formation is a subaerial exposure surface and paleoshore platform over which younger marine sediment onlapped during subsequent marine transgression. These new age constraints provide an improved age datum, of around 28.7 Ma, for the base of the Janjukian Stage. This age is very close to the European Rupelian-Chattian boundary, which is the accepted boundary between the Early and Late Oligocene with an estimated age of 28.4 Ma. [ABSTRACT FROM AUTHOR]

The largest component of the basaltic Newer Volcanic Province of Victoria, Australia, the Western Plains subprovince, comprises extensive tholeiitic to alkalic lava flows and pyroclastics. The subprovince is subdivided into eastern and western sectors with distinctive Sr isotopic signatures; the isotopic discontinuity corresponds to an underlying Paleozoic tectonic boundary. Fifty-one new K-Ar ages plus previously published results encompass the geographic, lithological and isotopic variants of the subprovince, and are used to date extensive flow units, provide time calibration of regolith mapping units, and to compile eruptive and petrogenetic histories. The onset of volcanism at 4.6 Ma, the peak of activity at 3.0-1.8 Ma, and the progression from tholeiitic to alkalic compositions towards the present day are subprovince-wide, indicating that the basis of activity is independent of the lithospheric substrate. Local activity appears to have little pattern: some subareas have a conservative aspect, erupting similar tholeiites over much of the time span of the field; others show the complete range of lithological and isotopic composition. Peak activity (3.0-1.8 Ma) produced the maximum range of rock and isotopic compositions erupted subprovince-wide: the western sector has alkalic basalt to tholeiite (87Sr/86Sr 0.7041-0.7044), and the eastern sector transitional basalt to basaltic icelandite with much larger isotopic variation (87Sr/86Sr 0.7042-0.7055). In contrast to this apparently erratic behaviour, plots of 87Sr/86Sr vs time reveal a coherent system with the distinct eastern and western sectors expressing parallel isotopic change, a progressive decline in 87Sr/86Sr as magma compositions change from tholeiitic to more alkalic. Potential explanations, mantle chemical heterogeneity and crustal contamination, are examined in the light of chronological constraints. The tholeiitic to alkalic progression is analogous to that of oceanic islands and considered characteristic of intraplate volcanism; it implies that the most recent phase of Cenozoic volcanic activity in Victoria is coming to a close. [ABSTRACT FROM AUTHOR]

Quantitative geochronology in Victoria began over 50 years ago with the setting up of a Radiocarbon Dating Laboratory at the Museum of Applied Science in Melbourne. Although this embryonic laboratory struggled throughout its life, and was eventually closed in 1970, it was followed by a new and much stronger phase centred on the development of fission-track dating initially at the University of Melbourne and Rb-Sr dating at La Trobe University. The most recent phase, since about 1990, has involved extensive collaboration between the universities in Melbourne and the joint development of a breadth of additional geochronological facilities and methods, mostly centred on noble-gas mass spectrometry, TIMS and, most recently, multi-collector ICP-MS. Today, geochronology in Victoria based on this extensive arsenal is applied to an ever-widening range of applications across the geological sciences. [ABSTRACT FROM AUTHOR]

Victorian granites containing more than 750 ppm Ba are almost entirely confined to the region between a line from Geelong to Swan Hill and the Wonnangatta Fault Zone. Granite Ba contents normalised to 70% SiO2 range from 620 to 733 ppm in western Victoria, 719 to 1560 ppm in central Victoria and 493 to 689 ppm in eastern Victoria. Melting of Ba-rich (meta)sedimentary rocks in the lower - middle crust is implicated in the petrogenesis of central Victorian S types, at least. Thus, granite geochemistry supports the concept of some sort of (largely concealed) Ba-rich Selwyn Block beneath central Victoria, although the boundaries that have been proposed for the block are modified here. There is a strong possibility that the Selwyn Block is an exotic terrane emplaced by northwest-directed strike-slip movement during the Bindian Orogeny. Such movement appears to have been controlled by the previously postulated Baragwanath Transform and another fundamental fault here called the Ulrich Transform. Asthenospheric upwelling related to movement on the Ulrich and Baragwanath Transforms may be the explanation for the twin belts of 400 Ma plutonism occurring to the west of the former and to the east of the latter. The southern extension of the Ulrich Transform may be the Tamar - Tiers structure in Tasmania. Plate-tectonic models suggesting Ordovician - Silurian subduction in Victoria need to be carefully revisited given the possibility of Siluro-Devonian exotic terrane emplacement. [ABSTRACT FROM AUTHOR]

Laser incremental-heating 40Ar/39Ar geochronology of seven leucitites from southeastern Australia indicates that leucite-bearing lavas in individual geographic clusters were erupted in one million years or less. The eruption ages range from 17.9 ± 0.3 Ma (2σ) at El Capitan in northern-central New South Wales to 8.9 ± 0.2 Ma (2σ) at Cosgrove in northern Victoria. The 40Ar/39Ar results demonstrate that the southward migration of leucite-bearing lavas was near-contemporaneous with age-progressive central-volcano magmatism in southeastern Australia. As such, the 40Ar/39Ar results are consistent with a hotspot-related origin for the leucitites. However, the question of whether single or multiple hotspots are required to explain these volcanic chains, which are separated by a distance of about 300 km, awaits a more complete geochronological picture of the onset, duration and migration of leucitite and central-volcano magmatism in eastern Australia. [ABSTRACT FROM AUTHOR]

The Devonian Lake Boga Granite in northern Victoria, while almost entirely under thin Murray Basin cover, is one of the largest plutons in the western Lachlan Fold Belt. Its only exposure is a quarry penetrating the Cenozoic sediments. In the quarry, prominent pod pegmatites and miarolitic cavities suggest a high level of emplacement. The granite, a non-magnetic, fractionated S-type, contains a large range of accessory minerals, including primary uranium- and REE-bearing phosphates and oxides, and primary copper sulfides. Monazite-series minerals show an exceptionally wide range of compositions, from normal monazite-(Ce) through cheralite (Ca - Th-rich) to rare huttonitic monazite (Th-rich) and brabantite; U contents in monazite also vary widely (0 - 7.9 wt%). Primary low-Ca uraninites are well preserved and are unusual in having low Th/U and LREE. Late-stage cavity fluorapatite crystals up to several centimetres across show intricate elemental zoning patterns with extreme U gradients (<10 - 6900 ppm) in some crystals. New 40Ar - 39Ar ages for magmatic biotite, muscovite and K-rich feldspar average 365 ± 3 Ma, which approximates the emplacement age of the granite. This is supported by a 377 ± 12 Ma U - Th - Pb (CHIME) age for primary uraninite. New whole-rock geochemical data support earlier observations: the granite is strongly fractionated (SiO2 70.7 - 76.0 wt%; 4.2 - 0.6 wt% FeOt) and peraluminous (ASI = 1.23 - 1.45), and has slightly elevated Na2O and P2O5 (0.30 wt%) contents compared with other fractionated S-type granites from the Lachlan Fold Belt. Trace-element abundances are typical of fractionated granites, although U and Cu concentrations vary strongly and reach >60 and ≈1400 ppm, respectively. REE patterns also vary strongly, from LREE-enriched with moderate Eu depletion, to flat with strong Eu depletion. The flattest of the REE patterns, in samples with FeOtotal < 1%, are characterised by M-type tetrad effects. These and other samples also show low (subcrustal average) and variable Zr/Hf (35 - 16) and Nb/Ta (8 - 4) ratios; these and other unusual elemental fractionations are related to changes in elemental partitioning during the late magmatic stage, when felsic peraluminous magma and high-temperature magmatic fluid coexisted. [ABSTRACT FROM AUTHOR]

The Bushy Creek Igneous Complex includes rock types ranging from greenstone through diorite, granodiorite and granite to aplite and was emplaced in the Grampians - Stavely Zone 30 km southeast of the Grampians Ranges at ca 495 Ma. Striking correlations with varying silica contents are observed in major- and trace-element abundances which, coupled with a close spatial and temporal association, suggests a strong petrogenetic link between all rock types. It is proposed that these trends reflect the operation of fractional crystallisation processes dominated by hornblende (and/or clinopyroxene) and plagioclase at lower silica levels, changing to an assemblage dominated by plagioclase and accessory phases in the felsic lithologies. However, it is apparent from Sr - Nd isotope data that some form of two-component mixing also played a key role in the evolution of these magmas prior to the final episode of fractional crystallisation. The most contaminated magma (a granodiorite) requires approximately equal contributions from each of two end-members, one similar to the greenstones and the other comparable to Adelaide Fold Belt S-type granites derived from Kanmantoo Group metasediments. Such a model also accounts for the broad major- and trace-element characteristics of the various rock types of the complex. Although supporting the view that the Adelaide Fold Belt extends to the Moyston Fault, the proposed model for the origin of the Bushy Creek magmas also has considerable bearing on the petrogenesis of Devonian granites of the Victoria Valley Complex (Mafeking Suite) in the Grampians Ranges. These were emplaced nearly 100 million years later yet share very similar chemical traits, consistent with a derivation via the partial re-melting of Bushy Creek-like lithologies. [ABSTRACT FROM AUTHOR]

A diverse fossil vertebrate record in the Devonian of coastal southeastern Australia includes at least 30 genera and species representing all three major groups of extant jawed vertebrates (bony and cartilaginous fishes, and tetrapods), and both extinct groups (placoderm and acanthodian fishes). A bone recorded by W. B. Clarke in 1860 from Twofold Bay is the first published record of a Devonian vertebrate from the Southern Hemisphere. Abundant plant remains at some localities include large impressions of arborescent lycopsids, indicating one of the earliest forest environments for the Gondwana supercontinent. An early terrestrial invertebrate fauna is evidenced by fossil millipede remains. A review of the evidence for age control using paleontology and isotopic dating suggests that the lower part of the Devonian sequence (Bunga beds, Boyd Volcanic Complex) may be somewhat older (Emsian - Eifelian) than recently proposed; the tetrapod trackways at Genoa River in Victoria (lower part of the Combyingbar Formation) may correlate with the lowermost Twofold Bay Formation of the Merrimbula Group, which is overlain by the marine Bellbird Creek Formation, a manifestation of the global late Frasnian marine transgression and extinction event. Abundant placoderm remains at all higher fossil fish horizons in the overlying Worange Point Formation indicate a latest Devonian rather than Carboniferous age for the upper Merrimbula Group. [ABSTRACT FROM AUTHOR]

Investigation of coarse (>2 mm) heavy-mineral concentrates from the White Hills Gravel near St Arnaud in western Victoria provides new evidence for the age and provenance of this widespread palaeoplacer formation. A prominent zircon - sapphire - spinel assemblage is characteristic of Cenozoic basaltic-derived gemfields in eastern Australia, while a single diamond shows similar features to others found in alluvial deposits in northeastern Victoria and New South Wales. Dating of two suites of zircons by fission track and U - Pb (SHRIMP) methods gave overlapping ages between 67.4 ± 5.2 and 74.5 ± 6.3 Ma, indicating a maximum age of Late Cretaceous for the formation. Another suite of minerals includes tourmaline (schorl - dravite), andalusite, rutile and anatase, which are probably locally derived from contact metamorphic aureoles in Cambro-Ordovician basement metapelites intruded by Early Devonian granites. U - Pb dating of rutile grains by laser ablation ICPMS gave an age of 393 ± 10 Ma, confirming an Early Devonian age for the regional granites and associated contact metamorphism. Other phases present include pseudorutile, metamorphic corundum of various types, maghemite and hematite, which have more equivocal source rocks. A model to explain the diverse sources of these minerals invokes recycling and mixing of the far-travelled basalt-derived suite with the less mature, locally derived metamorphic suite. Some minerals have probably been recycled from Mesozoic gravels through Early Cenozoic (Paleocene - Eocene) drainage systems during various episodes of weathering, ferruginisation and erosion. Comparison between heavy-mineral assemblages in occurrences of the White Hills Gravel may allow distinction between depositional models advocating either separate drainage networks or coalescing sheets. Such assemblages may also provide evidence for the present-day divide in the Western Uplands being the youngest expression of an old (Late Mesozoic - Early Cenozoic) stable divide separating north- and south-flowing streams or a much younger feature (ca 10 Ma) which disrupted mainly south-flowing drainage. [ABSTRACT FROM AUTHOR]