Apatite fission track analysis and vitrinite reflectance data from outcrop and well samples in the Hodgkinson Province and Laura Basin reveal regional Cretaceous cooling. Apatite fission track analysis appears to define two discrete cooling episodes, in the mid‐Cretaceous (110–100 Ma) and Late Cretaceous (80–70 Ma), although in most samples data allow only definition of a single episode. Rocks now at outcrop cooled from Cretaceous palaeotemperatures generally between 50 and 130°C in the south of the region, and from >100°C in the north. Some samples from the Hodgkinson Province also show evidence for an Early Jurassic cooling episode, characterised by maximum palaeotemperatures varying from at least 95°C (from apatite fission track analysis) to ∼200–220°C (from vitrinite reflectance), with cooling beginning at around 200 Ma. Apatite fission track analysis data do not reveal the earlier event in the Laura Basin, but on the basis of vitrinite reflectance data from Permian? units this event is also inferred to have affected the pre‐Jurassic basin units in this region. The regional extent of the Cretaceous cooling episode in the Hodgkinson Province suggests that the elevated palaeotemperatures in this region were most likely due to greater depth of burial, with subsequent cooling due to kilometre‐scale denudation. For a palaeogeothermal gradient of 30°C/km and a palaeosurface temperature of 25°C the total degree of Cretaceous cooling experienced by the samples corresponds to removal of between ∼0.8 and >3.0 km of Triassic and younger section removed by denudation, beginning some time between ca 110 and 80 Ma. Higher palaeogradients would require correspondingly lower amounts of removed section. The geology of the Laura Basin suggests that an explanation of the observed Cretaceous palaeotemperatures in this region in terms of deeper burial may be untenable. Heating due to hot fluid flow may be a more realistic mechanism for producing the observed Cretaceous palaeothermal effects in the Laura Basin.
Four species of Phyllopteroides are present in the Lower Cretaceous of eastern Australia: P. laevis sp. nov., P. serrata sp. nov., P. dentata Medwell 1954a and P. lanceolata (Walkom) Medwell 1954b. The first three of these species occur in successive stratigraphic horizons in the Victorian Lower Cretaceous; they form a probable evolutionary lineage showing progressive changes in leaf morphology, particularly shape and dentation. Well-preserved osmundaceous fructifications (Cacumen expansa gen. et sp. nov.) occur closely associated with P. dentata leaves, strongly suggesting that Phyllopteroides is an osmundaceous fern. The Lower Cretaceous megafloral zonation of Douglas (1969) is revised by defining the zone boundaries as the first occurrence of the nominate Phyllopteroides species for each zone. This modified zonation can be correlated with the Lower Cretaceous microfloral zonation, and is applicable to Lower Cretaceous floras throughout eastern Australia.
Curvilinear Th/U anomalies evident on radiometric imagery over southwestern Victoria were plotted on topographic maps and the corresponding exposures investigated in the field. These exposures were found to be dominated by quartz‐rich ferruginous duricrusts, which, based upon detailed profile descriptions and petrographic studies, are interpreted as marginal‐marine sediments of the Dorodong Sands and Loxton–Parilla Sands. Similarly oriented linear magnetic anomalies evident on aeromagnetic imagery for the Branxholme area are coincident with concentrations of detrital ferruginous magnetic gravels. These are interpreted as having been derived from weathering profiles developed in the Dorodong Sands and may have subsequently been concentrated in swales between Dorodong Sands ridges. Thus the curvilinear Th/U anomalies and magnetic anomalies are both spatially and genetically related to prior shorelines or strandlines. Some of the strandlines are covered by 4 Ma basalts, and were deposited between 6.6 and 4 Ma, i.e. very Late Miocene to Early Pliocene. Elsewhere the strandlines have a more extended time range, with the youngest strandline allocated a ca 3.5 Ma age. Strandlines at Branxholme are also younger and are regarded as Mid‐Pliocene in age. The extent of the older shorelines suggests that the major Late Miocene – Pliocene marine transgression into the Murray Basin also covered most of southwestern Victoria, much further inland than previously recognised. The variable orientation of the oldest strandlines appears to reflect a phase of syndepositional uplift of the Dundas Tablelands, causing changes in the configuration of the regressing Pliocene shoreline. The strandlines can be used as a datum for Neogene tectonism, and indicate fault displacements of up to 180 m and total uplift of up to 300 m, although faulting in the north was less intense. Most tectonism was relatively short‐lived and probably occurred between 6 and 4 Ma, i.e. Early Pliocene; the 4 Ma basalts overlying the strandlines show no evidence of faulting. Earlier faulting in the Late Miocene initiated subsidence of the prominent Hamilton Graben (identified by subsurface mapping underneath the basalt), with further movement in the Early Pliocene during deposition of the Dorodong Sands.
Although Aboriginal occupation dates to the Pleistocene, silcrete artifacts only begin to dominate archaeological sites found in the Cumberland Plain of western Sydney, eastern Australia, during the mid‐late Holocene. These assemblages routinely include large numbers of backed artifacts. A geoarchaeological survey of five potential silcrete source areas found that silcrete is irregularly distributed along ridgelines within gravel patches that represent paleochannel remnants. Thus, particular parts of the landscape, dictated by the geological history, were more likely to be targeted for stone procurement. The quality of silcrete differs between and within the sources due to variations in grain‐size, degree of silicification, and presence of inclusions/fractures, as well as the destructive influence of bush fires on exposed cobbles, resulting in an overall low abundance of high‐quality silcrete across the Cumberland Plain. Furthermore, a rind or chalky weathering cortex on many cobbles means that testing was required to assess the internal raw material quality. The difficulty in locating suitable raw material for artifact manufacture meant that when good silcrete sources were found, they were heavily targeted. This contextual information provides an essential backdrop in which to understand the archaeology of the Cumberland Plain and broader patterns of landscape use.
The substitution of aluminium for silicon in the opal-A structure results in a shift in the position of the broad, prominent, low-angle P,eak in the opal-A X-ray diffraction pattern.There is an almost linear decrease o£ the 'd spacir:g' with increasing alumin.iumcontent, from abo~t 4.0 A for allliuinium-£ree opal down to 3. 7 A
Six minor sulphide occurrences hosted by the Lower Devonian Buchan Group have been investigated. Sulphide minerals and associated phases are hosted by both dolostone and limestone lithologies along stylolitized bedding planes, cross‐cutting fractures, low angle minor faults and in cavities. Mineralization was closely associated with minor structures of inferred Tabberabberan age (Middle Devonian), which it appears to have post‐dated, but was largely strata‐bound in nature. The mineralogy of the occurrences is simple and characterized by the following generalized paragenesis which reflects the increasing oxidation state and pH of the mineralizing fluids: pyrite (pseudomorphous after marcasite)‐galena ± sphalerite ± pyrite‐dolomite ± barite‐calcite ± fluorite ± dolomite. The sulphur isotope composition of sulphide minerals varies from ‐32.1 to +4.1‰, with iron and base metal sulphide minerals forming two distinct populations around ‐25‰ and 0‰, respectively. A single barite sample gives a sulphur isotope composition of +22.4‰, which is similar to that estimated for Early Devonian seawater. Fluid inclusions in fluorite and calcite homogenize at temperatures in the range 160 to 212° C and have average salinities of approximately 10 wt% NaCI eq. Sphalerite contains up to 1.81 wt% iron which correlates with colour, and up to 1.43 wt% cadmium. The Pb isotopic pattern of galena suggests a source region with U/Pb(= μ) lower than the crustal average, and a high Th/U. A genetic hypothesis is proposed which involves the circulation of saline fluids through the Snowy River Volcanics, which directly underlie the Buchan Group, during or at some time after the Tabberabberan Orogeny. Although the Buchan occurrences show features characteristic of both Mississippi Valley‐type and stratiform ore deposits, they are most directly comparable to the epigenetic zones of Irish carbonate‐hosted base‐metal deposits. However, Pb‐Zn sulphide mineralization at Buchan appears to have been associated with minor compressional structures, suggesting that a simple correlation with the Irish deposits is not directly applicable. Key words: Buchan GroupDevonianore genesisPb‐Zn sulphide minerals
'/%3e%3c/defs%3e%3cpath fill='%23012169' d='M0 0h10080v5040H0z'/%3e%3cpath stroke='%23fff' stroke-width='.6' d='m0 0 6 3m0-3L0 3' clip-path='url(%23b)' transform='scale(840)'/%3e%3cpath stroke='%23e4002b' stroke-width='.4' d='m0 0 6 3m0-3L0 3' clip-path='url(%23c)' transform='scale(840)'/%3e%3cpath stroke='%23fff' stroke-width='840' d='M2520 0v2520M0 1260h5040'/%3e%3cpath stroke='%23e4002b' stroke-width='504' d='M2520 0v2520M0 1260h5040'/%3e%3cg fill='%23fff'%3e%3cuse xlink:href='%23d' x='2520' y='3780'/%3e%3cuse xlink:href='%23a' x='7560' y='4200'/%3e%3cuse xlink:href='%23a' x='6300' y='2205'/%3e%3cuse xlink:href='%23a' x='7560' y='840'/%3e%3cuse xlink:href='%23a' x='8680' y='1869'/%3e%3cuse xlink:href='%23e' x='8064' y='2730'/%3e%3c/g%3e%3c/svg%3e)
'%3e%3cpath fill='%23012169' d='M0 0v30h60V0z'/%3e%3cpath stroke='%23fff' stroke-width='6' d='m0 0 60 30m0-30L0 30'/%3e%3cpath stroke='%23C8102E' stroke-width='4' d='m0 0 60 30m0-30L0 30' clip-path='url(%23b)'/%3e%3cpath stroke='%23fff' stroke-width='10' d='M30 0v30M0 15h60'/%3e%3cpath stroke='%23C8102E' stroke-width='6' d='M30 0v30M0 15h60'/%3e%3c/g%3e%3c/svg%3e)