Evolution of central eastern Australia during the late Palaeozoic and early Mesozoic

Palaeogeographic reconstructions and structural analysis of the Late Carboniferous to Triassic of central eastern Australia indicate that sedimentation and deformation were responses to the prolonged application of a dextral rotational force couple to the craton margin and to eustatic sea‐level changes. The force couple distorted the craton margins and adjacent Yarrol‐New England geosyncline and orogen into an incipient coupled orocline. The influence of the couple commenced in the Late Devonian and continued with varying effect until the Late Triassic, when it reversed to a sinistral system, part of a completely different stress regime that controlled sedimentation and structure during the Early Jurassic. Within the craton, deformation mainly took the form of a series of en echelon depressions, such as the Drummond Basin, Koburra, Denison and Taroom Troughs. A lineament between Longreach and Roma marks the southern boundary of this type of strain, although crust beyond its limit was not so rigid as to be unaffected by the force couple. The Yarrol‐New England region during the Devonian and the Early Carboniferous was the site of geosynclinal deposition where a thick and typically volcanogenic wedge lay along the eastern border of the craton. During the Late Carboniferous and Early Permian comparable wedges were formed farther to the east, in effect building outwards into the geosyncline. The same tensional regime that created the geosyncline is seen as the means for thinning crust below the sediment wedge and thus provided thermal instability, and for the igneous diapirism expressed as both intrusion and extrusion that characterizes the orogen from the Late Carboniferous onwards. The dextral force couple was responsible for most of the deformation and for controlling final emplacement of plutons. Sea‐level rises were marked in the late Early Permian and again in the early Late Permian.     

Propagation of pressure change through thick clay sequences: an example from Liverpool Plains, NSW, Australia

In-situ hydraulic conductivity and specific storage measurements are derived from an analysis of pore-water pressure changes in a nest of piezometers installed in a 40-m-thick succession of smectitic clay on the Liverpool Plains of northern New South Wales, Australia. The cumulative response to the rainfall events that typically occurs during winter or early spring is propagated through the clay with measurable loss of amplitude and increasing phase lag. Five major rainfall events occurred over the four years of detailed monitoring. The phase lag at the base of the clay varied between 49 and 72 days. Barometric efficiency (BE) measurements for the clay sequence (BE = 0.07) and the underlying confined aquifer (BE = 0.10) were used, with a known porosity of 0.567, to derive specific storage values of 3.7×10^–5 and 6.8×10^–6 m^–1 respectively. Vertical hydraulic conductivity (K_v) of the clay sequence derived from observed amplitude and phase changes, resulted in an average value of 2.8×10^–9 m/s. These in-situ-derived values indicate that previous estimates of vertical hydraulic conductivity of the clays, made on core samples, are unrealistically high. The instantaneous response to individual rainfall events transmitted through the clay succession (tidal efficiency of 0.93) is also described.

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Resumen Se han derivado mediciones in-situ de conductividad hidráulica y almacenamiento específico a partir de un análisis de cambios de presión intersticial en una red de piezómetros instalados en una secuencia gruesa de 40 m de arcilla esmectítica en las Planicies Liverpool del norte de Nueva Gales del Sur, Australia. La respuesta acumulativa de los eventos de lluvia que típicamente ocurren en invierno y principio de primavera se propaga a través de la arcilla mediante pérdida de amplitud y un incremento en retraso de fase. Durante los cuatro años de monitoreo detallado ocurrieron cuatro eventos de lluvia principales. El retraso de fase en la base de la arcilla varió de 49 a 72 días. Las mediciones de eficiencia barométrica (BE) para la secuencia arcillosa (BE = 0.07) y el acuífero confinado subyacente (BE = 0.1) se utilizaron, con una porosidad conocida de 0.567, para derivar valores de almacenamiento específico de 3.7×10^–5 y 6.8×10^–6 m^–1, respectivamente. La conductividad hidráulica vertical (Kv) de la secuencia arcillosa derivada de cambios observados en amplitud y fase dio por resultado un valor promedio de 2.8×10^–9 m/s. Estos valores derivados in-situ indican que los estimados previos de conductividad hidráulica vertical de las arcillas, hechos en muestras de núcleo, son muy altos y poco confiables. También se describe la respuesta instantánea de eventos de lluvia individuales transmitidos a través de la secuencia arcillosa (eficiencia de marea de 0.93).

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Résumé Des mesures in-situ de conductivité hydraulique et demmagasinement spécifique sont obtenues à partir de lanalyse de la variation des pressions interstitielles telles que mesurées au droit dun nid de piézomètres interceptant une épaisseur de 40 m dargile smectique dans les plaines de Liverpool, région nord du New South Wales, Australie. La réponse cumulative aux évènements pluviométriques qui surviennent typiquement durant lhiver ou le début du printemps, se propage dans les argiles suite à une diminution damplitude (des pressions interstitielles) et à un accroissement du pas de temps.. Cinq évènements pluviométriques sont survenus lors dun suivi détaillé effectué sur quatre (4) années. Le pas de temps à la base de largile a varié de 49 à 72 jours. Les mesures defficience barométrique (EB) de lassemblage argileux (BE = 0.07) et de la nappe aquifère confinée sous-jacente (BE = 1.10) ont été utilisées, avec une porosité connue de 0,567, afin dobtenir des valeurs demmagasinement spécifique de respectivement 3.7×10^–5 et 6.6×10^–6 m^–1. La conductivité hydraulique verticale (Ky) de lassemblage argileux a été déterminée à partir de la mesure des variations damplitudes ainsi que des pas de temps, permettant la détermination dune valeur moyenne de 2.8×10^–9 m/s. Ces valeurs obtenues in-situ montrent que les estimations de conductivité hydraulique verticale des argiles telles quobtenues à partir déchantillons non remaniés sont surestimées. La réponse instantanée à des évènements pluviométriques individuels transmis au travers de la succession dargile (facteur defficacité de marée de 0.93) est également décrite.

     

Shallow marine sand bar sequences: an example from the late Precambrian of North Norway

Five coarsening upward shallow marine sandstone sequences (2–10 m thick), are described from the late Precambrian of North Norway, where they occur in a laterally continuous and tectonically undeformed outcrop. The sequences consist of five facies with distinct assemblages of sedimentary structures and palaeocurrent patterns. Each facies is the product of alternate phases of sedimentation during relatively high- and low-energy periods. Facies 1 to 4 are interpreted as representing prograding, subtidal sand bars. Sand bar progradation occurred during the highest energy periods when unidirectional currents flowed to the northwest, depositing trough cross-bedded sandstones (facies 3 and 4) on the bar crests and flanks, and sheet sandstone beds (facies 1 and 2) in the offshore environments. Weaker northwesterly flowing currents continued during moderate energy fair weather periods. Low energy fair weather periods were dominated by wave processes, which formed largescale, low-angle, westerly inclined surfaces on the bar flanks (facies 4) and wave rippled sandstone beds (facies 2) and flat laminated siltstone layers (facies 1) in the offshore environments. One sand bar was dissected by channels and infilled by tabular cross-bedded sandstones (facies 5). Bipolar palaeocurrent evidence, with two modes separated into two laterally equivalent channel systems, suggests deposition by tidal currents in mutually evasive ebb and flood channels. The inferred processes of these sand bars are compared with those associated with modern storm-generated and tidal current generated linear sand ridges. Both are influenced by the interaction of relatively low and high energy conditions. The presence of the tidal channel facies, however, combined with the inferred strong bottom current regime, is more analogous to a tidal current hydraulic regime.     

Significance of late Precambrian turbidite sequences bordering the East Antarctic Shield

A late Precambrian turbidite succession is described from North Victoria Land, Antarctica, in its sedimentological, biological and tectonic aspects. Similar sequences occurring across the whole continent along the present Transantarctic Mountains are discussed. The geotectonic significance of this flysch belt is analysed in its relation to the East Antarctic Shield, the Gondwana craton and a proto-Pacific.

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Zusammenfassung Aus dem Nord-Viktoria-Land, Antarktis, wird eine spätkambrische Turbiditfolge mit ihren sedimentologischen, biologischen und tektonischen Kriterien beschrieben. Ähnliche Sequenzen, die quer durch den ganzen Kontinent im Zuge des Transantarktischen Gebirges vorkommen, werden diskutiert. Die geotektonische Bedeutung dieses Flysch-Gürtels wird analysiert in seiner Beziehung zum ostantarktischen Schild, zum Gondwana Kraton und zu einem Proto-Pazifik.

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Résumé L'auteur décrit, avec ses critères sédimentologiques, biologiques et tectoniques, une série de turbidites du Précambrien supérieur dans le Nord de la Terre Victoria de l'Antarctique. Il discute des séries analogues se présentant dans tout le continent suivant la présente Chaîne transantarctique. Il analyse la signification géotectonique de cette ceinture de flysch dans sa relation avec le bouclier de l'Antarctique oriental, du craton du Gondwana et d'un Proto-Pacifique.

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Mechanisms to explain the loss of heavy minerals from the Upper Palaeozoic tillites of South Africa and Australia and the late Precambrian tillites of Australia

Heavy mineral studies on Pleistocene tills from North America, Upper Palaeozoic tillites of South Africa and Australia, and late Precambrian tillites of South Australia show that the heavy mineral suites of the Pleistocene tills are dominated by amphiboles, the Upper Palaeozoic tillites by garnet, and the late Precambrian tillites by zircon and tourmaline. About half of the garnets in the Upper Palaeozoic tillites show evidence of having been rounded, and retain delicate surface chattermark trails, which indicates that these garnets have not undergone chemical attack since deposition. Although the remainder of the garnets show, by way of etching, that intrastratal solutions were active in the sediments, it is suggested that amphiboles, pyroxenes and epidote, which must have been present in the original Upper Palaeozoic heavy mineral suites, were lost primarily by the action of sorting and mechanical abrasion in beach environments prior to, and during interglacial periods. The absence of garnet and the etching of tourmaline and zircon in the late Precambrian tillites is attributed to the action of alkaline intrastratal solutions over the long time interval during which the tillites were buried in the Adelaide Geosyncline.     

A large-scale meandering submarine canyon: outcrop example from the late Proterozoic Adelaide Geosyncline, South Australia

The late Proterozoic Adelaide Geosyncline, along with overlying Cambrian strata, comprises a thick sequence of sediments and sparse volcanics which accumulated in a major rift and passive margin setting. During late syn-rift or early post-rift phases, large volumes of terrigenous and carbonate sediments of the late Proterozoic Umberatana and Wilpena Groups and Cambrian Hawker Group filled the rift. Submarine canyon development was related to at least four of these depositional cycles, the most notable of which resulted in incision and subsequent filling of the major (several kilometres in width and up to 1.5 km deep) submarine canyons by the Wonoka Formation. The Wonoka Formation canyons are not obviously fault controlled. They are interpreted to have been eroded by turbidity currents during a relative low-stand of sea-level. They were subsequently filled by a fining-upwards suite of sediments which reflects subsequent relative rise of sea-level and carbonate platform development. Ultimately the canyon complex was buried by north-westerly progradation of overlying fluvial and slope sequences (Billy Springs Beds and possibly correlative upper Pound Subgroup). It is considered likely that more distal elements of this prograding clastic wedge provided the necessary material for canyon erosion, prior to canyon filling and ultimate burial by what may have been elements of the same depositional cycle. It is considered possible that the series of isolated outcrops of canyon cross-sections within the Wonoka Formation are sections of a single canyon thalweg developed within a considerably broader zone of slope degradation. If this interpretation is correct, then the gorge-like Patsy Springs Canyon lies in more proximal regions of the basin-slope, whereas 40 km to the north-east the lower slope is cut by the Fortress Hill Canyon Complex. Palaeocurrent analyses of channel-fill turbidites within the canyons imply that the Fortress Hill Complex is in fact the outcropping western edge of a sinuous, incised canyon thalweg. The Wonoka Formation canyons, containing basal sedimentary breccias but only minor conglomerates, are considered typical of passive margin canyon development. They are contrasted with the generally highly conglomeratic channel-fills observed in outcropping Tertiary and Cretaceous examples of active margin canyons and upper fan valleys.     

Carbonate turbidite sequences deposited in rift-basins of the Jurassic Tethys Ocean (eastern Alps, Switzerland)

Syn-rift sediments in basins formed along the future southern continental margin of the Jurassic Tethys ocean, comprise, in the eastern Alps of Switzerland, up to 500 m thick carbonate turbidite sequences interbedded with bioturbated marls and limestones. In the fault-bounded troughs no submarine fans developed; in contrast, the fault scarps acted as a line source and the asymmetric geometry as well as the evolution of the basin determined the distribution of redeposited carbonates. The most abundant redeposits are bio- and lithoclastic grainstones and packstones, with sedimentary structures indicating a wide range of transport mechanisms from grain flow to high- and low-density turbidity currents. Huge chaotic megabreccias record catastrophic depositional events. Their main detrital components are Upper Triassic shallow-water carbonates and skeletal debris from nearby submarine highs. After an event of extensional tectonism, sedimentary prisms accumulated in the basins along the faults. Each prism is wedge-shaped with a horizontal upper boundary and consists of a thinning- and fining-upward megacycle. Within each megacycle six facies associations are distinguished. At the base of the fault scarp, an association of breccias was first deposited by submarine rockfall and rockfall avalanches. A narrow, approximately 4000 m wide depression along the fault was subsequently filled by the megabreccia association, in which huge megabreccias interfinger with thin-bedded turbidites and hemipelagic limestones. The thick-bedded turbidite association covered the megabreccias or formed, farther basinward, the base of the sedimentary column. Within the thick-bedded turbidites, thinning- and fining-upward cycles are common. The overlying thin-bedded turbidite association shows nearly no cyclicity and the monotonous sequence of fine-grained calciturbidites covers most of the basin area. With continuous filling and diminishing sediment supply, a basin-plain association developed comprising fine-grained and thin-bedded turbidites intercalated with bioturbated marls and limestones. On the gentle slopes opposite the fault escarpment, redeposited beds are scarce and marl/limestone alternations as well as weakly nodular limestones prevail.     

Mudstones of the Tanqua Basin, South Africa: an analysis of lateral and stratigraphic variations within mudstones, and a comparison of mudstones within and between turbidite fans

Permian deep‐water mudstones in the Tanqua Basin, South Africa, have been studied using geochemical and spectral gamma ray techniques. The mudstones occur as thick sequences between sand‐rich submarine fans, but also occur as thinner mud‐rich units within each fan. The interfan mudstones are interpreted to have accumulated during transgression and the consequent period of relatively high sea‐level, while the submarine fans and their intrafan mudstones were deposited during regression and relatively low sea‐level. Geochemical analyses revealed systematic differences between interfan and intrafan mudstones because the two types of mudstones have slightly different source lithologies. Differences between the two types of mudstone suggest that changes in relative sea‐level played a role in controlling exposure of sediment source areas. There are geochemical signals that display systematic stratigraphic trends within both interfan and intrafan mudstones. These are best explained by gradual denudation, exposure and weathering of different lithologies within a single sediment source area. Both interfan and intrafan mudstones have uniform geochemical signals along the flow direction except for the relative amount of uranium. It is most likely that the basinward increase in uranium in the mudstones is the result of reduced clastic dilution of uranium‐bearing pelagic fallout.     

Palaeozoic suturing of eastern and western Tasmania in the west Tamar region: Implications for the tectonic evolution of southeast Australia

A new tectonic model for Tasmania incorporates subduction at the boundary between eastern and western Tasmania. This model integrates thin‐ and thick‐skinned tectonics, providing a mechanism for emplacement of allochthonous elements on to both eastern and western Tasmania as well as rapid burial, metamorphism and exhumation of high‐pressure metamorphic rocks. The west Tamar region in northern Tasmania lies at the boundary between eastern and western Tasmania. Here, rocks in the Port Sorell Formation were metamorphosed at high pressures (700–1400 MPa) and temperatures (400–500°C), indicating subduction to depths of up to 30 km. The eastern boundary of the Port Sorell Formation with mafic‐ultramafic rocks of the Andersons Creek Ultramafic Complex is hidden beneath allochthonous ?Mesoproterozoic turbidites of the Badger Head Group. At depth, this boundary coincides with the inferred boundary between eastern and western Tasmania, imaged in seismic data as a series of east‐dipping reflections. The Andersons Creek Ultramafic Complex was previously thought of as allochthonous, based mainly on associations with other mafic‐ultramafic complexes in western Tasmania. However, the base of the Andersons Creek Ultramafic Complex is not exposed and, given its position east of the boundary with western Tasmania, it is equally likely that it represents the exposed western edge of autochthonous eastern Tasmanian basement. A thin sliver of faulted and metamorphosed rock, including amphibolites, partially separates the Badger Head Group from the Andersons Creek Ultramafic Complex. Mafic rocks in this package match geochemically mafic rocks in the Port Sorell Formation. This match is consistent with two structural events in the Badger Head Group showing tectonic transport of the group from the west during Cambrian Delamerian orogenesis. Rather than being subducted, emplacement of the Badger Head Group onto the Andersons Creek Ultramafic Complex indicates accretion of the Badger Head Group onto eastern Tasmania. Subsequent folding and thrusting in the west Tamar region also accompanied Devonian Tabberabberan orogenesis. Reversal from northeast to southwest tectonic vergence saw imbricate thrusting of Proterozoic and Palaeozoic strata, possibly coinciding with reactivation of the suture separating eastern and western Tasmania.     

Seismic evidence for thick and underplated late Archaean crust of eastern Dharwar craton

The deep crustal structure of eastern Dharwar craton has been investigated through τ-p extremal inversion of P-wave travel times from a network of seismographs recording quarry blasts. Travel times have been observed in the distance range 30–250 km in a laterally homogeneous lithospheric segment Main features of the inferred velocity-depth relationship include: (a) 29 km thick combined upper and middle crust velocity varying from 6 km/s to 7 km/s, with no observable velocity discontinuity in this depth range; (b) a lower crust (∼ 29–41 km) with velocity increasing from 7.0 to 7.3 km/s; (c) an average upper mantle velocity of 8.1 km/s; and (d) presence of a 12 km thick high velocity crustal layer (7.4 – 7.8 km/s) in the depth range 41–53 km, with a distinct velocity gradient marking a velocity increase of 0.4 km/s. The anomalous 53 km thick crust is viewed as a consequence of magmatic underplating at the base of the crust in the process of cratonization of the eastern Dharwar craton during late Archaean. The underplated material reflects here with the velocity of 7–3 to 7–8 km/s below the depth of 40 km. Our proposition of magmatic underplating is also supported by the presence of large scale I-granitoid, a product of partial melting of the upper mantle material.     

Lithological stratification in supraglacial sediments: an example from western Tasmania, Australia

Pebble counts of the lithology of glacial sediments in the King Valley show that the content of distantly derived erratics of many sections decreases upwards in near surface sediments. Two factors that contribute to this lithological stratification are dilution of the erratic content of surface sediments by locally derived rocks and lithological stratification of debris within the Pleistocene King Glacier. The common diluting mechanism appears to have been slope detritus derived from the valley sides and small hills that crop out on the valley floor. Lithological stratification of debris in the King Glacier resulted from the altitude of the equilibrium line of the King Glacier relative to the position and altitude of the rock source areas and the thermal regime at the ice-bed interface. The Jurassic dolerite and Permian sediments that crop out above the equilibrium line altitude were transported in subglacial and englacial positions. In contrast, below the equilibrium line sediments that accumulated and were transported in a supraglacial position contained no erratic lithologies. When deposited, the supraglacial sediments formed a siliceous, non-erratic cover over sediments that were transported in subglacial and englacial positions. The model of the mode of sediment transport in the King Valley may have application to areas of alpine glaciation where the distribution of some rock types is restricted to areas above the equilibrium lines of glaciers.     

Petrographic variations among carbonaceous chondrites of the Vigarano type

The Vigarano subtype is a petrographically complex class of meteorites. Oxidized and reduced groups can be distinguished on the basis of metal vs magnetite abundances and Ni contents of sulfide minerals. These meteorites also differ in the proportions of matrix and chondrules and in polymict character. Slight bulk chemical differences correlate with the recognized petrologic groupings. It is likely that the Vigarano subtype includes several previously unrecognized subgroups. Metamorphism has affected Coolidge, Mulga (West) and, to a lesser extent, Allende, as evidenced by ferromagnesian mineral equilibration, Fe-enrichment of fine-grained inclusions, and loss of some volatile gases. Because of the metamorphic effects in the Allende chondrite (the only meteorite of the group that has been intensively studied) and the petrographie differences among all meteorites of the Vigarano subtype, it is suggested that Allende alone may not adequately reflect the wide spectrum of properties in this important class of meteorites.     

Pillow-beds: a new type of seismites? An example from an Oligocene turbidite fan complex,Alicante, Spain

A new type of soft sediment deformed beds, termed ‘pillow-beds’, originally a packet of calcarenitic turbidites, show sand-in-sand loading with plane lower and upper surfaces. The pillow-beds have some resemblance to ball and pillow structures and they are considered to have formed as a result of dewatering. The pillow-beds are Oligocene in age and are part of turbidite lobes in a submarine fan system, which was fed through a canyon along the southern border of the ancient Iberian continent. Three hypotheses may explain their occurrence: overloading, sliding or seismic shock. A seismic origin is preferrred on the basis of the flat, undeformed lower surface of the pillow-beds, excluding dewatering of—and loading into—the previously deposited beds. Furthermore, the pillow-bed structures are related to grain size and may show repetitions, thus excluding sliding. The scale and nature of the pillow-beds suggest they could be the result of an earthquake with a magnitude of 6–7 on the Richter scale.     

Multiple-step recrystallization within massive ancient dolomite units: an example from the Dinantian of Belgium

South of the Caledonian Brabant-Wales Massif a more than 200 m thick Tournaisian to Lower Visean replacive dolomite unit can be followed for several hundred kilometres from the Boulonnais (France) to Aachen (Germany). Field observations, of features such as karst cavities occurring at the top of the Lower Visean dolomite which are filled by Lower Visean crinoidal limestone, indicate that dolomitization and karstification took place during the Early Visean. This early development of the dolomite is in agreement with the presence of stylolites cutting the dolomite fabric. The minor element composition of the majority of the dolomites remains almost uniform throughout the entire studied area. Values for Fe, Mn, Na and Sr are normally in the range 700–4700 ppm, 15–400 ppm, 80–300 ppm and 50–200 ppm, respectively. The δ¹³C values (range-0.72 to +5.31%_o) mainly reflect the carbon isotopic composition of the precursor limestones. The δ¹⁸O values, in contrast, are highly variable: ranging from-19.15 to +0.85%_o. This rather large range of δ¹⁸O values is explained by multiple-step re-equilibration/recrystallization during progressive burial and subsequent uplift of the dolomites. These processes are also responsible for the high ⁸⁷Sr/⁸⁶Sr values of the dolomites which range from about 0.7088 to 0.7098. They are distinctly more radiogenic than Lower Visean marine carbonates (0.7076–0.7078). Correlation, however, of δ¹⁸O values or ⁸⁷Sr/⁸⁶Sr ratios with dolomite and/or cathodoluminescenec (CL) textures has not been very successful. This suggests that recrystallization may remain unrecognized if only petrographic techniques are used. Nevertheless, certain CL textures can be related to specific interactions with the ambient recrystallizing fluids.     

Temporal variations in the trace-element content of stream sediments: an example from the seasonally wet-dry tropics

Zimbabwe has a markedly seasonal climate with a dry period from May to November in which virtually no rain falls. Two streams draining known bedrock and soil geochemical anomalies from unexploited mineralization were sampled at times spanning both wet and dry seasons.     

Correlation of late precambrian and early Palaeozoic sequences by eustatic sea-level changes and the selection of the Precambrian-Cambrian boundary

The author argues for a pragmatic approach to the problem of defining the Precambrian-Cambrian boundary. A boundary at first occurrence of shelly fauna as recommended by the Precambrian-Cambrian working group will create severe difficulties in worldwide correlation. By placing the boundary stratigraphically somewhat higher (i.e. the Fallotaspis zone) with a more abundant fauna, there is a much better basis for biostratigraphic correlation.Eustatic sea-level changes may be another important tool for late Precambrian-early Palaeozoic correlation, and more detailed facies interpretation of such sequences should be carried out in order to recognise these eustatic sea-level changes.