R.A.F. Cas

Gravity currents descending a ramp in a stratified tank

This paper describes experiments and numerical simulations of a gravity current flowing down a ramp in a tank stratied in two layers. We study the dynamics of the conguration for dierent densities of the gravity current and dierent ramp angles. The experiments show that waves of large amplitude can be generated easily and that, depending on the density of the gravity current, the initial gravity current splits into a gravity current along the interface of the stratied layers, and a gravity current along the bottom of the tank. We also describe numerical simulations which give results in agreement with the results of the experiments and enable us to study three-fluid congurations with wider ranges of density than is possible in the laboratory.

Stratigraphy of the Bandas del Sur Formation: an extracaldera record of Quaternary phonolitic explosive eruptions from the Las Cañadas edifice, Tenerife (Canary Islands)

Explosive volcanism has dominated the large phonolitic shield volcano of Tenerife, the Las Canadas edifice, for the last 1.5 m.y. Pyroclastic deposits of the Bandas del Sur Formation are exposed along the southern flanks, and record the last two of at least three long-term cycles of caldera-forming explosive eruptions. Each cycle began with flank fissure eruptions of alkali basalt lava, followed by minor eruptions of basanite to phonotephrite lavas. Minor phonotephritic to phonolitic lava effusions also occurred on the flanks of the edifice during the latter stages of the second explosive cycle. Non-welded plinian fall deposits and ignimbrites are the dominant explosive products preserved on the southern flanks. Of these, a significant volume has been dispersed offshore. Many pyroclastic units of the second explosive cycle exhibit compositional zonation. Banded pumice occurs in most units of the third (youngest) explosive cycle, and ignimbrites typically contain mixed phenocryst assemblages, indicating the role of magma mixing/mingling prior to eruption. At least four major eruptions of the third cycle began with phreatomagmatic activity, producing lithic-poor, accretionary lapilli-bearing fallout and/or surge deposits. The repeated, brief phase of phreatomagmatism at the onset of these eruptions is interpreted as reflecting an exhaustive water supply, probably a small caldera lake that was periodically established during the third cycle. Accidental syenite becomes an increasingly important lithic clast type in ignimbrites up-sequence, and is interpreted as recording the progressive development of a plutonic complex beneath the summit caldera. Successive eruptions during each explosive cycle increased in volume, with the largest eruption occurring at the end of the cycle. More than ten major explosive eruptions vented moderately large volumes (1−[ges ]10 km 3 ) of phonolitic magma during the last two cycles. Culminating each explosive cycle was the emplacement of relatively large volume (>5−10 km 3 ) ignimbrites with coarse, vent-derived lithic breccias, interpreted to record a major phase of caldera collapse. In the extracaldera record, explosive cycles are separated by ∼0.2 m.y. periods of non-explosive activity. Repose periods were characterized by erosion, remobilization of pyroclastic deposits by discharge events, and pedogenesis. The current period of non-explosive activity is characterized by the construction of the Teide-Pico Viejo stratovolcanic complex within the summit caldera. This suggests that eruptive hiatuses in the extracaldera record may reflect effusive activity and stratovolcano or shield-building phases within the summit caldera. Alternating effusive and explosive cycles have thus been important in the volcanic evolution of the Las Canadas edifice.

Overthrust terranes in the Lachlan fold belt, southeastern Australia

The Paleozoic history of the Lachlan fold belt involves terrane translation and accretion of discrete allochthonous fragments. Four major terranes developed adjacent to the Gondwana continental margin and were overthrust in the Middle Devonian to form a partly composite crust for southeastern Australia.

Deformational style of the Castlemaine area, Bendigo‐Ballarat Zone: Implications for evolution of crustal structure in central Victoria

The Early Ordovician quartz‐wacke turbidite sequence (Castlemaine Supergroup) in the Castlemaine area of central Victoria has been folded about upright to steeply west‐dipping axial surfaces producing close to tight folds with long subplanar limbs and narrow hinge zones. Fold wavelengths range from 150 m to 500 m. The regional enveloping surface is gently north‐dipping. Fold growth has been accompanied by the development of a mesoscopically penetrative cleavage, particularly in pelitic to semi‐pelitic units. Convergent fanning solution cleavage is pronounced in arenite units in fold hinge zones. Folding has been associated with the development of steeply to moderately west‐dipping and east‐dipping reverse faults. Many of the smaller faults are accommodation structures formed during tightening of major chevron‐style folds. However, major west‐dipping reverse faults cut through folds and are responsible for nearly 2 km of vertical displacement over a strike‐normal section of 9 km. Gold mineralization and as...

Subaqueous, rhyolitic dome-top tuff cones: a model based on the Devonian Bunga Beds, southeastern Australia and a modern analogue

The relics of two Late Devonian subaqueous rhyolitic dome-top tuff and pumice cone successions are preserved in the Bunga Beds outlier of the Boyd Volcanic Complex, southeastern Australia. These cone successions and other rhyolitic volcanics of the Bunga Beds are associated with turbidite and other deep-water massflow sedimentary rocks. The two cone successions have a generally similar stratigraphy. At the base, flow-banded, variably autobrecciated and quench-fragmented rhyolite, representing an intrusive to extrusive dome, is overlain by rhyolitesediment breccia, representing extrusion of the dome through the deep-water sediment pile and resedimentation down its flanks. In the northern cone succession an overlying, succession of bedded pumiceous crystal-rich to crystal-poor tuffs represents the onset of pyroclastic activity and growth of a tuff cone. An overyling debris flow deposit represents degradation of part of the cone. The topmost unit, a stratified pumice succession, is thought to represent another cone-building eruptive phase, and is separated from the underlying strata by a major slide surface. The southern cone succession contains less tuff and abundant pumice, and is also terminated by a debris-flow deposit, indicating cone degradation. A modern analogue for the inferred eruptive style and sequence is the 1953–1957 rhyolite eruption that formed the Tuluman Island lava-tuff cone complex in the Bismarck Sea. The eruptions were often cyclical consisting of an initial inferred submarine-lava-forming stage, passing into a pumicecone-forming stage, in some cases a subaeriallava-forming stage, and a final stage, following the cessation of volcanism, during which the cones collapsed gravitationally or were destroyed by wave erosion. Using observations from both the Tuluman Island eruptions and the preserved stratigraphies of the Devonian tuff cones, a dynamic model is proposed for the formation of subaqueous rhyolitic dome-top tuff and pumice cones.

Ordovician palaeogeography of the Lachlan Fold Belt: A modern analogue and tectonic constraints

Abstract Re‐examination of Late Ordovician facies patterns and sediment movement patterns suggests a palaeogeography trending northwest for part of the Lachlan Fold Belt during this time. From southwest to northeast, the palaeogeography consists of a continental shoreline and shelf in western New South Wales, Victoria and Tasmania, a marginal sea in central New South Wales and Victoria, and a line of volcanic centres running southeast from northwestern New South Wales towards the south coast of New South Wales. The present‐day Andaman‐Nicobar region of the northeastern Indian Ocean has many similarities to this Late Ordovician palaeogeography and provides an important scale perspective. Although these two systems are useful geographic analogues they are not necessarily tectonic analogues.

The upper Devonian Boyd Volcanic complex, Eden, New South Wales

Abstract Intrusive and extrusive silicic rocks intertongue with mafic volcanics and sediments in the southern part of the Eden‐Comerong‐Yalwal Rift Zone. Late Devonian fish occur in the sediments. The present stratigraphic scheme of the ‘Eden Rhyolite’ and the ‘Lochiel Formation’ separated by an unconformity or disconformity is rejected. In its place, we rename the assemblage the Boyd Volcanic Complex, formed in a terrestrial zone of extension. The recognition that the alkaline Gabo Island Granite intrudes the Boyd Volcanic Complex, and that both are overlain by the Upper Devonian Merrimbula Group, provides a tight stratigraphic bracket on a potential point on the radiometric time scale.

AN INTRUSIVE ORIGIN FOR THE KOMATIITIC DUNITE-HOSTED MOUNT KEITH DISSEMINATED NICKEL SULFIDE DEPOSIT, WESTERN AUSTRALIA

The MKD5 nickel deposit at Mount Keith is hosted within the Mount Keith Ultramafic Complex, a thick komatiitic dunite body previously interpreted as either a large volume lava flow or as a dikelike intrusion. The upper contact relationships of the dunite body are critical for the evaluation of an extrusive versus intrusive origin. New drill core examined during this study has revealed preserved upper contact relationships between the Mount Keith Ultramafic Complex and the enclosing dacitic volcanic rocks. These contacts have lobate geometries with apophyses of the ultramafic material intruding the overlying dacite and dacitic xenoliths within the ultramafic rock along all observed margins. These contact features indicate an intrusive relationship between the Mount Keith Ultramafic Complex and the enclosing stratigraphy, which is consistent with the lack of definitive extrusive features. Our new interpretation of the Mount Keith Ultramafic Complex suggests that thick komatiitic dunite bodies may be regarded as subvolcanic sills emplaced within and below an extrusive komatiite pile. Importantly this model implies that komatiitic dunite bodies are not an integral or even necessary feature of a komatiite flow field.

A monogenetic, Surtla-type, Surtseyan volcano from the Eocene-Oligocene Waiareka-Deborah volcanics, Otago, New Zealand: a model

The relics of a small, monogenetic, continental-shelf, Surtseyan volcano are preserved on the North Otago coast, South Island, New Zealand, in the late Eocene-early Oligocene Waiareka-Deborah volcanics. The succession consists of two parts, i. e. a lower interval of bedded lapilli tuffs and lapillistones, representing the eruptive, aggradational-cone-building phase, and an upper epiclastic sequence, representing the post-eruptive degradational phase. All of the preserved succession appears to have been deposited below storm wave base. The lapilli tuffs and lappillistones are subaqueous fall deposits, modified contemporaneously by downslope grain flow and turbidity current redeposition, and perhaps by local reworking caused by turbulent thermal eddies. The absence of major discordances in the lapilli tuffs suggests that the active eruptive period was very short-lived, perhaps lasting only a few days. The epiclastic succession consists of redeposited volcanic, skeletal, lime mud and glauconitic detritus, transported by debris flows and other mass flows. The initial epiclastic unit, a debris flow, appears to represent the sector collapse of a significant part of the cone. The appearance of fossils and rounded clasts low in the epiclastic succession coincides with stabilisation of the top of the submarine volcanic edifice, development of a wave-planed top, and its colonisation by a diverse fauna. Periodic storm activity swept material off the platform, redepositing it as marginal talus ramps. Surtla, a wholly submarine satellite volcanic centre of the 1963–1967 eruptive activity of Surtsey, is an excellent modern analogue for both the eruptive and post-eruptive phases of the Bridge Point-Aorere Point volcanic centre. By analogy with Surtla, the 120 metres of lapilli tuffs and lapillistones exposed on Bridge Point and Aorere Point accumulated in only several days. The 25 metres of reworked, glauconitic and fossiliferous volcaniclastics, represent thousands of years based on the time required for glauconite to form.

Valley pond and ignimbrite veneer deposits in the small-volume phreatomagmatic 'Peperino Albano' basic ignimbrite, Lago Albano maar, Colli Albani volcano, Italy: influence of topography

Abstract The ca. 23-ka, small-volume, basic phreatomagmatic Peperino Albano ignimbrite, from the polygenetic Albano maar (Colli Albani volcano, central Italy) shows valley pond facies as well as veneer deposits along the maar rim and along topographic ridges. Valley pond facies is characterised mainly by massive structure and chaotic texture and can be up to 30 m thick. Veneer deposit facies is characterised by parallel to low-angle cross-stratified bedforms alternating fines-depleted lapilli-sized layers, and massive, matrix-supported beds. Occurrence of uncharred wood remains and accretionary lapilli suggests temperature of emplacement comprised between 246° and 100°C. We have interpreted the lateral facies variations in terms of temporal and spatial variations of the sediment supply from the transport system to the depositional system of the pyroclastic flow. Ignimbrite veneer facies at the maar rim may reflect pulsatory eruption dynamics, whereas valley pond facies may reflect the bulking of the pyroclastic flow inside the valleys and consequent high sedimentation rates. Ignimbrite veneer facies at topographic ridges has been interpreted to reflect detachment processes of more concentrated undercurrents draining within valleys from the more dilute upper part of the pyroclastic flow that climbs the ridges. The present interpretation suggests that processes of pyroclastic flow transformation downcurrent and induced by topography are not necessarily peculiar of hot, high-mobility pyroclastic density currents. The more likely source of water interacting with magma is interpreted to be groundwater contained within the karstic aquifer located at approximately 1 km below the ground level. This is inferred by both the large amount of limestone xenoliths present in the Peperino Albano and the absence of vesicular juvenile clasts, the latter implying that magma–water interaction occurred before gas exolution processes were significant.