Volcanic Clasts Research Articles

Origin of the supergiant Olympic Dam Cu-U-Au-Ag deposit, South Australia: Was a sedimentary basin involved?

The supergiant Olympic Dam Cu-U-Au-Ag ore deposit of South Australia occurs in a tectonic-hydrothermal breccia complex that is surrounded by Mesoproterozoic granite. The breccia is composed mainly of granite clasts and minor amounts of Mesoproterozoic volcanic clasts. Very thick (>350 m) sections of bedded sedimentary facies that occur in the breccia complex include laminated to very thin planar mudstone beds, thin to medium internally graded sandstone beds, and thick conglomerate beds. The bedded sedimentary facies extend continuously across a 1.5 km × 0.9 km area and are not limited to small separate maar craters, as previously thought. Detrital chromite and volcanic quartz in the bedded sedimentary facies cannot be matched with local sources, and imply that the provenance extended beyond the area of Olympic Dam. The lateral continuity, provenance characteristics, great thickness, below-wave-base lithofacies, and intracontinental setting suggest that the bedded sedimentary facies are remnants of a sedimentary basin that was present at Olympic Dam prior to formation of the breccia complex. We conclude that the Olympic Dam hydrothermal system operated beneath and partly within an active sedimentary basin, was not confined to maar craters, and did not vent. This new view of the setting raises the possibility that sedimentary processes were involved in ore genesis.

제주도 서부 하모와 협재 해빈 퇴적물의 계절 변화

The Hamo and Hyeopjae beaches in the western part of Jeju Island were studied in terms of seasonal variations of surface sediment and beach profile. Seasonal drift direction of the beach sands also was investigated. The Hamo beach of 7.3 o -10.8 o steep is composed of medium sand containing volcanic clasts and shell fragments. The Hyeopjae beach of 2.8 o -6.5 o steep is composed of coarse shelly sand. Hamo beach deformation is probably caused by the jetties constructed in the western part of the beach. In the Hyeopjae beach, surface sands were drifted into the dune side by the northwestern stormy wind during winter season.

Early Arc ,agmatism: geochemical characteristics of volcanic clasts from Punta Sámara, Costa Rica

The geochemical characteristics of volcaniclasts from Punta Sámara are used to infer magmaticprocesses that generated Late Cretaceous – Middle Paleocene arc lavas in the western margin of the Caribbeanplate. The Ba/La ratios of the volcaniclasts are higher than the ratios for any of the modern lavas from Costa Rica, and the La/Yb ratios are lower; suggesting that the lavas from the primitive arc were generated by greatercontribution of slab material than the modern arc lavas. Furthermore, the lower La/Yb ratios imply that thedegree of melting was significantly higher and/or that the mantle source was more depleted than in modernCosta Rica. The low Zr/Nb ratios for the modern lavas from central Costa Rica imply a more enriched mantlesource (OIB-like), whereas the higher ratios for the primitive arc volcaniclasts imply a more depleted mantlesource (MORB-like). We use the geochemical information to infer the conditions of subduction during the earlystages of arc magmatism in the southern part of the Central American arc. We infer that the primitive arc wasassociated with a subducting slab that dip at a steeper angle than the present. The steeper angle would imply thatthe subducting Farallon plate was older, colder and denser than the present Cocos plate.

Chapter 64 Neoproterozoic glacial deposits of Tasmania

Abstract In Tasmania, Neoproterozoic glaciogenic deposits were laid down in one or more epicratonic basins, probably situated at the eastern margin of the Australian–Antarctic craton. Rifting and volcanism took place in the late Cryogenian to early Ediacaran. On King Island, north of Tasmania, the Cottons Breccia consists of 50–200 m of diamictite, conglomerate and sandstone. Limestone and dolostone clasts are abundant in the diamictite, although carbonate is unknown in the underlying successions. The Cottons Breccia is overlain by 10 m of laminated dolostone and limestone with a negative, upward-decreasing δ 13 C profile. Rift volcanics and shallow intrusives higher in the sequence are dated at c. 575 Ma. In NW Tasmania, two diamictite units are found in the Togari Group. The Julius River Member, 200 m thick, contains dominantly dolostone clasts and overlies a shallow-marine dolostone unit with vase-shaped microfossils and C-isotopes consistent with a mid-Cryogenian age. Some clasts in the Julius River Member contain a stromatolite ( Baicalia cf. B. burra ) very similar to a form that is abundant in the middle part of the Burra Group, Adelaide rift basin. The Julius River Member is immediately overlain by black shale and impure carbonate dated by Re–Os at 641±5 Ma. The younger diamictite in the Togari Group is the Croles Hill Diamictite, 70 m thick, with predominantly volcanic clasts, underlain by a shale and mafic-volcaniclastic succession and overlain by thin mudstone followed by thick rift tholeiites. At one locality this diamictite is underlain by a rhyodacite flow dated at 582±4 Ma. In southern Tasmania, diamictites are found in the Wedge River Beds and in the Cotcase Creek Formation (Fm.) (Weld River Group). Laminated siltstone with dropstones is associated with the diamictites in the Cotcase Creek Fm. The southern Tasmanian deposits are poorly constrained in age. Chronometric and other evidence suggests correlation of the Julius River Member, Cottons Breccia and Croles Hill Diamictite with the Sturt, Elatina and Gaskiers glacial phases, respectively. However, a glacial origin for the Julius River Member and Croles Hill Diamictite remains uncertain.

Rounding of pumice clasts during transport: field measurements and laboratory studies

Volcanic clasts in many pyroclastic density current deposits are notably more round than their counterparts in corresponding fall deposits. This increase in roundness and sphericity reflects different degrees of comminution, abrasion and breakup during transport. We performed experimental measurements to determine an empirical relationship between particle shape and mass loss caused by particle–particle interactions. We consider, as examples, pumice from four volcanoes: Medicine Lake, California; Lassen, California; Taupo, New Zealand; Mount St Helens, Washington. We find that average sample roundness reaches a maximum value once particles lose between 15% and 60% of their mass. The most texturally homogeneous clasts (Taupo) become the most round. Crystal-rich pumice abrades more slowly than crystal-free pumice of similar density. Abrasion rates also decrease with time as particles become less angular. We compare our experimental measurements with the shapes of clasts in one of the May 18, 1980 pyroclastic density current units at Mount St Helens, deposited 4–8 km from the vent. The measured roundness of these clasts is close to the experimentally determined maximum value. For a much smaller deposit from the 1915 Lassen eruption, clast roundness is closer to the value for pumice in fall deposits and suggests that only a few volume percent of material was removed from large clasts. In neither field deposit do we see a significant change in roundness with increasing distance from the vent. We suggest that this trend is recorded because much of the rounding and ash production occur in proximal regions where the density currents are the most energetic. As a result, all clasts that are deposited have experienced similar amounts of comminution in the proximal region, and similar amounts of abrasion as they settle through the dense, near-bed region prior to final deposition.

Nature and significance of the diachronous contact between the Hazelton and Bowser Lake groups (Jurassic), north-central British Columbia

Abstract In north-central British Columbia, the contact between the Lower to Middle Jurassic Hazelton Group and the Middle Jurassic to Cretaceous Bowser Lake Group is an upward gradational change from fine-grained, commonly tuffaceous sedimentary lithofacies to commonly coarser and more heterogeneous lithofacies assemblages that include abundant chert clasts and generally lack distinct tuff beds. Paleontologic dating reveals that the lowest Bowser Lake Group is generally older (Bajocian) in north-most exposures and younger (Early Oxfordian) to the southwest toward the centre of the Bowser Basin. Interpretation of stratigraphic changes relies upon age data from ammonites, and to a lesser extent marine bivalves; a new bivalve species with biostratigraphic significance, Myophorella richardsi n. sp., is named herein. In many places the contact between the Hazelton and Bowser Lake groups is younger than previously understood. Earlier treatments of this boundary over-emphasized paleontologic content and time correlations rather than lithologic criteria. The diachroneity of the base of the Bowser Lake Group resulted from: southwestward migration of the initial front of coarse, chert-bearing, clastic detritus derived from the Cache Creek terrane, which started in the Bajocian; and subsequent northward migration of coarse facies containing volcanic clasts shed from a volcanic highland near the south margin of Bowser Basin. Although volcanic activity in the immediate vicinity of the Bowser Basin diminished significantly in the Bajocian, volcanism continued to the south of the basin into the latest Jurassic. This southern volcanism was likely the source for tuff beds in the uppermost Hazelton Group in the immediate vicinity of Bowser Basin. Continued southern volcanism and input of ash to the Bowser Lake Group is recorded by U-Pb ages of detrital zircons that are coeval with depositional ages inferred from fossils. The large volume of clastic detritus in the Bowser depositional systems, however, diluted the influx of ash and prevented the accumulation of distinct tuff beds within the Bowser Basin proper.

Behavior of halogens during the degassing of felsic magmas

Residual concentrations of halogens (F, Cl, Br, I) and H2O in glass (matrix glass and melt inclusions) have been determined in a series of volcanic clasts (pumice and lava‐dome fragments) of plinian, vulcanian and lava dome‐forming eruptions. Felsic magmas from calc‐alkaline, trachytic and phonolitic systems have been investigated: Montagne Pelée and Soufrière Hills of Montserrat (Lesser Antilles), Santa Maria‐Santiaguito (Guatemala), Fogo (Azores) and Vesuvius (Italy). The behavior of halogens during shallow H2O degassing primarily depends on their incompatible character and their partitioning between melt and exsolved H2O vapor. However, variations in pre‐eruptive conditions, degassing kinetics, and syn‐eruptive melt crystallization induce large variations in the efficiency of halogen extraction. In all systems studied, Cl, Br and I are not fractionated from each other by differentiation or by degassing processes. Cl/Br/I ratios in melt remain almost constant from the magma reservoir to the surface. The ratios measured in erupted clasts are thus characteristic of pre‐eruptive magma compositions and may be used to trace deep magmatic processes. F behaves as an incompatible element and, unlike the other halogens, is never significantly extracted by degassing. Cl, Br and I are efficiently extracted from melts at high pressure by H2O‐rich fluids exsolved from magmas or during slow effusive magma degassing, but not during rapid explosive degassing. Because H2O and halogen mobility depends on their speciation, which strongly varies with pressure in both silicate melts and exsolved fluids, we suggest that the rapid pressure decrease during highly explosive eruptions prevents complete equilibrium between the diverse species of the volatiles and consequently limits their degassing. Conversely, degassing in effusive eruptions is an equilibrium process and leads to significant halogen output in volcanic plumes.

Heat transfer coefficients of natural volcanic clasts

Heat transfer coefficients used in numerical simulations of volcanic eruptions are typically borrowed from industrial settings where the coefficients are well determined for non-permeable, machined (spherical) materials. Volcanic clasts, in contrast, are permeable and have irregular shapes. We performed a series of laboratory experiments to determine heat transfer coefficients for natural volcanic particles. We measured the surface and interior temperatures during cooling at wind speeds ranging from 0 to 10 m/s. We also measured the permeability and density of the particles. We find that the permeability of the particles has little effect on clast cooling. In the absence of any wind, heat loss occurs by free convection, and we find no relationship between the heat transfer coefficient and particle density. However, for non-zero Reynolds numbers (finite wind speed), the heat transfer coefficient decreases with increasing porosity. We obtain a correlation for the dimensionless heat loss, or Nusselt number, of the form Nu = 2 + aRe 1/2Pr 1/3 where a is a density dependent coefficient given by a = 0.00022ρ + 0.31, with ρ in kg/m 3, and Re and Pr are the Reynolds number and Prandtl number, respectively. Compared with non-porous particles, heat transfer coefficients for natural pumice clasts are reduced by a factor of 2–3 for particles with similar Re. Numerical simulations show that this leads to an increase in depositional temperature by 50–90 °C.

Petrology and geochemistry of greywackes from Goa-Dharwar sector, western Dharwar Craton: Implications for volcanoclastic origin

Late Archaean Supracrustals of the Goa-Dharwar sector (GDS) are composed of a thick sequence of greywacke sequence with narrow intercalations of quartzite, BIF and carbonates. Mafic volcanics occupy the base of the sequence. The greywackes are predominantly tuffacious containing chlorite-sericite and hornblende. Arkosic variations containing biotite dominate the western part of the sector. Fine-grained variations occur as isolated narrow lenses within other types of greywackes. The conglomeratic greywackes are localized along the western and the eastern margins of the sector. All of the greywackes are all typically immature containing coarser clasts of mostly plagioclase (18–23%) and quartz (32–34%). Lithic fragments of felsic volcanic rocks are common. The matrix is dominated by mafic material. Biotite and amphibole are related to metamorphic recrystallization. Chlorite, sericite, epidote, carbonate and chert are products of the interplay of diagenesis and low-grade metamorphism. Fe-Ti oxide, sphene, apatite and zircon are usual accessories. But for slight enrichment in K2O, the major element chemistry of the GDS greywackes is similar to the chemistry of Late Archaean greywackes. They also compare in respect of V, Co, Hf contents, K2O/Na2O, SiO2/Al2O3, Na2O/Al2O3, Ba/Rb, Th/U, La/Th, Sm/Nd ratios, steep REE patterns with distinct LREE enrichment and HREE depletion. The GDS greywackes however are distinctly enriched in Rb, Ba, Sr, Th, U, Cu, Zr, Ce/Ce* and depleted in Cr, Ni, and Zn. The conglomeritic and biotite bearing verities contain considerable proportions of clasts derived from the basement tonalitic/granitic terrain. The common tuffacious greywackes containing hornblende and biotite-sericite however include only volcanic clasts and bear evidence of derivation from submarine weathering of predominantly felsic volcanics erupted on a large scale to form a magmatic arc in the later stages of geosynclinal deposition. Geochemical data suggest that the GDS greywackes were laid down in progressively changing basin geometry from a passive to active continental margin and island arc setting.

Tectonics of an Early Carboniferous forearc inferred from a high‐P/T schist‐bearing conglomerate in the Nedamo Terrane, Northeast Japan

AbstractThe Nedamo Terrane, an Early Carboniferous accretionary complex, is the oldest biostratigraphically dated accretionary complex in Japan. The purpose of this study is to describe and interpret a conglomerate from the Nedamo Terrane that contains clasts of high‐pressure/low‐temperature (high‐P/T) schist (mainly garnet‐bearing phengite schist) and ultramafic rock, and to infer the tectonics of an Early Carboniferous arc–trench system at the eastern margin of the paleo‐Asian continent. Clasts of high‐P/T schist and ultramafic rock within the conglomerate make up 8.4 and 6.7% of the total clasts, respectively, based on modal counts. These clasts are subangular to subrounded, whereas volcanic clasts are well rounded. The source of the schist clasts, which yield a radiometric age of 347–317 Ma, is considered to be the Renge Metamorphic Rocks of Southwest Japan or equivalent rocks. Based on the chemical composition of chromian spinel, the source of ultramafic clasts is inferred to be the island‐arc‐type Ordovician Miyamori and Hayachine ultramafic complexes in the Kitakami Massif. The conglomerate records multiple provenance regions, including an island arc (South Kitakami Terrane) and a forearc ridge; the high P/T schist and ultramafic rocks were exhumed in the forearc region. The duration of the interval from the early stages of exhumation of the schist to its deposition in the trench as clasts is estimated to have been less than 30 my.

Late Cenozoic oscillations of Antarctic ice sheets revealed by provenance of basement clasts and grain detrital modes in ANDRILL core AND-1B

Petrological investigations of the sand fraction and of granule- to cobble-sized clasts in the Plio-Pleistocene sedimentary cycles of the AND-1B drill core at the NW edge of the Ross Ice Shelf (McMurdo Sound) highlight significant down-core modal and compositional variations. These variations provide: (i) direct information about potential source regions during both glacial maxima and minima; and (ii) evidence of an evolving provenance, documented by long-term shifts in compositional patterns that can be interpreted as reflecting variations in ice volume and ice sheet thermal regimes and changes in paleogeography related to the emergence of several volcanic centres during the deposition of the drill core over the past ca. 3.5Ma.The most significant change in diamictite provenance (identified at 82.7metre below the sea floor, mbsf), coincides with a change in sedimentary cycle architecture from sequences that are dominated by diamictites (Mid-Late Pleistocene, above 82.7mbsf) to sequences characterised by cycles of diamictite (subglacial) and diatomite (open-marine) deposition (Pliocene, below 82.7mbsf). In the Mid-Late Pleistocene glacial/interglacial cycles diamictites show high amounts of Skelton–Mulock sourced clasts, supplied from both basement and overlying Beacon and Ferrar supergroups, and they also include a variable contribution from reworking of glacial sediments that were deposited during earlier glacial activity.In the Pliocene to early Pleistocene diatomite–diamictite cycles basement clast compositions indicate the same provenance (Mulock–Skelton) but the main debris load was picked up from volcanic centres in the McMurdo Sound area. Similarly, associated glacial minima sediments (i.e., diatomite intervals) are dominated by volcanic clasts suggesting calving of glaciers from Ross Island or the Koettlitz Glacier region during interglacials.In agreement with previous glaciological reconstructions and numerical ice sheet models, the provenance of glacially transported material is firmly identified in the region between Ross Island and the Skelton–Mulock glacier area (South Victoria Land). The reconstructed ice directions and ice dynamic model are comparable to the configurations proposed for the grounded ice expansion within the McMurdo Sound during the Last Glacial Maximum, and they are also consistent with ice-flow patterns previously reconstructed for Pliocene and Pleistocene glacial settings in the region.

Strain analysis and rheology contrasts in polymictic conglomerates: An example from the Seine metaconglomerates, Superior Province, Canada

Deformed conglomerates typically display contrasting strains between different lithological clast populations and matrix, which can be exploited to provide information about effective viscosity contrasts. We analyzed three different clast populations (granitoid, mafic volcanic, and felsic volcanic) across a strain gradient in the Seine metaconglomerates of Ontario, Canada. We calculated finite strain using the R f/ φ method and bootstrap statistics. Granitoid clasts exhibited the least amount of strain, while the mafic and felsic clasts had similar, greater strains. We assumed that the bulk finite strain was recorded by the mafic clasts, given their large proportion of the rock and their similar composition to the matrix. We calculated effective viscosity contrasts using the finite strain information, assuming Newtonian viscous rheology. The two volcanic clast populations have similar effective viscosities, regardless of strain magnitude, with ratios of felsic/mafic ranging from 0.27 to 2.12 and average ratio of 1.19. The granitoid clasts are significantly more competent than the mafic clasts with effective viscosity ratios ranging from 2.35 to 12.39 and average ratio of 5.61. The results are consistent with a qualitative competence hierarchy of granitoid > felsic ≥ mafic, although the quantitative effective viscosity ratios change with strain magnitude. Consequently, the effective viscosity contrast between clast types is strain dependent and possibly deformation path dependent.

Provenance and palaeogeographic implications of Eocene–Oligocene sedimentary rocks in the northwestern Basin and Range

A thick sequence of uppermost Eocene to lower Oligocene volcaniclastic and sedimentary rocks is exposed at the base of the Warner Range in northeastern California. This isolated exposure provides insight into the palaeogeographic setting of the northwestern Basin and Range during this time period. Significant thinning of the unit over 35 km of lateral exposure and predominantly volcanic clast compositions suggest that the sequence was deposited in an alluvial plain adjacent to a volcanic arc. Palaeocurrent indicators in the conglomerates define a NNE transport direction. Detrital zircon analysis on coarse sandstones and dating of individual granite cobbles show a range of ages consistent with a local, volcanic source area primarily from the SSW with some far-travelled input from northern Nevada; the far-travelled component increases in influence as the unit thins to the north. Comparison with other sedimentary sequences of Eocene age and integration with palaeofloral and geophysical data help to define drainage divides, and suggest that this sequence accumulated in a relatively isolated, intra-arc basin. This localized accumulation differs markedly from contemporaneous drainages to the south that transported material westwards from central Nevada to the palaeoshoreline, and suggests that ongoing volcanism had a strong influence on palaeogeography in this region during the Eocene and Oligocene.

Aspectos geohidrológicos y sedimentológicos de los flujos de lodo asociados al terremoto de Cinchona (Mw 6,2) del 8 de enero del 2009, Costa Rica

The Cinchona earthquake (Mw 6.2) located 6.5 km east of the active crater of Poás, triggered several landslides (rock fall, rotational and translational slides), which in combination with existing pore water within the landslide mass in intense tropical weathering volcanic soils, local aquifers deformation, and mixing of fluvial waters into the moving sediment mass, produced high destructive debris flows along the Seco, María Aguilar, Cariblanco, Ángel, La Paz, Sarapiquí, Mataste and Tigre rivers. The deposits totaling 0.5-3 m in thickness, consisting of angular to rounded volcanic clasts, matrix-supported, very poorly sorted gravels, interpreted as a non-eruptive lahar or debris flow diamicton (volcanic muddy gravelly sands) deposits, specifically as a cohesive (5-7% clay) woody mud flows. Logs and other organic debris are often incorporated in the levees and the preserved front flows were well defined and 1.5 m high, suggesting a relative moderate viscosity flow. Megablocks of ignimbrite (60-200 m3) from the bedrock were incorporated and broken up into individual fragments. The proximal and medial cannon-side facies are represented by thin (0.1-1 m) veneers deposits on steep slopes with abundant evidences of marks peak-flow levels (mudline, striations) and bulking (incorporation of boulders, anthropogenic and log debris). In open spaces of the piedmont and alluvial plain, instead, the flow was of hypenconcentrate type due mixing with the stream water downstream, and present a bouldery flow channel deposit surface gradating to smooth flow deposit surface, and abundance evidence of cataclasis (breakage of clast owing to collisions). The velocity of the flows were estimated for several rivers variable between 13.3 and 4.8 m/s (48-17.5 km/h), with super-elevations in the channel rivers up to 35 m as a steep-fronted head of at least 5-15 m, but after the piedmont, it was only 1-0.5 m high and it move at relative slower velocities of 2.5-10 m/s (9-36 km/h). The flow pushes some stream water along the channel in front of it, a phenomenon termed miscible displacement. The total volume of sediments is crudely estimated in at least 2.5-3.5 x 106 m3. The apparent coefficient of friction or fahrböschung number was for the debris and hypoconcentrated flows of 0.09 and 0.06, respectively. Six bridges, several pasture land, the electromechanical equipment at the Cariblanco powerhouse were destroyed. The caudal was estimated in the Sarapiquí river at Cariblanco power house in 4374-5994 m3/s. The strong motion equipment registered peak accelerations of 25.64 cm/s2 due the debris flow, and the spectrum of seismic record produced is characterized by frequencies between 7 and 37 Hz. The protection dike at the Cariblanco power house produce a change in the hydrologic and reologic condition of the flow, from a supercritic flow with antidunes to a subcritic flow (Froude number: 0.48-0.84) with lower velocity of 1.6-3.2 m/s (6.1-11.7 km/h), favoring the loss its bedload zone transported as traction carpet, and therefore with a less destructive effects on the buildings due a relatively diluted overlying flow zone (tranquil flow) carrying suspended sediment and tree debris. The mudflows produced strong vertical and lateral erosion, killed all the fluvial fauna, resulted in profound changes in the hydrologic stability for years of the Sarapiquí drainage basin.

New biostratigraphic and petrographic data from the Poggio Carnaio Sandstone Fm (Val Marecchia Nappe): insights into the tectonic evolution of the Northern Apennines

In the Northern Apennines, the Poggio Carnaio Sandstone Formation consists of sandy-clayey turbidites, cropping out in the northernmost corner of the Val Marecchia Nappe. The formation has been considered Oligocene in age and is commonly interpreted as an Epiligurian unit, unconformably deposited above the Val Marecchia Nappe during its transport towards the Adriatic foreland. The Poggio Carnaio Sandstone Fm rests on the Argille Varicolori Fm of the Val Marecchia Nappe, but field data do not allow it to be recognized wherever it abruptly replaces the pelagic sediments of the Argille Varicolori Fm, thus testifying to the foredeep evolution of the basin, or where it unconformably overlies this latter formation. Nannofossil assemblages are characterized by abundant reworked Cretaceous and Paleogene taxa and by some taxa, whose first occurrence is reported in the upper part of the NN4 Zone = upper part of the CN3 Zone. Therefore the formation must be considered not older than Langhian. Detrital modes of arenites revealed a quartz-feldspathic composition and the lithic component includes mainly metamorphic fragments and minor plutonic, sedimentary, ophiolithic and volcanic clasts. The presence of clasts of garnet, sillimanite, hornblende and glaucophane is significant. Biostratigraphic as well as petrographic data agree with the interpretation of the Poggio Carnaio Sandstone Fm as an Epiligurian succession. Rock fragments indicate source areas characterized by Ligurian-, Pennidic- and Australpine-type units. Sedimentary facies and textural features of arenites, revealing a rapid erosion and deposition of clasts in a basin close to the source area of the clastic supply, indicate that the Alps cannot be considered as the source area of these arenites, as frequently argued for many North-Apennine clastic formations. Ligurian-, Pennidic- and Australpine-type units were located close to the Poggio Carnaio Sandstone basin, probably representing the geometrically highest units of the Palaeo-Apennine Chain.

Glassy fragmental rocks of Macquarie Island (Southern Ocean): Mechanism of formation and deposition

Glassy fragmental rocks are interlayered with pillow basalt and tabular basalt on Macquarie Island (54°30′ S, 158°54′ E). These facies formed along the Proto-Macquarie Spreading Ridge between 6 and 12 Ma and have since been uplifted and exposed on the apex of the Macquarie Ridge Complex. Through a combination of field and microscopic analyses, we investigate the submarine production, transportation, deposition and lithification of basalt and sideromelane clasts within a spreading-ridge environment. The findings of this study indicate that these glassy grains form predominantly by cooling-contraction granulation of pillow lava rinds while crystalline basalt clasts are derived from the fragmentation of pillows along concentric and radial cooling joints. Hyaloclastite breccias consist of crystalline volcanic clasts in a matrix of glassy fragments, and are termed pillow-fragment breccias when clasts identifiable as pillows account for > 25% of the cobble-sized fraction. This glassy fragmental sediment was transported predominantly by short-lived grain flows and deposited as a result of syn-eruptive talus accumulation. The above interpretations culminate in the production of a depositional model: these glassy fragmental rocks formed on the slopes of pillow cones following gravitational collapse of a destabilised cone flank along the Proto-Macquarie Spreading Ridge. Scanning electron microscopy reveals that palagonite alteration rims on glassy grains lithify the sediment. The findings may be used as an analogue for the formation of glassy fragmental rocks along past and present mid-oceanic ridges.

Pore structure of volcanic clasts: Measurements of permeability and electrical conductivity

The pore structure of volcanic clasts is examined using measurements of porosity, permeability, and electrical properties. Permeability varies by several orders of magnitude among volcanic clasts and does not depend solely upon porosity. Electrical property measurements of saturated volcanic samples illustrate the influence of pathway tortuosity and pore shape on permeability. For equivalent eruption conditions, silicic samples show higher tortuosities, smaller vesicle sizes, and lower permeabilities than mafic samples. These differences are largely due to variations in vesiculation and crystallization history. Differences between explosive and effusive samples reflect the relative ability of bubbles to form and maintain connected pathways during bubble expansion and collapse. Isotropic samples (variably expanded breadcrust bombs and most pumice fall samples) have pore pathways that simplify with increasing porosity. Highly vesicular anisotropic samples (e.g., tube pumice) have high permeabilities and low tortuosities parallel to pore elongation and low permeabilities and high tortuosities perpendicular to elongation. These pathways simplify with increasing deformation (i.e. tortuosity decreases as porosity decreases), until pore geometries collapse sufficiently to form intersecting cracks. More generally, Archie's Law (power law) relationships between electrical conductivity formation factor ( F) and porosity ( ϕ) have an Archie's exponent, m, between 1 and 4 (where F = ϕ − m ) for low porosity volcanic clasts. However, samples with higher connected porosities (> 20% for silicic samples and > 50% for mafic samples) have m values that increase with increasing porosity, reaching up to 15. We also find that a single Archie's Law fit to a suite of samples is not appropriate either for sample suites with widely varying porosities or for anisotropic samples with a directional variation in measured properties. These measurements caution against simple application of cross-property relationships derived from sedimentary rocks to models of permeability in volcanic samples.

Petrology and geochronology of the volcaniclastic and volcanogenic Mesozoic Loch Burn Formation in eastern Fiordland, New Zealand

The Loch Burn Formation in eastern Fiordland is the metamorphosed and eroded effusive product of a long‐lived Jurassic to Early Cretaceous volcanic arc. Relict sedimentary features within meta‐volcaniclastic components indicate sedimentation in a mostly terrestrial or shallow‐water environment that was fed by debris flows from proximal granitic and volcanic high points. In the Murchison Mountains, deposition is constrained by a c. 342 Ma granite, which is unconformably overlain by the Loch Burn Formation, and a c. 158 Ma quartz diorite that intrudes the Loch Burn Formation. This latter age is 8–9 m.y. older than a volcanic clast from Loch Burn Formation and a sandstone horizon previously dated from the Stuart Mountains, and therefore supports previous suggestions that the Loch Burn Formation is a long‐lived and time‐transgressive unit. The Carboniferous basement provides a potential source for detrital zircon in metasediment, inherited zircon in the intrusive quartz diorite, and some granitoid clasts within the formation. Geochemical similarities between the quartz diorite, volcanic clasts within the Loch Burn Formation, and nearby Jurassic to Early Cretaceous Darran Suite plutons imply that the Loch Burn Formation is the volcanic equivalent of the Darran Suite. The distinctive lithological, geochemical, age, and internal relationships of the Loch Burn Formation are also seen in the volcano‐sedimentary Largs Group of northern Fiordland and Paterson Group of Stewart Island, suggesting that these three units are lithological and chronological equivalents to one another. The Loch Burn Formation provides a comprehensive record of the tectonic evolution of eastern Fiordland, with several episodes of uplift and burial. These are: uplift and erosion of a Carboniferous plutonic basement by c. 195 Ma; deposition of the older part of the Loch Burn Formation sequence in the Jurassic before burial and intrusion by Darran Suite plutons at 158 Ma; deposition continuing until at least 148 Ma; metamorphism of the entire Loch Burn Formation at greenschist and amphibolite facies conditions; uplift, erosion, and then deposition of the overlying Eocene sediments; reburial to zeolite facies depths beneath the Tertiary Te Anau and Waiau Basins; final uplift in the Pliocene.

A possible early age for a diprotodon (Marsupialia: Diprotodontidae) fossil from the Papua New Guinea highlands

MENZIES, J., DAVIES, H.L., DUNLAP, W.J. & GOLDING, S.D., June, 2008. A possible early age for a diprotodon (Marsupialia: Diprotodontidae) fossil from the Papua New Guinea highlands. Alcheringa 32, 129–147. ISSN 0311-5518. A fossil diprotodon jawbone coated and impregnated with a well-cemented fine breccia or tuff was recovered from weakly consolidated Pleistocene lacustrine sediments near Yonki in the Papua New Guinea highlands. The fine breccia includes angular rock and mineral fragments derived from country rock, accretionary lapilli and clay minerals. It does not include any identifiable primary volcanic material. The presence of accretionary lapilli and lack of volcanic clasts suggests an origin by phreatic eruption—an explosive eruption driven by the violent escape of gas. Minerals in the fine breccia have an age of 13.2 ± 0.2 Ma, middle Miocene, as indicated by 40Ar/39Ar analysis. This is the age of the country rock that was blasted by the phreatic eruption. Igneous activity in the Yonki area is thought to have ceased at 7.4 Ma (younger age limit of Elandora Porphyry; late Miocene), and so it is likely, but not certain, that the phreatic eruption occurred not later than 7.4 Ma. The jawbone, as far as can be told from its poor condition, is dentally similar to the late Pliocene and possibly Pleistocene ‘Kolopsis’ watutense recovered from other sites in New Guinea. Probably, the jawbone, or the living marsupial, was buried in the fine breccia at the time of the phreatic eruption, and its remains were subsequently reworked by river erosion and redeposited in the lacustrine sediments. Recrystallization and loss of primary texture in some of the bone may be a result of heating at the time of, or preceding, the eruption.

Holocene recurrent explosive activity at Chimborazo volcano (Ecuador)

Ice-capped Chimborazo is one of the large composite Ecuadorian volcanoes whose recent eruptive activity is poorly known. This paper presents the characteristics and the ages of a newly discovered Holocene sequence of pyroclastic deposits on the east and north sides of the cone. Lying upon a moraine of the Late-Glacial period, the most complete section of ~ 4.5 m in thickness is located 5 km from the present summit crater. It consists of seven massive or diffusely stratified ash flow layers and four fallout layers interbedded with seven paleosoils. Based on field study, most flow deposits were assessed as surge layers, and six radiocarbon analyses obtained from charcoal fragments and paleosoils indicate that eruptions occurred at quite regular intervals between about 8000 and 1000 years ago. The first two and most potent events generated thick lahars over the north and west flanks of the cone. Surface textures of volcanic clasts were analysed by scanning electron microscopy. Blocky and blocky/fluidal vitric clasts indicate fragmentation during vulcanian explosions of a quite solidified shallow magma body. In addition, aggregates either cemented at a cooling stage (with surface fluidal textures), or consisting of fine particles (moss-looking aggregates), form a large part of the surge deposits. These characteristics indicate powerful explosions and intense fragmentation due to phreatic water reaching the conduit, probably from the ice-cap. Since the last eruption occurred between the early part of the 5th century (~ AD 420) and the end of the 7th century, these results highlight that Chimborazo is a potentially active volcano. Given its dominating presence over the densely populated Ambato and Riobamba basins, and owing its large ice-cap, Chimborazo should be considered a dangerous volcano.