Subaqueous Deposits Research Articles

Eruptive origins of a lacustrine pyroclastic succession: insights from the middle Huka Falls Formation, Taupo Volcanic Zone, New Zealand

Current and ancestral lakes within the central Taupo Volcanic Zone (TVZ) provide depocentres for pyroclastic deposits, providing a reliable record of eruption history. These lakes can also be the source of explosive eruptions that directly feed pyroclast-rich density currents. The lithofacies characteristics of pyroclastic deposits allow discrimination between eruption-fed and resedimented facies. The most frequently recognised styles of subaqueous eruptions in the TVZ are shallow-water phreatomagmatic and phreatoplinian eruptions that form subaerial eruption columns. However, deeper source conditions (>150 m water depth) could generate subaqueous explosive eruptions that feed water-supported pyroclast-rich density currents, similar to neptunian eruptions. Such deep-water eruptions have not previously been recognised in the TVZ. Here we study a subsurface deposit, the middle Huka Falls Formation (MHFF), in the Wairakei–Tauhara Geothermal Fields (Wairakei–Tauhara), TVZ, which we interpret to be the product of a relatively deep-water pyroclastic eruption (150–250 m). The largely subsurface Huka Falls Formation records past sedimentary and volcaniclastic deposition in ancient Lake Huka. Deposits examined from eight drill cores reveal a lithic-rich lower unit, a middle volumetrically dominant pumice lapilli-tuff and an upper thinly bedded suspension-settled tuff unit. A coarse lithic lapilli-tuff within the lower unit is locally thick and coarse near well THM12, suggesting proximity to a source located beneath Lake Huka. This research provides an understanding of the origin of the MHFF deposit and offers insights for evaluating and interpreting the diversity of subaqueous volcanic lake deposits elsewhere.

Influence of subaqueous processes on the construction and accumulation of an aeolian sand sheet

ABSTRACTIn aeolian sand sheets the interaction between aeolian and subaqueous processes is considered one of the principal factors that controls this depositional environment. To examine the role played by the subaqueous processes on the construction and accumulation of sand sheets, surface deposits and subsurface sedimentary sections of a currently active aeolian sand sheet, located in the Upper Tulum Valley (Argentina), have been examined. On the sand sheet surface, airflows enable the construction of nabkhas, wind‐rippled mantles (flattened accumulations of sand forming wind ripples), megaripples, and small transverse dunes. Subaqueous deposits consist of sandy current ripples covered by muddy laminae. The latter are generated by annual widespread but low‐energy floods that emanate from the nearby mountains in the aftermath of episodes of heavy precipitations. Deposits of subaqueous origin constitute 5% of the accumulated sand sheet thickness.The construction of the sand sheet is controlled by meteorological seasonal changes: the source area, the San Juan river alluvial fan, receives sediment by thaw‐waters in spring–summer; in fall–winter, when the water table lowers in the alluvial fan, the sediment is available for aeolian transport and construction of the sand sheet area. The flood events play an important role in enabling sand sheet accumulation: the muddy laminae serve to protect the underlying deposits from aeolian winnowing. Incipient cement of gypsum on the sand and vegetation cover acts as an additional stabilizing agent that promotes accumulation. Episodic and alternating events of erosion and sedimentation are considered the main reason for the absence of soils and palaeosols. Results from this study have enabled the development of a generic model with which to account for: (i) the influence of contemporaneous subaqueous processes on the construction and accumulation in recent and ancient sand sheets; and (ii) the absence of developed soils in this unstable topographic surface. Copyright © 2013 John Wiley & Sons, Ltd.

Pyroclast characteristics of a subaqueous to emergent Surtseyan eruption, Black Point volcano, California

We present the first-ever quantitative analysis of pyroclast textures from subaqueous deposits of a Surtseyan emergent volcano. The volcano, Black Point, formed in Mono Lake's Pleistocene precursor, Lake Russell, at ~13,300±500 BP, from an explosive basaltic eruption involving the interaction of shallow lake water (~105m) with vesiculating magma. The bulk of the edifice comprises unconsolidated tephra and represents submerged stages of eruption. Thin summit deposits formed during a final phase of emergent eruption and produced a group of low tuff rings. Lapillus clasts (16mm to 32mm) and fine ash grains (4Φ, 0.0625mm) from the subaqueous deposits have textures that reveal a conduit history of bubble nucleation, growth, collapse, intense microlite crystallisation, and magma–water interaction. Lapilli vesicularity ranges broadly to high values (up to 89%), while vesicle number densities (5.4×102–4.8×104 vesicles per mm3) are comparable to those published for Strombolian and Plinian basaltic eruptions (>1×104 vesicles per mm3). Vesicle size and volume distributions suggest continuous bubble nucleation, but the suite of data as a whole indicates that a late-stage nucleation event played a significant role in shaping the eruption. Ash grain morphology and surface features indicative of magma–water interactions attest to water's role in fragmentation. Overall the pyroclast textures are consistent with water modification of a rather intense basaltic eruption, which might have been of a violent Strombolian or Plinian style if the eruption had been “dry” and occurred subaerially.

VOLCANIC AND VOLCANICLASTIC ROCKS OF THE MESOARCHAEAN PONGOLA SUPERGROUP IN SOUTH AFRICA AND SWAZILAND: DISTRIBUTION, PHYSICAL CHARACTERISTICS, STRATIGRAPHY AND CORRELATIONS

The Mesoarchaean Pongola Supergroup is an exceptionally well-preserved succession of volcanic and sedimentary rocks that extends for 270 km close to the eastern margin of the Kaapvaal Craton in South Africa and Swaziland. It represents a volcano-sedimentary continental or epicontinental deposit that is one of the most extensive coherent Archaean terrains in the world, and the oldest of this extent. Its characteristics are unique amongst supracrustal terrains of this age and are unlike those of other Archaean greenstone belts. Its unique nature is that it marks the transition in southern African crustal development from preceding early greenstone belts, such as observed in the Barberton and Nondweni greenstone belts, to late Archaean basin formation on stable continental crust. The volcano-sedimentary succession of the Pongola Supergroup is divided into the lower, predominantly volcanic, Nsuze Group, which contains subsidiary but nonetheless important sedimentary formations, and the upper, dominantly sedimentary, Mozaan Group, which also contains several volcanic successions of highly variable forms and thicknesses. A basement high separates the Pongola Supergroup into a Northern Domain, which is relatively undeformed, and the Southern Domain, which is strongly deformed and folded. The focus of this paper is on the occurrence, field relations (with detailed maps), description and correlation of the volcanic rocks in both the Nsuze and Mozaan Groups in all areas of the Pongola Supergroup, focusing on 11 key areas in South Africa and Swaziland. The compositional range from basalt to rhyolite is remarkable, and this paper focuses on the distribution of different magma types and styles of eruption throughout the belt, including the first record of ultramafic compositions in the Pongola Supergroup. Ultramafic pyroclastic rocks in the Southern Domain contain highly vesicular fragments that may explain the absence of lava flows of the same composition. Volcanic styles transgress the entire length of the belt, with subaerial sheet flows and minor pillow development in the southern Nkandla region through to a possible continental marginal setting with both subaqueous and subaerial volcanism in the south of the Northern Domain and, finally, into a bimodal suite of felsic and mafic to intermediate volcanic rocks in the northernmost regions. Felsic ignimbrite volcanic rocks with abundant lithophysae are present in the central region. A 770 m-thick section of volcanic rocks that includes andesite and dacite compositions as well as fragmental and volcaniclastic units, including an ultramafic volcaniclastic deposit, is observed in a borehole section of the Tobolsk Formation of the Mozaan Group, that is completely overlain by Karoo sediments in the east. Cyclical episodes of volcanism, many of which were of an explosive nature, were interspersed with periods of rapidly deposited coarse-grained sediments in alluvial fans and braided rivers close to the continental margin. Carbonate formation, together with the development of stromatolites, took place during periods of volcanic quiescence, or reduced volcanic activity, in highly protected surrounds and in intertidal zones. Synchronous volcanism, sedimentation and deep erosion gave rise to the marked lateral variations in both subaerial and subaqueous deposits and marked lithologic variability. The largely volcaniclastic (with a high proportion of ultramafic component) Ndikwe Formation in the Southern Domain is enigmatic with contrasting suggested settings of an early phase of Pongola formation, or a later stage nappe structure, with a possible Mozaan Group time equivalence. No single tectonic setting, on modern day analogues, can explain all features of the Pongola volcano-sedimentary belt. Current evidence for the Nsuze Group is most consistent with a continental arc migrating into a continental rifted environment to the north. The overlying Mozaan Group is inferred to have formed in a cratonic basin following thermal relaxation after the preceding cycles of voluminous volcanism.

Spatial Change of Grading Pattern of Subaqueous Flood Deposits In Lake Shinji, Japan

Abstract We acquired three geo-slicer cores along a longitudinal profile on the subaqueous portion of the Hii River delta built since the 1630s in Lake Shinji, western Japan. Grading patterns of 26 individual sand beds in the cores vary spatially. We interpret these sand beds as subaqueous flood deposits from their sedimentary structures, the geomorphology of the delta lobe, and the absence of hydrodynamic influences other than river floods entering the lake. Most sand beds at shallow depths (< 4 m below the water surface) display triple stacks of inverse-to-normal grading. Single inverse-to-normal grading appears between 4 and 5 m depth, and normal grading dominates in deeper beds. Inverse and normal grading could reflect the waxing and waning of the parent flow, respectively. We explain the occurrence of triple stacks of inverse-to-normal grading in shallower horizons and single inverse-to-normal grading in deeper horizons as consequences of lateral movement of the plunge point of flood plumes during the course of flood events. Spatially decelerating sediment-laden river plumes steeply increase their velocity after they plunge beneath the water surface. In depth-limited proximal areas of a subaqueous delta, back-and-forth translation of the plunge point over a fixed point due to the waxing and waning of river discharge leads to three cycles of waxing and waning of flow velocity, and the resulting deposits at the fixed point would have triple stacks of inverse-to-normal grading. In the distal parts of the delta, velocity of fully plunged hyperpycnal flow increases and then decreases, reflecting directly the waxing and waning of river discharge, and the resulting deposits would be composed of single inversely-to-normally graded beds. In the most distal parts, the slower initial part of the hyperpycnal flow may be overtaken by the succeeding faster part to yield a monotonically waning flow at a fixed point. This process may have resulted in sand beds with uniformly normal grading in lower (distal) horizons. Assuming that flood hydrographs for the Hii River have not changed since the 17th century, these findings mean that the momentum of river flow during floods propagates to the outflow differently at different distances from the river mouth. Our results show that in parts of the delta-front slope, the outflow of river floods accounts for the inverse-to-normal grading considered to be an important basis for identifying hyperpycnal flow turbidites. However, in other parts of this environment, grading patterns of flood deposits do not necessary reflect the flood hydrograph. Overdependence on inverse-to-normal grading in identifying hyperpycnal flow deposits may lead to inaccurate estimations of their frequency in the sedimentary record. On the other hand, special care is needed for the reconstruction of paleo-flood hydrographs using sedimentary records.

Spatial Change of Grading Pattern of Subaqueous Flood Deposits In Lake Shinji, Japan

Spatial Change of Grading Pattern of Subaqueous Flood Deposits In Lake Shinji, Japan

An accurate record of volcanic ash fall deposition as characterized by dispersed organic matter in a lower Permian tonstein layer (Faxinal Coalfield, Paraná Basin, Brazil)

For the first time, the dispersed organic matter in the tonstein layer interbedded with a coal seam in the Faxinal Coalfield (Sakmarian, Southern Parana Basin, Brazil) is characterized. The deposition of clusters of pollen grains was highly influenced by the intense ash fall process that probably occurred during seasonal dehiscence of reproductive structures. The well-preserved phytoclasts with their upper and lower leaf cuticles stuck together indicate that the rapid fall of ash on this material hindered organic biodegradation. The preservation of seemingly autochthonous Botryococcus colonies at the top of the tonstein layer is evidence of the subaqueous deposition of this layer. The darkening in cuticles and xylem phytoclasts can be attributed to different causes: the thermal influence of ash fall during deposition, chemical effects of the ash, prolonged oxidation of organic matter in low water level conditions or the burning of plant organs by wildfires. Analyses of dispersed organic matter along the tonstein layer showed that the organic matter succession reflects the composition of different plant strata (herbaceous pteridophytes and arboreal glossopterids-cordaitaleans) around the deposition site.

New insights into the deformation of a Middle Pleistocene glaciotectonised sequence in Norfolk, England through magnetic and structural analysis

Abstract North Norfolk is a classic area for the study of glacial sediments with a complex glaciotectonic deformational history, but the processes leading to the formation of some structures can be ambiguous. Anisotropy of magnetic susceptibility (AMS) analyses, providing quantitative fabric data, have been combined with the analysis of visible structures and applied to the Bacton Green Till Member, exposed at Bacton, Norfolk. Thermomagnetic curves, low temperature susceptibility and acquisition of isothermal remanent magnetism (IRM) reveal that the magnetic mineralogy is dominated by paramagnetic phases. The magnetic foliation is parallel to fold axial planes and weakly inclined to bedding, whilst the magnetic lineation is orientated parallel to stretching, indicated by the presence of stretching lineations and the trend of sheath folds. Variations in the orientation of the magnetic lineation suggest that the Bacton section has been subject to polyphase deformation. After subaqueous deposition, the sequence was overridden by ice and glaciotectonically deformed which involved stretching initially north–south, then east–west. These results show that AMS can be used to detect strain in three dimensions through a glaciotectonite where paramagnetic mineralogy is dominant. This approach therefore provides further support to the use of AMS as a fast, objective and accurate method of examining strain within deformed glacial sediments.

Permian dust in Oklahoma: Source and origin for Middle Permian (Flowerpot-Blaine) redbeds in Western Tropical Pangaea

Analogous to many Permian units globally, the Middle Permian of Oklahoma (Flowerpot Shale and Blaine Formation) contains voluminous fine-grained redbeds. These units have long been interpreted to record marine to marginal-marine deposition owing to minor evaporite/dolomite strata; this interpretation, however, disregards the predominant siliciclastic material. Siltstone predominates, and all siliciclastic material is of inferred aeolian origin owing to the fine and remarkably uniform grain size, internally massive structure, blanket-like geometry, and common palaeosols, especially in the Flowerpot Shale. Previously suggested alternative environments for such abundant fine-grained material, such as distal deltaic deposition, are inconsistent with the absence of key sedimentary structures (e.g., graded beds), associated facies (e.g., channelised units), and vertical or lateral trends (e.g., upward coarsening). The minor claystone and associated evaporite and dolomite facies of the Blaine Formation exhibit evidence for subaqueous deposition, but with aeolian delivery of the siliciclastic component. An aeolian dust origin for the siliciclastic material reinforces the interpretation of generally semiarid conditions for this equatorial region of western Pangaea. Whole-rock geochemical and detrital-zircon geochronological data on the siliciclastic units indicate a mixed provenance that includes a mafic component exhibiting a composition similar to reference populations from the Ouachita orogen. The dominant zircon populations reflect transport from easterly/southeasterly directions, with fewer grains likely derived from basement located to the west. Combining an aeolian delivery with the provenance signal indicates predominant equatorial easterlies during deposition of the study units, and subordinate westerlies, consistent with Pangaean monsoonal circulation. Permian redbeds preserved in many parts of former low-latitude Pangaea bear attributes similar to those of the units documented here, suggesting a possible greater role for dust deposition during this time than previously appreciated.

Glacitectonic deformation in the Chuos Formation of northern Namibia: implications for Neoproterozoic ice dynamics

Abstract The Chuos Formation is a diamictite-dominated succession of Cryogenian age, variously interpreted as the product of glaciomarine deposition, glacially related mass movement, or rift-related sediment remobilisation in a non-glacial environment. These interpretations have wide ranging implications for the extent of ice cover during the supposedly pan-global Neoproterozoic icehouse. In the Otavi Mountainland, northern Namibia, detailed analysis of soft-sediment deformation structures on the macro- and micro-scale support glacitectonic derivation in response to overriding ice from the south/south-east. Overall, the upward increase in strain intensity, predominance of ductile deformation features (e.g. asymmetric folds, rotational turbates and necking structures, clast boudinage, unistrial plasmic fabrics) and pervasive glacitectonic lamination support subglacial deformation under high and sustained porewater pressures. In contrast, soft-sediment structures indicative of mass movements, including flow noses, tile structures, and basal shear zones, are not present. The close association of subglacial deformation, abundant ice-rafted debris and ice-contact fan deposits indicate subaqueous deposition in an ice-proximal setting, subject to secondary subglacial deformation during oscillation of the ice margin. These structures thus reveal evidence of dynamic grounded ice sheets in the Neoproterozoic, demonstrating their key palaeoclimatic significance within ancient sedimentary successions.

Characteristics of submarine pumice-rich density current deposits sourced from turbulent mixing of subaerial pyroclastic flows at the shoreline: field and experimental assessment

This study investigates the types of subaqueous deposits that occur when hot pyroclastic flows turbulently mix with water at the shoreline through field studies of the Znp marine tephra in Japan and flume experiments where hot tephra sample interacted with water. The Znp is a very thick, pumice-rich density current deposit that was sourced from subaerial pyroclastic flows entering the Japan Sea in the Pliocene. Notable characteristics are well-developed grain size and density grading (lithic-rich base, pumice-rich middle, and ash-rich top), preponderance of sedimentary lithic clasts picked up from the seafloor during transport, fine ash depletion in coarse facies, and presence of curviplanar pumice clasts. Flume experiments provide a framework for interpreting the origin and proximity to source of the Znp tephra. On contact of hot tephra sample with water, steam explosions produced a gas-supported pyroclastic density current that advanced over the water while a water-supported density current was produced on the tank floor from the base of a turbulent mixing zone. Experimental deposits comprise proximal lithic breccia, medial pumice breccia, and distal fine ash. Experiments undertaken with cold, water-saturated slurries of tephra sample and water did not produce proximal lithic breccias but a medial basal lithic breccia beneath an upper pumice breccia. Results suggest the characteristics and variations in Znp facies were strongly controlled by turbulent mixing and quenching, proximity to the shoreline, and depositional setting within the basin. Presence of abundant curviplanar pumice clasts in submarine breccias reflects brittle fracture and dismembering that can occur during fragmentation at the vent or during quenching. Subsequent transport in water-supported pumiceous density currents preserves the fragmental textures. Careful study is needed to distinguish the products of subaerial versus subaqueous eruptions.

A Neoproterozoic continental rift succession: The volcano-sedimentary Koivib Mountains deposits of Namibia

Neoproterozoic volcanic and sedimentary rocks compose the 750m thick Koivib Mountains succession, which forms part of the Rosh Pinah Formation of southwest Namibia. Detailed sedimentary facies analysis and physical volcanology enabled determination of four lithofacies and component twelve facies that record subaqueous deposition in a tectonically controlled shallow sea. The mafic volcanic lithofacies is composed of mafic: (1) flow, (2) volcaniclastic, and (3) intrusive facies characteristic of seamount building by effusive volcanic processes. The felsic volcanic lithofacies containing felsic: (4) flow and (5) volcaniclastic facies displays features characteristic of proximal to distal and marginal regions of subaqueous dome–flow complexes. The siliciclastic lithofacies composed of (6) shale, (7) thin- to medium-bedded sandstone, (8) thick-bedded sandstone, and (9) sedimentary breccia facies is consistent with a submarine fan environment in which subaqueous density current and turbidity flow processes were predominant. The silicilastic–carbonate lithofacies, which contains (10) massive bedded carbonate, (11) laminated to cross-bedded calcarenite, and (12) graded bedded calcarenite, formed mainly through erosion of primary carbonate and transport of material from a platform to a slope setting. The overall stratigraphy, combined with geochemical results indicating bimodal volcanism in a within-plate tectonic setting, signifies that the Koivib Mountains succession developed as part of a continental rift sequence associated with Neoproterozoic break-up of the Rodinia supercontinent. The structural geology of the Koivib Mountains revealed two generations of folds, including (i) NW trending tight, isoclinal folds (F1) and (ii) NNW trending open folds (F2). The second deformation event refolded F1 folds, producing periclinal structures. The deformation features are consistent with SE transpression associated with the continental collision that occurred during closure of the Adamastor Ocean and formation of Gondwana.

Stratigraphic architecture and sedimentology of a Late Pleistocene subaqueous moraine complex, southwest Ireland

AbstractGlacigenic sediments exposed in coastal cliffs cut through undulatory terrain fronting the Last Glacial Maximum laterofrontal moraine at Waterville on the Iveragh Peninsula, southwest Ireland, comprise three lithofacies. Lithofacies 1 and 2 consist of interdigitated, offlapping and superimposed ice‐proximal subaqueous outwash and stacked sequences of cohesionless and cohesive subaqueous debris flows, winnowed lag gravels and coarse‐grained suspension deposits. These are indicative of sedimentation in and around small grounding line fans that prograded from an oscillating glacier margin into a proglacial, interlobate lake. Lithofacies 3 comprises braided river deposits that have undergone significant syn‐sedimentary soft‐sediment deformation. Deposition was likely related to proglacial outwash activity and records the reduction of accommodation space for subaqueous sedimentation, either through the lowering of proglacial water levels or due to basin infilling. The stratigraphic architecture and sedimentology of the moraine at Waterville highlight the role of ice‐marginal depositional processes in the construction of morphostratigraphically significant ‘end moraine’ complexes in Great Britain and Ireland. Traditional ‘tills’ in these moraines are often crudely stratified diamictons and gravelly clinoforms deposited in ice‐proximal subaqueous and subaerial fans. Copyright © 2011 John Wiley &amp; Sons, Ltd.

Lateglacial to early Holocene multiproxy record from Loch Assynt, NW Scotland

A core, recovered from a water depth of 53m in Loch Assynt, North-West Scotland, has yielded a 9m sequence comprising two distinct units, an upper, organic-rich unit (Unit I, ca. 6m) overlying a sequence of laminated clays, silts and sands (Unit II, ca. 3m). The upper unit is essentially Holocene in age based upon three bulk AMS radiocarbon dates while a fourth radiocarbon date from Unit II confirms a late-glacial age for that interval and supports a broadly linear age–depth relationship. Distinct variations in the magnetic susceptibility record of the lower unit can be visually correlated to major changes in the Greenland ice core (GISP2), this together with pollen evidence supports the radiocarbon dating suggesting an age of approximately 11,000 to around 17,000cal. BP for Unit II, with evidence for the Younger Dryas (Loch Lomond) stadial and the Bolling–Allerød climatic phases. Variations in the magnetic susceptibility record of the late-glacial sediments are thought to relate to climatically driven changes in soil cover and erosion rates. The multiproxy record from Loch Assynt indicates relatively continuous, sub-aqueous sedimentation during the last ∼17,000 years, providing an approximate age for the initiation of modern Loch Assynt and supporting recent dates of moraine retreat lines in the Loanan Valley from about 14–15 ka BP. Pollen and chironomid sampling provides further insights to the history of this relatively deep water body and compliment existing high-resolution palaeo-precipitation records for the mid to late Holocene interval from speleothem archives within the loch catchment.

Physicochemical properties of concentrated Martian surface waters

[1] Understanding the processes controlling chemical sedimentation is an important step in deciphering paleoclimatic conditions from the rock records preserved on both Earth and Mars. Clear evidence for subaqueous sedimentation at Meridiani Planum, widespread saline mineral deposits in the Valles Marineris region, and the possible role of saline waters in forming recent geomorphologic features all underscore the need to understand the physical properties of highly concentrated solutions on Mars in addition to, and as a function of, their distinct chemistry. Using thermodynamic models predicting saline mineral solubility, we generate likely brine compositions ranging from bicarbonate-dominated to sulfate-dominated and predict their saline mineralogy. For each brine composition, we then estimate a number of thermal, transport, and colligative properties using established models that have been developed for highly concentrated multicomponent electrolyte solutions. The available experimental data and theoretical models that allow estimation of these physicochemical properties encompass, for the most part, much of the anticipated variation in chemistry for likely Martian brines. These estimates allow significant progress in building a detailed analysis of physical sedimentation at the ancient Martian surface and allow more accurate predictions of thermal behavior and the diffusive transport of matter through chemically distinct solutions under comparatively nonstandard conditions.

New chelid turtles of the lower section of the Cerro Barcino formation (Aptian-Albian?), Patagonia, Argentina

A new chelid species ( Prochelidella cerrobarcinae nov. sp.) are described from the Aptian-Albian? Puesto La Paloma Member, Cerro Barcino Formation, northern of Chubut Province, Argentina. The basal section of this member, which bears the turtle remains, is composed of tuffaceous mudstones with plane parallel lamination, asymmetrical ripples and a chert intercalation suggesting sub-aqueous deposition in a relatively shallow lacustrine environment. Pr. cerrobarcinae nov. sp. is represented by post-cranial remains of several specimens that not only represents the oldest pleurodiran chelid record in the world but, together with the chelid remains of Albian Lightning Ridge, New South Wales, Australia, indicates that chelid diversification began well before the final fragmentation of southern Gondwana.

Palaeoproterozoic metavolcanic and metasedimentary succession hosting the Dannemora iron ore deposits, Bergslagen region, Sweden

The Dannemora supracrustal inlier is located in the north-eastern part of the Bergslagen region in south-central Sweden and hosts the second largest iron ore deposit in the region. The metasupracrustal succession of the inlier consists of c. 1.9 Ga Palaeoproterozoic rocks that are mainly sub-alkaline, rhyolitic to dacitic, pyroclastic deposits, reworked pyroclastic deposits and metalimestone. It is c. 700–800-m thick and termed the Dannemora Formation. The formation is divided into lower and upper members and the former is in turn subdivided into subunits 1 and 2. The great thickness of individual pyroclastic deposits indicates deposition within a caldera. The rocks show characteristics of a pyroclastic origin by containing abundant pumice, cuspate and Y-shaped former glass shards, and fragmented crystals of quartz and subordinate feldspars. Scattered spherulites and lack of welding-compacted fiamme suggest that the lower member was slightly welded, where as the upper member contains sericite-replaced glass shards with preserved primary shapes indicating no welding. Undisturbed layers of ash-siltstone with normal grading and fluid–escape structures are attributed to subaqueous deposition below storm wave base in the eastern part of the inlier, where as erosion channels and cross-bedding in some of the volcaniclastic deposits imply deposition and reworking above wave base in the central part of the inlier. Epidote spots, previously interpreted as altered limestone fragments and an indicator for subaquatic deposition, are here reinterpreted as the result of selective alteration related to the intrusion of mafic dykes and to Ca release during dolomitisation of limestone.

Geochemical reconstruction of the provenance, weathering and deposition of detrital-dominated sediments in the Perth Basin: The Cretaceous Leederville Formation, south-west Australia

The deep (> 10 km) Permian to present day Perth Basin in southwest Western Australia is composed of sedimentary successions of continental to shallow marine origin. This study examines the compositional variation, weathering and provenance of the early Cretaceous Leederville Formation within the Perth Basin. Over 200 sediment samples were retrieved from two deep boreholes of ~ 350 to 530 m in depth encompassing the Leederville Formation, and analysed for major and trace element geochemistry and mineralogy. The Leederville Formation is composed of subarkosic sands, predominantly composed of quartz (64%) and K-feldspars (27%); silts and clays composed of kaolinite (24–54%), K-feldspars (20–29%) and quartz (18–40%), with their compositional variation reflecting weathering, hydraulic sorting during transport, mostly subaqueous deposition and subsequent diagenesis. Most trace elements are associated with fine grained lithologies due to their occurrence within clays, sulphide or zircon. Major and trace element geochemistry suggests a typical passive margin origin with minimal intrabasinal sediment recycling. The regionally extensive Yilgarn Craton, appears to be the main sediment source for the Leederville Formation within the study area.

Explosive rhyolite tuya formation: classic examples from Kerlingarfjöll, Iceland

Rhyolite eruptions in Iceland mostly take place at long-lived central volcanoes, examples of which are found associated with each of the present-day rift-zone ice caps. Subglacial eruptions at Kerlingarfjöll central volcano produced rhyolite tuyas that are notable for their exposures of early-erupted pyroclastic material. Observations from a number of these edifices are synthesised into a general model for explosive rhyolite tuya formation. Eruptions begin with violent phreatomagmatic explosions that generate massive tuff (mT), but the influence of water quickly declines, leading to the formation of massive lapilli-tuffs (mLT) containing magmatically-fragmented vesicular pumice and ash. These are deposited rapidly near the vent, probably by moist pyroclastic density currents, confined by ice but not within a meltwater lake. The explosive-effusive transition is controlled by the ascent rate and gas content of the magma. An unusual obsidian-rich massive lapilli-tuff lithofacies (omLT) is identified and interpreted as pyroclastic material that was intruded into gas-fluidised deposits at the explosive-effusive transition. The effusive phase of eruption involves the emplacement of intrusions and lava caps. Intrusions of lava into the early-erupted phreatomagmatic deposits are characterised by peperitic margins and the formation of hyaloclastite. Intrusions into stratigraphically higher levels of the pyroclastic material show more limited interaction with the host tephra and have microcrystalline cores. Large lava bodies with columnar-jointed margins cap the tuyas and have intrusive basal contacts with the tephras. The main influence of the ice is to confine the rhyolite eruptive products to immediately above the vent region. This is in contrast to subglacial basaltic tuya-forming eruptions, which are characterised by the formation of meltwater lakes, phreatomagmatic fragmentation and subaqueous deposition. The lack of meltwater storage may reduce the potential for large jökulhlaups.

Subaerial and subaqueous deposition of loess: Experimental assessment of detrital remanent magnetization in Chinese loess

Loess and intercalated paleosols on the Chinese Loess Plateau (CLP) are the most extensive and continuous terrestrial archive of geomagnetic and paleoclimate variations covering the last 2.6 Ma. A first-order time-frame has been established for these sequences using paleomagnetic polarity boundaries. Major inconsistencies between the positioning of the Matuyama/Brunhes polarity boundary in Chinese loess/paleosol sequences and marine records, previously accounted for by assuming magnetization lock-in depths in loess of up to 2–3 m, have recently been resolved using a paleoclimatic indicator insensitive to pedogenesis that does require any major post-depositional acquisition of a remanent magnetization. This resolved controversy accentuates the lack of an adequate understanding of how Chinese loess acquires and retains a stable remanent magnetization. We report on laboratory re-deposition experiments with subaerial and subaqueous simulations of natural loess to elucidate factors controlling the acquisition of magnetization in loess. Our work complements and extends recently published results from comparable laboratory experiments by Zhao and Roberts (2010). Our major finding is that a stable magnetization is acquired and retained after initial wetting of dry-deposited loess with no significant time lag, in general accord with recent findings.