Volcaniclastic Layers Research Articles

U-Pb zircon age of volcaniclastic layers in Middle Triassic platform carbonates of the Austroalpine Silvretta nappe (Switzerland)

We present precise U-Pb age determinations from two volcaniclastic layers within Middle Triassic carbonates in the Upper Austroalpine Silvretta nappe near Davos (Switzerland). The two volcaniclastic layers were dated using annealing-leaching techniques and yielded ages of 240.91 ± 0.26 Ma (Prosanto Formation) and 239.89 ± 0.21 Ma (Altein Formation), respectively. The high resolution ages allow comparison of the Upper Austroalpine record of the Ducan with sections in the Southern Alps. The upper Prosanto Formation is, thus, equivalent to the middle part of the Buchenstein Formation (Middle Pietra Verde, Earliest Ladinian), and the Altein Formation is equivalent to the upper part of the Buchenstein Formation in the section with the Global boundary Stratotype Section and Point (GSSP) for the base of the Ladinian (Bagolino, northern Italy). This study demonstrates that we can use precise, accurate and carefully intercalibrated U-Pb zircon ages from volcaniclastic layers to infer the stratigraphic position of their host sediments on zone level. The older volcaniclastic layer (240.91 ± 0.26 Ma) allows a precise age determination (earliest Ladinian) for the marine vertebrate beds in the upper Prosanto Formation.

Integrated stratigraphy of the Oligocene pelagic sequence in the Umbria-Marche basin (northeastern Apennines, Italy): A potential Global Stratotype Section and Point (GSSP) for the Rupelian/Chattian boundary

The Oligocene represents an important time period from a wide range of perspectives and includes significant climatic and eustatic variations. The pelagic succession of the Umbria-Marche Apennines (central Italy) includes a complete and continuous sequence of marly limestones and marls, with volcaniclastic layers that enable us to construct an integrated stratigraphic framework for this time period. We present here a synthesis of detailed biostratigraphic, magnetostratigraphic, and chemostratigraphic studies, along with geochronologic results from several biotite-rich volcaniclastic layers, which provide the means for an accurate and precise radiometric calibration of the Oligocene time scale. From this study, the interpolated ages for the Rupelian/Chattian stage boundary, located in the upper half of Chron 10n at meter level 188 in the Monte Cagnero section, and corresponding to the O4/O5 planktonic foraminiferal zonal boundary, are 28.36 Ma (paleomagnetic interpolation), 28.27 ± 0.1 Ma (direct radioisotopic dating), and 27.99 Ma (astrochronological interpolation). These ages appear to be slightly younger than those reported in recent chronostratigraphic time scale compilations. The Monte Cagnero section is a potential candidate for defining the Chattian Global Stratotype Section and Point (GSSP) and some reliable criteria are here proposed for marking the Rupelian/Chattian boundary according to International Union of Geological Sciences (IUGS) recommendations.

Vertical tectonics of the High Plateau region, Manihiki Plateau, Western Pacific, from seismic stratigraphy

The Manihiki Plateau is an elevated oceanic volcanic plateau that was formed mostly in Early Cretaceous time by hotspot activity. We analyze new seismic reflection data acquired on cruise KIWI 12 over the High Plateau region in the southeast of the plateau, to look for direct evidence of the location of the heat source and the timing of uplift, subsidence and faulting. These data are correlated with previous seismic reflection lines from cruise CATO 3, and with the results at DSDP Site 317 at the northern edge of the High Plateau. Seven key reflectors are identified from the seismic reflection profiles and the resulting isopach maps show local variations in thickness in the southeastern part of the High Plateau, suggesting a subsidence (cooling) event in this region during Late Cretaceous and up to Early Eocene time. We model this as a hotspot, active and centered on the High Plateau area during Early Cretaceous time in a near-ridge environment. The basement and Early Cretaceous volcaniclastic layers were formed by subaerial and shallow-water eruption due to the volcanic activity. After that, the plateau experienced erosion. The cessation of hotspot activity and subsequent heat loss by Late Cretaceous time caused the plateau to subside rapidly. The eastern and southern portions of the High Plateau were rifted away following the cessation of hot spot activity. As the southeastern portion of the High Plateau was originally higher and above the calcium carbonate compensation depth, it accumulated more sediments than the surrounding plateau regions. Apparently coeval with the rapid subsidence of the plateau are normal faults found at the SE edge of the plateau. Since Early Eocene time, the plateau subsided to its present depth without significant deformation.

Widespread syn-eruptive volcaniclastic deposits in the Pleistocenic basins of South-Western Calabria

Since late Tortonian, depositional sequences developed along the Southern Calabrian forearc, inside the half-graben depressions of the Mesima-Gioia Tauro and Reggio Calabria basins. In these basins volcaniclastic sedimentation took place during lower to middle Pleistocene. Volcaniclastic deposits consist of isolated pumice swarms, ash layers and thick successions of lapilli and ash. In this paper we describe and discuss the depositional characteristics of these volcanogenic sediments and the morphology, mineralogy and chemistry of pumices, shards, lithics and crystal fragments. These deposits all have the characteristics of a proximal tephra fall deposit from a Plinian plume, which has been reworked in a shore environment. In the Mesima basin the deposits reach 6 m in thickness and in the Gioia Tauro plain 21 m of pumiceous sediments have been cored in a drill hole. The compositional homogeneity of fragments and the absence of non-volcanic detritus as well as of soil or organic matter suggest that the deposits were quickly reworked after their primary emplacements. All the tephra deposits exhibit calc-alkaline affinity and dacitic to rhyolitic composition. The mineralogical assemblage, including orthopyroxene as phenocryst, indicates provenance from magmatic-arc volcanism. ICP-MS and LA-ICP-MS analyses of crystals and glass point to a common origin, probably from a single eruption. This makes these volcaniclastic layers a very useful tool for stratighaphic reconstructions at a regional scale. The geological setting of the deposits, their petrological and geochemical signature and the occurrence of large-size clasts suggest a provenance from explosive eruptions preceding the emplacement of the 50 ka dacitic lava dome of the island of Panarea (Aeolian Archipelago). However a provenance from a different calc-alkaline volcanic system cannot be excluded. These deposits can therefore represent the only evidence of a large explosive eruption in the Southern Tyrrhenian Sea whose very proximal equivalent is not preserved.

Geometry and chronology of growth and drowning of Middle Triassic carbonate platforms (Cernera and Bivera/Clapsavon) in the Southern Alps (northern Italy)

The depositional architecture and the geometric relationships between platform-slope deposits and basinal sediments along with paleontological evidence indicate the time interval of the younger Anisian Reitziites reitzi ammonoid zone to largely represent the main stage of platform aggradation at the Cernera and Bivera/Clapsavon carbonate platforms. Published and new U-Pb age data of zircons from volcaniclastic layers bracketing the stratigraphic interval of platform growth constrain the duration of platform evolution to a time span shorter than 1.8±0.7m.y., probably in the order of 0.5-1m.y., reflecting fast rates of vertical platform aggradation exceeding 500 m/m.y. In the range of growth potentials for shallow-water carbonate systems estimated in relation to the time span of observation, this high rate is in agreement with values for short intervals of 105-106yrs (e.g., Schlager 1999). After drowning, the platforms at Cernera and Bivera/Clapsavon were blanketed by thin pelagic carbonates. On the former platform flanks the draping sediments in places comprise red nodular pelagic limestones (Clapsavon Limestone) similar in facies to the Han Bulog Limestones occurring elsewhere in Middle Triassic successions of the Mediterranean Tethys. The drowning of vast areas of former carbonate platforms possibly triggered the onset of bottom-water circulation in adjacent basins as suggested by the abrupt transition from laminated to bioturbated pelagic nodular limestones in the Buchenstein Formation which occurred close to the time of initial platform submergence. During the Late Ladinian the topographic features of the drowned platforms were onlapped by rapidly deposited, predominantly clastic successions including coarse breccias and volcanic rocks sealing and preserving the peculiar stratigraphic setting.

Timing of Late Pliocene to Middle Pleistocene tectonic events in Rhodes (Greece) inferred from magneto-biostratigraphy and 40Ar/ 39Ar dating of a volcaniclastic layer

We discovered a volcaniclastic layer in the Plio-Pleistocene coastal sequences on the island of Rhodes (Aegean fore-arc, Greece). Here, we present an integrated isotopic, magnetostratigraphic, and biostratigraphic (planktonic foraminifers) study for the Haraki section, where this layer is found intercalated in several meters of sedimentary rocks corresponding to the Lindos Bay clay Member of the Rhodes Formation. 40Ar/ 39Ar dating of the volcaniclastic layer provides an age of 1.89 ± 0.09 Ma, which is consistent with our planktonic foraminiferal data. Magnetostratigraphic results show that the entire Haraki section is of normal polarity and according to the isotopic results this corresponds to the Olduvai subchron (1.95–1.77 Ma). The new age determination provides severe constraints for deciphering the sedimentary and tectonic evolution of Rhodes since the Late Pliocene, which can be summarized in the following: (1) 500 to 600 m drowning during the latest Pliocene that could be related to the westward motion of the Anatolian Plate; (2) at least 520 m of uplift at around 1.4–1.3 Ma related to activity of the sinistral strike-slip of the Pliny Trench, the deep Rhodes basin being separated from the island of Rhodes; (3) a counterclockwise rotation of Rhodes, younger than 1.2–1.1 Ma, and possibly synchronous with the young clockwise rotation of the western Aegean arc.

Stratigraphy and Detrital Modes of Upper Messinian Post-evaporitic Sandstones of the Southern Apennines, Italy: Evidence of Foreland-Basin Evolution during the Messinian Mediterranean Salinity Crisis

During the Messinian, the southern Apennines thrust belt experienced a period of strong tectonic rearrangement and accretion, activation of overthrusts, and consequent migration of depocenters. The upper Miocene successions cropping out in the northern segment of the southern Apennine thrust belt have good potential for improving our understanding of the interplay between Messinian salinity-crisis events and foreland-basin evolution. The local Messinian stratigraphy includes: (1) pre-evaporitic thin-bedded euxinic marly clay, interbedded with diatomaceous marls; (2) evaporitic limestone, crystalline gypsum, and reworked gypsum; (3) post-evaporitic deposits subdivided into two main units: the Torrente Fiumarella unit and the Anzano Molasse Formation that grade upward into ostracod-rich deposits (Lago-Mare facies). The evaporitic and post-evaporitic sequences are separated by an angular unconformity. This paper deals with the stratigraphic and petrographic study of the post-evaporitic deposits. The Torrente Fiumarella unit includes lacustrine and alluvial conglomerates, quartzolithic sandstones containing abundant carbonate detritus, shale, and reworked clastic gypsum. The Anzano Molasse Formation includes thick-bedded deltaic to turbiditic conglomerates and sandstones passing upward to thin-bedded turbidite sandstones and marlyclayey siltstones. Sandstones are quartzofeldspathic with variable proportions of sedimentary (both carbonate and siliciclastic) and plutonic detritus. In particular, two populations are present, plutonic-rich and mixed plutonic-sedimentary. Volcaniclastic layers, composed of dominantly vitric particles (shards and pumice), are also interbedded within Anzano Molasse sandstones. The Anzano succession includes rare freshwater ostracods that increase in abundance in the uppermost Lago-Mare facies. The Lago-Mare facies deposits are represented by silty-marly clay with abundant Ostracoda shells (Ilyocypris gibba, Cyprideis torosa and Candona sp.) and intrarenite having abundant intrabasinal carbonate particles (ooids, peloids, and bioclasts) and subordinate extrabasinal noncarbonate and carbonate particles. The post-evaporitic sequences represent an infilled foredeep basin, with a lacustrine environment progressively deepening and experiencing gravity resedimentation. Detrital modes document complex provenance relations from upper Messinian accreted terranes of the southern Apennines thrust belt. Post-evaporitic sandstones in the Irpinia-Daunia sector of the southern Apennines foreland-basin system record both the effects of the foreland tectonic evolution and the Messinian Mediterranean salinity crisis. They may represent alternative models for foreland-basin evolution during a restricted time in late Messinian, which can be applicable also in other portions of the circum-Mediterranean orogen.

Two new Emsian (Early Devonian) U–Pb zircon ages from volcanic rocks of the Rhenish Massif (Germany): implications for the Devonian time scale

So far, only few biostratigraphically well-constrained isotopic ages constitute the geochronological framework of the Devonian period. To expand this sparse dataset, two new U–Pb isotope dilution–thermal ionization mass spectrometry (ID-TIMS) zircon ages were obtained from volcanic rocks of the Rhenish Massif (Germany). These rocks are interbedded in Emsian (Lower Devonian) marine sedimentary successions that are well documented by index fossils. Given that the U–Pb zircon data represent primary crystallization and eruption ages, they provide an important contribution to the Devonian time scale. The older sample, a stratiform volcaniclastic layer (Hans-Platte) is collected from Bundenbach (Hunsrück), the type locality of the Lower Emsian Hunsrück Slate. A cluster of five coherent single-zircon analyses obtained from this layer yielded a weighted mean 206 Pb/ 238 U age of 407.7 ± 0.7 Ma, which is regarded as the time of eruption of the pyroclastic rock. Biostratigraphically, the layer can be assigned to the upper part of the Polygnathus excavatus conodont zone. The younger U–Pb age was obtained from the Global Stratotype Section and Stratigraphic Point section at Wetteldorf (Eifel), where the K-bentonite ‘Hercules I’ is intercalated 13 m below the Lower–Middle Devonian boundary in the uppermost part of the Polygnathus costatus patulus conodont zone. Some of the 19 single-zircon analyses obtained from this bentonite appear to document significant influences of inheritance. The 13 youngest points are, however, concordant and form an elongated cluster along concordia with 206 Pb/ 238 U ages between 392.2 and 407.7 Ma. Given the (risky) assumption that this age scattering is exclusively caused by varying amounts of inheritance, the youngest point of this cluster represents the analysis with the least or no inheritance. In this case, its 206 Pb/ 238 U age of 392.2 ± 1.5 Ma should approach the eruptional age of the bentonite. The ages presented here in combination with U–Pb ID-TIMS zircon and monazite ages from other workers allow a recalibration of the early Emsian to early Eifelian interval of the Devonian time scale. The result is a duration of the Emsian stage (excluding the basal Emsian P. kitabicus conodont zone) of more than 17.2 Ma and an Emsian–Eifelian (= Early–Middle Devonian) boundary age of 391.8 ± 0.4 Ma.

Debris avalanche deposits associated with large igneous province volcanism: An example from the Mawson Formation, central Allan Hills, Antarctica

An up-to-180-m-thick debris avalanche deposit related to Ferrar large igneous province magmatism is observed at central Allan Hills, Antarctica. This Jurassic debris avalanche deposit forms the lower part (member m 1 ) of the Mawson Formation and is overlain by thick volcaniclastic layers containing a mixture of basaltic and sedimentary debris (member m 2 ). The m 1 deposit consists of a chaotic assemblage of breccia panels and megablocks up to 80 m across. In contrast to m 2 , it is composed essentially of sedimentary material derived from the underlying Beacon Supergroup. The observed structures and textures suggest that the breccias in m 1 were mostly produced by progressive fragmentation of megablocks during transport but also to a lesser extent by disruption and ingestion of the substrate by the moving debris avalanche. The upper surface of the debris avalanche deposit lacks large hummocks, and sandstone breccias dominate volumetrically over megablocks within the deposits. This indicates pervasive and relatively uniform fragmentation of the moving mass and probably reflects the weak and relatively homogenous nature of the material involved. The avalanche flowed into a preexisting topographic depression carved into the Beacon sequence, and flow indicators reveal a northeastward movement. The source area is probably now hidden under the Antarctic ice sheet. Sparse basaltic bodies, which were hot and plastic during transport in m 1 , reveal the role of Ferrar magmatism in triggering the avalanche, possibly in relation to the emplacement of large subsurface intrusions. The documented deposits indicate that debris avalanches are among the various phenomena that can accompany the early stages of large igneous province magmatism, despite the common absence of large central volcanic edifices. Where large igneous provinces develop in association with faulting or slow preeruptive uplift accompanied by deep valley incision, there is a high probability that feeder dikes will approach the surface in areas of steep topography, allowing volcano-seismicity and fluid overpressures associated with intrusion to effectively trigger avalanches.

Overview of the Montalbano Jonico area and section: a proposal for a boundary stratotype for the lower–middle Pleistocene, Southern Italy Foredeep

In the southernmost part of the Southern Apennine Foredeep crops out a well-exposed and continuous succession, about 500 m thick, of muds and muddy silts and several interbedded volcaniclastic layers. On the basis of nannofossil biostratigraphy it ranges from “large” Gephyrocapsa to Pseudoemiliania lacunosa Zones that indicate the upper part of the Lower Pleistocene and the lower part of the Middle Pleistocene. The vertical distribution of both invertebrate fossil assemblages and teleostean otoliths points out changes in the primary environmental parameters, particularly bathymetry, sedimentation rate and oxygen concentration. The observations allowed the reconstruction of several deepening–shallowing cycles within a general regressive framework. The Montalbano Jonico composite section is likely to represent a regressive portion of a third-order cycle in which relevant tectonic pulses should be the main reason for the general shallowing, possibly related to the beginning of the filling-up phase of the southern part of the Apennines Foredeep. The two recognized fourth-order cycles could be related to tectonoeustatic causes. The higher-frequency cyclicity, represented by several fifth-order cycles, seems to be mainly connected with astronomical relationships. The Montalbano Jonico section could be suitable for defining the Lower/Middle Pleistocene boundary stratotype: the potential GSSP could be located close to the “small” Gephyrocapsa/P. lacunosa zonal boundary, near a biohorizon rich in Delectopecten vitreus, a muddy dark-grey sapropelitic layer rich in Chondrites ichsp. and in oxygen isotopic stage 25. Another potential location of the GSSP of Middle Pleistocene could be near the volcaniclastic horizon V4, close to the isotopic stage 19 boundary.

The lower and middle Pleistocene geological record of the San Lorenzo lacustrine succession in the Sant’Arcangelo Basin (Southern Apennines, Italy)

A facies analysis and preliminary palaeoclimate and biochronology investigation, are presented for the San Lorenzo lacustrine deposits, outcropping in the Pliocene to Pleistocene satellite Sant’Arcangelo Basin (Southern Apennines). Facies analysis shows that sedimentation developed in the inner zone of a terrigenous-dominated fresh water lake. The pollen record shows repeated alternations between two distinct vegetational assemblages, one dominated by steppe taxa, and the other one by forest taxa. The faunal assemblage is indicative of a late Biharian mammal age. The palaeomagnetic survey yielded three polarity intervals throughout the succession; the middle one is of reversed polarity (and is associated with a volcaniclastic layer radiometrically dated at approximately 1 Ma), and thus the two normal polarity intervals are identified as the Jaramillo subchron and the base of the Brunhes chron. The detailed geological and paleontological analyses as well as the preliminary results from the palynological and magnetostratigraphical investigation, indicates that these deposits may contain a continuous record of both climatic changes and tectonic activity within the Sant’Arcangelo Basin during the early and middle Pleistocene.

Integrated Anisian–Ladinian boundary chronology

We report magnetostratigraphic and biostratigraphic data from the Seceda core and the correlative outcrop section from the Dolomites of northern Italy. The Seceda rock succession consists of Tethyan marine limestones and radiometrically dated volcaniclastic layers of the Buchenstein Beds of Middle Triassic age (∼238–242 Ma). The Seceda outcrop section was correlated to coeval sections from the literature using magnetic polarity reversals and a selection of laterally traceable and isochronous lithostratigraphic marker beds. This allowed us to import the distribution of age-diagnostic conodonts, ammonoids, and daonellas from these sections into a Seceda reference stratigraphy for the construction of an integrated biochronology extending across a consistent portion of the Anisian–Ladinian boundary interval. Among the three options selected by the Subcommission for Triassic Stratigraphy to establish the Ladinian Global Stratigraphic Section and Point, we propose to adopt the level containing the base of the Curionii ammonoid Zone at Bagolino (Southern Alps, Italy) because this level is closely associated with a global means of correlation represented by the base of polarity submagnetozone SC2r.2r. The first occurrence of Neogondolella praehungarica in the Dolomites predates slightly the base of the Curionii Zone and can be used to approximate the Anisian–Ladinian boundary in the absence of ammonoids.

Late Cenozoic volcanism in the western Woodlark Basin area, SW Pacific: the sources of marine volcanic ash layers based on their elemental and Sr–Nd isotope compositions

Tephra fallout layers and volcaniclastic deposits, derived from volcanic sources around and on the Papuan Peninsula, form a substantial part of the Woodlark Basin marine sedimentary succession. Sampling by the Ocean Drilling Program Leg 180 in the western Woodlark Basin provides the opportunity to document the distribution of the volcanically-derived components as well as to evaluate their chronology, chemistry, and isotope compositions in order to gain information on the volcanic sources and original magmatic systems. Glass shards selected from 57 volcanogenic layers within the sampled Pliocene–Pleistocene sedimentary sequence show predominantly rhyolitic compositions, with subordinate basaltic andesites, basaltic trachy-andesites, andesites, trachy-andesites, dacites, and phonolites. It was possible to correlate only a few of the volcanogenic layers between sites using geochemical and age information apparently because of the formation of strongly compartmentalised sedimentary realms on this actively rifting margin. In many cases it was possible to correlate Leg 180 volcanic components with their eruption source areas based on chemical and isotope compositions. Likely sources for a considerable number of the volcanogenic deposits are Moresby and Dawson Strait volcanoes (D'Entrecasteaux Islands region) for high-K calc-alkaline glasses. The Dawson Strait volcanoes appear to represent the source for five peralkaline tephra layers. One basaltic andesitic volcaniclastic layer shows affinities to basaltic andesites from the Woodlark spreading tip and Cheshire Seamount. For other layers, a clear identification of the sources proved impossible, although their isotope and chemical signatures suggest similarities to south-west Pacific subduction volcanism, e.g. New Britain and Tonga–Kermadec island arcs. Volcanic islands in the Trobriand Arc (for example, Woodlark Island Amphlett Islands and/or Egum Atoll) are probable sources for several volcaniclastic layers with ages between 1.5 to 3 Ma. The Lusancay Islands can be excluded as a source for the volcanogenic layers found during Leg 180. Generally, the volcanogenic layers indicate much calc-alkaline rhyolitic volcanism in eastern Papua since 3.8 Ma. Starting at 135 ka, however, peralkaline tephra layers appear. This geochemical change in source characteristics might reflect the onset of a change in geotectonic regime, from crustal subduction to spreading, affecting the D'Entrecasteaux Islands region. Initial 143Nd/144Nd ratios as low as 0.5121 and 0.5127 for two of the tephra layers are interpreted as indicating that D'Entrecasteaux Islands volcanism younger than 2.9 Ma occasionally interacted with the Late Archean basement, possibly reflecting the mobilisation of the deep continental crust during active rift propagation.

Deep-sea benthic foraminiferal recolonisation following a volcaniclastic event in the lower Campanian of the Scaglia Rossa Formation (Umbria–Marche Basin, central Italy)

We studied the distribution of deep water agglutinated foraminiferal (DWAF) assemblages across a 15-cm-thick volcaniclastic layer in the lower Campanian Scaglia Rossa limestones of the Umbria–Marche Basin. Above the volcaniclastic layer, which is devoid of foraminifera, a remarkable pattern of recovery among DWAF has been observed. The complete recovery of DWAF in terms of trophic groups and complexity of assemblages is observed in the first 5 cm above the volcaniclastic layer, representing 4.8 kyr based upon the mean sedimentation rate of the Campanian Scaglia Rossa Formation. In its initial stage, the recovery pattern is remarkably similar to that observed following the 15 June 1991 Mount Pinatubo ashfall in the abyssal South China Sea where various species of Reophax, a small organically cemented species of Textularia, and the calcareous species Quinqueloculina seminula and Bolivina difformis are the earliest recolonisers on top of the tephra layer. Such similarities between modern and fossil analogues strengthens the reliability of environmental reconstructions based on DWAF.

Passage of debris flows and turbidity currents through a topographic constriction: seafloor erosion and deflection of flow pathways

Some of the Earth's largest submarine debris flows are found on the NW African margin. These debris flows are highly efficient, spreading hundreds of cubic kilometres of sediment over a wide area of the continental rise where slopes angles are often <1°. However, the processes by which these debris flows achieve such long run‐outs, affecting tens of thousands of square kilometres of seafloor, are poorly understood. The Saharan debris flow has a run‐out of ≈700 km, making it one of the longest debris flows on Earth. For its distal 450 km, it is underlain by a relatively thin and highly sheared basal volcaniclastic layer, which may have provided the low‐friction conditions that enabled its extraordinarily long run‐out. Between El Hierro Island and the Hijas Seamount on the continental rise, an ≈25‐ to 40‐km‐wide topographic gap is present, through which the Saharan debris flow and turbidites from the continental margin and flanks of the Canary Islands passed. Recently, the first deep‐towed sonar images have been obtained, showing dramatic erosional and depositional processes operating within this topographic `gap' or `constriction'. These images show evidence for the passage of the Saharan debris flow and highly erosive turbidity currents, including the largest comet marks reported from the deep ocean. Sonar data and a seismic reflection profile obtained 70 km to the east, upslope of the topographic `gap', indicate that seafloor sediments to a depth of ≈30 m have been eroded by the Saharan debris flow to form the basal volcaniclastic layer. Within the topographic `gap', the Saharan debris flow appears to have been deflected by a low (≈20 m) topographic ridge, whereas turbidity currents predating the debris flow appear to have overtopped the ridge. This evidence suggests that, as turbidity currents passed into the topographic constriction, they experienced flow acceleration and, as a result, became highly erosive. Such observations have implications for the mechanics of long run‐out debris flows and turbidity currents elsewhere in the deep sea, in particular how such large‐scale flows erode the substrate and interact with seafloor topography.

40Ar/ 39Ar geochronology of Middle Miocene calcareous nannofossil biohorizons in Central Japan

The Middle Miocene marine sequence in the Karasuyama area, Central Japan, is interbedded with numerous volcaniclastic layers and contains calcareous nannofossils and planktonic foraminifera. This succession is thus of great interest for both age calibration of biostratigraphic events and for refinements to the geologic time scale. The volcaniclastic layers contain various minerals suitable for isotopic dating and six purified mineral separates of biotite, hornblende, sanidine, and plagioclase were analysed by the conventional step heating 40Ar/ 39Ar technique. Geochronological results for the calcareous nannofossil zonal boundaries CN4/CN5a (last occurrence [LO] of Sphenolithus heteromorphus) and CN5a/CN5b (LO of Cyclicargolithus floridanus) have been determined at 13.7 (13.74±0.16, 2 σ) and 12.0 Ma (11.99±0.18, 2 σ), respectively, which are valid for Central Japan. The age estimate for the LO of S. heteromorphus of 13.7 Ma is analytically indistinguishable to results determined for the same biohorizon in Italy, thus, indicating a potentially significant biological time horizon occurring on a global scale.

Integrated stratigraphy of the lower part of the Miocene Karasuyama sequence, central Japan.

K-Ar ages were determined for four volcaniclastic layers, interbedded in a middle Miocene marine sequence of the Arakawa Group in the Karasuyama area, central Japan. Combining with the previously reported K-Ar ages of four other tuff layers, the numerical ages of some important biohorizons of calcareous nannofossils and planktonic foraminifera, defined by previous studies, were geochronologically estimated. The CN4/CN5a and CN5a/CN5b calcareous nannofossil zone/subzone boundaries (Okada and Bukry, 1980), defined by the last occurrences (LO s ) of Sphenolithus heteromorphus and that of Cyclicargolithus floridanus, were estimated as 13.7 Ma and 11.8Ma, respectively. Both ages coincide with previous estimations (e.g. Berggren et al., 1995; Montanari et al., 1997; Odin et al., 1997). On the other hand, the N.13/N.14 zone boundary of planktonic foraminifera (Blow, 1969), defined by the first occurrence (FO) of Globigerina nepenthes, was estimated as 11.6 Ma. This age almost coincides with the results of the Tomioka sequence in Japan (11.5Ma; Odin et al., 1995 recalculated) and the Italian section (11.5Ma; Odin et al., 1995). The integrated stratigraphic results of the Karasuyama sequence should constitute data sets in the refinement of a global geologic time scale.

The influence of surface volcaniclastic layers from Haleakala (Maui, Hawaii) on soil water conservation

This study focuses on the soils and surficial volcaniclastic layers of Haleakala's crater (Maui, Hawaii). The main objective is to assess the effects of covers with fragments of various sizes (ash, cinder, lapilli) on soil water conservation. Soil and gravel samples were collected in Haleakala National Park from a site at 2505 m where the Hawaiian silversword, a giant rosette-plant, grows densely on pyroclastic materials. An evaporation experiment lasting 22 days showed a gradual drop in seven pairs of soil samples initially at field capacity. One set of samples was left bare, the other was covered by gravel mulches (GM); these resulted in lengthening of T 100% (total desiccation time) by a factor of 2.3–5.2. Bare soils dried after 70–140 h, but drying time under gravel was 246–509 h. Mean grain size ( M z) and sorting ( φ σ ) had the greatest influence on evaporation rates. Coarse lapilli ( M z: 13.8 mm) were less effective than fine ash ( M z: 2.9–3.8 mm) in preventing water losses, while medium-grained cinder ( M z: 4–5.2 mm) produced the greatest water savings. Lapilli (mean φ σ : 0.48) and ash ( φ σ : 0.6) were moderately- to well-sorted; cinders, with a broader grain-size range ( φ σ : 1.13), were poorly sorted. This allowed infilling of large interstices between coarse fragments by smaller grains, effectively reducing pore size and therefore evaporation rates. A second experiment determined water storage and rates of water loss by mulches. Wetting occurred swiftly, within 3–4 min. All gravel types were dry within 26 h. This drying process is important for water conservation, as it effectively prevents further water loss from the mulch surface. Larger fragments stored less water. This is related to their `surface area/weight' ratio, which increases for smaller particles. Thus, fine ash or cinder grains, with a high total area, intercept more water than larger lapilli. A 5-cm thick layer of ash intercepted an average of ∼6.8 mm of rain, slightly more than the same depth of cinder (6.3 mm), but lapilli retained only 4.7 mm. Therefore, light rainfall events are more likely to contribute water to soil under lapilli than below finer pyroclastic material. Volcaniclastic covers serve an important ecological role in Haleakala by prolonging periods of water availability to plants, thus allowing Hawaiian silverswords to grow for longer time spans.

Distinguishing primary and resedimented vitric volcaniclastic layers in the Burdigalian carbonate shelf deposits in Monferrato (NW Italy)

A multidisciplinary study, including stratigraphic, sedimentological, biostratigraphic, petrographic, magnetic fabric and SEM analyses, has been performed on six volcaniclastic layers (VLs) interbedded in the Burdigalian shelf succession of the Monferrato (NW Italy). The aim was to distinguish between the volcanic and sedimentary processes that produced these deposits, to suggest a depositional model for volcaniclastic sedimentation in a shelf environment and to discuss the use of VLs for stratigraphic correlations. Two kinds of VLs have been distinguished: single volcaniclastic layers (SVLs) and multiple volcaniclastic layers (MVLs). SVLs are single beds of well sorted vitric siltites, mainly consisting of volcanic components and minor terrigenous and intrabasinal grains; the vitric fraction mainly consists of blocky fragments. They show a very low magnetic anisotropy degree and a prominent magnetic lineation. These VLs are interbedded in outer shelf marls and are interpreted as primary pyroclastic fall deposits. MVLs, which can be up to 10 m thick, show limited lateral continuity and are made up of several-decimetre-thick graded beds, separated by erosional surfaces and consisting of vitric arenites and siltites with about 15% non-volcanic components. Two kinds of MVLs have been distinguished: (1) Type 1 MVLs, interbedded in storm-dominated glaucony-rich calcarenites and showing rough, low-angle cross-stratification (hummocky cross stratification), water escape and load structures. These deposits are characterized by a slightly foliated magnetic fabric and are interpreted as storm layers, deposited between fairweather and storm wave base. (2) Type 2 MVLs are interbedded in outer shelf marls, and are characterized by parallel lamination and by a well developed magnetic foliation. They are interpreted as storm-induced, distal shelf turbidites, triggered by storm activity acting in the more internal part of the shelf. The Monferrato VLs resulted from explosive eruptions of volcanic edifices, located outside of the basin, that produced an extensive tuff blanket that was uniformly distributed on a carbonate-dominated shelf. Above storm wave base the VLs were immediately reworked by storm activity, and the resulting deposits are type 1 MVLs. Below storm wave base, primary pyroclastic fall deposits were preserved, corresponding to SVLs. Storm-induced turbidity currents gave rise to type 2 MVLs, that were deposited below storm wave base. Preservation of VLs in a shelf environment is hampered by the high-energy conditions of the shelf. Consequently, these deposits are characterized by a restricted lateral continuity and their use as correlation tools may be misleading.

Volcanogenic sedimentation in the Iceland Basin: influence of subaerial and subglacial eruptions

Cores recovered from the Iceland Basin show evidence of transport and deposition of volcaniclastic sediment from the Eastern Volcanic Zone of Iceland during the Holocene and last glacial period. Three types of deposits have been identified: tephra fall, sediment gravity flows, and bottom-current-controlled deposits. Tephra fall layers contain basaltic glass of composition that suggests Katla volcano as the major source. A chronology of the volcano activity is reconstructed, back to isotopic stage 5d (120,000 yr). Glass chemistry of tephra in sediment gravity flows deposited south of Myrdalsjökull Canyon indicates a source in the Grı́msvötn–Lakagı́gar volcanic system. These volcaniclastic gravity flows were most likely derived from jökulhlaups or large glacial floods, at a time of a more extensive ice cover over the volcanic zone. Deposition of the sediment gravity flows has created a deep-sea fan south of the canyon. Basalt glass composition, age, and depositional environment suggest that one early Holocene turbidite sequence was derived from a large jökulhlaup of the Grı́msvötn area. The volcanogenic sediment gravity flows were influenced by a strong contour current, moving across the Katla sediment ridges. The contour current has winnowed the silt fraction and transported it downstream as suspended load. The recovery of numerous silty volcaniclastic layers, enriched in detrital crystals, indicates that they contributed to the sedimentation of contourite drifts.