Electron microprobe analysis of glass shards from tephra assigned to set W,Mount St. Helens,Washington

We have extended the fallout areas for each of two members of tephra-set W, erupted from Mount St. Helens about 1500 ad, by several hundred kilometers beyond the limits mapped in 1975. We traced one member (We) east into Idaho, and the other (Wn) northeast into British Columbia. After using stratigraphic and petrographic observations to assign more than 100 tephra samples to set W, we found 26 of these, selected for chemical analysis, to be closely similar in content of Ca, Fe, and K in glass shards. But improved homogeneity was evident when the 26 sampling localities for tephra W were segregated geographically, east vs. northeast of the volcano. When Ca:Fe:K proportions were plotted on a ternary diagram, there was no overlap of the plotting areas for these two groups of tephra W samples. Without such data, tephra layers We and Wn are currently separable only from stratigraphic and geographic information. Partial glass analysis is also an aid, along with stratigraphic position and petrographic characteristics, in distinguishing tephra W from associated tephra layers. These include tephra layers T and Yn from Mount St. Helens, as well as older tephra layers from Mount Mazama and Glacier Peak.     

Mid-Holocene Glacier Peak and Mount St. Helens We Tephra Layers Detected in Lake Sediments from Southern British Columbia Using High-Resolution Techniques

A Glacier Peak tephra has been found in the mid-Holocene sediment records of two subalpine lakes, Frozen Lake in the southern Coast Mountains and Mount Barr Cirque Lake in the North Cascade Mountains of British Columbia, Canada. The age–depth relationship for each lake suggests an age of 5000–5080 ¹⁴C yr B.P. (5500–5900 cal yr B.P.) for the eruption which closely approximates the estimated age (5100–5500 ¹⁴C yr B.P.) of the Dusty Creek tephra assemblage found near Glacier Peak. The tephra layer, which has not been reported previously from distal sites and was not readily visible in the sediments, was located using contiguous sampling, magnetic susceptibility measurements, wet sieving, and light microscopy. The composition of the glass in pumice fragments was determined by electron microprobe analysis and used to confirm the probable source of this mid-Holocene tephra layer. Using the same methods, the A.D. 1481–1482 Mount St. Helens We tephra layer was identified in sediments from Dog Lake in southeastern British Columbia, suggesting the plume drifted further north than previously thought. This high-resolution method for identifying tephra layers in lake sediments, which has worldwide application in tephrachronologic/paleoenvironmental studies, has furthered our knowledge of the timing and airfall distribution of Holocene tephras from two important Cascade volcanoes.     

Posteruption glacier development within the crater of Mount St. Helens, Washington, USA

The cataclysmic eruption of Mount St. Helens on May 18, 1980, resulted in a large, north-facing amphitheater, with a steep headwall rising 700 m above the crater floor. In this deeply shaded niche a glacier, here named the Amphitheater glacier, has formed. Tongues of ice-containing crevasses extend from the main ice mass around both the east and the west sides of the lava dome that occupies the center of the crater floor. Aerial photographs taken in September 1996 reveal a small glacier in the southwest portion of the amphitheater containing several crevasses and a bergschrund-like feature at its head. The extent of the glacier at this time is probably about 0.1 km². By September 2001, the debris-laden glacier had grown to about 1 km² in area, with a maximum thickness of about 200 m, and contained an estimated 120,000,000 m³ of ice and rock debris. Approximately one-third of the volume of the glacier is thought to be rock debris derived mainly from rock avalanches from the surrounding amphitheater walls. The newly formed Amphitheater glacier is not only the largest glacier on Mount St. Helens but its aerial extent exceeds that of all other remaining glaciers combined.     

Post-glacial loess deposition in a montane environment: South Thompson River valley,British Columbia,Canada

In the late Wisconsinan, the South Thompson River valley, British Columbia, was occupied by an ice-dammed lake. After the lake drained, the exposed lacustrine silt became the source material for a Holocene loess. The purpose of this paper is to establish the stratigraphic, depositional and geomorphic framework of loess occurring along the South Thompson River valley immediately east of Kamloops, British Columbia. This montane environment of loess deposition was characterised by active slope and fluvial processes depositing sediments contemporaneously with the accumulation of loess. The loess reaches an average of 4 m in thickness in the central part of the valley and thins towards the valley sides. Two tephras—Mount St Helens Y (3.4 ka) and Mount Mazama (6.8 ka)— occur in the loess and are invaluable stratigraphic markers. Most of the loess was probably deposited between 8.2 ka and 3.4 ka, a period coinciding with mid-Holocene increased summer temperatures and decreased precipitation in south-central British Columbia. Debris flows and small streams, originating on the valley sides, flowed out on to the loess depositing sand and gravel beds. These deposits form a definite proximal—distal relation across valley with the slope-derived sediments decreasing and the loess increasing in thickness towards the centre of the valley. The lactustrine silt particles were mobilised by diurnal mountain and valley, gravity, and canalised winds flowing within the South Thompson valley. An analysis of contemporary wind-flow data was undertaken to provide a possible analogue for valley wind flows in the mid-Holocene.     

Electron microprobe data for tephra attributed to Glacier Peak,Washington

Reference samples of three prominent pumice units of Glacier Peak tephra collected east of the volcano within a distance of 100 km are similar petrographically to units described by earlier workers. Glass shards isolated from these samples were analyzed by electron microprobe to determine the content of Ca, Fe, and K. Resulting data, plus those published for two other references samples, provide a basis for attributing certain outlying tephra layers from 14 locations in eastern Washington, Idaho, Wyoming, and Montana to eruptions of Glacier Peak. Ten of the samples have properties of both Glacier Peak tephra and Mount St. Helens set J tephra, but proportions of Ca:Fe:K in glass shards indicate that 9 of the 10 outlying samples came from Glacier Peak, whereas one is assigned to Mount St. Helens set J. The remaining six outlying samples, all from southeastern Washington, contain cummingtonite phenocrysts and are chemically similar to some parts of Mount St. Helens tephra sets that are older than 12,000 BP.     

Petrologic diversity in Mount St. Helens dacites during the last 4,000 years: implications for magma mixing

Mount St. Helens has explosively erupted dacitic magma discontinuously over the last 40,000 years, and detailed stratigraphic data are available for the past 4,000 years. During this last time period the major-element composition of the dacites has ranged from mafic (62–64 wt% SiO₂) to felsic (65–67 wt% SiO₂), temperature has varied by about 150°C (770°–920°C), and crystallinity has ranged between 20% and 55%. Water content of these dacites has also fluctuated greatly. Although the source for the dacitic magmas is probably partial melting of lower crustal rocks, there is strong physical evidence, such as banded pumices, thermal heterogeneities in single pumices, phenocryst disequilibrium, contrasts between compositions of glass inclusions and host matrix glass, and amphibole reaction rims, that suggests that magma mixing has been prominent in the dacitic reservoir. Indeed, we suggest that the variations in major- and trace-element abundances in Mount St. Helens dacites indicate that magma mixing between felsic dacite and mafic magma has controlled the petrologic diversity of the dacitic magmas. Magma mixing has also controlled the composition of andesites erupted at Mount St. Helens, and thus it appears that the continuum of magmatic composition erupted at the volcano is controlled by mixing between felsic dacite, or possibly rhyodacite, and basalt. The flux of the felsic endmember to the reservior appears to have been relatively constant, whereas the flux of basalt may have increased in the past 4,000 years, as suggested by the apparently increased abundance of mafic dacite and andesite erupted in this period.     

Energy-dispersive X-ray fluorescence analysis as a rapid method for identifying tephras

The use of energy-dispersive X-ray fluorescence analysis (XES) for the routine identification of three tephras (Mazama, Bridge River, Mount St. Helens Yn) commonly found in archeological sites in British Columbia has been investigated. Researchers have often assumed that chemical analysis of bulk samples of glass separates would be hampered by contamination and weathering effects. Our results indicate that XES of bulk glass separates provides a very reliable method for rapidly identifying the three tephras in question, even with a very simple sample preparation. This should enable persons not skilled in geology or in tephrochronology to collect and to identify samples of these tephras. Finally, as a part of the study, similar measurements were made on the separated glass portions of these three tephras and of three others (Glacier Peak B and G, White River) from northwest North America. The results suggest that this method may provide tephrochronologists with a useful additional tool for studying tephras in other regions.     

Isotopic and trace element constraints on the petrogenesis of lavas from the Mount Adams volcanic field,Washington

Strontium, Nd, Pb, Hf, Os, and O isotope compositions for 30 Quaternary lava flows from the Mount Adams stratovolcano and its basaltic periphery in the Cascade arc, southern Washington, USA indicate a major component from intraplate mantle sources, a relatively small subduction component, and interaction with young mafic crust at depth. Major- and trace-element patterns for Mount Adams lavas are distinct from the rear-arc Simcoe volcanic field and other nearby volcanic centers in the Cascade arc such as Mount St. Helens. Radiogenic isotope (Sr, Nd, Pb, and Hf) compositions do not correlate with geochemical indicators of slab-fluids such as (Sr/P) _n and Ba/Nb. Mass-balance modeling calculations, coupled with trace-element and isotopic data, indicate that although the mantle source for the calc-alkaline Adams basalts has been modified with a fluid derived from subducted sediment, the extent of modification is significantly less than what is documented in the southern Cascades. The isotopic and trace-element compositions of most Mount Adams lavas require the presence of enriched and depleted mantle sources, and based on volume-weighted chemical and isotopic compositions for Mount Adams lavas through time, an intraplate mantle source contributed the major magmatic mass of the system. Generation of basaltic andesites to dacites at Mount Adams occurred by assimilation and fractional crystallization in the lower crust, but wholesale crustal melting did not occur. Most lavas have Tb/Yb ratios that are significantly higher than those of MORB, which is consistent with partial melting of the mantle in the presence of residual garnet. δ ¹⁸O values for olivine phenocrysts in Mount Adams lavas are within the range of typical upper mantle peridotites, precluding involvement of upper crustal sedimentary material or accreted terrane during magma ascent. The restricted Nd and Hf isotope compositions of Mount Adams lavas indicate that these isotope systems are insensitive to crustal interaction in this juvenile arc, in stark contrast to Os isotopes, which are highly sensitive to interaction with young, mafic material in the lower crust.     

Regional significance of an early Holocene moraine in Enchantment Lakes basin,North Cascade Range,Washington

The upper Enchantment Lakes basin in the North Cascade Range of Washington displays two moraine belts, each recording an episode of glacier advance after the end of the last glaciation. The inner belt, the Brynhild, 0.1 to 0.5 km beyond existing glaciers, postdates Mount St. Helens Wn tephra (～450 yr old), which lies only beyond the moraines. The morainal surface is only slightly weathered, is almost barren of lichens, and is devoid of soil, evidence suggesting that the Brynhild moraines are no more than a century old. The outer moraine, the Brisingamen, 0.3 to 0.7 km beyond existing glaciers, is weathered and is covered with large lichens. On and behind the Brisingamen moraine the Mazama ash (6900 yr old) is present beneath the Mount St. Helens Yn and Wn tephras. Despite more than 7 millennia of weathering, the rock surface behind the Brisingamen moraine is measurably less weathered than the surface beyond, which was last glaciated during the Rat Creek advance about 13,000 yr ago. The age of the Brisingamen moraine therefore is probably early Holocene. The Brisingamen moraine evidently correlates with moraines near Glacier Peak, near Mount Rainier, in northeastern and central Oregon, in the southern Canadian Rockies, and in the northern U.S. Rocky Mountains. These regional effects suggest that a climatic episode of cooling or increased snowfall affected the entire region some time during the early Holocene.     

Age of the last major scabland flood of the Columbia Plateau in eastern Washington

Pumice layers of set S from Mount St. Helens can be correlated with certain ash beds associated with young flood deposits of the channeled scabland. The correlation points to an age of about 13,000 ¹⁴C yr B.P. for the last major flood to have crossed the scabland. Until recently, the last major episode of flooding was thought to be closer to 20,000 yr B.P., an age inferred chiefly from the relation of the flood to glacial events of the northern Rocky Mountains. Several investigations within the last few years have suggested that the last major flood occurred well after 20,000 yr B.P. Tentative correlations of ash beds of the scabland with set S pumice layers, the relations of flood and glacial events along the northwestern margin of the Columbia Plateau, and a radiocarbon date from the Snake River drainage southeast of the plateau all indicate an age much younger than 20,000 yr. The postulated age of about 13,000 yr B.P. is further supported by a radiocarbon date in the Columbia River valley downstream from the scabland tract. Basal peat from a bog on the Portland delta of Bretz, which is a downvalley deposit of the last major scabland flood, has been dated as 13,080 ± 300 yr B.P. (W-3404).     

A report on the macroinvertebrates of the Columbia River estuary found in deposits of volcanic ash from the May 18, 1980 eruption of mount St. Helens

Following the May 18, 1980 eruption of Mount St. Helens, up to 11.5 cm of volcanic ash was deposited in sections of the upper Columbia River estuary. A survey of the benthic infauna of this area indicates that most taxa were able to inhabit the ash, suggesting that the material is nontoxic to most groups. However, the abundance of the taxa examined was dependent on the distribution of the ash within the sediment column. Except for the oligochaetes, animal densites were reduced in areas where volcanic ash lay atop the sediment surface as compared to areas where the ash layer had been buried beneath, or mixed with sands and/or muds. The ash apparently affects the infauna through some physical means, possibly related to its fine grain size.     

Ascent-driven crystallisation of dacite magmas at Mount St Helens, 1980–1986

We introduce a novel scheme that enables natural silicic glasses to be projected into the synthetic system Qz-Ab-Or-H₂O in order to relate variations in volcanic glass chemistry to changing pressure (P) and temperature (T) conditions in the sub-volcanic magma system. By this means an important distinction can be made between ascent-driven and cooling-driven crystallisation under water-saturated or undersaturated conditions. In samples containing feldspar and a silica phase (quartz or tridymite), quantitative P-T estimates of the conditions of last equilibrium between crystals and melt can be made. Formation of highly silicic melts (i.e. >77 wt% SiO₂) is a simple consequence of the contraction of the silica phase volume with decreasing pressure, such that high silica glasses can only form by crystallisation at low pressure. Resorption of quartz crystals appears to be a further diagnostic feature of decompression crystallisation. Groundmass and inclusion glasses in dacites from the 1980-1986 eruption of Mount St Helens volcano (WA) span a wide range in SiO₂ (68-80 wt%, anhydrous). The compositions of the least evolved (SiO₂-poor) inclusions in amphibole phenocrysts record entrapment of silicic liquids with Е.4 wt% water, corresponding to a water saturation pressure of ~200 MPa at 900 °C. The compositions of more evolved (higher SiO₂) plagioclase-hosted inclusions and groundmass glasses are consistent with extensive ascent-driven fractional crystallisation of plagioclase, oxide and orthopyroxene phenocrysts and microlites to low pressures. During this polybaric crystallisation, plagioclase phenocrysts trapped melts with a wide range of dissolved water contents (3.5-5.7 wt%). Magmas erupted during the Plinian phase of the 18 May 1980 eruption were derived from a large reservoir at depths of ̈́ km. Subsequent magmas ascended to varying depths within the sub-volcanic system prior to extraction. From glass chemistry and groundmass texture two arrest levels have been identified, at depths of 0.5-1 and 2-4 km. A single dome sample from February 1983 contains groundmass plagioclase, tridymite and quartz, testifying to temperatures of at least 885 °C at 11 MPa. These shallow storage conditions are comparable to those in the cryptodome formed during spring 1980. The corresponding thermal gradient, А.2 °C MPa^-1, is consistent with near-adiabatic magma ascent from ~8 km. We argue that the crystallisation history of Mount St Helens dacite magma was largely a consequence of decompression crystallisation of hot magma beyond the point of water saturation. This challenges the conventional view that phenocryst crystallisation occurred by cooling in a large magma chamber prior to the 1980-1986 eruption. Because the crystallisation process is both polybaric and fractional, it cannot be simulated directly using isobaric equilibrium crystallisation experiments. However, calculation of the phase proportions in water-saturated 910ᆣ °C experiments by Rutherford et al. (1985) over the pressure range 220-125 MPa reproduces the crystallisation sequence and phenocryst modes of Mount St Helens dacites from 18 May 1980. By allowing for the effects of fractional versus equilibrium crystallisation, entrained residual source material, and small temperature differences between nature and experiment, phase compositions can also be matched to the natural samples. We conclude that decompression of water-saturated magma may be the dominant driving force for crystallisation at many other silicic volcanic centres.     

Petrological cannibalism: the chemical and textural consequences of incremental magma body growth

The textures of minerals in volcanic and plutonic rocks testify to a complexity of processes in their formation that is at odds with simple geochemical models of igneous differentiation. Zoning in plagioclase feldspar is a case in point. Very slow diffusion of the major components in plagioclase means that textural evidence for complex magmatic evolution is preserved, almost without modification. Consequently, plagioclase affords considerable insight into the processes by which magmas accumulate in the crust prior to their eventual eruption or solidification. Here, we use the example of the 1980–1986 eruptions of Mount St. Helens to explore the causes of textural complexity in plagioclase and associated trapped melt inclusions. Textures of individual crystals are consistent with multiple heating and cooling events; changes in total pressure (P) or volatile pressure ( $P_{{{\text{H}}_{ 2} {\text{O}}}}$ P H 2 O ) are less easy to assess from textures alone. We show that by allying textural and chemical analyses of plagioclase and melt inclusions, including volatiles (H₂O, CO₂) and slow-diffusing trace elements (Sr, Ba), to published experimental studies of Mount St. Helens magmas, it is possible to disambiguate the roles of pressure and temperature to reconstruct magmatic evolutionary pathways through temperature–pressure–melt fraction (T– $P_{{{\text{H}}_{ 2} {\text{O}}}}$ P H 2 O –F) space. Our modeled crystals indicate that (1) crystallization starts at $P_{{{\text{H}}_{ 2} {\text{O}}}}$ P H 2 O  > 300 MPa, consistent with prior estimates from melt inclusion volatile contents, (2) crystal cores grow at $P_{{{\text{H}}_{ 2} {\text{O}}}}$ P H 2 O  = 200–280 MPa at F = 0.65–0.7, (3) crystals are transferred to $P_{{{\text{H}}_{ 2} {\text{O}}}}$ P H 2 O  = 100–130 MPa (often accompanied by 10–20 °C of heating), where they grow albitic rims of varying thicknesses, and (4) the last stage of crystallization occurs after minor heating at $P_{{{\text{H}}_{ 2} {\text{O}}}}$ P H 2 O  ~ 100 MPa to produce characteristic rim compositions of An₅₀. We hypothesize that modeled $P_{{{\text{H}}_{ 2} {\text{O}}}}$ P H 2 O decreases in excess of ~50 MPa most likely represent upward transport through the magmatic system. Small variations in modeled $P_{{{\text{H}}_{ 2} {\text{O}}}}$ P H 2 O , in contrast, can be effected by fluxing the reservoir with CO₂-rich vapors that are either released from deeper in the system or transported with the recharge magma. Temperature fluctuations of 20–40 °C, on the other hand, are an inevitable consequence of incremental, or pulsed, assembly of crustal magma bodies wherein each pulse interacts with ancestral, stored magmas. We venture that this “petrological cannibalism” accounts for much of the plagioclase zoning and textural complexity seen not only at Mount St. Helens but also at arc magmas generally. More broadly we suggest that the magma reservoir below Mount St. Helens is dominated by crystal mush and fed by frequent inputs of hotter, but compositionally similar, magma, coupled with episodes of magma ascent from one storage region to another. This view both accords with other independent constraints on the subvolcanic system at Mount St. Helens and supports an emerging view of many active magmatic systems as dominantly super-solidus, rather than subliquidus, bodies.     

A refined understanding of the paleoenvironmental history recorded at the Okanagan Centre section,an MIS 4 stratotype,south‐central British Columbia,Canada

The Okanagan Centre section is the stratotype for marine oxygen isotope stage (MIS) 4 sediments (Okanagan Centre Drift) in the southern interior of British Columbia, Canada. Previous work suggested that these sediments record two glacial and two interglacial cycles. This study reports on detailed sedimentological and geochronological investigations of lithostratigraphic units comprising the Okanagan Centre sequence, revealing successive deposition of subaqueous and subaerial outwash, a subglacial till and glaciolacustrine sediments during MIS 4. A limiting optical age of 113 ± 8 ka defines the base of this sequence. Sedimentological, paleopedological, optical dating and tephrochronological data from sediments near the middle of the sequence reveal soil development (MIS 3) in eolian sediments deposited on a river terrace overlying a deglaciated surface. Within these sediments, identification of Mt. St. Helens set C tephra suggest sedimentation between 50 and 35k ¹⁴C a BP. Optical dating corroborates the tephrochronology and suggests that this surface formed after ～52 ± 7 ka. The record of MIS 2 glaciation is restricted to deglaciation, and overlies MIS 3 sediments above an unconformity possibly related to regional subglacial meltwater erosion. Eolian sediments containing Mt Mazama set O tephra (～7.62k cal a BP) cap the sequence.     

Monitoring of geomorphic effects of the 1980 eruptions of Mount St. Helens, Washington, USA

On 18 May 1980, Mount St. Helens erupted explosively with a blast that devastated a 410 km² area, and triggered a debris avalanche exceeding 2.5 billion m³ into the North Fork Toutle River valley. In addition, mudflows radiated out from the stratovolcano cone into all of the major drainages, destroying structures and filling stream channels with sediment. This paper examines the use of geomorphology in the creation of volcanic hazards maps prior to this eruption, the mitigation strategies used, and the subsequent role of geomorphology in subsequent recovery efforts. A sediment budget is presented that summarizes the yield estimated from many geomorphic sources, based on post-eruption aerial monitoring and ground measurements.     

New tree-ring dates for recent eruptions of Mount St. Helens

Distinctive patterns of growth rings in increment cores from old-growth Douglas-fir (Pseudotsuga menziesii) stands identify A.D. 1800 as a more precise date for the eruption of tephra layer T by Mount St. Helens, Washington. Layer T was previously inferred to date to about A.D. 1802. Growth patterns also establish A.D. 1480 as the date of eruption of the earlier layer Wn, previously estimated as dating to about A.D. 1500. The timing of radial tree growth places a small limitation on the seasonal resolution of these new tree-ring dates.     

Mineralogy and phase chemistry of Mount St. Helens tephra sets W and Y as keys to their identification

Voluminous and widespread tephras were produced frequently during the last 36,000 yr of volcanic activity at Mount St. Helens. Numerous tephra sets have been defined by D. R. Mullineaux, J. H. Hyde, and M. Rubin (1975, U.S. Geological Survey Journal of Research, 3, 329–335) on the basis of field relations, FeMg phenocryst assemblage, and ¹⁴C chronology and are valuable marker beds for regional stratigraphic studies. In this study modal abundances and mineral compositions were determined (via petrographic and electron microprobe techniques) for numerous samples of individual layers within tephra sets W and Y to evaluate the degree of compositional variability within and between tephra layers and criteria by which to distinguish among Mount St. Helens and other Pacific Northwest tephras. Although individual layers within a set (e.g., We, Wn) cannot be distinguished from each other on the basis of mineralogic characteristics examined, mineral compositions allow distinction among layers W and Y and other Pacific Northwest tephras (e.g., Mazama, Glacier Peak). FeTi oxide compositions and T-f_O2 estimates derived using coexisting magnetite-ilmenite are especially useful due to the compositional homogeneity of these minerals both within and between samples of a given unit over a wide geographic area. The silicates show more compositional variability than the oxides, but $\text{iO}{}_2\text{Al}{}_2\text{O}{}_3$ contents in hornblende and Fe/Mg ratios in hypersthene aid in distinguishing among Pacific Northwest tephras.     

Pashukanis at Mount Polley: Law,eco-social relations and commodity forms

On August 4, 2014 the tailings pond failed at the Mount Polley copper, gold, and silver mine in British Columbia. The dam failure was amongst the largest recorded, and led to widespread debate in the province concerning weak environmental law and the effects of deregulation. This paper examines the changing role of the law in British Columbia around mining and the environment in relationship to the Mount Polley disaster. It draws on the work of the early Soviet legal theorist Evigny Pashukanis to help understand law’s role in the commodification of nature. Pashukanis suggests a legal analysis of the commodity form and a study of laws role in commodification. However, contemporary law departs from the rigid and formal property and contract principles that Pashukanis considered, and now responds to shifting social conditions, technologies and environmental concern. Yet even today, Pashukanis remains relevant, and provides a starting point for analysis of how nature is commodified. His work points to a study of the multiplicities of, and variegated legal geographies of, commodity forms.     

Grain-size distributions of pyroclastic deposits

A multivariate statistical technique (principal component analysis) has been used to investigate the granulometric features of 414 samples of pyroclastic rocks: 216 samples were collected from areas of recent and active volcanism in Central and Southern Italy; data for the remaining 198 samples were obtained from the literature and relate to products from Somma-Vesuvio, Mount St Helens and the Japanese volcanic arc. The analysis offers an improved means of discriminating pyroclastic deposits using their grain-size characteristics.     

New stratigraphic markers in the late Pleistocene Palouse loess: novel fossil gastropods, absolute age constraints and non-aeolian facies

Four stratigraphic sections in the southern part of the Columbia Basin preserve a sequence of aeolian and non-aeolian sediments ranging in age from 9·43 to >47·0 ¹⁴C ka based on accelerator mass spectrometry radiocarbon dating of fossil molluscs, geochemistry of Cascade Mountain-sourced tephra and association with formally recognized pedostratigraphic units (the Washtucna and Old Maid Coulee soils). Study sections are interpreted as representing concurrent deposition of loess and distal Missoula Flood rhythmites in valleys tributary to main drainages backflooded during the Missoula Floods, and formation of carbonate and iron-rich soils. Sediments belong to the formally recognized L-1 and L-2 loess units established for the Palouse loess, which were deposited in the Columbia Basin subsequent to events of glacial outburst flooding. Sediments associated with the Mount Saint Helens set S and set C tephras in the study sections preserve a fauna of five species of gastropod mollusc which have not been reported previously from sediments of late Pleistocene age in the Palouse region. The fossils comprise two distinct faunules stratigraphically separated by the Mount Saint Helens So tephra. Accelerator mass spectrometry radiocarbon dating of the fossils collected above the tephra in two of the sections yielded ages of 12·48 ± 0·06 and 9·43 ± 0·05 ¹⁴C kyr. These ages suggest that independent determinations of the 13·35 ¹⁴C kyr age of the So tephra in other areas where Missoula Flood sediments are preserved are probably accurate, and help to refine the age of the latest events in the most recent sequence of catastrophic glacial outburst flooding.