Sedimentology of proximal to distal volcaniclastics dispersed across an active foldbelt: Ellensburg Formation (late Miocene), central Washington

Late Miocene volcaniclastics of the Ellensburg Formation (Washington, northwestern USA) are exposed in basins within an integrated palaeodrainage over the depositional reach from 15 to 120 km eastward from a Cascade Range source area. Two facies associations are recognized. The first is composed of laterally restricted, well-sorted, polymictic conglomerates representing a gravel-bedload regime during inter-eruption periods. The second comprises laterally extensive sheets, bounded by deeply-developed paleosols, composed of monomictic pebbly, pumiceous dacitic sandstones with intercalated debris-flow and hyperconcentrated flood-flow deposits. These sheets aggraded in response to eruptive events that are sometimes recorded by air-fall tephras at the bases of sequences. Debris-flow and hyperconcentrated flood-flow deposits occur as far as 120 km from source, but are uncommon beyond 50 km. Hyperconcentrated flood-flow deposits initially increase in abundance away from source as debris-flow deposits diminish, suggesting formation of hyperconcentrated flood flows by dilution of debris flows. Sandy facies form broad sheets dominated by scour-fill bedding, in proximal and medial settings, and grade to narrower, trough cross-bedded sheets in distal settings, suggesting moderation of flood discharges with distance. Base-level changes associated with episodic sediment influx caused incision-aggradation cycles in mainstream settings, and episodic impoundment of tributaries to form lakes or rapidly aggrading, poorly drained floodplains. Although volcanism was the primary control on depositional style, concurrent development of the Yakima fold belt produced a structure-consequent drainage pattern that determined sediment dispersal, and basin subsidence permitted preservation of both syn-eruption and inter-eruption facies. Detritus from rising anticlines was generally diluted by the volumetrically superior extrabasinal volcaniclastics, but dominates deposits of small tributary streams flowing toward, rather than from, the Cascades.     

THE TIME-DOMAIN ELECTROMAGNETIC RESPONSE OF POLARIZABLE BODIES: AN APPROXIMATE CONVOLUTION ALGORITHM1

It is now believed that the negative transients observed in coincident-loop transient electromagnetic (TEM) measurements are caused by polarizable bodies (bodies whose conductivity increases as a function of frequency). Ordinarily the TEM response of polarizable bodies is obtained by calculating the frequency-domain response at many frequencies and transforming it to the time domain via Fourier, Laplace or Hankel transforms. This is normally a computationally laborious task. However, for some simple non-polarizable bodies the time-domain response is analytical and can be computed easily. When these simple bodies are weakly polarizable an approximate response can be obtained by convolving the easily-calculated, non-polarizable response with the impulse response of the polarization. The approximate response is found to be very similar to the exact response for the polarizabilities normally seen in geological materials.     

Sedimentation and palaeoenvironments of Late Cretaceous crater-lake deposits in Bushmanland, South Africa

A large diameter borehole core from an epiclastic kimberlite remnant on the farm Stompoor in the Prieska district, Cape Province, contains a continuous 76 m section of fossiliferous sediments interpreted as having accumulated within a crater-lake during the Late Cretaceous. Three distinct facies associations reflect depositional processes that prevailed in offshore areas of the original lake. Facies Association A: matrix-supported pebble conglomerates comprising a chaotic assemblage of pyroclastic, basement and country rocks set in a fine-grained matrix. Flat, non-erosional basal surfaces with ‘frozen’ rip-up clasts, the protrusion of matrix-supported clasts above the upper surfaces and a direct relationship between maximum clast size and bed thickness suggest deposition from debris flows that originated subaerially on pyroclastic talus cones surrounding the crater. Facies Association B: alternating thin beds of matrix-supported granule conglomerate, structureless fine-grained sandstone and parallel laminated mudrock. Small fining-upward sequences within these beds are comparable to turbidite Bouma Tade, Tde. Numerous partings display petrified fish and frog skeletons, as well as bivalve, gastropod and ostracode shells, leaf impressions, insect wings and a possible bird bone. These beds were deposited by thin debris-flows and turbidity underflows interspersed with periods of ‘pelagic’ sedimentation. Facies Association C: microlaminated mudstone beds containing scattered ‘dropstone lapilli’. The lamination is imparted by alternating Ca-rich/Ca-poor layers which may reflect climatic seasonality. They are interpreted as the result of seasonally influenced suspension settling through a thermally stratified water column. Short-term periodicities in conglomerate bed thicknesses are interpreted as the result of successive block caving of a slump scar giving rise to several debris flows from the same source area. Seismic shock from nearby volcanism may have simultaneously triggered slumps on both subaerial and subaqueous slopes. Dropstone lapilli in Type C beds and the preponderance of load casting in Type B beds support this interpretation. An estimate of the time span involved in accumulating 76 m of crater lake sediments based on the possible seasonal imprint of Type C beds gives a figure of some 220,000 yr.     

Recommended Radium-226 Values for Four CCRMP Uranium Reference Materials

The Canadian Certified Reference Materials Project announces recommended values for radium-226 activity in four reference materials previously certified for uranium. Thirteen laboratories participated in the interlaboratory program for DL-la, DH-la, BL-4a and BL-5. Four measurement techniques were applied. Procedural and instrument calibrations were performed with certified radium solutions from the National Bureau of Standards, thereby establishing traceability of the recommended values for radium-226 in DL-la, DH-la, BL-4a and BL-5. These reference ores are also shown to be close to, if not in, radioactive equilibrium.     

THERMAL RESPONSE CHARACTERISTICS OF STONE: IMPLICATIONS FOR WEATHERING OF SOILED SURFACES IN URBAN ENVIRONMENTS

Soiling of stone surfaces by particulate deposition increases absorption of radiant energy, raises surface/subsurface temperature gradients and accentuates rates of surface temperature change. Short-term fluctuation of raised surface temperatures, in response to variations in windspeed and cloud cover, may ultimately contribute to stone breakdown through ‘fatigue’ effects which reduce cohesive strength of intergranular bonds and initiate microfracture development. The effects of soiling are particularly marked for stone with low thermal conductivity and high albedo when clean. Albedo change has implications for the effectiveness of weathering processes and the durability of building stone by creating microenvironmental conditions which are more severe than those indicated by macroenvironmental regimes.     

Relative sea-level trends during the early–middle Holocene along the eastern coast of mainland Scotland, UK

Previously published radiocarbon-dated horizons relating to early and middle Holocene relative sea-level change along the eastern coast of mainland Scotland are examined and trends determined. The data are modified to ensure comparability and are compared against the pattern of glacio-isostatic uplift in the area. Results show that the rate of relative sea-level rise during the Main Postglacial Transgression in the middle Holocene becomes greater towards the edge of the uplifted area, whilst the age of the Main Postglacial Shoreline becomes younger in the same direction. Linear and quadratic regression analyses disclose trends which indicate that at the 0 m HWMOST isobase of the Main Postglacial Shoreline the rate of relative sea level rise between c. 8400 and c . 7000 14 C years BP ( c . 9500 to c . 7900 cal. BP) was 5-11 mm/radiocarbon year or 6-11 mm/calibrated year, whilst at the same isobase the Main Postglacial Shoreline was reached between 5500 and 6100 14 C years BP (between 6300 and 7000 cal. BP). The relative sea-level changes identified are compatible with a rising sea surface level offshore, which may have involved three episodes, possibly related to regional and wider deglaciation.     

Petrology and Geochemistry of Intraplate Basalts in the South Auckland Volcanic Field, New Zealand: Evidence for Two Coeval Magma Suites from Distinct Sources

The South Auckland Volcanic Field is a Pleistocene (1·59–0·51Ma) basaltic intraplate, monogenetic field situated south ofAuckland City, North Island, New Zealand. Two groups of basaltsare distinguished based on mineralogy and geochemical compositions,but no temporal or spatial patterns exist in the distributionof various lava types forming each group within the field: GroupA basalts are silica-undersaturated transitional to quartz-tholeiiticbasalts with relatively low total alkalis (3·0–4·6wt %), Nb (7–29 ppm), and (La/Yb)_N (3·4–7·6);Group B basalts are strongly silica-undersaturated basanitesto nepheline-hawaiites with high total alkalis (3·3–7·9wt %), Nb (32–102 ppm), and (La/Yb)_N (12–47). GroupA has slightly higher ⁸⁷Sr/⁸⁶Sr, similar _Nd, and lower ²⁰⁶Pb/²⁰⁴Pbvalues compared with Group B. Contrasting geochemical trendsand incompatible element ratios (e.g. K/Nb, Zr/Nb, Ce/Pb) areconsistent with separate evolution of Groups A and B from dissimilarparental magmas derived from distinct sub-continental lithosphericmantle sources. Differentiation within each group was controlledby olivine and clinopyroxene fractionation. Group B magmas weregenerated by <8% melting of an ocean island basalt (OIB)-likegarnet peridotite source with high ²³⁸U/²⁰⁴Pb mantle (HIMU)and enriched mantle (EMII) characteristics possibly inheritedfrom recycled oceanic crust. Group A magmas were generated by<12% melting of a spinel peridotite source also with HIMUand EMII signatures. This source type may have resulted fromsubduction-related metasomatism of the sub-continental lithospheremodified by a HIMU plume. These events were associated withMesozoic or earlier subduction- and plume-related magmatismwhen New Zealand was at the eastern margin of the Gondwana supercontinent. KEY WORDS: continental intraplate basalts; geochemistry; HIMU, EMII; Sr, Nd, and Pb isotopes; South Auckland; sub-continental lithospheric sources     

A procedure for determining the nature of Mercury's core

Abstract— We review the assertion that the precise measurement of the second degree gravitational harmonic coefficients, the obliquity, and the amplitude of the physical libration in longitude, C₂₀, C₂₂, θ, and φ₀, for Mercury are sufficient to determine whether or not Mercury has a molten core (Peale, 1976). The conditions for detecting the signature of the molten core are that such a core not follow the 88‐day physical libration of the mantle induced by periodic solar torques, but that it does follow the 250 000‐year precession of the spin axis that tracks the orbit precession within a Cassini spin state. These conditions are easily satisfied if the coupling between the liquid core and solid mantle is viscous in nature. The alternative coupling mechanisms of pressure forces on irregularities in the core‐mantle boundary (CMB), gravitational torques between an axially asymmetric mantle and an assumed axially asymmetric solid inner core, and magnetic coupling between the conducting molten core and a conducting layer in the mantle at the CMB are shown for a reasonable range of assumptions not to frustrate the first condition while making the second condition more secure. Simulations have shown that the combination of spacecraft tracking and laser altimetry during the planned MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, Ranging) orbiter mission to Mercury will determine C₂₀, C₂₂, and θ to better than 1% and φ₀ to better than 8%—sufficient precision to distinguish a molten core and constrain its size. The possible determination of the latter two parameters to 1% or less with Earth‐based radar experiments and MESSENGER determination of C₂₀ and C₂₂ to 0.1% would lead to a maximum uncertainty in the ratio of the moment of inertia of the mantle to that of the whole planet, C_m/C, of ?2% with comparable precision in characterizing the extent of the molten core.