Anatomy of turbidites and linked debrites based on long distance (120 × 30 km) bed correlation, Marnoso Arenacea Formation, Northern Apennines, Italy

Much of our understanding of submarine sediment‐laden density flows that transport very large volumes (ca 1 to 100 km³) of sediment into the deep ocean comes from careful analysis of their deposits. Direct monitoring of these destructive and relatively inaccessible and infrequent flows is problematic. In order to understand how submarine sediment‐laden density flows evolve in space and time, lateral changes within individual flow deposits need to be documented. The geometry of beds and lithofacies intervals can be used to test existing depositional models and to assess the validity of experimental and numerical modelling of submarine flow events. This study of the Miocene Marnoso Arenacea Formation (Italy) provides the most extensive correlation of individual turbidity current and submarine debris flow deposits yet achieved in any ancient sequence. One hundred and nine sections were logged through a ca 30 m thick interval of time‐equivalent strata, between the Contessa Mega Bed and an overlying ‘columbine’ marker bed. Correlations extend for 120 km along the axis of the foreland basin, in a direction parallel to flow, and for 30 km across the foredeep outcrop. As a result of post‐depositional thrust faulting and shortening, this represents an across‐flow distance of over 60 km at the time of deposition. The correlation of beds containing thick (> 40 cm) sandstone intervals are documented. Almost all thick beds extend across the entire outcrop area, most becoming thinly bedded (< 40 cm) in distal sections. Palaeocurrent directions for flow deposits are sub‐parallel and indicate confinement by the lateral margins of the elongate foredeep. Flows were able to traverse the basin in opposing directions, suggesting a basin plain with a very low gradient. Small fractional changes in stratal thickness define several depocentres on either side of the Verghereto (high) area. The extensive bed continuity and limited evidence for flow defection suggest that intrabasinal bathymetric relief was subtle, substantially less than the thickness of flows. Thick beds contain two distinct types of sandstone. Ungraded mud‐rich sandstone intervals record evidence of en masse (debrite) deposition. Graded mud‐poor sandstone intervals are inferred to result from progressive grain‐by‐grain (turbidite) deposition. Clast‐rich muddy sandstone intervals pinch‐out abruptly in downflow and crossflow directions, in a fashion consistent with en masse (debrite) deposition. The tapered shape of mud‐poor sandstone intervals is consistent with an origin through progressive grain‐by‐grain (turbidite) deposition. Most correlated beds comprise both turbidite and debrite sandstone intervals. Intrabed transitions from exclusive turbidite sandstone, to turbidite sandstone overlain by debrite sandstone, are common in the downflow and crossflow directions. This spatial arrangement suggests either: (i) bypass of an initial debris flow past proximal sections, (ii) localized input of debris flows away from available sections, or (iii) generation of debris flows by transformation of turbidity currents on the basin plain because of seafloor erosion and/or abrupt flow deceleration. A single submarine flow event can comprise multiple flow phases and deposit a bed with complex lateral changes between mud‐rich and mud‐poor sandstone.     

An experimental investigation of sand–mud suspension settling behaviour: implications for bimodal mud contents of submarine flow deposits

The settling behaviour of particulate suspensions and their deposits has been documented using a series of settling tube experiments. Suspensions comprised saline solution and noncohesive glass‐ballotini sand of particle size 35·5 μm < d < 250 μm and volume fractions, φ_s, up to 0·6 and cohesive kaolinite clay of particle size d < 35·5 μm and volume fractions, φ_m, up to 0·15. Five texturally distinct deposits were found, associated with different settling regimes: (I) clean, graded sand beds produced by incremental deposition under unhindered or hindered settling conditions; (II) partially graded, clean sand beds with an ungraded base and a graded top, produced by incremental deposition under hindered settling conditions; (III) graded muddy sands produced by compaction with significant particle sorting by elutriation; (IV) ungraded clean sand produced by compaction and (V) ungraded muddy sand produced by compaction. A transition from particle size segregation (regime I) to suppressed size segregation (regime II or III) to virtually no size segregation (IV or V) occurred as sediment concentration was increased. In noncohesive particulate suspensions, segregation was initially suppressed at φ_s ～ 0·2 and entirely inhibited at φ_s ≥ 0·6. In noncohesive and cohesive mixtures with low sand concentrations (φ_s < 0·2), particle segregation was initially suppressed at φ_m ～ 0·07 and entirely suppressed at φ_m ≥ 0·13. The experimental results have a number of implications for the depositional dynamics of submarine sediment gravity flows and other particulate flows that carry sand and mud; because the influence of moving flow is ignored in these experiments, the results will only be applicable to flows in which settling processes, in the depositional boundary, dominate over shear‐flow processes, as might be the case for rapidly decelerating currents with high suspended load fallout rates. The ‘abrupt’ change in settling regimes between regime I and V, over a relatively small change in mud concentration (<5% by volume), favours the development of either mud‐poor, graded sandy deposits or mud‐rich, ungraded sandy deposits. This may explain the bimodality in sediment texture (clean ‘turbidite’ or muddy ‘debrite’ sand or sandstone) found in some turbidite systems. Furthermore, it supports the notion that distal ‘linked’ debrites could form because of a relatively small increase in the mud concentration of turbidity currents, perhaps associated with erosion of a muddy sea floor. Ungraded, clean sand deposits were formed by noncohesive suspensions with concentrations 0·2 ≤ φ_s ≤ 0·4. Hydrodynamic sorting is interpreted as being suppressed in this case by relatively high bed aggradation rates which could also occur in association with sustained, stratified turbidity currents or noncohesive debris flows with relatively high near‐bed sediment concentrations.     

Anomalous arboreal pollen assemblages in Late Devensian and Early Flandrian deposits at Creich Castle, Fife, Scotland

Pollen analysis of a radiocarbon-dated core from a shallow basin in volcanic hills in eastern Scotland revealed the existence of Late Devensian and Flandrian deposits. Much of the vegetation history of the Flandrian proved to be missing due to a hiatus in the profile. The Late Devensian pollen record included anomalous arboreal taxa of three possible origins: contamination, long-distance wind transport and reworking of interstadial material. Because of lacking information on Late Devensian climates and the location of thermophilous arboreal refugia in Europe a re-examination of supposedly contaminated British Late Devensian pollen records might well be rewarding.     

THREE-DIMENSIONAL OPTIMUM MULTICHANNEL VELOCITY FILTERS*

Design procedures and characteristics of three stacking filters are discussed which may find application in various three-dimensional velocity filtering problems. These filters are derived in the time-domain as optimum multichannel Wiener filters. Random stationary functions are assumed as stochastic models for the seismic traces. All power and crosspower spectra which are the basic elements of the multichannel normal equations are statistically averaged according to specific three-dimensional considerations. Various properties of the input traces may be incorporated in the design of the optimum filters. With fairly general assumptions about the input these filters are deterministic in the sense that they are applicable to a broad class of input traces with similar statistics in amplitudes and arrival times of signals.     

Origin of Paleoproterozoic Komatiites at Jeesiorova, Kittila Greenstone Complex, Finnish Lapland

Komatiites from the 2 Ga Jeesiörova area in Finnish Laplandhave subchondritic Al₂O₃/TiO₂ ratios like those in Al-depletedkomatiites from Barberton, South Africa. They are distinct inthat their Al abundances are higher than those of the Al-depletedrocks and similar to levels in Al-undepleted komatiites. Moderatelyincompatible elements such as Ti, Zr, Eu, and Gd are enriched.Neither majorite fractionation nor hydrous melting in a supra-subductionzone setting could have produced these komatiites. Their highconcentrations of moderately incompatible elements may haveresulted from contamination of their parental melt through interactionwith metasomatic assemblages in the lithospheric mantle or enrichmentof their mantle source in basaltic melt components. Re–Osisotope data for chromite from the Jeesiörova rocks yieldan average initial ¹⁸⁷Os/¹⁸⁸Os of 0·1131 ± 0·0006(2), _Os(I) = 0·1 ± 0·5. These data, coupledwith an initial _Nd of +4, indicate that melt parental to thekomatiites interacted minimally with ancient lithospheric mantle.If their mantle source was enriched in a basaltic component,the combined Os–Nd isotopic data limit the enrichmentprocess to within 200 Myr prior to the formation of the komatiites.Their Os–Nd isotopic composition is consistent with derivationfrom the contemporaneous convecting upper mantle. KEY WORDS: Finnish Lapland; Jeesiörova; komatiites; mantle geochemistry; petrogenesis; redox state; Re/Os isotopes; Ti enrichment     

20th-CENTURY CHANGES OF TEMPERATURE IN THE MOUNTAIN REGIONS OF CENTRAL EUROPE

Daily maximum and minimum temperatures from 29 low-lying and mountain stations of 7 countries in Central Europe were analyzed. The analysis of the annual variation of diurnal temperature range helps to distinguish unique climatic characteristics of high and low altitude stations. A comparison of the time series of extreme daily temperatures as well as mean temperature shows a good agreement between the low-lying stations and the mountain stations. Many of the pronounced warm and cold periods are present in all time series and are therefore representative for the whole region. A linear trend analysis of the station data for the period 1901–1990 (19 stations) and 1951–1990 (all 29 stations) shows spatial patterns of similar changes in maximum and minimum daily temperatures and diurnal temperature range. Mountain stations show only small changes of the diurnal temperature range over the 1901–1990 period, whereas the low-lying stations in the western part of the Alps show a significant decrease of diurnal temperature range, caused by strong increase of the minimum temperature. For the shorter period 1951–1990, the diurnal temperature range decreases at the western low-lying stations, mainly in spring, whereas it remains roughly constant at the mountain stations. The decrease of diurnal temperature range is stronger in the western part than in the eastern part of the Alps.     

Saharan dust absorption and refractive index from aircraft-based observations during SAMUM 2006

During the Saharan Mineral Dust Experiment (SAMUM) conducted in summer 2006 in southeast Morocco, the complex refractive index of desert dust was determined from airborne measurements of particle size distributions and aerosol absorption coefficients at three different wavelengths in the blue (467 nm), green (530 nm) and red (660 nm) spectral regions. The vertical structure of the dust layers was analysed by an airborne high spectral resolution lidar (HSRL). The origin of the investigated dust layers was estimated from trajectory analyses, combined with Meteosat 2nd Generation (MSG) scenes and wind field data analyses. The real part n of the dust refractive index was found almost constant with values between 1.55 and 1.56, independent of the wavelength. The values of the imaginary part k varied between the blue and red spectral regions by a factor of three to ten depending on the dust source region. Absolute values of k ranged from 3.1 × 10⁻³ to 5.2 × 10⁻³ at 450 nm and from 0.3 × 10⁻³ to 2.5 × 10⁻³ at 700 nm. Groupings of k values could be attributed to different source regions.     

REE Variations Across the Peninsular Ranges Batholith: Implications for Batholithic Petrogenesis and Crustal Growth in Magmatic Arcs

Rare earth element (REE) patterns of plutonic rocks across theCretaceous Peninsular Ranges batholith vary systematically westto east, transverse to its long axis and structural trends andgenerally parallel to asymmetries in petrologic, geochronologicand isotopic properties. The batholith can be ivided into threedistinct parallel longitudinal regions, each defined by distinctREE pattern types. An abrupt transition occurs between rockswith slightly fractionated REE patterns in the western (coastal)region and rocks with middle to heavy REE fractionated and depletedpatterns in the central region. Further to the east a secondtransition to strongly light REE enriched rocks occurs. Theslopes of the REE patterns within each of these regions arelargely independent of rock type. The first REE transition isclosely coupled to regional discontinuities in other parameters:elimination of negative Eu anomalies, an increase in Sr content,and a marked restriction in petrologic diversity. This transitionoccurs over a range of initial ⁸⁷Sr/⁸⁶Sr ratios and ¹⁸O values,but approximately correlates to a major shift in the emplacementstyle of the batholith from a stationary arc to a rapidly eastward-migrating(cratonward) arc. The sense of the regionally consistent REE trends cannot beexplained by crystallization, assimilation, combined crystallization-assimilation,or mixing processes. The consequences of assimilation and high-leveldifferentiation are not observed generally, despite the sensitivityof the REE to these processes. Geochemical and petrologicalfeatures argue that the partial melting of mafic source rocksin which plagioclase-rich (gabbroic) residual assemblages abruptlygave way laterally and downward to garnet-bearing (eclogitic)residual assemblages produced all the changes associated withthe first REE transition. The change in residual assemblagesfrom gabbroic to eclogitic was superimposed on source regionsalready zoned in light REE abundances, ⁸⁷Sr/⁸⁶Sr and ¹⁸O. Temperatureand pressure constraints on the source regions place them ina subcrustal location. The calcic nature of the batholith andthe dominance of tonalite and low-K²O granodiorite in all itsregions argue that the source materials are broadly basalticin composition. Experimental studies are consistent with thegeneration of the abundant tonalitic magmas by the partial meltingof basalt under both low and high pressure conditions. Arc basaltssuch as those commonly erupted in modern island arcs and continentalmargins are inferred to have provided much of the source materialand the heat. Additional high-¹⁸O components are needed in themore easterly source regions. These materials must be distributedso as to contribute equally to the range of magmas that occurin a given local region, and must preserve the calcic natureof batholithic sources. Altered basalts of ancient oceanic crustand possibly their associated metasediments, previously incorporatedinto the lithosphere beneath the continental margin during earliercycles of subduction, most readily satisfy these constraints. The REE geochemistry of the central and eastern regions of thebatholith differs from that of oceanic island arcs in the presenceof strongly heavy REE depleted and fractionated magmas. A modelis proposed in which arc basalts accumulate beneath a crustallayer. Melting of accumulated material at low pressure producesmagmas of the western region. Where thickening of the basalticunderplate is sufficient to form eclogitic assemblages, eclogite-derivedmagmas of the central and eastern region are produced. The abrupttransition to eclogite-derived magmas that suggests a processdriven by a density instability is responsible for their origin. The Peninsular Ranges batholith appears to be representativeof a major differentiation process in which mantle-derived basaltis remelted, contributing its more sialic fractions to the continentalcrust and leaving its mafic to ultramafic residues in the mantle.This process preserves the sialic character of the continentalcrust and may play a significant role in its growth and evolution.The batholith and the processes that produced it may be a moreappropriate basis than immature oceanic island arcs on whichto construct models of continental growth and evolution.     

Metasomatic Mantle Xenoliths from the Bismarck Microplate (Papua New Guinea)--Thermal Evolution, Geochemistry and Extent of Slab-induced Metasomatism

A suite of ultramafic mantle xenoliths from the TUBAF and EDISONseamounts in the Bismarck Archipelago NE of Papua New Guineawas sampled by video-guided grab. The xenoliths, which weretransported to the sea floor by rift-related, Quaternary trachybasalts,mainly represent part of the oceanic mantle. Mineral zoningin peridotite xenoliths testifies to slow cooling after mantleformation at a mid-ocean ridge. Cooling rates in the range of1°C/Ma were calculated from zoning of Ca in olivine usingthe Lasaga algorithm. Subsequent to this cooling, a strong metasomatismaffected the mantle peridotites when metasomatic agents emergedfrom the underlying slab of a subduction zone, which was stalledabout 15 my ago. This resulted in the formation of orthopyroxene-,clinopyroxene-, phlogopite- and hornblende-bearing veins crosscuttingspinel peridotites and olivine clinopyroxenites, as well aspervasively metasomatized plagioclase lherzolites. The metasomaticxenoliths reveal strong chemical disequilibria between the metasomaticminerals and the adjacent, unaltered host rock minerals, whichare especially prominent in the veined samples. Temperaturesduring the metasomatic overprint, estimated using spinel–olivinethermometry, range between 660 and 950°C. Oxygen barometryreveals an elevated oxygen fugacity, with     

The Cenozoic Basaltic Rocks of Eastern China: Petrology and Chemical Composition

The Cenozoic volcanicity of eastern China is entirely basalticand occurred as relatively small eruptions widely dispersedin space and time, closely associated with graben basins andtheir regional bounding faults. Samples (157) from over 30 sitesin eastern China have been studied. They are predominantly alkalinebasalts, but vary in composition from olivine nephelinites andleucitites to quartz tholeiites. The majority are aphyric butsome contain olivine and clinopyroxene phenocrysts. Whole-rockanalyses (X-ray fluorescence) of all samples for the major and13 trace elements are used, as are the compositions of all themajor mineral phases determined by electron microprobe. It is argued that the most primitive basanites, alkali olivinebasalts, and olivine tholeiites represent primary or near-primarymagmas which were formed by different degrees of partial meltingof the upper mantle at different depths. The olivine tholeiitesrepresent larger degrees of partial melting (8–9%) ofa spinel peridotite at depths of <66 km. The alkalic basaltscarry xenoliths of spinel and garnet peridotite and appear tohave been derived by 1–7% partial melting of a garnetlherzolite (50% ol, 25% opx, 15% cpx, 10% garnet) at depths> 79 km. The olivine nephelinite may have formed by evensmaller degrees of partial melting. Most flows are not primary; the variations in their compositionsare consistent with fractional crystallization from the spectrumof primary parents created by varying degrees of partial meltingof a mineralogically heterogeneous source. The tholeiites havefractionated by the removal of clinopyroxene and some olivine;the alkali basalts by the removal of clinopyroxene with a smallerproportion of olivine. The incompatible behavior of Sr impliesthe absence of plagioclase from any of the fractionating assemblagesand, together with the high Al content of the pyroxene phenocrysts,suggests that much of the fractionation occurred at mantle depthsand pressures. The Cenozoic magmatism of eastern China is seen as a typicalexample of volcanism associated with continental extension.That is, small volumes of predominantly alkalic basalts andolivine tholeiites erupted over a prolonged period and associatedwith extensional basins and their bounding faults. As such,the province is distinct from continental flood basalt provinces.