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

LAWRENCE A. AMY  PETER J. TALLING 《Sedimentology》2006,53(1):161-212

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.  相似文献   

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

LAWRENCE A. AMY  PETER J. TALLING  VICTORIA O. EDMONDS  ESTHER J. SUMNER  ANNE LESUEUR 《Sedimentology》2006,53(6):1411-1434

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.  相似文献   

REE Variations Across the Peninsular Ranges Batholith: Implications for Batholithic Petrogenesis and Crustal Growth in Magmatic Arcs   总被引：10，自引：1，他引：10  

GROMET  PETER; SILVER  LEON T. 《Journal of Petrology》1987,28(1):75-125

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.  相似文献   

Anomalous arboreal pollen assemblages in Late Devensian and Early Flandrian deposits at Creich Castle, Fife, Scotland   总被引：3，自引：0，他引：3  

PETER CUNDILL  GRAEME WHITTINGTON 《Boreas: An International Journal of Quaternary Research》1983,12(4):297-311

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.  相似文献   

THREE-DIMENSIONAL OPTIMUM MULTICHANNEL VELOCITY FILTERS*     

PETER HUBRAL 《Geophysical Prospecting》1972,20(1):28-46

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.  相似文献   

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

GANGOPADHYAY  AMITAVA; WALKER  RICHARD J.; HANSKI  EERO; SOLHEID  PETER A. 《Journal of Petrology》2006,47(4):773-789

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  相似文献   

EFFECTS OF CLIMATE CHANGE ON FRESHWATER ECOSYSTEMS OF THE SOUTH-EASTERN UNITED STATES AND THE GULF COAST OF MEXICO     

PATRICK J. MULHOLLAND  G. RONNIE BEST  CHARLES C. COUTANT  GEORGE M. HORNBERGER  JUDY L. MEYER  PETER J. ROBINSON  JOHN R. STENBERG  R. EUGENE TURNER  FRANCISCO VERA-HERRERA  ROBERT G. WETZEL 《水文研究》1997,11(8):949-970

The south-eastern United States and Gulf Coast of Mexico is physiographically diverse, although dominated by a broad coastal plain. Much of the region has a humid, warm temperate climate with little seasonality in precipitation but strong seasonality in runoff owing to high rates of summer evapotranspiration. The climate of southern Florida and eastern Mexico is subtropical with a distinct summer wet season and winter dry season. Regional climate models suggest that climate change resulting from a doubling of the pre-industrial levels of atmospheric CO₂ may increase annual air temperatures by 3–4°C. Changes in precipitation are highly uncertain, but the most probable scenario shows higher levels over all but the northern, interior portions of the region, with increases primarily occurring in summer and occurring as more intense or clustered storms. Despite the increases in precipitation, runoff is likely to decline over much of the region owing to increases in evapotranspiration exceeding increases in precipitation. Only in Florida and the Gulf Coast areas of the US and Mexico are precipitation increases likely to exceed evapotranspiration increases, producing an increase in runoff. However, increases in storm intensity and clustering are likely to result in more extreme hydrographs, with larger peaks in flow but lower baseflows and longer periods of drought. The ecological effects of climate change on freshwaters of the region include: (1) a general increase in rates of primary production, organic matter decomposition and nutrient cycling as a result of higher temperatures and longer growing seasons: (2) reduction in habitat for cool water species, particularly fish and macroinvertebrates in Appalachian streams; (3) reduction in water quality and in suitable habitat in summer owing to lower baseflows and intensification of the temperature–dissolved oxygen squeeze in many rivers and reservoirs; (4) reduction in organic matter storage and loss of organisms during more intense flushing events in some streams and wetlands; (5) shorter periods of inundation of riparian wetlands and greater drying of wetland soils, particularly in northern and inland areas; (6) expansion of subtropical species northwards, including several non-native nuisance species currently confined to southern Florida; (7) expansion of wetlands in Florida and coastal Mexico, but increase in eutrophication of Florida lakes as a result of greater runoff from urban and agricultural areas; and (8) changes in the flushing rate of estuaries that would alter their salinity regimes, stratification and water quality as well as influence productivity in the Gulf of Mexico. Many of the expected climate change effects will exacerbate current anthropogenic stresses on the region's freshwater systems, including increasing demands for water, increasing waste heat loadings and land use changes that alter the quantity and quality of runoff to streams and reservoirs. Research is needed especially in several critical areas: long-term monitoring of key hydrological, chemical and biological properties (particularly water balances in small, forested catchments and temperature-sensitive species); experimental studies of the effects of warming on organisms and ecosystem processes under realistic conditions (e.g. in situ heating experiments); studies of the effects of natural hydrological variation on biological communities; and assessment of the effects of water management activities on organisms and ecosystem processes, including development and testing of management and restoration strategies designed to counteract changes in climate. © 1997 John Wiley & Sons, Ltd.  相似文献   

A direct way of specifying flow-dependent background error correlations for meteorological analysis systems   总被引：1，自引：0，他引：1  

LARS PETER RIISHØJGAARD 《地球，A辑:动力气象学与海洋学》1998,50(1):42-57

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Variations in the discharge and organic matter content of stalagmite drip waters in Lower Cave,Bristol     

ANDY BAKER  WILLIAM L. BARNES  PETER L. SMART 《水文研究》1997,11(11):1541-1555

Six drip waters, which were actively depositing stalagmites in Lower Cave, Bristol, were analysed both for discharge and luminescence properties. Drip discharges were determined for two different years, and show a complex response to surface precipitation variations. Inter annual variability in drip discharge is demonstrated to be significantly higher than intra-annual variability, and discharge was demonstrated both to increase and decrease non-linearly with increased precipitation. Drip waters demonstrate a correlation between their luminescence intensity and drip discharge, with increased luminescence in winter as more organic matter is flushed through the aquifer. The strength of the relationship between luminescence intensity and discharge increases with increased discharge. The results presented here have implications for the palaeoenvironmental interpretation of annual growth laminae and the growth rates of stalagmite samples. © 1997 John Wiley & Sons, Ltd.  相似文献   

Book Review: SOILS AND ENVIRONMENT by S. Ellis and A. Mellor. Routledge, 1995. 364 pp. £55.00 ($82.50) (hbk), £16.99 ($25.50) (pbk). ISBN 0 415 06887 8 (hbk); 415 06888 8 (pbk)     

PETER JAMES 《Geological Journal》1997,32(2):194-194

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