Editorial Note     

MICHAEL BROWN 《Journal of Metamorphic Geology》1984,2(2):73-73

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Gondwana and Tethys     

MICHAEL AUDLEY-CHARLES 《Geology Today》1986,2(5):136-138

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Spatial and temporal variability of ⁷Be surface concentrations     

DOROTHY M. KOCH  MICHAEL E. MANN 《Tellus. Series B, Chemical and physical meteorology》1996,48(3):387-396

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Controls on Holocene deep‐water sedimentation in the northern Gioia Basin,Tyrrhenian Sea     

FABIANO GAMBERI  MICHAEL MARANI 《Sedimentology》2008,55(6):1889-1903

The northern Gioia Basin of the south‐east Tyrrhenian Sea is a slope basin, ～ 20 km wide and ～ 50 km long, with a bathymetry of ≤ 1300 m, bounded by the Calabro‐Sicilian landmass and the Aeolian Island Arc. Coarse sediment is supplied from the Calabrian margin, where the shelf is very narrow to non‐existent, whereas the wider shelf on the Sicilian margin prevents supply by storing river‐fed sediments. The basin is dominated by the Gioia–Mesima canyon/channel system paralleled by a tongue‐shaped depositional lobe. Multibeam bathymetric surveys, sea floor reflectivity data and airgun seismic profiles reveal the recent evolution of the submarine system. Slope canyons and basin‐floor levéed channels formed where major rivers built deltas at the shelfless Calabrian margin and strong hyperpycnal flows predominated. The channels are a few hundred metres wide and a few tens of metres deep, with a downslope change from a straight to meandering pattern where the slope gradient decreases from 3·2% to 1·7%. The Mesima Channel has its lower segment abandoned because of avulsion and crevasse‐splay formation at an upslope bend. The adjacent Gioia Channel has had its upper segment straightened and lower segment entrenched because of erosional deepening of the Stromboli Valley into which it debouches and which acts as the local base level. Overbank features include levées, coalescent splays and ‘yazoo’ channels; their nature and surface characteristics depend upon the magnitude and sediment grain‐size of spill‐over flows. On an adjoining narrow shelf sliver of the Calabrian margin, in contrast, the coalescing plumes of sediment suspension supplied by an array of smaller coastal streams were apparently spilling over the shelf edge, scouring a funnel‐shaped bypass depression with chutes and forming an elongate, non‐channellized depositional lobe at the slope base. The study demonstrates the impact of sediment source type, shelf width, basin‐floor gradient and base‐level change on the style of deep‐water sedimentation.  相似文献   

A comparison of grain‐size analysis methods for sand‐dominated fluvial sediments     

MICHAEL D. CHEETHAM  ANNABELLE F. KEENE  RICHARD T. BUSH  LEIGH A. SULLIVAN  WAYNE D. ERSKINE 《Sedimentology》2008,55(6):1905-1913

Grain‐size distribution is a fundamental tool for interpreting sedimentary units within depositional systems. The techniques assessed in this study are commonly used to determine grain‐size distributions for sand‐dominated sediments. However, the degree of consistency and differences in interpretation when using a combination of grain‐size methods have not yet been assessed systematically for sand‐dominated fluvial sediments. Results obtained from laser diffraction, X‐ray attenuation and scanning electron microscopy grain‐size analysis techniques were compared with those obtained from the traditional sieve/hydrometer method. Scanning electron microscopy was shown to provide an inaccurate quantitative analysis of grain‐size distributions because of difficulties in obtaining representative samples for examination. The X‐ray attenuation method is unsuitable for sand‐dominated sediments because of its upper size range of only 300 μm. The consistently strong correlation between the laser diffraction results and the sieve/hydrometer results shows that these methods are comparable for sand‐dominated fluvial sediments. Provided that sample preparation is consistent, the latter two methods can be used together within a study of such sediments while maintaining a high degree of accuracy. These results indicate that data for sand‐dominated fluvial sediments gained from the long‐established sieve/hydrometer method can be compared with confidence to those obtained by modern studies using laser diffraction techniques.  相似文献   

Responsible Geographies: Co-oreating Knowledge in Aotearoa     

ROBERT MCCLEAN  LAWRENCE D. BERG  MICHAEL M. ROCHE 《New Zealand geographer》1997,53(2):9-15

We examine in this paper the potential for (reforming the relations of social scientific research in Aotearoa. Drawing upon our experience of undertaking historical geographical research with and for Ngati-Toa Rangatira, we argue for the need to develop responsible geographies in Aotearoa. We suggest that the metaphors of ‘crossing borders’ and ‘building structures’ might offer one route into the development of such a responsible geography.  相似文献   

Physical compaction of vertic palaeosols: implications for burial diagenesis and palaeo-precipitation estimates     

MICHAEL R. CAUDILL  STEVEN G. DRIESE  CLAUDIA I. MORA 《Sedimentology》1997,44(4):673-685

Burial compaction is one of several major obstacles to estimating palaeoprecipitation from depth to pedogenic carbonate in favourably preserved palaeosols. Palaeosols must be decompacted and the preburial depth to the pedogenic carbonate obtained. Vertic palaeosols may be particularly good candidates for palaeoprecipitation estimates, because of their increased likelihood of preserving clastic dykes, one of the best features for estimating burial compaction. Compaction estimates from clastic dykes and literature-based depth of burial estimates suggest vertic palaeosols undergo significantly less burial compaction than may be commonly assumed. Late Carboniferous vertic palaeosols, buried to 2·5–3·0 km, compacted to 93% of their original thickness. In contrast, clastic dykes in a nonpedogenic shale directly underlying one of the Late Carboniferous palaeosols records compaction to 70% of original thickness. Similarly obtained burial compaction and burial depth estimates for Early Carboniferous, Ordovician, and Proterozoic vertic palaeosols were used to test a burial compaction curve and equation specific to vertic palaeosols. Results suggest this ‘vertic-calibrated’curve and equation can be used to estimate burial compaction for vertic palaeosols lacking clastic dykes, but additional testing is needed. Naturally high bulk densities may have limited the compactibility of vertic palaeosols. Likewise, high initial bulk density and an abundance of swelling clays may have severely limited the transmissivity of some vertic palaeosols as they passed from pedogenic to burial environments. Upon burial these vertic palaeosols may have behaved as closed systems, which has implications for understanding their diagenetic modification. Additional efforts to understand burial compaction of vertic palaeosols also promises to improve our understanding of aquifer/aquiclude and hydrocarbon reservoir/seal relationships in sedimentary basins containing intercalated palaeosols.  相似文献   

Corundum-bearing Garnet Clinopyroxenites at Beni Bousera (Morocco): Original Plagioclase-rich Gabbros Recrystallized at Depth within the Mantle?   总被引：7，自引：5，他引：7  

KORNPROBST  JACQUES; PIBOULE  MICHEL; RODEN  MICHAEL; TABIT  ABDELHALIM 《Journal of Petrology》1990,31(3):717-745

The Beni Bousera ultramafic massif, Morocco, is composed ofperidotite with subordinate garnet pyroxenitc units which belongto two different families: (1) the Type I pyroxenites, whichare characterized by an Fe-enrichment trend; and (2) the TypeII pyroxenites, which are characterized by high but nearly constantMg/Fe ratios and highly variable concentrations of Ca and Al;the latter family includes corundum-bearing garnet pyroxeniteswhich resemble the peraluminous eclogites and grospydites describedas xenoliths in kimberlite diatremes. The Type II pyroxenites appear as layered sheets in the peridotite,and have granuloblastic metamorphic texture. They contain aprimary association of a coarse-grained assemblage (cpx + gt;cpx + gt + sp; cpx + gt + co), and a variety of secondary andtertiary associations includ ng clinopyrox-ene, orthopyroxene,olivine, spinel, corundum, sapphirine, plagioclase, and amphibole.The primary assemblage in the corundum-bearing pyroxenite ischaracterized by clinopyroxene rich in A1₂O₃ (up to 20 wt%),and poor in Na₂O (generally less than 2 wt.%). The clinopyroxenephase is therefore richer in the Ca-Ts molecule than in thejadeite molecule. On the other hand, the composition of theprimary and secondary clinopyroxene and garnet phases showsstrong variation across the pyroxenite sheets. These variationsexpress compositional variations of the rock system across thesheets. The cpx-gt associations indicate high temperatures (1200–1350?C) in the central parts of the sheets. The crystallizationpressure may have reached at least 20 kb in the corundum-bearingassemblages. The bulk-rock composition and the compatible element's behaviourin the Type II pyroxenite sheets suggest that the modal andcryptic layering mainly resulted from igneous fractionationprocesses. The REE patterns of corundum-bearing Type II pyroxeniteare characterized by low concentrations of HREE and by significantEu anomalies. These, together with the high bulk-rock Sr/Ndratios, suggest that plagioclase segregation may have playeda significant part in the rock genesis. These geochemical featuresare similar to those described, in the literature, in some low-pressure,plagioclase-bearing adcumulates (e.g., in the crustal sequenceof the Oman ophiolite). They are quite different from thoseobserved in the Type I pyroxenite sheets in the Beni Bouseramassif, whose geochemistry suggests that plagioclase playedno part in the fractionation process, whereas garnet probablyfractionated as an early igneous phase. The Type II pyroxenitesheets have a primary isotopic signature similar to MORB, basedon the composition of leached clinopyroxene. It is concluded that the Mg-rich Type II pyroxenite sheets resultedultimately from the fractionation of a basaltic melt at lowpressure, and from the accumulation of olivine, clinopyroxene,and plagioclase along dykes cross-cutting the surrounding peridotite.The close similarities with the geochemical features in theOman ophiolite lead us to suggest that these processes may havebeen operative in an oceanic crustal environment. The high-pressureand high-temperature crystallization of the ‘primary’cpx+gt + co assemblage was achieved deep in the mantle, aftersubduction and/or dragging down in convection currents of thisparticular piece of the (oceanic?) lithosphere. Further ascentmay have resulted in partial melting of peridotite and/or pyroxenite,and in the emplacement of the Type I pyroxenite sheets.  相似文献   

Origin of Mafic Enclaves in the Dinkey Creek Pluton, Central Sierra Nevada Batholith, California   总被引：4，自引：1，他引：4  

DORAIS  MICHAEL J.; WHITNEY  JAMES A.; RODEN  MICHAEL F. 《Journal of Petrology》1990,31(4):853-881

Two types of mafic enclaves occur in the Dinkey Creek pluton:ubiquitous microgranular enclaves, and rare gabbroic enclaves.Common petrographic features of the microgranular enclaves are:(1) fine grain-size, (2) abundant acicular apatite, and (3)plagioclase zoned from bytownitic cores to andesine-labradoriterims, with sharp boundaries between these main zones. Subordinateoscillatory variations are commonly superimposed on both coresand rims. It has been found by secondary ion mass spectrometrythat the rims are identical in major and trace element compositionto plagioclase in the tonalite, which suggests crystallizationfrom the same or similar magmas. The gabbroic enclaves are composedpredominantly of hornblende (50–85%) and appear to bemagmatic segregations. The microgranular enclaves and host rocks display two convergingtrends on silica variation diagrams for Fe₂O₃, TiO₂, Al₂O₃,Zn, and Zr. The dominant trend is defined by small microgranularenclaves, by samples from a large (20 m?30 m) microgranularenclave, and by the Dinkey Creek tonalites and granodiorites.The subordinate trend covers tholeiltic dikes and tonalitich and converges with the Dinkey Creek host rocks at 61 wt.%SiO₂ Alkali and alkaline earth elements exhibit greater variabilitythan the above constituents and appear to be either enrichedor depleted as required for equilibrium with the host rocks.Low CaO and Sr concentrations in small enclaves (<30 cm)apparently reflect a lower modal abundance of calcic plagioclaseand more sericitization of this feldspar as compared with theplagioclase of the large microgranular enclave. The large enclaveis also richer in MgO than the small enclaves. With the exceptionof the alkali elements, the major element compositions of themicrogranular enclaves approach high-Al basaltic to andesiticcom positions. In one analyzed microgranular enclave, low La/Cerelative to chondrites and more abundant HREE than in othermicrogranular samples suggest that it may also contain minorcumulus hornblende. The petrographic and whole-rock geochemical relations, and theplagioclase compositions in the microgranular enclaves and theirhost rocks, indicate that the microgranular enclaves representmixtures of quenched basalts and Dinkey Creek tonalites. Itappears that dikes of high-alumina basalt were intruded intothe lower, tonalitic portions of the Dinkey Creek pluton, wherethey were partially quenched along an interface with overlyingtonalitic magma. Large portions of residual liquid in the partiallyquenched basalts permitted mixing with the overlying magma toform a hybrid zone. This zone was then disaggregated, yieldingthe enclaves, and they were dispersed throughout the upper partof the Dinkey Creek magma chamber. Subsequent crystallizationof tonalitic melt within the enclaves produced the zoned plagioclaseand re-equilibrated hornblende and biotite in the enclaves tothe Dinkey Creek magmatic conditions. Scouring disrupted hornblende-richmagmatic segregations and produced the gabbroic enclaves.  相似文献   

The Petrogenetic Significance of Interstratified High- and Low-Ti Basalts in Central Nicaragua   总被引：3，自引：0，他引：3  

WALKER  JAMES A.; CARR  MICHAEL J.; FEIGENSON  MARK D.; KALAMARIDES  RUTH I. 《Journal of Petrology》1990,31(5):1141-1164

Basalts erupted from recent volcanoes in central Nicaragua canbe divided into distinct high-and low-Ti suites. Low-Ti basaltshave higher concentrations of LILE and LREE than high-Ti basalts.In addition, low-Ti basalts have obviously higher Ba/La, La/Sm,and ⁸⁷Sr/⁸⁶Sr, and lower Ti/Zr, than high-Ti basalts. In contrast,there are no mineralogical or petrographic differences betweenthe two suites. The differences between the high-and low-Ti basalts of centralNicaragua are inherited from their source regions. The primarymagmas of both are generated in the mantle wedge. However, low-Tiprimary magmas come from parts of the wedge which bear a strongsubduction zone signature, including that of subducted pelagicsediment. On the other hand, the primary magmas of the high-Tibasalts are generated in parts of the wedge relatively freeof subduction zone influence. Subducted pelagic sediment can therefore be a key source componentat active continental margins as well as at island arcs. Pelagicsediment could also be responsible for subtle high-field-strengthelement fractionations within subduction zone magmas. The mantlewedge beneath Nicaragua, which is variably modified by the subductingplate, is relatively enriched suboceanic mantle.  相似文献