Upslope flow of turbidity currents on the northwest flank of the Cearà Rise: western Equatorial Atlantic*     

JOHN E. DAMUTH  ROBERT W. EMBLEY&#x; 《Sedimentology》1979,26(6):825-834

A piston core (RC16-57) raised from the northwestern flank of the Ceará Rise contained several turbidites up to 62 cm thick with grain sizes ranging from clay to coarse sand. These turbidites were similar in composition to terrigenous turbidites found throughout the Amazon Cone, continental rise and abyssal plains of the western Equatorial Atlantic. The core site (RC16-57) on the Ceará Rise, however, was 156 m above the level of the adjacent Amazon Cone (the source of the turbidites). Thus the turbidity currents which deposited these beds apparently had to flow upslope for 17 km to reach the core site. Sub-bottom reflectors observed on a 3.5 kHz echogram that extended from the Amazon Cone upslope past the core site suggested that these and deeper turbidites extended from the cone up the rise flank to distances of up to 40 km from the cone/rise boundary and to elevations up to 400 m above the level of the cone at the base of the rise. An equally plausible explanation could be that the turbidity currents that deposited these sediments were in excess of 400 m in thickness and thus would not require uphill flow to reach their observed location on the rise flank. The absence of terrigenous turbidites from the bases of topographic knolls on the continental rise and abyssal plains throughout the western Equatorial Atlantic indicated, however, that turbidity currents were normally less than 100 m thick and hence would seem to rule out this explanation. The average gradient of the rise flank in this region was about 1 : 1000 (\sim 0.5°).  相似文献   

Trench-slope channels from the New Zealand Jurassic: the Otekura Formation, Sandy Bay, South Otago     

ROBERT M. CARTER 《Sedimentology》1979,26(4):475-496

The Otekura Formation (Early Jurassic, Pseudaucella zone) at Sandy Bay comprises part of a 10+ km thick, regressive, forearc shelf and slope sequence, the Hokonui facies belt of the Rangitata Geosyncline. The Otekura Formation is dominantly fine grained, being mostly mudstone, silty mudstone and siltstone. The sediments are volcanogenic throughout. The upper 150 m of the formation contains two 20 m thick, channelized bodies of medium-thick bedded sandy flysch, each associated with thin bedded muddy flysch interpreted as overbank turbidites. Directional indicators within the channel sequence indicate emplacement from the south-southwest. In contrast, rare turbidites that occur below the channel sequence, within the background mudstone sediment, were emplaced from the east, i.e. at right angles to the channelized flows. The immediately overlying Omaru Formation contains more abundant macrofossils, intraclastic conglomerates, and appreciable amounts of traction-emplaced cross-bedded sand. Bioturbated calcareous siltstones with an in situ molluscan fauna follow (Boatlanding Formation), and are of shelf origin. The Omaru Formation is therefore interpreted as a shelf-slope break deposit, and the Otekura Formation as an upper slope facies. Reconnaissance studies indicate that the Otekura Formation is underlain by several kilometres of dominantly fine grained, deep water slope sediments, containing occasional sand and conglomerate filled channels similar to those here described in detail from the Otekura Formation. Such channels are inferred to form when a mass-transported sand, derived from failure higher on the slope, ploughs erosively into the sea floor. After their incision, the channels served for a short time as conduits for downslope transport of sediment, the redeposited deposits of which are found filling each channel. Both channel fills at Sandy Bay are capped by thin-bedded turbidites inferred to have overspilled from similar channels nearby on the slope.  相似文献   

RUNAWAY FIRES,SMOKE‐HAZE POLLUTION,AND UNNATURAL DISASTERS IN INDONESIA*     

S. ROBERT AIKEN 《Geographical review》2004,94(1):55-79

ABSTRACT. Fires on Sumatra and Kalimantan have taken a heavy toll on Indonesia' remaining tropical forests. Drought exacerbates the fire hazard, but it does not cause the fires, most of which have been the result of inappropriate land‐use policies and practices. Peatland fires have emitted vast quantities of smoke that periodically blanket large parts of insular Southeast Asia, impairing visibility, disrupting travel, hampering economic activity, and posing serious health risks. The development agenda of Indonesia' New Order regime paid scant attention to forest management, including the need to detect, control, and suppress unwanted fires.  相似文献   

Fukutoku-oka-no-ba Volcano: A new perspective on the Alkalic Volcano Province in the Izu–Bonin – Mariana arc     

CHIH-HSIEN  SUN  ROBERT J.  STERN  TAKEYOSHI  YOSHIDA & JUN-ICHI  KIMURA 《Island Arc》1998,7(3):432-442

Pumice samples from Fukutoku-oka-no-ba in the Izu–Bonin – Mariana (IBM) arc were analysed for 40 trace elements and Sr, Nd, and Pb isotopic compositions. These samples are shoshonites (59.4–61.8 wt% SiO₂), characterized by high contents of K₂O (3.74–4.64 wt%), Ba (1274–1540 p.p.m.), Rb (91–105 p.p.m.), and light rare earth elements. The characteristics of alkali-element enrichment are similar to those of other parts of the Alkalic Volcano Province (AVP) in the northern Mariana and southernmost Volcano arcs. Sr (⁸⁷Sr/⁸⁶Sr = 0.7036–0.7038) and Pb isotopic compositions (²⁰⁶Pb/²⁰⁴Pb = 19.08–19.11, ²⁰⁷Pb/²⁰⁴Pb = 15.62–15.63, ²⁰⁸Pb/²⁰⁴Pb = 38.85–38.91) of Fukutoku-oka-no-ba pumice are relatively radiogenic, whereas Nd is unradiogenic (¹⁴³Nd/¹⁴⁴Nd = 0.51283–0.51286). Fukutoku-oka-no-ba is isotopically distinct from Iwo Jima and is similar to the Hiyoshi Volcanic Complex, suggesting that Fukutoku-oka-no-ba might have a magma source similar to that of the Hiyoshi volcanic complex. Plots of Pb and Nd isotopes for AVP lavas trend toward the fields of ocean island basalt (OIB) source and pelagic sediments, which are possible sources of AVP enrichments.  相似文献   

Melt Enrichment of Shallow Depleted Mantle: a Detailed Petrological, Trace Element and Isotopic Study of Mantle-Derived Xenoliths and Megacrysts from the Cameroon Line   总被引：4，自引：4，他引：4  

LEE  DER-CHUEN; HALLIDAY  ALEX N.; DAVIES  GARETH R.; ESSENE  ERIC J.; FITTON  J. GODFREY; TEMDJIM  ROBERT 《Journal of Petrology》1996,37(2):415-441

Major element, trace element and Sr–Nd–Pb isotopiccompositions of ultramafic xenoliths and megacrysts from thecontinental Cameroon line provide evidence for metasomatismof the upper most lithospheric mantle by enriched melts duringthe Mesozoic The megacrysts probably crystallized within thelower continental crust from melts similar to the host magmas.All the xenoliths originated as depleted residues after theextraction of basaltic melts, but some indicate evidence ofinteraction with enriched partial melts before entrainment.The U–Pb isotopic data on garnet are consistent with coolingthrough >900C at >300 Ma. The Sm–Nd isotope systematicsin constituent phases appear to have been in equilibrium ona xenolith scale at the time of entrainment, indicating derivationfrom mantle that remained at temperatures >600C until eruption.Spinel therzolies that show simple light rare earth element(LREE) depletions are characterized by isotopic compositionsthat are comparable with, but slightly more depleted than AtlanticN-MORB, suggesting that the unmetasomatized sub-continentallithosphere of the Cameroon line may be isotopically similarto that of sub-oceanic lithosphere. The Nd-depleted mantle modelages of these xenoliths are consistent with late Proterozoicdepletion, similar in age to much of the overlying continentalcrust. In contrast, samples that have LREE-enriched clinopyr-oxenes(La/Yb =4.7–9.4) contain trace amounts of amphibole, areenriched in U and have more radiogenic Pb and Sr. These xenolithsyield U–Pb and Sm–Nd model ages consistent withMesozoic enrichment, in agreement with the age of enrichmentof the source regions of the basalts, as deduced from Pb isotopiccompositions. Clinopyroxenes record three orders of magnitudeenrichment in U and LREE accompanied by progressive K depletionassociated with the growth of trace amphibole, with K/U ratiosthat range from 12000 to 1. The ratios of the trace elementsthought to have similar bulk D in mantle melting, Ce/Pb, Ba/Rband Nd/Sr ratios, display regional variations related to thetime integrated history of enrichments indicated by Nd isotopiccompositions. Mass balance calculations suggest that the meltsresponsible for the most recent enrichment of the lithospherehad higher La/Yb and U/Pb than Cameroon line host magmas, andwere probably the product of small degrees of partial meltingassociated with the earliest stages of the breakup of Pangea. KEY WORDS: Cameroon line; mantle xenoliths; megacrysts; REE; isotopic composition; trace element  相似文献   

Morphology and internal structure of sandwaves in the Bay of Fundy   总被引：1，自引：0，他引：1  

ROBERT W. DALRYMPLE 《Sedimentology》1984,31(3):365-382

Intertidal sandwaves in the Minas Basin and Cobequid Bay, Bay of Fundy, occur under a wide range of conditions (mean grain size 0.274-1.275 mm; velocity strength index (V¹)0.46-3.34; and velocity symmetry index (V²) 0.011-0.294), and they vary from symmetrical to strongly asymmetrical in cross-section. Heights and wavelengths average 0.81 and 37.9 m respectively. They are straight to weakly sinuous and laterally continuous in plan, occasionally show crestal branching reminiscent of wave ripples, and are commonly skewed relative to the strongest currents because of differential migration rates along their length. The average migration rate is 0.11 m/tidal cycle. Megaripples occur on each sandwave crest, at least during spring tides, but the areal extent, sinuosity and size of the megaripples increases as the dominant current speed increases. The megaripples have heights averaging 24% of the sandwave height, are oriented perpendicular to the fastest dominant currents, and have life spans of several tidal cycles. They are believed to be in quasi-equilibrium with the sandwaves and play a key role in sandwave dynamics and internal structure formation: periods of lee face steepening and rapid forward migration (megaripple crest at sandwave brink) alternate with times of non-deposition or erosion and slowed or reversed migration (trough at brink). Dominant-current cross-bedding predominates in the two intergradational varieties of translation structure observed: Inclined Cross-Bedding—decimetre-scale cross-beds separated by gently inclined (9° average) erosional surfaces; and Large-Scale Foresets—cross-beds with thicknesses greater than half the sandwave height, interrupted by weakly erosional to conformable discontinuity surfaces. These are overlain by a vertical growth or repair structure, Complex Cross-Bedded Cosets, that consists of nearly equal volumes of dominant- and subordinate-current cross-beds stacked without a preferred set-boundary dip. The translation structures correspond well to forms predicted by Allen (1980a, fig. 8) but the inclined set boundaries and discontinuity surfaces (master bedding planes) are produced by megaripple troughs rather than by current reversals. Consequently, Allen's regime diagram is unable to predict structure occurrences. The repair structures suggest that ‘curvature-related mass-transport’ (Allen, 1980a, b) is important in tidal sandwave maintenance, although it is not necessarily responsible for sandwave initiation.  相似文献   

Mineralogy, Chemistry, and Genesis of the Boninite Series Volcanics, Chichijima, Bonin Islands, Japan   总被引：12，自引：1，他引：11  

TAYLOR  REX. N.; NESBITT  ROBERT W.; VIDAL  PHILLIPE; HARMON  RUSSELL S.; AUVRAY  BERNARD; CROUDACE  IAN W. 《Journal of Petrology》1994,35(3):577-617

The Bonin archipelago represents an uplifted fore-arc terrainwhich exposes the products of Eocene supra-subduction zone magmatism.Chichijima, at the centre of the chain, represents the typelocality for the high-Mg andesitic lava termed boninite. Therange of extrusives which constitute the boninite series volcanicsare present on Chichijima, and are disposed in the sequenceboninite-andesite-dacite with increasing height in the volcano-stratigraphy.Progression to evolved compositions within the Chichijima boniniteseries is controlled by crystal fractionation from a boniniteparental magma containing 15% MgO. Olivine and clinoenstatiteare the initial liquidus phases, but extraction of enstatiticorthopyroxene, followed by clinopyroxene and plagioclase, isresponsible for the general evolution from boninite, throughandesite, to dacite. Some andesites within the overlying MikazukiyamaFormation are petrographically distinct from the main boniniteseries in containing magnetite phenocrysts and a high proportionof plagioclase. As such, these andesites have affinities withthe calc-alkaline series. Major and trace element data for 74 boninitic series rocks fromChichijima are presented. Although major element variation isdominantly controlled by high-level crystal fractionation, thelarge variations in incompatiable trace element concentrationsat high MgO compositions cannot be explained by this mechanism.Nd, Pb, and Sr isotopic data indicate the following: (1) a strongoverprint on ⁸⁷Sr/⁸⁶Sr by seawater alteration; (2) Pb isotopeslie above the northern hemisphere reference line (NHRL) andare thus similar to the <30-Ma are and basin lavas of theIzu—Bonin system, and (3) _Nd(40 Ma) ranges between 2.8and 6.8 within the boninite series volcanics. Differences inrare-earth elements (REE), Zr, Ti, and ¹⁴³Nd/¹⁴⁴Nd at similardegrees of fractionation can be explained by the addition ofa component of fixed composition from the down-going oceaniccrustal slab to a variably depleted source region within theoverlying wedge. Data presented for Sm/Zr and Ti/Zr indicatethat boninite series volcanics are characterized by low valuesfor both of these ratios. In particular, boninites appear tohave uniquely low Sm/Zr ratios. These characteristics may bethe result of slab melting in the presence of residual amphibole;the resultant melt could combine with typical slab dehydrationfluids and infiltrate the overlying mantle wedge. Such a fluid—meltcomponent could mix either with shallow mantle or directly withprimitive melts from depleted mantle. Trace elements, REE, andisotope data thus point to a model for boninite genesis whichrequires tightly constrained pressure—temperature conditionsin the slab combined with melting of a variably depleted sourcein the overlying wedge. Such constraints are rarely met exceptduring the subduction of juvenile oceanic crust beneath a young,hot overriding plate.  相似文献   

A note on the significance of lamination in stromatolites     

ROBERT PARK 《Sedimentology》1976,23(3):379-393

The lamination found in recent stromatolites, an intrinsic feature of many of them, comprises a number of sediment-rich and organic-rich couplets. The significance of this lamination has figured in at least two avenues of research. In one is its possible influential role upon geochemical and diagenetic trends, in the other its possible calendar role. Algal growth follows a diurnal pattern but sedimentation, which is largely marine, is far less regular or predictable. The monitoring of modern algal mat surfaces has yielded much useful and accurate information concerning rates of mat accretion. Millimetre-scale lamination may be produced on a number of interacting processes but as yet no one has been able to determine with any great assurance whether it represents a daily, monthly, annual or some other regular periodic growth. The evidence from the Trucial Coast tends to show, rather, that burial, subsequent compaction, distortion and, in the case of sabkha-type sections, destruction owing to evaporite mineral growth, all tend to obliterate or at best mask any regular patterns of growth. In conclusion, one is forced to acknowledge that any computations concerning the calendar role of stromatolites are to be regarded with the utmost suspicion.  相似文献   

Continental sedimentation, soil genesis and marine transgression in the basal beds of the Cretaceous in the east of the Paris Basin     

ROBERT MEYER 《Sedimentology》1976,23(2):235-253

In the eastern Paris Basin, on the boundary of two French ‘départements’, Marne and Meuse, the Cretaceous deposits begin with Wealden facies assigned to the Valanginian. This is overlain by marine Hauterivian and Lower Barremian. Several techniques have been used for studying these deposits: light-microscope, scanning electron microscope, grain-size analysis, chemical and mineralogical analysis. Environmental interpretations of the various facies are based on sedimentological, geochemical and pedological criteria (fossil soils are present). Clay minerals are dominantly illite and kaolinite. In the soil horizons kaolinite is dominant. A mixed-layer illite-smectite appears at the base of the transgression. Under marine influence a more stable illite dominant suite is formed. The Wealden sediments appear to have been deposited in a very mature flood-plain. Part of the sequence was subjected to weathering in situ by soil processes under a warm climate with repeated wetting and drying. Paraconformably above, the marine Hauterivian is the product of a shallow sea that tended to form lagoons. A true marine environment was established in the Lower Barremian, when mixing homogenized the sediments.  相似文献   

A Time Frame for Construction of the Kerguelen Plateau and Broken Ridge   总被引：8，自引：10，他引：8  

DUNCAN  ROBERT A. 《Journal of Petrology》2002,43(7):1109-1119

A key element in achieving Ocean Drilling Program Leg 183 scienceobjectives is determining the age of volcanism at differentlocations across the Kerguelen Plateau and Broken Ridge. Thispaper reports crystallization ages derived from ⁴⁰Ar–³⁹Arincremental heating experiments, for whole rocks and feldsparsseparated from basement units recovered at Sites 1136, 1137,1138, 1139, 1140, 1141 and 1142. The subaerial environment oferuption at most sites and the generally evolved, high K contentof these lava flows contributed to precise and reproducibleage determinations. Volcanic activity at southern KerguelenPlateau Site 1136 occurred at 118–119 Ma; at Elan BankSite 1137, 107–108 Ma; at central Kerguelen Plateau Site1138, 100–101 Ma; at Skiff Bank Site 1139, 68–69Ma; at northern Kerguelen Plateau Site 1140, 34–35 Ma;and at Broken Ridge Sites 1141 and 1142, 94–95 Ma. Thenew ages allow calculation of melt production rates throughthe  相似文献