Late Wisconsinan glacial landsystems on Atlantic Canadian shelves: New evidence from multibeam and single-beam sonar data     

JOHN SHAW  BRIAN J. TODD  DENISE BRUSHETT  D. RUSSELL PARROTT  TREVOR BELL 《Boreas: An International Journal of Quaternary Research》2009,38(1):146-159

Multibeam sonar surveys in the past decade, augmented by single-beam data from the OLEX charting system, reveal landsystems on Atlantic Canadian shelves that are diagnostic of Late Wisconsinan ice-sheet dynamics. Four landsystems are described. (1) The Bay of Fundy landsystem comprises two contrasting sets of bedforms, and is interpreted as evidence of topographically controlled fast-flowing ice adjacent to slower-moving ice. (2) The German Bank landsystem off southwest Nova Scotia is comprised of glacially fluted terrain overprinted by De Geer moraines and arcuate recessional moraines. We infer that a flow of grounded glacial ice out of the Bay of Fundy was followed by steady retreat, punctuated by at least one major re-advance. (3) The Placentia Bay landsystem consists of a convergent field of streamlined landforms with superimposed De Geer moraines, overprinted in one area by flutings. We infer that this landsystem was formed in the onset zone of fast-flowing ice, and that overprinting was due to a re-advance of ice from offshore. (4) The south coast of Newfoundland landsystem, which includes arcuate, fjord-mouth moraines and a coast-parallel, fluted moraine, indicates strong topographic control on a retreating marine ice margin as it reached a fjord coastline. These submarine glacial landsystems are not inconsistent with a conceptual model showing Late Wisconsinan ice advance to shelf edges, rapid calving retreat along deepwater channels and slower retreat of ice margins grounded in shallow water. The re-advances documented two of the study areas have parallels in the Last British Ice Sheet, confirming that the reorganization of marine-based ice sheets, caused by calving in embayments, led to internally forced re-advances.  相似文献   

Sensitivity of incipient particle motion to fluid flow penetration depth within a packed bed     

BENJAMIN A. YERGEY  MARIA‐LAURA BENINATI  JEFFREY S. MARSHALL 《Sedimentology》2010,57(2):418-428

A discrete element method is applied to a three‐dimensional analysis related to sediment entrainment on a micro‐scale. Sediment entrainment is the process by which a fluid medium accelerates particles from rest and advects them upward until they are either transported as bedload or suspended by the flow. Modelling of the entrainment process is a critically important aspect for studies of erosion, pollutant resuspension and transport, and formation of bedforms in environmental flows. Previous discrete element method studies of sediment entrainment have assumed the flow within the particle bed to be negligible and have only allowed for the motion of the topmost particles. At the same time, micro‐scale experimental studies indicate that there is a small slip of the fluid flow at the top of the bed, indicating the presence of non‐vanishing fluid velocity within the topmost bed layers. The current study demonstrates that the onset of particle incipient motion, which immediately precedes particle entrainment, is highly sensitive to this small fluid flow within the topmost bed layers. Using an exponential decay profile for the inner‐bed fluid flow, the discrete element method calculations are repeated with different fluid penetration depths within the bed for several small particle Reynolds numbers. For cases with slip velocity corresponding to that observed in previous experiments with natural sediment, the predicted particle velocity is found to be a few percent of the fluid velocity at the top of the viscous wall layer, which is a reasonable range of velocities for observation of incipient particle motion. This method for prescribing the fluid flow within the particle bed allows for the current discrete element method to be extended in future studies to the analysis of sediment entrainment under the influence of events such as turbulent bursting. Additionally, predictions for the slip velocities and fluid flow profile within the bed suggest the need for further experimental studies to provide the data necessary for additional improvement of the discrete element method models.  相似文献   

Genesis of large siliceous stromatolites at Frying Pan Lake, Waimangu geothermal field, North Island, New Zealand     

BRIAN JONES  ROBIN W. RENAUT†  KURT O. KONHAUSER 《Sedimentology》2005,52(6):1229-1252

Lilypad stromatolites, up to 3 m long and 1·5 m wide, were found to be actively growing in the shallow marginal waters of Frying Pan Lake and its outflow channel. These stromatolites, composed of Phormidium (> 90%), Fischerella, and a variety of other microbes, develop through a series of distinct growth stages. Dark green microbial mats cover the floor of the outflow channel and give rise to columns of various sizes and shapes in the shallower marginal waters. Once the columns reach the water level, the mats spread laterally to form a lilypad stromatolite. The lilypads are characterized by a raised, dark green rim, 4–5 mm high, that encircles a flat interior covered with a distinctive orange-red mat. The microbes forming the columns and lilypad plate are being actively silicified. The stromatolites are formed of: (i) flat-lying Phormidium filaments (P-laminae), (ii) upright filaments of Phormidium that are commonly associated with Fischerella (U-laminae), and (iii) mucus, diatoms and pyrite framboids (M-laminae). P-laminae dominate most of the columns, with tripartite cycles of P-, U-, to M-laminae being found mostly in the upper parts of the stromatolites. The transition from the P- to U-laminae is marked by a change in the growth pattern of the Phormidium and branching of Fischerella, which was probably triggered by a change in environmental conditions. In the Frying Pan Lake outflow channel, this change may be related to fluctuations in water level and flow rates that are caused by periods of heavy rain, seasonal changes, long-term variations in rainfall, and/or the unique 40-day hydrological cycle that exists between Frying Pan Lake and Inferno Crater, which is a nearby hydrothermal crater lake.  相似文献   

Developing KaNgwane: geographies of segregation and integration in the new South Africa     

BRIAN H KING 《The Geographical journal》2007,173(1):13-25

This paper examines social, political and economic processes within the former KaNgwane bantustan to understand the changing relationships between society and space in the post-apartheid era. Research on rural development and reconstruction in South Africa attest to the spatial legacy of apartheid while suggesting that dynamic transformations are occurring within the former bantustans. A central concern of this paper is the ways the apartheid government constructed and presented KaNgwane as a development project in order to justify racial segregation and control. While the bantustans have been effectively erased from the popular imagination, these spaces continue to be framed developmentally in ways that provide limited attention to local context and change. In order to consider the shifts in environment and development discourses within these territories, a case study is employed to evaluate livelihood production systems, environmental change, and governance institutions. It is argued that these patterns reveal the simultaneously static and dynamic nature of the bantustans while demonstrating that their reincorporation will remain an ongoing process in the post-apartheid era.  相似文献   

Microscopic calcite dendrites in cold-water tufa: implications for nucleation of micrite and cement   总被引：3，自引：1，他引：2  

ELIZABETH C. TURNER  BRIAN JONES 《Sedimentology》2005,52(5):1043-1066

Dendritic calcite forms in an active cold-water tufa system in association with extracellular polymeric substances (EPS) that discontinuously coat bryophytes and cyanobacteria. Dendrites consist of 100–200 nm thick calcite fibres that form 3D lattice-like domains. In each dendrite domain, fibres have three structurally equal orientations, which correspond in disposition to radii from the centre of a calcite unit cell to the convex triple face junctions on its surface. Fibres do not form in the orientation of the c-axis. The external form of each dendrite has the shape of half of a shortened octahedron, with an upper triangular surface parallel to the substrate. Dendrite nucleation takes place on or in microbial EPS, whether microbial cells are present or not, and is probably effected by attraction of Ca²⁺ cations to negatively charged EPS, together with CO₂-degassing and concomitant pH increase of supersaturated spring water in stream splash zones. Ensuing dendrite growth is abiogenic and controlled by diffusion. Dendrite c-axes are perpendicular to the substrate, probably because the negative charge of EPS forces the orientation of Ca²⁺ and CO planes within the developing dendrite crystal to be parallel to the EPS film surface. Dendrites are eventually filled and overgrown by solid, syntaxial calcite, which gradually and completely obliterates the dendrites as more familiar calcite crystal forms develop. No trace of the dendritic nucleus remains in the rock record. Calcite crystal nucleation may take place by this mechanism in many marine and meteoric settings, given that microbial EPS is now assumed to be virtually ubiquitous in these environments. This phenomenon could contribute to the development of familiar fabrics such as marine micrite cement and fibrous calcite cement, radial ooids, peloids, ‘abiogenic’ stromatolites, sea floor precipitates, microbialites, tufa, travertine, speleothems, and some meteoric cements. It may also contribute to the substrate-normal orientation of c-axes of common cement fabrics.  相似文献   

Interstadial and last interglacial deposits covered by till in Scotland: a reply     

J. BRIAN SISSONS 《Boreas: An International Journal of Quaternary Research》1982,11(1):123-124

Caseldine & Edwards ( Boreas , Vol. 11, pp. 119–122) have misrepresented assessments of published evidence in the paper by Sissons 1981 ( Boreas , Vol. 10, No. 1). They have not used radiocarbon dates objectively and argue illogically. A different interpretation for the Teindland site is suggested.  相似文献   

Ultramafic Xenoliths from the Bearpaw Mountains, Montana, USA: Evidence for Multiple Metasomatic Events in the Lithospheric Mantle beneath the Wyoming Craton   总被引：6，自引：1，他引：6  

DOWNES  HILARY; MACDONALD  RAY; UPTON  BRIAN G. J.; COX  KEITH G.; BODINIER  JEAN-LOUIS; MASON  PAUL R. D.; JAMES  DODIE; HILL  PETER G.; HEARN  B. CARTER  JR 《Journal of Petrology》2004,45(8):1631-1662

Ultramafic xenoliths in Eocene minettes of the Bearpaw Mountainsvolcanic field (Montana, USA), derived from the lower lithosphereof the Wyoming craton, can be divided based on textural criteriainto tectonite and cumulate groups. The tectonites consist ofstrongly depleted spinel lherzolites, harzburgites and dunites.Although their mineralogical compositions are generally similarto those of spinel peridotites in off-craton settings, somecontain pyroxenes and spinels that have unusually low Al₂O₃contents more akin to those found in cratonic spinel peridotites.Furthermore, the tectonite peridotites have whole-rock majorelement compositions that tend to be significantly more depletedthan non-cratonic mantle spinel peridotites (high MgO, low CaO,Al₂O₃ and TiO₂) and resemble those of cratonic mantle. Thesecompositions could have been generated by up to 30% partialmelting of an undepleted mantle source. Petrographic evidencesuggests that the mantle beneath the Wyoming craton was re-enrichedin three ways: (1) by silicate melts that formed mica websteriteand clinopyroxenite veins; (2) by growth of phlogopite fromK-rich hydrous fluids; (3) by interaction with aqueous fluidsto form orthopyroxene porphyroblasts and orthopyroxenite veins.In contrast to their depleted major element compositions, thetectonite peridotites are mostly light rare earth element (LREE)-enrichedand show enrichment in fluid-mobile elements such as Cs, Rb,U and Pb on mantle-normalized diagrams. Lack of enrichment inhigh field strength elements (HFSE; e.g. Nb, Ta, Zr and Hf)suggests that the tectonite peridotites have been metasomatizedby a subduction-related fluid. Clinopyroxenes from the tectoniteperidotites have distinct U-shaped REE patterns with strongLREE enrichment. They have ¹⁴³Nd/¹⁴⁴Nd values that range from0·5121 (close to the host minette values) to 0·5107,similar to those of xenoliths from the nearby Highwood Mountains.Foliated mica websterites also have low ¹⁴³Nd/¹⁴⁴Nd values (0·5113)and extremely high ⁸⁷Sr/⁸⁶Sr ratios in their constituent phlogopite,indicating an ancient (probably mid-Proterozoic) enrichment.This enriched mantle lithosphere later contributed to the formationof the high-K Eocene host magmas. The cumulate group rangesfrom clinopyroxene-rich mica peridotites (including abundantmica wehrlites) to mica clinopyroxenites. Most contain >30%phlogopite. Their mineral compositions are similar to thoseof phenocrysts in the host minettes. Their whole-rock compositionsare generally poorer in MgO but richer in incompatible traceelements than those of the tectonite peridotites. Whole-rocktrace element patterns are enriched in large ion lithophileelements (LILE; Rb, Cs, U and Pb) and depleted in HFSE (Nb,Ta Zr and Hf) as in the host minettes, and their Sr–Ndisotopic compositions are also identical to those of the minettes.Their clinopyroxenes are LREE-enriched and formed in equilibriumwith a LREE-enriched melt closely resembling the minettes. Thecumulates therefore represent a much younger magmatic event,related to crystallization at mantle depths of minette magmasin Eocene times, that caused further metasomatic enrichmentof the lithosphere. KEY WORDS: ultramafic xenoliths; Montana; Wyoming craton; metasomatism; cumulates; minette  相似文献   

Changing Places in Focus: Changing Places in New Zealand: A Geography of Restructering     

BRIAN EASTON 《New Zealand geographer》1993,49(1):48-48

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The effects of fluctuating base level on the structure of alluvial fan and associated fan delta deposits: an example from the Tertiary of the Betic Cordillera, Spain   总被引：2，自引：0，他引：2  

JUAN FERNÁNDEZ  BRIAN JOHN BLUCK†  CÉSAR VISERAS 《Sedimentology》1993,40(5):879-893

The Villanueva Complex is a lacustrine alluvial fan - fan delta system which accumulated on the northern margin of the Guadix Basin during the Pliocene. Five transitional zones can be distinguished from proximal to distal areas: proximal alluvial fan, mid-fan, fan fringe (transition zone), proximal fan delta and distal fan delta (lacustrine). This paper focuses on sedimentation in the fan fringe and the proximal fan delta areas, where the effects of fluctuations in the base level (lacustrine level) are more easily observed. The stratigraphical succession is here characterized by an alternation of fine lacustrine sediments (mudstones and siltstones) and gravels. The gravels appear as isolated channels, stacked channels, lenses and sheets. The isolated channels, which have a V-shaped profile and better developed wings towards the top, appear in the fan fringe zone and in the fan delta. The stacked channels originate in the fan fringe zone and evolve laterally and distally to isolated channels. The lenticular gravel bodies (lenses) are well represented throughout the fan delta and present small channels at the base, indicating a radial flow pattern. Finally, the sheets are characteristic of the fan fringe zone. The first episodes of channel incision occurred at lowstand lake level. The channels and lenses developed in rising lake level conditions, and sheet deposit took place at highstand lake level. Although the final form of the lithosome was heavily controlled by the fluctuations in lake level, it depended on the relative proportions of gravel and silt sedimentation. The gravel-siltstone interface therefore represents an equilibrium surface between the proportions of sedimentation of these lithologies. The bodies mentioned above are organized in coarsening and thickening upwards sequences tens of metres thick, in which a distal to proximal evolution can be observed from isolated channels to lenses or from stacked channels to sheets. The building and thickness of these sequences were controlled by fluctuations in the lake level. An initial fall in base level caused lengthening of the channels and entrenchment from distal to proximal areas, and a continuous increase in supply due to erosion in the drainage basin installed on the subaerial fan. When the lake level rose, lobes were formed at the channel endings and overbank processes were made possible, thus generating radial channels, whose levees were formed by amalgamation with the levees of the main lens channels. At the same time, sheets were formed in proximal areas, where the available amount of sediment was greater.  相似文献   

Dynamics and facies model of a macrotidal sand-bar complex, Cobequid Bay—Salmon River Estuary (Bay of Fundy)     

ROBERT W. DALRYMPLE  R. JOHN KNIGHT  BRIAN A. ZAITLIN  GERARD V. MIDDLETON 《Sedimentology》1990,37(4):577-612

The 40-km-long, Cobequid Bay—Salmon River estuary has a maximum tidal range of 16·3 m and experiences limited wave action. Sediment, which is derived primarily from areas seaward of the estuary, is accumulating faster than the high-tide elevation is rising, and the system is progradational. The deposits consist of an axial belt of sands, which is flanked by mudflats and salt marshes in the inner half of the estuary where a funnel-shaped geometry is developed, and by erosional or non-depositional foreshores in the outer half where the system is confined by the valley walls. The axial sands are divisible into three facies zones: zone 1—elongate, tidal sand bars at the seaward end; zone 2—sand flats with a braided channel pattern; zone 3—the inner, single-channel, tidal—fluvial transition. Tidal current speeds reach a maximum in zone 2, but grain sizes decrease headward (from medium and coarse sand in zone 1, to fine and very fine sand in zones 2 and 3) because the headward termination of the major flood channels prevents the coarse, traction population from entering the inner part of the estuary. Longitudinal progradation will produce a 20-m-thick, upward-fining succession, the lower 1/2–2/3 of which will consist of cross-bedded, medium to coarse sand deposited on the zone 1 sand bars. The ebb-dominated portion of this unit will be finer grained than the flood-dominated part, and will contain trough crossbedding produced by 3-D megaripples; the flood-dominated areas, by contrast, will consist mainly of compound cross-bedding created by sandwaves with superimposed megaripples. Headward migration of swatchways (oblique channels that link the ebb- and flood-dominated areas) will create packages of ebb cross-bedding that is orientated at a high angle to the long axis of the estuary and that contains headwardinclined, lateral-accretion surfaces. The overlying fine and very fine sands of zones 2 and 3 will be composed mainly of upper-flow-regime parallel lamination. The succession will be capped by a 4-m-thick unit of mixed flat, mudflat and salt marsh sediments. A review of other macrotidal estuaries with tidal ranges greater than 10 m suggests that the major elements of the model have general applicability.  相似文献