The upper Hawkesbury River, New South Wales, Australia: a Holocene example of an estuarine bayhead delta   总被引：1，自引：0，他引：1  

SCOTT L. NICHOL  BRIAN A. ZAITLIN†  BRUCE G. THOM‡§ 《Sedimentology》1997,44(2):263-286

Holocene deposits of the Hawkesbury River estuary, located immediately north of Sydney on the New South Wales coast, record the complex interplay between sediment supply and relative sea-level rise within a deeply incised bedrock-confined valley system. The present day Hawkesbury River is interpreted as a wave-dominated estuarine complex, divisible into two broad facies zones: (i) an outer marine-dominated zone extending 6 km upstream from the estuary mouth that is characterized by a large, subtidal sandy flood-tidal delta. Ocean wave energy is partially dissipated by this flood-tidal delta, so that tidal level fluctuations are the predominant marine mechanism operating further landward; (ii) a river-dominated zone that is 103 km long and characterized by a well developed progradational bayhead delta that includes distributary channels, levees, and overbank deposits. This reach of the Hawkesbury River undergoes minor tidal level fluctuations and low fluvial runoff during baseflow conditions, but experiences strong flood flows during major runoff events. Fluvial deposits of the Hawkesbury River occur upstream of this zone. The focus of this paper is the Hawkesbury River bayhead delta. History of deposition within this delta over the last c. 12 ka is interpreted from six continuous cores located along the upper reaches of the Hawkesbury River. Detailed sedimentological analysis of facies, whole-core X-ray analysis of burrow traces and a chronostratigraphic framework derived from 10 C-14 dates reveal four stages of incised-valley infilling in the study area: (1) before 17 ka BP, a 0–1 m thick deposit of coarse-grained fluvial sand and silt was laid down under falling-to-lowstand sea level conditions; (2) from 17 to 6·5 ka BP, a 5–10 m thick deposit composed of fine-grained fluvial sand and silt, muddy bayhead delta and muddy central-basin deposits developed as the incised valley was flooded during eustatic sea-level rise; (3) during early highstand, between 6·5 and 3 ka BP, a 3–8 m thick bed of interbedded muddy central-basin deposits and sandy river flood deposits, formed in association with maximum flooding and progradation of sandy distributary mouth-bar deposits commenced; (4) since 3 ka BP, fluvial deposits have prograded toward the estuary mouth in distributary mouth-bar, interdistributary-bay and bayhead-delta plain environments to produce a 5–15 m thick progradational to aggradational bayhead-delta deposit. At the mouth of the Hawkesbury estuary subaqueous fluvial sands interfinger with and overlie marine sands. The Hawkesbury River bayhead-delta depositional succession provides an example of the potential for significant variation of facies within the estuarine to fluvial segment of incised-valley systems.  相似文献   

Hurricane control on shelf-edge-reef architecture around Grand Cayman   总被引：3，自引：0，他引：3  

PAUL BLANCHON  BRIAN JONES 《Sedimentology》1997,44(3):479-506

Rimming the outer shelf of Grand Cayman is a submerged, 87 km long shelf-edge reef that rises to within 12 m of mean sea level. It consists of an array of coral-armoured buttresses aligned perpendicular to shore and separated by steep-sided sediment-floored canyons. Individual buttresses have a diverse coral-dominated biota and consist of three architectural elements: a shield-like front wall colonized by platy corals, a dome-shaped crown colonized by head corals, and a shoreward-projecting spur covered by varying amounts of branching coral. Buttresses are commonly fronted by coral pinnacles that, in some areas, have amalgamated with buttress walls to produce pinnacle-and-arch structures. As margin orientation changes, shelf-edge-reef architecture shows systematic variations that are consistent with changes in fetch and height of hurricane waves. Along margins exposed to fully developed storm waves, shelf-edge-reef buttresses are deep, have large amplitudes, and are dominated by robust head corals. These characteristics are consistent with hurricane-induced pruning of branching corals and the flushing of significant quantities of sand from buttress canyons by return flows. Along margins impacted by fetch-limited storm waves, reef buttresses are shallower, have intermediate-amplitudes, and have a significantly higher proportion of branching corals. These characteristics are consistent with less coral pruning and sand flushing by weaker hurricane waves. Along margins fully protected from storm waves, the buttresses-canyon architecture of the shelf-edge reef breaks down producing a series of shallow, undulating, branching-coral-dominated ridges that merge laterally into an unbroken belt of coral. These characteristics correspond with negligible amounts of pruning and flushing during hurricanes. In addition to differences between margins, local intra-marginal changes in shelf-edge reef architecture are consistent with changes in the angle of hurricane-wave approach. Open sections of the shelf-edge reef, which face directly into storm waves, are pruned of branching corals and the fragments swept back onto the shelf producing extensive spurs. By contrast, on more sheltered, obliquely orientated sections, storm-waves sweep debris along and off shelf producing little or no spur development. Instead, the debris shed seawards accumulates in front of the buttress walls and initiates the development of coral pinnacles. Over time, repeated buttress pruning and canyon flushing during hurricanes not only controls reef architecture but may also influence accretion patterns. Vertical accretion is limited by the effective depth of storm-wave fragmentation. Once this hurricane-accretion threshold is reached the reef moves into a shedding phase and accretes laterally via pinnacle growth, amalgamation, and infilling. Consequently, the reef steps out over its own debris in a kind of balancing act between lateral growth and slope failure — a pattern widely recognized in ancient reefs.  相似文献   

Architecture of fluvial-dominated valley-fill deposits in the Cretaceous Fall River Formation   总被引：1，自引：0，他引：1  

BRIAN J. WILLIS 《Sedimentology》1997,44(4):735-757

The Fall River Formation is a 45 m thick layer of fluvial-dominated valley-fills and shore-zone strata deposited on the stable cratonic margin of the Cretaceous Western Interior Seaway. Fall River deposits in Red Canyon, in the south-west corner of South Dakota (USA), expose a cross-section of a 3.5 km wide valley-fill sandstone and laterally adjacent marine deposits. The marine deposits comprise three 10 m thick upward-shoaling sequences; each composed of multiple metres-thick upward-coarsening successions. The lower two of these sequences are laterally cut by the valley-fill sandstone, and are capped by metres-thick muddy palaeosols. The upper sequence spans the top of the valley-fill sandstone, and is overlain by the Skull Creek Shale. The 30 m thick valley sandstone is partitioned into four distinct fills by major erosion surfaces, and each of these fills contain many metres-thick channel-form bodies. Deposits in the lower parts of these fills are sheet-like, top-truncated channel bodies, whereas deposits in the upper parts of fills are upward-concave, laterally amalgamated channel bodies, more completely preserved heterolithic channel bodies, or wave-deposited sheets. Each valley-fill basal erosion surface records an episode of valley incision and relative sea-level fall, and the gradual progression from fluvial to more estuarine deposits upwards within each fill records relative sea-level rise. All fills are dominantly channel deposits and are capped by marine flooding surfaces. The dominance of channel deposits, the gradual change to more estuarine facies in the upper parts of fills, and the location of flooding surfaces at valley-fill tops all suggest that sediment supply initially kept pace with relative sea-level rise and valleys filled during late marine lowstand and transgression, not during subsequent highstands. Recently proposed facies models have focused on variations in the relative strength of tide, wave and river currents as controls on valley-fill deposits. However, relative rates of sediment supply and basin accommodation change, and the shift in this ratio along the depositional profile during multiple-scale cycles in relative sea-level, are equally important controls on the style of valley-fill deposits.  相似文献   

Assessing a local ensemble Kalman filter: perfect model experiments with the National Centers for Environmental Prediction global model   总被引：1，自引：0，他引：1  

ISTVAN SZUNYOGH  ERIC J. KOSTELICH  G. GYARMATI  D. J. PATIL  BRIAN R. HUNT  EUGENIA KALNAY  EDWARD OTT  JAMES A. YORKE 《地球，A辑:动力气象学与海洋学》2005,57(4):528-545

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The fractal dimension of drifter trajectories and estimates of horizontal eddy-diffusivity   总被引：2，自引：0，他引：2  

BRIAN G. SANDERSON  DAVID A. BOOTH 《地球，A辑:动力气象学与海洋学》1991,43(5):334-349

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The fractal dimension of relative lagrangian motion   总被引：1，自引：0，他引：1  

BRIAN G. SANDERSON  ALLAN GOULDING  AKIRA OKUBO 《地球，A辑:动力气象学与海洋学》1990,42(5):550-556

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REVISITING RIO DE JANEIRO AND SÃO PAULO*     

BRIAN J. GODFREY 《Geographical review》1999,89(1):94-121

ABSTRACT. Contrasting models of urban development characterize the historical-geographical evolution of Brazil's two leading cities, viewed in an updating of the seminal 1933 article by Preston James. Native to Rio de Janeiro is a distinctive Luso-Brazilian style of irregular coastal urbanization, whereas SÃo Paulo displays a more uniform, modern type of inland commercial-industrial expansion. Even as Rio de Janeiro and SÃo Paulo sprawl today to form a virtual megalopolis in southeastern Brazil, they retain distinguishing roles in the national city system. Both metropolises are experiencing increasing functional decentralization and socioeconomic polarization, but in their own characteristic fashions. Generalized models of “Latin American city structure” are of limited value, unless they take into account such significant historical-geographical variations in urban form.  相似文献   

GLOBAL AND LOCAL CHANGE ON THE PORT‐CITY WATERFRONT*     

BRIAN HOYLE 《Geographical review》2000,90(3):395-417

ABSTRACT. Successful waterfront redevelopment requires an understanding of global processes and an appreciation of the distinctiveness of port‐city locations. Waterfront revitalization occurs at the problematic and controversial interface between port function and the broader urban environment. It reflects varied forces and trends, involves community attitudes and environmental sensitivities, and influences transport evolution and urban change. The revitalization phenomenon is examined using community attitudes in Canada and urban regeneration in East Africa to illustrate retrospective and prospective dimensions.  相似文献   

Carbonate platform dominated by peloidal sands: Lower Ordovician La Silla Formation of the eastern Precordillera,San Juan,Argentina     

BRIAN R. PRATT  MARIANA M. RAVIOLO  OSVALDO L. BORDONARO 《Sedimentology》2012,59(3):843-866

The Lower Ordovician La Silla Formation of the Precordillera of west‐central Argentina is part of the west‐facing early Palaeozoic, tropical carbonate platform succession that comprises the core of the Cuyania terrane. Up to 360 m thick, it is exposed in several thrust sheets over a distance of some 250 km along and across depositional strike over a palinspastically unrestored distance of about 35 km. La Silla Formation is a strikingly pure limestone with subordinate finely crystalline dolomite and rare chert. It accumulated on a more or less uniformly subsiding passive margin. Copious precipitation of microcrystalline calcite, probably influenced by microbial activity to varying degrees, led to the generation of peloids, ooids and aggregates of these grains, as well as small amounts of lime mud, intraclasts, stromatolites and thrombolites. Rare bioclasts are limited mostly to scattered gastropods and trilobite sclerites; bioturbation is present locally. The array of carbonate rock types is grouped into eight recurring lithofacies, in order of decreasing abundance: (i) peloidal grainstone; (ii) laminated dolostone; (iii) intraclastic rudstone; (iv) microbial laminite; (v) peloidal packstone; (vi) ooidal grainstone; (vii) thrombolite boundstone; and (viii) mudstone. These facies represent sediments that formed solely in a shallow subtidal marine environment, with no evidence of restricted conditions, hypersalinity or subaerial exposure. No events of eustatic sea‐level change are recorded. By far the dominant facies is grainstone composed of well‐sorted, fine sand‐sized peloids and peloidal aggregates in homogeneous, tabular to gently undulating, medium to thick beds; cross‐lamination is scarce. Clusters of sub‐metre‐sized microbial patch reefs developed sporadically. The shallow platform is envisaged to have been covered by extensive peloidal sand flats and low‐relief banks, and little lime mud was generated. The setting was probably microtidal and may not have been affected by strong trade winds. It was washed by frequent, relatively gentle wave action but without experiencing powerful storms. In the middle member, anomalous lenses of intraclastic rudstone and laminated dolostone occur as graded beds overlying sharply downcut scoured surfaces up to 20 cm deep; these are interpreted to indicate a phase when accretion was punctuated occasionally by tsunamis generated from rift‐faulting seaward of the platform margin. The remarkably uniform peloidal grainstone composition over a broad area shows that, given the appropriate combination of climate, environmental and ecological factors, large portions of some early Palaeozoic platforms were dominated by grainy sediment and remained under well‐agitated conditions within fair‐weather wave‐base, without distinct lateral facies differentiation or tidal‐flat aggradation.  相似文献   

The Himalayas     

BRIAN F. WINDLEY 《Geology Today》1985,1(6):169-173

After splitting from Gondwanaland, India drifted northwards to collide with the Asian landmass about 40 million years ago. The intervening Tethys ocean was closed by northwards subduction beneath southern Tibet, and the collision created the Himalayan orogenic belt. Continuing northward movement of India at a rate of about 5 cm per year over the last 40 million years has caused it to indent Asia, and the resultant massive shortening is expressed by thrusting of the northern margin of India, by faulting and earthquakes in the Himalayas and China, by rifting and faulting in Tibet, and by the uplift of the Himalayas which is still continuing at rates of up to several millimetres per year.  相似文献