Ribbon sandstones in the Pennsylvanian Waddens Cove Formation, Sydney Basin, Atlantic Canada: the influence of siliceous duricrusts on channel-body geometry   总被引：1，自引：0，他引：1  

MARTIN R. GIBLING  BRIAN R. RUST 《Sedimentology》1990,37(1):45-66

Channelized bodies up to 9 m thick in the Pennsylvanian Waddens Cove Formation show low width: thickness ratios (8:1 to 14:1), up to four vertically stacked storeys, and deep incision into associated floodplain strata. The presence of lateral accretion sets up to 6 m thick indicates that the migration of bank-attached bars was a major channel-filling process. The progressive rise in elevation of the basal part of the accretion surfaces where the sets adjoin the steeply inclined sides of the channel bodies shows that the channel floor aggraded progressively as the bars approached the channel sides. In planform, the river system probably showed a low-sinuosity, incised valley within which the channel followed a more sinuous and partially confined course. The alluvium surrounding the channel bodies includes sandstone sheets attributed to crevasse splay and levee accumulation. The upper strata of many sheet's and some channel bodies contain siliceous duricrusts (ganisters) which formed by pedogenic lithification of sandy material commencing shortly after the landforms became inactive. The channel bodies contain duricrust-bearing slumpblocks and show stepped margins over duricrusts. The stepped nature of the banks suggests that duricrusts were an important factor in confining the reach, and we suggest that channel-body geometry was influenced, although not necessarily controlled, by the presence of the lithified layers. Alternation in the Waddens Cove Formation of grey, coal-bearing zones with red, duricrust-rich zones is attributed to water-table fluctuation in response to base-level changes. The low width: thickness ratios and aggradation mode'of the channel bodies may in part reflect channel incision and subsequent aggradation resulting from these allocyclic events.  相似文献   

Environment and development in the former South African bantustans     

BRIAN H KING  BRENT MCCUSKER† 《The Geographical journal》2007,173(1):6-12

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The Composition of Garnets from Pelitic Schists in relation to the Grade of Regional Metamorphism   总被引：1，自引：0，他引：1  

STURT  BRIAN A. 《Journal of Petrology》1962,3(2):181-191

Analyses of garnets are presented from a wide variety of metamorphicterrains, in relation to which there appears to be a regularpattern of substitution of (FeO+MgO) for (CaO+MnO), the valuesof these components reflecting the metamorphic grade of thepelitic schists. This provides a method for indicating variationsin grade in metamorphic terrains and for comparing the variationsin different areas. In areas of repeated metamorphism this wouldappear to have a particular value in establishing the gradeof the various episodes of metamorphism. It is also demonstratedthat by the determination of selected physical propertics (unitcell edge a and refractive index) of garnets in the peliticschists, and evaluating the significance of their variations,it is possible to work out the local changes of metamorphicgrade within a particular area.  相似文献   

Glacial landforms on German Bank, Scotian Shelf: evidence for Late Wisconsinan ice-sheet dynamics and implications for the formation of De Geer moraines   总被引：1，自引：1，他引：1  

BRIAN J. TODD  PAGE C. VALENTINE  ODDVAR LONGVA  JOHN SHAW 《Boreas: An International Journal of Quaternary Research》2007,36(2):148-169

The extent and behaviour of the southeast margin of the Laurentide Ice Sheet in Atlantic Canada is of significance in the study of Late Wisconsinan ice sheet-ocean interactions. Multibeam sonar imagery of subglacial, ice-marginal and glaciomarine landforms on German Bank, Scotian Shelf, provides evidence of the pattern of glacial-dynamic events in the eastern Gulf of Maine. Northwest-southeast trending drumlins and megaflutes dominate northern German Bank. On southern German Bank, megaflutes of thin glacial deposits create a distinct northwest-southeast grain. Lobate regional moraines (>10km long) are concave to the northwest, up-ice direction and strike southwest-northeast, normal to the direction of ice flow. Ubiquitous, overlying De Geer moraines (<10 km long) also strike southwest-northeast. The mapped pattern of moraines implies that, shortly after the last maximum glaciation, the tidewater ice sheet began to retreat north from German Bank, forming De Geer moraines at the grounding line with at least one glacial re-advance during the general retreat. The results indicate that the Laurentide Ice Sheet extended onto the continental shelf.  相似文献   

Subtropical to temperate facies from a transition zone, mixed carbonate–siliciclastic system, Palaeogene, North Carolina, USA     

BRIAN P COFFEY  J. FRED READ† 《Sedimentology》2007,54(2):339-365

Palaeogene passive margin sediments on the US mid‐Atlantic coastal plain provide valuable insight into facies interaction and distribution on mixed carbonate–siliciclastic shelves. This study utilizes well cuttings, outcrop, core, and seismic data to document temporal and spatial variations in admixed bryozoan‐rich skeletal carbonates and sandy siliciclastic units that were deposited on a humid passive margin located in the vicinity of a major marine transition zone. This zone was situated between north‐flowing, warm waters of the ancestral Gulf Stream (carbonate dominated settings) and south‐flowing, cold waters of the ancestral Labrador Current (siliciclastic dominated settings). Some degree of mixing of carbonates and siliciclastics occurs in all facies; however, siliciclastic‐prone sediments predominate in nearshore settings, while carbonate‐prone sediments are more common in more open marine settings of the inner shelf break and deep shelf. A distinctive dual‐break shelf depositional profile originated following a major Late Cretaceous to Palaeocene transgression that drowned the earlier shallow platform. This profile was characterized by prominent mid‐shelf break dividing the shallow shelf from the deep shelf and a major continental shelf/slope break. Incomplete filling of available accommodation space during successive buildup of the shallow shelf preserved the topographic break on this passive margin. Storm wave base also contributed to the preservation of the dual‐break shelf geometry by beveling shallow shelf sediments and transporting them onto and seaward of the mid‐shelf break. Sediment fines in deep shelf facies were produced in place, transported downdip from the shallow shelf by storm ebb currents and boundary currents, and reworked from adjacent areas of the deep shelf by strike‐parallel boundary currents. Regional climate and boundary currents controlled whether carbonate or siliciclastic material was deposited on the shelf, with warmer waters and more humid climates favouring carbonate deposition and cooler, more arid conditions favouring glaucony and siliciclastic dominated deposition. Continuous wave and current sweeping of the shallow shelf favoured deposition of mud‐lean facies across much of the shallow shelf. Skeletal components in much of the carbonate‐rich strata formed in warm, nutrient‐rich subtropical waters, as indicated by widespread occurrences of larger benthic foraminifera and molluscan assemblages. These indicators of warm water deposition within the bryozoan‐mollusk‐rich carbonate assemblage on this shelf provide an example of a warm water bryomol assemblage; such facies generally are associated with cooler water depositional settings.  相似文献   

Sheetflow fluvial processes in a rapidly subsiding basin, Altiplano plateau, Bolivia   总被引：2，自引：0，他引：2  

BRIAN A. HAMPTON  BRIAN K. HORTON† 《Sedimentology》2007,54(5):1121-1148

Although facies models of braided, meandering and anastomosing rivers have provided the cornerstones of fluvial sedimentology for several decades, the depositional processes and external controls on sheetflow fluvial systems remain poorly understood. Sheetflow fluvial systems represent a volumetrically significant part of the non‐marine sedimentary record and documented here are the lithofacies, depositional processes and possible roles of rapid subsidence and arid climate in generating a sheetflow‐dominated fluvial system in the Cenozoic hinterland of the central Andes. A 6500 m thick succession comprising the Late Eocene–Oligocene Potoco Formation is exposed continuously for >100 km along the eastern limb of the Corque syncline in the high Altiplano plateau of Bolivia. Fluvial sandstone and mudstone units were deposited over an extensive region (>10 000 km²) with remarkably few incised channels or stacked‐channel complexes. The Potoco succession provides an exceptional example of rapid production of accommodation sustained over a prolonged period of time in a non‐marine setting (>0·45 mm year⁻¹ for 14 Myr). The lower ≈4000 m of the succession coarsens upward and consists of fine‐grained to medium‐grained sandstone, mudstone and gypsum deposits with palaeocurrent indicators demonstrating eastward transport. The upper 2500 m also coarsens upward, but contains mostly fine‐grained to medium‐grained sandstone that exhibits westward palaeoflow. Three facies associations were identified from the Potoco Formation and are interpreted to represent different depositional environments in a sheetflow‐dominated system. (i) Playa lake deposits confined to the lower 750 m are composed of interbedded gypsum, gypsiferous mudstone and sandstone. (ii) Floodplain deposits occur throughout the succession and include laterally extensive (>200 m) laminated to massive mudstone and horizontally stratified and ripple cross‐stratified sandstone. Pedogenic alteration and root casts are common. (iii) Poorly confined channel and unconfined sheet sandstone deposits include laterally continuous beds (50 to >200 m) that are defined primarily by horizontally stratified and ripple cross‐stratified sandstone encased in mudstone‐rich floodplain deposits. The ubiquitous thin‐sheet geometry and spatial distribution of individual facies within channel sandstone and floodplain deposits suggest that confined to unconfined, episodic (flash) flood events were the primary mode of deposition. The laterally extensive deposition and possible distributary nature of this sheetflow‐dominated system are attributed to fluvial fan conditions in an arid to semi‐arid, possibly seasonal, environment. High rates of sediment accumulation and tectonic subsidence during early Andean orogenesis may have favoured the development and long‐term maintenance of a sheetflow system rather than a braided, meandering or anastomosing fluvial style. It is suggested here that rapidly produced accommodation space and a relatively arid, seasonal climate are critical conditions promoting the generation of sheetflow‐dominated fluvial systems.  相似文献   

Aspects of the large-scale tropical atmospheric circulation     

BRIAN HOSKINS  RICHARD NEALE  MARK RODWELL  GUI-YING YANG 《Tellus. Series B, Chemical and physical meteorology》1999,51(1):33-44

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Middle and late Cenomanian oceanic anoxic events in shallow and deeper shelf environments of western Morocco     

BRIAN GERTSCH  THIERRY ADATTE  GERTA KELLER  ABDEL AZIZ A.M. TANTAWY  ZSOLT BERNER  HAYDON P. MORT  DOMINIK FLEITMANN 《Sedimentology》2010,57(6):1430-1462

The response of shallow‐water sequences to oceanic anoxic event 2 and mid‐Cenomanian events 1a and 1b was investigated along the west African margin of Morocco north of Agadir (Azazoul) and correlated with the deep‐water sequence of the Tarfaya Basin (Mohammed Beach) based on biostratigraphy, mineralogy, phosphorus and stable isotopes. In the deeper Mohammed Beach section results show double peaks in δ¹³C_org for mid‐Cenomanian events 1a and 1b (Rotalipora reicheli biozone, lower CC10a biozone), the characteristic oceanic anoxic event 2 δ¹³C excursion (Rotalipora cushmani extinction, top of CC10a biozone) and laminated (anoxic) black shale. In the shallow environment north of Agadir, a fluctuating sea‐level associated with dysoxic, brackish and mesotrophic conditions prevailed during the middle to late Cenomanian, as indicated by oyster biostromes, nannofossils, planktonic and benthonic foraminiferal assemblages. Anoxic conditions characteristic of oceanic anoxic event 2 (for example, laminated black shales) did not reach into shallow‐water environments until the maximum transgression of the early Turonian. Climate conditions decoupled along the western margin of Morocco between mid‐Cenomanian event 1b and the Cenomanian–Turonian boundary, as also observed in eastern Tethys. North of Agadir alternating humid and dry seasonal conditions prevailed, whereas in the Tarfaya Basin the climate was dry and seasonal. This climatic decoupling can be attributed to variations in the Intertropical Convergence Zone and in the intensity of the north‐east trade winds in tropical areas.  相似文献   

Drilling Blake Nose: the search for evidence of extreme Palaeogene–Cretaceous climates and extraterrestrial events     

KROON  NORRIS  KLAUS  IAN T. ALEXANDER  LEON PAUL BARDOT  CHARLES E. BARKER  JEAN-PIERRE BELLIER  CHARLES D. BLOME  LEON J. CLARKE  JOCHEN ERBACHER  KRISTINA L. FAUL  MARY ANNE HOLMES  BRIAN T. HUBER  MIRIAM E. KATZ  KENNETH G. MACLEOD  SANDRA MARCA  FRANCISA C. MARTINEZ-RUIZ  ISAO MITA  MUTSUMI NAKAI  JAMES G. OGG  DOROTHY K. PAK  THOMAS K. PLETSCH  JEAN M. SELF-TRIAL  NICHOLAS J. SHACKLETON  JAN SMIT  WILLIAM USSLERIII  DAVID K. WATKINS  JOEN WIDMARK  & PAUL A. WILSON 《Geology Today》1998,14(6):222-226

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Towards a PV-θ view of the general circulation     

BRIAN J. HOSKINS 《地球，A辑:动力气象学与海洋学》1991,43(4):27-35

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