摘要:

ABSTRACTVarious types of progradation of Triassic carbonate platforms are described from the Dolomites of the Southern Alps. The internal and external geometric relationships are exposed in spectacular natural sections and, moreover, their scale (500–1000 m of thickness) is such that they can be compared with features found in seismic profiles.The different types of progradation are controlled by a number of factors which, normally, interact with each other. These factors include: rate of basinal sedimentation, rate of subsidence, width of the platform, depth of the surrounding basin and eustatic variations of sea‐level.Progradation is not a continuous process but episodic. Moments of massive debris input, during which the platform advances, alternate with long periods of negligible progradation, during which basinal sediments accrete and onlap the toe of slope. Upper boundary relationships of the prograding platforms include offlap, toplap and erosional truncation. Lower boundary relationships are horizontal, climbing and descending progradations. A variety of phenomena and circumstances have caused the cessation of progradation of the Triassic platforms. They include volcanism, collapse of margins, drowning (rapid relative rise of sea‐level), subaerial exposure (relative fall of sea‐level) and, probably, a natural decay of the system.In the Triassic of the Dolomites, two main progradation models can be put forward: in theLadinian model, progradation took place simultaneously with aggradation (relative rise of sea‐level), whereas the characteristic feature of theCarnian modelis toplap (relative stillstand of s

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1984.tb00720.x

出版商:Blackwell Publishing Ltd    

年代:1984

数据来源: WILEY

摘要:

ABSTRACTThe Upper Ludlow Douro Formation contains the first reported Silurian sponge reefs. These relatively small (5–35 m diameter), mound‐shaped structures contain, on average, 35% lithistid demosponges. Reefs are surrounded by irregular haloes of crinoid debris; abundance and diversity of all fossil groups decreases away from the reefs. Each reef is underlain by a lens of crinoid wackestone to grainstone rich in crinoid holdfasts; trepostomate bryozoans, solenoporacean algae and rhynchonellid brachiopods are locally common. The bulk of each reef consists of lime mudstone with abundant lithistid sponges. This is capped by a thin layer of wackestone with abundant tabulate and rugose corals and fewer lithistid sponges, calcareous algae, trepostomate bryozoans and stromatoporoids. This zonation, in which a sponge colonization community was replaced by a coral diversification community, is similar to that reported from some Middle Ordovician, Upper Jurassic and Holocene sponge reefs.The Douro sponge reefs were relatively low structures, with about 3 m maximum topographic relief. They grew on a broad carbonate platform, probably in warm, tranquil, turbid waters of normal or near‐normal marine salinity. Periodic influxes of terrigenous mud adversely affected reef size, and caused biotic changes. Some of the reef lime mud was derived from non‐reef sources, but significant quantities were also produced on the reefs. Reefs underwent synsedimentary lithification, bioerosion and minor storm erosion. Fabrics and compositions of sparry calcite in cavities record three generations of meteoric cementation. Originally siliceous spicules of the lithistid sponges were dissolved and the moulds later filled with sparry calcite. Early dissolution of siliceous spicules is common in reef environments, and may have caused fossil sponges to be under‐represented in anci

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1984.tb00721.x

出版商:Blackwell Publishing Ltd    

年代:1984

数据来源: WILEY

摘要:

ABSTRACTObservations of the threshold of movement of loosely packed gravel in a tidal current are described. For gravel with equivalent ‘spherical’ diameters D in the range 0.2 ≲D≲ 5.0cm the critical friction velocityu*c, corresponding to the initiation of sediment transport, is given byu*c=7.0D0.2. At large values ofDwithin the quoted range, the valueu*cis significantly lower than would be obtained by a Shields experiment (u*c∞D0.5). By comparing our values ofu*cwith those obtained under well‐controlled laboratory conditions, the discrepancy with Shields is shown to be due to the open spacing between, and exposure of, individual pebbles on the seabed. By comparing our results with those from upland gravel streams and flume experiments, it is suggested that Shields assumed an excessively large water depth to particle size ratio as a constraint within which the critical sediment entrainment number

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1984.tb00722.x

出版商:Blackwell Publishing Ltd    

年代:1984

数据来源: WILEY

摘要:

ABSTRACTA remarkable exposure of Miocene marine molasse in western Switzerland records the evolution of a tidal sandwave over a period of approximately 2 1/2 months. The sandwave is composed of tidal ‘bundles’ in which a sandwave reactivation stage and full vortex stage can be recognized for the dominant flow (ebb tide) and a rippled flood apron overlain by high water drape for the reversed flow. Bundle thicknesses vary systematically through neap–spring cycles, with a periodicity of 27 demonstrating the semi‐diurnal lunar control of sedimentation. Waves were an additional component, especially when superimposed on flood tides, producing near‐symmetrical combined‐flow ripple marks in the flood apron.Tidal current velocities are estimated using critical shear velocities for entrainment, the ripple‐dune transition and the dune‐plane bed transition. Using appropriate estimates of roughness lengths and a logarithmic velocity law, maximum tidal speeds at 1 m above the bed were approximately 0·6 m sec−1for ebbs and up to 0·5 m sec−1for floods. The enhancement by waves of bed shear stress (τwc/τ of approximately 2 for 1 m high waves) under flood currents implies flood tidal velocities closer to 0·2–0·3 m sec−1.Peak instantaneous bedload sediment transport rates using a modified Bagnold equation are nearly 5 times greater under ebb tides than floods. The average net sediment transport rate at springs (0·04 kg m−1sec−1) is over 10 times greater than at neaps (0·002 kg m−1sec−1). Comparison with transport rates in modern tidal environments suggests that the marine molasse of Switzerland was deposited under spatially confined and relatively swift tidal flows not dissimilar to tho

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1984.tb00723.x

出版商:Blackwell Publishing Ltd    

年代:1984

数据来源: WILEY

摘要:

ABSTRACTSand samples from piston cores taken in the Peru‐Chile Trench and adjacent areas were examined petrographically in an attempt to characterize sands from the oceanward side of a continent‐margin volcanic arc. Framework composition averaged Q18F44L38, close to the average reported for this type of setting, but showed wide variation in the ratio of feldspar to lithic fragments; quartz was uniformly low, less than 30%. Samples rich in lithic grains were from sites that lie off areas of recent volcanism on the adjacent continent. Samples from off southern Central America had compositions more like those found in island‐arc settings, averaging Q3F21L76, whereas those from the Straits of Magellan increased in quartz and decreased in lithic fragments toward the Atlantic. Plagioclase was greatly predominant over K‐feldspar in all samples, and averaged An28. A considerable portion of the plagioclase was close to end‐member albite, suggesting recycling from pre‐existing sedimentary rocks. Climate had no obvious effect on the framework proportions or the feldspar composition, but the volume of sediment in the trench increases with increasing rainfall on th

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1984.tb00724.x

出版商:Blackwell Publishing Ltd    

年代:1984

数据来源: WILEY

摘要:

ABSTRACTBasal Stephanian deposits of early Cantabrian age infill a deeply incised fossil topography at the western edge of the post‐Leonian basin of NE Leon. Seven low sinuosity palaeovalleys, trending NNW–SSE range in depth of incision from 25 to 450 m. The fill is composed of: (i) Red bed succession: a proximal alluvial valley‐fill association of debris flow and stream deposits in which seven mesosequences and two fining upwards megasequences are recognized on the basis of maximum clast sizes. (ii) Grey bed succession: two megasequences are distinguished. The lower megasequence comprises distal alluvial valley‐fill deposits (sheet sandstone, lacustrine, swamp deposits). The upper megasequence comprises a mid‐fan association of debris flow and interlobe/floodplain deposits.The palaeovalleys are considered in the context of previously described palaeovalleys and fill. These are categorized according to the degree of tectonic control and climatic setting.The lower Cantabrian examples may serve as a model for valley‐fill sedimentation in a tectonically active area and a humid clima

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1984.tb00725.x

出版商:Blackwell Publishing Ltd    

年代:1984

数据来源: WILEY

摘要:

ABSTRACTWell sorted fine sand pretreated with varying concentrations of two soluble salts (MgCl2and CaCl2) was tested for threshold shear velocity in a recirculating wind tunnel. These tests demonstrate that even relatively low surface salt concentrations can significantly increase threshold shear velocity. However, the relationships between surface salt concentration and threshold shear velocity for the two salts were not significantly different, which is similar to the results for monovalent chlorides presented by Nickling&Ecclestone. A general relationship between threshold shear velocity and total, surface soluble salt content is presented.

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1984.tb00726.x

出版商:Blackwell Publishing Ltd    

年代:1984

数据来源: WILEY

摘要:

ABSTRACTThis paper provides some additional evidence supporting the necessary (but insufficient) condition for the formation of stream meandering proposed by Nakagawa:whereMeis a non‐dimensional parameter, τs, and τbare the average bank and bed shear stress respectively,psandpbare the average bank and bed wetted perimeter of a half‐channel respectively, andK≃0.2 is the critical value of the parameter estimated from experimental data.Provided the criterion is satisfied, the main thread of the stream meanders in the non‐erodible channel, and the maximum amplitudeamax, the angle a between the channel central axis and oblique crest line of the surface wave, and the mean wavelength L of the main thread decrease as the non‐dimensional parameter M

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1984.tb00727.x

出版商:Blackwell Publishing Ltd    

年代:1984

数据来源: WILEY

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1984.tb00728.x

出版商:Blackwell Publishing Ltd    

年代:1984

数据来源: WILEY

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1984.tb00729.x

出版商:Blackwell Publishing Ltd    

年代:1984

数据来源: WILEY