摘要:

ABSTRACTWithin high‐density flood flows a prominent mechanism of gravel transport and deposition is by stream‐driven, high‐density traction carpet (with a rheology similar to grain flow). These gravel carpets are envisaged to form the basal portion of a bipartite high‐density flood flow, decoupled from an overlying sand‐ and silt‐laden turbulent flow. Several examples already documented in the literature are reviewed and an additional case from the Lower Old Red Sandstone of southwest Ireland is presented. Two mechanisms of traction carpet initiation are discussed: by rapid entrainment of gravel into suspension on rising stage, followed by settling into the gravel traction carpet at peak and falling stage; and by overconcentration of a ‘normal’, low‐density bedload. Gravel entrainment, suspension and traction carpet development are significantly easier if the flood water already carries a high concentration of sand and silt in suspension. Theoretical consideration further shows that gravelly traction carpets can be maintained in channels of relatively low gradient by the shear stress exerted by the high‐density, sand‐bearing turbulent flood flow above. This tangential shear stress is converted to dispersive pressure, which aids buoyancy and quasi‐static grain‐to‐grain contacts in the support of the clasts within the gravel carpet. The carpet is thought to have a quasi‐plastic rheology but behave much like a viscous fluid at high shear rates. Stream‐driven gravelly traction carpets are expected to produce sheet‐like units of clast‐ to matrix‐supported conglomerate, characterized by a parallel or ana(p)a(i) clast fabric. These units may be ungraded, normally or inversely graded, depending on the rate of shear, the viscosity of th

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1989.tb02083.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

ABSTRACTThe Late Proterozoic Bakoye 3 Formation is a predominantly aeolian unit deposited in the glacially influenced cratonic Taoudeni Basin of western Africa. The Bakoye 3 can be divided into five distal units, two proximal units, and a local upper massive sandstone. The basal Unit 1 shows a complex interfingering of aeolian and subaqueous structures, and is interpreted as the precursor of the overlying erg sequences. Unit 2 consists of compound, trough cosets of aeolian cross‐strata dominated by grain‐flow strata. The unit is interpreted to represent draas with superimposed, small, crescentic dunes. A super bounding surface marks the termination and planation of the erg. Unit 3 is distinguished from the underlying Unit 2 by its larger, overall simple sets of trough cross‐strata, interpreted to represent simple, large, crescentic dunes. Unit 4 occurs only locally in laterally discontinuous, large troughs. In one case the trough is filled by small sets of tabular cross‐strata dominated by grain‐flow deposits. At another section, wedges of coarse‐grained wind‐ripple strata fill the trough. Unit 4 may represent remnants of ergs or, more likely, local deposition in depressions. The depressions, in the latter scenario, formed with the development of a second super surface that truncates Unit 3. Unit 5 consists of very large sets of wind‐ripple cross‐strata with less common sets of grain‐flow deposits. These deposits are believed to represent enormous dunes with large plinths and subordinate slip face development. A third super surface separates Unit 5 from overlying marine deposits. Together, Units 1–5 represent the core of the ergs in a distal position relative to adjacent upland source areas. Proximally, aeolian deposits are simple, smaller, trough sets interpreted as moderate sized crescentic dunes. Coarse‐grained braided stream deposits are prominent. Locally, the top of the Bakoye 3 is marked by channelized mass‐flow deposits containing aeolian blocks, and is believed to have resulted from iceberg grounding. An overall environment for the Bakoye 3 is one of uplands marked by ice sheets, with outwash plains extending distally to aeolian ergs. Super surfaces, all marked by polygonal fractures and coarsegrained sediment, represent periods of erg termination that may be linked to glaci

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1989.tb02084.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

ABSTRACTThe morphology and dynamics of modern gravel shorefaces are poorly documented. This hinders the interpretation of possible ancient counterparts. A comparative study of a modern (Chesil Beach, England) and an ancient (Baytree Member of the Cardium Formation, Alberta) gravel shoreface shows that the two systems are very similar close to and above sea‐level, with a high (about 1 m) gravel plunge step lying below plane‐bedded sands and gravels of the beachface. The shoreface at Chesil Beach is dominated by asymmetrical gravel wave ripples. These are oriented offshore near the toe of the shoreface, and onshore in shallower depths. This may reflect offshore movement during storms and landward reworking during fair weather. The Baytree Member is over 12 m thick and comprises over 80% conglomerate. Conglomerate is decimetre‐bedded, massive or cross‐bedded, with sets over 60 cm thick produced by gravel bedforms migrating alongshore. It is interbedded with discontinuous cm‐ to dm‐bedded sandstones which may be cross‐bedded. Pebble fabric and cross‐bed orientation both indicate strong alongshore sediment transport. Near the base of the section, pebble orientations suggest that gravel wave‐ripples developed below the zone of strong longshore flows. Differences between these two examples may be attributed to different directions

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1989.tb02085.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

ABSTRACTThe abrupt transition from coastal and shallow shelf sediments to bathyal sediments provides a record of rapid subsidence and deepening of the early Miocene Waitemata basin. Basal shallow marine strata (Kawau Subgroup) accumulated upon a highly dissected surface that overlies deformed Mesozoic metagreywacke. The early Miocene coast was characterized by an embayed and cliffed shoreline with numerous sea stacks and islands. Kawau Subgroup lithofacies, which include pocket beach, shallow shelf and base‐of‐cliff talus deposits, reflect rapidly changing coastline configuration and water depths as the rugged bedrock surface was buried.The response to continued rapid subsidence and transgression in Waitemata basin was a decrease in the supply of coarse clastic sediment. Beach gravels were locally displaced to greater water depths by avalanching down steep bedrock slopes. The first bathyal turbidite facies, which abruptly overlie the shallow‐water Kawau Subgroup, include locally derived sediment gravity flows commonly ponded by remnant bedrock submarine highs. When this local supply of sediment had been exhausted, coarse sediment starvation ensued and bathyal muds accumulated. With the resumption of sediment supply and gradual burial of submarine bedrock relief, submarine fans coalesced and increased in lateral extent.Subsidence of the Waitemata basin to bathyal depths is thought to have occurred in less than a million years. From the above hypothesis, a general model of sedimentation is pro

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1989.tb02086.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

ABSTRACTThis paper describes recent proglacial lacustrine sediments exposed by the drainage of a small (probably never more than 0·03 km2) ice‐dammed lake basin at Leirbreen, central Jotunheimen, Norway. The dominant facies include ripple‐laminated, massive and horizontally‐stratified sands, massive and horizontally‐laminated silts, and irregularly‐laminated fine sands and silts. The major control on lake circulation and the nature and distribution of these facies was an underflow driven by a subglacial meltwater stream which formed the major lake input. Although much of the sedimentary sequence indicates a pulsatory input, the proximal character of this small lake prevented the development of classic varved silts. Compressional deformation of shoreline sediments was due to winter lake ice push. Other deformational processes included the grounding of icebergs, water escape and syn‐sedimentary downslope collapse. Observations from an adjacent small ice‐marginal lake at Leirbreen provide support for several of the inferences drawn from the sediments of the former i

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1989.tb02087.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

ABSTRACTThe Late Proterozoic Conception Group, exposed on the Avalon Peninsula in Newfoundland, Canada, is a 4 km thick turbidite succession containing a conformable 300 m thick sequence of diamictites (the Gaskiers Formation) near the base. Massive and crudely‐stratified diamictites form beds up to 25 m thick which have a tabular geometry with slightly erosive basal contacts and are interbedded with mudstones and fine‐grained, thin‐bedded turbidites. These diamictites are interpreted as submarine debris flow deposits. Disrupted diamictites form strongly deformed units that contain large, complexly folded rafts of mudstone and turbidite facies. These diamictite units are interpreted as submarine slumps. Diamictites contain glacially‐striated and faceted clasts; clasts and matrix are predominantly of volcanic provenance. One outcrop shows interbedded volcanic agglomerate and diamictite, and volcanic bombs can also be identified.The interbedding of diamictites with turbidites and the stratigraphic context provided by the thick sequences of turbidites below (Mall Bay Formation) and above (Drook Formation) indicate a deep marine slope setting of diamictite deposition. Diamictite facies record remobilization and downslope transfer of large volumes of unstable volcanic and glacial debris initially deposited in a shallower water marginal marine zone. The regional tectonic framework suggests the Conception Group accumulated in a deep, southward‐opening ensialic rift basin with active but waning volcanic centres to the north.The Gaskiers Formation may be representative of other Late Precambrian glacially‐influenced diamictite sequences that were deposited around the North Atlantic region and in Europe. These deep marine diamictite sequences characterized by debris flows, turbidites, and slump deposits, can be contrasted with more extensive shallow marine shelf diamictite sequences found in association with dolomites and tidally influenced shallow water facies in other basin

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1989.tb02088.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

ABSTRACTBlackband iron formations are essentially thin (approximately 0·75 m thick) siderite‐rich (total iron up to 40%), carbonaceous, laminated mudrocks which commonly occur in grey coal bearing sequences in close proximity to coal seams. They exhibit a conspicuous laminated macrotexture made up of carbonaceous and siderite‐rich laminae together with primary textures such as root‐disturbed laminae, sideritized unflattened spores and preservation of plant cell detail. These all point to formation of siderite soon after deposition, in some cases before significant compaction. Their enclosing sediments clearly show that they were deposited in environments intermediate between delta top alluvial floodplains and coastal plain swamps in which small areas were subjected to periods of lacustrine deposition. The presence of varved mudrocks and oil shales in the beds directly above many of the blackband iron formations and the distinctive fine lamination of the blackbands are evidence of such depositional environments. The blackband iron formations are considered to have been formed in a similar way to Recent bog iron ores and are therefore interpreted as fossil bog iron ores. A model for their mode of formation is pre

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1989.tb02089.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

ABSTRACTEarly diagenetic K‐feldspar in the Triassic Buntsandstein of the Iberian Range (Spain) occurs as pseudomorphs after detrital K‐feldspar (Or<93) and plagioclase (Ab99), untwinned, commonly heavily clouded by vacuoles and tiny inclusions, dark‐luminescing and are composed of numerous fine euhedral crystals of K‐feldspar. The latter property suggests that the pseudomorphs form via dissolution of detrital K‐feldspar and plagioclase and precipitation of authigenic K‐feldspar. X‐ray diffraction analysis shows that the authigenic K‐feldspar is intermed

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1989.tb02090.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

ABSTRACTThe Codocedo Limestone Member is a thin but laterally persistent lacustrine sequence within the red beds of the Upper Jurassic‐Lower Cretaceous Quebrada Monardes Formation, in the Atacama region of northern Chile. The thick succession of clastic terrigenous sediments of the Quebrada Monardes Formation was deposited in an arid to semi‐arid environment. Sedimentary facies are indicative of deposition of aeolian dunes, alluvial fans and braided streams, playa‐lake mudflats, and saline lakes and coastal lagoons. The strata accumulated in a N‐S elongated extensional back‐arc basin on the landward side of an active volcanic arc.The 3 m thick Codocedo Limestone Member marks striking facies changes within the Quebrada Monardes Formation. It is underlain by a thick sequence of conglomerates and sandstones, deposited on alluvial fans. The limestone itself is characterized by evaporite minerals and laterally continuous laminations, indicative of deposition by vertical accretion in a perennial saline lake. The overlying siltstones and fine sandstones contain geodes and gypsum pseudomorphs and were deposited on playa‐lake mudflats. The limestone therefore represents a relatively short period of lacustrine deposition within an essentially terrigenous succession. The lake was possibly formed quite suddenly, for example by damming of the basin by a lava flow.Sedimentation in the perennial lake was predominantly cyclical. Seasonal planktonic algal blooms produced millimetre‐scale laminations. Interbedded with these laminites are centimetre‐scale beds of evaporitic gypsum, anhydrite and minor halite. The evaporite minerals have been largely replaced by calcite, chalcedony and quartz. The centimetre‐scale cycles may have resulted from periodic freshwater input into the lake. After a period of about 3000 yr the lake dried up, to be replaced by extensive playa‐lake mudflats.The Codocedo Limestone Member possibly formed a plane of detachment during an early Tertiary phase of E‐W directed regional compression. The limestones and evaporites were folded and extensively brecciated. This deformation probably resulted from simple shear along the bedding plane of the relatively weak evaporite minerals prior to their replacement

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1989.tb02091.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

ABSTRACTThe Berriasian Rupelo Formation of the W Cameros Basin consists of a 2–200 m thickness of marginal and open lacustrine carbonate and associated deposits. Open lacustrine facies contain a non‐marine biota with abundant charophytes (both stems and gyrogonites), ostracods, gastropods and rare vertebrates. Carbonate production was mainly biogenic. The associated marginal lacustrine (‘palustrine’) facies show strong indications of subaerial exposure and exhibit a wide variety of pedogenic fabrics. Silicified evaporites found near to the top of the sequence reflect a short hypersaline phase in the lake history. The succession was laid down in a low gradient, shallow lake complex characterized by wide fluctuations of the shoreline.Carbon and oxygen stable isotope analyses from the carbonates show non‐marine values with ranges of δ13from − 7 to − 11‰and δ18from − 3 to − 7.5‰. Differences in the isotopic composition of open lacustrine carbonates are consistent with sedimentary evidence of variation in organic productivity within the lake. Analyses from the entire sample suite plot on a linear trend; isotopic compositions become lighter with increasing evidence of pedogenic modification. This suggests progressive vadose zone diagenesis and influence of meteoric waters rich in soil‐derived CO2. The stable isotope data thus support evidence from petrography and facies relations that ‘palustrine’limestones form through pedogenic mo

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1989.tb02092.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY