摘要:

ABSTRACTAn empirical model is constructed for the development and equilibrium dimensions of small scale, unidirectional bedforms in sand with a median grain size of 0·095 mm, based on a series of steady flow experiments in a flume. Current ripples always attain a linguoid plan morphology with constant average height (13·1 mm) and wavelength (115·7 mm), provided that sufficient time is allowed for their formation. The development pattern of these ripples on a flat bed is independent of flow velocity, and involves four stages: (1) incipient ripples; (2) straight and sinuous ripples; (3) non‐equilibrium linguoid ripples, and (4) equilibrium linguoid ripples. Straight and sinuous ripples are non‐equilibrium bedforms at all flow velocities. The time needed to reach equilibrium dimensions is related to the inverse power of flow velocity and ranges from several minutes to more than hundreds of hours. At flow velocities where washed ripples are stable, the equilibrium wavelength is similar to that of equilibrium linguoid ripples, but the equilibrium height rapidly decreases from 13·1 mm to zero towards upper stage plane bed conditions.The results of the flume experiments correspond reasonably well with those of previous studies, provided that various complicating factors, such as different experimental methods, different sediment characteristics, shallow flow depths and non‐equilibrium runs, are acco

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1994.tb01400.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTCurrent crescents are well preserved on the upper surfaces of quartzite beds in the Lower Cambrian Herreria Formation of the Cantabrian Mountains in northern Spain. Within a sequence of nearshore deposits, the current crescents are interpreted as having formed by flow separation of wave backwash around obstacles on a beach. The obstacles are not preserved and are inferred to have been perishable organic material, such as stranded algae or jellyfish, which subsequently decayed.

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1994.tb01401.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTThe Oligocene and Miocene Valley Springs Formation is one element in the middle to late Cenozoic alluvial fill of the Central Valley of California and represents a large fluvial depositional system that extended westward from sediment‐filled palaeovalleys in the high Sierra Nevada to a piedmont alluvial plain under the present Central Valley. The Valley Springs Formation consists largely of tuffaceous mudrock, tuffaceous sandstone, polymict conglomerate and rhyodacitic tuff. The most prominent lithofacies in the formation is yellowish grey or greyish yellow, tuffaceous mudstone and claystone characterized by crude, commonly wavy layering or bedding, an irregular fracture, and common clay‐lined partings, fissures, and small branching tubules.Petrographic and X‐ray diffraction analyses of the composition, texture, fabric, mineralogy and organic content of an 8 m thick section of the tuffaceous mudrock lithofacies have provided evidence for an origin quite different from the earlier interpretations of this lithofacies as altered tuffs. Numerous rounded mudstone or claystone clasts in a detrital mud matrix, together with abundant freshwater microfossils (megaspores, chrysophyte cysts, diatoms and sponge spicules), indicates that the lithofacies is a mostly epiclastic sedimentary deposit. Root traces, fissuring, orientated clay skins lining pores and secondary surfaces, and mineralogical data are evidence of periodic exposure and desiccation.The tuffaceous mudrock lithofacies of the Valley Springs Formation, interpreted in the context of the middle to lower regions of a piedmont alluvial system, most probably represents a complex of ephemeral lake and marsh environments on a low gradient alluvial plain. The inferred abundance of shallow lakes, ponds and marshes implies a climate that was wetter than the semi‐arid climate of the regio

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1994.tb01402.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTExperiments demonstrate that fluid escape structures can be produced as a result of unstable fluidization behaviour where a lower base layer of granular material is inhibited from fluidizing by the presence of an overlying non‐fluidizing top layer. Before the base layer can fluidize the weight of the overlying material must be balanced, and this is accomplished by base layer material pressing against the bottom surface of the confining top layer forming a static layer. This static layer allows the top layer to lift away from the base layer which is then free to fluidize. A water‐filled crack forms below the static layer and, as this grows, instability causes the static layer and top layer to bend and conical voids to form below the antiformal sections. Rupture occurs at the apex of the water void, allowing the underlying water and fluidizing material to burst out through the top layer. The fluidized base layer material then flows through the rupture until all of this material, except that in the static layer, is deposited above the previously overlying layer and a stable fluidization system results. The top layer material is bent upwards around the rupture, and the resulting pillar‐type escape structure is preserved if flow then ceases. The vigour of the burst‐out is greatest when the base layer material has a grain size 15% of the top layer material. If the base layer grain size is less than 8% of the top layer then base layer material will pass through the top layer pore spaces, without forming an escape structure. If cohesive material is present, escape structures form when a layer of fine grained cohesive material overlies a layer of cohesionless material. At low flow rates small pipes with scattered angular bends pierce the top layer, and base layer material passes through them. The base layer material is ejected on to the top layer and builds up around the mouth of each pipe to form constructional structures, sand volcanoes. This is in contrast to the cohesionless experiments, where the weight of material being deposited on the top layer caused an ejecta‐filled depression to form around the rupture. If flow then ceases both the pipes and the sand volcanoes are preserved. At high flow rates, where the base layer fluidizes, the top cohesive layer becomes fragmented. Small fragments circulate within the fluidizing base layer and are preserved as floating clasts. Large fragments sink to the bottom of the fluidizing base layer. Erosion of the bottom surface of these larger fragments causes this surface to become convex downward. The experimentally derived structures are similar to pillar‐type structures observed in the field and the processes described can be used to investigate the development of these structures. Fluidization experiments also demonstrate the genesis of dish structures, and the cohesive behaviour can be applied to the deformation of these structures after initia

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1994.tb01403.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTStromatactis is a spar network whose elements in cross section have flat to undulose lower surfaces and digitate upper surfaces. The network is composed principally of isopachous crusts of centripetal cement and commonly occurs embedded in finely crystalline limestone. It is the cement filling of interconnected cavities. Stromatactis of Upper Silurian red stromatactis limestone from Gaspé Peninsula, Québec Appalachians, exhibits two types of cements: (1) an isopachous cement that lined the walls of the conduits and is interpreted as early marine; and (2) a later blocky cement that occupies the centres of cavities. The first cement is composed exclusively of non‐ferroan calcite, whereas the second cement is mixed non‐ferroan and ferroan calcite. The early isopachous cement is white on polished slabs and has a turbid aspect under transmitted light. In a few samples, the relative homogeneity of this early cement is broken by the presence of distinctive grey clear calcite. Under cathodoluminscence, the grey clear calcite is non‐luminescent and exhibits well defined bladed crystal shapes, whereas the white turbid cement has a dull orange luminescence and indistinct crystal shapes. The relationships between the two cements indicate that the dull luminescent cement is a secondary form of the non‐luminescent cement, and it is concluded that the dull cement is the product of alteration of the non‐luminescent cement by burial or meteoric fluids. The later blocky cement has the same dull luminescence as the white turbid cement and is thought to have been precipitated from the same fluids as those responsible for the alteration of the early marine cements. Oxygen isotopic values of the dull cement of the early isopachous crusts (mean δ18O= ‐6.8%oare intermediate between those of the non‐luminescent early marine cement (mean δ18O= ‐5.3%o) and the dull luminescent blocky cement (mean δ18O= ‐11.8%o), while carbon isotopic values do not differ significantly (δ13C=+2.9, +2.4 and +2.6%o,respectively). The alteration also has affected the distribution of some trace elements, particularly Mg. Both unaltered and altered cements contain less than 1% microdolomite inclusions, but the Mg content of the background calcite of unaltered cement is three times that of altered cement (14171 vs. 5502 ppm). Precursor early marine cement is thought to have been low‐Mg calcite. The mean δ18O value (− 5.3%o) of unaltered early marine cement is higher than values for the oxygen isotopic signature of Silurian oceans p

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1994.tb01404.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTExperimental results show that the grain size distribution of suspended material is related to flow parameters and grain size distribution in the bed. A theoretical model has been developed to compute the suspension grain size distribution on the basis of diffusion equations, taking into account the effect of hindered settling due to the increased concentration in suspension. Fluid velocity closest to the bed is estimated by using the concept of migration velocities of particles in the bed layer. Comparisons of data computed by the proposed method and data from actual observations show generally good agreement.

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1994.tb01405.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTClast fabric and morphological data have been used to determine the origin of fluted subglacial tills exposed by recent retreat of the Slettmarkbreen glacier, Norway. A new method for the interpretation of clast fabric data allows aspects of the strain and depositional history of the till to be reconstructed. The till formed by a combination of lodgement and subsole deformation by slip along discrete shear planes. Lodgement was dominant for the larger size fractions (>125 mm), while the smaller material was more susceptible to deformation. The fluted till surface reflects the tendency for the till matrix to deform into regions of low confining pressure in the lee of lodged boulders. Downglacier components of till flow are thought to have resulted in significant sediment transfer towards the margin.

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1994.tb01406.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTDetailed petrographic analyses along a depositional transect from a carbonate platform to shale basin reveals that dolomite is the principal burial diagenctic mineral in the Maryville Limestone. This study examines the role of burial dolomitization of subtidal carbonates. Dolomite occurs as a replacement of precursor carbonate and as inter‐ and intraparticle cements. Four different types of dolomite are identified based on detailed petrographic and gcochemical analyses.Type I dolomite occurs as small, irregular disseminations typically within mud‐rich facies.Type II dolomite typically occurs as inclusions of planar euhedral rhombs (ferroan), 5–300 μm in size, in blocky clear ferroan calcite (meteoric) spar. Type II dolomite is non‐luminescent. Type I and II dolomite formed during shallow to intermediate burial diagenesis. Type III dolomite consists of subhedral to anhedral crystals 10–150 μm in size occurring as thin seams along stylolites and as thick bands a few millimetres in width. This dolomite consists of dominantly non‐luminescent rhombs and, less commonly, orange luminescent and zoned rhombs. Type IV dolomite consists of baroque or saddle‐shaped, 100–1500 μm crystals, and is non‐luminescent. Type IV dolomite formed during the period of maximum burial.Types III and IV dolomite increase in abundance downslope. Type III dolomite contains 1.2–2.6 wt% Fe and a maximum of 1000 ppm Mn. The distribution of these elements displays no distinct vertical or lateral trends. In contrast, Fe and Mn distributions in Type IV dolomite exhibit distinct spatial trends, decreasing from 3.5–4.5 wl% Fe and 0.1–0.3 wt% Mn in the west (slope/basin) to 1.5–2.5 wt% Fe and less than 600 ppm Mn in the east (shelf margin), a distance of approximately 60 km. Spatial trends in Fe and Mn distributions in Type IV saddle dolomite, suggest a west‐east fluid flow during late burial diagenesis. Types III and IV dolomite have a mean δ18O value of ‐ 7.8%00and a mean δ13C value of + 1.1%00(relative to the PDB standard). Based on a range of assumed basinal water composition of 2.8%00SMOW, temperatures calculated from δ18O values of Types III and IV dolomite range between 75 and 160°C.87Sr/86Sr data for Types III and IV dolomite range from 0.7111 to 0.7139. These values are radiogenic when compared to Cambrian marine values and are consistent with the presence of a diagenetic fluid that interacted with siliciclastic sediments.The distribution of Palaeozoic facies in the southern Appalachians indicates a Cambrian shale source for the fluids, whilst burial curves suggest a Middle Ordo

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1994.tb01407.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTExperimental data are presented demonstrating the influence of boundary layer flow conditions on aerodynamic entrainment of grains in the absence of intersaltation collisions. New methods are proposed for (1) the unambiguous determination of aerodynamic threshold for any grain population and (2) approximation of the probability density function (PDF) distributions of threshold shear velocity for aerodynamic entrainment.In wind tunnel experiments, the orderly spatial development of flow conditions within a developing boundary layer over the roughened surface of a flat plate constrains the aerodynamic threshold condition in terms of both mean and fluctuating values. Initial grain dislodgements and subsequent erosion from narrow strips of loose, finely fractionated ballotini were recorded photographically as wind speed was increased. Boundary layer parameters, including average threshold shear velocity (U*t), were calculated using the momentum integral method.Direct observations show that sporadic oscillation of grains preceded dislodgement. At slightly higher velocities most grains rolled over their neighbours before entering saltation. Initial entrainment in spatially semi‐organized flurries of 50 or more grains was followed by quiescent periods at airflow velocities close to threshold. These observations provide strong circumstantial evidence linking both the nature and spatial pattern of initial grain motions to sweep events during the fluid bursting process.For each grain fraction, values ofU*twere found to span an unexpectedly wide range and to decrease downwind from the leading edge of the plate as turbulence intensity increased. A probabilistic entrainment model is applied to the aerodynamic threshold condition so as to incorporate the effects of changing turbulent flow regimes over the plate. Analysis of strip erosion curves gives both an objective definition of the threshold condition and usable approximations of the PDF forU*trequired by the model and for future stochastic treatment of the threshold conditio

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1994.tb01408.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTLithofacies analysis, pollen assemblages and radiocarbon age dates of 20 stratigraphic drill holes are used to develop an evolutionary history for late Quaternary sedimentation in two coastal embayments landward of the central Great Barrier Reef. Different physiographic settings of the embayments result in two contrasting styles of sedimentary sequence: (a) an exposed, moderate energy, beach barrier‐lagoon system (Wyvuri Embayment) and (b) a protected, low energy, muddy inlet fill sequence (Mutchero Inlet). Despite sharp contrast in sequence style, similar depositional cycles occur in both embayments in response to late Quaternary sea level fluctuations including: (1) a last interglacial highstand (+2 m;c.125 000 yrbp) beach barrier (Wyvuri); (2) an early to mid‐Holocene (8000–6100 yrbp) transgressive beach barrier‐lagoon (Wyvuri) and estuarine infill (Mutchero); and (3) mid‐Holocene to present highstand beach barrier (Wyvuri) and estuarine (Mutchero) progradation. Preservation of such cycles in the stratigraphic record would produce a series of vertically stacked and offset linear barrier sands surrounded by lagoonal mud and fine grained shoreface sediment juxtaposed to muddy, estuarine infills.Sea level elevations are well recorded by the upward transition fromRhizophora‐dominated intertidal mangrove mud to freshwater swamp (clearly identified by pollen analysis) and by the basal contacts of beach barrier sediments which sharply overlie the upper shoreface. Transgressive sedimentation is interrupted in both embayments by a constructional beach barrier (Wyvuri) and abbreviated progradation (Mutchero) corresponding to a — 5 m pause in relative sea level rise atc.6800 yrbp. Sea level control of fine scale coastal sedimentation patterns is beginning to be widely recognized and provides an accurate analogue for stacked anci

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1994.tb01409.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY