摘要:

ABSTRACTThe ability of mud aggregates to form depositional bedforms is of considerable sedimentological importance for explaining the geomorphology of theChannel Countryof central Australia as well as for understanding the depositional environment of certain argillaceous fluvial sequences in the rock record. The sediment transport and bedform development of mud aggregates from the floodplain of Cooper Creek, central Australia, was examined in a laboratory flume over a range of flow conditions. The aggregates were found to be clay‐rich (>60% clay), nonsaline (<0·02%), fine sand‐sized (meand50=0·13 mm), low density (2300 kg m−3) and water‐stable. Three wetting rates were applied to the sediment in the laboratory prior to wet sieving to replicate various field conditions and results in three mean aggregate sizes. Immersion wetting (no tension) represents inundation of the sediment by overland flow and results in aggregates of 0·13 mm. Tension wetting at 20 and 50 mm corresponds to high‐ and low‐intensity rainfall and results in meand50sizes of 0·75 and 0·70 mm, respectively. Immersion wetting is the most applicable wetting mode for hydraulic transport of aggregated sediment on the Cooper Creek floodplain. Considerable variability in sediment transport rates in the field could result from differences in pre‐wetting of the aggregated sediment. The dominance of smectite in the clay mineralogy of the sediment is an important factor in the development of the aggregates; disaggregated sediment reaggregated in a laboratory after 2–3 wetting/drying cycles. In flume experiments, bedforms of aggregated mud ranging from lower‐regime plane beds to upper‐regime antidunes were observed. The aggregates moved predominantly as bedload with measured peak bedload concentrations being high compared with other flume studies. The highly mobile nature of this sediment in the field is due to the ready entrainment of low‐density aggregates in the form of self‐mulching vertisols across extensive floodplains. The occurrence of low‐sinuosity braid‐like channels on this extensive low‐gradient semi‐arid floodplain can be attributed to: (a) the passage of floodwaters across a floodplain with steeper gradients than adjacent more sinuous anastomosing channels; (b) the highly mobile nature of the low‐density sediment aggregates; (c) the ability of the aggregates to be transported as bedload; and (d)

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1996.tb01501.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

ABSTRACTA new aeolian sediment trap is described which can give up to 1 Hz measurement frequency in field conditions. The trap adopts a circular, horizontal trap design with a load cell connection to give continuous, unobtrusive trap measurement of sediment flux. Simultaneous velocity recording is carried out using an anemometer. Trap construction costs are approximately £200. Initial results in field conditions using a direct comparison of wind velocity data, sampled at an equivalent frequency, have given a first‐order relationship between sediment flux and velocity. The trap enables simultaneous sampling of wind velocity and sediment flux at a sufficiently short interval to enable investigation of sediment transport dynamics under a variety of field conditio

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1996.tb01502.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

ABSTRACTThe classification of channel styles has to date proved difficult, with most schemes having been constructed around a series of geomorphological end‐members which are inherently unable to describe the continuum of channel morphologies. We present a framework that accommodates all possible channel‐interchannel configurations and facilitates the continued use of existing channel classification schemes. By considering the relative rates of channel and interchannel accumulation, a model has been derived with the potential to predict bed geometry and preservation potential for the entire spectrum of aggrading and eroding channels. The model identifies stability fields for all potential configurations of channel and interchannel areas. Significantly, the model predicts that the transition from aggrading to eroding systems need not be accompanied by a change from raised to incised chann

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1996.tb01503.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

ABSTRACTThe Seven Rivers Formation exposed in Slaughter Canyon, Guadalupe Mountains, New Mexico, reveals complex relations between long‐ and short‐term relative changes in sea‐level, shelf configuration and sedimentation, which interacted to create a distinct toplap geometry. At least five sandstones diverge basinward from a prominent boundary unit marking the surface of toplap at the top of the Seven Rivers Formation and create a series of prograding, shingled clinoforms. The boundary unit is a horizontal, well‐sorted, quartz arenite underlain across the shelf by peritidal carbonate or by other merging sandstones. Preserved palaeotopography is indicated by facies changes downdip and the presence of horizontal geopetal indicators in inclined beds. Near the boundary unit (updip), merging sandstones contain rare sedimentary structures including evaporite moulds and irregular fenestrae and are bounded above and below by peritidal carbonate with microbial laminae, fenestral fabrics and mudcracks. Laterally (downdip), the sandstone‐bounding peritidal carbonate facies pass into subtidal carbonate facies (ooid‐peloid‐fusulinid‐dasyclad‐mollusc pack‐ and grainstone) and interbedded sandstones contain sedimentary structures such as ripple marks and trough to planar cross‐stratification, as well as ooids, fusulinids and other carbonate grains. Toplap is interpreted to have developed by sediment bypass across a subaerially exposed shelf while sedimentation continued in still‐submerged areas downdip from the shelf crest, and hence represents depositional toplap. Physical tracing of subaerial exposure surfaces suggests that the shoreline migrated up and down palaeoslope several times. The vertical component of five short‐term shoreline migrations decreased during formation of the toplap geometry. Sea‐level rose to approximately the same position following each fall to create the toplap geometry. This depositional toplap is the stratigraphic result of high‐‘frequency’ relative changes of sea‐level that combined to produce the larger‐scale geometry. We suggest that changing amplitudes of relative sea‐level may play a significant role in the stratigraphic evolution of platforms and that separating ‘short‐term’and ‘long‐term’re

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1996.tb01504.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

ABSTRACTA superbly exposed stromatolite reef complex occurs in the Victor Bay Formation near Strathcona River on northern Baffin Island. Individual reefs are up to 130 m thick and nearly 1 km in length, and their development was clearly related to their position in the facies spectrum and to sea‐level dynamics. In the first sea‐level cycle, metre‐scale reefs grew amongst mid‐ramp calcarenites and outer‐ramp shales during slow sea‐level rise; a 25‐m‐thick oblate reef tract, separating mid‐ramp and outer‐ramp facies, formed during the highstand. The greatest period of reef growth was during the second sea‐level cycle. Pinnacle reefs nucleated on the karsted upper surface of the oblate reef tract and aggraded rapidly in response to rising sea‐level, producing structures with more than 75 m of depositional relief. A gradual symmetrical succession of stromatolite growth forms, from stratiform to cylindrical columns to conical columns and then back through cylindrical columns to stratiform, is mirrored by evidence in offreef deposits for deepening to a maximum flooding surface and then shallowing. The tops of these high‐standing reefs were karsted during the following regression, while dolomite ‘cryptodomes’ grew as sheets on their submerged flanks and as progradational tongues extending basinward of the reefs. Continued sea‐level fall resulted in subaerial exposure of the entire reef complex and the extensive formation of surface and subsurface karst.These Proterozoic slope buildups are similar to Phanerozoic deep‐water reefs in size, shape, prevalence of synsedimentary lithification, presence of Neptunian dykes and in their well‐developed vertical zonation of reefbuilders. However, they differ in being constructed exclusively by stromatolites rather than being mud mounds with small skeletal elements, and in lacking halos of perireefal sand‐ and gravel‐sized calcareous debris. Their responses to changes in sea‐level were strikingly similar to those shown by their younger counterparts, and suggest that sequence‐stratigraphic concepts derived from studies of Phanerozoic r

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1996.tb01505.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

ABSTRACTLarge symmetric and asymmetric dunes occur in the Fraser River, Canada. Symmetric dunes have stoss and lee sides of similar length, stoss and lee slope angles<8°, and rounded crests. Asymmetric dunes have superimposed small dunes on stoss sides, sharp crests, stoss sides longer than lee sides, stoss side slopes<3° and straight lee side slopes up to 19°. There is no evidence for lee side flow separation, although intermittent separated flow is possible, especially over asymmetric dunes. Dune symmetry and crest rounding of symmetric dunes are associated with high sediment transport rates. High near‐bed velocity and bed load transport near dune crests result in crest rounding. Long, low‐angle lee sides are produced by deposition of suspended sediment in dune troughs. Asymmetric dunes appear to be transitional features between large symmetric dunes and smaller dunes adjusted to lower flow velocity and sediment transport conditions. Small dunes on stoss sides reduce near‐bed flow velocity and bed load transport, causing a sharper dune crest. Reduced deposition of suspended sediment in troughs results in a short, steep lee slope. Dunes in the Fraser River fall into upper plane bed or antidune stability fields on flume‐based bedform phase diagrams. These diagrams are probably not applicable to large dunes in deep natural flows and care must be taken in modelling procedures that use phase diagram relations to predict bed configuration in

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1996.tb01506.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

ABSTRACTThe dynamic behaviour of sediment‐laden underflows was examined in Peyto Lake, Alberta, Canada, which contains a midlake sill 7 m high. Sediment‐laden underflows are driven by the downslope component of negative buoyant gravity multiplied by the current's thickness. Our measurements of wind, lake currents and water properties indicate that underflows pass over the sill due to the active storage of turbid suspension near the bottom in the deepest proximal region. Sill overflows occurred only when a hydrological threshold of the inflowing river was exceeded, causing quasicontinuous underflow and associated sedimentation in the distal region of the lake ba

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1996.tb01507.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

ABSTRACTLower Cretaceous (Hauterivian) bioclastic sandstone turbidites in the Scapa Member (North Sea Basin) were extensively cemented by low‐Mg calcite spars, initially as rim cements and subsequently as concretions. Five petrographically distinct cement stages form a consistent paragenetic sequence across the Scapa Field. The dominant and pervasive second cement stage accounts for the majority of concretions, and is the focus of this study. Stable‐isotope characterization of the cement is hampered by the presence of calcitic bioclasts and of later cements in sponge spicule moulds throughout the concretions. Nevertheless, trends from whole‐rock data, augmented by cement separates from synlithification fractures, indicate an early calcite δ18O value of+0·5 to ‐1·5‰ PDB. As such, the calcite probably precipitated from marine pore fluids shortly after turbidite deposition. Carbon isotopes (δ13C=0 to ‐2‰ PDB) and petrographic data indicate that calcite formed as a consequence of bioclastic aragonite dissolution. Textural integrity of calcitic nannoplankton in the sandstones demonstrates that pore fluids remained at or above calcite saturation, as expected for a mineral‐controlled transformation.Electron probe microanalyses demonstrate that early calcite cement contains<2 mol% MgCO3, despite its marine parentage. Production of this cement is ascribed to a combination of an elevated aragonite saturation depth and a lowered marine Mg2+/Ca2+ratio in early Cretaceous ‘calcite seas’, relative to modern oceans. Scapa cement compositions concur with published models in suggesting that Hauterivian ocean water had a Mg2+/Ca2+ratio of ≤1. This is also supported by consideration of the spatial distribution of early calcite cement in terms of concretion growth kinetics. In contrast to the dominant early cement, late‐stage ferroan,18O‐depleted calcites were sourced outwith the Scapa Member and precipitated after 1–2 km of burial.Our results emphasize that bioclast dissolution and low‐Mg calcite cementation in sandstone reservoirs should not automatically be regarded as evidence f

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1996.tb01508.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

ABSTRACTThe microped size distribution of erodible soils is one of the main factors controlling the emissions of mineral aerosol. To date, the microped size distributions of desert soils in natural field conditions are not well known and thus the existing dust source models cannot be applied to natural sites. Based on dry sieving and elemental analysis of 26 soil samples collected in arid and semiarid regions, it is shown that the microped size distributions of these desert soil samples involve a combination of four major mass size populations. These four size populations correspond to salty soils, fine quartz sand, coarse dune sand and aluminosilicate silt. We show that the microped size distributions of the various desert soils, according to their mineralogical characteristics, can be calculated by a mixture involving two or three of these four populations. The main field of application for this scheme is the characterization of the erodible soil size distributions for modelling the atmospheric dust cycle.

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1996.tb01509.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1996.tb01510.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY