摘要:

ABSTRACTA new type of soft sediment deformed beds, termed ‘pillow‐beds’, originally a packet of calcarenitic turbidites, show sand‐in‐sand loading with plane lower and upper surfaces. The pillow‐beds have some resemblance to ball and pillow structures and they are considered to have formed as a result of dewatering. The pillow‐beds are Oligocene in age and are part of turbidite lobes in a submarine fan system, which was fed through a canyon along the southern border of the ancient Iberian continent. Three hypotheses may explain their occurrence: overloading, sliding or seismic shock. A seismic origin is preferrred on the basis of the flat, undeformed lower surface of the pillow‐beds, excluding dewatering of—and loading into—the previously deposited beds. Furthermore, the pillow‐bed structures are related to grain size and may show repetitions, thus excluding sliding. The scale and nature of the pillow‐beds suggest they could be the result of an earthquake with a magnitude of 6

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1992.tb02148.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

ABSTRACTHeterogeneous coarse grained channels are often characterized by local transitions in bed surface roughness. Distinct spatial zones in terms of grain size have been reported, for example sand ribbons and bedload sheets. The transition from areas of finer to coarser grained surface sediment is often abrupt. However, the effects of these transitions on the shape of the velocity profile and associated shear velocity and roughness length estimates have not been investigated in detail in coarse grained channels. This paper therefore examines the combined effects of a sudden change in surface roughness and of superimposed scales of resistancé on the structure of the turbulent boundary layer. Measurements along roughness transitions from smooth to rough beds were conducted in a flume using artificial roughness features and in a natural gravel bed river. Immediately at the transition from a zone of close packed roughness to a rougher section dominated by obstacles superimposed on the more or less uniform roughness surface, boundary shear stress and roughness length increase considerably. Downstream from this transition, velocity profiles become concave upwards. Downstream and upstream sections show significant differences in terms of near bed velocities (deceleration downstream of the transition), velocity gradient and turbulence intensity of the streamwise velocity component. Comparing the mean velocity profiles corresponding to these two different roughness surfaces gives some indication of the proportion of total shear velocity (or shear stress) associated with the pressure drag produced by large and isolated obstacles

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1992.tb02149.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

ABSTRACTExperimental studies of subcritical, unidirectional flow over upper stage plane beds of medium grained sand reveal the ubiquitous presence of low amplitude bedwaves. Flow depth was 0·11 m, mean flow velocities were 0·86–1·0 m s−1, shear velocities were 0·058–0·71 m s−1and dimensionless shear stresses were 0·56–0·87. Bedwaves are asymmetrical in profile and range from 0·75 to 11 mm in height (mainly 2–6 mm), from 0·7 to 1·3 m in wavelength and have mean celerities of 10 mm s−1. Flow records suggest that the bedwaves are associated with accelerating flow over the bedwave crests and flow which decelerates and diverges laterally over the troughs. High resolution bed profiling during aggradation of the bed combined with subsequent box coring illustrates that these bedwaves are responsible for the planar laminae characteristic of upper stage plane beds. Lamina preservation is dependent upon the mean aggradation rate and the sequence of bedwaves of different height crossing any point; individual laminae are more readily preserved at higher aggradation rates where the possibility of reworking by later bedwaves is reduced. Laminae are recognized by small changes in grain size and commonly a fining upwards at the top of laminae which is generated by fine grained material infiltrating a lower lamina in t

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1992.tb02150.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

ABSTRACTThe Upper Ordovician rocks of Hadeland, Norway, form a sequence of thin bedded nodular limestones (wackestones) and shales, hosting five distinctive sedimentary breccia complexes. These breccias contain blocks of varying sizes and shapes in a wackestone and grainstone matrix. Blocks differ in lithology, and in their included biotas and cement sequences.The thin bedded limestones are interpreted as turbidites, deposited against a background of hemipelagic calcareous shales. The breccias occupy channels cut into this sequence. The lithologies and biotas of blocks in the breccias record deposition in differing sedimentary environments, whereas their cements are the results of contrasting diagenetic histories. Blocks were eroded from a diverse and mature carbonate platform, close to sea level, which probably lay 5–10 km east of Hadeland. The breccias are interpreted as debris flow deposits, transported as channellized flows. Following channel cutting events, perhaps triggered by sea level change, channels were characterized by deposition rather than erosion. Wackestones and grainstones associated with the breccias also reflect resedimentation, their less diverse biota suggesting local derivation on the slope. The reworking of calcarenaceous muds locally produced clean washed calcarenites (now grainstones).A fall in sea level resulted in emergence of the upper slope and erosion of the debris flow complex to form caverns and fissures. As sea level rose again crinoidal calcarenites, now grainstones, were deposited within these cavities.Cement sequences in blocks record early marine and burial conditions on the shelf, and also precipitation of new marine cements following downslope transport. Those cements in lithologies formed in situ document later shallowing, culminating in emergence. The localized dissolution of cements in both blocks and associated grainstones reflects the infiltration of ‘aggressive’meteoric waters through permeable channel deposits. A subsequent rise in sea level is recorded in the generation of an additional marine cement with final burial reflected in the deposition of blocky calcite. The debris flow deposits therefore maintained their distinctive character from deposition through diage

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1992.tb02151.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

ABSTRACTData from a moderate energy, meso‐tidal beach on the east side of Delaware Bay, New Jersey, USA, revealed the significance of both beach width as a source for aeolian transport and the effect of tidal rise on source width. Wind speeds averaged over 17·1 min, recorded 6 m above the crest of a 0·5 m high dune, ranged from 11·6 to 12·7 m s−1during the experiment. The highest observed rate of transport on the beach was 0·0085 kg m−1s−1, monitored at rising low tide when the average wind speed was 11·6 m s−1across 0·35 mm diameter surface sediments. The wind direction was oblique to the shoreline, creating a source width of 34 m. The reduction in the width of the beach as a source for aeolian transport during rising tide was approximately arithmetic, whereas the reduction in volume of sediment trapped was exponential. Aeolian transport effectively ceased when source width was less than 8 m. Wind conditions, moisture content of the surface sediments and presence of binding salts did not appear to vary dramatically, and no coarse grained lag deposit formed on the surface of the beach. The decrease in rate of sediment trapped through time in the tidal cycle is attributed to differences in source width. Sediment deposited in the litter behind the active beach by strong winds during the rising tide was eroded during the high water period by the high waves and storm surge generated by these winds, and net losses of sediment were observed despite initial a

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1992.tb02152.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

ABSTRACTA study of the avulsion of a major distributory channel on the alluvial fan (22 000 km2in area) of the Okavango River in northern Botswana has revealed that channels serve as arterial systems distributing water which sustains large areas of permanent swamp. The channels are vegetatively confined. A primary channel, defined here as a channel which receives water and sediment directly from the fan apex, aggrades vertically as a result of bedload deposition. The rate of aggradation increases downchannel and may exceed 5 cm yr−1in the distal reaches. Rapid aggradation is associated with a decline in flow velocity. This initiates a series of feedback mechanisms involving invasion of the channel by aquatic plants which trap floating plant debris, further reducing flow rate and causing the channel water surface to become elevated, thereby increasing rate of water loss from the channel, accelerating blockage and aggradation. The channel ultimately fails. Enhanced water loss from the channel promotes the growth of flanking swamp vegetation, which confines the failing channel. Increased flow through the swamp erodes pre‐existing hippopotamus trails, producing a secondary channel system which overlaps but does not connect directly to the failing reach of the primary channel. The region of failure of the primary channel migrates upstream, accompanied by headward propagation of the secondary channel system. The swamp distal to the failed primary channel dessicates and is destroyed by peat fires. Secondary channels are stable and not prone to blockage. Comparison with avulsions described in other river systems indicates that the influence of plants in the Okavango River system is exceptionally str

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1992.tb02153.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

ABSTRACTSediment entrainment and the initiation of bed defects has commonly been ascribed to the impact of high velocity sweeps upon mobile sand beds. Results of visualization experiments suggest that these sweep impacts are grouped and may define ‘patches’of entrainment upon a mobile bed that may be wider than an individual sweep impact. Additionally, the presence of longitudinal ribs of sediment generated by sweep impacts may stabilize the position of sweeps and low speed streaks. These observations are interpreted in the light of recent boundary layer research which suggests the formation of multiple hairpin shaped vortices within turbulent flows: these are postulated to generate multiple sweeps that manifest themselves as entrainment patches on a mobile bed. These features of the turbulent boundary layer and its modification by sweep generated sediment ridges can be used to propose a model for bed defect formation and the subsequent development of current ripp

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1992.tb02154.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

ABSTRACTMiddle to Late Ordovician subtidal carbonates in the Manitoulin Island area of Ontario are predominantly limestone in composition, but non‐ferroan and ferroan dolomite is a common cement as well as a selective or locally pervasive replacement phase. Integration of field, petrographic, geochemical (δ13C, δ18O) and fluid inclusion data indicates that lithification of these carbonates occurred during burial diagenesis, with much of the alteration controlled by regional fracturing and hydrothermal influences.Aqueous (type 1) fluid inclusions in early calcite (pre‐dolomite) and dolomite are saline (>29 wt% NaCl eq.) solutions with Ca and/or Mg in excess of Na and display homogenization temperatures with modes of 95 and 101°C, respectively. These temperatures can be explained by significantly more burial than can be accounted for either by the available stratigraphic information or by an unusually high palaeogeothermal gradient, which also is not well supported. The fluid inclusion temperatures are interpreted to have resulted from hydrothermal fluids which circulated during the burial diagenesis of these strata. Type 1 inclusions in late (post‐dolomite) calcite are less saline (<19 wt% NaCl eq.) and have a bimodal distribution of homogenization temperatures with a relatively well defined low temperature peak similar to those in early calcite and dolomite and a broad higher temperature grouping with a mode at 183°C. A small proportion of methane and light hydrocarbon‐bearing fluid inclusions (type 2) are present in all stages of carbonate.Dolomitizing fluids were derived from burial compaction of argillaceous sediments in the more central parts of the Michigan Basin and the updip migration of these brines along fractures to the basin margin where the carbonates of the Manitoulin Island area were dolomitized. Alternatively, migration of dolomitizing brines downward from the overlying pervasively dolomitized Silurian sequence into fractures in the Ordovician carbonates may have occurred. Integration of the aqueous fluid inclusion data into the diagenetic history of these carbonates remains equivocal because most of the inclusions are secondary or indeterminate in origin. Nevertheless, high salinities resulting from interaction with evaporitic strata and hydrothermal effects are clearly implicated although the origin of the latter remains unclear. The alteration styles of the Ordovician carbonates in the Manitoulin area are similar to those of Ordovician hydrocarbon reservoirs described from other parts of the Michigan Basin. They indicate that fracture‐related diagenesis occurred on a basin‐wide scale and that hydrothermal effects

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1992.tb02155.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

ABSTRACTThe Taveyannaz sandstones of eastern Switzerland are a succession of turbidites found within the Tertiary North Helvetic Flysch system; they represent a portion of the early, underfilled stage of the North Alpine Foreland Basin. The Taveyannaz sandstones were deposited in two sub‐basins (Inner and Outer basins) separated by a topographic high trending ENE‐WSW (parallel to the subsequent structural strike of the region), interpreted as an emergent thrust tip that propagated into the basin. The southerly Inner basin is therefore considered as a ‘piggy‐back’basin comprising a 140 m thick succession dominated by approximately 12 very thick bedded sandstones with thick mudstone caps; these very thick bedded sandstone‐mudstone couplets are interpreted as having resulted from the ponding of megaturbidite flows in the topographically confined Inner basin. Intercalated with the very thick bedded sandstones are thin to medium bedded sandstones. The Outer (northerly) basin comprises at least 240 m of turbidites characterized by sandstone packets (5–50 m thick) with extensive amalgamation of beds and a dominantly symmetrical vertical bed thickness and grain size profile. Intercalated between the sandstone packets are laminated graded siltstones and mudstones.The Inner basin sediments underwent localized deformation on the sea floor, generating an irregular surface topography which was then capped by a mud sheet emplaced by superficial sliding. During the emplacement of the mud sheet, large sandstone blocks (up to 130 m across) were incorporated from the underlying succession. The resultant geometry of the upper surface of the Inner basin sandstones exhibits vertical walls which truncate, and are perpendicular to, the underlying beds.The depositional style and structural control of the Taveyannaz sandstones, in association with the emplacement of superficial mud sheets, reflect processes that are highly analogous to those occurring in modern accretionary wedge environments.The sandstone packets of the Outer basin reflect a cyclical pattern of sedimentation alternating between deposition of sandstones and mudstones. The autocyclical or allocyclical controls on these high frequency alternations are difficult to interpret; likely mechanisms include lobe switching, climatic variations, eustatic sea level fluctuations and changes of horizontal in‐plane deviatoric stress on the lithosphere. In this example, an alternative mechanism is speculated upon. This is based on the analogy with accretionary wedge processes. In this hypothesis, it is proposed that high frequency fluctuations in the accommodation space available on the shelf may result from fluctuations in the topographic slope of an accretionary wedge around its critical taper. Hence, during periods of accelerated frontal accretion, the taper angle of the thrust wedge becomes subcritical resulting in a broad, low angle topographic slope and increased shelfal accommodation. Consequently, sediment becomes trapped in a relatively landward position. The necessary rejuvenation of the surface slope of the thrust wedge to a critical taper is achieved through internal reactivation resulting in tectonic uplift and hence a relative fall in sea level; this leads to the reworking of sediment to the base of slope or outer trench. Repeated alternations of relative sea level between a subcritical highstand and a supercritical lowstand are considered to be sufficient to generate the observed alternations between sandstone and mudstone packages in the

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1992.tb02156.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

ABSTRACTCaves, fossil mouldic cavities, sinkholes and solution‐widened joints are common in the Cayman and Pedro Castle members of the Bluff Formation (Oligocene‐Miocene) on Grand Cayman and Cayman Brac because they have been subjected to repeated periods of karst development over the last 30 million years. Many voids contain a diverse array of sediments and/or precipitates derived from marine or terrestrial environs, mineral aerosols, and groundwater. Exogenic sediment was transported to the cavities by oceanic storm waves, transgressive seas, runoff following tropical rain storms and/or in groundwater.At least three periods of deposition were responsible for the occlusion of voids in the Cayman and Pedro Castle members. Voids in the Cayman Member were initially filled or partly filled during the Late Oligocene and Early Miocene. This was terminated with the deposition of the Pedro Castle Member in the Middle Miocene. Subsequent exposure led to further karst development and void‐filling sedimentation in both the Cayman and Pedro Castle members. Speleothems are notably absent. The void‐filling deposits formed during these two periods, which were predominantly marine in origin, were pervasively dolomitized along with the host rock 2–5 million years ago. The third period of void‐filling deposition, after dolomitization of the Bluff Formation, produced limestone, various types of breccia, terra rossa, speleothemic calcite and terrestrial oncoids. Most of these deposits formed since the Sangamon highstand 125 000 years ago. Voids in the present day karst are commonly filled or partly filled with unconsolidated sediments.Study of the Bluff Formation of Grand Cayman and Cayman Brac shows that karst terrains on isolated oceanic islands are characterized by complex successions of void‐filling deposits that include speleothems and a variety of sediment types. The heterogenetic nature of these void‐filling deposits is related to changes in sea level and climatic conditio

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1992.tb02157.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY