Slack Water Periods Research Articles

Contrasting phytoplankton distributions controlled by tidal turbulence in an estuary

The vertical distribution of three phytoplankton species was monitored over a complete tidal cycle during daylight hours, with specific reference to estuarine hydrodynamics. The data suggest that pelagic diatoms ( Coscinodiscus spp.) and dinoflagellates ( Prorocentrum micans and Peridinium trochoideum) are able to co-exist by utilising contrasting properties of tidal mixing to develop and reside within the partially mixed estuary, Southampton Water, UK. The data imply that the stability within the water column during slack water periods, permits surface aggregation of dinoflagellates which become homogeneously distributed when turbulence intensifies during ebb and flood currents. Diatoms, conversely, rely on the periods of increased turbulent kinetic energy (TKE) to ensure entrainment into the upper water column and to prevent sinking from the photic zone during stable intervals. The unusual tidal regime for this estuary provides a unique environment to investigate the hydrodynamic properties which influence different phytoplankton groups.

Suspended sediment fluxes in the tidal Ouse, UK

A strong turbidity maximum (TM) of suspended particulate matter (SPM) was observed in the upper Humber and lower Ouse during both spring and neap tides of May 1994. Near-bed concentrations within the TM sometimes exceeded fluid mud levels following slack water periods. SPM within the TM comprised very fine-grained material. Its low organic content demonstrated that the SPM was essentially mineral, clastic sediment. Generally, tidal advection of SPM was the dominant flux mechanism and the pronounced flood-ebb asymmetry in the tidal currents was reflected in these fluxes. However, the presence of fluid mud near the down-estuary margins of the TM on the early ebb resulted in a strong, up-estuary shear flux that opposed the ebb-directed advection. This mechanism therefore acted to maintain fine sediment in the TM region. SPM concentrations in the inflowing fresh water at Naburn Weir were much less than those observed within the TM region ( 70 000 mg l -1 ). The estimated mean SPM transport into the tidal Ouse across Naburn Weir was about 4 kg s -1 (>2 and <11 kg s -1 ) during 1994. In the TM area, the SPM transported during the course of a single spring tide flood was roughly equivalent to 30 months of Naburn SPM inflows at average 1994 levels. The tidally averaged SPM transport in the TM region was directed into the estuary and, per tide, was roughly equivalent to three months of Naburn inputs.

Energy balance and time-scales of mixing and stratification in the Jaboatão estuary, NE-Brazil

Estuarine systems undergo different physical processes that simultaneously control their stratification and mixing dynamics. This energy balance determines both, the estuarine hydrodynamics and the dynamics of water. This article presents a quantitative and comparative analyses between the forces maintaining stratification (surface heating; rainfall precipitation; and differential advection of the longitudinal density gradient due to the vertical velocity field) and those responsible for the vertical mixing (mechanical stirring of bottom tidal stress; mechanical stirring ofsurface wind stress; and surface evaporation) in the lower estuary ofthe Jaboatão River (JE), Pemambuco, NE-Brazil. The energy available to mix the water column at the' lower Jaboatão was 2.2 and 2.0-fold greater than that available to promote stratification, during the dry and rainy seasons, respectively. The bottom shear turbulence caused by the tides was the major source of energy for the vertical mixing. A theoretical analyses revealed that the turbulence decay time-scale io both seasons was much greater (dry=29min; rainy=25 min) than the stratification time-scale (dry=8 mio; rainy=7 min) and than the slack water time-scale (15 min). Thus the estuary was vertically well-mixed even during slack water periods during both, dry and rainy seasons. Theoretical results were confirmed by field data and are in agreement with earlier numerical simulations.

Behaviour of Turbidity Maxima in the Tamar (U.K.) and Weser (F.R.G.) Estuaries

The Tamar (England) and Weser (Germany) Estuaries exhibit strong turbidity maxima in their low-salinity reaches. Whereas their morphologies and freshwater inflows are different, the estuaries have similar mean tidal ranges and mean tidal current speeds. Despite the differences, the turbidity maxima show similar intratidal, spring–neap and seasonal behaviour for the two estuaries. On an intratidal time scale, strong suspended particulate matter (SPM) variability occurs in both turbidity maxima regions due to deposition over slack-water periods, subsequent re-suspension and depletion of easily erodible bed-source sediment during the following flood or ebb, and transport by tidal currents. Over a spring–neap cycle, the spring-tide SPM levels are greater than those during neap tides owing to enhanced re-suspension in the stronger spring-tide currents. Both estuaries exhibit a hysteresis in SPM during the falling and rising spring–neap cycle. On a seasonal basis, the occurrences of river spates in both the Tamar and the Weser move both the limits of saline intrusion and the turbidity maxima further down-estuary. Following the return to normal freshwater inflows, the saline intrusion moves rapidly up-estuary, whereas the turbidity maximum exhibits a time lag.

Adaptation of focused beam reflectance measurement to in-situ particle sizing in estuaries and coastal waters

Recent advances in focused beam reflectance technology have resulted in new instruments with the potential for particle size determination in natural waters. A commercially manufactured instrument based upon this principle has been adapted to allow the size distribution of suspended particulate matter in estuaries and coastal waters to be determined in-situ. After calibration, the standard instrument has been shown to be capable of providing accurate, undisturbed particle size distribution data over a wide range of particle concentrations (10 mg l −1–50 g l −1), irrespective of current speed and, to a large extent, particle composition and density. Data obtained with the in-situ instrument in the Humber Estuary have revealed large temporal and spatial variations in the size distribution of suspended particles over a tidal cycle. Particle sizes were largest near the bed during periods of slack water where median diameters exceeded 500 μm. This was attributed to the sedimentation of large aggregates. Rapid reductions in median particle size coincided with periods of strong tidal current acceleration after low water slack current. At maximum current velocity on the flood tide, mobilisation of sediment from the bed gave rise to larger particles in the water column. Multiple regression analysis indicated that a combination of current velocity and particle concentration provides the best predictor of particle size.

The mechanism of production enhancement in coral reef carbonate systems: model and empirical results

Coral reefs are characterized by enormous carbonate production of the organisms. It is known that rapid calcification is linked to photosynthesis under control of the carbonate equilibrium in seawater. We have established a model simulating the coexisting states of photosynthesis and calcification in order to examine the effects of photosynthesis and calcification on the carbonate system in seawater. Supposing that the rates of photosynthesis and calcification are proportional to concentrations of their inorganic carbon source, the model calculations indicate that three kinds of unique interactions of the organic and inorganic carbon productions are expected. These are photosynthetic enhancement of calcification, calcification which benefits photosynthesis and carbonate dissolution induced by respiration. The first effect appears when the photosynthetic rate is more than approximately 1.2 larger than that of calcification. This effect is caused by the increase of CO32− content and carbonate saturation degree in seawater. If photosynthesis use molecular carbon dioxide, the second effect occurs when the calcification rate is more than approximately 1.6 times larger than that of photosynthesis. Time series model experiments indicate that photosynthesis and calcification potentially enhance each other and that organic and inorganic carbon is produced more efficiently in the coexisting system than in the isolated reactions.These coexisting effects on production enhancement of photosynthesis and calcification are expected to appear not only in the internal pool of organisms but also in a reef environment which is isolated from the outer ocean during low tide. According to the measurements on the fringing type Shiraho Reef in the Ryukyu Islands, the diurnal change of water properties (pH, total alkalinity, total carbon dioxide and carbonate saturation degree) were conspicuous. This environment offers an appropriate condition for the appearance of these coexisting effects. The photosynthetic enhancement of calcification and the respiratory inducement of decalcification were observed during day-time and night-time slack-water periods, respectively. These coexisting effects, especially the photosynthetic enhancement of calcification, appear to play important roles for fluorishing coral reef communities.

Rhythmic shelf storm beds: Proterozoic Kaimur Formation, India

The shallow-marine, turbidite-like storm beds of the Proterozoic Kaimur Formation exhibit intrabed rhythmic vertical variations in sedimentary structure and texture attributable to different patterns of velocity pulsations within an event of storm current deposition. The alternate repetition of parallel-laminated (S b) and ripple cross-laminated (S c) divisions (S bcbc…) within individual storm beds has been interpreted to represent periodic fluctuation in flow intensity beyond the stability limits of ripple and upper flat-bed during sedimentation. Sandstone beds characterised by successively occurring thin, internal mud partings indicate pulsing discontinuous flows with alternate periods of slack water within an event of storm current deposition. Successive mud drapes occurring within the ripple cross-laminated divisions (S c) of some of the storm beds also represent pulsing discontinuous flows. Palaeoccurrent analyses of these storm beds of the Kaimur Formation reveal a northwesterly current mode against an E—W trending shoreline. These storm beds thus may be interpreted to have deposited from geostrophic storm currents that were pulsating in nature with a periodicity of the order of tens of hours similar to that observed on present-day storm-dominated shelves.

Bulk properties of intertidal sediments in a muddy, macrotidal estuary

Measurements are presented of the bulk and mineralogical properties of intertidal sediments along the axis of the Tamar Estuary, southwest England. Bulk and dry density data indicate a significant increase in consolidation of the surface layer of the intertidal mudflats progressing down-estuary from the turbidity maximum region to the mouth. There are very significant trends of both density and estimated critical eroison shear stress with distance. Cross-sectionally averaged, bed shear stresses due to tidal currents are computed using a hydrodynamical model. These tidal stresses generally increase along the axis of the estuary from mouth to head and reach a maximum in the upper reaches. The increasing consolidation towards the mouth appears to result from the small bed shear stresses due to tidal currents in the lower reaches and the increasing proportion of coarse material there. Bed shear stresses are large in the upper reaches and regular intratidal resuspension and transport occur during spring tides, with little time available for consolidation following deposition during slack-water periods. The silt and clay fraction of the intertidal sediments increases from the mouth to the turbidity maximum region near the head (60 to > 99% dry weight). The particulate organic carbon (POC) content, assuming this to be approximated by loss on ignition, similarly increases from about 2% of dry weight near the mouth to about 8% in the turbidity maximum region. The POC content of a sample is largely dependent on the proportion of fine sediment within the sample, regardless of its position. The lower estuary is associated with intertidal sediments having relatively low silt and clay content and low associated organic material. It appears to be a fairly stable zone between the marine and estuarine environments. The upper estuary, in the turbidity maximum region, appears to be almost homogeneous in both POC (8.1 ±0.6%) and silt content (95±5%) during summer. The central region is an area of great variability in the size fraction, sediment type and POC content.

Tidal bedding and tidal cyclicities within the intertidal sediments of a microtidal estuary, Dyfi River Estuary, west Wales, U.K.

In the microtidal Dyfi River Estuary, west Wales, the characteristic sedimentary structure within the intertidal sediments is tidal bedding. This consists of sand-mud couplets. By means of thickness measurements and detailed observations, the following conclusions can be drawn. (1) The thicknesses of, and structures in, both mud layer and sand layer of each couplet vary systematically downward in the studied cores and trenching profiles. Variations are mainly due to differences in the current speed and tidal range between tides, to difference of individual neap-spring tides, and to the possible effect of ‘filtering’ of currents due to being in the intertidal zone. (2) The patterns of sand-mud couplet thickness variations reflect two main types of tidal cyclicities: diurnal inequality, and the neap-spring-neap tidal cycle, which is in turn related to the lunar cycle. (3) The mud layer of each couplet represents deposition from suspension at times of tidal slack (no ebb slack water period during low tide in the intertidal zone, thus, no ‘ebb’ mud deposition during low water), whilst the sand layer is deposited by asymmetrical bidirectional tidal currents in either the ebb or flood half of a tidal cycle. (4) The tidal regime derived from these variation patterns is characterized by a mixed, predominantly semi-diurnal form, which is comparable with the present tidal regime. (5) Finally, it can be concluded that tidal bedding patterns are the structural expressions of tidal cyclicity.

Preservation of Daily Tidal Cycles and Stacked Alluvial Swamp Deposits: Depositional Response to Early Compaction of Buried Peat Bodies: ABSTRACT

The character of the clastic depositional environments represented in the lower Mary Lee coal zone of the Pennsylvanian Pottsville Formation in the Warrior basin Alabama (tidally influenced mud flats and alluvial swamps) was controlled by the compaction of buried peat bodies. The lowest mineable coal in the Mary Lee coal zone, the Jagger, is overlain by laminated shale and sandstone exhibiting pronounced cycle bedding. This bedding records daily tidal cyclicity in the form of sand-mud couplets. These correspond to flood-current deposition of the coarser fraction followed by fallout of the finer grained fraction during ensuing slack-water periods. These couplets are cyclically bundled-sandier bundles corresponding to spring tides and muddier bundles to neap tides (lamination counts suggest a 24-30-day cycle). The clastic sequence above the overlying Blue Creek coal is characterized by a series of stacked alluvial swamp horizons. These can be identified by autochthonous fossil plants and pedological features indicative of gleyed paleosols. Catastrophic flooding buried and preserved these horizons. The rapid, early compaction of the buried Jagger and Blue Creek peat bodies created accommodation space that allowed both the preservation of tidalites in the Jagger coal to Blue Creek coal interval and the stacking of alluvial swamp paleosols abovemore » the Blue Creek seam. Carboniferous peats were comprised of highly compressible plant parts and hence, were sensitive to sediment loading. Once the peat bodies had compressed to a certain extent, stability of the overlying sediment surface created conditions amenable to resumption of peat accumulation.« less

The negative role of turbulence in estuarine mass transport

It is competition between the various stratifying and mixing influences which determines the character of stratification in an estuary. Borrowing concepts which have been successfully applied to the discussion of stratification in shelf seas, a quantitative basis for determining the potential energy associated with vertical structure in estuaries is derived. The formulation, along similar lines to that of Bowden (1981), provides a simple but comprehensive method of incorporating many relevant stratifying and mixing influences in a given problem, and is also shown to be capable of rearrangement into forms akin to the estuarine Richardson number which is commonly found in discussions of estuarine statification. The paper argues, based on a survey of the literature, that in wide, relatively well-mixed estuaries, the greatest longitudinal mass flux occurs at times when stratification is most developed, that is, when the turbulent kinetic energy in the water column is at a minimum. Modulation of turbulence, principally at various tidal frequencies, causes a pulsing of the mass flux in which the contribution of each pulse increases non-linearly as the period of the modulation increases. Some, possibly significant, changes to the state of stratification and to the corresponding mass transport may occur in association with slack water periods. However, the spring-neap cycle is proposed to have a far greater influence on stratification, mass transport and the long-term mass balance in estuaries, and recent observational studies lend support to this position.

Fluvial, tidal and storm sedimentation in the Chilhowee Group (Lower Cambrian), northeastern Tennessee, U.S.A.

ABSTRACTThe Lower Cambrian Chilhowee Group of northeastern Tennessee consists of the Unicoi, Hampton and Erwin Formations, and is divided into four facies. The conglomerate facies occurs only within the lower 200 m of measured section (the Unicoi Formation) and consists of fine‐grained to pebbly quartz wacke with rare thin beds of laminated siltstone. Low‐angle to horizontally laminated, fine‐grained sandstone with laminae and lenses of granules and pebbles represents upper flow‐regime, overbank deposition within a braided stream system that was close to a coastline. Medium‐scale, planar‐tabular cross‐stratified conglomerate in which megaripple bedforms are preserved is interpreted as representing deposition in interbar pools of braided channels, as flood stage waned and larger bedforms ceased to migrate. Large‐scale, planar‐tabular cross‐stratified conglomerate beds represent migration of large transverse bars within a broad braided stream channel during high flood stage.The sandstone facies occurs throughout the Chilhowee Group, and is therefore interbedded with all other facies. It consists of mainly medium‐ to very coarse‐grained, subarkosic to arkosic arenite. Thinly interbedded, laminated siltstone and sandstone, which may exhibit wavy or lenticular bedding, represents deposition during slack water periods between ebb and flood tides. Large‐scale planar‐tabular and trough cross‐stratification reflects deposition within the deepest areas of subtidal channels, whereas medium‐scale cross‐stratification represents deposition in shallower water on shoals separating channels. Fining‐ and thinning‐upward sequences most likely resulted from the longshore migration of channels and shoals.The hummocky facies occurs only in the Erwin Formation and consists of horizontally laminated to hummocky stratified, fine‐grained arkosic to subarkosic arenite interbedded with equal amounts of bioturbated mudstone. It represents deposition between storm and fairweather wave‐base by combined‐flow storm currents.The quartz arenite facies is characterized by an absence of fine‐grained units and lithologically consists of a super‐mature, medium‐ to coarse‐grained quartz arenite. Large‐scale planar‐tabular cross‐stratification and abundant low‐angle cross‐stratification with rare symmetrical ripples (lower quartz arenite facies) occurs interbedded with the braided fluvial conglomerate facies, and was deposited within either a ridge‐and‐runnel system or a system of nearshore bars. Large‐scale, planar‐tabular cross‐stratification (upper quartz arenite facies), which forms the top of two 40 m‐thick coarsening‐upward sequences of the type: hummocky faciessandstone faciesquartz arenite facies, probably represents deposition on sand ridges that formed on a sand‐starved shelf as transgression caused the detachment and reworking of shoreface channel‐shoal couplets.Palaeocurrent data for the Chilhowee Group are unimodal but widely dispersed from 0° to 180°, and exhibit a minor mode to the west. The data are interpreted to reflect the influence of longshore, tidal and storm currents. The ichnofossil assemblage changes upsection from one characterized only by Paleophycus to a Skolithos ichnofacies and finally to a Cruziana ichnofacies. The facies sequence, biogenic and palaeocurrent data reflect the interaction through time of (I) non‐marine and marine processes; and (2) transgression coupled with shoreline progradation. The Chilhowee Group represents an overall deepening from terrestrial deposition to a marine shoreface that experienced both longshore and tidal currents, and finally to a storm shelf environment that periodically shoaled upward.

A tidal origin for coarsening-upward sequences above two Wilcox lignites in East Texas

Conventional palaeogeographic reconstructions show a large river system occupying the East Texas Embayment during the deposition of the Wilcox Group. Coarsening-upward sequences above Wilcox lignites in East Texas have been interpreted as floodplain splays. However, the bedding types, sedimentary structures and shallow scour surfaces in a coarsening-upward sequence above the main seam at the Martin Lake Mine in Panola County indicate deposition in water stirred by tidal and wave activity. The bedding types include sand streaks in mud and lenticular, wavy and flaser bedding. The sedimentary structures include abundant wave-ripple lamination and large-scale crossbeds with thin, closely-spaced mud drapes. These crossbeds suggest traction currents of short duration alternating with periods of slack water. Pollen from broadleaf hardwood trees is the most common element of the palynoflora from the coarsening-upward sequence at the mine. The upper reaches of riverine estuaries along the SE coast of the United States provide a modern analogue for the environment of deposition of the Wilcox sediments at the Martin Lake Mine. Here tidal currents extend inland to places where freshwater aquatic plants and forests border the estuaries. The same bedding types and sedimentary structures that occur in the sediments at the Martin Lake Mine occur in a coarsening-upward sequence above a lignite at the Big Brown Mine in Freestone County. Recognition of the tidal origin of the coarsening-upward sequences suggests an embayed coast or a shoreline within the East Texas Embayment during the deposition of the strata above the lignites.

A study of the effectiveness of antifouling electrodes in the marine environment

Antifouling systems based upon Cu/Al and Cu/Fe electrodes were tested both in enclosed pipes and chambers (simulating cooling systems), and upon panels and metal structures exposed to the full influence of tides and wave action. Fouling organisms failed to settle/grow in treated enclosed systems, while electrode-protected glass fibre panels/steel structures mounted intertidally in the Menai Strait remained almost devoid of biological fouling during the summer months when control installations became heavily fouled. Laboratory tests established that the antifouling action of electrodes was due to the dissolution of copper (in the ppb range), and not to the formation of a colloidal copper complex between copper and aluminium hydroxide. The unexpected effectiveness in non enclosed situations is believed to be due to high local copper concentrations maintained near the electrodes during slack water periods, and suggests a novel function for such electrodes—the localized protection of structures or instruments which cannot be treated with antifouling paints.

Suspended sediment transport in an estuarine tidal channel within San Francisco Bay, California

A recently developed instrumentation system has been used to monitor simultaneously flow conditions and suspended sediment distribution in the bottom boundary layer of a tidal channel within San Francisco Bay, California. Measurements were made every 15 min over six successive flood and ebb tidal cycles. They included mean velocity profiles from four electromagnetic current meters within 1 m of the seabed; mean suspended sediment concentration profiles from seven miniature nephelometers placed within 1 m of the seabed; near-bottom pressure fluctuations; vertical temperature gradient; and bottom photographs. Additionally, suspended sediment was sampled from four levels within 1 m of the seabed three times during the tidal cycle. The instrument system was retrieved during each slack water period to exchange suspended sediment sample bags. While the instrument was deployed STD-nephelometer measurements were made thoughout the water column and water samples were collected each 1–h and bottom sediment was sampled at the deployment site. Size distributions of the suspended sediment samples, estimates of particle settling velocity ( μ s ), friction velocity ( U ∗ ), and reference concentration ( C a ) at z = 20 cm were used in the suspended sediment distribution equations to evaluate their ability to predict the observed suspended sediment profiles. Three suspended sediment particle conditions were evaluated: (1) individual particle sizes in the 4–11 Φ (62.5-0.5 μm) size range with the reference concentration C a at z = 20 cm ( C Φ ); (2) individual particle sizes in the 4–6 Φ size range, flocs representing the 7–11 Φ size range with the reference concentration C a at z = 20 cm ( C f ); and (3) individual particle sizes in the 4–6 Φ size range, flocs representing the 7–11 Φ size range with the reference concentration predicted as a function of the bed sediment size distribution and the square of the excess shear stress. An analysis was also carried out on the sensitivity of the suspended sediment distribution equation to deviations in the primary variables μ s , U ∗ , and C a . In addition, computations of mass flux were made in order to show vertical variations in mass flux for varying flow conditions.

Behaviour of rainbow trout, Salmo gairdneri Richardson, in marine cages

Abstract. Underwater television was used to observe the distribution and swimming behaviour of groups of farmed rainbow trout, Salmo gairdneri Richardson, in marine net cages. Undisturbed rainbow trout (mean total length 240 mm) aggregated near the surface, remaining motionless in slack water and formed polarized shoals at higher current velocities. Larger fish (mean total length 317 mm) occupied deeper parts of the cage and were more active during periods of slack water. The results are compared with previous work on farmed Atlantic salmon, Salmo salar L., and discussed in relation to the biology of wild rainbow trout and recent work on exercise, growth and condition in salmonids.

Characteristics of Suspended Cohesive Sediment of the Severn Estuary, U.K.

Characteristics of suspended cohesive sediment from the benthic boundary layer of the Bristol Channel/Severn Estuary were examined at one location (51°27.66′N. 2°53.20′W). Samples were collected in vertical profiles over the bottom 2 m of the water column at times near to, and at low and high slack water periods on spring and neap tides. Primary particle size distributions obtained using a Coulter Counter (range 1.59–50 μm) and electron microscopy (range 0.1–20 μm) reveal bimodal (volume) frequency distributions with a lower mode fixed at 1.59 μm and a higher mode variable between 6 and 30 μm. When these data are converted to number frequency distributions, the higher mode effectively vanishes, while the lower is retained. Suspended sediment mineralogy, of dispersed samples, was determined by individual and multiple particle examination using analytical electron microscopy. Average suspended sediment composition was found to be illite 44%, quartz 23%, chlorite 19%, calcium carbonate 5%, kaolinite 4%, and organic carbon 3% (by weight). Floc diameters estimated from hindered settling experiments were ≈ 140 μm possessing a sedimentation velocity in the range 1.8–2.0 mm∙s−1, in good agreement with field observations of temporal changes in the vertical distribution of turbidity.Key words: suspended cohesive sediment, flocculation, Bristol Channel.

Neap-spring cycles reflected in Holocene subtidal large-scale bedform deposits: A preliminary note

Research Article| November 01, 1980 Neap-spring cycles reflected in Holocene subtidal large-scale bedform deposits: A preliminary note M. J. Visser M. J. Visser 1Comparative Sedimentology Division, Institute of Earth Sciences, Budapestlaan 4, Postbox 80021, 3508 TA Utrecht, Netherlands Search for other works by this author on: GSW Google Scholar Geology (1980) 8 (11): 543–546. https://doi.org/10.1130/0091-7613(1980)8<543:NCRIHS>2.0.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation M. J. Visser; Neap-spring cycles reflected in Holocene subtidal large-scale bedform deposits: A preliminary note. Geology 1980;; 8 (11): 543–546. doi: https://doi.org/10.1130/0091-7613(1980)8<543:NCRIHS>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Sub-Holocene subtidal channel-fill deposits in a tidal inlet in the southwestern part of the Netherlands contain features recognized as due to individual ebb, flood, and slack-water events. Thicknesses of the cross-strata deposited during successive ebbs (the dominant current at the particular place) vary in a sinusoidal way, reflecting the tidal neap-spring cycle. Mud layers deposited during slack-water periods formed in couplets that constitute a unique criterion for distinguishing between subtidal and intertidal environments and facies. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Ripple-Drift Cross-Lamination in Tidal Deposits: Examples from the Precambrian Bhander Formation of Maihar, Satna District, Madhya Pradesh, India

Ripple-drift cross-laminations are described from tidal deposits of the Precambrian Bhander Formation, which belongs to the Vindhyan Supergroup of central India. Considering the transport processes operative in modern midflats, ripple-drift cross-laminations should be common in ancient counterparts. The generation of ripple-drift cross-laminations is, however, usually hindered by the absence of persistent, unidirectional current flow. These structures can only develop in areas of the midflat where there is persistent, unidirectional current. Because of periodic velocity changes of tidal currents, rhythmically size-differentiated foreset laminae appear to be a diagnostic feature of tidal ripple-drift cross-laminations. Size differentiation is normally absent in the ripple-drift cross-laminations of turbidites and fluvial sequences, for in the environments responsible, there exists no cyclic pattern of water movement (that is, currents that alternate with periods of slack water) as is found in the tidal-flat environment.

EXPERIMENTS ON THE ORIGIN OF FLASER, LENTICULAR AND SAND‐CLAY ALTERNATING BEDDING

In Dutch sub-tidal waters, mud settling from suspension can produce a layer with a thickness of only some millimetres in one slack-water period. However, consolidated mud layers of 10–25 mm thickness in sub-tidal deposits are often observed. This indicates that mud may be preserved over several slack-water periods. This is only possible if the critical shear velocity (U*c) for the initiation of sediment movement is not surpassed after slack water. Tube and flume experiments reveal: 1 Thicker mud layers have smaller initial consolidation rates and lower values of U*c. 2 A higher sand content of the mud always gives a more rapid initial consolidation. The U*c-values increase to a maximum which is reached at a sand content of 40%. Higher sand contents lead to decreasing values of U*c. 3 For one particular mud sample the U*c-values increase linearly with the mud concentration of the bed material. 4 After three hours of consolidation a mud layer may carry a sharply bounded sand layer 0.5 cm thick. By application of the results of the experiments to natural conditions in tidal water some tentative explanations are possible of the formation of flaser, lenticular and sand-clay alternating bedding.