Sediment discharged into the portion of the Southern California Bight extending from Santa Barbara to Dana Point enters a complex system of semi-isolated coastal cells, narrow continental shelves, submarine canyons, and offshore basins. On both the Santa Monica and San Pedro margins, 210 Pb accumulation rates decrease in an offshore direction (from ~0.5 g cm −2 yr −1 to 0.02 g cm −2 yr −1 ), in concert with a fining in sediment grain size (from 4.5φ to 8.5φ), suggesting that offshore transport of wave-resuspended material occurs as relatively dilute nepheloid layers and that hemiplegic sedimentation dominates the supply of sediment to the outer shelf, slope, and basins. Together, these areas are effectively sequestering up to 100% of the annual fluvial input. In contrast to the Santa Monica margin, which does not display evidence of mass wasting as an important process of sediment delivery and redistribution, the San Pedro margin does provide numerous examples of failures and mass wasting, suggesting that intraslope sediment redistribution may play a more important role there. Basin deposits in both areas exhibit evidence of turbidites tentatively associated with both major floods and earthquakes, sourced from either the Redondo Canyon (San Pedro Basin) or Dume Canyon (Santa Monica Basin). On the Palos Verdes shelf, sediment-accumulation rates decrease along and across the shelf away from the White's Point outfall, which has been a major source of contaminants to the shelf deposits. Accumulation rates prior to the construction of the outfall were ~0.2 g cm −2 yr −1 and increased 1.5–3.7 times during peak discharges from the outfall in 1971. The distal rate of accumulation has decreased by ~50%, from 0.63 g cm −2 yr −1 during the period 1971–1992 to 0.29 g cm −2 yr −1 during the period 1992–2003. The proximal rate of accumulation, however, has only decreased ~10%, from 0.83 g cm −2 yr −1 during the period 1971–1992 to 0.73 g cm −2 yr −1 during the period 1992–2003. Effluent-affected sediment layers on the Palos Verdes shelf can be identified in seabed profiles of naturally occurring 238 U, which is sequestered in reducing sediments. The Santa Clara River shelf, just north and west of the Santa Monica and San Pedro margins, is fine-grained and flood-dominated. Core profiles of excess 210 Pb from sites covering the extent of documented major flood deposition exhibit evidence of rapidly deposited sediment up to 25 cm thick. These beds are developing in an active depocenter in water depths of 30–50 m at an average rate of 0.72 g cm −2 yr −1 . Budget calculations for annual and 50-yr timescale sediment storage on this shelf shows that 20%–30% of the sediment discharge is retained on the shelf, leaving 70%–80% to be redistributed to the outer shelf, slope, Santa Barbara Basin, and Santa Monica Basin.

Abstract Sedimentation in tidal environments covers a broad spectrum of conditions, and produces a wide range of tidal signatures. These sediment sequences that have accumulated as the result of significant tidal influence are called tidalites. Over the past few decades, several models have been developed that are useful in interpreting tidal sequences in the ancient stratigraphic record. The models include tidal bedding, tidal bundles, and the relationships of cycles with various time scales to these tidal deposits. Although the efforts of sedimentologists studying both modern and ancient sequences have provided techniques for detailed analysis of tidalites, there is still much to be learned about tidal sedimentation and the resulting sediment deposits. This volume is a step toward that end. This introductory paper will provide the reader with a brief history of the organized effort in tidal sedimentology along with a synopsis of this volume.

Abstract Research on tidalites, sediments deposited by tidal currents, evolved through four phases during the last half century: PHASE I, Facies mapping of Holocene tidalites in Germany, Holland, the United Kingdom, and Canada identified the seaward-coarsening pattern of sediment distribution, a distinct zonation of sedimentary structures, and provided a fining-upward facies model used to recognize ancient counterparts. Mapping in subtidal areas showed that extensive sheets of tidally molded-and-deposited sand accumulations characterized continental shelves that were both wide, and funnel-shaped in plain view. Similarly, extensive work was completed on carbonate tidalites, although it is not discussed herein. PHASE II. Study of sedimentary structures was followed by a detailed analysis of sediment transport dynamics on intertidal sand bodies in Canada, where time-velocity asymmetry is the major factor controlling sand body geometry, orientation of bedforms, grain size distribution, sediment dispersal, and sand body orientation. Parallel work in tide-dominated continental shelves of the Yellow Sea of Korea and the southern North Sea showed similar patterns. These studies confirmed that tidal sand bodies are likely to be preserved in the rock record and provide a counterpart facies that is likely to dominate ancient cratonic seas. PHASE III. In tide-dominated estuaries of The Netherlands, cross-bedded units were observed to be organized into discrete bundles that were correlated to neap and spring tides. These observations were replicated in ancient counterparts. PHASE IV. Detailed analysis of the Schelde Estuary, The Netherlands, demonstrated that parallel-bedded couplets of sand and mud (tidal bedding) could be correlated directly to neap-spring tidal cycles. Recognition of such couplets, particularly in Mississippian and Pennsylvanian sediments of the midcontinent of North America, can be correlated to lunar dynamics and tidal patterns. ALL of these studies demonstrated that tidalites accumulated rapidly and were preserved widely. Where preserved in the stratigraphic record, tidalites represent accumulation during very short time intervals. Consequently, in many sequences where such facies are preserved, the time gaps in the stratigraphic record were far longer than previously interpreted.

Abstract Sedimentological studies have recently been carried out in the Spiekeroog back-barrier area (southern North Sea coast of Germany) to explore the interrelationship between the surficial sediment distribution patterns, energy levels, and transport processes. The sediment distribution patterns show that the sediment generally becomes finer landwards (north-south), irrespective of the tidal channel orientations (east-west). a closer examination, however, shows two distinct patterns. The first pattern is a general landward-fining of the sediments within the inlet from about 1.0 phi (0.50 mm) in the inlet throat to about 3.5 phi (0.088 mm) on the landward reaches of the inlet. This pattern is a result of the decrease in current velocity from the inlet throat landwards. The second pattern, which is the most conspicuous on the mean grain-size map, shows a distinct shore-normal (north-south), landward sediment fining across the tidal flats from about 2.0 phi (0.25 mm) on the islands to 2.5–3.0 phi (0.25–0.125 mm) on the tidal flats to as fine as 3.5 phi (0.088 mm) along the dike (mainland coast). This shore-normal sediment fining has been found to be a result of the shore-normal energy gradient (flow velocity) associated with overbank flow from one channel to the next. The analysis of skewness evolution has been shown to be a powerful tool for the interpretation of transport pathways in a tidal environment. Skewness distribution patterns indicate that the inlet areas act as sources of sediment from which sediment is transported landwards during the flood tide and seawards during the ebb phase. In the whole area, however, the fine sediment fraction (population) whose energy niche is the landward margin of the backbarrier areas, and the coarse fraction characterizing the inlet throat areas, undergo a range of population mixing as well as progressive sorting of the individual populations. Progressive sorting appears to be dominant normal to the shore and across the tidal flats whereas mixing processes are more pronounced along the main channel of the tidal inlets.

Abstract A survey of tidal flat structure in the back-barrier area of Spiekeroog Island, Germany, southern North Sea, has provided a variety of morphologically distinct sediment bodies which, however, reveal only a limited number of definite structural units. These exhibit specific facies profiles visible in sediment cores, particularly in relief casts. The most conspicuous structural and facies units, as seen from the margins at low water line towards the central parts of the tidal flats, are: (1) fine-sandy edges and spits where strong physical reworking by tidal currents results mainly in laminated stratification and complete absence of bioturbation; (2) edge-gully zones with channel fill structures; (3) trough channels with intense erosion of Mya arenaria shells originating from ancient colonies; (4) sandy tidal flats with ripple stratification showing sporadic and transient bioturbation by Arenicola marina; (5a) sandy tidal flats with ripple stratification, sporadic Arenicola bioturbation and sparsely spaced tubes of Lanice conchilega; (5b) sandy platforms with densely spaced tubes of Lanice conchilega and indistinct remnants of physical stratification; (6) Mytilus beds underlain by shell layers representing remnants of former Mytilus colonies; (7) muddy channel zones crossing the elevated watershed area of the back-barrier system totally bioturbated by Heteromastus filiformis, additionally populated by Mya arenaria; and (8) old semi-consolidated mud banks totally bioturbated and with shells of Mya arenaria in life position, representing ancient deposits in former watershed channel zones. The time covered by the facies profiles increases from the marginal zones towards the central platforms of the tidal flats, that is, from several tides to several decades or even centuries. In addition to the succession of facies types in the horizontal direction, vertical successions are visible, as well, according to Walther's facies rule.

Abstract Theoretical considerations suggest that, while changes in hydrodynamic conditions are an important factor in mobilizing and resuspending sediments, seasonal shifts in spatial sediment distributions in shallow water environments of intermediate to high latitudes should be influenced by variations in settling velocities and critical shear strengths induced by changes in water temperature and fluctuations in salinity. In the Wadden Sea of the southern North Sea, water temperatures range from <4°C in January to almost 24°C in July, and salinities from 2.6% to 3.3%. In accordance, the kinematic viscosity of sea water varies from 1.5728–0.9096 cSt (centi-Stokes). Under these circumstances sediment particles respond to the changes in viscosity with corresponding changes in settling velocity and particles of a given size thus have as many settling velocities as there are viscosity changes. In hydraulic terms a particle can therefore have many effective grain sizes. To facilitate the calculation of critical shear velocity on the basis of settling velocity, the following equation was developed: U *cr = (0.482 [((δ S – δ f )/δ f ) v g] 0.282 ) * (0.15 w ) + 0.61. The back-barrier tidal basin investigated in this study is characterized by a shoreward-decreasing energy gradient, as documented by progressively decreasing grain sizes and the resulting landward-fining sequence of shore-parallel facies belts. Applying the viscosity principle, one should expect the facies belts to show a seasonal temperature adjustment. Time-series studies, however, have demonstrated that the sediment zonation pattern remains stable throughout the year; that is, a seasonal temperature-induced shift of the belts could not be confirmed. This apparant contradiction can be explained if one assumes that the sediment distribution pattern is permanently adjusted to winter conditions at which the settling velocities are lowest (the sediment is hydraulically finest) and the energy input is highest. Subsequent summer conditions are unable to reverse the depositional pattern produced in winter because settling velocities are higher, making the sediment hydraulically coarser. This interpretation is supported by the observed seasonal dynamics of suspended matter. During winter, muddy sediments occupy relatively small areas adjacent to the mainland dike. In summer, on the other hand, mud contents of local sediments were found to increase and muddy sediments occupied much larger areas. This pattern is explained by the higher settling velocities of suspended matter in summer as required by the viscosity principle. Field data therefore support the theoretical considerations and demonstrate that the kinematic viscosity of the fluid plays an important role in depositional processes.

Abstract In the main channel of the typical T-shaped tidal area of Grådyb (mean tidal range ≈ 1.5 m), the maximum tidal currents are highest in both ebb (V 1% ≈ 1.5 m/s; V 50% ≈ 0.75 m/s) and flood (V 1% ≈ 1.2 m/s; V 50% ≈ 0.65 m/s) in the central part where the inlet splits up into two main tributaries behind the barriers. The ebb dominates over the flood in the outer parts and visa versa. The transport of fine-grained sediment in the area is extremely dependent on the weather conditions. In general, during windy periods the concentration level is above 40 mg/1 with mean values over the tidal period reaching a maximum of approximately 150 mg/1. In fair-weather periods the typical mean concentration in the main channel is between 15 mg/1 and 30 mg/1 and the in situ median grain size is surprisingly stable with a mean value of 26 µm. Recordings of the transport of suspended fine-grained sediment over 180 tidal periods, covering all seasons with typical weather conditions, showed that the important exchange of fine-grained material between the Wadden Sea and the open North Sea, is episodic. The investigated tidal area is exporting during stormy periods, concentrated in the winter term, where large amounts of fine-grained material are mobilized and apparently lost through the exchange of local turbid water with relatively clean water from the North Sea, The tidal area is importing during and after windy periods, following long periods of calm weather. This is speculated to be the outcome of fine-grained sediment settling on the shelf during long periods of calm weather, which increase the potential for high concentrations and large fall-velocities of suspended sediment. During subsequent windy periods those deposits are reworked and brought into the tidal area by the flood current. These conditions are most likely to appear in the summer.

Abstract Concentration of fine-grained suspended sediment in a microtidal estuarine environment (northern part of the European Wadden Sea; mean tidal range = 1.5 m) is examined in order to clarify how this is influenced by flood/ebb, diurnal inequality and neap/spring during calm weather conditions. The diurnal inequality was reflected in the fine-grained suspended sediment concentration as an alternation of the maximum and minimum concentrations caused by the variation in advection as well as an alternation in the resuspension strength. The onset of the ebb current generally caused higher resuspension than the onset of the flood current, which presumably was due to the fact that a larger portion of the tidal flats was affected by the ebb current than by the flood current because of the difference in water level. The variability of the concentration of fine-grained suspended sediment over a neap/spring/neap cycle is found to be controlled primarily by the changes in the mean water level. A decrease in mean water level caused a rise in the concentration level because of a seaward extension of the turbidity maximum. An increase in mean water level could also cause a rise in the concentration level, presumably because of exposure of the tidal flats to the current. Because of the relatively low tidal range, however, the resulting long-term tide is strongly modified by the wind effects even in relatively calm periods.

Abstract Cyclic rhythmites (tidal rhythmites) provide fine-scale resolution of deposition within specific types of tidal systems. Rhythmites of this kind have been reported from many sites throughout geologic time; nonetheless, they are particularly widespread and common within the Carboniferous basins of the eastern and mid-continental United States. These rhythmites have great potential for providing information regarding the local, regional, and global controls on their deposition. Widespread occurrence of cyclic rhythmites within these basins outlines a hierarchy of parameters that controlled deposition. These controls include: (1) local and regional depositional setting, (2) elevated paleotidal ranges, (3) sequence stratigraphic and basinal conditions, (4) a unique paleoceanographic configuration, and (5) specific conditions of astrophysical alignments.

Abstract Marsh deposits frequently consist of a succession of packages interpreted to be annual cycle records. Annual sedimentary cycles are usually recognized because of a contrasted seasonal differentiation of sedimentation. In the inner estuary of the Bay of Mont-Saint-Michel, detailed lamination analysis of some marsh packages that are not heavily disturbed by root traces demonstrates that they can be made of planar silty-mud couplets that thicken and thin systematically. In terms of facies and lamina thickness evolution, these sedimentary cycles are similar to neap-spring-neap tidal rhythmites. However, the number of couplets they contain is inconsistent with the number of tides that are able to reach the supratidal domain during a single fortnightly cycle, but it fits the average number of tides that inundate the marshes during one year. The problem with the occurrence of such annual records arises from their possible misinterpretation as neap-spring-neap cycle records in ancient tidal facies. This confusion could lead to erroneous conclusions about sedimentation rate, tidal regime, environmental context, and orbital parameters, conclusions that are usually inferred from the analysis of neap-spring-neap tidal rhythmites.

Abstract Three occurrences of rhythmites in Carboniferous tidal channels from the Appalachian and Illinois Basins were compared in order to assess the controls on the rapid accumulation of tidal sedimentation in paleoenvironments in which channelized flow and daily reworking normally preclude such preservation. In all three channels, rhythmites consist of stacked composite bedsets. Daily tidal sedimentation in these rhythmites is marked by submillimeter- to millimeter-scale laminae couplets. Bundles of 12 or fewer couplets represent fortnightly neap-spring deposition, alternating thick and thin neap-spring bundles represent perigeen and apogeen cycles, and the association of 24 or fewer neap-spring bundles in each bedset represents annual sedimentation cycles. The most complete annual bedsets are often divided into sand- and shale-dominated halves, which are inferred to represent seasonal differences in current energy during aggradation. Interpreting the various orders of tidal cyclicity is complicated in many bedsets because of the thinness of daily laminae couplets and typical incompleteness of continuous daily sedimentation records in neap-spring bundles of the three channels studied. In many cases, bundles of laminae or ripples with foreset drapes are superficially very similar to laminae couplets. In such examples, fortnightly cycles can be misinterpreted as daily deposits. Furthermore, annual cycles can be misinterpreted as monthly deposits because 24 weekly cycles look like 24 daily deposits. In all three channel-fill deposits, annual bedsets thin upward, through (1) a vertical decrease in the number of daily couplets per neap-spring bundle, (2) loss of neap bundles per monthly pair, and (3) loss of monthly bundle pairs in the shalier, low-energy, seasonal parts of each bedset. These vertical changes reflect a strong accommodation control, especially considering the lack of rhythmic sedimentation in lateral deposits. In each of the channels, the most complete tidal records were recorded toward the base and axes of the fills, and daily, neap-spring, monthly, seasonal, and annual cycles amalgamated upward and laterally in response to shallowing. This is significant because accommodation-controlled thinning and amalgamation causes substantial changes in the rhythmicity of the fill and results in subtle hiatal surfaces, which could lead to incorrect inferences of temporal duration spatially across the channel, even within the same bedset.