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.

Abstract The Magoffin Member of the Four Corners Formation (Breathitt Group) outcrops in eastern Kentucky as a predominantly coarsening-upward succession of rhythmically interstratified sandstone, siltstone, and mudstone with minor occurrences of limestone at its base. Primary sedimentary structures, trace fossils, vertical successions of facies, and sediment body geometries suggest that these rhythmically bedded sediments were deposited in a delta-front/distributary-mouth-bar setting. The overall thickness of the Magoffin Member is highly variable. Within the study area, it ranges in thickness from less than 25 m to greater than 40 m. Evidence indicates that in areas where the Magoffin Member is thickest, it tends to be sandier and contain rhythmite intervals that are thicker and more complete than in areas where it is thin. The member displays several orders of centimeter- to decimeter-scale cycles that are consistent with semi-diurnal, diurnal, semi-monthly, and monthly tidal periodicities. Halfsynodic (semi-monthly neap-spring-neap) and anomalistic (monthly) lunar periodicities are manifested by the systematic thickening and thinning of the shorter duration cycles. The rhythmite interval ranges in thickness from less than 5 m up to 20 m and records up to four months of deposition. Accumulation rates for the rhythmites typically ranged from 20–100 cm per neap-spring-neap event, but reached rates of over 30 cm per day in areas where the Magoffin Member is thickest and the most proximal deltaic facies are preserved. The nature of tidal bundling suggests that these rhythmites accumulated in a mixed, dominantly semi-diurnal tidal system where both lunar phases and lunar declination influenced tidal cyclicity.

Abstract Well-developed tidal rhythmites associated with tidal mudflats overlie some of the lowest-sulfur Pennsylvanian coals in the eastern part of the Illinois Basin, U.S.A. Such an observation is seemingly at odds with the traditionally held view that low-sulfur coals are not typically associated with marine-influenced roof rock in the Illinois Basin and elsewhere. This association can be explained, however, if the tidal mudflats formed in a freshwater or low-salinity marine setting. Such deposits are known from modern systems but have not been adequately documented from the rock record. Geochemical, petrographic, and sedimentologic analyses of tidal deposits immediately above the low-sulfur (sulfur values <1%) Lower Block Coal Member of the Pennsylvanian Brazil Formation (Atokan), Daviess County, Indiana, confirm the potential for the preservation of extensive (>480 km 2 area) freshwater tidal flat deposits in the rock record. A strong tidal (marine) signal, manifested as tidal rhythmites preserving small-scale neap-spring cycles, is preserved within the laminated mudstone and interbedded sandstone and mudstone immediately above the Lower Block Coal. These cycles indicate deposition within a mixed, predominantly diurnal tidal system in which sedimentation rates were as high as 1 m/yr. Carbon to sulfur ratios, macerai types, and the dominance of terrestrial organic markers within the rhythmite facies reveal that the Lower Block peat (coal) was initially onlapped by a freshwater (low salinity) tidal flat. The presence of tidal rhythmites indicates that at least mesotidal conditions prevailed during transgression. The areal distribution, sedimentology, stratigraphy, petrography, and geochemistry of the succession from the Lower Block Coal to the next younger coal (Upper Block Coal Member of the Brazil Formation) suggest that the tidal flat facies formed within an embayed coastal setting that experienced significant rainfall and runoff typical of an ever-wet climate. The embayments likely formed by the collapse and transgression of the coastal peat mires (Lower Block Coal) and ultimately filled with mudflat and mixed sandflat and mudflat facies. Very high rainfall and associated runoff resulted in the formation of coast-hugging freshwater plumes. These plumes pushed the salt water wedge offshore and prevented it from entering the embayments until the coastal peats had already been covered by at least a meter of tidal flat mud. The results of this study may have implications for paleogeographic reconstructions in other Carboniferous basins. Similar tide-dominated facies may have gone unrecognized in some Carboniferous successions in Europe, and elsewhere, that are currently interpreted to be ancient nonmarine mud-dominated tropical systems. Such recognition would have important sequence stratigraphic and paleogeographic implications.

Abstract Climbing ripples characterize a variety of sedimentary depositional settings in which suspension sedimentation exceeds the rate of traction transport, but are poorly documented from tidal environments. Research within a modern macrotidal estuary (Bay of Mont-Saint-Michel, France) in comparison with a Carboniferous example of climbing ripples from tidally influenced sedimentary rocks (Tonganoxie Sandstone, eastern Kansas, USA), demonstrates that this form of stratification is very common and also closely associated with tidal dynamics along the fluvio-tidal transition zone of macrotidal estuaries. In the modern example, flood- and ebb-dominated climbing-ripple facies (CRF) have been distinguished. Successive climbing-ripple units are up to 10 cm in thickness. Flood dominated CRF are associated with tidal channel levees found in the inner/straight channel zone of the fluvio-estuarine transition. In thicker CRF units, sedimentary structures indicate very high suspended sediment loads and rapidly decelerating flow velocity. Ebb-dominated CRF are found in chute channels and chute bars associated with the meandering zone of the fluvial-estuarine transition. The role of tidal dynamics in the formation of these CRF, both flood- and ebb-dominated, is indicated by the vertical organization and thickness evolution of the successive climbing-ripple units. They are frequently arranged in packages of strata that thicken and thin progressively. These packages are tidal rhythmites and correspond to the sedimentary record of the neap-spring-neap cycle. The increasing energy from neap to spring tides is indicated by an overall decreasing angle of climb in the generalized bedding sequence (progradation is dominant), whereas the decreasing energy from spring to neap is evidenced by an increase of this angle (vertical accretion is dominant). Facies within the Tonganoxie Sandstone (Carboniferous) have identical climbing-ripple successions and similar vertical progressive thickening and thinning of strata in outcrop and in core, indicating a strong tidal influence on sediment deposition. Single sedimentation units in the Tonganoxie are up to 16 cm in thickness, but show an identical vertical progression of sedimentary structures as those from the modern facies in Mont-Saint-Michel. The physical sedimentary structures of both the modern and ancient are strongly comparable on a hydrodynamic basis insofar as silt-sized sediments dominate both systems. Furthermore, a variety of additional physical and biogenic sedimentary structures that require periodic or episodic exposure have been described from both the modern and the ancient. Sedimentation patterns in both systems suggest relatively rapid aggradation within the existing accommodation space, with soils and rooted horizons capping the units once this accommodation space is filled.

Abstract Previous lithostratigraphic studies of incised, valley-fill systems on the mid-Atlantic coast of the United States indicate a continuous Holocene sea-level rise. Likewise, the lithostratigraphic evaluation of vibracores from Albemarle Sound, North Carolina, can be interpreted as a simple infill history resulting from a continuous Holocene transgression. However, an integrated litho-, high-resolution seismic-, and chronostratigraphic approach suggests that the infill record of Albemarle Sound may be the result of several relative sea-level oscillations during the Holocene. The Holocene section of Albemarle Sound contains at least three depositional sequences (ASDS-1 to ASDS-3), each consisting of lithologically similar successions of depositional environments. Basal sediments from ASDS-1 were deposited in a restricted-estuarine environment behind a continuous barrier island system between 8.1 ka and 5.9 ka. For some interval between 5.9 ka and 5.5 ka sea level dropped (<6 m), producing an extensive erosional surface throughout the middle- and outer-estuarine zone. Deposition resumed as sea level rose to form ASDS-2. Basal sediments of ASDS-2 were deposited in an open-estuarine system between 5.5 ka and 2.9 ka, suggesting that the previous barrier island complex was extensively breached. ASDS-2 is capped with closed-estuarine sediments that date from 2.9 ka to 1.5 ka, suggesting the reformation of a continuous barrier island complex. Another small drop of sea level occurred for some interval between 1.6 ka and 0.4 ka, which truncated ASDS-1 and ASDS-2. This drop was only between 1–2 m based on the absence of basal open-estuarine sediments in the overlying depositional sequence (ASDS-3), suggesting that the barrier island system was preserved and active during this interval. ASDS-3 consists of closed-estuarine sediments that are being deposited during the ongoing sea-level transgression and behind the modern, well developed barrier island system.

Abstract The Virgelle Member of the Milk River Formation, Alberta, Canada represents a sandy progradational depositional systems tract that contains linkages between offshore, estuarine, and coastal plain environments. Distinct upward-shoaling depositional successions include regional erosion surfaces that punctuate transitions from storm- and fair-weather-dominated deposition to tidal sand bars and estuarine channel complexes that developed as the systems tract prograded basinward. The lower part of the Virgelle Member is characterized by hummocky and swaley cross-bedded sandstones depicting the transition from offshore to storm-dominated middle shoreface. A sharp, regionally flat erosion surface separates middle shoreface deposits below from two end-member upper shoreface/foreshore lithofacies associations above: (1) rare fair weather wave-reworked deposits or, (2) common tidally reworked deposits represented by outer estuarine tidal bars. The upper shoreface unconformity is thus dominantly a tide-cut source diastem (TSD), overlain by bathymetrically equivalent subtidal to intertidal sand bars typified by planar-bound, herringbone, cross-bedded sandstone. The erosive base of extensive, laterally accreted estuarine channels (ECh) cuts into middle shoreface deposits and truncates the flat disconformity and the above-mentioned shoreface and esmarine successions. Tidal influence within the channels is recorded by carbonaceous bundles and couplets, reactivation surfaces, and subordinate, flood-directed, three-dimensional dunes. In addition, a restricted ichnofauna documents the influence of an estuarine environment. The resulting depositional model depicts a west-northwest/east-southeast trending estuarine system, open to the east, that truncates the storm-dominated middle shoreface, and is itself cut by a belt of meandering estuarine channels merged into overlying supralidal coastal plain mudstones. The genera] distribution of palynomorphs is consistent with the progression of marine dinoflagellate-rich assemblages in outer estuarine tidal bars to mostly terrestrial assemblages in ebb-dominated estuarine channels. A qualitative analysis of depositional regime variables Q, M, D and R within the supply-dominated depositional systems tract of the Virgelle Member highlights the critical importance of the sediment dispersal function D, in this case controlled by tides and storms. The corresponding relationship may be expressed as Q M ≥ D R. The proposed model for a progradational estuary contrasts with sequence stratigraphic models of transgressive estuaries because it is not restricted to specific relative sea-level stages, and because it arises from the linkage of depositional processes along the entire systems tract, from offshore to coastal plain.

Abstract Upper Cambrian Sawatch and Peerless formations exposed in the vicinity of Manitou Springs, Colorado, U.S.A., are part of the Cambrian inner detrital belt of North America. Transgressive systems tract (TST) deposits composed of quartz-rich shoreline and nearshore sandstone (Sawatch Formation) are overlain by coarse-grained, glauconite-rich, tidal dune deposits at the base of the Peerless Formation. These compound cross-bedded sandstone units include spectacular complete formsets up to 3.5 m thick and 3- to 5-m-thick co-sets, the latter composed of two or three stacked-to-shingled, cross-stratified sets. The formsets have near-symmetrical cross-sectional shapes and low stoss and foreset dips, which, in conjunction with their internal structure, indicate deposition under strong but nearly symmetrical tidal currents. Slight asymmetry in current strength is, however, indicated by the dip directions of large-scale foresets, which are consistently from north-northeast to northwest. Centimeter- to decimeter-scale internal cross-bedding shows bimodal to polymodal paleocurrent orientations and reflects the migration directions of superimposed dunes. Sedimentological and stratigraphic analysis indicates that the dunes are part of condensed deposits that formed as latest TST deposits. The underlying Sawatch Formation provides insight into the nature of TST's in epicontinental settings. The lack of valley fill or coastal plain deposits at the base of the Sawatch suggests transgression over a locally unchannelized, low-relief hinterland. This quartz-rich unit presumably formed from reworking of a thick regolith that sat on deeply weathered Precambrian crystalline rock. No remnants of this regolith remain in the Manitou Springs region. The extremely thin nature of the TST and the presence of numerous ravinement and marine erosion surfaces is the expected signature of TST deposits in epicratonic settings.

Abstract Most of the economic gold placers of the late Archean Witwatersrand Supergroup of South Africa have been found to be of fluvial origin. A downstream transition from the fluvial paleoenvironment to coastal environments is expected, but such facies changes have not been observed within the previously studied areas. In this paper, the sedimentology of the uppermost formations of the Supergroup in a distal part of the preserved basin, namely the Vredefort structure, is described and evidence for transition from fluvial through tidal to shallow marine depositional conditions is submitted. The study area comprises the exposed arcuate northeastern to southwestern sector of the Vredefort structure, where overturned strata dip radially towards its center. In the Vredefort structure, the upper subgroup of the Witwatersrand Supergroup, the Turffontein has been subdivided into five formations, for which the codes Rt1 through Rt5 are used. The succession between formations Rt1 through Rt3 is the subject of this study. Only the upper part of the basal formation, Rt1, is exposed in the study area and comprises coarse-grained, trough and planar cross-bedded arenites, which are texturally moderately mature. Paleocurrent distributions are mostly unimodal, except in the extreme eastern region, where the distribution is polymodal. Formation Rt2 disconformably overlies formation Rtl and consists of interbedded conglomerate and coarse-grained, cross-bedded arenites. Pebble size is largest in the northwestern parts of the study area and decreases towards the east. Paleocurrent distributions are unimodal in the western parts of the study area, but bimodal to polymodal in the central and eastern parts. Formation Rt3 conformably overlies formation Rt2 and comprises predominantly coarse-grained, moderately mature, trough and planar cross-bedded quartz wackes, with scattered small pebbles and thin conglomerates. Paleocurrent distributions of this formation, too, are unimodal in the western part of the study area and mostly bimodal in the remainder. A number of planar cross-bed sets with foreset packages separated by mudstone or immature sandstone laminae were found in the outcrops of the central and eastern parts of the study area. In some cases the thicknesses within series of consecutive foreset packages vary cyclically. A planar cross-bed set with mud-draped foreset packages was found towards the top of the formation in the eastern parts of the study area. The mudstone-bounded foreset packages described are interpreted as tidal bundles and indicate a tidal setting for those paleogeographic parts of the formations where they occur. Bimodal paleocurrent distributions provide additional evidence for tidal deposition. West of an imaginary point near the center of the study area, no indicators of tidal sedimentation have been found and all the formations were probably deposited by fluvial processes here. However, to the east of this point, which indicates the position of a paleo-bayline, formations Rt2 and Rt3 exhibit characteristics indicative of tidal deposition. Paleocurrents, and the decrease in pebble size of the conglomerates of formation Rt2 towards the east, suggest that its sediment was derived from a western source area, most likely in response to tectonic uplift and tilting here, and transported into a tidal environment by fluvial processes. The basal disconformity of this formation is thought to be a type 2 unconformity and the result of progradation of coarse-grained gravel across a paleo-surface comprising sand.

Abstract Tidalites: Processes and Products - Tidal processes are important agents of sediment transport and deposition in most marine coastal zones. Recent development of better tools for recognizing tidal deposits (e.g., tidal bundles and tidal rhythmites) has also revealed that such deposits are much more common and extensive than previously thought. This volume presents the latest ideas on tidal sedimentation, including exciting new work concerning the tidal flats of the Wadden Sea, the area where modern-day studies of tidal sedimentation began; the origin, interpretation, and uses of tidal rhythmites; and the sequencestratigraphic context of tidal sediments.