Ichnologic data reveal the complexity of the fluvial–tidal transition in terms of the interaction of riverine and marine processes and changes in the salinity conditions. The ichnofaunas of the fluvial–tidal transition are mainly controlled by the salinity limit that separates two environmental zones, seaward of this limit brackish-water ichnofaunas occur, whereas landward terrestrial/freshwater ichnofaunas are present. The ichnology of the fluvial–tidal transition is summarized based on a series of case studies, spanning the Carboniferous to the Miocene. Freshwater trace-fossil assemblages are widespread in the fluvial–tidal setting and are characterized by moderate to relatively high diversity of surface trails and meniscate trace fossils typically present in continental environments, whereas brackish-water trace-fossil assemblages are typified by common monospecific to low-diversity suites of diminutive simple marine trace fossils. In addition, a comparison of freshwater ichnofaunas at the fluvial–tidal transition through time reveals a remarkable evolutionary control. Whereas Paleozoic ichnofaunas are dominated by shallow-tier grazing trails and arthropod trackways, post-Paleozoic examples are dominated by deeper-tier meniscate trace fossils, which significantly contributed to a remarkable increase in intensity of bioturbation of deposits formed in fluvial–tidal transition. Based on information from the stratigraphic record of freshwater ichnofaunas, these changes could be linked to the Mesozoic Lacustrine revolution, an evolutionary event representing a breakthrough for freshwater biotas. The increase in depth and extent of bioturbation obliterated shallow-tier traces and allowed overprinting of previously emplaced brackish-water suites, resulting in the formation of composite ichnofabrics.
Depositional environment controls the architecture, heterogeneity, and ultimately the quality of oil reservoirs and is therefore one of the most important considerations in the development of any enhanced oil recovery program (EOR). Detailed characterization of the Pennsylvanian Bridgeport sandstone reservoirs in Lawrence Field, Illinois Basin, USA, has revealed juxtaposed deltaic and incised valley fill sediments that were deposited in the fluvial–tidal transition zone. Fluvial–tidal transition zone sediments are some of the most complex deposits known, but it has been shown through detailed facies analysis how two seemingly stratigraphically equivalent reservoirs can exhibit different reservoir properties depending on their position within the fluvial–tidal transition zone with important implications for operators considering EOR techniques, including chemical or carbon dioxide (CO2) EOR and geologic storage. The thinner, finer grained, and more compartmentalized Griggs sandstone was deposited in a low accommodation tidally influenced deltaic setting located in a seaward position within the fluvial–tidal transition zone. The thicker, coarser grained, and younger Robins sandstone was deposited as a fluvial system in an incised valley system that transitioned upward to estuarine conditions during transgression but remained in the landward portion of the fluvial–tidal transition zone. In the Robins sandstone, preserved primary intergranular porosity in the largely fluvial sandstone has resulted in high-quality, largely homogeneous reservoirs. Whereas in the Griggs sandstone, porosity and permeability are lower because of finer grained, more heterolithic deltaic reservoir facies that have had porosity and permeability reduced to a greater degree by diagenetic alteration. Only through detailed reservoir characterization were the differences in these reservoirs realized.
Sedimentation in the fluvial–marine transition is governed by the interaction of river and tidal currents. Tidal currents act continuously, albeit with small variations in strength as a result of neap–spring cyclicity and modulation by changes in river discharge. River currents, by contrast, commonly change more dramatically because of the presence of river floods. The superposition of river floods on the tides causes depositional conditions to vary temporally from less fluvially influenced/dominated (and more tidally influenced/dominated) during the times between river floods (the interflood periods) to more fluvially influenced/dominated (and less tidally influenced/dominated) during river floods. These temporal variations in the relative importance of river and tidal currents are recorded within individual beds in the point-bar and mouth-bar deposits of the fluvial–marine transition, creating a spectrum of possible deposit types depending on the longer-term ratio of river flood to tidal energy. In areas of fluvial dominance, tidal action is only present in the interflood deposits, if present at all, whereas in areas of tidal dominance, river-flood sedimentation can become cryptic and is indicated by intervals with coarser sand and a greater abundance of fluid-mud deposits. The ichnological character of the deposits preferentially reflects interflood conditions. A detailed analysis of the deposit characteristics allows deductions to be made about the strength of the tidal currents, the intensity of river floods, and the relative position of a given deposit within the fluvial–marine transition. Discrepancies between the proximality indicated by the nature of the river-flood deposits and the ichnology of the interflood intervals can give an indication of the relative magnitude of river-discharge fluctuations.
The tidal–fluvial transition (TFT) is a complex depositional zone in rivers, where tidal- and river-flow interact. However, subdivision of the TFT into discrete zones is not well established. Studies of the lower Fraser River, Canada, reveal criteria for subdividing the TFT into zones that experience similar hydrodynamic and salinity variations from year-to-year, and three zones are identified: (1) mixed tidal–fluvial with persistent brackish-water zone (mixed tidal–fluvial); (2) fluvially dominated and tide influenced, freshwater to brackish water transition zone (turbidity maximum); and, (3) fluvially dominated and tidally modulated, freshwater zone (tidal backwater). Of the four main datasets (sedimentological, ichnological, palynological, and geochemical) evaluated across the TFT of the Fraser River, only sedimentological and ichnological datasets can be used reliably to distinguish between depositional zone within the TFT. Mud volume is highest in the turbidity maximum zone and decreases in both the landward and seaward direction. Rhythmic (tidal) bedding is common in the mixed tidal-fluvial zone, and sand–mud interbedding occurs in both the turbidity maximum and mixed tidal–fluvial zones. Significant sand–mud interbedding is not expected in the tidal backwater, nor landward of the tidal backwater. Higher salinity and longer residence times of saline water at the bed are manifested ichnologically in larger diameter burrows, higher bioturbation intensities, and a more diverse trace assemblage. Variations in river discharge result in a heterogeneous distribution of burrows, where burrows are mainly concentrated in mud beds. Increased fluctuations in depositional energy and increasing sedimentation rate reduce trace density. The depositional trends presented from the lower Fraser River are intended as a basis for comparison to trends defined from the TFTs of other major river systems. This work demonstrates that it is possible to predict relative depositional position across the TFT, and that a comprehensive model for TFT deposits can be constructed.
A series of Late Pliocene tidally influenced channels associated with both palaeo-Orinoco delta lobes and estuaries are documented in outcrops along the Trinidad coast. The Morne L'Enfer phase (ca. 3.6 Ma) of palaeo-Orinoco stratigraphy has been chosen because its regressive delta lobes, and the associated transgressive estuaries, were strongly influenced by tidal processes. This study describes deposits of the Orinoco shorelines as they transited both the pre-existing inner shelf (Cedros and Erin Bays, southwest Trinidad) and the coeval outermost shelf (Matura Bay in east Trinidad) reaches of the system. The criteria used to identify tidal processes within the channels and associated deposits include (1) fluid mud layers in the base of subtidal channels; (2) bidirectional palaeoflow indicators in cross-strata and ripple laminae; (3) heterolithic rhythmites in ponded or other aggrading subenvironments; (4) repeated tidal bundles with double mud drapes reflecting tidal cycles and tidal current asymmetry; (5) tidal bundling in dunes, showing alternation of thicker, spring–tide foresets with thinner, neap tide foresets over fortnightly periods; and (6) flaser (frequent mud drapes), wavy, lenticular, and "pin-stripe" bedding. The deltaic distributary channels show a greater (up to 20 m) channel thickness but are less sand rich (55%) and finer grained (mainly upper very fine sand), whereas the slightly shallower estuarine channels (average < 10 m) are more sandy (65% sand) and coarser grained (lower-fine to upper-fine sand) with a high diversity and density of bioturbation. The regressive–transgressive cross-shelf transits (100–150 km) of the Orinoco palaeo-delta, of which these channel deposits are a component, had a duration of some 100 ky showing three characteristic stages: (i) an early river-tide-dominated delta stage with strong progradation during eustatic sea-level fall and enhanced tides due to wide shelf resonance, (ii) a later regressive delta stage with strong storm-wave reworking as the delta lobes approached the outer shelf and shelf edge where open ocean waves dominated, and (iii) a transgressive backfilling estuarine stage that also was tide dominated.
Although three-dimensional bend flow within fluvial meanders is well known and the linkages between the flow field, bend morphodynamics, and resultant floodplain sedimentology well connected, there is limited knowledge on the dynamics of flows in bends that are subject to tidal forcing or tidal influence. This chapter presents measurements of three-dimensional flow around a tightly curved meander bend in the tidally influenced fluvial zone (TIFZ) within the River Severn, UK. Repeat measurements were taken at two hydrological conditions: (i) high river flows and neap tides and (ii) lower fluvial discharge and spring tide. These two cases thus highlighted the end members of the flow forcing distributions experienced at the bend, with the former showing the maximum fluvial influence and the latter the maximum tidal influence on bend flow processes. Results show that during a period of higher river flow at neap tides, there was very little tidal influence experienced at the bend. During the second set of measurements, made during a period of low river flow and at high spring tides, a full flow reversal occurred, indicating that the tidal–fluvial transition had moved landward of the bend. In both cases, the maxima primary flow velocities measured were of a similar magnitude, even though flow was fully reversed during the spring tides. However, the secondary flow velocities increased notably during the flow reversal at spring tide. These flow patterns provide an explanation of the often observed stability of meander bends within the TIFZ, with the location and focus of maximum flow shear migrating around the bend during reversals, likely hindering bar push processes that can drive meander migration processes.
Although they form important resources in many basins, exploitation of petroleum reservoirs that were deposited in the fluvial/tidal transition is challenging because tidal influence during deposition results in pervasive lithologic heterogeneity and compartmentalization. Recognition of tidal influence on deposition from subsurface data (primarily cores, although well logs and seismic data may support interpretation) is the first challenge. Sedimentologic records of Earth–Solar–Lunar orbital cycles, such as tidal rhythmites, are definitive, but rarely preserved. More common are apparent paleocurrent bidirectionality and an abundance of mud drapes, which are indicative of tide influence, but not definitively diagnostic. Reservoirs deposited in the fluvial/tidal transition occur in two broad stratigraphic settings: (1) incised valley fills (IVFs) that include updip tidally-influenced point bars, bay-head deltas, mid-estuarine tidal bars, and tidal flat/channel fills and (2) tidal deltas that have a wide range of potential reservoirs including distributary channel fills, mouth bars, and delta front deposits, which may be reworked into tidal bars. Predictive models for the occurrence of tidal facies are much generalized at present, although some broad trends in reservoir presence, stratal geometry, and quality can be gleaned from a comparison of modern and subsurface case studies, and viewing the tidal facies within the context of their sequence stratigraphic architecture. Preliminary analysis from the Niger Delta suggests that the coastal lowstand sequence sets are far more likely to have tidally-influenced reservoirs than the highstand sequence sets. The lowstand reservoirs include both IVFs and shelf edge deltas.
An outcrop of the McMurray Formation along the Christina River (Alberta, Canada) has been investigated to better understand depositional processes and setting. The succession is formed by large-scale tabular sets of unidirectional trough cross-stratification. Many of these sets are characterized by profusely ripple-laminated and thick, laterally persistent bottomset intervals at their base. Additionally, reactivation surfaces and infrequent set climbers occur in the foresets. The bottomsets almost entirely consist of backflow cross-lamination. Available knowledge indicates that this points to a rather strong vortex circulation and related strong and persistent main flow velocity. The observed bottomset succession is discussed within the range of variation in bottomset architecture that results from the structure and strength of the flow in the wake behind dunes and related strength of the main flow. Sets descend along a gentle slope, suggesting that dunes filled a preexisting depression, thus representing conditions of a vertically expanding and decelerating flow. This means that aggradation rate was high, which is in accordance with the thickness of the preserved sets. Systematic changes in flow strength are documented by downstream cyclic variations in organic debris, bottomset thickness, and foreset dip. The periodic increase of flow velocity is interpreted as being produced by the increased strength of the river flow during the ebbing tide on the days around spring tide. Apart from these subtle variations, the area experienced large changes in flow strength due to seasonal differences in fluvial discharge. The turbidity maximum zone was located downstream of the study site since thick slackwater mud drapes that characterize the seaward part of the fluvial to tidal transition zone are not present; only a few thin mud drapes are found at the study locality. Therefore, it is concluded that deposition took place in the most landward part of this zone. This new interpretation of this facies in the Christina River area is in line with the inferred depositional setting of the transition to the overlying thick point bar units formed by inclined heterolithic stratification.
The sedimentary facies and architecture of Late Campanian fluvial–tidal channels and channel belts of the Neslen Formation are described from the Floy area of the upper Book Cliffs in northeastern Utah using outcrop data. Vertical sedimentary sections in multiple closely spaced canyons, combined with photomosaics and LIDAR data, describe in detail the deposits over an area of about 3 km by 2 km. The tidal–fluvial channels and channel belts fed sediments to embayments with bay-head deltas and to shorelines to the east. The Neslen distributary system (known to deposit diachronously, younger deposits towards east) fed the mixed energy (tidal and wave) shorelines of the Sego, Corcoran, and Cozette systems of eastern Utah and western Colorado. Above the inclined heterolithic strata (IHS) units of the Neslen Formation, there are more proximal alluvial plain deposits of the Blue Castle Tongue Sandstone, so that the succession is overall progradational. Focus is on the up to 8-m-thick channel units of IHS of Neslen Formation, that stack in successions with four to five stories and are interpreted as tidally influenced fluvial channel belts. The channel interpretation is supported by the presence of sharp erosional boundaries between the IHS units and overall “lenticular” geometry of these units. The tidal interpretation is based on facies characteristics such as bioturbation (Skolithos, Palaeophycus, Teichichnus, Teredolites), tidal rhythmites, occasional bidirectional paleocurrents, mud drapes, and examples of ripple and dune cross-strata that climb high upward on the point-bar accretion surfaces. The architecture and facies of the Neslen fluvial–tidal point bars show some specific characteristics that are different from previously described IHS strata such as a (1) dominance of dm-thick rippled sandstone beds within the heterolithic strata throughout the channel-fill deposits (little vertical or lateral variability), (2) no consistent trend of upward fining or coarsening from bottom to top of the point-bar units, and (3) lack of any channel abandonment “mud plug” deposits. The resulting model for Neslen fluvial–tidal channels is an erosive basal boundary with a “circular” or “draping” fill of the channels under similar flow conditions from bottom to top. Despite almost continuous exposure, correlation of the individual IHS units over a distance > 2–3 km is difficult because of multiple incisions and change in the character (thickness variability of sandstone beds) of individual IHS units.
Three economically important case histories serve as illustrations of the integration of analyses of depositional environments, sequence stratigraphic architecture, and porosity evolution during diagenesis, as a means of maximizing effectiveness of reservoir production and/or modelling: (1) the Paleozoic Madison Formation of central Wyoming, (2) the Upper Jurassic Smackover Formation of the central Gulf of Mexico, and (3) the Tertiary Malampaya buildup, offshore Philippines. The three embody a broad range of geologic contexts (e.g., icehouse versus greenhouse during deposition) and different approaches for optimizing development programs (e.g., use of surface analogs, 3D seismically based reservoir modelling). High drilling costs during development of the deep (23,000 ft.) Madden Field in the Wyoming Madison Formation (due to high temperature, pressure, and H2S content of the gas) mandated high efficiency during development. Meticulous evaluation of a surface outcrop analog and maximized collection of analog data were the primary means of assuring optimal reservoir development. The Upper Jurassic Smackover trend in the central Gulf of Mexico illustrates revitalization of a mature petroleum fairway through application of sequence stratigraphic interpretation. Previously overlooked lowstand siliciclastic slope fans become geographically and stratigraphically predictable reservoir targets when understood in their proper sequence stratigraphic framework. The 3D seismic grid over the drowned isolated Oligocene–Miocene Malampaya platform, offshore Philippines, is integrated with geologic and petrophysical data from sparse well control and field-wide depositional and diagenetic models in order to develop a reservoir simulation model of the reservoir.
Cover beds are usually not regarded of use for relative dating. The examples discussed in this chapter demonstrate otherwise. Central European cover beds usually are of Pleistocene age, and they can be utilized for distinguishing older landforms, such as slope failures, covered by one or more cover beds, from those which are not covered by periglacial deposits. In the western USA, where intervening soil-forming episodes provide a stratigraphic framework for such deposits, the stratigraphic value of cover-bed and soil successions is tested on various types of landforms. However, dating landforms relatively by overlying cover beds calls for due consideration of erosion-induced hiatuses and of tectonically induced processes out of phase with those driven by climate.
Slope deposits, which veil entire slopes or large parts of them in a rather uniform manner (cover beds), are ubiquitous in the subdued mountains of Central Europe. Here we provide an overview of the current state of knowledge on these deposits. The Central European cover beds are divided into (1) the upper layer that is ubiquitously distributed and displays a relatively constant thickness; (2) the intermediate layer the distribution of which is mainly restricted to flat relief, to slope depressions, and to lee-ward facing slopes; and (3) the basal layer, which is rather widespread again. Both the upper and intermediate layer contain intermixed loess, whereas the basal layer is free of loess and typically has a high bulk density. Aside from the loess content, the composition of the layers differs, reflecting varying portions of crushed and chemically weathered rock allocated from up-slope. This causes notable diversity depending on bedrock and, thus, induces remarkable regional differences. Cover beds were mainly formed by periglacial gelifluction. The upper layer formed in the Late Glacial possibly during several short episodes of activity. In contrast, the underlying layers may be diachronous; nevertheless, they display recurring vertical sequences. This is probably due to the fact that loess-free layers usually could not deposit as long as there was loess in the environs, which may have been inherited from older deposits. Thus, the last phase of surface wash, during which older loess was removed, determines the age of the lower layers.
Slope deposits, which veil entire slopes or large parts of them in a rather uniform manner (cover beds), are ubiquitous in the subdued mountains of Central Europe (e.g., Kleber, 1992a). Here, we show that successions of cover beds are not restricted to this area but occur in many other regions of rather different natural inventories, such as the European Alps, the Russian Plain, south-central Turkey, and the western USA (Great Basin and Rocky Mountains). Cover beds usually form sequences of two or more distinct layers, and their distribution depends on the geomorphic, climate-driven processes of their formation. As they influence pedogenesis, they contribute to the understanding of soil properties and soil distribution: horizon boundaries occur at depths where cover-bed properties change. The properties of the layers and of the soils developed in them are different per region: in humid areas layers free of admixed loess components, thus being solely influenced by weathered local materials, are frequent, whereas in dry regions such layers have not yet been reported. In several areas studied in this chapter, paleosols either occur within cover-bed successions or have been reallocated and incorporated into the cover beds. This provides handles to the ages of layers. The layer successions slowly change with elevation but show a drastic break at around the timberline where Holocene rather than Pleistocene periglacial slope processes gained supremacy.
