Fluvial-tidal Transition Research Articles

Fluvial character and architecture of an outcrop using sedimentology combined with UAV-based modeling, Cretaceous McMurray Formation, NE Alberta, Canada

ABSTRACT It is widely accepted that most occurrences of inclined heterolithic stratification (IHS) in the rock record form by laterally accreting point bars in freshwater fluvial, tidally influenced fluvial, or tidally dominated estuary channels. Despite the widespread distribution of IHS in the subsurface and outcropping strata of the lower Cretaceous McMurray Formation, the large-scale depositional architecture and lateral facies variability of these deposits remains controversial. The relatively limited lateral extent of many of the outcrops is a challenge, particularly when point-bar deposits on the scale of hundreds of meters to kilometers are interpreted in outcrops spanning anywhere from 100 to 300 meters laterally. This has in turn led researchers to leverage other datasets such as 3-D seismic to analyze the large-scale depositional architecture of the IHS, leading to two main interpretations for the IHS in the McMurray Formation: 1) a fluvially dominated environment owing to geomorphological features comparable to those in large modern fluvial systems, or 2) an estuarine environment owing to the presence of trace fossils characteristic of marine-derived faunal colonization in brackish-water settings and strong evidence of tidal modulation. The purpose of this study is to investigate the sedimentology and depositional architecture of IHS in a unique, kilometer-wide outcrop exposure of McMurray Formation strata and compare it to IHS observed at other McMurray Formation outcrops previously interpreted as estuarine channels. This is achieved by combining traditional field-based methods with Unmanned Aerial Vehicle-based outcrop modeling to create a 3-D outcrop model to visualize and analyze large point-bar geobodies deposited in a channel upwards of 25 meters deep and 750 meters wide exposed in outcrop at Crooked Rapids of the Athabasca River, west of the City of Fort McMurray. Importantly, this methodology uses bed orientation trends, paleocurrent data, and sedimentological observations to identify and map architectural elements, which constitute an eastward-accreting point bar crosscut by a southwestward-accreting counter point bar in the outcrop. The results strongly suggest that the IHS at Crooked Rapids was deposited in a freshwater fluvial environment. When compared to IHS deposited in estuarine depositional environments, fluvial IHS is driven by seasonal river discharge as opposed to the interplay between river discharge and the extent of the tidal prism. Therefore, fluvial IHS is: 1) dominantly sandstone with very minor waning-flow siltstone interbeds resulting from erosion by the succeeding freshet phase, and 2) completely devoid of bioturbation until flat-lying bar top or overbank strata is encountered. Using 3-D outcrop modeling to supplement sedimentological and ichnological observations strengthens the interpretation of complex fluvial geobodies and increases the overall understanding of the large-scale depositional architecture of point bars across the tidal–fluvial transition zone.

Sedimentology and stratigraphic evolution of fluvial–tidal transition reservoirs: an outcrop analog for the hydrocarbon-bearing Bahariya Formation, Western Desert, Egypt

ABSTRACT The deposits of the tidal–fluvial transition zone are one of the most significant and complicated components of marginal marine systems. Sedimentological studies of these deposits are necessary due to their heterogeneous nature, which is controlled by competing tidal and fluvial parameters. Outcrop studies are required to understand the architecture, sedimentology, and evolution of tidal–fluvial deposits. The Cenomanian upper unit of the Bahariya Formation in the northern part of the Western Desert of Egypt is a tide-dominated fluvio-estuarine deposit sourced from crystalline basement and Early Cretaceous siliciclastic sedimentary rocks that lie to the southeast and south. Based on sedimentary facies analysis and paleocurrent data, the upper Bahariya Formation is composed of six main architectural elements: 1) river-dominated, tide-influenced point bar, 2) tide-dominated, river-influenced point bar, 3) floodplain, 4) crevasse splay, 5) crevasse channel, and 6) mud plug. These elements are stacked in a multistory tidal–fluvial channel complex and associated depositional elements. The reconstructed paleochannels trend from southeast to northwest, and migrated to the east and southeast. The relative contribution of fluvial processes decreased upwards through the stacked stories, with a corresponding increase in the contribution of tidal processes that were associated with transgression. An understanding of the architecture and sedimentology of the tidal–fluvial transition from outcrop successions allows the improved characterization of tidal–fluvial point-bar reservoirs and associated elements.

Tidal and seasonal controls on the stratigraphic architecture of blind tidal channel deposits in the fluvial-tidal transition of the macrotidal Sittaung River estuary, Myanmar

The morphodynamics and sedimentology of tide-dominated estuaries are controlled by the interaction of tidal currents and river processes. Decoupling tidal and river processes is challenging and rarely documented for the distal part of the estuary, where river influences are cryptic due to the large tidal prism. Dendritic blind tidal channels drain the inshore tidal flats and saltmarshes in the distal part of the fluvial-tidal transition zone (FTZ) of the macrotidal Sittaung River estuary, Myanmar. The present study details the sedimentology and stratigraphic architecture of point bars of the blind tidal channels to evaluate the relative importance of tidal and seasonal river processes in the distal part of the FTZ. The point bars consist of silty tidal rhythmites that display hierarchical tidal cycles manifested in the layer thicknesses. The rhythmites formed during the dry season are slightly muddier and more thinly laminated with accentuated neap-spring tidal cycles, whereas those formed during the wet season exhibit anomalously thick layers with indistinct tidal cycles and abundant subcritical climbing ripples. The wet-season rhythmites are commonly associated with bank-normal, high-relief truncation surfaces presumably formed by heavy rainfall at low tides during river floods. Chevron-like upbuilding of the tidal rhythmites overlies the surfaces in the upper part of tidal point bars, attesting to the rapid removal of accommodation space by sedimentation during the river floods with sustained high sediment loads. A time series of digital elevation models exhibited a rapid infill of the blind tidal channels, suggesting high sediment supply onto the tidal flats throughout the year. Despite being located in the seaward end of the FTZ of the estuary, where tidal amplification occurs, the blind tidal channels evolve in response to seasonal river floods as well as tidal currents, accounting for the dynamic geomorphic changes of the inshore tidal flats and saltmarshes in the Sittaung River estuary.

Erosional and Tectonic Evolution of a Retroarc Orogenic Wedge as Revealed by Sedimentary Provenance: Case of the Oligocene – Miocene Patagonian Andes

Sedimentary provenance techniques have been widely applied in foreland basin settings to understand tectonic and magmatic processes by tracking the exposure and erosion of distinct sediment source areas through time. We present a case example from the Magallanes-Austral retroarc foreland basin of Chile and Argentina (51°30’S), where modal sandstone and conglomerate compositional data, detrital zircon U-Pb geochronology, and sedimentology data from the Oligocene-Miocene Rio Guillermo Formation document a change in source areas during an important stage of orogenic development. In particular, our results from the ~24.3-21.7 Ma Rio Guillermo Formation record an abrupt shift from transitional to undissected arc provenance that indicate rejuvenated magmatism within the contemporary arc. Minor components of lithic grains suggest a subordinate source of recycled sediments that we interpret may have been derived from the intervening external fold-and-thrust belt, rather than directly from sources in the hinterland thrust domain. Detrital zircon U-Pb geochronology data show mostly Neogene (~20-40 Ma) and Cretaceous (~70-110 Ma) age groups, with minor amounts of Jurassic (~145-155 Ma) and Paleozoic (~260-540 Ma) age groups, which are consistent with a syndepositional arc and recycled external fold-and-thrust belt sources. Stratigraphic data suggest a vegetated, channelized braidplain environment developed above an erosional unconformity with the underlying shallow-marine Rio Turbio Formation. Upsection, the Rio Guillermo Formation locally transitions to a low-energy, organic-rich floodplain setting located within the upper reaches of a fluvial-tidal transition zone of the coastal plain, and the uppermost part of the formation is characterized by a coarse-grained sandy channelized braidplain environment along the foreland basin margin. Moderate sediment accumulation rates and coastal plain progradation during this period is consistent with sustained sediment flux from the Patagonian Andes and tectonic subsidence along the basin margin. Taken collectively, we propose that the abrupt provenance shift dominantly records erosion of the rejuvenated mafic volcanic arc, despite coeval changes in orogenic wedge dynamics brought about by increased plate convergence rates that drove uplift of the intervening external-fold-and-thrust belt along reactivated deep-seated high-angle basin structures.

Using a modern analogue to interpret depositional position in ancient fluvial-tidal channels: Example from the McMurray Formation, Canada

The fluvial–tidal transition (FTT) is a complex depositional zone, where fluvial flow is modified by tides as rivers approach a receiving marine basin. Variations in the relative importance of tidal versus fluvial processes lead to a distinctive distribution of sediments that accumulate on channel bars. The FTT generally consists of three broad zones: (1) a freshwater-tidal zone; (2) a tidally influenced freshwater to brackish-water transition; and (3) a zone of relatively sustained brackish-water conditions with stronger tides. A very common type of deposit through the fluvial–tidal transition, especially on the margins of migrating channels, is inclined heterolithic stratification (IHS). At present, a detailed account of changes in the character of IHS across the FTT of a paleo-channel system has not been reported, although a number of modern examples have been documented. To fill this gap, we quantitatively assess the sedimentology and ichnology of IHS from seven cored intervals in three geographic areas situated within the youngest paleovalley (“A” Valley) in the Lower Cretaceous McMurray Formation of Alberta, Canada. We compare the data to trends defined along the FTT in the present-day Fraser River in British Columbia, Canada to interpret paleo-depositional position in the ancient fluvial–tidal channels.Analysis determined that the mean mudstone thickness is 8.2 cm in the southern study area (SA). Mean thickness increases to 11 cm in the central study area (CA), and decreases again to 4.4 cm in the northern study area (NA). The proportion of mudstone is 31% in SA, 44% in CA, and 27% in NA. Thickness-weighted mean bioturbation intensity in sands varied from 0.29 in SA and CA, to 0.28 in NA. On the other hand, thickness-weighted mean bioturbation intensity (BI) in mudstone increases from 1.46 in SA, to 1.77 in CA, and is 1.94 in NA. The ichnological diversity also increased from south to north.Sedimentological results show similar trends to those of the Fraser River, enabling the identification of a freshwater to brackish-water transition zone with tidal influence. The interpreted position of the transition is underpinned by the bioturbation intensity and trace-fossil diversity trends, indicating periodic brackish-water conditions throughout SA in the McMurray Formation during low river flow conditions. Together, these data suggest that a broad FTT existed in the “A” Valley, with fluvial-dominated channels to the south that experienced seasonal brackish-water inundation during base flow, and channels experiencing increasing brackish-water influence lying further north towards a turbidity maximum zone. The FTT zone appears to have extended for several hundred kilometers from south to north.Based on the sedimentological and ichnological data, as well as estimations of lateral accretion rates, we refute the commonly applied Mississippi River depositional analogue for McMurray Formation channels. Rather, we show that while not a perfect fit, the tidally influenced Fraser River shows much greater agreement with the depositional character recorded in McMurray Formation IHS. Future work on the McMurray system should focus on characterizing tide-dominated deltaic and estuarine systems, such as the Ganges-Brahmaputra, and on forward-modeling the evolution of tide-dominated and tide-influenced river systems.

Morphological effects of vegetation on the tidal–fluvial transition in Holocene estuaries

Abstract. Vegetation enhances bank stability and sedimentation to such an extent that it can modify river patterns, but how these processes manifest themselves in full-scale estuarine settings is poorly understood. On the one hand, tidal flats accrete faster in the presence of vegetation, reducing the flood storage and ebb dominance over time. On the other hand flow-focusing effects of a tidal floodplain elevated by mud and vegetation could lead to channel concentration and incision. Here we study isolated and combined effects of mud and tidal marsh vegetation on estuary dimensions. A 2-D hydromorphodynamic estuary model was developed, which was coupled to a vegetation model and used to simulate 100 years of morphological development. Vegetation settlement, growth and mortality were determined by the hydromorphodynamics. Eco-engineering effects of vegetation on the physical system are here limited to hydraulic resistance, which affects erosion and sedimentation pattern through the flow field. We investigated how vegetation, combined with mud, affects the average elevation of tidal flats and controls the system-scale planform. Modelling with vegetation only results in a pattern with the largest vegetation extent in the mixed-energy zone of the estuary, which is generally shallower. Here vegetation can cover more than 50 % of the estuary width while it remains below 10 %–20 % in the outer, tide-dominated zone. This modelled distribution of vegetation along the estuary shows general agreement with trends in natural estuaries observed by aerial image analysis. Without mud, the modelled vegetation has a limited effect on morphology, again peaking in the mixed-energy zone. Numerical modelling with mud only shows that the presence of mud leads to stabilisation and accretion of the intertidal area and a slight infill of the mixed-energy zone. Combined modelling of mud and vegetation leads to mutual enhancement with mud causing new colonisation areas and vegetation stabilising the mud. This occurs in particular in a zone previously described as the bedload convergence zone. While vegetation focusses the flow into the channels such that mud sedimentation in intertidal side channels is prevented on a timescale of decades, the filling of intertidal area and the resulting reduction in tidal prism may cause the infilling of estuaries over centuries.

Dimensions of fluvial-tidal meanders: Are they disproportionally large?

Many of the world’s major river systems seemingly have one or a few disproportionally large meanders, with tight bends, in the fluvial-tidal transition (e.g., the Thames in the UK, and the Salmon River in Canada). However, quantitative studies on meanders have so far primarily focused on rivers without tidal influence or on small tidal meanders without river inflow, providing relations between channel geometry and meander characteristics (length, amplitude, and sinuosity). Physics-based predictions of meander size and shape for the fluvial-tidal transition zone remain untested for a lack of data. Therefore, it remains unclear whether the dimensions of meanders in the fluvial-tidal transition zone are indeed disproportionally large, and whether meander characteristics can be used as an indicator for tidal influence. Here, data from 823 meanders in 68 fluvial-tidal transition zones worldwide are presented that reveal broad-brush relations between channel geometry and meander dimensions. Our results show that fluvial-tidal meanders indeed become larger in the seaward direction, but the dimensions are proportional to local channel width, as in rivers. Sinuosity maxima are an exception, rather than the rule, in the fluvial-tidal transition zone. Surprisingly, the width of the upstream river correlates with estuarine channel width and tidal meander size even though river discharge constitutes only a fraction of the tidal prism. The new scaling relations can be used to constrain dimensions of rivers and estuaries and their meanders.

Integrated sedimentology and ichnology of Late Jurassic fluvial point‐bars – facies architecture and colonization styles (Lourinhã Formation, Lusitanian Basin, western Portugal)

AbstractIntegrated sedimentological and ichnological case studies of ancient meandering river systems have, for the most part, focused on the deposits of the fluvial–tidal transition zone; much less emphasis has been placed on the purely fluvial realm above the landward limit of tidal effects. This problem needs to be addressed so that in future the defining sedimentological and ichnological criteria assigned to ancient fluvial reaches are sufficiently well‐established to enable their separation from the down‐dip fluvial–tidal transition zone. Accordingly, a case study has been carried out on a well‐exposed meander belt deposit from the Late Jurassic Lourinhã Formation of the Lusitanian Basin, western Portugal (Praia Do Valmitão, Ribamar). Analysis indicates that the meander belt here comprised mixed‐load fluvial channels traversing a vegetated floodplain subject to a seasonal winter wet/summer dry palaeoclimate. This setting facilitated the development of both calcic palaeosols and shallow lakes on the adjacent floodplain. Critically, there is no evidence of the effects of tidal modulation on bedding structures, thereby establishing purely fluvial conditions. Heterolithic point‐bar deposits generated in this setting are bioturbated extensively by a trace fossil assemblage dominated by the meniscate traceTaenidium barretti, withSkolithos linearis,Planolites beverleyensisandCylindricumisp. also recognized. A number of factors suggest that theTaenidium barrettiproducer in this case was a subaquatic organism living in an active fluvial channel setting (i.e. not colonizing subaerially exposed channel‐margin/floodplain deposits). Accordingly, there are some implications for current ichnofabric/ichnofacies models in the continental realm. Firstly,Taenidium‐dominated ichnofabrics need not necessarily be confined to colonization beneath subaerially exposed surfaces; they might also be produced within submerged substrates. Secondly, there is scope to extend the range of theScoyeniaichnofacies to include active fluvial channels and not simply those channels that were inactive or abandoned.

Channel geomorphology along the fluvial-tidal transition, Santee River, USA

Channel geomorphology along the fluvial-tidal transition, Santee River, USA

Recognizing seasonal fluvial influence in ancient tidal deposits

Abstract Seasonal changes in rainfall affect water and sediment discharges in fluvial systems. This is reflected within the rock record by changes in (1) the flow processes and structures created and (2) the composition and volume of sediment. This study demonstrates that high-resolution sedimentological studies of rhythmites in ancient tidal bar deposits reveal a seasonal fluvial influence. The rocks studied belong to the Plio-Pleistocene Erin Formation on Trinidad, deposited under tropical seasonal conditions in the fluvial–tidal transition zone and tidally dominated palaeoenvironments. Time-series and stratigraphic analyses were undertaken of two rock subsections separated by an erosional surface. Spring–neap, monthly and semi-annual cycles were identified within the deposits. Sand-rich and mud-rich couplets showed neap–spring cyclicity, with gradual transitions between flaser, wavy and lenticular bedding. Seasonal changes in fluvial discharge were recognized by minor scours that marked periods of relatively higher discharge. The content and distribution of organic material varied through the seasonally influenced cycle. Rare and dwarfed trace fossils and bioturbated beds also showed a periodic pattern, interpreted as representing changes in palaeoenvironmental conditions due to seasonal freshwater discharge. As a result, reservoirs containing similar deposits may have porosity and permeability variations with seasonal depositional periodicity. Supplementary material: The full set of measured time-series data is available at https://doi.org/10.6084/m9.figshare.c.3283376

Ichnofauna from coastal meandering channel systems (Upper Cretaceous Tremp Formation, South-Central Pyrenees, Spain): delineating the fluvial-tidal transition

AbstractThe Upper Cretaceous “redbeds” of the lower Tremp Formation (South-Central Pyrenees, Spain) contains an ichnofauna consisting ofTaenidium barretti,Taenidium bowni,Loloichnusisp.,Arenicolitesisp.,Planolitesisp., andPalaeophycusisp. This ichnofauna occurs in deposits formed in tide-influenced meander loops and their associated overbank mudflats. Evaluation of the taphonomic controls on the Tremp ichnofauna shows that (1) two morphotypes ofTaenidium barrettiare controlled by the substrate consistence, (2)Arenicolitesmay be enlarged by erosion processes, and (3)Taenidium barrettiandPlanolitesisp. are not the same ichnotaxa showing different types of preservation. The meniscate fill inTaenidium barrettisuggests that this structure was produced by deposit feeders. The Tremp ichnofauna is grouped into two trace-fossil assemblages, a depauperate subaquatic monospecificPlanolitessuite and an assemblage representing theScoyeniaIchnofacies. Trace-fossil distribution reflects paleoenvironmental changes in the meandering channels along the stratigraphic section with thePlanolitessuite in the lowermost part of the lower interval and theScoyeniaIchnofacies in the middle and upper intervals. The lowermost suite may be likely formed seaward of the maximum salinity limit, under extreme brackish-water conditions, whereas theScoyeniaIchnofacies records a freshwater assemblage that was formed landward of the maximum salinity limit, reflecting deltaic progradation.

A Synthesis of Depositional Trends In Intertidal and Upper Subtidal Sediments Across the Tidal–Fluvial Transition In the Fraser River, Canada

Abstract Sedimentological, neoichnological, palynological, and geochemical trends from upper subtidal and intertidal positions on channel bars in the lower Fraser River, Canada are synthesized into a single, coherent framework. From these data we define criteria for determining depositional position in shallow water depths in tide-influenced rivers. Three sedimentological trends are observed from the river mouth, through the locus of mud deposition (within the turbidity maximum zone (TMZ)), and into the freshwater-tidal zone. (1) The recurrence (per meter) and thickness of mud beds increase towards the TMZ and tapers in both the landward and seaward directions. (2) Muddy current ripples and graded current ripples are most abundant in the TMZ; they are less common with decreasing brackish-water influence and are absent in the freshwater river reach. (3) Heterolithic bedding (i.e., flaser, wavy, and lenticular) is common in the TMZ, less common seaward, and absent from the freshwater realm. In addition to the sedimentological trends, four ichnological trends parallel decreasing water salinity. With decreasing salinity, there is: (1) a decrease in bioturbation intensity from BI 2–3 to BI 0–1; (2) a decrease in the abundance of bioturbated beds; (3) a marked decrease in the diversity of traces from 5–6 forms to 1–2 forms; and, (4) a decrease in the diameter and length of traces. Traces are rare to absent in the tidal freshwater zone. Palynological and geochemical trends generally follow ichnological trends but are less obvious. Neither dinocyst abundance nor geochemical signature can be used to determine relative position in a tide-influenced river channel, although dinocyst abundances greater than 1% indicate significant marine influence, and 0–1% marine dinocysts indicate tidal influence. Although it is not feasible to determine exact depositional position within the tidal–fluvial transition, our results suggest that it is possible to determine where sediments were deposited relative to the TMZ. In turn, predicting relative depositional position can assist in unraveling stratigraphy and in recognizing nested channels in architecturally complex sedimentary successions.

New Discoveries of Diplocraterion and Tidal Rhythmites in the Upper Devonian Rocks of Grab-all Bay, Cork Harbour: Palaeoenvironmental Implications

Geological mapping of the Upper Devonian succession on the northern side of Grab-all Bay in Cork Harbour has led to a new interpretation of the local lithostratigraphy. Rocks previously mapped as the Ballyknock Member of the Gyleen Formation are now reassigned to the Toe Head Sandstone Formation which is here divided into two new members, the Rams Head and Grab-all Bay Members and to the Old Head Sandstone Formation. The axis of the Grab-all Bay Syncline is re-positioned 160m further to the north in Grab-all Bay, revealing a younger and previously undescribed interval of strata here assigned to the Old Head Sandstone Formation. Miospore assemblages recorded from the Toe Head Sandstone and the lower part of the Old Head Sandstone Formation are assigned to the Retispora lepidophyta—Knoxisporites literatus (LL) Miospore Biozone, indicating a late Devonian (Uppermost Famennian) age for the succession. New discoveries of U-shaped burrows, assignable to the trace fossil Diplocraterion, are recorded from several intervals in the Toe Head Sandstone Formation and the lower part of the Old Head Sandstone Formation. These monospecific ichnofaunas are believed to have colonised finely laminated sediments of the intertidal flat environment. In addition, rhythmically laminated sandstone and siltstone beds interpreted as tidal rhythmites are recorded for the first time in the Toe Head Sandstone Formation. The paleoenvironmental significance of the tidal rhythmites and Diplocraterion beds in the Toe Head Sandstone is important in understanding the nature of the fluvial–tidal transition zone in the Cork Harbour region in late Devonian times. It is proposed that in late Devonian (LL Biozone) times the upper Toe Head Sandstone Formation sediments in the Cork Harbour area were deposited in the upper reaches of an estuarine embayment within a low gradient fluvial coastal plain.

Evidences for a Paleocene marine incursion in southern Amazonia (Madre de Dios Sub-Andean Zone, Peru)

This article presents new biostratigraphic dating, facies analysis, organic geochemical data and Nd–Sr isotopic provenance from five outcrops of southern Amazonia (MD-85, MD-177 MD-184, MD-255 and MD-256) to document for the first time the presence of a shallow marine ingression in the Paleocene of southern Amazonia basin. The co-occurrence of a selachian assemblage encompassing Potobatis sp., Ouledia sp., and Pristidae indet. with the ostracod Protobuntonia sp. and the charophytes Peckichara cf. varians meridionalis, Platychara perlata, and Feistiella cf. gildemeisteri suggests a Paleocene age for the studied deposits (most likely Thanetian but potentially Danian). Fifteen facies have been recognized and have been grouped into three facies assemblages. Facies association A corresponds to the sedimentary filling of a tide-influenced meandering channel formed in the fluvial–tidal transition zone. Facies association B is related to more distal tidal-flats, little channelized tidal inlets and saltmarsh deposits. Facies association C corresponds to a stressed shallow marine environment such as a bay or a lagoon. The δ13CTOC value (−23.4‰) of MD-184 is enriched in 13C compared to the other samples suggesting the presence of substantial amounts of marine organic matter in MD-184. The δ13CTOC values of samples from other outcrops (−27.3 to −29.8‰) indicate a mixed organic matter origin, from terrestrial to brackish environments. The analyzed sediments have similar Nd–Sr isotopic compositions as those of the Cenozoic sediments of the Altiplano (εNd(0) values from −6.2 to −10.7 and 87Sr/86Sr compositions from 0.712024 to 0.719026) indicating a similar volcanic source. This multidisciplinary dataset documents the presence of a tide-dominated estuary sourced by the proto-Western Cordillera debouching into a shallow marine bay during Paleocene times. This transgression might be explained by subsidence created in response to the proto-Western Cordillera loading. Similar to Miocene marine incursions affecting the Pebas megawetland, Paleogene marine incursions in the Amazonian foreland basin associated with Andean uplift may have played a role in the Neotropical biodiversity dynamics in favoring biogeographical isolation and promoting allopatric speciation for terrestrial organisms.

Palynology and geochemistry of channel-margin sediments across the tidal–fluvial transition, lower Fraser River, Canada: Implications for the rock record

In the tidally influenced Fraser River, Canada, palynological and carbon isotope (δ13Corg) signatures of channel-margin sediments are compared to environmental parameters (e.g., grain size, water salinity) to establish how the signatures vary across the tidal–fluvial transition. Palynological assemblages in the Fraser River are dominated by tree pollen, which constitutes between 85% and 95% of all assemblages. Dinocyst abundances do not exceed 2% of the total palynological assemblage, and the number and diversity of dinocysts gradually decreases landward. The calculated landward limit for dinocysts is at approximately 83 river km, which is relatively close to the upstream limit of the tidal backwater (at ∼100 km). δ13Corg values show minimal variability across the tidal–fluvial transition, and the average value is approximately −26‰. The δ13Corg signature of river sediments indicates a dominance of terrestrially sourced organic matter regardless of brackish-water and tidal influence on sediment deposition.Six palynological and geochemical trends are identified as relevant to the rock record. 1) In deltaic environments, palynological and geochemical characteristics are less useful than sedimentological and ichnological characteristics for establishing depositional conditions. 2) In marginal-marine settings, low abundances and low species diversities of dinocysts, coupled with a “terrestrial” geochemical signature (δ13Corg < −25‰) do not necessarily indicate deposition in a terrestrial environment. 3) Dinocyst abundances above 1% of the total palynomorph population can indicate a significant marine influence on sediment deposition. 4) Mud beds, preferably bioturbated, should be preferentially sampled in order to maximize palynomorph recovery. 5) Marine palynomorphs can occur, albeit in very low concentrations, to the landward limit of the tidal–backwater zone. 6) Palynological and geochemical data should be compared across the paleo-depositional environment in order to establish general trends and remove local variations caused by biases such as grain size.

Sedimentation Across the Tidal–Fluvial Transition in the Lower Fraser River, Canada

Sedimentation Across the Tidal–Fluvial Transition in the Lower Fraser River, Canada

Eemian Rhine delta architecture in The Netherlands: facies distribution, deposit characteristics and preservation potential in a near-coastal deltaic setting

Within near-coastal environments, the fluvial-tidal transition zone is one of the most complex zones due to mixture of processes and sediments of different source and depositional styles. Despite a large number of excellent Holocene fluvial-estuarine cases, transferring sedimentary concepts into the fluvial-estuarine palaeo-records remains a major challenge...

Tidal rhythmites in a mixed-energy, macrotidal estuarine channel, Gomso Bay, west coast of Korea

Well-developed tidal rhythmites are documented from the intertidal point bar of sinuous tidal channel in Gomso Bay, west coast of Korea. Associated with inclined heterolithic stratification, rhythmites constitute the upper part of fining-upward succession formed by the lateral migration of a point bar. Occurring between mean sea-level and mean high water level, rhythmites commonly overlie rhythmic climbing-ripple cross-laminations. Various rhythmicities are registered in laminae thicknesses, which reflect hierarchical tidal cycles ranging from semidiurnal to monthly tidal cycles. Inferred neap–spring tidal cycle (half-synodic) thins upwardly with a proportionately reduced number of preserved laminae in the cycle. Such vertical organization of rhythmites seems to reflect decreased accommodation space through deposition. Tidal rhythmites exhibit longitudinal variation within fluvial–tidal transition with best developed ones formed in a highly meandering tidal channel, where the influence of offshore waves is minimal due to confined channel morphology that is characterized by low energy level in association with high water turbidity. Comparison of spectral peaks between modern and simulated rhythmites further substantiates that tidal rhythmites can serve as a reliable indicator of paleotide, paleoelevation and paleogeography of mixed energy, tide-dominated estuaries.

Geomorphic Influence on Flood Hazards in a Lowland Fluvial-Tidal Transitional Area, Central Valley, California

Geomorphic processes influence flood hazards in the lowland fluvial-tidal transition area near the Cosumnes River-Dry Creek-Mokelumne River Confluence, Central Valley, California. Anthropogenic changes in the inherently flood-prone floodplain, flood basin, and Delta Island landscape increase flood hazards. Levee construction and flow regulation influence floods, increasing risks in both agricultural and rapidly urbanizing areas. Effective flood management requires a landscape scale approach that addresses both fluvial and tidal processes and that accommodates current and potentially higher future flood magnitudes and sea levels.

Diagnostic sedimentary structures of the fluvial-tidal transition zone – Evidence from deposits of the Rhine and Meuse

AbstractIn mesotidal settings the transition of a coastal plain estuary to the river is marked by the change of a multiple ebb and flood channel configuration to a single channel system. At high river discharge fluvial processes operate, whereas in periods of low discharge the flow is complicated by a tidal component and a landward intrusion of the salt wedge. These hydraulic and morphological characteristics make the transitional zone different from the ‘pure’ fluvial and estuarine environment. Inspection of published and unpublished data from a number of outcrops of Recent and Tertiary deposits of the Rhine reveals that also in a sedimentary sense a transitional zone can be recognized. In order to separate this zone from the upstream fluvial and downstream estuarine environment a sedimentological definition of the fluvial-tidal zone is proposed being the part of river that lies between the landward limit of observable effects of tidal-induced flow deceleration on fluvial cross-bedding at low river discharge and the most seaward occurrence of a textural or structural fluvial signature related to the high river stage.