Tidal Point Bars Research Articles

Paleogeographic reconstruction and sedimentary evolution of tidal-dominated estuarine depositional systems: Insights from the campanian M1 sandstone formation, Oriente Basin, Ecuador

The Oriente Basin is a back-arc basin located east of the Ecuadorian Andes in South America. Evidence suggests that during the Cretaceous period, the basin's margins received a terrigenous shallow marine sedimentation. Due to regional variations in accommodation and depositional controlling factors near a shoreline, the sedimentary environments in the fluvial-marine transitional zones exhibit considerable variability. In this study, we analyzed the sedimentological characteristics of the M1 Sandstone Formation unit (∼83 Ma-72 Ma), which comprises a set of ancient estuarine deposits preserved in the northern part of the basin. We quantified the impact of mixed hydrodynamic processes on the distribution of tidal-dominated estuarine facies and discussed the development conditions and evolutionary processes of an estuarine formation under the background of persistent transgression. Core, well log, and seismic data provide evidence for reconstructing the tidal-dominated estuarine environment. In the core section, we identified 14 lithofacies and 7 major facies association types. The frequent occurrence of sedimentary structures associated with tidal currents indicates that tidal processes extensively reworked the deposits. The spatial distribution trends of facies associations reveal the evolution of mixed hydrodynamic conditions dominated by tidal processes in the estuary and allow us to divide the M1 Sandstone Formation into four depositional periods: the initial development, rapid development, decline, and the post-infilling open coast tidal flat development periods. Furthermore, through a systematic analysis of hydrodynamic conditions and sediment distributions, we identified multiple depositional centers in the tidal-dominated estuary influenced by different hydrodynamic processes. These correspond to the upstream fluvial-dominated tidal point bar depositional region, the middle mixed-energy depositional interaction zone, and the downstream tidal-dominated tidal sand bar depositional region. In these regions, the substantial accumulation of sandy deposits, represented by tidal sand bars, reflects the high accommodation space characteristic of tidal-dominated estuaries. Our findings indicate that the slow subsidence of a broad and gently sloping coastal topography, stable material transport, and sea level change dominated by marine transgression ensured the necessary accommodation for the development of the estuary in the M1 Sandstone Formation. The sedimentological study of the M1 Sandstone Formation provides a case for understanding the sedimentary evolution in fluvial-marine transitional zones.

Elemental Geochemistry on Paleoenvironment Reconstruction: Proxies on Miocene-Pliocene of Marine to Fluvial Sediment in Serpong, Banten, Indonesia

Research of the depositional environment using geological mapping, petrography, gamma-ray (GR) log, palynology, and foraminifera fossils of the Bojongmanik Formation has led to the formation of several different conclusions about the transition to the marine environment, which are attractive to revisit. The expected results of this research are to determine the paleoenvironment of the Bojongmanik and Serpong Formations based on elemental geochemistry, the development of paleoenvironment proxies based on portable X-ray fluorescence (pXRF) in fluvial to transitional environments studies, and the contribution of paleoenvironment analysis to GR-log facies interpretation. The research methodology starts with GR-log facies analysis, Pearson’s correlation, paleoenvironment analysis based on elemental affinity and elemental ratio, and comparing the paleoenvironment with GR-log-based facies. The paleoenvironment analysis based on elemental geochemistry resulted in the Bojongmanik Formation in the research area deposited at the tidal point bar, lagoon, and shoreface, while the Serpong Formation was deposited at the fluvial point bar and floodplain. Compared to previous research, the Bojongmanik Formation in the research area could be stratigraphically related to the upper Bojongmanik Formation. Proxies based on elemental geochemical affinities of carbonate-associated, carbonate-productivity, terrigenous-associated elements, and redox-sensitive trace elements show contrast changes between facies. Proxies based on the specific ratio show a detailed paleoenvironment for paleoclimate (Sr/Cu), paleosalinity (Sr/Ba), paleoredox (Cu/Zn), paleo-hydrodynamics and water depth (Zr/Rb and Fe/Mn), sediment provenance (Cr/Zr), and siliciclastic-dominated (Zr + Rb)/Sr. Adding a geochemistry element-based paleoenvironment analysis benefits from a more specific justification for GR-log facies interpretation.

Tidal versus fluvial point bars: Key features from the integration of outcrop, core and wireline log information of Triassic examples

AbstractThe Triassic red beds of the Tabular Cover of the Iberian Meseta are an excellent reservoir outcrop analogue, a direct consequence of high‐quality exposures, which offer exceptional three‐dimensional outcrops, as well as a wide variability of depositional environments. Fluvial and transitional with tide‐influenced and wave‐influenced settings are recognised. Three point bar geobodies of similar scale, but influenced by different processes, were selected from this succession. Point bar geobody 1 was influenced by purely fluvial processes while geobodies 2 and 3 were tide‐influenced. Both types of geobody were developed as point bar deposits in sinuous channels. A fully integrated study was carried out on these geobodies, utilising both outcrop and subsurface‐based approaches, to characterise the key differences between fluvial and tidal point bars in the sedimentary record. The outcrop‐based component involved traditional field data collection methods alongside digital techniques and data capture, including the use of digital outcrop models. Additionally, subsurface‐based methods were employed, utilising core and wireline logs obtained from wells drilled in close proximity to the outcrop. The integration of these approaches aims to accurately differentiate the depositional settings of the three different geobodies, which while apparently very similar in many key respects also exhibit considerable differences when considered from the perspective of subsurface management of potentially similar geobodies. This study also emphasises the need to clearly distinguish high‐sinuosity deposits based on their depositional sub‐environment in order to properly evaluate their potential for subsurface management. Additionally, it highlights the presence and importance of internal baffles that may well influence fluid migration and indeed even compartmentalise geobodies. Three point bar geobodies of similar scale, but influenced by different processes, have been selected in this succession. A fully integrated study was carried out on these geobodies, utilising both outcrop‐based and subsurface‐based approaches, to characterise the key differences between fluvial and tidal point bars in the sedimentary record.

Morphodynamics of macrotidal channels in Korean tidal flats: implications for the role of monsoon precipitation and the stratigraphic architecture of tidal point bars

Abstract Tidal channels exert a crucial control on sediment transport and drive geomorphic changes in the tidal environment. Despite their ubiquitous occurrence, long-term morphodynamics and processes driving the morphologic changes remain less well understood than fluvial counterparts. Spanning from straight to dendritic, Korean tidal channels become more sinuous and densely populated with elevation due to higher mud content. Mutually evasive current patterns resulted in a cuspate meander bend, where a flood barb develops at the seaward side of the bend. Multiannual observation revealed that tidal channels migrate from up to 80 m per year in the lower intertidal zone of open-coast sandy tidal flats to nearly stationary for several years in the upper intertidal zone of protected muddy tidal flats. Migration rates are temporarily pronounced during the summer monsoon, when heavy rainfall-induced surface runoff intensifies ebb tidal asymmetry and promotes headward erosion. Meander bends are mostly landward-skewed and shift downstream, implying that ebb currents primarily drive the long-term channel morphodynamics. Tidal point bars commonly display ebbward dipping, inclined heterolithic stratification (IHS) dominated by bedforms formed by subordinate flood tidal currents. An overall ebb dominance and mutually evasive current patterns account for the counterintuitive stratigraphic architecture of point bars in Korean tidal channels.

Hydrodynamic controls on sedimentary facies of tidal point bars: A case study in the Georgia coastal plain, USA

AbstractTidal point bars are commonly developed in coastal plain meandering channels. They form by the same basic processes as fluvial point bars but are further modified by tidal action. This study analyses the sedimentary facies of six active tidal point bars and their relationships. The bars are situated along the Georgia coastal plain, USA, in the lower and upper reaches of the fluvial marine transition and in tidal channels without upstream river input. Based on millimetric logging of 31 cores, nine sedimentary facies were identified, which are proposed to be used for the characterization of tidal point bars in other depositional settings. These bars share common sedimentary facies; however, the internal organization of these facies and their associations differ appreciably in response to complex local tidal processes and fluvial–tidal interactions. The increase in tidal influence results in prominent heterogeneous architectures, with greater concentration of heterolithic stratification (mostly inclined), bioturbation and evidence of flow reversals. In contrast, when freshwater river discharge overcomes tidal stages in the fluvial marine transition bars, structureless coarse‐grained facies with attendant erosional surfaces and decreased bioturbation are predominantly developed. In the lower reach of the fluvial marine transition, along the estuarine turbidity maximum, alternations of fluvial and tidal sedimentation are evidenced by a bimodal facies association, where alternations of gravel lag and muddy facies (massive to heterolithic) dominate. Moreover, despite differences in local hydrodynamics, coarsening‐upward sequences, mostly composed of structureless coarser facies, are consistently distributed along the upper parts of the bars, which may indicate overprinting of regional events such as storm surge‐related floodings in the sedimentological record. This study provides a basis for better identifying the hydrodynamic processes that shape coastal systems, such as estuarine and backbarrier environments. Additionally, those deposits are analogues for certain hydrocarbon reservoirs, thereby increasing knowledge of their facies distribution and aiding production optimization.

Reservoir Architecture in Tide-Dominated Estuary: A Case Study of McMurray Formation Oil Sand Reservoir in the Athabasca Block, Canada

This paper presents a detailed description of the sedimentary characteristics and architecture model of the tidal-dominated estuary reservoir in the Lower Cretaceous McMurray formation. Based on a thorough study of the core data, 13 lithofacies were recognized. The lithofacies associations were divided into nine types of architecture elements, that is channel, salt marsh, fluvial point bar, tidal point bar, mud flat, mixed flat, sand flat, tidal bar and offshore. Through the combination of high resolution seismic data and well data, the architecture model was established. The boundary surfaces of each architecture elements are depicted and the logging characteristics is concluded. The cross-section is constructed to reveal the superimposition model of different architectural elements with the seismic profiles. The plane and vertical distribution of architectural elements are revealed and the sedimentary model is constructed. In the longitudinal section of estuary, the architecture elements is fluvial deposits (channel, fluvial point bar, and salt marsh), tidal flat (mud flat, mixed flat, and sand flat), tidal bar, tidal flat and offshore from land to sea. In the cross section, tidal bar, sand flat, mixed flat, mud flat, tidal point bar and salt marsh are distributed successively from the center to the sides. Tidal bars and sand flats are the best reservoirs in terms of physical properties. We focus on the analysis of their architecture and summarize their stacking patterns. We propose tidal bars as compounds are lateral accretion. Tidal bar has high-angle continued lateral accretion, with a dip angle of 6–12°. Sand flat also has lateral-migrated characteristics, but their dip angle is 2–6° which is smaller than tidal bars. As a result, the dimension and stack of a single tidal bar and sand flat are quantified. The tidal bar was lateral stacking or vertical stacking with lateral-migrated or forward-migrated sand flat and existed vertical and lateral erosion-filling phenomenon. Tidal bars have width of 100–550 m and length of 800–2,400 m, and sand flats are 300–2,500 m long. Tidal bars and sand flats made up the best reservoirs. The study provides some insights for future research of estuarine sedimentary patterns and reservoir architecture.

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.

Sedimentology of a hypertidal point bar (Mont‐Saint‐Michel Bay, north‐western France) revealed by combining lidar time‐series and sedimentary core data

AbstractIntertidal meanders developed on salt marshes are known to expand and produce inclined heterolithic stratification rich in fine‐grained sediments and to bear evidence for rhythmic deposition in the upper part of the inner meander bend (i.e. the upper part of the point bar). This occurs because the lower point‐bar deposits are washed by strong currents, which remove mud drapes and develop discontinuous record of tidal cycles. Although these criteria are widely accepted, facies models for tidal point bars still lack a three‐dimensional perspective and overlook the along‐bend variability of sediment distribution. The present study focuses on a hypertidal point bar belonging to the upper‐intertidal domain of the Mont‐Saint‐Michel Bay (France), and it analyses the sedimentology of a 3D time‐framed accretionary package formed between 28 March 2012 and 29 November 2012 by means of lidar topographic data, geomorphological field surveys and sedimentological core data. To define the 3D time‐framed accretionary package, data from thirteen lidar (light detection and ranging) topographic surveys were used to create the point‐bar synthetic stratigraphy. Data show that over the study period the point bar expanded alternating deposition along its seaward and landward sides, pointing out the occurrence of depositional patterns more complex than a simple progressive expansion of the bend. The thickest deposits were accumulated in the point‐bar‐apex zone, where the largest amount of mud was also stored. High sediment accretion in the bend‐apex zone is ascribed to the development of low‐energy conditions due to flow and bed configuration. High accretion rate of the point‐bar‐apex zone promoted also a better preservation of rhythmites, which are almost missing from deposits accumulated along the point‐bar sides. This study remarks that preservation of mud and tidal rhythmites within intertidal‐point‐bar deposits is controlled, not only by their elevation with respect to the tidal range, but also by their location along the point bar.

Fluvial-Tidal to Fluvial-Lacustrine Sedimentation of the Middle Miocene to Pleistocene Mapia Formation, Dogiyai, Papua (Indonesia)

A sedimentological and palynological investigation was carried out on outcropping sedimentary rocks at Dogiyai, Papua, proposed to be named as the Mapia Formation. The age range is from Middle Miocene to Pleistocene. The lower Mapia Formation was deposited at Metroxylon type to Nothofagus emarcida Zone, Middle Miocene to Early Pliocene. It is comprised of three facies associations: tidal channel, tidal point bar, and tidal flat deposits. A tidally dominated fluvially influenced depositional environment is suggested for the deposition of sediments of this unit. The upper Mapia Formation was deposited at Malvacipollis diversus Zone, Garcinia cuspidata type Zone, and Proteacidites sp. Zone, Late Pliocene to Pleistocene. It is comprised of five facies associations: delta front, slump, debrite, turbidite, and lacustrine mud deposits. A non-channelized deep-lacustrine slump and debris-flow dominated depositional environment is suggested for the deposition of sediments of this unit. The lower Mapia Formation was deposited as synorogenic clastic sediments at the beginning of Central Range orogeny event while the upper Mapia Formation was deposited in the piggyback basin at the major uplift event.

An integrated approach to determine three‐dimensional accretion geometries of tidal point bars: Examples from the Venice Lagoon (Italy)

AbstractLow rates of lateral migration (centimetres to decimetres per year) combined with relatively high rates of vertical accretion (millimetres to centimetres per year) recorded in microtidal channels of the Venice Lagoon (Italy) give rise to point‐bar geometries and internal facies arrangements that differ substantially from widely accepted models of point‐bar sedimentary architecture. In this study, field data from the Venice Lagoon are combined with a three‐dimensional forward stratigraphic model, the ‘Point‐Bar Sedimentary Architecture Numerical Deduction’ (PB‐SAND), to predict the stratal geometries of point bars formed in aggradational settings. The PB‐SAND uses a combined geometric and stochastic modelling approach that can be constrained by field evidence. The model applied determines the geometry of four point bars generated by 9 to 11 m wide channels cutting through salt marshes. An iterative best‐fit modelling approach has been used to obtain multiple simulations for each case study, each of which fits the observations derived from the analysis of time‐series historical aerial photographs and 44 sedimentary cores. Results demonstrate how the geometry of the bars is determined by the development of two key stratal surfaces: the point‐bar brink and channel‐thalweg surfaces. These surfaces are defined by the progressive translation and vertical shift of the point‐bar brink (i.e. break of slope between bar top and bar slope) and the channel thalweg (i.e. deepest part of the channel) during bar evolution. The approach is used to: (i) reconstruct three‐dimensional point‐bar geometries; (ii) propose alternative reconstructions; (iii) provide insight to drive the acquisition of additional data to better constrain the proposed models; and (iv) provide insight into the mechanism of bar growth for slowly migrating channels in settings subject to relatively high rates of aggradation. This study highlights how interaction between styles of planform transformation and latero‐vertical shifts of meandering channels can determine the geometry of related sedimentary bodies.

Heterolithic Lower Rewa Sandstone of the Neoproterozoic Rewa Group, Vindhyan Basin, U. P., India: An example of tidal point bar

The Lower Rewa Sandstone of the Rewa Group exposed in Drummondganj ghat area is about 15 m thick; it shows well developed features of tidal and wave origin. The sandstone succession characterized by Inclined Heterolithic Strata (IHS) is composed of three lithofacies associations namely, i) Sandstone lithofacies association (A), ii) Siltstone lithofacies association (B) and iii) Shale lithofacies association (C). These three lithofacies associations are repetitive and signify vertically stacked parasequences, which are further divisible into smaller hemicycles showing fining upward in terms of grains size and lithology. The studied section is characterized by heterolithic facies, lateral accretion elements (LA surfaces) with tide generated features such as cross-beddings with mudstone drapes, lenticular and wavy bedding, often of wave origin, suggesting a tidal point bar of a sinuous estuarine channel formed along coast line having momentous tidal action, superimposed by significant wave action. The upper part of the section, characterized by erosionally based cross-bedded sand-bodies with occasional mudstone drapes and current modified wave ripples, indicates translation of river-dominated inner part of an estuarine channel over the mixed-energy zone of the estuary in the sense of Dalrymple et al. (1992). The complete Lower Rewa Sandstone represents a 3rd order Highstand Systems Tract (HST). Palaeocurrent pattern is polymodal suggesting subtidal to intertidal sedimentation whereby sediment was moved mainly by tidal action with intermittent wave activity. Petrographic study reveals that the fine to very-fine grained sandstone is quartzarenite, which has been derived from a stable cratonic uplifted basement.

Piracy-controlled geometry of tide-dominated point bars: Combined evidence from ancient sedimentary successions and modern channel networks

Establishing critical comparisons between fluvial- and tidal-channel morphodynamics is a major goal in the study of coastal landscapes. Freely migrating meandering rivers are known to produce laterally extensive, sand-prone point-bar bodies which commonly exhibit width:thickness ratios up to 250. Meandering channels are widespread in tidal channel networks draining intertidal plains, where they exhibit planform dynamics similar to their fluvial analogues. However, tidal networks are characterized by a high channel density that likely hinders tidal meanders from migrating laterally for long distances without interacting with other channels. In order to better understand how the interaction between adjacent meandering channels controls the development of tide-dominated point bars, two point-bar bodies from the Eocene Castigaleu Formation (Spain) are investigated and compared with the geometry of a modern tidal point bar of the northern Venice Lagoon (Italy). The Eocene bars are characterized by a low width:thickness ratio (<30), interpreted as the result of premature deactivation of their parent-channel migration. Such deactivations were likely related to avulsive piracy operated by adjoining tide-dominated channels, and likely prevented the parent channel bend from depositing a laterally extensive, tabular sand body along its inner bank. This analysis is corroborated by direct time-lapse observations of similar dynamics in modern tidal networks, and from morphodynamic and sedimentological evidence from recent deposits of the Venice Lagoon. We conclude that, in densely drained tidal networks, meander bends often interact with adjacent channels, thereby triggering piracy events and premature channel abandonments that generate point-bar architectures distinct from those produced in fluvial meandering rivers.

Tidal inlets in the Anthropocene: Geomorphology and benthic habitats of the Chioggia inlet, Venice Lagoon (Italy)

AbstractAdopting a multidisciplinary approach, we mapped with unprecedented detail the seafloor morphology, sediment distribution and benthic habitats of a tidal inlet in the Venice Lagoon, Italy, which has been greatly impacted by human activity. Thanks to very high‐resolution multibeam data, we identified ebb and flood‐tidal deltas, a tidal point bar, active dune fields, pools and scour holes.The seafloor substrate of the inlet was investigated by integrating automatically classified multibeam backscatter data with sediment samples and underwater seafloor images. The sediment composition comprises four textural classes with 75% overall thematic accuracy. The particle size distribution of each morphological feature was assessed distinguishing, in particular, sediments over crests and troughs of small‐dune fields with wavelengths and heights of less than 4 m and 0.2 m, respectively.Adopting state‐of‐the‐art benthic habitat mapping procedures, we found seven distinctive benthic habitats that reflect spatial variability in hydrodynamics and sediment transport pathways. The dominant classes were Sand with sparse shell detritus (46%) and Bare sand (32%). The rip‐rap revetment used for the inlet jetties and for the hard structures, built in the inlet channel to protect Venice from flooding, created a new habitat that covers 5.5% of the study area. This study shows how combining geomorphological and ecological analyses is crucial for the monitoring and management of tidal inlets and coastal infrastructures. © 2019 John Wiley &amp; Sons, Ltd.

Point-bar brink and channel thalweg trajectories depicting interaction between vertical and lateral shifts of microtidal channels in the Venice Lagoon (Italy)

ABSTRACT Although in tidal point bars, the point-bar brink (i.e. the break-in-slope between bar top and bar slope) and the channel thalweg (i.e. the deepest part of the channel) are thought to shift horizontally toward the outer bank, the occurrence of stable to slow-migrating channels in high-aggradational settings, e.g. salt marshes, is likely to promote a mixed, latero-vertical shift of meandering channel systems. The present study focuses on the trajectories of point-bar brink and channel thalweg of eight point bars located in the salt marshes of the Venice Lagoon, in order to understand how vertical aggradation can interact with lateral migration to shape their stratal geometries. High resolution facies-analysis carried out on closely-spaced sediment cores, collected along the bar axis, allowed us to define specific trajectories, which were classified either as ascending or descending, and linear or non-linear. All the analyzed brink trajectories are ascending, and show evidence of lateral shift of the bar brink under aggradational conditions of surrounding marshes. Development of non-linear brink trajectories is linked with changes in the ratio between vertical and lateral shift rates of the brink, which is in turn dictated by changes in local base level due to substrate compaction. Conversely, the thalweg trajectories can be either ascending or descending, reflecting an interaction between rates of lateral shift and aggradation/degradation of the channel floor. The brink and thalweg can either shift consistently (e.g., both trajectories are ascending) or incongruously (e.g., ascending brink vs. descending thalweg trajectory), reflecting different attitudes of the channel to maintain or increase its cross-sectional area.

Morphodynamic evolution and stratal architecture of translating tidal point bars: Inferences from the northern Venice Lagoon (Italy)

AbstractThe widespread distribution of tidal creeks and channels that undertake meandering behaviour in modern coasts contrasts with their limited documentation in the fossil record, where point‐bar elements arising from the interaction between a mix of both fluvial and tidal currents are mainly documented. The sedimentary products of tidal channel‐bend evolution are relatively poorly known, and few studies have focused previously on specific facies models for tidal point bars present in modern settings. This study improves understanding of tidal channel meander bends through a multi‐disciplinary approach that combines analyses of historical aerial photographs, measurements of in‐channel flow velocity, high‐resolution facies analyses of sedimentary cores and three‐dimensional architectural modelling. The studied channel bend (12 to 15 m wide and 2 to 3 m deep) drains a salt marsh area located in the north‐eastern sector of the microtidal Venice Lagoon, Italy. Historical photographs show that, during the past 77 years, the bend has translated seaward ca 15 m. Results show that the channel bend formed on a non‐vegetated mud flat that was progressively colonized by vegetation. Seaward translation occurred under aggradational conditions, with an overall migration rate of 0·2 to 0·3 m year−1, and was promoted by the occurrence of cohesive, poorly erodible outer bank deposits. Ebb currents are dominant, and translation of the channel bend promotes erosion and deposition along the landward and seaward side of the bar, respectively. Tidal currents show a clear asymmetry in terms of velocity distribution, and their offset pattern provides a peculiar grain‐size distribution within the bar. During the flood stage, sand sedimentation occurs in the upper part of the bar, where the maximum flow velocity occurs. During the ebb stage, the bar experiences the secondary helical flow that accumulates sand at the toe of the bar. Lateral stacking of flood and ebb deposits has caused the formation of localized coarsening‐upward and fining‐upward sedimentary packages, respectively.

Depositional facies model and reservoir characterization of USANI field 1, Niger delta basin, Nigeria

The 3-D depositional environment was built using seismic data. The depositional facies was used to locate channels with highly theif zones and distribution of various sedimentary facies. The integration core data and the gamma ray log trend from the wells within the studied interval with right boxcar/right bow-shape indicate muddy tidal flat to mixed tidal flat environments. The bell–shaped from the well logs with the core data indicate delta front with mouth bar, the blocky box- car trend from the well logs with the core data indicate tidal point bar with tidal channel fill. The integration of seismic to well log tie display a good tie in the wells across the field. The attribute map from velocity analysis revealed the presence of hydrocarbons in the identified sands (A, B, C, D1, D2, D4, D5). The major faults F1, F2, F3 and F4 with good sealing capacity are responsible for hydrocarbon accumulation in the field. Detailed petro physical analysis of well log data showed that the studied interval are characterized by sand-shale inter-beds. Eight reservoirs were mapped at depth intervals of 2886m to 3533m with their thicknesses ranging from 12m to 407m. Also the Analysis of the petrophysical results showed that porosity of the reservoirs range from 14% to 28 %; permeability range from 245.70 md to 454.7md; water saturation values from 21.65% to 54.50% and hydrocarbon saturation values from 45.50% to 78.50 %. The by-passed hydrocarbons were identified and estimated in low resistivity pay sands D1, D2 at depth of 2884m – 2940m, sand D5 at depth of 3114m – 3126m respectively. The model serve as a basis for establishing facies model in the field.

Aggradation and lateral migration shaping geometry of a tidal point bar: An example from salt marshes of the Northern Venice Lagoon (Italy)

Although meanders are ubiquitous features of the tidal landscape, the architectural geometries of tidal point bar deposits are relatively unexplored and commonly investigated on the basis of facies models developed for their fluvial counterparts. The present study aims at improving current understanding of tidal point bar deposits developed in salt marsh settings, through a high-resolution investigation of an abandoned intertidal meander loop, located in the northern part of the Venice Lagoon (Italy). The study channel is 6m wide and was active until the 1950s, when it was deactivated as consequence of a neck cut-off. A total of 150 cores was recovered from the associated point bar. The bar erosionally overlies a subtidal platform consisting of sand and mud and is covered by both channel fill and salt marsh mud. The bar, floored by a shell-rich sandy lag, consists of stratified fine sand, grading upward into sandy mud. The outer bank of the bend is characterized by well-developed, sand-rich levee deposits and absence of crevasse splays, which represent a distinctive feature of alluvial sedimentation. Sediment grain size distributions suggesting that seaward and landward sides of the point bar experienced comparable changes of bed shear stress due to alternation between flood and ebb currents, highlighting a remarkable difference with the classical downstream-fining characterizing fluvial point bars. Spatial interpolation between key stratal surfaces shows an overall thickening of the bar from 1.2 to 1.7m in the direction of channel migration, associated with both lowering of the bar base and rising of its brink, which occurs in parallel with an increase in channel cross-sectional area, to progressively accommodate the increasing tidal prism. The bar top surface is characterized by a spoon-shaped geometry stemming out from a combination between lateral migration (8–10cm/yr) and vertical aggradation (2.5–3.0mm/yr) of the inner bank. In salt marsh settings, vertical aggradation plays, therefore, a major effect on point bar sedimentation, generating peculiar bar top geometries that are not common in fluvial meanders, where the high rate of lateral migration causes the cutoff to be reached before substantially thick deposits are accumulated on the bar top.

Paleoenvironmental reconstruction of the coastal Monte Léon and Santa Cruz formations (Early Miocene) at Rincón del Buque, Southern Patagonia: A revisited locality

Sedimentological, ichnological and paleontological analyses of the Early Miocene uppermost Monte León Formation and the lower part of the Santa Cruz Formation were carried out in Rincón del Buque (RDB), a fossiliferous locality north of Río Coyle in Santa Cruz Province, Patagonia, Argentina. This locality is of special importance because it contains the basal contact between the Monte Léon (MLF) and the Santa Cruz (SCF) formations and because it preserves a rich fossil assemblage of marine invertebrates and marine trace fossils, and terrestrial vertebrates and plants, which has not been extensively studied. A ∼90 m-thick section of the MLF and the SCF that crops out at RDB was selected for this study. Eleven facies associations (FA) are described, which are, from base to top: subtidal–intertidal deposits with Crassotrea orbignyi and bioturbation of the Skolithos-Cruziana ichnofacies (FA1); tidal creek deposits with terrestrial fossil mammals and Ophiomorpha isp. burrows (FA2); tidal flat deposits with Glossifungites ichnofacies (FA3); deposits of tidal channels (FA4) and tidal sand flats (FA5) both with and impoverish Skolithos ichnofacies associated; marsh deposits (FA6); tidal point bar deposits recording a depauperate mixture of both the Skolithos and Cruziana ichnofacies (FA7); fluvial channel deposits (FA8); fluvial point bar deposits (FA9); floodplain deposits (FA10); and pyroclastic and volcaniclastic deposits of the floodplain where terrestrial fossil mammal remains occur (FA11).The transition of the MLF–SCF at RDB reflects a changing depositional environment from the outer part of an estuary (FA1) through the central (FA2–6) to inner part of a tide-dominated estuary (FA7). Finally a fluvial system occurs with single channels of relatively low energy and low sinuosity enclosed by a broad, low-energy floodplain dominated by partially edaphized ash-fall, sheet-flood, and overbank deposits (FA8–11). Pyroclastic and volcaniclastic materials throughout the succession must have been deposited as ash-fall distal facies in a fluvial setting and also were carried by fluvial streams and redeposited in both estuarine and fluvial settings. These materials preserve most of the analyzed terrestrial fossil mammals that characterize the Santacrucian age of the RDB's succession. Episodic sedimentation under volcanic influence, high sedimentation rates and a relatively warm and seasonal climate are inferred for the MLF and SCF section.Lateral continuity of the marker horizons at RDB serve for correlation with other coastal localities such as the lower part of the coastal SCF south of Río Coyle (∼17.6–17.4 Ma) belonging to the Estancia La Costa Member of the SCF.

Tidal‐channel deposits on a delta plain from the Upper Miocene Nauta Formation, Marañón Foreland Sub‐basin, Peru

AbstractMiocene siliciclastic sediments of the Marañón Foreland Sub‐basin in Peru record the sedimentary response to regional marine incursions into Amazonia. Contrary to previous interpretations, the Late Miocene Nauta Formation provides evidence of the last known marine incursion before the current Amazonia river basin became established. Sedimentological, ichnological and palynological data from well‐exposed outcrops along a ca 100 km road transect suggest that the Nauta Formation represents a shallow, marginal‐marine channel complex dominated by tidal channels developed in the inactive, brackish‐water portions of a delta plain. The main facies associations are: FA1 – slightly bioturbated mud‐draped trough cross‐stratified sand; FA2 – locally, pervasively bioturbated inclined heterolithic stratification (IHS); and FA3 – moderately bioturbated horizontally bedded sand–mud couplets. These identify subtidal compound dunes, tidal point bars and shallow subtidal to intertidal flats, respectively. Bi‐seasonal depositional cycles are ascribed to the abundant metre‐ to decimetre‐scale sand–mud couplets that are found mainly in the IHS association: semi‐monthly to daily tidal rhythmicity is inferred from centimetre‐ and millimetre‐scale couplets in the mud‐dominated parts of the decimetre‐scale couplets. The ichnology of the deposits is consistent with brackish depositional conditions; the presence of Laminites, a variant of Scolicia, attests to episodic normal marine conditions. Trace fossil suites are assigned to the Skolithos, Cruziana and mixed Skolithos–Cruziana ichnofacies. Pollen assemblages related to mangrove environments (e.g. Retitricolporites sp., Zonocostites sp., Psilatricolporites maculosus, Retitricolpites simplex) support a brackish‐water setting. Uplift of the Mérida Andes to the North and the consequent closure of the Proto‐Caribbean connection, and the onset of the transcontinental Amazon drainage, constrain the deposition of the Nauta sediments with around 10 to 8 Ma, probably contemporaneous to similar marine incursions identified in the Cuenca (Ecuador), Acre (Brazil) and Madre de Dios (Southern Peru) (sub)basins, and along the Chaco‐Paranan corridor across Bolivia, Paraguay and Argentina.

Sedimentary structures of tidal flats: A journey from coast to inner estuarine region of eastern India

Sedimentary structures of some coastal tropical tidal flats of the east coast of India, and inner estuarine tidal point bars located at 30 to 50 kilometers inland from the coast, have been extensively studied under varying seasonal conditions. The results reveal that physical features such as flaser bedding, herringbone cross-bedding, lenticular bedding, and mud/silt couplets are common to both the environments. In fact, flaser bedding and lenticular bedding are more common in the point bar facies during the monsoon months than in the coastal tidal flat environments. Interference ripples, though common in both the environments, show different architectural patterns for different environmental domains. Interference ripples with thread-like secondary set overriding the earlier ripple-form, resembling wrinkle marks, are the typical features in estuarine point bars near the high water region. Because structures which are so far considered as key structures for near-coastal tidal flats are common to both the environments, caution should be exercised for deciphering palaeo-environments, particularly for Proterozoic rocks, where one has to depend only on physical sedimentary structures.