Shallow Shoals Research Articles

Deep vs shallow: GPS tags reveal a dichotomy in movement patterns of loggerhead turtles foraging in a coastal bay

BackgroundIndividual variation in movement strategies of foraging loggerhead turtles have been documented on the scale of tens to hundreds of kilometers within single ocean basins. Use of different strategies among individuals may reflect variations in resources, predation pressure or competition. It is less common for individual turtles to use different foraging strategies on the scale of kilometers within a single coastal bay. We used GPS tags capable of back-filling fine-scale locations to document movement patterns of loggerhead turtles in a coastal bay in Northwest Florida, U.S.A.MethodsIridium-linked GPS tags were deployed on loggerhead turtles at a neritic foraging site in Northwest Florida. After filtering telemetry data, point locations were transformed to movement lines and then merged with the original point file to define travel paths and assess travel speed. Home ranges were determined using kernel density function. Diurnal behavioral shifts were examined by examining turtle movements compared to solar time.ResultsOf the 11 turtles tagged, three tracked turtles remained in deep (~ 6 m) water for almost the entire tracking period, while all other turtles undertook movements from deep water locations, located along edges and channels, to shallow (~ 1–2 m) shoals at regular intervals and primarily at night. Three individuals made short-term movements into the Gulf of Mexico when water temperatures dropped, and movement speeds in the Gulf were greater than those in the bay. Turtles exhibited a novel behavior we termed drifting.ConclusionsThis study highlighted the value provided to fine-scale movement studies for species such as sea turtles that surface infrequently by the ability of these GPS tags to store and re-upload data. Future use of these tags at other loggerhead foraging sites, and concurrent with diving and foraging data, would provide a powerful tool to better understand fine-scale movement patterns of sea turtles.

Responses of hydrodynamic circulation to the climatic change of wind forcing in a subtropical estuary

Under the background of climate change, long-term trends in atmospheric-marine processes have been observed over the past decades. Being a subtropical estuary influenced by East Asian monsoon wind, the Pearl River Estuary (PRE) is chosen as a prototype to investigate the response of estuarine hydrodynamics to the local climatic regime shift in wind forcing. Analyses from ERA5 wind data and local meteorological observations have revealed that summer wind over PRE (prevailed by the up-estuary wind) was decreasing in magnitude and shifting anticlockwise from southwesterly to southeasterly over the past 42 years (1979–2020). Using a fully calibrated three-dimensional numerical model, the hydrodynamic circulation of PRE were simulated for 42 years with realistic daily wind forcing and repeated annual cycles for other forcing fields. Both the least-squares fitting and the EOF analysis demonstrate that the summertime longitudinal flow was strengthened while the lateral circulation was weakened over the past 42 years. Comparative analyses of the results obtained from the process-oriented numerical experiments revealed that the decreasing wind speed is the dominant factor for the strengthening of the longitudinal circulation, while the shifting wind direction is the dominant factor for the weakening of the lateral circulation. Using calculated long-term trends in wind forcing, wind conditions are predicted for 2100. The model results indicate that by 2100, there will be a further strengthening of the longitudinal circulation and a further weakening of the lateral circulation during the summer. The momentum budget reveals that the baroclinic pressure gradient and the diffusion term are the dominant terms modulating the changes of hydrodynamic circulation. The water mass in the PRE will become saltier, and the exchange between the deep channel and shallow shoals of the estuary will be slowed down in the future. The impact of these changes on brackish water habitats and local ecosystems needs further study.

Generation and Evolution of Internal Solitary Waves in a Coastal Plain Estuary

Abstract Large-amplitude internal solitary waves were recently observed in a coastal plain estuary and were hypothesized to evolve from an internal lee wave generated at the channel–shoal interface. To test this mechanism, a 3D nonhydrostatic model with nested domains and adaptive grids was used to investigate the generation of the internal solitary waves and their subsequent nonlinear evolution. A complex sequence of wave propagation and transformation was documented and interpreted using the nonlinear wave theory based on the Korteweg–de Vries equation. During the ebb tide a mode-2 internal lee wave is generated by the interaction between lateral flows and channel–shoal topography. This mode-2 lee wave subsequently propagates onto the shallow shoal and transforms into a mode-1 wave of elevation as strong mixing on the flood tide erases stratification in the bottom boundary layer and the lower branch of the mode-2 wave. The mode-1 wave of elevation evolves into an internal solitary wave due to nonlinear steepening and spatial changes in the wave phase speed. As the solitary wave of elevation continues to propagate over the shoaling bottom, the leading edge moves ahead as a rarefaction wave while the trailing edge steepens and disintegrates into a train of rank-ordered internal solitary waves, due to the combined effects of shoaling and dispersion. Strong turbulence in the bottom boundary layer dissipates wave energy and causes the eventual destruction of the solitary waves. In the meantime, the internal solitary waves can generate elevated shear and dissipation rate in local regions. Significance Statement In the coastal ocean nonlinear internal solitary waves are widely recognized to play an important role in generating turbulent mixing, modulating short-term variability of nearshore ecosystem, and transporting sediment and biochemical materials. However, their effects on shallow and stratified estuaries are poorly known and have been rarely studied. The nonhydrostatic model simulations presented in this paper shed new light into the generation, propagation, and transformation of the internal solitary waves in a coastal plain estuary.

Impact of tidal shear fronts on terrigenous sediment transport in the Yellow River Mouth: Observations and a synthesis

A tidal shear front (TSF) can be defined as an obvious horizontal gradient of flow velocity formed by opposing tidal currents (reversed TSF) or same-direction tidal currents (homogenous TSF) on both sides of the front with limited impact by river runoff. Observational and modeling studies have been conducted to investigate the impact of a reversed TSF on the transport of freshwater and sediment in the Yellow River Mouth (YRM). The reversed TSF existence period is only about 40% of a tidal cycle (around slack of flood and ebb); however, when this front disappears (around maximum flood and ebb), sediment transport across the frontal zone is still rare. This suggests that the dynamic mechanism of cross-frontal sediment transport in a tidal cycle remains unclear. In this study, synchronous observations of sea current and suspended sediment concentrations along the river mouth profiles at the YRM in 1995, 2003, and 2019 were used to investigate the long-term variation characteristics, mechanisms, and impacts of the TSF on cross-frontal sediment transport. The results indicate that in addition to the reversed TSF, there are also homogenous TSF during flood and ebb tides, with the ebb homogenous TSF being more significant due to tidal asymmetry in the YRM. The ebb homogenous TSF is located between shallow shoals and the slopes of the subaqueous delta front and lasts for about 3 h around the maximum ebb. Its maximum shear strength is similar to that of the reversed TSF, and the formation mechanism is also similar. The presence of the ebb homogenous TSF reduces the efficiency of cross-frontal sediment transport during the ebb. The reversed and homogenous TSFs alternately block sediment moving towards the sea. Thus, terrigenous sediment is difficult to cross the frontal zone to the central Bohai Sea by tidal currents solely. Over the past two decades of low discharge, with the Yellow River implementing artificial water and sediment regulation, the homogenous TSF during the ebb tide has become more and more important and cannot be neglected for cross-frontal sediment transport under normal conditions. Furthermore, from a long-term perspective, the shear strength of the TSF is positively correlated with the distance of river mouth siltation into the sea. Additionally, salinity and turbidity gradients are also positively correlated with the shear strength in the estuary. This indicates that the blocking of cross-frontal sediment transport by the TSF is a key process in river-sea interaction in the YRM.

Quantitative microfacies analyses of a carbonate ramp in the upper Cretaceous Mishrif Formation, A oilfield in Iraq, and paleogeography reconstruction of the North-eastern Arabian Plate

Sediment composition and microfacies (MF) types in the Cretaceous Mishrif Formation of the A oilfield in Iraq were quantitatively analyzed to identify changes in facies belts. Nine major constitutes were recognized via point-count group analysis and thin sections. The hierarchical cluster analysis of point-count groups indicated that the Mishrif Formation contained 12 MF types that were clustered into seven facies belts: protected-, open inner ramp, and shallow shoal, proximal and distal middle ramp, agitated outer ramp, and distal outer ramp. The quantitative analysis of MF enabled the interpretation of the water energy and continuity of the sub-environments. A low-energy homoclinal ramp was established by integrating the significant volumes of fine-grained carbonates that were present in almost all MF and low-relief green algal build-ups. Diagenetic discrepancy along Cenomanian-Turonian (C-T) unconformity is presented by extensive fabric-selective dissolution, classical karstification of dissolution-collapse breccia and paleosol deposits, and incisions and channels. Finally, a comprehensive comparison of the carbonate developments and diagenetic processes along unconformities in the Mishrif and its time-equivalent platform implied that the configuration of platforms associated intrabasins, and related diagenesis along unconformities were affected by the far-field effect of the tectonic activities that occurred in the northeast margin of the Arabian Plate. This study demonstrated that the quantitative analysis of carbonate MF is a powerful tool that can improve the reconstruction of a model of a low-energy carbonate ramp and provided important implications for the study of the paleogeography of the northeastern margin of the Arabian Plate during the Cenomanian–Middle Turonian.

Impact of Water Level Fluctuation on Microplastic Transportation and Redistribution in a Floodplain Lake System

Microplastic contamination is a challenge in aquatic systems. Among these, floodplains exhibit their cyclical hydrological patterns with substantial fluctuations in water levels caused by annual floods or discharges from rivers into lakes. The influence of water level fluctuation on microplastics within complex floodplain systems has received limited attention in existing studies. This study employs hydrological data and a physics-based hydrodynamic model to assess the effects of water level fluctuations on the transport and redistribution of microplastics within Poyang Lake from 2018 to 2025. High spatiotemporal distribution variability in microplastic concentrations was found within the flood and drought periods. Furthermore, the residence rate of microplastics was assessed based on the microplastic concentration variations in the computational time. A gradual drop in the average discharge rate of microplastics was assessed at the outlet of Poyang Lake. The microplastics are more likely to drain into the Yangtze River in the high water-level period. Meanwhile, under the influence of hydrodynamics, local topography, and water level fluctuation, an accumulation of microplastics appears on the shallow shoals of the western and eastern regions of Poyang Lake, especially in the Gan River. Overall, by means of numerical simulation, the aim of our study is to serve as a reference and advance our understanding of the transportation patterns of microplastics at the aquatic–terrestrial interface.

Long-Term Geomorphological Evolution of the Mouth Bar in the Modaomen Estuary of the Pearl River over the Last 55 Years (1964–2019)

Based on mass bathymetric data and remote sensing data in the Modaomen Estuary, this study explored the long-term evolutionary characteristics of the mouth bar in the Modaomen Estuary of the Pearl River from 1964 to 2019. In the past 55 years, due to the impact of human activities, such as shoal reclamation and estuarine regulation in the Modaomen Estuary, the river mouth moved out of the shallow sea covered by several islands and faced the South China Sea directly. Therefore, the mouth bay became a siltation center in the estuarine region and expanded outwards, gradually evolving a geomorphic pattern with three shallow shoals and two distributary branches; a west branch as the main branch accompanied by a small east branch. Over the past decade, high-intensity sand dredging activities in the mouth bar have led to a considerable deepening of the water depth and a significant refinement of bed sediments, forming a discharge pattern of a wide and shallow channel flowing into the sea. Therefore, the evolutionary characteristics of the mouth bar have become abnormal in recent years, so additional field bathymetric data and hydrological data are required for further research regarding the subsequent evolution of the mouth bar, against the background of a significant reduction of sediment discharge and high-intensity human activities.

Alluvial connectivity in multi‐channel networks in rivers and estuaries

Channels in rivers and estuaries are the main paths of fluvial and tidal currents that transport sediment through the system. While network representations of multi‐channel systems and their connectivity are quite useful for characterisation of braiding patterns and dynamics, the recognition of channels and their properties is complicated because of the large bed elevation variations, such as shallow shoals and bed steps that render channels visually disconnected. We present and analyse two mathematically rigorous methods to identify channel networks from a terrain model of the river bed. Both methods construct a dense network of locally steepest‐descent channels from saddle points on the terrain, and select a subset of channels with a certain minimum sediment volume between them. This is closely linked to the main mechanism of channel formation and change by displacement of sediment volume. The two methods differ in how they compute these sediment volumes: either globally through the entire length of the river, or locally. We compare the methods for the measured bathymetry of the Western Scheldt estuary, The Netherlands, over the past decades. The global method is overly sensitive to small changes elsewhere in the network compared to the local method. We conclude that the local method works best conceptually and for stability reasons. The associated concept of alluvial connectivity between channels in a network is thus the inverse of the volume of sediment that must be displaced to merge the channels. Our method opens up possibilities for new analyses as shown in two examples. First, it shows a clear pattern of scale dependence on volume of the total network length and of the number of nodes by a power law relation, showing that the smaller channels are relatively much shorter. Second, channel bifurcations were found to be predominantly mildly asymmetrical, which is unexpected from fluvial bifurcation theory.

Benthic foraminifera distribution and sedimentary environmental evolution of a carbonate platform: A case study of the Guadalupian (middle Permian) in eastern Sichuan Basin

Benthic foraminifera are significant indicators of habitat changes and are useful for paleoenvironmental reconstruction. During the Permian period, the variation in species, individual sizes, and morphological characteristics of the different assemblages, such as the fusulinids, is a clue to paleoecological and paleoenvironmental interpretations. The eastern part of the Sichuan Basin, located near the equator in the middle Permian, Guadalupian period, was precipitated by thick beds of marine carbonate rocks with numerous benthic foraminifera. In this study, we record the species and calculate the frequency of benthic foraminifera from 445 thin sections of Guadalupian (Maokou Formation) marine carbonate rocks. Seven types of benthic foraminiferal biofacies are recognized and associated with five sedimentary microfacies in the Erya and Huilongchang sections, eastern Sichuan area. During the early Guadalupian period, small uniserial nodosariids and Endothyrida dominated the benthic foraminiferal assemblages; in the middle Guadalupian period, species of Miliolida, Ammodiscidae, and uniserial nodosariids were common; however, the species of Fusulinida, such as Schwagerina sp. and Verbeekina sp., were abundant in the late Guadalupian period. The associated sedimentary microfacies indicates a shallow open platform to marginal platform. In our study area, the early Guadalupian was deposited in an open platform to grain shoals. This was followed by gradual shallowing as the sedimentary environment shifted to marginal shallow shoals from the middle to late Guadalupian period until the paleoweathering abruptly terminated the deposition at the end of the Guadalupian period.

High and Variable Drag in a Sinuous Estuary With Intermittent Stratification

AbstractIn field observations from a sinuous estuary, the drag coefficient based on the momentum balance was in the range of , much greater than expected from bottom friction alone. also varied at tidal and seasonal timescales. was greater during flood tides than ebbs, most notably during spring tides. The ebb tide was negatively correlated with river discharge, while the flood tide showed no dependence on discharge. The large values of are explained by form drag from flow separation at sharp channel bends. Greater water depths during flood tides corresponded with increased values of , consistent with the expected depth dependence for flow separation, as flow separation becomes stronger in deeper water. Additionally, the strength of the adverse pressure gradient downstream of the bend apex, which is indicative of flow separation, correlated with during flood tides. While generally increased with water depth, decreased for the highest water levels that corresponded with overbank flow. The decrease in may be due to the inhibition of flow separation with flow over the vegetated marsh. The dependence of during ebbs on discharge corresponds with the inhibition of flow separation by a favoring baroclinic pressure gradient that is locally generated at the bend apex due to curvature‐induced secondary circulation. This effect increases with stratification, which increases with discharge. Additional factors may contribute to the high drag, including secondary circulation, multiple scales of bedforms, and shallow shoals, but the observations suggest that flow separation is the primary source.

Exchange Flow and Material Transport Along the Salinity Gradient of a Long Estuary

AbstractMost estuaries are characterized by non‐uniform axial topography with shallow shoals near the mouth. Previous studies have addressed the impacts of the axial topographic variations on mixing and estuarine circulations yet seldom on material transport and retention. This study investigates the longitudinal structure and mechanisms of exchange flow and material transport of Chesapeake Bay (CB), featuring a shallow sill in the lower bay, by applying total exchange flow (TEF) algorithm, tracer experiments, and partial residence time (PRT) using a validated 32‐years numerical model simulation. A retention coefficient was adopted to quantify the material retention rate using two characteristic PRTs: with and without incorporating water parcels returning to a concerned region. It is found that shoaling from the Rappahannock Shoal to the mouth causes persistent downwelling, strong reflux, and the highest material retention rate in the middle of the bay. The gravitational circulation and the river outflow dominate the transport of salt and riverine dissolved materials (RDMs), whereas the contribution of the tidal oscillatory process is localized near the mouth. The dominance of river outflow over the gravitational circulation for transporting RDMs is confined within the upper bay, where PRTs exhibit distinct seasonality. PRTs show small seasonality in the middle to the lower bay controlled by the exchange flow. The present analysis combining TEF, efflux/reflux theory, and PRT is applicable to other coastal aquatic ecosystems to characterize the water exchange and renewal efficiency along the salinity gradient and understand the contributions of transport to biogeochemical processes.

Benthic Flow and Mixing in a Shallow Shoal Grass (Halodule wrightii) Fringe

Mean flow and turbulence measurements collected in a shallow Halodule wrightii shoal grass fringe highlighted significant heterogeneity in hydrodynamic effects over relatively small spatial scales. Experiments were conducted within the vegetation canopy (~4 cm above bottom) for relatively sparse (40% cover) and dense (70% cover) vegetation, with reference measurements collected near the bed above bare sediment. Significant benthic velocity shear was observed at all sample locations, with canopy shear layers that penetrated nearly to the bed at both vegetated sites. Turbulent shear production (P) was balanced by turbulent kinetic energy dissipation (ϵ) at all sample locations (P/ϵ≈1), suggesting that stem-generated turbulence played a minor role in the overall turbulence budget. While the more sparsely vegetated sample site was associated with enhanced channel-to-shore velocity attenuation (71.4 ± 1.0%) relative to flows above bare sediment (51.7 ± 2.2%), unexpectedly strong cross-shore currents were observed nearshore in the dense canopy (VNS), with magnitudes that were nearly twice as large as those measured in the main channel (VCH; VNS/VCH¯ = 1.81 ± 0.08). These results highlight the importance of flow steering and acceleration for within- and across-canopy transport, especially at the scale of individual vegetation patches, with important implications for nutrient and sediment fluxes. Importantly, this work represents one of the first hydrodynamic studies of shoal grass fringes in shallow coastal estuaries, as well as one of the only reports of turbulent mixing within H. wrightii canopies.

Alteration of estuarine circulation pattern due to channel modification in the North Passage of the Changjiang River Estuary

The exchange flow structure was examined in the North Passage of Changjiang River Estuary, where a deep waterway project (DWP) was carried out to improve the navigability. Before the construction of the DWP, the friction effect played a significant role in shaping the transverse structure of the exchange flow. The turbulent eddy viscosity generated near the seabed can be transferred to the upper water column, which facilitated vertical momentum exchange. As a result, the landward inflow extended to −2 m below the water surface and the seaward outflow was concentrated on the shallow shoal on the southern side of the cross section. After the construction of the DWP, the turbulent mixing was suppressed as a result of density stratification. The friction felt by the water was constrained in the lower half of the water column and the vertical momentum exchange was reduced. Meanwhile, the channel became dynamically narrowed with a Kelvin number of 0.52. Therefore, the Coriolis played a minor role in shaping the transverse structure of the exchange flow. As a consequence, the exchange flow featured a vertically-sheared pattern, with outflow at the surface and inflow underneath. Additionally, the gravitational circulation was enhanced due to increase in along-channel density gradient and stratification. The exchange flow components associated with the lateral processes (residual currents induced by eddy viscosity-shear covariance and lateral advective acceleration) were reduced, which suggests that lateral processes played a minor role in modifying the along-channel dynamics when the estuary becomes dynamically-narrowed.

Estuarine Habitat Use by White Sturgeon (Acipenser transmontanus)

White Sturgeon (Acipenser transmontanus), a species of concern in the San Francisco Estuary, is in relatively low abundance due to a variety of factors. Patton et al. sought identify the estuarine habitat used by White Sturgeon to aid in the conservation and management of the species locally and across its range. By seasonally sampled sub-adult and adult White Sturgeon in the central estuary using setlines across a habitat gradient representative of three primary structural elements, the authors found that the shallow open-water shoal and deep open-water channel habitats were consistently occupied by White Sturgeon in spring, summer, and fall across highly variable water quality conditions, whereas the shallow wetland channel habitat was essentially unoccupied. In summary, sub-adult and adult White Sturgeon inhabit estuaries in at least spring, summer, and fall and small, shallow wetland channels are relatively unoccupied.

Mechanisms of Exchange Flow in an Estuary With a Narrow, Deep Channel and Wide, Shallow Shoals

AbstractDelaware Bay is a large estuary with a deep, relatively narrow channel and wide, shallow banks, providing a clear example of a “channel‐shoal” estuary. This numerical modeling study addresses the exchange flow in this channel‐shoal estuary, specifically to examine how the lateral geometry affects the strength and mechanisms of exchange flow. We find that the exchange flow is exclusively confined to the channel region during spring tides, when stratification is weak, and it broadens laterally over the shoals during the more stratified neap tides but still occupies a small fraction of the total width of the estuary. Exchange flow is relatively weak during spring tides, resulting from oscillatory shear dispersion in the channel augmented by weak Eulerian exchange flow. During neap tides, stratification and shear increase markedly, resulting in a strong Eulerian residual shear flow driven mainly by the along‐estuary density gradient, with a net exchange flow roughly 5 times that of the spring tide. During both spring and neap tides, lateral salinity gradients generated by differential advection at the edge of the channel drive a tidally oscillating cross‐channel flow, which strongly influences the stratification, along‐estuary salt balance, and momentum balance. The lateral flow also causes the phase variation in salinity that results in oscillatory shear dispersion and is an advective momentum source contributing to the residual circulation. Whereas the shoals make a negligible direct contribution to the exchange flow, they have an indirect influence due to the salinity gradients between the channel and the shoal.

Effects of Wind-Driven Lateral Upwelling on Estuarine Carbonate Chemistry

Estuaries are productive ecosystems that support extensive vertebrate and invertebrate communities, but some have suffered from an accelerated pace of acidification in their bottom waters. A major challenge in the study of estuarine acidification is strong temporal and spatial variability of carbonate chemistry resulting from a wide array of physical forces such as winds, tides and river flows. Most past studies of carbonate system dynamics were limited to the along channel direction, while lateral dynamics received less attention. Recent observations in Chesapeake Bay showed strong lateral asymmetry in the partial pressure of carbon dioxide (pCO2) and air-sea CO2 flux during a single wind event, but comparable responses to different wind events has yet to be investigated. In this work, a coupled hydrodynamic-carbonate chemistry model is used to understand wind-driven variability in the estuarine carbonate system. It is found that wind-driven lateral upwelling ventilates high DIC (Dissolved Inorganic Carbon) and CO2 deep water and raises surface pCO2, thereby modifying the air-sea CO2 flux. The upwelling also advects low pH water onto the adjacent shoals and reduces the aragonite saturation state Ω_arag in these shallow water environments, producing large temporal pH fluctuations and low pH events. Regime diagrams are constructed to summarize the effects of wind events on temporal pH and Ω_arag fluctuations and the lateral gradients in DIC, pH and pCO2 in the estuary. This modeling study provides a mechanistic explanation for the observed wind-driven lateral variability in DIC and pCO2 and reproduces large pH and Ω_arag fluctuations that could be driven by physical forcing. Given that current and historic mainstem Bay oyster beds are located in shallow shoals affected by this upwelling, a large fraction of the oyster beds (100-300 km2) could be exposed to carbonate mineral under-saturated 〖(Ω〗_arag<1) conditions during wind events. This effect should be considered in the management of acidification-sensitive species in estuaries.

Sediment‐Induced Stratification in an Estuarine Bottom Boundary Layer

AbstractWe took field observations on the shallow shoals of South San Francisco Bay to examine how sediment‐induced stratification affects the mean flow and mixing of momentum and sediment throughout the water column. A Vectrino Profiler measured near‐bed velocity and suspended sediment concentration profiles, which we used to calculate profiles of turbulent sediment and momentum fluxes. Additional turbulence statistics were calculated using data from acoustic Doppler velocimeters placed throughout the water column. Results showed that sediment‐induced stratification, which was set up by strong near‐bed wave shear, can reduce the frictional bottom drag felt by the mean flow. Measured turbulence statistics suggest that this drag reduction is caused by stratification suppressing near‐bed turbulent fluxes and reducing turbulent kinetic energy dissipation. Turbulent sediment fluxes, however, were not shown to be limited by sediment‐induced stratification. Finally, we compared our results to a common model parameterization which characterizes stratification through a stability parameter modification to the turbulent eddy viscosity and suggest a new nondimensional parameter that may be better suited to represent stratification when modeling oscillatory boundary layer flows.

Depositional sequences and microfacies of Lower Carboniferous strata in the Marsel block of the Chu-Sarysu Basin, Southern Kazakhstan

The sequence development and evolution of Lower Carboniferous strata in the Marsel block of the Chu-Sarysu Basin, Kazakhstan have been established through detailed sedimentological analysis of cores, petrophysical data, and new seismic interpretations. Four depositional sequences and eighteen parasequences were classified in the Lower Carboniferous strata. Each depositional sequence is composed of a transgressive systems tract (TST) and highstand systems tract (HST), roughly coinciding, respectively, with the following four stratigraphic intervals (1) lower Tournaisian (C1t) through lower Visean (C1v-1) strata; (2) middle Visean (C1v-2) strata; (3) upper Visean (C1v-3) strata; and (4) Serpukhovian (C1sr) strata. Parasequences are bounded by an instantaneous drowned punctuated surface (C1t and C1v) or instantaneous exposed punctuated surface (C1sr). In addition, petrographic studies have led to the recognition of 20 microfacies, distributed on the major depositional facies zones of the outer ramp (Mf1, Mf6, Mf20), middle ramp (Mf3–5, Mf8–13), and inner ramp (Mf2, Mf4, Mf7, Mf9, Mf12, Mf14–19). They are grouped into seven facies associations corresponding to widespread deposits of tidal flats (MA3, MA5), shallow shoals (MA6), and shallow lagoons (MA4) in the inner ramp; deposits of restricted lagoons (MA1) and local patch shoals (MA7) in the middle ramp and a relatively small outer ramp deposits (MA2). SQSQ All the microfacies and their associations are grouped into the following 6 major facies belts: outer ramp; restricted lagoon, and patch shoals in the middle ramp; and shallow shoals, restricted lagoon, subtidal to intertidal and supratidal in the inner ramp. The interpretation of these facies belts suggests a shallow warm-water tropical-subtropical depositional environment on a passive continental margin ramp. Facies correlation between wells in sequence framework suggests that the strata may be divided into four sequences (SQ1, SQ2, SQ3, SQ4). SQ1 and SQ2 are dominated by lagoon facies and outer ramp facies in the northwest area in the TST, and tidal flat facies in the HST; SQ3 is characterized by restricted to semi-restricted lagoon facies in the TST and local patch shoal facies and mixed tidal flat facies in the HST; SQ4 is marked by restricted lagoon facies in the TST and evaporitic lagoon, tidal flat, and shallow shoal facies in the HST. The research results can be used for investigating the development and distribution of potential reservoirs in the Lower Carboniferous strata of the Chu-Sarysu Basin; the reservoirs in the HST of late Visean and Serpukhovian in the northern and southeastern part of the platform interiors may be of premium quality.

High-resolution observations of a shear front in a tidal channel

The characteristics of a shear front system in a tidal channel have been investigated through a field study. High-resolution sampling, including three-dimensional measurements of current, temperature, salinity, and turbidity, was performed on repeated transects across the frontal zone both with fronts apparent and with fronts absent. The density measured using conductivity/temperature/depth sensors provides a field proof that the shear front is present whether a density gradient exists. It is distinguishable from conventional buoyant shear fronts, which require a strong along-channel density gradient. The velocities measured using an Acoustic Doppler Current Profiler (ADCP) and trapping Global Position System (GPS) drifters indicate that shear front evolution has a four-stage dynamic process in a tidal cycle: (1) a directional shear front of shoal-flood and channel-ebb during the slack before flood and early flood, (2) a speed shear front of shoal-quick and channel-slow after the early flood, (3) a directional shear front of shoal-ebb and channel-flood during the slack before ebb and early ebb, and (4) a speed shear front of shoal-slow and channel-quick ebb during the late ebb. Thus, the velocity shear, a variation in the current speed and direction on either side of the front, is the generation mechanism of the observed shear front system. It is caused by the unequal tidal durations between deep channels and shallow shoals in the channel discussed herein. The Coriolis effect is found to be negligible through the momentum balance analysis. The unequal tidal durations can be caused by the following alternative mechanisms, according to the analysis and model simulation: (1) differential friction, owing to the steep bathymetry between the shallow shoal and the deep channel, and (2) circulation, owing to the coastline bend of the shallow shoal.

Extreme wave statistics of long-crested irregular waves over a shoal

We report laboratory experiments of long-crested irregular water surface waves propagating over a shoal. For a sufficiently shallow shoal we find that the surface elevation can have a local maximum of skewness and kurtosis above the shallower part of the shoal close to the edge on the incoming side, and a local minimum of skewness over the downward slope on the lee side of the shoal. We find that the horizontal fluid velocity can have a local maximum and minimum of skewness at the same locations as those for the surface elevation. However, the kurtosis of the horizontal fluid velocity can have a local maximum over the downward slope on the lee side of the shoal, different from the location of the maximum of kurtosis of the surface elevation.