Sand Wave Migration Research Articles

Sediment remobilization over subaqueous sand waves: In-situ observation in the northern South China Sea

High-resolution tripod observations were conducted in a subaqueous sand-wave field in the northern South China Sea. The objective was to gain insight into the dynamic mechanism by which sandy sediments are remobilized by oceanic dynamic processes, in particular internal solitary waves and internal tides. Daily-recurring high suspended sediment concentrations were observed, likely due to lateral transport of sediments resuspended by critical reflection of diurnal internal tides at the shelf break. The passage of extreme internal solitary waves results in resuspension of sediments from the seabed and ejection of these sediments out of the boundary layer. The resuspension of sediments from the seabed is controlled by current-induced strong bed shear stress, while the ejection of sediments out of the bottom boundary layer is driven by the compression and subsequent expansion of the mixing layer, together with the developments of the separation bubbles behind the internal solitary waves. These findings provide new insights into understanding the dynamic mechanism of sediment remobilization by internal solitary waves. Moreover, they contribute to our understanding of the migration of subaqueous sand waves.

Investigation of the local scour depth of a pile foundation on the migrating sand waves seabed in the western South China Sea

Investigation of the local scour depth of a pile foundation on the migrating sand waves seabed in the western South China Sea

Impact of Hurricane Irma on coral reef sediment redistribution at Looe Key Reef, Florida, USA

Abstract. Understanding event-driven sediment transport in coral reef environments is essential to assessing impacts on reef species, habitats, restoration, and mitigation, yet a global knowledge gap remains due to limited quantitative studies. Hurricane Irma made landfall in the Lower Florida Keys with sustained 209 km h−1 winds and waves greater than 8 m on 10 September 2017, directly impacting the Florida Reef Tract (FRT) and providing an opportunity to perform a unique comprehensive, quantitative assessment of its impact on coral reef structure and sediment redistribution. We used lidar and multibeam derived digital elevation models (DEMs) collected before and after the passing of Hurricane Irma over a 15.98 km2 area along the lower FRT including Looe Key Reef to quantify changes in seafloor elevation, volume, and structure due to storm impacts. Elevation change was calculated at over 4 million point locations across 10 habitat types within this study area for two time periods using data collected (1) approximately 1 year before the passing of Irma and 3 to 6 months following the storm's impact as well as (2) 3 to 6 months after and up to 16.5 months after the storm. Elevation change data were then used to generate triangulated irregular network (TIN) models in ArcMap to calculate changes in seafloor volume during each time period. Our results indicate that Hurricane Irma was primarily a depositional event that increased mean seafloor elevation and volume at this study site by 0.34 m and up to 5.4 Mm3, respectively. Sediment was transported primarily west-southwest (WSW) and downslope, modifying geomorphic seafloor features including the migration of sand waves and rubble fields, formation of scour marks in shallow seagrass habitats, and burial of seagrass and coral-dominated habitats. Approximately 16.5 months after Hurricane Irma (during a 13-month period between 2017 and 2019), net erosion was observed across all habitats with mean elevation change of −0.15 m and net volume change up to −2.46 Mm3. Rates of elevation change during this post-storm period were 1 to 2 orders of magnitude greater than decadal and multi-decadal rates of change in the same location, and changes showed erosion of approximately 50 % of sediment deposited during the storm event as seafloor sediment distribution began to re-equilibrate to non-storm sea-state conditions. Our results suggest that higher-resolution elevation change data collected over seasonal and annual time periods could enhance characterization and understanding of short-term and long-term rates and processes of seafloor change.

UNDERSTANDING 3D SAND WAVE DYNAMICS FOR ENGINEERING PURPOSES

Due to ambitious green energy goals the pressure on the offshore area is increasing at an unprecedented pace. These offshore developments, such as wind farms, require detailed bed level predictions. However, the uncertainty in these predictions increases dramatically when dynamic bed forms, such as sand waves, are present. This is the case in many areas, such as the North Sea, Taiwan Strait and the Banks Strait Australia. Sand waves can grow up to 25 percent of the water depth, have wavelengths of hundreds of meters and migrate with several meters per year. Due to their dynamic nature and size they may pose a threat to offshore activities, such as the construction and maintenance of wind farms and ship navigation. Cables and pipelines may become exposed and foundation could become unstable, due to the migration and/or shape changes of sand waves. Currently data-driven methods are applied to predict future bed levels. The use of process-based numerical models could improve the accuracy of these predictions and help quantifying uncertainties over time. Additionally, these models give insight into the effects of extreme events and human interventions, and provide a solution for data-scarce areas.

Variational RANS modeling of hydraulic jumps

The hydraulic jump is a phenomenon which frequently appears in nearshore, estuarine and riverine regions; it plays a major role for example in the development and migration of sand waves in mobile beds, and in controlling the propagation of infra-gravity water waves from the nearshore into the river through bar-built, shallow estuaries. Although the prediction of these natural flows becomes crucial in many circumstances, it is difficult because hydraulic jumps are complex flows, and their modeling is far from simple. The steady-state hydraulic jump may be undular or broken, and their physical properties differ significantly. Undular jump profiles are dominated by the deviation of the fluid pressure from hydrostatic conditions, with turbulence playing a secondary role. In contrast, broken hydraulic jumps are determined by the differential advection of momentum due to breaking, and turbulence stresses. Between both types, there is a complex transition involving the interaction of non-hydrostaticity and turbulence, which is difficult to mimic with depth-averaged models. Simulations of these flows are typically conducted by resorting to vertically-averaged models either using the Saint-Venant or the Serre-Green-Naghdi equations, but none of them are fully satisfactory. In this work, an alternative approach for hydraulic jumps is proposed by developing a variational Reynolds-Averaged Navier-Stokes (RANS) model which uses Kantorovich-Krylov expansions of the flow variables, and a novel depth-averaged, eddy viscosity approach suitable for non-hydrostatic flows. The model is validated through results of simulations of both undular and broken jumps compared against detailed experiments. New experiments were conducted in a simplified bar-built shallow river inlet; the variational RANS model was found to reproduce well the flow profile over the bar, the hydraulic jump position and dynamic pressures over the bar surface.

Sand wave migration near the southeastern corner of Martha's Vineyard, Massachusetts, USA

Sand waves of approximately 2 m in height were observed to migrate nearly 40 m with counterclockwise rotation between two bathymetric surveys performed three months apart near the southeastern corner of Martha's Vineyard, Massachusetts. The region is characterized by strong tidal currents, intermittent energetic surface wave events, and shallow water with local depth ranging from 2 to 7 m. This study uses the process-based model, Delft3D, with a three-dimensional approach to examine the sand wave dynamics by incorporating surface waves, winds, currents, and bathymetric observations. The model successfully simulates sand wave migration in comparisons to observations. Model sensitivity analyses show that the sand wave migration reduces by 65% with the absence of the surface waves. The modeled sand wave migration speed is correlated with the tidal current Shields parameter, and sharp increases in migration speed occur when the wave-driven Shields parameter increases in response to energetic surface wave events. The combined effect of tides, surface waves, and bathymetry is the origin of the rotational aspect of the sand wave, using the Shields parameter as an indicator of tidal currents and surface wave influence on sand wave dynamics.

A Numerical Simulation of Internal Wave Propagation on a Continental Slope and Its Influence on Sediment Transport

Significant current velocity near the sea bottom can be induced by internal waves, even for water a few hundred meters in depth. In this study, a nonhydrostatic ocean model was applied to simulate the generation and propagation of internal waves on the continental slope of the northern SCS. Based on the analyses of the vertical profiles of the currents, the propagation of internal waves along the continental slope can be categorized into six modes. The bed shear stress and the bedload transport were calculated to analyze the general characteristics of sediment transport along the continental slope of the northern SCS. Generally, there was no sediment transport on the sea bottom induced by the internal waves when the water depth was deeper than 650 m or shallower than 80 m. The downslope sediment transport dominated the slope at a water depth range of 200~650 m, while the upslope sediment transport dominated the slope at a water depth range of 80~200 m. The predicted directions of the bedload transport are coincident with the field observations of sand wave migration on the continental slope, which further confirms that the main cause of the generation and formation of sand waves on the continental slope of the northern SCS is the strong bottom current induced by the shoaling process of internal waves.

Shelf-wide analyses of sand wave migration using GIS: A case study on the Netherlands Continental Shelf

Shelf seas often display rhythmic bedforms, of which tidal sand waves are the most relevant to study from an engineering point of view due to their dimensions and dynamic nature. Environmental conditions such as water depth, flow velocities and grain size vary across entire shelf regions, consequently influencing the characteristics of tidal sand waves, including migration rates. However, as opposed to site-specific analyses, shelf sea-wide analyses on sand wave migration are scarce. Through the use of timeseries of bathymetric data, we investigate the spatial variability of sand wave migration on the scale of the Netherlands Continental Shelf (NCS). Two existing data-driven techniques, Pairs of Source and Target Points (PSTP) and Spatial Cross Correlation (SCC), were implemented in a Geographic Information System (GIS). Through testing at four sites on the NCS, PSTP was deemed the preferred technique for a shelf-wide analysis and was hence used to quantify the NCS-wide migration rates and directions. To illustrate the spatial variability, the quantified migration rates and directions were averaged per km2. For the NCS in general, migration to the south-west or north-east at typical rates between 0 and 3 m/year was found. The south-western region of the NCS demonstrated migration rates below 0.5 m/year with migration directions alternating over short distances. The north-eastern region exhibited uniform migration to the north-east at rates which gradually increased when in closer proximity to the Wadden Islands. Typical rates in this region were found to be between 2 and 8 m/year. The highest migration rates were observed near coastal regions and on top of sandbanks. The presence of these sandbanks strongly deflects the migration directions of superimposed sand waves at a number of locations. The analysis performed within this study demonstrates valuable insights into methods that can be used to evaluate shelf-wide regions of interest. A deeper understanding of the dynamic nature of the North Sea bed assists in optimising hydrographic re-survey policies, critical infrastructure planning and subsequent monitoring. Furthermore, this study may offer a wider set of tools that could be used for correlating sand wave migration to its governing physical mechanisms and environmental parameters.

The role of idealized storms on the initial stages in sand wave formation: A numerical modeling study

Sand waves are rhythmic bedforms observed in many shallow shelf seas. By adopting a process-based numerical morphodynamic model, Delft3D, the effect of the storm-related processes on the growth and migration of sand waves was studied systematically in this paper. In our model, the storm effect was included by adopting the shear stresses due to the nonlinear wave effect, wave-current interaction, and wind-driven current. A series of numerical experiments were performed, and it was found that accounting for the wave nonlinearity is important in predicting the growth rate. Moreover, the effect of wave-current interaction on sand wave growth and migration is significant. The asymmetric velocity of water particles induced by wave-current interaction leads to net sediment transport rates which resulted in the migration of sand waves. The wind-driven current effect can breach the symmetry of the tidal current and cause the migration of sand waves. In our specified cases, the effect of wind-driven current or wave action on the LFGM was not significant, while the combination of these two effects causes a larger wavelength of sand waves. The model results show that the storms influence sand wave dynamics in the formation stage significantly and are therefore important to account for in designing offshore engineering operations in sand wave fields.

Modelling the influence of Tidal Energy Converters on sediment dynamics in Banks Strait, Tasmania

As the tidal energy industry grows, developers are interested in the influence that large scale arrays might have on their surrounding environment including hydrodynamic and sediment. Understanding of local and regional sediment dynamics is an important element for environment impact and site assessments, as changes in sediment processes could alter bed ecosystems. In Australia, the Banks Strait located in the Northeast of Tasmania has been identified as a promising site for tidal energy. Very large sand waves have been observed near the area suitable for tidal turbines installation and they may interfere with human activities. Building from previous work, a 2D regional scale model investigated the influence of different tidal farms on the morphology of Banks Strait and especially the sand waves. Results showed that tidal farm of more than 49 MW rated power will alter the circulation of residual currents in Banks Strait leading to significant changes in the sediment dynamics, and farms less than 24.5 MW will have negligible influence on the migration of sand waves.

Upslope migrating sand waves on sediment-starved shelves: An example from the southeastern continental margin of the Korean Peninsula

An uncharted field of sand waves was discovered in a low-relief submarine canyon incised in the outer shelf on the southeastern continental margin of the Korean Peninsula in water depths of 180–190 m. We characterize the nature and origin of the waves and the sand forming them using sub-bottom chirp profiles, eXpendable bathythermograph (XBT) profile, multibeam echosounder (MBES) data, and sediment samples from four piston cores. Two types of sand waves characterized by distinct height versus wavelength relationships were found in the study area. The sand waves in the upper, narrower part of the shelf-incised canyon are sinuous-crested, with amplitudes of 0.3–2.1 m (mean: ~1 m) and wavelengths of 10–45 m (mean: ~24 m). Their asymmetry indicates migration upslope in a southwesterly direction, opposite to the surface currents. In contrast, the lower part of the canyon that is wider and closer to the margin of the continental shelf hosts nine long (ca. 1 km) curvilinear-crested sand waves with symmetrical crests; these waves likely reflect transient bedforms forming under fluctuating current conditions.The sediment of the sand waves consists of a variable mixture of siliciclastic and carbonate materials. The carbonate fraction (~22–55%; mean: ~34%) is derived mainly from the remains of bryozoans, bivalves, echinoderms, foraminifers, gastropods, and serpulids. Six bioclasts were dated by the radiocarbon method between ca. 41.3 and 11.8 ka BP. These relatively old ages and palaeontological data supports reworking from a shallow-marine environment during the last glacial transgression and limited sedimentation/sediment supply in the study area. The siliciclastic fraction (~44–79%; mean: ~37%) is composed of rounded to subrounded quartz and feldspar of moderate to good sorting and a mean grain size of ~1.3 phi (medium sand). The uppermost ~30 cm of all the sand wave cores reveals a decrease in the grain size of the siliciclastic fraction coupled with an increase in the carbonate/siliciclastic ratio, suggesting episodic sediment reworking and migration of the sand waves in response to fluctuating bottom currents. The coarser sediment that forms the core of the sand waves records bedload transport during periods of stronger currents. Finer carbonate-rich pelagic sediment (i.e., plankton) accumulated at the top of the sand waves during periods of weaker bottom currents. Significantly, our results show that the grain size and mineralogy of the sediment composing the sand waves are controlled by changes in hydrodynamic conditions. Our study provides novel geomorphological evidence for the influence of SW-flowing cold-water incursions (Korean Strait Bottom Cold Water) on the seafloor sediments.

Effect of topographic background on sand wave migration on the eastern Taiwan Banks

Observational analysis has revealed an apparent immobility of giant sand waves on the Taiwan Banks, while sand waves on the eastern Taiwan Banks show distinct dynamics influenced by the underlying larger-scale sand ridge bedforms. In this study, the effects of these sand ridge bedforms on sand wave migration and the local hydrodynamic environment were investigated using bathymetric and hydrodynamic datasets from 2016 and 2018. Upon separating the NNW-SSE sand ridge crest, the opposing sand wave migration and complementary sediment loss/gain on either side of the sand ridge are interpreted as convergence processes that serve to maintain the shape of the sand ridge. Analysis of the hydrodynamics revealed that, during the flood phase, the northward directed flow on the western side of the ridge crest was enhanced due to rapid shoaling. This caused the development of a flow shadow zone on the eastern side of the ridge crest, where the flow was rotated and weakened. During a tidal cycle, the dominance of the flood current on the western side of the ridge resulted in northward migration of sand waves. Conversely, due to the weakened flood current on the eastern side of the ridge, sand waves migrated southward in response to the stronger ebb current. This study thus provides insights into bedform dynamics in complex morphological settings and the maintenance mechanism of morphodynamic bedform systems.

A physically-modified spectral model for prediction of sand wave migration

Sand waves are rhythmic bedforms existing in shallow seas and typically induced by the interaction of currents and topography. In this study, a physically-modified spectral model for simulating small-scale sand wave migration is proposed by integrating non-linear spectral analysis, typhoon wind field and sediment transport theory. We introduce a power-average velocity to modify the spectral analysis, which gives explicit physical meanings to the morphological method and improves its stability and reliability. Then, the effects of typical and extreme weather are decoupled and the short-term migration caused by typhoons is removed from the total migration. Results show that a single typhoon will cause large sand wave migration, but the migration caused by multiple typhoons in a long period of time will cancel each other out in the measured data. Moreover, the bilateral reverse migration trend of one-group sand waves in a small scale (∼3 km) is captured, which is consistent with field data and previous researches. Compared with existing researches, the proposed model is able to identify the influences of extreme conditions on sand wave migration and provide a feasible and high efficiency way to predict sand wave migration when the available hydrodynamic data is limited.

Modelling Morphological Changes and Migration of Large Sand Waves in a Very Energetic Tidal Environment: Banks Strait, Australia

Banks Strait, Tasmania, Australia, has been identified as a potential site for the deployment of tidal turbines. In this study, the characterization of sediment transport and large sand waves for this site is performed. Observations of bed level change collected from surveys in 2018 showed a migration of large sand waves over a period of nine months. Migration rates in an excess of one hundred meters for nine months were found, which are large compared to the rate reported at other coastal sites, by several meters per year. A validated hydrodynamic model is coupled with a morphodynamic model to perform sensitivity tests and identify what parameters influence migration to better understand sediment dynamic in the Banks Strait. Numerical analysis showed a constant shift of the sand waves profile in an eastward direction, consistent with the observations. This migration was strongly linked with tidal asymmetry, with a residual current flowing towards the east. The principal parameters driving the migration of sand waves in the Banks Strait were found to be sediment sorting, bed friction and residual current. This study gives new insights for the seabed of Banks Strait and provides an assessment of the natural variability of sediment for futures tidal farms deployments.

Horizontal and Vertical Sediment Sorting in Tidal Sand Waves: Modeling the Finite‐Amplitude Stage

The bed of coastal seas displays a large number of rhythmic bed features, of which sand waves are relevant to study from an engineering perspective. Sediments tend to be well sorted over these bed forms, which is so far poorly understood in terms of modeling of finite‐amplitude sand waves. Using Delft3D, we employ bed stratigraphy and consider four different grain‐size classes, which are normally distributed on the phi scale. The standard deviation (sortedness) is then varied, whereas the geometric mean grain size is kept constant. The results show that, typically, the crests of sand waves are coarser than the troughs. Residual flow causes net sedimentation on the leeside of the crest, and, consequently, the general sorting pattern is distorted. Since larger grains experience a larger settling velocity, they are deposited on the upper lee slope, whereas the smaller grains are found on the lower lee slope. Due to sand wave migration, also, the internal structure of the sand wave is revealed, which follows the same sedimentation pattern as the lee slope surface. These results qualitatively agree with sorting patterns observed offshore. The sorting processes lead to longer wavelengths and lower wave heights, as a function of standard deviation. This relates to the dampening effect of suspended sediment transport for fine grains. Finally, it appears that the modeled wave heights fall in the same range as observations in the North Sea. These results are valuable for, for example, predicting the morphological response after engineering activities and determining suitable aggregates for sand extraction.

Numerical modelling of the migration direction of tidal sand waves over sand banks

Tidal sand waves are large-scale bed forms found in shallow sandy seas, which show a migration of several meters per year. Field data from the Dutch part of the North Sea revealed a migration pattern causing bidirectional migration of sand waves over a sand bank, resulting in sand wave migration uphill from both sides of the sand bank. In order to understand the physical mechanisms behind this migration behaviour, we study the inclusion of a sand bank under a sand wave field using the numerical model Delft3D. First, the schematized model set-up (i.e. schematized bathymetry and simple tidal forcing) showed that the alteration of the tidal flow by a sand bank resulted in tide-averaged horizontal flow towards the top of the sand bank, causing the bidirectional migration of sand waves over the sand bank. Second, a case study was performed in which a more complex model set-up was used (bathymetry and tidal forcing identical to a study site in the North Sea where an offshore wind farm is developed). Here, the model results revealed migration directions comparable to field observations. The results open opportunities to explore modelling migration patterns of sand wave fields for offshore wind farm development in areas with complex bathymetric environments.

Bidirectional bedform fields at the head of a submarine canyon (NE Atlantic)

Submarine canyons are known to force ocean mesoscale circulation and local hydrodynamics. Alternate up- and down-canyon near-bottom flows have been widely documented along the upper reaches, connecting the canyon heads with the contiguous outer shelves and vice versa. Nonetheless, we still miss clear evidence of bedform fields expressing these complex patterns. In this study, through a multi-scale analysis in both space and time, we document rare asymmetric bedforms, up to 880 m long and 10 m high, developing within a depth range of 168-220 m at the head of the Whittard Canyon (NE Atlantic). One field of well-developed sandwaves has an atypical up-slope asymmetry, with the steeper slope facing the shallower regions of the shelf, and contrasting with surrounding down-slope sandwaves facing the canyon. The bedforms are interpreted to represent both up-slope and down-slope bottom currents connecting the upper reaches of the canyon to the outer shelf on the southern Celtic Margin, in the Bay of Biscay. The sandwaves were surveyed with shipboard Multibeam bathymetry (5 m grid cell resolution), AUV sidescan sonar (0.15 m grid cell resolution) and ROV footage, and sampled with three ROV-mounted vibro-cores and two box-cores. Sidescan sonar mosaics groundtruthed by sediment samples and ROV footage show with unprecedented detail spectacular trains of fresh overprinting megaripples, previously undocumented sand peaks and bowl-shaped depressions on the crests of the tallest sandwaves. Differences in sedimentary settings and benthic habitats indicate that these features are currently active in particularly dynamic areas, allowing for very slow migration of sandwaves. Numerical modelling together with concurrent hydrographic observations suggest large-amplitude semi-diurnal internal tides, possibly transitioning to asymmetric internal bores, as the main mechanism maintaining the mapped up-slope sandwaves. This work highlights the importance of uncommon sediment dynamics in canyon head environments and adds insight to the traditional notions of gravity-driven processes, being dominant in these environments, envisaging implications for improving geo-hazard assessment of mobile substrates and quantification of offshore sediment and carbon fluxes.

Pressure Sensing Technique for Observing Seabed Deformation Caused by Submarine Sand Wave Migration

Long-term, continuous in-situ observation of seabed deformation plays an important role in studying the mechanisms of sand wave migration and engineering early warning methods. Research on pressure sensing techniques has examined the possibility of using the temporal characteristics of the vertical deformation of the seafloor to identify important factors (e.g., wave height and migration rate) of submarine sand wave migration. Two pressure sensing tools were developed in this study to observe the seabed deformation caused by submarine sand wave migration (a fixed-depth total pressure recorder (TPRFD) and a surface synchronous bottom pressure recorder (BPRSS)), based on the principle that as a sand wave migrates under hydrodynamic forcing, the near-bottom water pressure, bottom pressure and total fixed pressure synchronously change with time. Laboratory flume experiments were performed, using natural sandy sediments taken from the beach of Qingdao, China, to better present and discuss the feasibility and limitations of using these two pressure sensing methods to acquire continuous observations of seabed deformation. The results illustrate that the proposed pressure sensor techniques can be effectively applied in reflecting elevation caused by submarine sand wave migration (the accuracy of the two methods in observing the experimental bed morphology was more than 90%). However, an unexpected step-like process of the change in sand wave height observed by BPRSS is presented to show that the sensor states can be easily disturbed by submarine environments, and thus throw the validity of BPRSS into question. Therefore, the TPRFD technique is more worthy of further study for observing submarine sand wave migration continuously and in real-time.

Morphodynamic response of sand waves in the Taiwan Shoal to a passing tropical storm

Data from three repeated multi-beam echo-sounder surveys of pre-storm, post-storm, and one year after the storm were used to study the response of sand waves in the Taiwan Shoal to the tropical storm, Talim. The measurements showed significant decreases in wave height of giant sand waves between post-storm and pre-storm, and moderate increases in one year period after the storm. The maximum decrease in wave height is more than 2 m and the averaged decrease is 1.2 m along the measured profile. The linear regression analysis shows an increasing trend of wave height changes from northwest to southeast along the profile. The uneven evolution pattern was possibly caused by the asymmetry of the currents induced by the cyclone winds. The analysis based on the Morlet wavelet function showed that the storm smeared out small-scale sand waves and eroded the giant sand wave crests, and recovery of bedforms after the storm. Two-dimensional features of storm effects were analyzed using the imagery of a Digital Terrain Model processed in a 2D survey area for pre-storm, post-storm and one year after the storm. The analysis revealed sand wave migration processes during the storm and the growing-back feature of the eroded wave crests after the storm.

BIOSTRATIGRAPHY AND DEPOSITIONAL SETTING OF PUNTA MANNA MEMBER TYPE-SECTION (NEBIDA FORMATION, LOWER CAMBRIAN, SW SARDINA, ITALY)

Punta Manna Member is the younger unit of the Lower Cambrian Nebida Formation in southwestern Sardinia. Sedimentological analyses aim to characterize the type-section, formally and and briefly described by Rasetti in 1972.
 The sequence has been divided into three units on the basis of its lithological features.
 The lower unit (Oolitic Unit) mainly consists of Girvanella - archaeocyathan mounds alternated with oolitic grainstones and/or sandstones containing scattered accumulations of trilobite debris. The depositional setting corresponds to a littoral environment characterized by the alternated input of land and sea-derived sands, and where the mounds were periodically destroyed or buried by migrating sand waves.
 In the middle unit bioconstructions and archaeocyaths virtually disappear whereas trilobites were accumulated by currents and waves. Siliciclastic and oolitic sands prevail but are gradually replaced by quartzitic sands and early dolomitic muds. The appearance of these latter suggests a climatic trend towards arid condition in a depositional environment evolving to a tidal lagoon.
 In the upper unit the development of early dolomitic algal mats alternating with quartzitic sandstones, siltstones and shales, attests the establishment of a sabkha complex at first influenced by siliciclastic input coming from an arid continent.
 The low diversity of the fauna throughout the section appears to reflect the severe conditions of life resulting from the high variability in energy and salinity of the depositional setting.