A sedimentary model of the Brussels Sands, Eocene, Belgium

New observations in the central Belgian Brussels Sands (transition Lower to Middle Eocene) allow to build a consistent model of the sedimentary processes and facies relationships of the deposit. The Brussels Sands fill a 120 km long and 40 km wide lowstand complex valley incision, correlated with the Yp10 50.0 Ma global lowstand. A marine transgression penetrated into the incised valleys transforming them first into estuaries and then into a tidal marine embayment. The sediment fill has a highstand signature. Filling started at the western bank, probably fed by a continuous coastal drift inferred to have existed along the southern North Sea coast. The West to East lateral progradation of the embayment shore created the “westerly lateral accretion” arrangement which dominates the Brussels Sands sedimentary record. This arrangement contains many gravity flow deposits, thought to be caused by breach failure. That mechanism conveyed coarser-grained sand from the embayment shore environment to the embayment floor. As the hydraulic section of the embayment narrowed, “flow-section restriction” events became more frequent, on either local or regional scale, depending on the presence and magnitude of intraformational relief and inherited paleorelief highs. Due to the section restriction, flow currents were increased locally causing lateral and vertical erosion to generate a short-lived accommodation space increase. Sedimentation outpaced the space creation rate and the narrowing channels and scour pits were filled with thick cross beds. Each time after filling the local space, the westerly lateral accretion resumed. The embayment closed with the deposition of the coarse glauconite sands in the East of the basin. A successive sea-level rise, following Lu1 at 48.1 Ma, caused a marine ravinement to truncate at least 10 to 20 metres of the top of the Brussels Sands

Evaluation of a shoreface nourishment in De Haan: analysis of 20 years of data

In the framework of the implementation of the Master Plan for Coastal Safety and Vlaamse Baaien project, an assessment of the added value of shoreface nourishment as coastal protection measure and as alternative to classic procedures for beach nourishment maintenance will be performed. This project, entitled “Shoreface nourishments as coastal protection measure”, is carried out at Flanders Hydraulics Research with as central activity the monitoring of a pilot shoreface nourishment that will take place in 2013 in Mariakerke.The only antecedent of a shoreface nourishment in Belgium is the nourishment performed in De Haan in 1990. This document brings together results from the monitoring carried out in the 90’s as well as new results obtained from later surveys in the study area. The results is a more than twenty years analysis of the morphological evolution of the coast at the Haan. The interpretation of the results is not straight forward due to the various nourishments performed since then in neighboring areas. However, it is possible to conclude that after a local cross-and long-shore redistribution of the sand nourished in the shoreface, this sand has not been transported and therefore has remain in the system acting as sand supply for the beach.A general reduction of the background erosion has also been detected during this analysis but the fact that during the study period no severe storms occurred makes it difficult to draw a definitive conclusion in this regard.Taking into account the difficulties encountered to draw conclusions on the evolution of the beach and shoreface in De Haan during this study and the goals of the new project it is recommended that during the monitoring of the pilot shoreface nourishment to be started in 2013 no other nourishments on the beach and/or neighboring shoreface and beaches are executed

Sand dynamics along the Belgian coast based on airborne hyperspectral data and lidar data

The goal of this project was to explore the possibilities of airborne hyperspectral data and airborne lidar data to study sand dynamics on the Belgian backshore and foreshore. The Belgian coast is formed by a sandy strip at the southern edge of the North Sea Basin which is commonly known as the Southern Bight. Since the beach is prone to structural and occasional erosion, it is very important to obtain a better understanding of the processes controlling it. The combination of multi-temporal hyperspectral data and lidar data provides a suitable tool for follow-up of the Belgian coastline, and sandy coastlines in general. Hyperspectral imagery generates a reflectance spectrum for each pixel in the image. The shape of this spectrum is influenced by the composition of the topsoil of the beach, being mainly the mineralogical composition and the grain size. A Spectral Angle Mapper (SAM) algorithm was used to perform a supervised classification of the hyperspectral images in order to distinguish between different sand types. Digital terrain models (DTM’s) with a mean vertical accuracy of 5 cm were generated from lidar data. By differencing a DTM from September 2000 and one from September 2001 a map with sedimentation and erosion zones was generated. By combining the erosion/sedimentation map with the classified hyperspectral images, dating from August 2000 and August 2001, an appropriate and cost-effective method was found for studying the processes of sand transport along the Belgian coastline

Vergelijkende studie van de afzettingsstruktuur van getijzanden uit het Eoceen en van de huidige Vlaamse Banken = Comparative study of the depositional structures of tidal sands from the Eocene and from the modern Flemish Banks

It has been shown in the recent past that the study of sedimentary structures (the type and geometry of the units that build up a sedimentary rock) considerably contributes to our knowledge of the conditions that prevailed during the deposition. This monograph brings a comparative study of sedimentary structures of three shallow marine sand deposits in Belgium: those of a modern, active environment (the Flemish Banks off the French-Belgian North-Sea coast) and two ancient examples (the late Lower Eocene Vlierzele Sands and the early Middle- Eocene Brussels Sands). The introductory literature section discusses the interpretation of sedimentary structures in loose, clastic sediments of the shallow-marine clastic tidal environment. It puts together some relevant reviews from recent dynamic and experimental studies, bedform morphology, and the classification and interpretation of sedimentary structures. It appears that, especially in the subtidal environment, there is quite a lot of confusion in terminology and a lack of study concerning the dynamic interpretation of bedforms occurring and the internal structures resulting. For the purpose of this study, the following main definitions are adopted. A megaripple is a two-dimensional bedform generated by a unidirectional current, that flows sufficiently long over a bed of loose sand grains and applies bottom shear stresses, greater than those needed for the migration of (common) small-current ripples, but smaller than those required for the flattening of the bed morphology (that would result in the upper flow regime plane beds). Megaripples have straight to sinuous crestlines oriented perpendicular to the flow direction. In cross-section, they are asymmetric, their steeper (downstream) lee faces sloping between 20° and 35° . Megaripples are higher than 5 cm and their spacing (wavelength) is greater than 60 cm. Sandwaves are also flow-transverse bedforms, essentially of larger dimensions than megaripples. Megaripples always cover the active sandwave flanks and it is the megaripple migration that causes the movement of the sandwaves. Sandwaves are typically associated with the tidal environment. Their height ranges from 1 to 10 m and more, while their spacing (wavelength) attains 80 to 500 m and more; the sandwave flanks have normally slopes of only 1 to 7°. Their cross-profile may vary from asymmetric to symmetric. From our literature survey and from our own data, we believe that sandwaves occur where at the same time there is a sufficient supply of sand, and where both the ebb and flood currents are strong enough to create megaripples. In the literature review furthermore some recent publications on the relationship between tidal hydrodynamics and megaripple morphology are discussed. Special attention is also paid to the recent progress in bedding (stratification) characteristics of sediments deposited in highly dynamic tidal nearshore and inshore environments. Chapter 3 opens with a literature discussion concerning actual North Sea tidal sandbank (tidal current ridge) morphology and dynamics. The Oost Dijk and Buiten Ratel sandbanks are two typical sand ridges of Ch There is no overall model yet to explain the sandbank origin or to predict its internal structures. Our approach to contribute in this research was to obtain undisturbed surface sediment samples (using a small Reineck boxcorer) on relevant morphological areas of the Oost Dijk and Buiten Ratel sandbanks and to study the sedimentary structures made clear on lacquer peels. From the 37 samples three main sediment structure facies could be recognized (megaripple cross-bedding X, horizontal bedding H, bioturbated sands B) and further subdivided into eight subfacies. The undisturbed megaripple cross-bedded sand was interpreted as the deposition, caused by the migration of active, tidal megaripples of only a few decimetres height. The horizontally bedded samples are produced either directly by surface wave action, or as the result of storm wave deposition. The bioturbated structures are mainly attributed to the action of sea urchins and Polychaeta worms. The presence of these species indicates a tidal flow that is too weak to produce bed-load sand transport, but strong enough to prevent the settling of mud. The kind and occurrence of sedimentary facies is closely related to the position of the sampling stations with respect to the sandbank morphology. This consistent relationship is, together with re-interpreted published data on hydrodynamics, bed morphology, and grain-size characteristics, incorporated into a preliminary, qualitative modelof fair-weather sediment dynamics on the Flemish Banks. It is thought that though the near-bed sediment transport paths converge to the sandbank crest area, the resulting sand transport is partly counteracted by transport in suspension from theshallowest parts of the sandbank to the surrounding deeper areas. The main points of our model are: (1) sand sedimentation from suspension is the main source of sediment in the inter-bank channels and on the lower mild, landward sandbank slopes. (2) the steeper slopes, facing the regionally dominant flood currents, are erosional. (3) continued action of only the fair-weather tidal sediment processes would ultimately result in a landward shift of the sandbanks. As there is no measurable shift of the sandbank position over the past century, it is supposed that storm-wave effects combined with storm-enhanced tidal currents would compensate for the landward sediment transport. How this mechanism exactly works still remains to be elucidated. Sedimentary structures of the Brussels and Vlierzele Sands were studied in outcrops throughout Central Belgium. Both clastic tidal deposits appear in an area, little affected by tectonics, and occupy nearly the same position in the biostratigraphic scale (transition Lower-Middle Eocene); but as they don't overlap in outcrops, and as both deposits are restricted to a limited area, their relative position still is in discussion. The Brussels Sands are characterized by an irregular base, while their top surface is very smooth. Thicknesses may locally amount to up to 80 m. The erosive character of the Brussels Sands base is thought to be related to a strong SSW-NNE tidal current, that eventually produced longitudinal scour troughs. One remarkable, well-localized, highly glauconiferous, depositional facies of the Brussels Sands, characterized by thick trough-shaped (XI) and, mostly, tabular (X2) cross-beds, was studied in detail. lts sedimentary structures include: thick cross-beds with thick bottomsets and eroded top; many unidirectional reactivation surfaces; mud-clad burrows; mud drapes, many of which are believed to be incompletely developed. As to grain size, the facies consists of a mixture of fine and medium sands; but also a coarse to very coarse subpopulation is present. The deposit is the result of the migration of asymmetric, mostly straight-crested transverse bars fiIling up quite narrow (1 km?), shifting, tidal channels. The dominant tidal flow was from SSW to NNE. Sand supply was abundant; due to the presence of transported glauconite, it is most probably derived from the open shelf. The other facies determined in the Brussels Sands are all part of a continuous succession X3-XB-B/HB. Master bedding is conform with the (often erosive and sloping) lower surface of X3, and graduaIly becomes horizontal. At the same time, the amount of (tidal mud-draped) cross-bedding diminishes, while grain size decreases, and the degree of bioturbation and the content of carbonates increase. The homogeneous to faintly parallel-laminated, medium-sized and very well sorted facies H is several times intercalated within this succession. Due to its highly erosive base, its good sorting, its lack of any bioturbation traces and its vague parallellamination, the facies H is interpreted as a storm deposit, that periodically interrupts the lateral accretion of the X3-XB-B/HB succession. The lateral relations between the different Brussels Sands facies are very complex and some still remain hypothetical. In general, the fine-grained, carbonate-rich facies (B and HE) are the only facies to occupy the top part of the Brussels basin fill. Our paleogeographical reconstruction should therefore be regarded in a preliminary context. The reconstruction assumes (i) at the transition Ypresian-Lutetian there is a global low sea level; (ii) at the time of the beginning deposition of the Brussels Sands, there is no connection between the North-Sea basin and the Paris Basin via a narrow seaway over the Ardennes-Artois tectonic high, so the "Brussels" transgression came from the north; (iii) no local tectonic events, other than a regional, slow subsiding and tilting, influence the sedimentation of the Brussels Sands.There is a schematic representation of the different stages thought to correspond to the different Brussels Sands depositional history: 1. Low sea level during transition Ypresian-Lutetian. Low or moderate tidal range. Some consequent river valleys are developed in a relatively flat landscape. Glauconite is being formed on the North-Sea shelf. 2. Rapid sea-level rise. The sea extends into the Brabant river mouths that are transformed into estuaries and/or tidal inlets. Due to their favourable dimensions with respect to the tidal system supposed to exist in the open North Sea, a situation of tidal resonance (enhancement) is installed in the Brabant tidal inlets. 3. Considerable, tidal erosion and incision in the Brabant area. Separate tidal channels are installed. They have local deeps of up to 80 m. During decreasing sea-leve1 rise, large amounts of clastic sediment mixed with glauconite are carried from the open North Sea into the estuaries or tidal channels. They are deposited as transverse bars, that migrated to the north in ebb-dominated, rapidly shifting channels. 4. Decreasing sea-level rise. Supply of coastal-reworked clastics goes on, under decreased tidal energy. Supply of glauconite terminated. In cut-off tidal channels, there is deposition of fine sand and a limited input of carbonate mud of local origin. There is now also a higher production of biogenous silica, that will be the source of the siliceous cement of the typical Brussels Sands concretions. The Brabant area develops into a semi-enclosed bay in which occasional catastrophic wave events produce the enigmatic facies H. 5. High sea-level stand. Tidal range reduced to moderate or low. Deposition of fine-grained, carbonate-rich facies B and RB. 6. Slight regression (possibly linked with sea-level fall) before the deposition of the Lede Sands. In the Vlierzele Sands, four main sedimentary facies were recognized. The lower one (B), cbaracterized by a dominance of fine-grained, bioturbated sand, was only found in the type section at Vlierzele. Three other facies alternate in a succession of up to 10 m thickness, whose units are no more than lor a few metres thick and laterally pinch out. Facies X has decimetre-scale megaripple cross-bedding, often with herringbones and mud drapes. Facies P is characterized by low-angle beds of parallel lamination, thin megaripple cross-beds and small-ripple crossbedding. Facies H consists of massive structured sand or faintly visible, parallellamination. The geometry of the facies described is illustrated. In many of the outcrops of the Vlierzele type area, the top of units of facies X shows completely preserved megaripple form sets and even of partly preserved, 1 m high sandwaves. The megaripples apparently were part of rather extensive, slightly sloping megaripple fields, that were partly eroded and partly left untouched by the event that occasioned the covering mass of facies H. Three of these megaripple field levels were progressively measured and mapped as excavation works in the outcrops proceeded. From these maps and our outcrop surveys, it is concluded that the megaripples are generally 25 cm high, have spacings of 5- 8 m, and are relatively straight-crested. They form megaripple fields covering slopes of 2° (locally up to 6°) dip to the north (the open sea). Their crestlines are not completely perpendicular to the megaripple field contour lines; instead, they form angles of 10-15°. All megaripples belonging to one field are in a similar evolution stage with respect to their morphology, shaped by an almost symmetrical, strong tidal flow; thus they were all preserved at the same time. As to their sedimentological environment, it is put forward that the different facies described could have developed in a clastic shelf tidal current ridge environment. The very active, tidal megaripples represent dynamic sediment movements at the steeper flanks of the sand ridges. The fine sand fractions are winnowed out and carried to the top and the landward mild slopes of the sandbanks, thus forming the facies P and B. The megaripple fields of the steep sandbank slopes were "fossilized" more than once, under a cover of unstructured sand (facies H), that is interpreted as a rapid deposition of suspended sand. The suspension was probably due to the extremely powerful water movements, occasioned by high waves breaking on the sandbanks. The Vlierzele Sands are considered as a fossil equivalent of the Flemish Banks depositional environment, whereas the Brussels Sands are thought to be the product of the evolution from a tidal inlet or estuary environment to a more open shelf bay. These sedimentary conditions don't necessarily exclude a contemporaneous, lateral setting