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. |
