Bernadette Tessier

Upper intertidal rhythmites in the Mont-Saint-Michel Bay (NW France): perspectives for paleoreconstruction

Abstract The Mont-Saint-Michel Bay constitutes a good present-day example of sedimentation and facies in an upper intertidal domain. The tangue , a specific sediment of this area, displays an alternated structure of sandy and silty-muddy beds generated by the dynamics of tidal currents. This tidal bedding is frequently characterized by a vertical organization determined by the cyclic evolution of tidal energy during a neap/spring cycle. The organisation mainly consists in thickness variation of the successive sand/mud couplets which represent the deposit of each semi-diurnal tidal cycle. This record of tidal cyclicity can be observed in a wide range of bedding from planar bedding to climbing ripple bedding. The investigations performed allowed to clarify the conditions of deposition and of short-term preservation of these tidal rhythmites. Most well recorded neap/spring cycles develop within protected environments and necessarily require high suspended sediment concentrations. Characteristic features of these modern tidal rhythmites, such as the number of tidal event recorded per neap/spring cycle, are discussed in order to select criteria for the recognition of tidal dynamics in ancient sediments. The use of such criteria should constitute a way to discriminate subtidal to upper intertidal deposits and to propose paleogeographic reconstructions.

Surficial sedimentology of the Middelkerke Bank (southern North Sea)

Detailed surficial investigations over the Middelkerke Bank, a tidal sand bank in the southern North Sea, revealed the relationship between morphology, surficial structures and grain-size parameters. Data from 85 grab samples all over the bank show that on a bank normal profile, the coarser, CaCO3 rich and badly sorted sediments are generally located near the highest point of the bank, seaward at the northern end and landward at the southern end. Sedimentary structures were studied from 239 boxcores sampled on all the morphological units of the bank: crest, flanks and adjacent channels. In the shallower parts, foreset beds are preserved while in the deeper zones, intense bioturbation arises and destroys any structure. The combination between these data and the virtual absence of wave-induced structures indicates that the main agents responsible for the bank shaping are the tidal currents.

The Celtic Sea banks: an example of sand body analysis from very high- resolution seismic data

Very high-resolution seismic data from the Kaiser-I-Hind sand bank (southern Celtic Sea) recently highlighted the internal structure of the enigmatic Celtic Banks, which are among the deepest and largest shelf sand ridges. The main body of the bank is made up of 4 seismic/depositional units which reflect a transgressive evolution. New data on the detailed architecture of two of these units allow discussion of bank growth in terms of either (1) a channel–levee system preserved both by lateral migration and aggradation of the channels, or (2) a package of large offshore tidal sediment bodies (bar chains and/or giant dunes). Careful geometrical observations of seismic discontinuities make the second hypothesis more likely. The unit architecture is analysed in terms of long- to short-term processes of build-up. Long-term processes are evinced by the landward stacking of erosive sub-units in response of the last post-glacial sea-level rise, whereas short-term processes control the seaward progradation of sand bodies and fills due to the ebb predominance of the Western Channel Approaches.

Field and Seismic Images of Sharp-Based Shoreface Deposits: Implications for Sequence Stratigraphic Analysis

ABSTRACT Sharp-based shoreface sandstones are of considerable interest because of their potential as hydrocarbon reservoirs and because they play an important role in the stratigraphic analysis of basin fills. The sharp-based shoreface sandstones studied herein are Upper Jurassic (Kimmeridgian-Tithonian) and exposed along the coastal cliffs of the Dover Strait in northwestern France. These series consist of tens of meter-thick alternations of sandstones bodies and organic-rich shales that can be correlated for over 30 km along coastal cliff exposures and tied to high-resolution (ca. 1 m of sediment) marine-seismic profiles obtained several hundred meters offshore. The units described here comprise two sharp-based sandstone bodies. Each is composed of a basal progradational set of shoreface parasequences overlain by a progradational-aggradational shoreface succession. Each sharp-based sand body lies on a marine regressive surface of erosion and is truncated by a marine transgressive surface of erosion, which in turn is overlain by a thin retrogradational ravinement lag or coarse-grained, planar-laminated bedset. The two progradational packages are separated by a third surface, a subaerial exposure surface that is interpreted as a sequence boundary. Two distinct types of seismic units, referred to as type A and type B, have been identified in the study area. Seismic unit A has conformable upper and lower boundaries and parallel (aggradational) configurations; seismic unit B is characterized by downlap and toplap boundaries and simple or compound, sigmoid and oblique-tangential (progradational) configurations. A single sharp-based shoreface sandstone body makes up the type B unit and typically consists of two compound superimposed progradational sets (B1, B2). The lowermost set, B1, corresponds in the field to the progradational set sensu stricto, whereas B2 corresponds to a progradational-aggradational set. B1 and B2 are separated in outcrop by a sequence boundary. These observations led us to reevaluate the sequence stratigraphic interpretation of sharp-based shoreface sandstones. It is proposed here that complete, single, sharp-based shoreface sandstones bodies can be separated into two different systems tracts: (1) a progradational set (B1 seismic body) at the base, which corresponds to the forced regressive wedge systems tracts (FRWST) of Hunt and Tucker (1992), and (2) a progradational-aggradational set (B2) at the top, above the sequence boundary, which corresponds to the lowstand systems tract (LST) of Posamentier et al. (1992). A complete sharp-based shoreface sandstone body is bounded below the FRWST by a regressive surface of marine erosion caused by the downward shift of wave base, and by a transgressive surface of marine erosion, or ravinement surface, at the top of the LST.

Morphodynamique d'une plage macrotidale à barres

The topography, currents, waves, and sediment dynamics of a macrotidal ridge and runnel beach were monitored from April 24 to May 4, 1997 in order to characterise the morphodynamics of this beach type under fair-weather and storm conditions. Sand tracer experiments show moderate longshore and cross-shore transport components while profile surveying highlights a remarkably stable system. These trends are not in agreement with the current pattern, which is dominated by an essentially tidally driven longshore component. The stability of the ridges reflects the high rates of migration of the breaker/swash zone in response to the important vertical tidal excursion across a wide, low-gradient beach.

Comparison between high-resolution seismic and sequence stratigraphic approaches applied to the Upper Jurassic deposits of the Dover Strait area (Northern France)

Abstract This study is an attempt to interpret very high resolution seismic data, usually devoted to shallow marine surveys, in terms of very high resolution seismic and sequence stratigraphy at the scale of the reservoir. Seismic data are compared with outcrop-based observations in order to discuss the geometrical relationships between rock bodies, and the nature of the seismic reflections in terms of seismic facies related to type lithologies. Two high-resolution seismic surveys, using a sparker, were performed in the Dover Strait area some 100s of metres away from the Boulonnais coastal cliffs. The aim was to look to the 130 m-thick, mixed siliciclastic-carbonate deposits of Upper Jurassic age (Kimmeridgian-Tithonian) cropping out at the sea floor and to compare them to the coastal cliffs exposures. The 100 m apart seismic profiles exhibit a 100 m deep penetration and 1 m in vertical resolution (upper 30 ms part of the profiles). Four seismic facies are recognised on the seismic profiles. They comprise ten seismic units which can be grouped into three broad families. Group 1 has a parallel/aggradational reflection configuration and conformable boundaries which may correspond to the transgressive and highstand systems tracts. The seismic facies exhibit either a high continuity-high amplitude (corresponding to clayey-mudstone lithologies of the mid ramp environment) or a high continuity-low amplitude (corresponding to shaly lithologies of the outer ramp environment). Group 2 has a sigmoid-progradational reflection configuration, bounded at the base by a downlap surface and at the top by a toplap surface. These boundaries correspond respectively in the field to a marine regressive surface of erosion and a marine transgressive surface of erosion. The seismic facies is cross-layered and exhibits a moderate continuity, low to moderate amplitude and moderate frequency. Group 2 corresponds to the bioclastic, sand-prone facies of the inner ramp environment. It is interpreted as lowstand sharp-based shorefaces. Group 3 exhibits a progradational, complex to chaotic configuration. It lies on an erosional unconformity with paleogullies. The seismic facies shows discontinuous reflections with moderate amplitude and high to moderate frequency. It corresponds in the field to cross-bedded sandstones in channel fill arrangements of fluvial origin (Wealdian deposits) marking the top of the section. The different systems tracts, sequence boundaries and depositional sequences identified onshore can be recognised as well on the seismic profiles, but the comparison demonstrates that seismic data complement the onshore sequence stratigraphic analysis by providing the geometrical relationships between systems tracts (onlap, downlap, toplap …) that cannot be observed onshore. The maximum flooding surfaces however present no specific seismic signature on such a low angle ramp profile. It can only be identified precisely in the field, which also help to relate lithologies to seismic facies. This study represents one of the first attempts to interpret very high resolution seismic data at the outcrop scale. It will be improved again in the future by the collection of impedance logs and synthetic seismic traces from cored sections on the coastal cliffs for comparison with the seismic data.

Stratigraphie sismique et séquentielle haute résolution des formations du Jurassique supérieur du Boulonnais (Nord de la France)

Abstract Very high-resolution seismic data were obtained from a Kimmeridgian-Tithonian submarine series located off the Boulonnais (northern France). The data were compared with high-resolution sequence stratigraphic results obtained on the same formations cropping out on the adjacent coastal cliffs. The seismic data provide better defined geometrical relationships between the sequences and the surfaces (‘toplap’, ‘downlap’) than the ones identified in the field. Based on the identification of downlap and toplap surfaces, one of the major contributions of the seismic data is to point out that the sandstone bodies isolated in offshore shales are sharp-based shoreface deposits induced by forced regression.

Contributions to Modern and Ancient Tidal Sedimentology: Proceedings of the Tidalites 2012 conference

Description: Tidal deposits have been a specific research topic for about 40 years, and whilst this has resulted in a proliferation of papers in scientific journals, there have only been a few book–length syntheses. Over the years, tidal sedimentology has been reinforced by fluid mechanics and numerical modelling but has remained rooted in facies and stratigraphic studies. Recent developments in tidal sedimentology lean toward a more quantitative assessment of the imprint of tides in the facies record of intertidal and shallow subtidal areas. They highlight the increasing relevance of tidal deposits studies, from high resolution subsurface reservoir geology to climate change and sea–level rise. This volume gathers 17 contributions to the Tidalites 2012 congress held in Caen, France. It reflects current advances in the sedimentology and stratigraphy of tidal deposits, in both ancient and modern environments. It shows the current diversity of this field of research, through a wide spectrum of methods including remote sensing, in–situ hydrodynamical measurements, and ichnology, in addition to classic field studies and petrography.