This chapter contains sections titled: Introduction Clay Mineral Associations of Late Eocene-Early Oligocene Age Clay Mineral Variations in the South Atlantic and Southern Ocean Clay Mineral Variations in the Northern Hemisphere Global Evolution of Climate and Circulation at the Eocene/Oligocene Transition Conclusion and Possible Scenario for Climatic and Oceanographic Development at the Eocene/Oligocene Transition

Detailed mineralogical and geochemical studies were performed on samples from selected time intervals recovered during Leg 79 on the Mazagan Plateau. The uppermost Albian and Cenomanian sediments of Sites 545 and 547 can be correlated on the basis of mineralogy and geochemistry; these sediments illustrate differential settling processes and the existence of hot climates with alternating humid and dry seasons in the African coastal zone. The upper Aptian to Albian black shales of Site 545 point to an irregular alternation of tectonic activity and relaxation stages, allowing different behaviors in the reworking of soils, crystalline rocks, and sediments born in peri-marine basins. The barren lower Mesozoic reddish sediments and evaporitic series of Sites 546 and 547 are characterized by a strong physical erosion of sialic landscapes, without clear evidence of post-depositional metamorphic events. At Site 546 strong early diagenetic processes in a confined evaporitic environment affect both the mineralogy and the geochemistry of pre-Miocene rocks.

Detailed investigation (630 samples) of clay mineral assemblages and benthic foraminifer accumulation rates (BFAR) as a proxy for paleoproductivity has been conducted on two late Eocene–Oligocene sequences drilled at intermediate water depths of the Kerguelen Plateau and Maud Rise (Southern Ocean). Distinct changes in clay assemblages and BFAR – that are in general a factor two higher on the Maud Rise compared to the Kerguelen Plateau – unravel a step-by-step evolution of marine and continental Antarctic environments that eventually resulted in permanent ice coverage and dense cold water formation: (1) late Eocene dominance of chemical weathering was associated with intermittent erosion of Antarctic soils and substrates. Main erosional events are recorded at 36 and 34 Ma, prior to the major earliest Oligocene glacial event, and are especially noticeable in the Indian sector of the Southern Ocean; (2) physical weathering progressed in continental areas adjacent to the Maud Rise and intermediate water production intensified in the Indian sector of the Southern Ocean throughout the Eocene–Oligocene transition, as the Tasmanian seaway developed. Environmental changes and a strong 600 kyr lasting peak of paleoproductivity seem closely associated with the earliest Oligocene isotope shift and the accumulation of ice-rafted detritus on the Kerguelen Plateau; (3) aridity and physical weathering progressed in East Antarctica near the Maud Rise (Site 689) at 33.2–33.15 Ma, as polar transfer of heat within the deep ocean increased slightly. This may have prevented extensive ice development during at least part of the early Oligocene; (4) increased productivity, vertical mixing and turnover at Site 744 on the Kerguelen Plateau, followed by a two-step increase of physical weathering on the adjacent continent from 32 to 31.2 Ma, suggest progressive homogenization of climates and circulation as the passageway South of Tasmania widened and deepened; (5) physical weathering reached a maximum in East Antarctic areas adjacent to the Maud Rise and a hiatus developed on the Kerguelen Plateau during the mid-Oligocene Oi2–Oi2a interval (31–26.5 Ma), in possible relation with opening processes of the modern Drake Passage and Scotia Sea and a further step in the development of oceanic circumpolar circulation; (6) near-400-kyr cycles of productivity and clay mineral composition throughout the two sequences suggest that orbital frequencies played a role in the small-scale variations of both continental and marine Antarctic environments of the late Paleogene.

Abstract The clay mineralogy of Pleistocene sediments of eleven sediment cores at three ODP sites from 30°N to 60°N northwestern Atlantic Ocean has been investigated. The sediments are characterized by the presence of random illite-vermiculite mixed layers (I-V) (up to 32% of the clay mineral assemblage). The I-V clays are much more abundant during interglacial periods than during glacial ones. They are attributed to detrital supply through erosion of high-latitude continental areas from which they are derived mainly from chemical weathering of micaceous phyllosilicates. Their spatial distribution and the specific conditions for their formation through weathering suggest that I-V mixed layers are mainly derived from the Canadian Shield. They were transported from their source to marine deposition areas by rivers, deep water masses or nepheloid layers. Due to the specific conditions required for the formation of I-V, its occurrence and abundance are used as a palaeoclimate and palaeocirculation proxy for northwestern Atlantic Pleistocene sediment.

The present paper deals with dune dynamics in a zone of the Strait of Dover located in the sea lane running into the North Sea. The dunes, widespread in this 35-m depth area, are mobile sedimentary structures (up to 40 m.yr(-1)) that culminate at a maximum of 22 m depth and endanger navigation as well as submarine man-made structures (cables, pipelines). Single- and multibeam bathymetric data, coupled with seismic data, allow us to follow dune displacements over different time scales. A net bedload parting zone has been displayed and divides the area into two parts, SE and NW. However, according to the considered time-scale, dune movements present Variations in intensity and direction. Over a long-term period (decades), sedimentary dynamics fit the regional scheme of the residual tidal currents that induce transport toward the SW and the NE in the respective SE and NW parts of the studied area. Over a medium-term period (several years), meteorological data show that a high frequency of storm winds alters the residual sand transport characteristics by strengthening, slowing or reversing the effect of the tidal currents and can lead to the reversal of dune asymetry. (C) 2000 Ifremer/CNRS/IRD/Editions scientifiques et medicales Elsevier SAS.

Clay mineral investigations have been performed on more than 500 limestones and shales sampled in Lower Devonian (Emsian) to Lower Carboniferous (Namurian) outcrops in the Dinant and Avesnes Basins (Ardenne Massif, NW Europe). Clay mineral data have been placed in the palaeoenvironmental and structural histories documented by previous lithological, stratigraphical, palaeontological, diagenetic and tectonic contexts. The clay associations are dominated by illite and chlorite derived partly from the erosion of land masses surrounding the marine domain. The geothermal gradient estimated from correlation with conodont colour alteration index ranges between 40 and 70°C/km. A diachronous northwards migration of the diagenesis/metamorphism interface links to uplift caused by Late Carboniferous compressional folding and overthrusting. Associated clay minerals include smectite, locally preserved from diagenetic changes mainly by early pore closure, that reflect lagoonal or quiet offshore marine conditions. Smectite and subordinate kaolinite abundances decrease upwards during the Devonian in three successive intervals suggesting alternations of sub-arid to drier climates. The local occurrence of corrensite (ordered chlorite–smectite mixed-layer) is attributed to the moderate diagenetic transformation of pre-existing smectite

TESSIER, B.; CORBAU, C.; CHAMLEY, H., AND AUFFRET, J-P., 1999. Internal Structure of Shoreface Banks Revealed by High-Resolution Seismic Reflection in a Macrotidal Environment (Dunkerque Area, Northern France). Journal of Coastal Research, 15(3), 593-606. Royal Palm Beach (Florida), ISSN 0749-0208. The large-scale internal structure of shoreface banks located off Dunkerque (N France) in the Southern Bight of the North Sea is imaged in a very-high resolution seismic reflection study. An interpretation is proposed of the processes controlling the construction and migration of these banks located in a macrotidal coastal environment. The Dunkerque coastal system is characterized by strong, coast-parallel, tidal currents, and northerly moderate storms. The banks are coast-parallel and almost emergent during low-water spring tides. Their length, width and maximum height reach 8 km, 1.5 km and 12 m respectively. They have an asymmetrical profile with a steeper flank dipping either landward or seaward. Bathymetric investigations show that some parts of the banks migrate actively landward, while others tend to elongate or to migrate seaward. The steeper flank indicates the direction of the dominant migration. Seismic data reveal one main feature: the reflectors observed beneath the landward flank dip landward, and the reflectors observed beneath the seaward flank dip seaward. This large-scale internal structure conforms with the bank morphology and reflects the present-day migration mechanisms. According to bathymetric, hydrodynamic and seismic data, the landward migration component is induced by northern storm wave action and the longshore/ seaward component is tide-controlled. Seismic data show that this migration pattern was occasionally reversed, some parts of the banks that presently migrate longshore having migrated landward at an earlier time (and vice versa). The predominance of storm-induced landward migration over tide-induced longshore migration (and the reverse) is related to water depth and surrounding seabed morphology evolution. The banks are thought to be recent sedimentary features of a few centuries. They represent very active sand bodies the migration of which induces significant morphodynamic modifications of the coastal system, especially of the beach domain to which they tend to attach.

The clay mineral assemblages of the Cretaceous-Tertiary (K/T) transition recorded in southern Peten sedimentary rocks have been studied in sections dated by calcareous nannofossils and foraminifera. The deposits contain cosmic and impact remains (iridium, Ni-rich magnetite, tektite) and typical Cheto smectites resulting from weathered tektites (glass spherules) produced by the Yucatan impact. The clay mineral assemblages are used for describing the palaeoenvironmental and tectonic evolution of southern Peten near the K/T boundary.

The sedimentary response of deep shelf sediments to the interplay of tides and waves, based on the example of the Celtic Sea sand banks is described. Grab samples and 1500 km of multibeam and side scan data have been collected from the 300 km2 densely surveyed Kaiser bank area. The bank is 140–170 m deep, 30 m high and 60 km long, oriented perpendicular to the shelf edge and nearly parallel to the major axis of the tidal ellipse. Surficial sediments of the bank consist of medium to gravely biolithoclastic sands. They are swept by tidal currents that reach 0.9 m/s 1 m above the seabed, and under the occasional influence of waves. The mobile sands are commonly bedformed and rest on a highly backscattering lag. The flanks below −140 m mainly exhibit transverse tidal dunes and sand ribbons, whereas the top of the bank mostly displays discontinuous sand patches and wave ripples. The sand patches are interpreted as the remnants of tidal bedforms reworked by waves. Calculations of the threshold of motion of four modal grain sizes under various conditions at the seabed show that tidal bedforms are active during spring tides sometimes associated with waves, whereas only the largest annual waves may explain the observed wave ripples. As in active tidal banks, there is a reversal of tidal bedload transport from one bank side to the other. The sedimentation rate is very low on the Kaiser bank. In addition, there is a loss of sediment at both bank extremities and the tidal bedload cross-bank transfer is very small, probably controlled by shelf residual currents and long-term drift of sediment resuspended by waves. However, the present-day action of waves at the bank top is less intense than it was at lower sea levels during the Holocene, as evidenced by seismic studies.

Very high-resolution seismic data were obtained from a Kimmeridnian-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. ((C) Academie des sciences / Elsevier, Paris.).

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.

The clay mineral assemblages of the Cretaceous-Tertiary (K/T) transition recorded in southern Peten sedimentary rocks have been studied in sections dated by calcareous nannofossils and foraminifera. The deposits contain cosmic and impact remains (iridium,Ni-rich magnetite, tektite) and typical Cheto smectites resulting from weathered tektites (glass spherules) produced by the Yucatan impact. The clay mineral assemblages are used for describing the palaeoenvironmental and tectonic evolution of southern Peten near the K/T boundary.

Differential maps have been digitalized and compared from successive bathymetric and topographic surveys performed under different meteorological conditions from May 1992 to December 1994 in the shoreface-to-beach system of the 30 km long coastal area extending from Gravelines to the French-Belgian border in southernmost North Sea. The data, which concern a low-relief sandy coastal system exposed to a macrotidal regime with flood-driven eastward sediment transport, have been interpreted according to meteorological, hydrodynamical and aerodynamical characteristics.A balanced sediment budget was observed, which contradicts previous data suggesting a progressive long-term erosional trend. This result indirectly underlines the key-role on the sedimentary budget of exceptional events such as severe storms. The two types of erosion identified comprise the action of frontal waves and the combined interaction of tidal currents and wind.Five hydrodynamical cells have been recognized along the shoreface. The corresponding segmentation is attributed to the specific distribution of the wave energy along the coast due to the presence of submarine banks responsible for the deformation of the wave propagation.The morphological changes of the beach depend on the tide and swell action responsible for the construction/migration/destruction of the ridge and runnel system.

At four sites from the Eastern Mediterranean, three to five sapropel layers and surrounding oxygenated sediments from the late and early Quaternary and Pliocene were investigated by a multidisciplinary approach which included analyses of coarse fraction, carbon and sulfur contents, grain size, and clay-minerals. Clay-mineral associations are comparable at all sites and in all time periods, in all sapropels, and oxygenated sediments. There is no diagenetic alteration of clay minerals in the sapropels. The relative importance of the different clay species varies with location of the sites and with time. African and European provenance can be distinguished. In the sapropels clay-mineral associations show changes that suggest increased precipitation in Southern Europe (increase in chlorite, illite, and mixed-layer clays) and intensified atmospheric circulation over Northern Africa and the Middle East (increased amounts of palygorskite and sepiolite). In all sapropels silt-sized particles increase at the expense of clay-sized particles in relation to higher energy of circulation. Sand-sized terrigenous particles have maximum accumulation rates just prior to sapropel deposition. Export productivity during deposition of oxygenated sediments has been calculated from benthic foraminifer numbers. It is highest just prior to sapropel deposition than above the sapropels. Paleoproductivity decreases from west to east and from the Pliocene to the Quaternary. Values from the late Quaternary (30−60 gC/m2·yr) are comparable to values obtained from oceanographic measurements (Bethoux, 1989). On top of Eratosthenes Seamount values are much higher (130 −360 gC/m2·yr). The degree of carbonate dissolution reflects three types of sapropels: (1) nonbioturbated sapropels with excellent carbonate preservation; (2) increase in carbonate dissolution towards the top of the sapropel because of “burning down” phenomena; (3) bioturbated Pliocene sapropels with carbonate dissolution as strong as in surrounding oxygenated sediments.

In 1992, the Nautile went to a mud volcano field located east of the Barbados accretionary wedge near 13 ° 50N. Using nannofossil analysis on cores, we determined the sedimentation rate, and provided a new estimation of the age of the mud volcanoes (750,000 years for the oldest one). Six structures have been explored with the submersible Nautile, and manifestations of fluid venting (chimneys, carbonate cementation and chemosynthetic communities) were observed on all. Sedimentological analysis identifies two sources of diapiric mud. Most mud volcanoes expel mud containing Late Miocene to Quaternary faunae that have the same composition as sediments drilled above the Barbados wedge decollement. One volcano also contains older Oligocene taxa, with a mud composition corresponding to the sedimentary sequence below the decollement. We use diving observations to map the fine-scale morphology, the distribution of chemosynthetic fauna and define two end-member types of structures: mud-pies (flat topped mud volcanoes) and conical mounds. Mud-pies (Atalante and Cyclops) are characterised by the presence of a lake of highporosity mud (70% to 75%) in their central parts. Chemosynthetic benthic communities (Calyptogena colonies and sponge bushes) are concentrated in the outer parts. Contrasting morphologies of the two mud-pies indicate different stages of activity: Cyclops is growing whereas Atalante is collapsing. Expulsion of water and methane occurs mostly through the mud lake and may be stronger during the collapse phase. On conical mounds there are no mud lakes, fluid venting concentrates near the summit and occurs through carbonate cemented chimneys which form within the sediment. Viscosity measurements have been carried out on mud samples from the two mud-pies and one conical mound. All mud samples have a plastic fluid behaviour, the plastic threshold decreases with porosity, and thixotropy is observed for a porosity of more than 70%. An analogue experiment shows that for this thixotropic mud, shearing in the feeding conduit liquefies the mud which then spreads to form a mud-pie. Conical mounds form when the mud remains plastic. We show that the dissociation of methane hydrate is the cause of the high porosity in mud-pies and confirm that these structures are a consequence of large-scale dissociation of methane hydrate at the base of its stability field. Dissociation of hydrates before and during ascent is only slightly contributing to the pore fluid in conical mounds, but solid hydrates still present in the mud may contribute to its buoyancy.

At four sites from the Eastern Mediterranean, three to five sapropel layers and surrounding oxygenated sediments from the late and early Quaternary and Pliocene were investigated by a multidisciplinary approach which included analyses of coarse fraction, carbon and sulfur contents, grain size, and clay-minerals.Clay-mineral associations are comparable at all sites and in all time periods, in all sapropels, and oxygenated sediments.There is no diagenetic alteration of clay minerals in the sapropels.The relative importance of the different clay species varies with location of the sites and with time.African and European provenance can be distinguished.In the sapropels clay-mineral associations show changes that suggest increased precipitation in Southern Europe (increase in chlorite, illite, and mixed-layer clays) and intensified atmospheric circulation over Northern Africa and the Middle East (increased amounts of palygorskite and sepiolite).In all sapropels silt-sized particles increase at the expense of clay-sized particles in relation to higher energy of circulation.Sand-sized terrigenous particles have maximum accumulation rates just prior to sapropel deposition.Export productivity during deposition of oxygenated sediments has been calculated from benthic foraminifer numbers.It is highest just prior to sapropel deposition than above the sapropels.Paleoproductivity decreases from west to east and from the Pliocene to the Quaternary.Values from the late Quaternary (30-60 gC/m 2 •yr) are comparable to values obtained from oceanographic measurements (Bethoux, 1989).On top of Eratosthenes Seamount values are much higher (130-360 gC/m 2 •yr).The degree of carbonate dissolution reflects three types of sapropels: (1) nonbioturbated sapropels with excellent carbonate preservation; (2) increase in carbonate dissolution towards the top of the sapropel because of "burning down" phenomena; (3) bioturbated Pliocene sapropels with carbonate dissolution as strong as in surrounding oxygenated sediments.

Givetian deposits accumulated in a more than 300-m-thick succession on a carbonate platform developed on the passive margin of the so-called Rheic ocean. These rocks crop out in the Ardenne massif at Glageon in a structural context marked by small fault-controlled basins subjected to moderate overburden, tectonic and metamorphic constraints. Data on the clay mineral distribution, associated with detailed information on limestone facies/microfacies, as well as data on the depositional profile and sequence stratigraphic evolution of these rocks indicate strong carbonate diagenesis and moderate clay diagenesis, the latter being favoured by the early occurrence of the former, which prevented further fluid-rock interaction. The clay assemblage distribution, which roughly parallels the palaeomorphologic and palaeobathymetric shape of the Givetian continental margin, is organized in five successive zones indicating large-scale sequence stratigraphic evolution and the control of regional sea-level fluctuations. The clay and other sedimentary components provide additional information on the warm and variably humid climate, northern, relative to southern, terrigenous sources, the open sea relative to restricted depositional environments, the temporary tectonic rejuvenation of the continental hinterland and the chemical conditions allowing early diagenetic modifications.

The clay mineralogy of four 5.5‐ to 13.5‐m‐long cores sampled between 45° and 60°N in the North Atlantic Ocean has been investigated at high latitudes within a well‐constrained chronostratigraphic scale. Cross‐correlation spectral analyses have been performed on both clay mineral and δ18O planktonic records. Detrital clay minerals display strong signals which are coherent with the δ18O record, within the three main Milankovitch frequency bands (eccentricity, obliquity, and precession). The climatic control on clay mineral sedimentation largely depends on the latitudinal location of the sediment cores. The 100,000‐year signal occurs as a uniformly acting factor, whereas the 41,000‐year signal dominates clay sedimentation at high latitudes and the 23,000‐year signal dominates at midlatitudes. We suggest that the latitudinal variations of the orbital forcing on the detrital clay mineral distribution in the North Atlantic Ocean not only result from climatic control of the intensity of physical and chemical weathering, but also from latitudinal control on the detrital clay supply linked to influences of the high‐latitude wind‐driven and midlatitude ocean‐driven transportation processes, respectively.