Submarine pyroclastic deposits formed during the 20th May 2006 dome collapse of the Soufrière Hills Volcano, Montserrat

The 20th May 2006 lava dome collapse of the Soufrière Hills Volcano, Montserrat, had a total non-dense rock equivalent (non-DRE) collapse volume of approximately 115 × 10 6 m 3. The majority of this volume was deposited into the ocean. The collapse was rapid, 85% of the mobilized volume being removed in just 35 min, giving peak pyroclastic flow flux of 66 × 10 3 m 3 s -1. Channel and levee facies on the submarine flanks of the volcano and formation of a thick, steep-sided ridge, suggest that the largest and most dense blocks were transported proximally as a high concentration granular flow. Of the submerged volume, 30% was deposited from the base of this granular flow, forming a linear, high-relief ridge that extends 7 km from shore. The remaining 70% of the submerged volume comprises the finer grain sizes, which were transported at least 40 km by turbidity currents on gradients of <2°. At several localities, the May 2006 distal turbidity currents ran up 200 m of topography and eroded up to 20 cm of underlying substrate. Multiple turbidites are preserved, representing current reflection from the graben margins and deflection around topography. The high energy of the May 2006 collapse resulted in longer submarine run out than the larger (210 × 10 6 m 3) Soufrière Hills dome collapse in July 2003.

Evidence for carbonate platform failure during rapid sea-level rise; ca 14 000 year old bioclastic flow deposits in the Lesser Antilles

Bioclastic flow deposits offshore from the Soufrie`re Hills volcano on Montserrat in the Lesser Antilles were deposited by the largest volume sediment flows near this active volcano in the last 26 kyr. The volume of these deposits exceeds that of the largest historic volcanic dome collapse in the world, which occurred on Montserrat in 2003. These flows were most probably generated by a large submarine slope failure of the carbonate shelf comprising the south west flank of Antigua or the east flank of Redonda; adjacent islands that are not volcanically active. The bioclastic flow deposits are relatively coarse-grained and either ungraded or poorly graded, and were deposited by non cohesive debris flow and high density turbidity currents. The bioclastic deposit often comprises multiple sub-units that cannot be correlated between core sites; some located just 2 km apart. Multiple sub-units in the bioclastic deposit result from either flow reflection, stacking of multiple debris flow lobes, and/or multi-stage collapse of the initial landslide. This study provides unusually precise constraints on the age of this mass flow event that occurred at ca 14 ka. Few large submarine landslides have been well dated, but the slope failures that have been dated are commonly associated with periods of rapid sea-level change.

Submarine pyroclastic deposits formed at the Soufrière Hills volcano, Montserrat (1995–2003) : what happens when pyroclastic flows enter the ocean?

The Soufrière Hills volcano, Montserrat, West Indies, has undergone a series of dome growth and collapse events since the eruption began in 1995. Over 90% of the pyroclastic material produced has been deposited into the ocean. Sampling of these submarine deposits reveals that the pyroclastic flows mix rapidly and violently with the water as they enter the sea. The coarse components (pebbles to boulders) are deposited proximally from dense basal slurries to form steep-sided, near-linear ridges that intercalate to form a submarine fan. The finer ash-grade components are mixed into the overlying water column to form turbidity currents that flow over distances >30 km from the source. The total volume of pyroclastic material off the east coast of Montserrat exceeds 280 × 106 m3, with 65% deposited in proximal lobes and 35% deposited as distal turbidites.

Fine-grained Pleistocene deepwater turbidite channel system on the slope of Qiongdongnan Basin, northern South China Sea

Recently, as oil exploitation has become focused on deepwater slope areas. more multi-channel high resolution 2D and 3D seismic data were acquired in the deepwater part of the Qiongdongnan Basin, northern South China Sea. Based on 3D seismic data and coherence time slice, RMS and 3D visualization, a series of deepwater channels were recognized on the slope that probably developed in the late Quaternary period. These channels trend SW-NE to W-E and show bifurcations, levees, meander loops and avulsions. High Amplitude Reflections (HARs), typical for channel-levee complexes, are of only minor importance and were observed in one of the channel systems. Most of the detected channels are characterized by low-amplitude reflections, and so are different from the typical coarse-grained turbidite channels that had been discovered worldwide. The absence of well data in the study area made it difficult to determine the age and lithology of these channels. Using a neighboring drill hole and published data about such depositional systems worldwide, the lithology of these channels is likely to be dominated by mudstones with interbedded thin sandstones. These channels are formed by turbidity currents originated from the little scale mountain river of mid-Vietnam in SW direction and were probably accompanied by a relative sea level drop in the last glacial age. These channels discovered on the northern South China Sea slope are likely to be fine-grained, mud-dominant and low N:G deposits in a deepwater paleogeographic setting. (C) 2009 Elsevier Ltd. All rights reserved.

Geomorphology, sedimentary processes and development of the Zenisu deep-sea channel, northern Philippine Sea

The Zenisu deep-sea channel originates on the Izu-Ogasawara island arc, and disappears in the Shikoku Basin of the Philippine Sea. The geomorphology, sedimentary processes, and the development of the Zenisu deep-sea channel were investigated on the basis of swath bathymetry, side-scan sonar imagery, submersible observations, and seismic data. The deep-sea channel can be divided into three segments according to the downslope gradient and channel orientation. They are the Zenisu Canyon, the E-W fan channel, and the trough-axis channel. The sediment fill is characterized by turbidite and debrite deposition and blocky-hummocky avalanche deposits on the flanks of the Zenisu Ridge. In the Zenisu Canyon and the Zenisu deep-sea channel, sediment transport by turbidity currents generates sediment waves (dunes) observed during the Shinkai 6500 dive 371. The development of the Zenisu Canyon is controlled by a N-S shear fault, whereas the trough-axis channel is controlled by basin subsidence associated with the Zenisu Ridge. The E-W fan channel was probably affected by the E-W fault and the basement morphology.

A new Coniacian-Santonian conchostracan genus from the Bauru Group, south-east Brazil: Taxonomy, palaeobiogeography and palaeoecology

Abundant conchostracans occur in Coniacian-Santonian dark grey, argillaceous, lacustrine sediments of the Sao Carlos Formation, Bauru Group, Parana Basin, in the central part of São Paulo State, south-east Brazil. They are ascribed to a new genus and species, Bauruestheria sancarlensis, included in the family Jilinestheriidae. The new taxon is similar to some Late Cretaceous species from China and Mongolia. It probably evolved from a Late Jurassic-Early Cretaceous ancestral form (Migransia), which first lived in West Gondwana, and later dispersed to Europe and Asia, originating distinct parallel lineages with increasing ornamental complexity. The conchostracans probably lived in oxygenated marginal areas of a very calm, perennial lake with an anoxic bottom, and were transported in suspension to the depositional site by weak turbidity currents or storm-induced flows. Great concentrations of juvenile conchostracans in some thin layers can be related to mass mortality, episodes caused by convection and dispersion of anoxic water during storms. (c) 2005 Elsevier Ltd. All rights reserved.

Studio di fenomeni d'instabilità gravitativa sui fondali marini, con particolare riferimento all'isola di Stromboli

In the last decade the interest for submarine instability grew up, driven by the increasing exploitation of natural resources (primary hydrocarbons), the emplacement of bottom-lying structures (cables and pipelines) and by the development of coastal areas, whose infrastructures increasingly protrude to the sea. The great interest for this topic promoted a number of international projects such as: STEAM (Sediment Transport on European Atlantic Margins, 93-96), ENAM II (European North Atlantic Margin, 96-99), GITEC (Genesis and Impact of Tsunamis on the European Coast 92-95), STRATAFORM (STRATA FORmation on Margins, 95-01), Seabed Slope Process in Deep Water Continental Margin (Northwest Gulf of Mexico, 96-04), COSTA (Continental slope Stability, 00-05), EUROMARGINS (Slope Stability on Europe’s Passive Continental Margin), SPACOMA (04-07), EUROSTRATAFORM (European Margin Strata Formation), NGI's internal project SIP-8 (Offshore Geohazards), IGCP-511: Submarine Mass Movements and Their Consequences (05-09) and projects indirectly related to instability processes, such as TRANSFER (Tsunami Risk ANd Strategies For the European region, 06-09) or NEAREST (integrated observations from NEAR shore sourcES of Tsunamis: towards an early warning system, 06-09). In Italy, apart from a national project realized within the activities of the National Group of Volcanology during the framework 2000-2003 “Conoscenza delle parti sommerse dei vulcani italiani e valutazione del potenziale rischio vulcanico”, the study of submarine mass-movement has been underestimated until the occurrence of the landslide-tsunami events that affected Stromboli on December 30, 2002. This event made the Italian Institutions and the scientific community more aware of the hazard related to submarine landslides, mainly in light of the growing anthropization of coastal sectors, that increases the vulnerability of these areas to the consequences of such processes. In this regard, two important national projects have been recently funded in order to study coastal instabilities (PRIN 24, 06-08) and to map the main submarine hazard features on continental shelves and upper slopes around the most part of Italian coast (MaGIC Project). The study realized in this Thesis is addressed to the understanding of these processes, with particular reference to Stromboli submerged flanks. These latter represent a natural laboratory in this regard, as several kind of instability phenomena are present on the submerged flanks, affecting about 90% of the entire submerged areal and often (strongly) influencing the morphological evolution of subaerial slopes, as witnessed by the event occurred on 30 December 2002. Furthermore, each phenomenon is characterized by different pre-failure, failure and post-failure mechanisms, ranging from rock-falls, to turbidity currents up to catastrophic sector collapses. The Thesis is divided into three introductive chapters, regarding a brief review of submarine instability phenomena and related hazard (cap. 1), a “bird’s-eye” view on methodologies and available dataset (cap. 2) and a short introduction on the evolution and the morpho-structural setting of the Stromboli edifice (cap. 3). This latter seems to play a major role in the development of largescale sector collapses at Stromboli, as they occurred perpendicular to the orientation of the main volcanic rift axis (oriented in NE-SW direction). The characterization of these events and their relationships with successive erosive-depositional processes represents the main focus of cap.4 (Offshore evidence of large-scale lateral collapses on the eastern flank of Stromboli, Italy, due to structurally-controlled, bilateral flank instability) and cap. 5 (Lateral collapses and active sedimentary processes on the North-western flank of Stromboli Volcano), represented by articles accepted for publication on international papers (Marine Geology). Moreover, these studies highlight the hazard related to these catastrophic events; several calamities (with more than 40000 casualties only in the last two century) have been, in fact, the direct or indirect result of landslides affecting volcanic flanks, as observed at Oshima-Oshima (1741) and Unzen Volcano (1792) in Japan (Satake&Kato, 2001; Brantley&Scott, 1993), Krakatau (1883) in Indonesia (Self&Rampino, 1981), Ritter Island (1888), Sissano in Papua New Guinea (Ward& Day, 2003; Johnson, 1987; Tappin et al., 2001) and Mt St. Augustine (1883) in Alaska (Beget& Kienle, 1992). Flank landslide are also recognized as the most important and efficient mass-wasting process on volcanoes, contributing to the development of the edifices by widening their base and to the growth of a volcaniclastic apron at the foot of a volcano; a number of small and medium-scale erosive processes are also responsible for the carving of Stromboli submarine flanks and the transport of debris towards the deeper areas. The characterization of features associated to these processes is the main focus of cap. 6; it is also important to highlight that some small-scale events are able to create damage to coastal areas, as also witnessed by recent events of Gioia Tauro 1978, Nizza, 1979 and Stromboli 2002. The hazard potential related to these phenomena is, in fact, very high, as they commonly occur at higher frequency with respect to large-scale collapses, therefore being more significant in terms of human timescales. In the last chapter (cap. 7), a brief review and discussion of instability processes identified on Stromboli submerged flanks is presented; they are also compared with respect to analogous processes recognized in other submerged areas in order to shed lights on the main factors involved in their development. Finally, some applications of multibeam data to assess the hazard related to these phenomena are also discussed.

Grain size distribution in surface sediments samples of the continental shelf and slope of southeast Greenland

During "Meteor" Cruise 6/1966 in the northwest Atlantic a systematic survey of the bottom topography of the southeast Greenland continental margin was undertaken. Eighty-seven profiles transverse to the shelf edge at distances of 3-4 nautical miles and two longitudinal profiles parallel to the coast were carried out with the ELAC Narrow Beam Echo-Sounder giving a reliable record of even steep slopes. On the basis of the echo soundings the topography and morphology of the continental shelf and slope are evaluated. A detailed bathymetric chart and a serial profile chart were designed as working material for the morphological research. These maps along with the original echograms are morphometrically evaluated. The analysis of the sea bottom features is the basis of a subsequent morphogenetical interpretation, verified and extended by means of interpretation of magnetic data and sediment analysis (grain size, roundness, lithology). The results of the research are expressed in a geomorphological map. The primary findings can be summarized as follows: 1) The southeast Greenland shelf by its bottom topography can be clearly designated as a glacially formed area. The glacial features of the shelf can be classified into two zones nearly parallel to the coast: glacial erosion forms on the inner shelf and glacial accumulation forms on the outer shelf. The inner shelf is characterized by the rugged and hummocky topography of ice scoured plains with clear west/east slope asymmetry. On the outer shelf three types of glacial accumulation forms can be recognized: ice margin deposits with clearly expressed terminal moraines, glacial till plains and glaciomarine outwash fans. Both zones of the shelf can be subdivided into two levels of relief. The ice scoured plains, with average depths of 240 meters (m), are dissected to a maximum depth of 1060 m (Gyldenloves Trough) by trough valleys, which are the prolongations of the Greenland fjords. The banks of the outer shelf, with an average depth of 180 m, surround glacial basins with a maximum depth of 670 meters. 2) The sediments of the continental shelf can be classified as glacial due to their grain size distribution and the degree of roundness of the gravel particles. The ice margin deposits on the outer shelf can be recognized by their high percentage of gravels. On the inner shelf a rock surface is suggested, intermittently covered by glacial deposits. In the shelf troughs fine-grained sediments occur mixed with gravels. 3) Topography and sediments show that the southeast Greenland shelf was covered by an ice sheet resting on the sea floor during the Pleistocene ice-age. The large end moraines along the shelf edge probably indicate the maximum extent of the Wurm shelf ice resting on the sea floor. The breakthroughs of the end moraines in front of the glacial basins suggest that the shelf ice has floated further seaward over the increasing depths. 4) Petrographically the shelf sediments consist of gneisses, granites and basalts. While gneisses and granites occire on the nearby coast, basalt is not known to exist here. Either this material has been drifted by icebergs from the basalt province to the north or exists on the southeast Greenland shelf itself. The last interpretation is supported bythe high portion of basalt contained in the sediment samples taken and the strong magnetic anomalies probably caused by basaltic intrusions. 5) A magnetic profile allows the recognition of two magnetically differing areas which approximately coincide with the glacial erosion and accumulation zones. The inner shelf shows a strong and variable magnetic field because the glacially eroded basement forms the sea floor. The outer shelf is characterized by a weak and homogenous magnetic field, as the magnetized basement lies at greater depthy, buried by a thick cover of glacial sediments. The strong magnetic anomalies of the inner shelf are probably caused by dike swarms, similar to those observed further to the north in the Kangerdlugssuaq Fjord region. This interpretation is supported by the high basalt content of the sediment samples and the rough topography of the ice scoured plains which correlates in general with the magnetic fluctuations. The dike structures of the basement have been differentially eroded by the shelf ice. 6) The continental slope, extending from the shelf break at 313 m to a depth of 1270 m with an average slope of 11°, is characterized by delta-shaped projections in front of the shelf basins, by marginal plateaus, ridges and hills, by canyons and slumping features. The projections could be identified as glaciomarine sediment fans. This conclusion is supported by the strong decrease of magnetic field intensity. The deep sea hills and ridges with their greater magnetic intensities have to be regarded as basement outcrops projecting through the glaciomarine sediment cover. The upper continental rise, sloping seaward at about 2°, is composed of wide sediment fans and slump material. A marginal depression on the continental rise running parallel to the shelf edge has been identified. In this depression bottom currents capable of erosion have been recorded. South of Cape Farvel the depression extends to the accumulation zone of the "Eirik" sedimentary ridge. 7) By means of a study of the recent marine processes, postglacial modification of the ice-formed relief can be postulated. The retention effect of the fjord troughs and the high velocity of the East Greenland stream prevents the glacial features from being buried by sediments. Bottom currents capable of active erosion have only been found in the marginal depression on the continental rise. In addition, at the time of the lowest glacio-eustatic sea level, the shelf bottom was not situated in the zone of wave erosion. Only on the continental slope and rise bottom currents, sediment slumps and turbidity currents have led to significant recent modifications. Considering these results, the geomorphological development of the southeast Greenland continental terrace can be suggested as follows: 1. initial formation of a "peneplain", 2. fluvial incision, 3. submergence, and finally 4. glacial modification.

Sedimentology of cores from the continental rise west of the Antarctic Peninsula

The continental rise west of the Antarctic Peninsula includes a number of large sediment mounds interpreted as contourite drifts. Cores from six sediment drifts spanning some 650 km of the margin and 48 of latitude have been dated using chemical and isotopic tracers of palaeoproductivity and diatom biostratigraphy. Interglacial sedimentation rates range from 1.1 to 4.3 cm/ka. Glacial sedimentation rates range from 1.8 to 13.5 cm/ka, and decrease from proximal to distal sites on each drift. Late Quaternary sedimentation was cyclic, with brown, biogenic, burrowed mud containing ice-rafted debris (IRD) in interglacials and grey, barren, laminated mud in glacials. Foraminiferal intervals occur in interglacial stages 5 and 7 but not in the Holocene. Processes of terrigenous sediment supply during glacial stages differed; meltwater plumes were more important in stages 2-4, turbidity currents and ice-rafting in stage 6. The terrigenous component shows compositional changes along the margin, more marked in glacials. The major oxides Al2O3 and K2O are higher in the southwest, and CaO and TiO2 higher in the northeast. There is more smectite among the clay minerals in the northeast. Magnetic susceptibility varies along and between drifts. These changes reflect source variations along the margin. Interglacial sediments show less clear trends, and their IRD was derived from a wider area. Downslope processes were dominant in glacials, but alongslope processes may have attained equal importance in interglacials. The area contrasts with the East Antarctic continental slope in the SE Weddell Sea, where ice-rafting is the dominant process and where interglacial sedimentation rates are much higher than glacial. The differences in glacial setting and margin physiography can account for these contrasts.

Sedimentology and stable oxygen isotope ratios of foraminifera from cores of the Porcupine Seabight, Northeast Atlantic

Large carbonate mound structures have been discovered in the northern Porcupine Seabight (Northeast Atlantic) at depths between 600 and 1000 m. These mounds are associated with the growth of deep-sea corals Lophelia pertusa and Madrepra oculata. In this study, three sediment cores have been analysed. They are from locations close to Propeller Mound, a 150 m high ridge-like feature covered with a cold-water coral ecosystem at its upper flanks. The investigations are concentrated on grain-size analyses, carbon measurements and on the visual description of the cores and computer tomographic images, to evaluate sediment content and structure. The cores portray the depositional history of the past ~31 kyr BP, mainly controlled by sea-level fluctuations and the climate regime with the advance and retreat of the Irish Ice Sheet onto the Irish Mainland Shelf. A first advance of glaciers is indicated by a turbiditic release slightly older than 31 kyr BP, coherent with Heinrich event 3 deposition. During Late Marine Isotope Stage 3 (MIS 3) and MIS 2 shelf erosion prevailed with abundant gravity flows and turbidity currents. A change from glaciomarine to hemipelagic contourite sedimentation during the onset of the Holocene indicates the establishment of the strong, present-day hydrodynamic regime at intermediate depths. The general decrease in accumulation of sediments with decreasing distance towards Propeller Mound suggests that currents (turbidity currents, gravity flows, bottom currents) had a generally stronger impact on the sediment accumulation at the mound base for the past ~31 kyr BP, respectively.

Physical properties of 4 cores from the Bengal Fan

New stratigraphic and high-resolution seismic data from the Bengal Fan indicate that the world's largest fan shows active growth during the most recent sea-level rise and the recent highstand. This unique phenomenon contradicts common sequence-stratigraphic models, and the sediment preserved provides new insight into the sedimentological response of a fan system to sea-level rise, climatic terminations, and monsoon intensity during the past climatic cycle. We present a detailed dated sequence of turbidite sedimentation based on a core transect perpendicular to the active channel-levee system in the upper mid-fan area. Between the two major terminations 1a (12 800 14C yr B.P.) and 1b (9700 14C yr B.P.), and especially at the end of the Younger Dryas, a 13-km-wide channel built up levees 50 m high. With decreasing sediment supply, continued sea-level rise, and increasing monsoon intensity during the early Holocene, turbidity currents were confined to the channel and gradually filled it. The canyon "Swatch of No Ground," a shelf depocenter that serves as the source for frequent turbidity currents, and the channel-levee system provide the unique opportunity for studying an active highstand system. Many fans showed this behavior only during lowered sea-level.

Petrology and geochemistry of sedimentary Unit III of ODP Hole 135-840B

The sedimentary sequence recovered at Site 840, on the Tonga frontal-arc platform, is 597.3 m thick and is subdivided into three lithostratigraphic units. The lowermost, late Miocene Unit III is 336.8 m thick and consists of a sequence of volcaniclastic mass-flow deposits (predominantly turbidites) interbedded with pelagic/hemipelagic deposits. Unit III was deposited in the forearc basin of the Lau volcanic arc, probably on a slope dominated by mass flows that built eastward from the ridge front and across the forearc. Upward through the unit a thinning and fining of individual turbidites takes place, interpreted to reflect a reduced sediment supply and a change from large to smaller flows. Decreasing volcanic activity with time is inferred from a decrease in coarse-grained volcaniclastic content in the upper part of the unit. The majority of the turbidites show the typical Bouma-type divisions, although both high- and low-density turbidity currents are inferred. High-density turbidity currents were especially common in the lower part of the unit. Geochemical analyses of detrital glass lie mainly in the low-K tholeiite field with a compositional range from basalt to rhyolite. A coherent igneous trend indicates derivation from a single volcanic source. This source was probably situated on the rifted part of the Lau-Tonga Ridge, within the present Lau backarc basin. The initial opening of the Lau Basin may have been around 6.0 m.y. ago. The onset of more extensive rifting, approximately 5.6 m.y. ago, is reflected in an increase in the silica content of volcanic glass. At the boundary toward Unit II, at approximately 5.25 Ma, an influx of thicker bedded and coarser grained volcaniclastic material is interpreted to reflect increasing volcanism and tectonism during the final breakup of the Lau-Tonga Ridge.

Turbidite sedimentology and history at DSDP Site 89-585

At Site 585 in the East Mariana Basin, a 900-m section of Aptian-Albian to Recent sediments was recovered. The upper 590 m are pelagic components (carbonate, siliceous, and clay); small-scale graded sequences and laminations are common. The underlying sediments are volcaniclastic sandstones with a large proportion of shallow-water carbonate debris; sedimentary structures including complete Bouma sequences, cross-laminae, and scouring are common. These structures indicate that the entire section was deposited by turbidity currents. The change in lithology upward in the section reflects the evolution of the surrounding seamounts, from their growth stages during the middle of the Cretaceous to the later subsidence phases. Several black layers containing pyritized organic debris and associated turbidite structures were cored near the Cenomanian/Turonian boundary; this material has been transported from the flanks of the seamounts where it was deposited within a shallow anoxic zone. Seismic data extends the stratigraphy across the entire Basin, showing the reflectors onlapping the seamounts, and indicating at least 1200 m of sediment at Site 585. The crust is placed at 6900 m after correcting for sediment loading, and the subsidence curve indicates that the Basin has been deeper than 5500 m since before the Aptian.

Geochemistry at DSDP Hole 76-534A

At Site 534 in the Blake-Bahama Basin, western North Atlantic, an interval of 68 m of Maestrichtian (Upper Cretaceous) and upper middle to upper Eocene sediments consists of terrigenous siltstones, mudstones, and varicolored zeolitic claystones; minor recovery of micritic limestones, porcellanites, and quartzitic chert was made at this site as well. Comparisons with other Deep Sea Drilling Project (DSDP) sites in the western North Atlantic suggest that the following formations are present in this interval: Hatteras (Maestrichtian), Plantagenet (Maestrichtian and upper Eocene), Bermuda Rise (upper middle to upper Eocene), and the basal Blake Ridge Formation (upper middle to upper Eocene). Recognition of a Tertiary interval of the Plantagenet allows that formation to be divided into lower and upper informal units. Condensation makes this formal lithostratigraphic subdivision difficult. Together the formations record marked net condensed sedimentation (average rate ca. 2.5 m/m.y.) in strongly oxidizing bottom waters. From sedimentary structures and petrography, it is inferred that the terrigenous siltstones and micritic limestones were redeposited from the continental margin by turbidity currents. Chemical data plus petrography confirm relatively high plankton productivity during the upper Eocene. Much of the nonrecovered Eocene interval may represent chert and porcellanite. Fragments recovered were formed by replacement of relatively porous calciturbidites by opal-CT and quartz. Radiolarians in interbedded claystones rich in clinoptilolite show extensive dissolution. Relative to typical hemipelagic sediments, the claystones are enriched in many metals (Cu, Ni, Zn, Pb), particularly within manganese micronodules. The metal accumulation is related to a 30-m.y. period of slow net sediment accumulation, rather than to hydrothermal enrichment or to upward mobilization of metals from the underlying reduced Hatteras black shale facies. Elsewhere in the Blake-Bahama Basin, at Site 391, 22 km to the northwest, upper Eocene facies are missing, reportedly due to deep seafloor erosion of up to 800 m of the sedimentary succession. By contrast, the discovery that this interval is preserved at nearby Site 534 points to much less extensive seafloor erosion, possibly mostly in the Oligocene, which is missing at both DSDP Sites.

Holocene sedimentation in the Strait of Otranto, Mediterranean Sea

An extensive radiograph study of 24 undisturbed, up to 206-cm long box and gravity cores from the western part of the Strait of Otranto revealed a great variety of primary bedding structures and secondary burrowing features. The regional distribution of the sediments according to their structural, textural, and compositional properties reflects the major morphologic subdivisions of the strait into shelf, slope, and trough bottom (e.g., the bottom of the northern end of the Corfu-Kephallinia Trough, which extends from the northeastern Ionian Sea into the Strait of Otranto): (1) The Apulian shelf (0 to -170m) is only partly covered by very poorly sorted, muddy sands without layering. These relict(?) sands are rich in organic carbonate debris and contain glauconite and reworked (?Pleistocene) ooids. (2) The slope sediments (-170 to -1,000 m) are poorly sorted, sandy muds with a high degree of burrowing. One core (OT 5) is laminated and shows slump structures. An origin of these slumped sediment masses from older deposits higher on the slope was inferred from their abnormal compaction, color, texture, organic content, and mineral composition. (3) Cores from the northern end of the Corfu-Kephallinia Trough (-980 to -1,060 m) display a few graded sand layers, 2-5 cm (maximum 30 cm) thick with parallel and ripple-cross-laminations, deposited by oceanic bottom or small-scale turbidity currents. They are intercalated with homogeneous lutite. (4) Hemipelagic sediments prevail in the more southerly part of the Corfu-Kephallinia Trough and on the "Apulian-Ionian Ridge", the southern submarine extension of the Apulian Peninsula. Below a core depth of 160 cm, these cores have a laminated ("varved") zone, representing an Early Holocene (Boreal-Atlanticum) "stagnation layer" (14C age approximately 9,000 years). The terrigenous components of the surface sediments as well as those of the deeper sand layers can be derived from the Apulian shelf and the Italian mainland (Cretaceous Apulian Plateau and Gargano Mountains, southern Apennines, volcanic province of the Monte Vulture). Indicated by the heavy mineral glaucophane, a minor proportion of the sedimentary material is probably of Alpine origin. If this portion is considered to be first-cycle clastic material it reaches the Strait of Otranto after a longitudinal transport of 700 km via the Adriatic Sea. The lack of phyllosilicates in the coarse- to medium-grained shelf samples might be explained by the activity of the "Apulian Current" (surface velocities up to 4 knots) which in the past possibly has affected the bottom almost down to depths of the shelf edge. The percentage of planktonic organisms, and also the plankton: benthos ratio in the sediments is a useful indicator for bathymetry (depth zonation).