Winnowing Of Sediments Research Articles

Provenance and dispersal of glacial–marine surface sediments in the Weddell Sea and adjoining areas, Antarctica: ice-rafting versus current transport

Mineralogical and granulometric properties of glacial–marine surface sediments of the Weddell Sea and adjoining areas were studied in order to decipher spatial variations of provenance and transport paths of terrigenous detritus from Antarctic sources. The silt fraction shows marked spatial differences in quartz contents. In the sand fractions heavy-mineral assemblages display low mineralogical maturity and are dominated by garnet, green hornblende, and various types of clinopyroxene. Cluster analysis yields distinct heavy-mineral assemblages, which can be attributed to specific source rocks of the Antarctic hinterland. The configuration of modern mineralogical provinces in the near-shore regions reflects the geological variety of the adjacent hinterland. In the distal parts of the study area, sand-sized heavy minerals are good tracers of ice-rafting. Granulometric characteristics and the distribution of heavy-mineral provinces reflect maxima of relative and absolute accumulation of ice-rafted detritus in accordance with major iceberg drift tracks in the course of the Weddell Gyre. Fine-grained and coarse-grained sediment fractions may have different origins. In the central Weddell Sea, coarse ice-rafted detritus basically derives from East Antarctic sources, while the fine-fraction is discharged from weak permanent bottom currents and/or episodic turbidity currents and shows affinities to southern Weddell Sea sources. Winnowing of quartz-rich sediments through intense bottom water formation in the southern Weddell Sea provides muddy suspensions enriched in quartz. The influence of quartz-rich suspensions moving within the Weddell Gyre contour current can be traced as far as the continental slope in the northwestern Weddell Sea. In general, the focusing of mud by currents significantly exceeds the relative and absolute contribution of ice-rafted detritus beyond the shelves of the study area.

Silicon-isotope composition of diatoms as an indicator of past oceanic change

Silicon is essential for the growth of diatoms, a group of phytoplankton with opal (amorphous hydrated silica) shells. Diatoms largely control the cycling of silicon in the ocean1 and, conversely, diatom silica production rates can be limited by the availability of silicic acid2. Diatoms are biogeochemically important in that they account for an estimated 75% of the primary production occurring in coastal and nutrient-replete waters1, rising to more than 90% during ice-edge blooms such as occur in the Ross Sea, off Antarctica3. There are few means by which to reconstruct the history of diatom productivity and marine silicon cycling, and thus to explore the potential contribution of diatoms to past oceanic biogeochemistry or climate. Indices based on the accumulation of sedimentary opal are often biased by the winnowing and focusing of sediments and by opal dissolution4, 5, 6, 7. Normalization of opal accumulation records using particle-reactive natural radionuclides may correct for sediment redistribution artefacts and the dissolution of opal within sediments6,8, but not for opal dissolution before it arrives at the sea floor. Half of the opal produced in the euphotic zone may dissolve before sinking to a depth of 200 m (ref. 1), constituting a potentially large bias to both normalized and uncorrected records of opal accumulation. Here we exploit the potential that variations in the ratio of 30Si to 28Si in sedimentary opal may provide information on past silicon cycling that is unbiased by opal dissolution. Our silicon stable-isotope measurements suggest that the percentage utilization of silicic acid by diatoms in the Southern Ocean during the last glacial period was strongly diminished relative to the present interglacial.

Morphology and sedimentation on the Hebrides Slope and Barra Fan, NW UK continental margin

Abstract Rapidly deposited Neogene sands and Pleistocene glacigenic sediments originating from NW Britain were transported across the shelf and downslope to the Barra Fan depocentre. In contrast, the Holocene interglacial shelf environment is typified by sediment winnowing, sea-floor polishing and transport of relatively small volumes of sediment along the shelf to the northwest. Interpretation of swath bathymetry, 32 kHz side-scan sonar, 3.5 kHz pinger, sea-bed photography and near-bottom current survey data on the shelf edge and slope provides good morphological evidence for both downslope and alongslope sedimentary processes on this composite slope-front fan. On the northern fan a corrugated shelf edge gives rise to a network of channels on the upper slope, which incise into Pleistocene and older sediments and funnel sediment to middle and lower fan. An underpinning variable topography on the fan bulge originates from major and minor slide masses, sediment creep and debris flows. These are reworked by spatially and secularly variable strong-to-weak bottom currents and redistribute material across and down fan forming transverse and linear bedforms, draped sandy contourite sheets and moulded drifts.

Spatial and temporal variability in aggregated grain-size distributions, with implications for sediment dynamics

The grain-size distribution of bottom sediments has important implications for diverse aspects of sediment dynamics, including prediction of the critical boundary shear stress and calibration of suspended sediment sensors. Past sampling strategies to obtain estimates of the seabed grain-size distribution typically have not considered spatial and temporal variability, and have been insufficient to resolve potential millimeter-scale vertical variations in grain size. Moreover, laboratory analyses have been predicated on chemically and/or ultrasonically disaggregating the sediments before resolving particle diameter, therefore the more dynamically relevant in situ grain-size spectrum is not measured. To test for such effects, three sites on the northern California continental shelf comprising a cross-shelf transect from a sandy, inner-shelf (60 m) site, to a muddy, mid-shelf (90 m) site and a relict, outer-shelf (130 m) site were studied. Replicate box cores were collected over two winter field seasons, and multiple subcores from each box core were vertically sectioned at 2-mm intervals. Gentle wet sieving techniques were used to determine the mass fraction in the <20, 20–44, 45–62, 63–89, 90–179, 180–300 and >300,um size classes. In addition, a lesser number of standard disaggregated grain-size analyses were performed using a Coulter Counter. Results from the sandy, inner-shelf site indicate the presence of an ephemeral fine-grained (<20 μm) surface layer (0–2 mm) that substantively alters the grain-size distribution “seen” by the flow. In addition, there is evidence for a progressive and substantial winnowing of fine-grained sediment from the surface layer over the course of a winter storm season. At the deeper sites, the upper 2 mm of the bed contained 5–20% more material <20 μm than deeper portions of the sediment column. At all three sites, the level of small- (10s of centimeters) and intermediate- (10s of meters) scale spatial variability is modest. In addition, the disaggregated grain-size distribution at the two muddy sites is, in all cases, markedly finer than the in situ grain-size distribution. Therefore, calibrations and predictions based on a knowledge of the size distribution of the primary (i.e. disaggregated) particles could be in serious error.

Holocene ice retreat from the Lazarev Sea shelf, East Antarctica

Distinct facies types, classified in radiocarbon-dated sediments from the shelf of the Lazarev Sea, East Antarctica, reveal a detailed history of processes that have controlled sedimentation during the deglaciation over the last 10,000 yr. The ice retreat on this part of the Antarctic shelf started 9500 yr BP, marked by the deposition of laminated sediments, deposited from a floating ice shelf. These laminites, which occur on top of diamictons laid down from a grounded ice sheet, are the basal sediments of the postglacial sequence. The intensity of the Antarctic Coastal Current (ACC), directed by shelf morphology, controlled sedimentation of the postglacial facies. A residual glaciomarine sediment with the fine fraction winnowed by strong currents developed from 9000-8000 yr BP in the western part of the investigation area and from 9000-5000 yr BP in the eastern part, closer to the prominent ‘Fenno Deep’ trough. Current velocities apparently decreased between 8000 and 2000 yr BP due to a deflection of the ACC by advancing ice tongues to the east of the investigation area during the ‘Hypsithermal’. This led to a deposition of fine-grained sediments, and clay mineralogy suggests a continental source, possibly near the grounding line of the Nivl Ice Shelf, rather than a winnowing of sediments near the shelf break or advection from deeper water. Current velocities intensified after 2000 yr BP, removed fine material from these sediments and led to a relict sediment, consisting of coarse bryozoan and molluscan debris.

230Th ex flux into Norwegian-Greenland Sea sediments: Evidence for lateral sediment transport during the past 300,000 years

230Th was determined to investigate the influence of climatic cycles on the 230Th ex flux into Norwegian-Greenland Sea sediments. The F a F p ratios, i.e., the ratios between the measured 230Th ex flux ( F a) into the sediments and the production rate of 230Th in the water column ( F p), vary considerably in the four cores investigated. Systematic variations in the ratios with the climatic cycles could not be observed. Furthermore, variations in the F a F p ratios cannot be correlated between individual sites. It is argued that lateral sediment transport, due to deglaciation of ice sheets, bottom currents and/or downslope sediment transport, result in F a F p ratios that are not equal to 1. The results suggest that both sediment winnowing (and/or erosion) and sediment focusing have strong influences upon sediment accumulation in the Norwegian-Greenland Sea.

An undulatory discontinuity surface on a gently inclined muddy ramp: possible evidence of secondary helical flow from the geological record

An unusual, somewhat cryptic, undulatory discontinuity has been recognized near the basal contact of the Wanakah Shale Member of the Middle Devonian Ludlowville Formation in western New York. It is in part marked by undulating diagenetic limestone beds and concretionary horizons, but can be traced continuously by subtle surfaces of fossil and pyritic lags. This surface marks the top of parallel, elongate mud ridges trending approximately N-S, perpendicular to the inferred E-W depositional strike of the Appalachian Basin in western New York. These ridges are observed in the field as irregularly spaced mud swells which have up to 1.5 m of relief, and vary from 30 to 50 m or more in width. The intervening, broad, flat‘channel floors’ are characterized by dense concentrations of diminutive brachiopods with gastropods and styliolinids, which are gradually replaced by thin styliolinid horizons with abundant pyrite nodules and pyritized burrows over the tops of the swells. Traceable shelly horizons are observed to pinch out over the swells, suggesting a period of relatively undisturbed sediment-filling following the formation of the channellized surface. The interval of nearly barren shale below this irregular surface, however, contains concretionary horizons and rare styliolinid pavements which are themselves irregular, possibly indicating a changing topography for this surface through time. Previous taphonomic and palaeoecological studies of the lower Wanakah Shale Member have disclosed a well-defined regressive-transgressive cycle with more subtle superimposed subcycles. The undulatory surface described here occurs within the early regressive part of this cycle, and appears to record an accentuated regressive kick of a lower-order subcycle. The formation of this surface is therefore interpreted to be a consequence of regressive submarine erosion and sediment bypass, together with redeposition from suspension. The development of the‘channels’ and parallel longitudinal mud ridges was probably controlled by helical flow cells within offshore-directed bottom currents. They may be the first recorded instance of sedimentary furrows described from the geological record. Taphonomic and biofacies data indicate relative depths below average storm wave-base, suggesting that sediment winnowing and redeposition occurred at depths only rarely affected by the direct impact of storm wave agitation.

Continuous surface dwelling of manganese nodules on a hill on the Madeira Abyssal Plain during abrupt sedimentation changes

The accretion rate of manganese nodules and the sedimentation rate of the underlying sediment has been studied with 10Be and 14C on a nodule covered hill in the Madeira Abyssal Plain. The accretion rate of (3.2±0.6) mm/Ma for the nodules is considerably smaller than the average sedimentation rate of 12 mm/ka for the last 50–100 ka. The mechanism that kept the nodules at the surface in this specific environment during the last 40 ka is likely to be flotation. The discovery of a large manganese-covered concretion in a piston core points to a long period of sediment winnowing. The 14C analyses also seem to point to an increase in the sedimentation rate round 6500 yrs B.P.

Shoreface dynamics: Evidence from bathymetry and surficial sediments

A conceptual model for the maintenance of equilibrium shoreface textures is tested on the shoreface of Long Island, New York, by using empirical eigenfunction analysis on sediment grain-size data. A bluff, composed of morainal material in the headland section, is a possible external sediment source for the study area. Within the shoreface system, a remnant terminal moraine lobe complex, sediments associated with old tidal inlet outer channels, and drowned outwash plain deposits are permanent (irreversible) internal sediment sources. The present shoreface sediments are derived primarily from the drowned outwash plain sediments as a result of in-situ reworking by the present shoreface hydrodynamic regime. Other temporary (reversible) internal sediment sources are storm deposits, and modern shoreface depositional features such as nearshore bars, sand ridges and active ebb tidal deltas. As sea level rises, the shoreface will continue to migrate landward. Subsequently, the recessional shoreface will leave behind a sheet of relict transgressive sand, composed of mainly medium sands as the result of the winnowing of fine-grained sediments by hydrodynamic processes of the traversing shoreface.

Depositional Model for a Muddy Shelf-Sand Complex and Their Relationship to Reservoir Development: Mancos B"Interval of Late Cretaceous Mancos Shale, Northwest Colorado and Northeast Utah: ABSTRACT"

The Mancos B interval of the Mancos Shale is an unusually thick (up to 1345 ft or 410 m) assemblage of sandy mudstone and muddy sandstone deposited in the Western Interior seaway approximately 100 mi (161 km) offshore. The basic sedimentation unit for the Mancos B is a thin couplet consisting of a basal fine-grained sandstone component and an upper mud rock component. Individual couplets are lenticular and generally less than 0.3 ft (0.09 m) thick and 2.5 ft (0.8 m) across. The sand component has sharp upper and lower contacts and is characterized by ripple lamination, wavy lamination, flaser lamination, and graded lamination (rare), whereas the mud rock component has discontinuous horizontal lamination accentuated by thin sandy streaks. Paleocurrents taken from ripples are to the southeast. Sequential deposition of sand/mud couplets resulted in formation of muddy sandstone sequences that range in thickness from 25 to 90 ft (8 to 27 m). The gross sand content in a sequence increases from approximately 30% at the base to 50 to 80% at the top. Bioturbation, which is characterized by members of the Cruziana ichnofacies, increases proportionally with sand content. Sequences are usually capped by thin intervals of sandy Fe-dolomite (beds andmore » concretions). Sequences are typically stacked into packages, ranging in thickness from 175 to 480 ft (53 to 146 m). Four main packages are present in the Mancos B. A depositional model involving uplift, sea-bottom shoaling, and sediment winnowing by episodic shelf currents is proposed for the generation of the Mancos B shelf-sand complex.« less

Milankovitch cycles in Neocene deep‐sea sediment

Pelagic carbonate sediments from the world ocean basins commonly show cyclic variations in amount and/or degree of preservation of biogenic calcite, with periodicities of several tens to several hundreds of thousands of years. The direct causes of these cycles are fluctuations in noncarbonate dilution, carbonate production, carbonate dissolution, and/or current winnowing. The overall driving force, however, is variation in the earth's orbital characteristics (Milankovitch cycles) through their influence on global climate and depositional processes. The main inferred climatic effects of orbital perturbations are on global ice volume, global temperature, ocean circulation, and distribution of climatic patterns. Eustatic sea level is directly related to ice volume, as is the rate of erosion of clastic material from continental margins. Changes in volume of sea ice affect the volume and intensity of bottom water flow, which in turn may cause changes in the intensity of sediment winnowing by bottom currents, in the intensity of upwelling of nutrient‐rich bottom waters, and in the depth of carbonate dissolution. Change in productivity of calcareous plankton are difficult to prove as a cause of carbonate cycles but may have contributed to the formation of carbonate cycles off northwest and southwest Africa. Fluctuations in winnowing of fine‐grained components have been demonstrated as a cause of cyclic variations in the coarse‐fraction component of carbonate sediments in the southwest Pacific. Dilution of carbonate by clastic material probably was a major cause of fluctuations in carbonate content of deep‐sea sediments off northwest and southwest Africa. Carbonate dissolution cycles probably are the most common manifestation of fluctuations in bottom water flow. Dissolution cycles are common in Quaternary and Neogene sediments of the North Atlantic, Caribbean, and eastern equatorial Pacific. The main cause of the carbonate dissolution was shoaling of the carbonate compensation depth during the early Neogene in response to climatically induced fluctuations in the thickness of Antarctic Bottom Water.

Micromorphlogy of recent and buried soils in a semi-arid region of northwestern India

Profiles of four recent soils and of eleven soils buried under archaeological sites of known ages were investigated in an area between the Ghaggar Valley and the northern piedmont of the Arawalli Hills in northwestern India. The principal approach was through study of micromorphology. Data were also obtained on mineralogies of several size fractions. Characteristics shared by the recent and buried soils indicate that essentially the same processes operated in their formation. The oldest soils are buried under Protohistoric sites and thus were formed at least 5000 years ago. They are developed on sediments rich in sand-sized mica flakes which indicate an alluvial origin. Others are buried beneath historic sites with ages ranging from 500 B.C. to 1500 A.D. These soils and the recent ones have a silt fraction low in mice. Major features of both buried and recent soils are attributed to biological activity, reflected in extensive channeling and additions of excrements throughout profiles. Local distribution of clay, silt and carbonates indicates translocation of these components. Furthermore, the forms of calcitic features indicate modifications of various kinds. Evidence of biological activity as expressed in characteristics of the soils becomes progressively weaker from older to younger profiles. Soil development seems to have weakened over that period as well. These changes are correlated with the decline with time in the proportions of mica flakes in coarse fractions. The decline is attributed to lack of alluvial sedimentation since the Protohistoric period and to greater transport and winnowing of sediments by wind. We believe this has reduced fertility of the soils formed during the last 4000 years.

Sedimentary processes on the Atlantic Continental Slope of the United States

Until recently, the sedimentary processes on the United States Atlantic Continental Slope were inferred mainly from descriptive studies based on the bathymetry and on widely spaced grab samples, bottom photographs, and seismic-reflection profiles. Over the past 6 years, however, much additional information has been collected on the bottom morphology, characteristics of shallow-subbottom strata, velocity of bottom currents, and transport of suspended and bottom sediments. A review of these new data provides a much clearer understanding of the kinds and relative importance of gravitational and hydrodynamic processes that affect the surface sediments. On the rugged slope between Georges Bank and Cape Lookout, N.C., these processes include: (1) small scale mass wasting within submarine canyons and peripheral gullies; (2) density flows within some submarine valleys; (3) sand spillover near the shelf break; (4) sediment creep on the upper slope; and (5) hemipelagic sedimentation on the middle and lower slope. The area between Georges Bank and Hudson Canyon is further distinguished by the relative abundance of large-scale slump scars and deposits on the open slope, the presence of ice-rafted debris, and the transport of sand within the heads of some submarine canyons. Between Cape Lookout and southern Florida, the slope divides into two physiographic units, and the topography is smooth and featureless. On the Florida—Hatteras Slope, offshelf sand spillover and sediment winnowing, related to Gulf Stream flow and possibly to storm-driven currents, are the major processes, whereas hemipelagic sedimentation is dominant over the offshore slope along the seaward edge of the Blake Plateau north of the Blake Spur. Slumping generally is absent south of Cape Lookout, although one large slump scarp (related to uplift over salt diapirs) has been identified east of Cape Romain. Future studies concerning sedimentary processes on the Atlantic slope need to resolve: (1) the ages and mechanisms of mass wasting; (2) the accumulation rates and thicknesses of hemipelagic sediments; and (3) the causes and variability of offshelf sand spillover, sediment winnowing, and canyon transport.

Smackover Platform Sand Bodies: A Bahamas Model: ABSTRACT

Stave Creek field, a potential multimillion-barrel Smackover oil field in Clarke County, Alabama, owes much of its prolific production to an extensive, 100 ft (30 m) thick buildup of porous ooid and peloid grainstones. Seismic profiles and residual gravity maps indicate the field is located on the edge of a large, Jurassic, northeast tilted block of approximately 12 mi2 (31 km2). The block, which formed a shallow platform for Late Jurassic deposition, is bounded on the west by an older component of the Jackson fault, and slopes eastward into the present graben complex of the Gilbertown and West Bend faults. This location placed the platform in a position between the mouth of the Manila embayment and the eastern edge of the Mississippi interior salt basin. Tidal fluctuation between basins and the open marine conditions to the west had a strong influence on sedimentation across the platform. Cores from the area indicate a pronounced westward trend in increased carbonate grain stability and sediment winnowing. Smackover sediments from the deeper east side of the platform are composed primarily of peloidal lime mudstones and wackestones. Those of the central platform are peloidal and onchoidal wackestones and packstones. Those of the western edge form a high-energy facies of well-sorted ooid and onchoidal grainstones, e.g., the Stave Creek field reservoir. A modern analog to the deposition on this Jurassic platform is found along the margins of the Great Bahama Bank, and specifically the north Andros-Joulters Cays area. End_of_Article - Last_Page 1461------------

Eocene-Oligocene: A Time of Transition: ABSTRACT

The Eocene-Oligocene is a time of transition from a warm early Tertiary world with low vertical and latitudinal thermal gradients to the Neogene world with steep vertical oceanic gradients and high latitudinal gradients between equator and poles. The transition between these two regimes occurred primarily between the middle Eocene and middle Oligocene and can be observed in faunal and floral assemblage changes, associated paleotemperature changes, periodic current intensification as implied by increased carbonate dissolution and hiatuses, eustatic sea level changes, and the curious association of microtektites and iridium anomalies with several of these intervals. Population studies of planktonic foraminifers in 14 DSDP sites in the Atlantic, Pacific, and Indian Oceans indicate a general cooling trend between the middle Eocene and Oligocene. Major faunal changes indicating cooling episodes occur, however, at discrete intervals: middle Eocene 44 to 43 Ma (P13); middle/late Eocene boundary 41 to 40 Ma (P14/P15); late Eocene 39 to 38 Ma (P15/P16); Eocene/Oligocene boundary 37 to 36 Ma (P18); and late Oligocene 31 to 29 Ma (P20/P21). Each cooling episode resulted in the extinction of warmer water species and evolution and dominance of cooler water species. This trend is associated with the development of a steep vertical thermal gradient and resulting stratification in the upper water masses (0 to 300 m, 1,000 ft) in the latest Eocene. 18The presence of microtektites and iridium anomaly in latest Eocene sediments has resulted in the scenario of catastrophic extinctions due to a bolide impact. The present study reveals multiple microtektite occurrences at 43 Ma, 40 Ma, 38 Ma, 34 Ma, and 30 Ma. Moreover, these microtektite occurrences coincide with intervals of increased carbonate dissolution and/or hiatuses. This suggests that microtektites are concentrated as a result of carbonate dissolution and selective winnowing of sediments at these intervals. Consequently, a concentration of microtektites in deep-sea sediments may not always imply a bolide impact, nor is there any evidence of catastrophic extinctions during Eocene-Oligocene time. End_of_Article - Last_Page 494------------

Sedimentary and structural patterns on the northern Santa Rosa-Cortes Ridge, southern California

The Santa Rosa-Cortes Ridge is an 1800 m high, 180 km long feature lying approximately 90 km off the coast of southern California and directly south of the northern group of Channel Islands. Geophysical profiling and sampling cruises to the northern part of the ridge, an area of recent and future lease sales for petroleum development, provide a strong data base for interpretation of structural and sedimentary patterns and their relation to potential geologic hazards. The northern part of the ridge is a complexly folded and faulted northwest-trending anticlinorium composed mostly of lower and middle Miocene shale and mudstone. Erosional remnants of upper Miocene and Pliocene rocks unconformably overlie highly folded pre-upper Miocene strata. The major structure of the ridge developed almost continuously from Oligocene or early Miocene time to the end of the Tertiary; many small folds formed between post-late Miocene and pre-late Pleistocene time. Numerous small faults cut the rocks on the ridge top. Faults cut basement rocks on the west side of the ridge along the Ferrelo fault zone. Published data on historical epicenters and evidence of offsets on the seafloor from seismic reflection and side-scan profiles, including limited evidence of possible strike-slip movement along fault traces, indicate that the northern Santa Rosa-Cortes Ridge is tectonically active. The crest of the ridge was truncated as the result of repeated sea-level fluctuations in the Quaternary. A thin veneer of unconsolidated medium to fine sand, less than 3 m thick, mantles most of the ridge; locally in topographically low areas and on the upper flanks of the ridge, thickness exceeds 20 m. The meager sediment cover, coupled with available evidence of relatively strong currents flowing across the ridge top, indicate that the ridge is an area of nondeposition or winnowing of fine sediments. Modern sediments become thicker and finer-grained from the ridge top to the ridge flanks, suggesting active accretion in those areas. Slumps are present at various depths on the flanks, and it seems probable that the thicker deposits (> 20 m) of modern sediments on the upper flanks will be sites of future slumping.

Chronology of late Brunhes biostratigraphy and late Cenozoic disconformities in the Vema Channel (South Atlantic)

The stratigraphic ranges of two planktonic foraminiferal species provide a useful biostratigraphic zonation for late Brunhes sediments of the subtropical South Atlantic (30°S). By comparison with oxygen isotopic stratigraphy in two cores and calcium carbonate stratigraphy in nine cores, the range of G. hirsuta extends from isotope stage 7 to the Recent ( t = 240,000 years B.P. to Present Day) and that of G. crassaformis from the late Pliocene to isotope stage 5 (120,000 years B.P.). This biostratigraphic zonation was utilized to demonstrate that for the last 130,000 years sediment accumulation changes between water depths of 2,940–4,235 m reflect the transition between Antarctic Bottom Water (AABW) and North Atlantic Deep Water (NADW) in the Vema Channel. The combination of the paleomagnetic stratigraphy and planktonic foraminiferal biostratigraphy in two other cores enabled the detection of two late Cenozoic disconformities in the Vema Channel. The late Pliocene disconformity is marked by a buried manganese oxide crust and associated winnowing of slope sediments. The intensification of bottom currents which produced the Mn-crust ceased in the middle of the Gauss Epoch (2.90 m.y. B.P.). The late Pliocene disconformity may be related to the initial build-up of Northern Hemispheric ice sheets and/or Southern Hemispheric cooling. The second disconformity extends from the late Miocene to the middle of the Gilbert Epoch (3.8 m.y. B.P.).

Manganese Deposits of Variable Composition from North of the Indian-Antarctic Ridge

EXTENSIVE deposits of manganese nodules exist in the South Tasman Basin near the Indian-Antarctic Ridge1–4 and constitute the Tasman Manganese Pavement which is assumed to have formed as a result of increased bottom current velocity in the last 3.5 m.y. This current caused an increased winnowing of sediments from the area and created conditions favourable for manganese accretion. Here I report new data on the morphology and composition of manganese oxide deposits from two dredge hauls taken about 500 km north of the Indian-Antarctic Ridge system in the Southern Tasman Basin (Fig. 1) during Cruise 36 of the USNS Eltanin and donated to the New Zealand Oceanographie Institute. A fuller account will be presented elsewhere5.