Bottom Current Activity Research Articles

Pelagic carbonate ooze reworked by bottom currents during Devonian approach of the continents Gondwana and Laurussia

Abstract Givetian and lower Frasnian carbonates of pelagic carbonate-platform and distal slope-apron settings in the Harz Mountains of Germany (Herzyn Limestone Formation), the eastern Moroccan Central Massif (Ziar-Mrirt Nappe), and the Carnic Alps in Austria-Italy (Valentin and Pal Limestone Formation) show strong evidence for bottom-current activity during deposition. Calcarenites, laminated calcisiltites, and mottled calcisiltites and calcilutites can be distinguished, which are similar to recent calcareous bioclastic contourites. They combine faint structures caused by current action with pervasive bioturbation. Calcarenites are mostly represented by styliolinid grainstones to packstones with rarely preserved parallel lamination and ripple cross-lamination. Laminated calcisiltites are particularly rich in non-carbonate components with a higher density than calcite such as conodonts and phosphatic intraclasts. Relics of coarsening-upward to fining-upward micro-sequences a few centimetres thick are preserved in the Moroccan record. Erosional surfaces, hardgrounds and condensed phosphates are more typical of the Harz Mountains and the Carnic Alps. The bottom-current influenced facies build up strongly condensed and reduced sequences that occur at the same stratigraphic interval in different areas of central Europe and NW Africa. Variations in rate of accumulation, magnitude of erosion and microfacies, which are found across the three regions, are compatible with a contourite interpretation. The widespread current-induced reworking of calcareous sediments and phosphate formation during the Givetian and early Frasnian as well as the associated erosion marked by pronounced hiatuses all signal a major palaeocirculation event. Thermohaline currents were intensified by the acceleration of flows constricted in narrow oceanic passages between the approaching continental plates Laurussia and Gondwana. Areas affected were the southeastern Rhenish Sea shelf, which occupied the distal passive margin of Laurussia, the disintegrated northern continental margin of Gondwana, whose sedimentary record is now preserved in the Moroccan Meseta, and deep marginal plateaux of the Noric Terrane in the western part of the Prototethys. Thus, the occurrence of fossil calcareous contourites confirms a very advanced convergence between Gondwana and Laurussia and the minor terranes between during Middle and Late Devonian times.

Short-distance variability in slope bed-forms along the Southwestern Adriatic Margin (Central Mediterranean)

The slope of SW Adriatic Margin (SAM) is impinged today by two bottom-water masses, of thermohaline origin, that interact constructively along a preferred path: the Levantine Intermediate Water (LIW), generated in the eastern Mediterranean through intense evaporation and flowing along the contour, and the North Adriatic Dense Water (NAdDW), formed on the shallow north Adriatic and cascading episodically off the shelf. These two southward-flowing bottom-water masses impact the upper slope, which is characterized by an irregular physiographic setting, and together create the bottom-current deposits (e.g. fine-grained contourite drifts and sediment waves) and related erosional features (e.g. furrows, scours, comet marks). This study is focussed on a small open-slope area of the SAM (ca. 35 × 25 km), where high-resolution seismic data allow observe elongated contour-parallel scours, widespread erosion surfaces (accompanied by wide crescent-shape scours and furrow fields) and giant comet-marks spatially associated and genetically linked along a common current path. Further downslope, and away from this highly energetic current path, a field of sediment waves migrates upcurrent and shows bifurcated crests. In some cases the downcurrent limbs of the sediment waves display a well-defined erosional character on seismic profiles and furrows on side scan sonar mosaics. All bottom-current features observed are in equilibrium with the present day oceanographic regime, as documented by the available current-metre data. Moreover, sediment cores suggest that the sediment waves are actively migrating during the present interglacial, possibly implying that bottom waters formation was enhanced after the shallow Adriatic shelf was drowned during the late-Quaternary eustatic rise. The SAM slope represents, therefore, an ideal site to improve our understanding of the depositional impact of bottom currents along the Mediterranean margins and may provide useful hints on the activity of bottom currents elsewhere on the Mediterranean margins.

Seismic and oceanographic evidence of present-day bottom-water dynamics in the Lousy Bank—Hatton Bank area, NE Atlantic

High-resolution multi-channel seismic records from the eastern margin of the Iceland Basin, Northeast Atlantic, are used to infer regional patterns of bottom water dynamics from seafloor morphology and distribution of the most recent geological units deposited along the margin. This information is combined with results from oceanographic and hydrodynamic measurements made in the area. The study area is located between Lousy Bank and Hatton Bank, where deep-water currents are forced around structural highs, leaving a complex pattern of topographically controlled sediment transport pathways. At the top and upper flank of Lousy Bank, George Bligh Bank and Hatton Bank topographic forcing leads to considerable acceleration of the northward flowing North Atlantic Current. At greater water depth, seismic facies indicative of bottom current action are found to be widespread. In addition, seafloor morphology displays moat features extending over large distances. The occurrence of these moats is confined to specific water depths in the range from 700–1400 m and from 1800 to 2200 m depth range. These depth ranges correspond to the basal depth stratum of the North Atlantic Current and the depth range of the Deep Northern Boundary Current, respectively. Geological evidence suggests maximum near-bottom current speed of about 0.5 m/s for these moat areas. Using oceanographic data we suggest that formation of the moats is not likely to be associated with the presence of a persistent high-speed contour current core, but probably originates from the occurrence of solibores and thus may be related to the internal wave field of the Iceland Basin possible linked to atmospheric pressure variation.

The contourite depositional system of the Gulf of Cádiz: A sedimentary model related to the bottom current activity of the Mediterranean outflow water and its interaction with the continental margin

The present paper is mostly a synthesis of work conducted on the continental margin of the Gulf of Cadiz using a broad database collected by several cruises and projects supported by the Spanish Research Council and the NRL and Naval Oceanographic Office (USA), including: bathymetry, sidescan sonar imagery, seismic profiles, sediment cores, submarine photographs and physical oceanographic data. These data have enabled us to establish a detailed understanding of the morphologic development of the margin, its Pliocene and Quaternary stratigraphy, and a full characterization of the contourite depositional system (CDS) generated by the Mediterranean outflow water (MOW). The northern margin of the Gulf of Cadiz shows the following distinct features: (a) an active compressive framework where the “Cadiz Allocthonous Unit” provides an unstable substratum for Late Miocene, Pliocene and Quaternary sedimentation; (b) a relative lack of submarine canyons, except in the western area of the Algarve margin; (c) a very broad continental slope that lacks a marked continental rise; (d) a middle slope dominated by along-slope processes driven by the MOW, which has generated a complex CDS during the Pliocene and Quaternary; and (e) an irregular lower slope and abyssal plain region dominated by down-slope processes that is partly detached from an upper slope source region. The CDS is composed of both depositional and erosive features. The main depositional features are characterised by sedimentary wave fields, sedimentary lobes, mixed drifts, plastered drifts, elongated mounded, and separated drifts and sheeted drifts. The main erosive features are contourite channels, furrows, marginal valleys and moats. These various depositional and erosive features have a specific location along the margin, and their detailed distribution is essential to understand the present (and past) interaction of the MOW with the middle slope. Based on this distribution, five morphosedimentary sectors have been identified within the CDS, which from east to west are: (1) proximal scour and sand ribbons; (2) overflow sedimentary lobe; (3) channels and ridges; (4) active contourite drifts; and (5) submarine canyons. The development of the CDS has been controlled in general by the Pliocene and Quaternary environmental and paleoceanographic changes and by the morphology of the margin, but in detail the development of each of these sectors is related to systematic deceleration of the MOW as it flows westwards, to the interaction with the margin bathymetry, and to the effects of Coriolis force. Our comprehensive sedimentary model for the CDS defines the Gulf of Cadiz margin as a mixed contourite-turbidite system with a detached combined drift-fan morphology. This is different from many other contourite influenced margins, where the contourite processes are dominant on the middle slope, and separated from the down-slope processes, which are characteristic on the lower slope and abyssal plains.

Storm-induced Redistribution of Deepwater Sediments in Lake Ontario

High-resolution seismic reflection profiles, side-scan sonar profiles, and surface sediment analyses for grain size (% and, silt & clay), total organic carbon content, and carbonate content along shore-perpendicular transects offshore of Olcott and Rochester in Lake Ontario were utilized to investigate cm-thick sands or absence of deepwater postglacial sediments in water depths of 130 to 165 m. These deepwater sands were observed as each transect approached and occupied the “sills,” identified by earlier researchers, between the three deepest basins of the lake. The results reveal thin (0 to 5-cm) postglacial sediments, lake floor lineations, and sand-rich, organic, and carbonate poor sediments at the deepwater sites (> 130 m) along both transects at depths significantly below wave base, epilimnetic currents, and internal wave activity. These sediments are anomalous compared to shallower sediments observed in this study and deeper sediments reported by earlier research, and are interpreted to indicate winnowing and resuspension of the postglacial muds. We hypothesize that the mid-lake confluence of the two-gyre surface current system set up by strong storm events extends down to the lake floor when the lake is isothermal, and resuspends and winnows lake floor sediment at these locations. Furthermore, we believe that sedimentation is more likely to be influenced by bottom currents at these at these sites than in the deeper basins because these sites are located on bathymetric highs between deeper depositional basins of the lake, and the bathymetric constriction may intensify any bottom current activity at these sites.

3D seismic analysis of the West Shetland Drift system: Implications for Late Neogene palaeoceanography of the NE Atlantic

Detailed seismic stratigraphic analysis of the West Shetland Drift, located on the Shetland Margin of the Faeroe–Shetland Channel, has been undertaken using commercial 3D seismic data. Emphasis was placed on the interpretation of a significant plastered contourite drift body, the West Shetland Drift–Slope section, with the aim of reconstructing the palaeoceanographic regime of the West Shetland slope during the past 5 Ma. Although limited in areal extent, the West Shetland Drift Slope section provides a key record of the depositional environment and bottom current activity on the West Shetland slope during the Late Neogene. The drift body was divided into three seismic–stratigraphic units and depositional processes responsible for each unit were interpreted, resulting in proposal of an evolutionary model in which a mixed downslope–alongslope depositional system was succeeded by alongslope current derived contourite drift deposition during the Early Pliocene to Mid-Pleistocene. During this time SW flowing alongslope currents impinged upon the West Shetland slope between the basin floor and a mid- to upper slope position. The final stage of deposition consisted of slope wedge progradation associated with Mid- to Late Pleistocene glaciation. Contourite deposition on the West Shetland Slope was punctuated by periods of glaciation activity, during which alongslope current activity appears to have been disrupted, suggesting a link between alongslope current activity and glaciation. Detailed mapping of key, smaller scale features, including a spectacular bottom current sediment wave field and iceberg ploughmarks, both of which were identified for the first time during this study, was instrumental in constraining the palaeocurrent regime, and would not have been possible without the high spatial resolution of the 3D seismic data. This study therefore highlights the efficacy of 3D seismic data as a tool for large scale spatial and temporal analysis of contourite drifts palaeoceanographic research.

The Holocene record of Loch Etive, western Scotland: Influence of catchment and relative sea level changes

Two sediment cores from inner Loch Etive, a deep fjord basin on the west coast of Scotland, reveal a continuous sediment sequence spanning the last 10,000 yr. Benthic foraminiferal assemblages indicate that marine conditions prevailed in Loch Etive throughout the Holocene. However, changes in sediment grain size composition and magnetic susceptibility suggest that the strength and frequency of deep water renewal events has varied through time. This exchange with coastal water is controlled mainly by relative sea level changes (influencing sill depth) and the influx of freshwater to the loch. The very early Holocene sediments are well-sorted coarse silt–fine sands, with a diverse assemblage of calcareous foraminifera indicating frequent renewal of bottom waters. During this period, immediately following the withdrawal of Younger Dryas glaciers, denudation rates were high in the catchment area. The period 9–7 ky BP, during which relative sea level rose in this part of W Scotland, is characterised by highly variable environments. The grain size data indicate an enhanced transport of sandy bed load material from the ‘Bonawe Sill’ region to the basin deep during the mid-Holocene relative sea level high (8–9 m higher than present). High relative sea levels at this time would explain the occurrence of frequent, vigorous deep water renewal events and an increased tidal current regime in the loch. The late Holocene record is characterised by an upward fining of sediment grain size, disappearance of calcareous benthic foraminifera and increasing organic content; suggesting a development towards a more restricted fjordic circulation in response to the progressive lowering of relative sea level. A sudden acceleration in this late Holocene trend took place during the last millennium, during which time human land use and deforestation have influenced the sediment and organic material flux to Loch Etive. It appears that average Holocene sediment accumulation rates in the ‘Bonawe Deep’ were about 0.01–0.03 g cm − 2 yr − 1 , which is far below modern estimates for the site. This can be explained by vigorous bottom current activity during much of the Holocene, leading to a low net depositional rate.

Neogene sediment structures in Bounty Trough, eastern New Zealand: Influence of magmatic and oceanic current activity

New seismic reflection profiles have been interpreted to shed more light on the Neogene deposition in Bounty Trough, an aborted rift characterizing the eastern New Zealand continental margin. Two major processes influencing the deposition were identified. Magmatic activity led to the formation of basement highs, which deform the sedimentary sequences up to early and middle Miocene. The origin of the magmatism unfortunately cannot be solved with the new data presented here. It is difficult to say whether the magmatic activity is an on-going process. Drape structures in the youngest sedimentary unit argue against this. The Oligocene–Miocene represents a period of major change. Bottom current activity then took over as the most important depositional process. Strong cooling events in the late Miocene resulted in modification in the oceanographic regime east of New Zealand. This led to the formation of channels, sediment drifts, and sediment waves. At least since the Miocene, bottom current activity has been the dominating depositional process.

Petit-Lac (western Lake Geneva) environment and climate history from deglaciation to the present: a synthesis

During the past decade, the presentation of seismic and sedimentological data has allowed reconstruction of the environment and climate history of the Petit-Lac (western Lake Geneva). Methods such as high-resolution seismics, sediment core analysis (macroscopic description, grain-size analysis, mineralogy) and palynology have been used to infer the changes in the lake's environment from deglaciation to the present. However, no final synthesis has been attempted to link this information in the development of a comprehensive evolution model of the Petit-Lac and its surrounding region. The Petit-Lac deglaciation occurred in three phases during the Rhône glacier retreat: the Geneva stage and the Coppet and Nyon re-advances. In the Versoix area, rivers developed just after the retreat of the Rhone glacier from the Nyon stage. The Nyon fan delta started at the end of the Bølling, and its lobe fluctuated in size and orientation in six phases from the Lateglacial to the present. The action of bottom currents (i.e. erosion, non-deposition surfaces) arising at the beginning of the Holocene indicates that the frequency and direction of strong wind regimes varied greatly. Lacustrine mass failures occurred at different time intervals: two between deglaciation and the end of the Oldest Dryas, two between the Bølling and the Younger Dryas, and four during the Holocene. From the Oldest Dryas to the Contemporary Epoch, the vegetation changed from a steppe to a climate-influenced forest, and finally to a mostly human-controlled forest.

Slope failure, mass flow and bottom current processes in the Rockall Trough, offshore Ireland, revealed by deep-tow sidescan sonar

The margins of the Rockall Trough, currently the focus of oil and gas exploration interest, incorporate an environmentallyimportant and lightly studied shelf region. To address the paucity of information within this region an extensive high resolution sidescan sonar survey was carried out in the Irish sector of the Rockall Trough in June/July 1998. This valuable natural laboratory contains information on slope stability, sediment transport, nutrient upwelling, bottom current, and biological activity at a shelf/slope/basin-floor transition. Detailed information on the nature of many of these processes can be provided from shallow geophysical techniques. The TRIM (TOBI Rockall Irish Margins) project (Shannon et al., 2001), funded by the Irish Petroleum Infrastructure Programme (Rockall Studies Group), acquired 3100 line-km of TOBI sidescan sonar and 3700 km of 3.5 kHz profiler data along the margins of the Rockall Trough (Figure 1). This project followed on from the earlier reconnaissance AIRS (Atlantic Irish Regional Survey) survey conducted over the entire southern Rockall Trough region, using the lower resolution surface towed sonar side-scan system known as GLORIA (Unnithan et al., 2001). The TOBI sidescan system, developed at the Southampton Oceanography Centre (Flewellen et al., 1993), is deep-towed at 300-400 m above the seafloor. It transmits at a frequency of 32 kHz and samples the acoustic response of the seabed and shallow sedimentary structure to a centimetre to metre depth scale. This acoustic response therefore reflects not only the backscattering properties of the seabed but also the shallow detailed structure of sediments formed by recent sedimentological processes.

Late Cenozoic prograding wedges on the NW European continental margin: their formation and relationship to tectonics and climate

During late Pliocene to Pleistocene times, prominent prograding wedges were deposited along the continental margin of NW Europe, resulting in seaward shelf break migration of up to 150 km. Much of the sediment accumulation occurred marginal to the former mid- to high-latitude ice sheets. The geographical distribution, and stratigraphical and chronological data may suggest that the instigation of the wedges was variously related to tectonic uplift as well as a response to the late Pliocene to Pleistocene climate deterioration and onset of major northern hemisphere glaciations. The onset of wedge growth on the NW UK and Irish margins was initiated at about 4 Ma in response to tectonic tilting of the margin in that region. However, glacially derived sediments here comprise a significant proportion of the wedges, especially since 0.44 Ma. For the Faroe margin, no detailed chronology is available; however, it may be inferred that onset of glacigenic wedge growth here did not post-date that observed on the NW UK and Irish margins. Offshore Norway, wedge growth has largely occurred since ca. 2.7 Ma in response to northern hemisphere glaciations, also recording a major change in sediments transport routes at 0.8–1.1 Ma (reflecting larger Fennoscandian Ice Sheets). Presently, it is uncertain whether the glacigenic wedge growth was preceded by a fluvial phase (in response to uplift) in this area. In the western Barents Sea, an early phase of wedge growth was (glacio) fluvial in character. Off western Spitsbergen, the development was similar to that of the Barents Sea although the glacigenic wedge-growth phase may have started somewhat earlier. The wedges commonly display gently inclined seaward prograding clinoforms, and transparent to chaotic internal acoustic facies. Sampling of their sediments reveals that they are mainly composed of glacigenic diamicton interbedded with marine and glaciomarine sediments that, to various extents, have been affected by bottom-current action. The clinoforms of these wedges vary in geometry from oblique to sigmoidal, and they also show varying degrees of aggradation throughout their development. The resulting stratal stacking pattern can be attributed to a combination of variations in sediment supply, sedimentary processes, and accommodation space, the latter being a function of tectonic movements and/or loading induced subsidence as well as eustatic sea-level fluctuations.

Cenozoic alongslope processes and sedimentation on the NW European Atlantic margin

Based on studies of sediment accumulations deposited from-and erode by-alongslope flowing ocean currents on the European continental margin from Porcupine (Ireland) to Lofoten (Norway), the evolution of the Cenozoic paleocirculation was reconstructed as part of the STRATAGEM project. There is evidence of ocean current-controlled erosion and deposition in the Rockall Trough, in the Faeroe-Shetland Channel and on the Vøring Plateau since the late Eocene, although the circulation pattern remains ambiguous. The late Palaeogene flow in the Rockall Trough was almost probably driven by southerly-derived Tethyan Outflow Water. The extent and strength of any northerly-derived flow is uncertain. From the early Neogene (early-mid-Miocene), there was a massive regional expansion of contourite drift development both in the North Atlantic and in the Norwegian-Greenland Sea. This was most probably related to the development of the Faroe Conduit, the opening of the Fram Strait and the general subsidence of the Greenland-Scotland Ridge. These may have combined to cause a considerable acceleration in the exchange and overflow of deep waters between the Arctic and Atlantic Oceans. An early late Neogene (late early Pliocene) regional erosional event has been ascribed to a vigorous pulse of bottom-current activity, most probably the result of a global reorganisation of ocean currents associated with the closure of the Central American Seaway. During the late Neogene, contourites and sediment drifts developed in deep-water basins, between units of glacigenic sediments as well as infill of several paleo-slide scars. These sediments were derived from areas of bottom-current erosion as well as from the development of Plio-Pleistocene prograding sediment wedges, incorporating the extensive sediment supply derived from shelf-wide ice sheets. Presently a profound winnowing prevails along the shelf and upper slope due to the inflowing currents of Atlantic water. Depocentres of sediments derived from the winnowing are located (locally) in lower slope embayments and in slide scars.

Late Quaternary sedimentary processes and variations in bottom-current activity in the Ulleung Interplain Gap, East Sea (Korea)

A detailed facies analysis of core sediments from the Ulleung Interplain Gap (UIG) reveals ten sedimentary facies which reflect a variety of sedimentary processes: volcaniclastic high-concentration sediment gravity flows, low-density turbidity currents, pelagic settling under either well or poorly oxygenated bottom-water conditions, marine fallout of tephra, hampered sedimentation by bottom currents, top-truncation of turbidites by bottom-current reworking, and Mn-carbonate precipitation under changing bottom-water oxygenation states. Distribution of the facies in the cores, integrated with echo characters of high-resolution subbottom profiles, delineates spatial and temporal changes in sedimentary processes, influenced by variations in bottom-current activity during the late Quaternary. The glacial (> ∼15 ka) sediment units from the Ulleung Interplain Channel (UIC), an erosional axial channel system in the UIG, consist mainly of manganiferous and muddy contourites, interbedded with coarse-grained volcaniclastic turbidites or debrites and non-bioturbated pelagites. This indicates prevalent activity of bottom currents, rarely interrupted by volcaniclastic high-concentration sediment gravity flows and deep-water stagnations which resulted in anoxic bottom-water conditions. On the other hand, the glacial units of the southeastern margin of the UIC, where upslope migrating sediment waves developed, are generally dominated by alternating fine-grained turbidites and bioturbated or non-bioturbated pelagites. This suggests that frequent turbidity currents derived from upslope mass wasting were responsible for the generation of the sediment waves. The post-glacial (< ∼15 ka) units from both the UIC and its southeastern margin consist mostly of muddy contourites or bioturbated pelagites with basal manganiferous contourites. They correspond to an acoustically transparent layer which occurs as an elongate mound parallel to the UIC in the southeastern margin. These features are suggestive of bottom-current-controlled sedimentation, hampered sedimentation in the UIC axis by stronger bottom-current activity and focused accumulation of resuspended sediments in the southeastern margin. The post-glacial bottom-current activity was initiated by renewed circulation with open ocean at ∼ 15 ka, probably more vigorous than that of the present. The bottom-current activity seems to have been augmented by enhanced bottom-water formation by colder sea-surface temperature.

Late Quaternary glacial history, flow dynamics and sedimentation along the eastern margin of the Antarctic Peninsula Ice Sheet

Geological and geophysical data from the NE Antarctic Peninsula continental shelf are used to reconstruct the glacial history, flow-dynamics and sedimentation of the Antarctic Peninsula Ice Sheet (APIS) along its eastern margin during the Late Quaternary. Ice advanced to the shelf edge during the Last Glacial Maximum (LGM) and deposited a stiff till across the shelf. The presence of highly attenuated bedforms indicates that fast-flowing outlets drained the APIS through cross-shelf troughs to the outer shelf after this ice advance. The bedforms are formed in deformation till in response to deforming bed processes. Deglaciation of Robertson Trough and the troughs of Prince Gustav Channel, Larsen-A and Larsen Inlet was continuous (and possibly rapid) on account of the absence of ice-margin recessional features. In contrast, grounding-zone wedges across the shelf of Northern Larsen-A and south of Prince Gustav Channel indicate that ice retreat was gradual and was punctuated by stillstands. The transition from a grounded ice sheet to ice shelf conditions was completed before 11–12 14C ka BP on the shelf south of Prince GustavChannel, and is marked across the shelf by a change from subglacial till to a transitional heterogeneous unit dominated by coarse-grained facies. Transitional sediments record mainly sub-ice shelf rain-out and restricted bottom current and sediment-laden plume activity, as well as localised debris flows. Meltwater-derived facies are largely absent indicating that release of meltwater was not significant beneath polar ice shelves, or during deglaciation of the APIS. In the broader context, the colder, eastern side of the APIS was extensive and was drained mainly through fast-flowing outlets (palaeo-ice streams). Therefore, the eastern APIS would have been an important contributor to sediment and iceberg flux to the Weddell Sea Embayment during the LGM and subsequent deglaciation.

Phosphogenesis and organic-carbon preservation in the Miocene Monterey Formation at Naples Beach, California—The Monterey hypothesis revisited

The middle part of the Miocene Monterey Formation at Naples Beach, west of Santa Barbara, California, is predominantly composed of organic-rich mudstone interstratified with phosphatic laminae. Minor lithologies include volcanic ash, dolomite, porcelanite and chert, and condensed phosphatic beds. Sediments dated as 14.3–13.5 Ma have average total organic carbon (TOC) values around 8.5 wt%, and organic carbon (OC) accumulation rates are around 565 mg/cm2/k.y. Sediments dated as 13.5–13 Ma are characterized by average TOC values of 12.6 wt% and OC accumulation rates of around 1130 mg/cm2/ k.y. The interval between 13 and 10.6 Ma is marked by condensation; average TOC values are around 8.6 wt%, and OC accumulation rates diminished to around 55 mg/cm2/k.y. The last interval studied is dated as 10.6–9.4 Ma, and average TOC values are around 6 wt%, whereas OC accumulation rates rose again to 320 mg/cm2/k.y. The presence of erosional surfaces, angular unconformities, and reworked clasts and nodules suggests that bottom-current activity and gravity-flow deposition have been instrumental in sediment accumulation. The phosphatic laminae were precipitated at a very early stage of diagenesis during periods of nonsedimentation. They formed less permeable sedimentary lids and may as such have contributed to enhanced OC preservation. Between 13 and 10.6 Ma, the thus-formed phosphatic laminae were frequently subjected to subsequent sediment winnowing and reworking, resulting in the formation of condensed phosphatic beds. Calculated P:C molar ratios suggest that (1) the measured section is highly enriched in phosphorus (P) relative to OC; (2) regeneration of organic P from organic-matter decomposition was negligible; and (3) the source of P was external, likely upwelled bottom water rich in inorganic P. In spite of good preservation conditions and correspondingly high TOC contents, the overall OC accumulation rates are moderate in comparison to those of actual high productivity areas, which is mainly due to the episodic character of depositional processes and the intervening long periods of nondeposition and sediment reworking. They preclude this section, and by extrapolation, the Monterey Formation in general from being an important OC sink during the middle Miocene. Alternatively, large OC sinks were probably created on the continent (lignite deposits) and in sedimentary depocenters, which received increasing amounts of detrital sediments due to a combination of climate change, spreading of grasslands, and the increasing importance of mountain chains such as the Himalaya. The associated high nutrient fluxes may have been involved in the backstepping and drowning of carbonate platforms and in the generation of widespread phosphat-rich deposits during the late early and early middle Miocene.

Miocene changes in bottom current regime recorded in continental rise sediments on the Pacific margin of the Antarctic Peninsula

A Fossil Mounded Sedimentary Body (MB) has been identified in the Miocene sedimentary record on the central continental rise west of Adelaide Island, Antarctic Peninsula, using multichannel seismic reflection profiles. The MB has an elongated NE trend away from a group of seamounts, and it developed between two troughs. We interpret it as a patch drift plastered against the NE (lee) side of an obstacle. The MB's depositional patterns provide the first clear evidence of Early Miocene bottom current activity on the central rise, and they suggest that flow was towards the NE, probably as part of the Antarctic Circumpolar Current. This segment of the rise is, however, presently affected by a SW‐flowing branch of Lower Circumpolar Deep Water. The change in bottom water flow took place during the Middle or Late Miocene, and we suggest that it is probably indicative of more widespread palaeoceanographic changes during this period.

A geophysical study of the northern Svalbard continental margin

SUMMARY In the summer of 1999, the first systematic seismic profiles were acquired across the northern Svalbard continental margin east of 15 ◦ E. Approximately 1470 km of multi-channel seismic reflection data as well as sonobuoy wide-angle data were collected up to 82 ◦ N. With few exceptions the signals imaged the whole sedimentary cover down to the acoustic basement. The uppermost sedimentary deposits of the inner shelf yield P-wave velocities of 2 km s −1 and higher, indicating erosion and compaction due to a former ice load. The inner shelf east of Hinlopen Strait has only a thin veneer of over-consolidated sediments above the acoustic basement. Beneath the outer shelf, up to 3.5 km of sedimentary deposits cover the down-faulted acoustic basement. The continental slope is heavily eroded due to bottom current activity and slumping. At about 30 ◦ E the morphology of the continental slope has a smooth appearance. Shelf progradation only in the vicinity of glacial troughs crossing the shelf (associated with submarine fans) indicates main sediment transport by ice streams during former glacial periods. The maximum sedimentary thickness in the Sophia Basin is more than 9 km, and the Nansen Basin has a sediment thickness of 4.5 km close to the margin. Gravity modelling along the seismic profiles was performed to constrain the position of the continent‐ocean transition. Existing sedimentary thickness and structural maps were extended over the area investigated. The new data provide no evidence for the presence of former extensive subaerial volcanic sequences (seaward-dipping reflectors), which would have been emplaced during the break-up along the margin. Thus, we consider this part of the margin as non-volcanic.

Late Quaternary sedimentation in the Ulleung Interplain Gap, East Sea (Korea)

The Ulleung Interplain Gap (UIG) is a deep (2300–2700 m) passage which has served as a conduit for deep-water circulation between the Ulleung and Japan basins. A detailed analysis of Chirp (2–7 kHz) subbottom profiles (ca. 6270 line-km) and nine sediment cores (8.6–11.4 m long) together with age data of tephra layers and four AMS 14C from the UIG and the adjacent areas reveals complex sedimentation caused by an interaction between bottom currents and mass flows during the last- and post-glacial periods. From high-resolution subbottom data, rock basement, slide/slump/rock-fall deposits, mass-flow chutes/channels, mass-flow deposits, bottom-current deposits, and a large-scale bottom-current channel system are recognized. Core sediments consist of various deposits of turbidites, muddy contourites, manganiferous contourites, and pelagic/hemipelagic sediments. Based on vertical distribution of sedimentary facies together with a chronostratigraphic framework, core sediments can be divided into Units I (<∼15 ka) and II (>∼15 ka). The extensive mass-flow deposits with slope failures on the entire slopes of topographic highs around the UIG and the dominant turbidites in Unit II (>∼15 ka) suggest that a relatively large amount of sediment was delivered into the UIG by frequent mass flows (recurrence intervals of ca. 250–500 years in the upper Unit II) during the last-glacial period. Erosion or hampered sedimentation by bottom currents is indicated by the truncated reflectors of channel walls and muddy/manganiferous contourites in the Ulleung Interplain Channel (UIC) along the UIG. Interbedded turbidites in the UIC floor reflect that some large-scale mass flows intermittently entered into the UIC. The UIC has an asymmetric channel-flank geometry. The southeastern flank shows a gentle, wide mound morphology of mass-flow deposits derived from large-scale slope failures on the slopes of the Oki Bank, reflecting a dominance of downslope gravitational processes over alongslope bottom currents. In contrast, the northwestern flank is characterized by a narrow, steep geometry of mass-flow deposits, where a relatively small amount of sediment derived from the slopes of the South Korea Plateau could not overcome bottom-current activity. The dominant muddy and manganiferous contourites with rare turbidites in Unit I (<∼15 ka) reflect intensified bottom currents and infrequent slope failures (recurrence intervals of ca. 1700–5000 years) during the post-glacial period. These conditions facilitated the formation of a thin, elongate mound of bottom-current drifts overlying mass-flow deposits on the southeastern UIC flank, and sustained erosion or hampered sedimentation in the UIC.

Sedimentary Structures of the La Luna, Navay and Querecual Formations, Upper Cretaceous of Venezuela

Other| October 01, 2003 Sedimentary Structures of the La Luna, Navay and Querecual Formations, Upper Cretaceous of Venezuela OLIVER MACSOTAY; OLIVER MACSOTAY 1Urbanización El Trigal Norte, Ave. Atlántico, no. 155-61B, Valencia, Venezuela Search for other works by this author on: GSW Google Scholar ROBERT N. ERLICH; ROBERT N. ERLICH 2Burlington Resources International, 400 North Sam Houston Parkway East, Suite 1200, Houston, TX 77060, rerlich@br-inc.com * Corresponding author. Search for other works by this author on: GSW Google Scholar TULIO PERAZA TULIO PERAZA 3Intevep-Laboratorios Generales, Sur 1, nivel 2, oficina 222, Los Teques, Edo. Miranda, Venezuela Search for other works by this author on: GSW Google Scholar Author and Article Information OLIVER MACSOTAY 1Urbanización El Trigal Norte, Ave. Atlántico, no. 155-61B, Valencia, Venezuela ROBERT N. ERLICH * Corresponding author. 2Burlington Resources International, 400 North Sam Houston Parkway East, Suite 1200, Houston, TX 77060, rerlich@br-inc.com TULIO PERAZA 3Intevep-Laboratorios Generales, Sur 1, nivel 2, oficina 222, Los Teques, Edo. Miranda, Venezuela Publisher: SEPM Society for Sedimentary Geology Accepted: 01 Jun 2003 First Online: 03 Mar 2017 Online Issn: 1938-5323 Print Issn: 0883-1351 Society for Sedimentary Geology PALAIOS (2003) 18 (4-5): 334–348. https://doi.org/10.1669/0883-1351(2003)018<0334:SSOTLL>2.0.CO;2 Article history Accepted: 01 Jun 2003 First Online: 03 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation OLIVER MACSOTAY, ROBERT N. ERLICH, TULIO PERAZA; Sedimentary Structures of the La Luna, Navay and Querecual Formations, Upper Cretaceous of Venezuela. PALAIOS 2003;; 18 (4-5): 334–348. doi: https://doi.org/10.1669/0883-1351(2003)018<0334:SSOTLL>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyPALAIOS Search Advanced Search Abstract The Late Cretaceous seas of Venezuela were characterized by deposition of organic carbon-rich limestones, cherts, and shales in the Maracaibo and Barinas/Apure Basins of western Venezuela (La Luna and Navay Formations) and in the Maturín and Guarico Sub-basins of eastern Venezuela (Querecual, Tigre, and San Antonio Formations). Deposition of these units typically occurred under low-energy, low-oxygen conditions that were punctuated by the episodic activity of bottom currents, debris flows, turbidites, syndepositional faulting, and by intermittent oxygenation. These events produced a characteristic set of sedimentary structures that can be used to provide more detailed interpretations of each depositional system. Four main types of sedimentary structures noted in this study are: (1) hydrodynamic—primarily produced through the actions of currents; (2) rheotropic—structures produced during loading and dewatering of unconsolidated sediments; (3) biogenic—formed through the actions of living organisms, such as trace fossils; and (4) diagenetic—structures and textures formed through near surface and burial diagenetic processes.Identification of these structures can provide new insights on the hydrodynamic and biologic settings of Venezuelan Cretaceous strata. Utilizing observations made in this study along with previously published works, bottom-water conditions in the La Luna depositional system are interpreted to have been maintained consistently in a state of anoxia or very low dissolved oxygen content. This contrasts with bottom waters in the Querecual depositional system, during which strong periods of anoxia or low dissolved oxygen content were punctuated by periods of intermittent oxygenation. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Seismic facies and sedimentary processes on the continental rise off Wilkes Land (East Antarctica): evidence of bottom current activity

Abstract The slope and rise areas off Wilkes Land (East Antarctica) encompass three large-scale composite sediment ridges dissected by two broad deep-sea valleys (Jussieu Canyon and WEGA Channel). Large-scale depositional architecture, revealed from multi-channel seismic profiles acquired during the WilkEs Basin GlaciAl History (WEGA) cruise, indicates a wide temporal and spatial variability in the dominant depositional processes delivering or redistributing sediments on the slope and rise. All three sediment ridges are built on a flat seafloor and resulted from the growth of a thick sediment section (at least 2 km) with a depositional relief in the order of hundreds of meters on the adjacent deep-sea valleys. The sediment ridges show their maximum internal complexity in areas shallower than 3000 m, where they are also substantially thicker (as much as 1–1.5 s. in two-way travel time). High amplitude sediment waves (typically 20–30 m and up to 50–70 m) have wavelengths ranging from 1500 to 3500 m and occur in the deepest sectors of the continental rise, where the sediment ridges toe out. In the lower rise, the sediment waves display a consistent decrease in wavelength with increasing water depth. On seismic profiles, the relief of the sediment waves decreases from older to younger sequences, possibly indicating a decrease in bottom current intensity throughout their formation. Sediment waves occur also at the seafloor, which is dominated elsewhere by irregular topography and widespread evidence of erosion. However, no conclusion can be drawn on the possible modern activity of these large-scale bedforms. In shallower, upper-rise areas, where the sediment ridges are thicker, sediment waves are evident in the upper stratigraphic units, while the remaining section is characterized by the development of channel-levee complexes (seen as small-scale erosional channels, bordered by depositional reliefs consisting of variable-amplitude reflector packages wedging out away from the channels). Channel-levee complexes are locally associated with poorly developed sediment waves. The origin of these features is likely related to transport by turbidity flows from the edge of the Antarctic shelf and implies the presence of a more temperate glacial regime than the one dominating since the Late Miocene. The formation of the sediment waves, both in this more proximal area and on the deep rise, is indicative of the impact of bottom-hugging currents during intervals of the Cenozoic history of the margin. No constrains are available at present on the nature of these currents nor on the timing of their activity; down-slope turbidity currents, downwelling dense waters or slope parallel contour currents are possible mechanisms. Despite this substantial uncertainty, the observed progressive upward attenuation of the wave amplitudes indicates that substantial variations of sediment delivery and dispersal occurred through time. We suggest that the interval of more complex depositional geometry in the proximal areas reflects higher bottom current velocity and/or increased sediment input perhaps during conditions of temperate, wet-based ice sheet on the continent. These conditions were conducive to high rates of melt-water production, formation of dense plumes, and enhanced sediment input.