Low-concentration Turbidity Currents Research Articles

Contrasting developments of Pleistocene calcareous clay units in the middle Bengal Fan and their implications for paleoenvironmental changes

This study details sedimentary and compositional characteristics of the Middle to Late and Early Pleistocene calcareous clay units recovered from Site U1453 in the middle Bengal Fan. The sedimentary characteristics reveal that the calcareous clay units consist of calcareous hemipelagic muds and terrigenous hemiturbidite muds which represent intervening periods of hemipelagic sedimentation sensu stricto and a slow sedimentation from suspension clouds of low-concentration turbidity currents on the fan surface, otherwise dominated by sand and mud turbidites. Variations in compositions, such as organic and inorganic carbon contents and δ13C of bulk organic matters of the calcareous hemipelagic muds, indicate the glacial-interglacial climate forcing of carbonate dissolution, surface marine productivity, and sources of particulate organic matters in the middle Bengal Fan during the Middle to Late Pleistocene. The glacial-interglacial climate forcing on hemipelagic sedimentation seems to be much obscure during the Early Pleistocene because of dilution by terrigenous clay inputs. Grain-size distribution of bulk sediments suggests the presence of coarse foraminifer and finer nannofossil modes in the calcareous clay units and the size of dominant foraminifer species could have been finer in the Early Pleistocene.

Sedimentology, stratigraphic context, and implications of Miocene intrashelf bottomset deposits, offshore New Jersey

Drilling of intrashelf Miocene clinothems onshore and offshore New Jersey has provided better understanding of their topset and foreset deposits, but the sedimentology and stratigraphy of their bottomset deposits have not been documented in detail. Three coreholes (Sites M27–M29), collected during Integrated Ocean Drilling Program (IODP) Expedition 313, intersect multiple bottomset deposits, and their analysis helps to refine sequence stratigraphic interpretations and process response models for intrashelf clinothems. At Site M29, the most downdip location, chronostratigraphically well-constrained bottomset deposits follow a repeated stratigraphic motif. Coarse-grained glauconitic quartz sand packages abruptly overlie deeply burrowed surfaces. Typically, these packages coarsen then fine upwards and pass upward into bioturbated siltstones. These coarse sand beds are amalgamated and poorly sorted and contain thin-walled shells, benthic foraminifera, and extrabasinal clasts, consistent with an interpretation of debrites. The sedimentology and mounded seismic character of these packages support interpretation as debrite-dominated lobe complexes. Farther updip, at Site M28, the same chronostratigraphic units are amalgamated, with the absence of bioturbated silts pointing to more erosion in proximal locations. Graded sandstones and dune-scale cross-bedding in the younger sequences in Site M28 indicate deposition from turbidity currents and channelization. The sharp base of each package is interpreted as a sequence boundary, with a period of erosion and sediment bypass evidenced by the burrowed surface, and the coarse-grained debritic and turbiditic deposits representing the lowstand systems tract. The overlying fine-grained deposits are interpreted as the combined transgressive and highstand systems tract deposits and contain the deepwater equivalent of the maximum flooding surface. The variety in thickness and grain-size trends in the coarse-grained bottomset packages point to an autogenic control, through compensational stacking of lobes and lobe complexes. However, the large-scale stratigraphic organization of the bottomset deposits and the coarse-grained immature extrabasinal and reworked glauconitic detritus point to external controls, likely a combination of relative sea-level fall and waxing-and-waning cycles of sediment supply. This study demonstrates that large amounts of sediment gravity-flow deposits can be generated in relatively shallow (~100–200 m deep) and low-gradient (~1°–4°) clinothems that prograded across a deep continental shelf. This physiography likely led to the dominance of debris flow deposits due to the short transport distance limiting transformation to low-concentration turbidity currents.

Deep-sea fan pinch-out geometries and their relationship to fan architecture, Tanqua Karoo basin (South Africa)

Exceptional exposures of Permian basin floor fans (fans 3, 4) and a slope fan (fan 5) in the Tanqua Karoo foreland basin of South Africa have enabled an investigation of the relation between the pinch-out geometries and fan architecture. The pinch-out geometry of fan 3 is characterized by the down dip transition from thin to medium bedded sheet deposits to pinch-out fingers, which are overlain by younger prograding sheet deposits. This geometry reflects the progradational stacking pattern of the fan. In contrast, the fan 4 pinch-out fingers consist of stacked channel fills in the same conduit. This pinch-out configuration relates to the dominant aggradational style observed on the mid and distal parts of fan 4. Fan 5 represents a slope fan comprising an axial channel conduit, which branches down slope into three distributary channels. The distal fan is characterized by larger channel fills, which may represent bypass channels to other basin floor fans. The very thick-bedded nature of the youngest channel fill unit suggests early bypass followed by retrogradation as indicated by the presence of thinner bedded heterolithic channel fill deposits along the axial conduit. Although some of the massive pinch-out channels exhibit basal scour, their depositional morphology suggests that they mainly originated due to the infill of subtle topographic depressions by low concentration turbidity currents. Instead of describing these features as channel fills, the use of the term pinch-out fingers is preferred.

A Formação Salinas na área-tipo, Ne de Minas Gerais: uma proposta de revisão da estratigrafia da Faixa Araçuaí com base em evidências sedimentares, metamórficas e idades U-Pb shrimp

The Salinas Formation has been considered to be a stratigraphic unit of the Macaubas Group. This group comprises the rift to passive margin sequences of the precursor basin of the Neoproterozoic Aracuai Orogen, eastern Brazil. However, new roadcuts show extensive and spectacular outcrops with very well-preserved sedimentary structures, allowing detailed studies in the type-locality of this formation, located in the Salinas town and surroundings, northeast Minas Gerais State. In its type-locality, the Salinas Formation consists of graywacke, pelite and clast-supported conglomerate, metamorphosed in the greenschist facies. The sedimentary lithofacies are grouped into three facies association (shelf, slope and deep-sea), indicating sedimentation from shelf deposits to deep-water turbidites. The shelf sedimentation was influenced by storm-wave during deposition. The slump and deep-sea deposits were generated by gravitational flows and high- to low-concentration turbidity currents. Shelf sandstone, clast-supported conglomerate and proximal to distal turbidites outline a submarine fan system. U-Pb SHRIMP data from detrital zircons of graywacke samples indicate a maximum sedimentation age of ca . 568 Ma. Thus, the Salinas Formation is much younger than the Macaubas Group, and represents late orogenic deposits ( ca . 568- 500 Ma). The distal, passive margin unit of the Macaubas Group is now called Ribeirao da Folha Formation ( ca . 800 Ma). Keywords : Salinas Formation, Macaubas Group, Aracuai Belt, sedimentary facies, depositional environment, U-Pb geochronology

Subaqueous eruption-fed density currents and their deposits

Density currents fed directly from subaqueous eruptions can be divided conceptually into three groups based on modes of fragmentation and transport: (I) explosive fragmentation, with deposition from a gas-supported current; (II) explosive fragmentation, with deposition from a water-supported current; (III) fragmentation of flowing lava, with deposition from a water-supported current (Fig. 1 ). Group I products include subaqueously emplaced welded ignimbrite and other high-temperature emplaced subaqueous pyroclastic flow deposits. Group II products are the most varied, and include representatives of both high- and low-concentration turbidity currents, grainflows and debris flows, and are termed eruption-fed aqueous density currents. Some clasts in such currents are transported and even deposited at high temperature, but the transporting currents, ranging from grain flows to dilute turbidity currents, are water dominated even though steam may be developed along large clasts’ margins. Group III products formed from lava flow-fed density currents tend to be weakly dispersed down-gradient along the seafloor, and generally consist largely of fragments formed by dynamo-thermal quenching and spalling. Bursting of bubbles formed by vapor expansion probably contributes to some Group III beds. Distinctive column-margin fall deposits may form in water-excluded zones that developed very locally around vents in association with Group I and II deposits, and are distinguished by heat retention structures, indicators of gas-phase transport and absence of current-formed depositional features.

Reconnaissance sedimentology and hydrocarbon biomarkers of Ediacarian microbial mats and acritarchs, lower Ungoolya Group, Officer Basin

Reconnaissance sedimentology and hydrocarbon biomarkers of Ediacarian microbial mats and acritarchs, lower Ungoolya Group, Officer Basin

The southeastern margin of the Cretaceous Yongdong Basin, Korea: a lacustrine fan-delta system

The Cretaceous (Neocomian to Aptian—Albian) Yongdong Basin is a NE-SW trending strike-slip basin (8–10×50 km2), filled with alluvial, fluvial, and lacustrine deposits. The southeastern margin of the basin includes ca. 700-m-thick siliciclastic sequence, consisting of conglomerate, (gravelly) sandstone, and reddish or dark gray mudstone. A detailed facies analysis reveals that the sequence can be represented by 13 sedimentary facies which can be organized into 3 facies associations (FA) representing three distinctive depositional environments. The FA I (stream-dominated alluvial fan) comprises decimeter- to 1-m-thick, granule-to-cobble conglomerates, horizontally stratified gravelly sandstone, and reddish mudstone, deposited by hyperconcentrated flows, stream flows, and overbank suspension fallout. The FA II (fan-delta slope) consists of decimeter- to 2-m-thick, pebble-to-boulder conglomerates with wedge or lobate geometry and decimeter- to 2.5-m-thick, (crudely) stratified (laminated), graded or massive (gravelly) sandstones. They were deposited by debris flows and turbidity currents, respectively. The FA III (prodelta/basin plain) comprises a few cm to 1.2-m-thick, horizontally stratified (laminated), massive or graded (gravelly) sandstones with wedge or sheetform geometry, and decimeter- to 1.5-m-thick mudstone and sandstone-mudstone couplet. They were deposited by high- or low-concentration turbidity currents and settling from suspended sediments in buoyant plumes. The FA I is distributed along the basin margin and passes into the FA II toward the basin center, where FA III is in gradational contact with FA II, forming a radial distributional pattern from the basin margin. These facies associations form a mass-flow-dominated, lacustrine fan-delta system (Ibawisan fan-delta system) with a vertically-/laterally-stacked, fining-upward megasequence. The Ibawisan system probably resulted from continuous faulting along the basin margin and was associated with high rate of sediment supply, and thus formed thick, coarse-grained basin margin deposits abruptly wedging out toward the basin axis. The development of Ibawisan fan-delta system is related to locally different tectonic setting within the basin, and the recognition of lacustrine fan-delta system along the southeastern basin margin is important to understand the tectonic evolution and sequential development of the Yongdong Basin through time.

Late Jurassic palaeogeography and anaerobic-dysaerobic sedimentation in the northern Antarctic Peninsula region

Late Jurassic anaerobic-dysaerobic mudstones crop out on both the Weddell Sea (back-arc) and Pacific (fore-are) margins of the northern Antarctic Peninsula. The only known occurrence on the Pacific margin of the Antarctic Peninsula is the Anchorage Formation of Livingston Island. This mudstone dominated unit comprises interbedded volcaniclastic sandstones, pyroclastic/epiclastic tuffs and radiolarian mudstones. The volcaniclastic sandstones are interpreted as representing deposition from turbidity currents. The tuffs represent sedimentation by both primary airfall processes and resedimentation by low concentration turbidity currents. The radiolarian mudstones represent suspen­sion sedimentation, and reveal an upward increase in bioturbation with a transition from anaerobic-dysaerobic conditions to dysaerobic-aerobic conditions. These facies and the observed vertical change in oxygenation conditions are similar to those seen in the Nordenskjold Formation on the Weddell Sea margin of the Antarctic Peninsula. However, biostratigraphical investigations show that the transition from dysaerobic to aerobic conditions occurred during the late Kimmeridgian-early Tithonian in the Anchorage Formation but late Tithonian or early Berriasian in the Nordenskjold Formation. This diachroneity is related to the palaeogeographical development of the Antarctic Peninsula magmatic arc. A wide epicontinental sea and subdued arc relief in the early Kimmeridgian was followed in the Tithonian by arc uplift, increasing oxygenation in the fore-are basin, and the development of a restricted basin in the hack-arc region. In latest Tithonian-earliest Berriasian times a substantial arc had developed which supplied volcaniclastic sediment to the fore-arc basin; only then was the back-arc basin undergoing the transition from dysaerobic to aerobic conditions. Anaerobic conditions initiated by regional upwelling and expansion of the oxygen minimum zone were perpetuated in a silled basin in the back-arc area, formed by the emergent arc.

CONTRIBUIÇÃO AO ESTUDO DA RESSEDIMENTAÇÃO NO SUBGRUPO ITARARÉ: TRATOS DE FÁCIES E HIDRODINÂMICA DEPOSICIONAL

This paper is a contribution to the resedimentation facies analysis in the Permo-Carboniferous Itarare Subgroup, Parana Basin, southeast Brazil, and it is complemented by a thematic field guide for Porto Feliz region, Sao Paulo State. Two facies tracts were identified. The confined tract (identified by erosional features at the base of the deposits) is composed by a transitional facies set typified by the following end-members: disorganized diamictites, organized diamictites or conglomerates, stratified sandstones, and massive sandstones. To the unconfined tract belong the following transitional spectrum end-members: disorganized diamictites, graded rhythmites, varvic rhythmites and their disturbed equivalents. The origin of resedimentation hydrodynamic process is essentially gravitational. It starts when the glaciogenic deposits are remolded upon flooding by marine transgressions. The unstable remolded deposits are submited to debris flows which evolve to high concentration turbidity currents under confined conditions. At the end of this continuous process the turbidity current interplays with traction, grain interaction and fluidization. The debris flows under unconfined conditions evolve directly to low concentration turbidity currents and from these to diluted suspensions. As the deposition may take place at any moment in those continuous flow spectra, the resultant deposits show continuous transition in their facies characteristics. The presence and abundance of resedimentation facies in the Itarare Subgroup attest that the glaciers were more important functioning as a supply agents than depositional agents.

Depositional sequences from high-concentration turbidity currents, Cretaceous Uhangri Formation (SW Korea)

A detailed sedimentary facies analysis of the middle part of the Uhangri Formation (Late Cretaceous), SW Korea, reveals that the dominant facies sequences consist of: (1) crudely stratified and inverse-to-normally graded gravelly sandstone, laminated sandstone, and sand-siltstone couplets; (2) inverse-to-normally graded gravelly sandstone, laminated sandstone, and sand-siltstone couplets; (3) stratified and normally graded gravelly sandstone and laminated sandstone. The lower units of each fining-upward sequence are characterized by crude stratification, probably formed under high-concentration and cohesionless flow conditions. A progressive freezing of the traction carpet causes crude stratification, whereas strong dispersive pressures at the base form inverse grading. The upper laminated sandstone and sand-siltstone couplets represent the overlying low-concentration turbidity currents.

The Miocene Doumsan Fan-Delta, Southeast Korea: A Composite Fan-Delta System in Back-Arc Margin

ABSTRACT The Miocene in the southwestern margin of the Ulleung Back-arc Basin comprises elements common to both Gilbert- and slope-type deltas, forming a composite fan-delta system. It contains an alluvial fan with laterally-discontinuous breccia and gravelstone, deposited by sheet floods, debris flows and stream flows. The Gilbert-topset is characterized by horizontal, cross-bedded and wedged gravelstones and sandstones, formed in shallow braided channels and bars, whereas the foreset (initial slope of ca. 20°) is dominated by stratified gravelstones formed by gravity sliding and avalanching. At the toe, slope failure caused deposition of mainly disorganized and graded gravelstones which are mostly penecontemporaneously deformed. Further downslope, deposition of well-s rted, disorganized and graded gravelstones are indicative of high-density turbidity currents and debris flows. Here, the bed geometry is suggestive of submarine channels and lobes. On the basin slope, high- or low-concentration turbidity currents, slide/slump and debris flows form debris aprons. On the basin plain, hemipelagic sedimentation was dominant with scarce low-concentration turbidity currents. The Doumsan fan-delta sequence shows a radial, lateral facies change, progressively fining and thinning away from the point of origin. It contains an entire spectrum of sedimentary environments that are characteristic of rapidly-subsiding intraslope basin formed by multi-lateral strike-slip faults.

Shallow marine sedimentation within an active margin basin, James Ross Island, Antarctica

The Santa Marta Formation exposed on northern James Ross Island, Antarctica, represents shallow marine shelf sedimentation within an active margin basin. The formation is approximately 1000 m thick, is of uppermost Santonian to Campanian age and represents part of a 5000–6000 m thick sedimentary sequence forming the Larsen Basin. The Larsen Basin represents either a back-arc basin, or a half-graben basin, developed on the extending margin of the Weddell Sea. Twelve sedimentary facies have been recognised, which can be subdivided into two groups: those which are controlled by shelf processes and those which are related to active arc volcanism. Background shelf processes include fair-weather suspension sedimentation, storm sands and rare tidal current reworking. Active arc processes include direct settling of airfall detritus and rapid resedimentation of volcaniclastic detritus by sandy debris-flows and both high- and low-concentration turbidity currents. In addition sheet conglomerates represent debris flows that evolved from syn-sedimentary slumps. Two facies associations representing a mid to outer shelf and an inner shelf depositional setting respectively can be distinguished, with an apparent regression in the ?mid to late Campanian.

The Upper Jurassic ‘Boulder Beds’ and related deposits: a fault-controlled submarine slope, NE Scotland

During the late Jurassic (Kimmeridgian) rifting in the North Sea, large-scale normal faults developed to form relatively deep marine basins. The north-western margin of one of these basins, defined by the Helmsdale Fault, is described to show the nature of sedimentation associated with a submarine fault scarp. Subsidence to the SE of the Helmsdale Fault led to a fault scarp down which various sediment mass flows moved, including debris flow, rockfall, high and low concentration turbidity currents and possible storm surge wash-over deposits. Redeposition by soft-sediment sliding was important as a slope accretion process. Deep channels were incised into the slope and infilled by coarse-grained elastics: contemporaneous, small-scale submarine fans, located farther off in the present North Sea, may have developed as a result of slope-bypassing sediment flows down such channels. The succession is characterized by extreme sediment heterogeneity compared to many deep water environments and it may be part of an ancient fan delta. The main influences upon slope accretion were the synsedimentary movements on the Helmsdale Fault and the nature of the available sediment input, such that the succession represents structurally controlled and sedimentologically modified slope development.

Sediment waves on the monterey fan levee: A preliminary physical interpretation

Sediment waves on the deep ocean floor occur mostly on the lower continental rise on slopes of 1° or less. Previous studies show that their amplitude and wavelength vary greatly, but little is known about their shape in plan. A detailed survey of a 30-km 2 area of abyssal-depth sediment waves associated with the levee of the Monterey fan valley shows a pattern of sinuous crests and troughs with parallel, well-bedded internal structure. Material in the upper 1 m of sediment consists predominantly of bioturbated, muddy coccolith ooze. A single thin, silty horizon can be correlated between adjoining waves. On the basis of measured wave dimensions and an estimate of flow velocity we use a simple two-layer model for water movement to infer approximate flow parameters. The sediment waves are considered to be formed most likely by low-velocity (10 cm/s), low-concentration turbidity flows approximately 100–800 m thick. This interpretation emphasizes the role of low-speed, low-concentration turbidity currents in the downslope movement of fine-grained material. This type of transport—deposition regime explains the formation of sediment waves very well except for certain waves occurring on depositional ridges in the deep ocean.