Dilute Turbidity Currents Research Articles

Late Quaternary sedimentation on the Portuguese continental margin: climate-related processes and products

The late Quaternary sedimentary history of the continental margin off Portugal was reconstructed from sediment gravity cores. Hemipelagic sedimentation (lithofacies A) was dominant during glacial times. It was interrupted periodically by deposition of shelf- and upper-slope-derived silty and sandy terrigenous material by dilute turbidity currents (lithofacies B and C), ice-rafted debris during distinct periods of breakdown of North Atlantic ice sheets (Heinrich events, lithofacies D) and large amounts of pteropods (lithofacies F). Settling of biogenic particulate material (lithofacies E) prevailed during the Holocene, when sea level and sea surface temperatures were high and terrigenous shelf-input was low. Downslope transport was dominant on the northern part of the Portuguese margin, culminating in frequent turbidity current transport between 35 and 70 ka. This may be due to a humid climate and a high fluvial input. Pteropod muds are confined to cores south of 41°N. Prominent peaks in pteropod concentration were radiocarbon dated at 17.8 and 24.6 ka. Layers rich in ice-rafted debris (IRD) were found along the entire margin. The base of these layers have been dated at 13.6–15.9 14C ka, 21.0–22.0 14C ka, 33.8 14C ka and ±64.5 ka, which correspond well with the ages of Heinrich events 1, 2, 4 and 6 in the central North Atlantic. Heinrich events 0 (10.5 ka), 3 (27 ka) and 5 (50 ka) rarely influenced sedimentation on the Portuguese slope. A mineralogical study of the IRD within Heinrich layers suggests that most icebergs were derived from the Laurentide Ice Sheet in the Hudson Strait and Hudson Bay area through the Labrador Current and the Canary Current and flowed in a southward direction along the margin. IRD from European ice sheets may have been mixed in during Heinrich event 6. On their way along the margin the icebergs lost much of their sediment load due to melting of the ice in a progressively warmer climate. The southernmost latitude studied (37°N) may be close to the southeastern extension of iceberg transport during Heinrich events.

Deep marine trace fossil assemblages from the Lower Carboniferous of Menorca, Balearic Islands, western Mediterranean

Sediments of Lower Carboniferous age in eastern Menorca, Balearic Islands contain a diverse and exceptionally well preserved ichnofauna, including Neonereites biserialis, Nereites isp., Arthrophycus isp., Dictyodora liebeana, two ichnospecies of Chondrites, several ichnospecies of Lophoctenium, two ichnospecies of Phycosiphon, Syncoprulus pharmaceus, annulated burrows and a vertical burrow. The host lithologies are conglomerates, sandstones, siltstones and mudstones; most are the product of debris flows, and of high concentration [Ta(bc) intervals] to relatively dilute (Tcd/Tde intervals) turbidity currents. The rocks are interpreted as deposits of an inner- to mid-fan palaeoenvironment. Channelized deposits, sequences of overbank deposition and interchannel deposits interspersed with the deposits of unconfined debris flows and high concentration turbidity currents are present. The ichnofauna is most frequently, and best, preserved within the Td interval of turbidities, which are interpreted as interchannel deposits, produced by low concentration turbidity flows. The distribution of the ichnofauna is partly controlled by the lithologies in which they are preserved. The rarity and poor preservation of trace fossils in the coarser grained facies contrasts with the detailed preservation of very delicate traces in the finer grained lithologies. However, the ichnofauna is also partitioned between different subenvironments of the mid-fan to produce a series of palaeoichnocoenoses. Thin intervals of interchannel deposits, separated by deposits of high concentration turbidity currents, repeatedly contain only Phycosiphon incertum and small (?juvenile)Dictyodora liebeana. These traces are interpreted as the products of opportunistic colonization of near-channel environments during episodes of quiescent deposition. Thicker intervals of interchannel deposits contain diverse assemblages of trace fossils characteristic of more stable environments, in which widespread colonization occurred. Overbank deposits at Cabo de Favaritz are medium- to thick-bedded, fine-grained beds. In these, the ichnofauna occurs in a simple, two-tier profile. The upper tier is dominated by Nereites isp.; this is underlain by a partially bioturbated layer characterized by large Dictyodora liebeana and Arthrophycus isp.

Contrasting sediment gravity flow processes in the late Llandovery, Rhuddnant Grits turbidite system, Welsh Basin

AbstractThe Rhuddnant Grits turbidite system was deposited within an elongate, fault‐bounded trough in the late Llandovery (Telychian) Welsh Basin. Two groups of sandstones are identified within the system: high‐matrix sandstones and laminated sandstones. The high‐matrix sandstones are medium to very thick bedded, fine to very coarse‐grained muddy sandstones. The high‐matrix sandstone beds are almost entirely structureless and have several features indicative of deposition from high density turbidity currents, probably undergoing late stage debris flow behaviour (e.g. grain size discontinuities, inverse grading, floating clasts). The laminated sandstones are thin to very thin bedded, fine‐grained and have a distinctive mud/silt lamination. Tractional structures and convolution are common in these beds. They were probably deposited by slow moving, dilute turbidity currents. Dissimilar palaeocurrent vectors and estimates of flow properties from the two types of sandstone support the contrasting nature of the depositing flows. A coarsening and thickening upwards trend is identified in the laminated sandstones of the Rhuddnant Grits Formation. This trend is not reflected in the high‐matrix sandstone beds. Although the high‐matrix sandstones appear in packets or groups within the laminated sandstone background, they were otherwise deposited in an entirely random manner throughout the exposed system. This may suggest that the two types of sandstone are the result of different triggering mechanisms at source, or of contrasting flow properties developed early in the flow histories.

Geometry of a Miocene submarine canyon and associated sedimentary facies in southeastern Calabria, southern Italy

Research Article| October 01, 1993 Geometry of a Miocene submarine canyon and associated sedimentary facies in southeastern Calabria, southern Italy WILLIAM CAVAZZA; WILLIAM CAVAZZA 1Dipartimento di Scienze Mineralogiche, Università di Bologna, Piazza di Porta San Donato 1, 40126 Bologna, Italy Search for other works by this author on: GSW Google Scholar PETER G. DECELLES PETER G. DECELLES 2Department of Geological Sciences, University of Rochester, Rochester, New York 14627 Search for other works by this author on: GSW Google Scholar Author and Article Information WILLIAM CAVAZZA 1Dipartimento di Scienze Mineralogiche, Università di Bologna, Piazza di Porta San Donato 1, 40126 Bologna, Italy PETER G. DECELLES 2Department of Geological Sciences, University of Rochester, Rochester, New York 14627 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1993) 105 (10): 1297–1309. https://doi.org/10.1130/0016-7606(1993)105<1297:GOAMSC>2.3.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation WILLIAM CAVAZZA, PETER G. DECELLES; Geometry of a Miocene submarine canyon and associated sedimentary facies in southeastern Calabria, southern Italy. GSA Bulletin 1993;; 105 (10): 1297–1309. doi: https://doi.org/10.1130/0016-7606(1993)105<1297:GOAMSC>2.3.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 SocietyGSA Bulletin Search Advanced Search Abstract Outcrops of the Stilo-Capo d'Orlando Formation (latest Oligocene-early Miocene) along the southeastern coast of Calabria (southern Italy) expose a cross section, orthogonal to paleoflow, of the proximal part of a turbidite depositional system. Major erosion surfaces cutting into basement rocks define several submarine paleocanyons. Paleocanyon fills consist of large, lenticular conglomerate bodies that are 200-580 m thick and 3-6 km wide.The best example of these paleocanyons is located near the town of Stilo, where the geometric relationships between paleocanyon fill and adjacent slope deposits are well exposed. At this locality, a 460-m-thick section of conglomerate cuts into the basement and is composed internally of smaller scale channel fills of poorly organized, clast-supported, cobble-to-boulder conglomerate, deposited mainly by high-density turbidity flows. The conglomerate is thickest near Stilo and becomes progressively thinner southward, where a wedge of mudrock intervenes between the basement and the conglomerate. The mudrock wedge is composed of poorly bedded, intensely bioturbated mudstone with slump structures and represents slope deposits. Bedding within the mudrock wedge is better defined in its stratigraphically higher part near the conglomerate body, where thin-bedded, normally graded sandstone layers with climbing ripples, horizontal lamination, and general thickening toward the canyon axis are present. These layers are the deposits of dilute turbidity currents that occasionally spilled over the canyon margins when the depression was nearly filled.The conglomeratic canyon fill and the adjacent muddy slope deposits are both overlain by a laterally continuous sequence, 160 m thick, composed of two units of fine-grained, thin-bedded turbidites alternating with two units of thicker sandstone and minor pebble-conglomerate beds.The paleocanyons probably originated as subaerial valleys in response to a major fall in relative sea level at 30 Ma and were later submerged by a combination of relative sea-level rise and concomitant tectonic activity. The sharp transition between coarse-grained, canyon-confined conglomerate and the overlying fine-grained, unconfined thin-bedded turbidites exists throughout southern Calabria and may represent the effect of a significant rise in relative sea level. Two other cycles of relative sea-level changes, probably resulting from local tectonic control, are indicated by the upper part of the Stilo-Capo d'Orlando Formation. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Entrainment in turbulent gravity currents

LABORATORY gravity currents are frequently used to model a range of environmental and industrial flows1. The manner in which these flows become diluted with distance by the surrounding fluid has important implications for turbidity currents2, pyroclastic flows3,4, avalanches5, accidental dense gas releases6, fire propagation7 and emission of industrial pollutants. Here we present an experimental technique for quantifying the entrainment of ambient fluid into the head of a gravity current propagating along a horizontal surface. The technique relies on the neutralization of an alkaline current by entrainment of acidic ambient fluid, and is visualized by using a pH indicator. Dimensional analysis indicates that the proportion of ambient fluid entrained into a gravity current head depends only on the initial volume of the current and distance from the release point, and is independent of the initial value of the density difference. This result is confirmed by the experimental data, which also show that little dilution of the head takes place during the slumping phase8,9. Thereafter the dilution increases with the downstream distance, in quantitative agreement with the predictions of a theoretical model which evaluates the volume of entrained fluid. We apply the results to show that sediment slumps of initially high sediment concentrations will become dilute turbidity currents owing to entrainment of sea water before they have propagated extensively over the floors of ocean basins.

Origin of fine‐grained carbonate and siliciclastic sediments in an Early Palaeozoic slope sequence, Cow Head Group, western Newfoundland

ABSTRACTThe Cow Head Group is an Early Palaeozoic base‐of‐slope sediment apron composed of carbonate and shale. Whereas coarse‐grained conglomerate and calcarenite are readily interpreted as debris‐flow and turbidite deposits, calcilutite (lime mudstone), calcisiltite, and shale combine to form three distinct lithofacies whose present attributes are a function of both sedimentation and early diagenesis. Shale is the most common lithology. Black, green, and red shale colour variations reflect the abundance of organic matter in the source area and oxygenation conditions of the sea bottom. In black and green shale, millimetre‐ to centimetre‐thick, alternating dark and light laminations represent terrigenous mud turbidites and hemipelagites, respectively. The calcisiltite/shale facies is uncommon and is composed of numerous graded carbonate‐shale sequences (GCSS) deposited from waning carbonate turbidites and fall‐out of terrigenous muds. Some of the characteristics of ribbon and parted lime mudstones in the calcilutite/shale facies can be explained by deposition of carbonate mud from dilute turbidity currents or hemipelagic settling. Other features are diagenetic in origin.The lack of micrite in GCSS and in the interbedded shales of the calcilutite/shale facies is interpreted to reflect early dissolution of the finer carbonate from these sediments. This remobilized carbonate was precipitated locally to: lithify lime mudstone turbidites or hemipelagites; form diagenetic lime mudstone beds and nodules; cement calcisiltites; and form dolomite. Many of the calcisiltites and calcilutites were, therefore, carbonate enriched at the expense of adjacent argillaceous sediments.These attributes characterize not only fine‐grained sediments of the Cow Head Group but many other Early Palaeozoic slope carbonates as well, suggesting that the model proposed here for depositionl diagenesis has wider application.

Deposition of muds in the ulleung marginal basin

Parallel laminated, graded, and homogeneous muds of turbidity current origin are the predominant facies in the non-fan slope-centered Ulleung marginal basin during the last glacial period. Dilute turbidity currents were probably generated from slumps, slides, and debris flows on the slope. A mid-slope core contains poorly sorted mud-clast muds of debris flow origin. During the period of 75,000 and 10,000 years BP, turbidity currents occurred approximately every 125 years, each depositing about 0.5 km3 of mud with an accumulation rate of up to 40 cm/103 years. The basin was largely suboxic with a rare incursion of bottom currents.

The dali ash, island of rhodes, greece: a problem in interpreting submarine volcanigenic sediments

In the southern part of Rhodes, Greece, rhyolitic subaqueous pyroclastic deposits are interbedded with Tertiary, deep water, marine sediments. The lowermost and best exposed of these deposits — the Dali Ash — is described here. The deposit has been previously described as a deep water welded subaqueous ignimbrite. This paper shows that there is no evidence of welding, and texture previously reported were misidentified. The Dali Ash consists of a lower massive unit (5 m thick), overlain by a sequence of ash-turbidites (2.5 m thick). The lower unit was deposited by a high concentration turbidity current and the ash-turbidites by dilute turbidity currents. Foraminifera are dispersed throughout the deposit and indicate that all the sedimentary gravity flows were cold water/particulate systems. A palaeomagnetic study also suggests they were deposited cold. The Dali Ash can be interpreted as the lateral equivalent of a subaerial pumiceous pyroclastic flow deposit (ignimbrite). The ash-turbidites then may be redeposited slumps off the submarine slope of the lower massive unit, or, may represent later, smaller pyroclastic flows in the eruption. Other alternatives for the origin of the Dali Ash are fully discussed to show the problems in interpreting submarine volcanigenic sediments. It is possible that the deposits are not even a primary eruptive product and are remobilized pyroclastic debris, slumped, for example, off the sides of a shallow marine rhyolitic tuff ring.

The Northwest Atlantic Mid‐Ocean Channel of the Labrador Sea: II. Deposition of parallel laminated levee‐muds from the viscous sublayer of low density turbidity currents

ABSTRACTVarve‐like parallel laminated and homogeneous muds on natural levees of submarine channels result from the spill‐over of the dilute upper parts of channelized turbidity currents. On the levees of the Northwest Atlantic Mid‐Ocean Channel, Labrador Sea, laminated turbidite muds show a systematic upward thickness variation of the granular silt laminae and the clay laminae. In many depositional units the former decrease logarithmically upwards in thickness while the latter increase logarithmically. The origin of parallel lamination in current‐deposited, fine‐grained sediments is interpreted in terms of the ‘burst‐and‐sweep model’ for the viscous sublayer of turbulent flows. Individual silt laminae of laminated turbidite muds form due to shear sorting during burst‐and‐sweep events which have a winnowing effect on the clay fraction. In contrast to sand‐transporting flows the viscous sublayer of silt‐ and clay‐carrying dilute turbidity currents cannot be permanently in a turbulent state. The presence of clay laminae suggests that the boundary layer relaminarizes periodically. It is speculated that this is caused by pressure waves with negative pressure gradients favouring the suppression of bursts and sweeps. The observed vertical thickness variations suggest that the periods of laminar boundary‐layer flow increase in duration as turbulence of the main current decays. The upward thickness decrease of the silt laminae is explained in terms of decreasing supply rates to the boundary layer from the main flow. A multiple bursting‐cycle mechanism is favored over a single bursting cycle mechanism for the formation of individual silt laminae by shear sorting.

Subaqueous liquefied and fluidized sediment flows and their deposits

ABSTRACTA clear distinction must be made between liquefied and fluidized systems. In liquefied beds and flows, the solids settle downward through the fluid, displacing it upward, whereas, in fluidized beds, the fluid moves upward through the solids, which are temporarily suspended without net downward movement. Many recent references to fluidized sediment gravity flows refer, in fact, to flows of liquefied debris.Most uniformly liquefied beds of well‐sorted sand‐ or gravel‐sized sediment will resediment as simple two‐layer systems. Liquefied flows can originate either by liquefaction followed by failure, as in many retrogressive flow slides, or by failure followed by liquefaction, as in the case of some slumps. Empirical and theoretical estimates of flow velocity, thickness, and travel distance suggest that natural laminar liquefied flows of fine‐grained sand will generally resediment after moving a kilometre or less. Laminar flows of coarse‐grained sand will resediment after moving only a few metres. Grain dispersive pressure is thought to be of little significance in the development or maintenance of liquefied flows.Many surficial submarine sand beds are apparently susceptible to liquefaction, including submarine canyon and continental rise deposits. Within submarine canyons and narrow fjords, steep slopes and channels promote the evolution of liquefied flows from slumps by liquefaction after failure and of high density turbidity currents from liquefied flows by the development of turbulence. Upon moving into the lower parts of submarine canyons or into proximal fan channels, liquefied flows will resediment and high density turbidity currents will tend to decline to flows transitional between liquefied flows and turbidity currents. The liquefied, coarser detritus within such transitional flows will be deposited while finer‐grained debris will remain in suspension and continue downslope as dilute turbidity currents. Resedimentation of the liquefied portions of such flows may be responsible for the deposition of the A‐subdivision of many turbidites and many thick, structureless ‘proximal turbidites’ or ‘fluxoturbidites’. Similar units can originate by liquefaction of the traction deposits of normal turbidity currents.Fluidized flows are probably uncommon, thin, and, where formed, originate through fluidization of the fine‐grained tops of liquefied graded beds.