Submarine Flows Research Articles

Density- and viscosity-stratified gravity currents: Insight from laboratory experiments and implications for submarine flow deposits

Vertical stratification of particle concentration is a common if not ubiquitous feature of submarine particulate gravity flows. To investigate the control of stratification on current behaviour, analogue stratified flows were studied using laboratory experiments. Stratified density currents were generated by releasing two-layer glycerol solutions into a tank of water. Flows were sustained for periods of tens of seconds and their velocity and concentration measured. In a set of experiments the strength of the initial density and viscosity stratification was increased by progressively varying the lower-layer concentration, C L. Two types of current were observed indicating two regimes of behaviour. Currents with a faster-moving high-concentration basal region that outran the upper layer were produced if C L < 75%. Above this critical value of C L, currents were formed with a relatively slow, high-concentration base that lagged behind the flow front. The observed transition in behaviour is interpreted to indicate a change from inertia- to viscosity-dominated flow with increasing concentration. The reduction in lower-layer velocity at high concentrations is explained by enhanced drag at low Reynolds numbers. Results show that vertical stratification produces longitudinal stratification in the currents. Furthermore, different vertical and temporal velocity and concentration profiles characterise the observed flow types. Implications for the deposit character of particle-laden currents are discussed and illustrated using examples from ancient turbidite systems.

Abrupt transitions in gravity currents

Pyroclastic flows and snow avalanches sometimes exhibit a rapid deceleration of their dense flow fronts and detachment of their dilute clouds. This behavior is also inferred for submarine flows and could explain stepped thickness patterns in their deposits. A similar “abrupt transition” process occurs in particle‐laden, lock release laboratory currents with relatively high concentrations. New experiments on nonparticulate, solute‐driven density currents were run to investigate the cause of abrupt transitions. Abrupt transitions occur in laboratory currents with Reynolds numbers (Re) less than 1000 and are interpreted, supported by theoretical scaling analysis, to signify a change in dynamic regime. Currents with high Re, which do not show abrupt transitions, undergo a downstream change in dynamic regime from (1) inertial slumping to (2) inertial‐buoyancy spreading to (3) viscous‐buoyancy spreading. In low Re currents that undergo abrupt transitions, however, the duration of the second regime is very short, and hence they appear to pass directly from the quickly moving slumping phase into the slowly moving viscous phase. Scaling analysis indicates that an abrupt transition should occur in currents below a critical value of Re of ∼10–5000 for currents with different initial aspect ratios. Given that natural flows typically have greater Reynolds numbers, we suggest that abrupt transitions in laboratory and natural currents are likely to be dynamically different. This work has important implications for the physical modeling of gravity flows.

Erosional processes and paleo-environmental changes in the Western Gulf of Lions (SW France) during the Messinian Salinity Crisis

Current interpretation of the Messinian Salinity Crisis (MSC) involves partial “desiccation” of the Mediterranean Sea coupled with the deposition of thick evaporites in the deep basins. New sets of seismic reflection profiles in the western part of the Gulf of Lions confirm the basinward extension of the Messinian erosion and enable the mapping of distinctive seismic markers indicating the Messinian Erosional Surface (or Messinian unconformity), the basin-margin detrital deposits, and the deep evaporite sequence. The geometrical relationship between these three elements and their relationship to the paleogeography of the margin during the MSC provide new information about the evolution of the study area during the Messinian. The Messinian Erosional Surface (MES), commonly correlated with the “desiccation” phase and the deposition of deep evaporites during the apogee of the event, is generally interpreted as a subaerial feature. In the Gulf of Lions, it is a complex diachronic polygenic erosional surface observed at the base of the prograding Plio-Quaternary sequence beneath the shelf and slope; it extends downslope beneath the deep basin Upper Evaporites and the Salt, and possibly correlates conformably with the base of the so-called deep Lower Evaporites. The whole morphology of the MES reflects a buried drainage pattern, supporting the interpretation of fluvial erosion driven by a substantial drop in sea level. Our results also suggest that large submarine gravity flows occurred prior to any significant accumulation of Salt in the basin and prior to the Upper Evaporites. Consequently, interbedded clastic deposits may partly account for the parallel reflectors of the Lower Evaporites. Since river erosion persisted throughout the MSC, the Salt and Upper Evaporite units may also contain a large amount of detrital sediments. The good quality of the new seismic data clearly reveals fan-shaped Messinian deposits in the downstream part of the main Messinian valleys (i.e., the Nile, Var, and Spanish rivers). The depositional scenarios generally involve a substantial sea-level fall coupled with deltaic/prodeltaic accumulations. A chaotic seismic unit (Unit D) filling Messinian lows and extending beneath the Salt within the study area is interpreted as a Messinian clastic unit. We propose a polyphase scenario of detrital fan deposition involving pre-, syn-, and post-Salt deposition in subaqueous/subaerial environments. In the Gulf of Lions, a late Miocene tectonic phase that affected the western shelf also played an important role in controlling (a) the pattern of the Messinian fluvial network, (b) the location of maximum erosion on the shelf, and (c) the location of the detrital fan depocentre downslope.

The40Ar/39Ar dating of core recovered by the Hawaii Scientific Drilling Project (phase 2), Hilo, Hawaii

The Hawaii Scientific Drilling Project, phase 2 (HSDP‐2), recovered core from a ∼3.1‐km‐thick section through the eastern flanks of Mauna Loa and Mauna Kea volcanoes. We report results of40Ar/39Ar incremental heating by broad‐beam infrared laser of 16 basaltic groundmass samples and 1 plagioclase separate, mostly from K‐poor tholeiites. The tholeiites generally have mean radiogenic40Ar enrichments of 1–3%, and some contain excess40Ar; however, isochron ages of glass‐poor samples preserve stratigraphic order in all cases. A 246‐m‐thick sequence of Mauna Loa tholeiitic lavas yields an isochron age of 122 ± 86 kyr (all errors 2σ) at its base. Beneath the Mauna Loa overlap sequence lie Mauna Kea's postshield and shield sequences. A postshield alkalic lava yields an age of 236 ± 16 kyr, in agreement with an age of 240 ± 14 kyr for a geochemically correlative flow in the nearby HSDP‐1 core hole, where more complete dating of the postshield sequence shows it to have accumulated at 0.9 ± 0.4 m/kyr, from about 330 to &lt;200 ka. Mauna Kea's shield consists of subaerial tholeiitic flows to a depth of 1079 m below sea level, then shallow submarine flows, hyaloclastites, pillow lavas, and minor intrusions to core bottom at 3098 m. Most subaerial tholeiitic flows fail to form isochrons; however, a sample at 984 m yields an age of 370 ± 180 kyr, consistent with ages from similar levels in HSDP‐1. Submarine tholeiites including shallow marine vitrophyres, clasts from hyaloclastites, and pillow lavas were analyzed; however, only pillow lava cores from 2243, 2614, and 2789 m yield reliable ages of 482 ± 67, 560 ± 150, and 683 ± 82 kyr, respectively. A linear fit to ages for shield samples defines a mean accumulation rate of 8.6 ± 3.1 m/kyr and extrapolates to ∼635 kyr at core bottom. Alternatively, a model relating Mauna Kea's growth to transport across the Hawaiian hot spot that predicts downward accelerating accumulation rates that reach ∼20 m/kyr at core bottom (DePaolo and Stolper, 1996) is also consistent with all reliable ages except the deepest.

Stratigraphic architecture of the northern Oman continental margin - Mesozoic Hamrat Duru Group, Hawasina complex, Oman

ABSTRACT The Triassic to Late Cretaceous deep-marine sediments of the Hamrat Duru Group, Oman Mountains, represent a subunit of the Hawasina nappe-complex which was deposited in a deep marine basin. During the Late Cretaceous SSW-directed obduction of the Semail Ophiolite, the Hawasina complex was emplaced onto the autochthonous cover of the Arabian basement, while the original configuration of the basin was destroyed. New lithostratigraphic results and high-resolution radiolarian and conodont biostratigraphy lead to a revised stratigraphic scheme of the Hamrat Duru Group which conforms with the standard stratigraphical nomenclature. The Hamrat Duru Group is divided into six formations: (1) The Early Triassic (Olenekian) to Late Triassic (Upper Norian) Zulla Formation (Limestone and Shale Member, Sandstone and Shale Member, Radiolarian Chert Member and Halobia Limestone Member); (2) The Late Triassic (late Norian to Rhaetian) Al Ayn Formation; (3) The Early Jurassic (late Pliensbachian) to Middle Jurassic (early Callovian) Guwayza Formation (Tawi Sadh Member and Oolitic Limestone Member); (4) Middle Jurassic (Callovian) to Late Cretaceous (Cenomanian?) Sid’r Formation (Lower Member, Upper Member); (5) Late Cretaceous (Cenomanian? to Santonian?) Nayid Formation; and (6) Late Jurassic (early Callovian) to Early (Late?) Cretaceous Wahrah Formation. Most of the lithostratigraphic units (formations and members) show isochronous boundaries between the different outcrop areas. The stratigraphic architecture of the Hamrat Duru Group megasequence is controlled by alternating siliciclastic and carbonate sedimentation possibly related to the second-order sea-level variations. The sediments accumulated on the continental rise of the Arabian margin mostly by submarine sediment-gravity flows and hemipelagic to pelagic rainout. A close relationship of the evolution of the Arabian Platform and the adjoining slope and basinal environments is evident. Changes in carbonate supply, oceanographic circulation and/or variations in silica productivity resulted in two distinct phases of radiolarian sedimentation. The first phase corresponds to the Triassic late Anisian-early Norian time interval; the second started in the Early Jurassic late Pliensbachian and lasted, with some interruptions, up to the Late Cretaceous Coniacian. The litho- and biostratigraphic similarities between the Mesozoic Hamrat Duru Basin of the northern/central Oman Mountains and the Mesozoic Batain Basin of northeastern Oman are seen as related to Neo-Tethys-wide palaeoceanographic changes and suggest a strong interdependence of the two basins with the evolution of the Arabian Platform.

A mechanism of pore pressure accumulation in rapidly sliding submerged porous blocks

The generation and dissipation of pore pressure excess due to the rapid, mutual sliding of two water-saturated blocks which are composed of glued arrays of cylindrical rods is herein theoretically examined. The mathematical model consists of three ordinary differential equations describing the motion of a solid mass coupled with the continuity of a fluid mass. These equations are numerically solved for the following conditions: (a) motion induced by the constant horizontal velocity of the sliding mass; (b) constant drag force acting in a horizontal direction; (c) gliding on the irregular, inclined surface that envelops the rods. Theoretical predictions for case (a) favorably compare with experimental results reported in the literature from an investigation carried out on a physical model. Primary results show that the pore pressure excess depends upon physical and geometrical parameters, and furthermore, that displacement of the rods and pore pressure excess are strongly coupled. Maximum pore water pressure occurs when “fluidization”, i.e. the complete balancing of submerged block weight by pore pressure excess, is achieved. The latter does not coincide with the rising of the upper block (hydroplaning). A cyclic motion is reached under condition (a), when dynamic pore water pressure increases from a negative to a maximum positive value and then falls again to a negative value. On the other hand, non-cyclical motion and monotonically increasing pore water pressure are obtained under sliding conditions (b) and (c). The response of the model under condition (c), which is of particular interest for run-out analyses of flow slides, is compared to the sliding response of ordinary submerged blocks on a slope, showing significant differences and pointing out the roles of particle diameter, layer extent and medium permeability. The occurrence of different pore pressure states for fluidization and hydroplaning is highlighted. This, hopefully, can open the way to a reliable treatment of the mechanics of sub-marine flow slide propagation.

Beds comprising debrite sandwiched within co‐genetic turbidite: origin and widespread occurrence in distal depositional environments

AbstractCo‐genetic debrite–turbidite beds occur in a variety of modern and ancient turbidite systems. Their basic character is distinctive. An ungraded muddy sandstone interval is encased within mud‐poor graded sandstone, siltstone and mudstone. The muddy sandstone interval preserves evidence of en masse deposition and is thus termed a debrite. The mud‐poor sandstone, siltstone and mudstone show features indicating progressive layer‐by‐layer deposition and are thus called a turbidite. Palaeocurrent indicators, ubiquitous stratigraphic association and the position of hemipelagic intervals demonstrate that debrite and enclosing turbidite originate in the same event. Detailed field observations are presented for co‐genetic debrite–turbidite beds in three widespread sequences of variable age: the Miocene Marnoso Arenacea Formation in the Italian Apennines; the Silurian Aberystwyth Grits in Wales; and Quaternary deposits of the Agadir Basin, offshore Morocco. Deposition of these sequences occurred in similar unchannellized basin‐plain settings. Co‐genetic debrite–turbidite beds were deposited from longitudinally segregated flow events, comprising both debris flow and forerunning turbidity current. It is most likely that the debris flow was generated by relatively shallow (few tens of centimetres) erosion of mud‐rich sea‐floor sediment. Changes in the settling behaviour of sand grains from a muddy fluid as flows decelerated may also have contributed to debrite deposition. The association with distal settings results from the ubiquitous presence of muddy deposits in such locations, which may be eroded and disaggregated to form a cohesive debris flow. Debrite intervals may be extensive (&gt; 26 × 10 km in the Marnoso Arenacea Formation) and are not restricted to basin margins. Such long debris flow run‐out on low‐gradient sea floor (&lt; 0·1°) may simply be due to low yield strength (≪ 50 Pa) of the debris–water mixture. This study emphasizes that multiple flow types, and transformations between flow types, can occur within the distal parts of submarine flow events.

Transient hydrogeologic models for submarine flow in volcanic seamounts: 2. Comparison of the Hawaiian, Canary, and Marquesas Islands

Large bathymetric gradients associated with volcanic seamounts can drive convective flow, while thick sedimentary aprons that typically surround volcanic edifices host compaction‐driven flow. In these submarine environments the interactions of compaction‐driven and buoyancy‐driven fluid flow lead to complex hydrogeologic regimes. We apply transient numerical models of coupled fluid flow and heat transport to the Hawaiian, Canary, and Marquesas Islands to examine the role of volcanic architecture on the evolution of fluid flow and pore pressure during volcanic building, lithospheric flexure, sedimentation, and compaction. The islands differ in edifice size, sedimentary apron structure, amount of lithospheric flexure, and sedimentation and volcanic growth rates. By comparing these variations, we examine how geometry and geologic history affect fluid flow and pore pressure patterns. Buoyancy‐driven flow is most influenced by edifice height and amount of lithospheric flexure. Compaction‐driven flow is altered primarily by thickness of prevolcanic sediment, the sedimentation rate, and the size of the volcanic edifice. In Hawaii, the area with the highest edifice and most flexure, flow velocities and excess pore pressures are greatest, with Darcy velocities of &gt;30 mm/yr and excess pressures of &gt;7 MPa during the beginning of volcanic building. High sedimentation rates in the Marquesas sedimentary apron increase fluid velocities in the later portion of volcanic building, with Darcy velocities &gt;12 mm/yr. In the Canary Islands, compaction‐driven flow occurs for an extended time period because of long, slower volcanic building, resulting in Darcy velocities &gt;10 mm/yr that endure over 5 Myr.

Transient hydrogeologic models for submarine flow in volcanic seamounts: 1. The Hawaiian Islands

Seamounts and volcanic islands are regions of the seafloor where pressure gradients, caused by temperature variations, are large enough to drive fluid flow. Compaction in surrounding sedimentary aprons, formed during mass wasting of these features, drives fluid flow as well. In seamount environments a complex flow pattern evolves as effects of compaction and free convection interact. We use transient finite element modeling to explore fluid flow and pore pressure in a volcanic edifice and its surrounding sedimentary apron to determine the hydrogeologic and pore pressure regime. Models are based on geophysical cross sections of the Hawaiian Islands and include the volcanic edifice, sedimentary apron, and underlying oceanic crust. Models simulate growth of the volcanic island, flexure of the oceanic crust due to loading, and formation of the sedimentary apron. High sedimentation rates of up to 1 mm/yr during volcanic building cause compaction in the sedimentary apron, and the weight of the edifice compacts the pelagic sediment beneath it; compaction‐driven flow dominates flow patterns. Excess pore pressures are highest during the beginning of volcanic construction and decline through the remainder of the simulation. As sedimentation decreases, compaction‐driven and buoyancy‐driven flow both control flow patterns. When sedimentation stops, pore pressure dissipates, and buoyancy‐driven flow dominates the flow regime. Pore pressure ratios indicate that under certain parameter conditions, pore pressure may exceed lithostatic, leading to unstable edifice conditions. Although fluid velocities are small (q &lt; 0.5 cm/yr), the hydrologic regime has important implications for slope stability, volcanic spreading, sedimentation processes, and geochemical transport.

A new ‘schlieren’ technique application for fluid flow visualization at cold seep sites

A new optical instrument for the investigation of submarine fluid flows based on a ‘schlieren’ technique was developed and successfully deployed at cold seep sites. With this application it is possible to visualize the discharge and distribution of fluids in the ambient bottom water and to resolve microstructures and mixing processes at a scale of centimeters. The system is sensitive to small refractive index anomalies caused by temperature and salinity variations. Density anomalies of Δ σ t =0.049 are detectable evaluated by in-situ temperature variations of Δ T=0.1°C and salinity variations of Δ S=0.045 psu. In flume experiments the smallest detectable density variation was even lower with Δ σ t =0.023. The technique has been successfully applied in two different environments. First field experiments were performed to observe submarine groundwater discharge in Eckernförde Bay (western Baltic Sea) at shallow water depths. Subsequently, the ‘schlieren’ technique was successfully brought to a cold seep location at the Cascadia convergent margin (800 m water depth). The discharge of fluids was recorded in both field experiments which enabled a qualitative seep site identification.

Quantitative constraints on the growth of submarine lava pillars from a monitoring instrument that was caught in a lava flow

Lava pillars are hollow, vertical chimneys of solid basaltic lava that are common features within the collapsed interiors of submarine sheet flows on intermediate and fast spreading mid‐ocean ridges. They are morphologically similar to lava trees that form on land when lava overruns forested areas, but the sides of lava pillars are covered with distinctive, evenly spaced, thin, horizontal lava crusts, referred to hereafter as “lava shelves.” Lava stalactites up to 5 cm long on the undersides of these shelves are evidence that cavities filled with a hot vapor phase existed temporarily beneath each crust. During the submarine eruption of Axial Volcano in 1998 on the Juan de Fuca Ridge a monitoring instrument, called VSM2, became embedded in the upper crust of a lava flow that produced 3‐ to 5‐m‐high lava pillars. A pressure sensor in the instrument showed that the 1998 lobate sheet flow inflated 3.5 m and then drained out again in only 2.5 hours. These data provide the first quantitative constraints on the timescale of lava pillar formation and the rates of submarine lava flow inflation and drainback. They also allow comparisons to lava flow inflation rates observed on land, to theoretical models of crust formation on submarine lava, and to previous models of pillar formation. A new model is presented for the rhythmic formation of alternating lava crusts and vapor cavities to explain how stacks of lava shelves are formed on the sides of lava pillars during continuous lava drainback. Each vapor cavity is created between a stranded crust and the subsiding lava surface. A hot vapor phase forms within each cavity as seawater is syringed through tiny cracks in the stranded crust above. Eventually, the subsiding lava causes the crust above to fail, quenching the hot cavity and forming the next lava crust. During the 1998 eruption at Axial Volcano, this process repeated itself about every 2 min during the 81‐min‐long drainback phase of the eruption, based on the thickness and spacing of the lava shelves. The VSM2 data show that lava pillars are formed during short‐lived eruptions in which inflation and drainback follow each other in rapid succession and that pillars record physical evidence that can be used to interpret the dynamics of seafloor eruptions.

The Early Palaeozoic Break-up of Northern Gondwana: Sedimentology, Physical Volcanology and Geochemistry of a Submarine Volcanic Complex in the Bavarian Facies Association, Saxothuringian Basin, Germany

Ordovician volcano-sedimentary successions of the Bavarian facies association in the Saxothuringian basin record the continental rift phase of the separation of the Saxothuringian Terrane from Gondwana. An 80 m succession from the Vogtendorf beds and Randschiefer Series (Arenig-Middle Ordovician), exposed along the northern margin of the Münchberg Gneiss Massif in northeast Bavaria, were subjected to a study of their sedimentology, physical volcanology and geochemistry. The Randschiefer series previously has been interpreted as lavas, tuffs, sandstones and turbidites, but the studied Ordovician units include four main lithological associations: mature sandstones and slates, pillowed alkali-basalts and derivative mass flow deposits, trachyandesitic lavas and submarine pyroclastic flow deposits interbedded with turbidites. Eight lithofacies have been distinguished based on relict sedimentary structures and textures, which indicate deposition on a continental shelf below wave base. The explosive phase that generated the pyroclastic succession was associated with the intrusion of dykes and sills, and was succeeded by the eruption of pillowed basalts. Debris flow deposits overlie the basalts. Ordovician volcanism in this region, therefore, alternated between effusive and explosive phases of submarine intermediate to mafic volcanism. Based on geochemical data, the volcanic and pyroclastic rocks are classified as basalts and trachyandesites. According to their geochemical characteristics, especially to their variable concentrations of incompatible elements such as the High Field Strength Elements (HFSE), they can be divided into three groups. Group I, which is formed by massive lavas at the base of the succession, has extraordinarily high contents of HFSE. The magmas of this group were probably derived from a mantle source in the garnet stability field by low (ca. 1%) degrees of partial melting and subsequent fractionation. Group II, which comprises the pillow lavas at the top of the sequence, displays moderate enrichment of HFSE. This can be explained by a slightly higher degree of melting (ca. 1.6%) for the primary magma. Group I and II melts fractionated from their parental magmas in different magma chambers. The eruption centres of Groups I and II, therefore, cannot be the same, and the volcanic rocks must have originated from different vents. The sills and pyroclastic flow deposits of Group III stem at least partly from the same source as Group I. Rocks of Group I most likely mixed together with Group II components during the formation of the Group III flows, which became hybridised during eruption, transportation and emplacement. The sedimentological and geochemical data best support a rift as the tectonic setting of this volcanism, analogous to modern continental rift zones. Hence, the rift-associated volcanic activity preserved in the Vogtendorf beds and Randschiefer Series represents an early Ordovician stage of rift volcanism when the separation of the Saxothuringian Terrane from Gondwana had just commenced.

A new model for submarine volcanic collapse formation

Collapse pits and an associated suite of collapse‐related features that form in submarine lava flows are ubiquitous on the global mid‐ocean ridge crest. Collapse pits, the lava tube systems they expose, and lenses of talus created by the collapse process combine to produce a permeable region in the shallow ocean crust and are thought to contribute significantly to the 100–300 m thick low velocity zone observed at intermediate to fast‐spreading mid‐ocean ridges. This horizon of low‐density, high‐porosity material is likely to be an important aquifer for the transfer of hydrothermal fluids in the upper ocean crust. In a recent survey of the East Pacific Rise at 9°37′N, we used photographs, video and observations from the submersible Alvin, and DSL‐120A side scan data to determine that 13% of the 720,000 m2 of seafloor imaged had foundered to form collapse pits. In 98% of the images collapse pits occurred in lobate flows, and the rest in sheet flows. On the basis of our observations and analyses of collapse features, and incorporating data from previous models for collapse formation plus laboratory and theoretical models of basalt lava behavior in the deep ocean, we develop a detailed multistage physical model for collapse formation in the deep ocean. In our model, lava extruded on the seafloor traps pockets of seawater beneath the flow that are instantly vaporized to a briny steam. The seawater is transformed to vapor at temperatures above 480°C with a 20 times expansion in volume. Bubbles of vapor rise through the lava and concentrate below the chilled upper crust of the lava flow, creating gas‐filled cavities at magmatic temperatures. Fluid lava from the cavity roofs drips into the vapor pockets to create delicate drip and septa structures, a process that may be enhanced by water vapor diffusing into the magma and reducing its melting point. As the vapor pocket cools, the pressure within it drops, causing a pressure gradient to develop across the upper crust. The pressure gradient often causes the roof crust to collapse during cooling, though vapor pocket geometry may be such that the roof remains intact during subsidence of the underlying lava. Alternatively, drainaway of the molten lava may cause collapse in locations where inflated lava roof crusts are not supported from below by bounding walls or lava pillars. Post‐eruption seismicity, lava movement, or hydrovolcanic explosions may cause continued collapse of the lava carapace after the eruption.

The Nickeliferous Late Archean Reliance Komatiitic Event in the Zimbabwe Craton––Magmatic Architecture, Physical Volcanology, and Ore Genesis

The widespread komatiitic Reliance lithostratigraphic unit of the late Archean Bulawayan Supergroup consists of many submarine flow fields directly or indirectly overlying the sedimentary Manjeri unit, which varies from a generally carbonaceous, sulfide-rich, deep-water, basinal facies in the north and west parts of the Zimbabwe craton to a generally sulfide poor, shallow-water, platformal facies in the south and east. The flow fields are associated with high-level subvolcanic feeder sills, intruded into pre-Manjeri basement made up of the conformable, sulfide-bearing, felsic Koodoovale unit and older, barren, granitoid-greenstone crust in the north and west, and south and east, respectively. The sills and flow fields together constitute largely discrete sill-flow complexes, very approximately 100 km in diameter, of which five are well preserved in the north and west, and three in the south and east parts of the craton. The Reliance sills, up to 40 km in diameter and 2,000 m thick, some composite and locally overlapping to slightly discordant, mostly comprise a thick lower dunite-peridotite and a relatively thin to intermittent upper pyroxenite-gabbro. Other larger, probably related, sills intruded at greater structural depths link adjacent complexes and form midcrustal magma chambers. The Reliance sills are interpreted as an interconnected system of flow-through feeders at different crustal levels. The Reliance flow fields are 33 to 85 km in diameter and up to 2,500 m thick, dominantly of komatiitic basalt composition, of very low height/breadth aspect ratio, locally linked to the sills via structures interpreted as fissure vents and partly extrusive cryptodomes, and thin out toward their lateral margins. Most display a simple, coherent, volcanological growth sequence (from sheet and/or channelized sheet flows and lava channel complexes in lower and proximal positions to marginal lava lobes in upper and distal positions), comparable in general organization and probable emplacement processes to those of Hawaiian and flood-basalt flow fields. Flow-field development was influenced by episodic eruption, preexisting topographies, and penecontemporaneous sedimentation and felsic and mafic volcanism, and some flow fields in the south and east were partly emergent. Sills in the north and west are weakly mineralized, but those in the south and east are barren. The principal Reliance nickel sulfide deposits are confined to flow fields in the north and west, occur within lava channel complexes (Trojan and Shangani mines, Damba-Silwane and Hunters Road prospects) and a fissure vent (Epoch mine), and include type 1 basal sequences of massive to matrix to disseminated sulfides (lowermost flows Trojan, Shangani) and type 2 central accumulations of weakly disseminated sulfides (Hunters Road, upper flows Trojan, Shangani). Epoch and Shangani occupy central flow-field positions, whereas Hunters Road and Damba-Silwane are proximal; Trojan may be either distal or proximal. Field evidence does not directly support the origin of the Reliance deposits solely by magmatic erosion of sulfidic substrate during flow emplacement, as proposed for komatiite-hosted nickel deposits elsewhere. Instead, the deposits’ origin and asymmetric distribution across the craton may be mainly due to assimilation of sulfidic felsic wall rocks during magma passage through the subvolcanic sills in the north and west, possibly combined with sulfur degassing, due to very shallow water lava emplacement in the south and east.

Evidence for textural and alteration changes in basaltic lava flows using variations in rock magnetic properties (ODP Leg 183)

The different effects of cooling and alteration on magnetic properties, in single thick flows from subaerial and submarine eruptions, cored and logged during Ocean Drilling Program (ODP) Leg 183 at Sites 1137 and 1140 (Kerguelen Plateau) are examined. Downhole logging data from both sites is supplemented by petrology and geochemistry of 32 samples from three subaerial lava flows at Site 1137 and two flows units at Site 1140, covering transects from fresh to highly altered basalts. Changes in magnetic properties have previously been observed in several ODP drill holes, which penetrate basaltic basement. In subaerial basalts, a typical trend is that of high magnetic susceptibility and natural remanent magnetization (NRM) values in the altered flow top, and lower values in the less-altered massive flow interior. In contrast, submarine lava flows display the opposite behavior in their magnetic properties. Altered pillow rims have lower susceptibility and NRM values than the fresh pillow interiors. It is concluded that rate of cooling and degree of alteration are the main factors influencing the magnetization and, hence, the distribution of iron oxides. The effects of low-temperature alteration are most noticeable in the distribution of more mobile elements, such as K. Consequently, the spectral gamma ray (SGR) log, which in basaltic basement is largely controlled by K concentration, is an excellent proxy to the downhole identification of alteration. The strong positive correlation observed for the subaerial basalts between the downhole total magnetic field (F tot) and SGR, suggest a potential link with alteration in the drilled sections. The alteration of the submarine basalts is not as pronounced and therefore no correlation is evident.

Volcanic investigations of the Puna Ridge, Hawai′i: relations of lava flow morphologies and underlying slopes

Previous investigators have demonstrated that submarine lava flow morphologies are the result of a complex interplay between lava rheology, effusion rate, and underlying slope. Here, we specifically investigate the relation between underlying slope and submarine lava flow morphology along three study sites of the Puna Ridge, Kilauea Volcano, Hawai′i. Using Electronic Still Camera (ESC) images and detailed bathymetry collected by ARGO-II, we can compare the lava morphology and the associated underlying slope. Our results indicate that rubble is most common on the steepest slopes (≥25°) and that pillowed and lobate flows dominate on slopes ≤20°. Sheet flows were only seen on relatively shallow slopes (≤15°). These observations disagree with the results obtained from previous laboratory analog experiments, suggesting that the laboratory results are directly applicable for lavas emplaced on relatively low (<15°) underlying slopes. We conclude from our results that the tensile strength of the basaltic lava crust plays a much greater role in determining submarine lava flow morphology than previously thought.

Aqueous flows carved the outflow channels on Mars

The role of water in carving the Martian outflow channels has recently been challenged by the hypothesis known as “White Mars.” This hypothesis claims that the channels were cut by CO2 gas‐supported debris flows that also resurfaced the northern plains. However, proposed analogs of “cryoclastic” flows are either inappropriate (i.e., submarine density flows) or are primarily depositional rather than erosional (i.e., pyroclastic flows). Subaerial mass movements on Earth do not carve long deep channels like those on Mars. I review runout efficiencies for mass movements on Earth and Mars. The efficiencies required for “cryoclastic” flows to resurface the northern plains of Mars are so large that they appear unattainable. White Mars seeks to resolve carbonate and floodwater “paradoxes” that probably do not exist. It is also doubtful whether reservoirs of CO2 could persist over geologic time in the crust. Overall, the CO2 hypothesis fails key tests and should be abandoned as a means to carve outflow channels. I present a new interpretation of the fluid source that created Aromatum Chaos and Ravi Vallis. Water, not liquid or gaseous CO2, was the causative fluid, and the source was an ice‐covered impoundment in ancestral Ganges Chasma. At that time the canyon had no eastern outlet, and groundwater flowed northward to discharge at Aromatum Chaos and Shalbatana Vallis. The presence of ice‐covered water bodies can help to calibrate models of volcanic‐hydrologic climaxes during Hesperian time. The outflow channels, like the spectacular landforms of the Channeled Scabland, are monuments to the erosive power of catastrophic aqueous floods.

Characteristic magnetic behavior of subaerial and submarine lava units from the Hawaiian Scientific Drilling Project (HSDP‐2)

This study presents rock magnetic properties and the magnetic mineralogy of subaerial and submarine lava flows of Mauna Loa and Mauna Kea volcanoes collected from the 3109 m deep HSDP‐2 drill hole in Hawaii. Three different groups of magnetic behavior are recognized in the subaerial lava flows related to the degree of high temperature oxidation during extrusion. Group 1 shows homogenous titanomagnetite with low Xmt, low Curie temperatures (TC: 100°–200°C) and weak median demagnetizing fields (&lt; 20 mT). Further subdivision into 1a and 1b subgroups is based on the low temperature behavior of magnetic susceptibility (MS) and hysteresis loops, which indicate a contribution from ferrimagnetic Cr‐Al spinel below ca. −160°C in the 1b‐type samples. Group 2 samples, with exsolution lamellae of ilmenite in the titanomagnetites, have higher TC (480°–580°C) and higher coercive forces (20–40 mT). Group 3, the highest oxidation stage, is characterized by titanohematite‐bearing assemblages with enhanced median demagnetizing fields (35–85 mT) and a significantly different low‐temperature MS behavior. MS core logging shows a systematic variation occurs in the subaerial lava flows, directly related to the degree of high temperature oxidation and their flow morphology. Aa lava flows have higher mean MS than other lava flow types. Besides these factors, MS appears to be also affected by the magma composition of the various shield‐building stages. Mauna Loa subaerial lava flows generally show lower mean susceptibilities (4.6 ± 3 × 10−3 SI) than subaerial Mauna Kea lava flows (9.8 ± 5 × 10−3 SI). As submarine lava flows show no group 3 assemblages no high temperature oxidation influenced these rocks. Some hyaloclastites and pillow breccias show low MS (&lt; 1 × 10−3 SI), small amounts of nearly pure magnetite (TC = 580°C) and high coercive forces up to 110 mT suggesting single domain and/or superparamagnetic behavior. The controlling mechanism of the magnetic properties in the submarine lava units is the cooling and quenching rate of lava flows, which creates large grain size variations in titanomagnetites of varying compositions. Hydrothermal alteration, as described from ocean floor or Icelandic basalts, is not an important process that influences the magnetic properties in the ocean island basalts from the HSDP‐2 drill hole.

Occurrence and origin of submarine plunge pools at the base of the US continental slope

High-resolution bathymetric data from the New Jersey and Californian continental margins show a marked depression running along parts of the base of the continental slope. Detailed analysis reveals that the depressions are a series of discrete ‘plunge pools’ with associated downslope topographic ramparts. We have used new bathymetric data to create our own data base (of over 150 examples) and systematically analyse plunge pool morphology and location. Previous observations of plunge pools have been sparse. Plunge pools are up to 1100 m wide and 75 m deep, with a mean diameter of 400 m and a mean depth of 21 m. Plunge pools only occur where there are sharp decreases in slope of more than 4°, and are well developed where changes in slope exceed 15°. We propose plunge pools can be created by two mechanisms. Firstly, they may be due to reduced bed shear stress downstream of hydraulic jumps in submarine sediment-laden density flows that causes the deposition of bedload and the creation of a sediment bar. This bar then defines the downslope margin of a pool. Secondly, the impact of high-momentum sediment-laden density flows can excavate a depression, as has been observed for subaerial snow avalanches. Sediment deposited downslope of these impact pools is very poorly sorted, and partly derived from erosion within the pool. Both mechanisms influence whether turbidity currents are generated from high-density sediment-laden density flows, influence whether depositional flows are channelised, and have implications for base-of-slope facies models.

Experimental constraints on shear mixing rates and processes: implications for the dilution of submarine debris flows

Abstract Submarine debris flows show highly variable mixing behaviour. Glacigenic debris flows travel hundreds of kilometres along the sea floor without undergoing significant dilution. However, in other locations, submarine slope failures may transform into turbidity currents before exiting the continental slope. Rates and processes of mixing have not been measured directly in submarine flow events. Our present understanding of these rates and processes is based on experimental and theoretical constraints. Significant experimental and theoretical work has been completed in recent years to constrain rates of shear mixing between static layers of sediment and overlying turbulent flows of water. This work was driven by a need to predict transport of fluid mud and the erosion of cohesive mud beds in shallow water settings such as estuaries, docks and shipping channels. These experimental measurements show that the critical shear stress necessary to initiate shear mixing (around 0.1 to 2 Pa) is typically several orders of magnitude lower than the yield strength of the debris. Shear mixing should initiate at relatively low velocities (about 10–200 cm s −1 ) on the upper surface of a submarine debris flow, at even lower velocities at its head (about 1–10 cm s −1 ), and play an important role in mixing over-ridden water into the debris flow. Addition of small amounts of mud (approximately 3% kaolin) to a sand bed dramatically reduces the rate of mixing at its boundary, and changes the processes by which sediment is removed. Estimates are presented for rates of shear mixing at a given flow velocity, and for the critical velocity necessary for hydroplaning or a transition from laminar to turbulent flow. Although these estimates are crude, and highlight the need for further experimental work, they illustrate the potential for highly variable mixing behaviour in submarine flow events.