Subaqueous Sediment Gravity Flows Research Articles

Hybrid sediment gravity flow deposits – Classification, origin and significance

The deposits of subaqueous sediment gravity flows can show evidence for abrupt and/or progressive changes in flow behaviour making them hard to ascribe to a single flow type (e.g. turbidity currents, debris flows). Those showing evidence for transformation from poorly cohesive and essentially turbulent flows to increasingly cohesive deposition with suppressed turbulence ‘at a point’ are particularly common. They are here grouped as hybrid sediment gravity flow deposits and are recognised as key components in the lateral and distal reaches of many deep-water fan and basin plain sheet systems. Hybrid event beds contain up to five internal divisions: argillaceous and commonly mud clast-bearing sandstones (linked debrite, H3) overlie either banded sandstones (transitional flow deposits, H2) and/or structureless sandstones (high-density turbidity currents, H1), recording longitudinal and/or lateral heterogeneity in flow structure and the development of turbulent, transitional and laminar flow behaviour in different parts of the same flow. Many hybrid event beds are capped by a relatively thin, well-structured and graded sand–mud couplet (trailing low-density turbulent cloud H4 and mud suspension fallout H5). Progressive bed aggradation results in the deposits of the different flow components stacked vertically in the final bed. Variable vertical bed character is related to the style of up-dip flow transformations, the distance over which the flows can evolve and partition into rheological distinct sections, the extent to which different flow components mutually interact, and the rate at which the flows decelerate, reflecting position (lateral versus distal) and gradient changes. Hybrid beds may inherit their structure from the original failure, with turbidity currents outpacing debris flows from which they formed via partial flow transformation. Alternatively, they may form where sand-bearing turbidity currents erode sufficient substrate to force transformation of a section of the current to form a linked debris flow. The incorporation of mud clasts, their segregation in near-bed layers and their disintegration to produce clays that can dampen turbulence are inferred to be key steps in the generation of many hybrid flow deposits. The occurrence of such beds may therefore identify the presence of non-equilibrium slopes up-dip that were steep enough to promote significant flow incision. Where hybrid event beds dominate the entire distal fan stratigraphy, this implies either the system was continually out of grade in order to freight the flows with mud clasts and clays, or the failure mechanism and transport path repeatedly allowed transmission of components of the initial slumps distally. Where hybrid beds are restricted to sections representing fan initiation, or occur more sporadically within the fan deposits, this could indicate shorter episodes of disequilibrium, due to an initial phase of slope re-adjustment, or intermittent tectonically or gravity-driven surface deformation or supply variations. Alternatively, changes between conventional and hybrid event beds may record changes in the flow generation mechanism through time. Thus the vertical distribution of hybrid event beds may be diagnostic of the wider evolution of the fan systems that host them.

The LIQSEDFLOW: Role of two-phase physics in subaqueous sediment gravity flows

The paper describes an extension of the computational code LIQSEDFLOW proposed by the authors. The salient features of the code lie in the capabilities to describe the multi-phased physics of subaqueous sediment gravity flows. Specifically, it combines Navier-Stokes/continuity equations and equations for advection and hindered settling of grains for a liquefied soil domain, with a consolidation equation for the underlying, progressively solidifying soil domain, via a transition layer that is characterized by zero effective stress and a small yet discernable stiffness. Evolutions of the flow and solidification surfaces are traced as part of solution by using a volume-of-fluid (VOF) technique. The predicted features of gravity flows of initially fluidized sediments with different concentrations conform to the observed performances in two-dimensional flume tests. The present results demonstrate the crucial role of two-phase physics, particularly solidification, in reproducing the concurrent processes of flow stratification, deceleration, and redeposition in subaqueous sediment gravity flows.

Pattern of sedimentation during the Late Paleozoic, Gondwanaland glaciation: An example from the Talchir Formation, Satpura Gondwana basin, central India

The Talchir Formation of Permian age is the lowermost lithostratigraphic unit of the Indian Gondwana successions preserving a record of the Late Paleozoic glaciation that affected the whole Gondwanaland. The formation unconformably overlies the Precambrian basement in all the Gondwana basins of India, and marks initiation of sedimentation after a long hiatus since the Proterozoic. The Talchir Formation of the Satpura Gondwana basin of central India shows features diagnostic of deposition under glacial regime such as the presence of diamictites, boulder pavements, bedding plane striation, faceted/striated gravels, bullet-shaped clasts, dropstone, cryoturbation, etc. The Talchir strata of the Satpura basin, in addition to tidal signatures, contain marine bivalve and trace fossils. A proglacial marine environment is thus inferred for their deposition. The dominant lithologies are conglomerate, pebbly sandstone, sandstone and shale with little limestone, which can be classified into the following major facies: • matrix-supported conglomerate, • clast-supported conglomerate, • cross-stratified, channel form pebbly sandstone, • hummocky, swaley or low-angle cross-stratified pebbly and non-pebbly sandstone, • turbidite/tempestite sheet sandstone, • massive to fissile, green to black claystone with carbonate layers and nodules. The conglomerates are interpreted as deposits of a spectrum of sub-aqueous sediment gravity flows including debris flow, hyperconcentrated/concentrated density flow and grain flow. The crossstratified pebbly sandstone bodies, on the other hand, represent braidplain delta deposits of glacial outwash origin. Hummocky, swaley, low-angle cross-stratified sediment bodies were deposited by storm-generated combined flows in a shoreface setting. In contrast, the sheet sandstone bodies are products of turbidity currents and storm-induced density currents in an inner shelf setting below fair weather wave base. The claystone facies is interpreted as a product of suspension fallout from hypopycnal sediment plumes in the outer shelf domain below storm wave base. All the lithofacies described above occur repetitively in the succession. Three facies-associations have been recognized in the Talchir Formation, representing ice-contact fandelta, outwash braidplain delta and a virtually ice-free, non-deltaic open marine realm. The different marine facies of the Talchir Formation define a number of shallowing-upward packages, which are arranged in the succession with a retrogradational trend to result in an overall fining- and deepening-upward succession. The overlying coal-bearing Barakar Formation has been interpreted to have formed in a tide-wave influenced

Palaeoclimate implications from deep drilling of Neoproterozoic strata in the Officer Basin and Adelaide Rift Complex of Australia; a marine record of wet-based glaciers

Two prominent episodes of Neoproterozoic glaciation in Australia are recorded in the Cryogenian (Sturtian < 725 Ma) and during the Ediacaran (Elatina < c. 580 Ma). Deep stratigraphic drilling and coring of Sturtian and Elatina glacial strata within the Adelaide Rift Complex (Blinman 2), the eastern Officer Basin (Nicholson 2) and the western Officer Basin (Vines 1 and Empress 1/1A) permits detailed sedimentary logging of lithofacies in more than 3000 m of continuous drill core. Glacial intervals in widely spaced drill sites show a commonality of facies types consisting of diamictites intimately interbedded with graded conglomerates, sandstones and siltstones that were deposited by subaqueous sediment gravity flows below wave base. Direct evidence for a glacial source of sediment is provided by striated and outsized clasts in fine-grained laminated facies. Soft-sediment deformation structures produced by downslope slumping, together with intraclasts of reworked sediment, are common. Facies suggest a depositional setting such as that found on submarine fans within Pleistocene glaciated basins strongly influenced by floating ice and the delivery of poorly sorted debris by the rainout of ice rafted debris, fall out of fine-grained suspended sediment and episodic downslope resedimentation. The large volume and great thickness of these subaqueous glacial facies within the Adelaide Rift Complex and Officer Basin indicates that Neoproterozoic glaciers in Australia delivered large fluxes of meltwater and sediment to offshore basins in similar fashion to wet-based glaciers typical of younger Phanerozoic glaciations. Evidence of meltwaters as a key agent of transportation is difficult to reconcile with climate models of a fully permafrozen planet where the hydrological cycle was shut down by extreme cold and normal sedimentary processes were suspended.

Stratigraphy of the Fungurume Group - evolving foreland basin succession in the Lufilian fold-thrust belt, Neoproterozoic-Lower Palaeozoic, Democratic Republic of Congo

The Lufilian arc (Neoproterozoic-Lower Palaeozoic) is a Pan-African orogen that lies between the Kalahari Craton in the south and the Congo Craton in the north. North vergent thrusting of its Katangan metasedimentary succession formed a fold-thrust belt in the northern sector of the arc. The belt contains prominent bodies of fragmental rocks including thrust sheets and megablocks of older Katangan strata, some of which contain Cu-Co mineralisation. These fragmental rocks were traditionally considered as tectonic friction breccias formed underneath the advancing Katangan nappes. This paper presents the stratigraphic implications of a recent reinterpretation of these breccias as sedimentary conglomerates in the fold-thrust belt of the Lufilian arc. The Katangan succession in Zambia and the Democratic Republic of Congo (DRC) is traditionally subdivided into four main lithostratigraphic units: Roan, Mwashya, Lower Kundelungu and Upper Kundelungu. It has been considered that that the Roan, Mwashya and Lower Kundelungu strata were deposited in a progressively opening rift basin, then folded and thrust northwards during Upper Kundelungu times. No syntectonic sedimentation associated with this orogenesis was previously recognised. Megabreccias within the Kundelungu succession in the northern part of the fold-thrust belt are interpreted here to be sedimentary complexes of synorogenic conglomerates derived from the orogenic front advancing from the south and deposited in a newly recognised foreland basin to the north. The sedimentary origin means that the ‘megabreccias’ should be classified as new lithostratigraphic units of the Katangan succession. The Fungurume Group (new unit), which unconformably overlies older strata, represents foreland basin fill and is subdivided into three formations. The Mutoshi Formation is characterised by continental red beds, previously called RAT. A facies association of hematite-rich, marginal marine, mixed clastic and carbonate rocks constitutes the Dipeta Formation. The Kambove Formation comprises olistostromes deposited by subaqueous sediment-gravity flows. The Mutoshi and Dipeta Formations were previously correlated with the rift-related Roan Group of Zambia. The Cu-Co-bearing Roan-derived strata, traditionally called the Mines Group, occur only as allochthonous blocks embedded in any of the above formations. Therefore, the Mines Group does not form a legitimate lithostratigraphic unit, and it is excluded herein from the revised stratigraphy of the Katanga Supergroup. The undeformed continental molasse of the Plateau Group fills the youngest foreland basin, extending ahead of the fold-thrust belt.

Catastrophic mass failure of a Neoproterozoic glacially influenced continental margin, the Great Breccia, Port Askaig Formation, Scotland

Abstract The Great Breccia is a thick (over 50 m) diamictite that occurs close to the base of the Neoproterozoic Port Askaig Formation in Scotland, and contains matrix-supported clasts up to 100 m across in a poorly sorted dolomitic matrix. Detailed logging and mapping of cliff and shoreline exposures through the Great Breccia on the Garvellach Islands show that it is not a simple ‘megabreccia’ but consists of three sedimentary units including a megabreccia (Unit 1), diamictites interbedded with dolomitic conglomerate, sandstone and mudstone (Unit 2) and a diamictite with metre-sized dolomite clasts (Unit 3). The sedimentological characteristics of the Great Breccia indicate that it formed by a variety of subaqueous sediment gravity flow processes, including matrix-rich debris flows and turbidites. Clasts up to 100 m across contained within the megabreccia unit record catastrophic collapse and failure of local calcareous siliciclastic deposits; other diamictite units were deposited by debris flows. Conglomerates were deposited from high-concentration turbulent flows, and sandstones and fine-grained facies also exhibit characteristics typical of turbidites. Continued downslope remobilization of sediment in the basin following deposition of the Great Breccia is recorded by overlying deformed diamictites, detrital dolomites and siltstones of the Disrupted Beds. The Great Breccia and associated facies within the lowermost part of the Port Askaig Formation are similar to those found in carbonate megabreccia successions that record catastrophic mass failure of carbonate platforms. Failure events in the Port Askaig Formation most likely relate to localized faulting and subsidence of the Dalradian Basin. Although the Port Askaig Formation is considered a Neoproterozoic ‘glacial’ succession, there is little direct evidence for glacial influence on sedimentation in the Great Breccia, making it difficult to fully assess paleoclimatic conditions.

The Maesan fan delta, Miocene Pohang Basin, SE Korea: architecture and depositional processes of a high‐gradient fan‐delta‐fed slope system

The Maesan fan‐delta‐fed slope system in the Miocene Pohang Basin occurs between two Gilbert‐type fan deltas. Detailed analysis of sedimentary facies and bed geometry reveals that the sequence is represented by 13 sedimentary facies. These facies can be organized into three facies associations, representing distinct depositional environments: alluvial fan (facies association I), steep‐faced slope (facies association II), and basin plain (facies association III). Subaerial debris flows and dense, inertia‐dominated currents were transformed into subaqueous sediment gravity flows in steep‐faced slope environments. Further downslope, these flows were channelized and formed lobate conglomerate and sandstone bodies at the terminal edge of the channels (or chutes). Interchannel and interlobe areas were dominated by homogeneous mudstone and muddy sandstone, deposited by suspension settling of fine‐grained materials. Part of the steep‐faced slope deposits experienced large‐scale slides and slumps. The chutes/channels, lobes and splays on the steep‐faced slope of the Maesan system are similar to those in modern subaqueous coarse‐grained fan‐delta systems.

Lacustrine fan-deltaic sedimentation in a Proterozoic rift basin: the Sopa-Brumadinho Tectonosequence, southeastern Brazil

The Sopa-Brumadinho Tectonosequence (100 to 200 m thick) is the fourth unit in ascending order of the up-to-2000 m thick Mesoproterozoic Espinhaço Supergroup, representing the depositional products of the third and main phase of the rift stage of the Espinhaço basin. The Sopa-Brumadinho Tectonosequence in the study area consists of diamond-bearing lacustrine fan-delta deposits, bounded by prominent angular and erosional unconformities. Detailed lithofacies studies were carried out in the central part of the southern Serra do Espinhatço, southeastern Brazil, to characterize the paleogeography and basin-fill architecture of the Sopa-Brumadinho Tectonosequence. Thirteen lithofacies are present: massive clast-supported conglomerates (lithofacies UGCS), inversely graded clastsupported conglomerates (lithofacies IGCS), normally graded clast-supported conglomerates (lithofacies NGCS), massive to crudely stratified clast-supported conglomerates (lithofacies CSCS), sandy matrix-supported conglomerates (lithofacies S-MS), muddy matrix-supported conglomerates (lithofacies M-MS), talus-breccia clast-supported conglomerates (lithofacies TB-CS), massive sandstones (lithofacies Sm), horizontal stratified sandstones (lithofacies Sh), trough cross-stratified sandstones (lithofacies St), planar cross-stratified sandstones (lithofacies Sp), graded-stratified sandstones (lithofacies Sg), and pelites (lithofacies P). They represent the products of several types of subaerial and subaqueous sediment gravity flows (ranging from turbidity currents to cohesive debris flows) and ephemeral braided streams. The deposits are usually arranged in coarsening- to fining-upward tectono-depositional intervals. Each tectono-depositional interval represents a progradational lobe, which starts at the base with lacustrine pelites and evolves with time from distal (subaqueous) to proximal (subaerial) characteristics. The fining-upward part represents the abandonment phase of the depositional lobes. The origin, characteristics, distribution and spatial arrangement of the various lithofacies, the interbedding of gravityflow and stream-flow deposits, the predominance of debris-flow conglomerates over aqueous current-formed ones, the semi-radial, fan-like dispersion pattern of the paleocurrents, and the recognition of associated subaerial and subaqueous depositional settings are all indicative of a lacustrine alluvial-fan/fan-delta environment, or a fan-delta system. An interplay between several intrabasinal and extrabasinal controls probably determined the fan evolution. The sedimentation of the lacustrine fan-delta systems of the Sopa-Brumadinho Tectonosequence probably took place in a N-S elongated continental rift basin, compartmentalized by N-S trending normal faults and W-E trending transfer faults, under a tropical semi-arid climate. The basin-fill architecture was probably determined by episodic subsidence, clockwise block tilting and asymmetric-graben evolution, as well as by inherited characteristics of the source area. Additionally, a new descriptive scheme for classification of conglomerate lithofacies is here proposed.

Fan-delta sedimentation on the basin margin slope of the Cretaceous, strike-slip Izumi Basin, southwestern Japan

The Late Cretaceous (Campanian-Maastrichtian) Izumi Basin is an E-W-trending, strike-slip basin, some 300 km long and 15 km wide, filled with turbidites and associated coarse clastic deposits. In the northeastern margin of the basin, steep-face fan deltas were built on a steep slope and prograded directly into deep water, forming conglomeratic slope aprons. The fan-delta deposits comprise debris-flow-dominated alluvial-fan conglomerates, delta-slope conglomerates originated from various subaqueous sediment-gravity flows, and prodelta mudstones with intercalations of thin sandstone and channel-fill pebbly sandstone.The most significant feature of the Izumi fan-delta system is a contrast between the gravel-dominated delta slope and the mud-dominated prodelta. The development of this fan-delta system was due to tectonic activity along the northern margin of the basin. Sedimentation took place due to vigorous uplift of the adjacent source terrane, which caused a high rate of supply of coarse detritus, and rapid basin subsidence which resulted in deep water depth.

Sedimentology of subaqueous volcaniclastic sediment gravity flows in the Neogene Santa Maria Basin, California

ABSTRACTSubaqueous tuff deposits within the lower Miocene Lospe Formation of the Santa Maria Basin, California, are up to 20 m thick and were deposited by high density turbidity flows after large volumes of ash were supplied to the basin and remobilized. Tuff units in the Lospe Formation include a lower lithofacies assemblage of planar bedded tuff that grades upward into massive tuff, which in turn is overlain by an upper lithofacies assemblage of alternating thin bedded, coarse grained tuff beds and tuffaceous mudstone. The planar bedded tuff ranges from 0.3 to 3 m thick and contains 1‐8 cm thick beds that exhibit inverse grading, and low angle and planar laminations. The overlying massive tuff ranges from 1 to 10 m thick and includes large intraclasts of pumiceous tuff and stringers of pumice grains aligned parallel to bedding. The upper lithofacies assemblage of thin bedded tuff ranges from 0.4 to 3 m thick; individual beds are 6‐30 cm thick and display planar laminae and dewatering structures. Pumice is generally concentrated in the upper halves of beds in the thin bedded tuff interval.The association of sedimentary structures combined with semi‐quantitative analysis for dispersive and hydraulic equivalence of bubble‐wall vitric shards and pumice grains reveals that particles in the planar bedded lithofacies are in dispersive, not settling, equivalence. This suggests deposition under dispersive pressures in a tractive flow. Grains in the overlying massive tuff are more closely in settling equivalence as opposed to dispersive equivalence, which suggests rapid deposition from a suspended sediment load. The set of lithofacies that comprises the lower lithofacies assemblage of each of the Lospe Formation tuff units is analogous to those of traction carpets and subsequent suspension sedimentation deposits often attributed to high density turbidity flows. Grain distributions in the upper thin bedded lithofacies do not reveal a clear relation for dispersive or settling equivalence. This information, together with the association of sedimentary features in the thin bedded lithofacies, including dewatering structures, suggests a combination of tractive and liquefied flows.Absence of evidence for elevated emplacement temperatures (e.g. eutaxitic texture or shattered crystàls) suggests emplacement of the Lospe Formation tuff deposits in a cold state closely following pyroclastic eruptions. The tuff deposits are not only a result of primary volcanic processes which supplied the detritus, but also of processes which involved remobilization of unconsolidated ash as subaqueous sediment gravity flows. These deposits provide an opportunity to study the sedimentation processes that may occur during subaqueous volcaniclastic flows and demonstrate similarities with existing models for sediment gravity flow processes.

Sedimentology and depositional model for glaciolacustrine deposits in an ice‐dammed tributary valley, western Tasmania, Australia

ABSTRACTQuaternary sedimentary successions are described from the Linda Valley, a small valley in western Tasmania that was dammed by ice during Early and Middle Pleistocene glaciations. Mapping and logging of exposures suggest that an orderly sequence of deposits formed during ice incursion, occupation and withdrawal from tributary valleys. Four principal sediment assemblages record different stages of ice occupation in the valley. As the glacier advanced, a proglacial, lacustrine sediment assemblage dominated by laminated silts and muds deposited from suspension accumulated in front of the glacier. A subglacial sediment assemblage consisting of deformed lacustrine deposits and lodgement till records the overriding of lake‐bottom sediments as the glacier advanced up the valley into the proglacial lake. As the glacier withdrew from the valley, a supraglacial sediment assemblage of diamict, gravel, sand and silt facies formed on melting ice in the upper part of the valley. A lacustrine regression in the supraglacial assemblage is inferred on the basis of a change from deposits mainly resulting from suspension in a subaqueous setting to relatively thin and laterally discontinuous laminated sediments, occurrence of clastic dykes, and increasing complexity of the geometry of deposits that indicate deposition in a subaerial setting. A deltaic sediment assemblage deposited during the final stage of ice withdrawal from the valley consists of steeply dipping diamict and normally graded gravel facies formed on delta foresets by subaqueous sediment gravity flows. The sediment source for the delta, which prograded toward the retreating ice margin, was the supraglacial sediment assemblage previously deposited in the upper part of the valley. A depositional model developed from the study of the Linda Valley may be applicable to other alpine glaciated areas where glaciers flowed through or terminated in medium‐ to high‐relief topography.

Subaqueous sediment gravity flow deposits: practical criteria for their field description and classification

ABSTRACTA new method for the description and classification of subaqueous sediment gravity flow deposits is proposed. The classification scheme employs a convenient letter code and divides deposits (individual beds) into descriptive categories of two hierarchical levels: facies and subfacies. Facies, as the higher rank categories, are distinguished chiefly on the basis of sediment type (i.e. bed grain size/texture). A total of 13 facies have been distinguished: G= gravel; GS = gravel‐sand couplet; GyS = gravelly sand; S = sand; SM = sand—mud couplet; MS = mud—sand couplet; TM = silt—mud couplet; MT = mud—silt couplet; M = mud; MyS = muddy sand; SyM = sandy mud; MyG = muddy gravel; GyM = gravelly mud. Subfacies, as the lower rank categories, are distinguished within the individual facies on the basis of the bed's internal structures. The number of subfacies is unlimited, and their labelling code includes particular facies symbols (see above) preceded by lower—case letters denoting specific sedimentary structures and their vertical arrangement. Subfacies thus refer to the bed's intervals, or divisions, which are labelled as follows: m = massive (unstratified and ungraded); g = graded (unstratified and graded); s = plane‐stratified; x=cross‐stratified; 1 = parallel– and/or cross‐laminated; q = liquefied. For example, subfacies gsG (graded to plane‐stratified gravel) are gravel beds that have a lower graded interval and an upper plane‐stratified interval; subfacies xG (cross‐stratified gravel) are gravel beds that are cross‐stratified throughout; subfacies slS (plane‐stratified to laminated sand) are sand beds that have a lower plane‐stratified interval and an upper laminated (parallel‐ and/or cross‐laminated) interval.

Sedimentology of the lower huronian supergroup (early proterozoic), Elliot lake area, Ontario, Canada

The Early Proterozoic Huronian Supergroup is a slightly metamorphosed sedimentary succession locally > 10 km thick, exposed to the north of Lake Huron, Ontario, Canada. The lower part of the supergroup, comprising the Matinenda, McKim, Ramsay Lake and Pecors formations, was studied in the Quirke Syncline area, Elliot Lake, and regions immediately to the west and south. Thirteen lithofacies are present: trough and planar cross-stratified sandstone, quartz pebble conglomerate, clay-rich sandstone, lenticular-bedded sandstone-siltstone-mudstone, laminated sandstone-siltstone-mudstone, massive sandstone- massive pebbly sandstone, massive diamictite, stratified matrix-supported diamictite, stratified clast-supported diamictite, graded diamictite, and massive siltstone. The lithofacies may be grouped into associations which correspond approximately to the mapped formations. The crossbedded sandstones and conglomerates comprise a fluvial association (Matinenda Formation), which was formed mainly in shallow braided channels. Clay-rich sandstone and the lenticular and laminated lithofacies, constituting portions of the McKim and Pecors Formations, were deposited on the strand and offshore areas of braid deltas, interfingering laterally and vertically with the Matinenda fluvial deposits. Four glacigenic subassociations are present in the Ramsay Lake Formation. An ice-proximal subassociation (I) consists of massive pebbly sandstone and diamictite which progressively truncates the Matinenda and McKim Formations to the north. The massive sandstone may be an ice-override deposit formed by erosion and reworking of the underlying Matinenda Formation, or a sub-ice shelf rain-out accumulation of locally eroded glacial debris. Subassociations II and III consist mainly of subaqueous sediment gravity flow and ice rain-out deposits. Subassociation IV is interpreted as an ice-proximal fluvial outwash deposit containing resedimented blocks of moraine material. Stratigraphic and paleocurrent data indicate a south-dipping paleoslope that tilted southeast during the deposition of the fluvial and deltaic units. A trough of relatively deep water extended NW-SE across the region, transverse to the E-W divergent continental margin hinge line that is assumed to have existed to the south of the project area. Analogies are suggested to Mesozoic-Recent basins on the Atlantic margin of Canada, where transverse paleogeographic trends reflect either landward extension of active oceanic fracture zones, or reactivation of earlier structural lineaments.

Field evidence for hydraulic jumps in subaqueous sediment gravity flows

Despite the significance of subaerial and subaqueous sediment-laden gravity flows as geological and geomorphic agents1–5, our understanding of such transport and depositional processes is limited. In particular, there is little known about the mechanisms involved in the conversion of higher-density subaqueous debris flows into lower-density turbidity currents. To address this issue, a detailed field study of this conversion process was undertaken on flows and their deposits within a reservoir. The results presented here provide field evidence indicating the occurrence of hydraulic jumps as a conversion mechanism in such flows, and the deposition of large quantities of sediment in the immediate down-jump area. The findings are significant both in terms of basic geological processes as well as in applications to problems of reservoir management in areas with high sediment inputs.

Channel formation and migration by mass‐flow processes in the Lower Carboniferous fluviatile Fell Sandstone Group, northeast England

ABSTRACTSmall, steep‐sided narrow channels, locally developed within the braided fluviatile Lower Carboniferous Fell Sandstone of northeast England, are attributed to mass‐flow processes initiated by channel‐bank slumping. The channels have sharp undisturbed margins and are filled with moderately well‐sorted medium to coarse sandstone identical to the sandstone which they cut. Most channel‐fills are structureless, but a few show diffuse concentric marginal laminations. Laminations also occur towards the top of channel fills, but they are less regular and show low‐amplitude trough‐like undulations. Channel margin inclinations range from 13° to 55°, and channel depths vary between 1·7 m and 2·5 m. The channels, which are only seen in profile, are orientated perpendicular to the local and regional palaeocurrent trend, and cut through structureless sandstone and planar cross‐stratified sandstone characterized by compound and compound‐complex bar bedforms. Only one channel has both margins preserved; and of the remainder (seven) only one has its northern margin preserved.Undercutting and retrogressive slumping of the sandy channel bank is thought to have initiated a subaqueous sediment gravity flow, which moved across the main channel along a transverse scour at the base of the slip face of a large composite bar bedform. Such scours have strong asymmetric profiles with the steeper slope along the base of the active slip face (northern side) and the lower slope on the downstream (southern) side of the scour. Thus, the northern channel margin was steeper, less stable and more prone to collapse than the southern margin, and is almost never preserved. Flow reconstruction indicates that on entering the channel a hydraulic jump developed at the change in slope, and the flow became more turbulent and erosive. Scoured sand incorporated into the flow increased concentration and shearing along the boundaries. Reverse shear was also exerted by the overlying current, on top of the flow, but the central part remained relatively unaffected. The presence or absence of marginal laminations is probably related to marginal shearing and flow dilution. Deposition occurred in response to deceleration and frictional freezing of the sand flow.

Petrology and provenance of pre–Late Devonian sandstones, Shoo Fly Complex, northern Sierra Nevada, California

Between Quartz Hill and Fall Creek Mountain, California, the Shoo Fly Complex includes the Poison Canyon and Red Hill formations. Field, biostratigraphic, and U-Pb zircon data indicate that both formations are most likely post-Cambrian and pre-Middle Devonian in age. Medium-grained and pebbly sandstone in both formations was deposited in submarine-fan environments by subaqueous sediment gravity flows laden with continentally derived detritus. The source of terrigenous detritus in the Poison Canyon and Red Hill formations included rocks that contained zircons as old as ∼2.09 b.y. The depositional basin in which the Poison Canyon and Red Hill formations accumulated was floored by oceanic lithosphere and was close enough to some continental landmass to have received sand-sized detritus derived from it. The continental landmass may have been located in western North America, but there is nothing in the data presented here that precludes the possibility that it was located on some foreign continent.