Depositional Lobes Research Articles

AN AGRICHNIAL FEEDING STRATEGY FOR DEEP-MARINE PALEOGENE OPHIOMORPHA GROUP TRACE FOSSILS

Abstract Ophiomorpha group trace fossils occur abundantly in a range of Eocene-aged deep-marine environments of deposition in the Basque basin, northern Spain. The morphology and dimensions of these trace fossils, observed in off-axis submarine lobe deposits, are discussed. The reported specimens display a highly organized and systematic burrowing behavior preserved on turbidite bed bases that display interconnecting Y-shaped (hexagonal-polygonal) morphologies. This observation, together with the cross-cutting relationship with tool marks, suggests construction of postdepositional agrichnial burrow networks. The networks probably harvested microbes that broke down cellulose-based organic matter providing an exploitable nutrient source for crustacean trace makers of Ophiomorpha. Therefore, the Ophiomorpha group–related traces discussed herein are postulated to represent an ethological response to changes in deep marine environmental conditions driven by global climate change during the early Paleogene, inc...

Paleocanais no megaleque do rio Taquari: mapeamento e significado geomorfológico

Abstract This paper presents the results of the first systematic mapping of the paleochannel network on the Taquari megafan (Pantanal, Brazil). We used remote sensing data collected by the ASTER sensor, which captures images with 15 m square pixels. A total of 34 scenes, acquired from 2001 - 2006, were used in the analysis. These data were processed using a decorrelation stretch technique in order to obtain a better visual identification of the paleochannels. The mapping procedure began with the overlay of a 1:50.000 IBGE articulation, in order to systematize the extraction of features and to reduce the subjectivity inherent to visual analysis methods. We mapped a total of 33,205 km of paleochannels. The density of these features is related to the relative ages of distinct geomorphological compartments, known as depositional lobes. The mapped features preserve avulsion events that occurred on the megafan. The results suggest that low-magnitude avulsions occur more frequently than larger events.

Paleocanais no megaleque do rio Taquari: mapeamento e significado geomorfológico

Abstract This paper presents the results of the first systematic mapping of the paleochannel network on the Taquari megafan (Pantanal, Brazil). We used remote sensing data collected by the ASTER sensor, which captures images with 15 m square pixels. A total of 34 scenes, acquired from 2001 - 2006, were used in the analysis. These data were processed using a decorrelation stretch technique in order to obtain a better visual identification of the paleochannels. The mapping procedure began with the overlay of a 1:50.000 IBGE articulation, in order to systematize the extraction of features and to reduce the subjectivity inherent to visual analysis methods. We mapped a total of 33,205 km of paleochannels. The density of these features is related to the relative ages of distinct geomorphological compartments, known as depositional lobes. The mapped features preserve avulsion events that occurred on the megafan. The results suggest that low-magnitude avulsions occur more frequently than larger events.

Architecture of a modern transient slope fan (Villafranca fan, Gioia basin–Southeastern Tyrrhenian Sea)

The high resolution of geophysical data applied to the study of the modern seafloor provides excellent details on the architecture of deep-water fans. Numerous researches have addressed the study of modern ponded and terminal deep-water fans. In contrast, thorough studies of present-day examples of transient slope fans, i.e. those fans that are connected to a deeper depositional area, are not available. Multibeam bathymetry and CHIRP profiles have been used in the study of the Villafranca transient slope fan located in the northeastern Sicilian margin. Hanging above the 300-m high erosional flank of the Stromboli axial valley, perpendicular to the fan elongation, it represents a particular kind of transient fan. The fan is composed of a leveed channel followed downslope by a depositional lobe. The relief of the eastern levee above the channel axis changes due to longitudinal variations in the rate of sediment overbanking; despite the low sinuosity of the channel, an increased rate of overbank flow is observed in the outside of the channel bends. Downslope from a knickpoint, an increase in levee relief is the result of the entrenchment of the central tract of the channel. The erosional deepening of the channel followed the failure of a mass-transport deposit (MTD) that lowered the distal slope portion, also causing a step along the channel course. The MTD is characterized by an extensional headwall domain and by a slightly deformed main body that has suffered very restricted downslope movement. It was caused by the gliding of the levee wedge as a consequence of its undermining, due to the excavation of the Stromboli valley in the distal slope. Retrogressive sediment failures are widespread in the eastern levee; they form high mobility MTDs such as a debris flow that spread in the eastern portion of the depositional lobe. The Villafranca depositional lobe has an internal architecture that largely differs from that of classic fan models. Straight, approximately 500-m-wide and 10 m deep channels and longitudinal bars are in fact the main elements that compose the Villafranca channel-mouth depositional lobe. The channels narrow and deepen downslope, pointing to a progressive establishment of lateral flow non-uniformity in approaching the step at the edge of the depositional lobe. Thick transparent layers or thin well layered deposits, sometimes arranged in smaller scale bedforms, accumulate on the bars. Conversely, prevailing erosional or bypass processes are responsible for reduced sedimentation in the channels. The spatial arrangement of channels and bars within the depositional lobe results in a downslope increase in stratigraphic complexity. The architecture of the Villafranca fan lobe has the potential to be applied as analog for seismic or outcrop-based studies of fans developed in topographically complex slopes and connected to a deeper base level.

Response of a proglacial delta to rapid high-amplitude lake-level change: an integration of outcrop data and high-resolution shear wave seismics

In this paper, we will present the stratigraphic evolution, internal facies architecture and geomorphology of the Middle Pleistocene Emme delta, controlled by rapid high-amplitude lake-level change. The Emme delta was deposited on the northern margin of glacial Lake Weser, located in north-west Germany. Rates of lake-level rise were probably >50 mm year−1 and rates of lake-level fall 30–50 m within a few days or weeks, due to the opening of lake outlets. We use digital elevation models, sedimentology and shear wave seismics to improve earlier reconstructions and investigate the influence of rapid base-level change on delta development. Shear wave seismic data resolve architectural elements in the range of metres and bridge the common gap between outcrop and conventional compression wave seismic data. The radial delta complex is about 2 km long, 1.8 km wide and up to 70 m thick, overlying a concave, up to 13° steep dipping ramp surface. It consists of vertically and laterally stacked delta lobes, caused by lobe switching during base-level change. During the lake-level rise, vertically stacked (Gilbert-type) delta systems formed. The decrease in thickness and lateral extent indicates a rapid upslope shift of depocentres. A high rate and magnitude of lake-level fall (50 m) promoted the development of a single incised valley and the deposition of forced regressive coarse-grained delta lobes in front of the valley. The incised valley was filled during decreasing rates of lake-level fall and low base-level, because the alluvial gradient was larger than the emergent lake profile. Attached sand-rich forced regressive aprons formed during lower magnitudes of lake-level falls in the range of 30–35 m. Valley incision occurred, but was limited to the uppermost portion of the delta, controlled by the steep slope. The incised valley related to the final lake drainage is associated with long-wavelength (60–90 m) bedforms at the downslope end, attributed to the formation of standing waves as a result of a hydraulic jump. Estimated palaeoflow depth during standing wave formation was ∼9–14 m and flow velocity was 10–12 m s−1. Because subsidence, waves or tides did not play a major role, the Emme delta can be used as an analogue-based predictive stratigraphical and sedimentological model for steep glacigenic deltas controlled by rapid base-level change and can help to understand better the facies distribution and three-dimensional geometry of these depositional systems.

Geomorphology of the Irish Rockall Trough, North Atlantic Ocean, mapped from multibeam bathymetric and backscatter Data

Please click here to download the map associated with this article. Multibeam echosounder data collected by the Irish National Seabed Survey (INSS) between 2000 and 2001 in the Rockall Trough have been used by many projects to analyze the geomorphology and the sedimentary processes of the margins of the Trough. However, due to the low spatial resolution of 150–250 m and the amount of artefacts still embedded in the publicly available data, many geomorphological features have been overlooked. This paper shows that the same multibeam dataset can provide more detailed information on seabed processes when the latest hydrographic and backscatter post-processing techniques are used to reduce data artefacts and improve data quality and spatial resolution. The resulting map, presented here at a scale of 1:450,000, is based on a semi-automatic and manual interpretation of geomorphological features highlighted using a new and improved 30 m bathymetric grid and a multibeam backscatter mosaic obtained using the GEOCODER post-processing algorithm. Overall, the map shows a complex sedimentological scenario where the Rockall Bank Mass Flow failure, the Donegal-Barra trough-mouth fan, along slope structures such as the Feni Contourite Drift and the Porcupine moat dominate the Rockall Trough flanks and basin floor. Smaller features such as canyon and gully systems, escarpments, coral mounds, volcanic mounds and iceberg scours have also been mapped in detail. In addition, the interpretation of the backscatter mosaic allows for the first time the identification of depositional lobes generated by downslope mass transport and canyon drainage systems, particularly where these features completely lose their bathymetric expression.

Validation of Hurst statistics: a predictive tool to discriminate turbiditic sub-environments in a confined basin

ABSTRACT Turbidite sequences within confined basins constitute important hydrocarbon reservoirs world-wide and, for this reason, the discrimination of sedimentary sub-environments based on an objective statistical method is of interest for pure and applied science. We investigated the potential use of the Hurst test as a statistical tool to discriminate sub-environments within geologically complex turbiditic units that fill a confined basin with well-exposed facies transitions and onlaps, at the scale of several stacked reservoirs (Cengio and Bric la Croce–Castelnuovo Turbidite Systems in the Tertiary Piedmont Basin, Oligocene, northern Italy). In vertical stratigraphic sections, the Hurst test determines the degree of clustering of low and high values of sedimentological variables, such as bed thickness, grain size and sand/mud ratio, which are dependent on sub-environments of deposition. We applied the Hurst test to depocentral and marginal sub-areas across the basin (parallel and perpendicular to the main palaeocurrent direction), documenting a different clustering of thick and thin beds, and of high and low values of the sand/mud ratio, in the depocentre–distal sector with respect to the onlap areas. A new field (onlap sub-environment) could thus be added to the classification diagram of turbidite settings based on the Hurst index. The Hurst phenomenon (clustering of high and low values of the selected variables) was also able to distinguish between proximal and distal (depocentral) lobe settings, and to recognize the fingerprint of the different depositional lobes (fully confined aggrading, prograding, backstepping). The map of turbidite sub-environments obtained by interpolation of the Hurst index is quite comparable to the field-observed facies map, providing impressive robust validation of the Hurst statistics. This method seems to represent a very promising predictive tool for subsurface studies of turbiditic oil fields based on core and log analyses.

Evidence for episodic alluvial fan formation in far western Terra Tyrrhena, Mars

A Late Noachian-aged alluvial fan complex within Harris Crater in far western Terra Tyrrhena, Mars, is comprised of two well-defined source regions and associated discrete depositional lobes. Three fan units were recognized based on common morphological characteristics, thermal properties and spectral signatures. Although the entire fan complex has been subjected to extensive erosional degradation, the preserved morphologies record episodic fan formation and indicate the type of flow processes that occurred; the bulk of the fan surface has morphology consistent with fluvial emplacement while one fan unit exhibits a rugged surface texture with boulders consistent with a debris flow. This transition from fluvial to late-stage debris flow(s) suggests a decline in available water and/or change in sediment supply. The thermal inertia values obtained for all three fan surface units (mean values ranged from 318 to 344 J m −2 K −1 s −1/2) are typical for coarse-grained and/or well-indurated materials on Mars, but subtle variations point to important distinctions. Variations in aeolian bedform coverage as well as the density of ridges (inferred inverted channels) and boulders contribute to these subtle fan thermophysical differences and likely reflect changes in the fan depositional mechanisms and variations in post-depositional modification histories. The majority of the alluvial fan surface has a spectral signature that is broadly similar to TES “Surface Type 2” (ST2), with some important exceptions at long wavelengths. However, a unique spectral component was identified in one of the fan units (unit 3), that likely reflects lithological differences from other fan materials. This spectral attribute of unit 3 matched locations within the western catchment providing confirmation of provenance and supporting the contention that sediment supply changed over time as the fan developed. Finally, we applied simple modeling to a well preserved subsection of the fan complex to quantify the developmental history. Using the computed eastern fan volume (32 km 3), significant water, likely from precipitation, was involved in fan construction (>50 km 3) and an extensive period of fan formation occurred over millennia or longer.

Intrinsic controls on the range of volumes, morphologies, and dimensions of submarine lobes

Submarine lobe dimensions from six different systems are compared: 1) the exhumed Permian Fan 3 lobe complex of the Tanqua Karoo, South Africa; 2) the modern Amazon fan channel–mouth lobe complex, offshore Brazil; 3) a portion of the modern distal Zaïre fan, offshore Angola/Congo; 4) a Pleistocene fan of the Kutai basin, subsurface offshore Indonesia; 5) the modern Golo system, offshore east Corsica, France; and 6) a shallow subsurface lobe complex , offshore Nigeria. These six systems have significantly different source-to-sink configurations (shelf dimension and slope topography), sediment supply characteristics (available grain size range and supply rate), tectonic settings, (palaeo) latitude, and delivery systems. Despite these differences, lobe deposits share similar geometric and dimensional characteristics. Lobes are grouped into two distinct populations of geometries that can be related to basin floor topography. The first population corresponds to areally extensive but thin lobes (average width 14 km × length 35 km × thickness 12 m) that were deposited onto low relief basin floor areas. Examples of such systems include the Tanqua Karoo, the Amazon, and the Zaïre systems. The second population corresponds to areally smaller but thicker lobes (average width 5 km × length 8 km × thickness 30 m) that were deposited into settings with higher amplitude of relief, like in the Corsican trough, the Kutai basin, and offshore Nigeria. The two populations of lobe types, however, share similar volumes (a narrow range around 1 or 2 km 3), which suggests that there is a control to the total volume of sediment that individual lobes can reach before they shift to a new locus of deposition. This indicates that the extrinsic processes control the number of lobes deposited per unit time rather than their dimensions. Two alternative hypotheses are presented to explain the similarities in lobe volumes calculated from the six very different systems. The first states that the wide range of starting flow volume and grain size across all systems enters the basin floor as a narrow range due to slope ‘filtering’ via more overspill and intra-channel deposition in larger systems. The second hypothesis is a result of the gradual decrease in downstream gradient from the distributive channel base to the lobe top during lobe growth. This is not sustainable as the channel will start to aggrade, and when a steeper lateral gradient is present, an avulsion will occur to an adjacent depositional low, which will be used for flows to fill and build a new lobe. This analysis of submarine lobe volumes indicates that the basin floor topography influences lobe geometry, but the fact that lobe volumes have a narrow range indicates a strong influence of intrinsic processes.

Flow-Deposit Interaction in Submarine Lobes: Insights from Outcrop Observations and Realizations of a Process-Based Numerical Model

Abstract Sediment gravity flows have a propensity to infill lows and build depositional relief, which influences subsequent flows. This flow–deposit interaction is intrinsic to the evolution of submarine fans at a range of scales. A novel approach is presented that assesses the interaction of turbidity currents with a subtle but evolving depositional topography. Conceptual models developed from outcrop observations are tested with a process-based numerical model. The outcrop dataset was collected from submarine lobe deposits extensively exposed in the Tanqua depocenter, SW Karoo Basin, South Africa. The process-based numerical model of turbidity-current flow and sedimentation (FanBuilder) is used to mimic the process sedimentology. Input parameters of flows are constrained by observations of the outcrop geology (sedimentology and depositional architecture). Modeling results are analyzed and compared with outcrop observations and, where necessary, lead to iterative refinement of the underlying conceptual process-sedimentological model. The model successfully developed characteristic features of the depositional architecture, such as finger-like geometries and stacking patterns, which are comparable in scale and geometry to those observed in outcrop. The results highlight that lobe deposits have intricate geometries that, when stacked, form a complicated internal stratigraphy, in contrast to simple models of lobes consisting of laterally extensive sheets. In addition, new insights into the processes of lobe growth have come from the modeling that can be tested at outcrop in the future. Process modeling indicates that the stratigraphic complexity can be controlled by a subtle and dynamic depositional surface that drives instability in the position of distributive channels and the site of deposition. As such, this research emphasizes the importance of autogenic controls on the depositional architecture of submarine fan systems.

The sieve lobe paradigm: Observations of active deposition

Sieve deposits were once considered to be one of the building blocks in alluvial fan stratigraphy. Later reinterpretation of sieve lobes as debris-flow deposits, favored because no visual records of active sieve deposition had been reported, undermined their significance, divided opinions, and left this issue unresolved. Here I document active deposition of sieve lobes in natural settings, in support of the original model. Sieve deposition can easily occur in natural settings such as proglacial outwash fans, small arid alluvial fans, or perennial streams when there is a scarcity of fine material, significant bedload, high slope, permeable ground, and discharges moderate enough to allow infiltration. The only hydrodynamic requirement for sieve deposition is a high rate of water loss promoted by permeable bed sediments. Under some circumstances alluvial fans can be built almost entirely of sieve deposits, as shown here. One effect of the rapid extraction of water is the creation of sigmoidal fan profiles. A gradation from sieve deposition to sheetflood occurs if sediment becomes progressively less permeable or if water flow increases, overcoming bed permeability. Sieve deposition is a universal depositional process based simply on infiltration, and it explains matrix-poor clast-supported gravels, while alternative hypotheses, such as matrix winnowing of debris flows used to dispute the sieve model, still need to be proven by observations in nature.

Monitoring the lateral channel movements on the alluvial fan of Wadi Feiran drainage basin, South Sinai, Egypt using multi-temporal satellite imagery

Human activities triggered various lateral channel movements over the alluvial fan of Wadi Feiran drainage basin, south Sinai, Egypt. Landsat TM images dated 1986, 1996, and 2001, along with a SPOT 4 XS image dated 2006 were acquired to monitor the lateral channel movement on the alluvial fan of Wadi Feiran through the last two decades. The lateral channel movements have been initiated by the channel avulsion of the incised channel of the alluvial fan as a result of obstructing the flow by developing a man-made barrier from the accumulated flood debris in the early years of 1990(s). The man-made barrier had evolved into an elevated concrete road of 3.75 m height with two culverts underneath directly south the fan apex by March 2007. These human activities enforced the incised channel to avulse in a new location leaving the original path and the active depositional lobe of the fan abandoned. Under the influence of successive flash flood events through the last twenty years, channel aggradations occurred resulting in channel braiding and channel widening processes in the reaches of the new formed distributary channels. The channel widening processes occurred on the expense of the inactive fan lobes located within and south the reaches of the new distributary channels. Observing the southward lateral growth of the reaches of the new formed distributary channels implies the possibility of forming a new active depositional lobe that could be merged with the abandoned depositional lobe of the fan.

東部南海トラフ海域のメタンハイドレート探査における堆積学的手法の適用

An integrated sedimentological workflow was applied to the methane-hydrate exploration project in the eastern Nankai Trough area, central Japan, as a useful tool to investigate the distributions and actual volume of methane hydrates. Since previous research revealed that most of the methane hydrates in the eastern Nankai Trough area occur in matrix pores of submarine-fan turbidite sandstones, the facies distribution of turbidite sandstones may be one of the important keys to evaluate the occurrence of methane hydrates. Accordingly, this study conducted a) core facies analysis, b) log facies analysis mainly using FMI, c) depositional sequence division, d) seismic facies analysis on the 2D/3D seismic sections, and e) seismic geomorphological analysis on the 3D seismic survey data to map submarine-fan turbidite facies distributions for 17 depositional sequence horizons in the targeted Pleistocene interval. The obtained facies maps reveal that submarine-fan depositional styles changed throughout Pleistocene from a braided channel type, through small radial fan, trough-fill fan, and muddy sheet fan types, to a channel-levee system type. As the next step, the facies maps of each depositional sequence were overlaid with bottom simulating reflector (BSR) distributions as a proxy of methane hydrates. The overlaid maps indicate that the BSRs occur on feeder channels, distributary channels, and proximal lobes of submarine fans, suggesting that methane hydrates selectively occur in coarser grained portions of a submarine fan. The facies maps were also used for calculation of average net-to-gross ratio of the methane hydrate occurrence zones, as the maps provide information on the spatial distributions of seismic facies class, which has individual value of net-to-gross ratio. Finally, this study analyzed reservoir-scale geologic bodies, such as depositional lobes and braided channels of a submarine fan, using 3D seismic survey data, and constructed a detailed geologic model for simulations of methane-hydrate generation and production.

Glacial and glacially-related features on the continental margin of northwest Ireland mapped from marine geophysical data

Please click here to download the map associated with this article. Despite over a century of research, reconstructions of the western margin of the last British-Irish Ice Sheet remain controversial. This is mainly because the ice sheet extended offshore onto the continental shelf and therefore significant portions of the glacial record are below present sea level. The availability of high resolution multibeam swath bathymetric and geophysical sub-bottom data for over ∼30,000 km2 of the northwest Irish continental margin (between 55°40′ and 54°18′ N, and 7°50′ and 11°4′ W) has made it possible to map glacial features on the continental shelf and glacially-related features on the continental slope and rise. The resulting map, presented here at a scale of 1:260,000, shows that the major glacial features consist of subglacial bedforms and nested arcuate moraines of different orientations across the width of the shelf. Distal to these moraines, the outermost shelf and upper slope are crossed by subparallel and cross-cutting furrows created by iceberg scouring and meltwater discharge at the ice margin. A well-developed system of gullies and canyons and frequent escarpments incise the continental slope. Several large depositional lobes, likely related to the delivery of vast amount of sediment at the ice margin are found in inter-canyon areas and at the base of the slope. Collectively these data show an extensive ice sheet margin across the entire shelf and sedimentation on the continental margin was greatly affected by the presence of the ice margin at the shelf edge.

Mesozoic deep-water carbonate deposits from the southern Tethyan passive margin in Iran (Pichakun nappes, Neyriz area): biostratigraphy, facies sedimentology and sequence stratigraphy

Abstract The objective of this work is to study the Mesozoic turbiditic sediments from the southern Tethys margin in Iran. These sediments are exposed as nappes in the Pichakun Mountains (i.e. the Zagros Mountains in the Neyriz area), which inverted during latest Cretaceous time. Radiolarians are used to both define and date four main lithostratigraphic formations: (1) the Bar Er Formation (undated, probably Late Triassic to Early Jurassic); (2) the Darreh Juve Formation (Aalenian–early Bajocian to middle Callovian–early Oxfordian); (3) the Imamzadeh Formation (middle Callovian–early Oxfordian to Aptian); (4) the Neghareh Khaneh Formation (late Aptian to Turonian–Coniacian). Most of the sediments are deep-sea gravity-flow lobe deposits. Channel deposits occurred during the Bajocian (i.e. the Darreh Juve Fm) and deeply incised channels (canyons?) occurred during the Albian (i.e. the Neghareh Khaneh Fm). Twenty-seven facies, grouped into eight facies associations, are defined. Based on a sequence stratigraphic study (i.e. the stacking pattern), five second-order cycles (10–30 Ma duration), defined between two successive distal facies time-intervals, are characterized: (1) the J2 (Toarcian?–middle Oxfordian, unconformity: Late Toarcian–Aalenian); (2) the J3 (middle Oxfordian–Berriasian, unconformity: middle? Tithonian); (3) the K1.1 (Berriasian–undated top); (4) the K1.2 (undated base–early Aptian, unconformity: late Hauterivian); (5) the K1.3 (early Aptian–at least Turonian–Coniacian, unconformity: Aptian–Albian boundary). The most important tectonic event recorded occurred at the Aptian–Albian boundary (a deposition of olistoliths, from a few metres to 100 m thick, in debris flows; related to Austrian deformations). The Arabian-scale late Toarcian and early Tithonian deformations have been recorded as unconformities. It is expected that another tectonic event occurred during the late Hauterivian. The unconformity of cycle K1.1 could be a late Valanginian eustatic fall of climatic origin.

Sedimentologic and volcanologic investigation of the deep tyrrhenian sea: preliminary result of cruise VST02

The VST02 cruise carried out in the summer of 2002 focused on sedimentologic and volcanologic research over selected areas of the deep portion of the Tyrrhenian Sea. Chirp lines and seafloor samples were collected from the Calabrian slope surrounding Stromboli Island, in the Marsili deep-sea fan, in the Vavilov Basin and in the Vavilov seamount. Submarine volcanic activity, both explosive and effusive, is occurring in the Stromboli edi- fice. Explosive submarine volcanism also affects the shallowest areas of the Vavilov seamount. Submarine car- bonate lithification has been observed on the sediment-starved flanks of the Vavilov seamount. Acoustic trans- parent layers make up the recentmost infill of the Gortani Basin, the easternmost portion of the Vavilov Basin. Channels comprised of a variety of architectural elements and depositional lobes are the main elements of the Marsili deep-sea fan where, apparently, sedimentation occurs mainly through debris flow processes.

Evolution, architecture and hierarchy of distributary deep‐water deposits: a high‐resolution outcrop investigation from the Permian Karoo Basin, South Africa

AbstractSea floor and shallow seismic data sets of terminal submarine fan lobes can provide excellent planform timeslices of distributive deep‐water systems but commonly only limited information on cross‐sectional architecture. Extensive outcrops in the Tanqua depocentre, south‐west Karoo Basin, provide these three‐dimensional constraints on lithofacies distributions, stacking patterns, depositional geometries and the stratigraphic evolution of submarine lobe deposits at a scale comparable with modern lobe systems. Detailed study (bed‐scale) of a single‐lobe complex (Fan 3) over a 15 km by 8 km area has helped to define a four‐fold hierarchy of depositional elements from bed through to lobe element, lobe and lobe complex. The Fan 3 lobe complex comprises six distinct fine‐grained sandstone packages, interpreted as lobes, which display compensational stacking patterns on a 5 km scale. Between successive lobes are thin‐bedded, very fine‐grained sandstones and siltstones that do not change lithofacies over several kilometres and therefore are identified as a different architectural element. Each lobe is built by many lobe elements, which also display compensational stacking patterns over a kilometre scale. Thickness variations of lobe elements can be extremely abrupt without erosion, particularly in distal areas where isopach maps reveal a finger‐like distal fringe to lobes. Lobe deposits, therefore, are not simple radial sheet‐dominated systems as commonly envisaged.

High-resolution analysis of submarine lobes deposits: Seismic-scale outcrops of the Lauzanier area (SE Alps, France)

The Lauzanier area represents the northernmost extension of the Annot Sandstone series and contains deposits between 650 and 900 m-thick. This basin was active from upper Bartonian or lower Priabonian to early Rupelian. It is composed of two superposed units separated by a major unconformity. The sediment supply is due to channelled flows coming from the south. Flow processes include mass flow to turbidity currents. The size of the particles and the absence of fine-grained sediment suggest a transport over a short distance. The Lower Unit is made of coarse-grained tabular beds interpreted as non-channelled lobe deposits. The Upper Unit is made of massive conglomerates interpreted as the channelled part of lobes. These lobe deposits settle in a tectonically confined basin according to topographic compensation that occurs from bed scale to unit scale. The abrupt progradation between the lower and the upper unit seems related to a major tectonic uplift in the area. This uplift is also suggested by a change in the petrographic nature of the source and an abrupt coarsening of the transported clasts. This field example allows providing high resolution analysis for depositional sedimentary sequences of terminal lobe deposits in a coarse-grained turbidite system. The outcrop analysis shows the lateral evolution of deposits and the system progradation allows a longitudinal analysis of facies evolution by superposing on the same outcrops the channelled lobe system and the non-channelled lobe system. These results of high-resolution outcrop analysis can be extrapolated to results obtained on sedimentary lobes in recent deep-sea turbidite system that are either restricted to cores, or with a lesser resolution (seismic).

An inverse problem to infer paleoflow conditions from turbidites

An inverse problem is here studied to infer paleoflow conditions from turbidites (i.e., from turbidity current deposits). Such a problem is constrained to outcrop data from the Laga Formation (Lower Messianian; central Apennines, Italy). Analytical solutions of layer‐averaged governing equations for net depositional turbidity currents flowing over a flat bottom are found. These allow recovering conditions for current velocity, thickness, and sediment concentration at the start of the net deposition by using the analytic solution of the Exner equation for net‐depositional turbidity currents. The Exner equation describes the along‐current profile of a sedimentary bed deposited by a single depositional event. For lobe deposits of the Laga Formation we obtain values of ∼1.5 m/s for layer‐averaged current velocities, 50–100 m for current thicknesses, and ∼0.5% for layer‐averaged sediment volume concentrations. The duration time of these flows is also computed and gives rather short time span values varying from 1 to 15 h. All of these values are in good agreement with those reported in literature.

The geomorphology and sedimentology of the Tista megafan, Darjeeling Himalaya: Implications for megafan building processes

This paper reports a study of the Tista megafan in the foothills of Darjeeling Himalaya. Spread over parts of India and Bangladesh, the megafan is bounded by the Mahananda River to its west and the Tista River to the east. The Atrai and Karatoya Rivers flow through its axial part. The megafan covers an area of ~ 18,000 km 2. Near its apex the surface slope is ~ 0.19°, that declines to 0.01° near the toe. The east–west transverse profile of the megafan is broadly upward convex, with gently sloping (~ 0.01°) flanks. Maximum width and the length of the megafan are about 145 km and 166 km respectively. The highest point near the megafan apex is ~ 150 m above the Brahmaputra alluvial plain. The Tista River flanking the megafan has an average annual discharge of 609 m 3/s with highest average monthly discharge exceeding 2000 m 3/s during monsoon. Most of the other channels currently traversing the megafan are plains-fed, and compared to Tista and Mahananda Rivers these channels have lesser discharge, higher sinuosity, and decreased widths. A radiating network of abandoned channel belts can be identified in satellite images of the megafan. Each of these major paleochannels is associated with numerous crevasse channels in the distal part of the megafan, forming an intricate network of radial drainage on the megafan surface. Three distinct depositional lobes can be recognised on the Tista megafan. Each of the lobes is identified by a set of ancient and modern radial drainage systems. The lobe boundaries are marked by discordance in drainage network of adjacent lobes. The relative ages of the lobes, as tentatively determined from the drainage discordances, indicate that the megafan first built up the eastern lobe (lobe 1), then shifted to the west to form lobe 2, and finally switched again eastward giving rise to the smallest lobe located close to the mountain front (lobe 3). Although broadly upward convex in cross profiles, subtle reflections of multiple lobes are apparent in some of the cross profiles. Study of the old maps published between 1794 and 1945 reveals that some of the present-day plains-fed rivers, like the Tangon, the Atrai and the Karatoya, were directly connected to the Himalayan catchment basins prior to the late eighteenth century. Archaeological excavations along the banks of these rivers appear to support this paleodrainage configuration. Eight facies and five facies associations were recognised in the uppermost megafan sediments. Facies associations suggest deposition from high-energy sandy streams in the proximal area and deposition from mixed load, sinuous streams and flanking marsh or lake in the distal part of the lobes. Paleocurrent data are consistent with a southwestward spreading paleochannel pattern recognised within the studied part of the megafan. Multiple lobes typify megafans ocurring in different parts of the globe. The existence of the mutiple accretionary lobes in Tista megafan, its radial drainage pattern, its concave-upward longitudinal and convex-upward transverse profile shape make it morphologically comparable with other high-gradient alluvial fans observed in nature and those produced in the laboratory. These morphological features denote the similarities between the small, high-gradient alluvial fans and large, low-gradient megafans, and that in turn may be indicative of the commoness in certain controlling factors between these two depositional systems.