Channel-lobe Transition Research Articles

Coarse-grained submarine channels: from confined to unconfined flows in the Colombian Caribbean (late Eocene)

Submarine channel mouth settings are hardly preserved in the stratigraphic record. Although they are still poorly known with respect to other segments of turbidite systems, conceptual models are being refined in the light of new discoveries in modern and ancient examples. Still, some questions such as the transition between expansion zones and the traditional Channel-Lobe Transition Zone (CLTZ) remains open in ancient systems. Upper Eocene deposits of the Colombian Caribbean (San Jacinto Fold Belt) are interpreted here as a fan-delta-fed, submarine, coarse-grained channel-lobe system. It displays a well-preserved channel inception stage in the shelf break represented by sigmoidal to lens-shaped gravels, and planar cross-stratified pebbly sandstones (foreset and backset) interpreted as cyclic steps in an expansion zone. In a later stage, a classical channel-levee complex was developed, represented by channel fill elements showing sharp- and erosional-based, fining-upward sequences that are meters thick, having basal massive matrix-supported pebble conglomerates (hard—extrabasinal—clasts, rip-up clasts, coastal bioclasts), vertically evolving to liquefied massive to planar-laminated coarse-grained sandstones with phytodetrital carbonaceous laminae. They are interpreted as concentrated flow deposits (high-density turbidites) coming from continental areas or from coastal systems (i.e., delta reworking). Undifferentiated channel belt thin-bedded turbidites associated with levees and terraces deposits are related to these confined systems. The channel-lobe transition zone is characterized by debrites from cohesionless debris flow in a channel-mouth bar setting, representing bypass processes that developed distally into low-angle, planar cross- and sigmoidally-stratified (upstream antidune) pebble-size to coarse-grained sandstones that fill low-angle scours (cut-and-fill structures) in an antidune field setting with supercritical conditions. When the currents lose channel confinement, the setting is characterized by changes from Froude supercritical to subcritical flow conditions in an inner lobe to lobe off-axis environment. Large seasonal fluctuations in precipitation favor high sediment concentrations, promoting the formation of volumetrically significant fan deltas and coarse-grained submarine channels with high erosive capacity; therefore, their record helps refine interpretations of depositional processes, providing criteria for recognizing areas of the turbiditic systems that are hardly preserved. The particular aggradational conditions for the preservation and stratigraphic characterization of the rare exhumed submarine channel mouth systems make it possible to decipher sediment dispersal patterns and thus connect the models proposed here, from supercritical systems to the traditional models of turbiditic systems.

The response of high density turbidity currents and their deposits to an abrupt channel termination at a slope break: Implications for channel–lobe transition zones

ABSTRACTThe transition between the slope and basin floor is typically marked by a slope break, in some cases causing channels to terminate and turbidity currents to undergo a loss of confinement. It is thus essential to understand how these slope breaks and losses of confinement influence the hydrodynamic evolution of turbidity currents and impact their depositional variability within natural scale channel mouth settings. Flume experiments, utilizing Shields scaling, are conducted to study how channel slope angle (3°, 6° and 9°) and initial suspended sediment concentrations (12 to 18% by volume) impact the hydrodynamics and deposit geometries of high density turbidity currents, subject to a simultaneous break of slope and loss of confinement. Measured velocity and concentration profiles indicate that turbidity currents are supercritical, with mean velocities between 0.80 m s−1 and 1.04 m s−1 and depth‐averaged basal concentrations between 9.2% and 23.9%, yielding bed shear velocities between 0.050 m s−1 and 0.064 m s−1. Upon encountering the slope break and loss of confinement, turbidity currents exhibit increases to their densimetric Froude numbers and shear velocities. This is due primarily to two factors: firstly, turbidity currents continue to accelerate during an initial period of velocity lag as their residual momentum gradually dissipates; and, secondly, expansion via flow relaxation collapses their structure towards the bed. The corresponding depositional geometries of these processes reveal that turbidity currents produce elongate channel–lobe transition zones that disconnect channel and basin deposits. The length to width ratios of channel–lobe transition zones decrease as the initial sediment concentrations of turbidity currents increase, while a reduction in the channel slope break angle reduces their length to width ratios. Corresponding, lobe elements are observed to increase in length, width and thickness with increasing initial sediment concentrations, while a reduction in channel slope break angle reduces their dimensions due to enhanced slope deposition.

Reconstructing sedimentary processes in a Permian channel–lobe transition zone: an outcrop study in the Karoo Basin, South Africa

AbstractTurbidity currents commonly bypass sediment in submarine channels on the continental slope, and deposit sediment lobes farther down-dip on the flat and unconfined abyssal plain. Seafloor and outcrop data have shown that the transition from bypass to deposition usually occurs over complex zones referred to as channel–lobe transition zones (CLTZs). Recognition of these zones in cores and outcrop remains challenging due to a lack of characteristic sedimentary facies and structures. This paper focuses on Unit E of the Permian Fort Brown Formation in the Karoo Basin, South Africa, in the Slagtersfontein outcrop complex, which has previously been interpreted as a CLTZ. This study integrates thin-section micrographs, sedimentary facies, bed-set and stratigraphic architecture, and palaeoflow directions to achieve a multiscale analysis of CLTZ features. A novel process-based facies scheme is developed to evaluate deposits in terms of the depositional or erosional tendencies of the flows that formed them. This scheme allows bypass to be distinguished from depositional zones by the spatial distribution of certain sediment facies. Areas of net sediment bypass were predominantly marked by erosive sediment facies and a larger variability in palaeoflow direction while depositional areas showed a lower variability in palaeoflow directions. Metre-scale structures in the bypass-dominated area reveal seafloor erosion and scour formation. Field relations suggest the presence of a ∼500 m long mega-scour in the CLTZ. The characteristic structures documented here are applicable for identifying CLTZs in sparse datasets such as outcrops with limited palaeogeographical context and sediment cores obtained from subsurface systems.

Submarine Channel Mouth Settings: Processes, Geomorphology, and Deposits

Observations from the modern seafloor that suggest turbidity currents tend to erode as they lose channel-levee confinement, rather than decelerating and depositing their sediment load, has driven investigations into sediment gravity flow behaviour at the mouth of submarine channels. Commonly, channel mouth settings coincide with areas of gradient change and play a vital role in the transfer of sediment through deep-water systems. Channel mouth settings are widely referred to as the submarine channel-lobe transition zone (CLTZ) where well-defined channel-levees are separated from well-defined lobes, and are associated with an assemblage of erosional and depositional bedforms (e.g., scours and scour fields, sediment waves, incipient channels). Motivated by recently published datasets, we reviewed modern seafloor studies, which suggest that a wide range of channel mouth configurations exist. These include traditional CLTZs, plunge pools, and distinctive long and flared tracts between channels and lobes, which we recognise with the new term channel mouth expansion zones (CMEZs). In order to understand the morphodynamic differences between types of channel mouth settings, we review insights from physical experiments that have focussed on understanding changes in process behaviour as flows exit channels. We integrate field observations and numerical modelling that offer insight into flow behaviours in channel mouth settings. From this analysis, we propose four types of channel mouth setting: 1) supercritical CMEZs on slopes; 2) plunge pools at steep slope breaks with high incoming supercritical Froude numbers; 3) CLTZs with arrays of hydraulic jumps at slope breaks with incoming supercritical Froude numbers closer to unity; and, 4) subcritical CLTZs associated with slope breaks and/or flow expansion. Identification of the stratigraphic record of channel mouth settings is complicated by the propagation, and avulsion, of channels. Nonetheless, recent studies from ancient outcrop and subsurface systems have highlighted the dynamic evolution of interpreted CLTZs, which range from composite erosion surfaces, to tens of metres thick stratigraphic records. We propose that some examples be reconsidered as exhumed CMEZs.

Depositional elements and evolution of gravity-flow deposits on Lingshan Island (Eastern China): An integrated outcrop-subsurface study

Understanding how subaqueous sediment gravity flows (SSGF) evolve in time and space, and how their deposits vary spatially, is a key research focus for gravity flow sedimentology. This study investigates depositional facies, depositional elements, and sediment transport processes of supercritical flows and hybrid event beds (HEBs) of the Lingshandao Formation in Lingshan Island, Eastern China. Three kinds of depositional elements were recognized: mass transport deposits (MTDs), channel-lobe transition zone (CLTZs), and lobe complexes. MTDs can be sub-divided into proximal and distal deposits. CLTZs are characterized by facies changes from massive coarse-grained pebbly sandstone to backset bedding sandstone in a down-slope direction, which are the deposits of supercritical turbidity currents. HEBs are common in both proximal and distal lobe settings. Tripartite structure in HEBs, which may be caused by up-slope substrate erosion, implies a relatively proximal origin compared with bipartite HEBs, perhaps caused by fluid fractionation in the research area. The succession is formed by a prograding lobe unit, followed by MTDs which are themselves overlain by deposits from a CLTZ. This vertical stacking pattern implies the potential longitudinal facies tract from CLTZ; to lobe complex deposits, is accompanied by emplacement of MTDs at the slope break. The flow types accompanied by sediment transport processes imply that erosion by supercritical turbidity currents associated with a hydraulic jump in the channel-lobe transition zone may be the main reason for mud-clasts and matrix addition to the flow. The large-scale addition of mud-clasts and suspended mud may dampen turbulence in proximal to medial lobe settings, and result in medium-to thick-bedded HEBs with common erosional features and tripartite structures. The remaining suspension flow (with abundant mud) may further transport down-dip and form medium-to thin-bedded HEBs with bipartite structures and rare mud-clasts. These findings may be applicable to other SSGF systems with supercritical flow deposits and hybrid event beds, emphasizing the downdip and lateral variation in depositional elements associated with gravity flow evolution.

Depositional Model for Turbidite Lobes in Complex Slope Settings Along Transform Margins: The Motta San Giovanni Formation (Miocene—Calabria, Italy)

Intraslope lobes, or perched lobes, are attracting scientific interest because they represent a key archive between the shelf and the deep basin plain when looking at a complete source-to-sink depositional system across a continental margin and can form significant offshore hydrocarbon plays. In this study, we focus on a detailed characterization of intraslope lobes of the Motta San Giovanni Formation (Miocene, Calabria), which were deposited in confined conditions during the Miocene along a transform margin. We determine the typical facies associations and stratigraphic architecture of these intraslope lobes using a 3D digital outcrop model resulting from a combined Uncrewed Aerial Vehicle (UAV) and walking acquisition, together with sedimentological logging and geological mapping. We propose recognition criteria for the identification of intraslope lobes, including facies and geometries, integrated within a depositional model. A comparison with other well-known intraslope and confined lobes, as well as basin floor lobes, is finally discussed, to highlight the peculiarities of intraslope lobes deposited along transform margins. The diagnostic depositional model for these types of intraslope lobes includes four main stages of evolution: 1) Stage 1—isolated detached lobe precursor in response to a flushed hydraulic jump, 2) Stage 2—prograding and aggrading lobe elements associated with a relatively stable and submerged hydraulic jump in the Channel-Lobe Transition Zone (CLTZ), 3) Stage 3—major bypass associated with lateral accretion and local aggradation interpreted as a renewal of a normal hydraulic jump in the CTLZ, and 4) Stage 4—erosion and bypass then abandonment. The development of intraslope lobes along active transform margins is allowed by tectonically induced slope segmentation and local confinement. In such a context, flow stripping and overspill processes occurred. Resulting lobes appear to be particularly small and relatively thin sandy deposits. They could be considered end-member in a lobe classification based on the Net-to-Gross content (high) and taking into account their thickness/width ratio (intermediate between 10:1 and 100:1 lines).

Sole marks reveal deep-marine depositional process and environment: Implications for flow transformation and hybrid-event-bed models

ABSTRACTDeposits of sediment gravity flows in the Aberystwyth Grits Group (Silurian, west Wales, United Kingdom) display evidence that sole marks are suitable for reconstructing depositional processes and environments in deep-marine sedimentary successions. Based on drone imagery, 3D laser scanning, high-resolution sedimentary logging, and detailed descriptions of sole marks, an outcrop 1600 m long between the villages of Aberarth and Llannon was subdivided into seven lithological units, representing: a) mudstone-poor, coarse-grained and thick-bedded submarine channel fills, dominated by the deposits of erosive high-density turbidity currents with flute marks; b) mudstone-rich levee deposits with thin-bedded, fine-grained sandstones formed by low-density turbidity currents that scoured the bed to form flute marks; c) channel–lobe transition-zone deposits, dominated by thick beds, formed by weakly erosive, coarse-grained hybrid events, with pronounced mudstone-rich or sandstone-dominated debritic divisions and groove marks below basal turbiditic divisions, and with subordinate amounts of turbidites and debris-flow deposits; d) tabular, medium- to thick-bedded turbiditic sandstones with flute marks and mixed sandstone–mudstone hybrid event beds mainly with groove marks, interpreted as submarine lobe-axis (or off-axis) deposits; and e) tabular, thin- to medium-bedded, fine-grained, mainly turbiditic sandstones mostly with flute marks, formed in a lobe-fringe environment. Both lobe environments also comprised turbidites with low-amplitude bed waves and large ripples, which are interpreted to represent transient-turbulent flows. The strong relationship between flute marks and turbidites agrees with earlier predictions that turbulent shear flows are essential for the formation of flute marks. Moreover, the observation as part of this study that debris-flow deposits are exclusively associated with groove marks signifies that clay-charged, laminar flows are carriers for tools that are in continuous contact with the bed. A new process model for hybrid event beds, informed by the dominance of tool marks, in particular grooves, below the basal sand division (H1 division of Haughton et al. 2009) and by the rapid change from turbidites in the channel to hybrid event beds in the channel–lobe transition zone, is proposed. This model incorporates profound erosion of clay in the channel by the head of a high-density turbidity current and subsequent transformation of the head into a debris flow following rapid lateral flow expansion at the mouth of the channel. This debris flow forms the groove marks below the H1 division in hybrid event beds. A temporal increase in cohesivity in the body of the hybrid event is used to explain the generation of the H1, H2, and H3 divisions (sensuHaughton et al. 2009) on top of the groove surfaces, involving a combination of longitudinal segregation of bedload and vertical segregation of suspension load. This study thus demonstrates that sole marks can be an integral part of sedimentological studies at different scales, well beyond their traditional use as indicators of paleoflow direction or orientation.

Evolution of deepwater turbidite bedforms in the Huaguang channel–lobe transition zone revealed by 3D seismic data in the Qiongdongnan Basin, South China Sea

The theoretical model of turbidite bedform evolution has been verified based on laboratory tests and outcrop surveys. However, the evolution of the bedforms associated with flow regime variation within the channel–lobe transition zone (CLTZ) is poorly documented and generally extremely poorly understood. In this study, a combination of seismic sedimentological and topographic analyses, based on high-resolution 3D seismic data within the Qiongdongnan Basin of the South China Sea, was used. Continuous bedforms varied with the depositional elements within the CLTZ of the Huaguang leveed channel system along the flow direction. The depositional elements within the CLTZ change from the main channel to the distributary channels as well as to the sub-distributary channels. Numerous bifurcation and confluence points of gravity flows illustrated the complexity of the drainage network within the CLTZ. The average asymmetry index of the bedforms increased from 0.8 to 1.6, with the average wavelength increasing from 803 m to 1545 m along the thalweg of the channels. Crevasse splays at the starting reach of the CLTZ are likely to form the initial state of the new CLTZ in the future, because the slope gradient starting from the avulsion point was higher than that inside the channel. The deep scours at the bifurcation points and high steps at the confluence points demonstrate the critical roles of supercritical flows and hydraulic jumps in reworking the bedforms of the CLTZ. The high steps at confluence points were simultaneously formed along the margin and thalweg of the main channel. The acceleration mechanisms of flow velocity forming the high steps were the increase of the elevation difference along the internal slope of the main channel margin and promotion of discharge along the main channel thalweg. This paper presents newly completed deepwater observations of the present seafloor bedforms based on the turbidity currents within the CLTZ. These observations update the evolution model of local supercritical flows and hydraulic jumps under the global decreasing trend of the Froude numbers within CLTZs.

On the termination of deep-sea fan channels: Examples from the Rhône Fan (Gulf of Lion, Western Mediterranean Sea)

The termination of a deep-sea turbiditic channel represents the ultimate sink of terrigenous sediment in the oceans or lakes. Such environment is characterized by rapid slope decrease and by loss of confinement of turbidity currents. It results in the deposition of Channel-Mouth-Lobes that can be separated from the channel mouth by an erosional (scoured) or by-pass dominated Channel-Lobe Transition Zone. Several factors can control the occurrence, extent and morphologic expression of the area such as the slope break angle, the upslope and downslope angle and the mud/sand ratio in flows. Disentangling these factors remains challenging due to the scarcity of outcrops and to the usual faint morphologies and low thickness of deposits. With bathymetric and seismic data we calculated the morphometric parameters of 8 channel-levees and their Channel-Mouth Lobes from the deepest area of the Rhone fan, a mud-sand rich system, and among which the youngest one (called neofan) was deposited at the end of the Last Glacial Maximum between 21.5 and 18.3 ka cal. BP. Emplacement and shape (finger-shaped or pear-shaped bulges) of Channel-Mouth Lobes is controlled by the seabed morphology (adjacent channel-levees and salt diapirs). A less prominent morphology of the neofan is attributed to premature quiescence related to the post sea-level rise sediment starvation. We show that the occurrence and expression of a Channel-Lobe Transition Zone is controlled by the gradient upstream of the channel mouth slope break. The extended Channel-Lobe Transition Zone and detached lobe of the neofan are attributed to the high upslope gradient (0.26°) while the less detached or attached lobes of other channel-levees is attributed to lower upslope gradient (0.13°). We show that scouring and scours concatenation into flutes at the Channel-Lobe Transition Zone is a major driver for the inception of channels and further confinement of turbidity current. For the first time we show that concatenation of scours in shingled disposition developed an incipient channel sinuosity at this very early stage of channel development. The channel-levee can extend downslope nearly instantaneously by tens of kilometers when isolated nascent channels connect to the channel mouth.

Reconstruction of bedform dynamics controlled by supercritical flow in the channel–lobe transition zone of a deep‐water delta (Sant Llorenç del Munt, north‐east Spain, Eocene)

AbstractStable supercritical‐flow bedform phases under two‐dimensional steady flow are geometrically simple and include long‐wavelength cyclic steps at high Froude numbers and antidunes characterized by in‐phase flow that is near critical. Less well understood are the transitional bedform phases at the boundaries of the stable bedform fields and bedforms developing in complex flow geometries like the channel–lobe transition zone. This complexity is exacerbated by the fact that natural flows are never steady. Stable antidune bedforms may be reworked by temporally increasing discharge into chute and pool, and cyclic step and chute and pool fields will be reworked into antidunes if discharge is sufficiently decreasing. In addition, the channel–lobe transition zone is continuously evolving in space and time due to the influence of solitary hydraulic jumps at the channel mouth on channel extension and back stepping. This detailed outcrop study of a deep‐water delta slope belonging to the Eocene Sant Llorenç del Munt clastic wedge exposed near El Pont de Vilomara (north‐east Spain), tackles the complex bedform architecture problems by applying a method previously developed for fluvial deposits. Analysis of surfaces traced on high‐definition, drone‐derived in‐strike images combines architectural studies with facies analysis. Set boundaries of the bedforms were thus established, revealing the upslope migration of hydraulic jump zones and the intricate stacking of antidunes and solitary, mouthbar related chute and pool like structures. Further analysis of the stacking of bedforms and bounding surfaces provide evidence that deposition occurred in a relatively short (few hundreds of metres) channel–lobe transition zone at the base of the delta slope. The usefulness of the bounding surface hierarchy approach for turbidite deposits lies in the careful evaluation of the spatial extent of bounding surfaces, which are easily overlooked in complex architectures such as those created in the channel–lobe transition zone.

Formation of detrital clay grain coats by dewatering of deep-water sands and significance for reservoir quality

ABSTRACTWell-developed detrital clay grain coats are observed in deep-marine sandstones of the Upper Cretaceous Springar Formation of the Vøring Basin in the Norwegian Sea. The detrital clay coats form thin and compact rims on individual sand grains and meniscus-shaped bridges between grains. These well-developed coats are found in high-density turbidites and proximal hybrid event beds with common to pervasive dewatering structures deposited in proximity to the base of a syndepositionally active basin high. Here, in one exploration well, detrital clay grain coats are common throughout a sandstone package 100 m thick. High-density turbidites and proximal and distal hybrid event beds drilled in mid- to distal-fan settings unaffected by seismically resolved seafloor topography show common dewatering features, but have only scattered detrital clay coats confined to individual dewatering pipes or dish structures. Hence, we propose that intense sediment dewatering has the potential to form detrital clay coats in deep-marine sandstones by a combination of elutriation and reorganization of clays during fluid escape from sediment bodies with pore fluid pressures significantly higher than the hydrostatic pressure. In submarine fan systems, deposition of sediment with coeval trapping of large volumes of interstitial pore fluid is most likely to occur where gravity flows undergo rapid deceleration in response to an abrupt decrease in confinement or gradient. Such environments include the channel–lobe transition and settings in proximity to seabed topography.The investigated sandstones are quartz arenites and subarkoses, with minor to moderate volumes of quartz cement (up to 6%). However, strongly to completely quartz-cemented intergranular pore space is observed where detrital clay coats or matrix does not cover quartz grains in the deepest part of the studied formation. Modeling of quartz cementation predicts that most intergranular macroporosity in the lower part of the Springar Formation would be quartz cemented if the sandstones were free of detrital clays. Based on our observations and modeling results we propose that intense sediment dewatering has the potential to form detrital clay coats, which can be important for retaining porosity in deeply buried sandstones and in basins with high present or past heat flow.

Architecture and development of the Magdalena Submarine Fan (southwestern Caribbean)

Submarine fans are the largest sediment accumulations on Earth and are key to study the sediment-transfer zone between terrestrial source area and deep-sea depositional sink. They provide information of past climate, tectonics, carbon global inventory, and can be prolific hosts for oil/gas resources. In this paper, we present, for the first time, an analysis of the entire Magdalena Submarine Fan (MSF), from the shelf break to the distal domain, based on ~224,000 km2 of multibeam bathymetry, as well as backscatter, subbottom profiler and regional/local seismic reflection data recently acquired in southwestern Caribbean Sea. The aim is to describe in detail the MSF morphology and depositional architecture, propose a fan segmentation landform assemblage, and determine its extent within the deep basin. Three fan segments are identified: upper, middle and lower fans. The upper fan extends from the coastline to a depth of ~3000 m. Its eastern sector is characterized by a canyon system off the present-day mouth of the Magdalena River, through which most of the sediments are transported downslope to be deposited in intra-slope basins bounded by structural highs of the South Caribbean Deformed Belt. The western upper fan is characterized by distinct channel-levee complexes that are partially removed by mass transport complexes that originated near the continental shelf edge. The upper to middle fan transition is marked by a decrease in regional gradient from 1° to about 0.3° and local relief, a change from large, irregular sediment waves to more regular waves that slowly decrease in amplitude downslope, and the appearance of erosional megascours. The middle fan bedform assemblage evidences the presence of a channel-lobe transition zone in this sector of the MSF, separating well-defined, upper fan channels from lower fan depositional lobes. The middle to lower fan transition occurs between ~3800 and ~3950 m and is defined by a further decrease in gradient and by a change from sediment waves to planar parallel-bedded reflectors on subbottom profiles. The lower fan extends to a maximum depth of ~4250 m and is characterized by an almost flat seabed (slope of 0.05°), interrupted by a few isolated bathymetric highs and low-relief channels (<5 m deep). The entire area of the lower fan is interpreted as an area of turbidity current deposition, with relatively sandy sediments in the south and west giving way to muddier sediments in the northeast, which could correspond to distal fan lobes. The identification of four regional seismic facies related to MSF sedimentation allowed us to constrain the fan extent in the southwestern sector of the Caribbean. Distally, within the Colombia Basin the fan terminates against areas of elevated seafloor (Kogi-Coyaima and Quimbaya seamounts, Aruzi and Ticuna hills, and Beata Ridge), which have acted as barriers to sedimentation. Nevertheless, it was determined that the MSF extends beyond the Colombia Basin and locally reaches the Lower Nicaraguan Rise. In this new scenario, we established that the MSF is much more extensive than previously recognized, with an area of ~237,000 km2, which puts it among the World's largest submarine fans.

Daily bathymetric surveys document how stratigraphy is built and its extreme incompleteness in submarine channels

Turbidity currents are powerful flows of sediment that pose a hazard to critical seafloor infrastructure and transport globally important amounts of sediment to the deep sea. Due to challenges of direct monitoring, we typically rely on their deposits to reconstruct past turbidity currents. Understanding these flows is complicated because successive flows can rework or erase previous deposits. Hence, depositional environments dominated by turbidity currents, such as submarine channels, only partially record their deposits. But precisely how incomplete these deposits are, is unclear. Here we use the most extensive repeat bathymetric mapping yet of any turbidity current system, to reveal the stratigraphic evolution of three submarine channels. We re-analyze 93 daily repeat surveys performed over four months at the Squamish submarine delta, British Columbia in 2011, during which time >100 turbidity currents were monitored. Turbidity currents deposit and rework sediments into upstream-migrating bedforms, ensuring low rates of preservation (median 11%), even on the terminal lobes. Large delta-lip collapses (up to 150,000 m3) are relatively well preserved, however, due to their rapidly emplaced volumes, which shield underlying channel deposits from erosion over the surveyed timescale. The biggest gaps in the depositional record relate to infrequent powerful flows that cause significant erosion, particularly at the channel-lobe transition zone where no deposits during our monitoring period are preserved. Our analysis of repeat surveys demonstrates how incomplete the stratigraphy of submarine channels can be, even over just 4 months, and provides a new approach to better understand how the stratigraphic record is built and preserved in a wider range of marine settings.

Quantification of Basin-Floor Fan Pinchouts: Examples From the Karoo Basin, South Africa

The topography of the seabed (orientation and gradient) and rheology of the flows greatly influences the character of basin-floor turbidity current deposits. Therefore, submarine fan pinchouts can help to constrain seabed topography and basin configurations at the time of deposition. Although the depositional architecture of submarine lobe pinchouts has been documented in various basin-fills, the quantification of the rates of change at pinchouts in different palaeogeographic positions and basin configurations has not been attempted previously. Here, we utilize extensive outcrops and research boreholes from the oblique up-dip pinchout of Fans 3 and 4 and the lateral pinchout of Fan 3 in the Tanqua depocentre, Karoo Basin, South Africa, to compare sedimentary facies and to quantify the rates of change in gross interval thickness. At the oblique up-dip pinchout, Fan 3 thins abruptly at a rate of 12 m/km, whilst Fan 4 thins at a rate of 4 m/km. Marked differences between Fan 3 and 4 in sedimentary facies and architecture towards the up-dip pinchout, with termination of lobes in Fan 3 and a channel-lobe transition zone and external levee in Fan 4, suggests progradation of the system. The thinning rate of the lateral pinchout of Fan 3 is 2 m/km with the presence of hybrid beds in the lower part of Fan 3, whilst the upper part is dominated by structured sandstones and thin-bedded heterolithics. The variations in facies suggest that at the lobe-scale frontal and lateral pinchouts are stacked at the lobe complex-scale lateral pinchout of Fan 3, highlighting the importance of a hierarchical understanding when studying basin-floor fan pinchouts. The quantified rates of change in fan thickness and sedimentology on the oblique up-dip and lateral fan pinchouts are markedly different. Contrasting pinchout architectures above slopes with subtle differences in gradient and orientation cautions against simple definition of reservoir input parameters for stratigraphic traps in submarine fan systems.

Provenance analysis of the Dezadeash Formation (Jurassic–Cretaceous), Yukon, Canada: implications regarding a linkage between the Wrangellia composite terrane and the western margin of Laurasia

The Mesozoic convergence of the allochthonous Wrangellia composite terrane (WCT) with the western margin of Laurasia coincided with the construction of the Chitina magmatic arc on the WCT, and the dispersal of volcanic flows and sediment gravity flows into an adjacent flysch basin. The basin, preserved as the Gravina–Nutzotin belt, includes the Dezadeash Formation in southwest Yukon, the Nutzotin Mountains sequence in southern Alaska, and the Gravina belt in southeastern Alaska. The Dezadeash Formation is a submarine fan system comprising stacked channel-lobe transition and lobe deposits interposed with overbank deposits. Conglomerate pebble-counts, sandstone point-counts, detrital zircon ages, and major element, trace element, rare earth element, and Sm–Nd isotopic geochemistry of sandstone, mudstone, and hemipelagite beds suggests that the deposits consist mainly of first-cycle volcanogenic detritus shed from the undissected Chitina arc, in addition to material eroded from the WCT. The arc was constructed of undifferentiated magma sourced from the depleted mantel, as well as older crustal material attributed to the WCT proxying for continental crust. The compositional provenance results, together with published paleocurrent data for the Dezadeash Formation and compositional and directional provenance indicators from the Nutzotin Mountains sequence and Gravina belt, does not require a sediment source from Laurasia. The provenance record is compatible with deposition of the Gravina–Nutzotin belt in a convergent plate margin setting.

Turbidite, debrite, and hybrid event beds in submarine lobe deposits of the Palaeocene to middle Eocene Kapit and Pelagus members, Belaga Formation, Sarawak, Malaysia

Turbiditic flysch units of the Rajang Group form a large crescentic belt of deformed strata in central Borneo. Unfortunately, our understanding of its bed type characteristics and depositional setting is very poor. Here, we present a detailed bed type analysis of the Palaeocene to middle Eocene Kapit and Pelagus members of the Belaga Formation, based on detailed investigations of recent road‐cut exposures around Sibu, Sarawak, Malaysia. Five bed types are identified from the studied sections, representing deposition from turbidity currents, debris flows, and flows that show turbulent and laminar characteristics. The Belaga Formation deposits are divided into four bed type associations based on bed type assemblages, bed geometry, degree of bed amalgamation, vertical grain size, and bed thickness trends, for example, lobe axis, lobe off‐axis, lobe fringe, and slump. Hybrid event beds (HEBs) are not restricted only to the lobe fringe but are also a common element of the lobe axis sub‐environment in the Belaga Formation system. Their common occurrence in a proximal location in the lobes is probably due to enhanced seafloor erosion and rapid deceleration due to loss of confinement at the channel lobe transition further up‐dip. Evidence such as bidirectional ripple cross‐lamination in thin‐bedded turbidites and variable palaeocurrent orientations suggest a complex depositional topography, which also suppressed flow turbulence and promoted deposition of hybrid event beds in proximal locations through deflection and deceleration of incoming flows by confining counter slopes. Similar bed type assemblages within the slump and lobe deposits indicate a local origin for the slumps. Localized failures can form on low gradient slopes developed in tectonically active basins with complex topography.

The Hypanis Valles delta: The last highstand of a sea on early Mars?

One of the most contentious hypotheses in the geological history of Mars is whether the northern lowlands ever contained an oceanic water body. Arguably, the best evidence for an ocean comes from the presence of sedimentary fans around Mars' dichotomy boundary, which separates the northern lowlands from the southern highlands. Here we describe the palaeogeomorphology of the Hypanis Valles sediment fan, the largest sediment fan complex reported on Mars (area >970 km2). This has an extensive catchment (4.6×105 km2) incorporating Hypanis and Nanedi Valles, that we show was active during the late-Noachian/early-Hesperian period (∼3.7 Ga). The fan comprises a series of lobe-shaped sediment bodies, connected by multiple bifurcating flat-topped ridges. We interpret the latter as former fluvial channel belts now preserved in inverted relief. Meter-scale-thick, sub-horizontal layers that are continuous over tens of kilometres are visible in scarps and the inverted channel margins. The inverted channel branches and lobes are observed to occur up to at least 140 km from the outlet of Hypanis Valles and descend ∼500 m in elevation. The progressive basinward advance of the channellobe transition records deposition and avulsion at the margin of a retreating standing body of water, assuming the elevation of the northern plains basin floor is stable. We interpret the Hypanis sediment fan to represent an ancient delta as opposed to a fluvial fan system. At its location at the dichotomy boundary, the Hypanis Valles fan system is topographically open to Chryse Planitia – an extensive plain that opens in turn into the larger northern lowlands basin. We conclude that the observed progradation of fan bodies was due to basinward shoreline retreat of an ancient body of water which extended across at least Chryse Planitia. Given the open topography, it is plausible that the Hypanis fan system records the existence, last highstand, and retreat of a large sea in Chryse Planitia and perhaps even an ocean that filled the northern plains of Mars.

The Tectonically Controlled San Gabriel Channel–Lobe Transition Zone, Catalina Basin, Southern California Borderland

Abstract High-resolution geophysical data across the Catalina Basin, offshore southern California, USA, reveal a complex channel–lobe transition zone (CLTZ) and provide an opportunity to characterize an entire seafloor CLTZ in a tectonically active and confined-basin setting. The seafloor morphology, distribution of depositional and erosional features, and location of depocenters in the CLTZ are controlled by shifting confinement and seafloor gradient related to inherited basement structures, active faults, and basin margins. Below a Holocene hemipelagic drape, the Catalina Basin is dominated by CLTZ and lobe sedimentation from the San Gabriel Channel, with lesser accumulations from local sediment sources limited to basin margins. The San Gabriel Channel is structurally confined as it enters the Catalina Basin and appears unable to avulse; it continues into the basin as a channel that rapidly widens, decreases in relief, and becomes scoured at its margins. A CLTZ is imaged between the confined San Gabriel channel and its terminal lobes deposited > 50 km into the basin. Narrow, apparently disconnected channels with knickpoints occur throughout the proximal and mid-CLTZ and are concentrated near basement highs and basin-bounding Quaternary-active dextral strike-slip faults. A field of small-scale erosional crescent-shaped scours (∼ 100 m length, ∼ 200 m width, up to ∼ 10 m relief across ∼ 30 km2 region) occurs above a partially buried basement high that creates perturbations in seafloor gradient. Likewise, above a buried basement structure that locally increases seafloor gradient (up to 0.4°), the distal CLTZ may contain sediment waves (∼ 2–4 m wave height and ∼ 200–300 m wavelength) that are smaller than many other CLTZ examples. This study of the San Gabriel CLTZ in Catalina Basin provides high-resolution geophysical data coverage of a complete CLTZ and illustrates a tectonically controlled end-member CLTZ from the modern seafloor.

Deep-water channel-lobe transition zone dynamics: Processes and depositional architecture, an example from the Karoo Basin, South Africa

Submarine channel-lobe transition zones separate well-defined channels from well-defined lobes and form morphologically complicated areas, commonly located at breaks in slope. These areas play a vital role in the transfer of sediment through deep-water systems. Extensive outcrop exposures in the Karoo Basin, South Africa, permit investigation of the depositional architecture and evolution of entirely exhumed dip transects of a channel-lobe transition zone for the first time. Furthermore, the excellent paleogeographic constraint allows correlation to genetically related updip channel-levee systems and downdip lobe deposits over 40 km, with strike control over 20 km. Unlike the single time slice afforded by modern systems, the Karoo example uniquely allows study of the temporal shifting of the channel-lobe transition zone and transfer into the stratigraphic record. Key lateral changes along the base of slope include the variation from an interfingering levee-lobe transition zone to a bypass-dominated channel-lobe transition zone over a width of 14 km. Key recognition criteria for channel-lobe transition zones in the ancient record include combinations of scours and megaflutes, composite erosional surfaces, mudstone clast/coarse-grained sediment lags, and remnants of depositional bed forms, such as sediment waves. Documented here in a single channel-lobe transition zone, these features are arranged in a zone of juxtaposed remnant erosional and depositional features. The zone reaches 6 km in length, formed by at least four stages of expansion/contraction or migration. Strike variations and changes in the dimensions of the channel-lobe transition zone through time are interpreted to be the result of physiographic changes and variations in flow dynamics across the base of slope. The dynamic nature of channel-lobe transition zones results in complicated and composite stratigraphy, with preservation potential generally low but increasing distally and laterally away from the mouth of the feeder channel system. Here, we present the first generic model to account for dynamic channel-lobe transition zone development, encompassing distinctive recognition criteria, fluctuations in the morphology and position of the zone, and the complex transfer into the sedimentary record.

Architecture and morphodynamics of subcritical sediment waves in an ancient channel–lobe transition zone

AbstractIn modern systems, submarine channel–lobe transition zones show a well‐documented assemblage of depositional and erosional bedforms. In contrast, the stratigraphic record of channel–lobe transition zones is poorly constrained, because preservation potential is low and criteria have not been established to identify depositional bedforms in these settings. Several locations from an exhumed fine‐grained base of slope system (Unit B, Laingsburg depocentre, Karoo Basin, South Africa) show exceptional preservation of sandstone beds with distinctive morphologies and internal facies distributions. The regional stratigraphic context, lack of a basal confining surface, wave‐like morphology in dip section, size and facies characteristics support an interpretation of subcritical sediment waves within a channel–lobe transition zone setting. Some sediment waves show steep (10 to 25°) unevenly spaced (10 to 100 m) internal truncation surfaces that are dominantly upstream‐facing, which suggests significant spatio‐temporal fluctuations in flow character. Their architecture indicates that individual sediment wave beds accrete upstream, in which each swell initiates individually. Lateral switching of the flow core is invoked to explain the sporadic upstream‐facing truncation surfaces, and complex facies distributions vertically within each sediment wave. Variations in bedform character are related to the axial to marginal positions within a channel–lobe transition zone. The depositional processes documented do not correspond with known bedform development under supercritical conditions. The proposed process model departs from established mechanisms of sediment wave formation by emphasising the evidence for subcritical rather than supercritical conditions, and highlights the significance of lateral and temporal variability in flow dynamics and resulting depositional architecture.