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Examples of slope and basin-floor topography and resultant deposits. (A) Simple slope profile with single break-in-slope changing from bypass dominated channel-levee system to depositional dominated basin-floor lobes, with potential for channel-lobe transition zone (CLTZ) development at base-of-slope. (B) Stepped slope profile with higher gradient ramps linking lower gradient steps. Formation of entrenched channel/channel levee systems on ramps and intraslope/basin-floor lobes on steps, with potential for CLTZ development at breaks-of-slope. (C) Topographically complex slope, encompassing varying magnitudes of topography. Development of several ramps within entrenched channel/channel levee systems, including a step on the basin floor. Intraslope and basin-floor lobe development on lower gradient steps. Formation of high magnitude topography leads to the creation of a tortuous corridor controlling channel levee systems, and development of mini-basins where 3-D closure occurs. Topography on slopes is generally of much greater magnitude than on the basin floor. MTD-mass transport deposit. (D) Perspective view of partially (perched) and fully (ponded) confined intraslope deposits from the Brazos-Trinity intraslope basins, Gulf of Mexico, linked by a higher gradient area dominated by sediment bypass (modified and republished with permission of John Wiley and Sons, Inc., from Basin Research, Prather et al., vol. 29, issue 3, 2017; permission conveyed through Copyright Clearance Center, Inc.). (E) Thickness map and dip seismic profile of intraslope basin fill from the Western Niger Delta slope, demonstrating the formation and healing of intraslope accommodation above mobile shale (modified from GSA Bulletin, Jobe et al., vol. 129, 2017; cropped image with minor changes to text).
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The effects of abrupt changes in slope angle and orientation on turbidity current behavior have been investigated in numerous physical and numerical experiments and examined in outcrop, subsurface, and modern systems. However, the long-term impact of subtle and evolving seabed topography on the stratigraphic architecture of deep-water systems requi...
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... Erosion features include isolated spoon and chevron shaped scours and large amalgamation scours (Wynn et al, 2002;Postma et al 2016). Depositional features, on the other hand are characterized by a thin stratigraphic expression, deposits of coarse-grained or mud clast lags, aggradational bedforms like sediment waves, soft-sediment deformation, interfingering of deposits from up-dip and down-dip, and the occurrence of sand-rich hybrid beds within proximal lobes (Wynn, 2002;Ito, 2008;Pemberton et al., 2016;Hofstra et al., 2015;Brooks et al., 2018aBrooks et al., , 2018bBrooks et al., , 2021. Based on both modern and ancient studies of CTLZs, can extend horizontally for several kilometers or more along the fan surface (Wynn, 2002;Ito, 2008;Brooks et al., 2018aBrooks et al., , 2018bBrooks et al., , 2021.Width can vary depending on the size and complexity of the channel system, ranging from a few hundred meters to several kilometers (Wynn, 2002;Ito, 2008;Pemberton et al., 2016;Hofstra et al., 2015;Brooks et al., 2018aBrooks et al., , 2018bBrooks et al., , 2021. ...
... Depositional features, on the other hand are characterized by a thin stratigraphic expression, deposits of coarse-grained or mud clast lags, aggradational bedforms like sediment waves, soft-sediment deformation, interfingering of deposits from up-dip and down-dip, and the occurrence of sand-rich hybrid beds within proximal lobes (Wynn, 2002;Ito, 2008;Pemberton et al., 2016;Hofstra et al., 2015;Brooks et al., 2018aBrooks et al., , 2018bBrooks et al., , 2021. Based on both modern and ancient studies of CTLZs, can extend horizontally for several kilometers or more along the fan surface (Wynn, 2002;Ito, 2008;Brooks et al., 2018aBrooks et al., , 2018bBrooks et al., , 2021.Width can vary depending on the size and complexity of the channel system, ranging from a few hundred meters to several kilometers (Wynn, 2002;Ito, 2008;Pemberton et al., 2016;Hofstra et al., 2015;Brooks et al., 2018aBrooks et al., , 2018bBrooks et al., , 2021. ...
... Depositional features, on the other hand are characterized by a thin stratigraphic expression, deposits of coarse-grained or mud clast lags, aggradational bedforms like sediment waves, soft-sediment deformation, interfingering of deposits from up-dip and down-dip, and the occurrence of sand-rich hybrid beds within proximal lobes (Wynn, 2002;Ito, 2008;Pemberton et al., 2016;Hofstra et al., 2015;Brooks et al., 2018aBrooks et al., , 2018bBrooks et al., , 2021. Based on both modern and ancient studies of CTLZs, can extend horizontally for several kilometers or more along the fan surface (Wynn, 2002;Ito, 2008;Brooks et al., 2018aBrooks et al., , 2018bBrooks et al., , 2021.Width can vary depending on the size and complexity of the channel system, ranging from a few hundred meters to several kilometers (Wynn, 2002;Ito, 2008;Pemberton et al., 2016;Hofstra et al., 2015;Brooks et al., 2018aBrooks et al., , 2018bBrooks et al., , 2021. ...
Recent data from modern and ancient submarine fans contradicts classical fan models in (1) the abundance of Froude supercritical flow deposits, and (2) avulsive rather distributive 1 processes of the fan feeder channel, where channels reach across the entire fan lengths and lobes occur as lateral and terminal splays also across the entire fan. The latter is in contrast to models that show distributive channels in proximal fans and large lobes without channels in distal fans. These data arise questions whether Froude supercritical flow and a high degree of channelization are common features even in distal basin-floor fans. Here we study the facies and architecture of the outcrops of the Point Loma Formation, San Diego, California, previously interpreted as distal fan deposits, using drone photography and measured sections. We show that channelized deposits occur intimately interbedded and laterally related to heterolithic as well as muddy lobe facies, and that even the most fine-grained facies include coarse sand and gravel lags. We further document a wide range of Froude supercritical flow faces. These results contradict the classical submarine fan models and are in agreement with avulsive behavior of submarine fan channels that reach across entire fans. 2.2 Introduction Submarine fans are the most important site of sand accumulation in deepwater settings, and include fans developed on both the basin-floor and base of slope. Menard (1955) introduced the term "submarine/deep-sea fans" as "fans" or "deltas" shaped like gently sloping cones and found at the mouths of many submarine canyons. Nomark (1970) established the first work on modern submarine fans from the seafloor observation of La Jolla Fan and the San
... The cyclicity recorded in: (i) moving average of bed thickness; and (ii) facies associations distribution (see Figs 2, 14A and 14B, the Gryb ow Beds), and on the other hand lack of bed thickness trends and randomness in facies distribution within individual cycles suggest that deposition of studied succession had not resulted in an intraslope lobe system (e.g. Spychala et al., 2015;Brooks et al., 2018). In terms of number of beds, oscillatory fluctuations at Szczawa (Fig. 14A and C) are of one order longer than upwardthickening or upward-thinning sequences in lobe systems, i.e. tens of beds in one cycle versus up to 20 beds in an individual sequence (see Fig. 16 for comparison at the scale of several tens of beds, SZ1, SZ2 and M1-M3, Kl1 sections, respectively). ...
This study demonstrates the effectiveness of sedimentological–statistical multivariate analysis of one‐dimensional‐section in the unravelling depositional processes and sedimentation patterns recorded in a contained and partially ponded succession. Turbidite deposition of a 100 m thick mud‐rich Oligocene‐age sequence at Szczawa, the Polish Outer Carpathians, was primarily controlled by topographic confinement and magnitude of incoming flows. Only the largest turbidite flows were subjected to the true flow ponding, while in smaller volume flows the silty/sandy part behaved as unconfined flow and only muddy suspension cloud developed a ponded character. True ponding of low‐density turbidity currents is interpreted for sandstones that show sedimentary structures attesting to flow reflections and combined‐flow processes and are associated with abnormally thick co‐genetic mudstones, which in turn results in low sandstone‐to‐mudstone ratio, irrespective of turbidite bed thickness. These features testify to pronounced interaction of flows with confining topography. Tractional structures associated with a considerable proportion of fines, namely banded structure and heterolithic bedding, are interpreted as produced during transitional (turbulent to laminar) flow phase in the condition of mud‐rich turbidite flow confinement. These structures are also considered here as indicators of proximity to source area and location on the slope. The co‐occurrence of banded sandstones, hybrid event megabeds and an olistostrome resulted from flows‐trapping, confined basin geometry, which precluded downslope propagation and further transformation of large‐volume flows. Therefore, a structurally‐controlled contained intraslope mini‐basin is proposed here as the most likely depositional setting. This work provides robust field and statistical evidence for ponding processes of low‐density turbidity currents in a structurally‐controlled mini‐basin. The results of this study are consistent with experimental data and recent field studies on ponded turbidites. Therefore, the Szczawa succession may serve as a new reference example of containment processes of low‐density turbidity currents and represents a valuable depositional model for intraslope turbidite succession.
... On slopes with complex localized topography, severe SBs can occur more often as a result of the surface expression of complex subsurface processes , leading to the formation of: (i) ponded accommodation (Spychala et al., 2015); (ii) slopes with stepped profiles, such as mid-slope terraces (Brooks et al., 2018a); (iii) seafloor topography resulting from geological folds (Howlett et al., 2019) and exposed faults (Ge et al., 2017); (iv) active salt diapirism (Howlett et al., 2020; at the base of steep continental margins (Lee et al., 2002). Localized topographic variations are therefore important as they induce rapid shifts in the hydrodynamics of TCs that alter their depositional patterns on both local (Amy et al., 2007;Patel et al., 2021) and regional (Soutter et al., 2019) scales. ...
The 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 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.
... 1800 m, based on the uncompacted thickness of the succession from basin-floor fans to the first delta deposits . Following the strike dip of the palaeoslope reconstructed by Brooks et al. (2018b), deepening direction was towards E. ...
Turbidity 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.
... Stepped profiles have been associated with low rates of slope deformation and average sedimentation rates (Hay, 2012;Meckel et al., 2002) and exhibit subtle gradient changes (Brooks et al., 2018;Mignard et al., 2019), where sedimentation rates outpace deformation rates (Adeogba et al., 2005;Deptuck et al., 2012;Hay, 2012;Pirmez et al., 2000;Prather, 2003). Accommodation creation has been attributed to several factors such as salt tectonics (Hay, 2012;Smith, 2004), mud diapirism (Adeogba et al., 2005;Barton, 2012;Deptuck et al., 2012;Jobe et al., 2017), scars of mass transport complexes (Spychala et al., 2015) and differential compaction and subsidence (Brooks et al., 2018;Jackson et al., 2008;Spychala et al., 2015). ...
... Stepped profiles have been associated with low rates of slope deformation and average sedimentation rates (Hay, 2012;Meckel et al., 2002) and exhibit subtle gradient changes (Brooks et al., 2018;Mignard et al., 2019), where sedimentation rates outpace deformation rates (Adeogba et al., 2005;Deptuck et al., 2012;Hay, 2012;Pirmez et al., 2000;Prather, 2003). Accommodation creation has been attributed to several factors such as salt tectonics (Hay, 2012;Smith, 2004), mud diapirism (Adeogba et al., 2005;Barton, 2012;Deptuck et al., 2012;Jobe et al., 2017), scars of mass transport complexes (Spychala et al., 2015) and differential compaction and subsidence (Brooks et al., 2018;Jackson et al., 2008;Spychala et al., 2015). In salt basins, stepped profiles have been related to complex topographic settings such as connected tortuous corridors formed by discontinuous salt-cored structures (Hay, 2012;Howlett et al., 2021;Oluboyo et al., 2014;Prather, 2003;Smith, 2004). ...
... In addition, the stratigraphic architecture of fill-and-spill cycles with vertical deformation has been modelled by several authors (e.g., Christie et al., 2021;Sylvester et al., 2015;Wang et al., 2017). In cases where fill-and-spill cycles are vertically stacked, the stratigraphic cyclicity is attributed to the interaction of sediment input and vertical accommodation rejuvenation created by dynamic seafloor deformation induced by fixed structural elements (e.g., Booth et al., 2003;Brooks et al., 2018;Hay, 2012;Spychala et al., 2015). However, accommodation patterns can be spatially variable during the lifespan of a turbidite system, for instance due to basinward tilting , lateral tilting (Kane et al., 2010(Kane et al., , 2012 and the emplacement of masstransport complexes (Wu et al., 2020). ...
The classic fill‐and‐spill model is widely applied to interpret topographic controls on depositional architecture and facies distributions in slope successions with complicated topography. However, this model implies a constant topographic configuration over the lifespan of a turbidite system. In contrast, the impact on patterns of erosion and deposition above dynamic slopes whose topographic configuration varies spatially over time remains poorly investigated. Here, using high‐resolution 3D seismic reflection data and more than 100 wells from a 40 km long stepped slope system (Campos Basin, offshore Brazil), we document the evolution of a sand‐prone turbidite system active during the Oligocene‐Miocene transition. This turbidite system was influenced by vertical and lateral deformation, and we propose a new stratigraphic model to explain the resultant depositional architecture. Two depocentres were identified as steps, with channels on the proximal step, and channel‐lobe complexes on the distal step, bounded by sediment bypass‐dominated ramps. Lateral stepping of channels on the proximal step, and oblique stacking of the down‐dip lobe complexes, each cut by through‐going channels, indicate multiple fill‐and‐spill cycles. A persistent NE‐ward stepping and thickening on the steps is interpreted to reflect lateral tilting of the seafloor driven by salt tectonics. The dynamic substrate prevented the establishment of a single long‐lived conduit across the proximal step, as recorded in systems with fixed topographic configurations. The filling of through‐going channels with mud at the end of each cycle suggests waxing‐to‐waning sediment supply cycles and periods of sand starvation, when the lateral tilting dominated and drove avulsion of the feeder channels towards topographic lows. This study demonstrates that subtle dynamic slope deformation punctuated by discrete sediment supply cycles results in complex stratigraphic patterns with multiple phases, and multiple entry and exit points, during repeated cycles of fill‐and‐spill, tilt‐and‐repeat (FaSTaR). These are likely to be present in other stepped slope systems.
... Flows were likely interacting with the developing structures, such as the Pōrangahau Ridge, with variable flow reflection, diversion and restriction apparent (e.g. Howlett et al., 2019;Soutter et al., 2021); one result of this topographic interaction is that the lobes in this interval appear to be disconnected from their feeder systems (Figure 8b; sensu Brooks et al., 2018). Fill of these basins was also supplied by activation of the Omakere System (SF7; Crisóstomo-Figueroa et al., 2021), which was interacting with MTCs (SF13) in the Omakere Trough (Figure 8b). ...
Deep‐water sedimentation on active margins often entails complex sediment transport pathways through slope accommodation. Sedimentation in such settings is commonly differentiated into ‘fill and spill’ vs. ‘tortuous corridor’ models. To investigate the utility of these models in convergent settings 15,344 km2 of 3D seismic data is used to investigate sedimentation and erosion patterns across the Hikurangi subduction margin. A series of thrust‐bound trench‐slope basins, each tens of kilometres long by kilometres wide, have been diachronously forming, filling, and deforming through the Neogene until today. Five primary input points delivered sediment to the basins along the studied part of the margin. Channels display both axial and transverse orientations, the run‐out lengths of which vary temporally. At various times, relatively coarse‐grained sediment was trapped in the interior basins, occasionally then to be cannibalised during landsliding or erosion of growing structures. At other times, coarse‐grained sediment was bypassed to distal basins or the trench. Multiple sediment input points and occasionally tortuous sediment dispersal corridors result in the evolution of convoluted depositional systems, often with similar styles of sedimentation occurring contemporaneously in proximal and distal basins, contrary to simple models of basin fill. A hierarchy of controls on sediment distribution can be distinguished. At the highest level, sediment distribution is controlled by external factors, e.g., glacio‐eustacy and tectonics. At basin scale, the interaction of sedimentary systems with local relief (e.g., evolving seafloor structures and landslides) dictates the location and style of deposition. At the lowest level, autocyclic factors (e.g., flow response to earlier deposits) influence the spatiotemporal variation in erosion and sedimentation. The complex interplay of these factors dictates whether basins were filling, spilling, or some combination at any point in time, whilst basins that were filled and spilled may subsequently resume filling due to changes in the bounding conditions. Hence simple use of ‘fill and spill’ or ‘tortuous corridor’ models to tectonically active margins is not advised. Furthermore, as sedimentation may influence structure growth, constraining the controls on sediment distribution may improve understanding of the broader evolution of convergent margins and their resource distribution.
... The architecture and depositional environments of the Laingsburg and Fort Brown formations are constrained by a robust three-dimensional stratigraphic framework, established through extensive mapping over 2500 km 2 ( Figure 1B,D) (Brooks et al., 2018a;Brunt et al., 2013aBrunt et al., , 2013bDi Celma et al., 2011;Figueiredo et al., 2010;Flint et al., 2011;Grecula et al., 2003aGrecula et al., , 2003bHodgson et al., 2011;Morris et al., 2016;Morris et al., 2014aMorris et al., , 2014bPoyatos-Moré et al., 2019;Sixsmith et al., 2004;Spychala et al., 2015Spychala et al., , 2017Van der Merwe et al., 2009, 2014. Palaeoflow was dominantly to the north-east and east, and the main sediment entry point was located to the south-west ( Van der Merwe et al., 2014). ...
... The stratigraphic framework of the Laingsburg (Units A, A/B, B) and Fort Brown (Units B/C, C, D, D/E, E, F, G) formations is based on the recognition and mapping of continuously exposed regional mudstone units intercalated between sandstone-prone units. Overall, mudstone units thin gradually basinward (to the east) forming a wedge-shape geometry (Brooks et al., 2018a;Brunt et al., 2013b;Poyatos-Moré et al., 2019), and locally infill and heal residual depositional topography such as channel-fills and levees (Morris et al., 2016) ( Figure 1D). The relative thicknesses of the mudstone units have been used as a tool to propose a hierarchical subdivision of the deep-water succession . ...
... The stratigraphic package encompassed by the two cores is indicated by the black rectangles. Redrawn after Brooks, Hodgson, Brunt, Peakall, Poyatos-Moré, and Flint (2018a bioturbation intensity observed at core scale. Mudstone facies were later refined by observations of microscopic features in thin sections. ...
Deep‐water mudstones overlying basin‐floor and slope sandstone‐prone deposits are often interpreted as hemipelagic drapes deposited during sand starvation periods. However, mud transport and depositional processes, and resulting facies and architecture of mudstones in deep‐water environments, remain poorly understood. This study documents the sedimentology and stratigraphy of basin‐floor and slope mudstones intercalated with sandstone‐prone deposits of the Laingsburg depocentre (Karoo Basin, South Africa). Sedimentologic and stratigraphic criteria are presented here to distinguish between slope and basin‐floor mudstones, which provide a tool to refine palaeogeographic reconstructions of other deep‐water successions. Several mudstone units were mapped at outcrop for 2500 km2 and investigated using macroscopic and microscopic core descriptions from two research boreholes. Basin‐floor mudstones exhibit a repeated and predictable alternation of bedsets dominated by low‐density turbidites, and massive packages dominated by debrites, with evidence of turbulent‐to‐laminar flow transformations. Slope mudstones exhibit a similar facies assemblage, but the proportion of low‐density turbidites is higher, and no repeated or predictable facies organisation is recognised. The well‐ordered and predictable facies organisation of basin‐floor mudstones suggest local point sources from active slope conduits, responsible for deposition of compensationally‐stacked muddy lobes. The lack of predictable facies organisation in slope mudstones suggests deposition took place in a more variable range of sub‐environments (i.e., ponded accommodation, minor gully/channel‐fills, levees). However, regional mapping of three mudstone units evidence basinward tapering and similar thicknesses across depositional strike. This geometry is consistent with the distal part of basin margin clinothems, and suggests laterally extensive mud delivery across the shelf edge combined with along‐margin transport processes. Therefore, the sedimentology and geometry of mudstones suggests that mud can be delivered to deep‐water dominantly by sediment gravity flows through point‐source and distributed regionally, during periods of up‐dip sand storage. These findings challenge the common attribution of deep‐water mudstones to periods of basin‐floor sediment starvation.
... Deepwater siliciclastic depositional systems can be simply divided into (i) a slope dominated by erosion and bypass processes (Galloway, 1998;McHargue et al., 2011;Prather, 2003;Stevenson et al., 2013) and (ii) a basin-floor dominated by depositional processes (Posamentier, 2003;Prélat et al., 2009;Sixsmith et al., 2004). At the critical zone of transition between these two areas, which typically coincides with the base-of-slope, deposits can either show characteristics typical of both the slope and basin-floor (Brooks et al., 2018a;Gardner et al., 2003;Van der Merwe et al., 2014), or are identified by their own separate recognition criteria (Brooks et al., 2018a(Brooks et al., , 2018bHofstra et al., 2015;Ito, 2008;Pemberton et al., 2016;. This area is referred to as the channellobe transition zone (CLTZ; Mutti & Normark, 1987, which has been defined as 'the region that, within any turbidite system, separates well-defined channels or channel-fill, from well-defined lobes or lobe facies' (Mutti & Normark, 1987). ...
... Ancient examples of CLTZs are challenging to identify and require a well-constrained palaeogeographic context (Mutti & Normark, 1987). Nevertheless, outcrop studies (Brooks et al., 2018a(Brooks et al., , 2018bHofstra et al., 2015;Ito, 2008;Pemberton et al., 2016;Pyles et al., 2014; Van der Merwe et al., 2014) have helped develop recognition criteria for CLTZs, which include: a thin stratigraphic expression; amalgamated erosional features; coarse-grained/mudclast lag deposits; aggradational bedforms (i.e. sediment waves); soft-sediment deformation; interfingering of up-dip and down-dip deposits; and sand-rich hybrid beds in proximal lobes (Brooks et al., 2018a;Mansor & Amir Hassan, 2021;Stevenson et al., 2015). ...
... For instance, the classic example of a CLTZ from the Navy Fan (Normark et al., 1979) was later discovered with higher resolution side-scan sonar data (Carvajal et al., 2017) to be a downstream widening, and shallowing channel-mouth transition zone. Despite this, the regional setting at the base-of-slope (Ito, 2008), with channel levee deposits up-dip and medial to distal lobes down-dip, together with the recognition of scour-fills, grain-size breaks, sand-rich hybrid beds and rip-up clasts and grooves (suggesting bypass of forerunning debris flows down-dip; Baas et al., 2021;Peakall et al., 2020) supports a sediment bypass-dominated setting (Brooks et al., 2018b) and interpretation of a CLTZ (Ito, 2008). Moreover, a mixed-foraminiferal assemblage that has previously been recorded in the debrites is dominated by species typical of the upper and middle bathyal zones (1000-1500 m palaeowater depth) similar to faunas in interbedded hemipelagites (Ito, 2008). ...
Channel‐lobe transition zones are dynamic areas located between deepwater channels and lobes. Presented here is a rare example of an exhumed channel‐lobe transition zone from an active‐margin setting, in the Kazusa forearc Basin, Boso Peninsula, Japan. This Plio‐Pleistocene outcrop exposes a thick (tens of metres) channel‐lobe transition zone succession with excellent dating control, in contrast to existing poorly‐dated studies of thinner (metres) deposits in tectonically‐quiescent settings. This high‐resolution outcrop permits the roles of climate and associated relative sea‐level changes on stratigraphic architecture to be assessed. Three development stages are recognised with an overall coarsening‐upward then fining‐upwards trend. Each stage is interpreted to record one obliquity‐driven glacioeustatic sea‐level fall‐then‐rise cycle, based on comparison with published data. Deposition of the thickest and coarsest strata, Stage 2, is interpreted to record the end of a period of relative sea‐level fall. The thinner and finer strata of Stages 1 and 3 formed during interglacial periods where the stronger Kuroshio Oceanic Current, coupled to increased monsoonally‐driven tropical cyclone frequency and intensity, likely resulted in inhibited downslope sediment transfer. A key aspect of channel‐lobe transition zone deposits in this case is the presence of a diverse range of hybrid beds, in contrast to previous work where they have primarily been associated with lobe fringes. Here hybrid bed characteristics, and grain‐size variations, are used to assess the relative importance of longitudinal and vertical segregation processes, and compared to existing models. Compared to channel‐lobe transition zones in tectonically‐quiescent basin‐fills, this channel‐lobe transition zone shows less evidence of bypassing flows (i.e. thicker stratigraphy, more isolated scour‐fills, fewer bypass lags) and has significantly more hybrid beds. These features may be common in active basin channel‐lobe transition zones due to: high subsidence rates; high sedimentation rates; and disequilibrium of tectonically‐active slopes. This disequilibrium could rejuvenate erodible mud‐rich substrate, leading to mud‐rich flows arriving at the channel‐lobe transition zone, and decelerating rapidly, forming hybrid beds.
... During the last decade, lobe beds to lobe complexes have been intensively studied (Kane et al., 2017;Spychala et al., 2017a;Brooks et al., 2018aBrooks et al., , 2018bHansen et al., 2019;Boulesteix et al., 2020a). Hodgson (2009) defines longitudinal (proximal, medial and distal) and lateral subdivisions (axis, off-axis and fringe) of turbiditic lobes. ...
Stratigraphic rift-basin successions have promoted a tremendous amount of tectonostratigraphic studies during the last decades. However, the syn-rift to post-rift transition is still a matter of debate. This PhD thesis presents the case of the Norwegian Sea continental margin, intending to reconstruct the evolution of depositional environments during the syn-rift and post-rift stages. The Norwegian Sea has been chosen for its atypically wide terrace domain across which a tremendous amount of data has been acquired, but never comprehensively integrated. In this study, the identification of fifteen stratigraphic sequences (S1 to S15), combined with a reappraised dinoflagellate cyst zonation, enables the studied stratigraphic interval to be divided into three major periods with (i) the syn-rift period (S1 to S7, dated from the Middle Bathonian to the Middle/Late Berriasian), (ii) the transitional rift-sag period (S8, dated from the Middle/Late Berriasian to the Aptian/Albian boundary) and (iii) the post-rift period (S9 to S15, dated between the Aptian/Albian boundary and the Early Coniacian). Those three major periods fit well into the Atlantic multiphased rift evolution. The syn-rift period is marked by an intense tectonic activity during which coarse-grained deltas have conjointly developed with wave-dominated coasts. The rift-sag stage is a transitional period during which tectonic activity waned between the terrace and the platform domains while fostered the development of deep-water turbidite lobe complexes. The rift-sag phase most probably corresponds to a period of migration of the deformation from the terrace domain to the deep domain. The post-rift stage is defined by a phase of tectonic quiescence coevally with the deposition of thick offshore marine mudstone successions.
... The sandstones that fill the erosion surface are thick-bedded, amalgamated, structureless to parallel laminated with no evidence for hybrid-bed prone facies. Furthermore, the lack of fine-grained heterolithics or bed tops suggest that the flows may have been stripped and finer grain-sizes deposited downdip, making these lobes more similar in character to intraslope lobes than basin-floor lobes (Spychala et al., 2015;Brooks et al., 2018c). Small-scale erosion surfaces towards the top of the sandstone-prone part of the succession are interpreted as either distributary channels where lags are present, or scour-fills, and are linked to a final phase of basinward progradation of the system ( Figure 11E). ...
Scours, and scour fields, are common features on the modern seafloor of deep-marine systems, particularly downstream of submarine channels, and in channel-lobe-transition-zones. High-resolution images of the seafloor have improved the documentation of the large scale, coalescence, and distribution of these scours in deep-marine systems. However, their scale and high aspect ratio mean they can be challenging to identify in outcrop. Here, we document a large-scale, composite erosion surface from the exhumed deep-marine stratigraphy of Unit 5 from the Permian Karoo Basin succession in South Africa, which is interpreted to be present at the end of a submarine channel. This study utilizes 24 sedimentary logs, 2 cored boreholes, and extensive palaeocurrent and thickness data across a 126 km² study area. Sedimentary facies analysis, thickness variations and correlation panels allowed identification of a lower heterolithic-dominated part (up to 70 m thick) and an upper sandstone-dominated part (10–40 m thick) separated by an extensive erosion surface. The lower part comprises heterolithics with abundant current and sinusoidal ripples, which due to palaeocurrents, thickness trends and adjacent depositional environments is interpreted as the aggradational lobe complex fringes. The base of the upper part comprises 2-3 medium-bedded sandstone beds interpreted as precursor lobes cut by a 3–4 km wide, 1–2 km long, and up to 28 m deep, high aspect ratio (1:100) composite scour surface. The abrupt change from heterolithics to thick-bedded sandstones marks the establishment of a new sediment delivery system, which may have been triggered by an updip channel avulsion. The composite scour and subsequent sandstone fill support a change from erosion- and bypass-dominated flows to depositional flows, which might reflect increasingly sand-rich flows as a new sediment route matured. This study provides a unique outcrop example with 3D stratigraphic control of the record of a new sediment conduit, and development and fill of a large-scale composite scour surface at a channel mouth transition zone, providing a rare insight into how scours imaged on seafloor data can be filled and preserved in the rock record.
