Mass Transport Complexes Research Articles

Axial and transverse deep‐water sediment supply to syn‐rift fault terraces: Insights from the West Xylokastro Fault Block, Gulf of Corinth, Greece

AbstractDeep‐water syn‐rift systems develop in partially‐ or transiently‐linked depocentres to form complicated depositional architectures, which are characterised by short transport distances, coarse grain sizes and a wide range of sedimentary processes. Exhumed systems that can help to constrain the tectono‐stratigraphic evolution of such systems are rare or complicated by inversion tectonics. Here, we document a mid‐Pleistocene deep‐water syn‐rift system fed by Gilbert‐type fan deltas in the hangingwall of a rift margin fault bounding the West Xylokastro Horst block, on the southern margin of the Gulf of Corinth, Greece. Structural and stratigraphic mapping combined with digital outcrop models permit observations along this syn‐rift depositional system from hinterland source to deep‐water sink. The West Xylokastro Fault hangingwall is filled by two distinct sediment systems; an axial system fed by coarse‐grained sediment gravity flows derived from fault‐tip Gilbert‐type fan deltas and a lateral system dominated by mass transport deposits fed from an evolving fault‐scarp apron. Abrupt changes in stratigraphic architecture across the axial system are interpreted to record changes in relative base level, sediment supply and tectonics. Locally, depositional topography and intra‐basinal structures controlled sediment dispersal patterns, from bed‐scale infilling of local rugose topography above mass transport complexes, to basin‐scale confinement from the fault scarp apron. These acted to generate a temporally and spatially variable, heterogeneous stratigraphic architecture throughout the basin‐fill. The transition of the locus of sedimentation from a rift margin to a fault terrace through the syn‐sedimentary growth of a basinward fault produced regressive surfaces updip, which manifest themselves as channels in the deep‐water realm and acted to prograde the system. We present a new conceptual model that recognises coeval axial and transverse systems based on the stratigraphic architecture around the West Xylokastro fault block that emphasizes the lateral and vertical heterogeneity of rift basin‐fills with multiple entry points.

Subaqueous debrites of the Grand ConglomÉrat Formation, Democratic Republic of Congo: A model for anomalously thick Neoproterozoic: “Glacial” diamictites

ABSTRACTVery thick (> 1 km) successions of matrix-supported conglomerates (diamictites) are a very distinctive component of many Neoproterozoic basins. Classically interpreted as glacially deposited sedimentary rocks, their thickness has been seen as requiring exceptional depositional conditions such as world-wide “panglacial” climates. The Neoproterozoic Grand Conglomérat Formation (GC) of Katanga Province, southeastern Democratic Republic of Congo, is a 1.8-km-thick diamictite succession hosting one of the world's largest stratiform copper deposits. Examination of more than 300 km of recently acquired large diameter (up to 4 inches, 10.2 cm) core identifies the diamictites of the GC as debrites that accumulated as part of a deep-water “mass-transport complex” in a tectonically active and volcanically influenced anoxic rift basin. Detailed sedimentological descriptions of debrite facies and their lateral and vertical variability permits new insights into processes of debris-flow formation and transport, and their wider paleoenvironmental and paleotectonic significance. A genetic model is herein presented that highlights the importance of slumping and subaqueous downslope mixing of basin-margin fan-delta gravels, fault breccias, volcanic debris from contemporaneous basaltic fissure eruptions with basinal muds, and the downslope ponding of flows in narrow fault-bounded depocenters. Preservation of such an exceptionally thick subaqueously deposited debrite-dominated mass transport complex lacking any evidence of shallow-water deposition, requires rapid subsidence. Any direct sedimentary evidence of a glacial influence on sedimentation in the wider basin hinterland (if present) has been destroyed and homogenized by mass flow. Indirect evidence of a cold-climate setting is possibly expressed as lonestones in laminated turbidite facies dropped by either glacial or seasonal ice, and by exceptionally rare scratched clasts that may have been striated subglacially. Descriptions and interpretations presented here provide clues to the origin of other unusually thick debrite and turbidite successions elsewhere in other Neoproterozoic basins; their primary paleoenvironmental significance is that they appear to record ponding and focusing of mass flows in narrow, rapidly subsiding fault-bounded depocenters, rather than any unique glacial paleoclimate.

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.

Large-scale mass wasting on the Miocene continental margin of western India

Abstract A giant mass-transport complex was recently discovered in the eastern Arabian Sea, exceeding in volume all but one other known complex on passive margins worldwide. The complex, named the Nataraja Slide, was drilled by International Ocean Discovery Program (IODP) Expedition 355 in two locations where it is ∼300 m (Site U1456) and ∼200 m thick (Site U1457). The top of this mass-transport complex is defined by the presence of both reworked microfossil assemblages and deformation structures, such as folding and faulting. The deposit consists of two main phases of mass wasting, each consisting of smaller pulses, with generally fining-upward cycles, all emplaced just prior to 10.8 Ma based on biostratigraphy. The base of the deposit at each site is composed largely of matrix-supported carbonate breccia that is interpreted as the product of debris-flows. In the first phase, these breccias alternate with well-sorted calcarenites deposited from a high-energy current, coherent limestone blocks that are derived directly from the Indian continental margin, and a few clastic mudstone beds. In the second phase, at the top of the deposit, muddy turbidites dominate and become increasingly more siliciclastic. At Site U1456, where both phases are seen, a 20-m section of hemipelagic mudstone is present, overlain by a ∼40-m-thick section of calcarenite and slumped interbedded mud and siltstone. Bulk sediment geochemistry, heavy-mineral analysis, clay mineralogy, isotope geochemistry, and detrital zircon U-Pb ages constrain the provenance of the clastic, muddy material to being reworked, Indus-derived sediment, with input from western Indian rivers (e.g., Narmada and Tapti rivers), and some material from the Deccan Traps. The carbonate blocks found within the breccias are shallow-water limestones from the outer western Indian continental shelf, which was oversteepened from enhanced clastic sediment delivery during the mid-Miocene. The final emplacement of the material was likely related to seismicity as there are modern intraplate earthquakes close to the source of the slide. Although we hypothesize that this area is at low risk for future mass wasting events, it should be noted that other oversteepened continental margins around the world could be at risk for mass failure as large as the Nataraja Slide.

Neogene-Quaternary architecture and sedimentary processes on an isolated carbonate-fed deep-water basin (Fairway Basin, Southwest Pacific)

The Fairway Basin is an intracratonic deep-water basin inherited from cretaceous rifting, perched between the Lord Howe Rise and the New Caledonia Basin in the Southwest Pacific. Its originality lies in its atypical present-day physiography and its carbonate-dominated sedimentary setting since the Miocene. Although very limited well controls are available, its overall structure, seismic stratigraphy and architecture are relatively well known. However, little is known on the detailed sedimentary processes controlling its recent infill. Its remote location from any major clastic sediment sources provides a great setting to study deep-water carbonate sedimentary processes. In this paper, we propose a detailed analysis of surface morphologies based on existing multibeam and subbottom echosounding data together with an analysis of the Neogene to Quaternary basin-fill based on seismic reflection data. This allows us to discuss active sedimentary processes, notably gravity flow processes which are thought to be active since the Middle Miocene. These processes are evidenced by both surface and subsurface features, such as submarine canyons, channels and lobes, composite scarps and mass transport deposits (MTDs). In the northern part of the basin, a 400 km-long axial turbidite system is developed close to the Lansdowne Bank, a partly drowned isolated carbonate platform, where present-day sediment remobilisation seems to be essentially sourced from intra-slope instabilities. These reworked sediments extend over an area of 16,000 km2. Along with this axial turbidite system, seismic data show two main mass transport complexes (covering an area of ca 20% of the basin's surface) originating from the slopes of the basin. The potential triggering mechanisms of MTDs of the northern complex might be directly attributed to overloading and high slope values of the Lansdowne Bank area (>1.5°). For the southern complex, surface slope values are lower (<1.4°), and the role of fluid migration and escape is discussed. The Fairway Basin indeed shows strong evidences for fluid generation (e.g. polygonal faulting, pockmarks, presence of a widespread Opal-A/Opal-CT diagenesis-related bottom simulating reflectors) and fluid overpressures.

Submarine channel network evolution above an extensive mass-transport complex: A 3D seismic case study from the Niger delta continental slope

Abstract A submarine channel network, named Abalama Channel System (ACS), has been recognised in the subsurface of the Niger Delta continental slope. It overlies a mass-transport complex (MTC) and consists of six channel segments, delimited by five avulsion points and one confluence point. High-resolution 3D seismic data are used to investigate the development of the ACS and to describe the interaction between the channels and the underlying MTC. The MTC mainly consists of highly disaggregated materials (MTC matrixes) and in plan-view has a very complex fingered geometry, characterised by the presence of erosional remnants (remnant blocks). The different character of the MTC matrixes compared to that of the remnant blocks likely resulted in a bathymetry characterised by negative and positive relief, which provided the initial confinement for the channels of the ACS. In areas where the MTC-induced confinement was weak or decreased abruptly, channels tended to develop higher sinuosity, increasing channels instability and ultimately causing avulsions. Three ideal categories of submarine channel avulsions are observed. Type 1 is characterised by parent and avulsion channel having similar size and maturity; Type 2 is characterised by a large, high-maturity parent channel and a small, low-maturity avulsion channel; Type 3 emphasizes the larger scale and higher maturity of the avulsion channel compared to the parent channel. In the distal part of the study area, topography related to mud diapirs provided lateral confinement that captured flows avulsed at different times resulting in a channel confluence phenomenon. Submarine channel network evolution recorded by avulsion and confluence points represents an important research theme in deep-water sedimentology, as it controls the final distribution of sediments and the extension of sands in the whole deep-water depositional system; hence this study can be used to guide hydrocarbon exploration in analogue systems.

Strain analysis of a seismically imaged mass‐transport complex, offshore Uruguay

AbstractStrain style, magnitude and distribution within mass‐transport complexes (MTCs) are important for understanding the process evolution of submarine mass flows and for estimating their runout distances. Structural restoration and quantification of strain in gravitationally driven passive margins have been shown to approximately balance between updip extensional and downdip contractional domains; such an exercise has not yet been attempted for MTCs. We here interpret and structurally restore a shallowly buried (c. 1,500 mbsf) and well‐imaged MTC, offshore Uruguay using a high‐resolution (12.5 m vertical and 15 × 12.5 m horizontal resolution) three‐dimensional seismic‐reflection survey. This allows us to characterise and quantify vertical and lateral strain distribution within the deposit. Detailed seismic mapping and attribute analysis shows that the MTC is characterised by a complicated array of kinematic indicators, which vary spatially in style and concentration. Seismic‐attribute extractions reveal several previously undocumented fabrics preserved in the MTC, including internal shearing in the form of sub‐orthogonal shear zones, and fold‐thrust systems within the basal shear zone beneath rafted‐blocks. These features suggest multiple transport directions and phases of flow during emplacement. The MTC is characterised by a broadly tripartite strain distribution, with extensional (e.g. normal faults), translational and contractional (e.g. folds and thrusts) domains, along with a radial frontally emergent zone. We also show how strain is preferentially concentrated around intra‐MTC rafted‐blocks due to their kinematic interactions with the underlying basal shear zone. Overall, and even when volume loss within the frontally emergent zone is included, a strain difference between extension (1.6–1.9 km) and contraction (6.7–7.3 km) is calculated. We attribute this to a combination of distributed, sub‐seismic, ‘cryptic’ strain, likely related to de‐watering, grain‐scale deformation and related changes in bulk sediment volume. This work has implications for assessing MTCs strain distribution and provides a practical approach for evaluating structural interpretations within such deposits.

Tectonic and oceanographic process interactions archived in Late Cretaceous to Present deep‐marine stratigraphy on the Exmouth Plateau, offshore NW Australia

AbstractDeep‐marine deposits provide a valuable archive of process interactions between sediment gravity flows, pelagic sedimentation and thermohaline bottom‐currents. Stratigraphic successions can also record plate‐scale tectonic processes (e.g. continental breakup and shortening) that impact long‐term ocean circulation patterns, including changes in climate and biodiversity. One such setting is the Exmouth Plateau, offshore NW Australia, which has been a relatively stable, fine‐grained carbonate‐dominated continental margin from the Late Cretaceous to Present. We combine extensive 2D (~40,000 km) and 3D (3,627 km2) seismic reflection data with lithologic and biostratigraphic information from wells to reconstruct the tectonic and oceanographic evolution of this margin. We identified three large‐scale seismic units (SUs): (a) SU‐1 (Late Cretaceous)—500 m‐thick, and characterised by NE‐SW‐trending, slope‐normal elongate depocentres (c. 200 km long and 70 km wide), with erosional surfaces at their bases and tops, which are interpreted as the result of contour‐parallel bottom‐currents, coeval with the onset of opening of the Southern Ocean; (b) SU‐2 (Palaeocene—Late Miocene)—800 m‐thick and characterised by: (a) very large (amplitude, c. 40 m and wavelength, c. 3 km), SW‐migrating, NW‐SE‐trending sediment waves, (b) large (4 km‐wide, 100 m‐deep), NE‐trending scours that flank the sediment waves and (c) NW‐trending, 4 km‐wide and 80 m‐deep turbidite channel, infilled by NE‐dipping reflectors, which together may reflect an intensification of NE‐flowing bottom currents during a relative sea‐level fall following the establishment of circumpolar‐ocean current around Antarctica; and (c) SU‐3 (Late Miocene—Present)—1,000 m‐thick and is dominated by large (up to 100 km3) mass‐transport complexes (MTCs) derived from the continental margin (to the east) and the Exmouth Plateau Arch (to the west), and accumulated mainly in the adjacent Kangaroo Syncline. This change in depositional style may be linked to tectonically‐induced seabed tilting and folding caused by collision and subduction along the northern margin of the Australian plate. Hence, the stratigraphic record of the Exmouth Plateau provides a rich archive of plate‐scale regional geological events occurring along the distant southern (2,000 km away) and northern (1,500 km away) margins of the Australian plate.

Mass Transport Complexes on a Cenozoic paleo-shelf edge, Gippsland basin, southeastern Australia

Abstract The Southern Platform of the Gippsland Basin of southeastern Australia contains a mixed carbonate-clastic succession deposited on the outer continental shelf in water depths of 50–200 m. Seismic geometry indicates the presence of likely carbonate build-ups formed in deeper water near the north-eastern edge of the Southern Platform by bryozoan mounds which pass to the northeast into distal bottom set deposits in shelf edge clinoforms. Mass Transport Complexes (MTCs) developed along the paleo-shelf edge and basin bounding faults through repeated failure from the Late Oligocene to the Mid Miocene. Two distinct Mass Transport Complexes are present. The Sailfish MTC affects a zone 7 km wide along the paleo-shelf edge in the SE of the area and features slumped paleo-shelf masses with listric faulting and toe thrusts in places. Some inversion of slump-related structures indicates continued movement after an initial failure event. The Devilfish MTC is up to 32 km wide and consists of a zone of extension which passes downslope into a broad zone of compressional structures which die out downslope without the development of a clear emergent slump mass or a terminal structure. Stratigraphic relationships suggest that it formed progressively by creep from the Late Oligocene to the Late Miocene. The difference in deformation styles between the two MTCs can be explained by proximity to major basin forming faults. The Devilfish MTC formed where the paleo–shelf edge sat adjacent to an underlying fault which juxtaposes deeper basin fill sediments against basement. This geometry provided abundant pore water to weaken the overlying sediments which failed under differential loading imposed by shelf edge build ups to form the MTC, possibly triggered by repeated earthquakes. Devilfish MTC differs from mass transport deposit described in the literature in comprising an extensional headwall zone and a main body affected by compressional structures which then die out downslope, probably because of thickening of the section and a return to normal porewater pressures basin-ward. This type of MTC could be termed “Frontally Diffuse” in contrast to the recognised Frontally Emergent and Frontally Confined MTC terminations.

Erosion and ponding of Thunder Horse deepwater turbidites by mass transport complexes in Mississippi Canyon based on image log sedimentology

Abstract Mass transport complexes are often associated with deepwater turbidite reservoirs, but typical subsurface data sets are of insufficient resolution or coverage to determine if and how the mass transport complexes have an impact on reservoir architecture. We present a case study of integrated, high resolution data (core, image logs, and seismic) from Thunder Horse, one of the largest deepwater oil and gas discoveries in the Gulf of Mexico, to illustrate how mass transport complexes can both compartmentalize and pond reservoirs. The key data type enabling these conclusions are image logs, which reveal erosive mass transport complexes whose emplacement removed the upper portion of one of the Thunder Horse sandstone reservoirs on the northwest side of the asset and resedimented the sand as multiple smaller compartments. Stacked seafloor landslide deposits interbedded with turbidite sandstone reservoirs are herein characterized in detail from image logs at Thunder Horse, located in lower Mississippi Canyon. Dips from bedding boundaries on image logs in the sandstone reservoirs show little variation in structural tilt ( Unconformable bedding contacts at the tops of the sandstone reservoirs in northern Thunder Horse suggest scouring and erosion, whereas basal sandstone contacts are in conformance with overall structural dip, suggesting amalgamation. On seismic data, the mass transport complexes stack and mound together, forming a paleo-topographic high with at least 30 m of relief. Core acquired in this mudrock-dominated deposit reveals convoluted and folded bedding capped by 0.5 m of highly bioturbated marl that is interpreted as a Glossifungites hiatal surface. The implication is that the mass transport complexes formed a topographic obstacle that diverted subsequent debris and turbidite flows, and later created localized ponding and thickening of the overlying reservoir. Thus, mass transport complexes were found to both erode and augment sandstone reservoirs. Biostratigraphic dating of the two reservoirs brackets their deposition as occurring in As a case study this type of integrated, high resolution data has wide applicability to other deepwater, sub-salt reservoirs, as improved structural and depositional interpretations inform reservoir performance and impact future well plans. Further, recognition of mass transport complexes as agents of reservoir compartmentalization and sandstone ponding allows for more accurate reserves estimation.

Coherence attribute applications on seismic data in various guises — Part 1

The iconic coherence attribute is very useful for imaging geologic features such as faults, deltas, submarine canyons, karst collapse, mass-transport complexes, and more. In addition to its preconditioning, the interpretation of discrete stratigraphic features on seismic data is also limited by its bandwidth, where in general the data with higher bandwidth yield crisper features than data with lower bandwidth. Some form of spectral balancing applied to the seismic amplitude data can help in achieving such an objective so that coherence run on spectrally balanced seismic data yields a better definition of the geologic features of interest. The quality of the generated coherence attribute is also dependent in part on the algorithm used for its computation. In the eigenstructure decomposition procedure for coherence computation, spectral balancing equalizes each contribution to the covariance matrix, and thus it yields crisper features on coherence displays. There are other ways to modify the spectrum of the input data in addition to simple spectral balancing, including the amplitude-volume technique, taking the derivative of the input amplitude, spectral bluing, and thin-bed spectral inversion. We compare some of these techniques, and show their added value in seismic interpretation, which forms part of the more elaborate exercise that we had carried out. In other work, we discuss how different spectral components derived from the input seismic data allow interpretation of different scales of discontinuities, what additional information is provided by coherence computed from narrow band spectra, and the different ways to integrate them.

Stratal architecture and evolution of a slope mass-transport complex, Isaac Formation, Neoproterozoic Windermere Supergroup, southern Canadian Cordillera, British Columbia, Canada

Abstract Detailed sedimentological and stratigraphic analyses of a c. 1500 m thick, siliciclastic-dominated slope succession in the Neoproterozoic Isaac Formation at the Castle Creek study area (southern Canadian Cordillera) reveals the occurrence of four well-preserved mass-transport complexes (MTCs) composed principally of slide/slump and debris-flow deposits. The stratigraphically lowest of these complexes is about 60 m thick and crops out for &gt;2.5 km laterally, consisting of slide and debrite. The slide has an irregular erosive base with ramp-and-flat geometry. This is overlain locally by boulder-sized blocks of slightly to moderately deformed strata, bounded by shear surfaces. The slide is overlain by a debrite that pinches and swells laterally, consisting of matrix-supported conglomerate with common metre-scale clasts of mudstone and coarse-grained sandstone embedded in a mudstone-rich matrix with dispersed, pebble quartz grains. Based on its stratigraphic position at the base of the slope, vertical stacking of slide-debrite, lithological distribution, considerable thickness and lateral extent, this MTC is interpreted to be associated with a major episode of continental slope instability and submarine mass-wasting. The close association between the MTC and underlying/overlying mixed carbonate-siliciclastic strata suggests that sea level most likely exerted a key control on sediment supply, which ultimately led to the emplacement of this MTC.

Episodic fluid flow as a trigger for Miocene‐Pliocene slope instability on the Utgard High, Norwegian Sea

AbstractMass wasting is triggered on many continental slopes by a number of mechanisms, including seismic shaking, high sedimentation rates, the presence of weak geological units and gas hydrate dissociation. In this study, the morphology of a Late Miocene–Early Pliocene mass‐transport complex (MTC) on the Utgard High is unravelled and discussed in relation to possible trigger mechanisms. The approach used here includes 3D seismic interpretation and the analysis of variance attribute maps. The interpreted MTC is located on the crest and flanks of the Utgard High and is composed of three mass‐transport deposits with seismic characters varying from transparent and chaotic seismic facies at the base to slightly deformed layers composed of mounds and rafted blocks in the middle and chaotic to transparent reflections at the top. Lithologically, the MTC consists predominantly of claystone with high gamma ray and low density and resistivity values, demonstrating that the associated mounds represent remobilized ooze sediments. A vertical stack of six magmatic sills emplaced from 55.6 to 56.3 Ma into the Upper Cretaceous shales is interpreted at depths of 3,000–5,500 ms two‐way travel time (TWTT). In association with these magmatic sills are several hydrothermal vent complexes that interacted with the top MTC horizon, signifying that episodic and secondary fluid‐venting events might be the principal mechanism facilitating mass wasting in the study area. In addition, the remobilization of ooze sediments into mounds is hypothesized to be dependent on fluids and clayey layers. As a corollary of this work, the importance of relict and recurrent episodes of fluid flow in the Vøring Basin and their influence on the geotechnical integrity of the overburden and later mass wasting is established.

Bottom currents, submarine mass failures and halokinesis at the toe of the Sigsbee Escarpment (Gulf of Mexico): Contrasting regimes during lowstand and highstand conditions?

In this study we explore the role of sediment supply, halokinesis and deep ocean circulation in promoting margin instability. The analysis was carried out on multibeam and high-resolution seismic data that allowed the imaging of mass failure deposits and current-driven depositional features along a portion of the lower continental slope and upper continental rise of the Sigsbee Escarpment (Gulf of Mexico). Different styles of deposition have been recognised during sea level lowstand (LST) and highstand (HST) conditions, due to alternating bottom current activity and salt tectonics. Lowstands are characterized by a reduced intensity of the Loop Current, as underlined by the lack of current-driven erosional features. On the contrary, highstands show a strengthened Loop Current that generates a fast bottom current circulation, as suggested by the presence of extensive furrow fields on the modern sea floor and on the Marine Isotope Stage 5e palaeo-sea floor horizon. Increased sediment load combined with changes in the intensity of deep water circulation are also responsible for the instability of the Sigsbee Escarpment, triggering mass failure phenomena with distinct morphology, size, location and timing of emplacement. Type 1 mass transport complexes (MTCs) form on the upper continental rise during sea level fall, and are genetically linked to the growth of deep water sediment drifts. Type 2 MTCs develop during sea level lowstands and originate along the slope of the Sigsbee Escarpment, triggered by oversteepening generated by halokinesis. Type 3 MTCs form during sea level rise to highstand conditions and mostly consist of debris flow deposits, generated in the lower portions of the Sigsbee Escarpment and then accumulated in the upper continental rise.

Entrainment and abrasion of megaclasts during submarine landsliding and their impact on flow behaviour

Abstract Many mass transport complexes (MTCs) contain up to kilometre-scale (mega)clasts encased in a debritic matrix. Although many megaclasts are sourced from the headwall areas, the irregular basal shear surfaces of many MTCs indicate that megaclast entrainment during the passage of flows into the deeper basin is also common. However, the mechanisms responsible for the entrainment of large blocks of substrate, and their influence on the longitudinal behaviour of the associated flows, have not been widely considered. We present examples of megaclasts from exhumed MTCs (the Neuquén Basin, Argentina and the Karoo Basin, South Africa) and MTCs imaged in three-dimensional seismic reflection data (Magdalena Fan, offshore Colombia and Santos Basin, offshore Brazil) to investigate these process–product interactions. We show that highly sheared basal surfaces are well developed in distal locations, sometimes extending beyond their associated deposit. This points to deformation and weakening of the substrate ahead of the flow, suggesting that preconditioning of the substrate by distributed shear ahead of, and to the side of, a mass flow could result in the entrainment of large fragments. An improved understanding of the interactions between flow evolution, seabed topography, and the entrainment and abrasion of megaclasts will help to refine estimates of run-out distances, and therefore the geohazard potential of submarine landslides.

Tectono-stratigraphic evolution of the western margin of the Levant Basin (offshore Cyprus)

Abstract Stratigraphic interpretation of twenty-four 2D seismic reflection profiles coupled with available well data from the Ocean Drilling Program (ODP) offshore Cyprus enabled identification of the main unconformities and the seismic packages in the western part of the Levant Basin and their correlation with major geodynamic events. The basic concepts of seismic stratigraphy were applied to improve our understanding of the tectonostratigraphy and the sedimentary architecture of Eratosthenes isolated carbonate platform. This platform is composed of dominant retrograding and aggrading Mesozoic carbonate build-ups and its evolution partly resembles that of the Egyptian margin. Deep-water mixed carbonates and siliciclastics probably prevailed in the basin during the Early Cretaceous. During the Late Cretaceous, these platforms were drowned. Following the convergence of the two plates, both the Eratosthenes Seamount (ESM) area and the Levant Basin progressively became part of a foreland basin along the Cyprus Arc. Different units in the Eratosthenes carbonate platforms (ECPs) and the transition to the deep Levant Basin are detailed here for the first time. We present a 3D conceptual model for the evolution of the western margin of the Levant Basin. We discuss the effect of the Miocene collision between the Cyprus Arc and Eratosthenes to understand the presence of upper Miocene Mass transport complexes (MTCs) in the deep basin and the tilting of the top of the adjacent ECPs. We briefly examine the deformation mechanisms of the Messinian salt near the paleo-shelves of the ECPs.

3D seismic investigation of the structural and stratigraphic characteristics of the Pagasa Wedge, Southwest Palawan Basin, Philippines, and their tectonic implications

The Pagasa Wedge is a poorly imaged deepwater orogenic wedge that has been variously interpreted as representing an accretionary prism, a former accretionary prism modified by thrusting onto a thinned continental margin, and a gravity-driven fold-thrust belt. This study, using 2D and 3D seismic data, together with well information indicates that at least the external part of the wedge is dominantly composed of mass transport complexes, capped by syn-kinematic sediments that have thrusts and normal faults superimposed upon them. Drilling shows that despite stratigraphic repetition of Eocene Middle Miocene units, there is stratigraphic omission of Oligocene and Early Miocene units. This absence suggests that mass transport processes have introduced the Eocene section into the wedge rather than tectonic thrusting. The accretionary prism stage (Oligocene) of the Central Palawan Ophiolite history appears to be marked by predominantly north-vergent deformation. The Deep Regional Unconformity (∼17 Ma) likely indicates the approximate time when obduction ceased in Palawan. The Pagasa Wedge is a late-stage product of the convergence history that was active in its final phase sometime above the top of the Nido Limestone (∼16 Ma) and the base of the Tabon Limestone in the Aboabo-A1X well (∼9 Ma). The top of the wedge is traditionally associated with the Middle Miocene Unconformity (MMU), However the presence of multiple unconformities, diachronous formation tops, local tectonic unconformities and regional diachronous events (e.g. migrating forebulges) all suggest simply giving a single age (or assigning a single unconformity, such as the MMU as defining the top of the Pagasa Wedge is inappropriate. The overall NE-SW trend of the wedge, and the dominant NW transport of structures within the wedge diverge from the more northerly transport direction determined from outcrops in Palawan, and also from the Nido Limestone in the SW part of the Pagasa Wedge. Possibly this NW transport direction is more related to gravity-driven structures responding to uplift of NE-SW Dangerous Grounds margin during the Middle Miocene (related to slab breakoff?) than it is to thrusting rooted in a plate boundary. The final modification of the wedge occurred when the effects of compression deformation on the wedge had largely ended, but gravity processes (in particular mass transport and normal faulting) still operated.

Sedimentary dynamics and high-frequency sequence stratigraphy of the southwestern slope of Great Bahama Bank

New geophysical data from the leeward slope of Great Bahama Bank show how contour currents shape the slope and induce re-sedimentation processes. Along slope segments with high current control, drift migration and current winnowing at the toe of slope form a deep moat. Here, the slope progradation is inhibited by large channel incisions and the accumulation of large mass transport complexes, triggered by current winnowing. In areas where the slope is bathed by weaker currents, the accumulation of mass transport complexes and channel incision is rather controlled by the position of the sea level. Large slope failures were triggered during the Mid-Pleistocene transition and Mid-Brunhes event, both periods characterized by changes in the cyclicity or the amplitude of sea-level fluctuations. Within the seismic stratigraphic framework of third order sequences, four sequences of higher order were identified in the succession of the upper Pleistocene. These higher order sequences also show clear differences in function of the slope exposure to contour currents. Two stochastic models emphasize the role of the contour currents and slope morphology in the facies distribution in the upper Pleistocene sequences. In areas of high current influence the interplay of erosional and depositional processes form a complex facies pattern with downslope and along strike facies alterations. In zones with lower current influence, major facies alternations occur predominately in downslope direction, and a layer-cake pattern characterizes the along strike direction. Therefore, this study highlights that contour currents are an underestimated driver for the sediment distribution and architecture of carbonate slopes.

Messinian canyons in the Turkish western Black Sea

Abstract Several canyons are observed along the Turkish margin of the western Black Sea that are associated with a prominent unconformity and interpreted to be the manifestations of the sea-level fall during the Messinian salinity crisis in the Mediterranean. In this study, their morphology, geometry and fill characteristics, as well as downslope evolution, are compared and contrasted using four 3D seismic surveys and some 2D regional seismic lines. Two types of canyon morphologies are observed in the study area: (1) shelf incising and (2) blind. Located in the western part of the study area and deeply incised into a wide shelf, the Karaburun Canyon extends roughly in a SW–NE direction. The fill of the canyon is almost absent on the shelf, where the canyon base is downlapped by a series of Pliocene clinoforms. A thin fill appears on the upper slope, which gets thicker towards the lower slope. The eastern part of the study area is dominated by a series of blind canyons (Boğaziçi canyons). They are typically confined to the continental rise, with their heads hardly reaching the lower slope. Their fill is entirely characterized by mass-transport complexes (MTCs). It is concluded that during the Messinian lowstand, the sediments within the Karaburun Canyon bypassed the wide shelf and were funnelled down to the continental rise and abyssal plain through the slope, which was followed by progradation of the basin margin during the relative sea-level rise in the Pliocene. A minimal imprint by tectonics in that particular area might have helped establish more stable conditions for the development of a relatively mature sediment dispersal system extending from the hinterland down to the basin centre. In this area, the shelf-slope morphology was dominantly shaped by the depositional geometries of the sedimentary packages. Being fully confined to the continental rise, the Boğaziçi canyons are situated in an area where shelf-slope morphology is governed by the Late Cretaceous volcanic arc. Parallel with the coastline, these volcanic edifices have created fairly steep dips; thus, leading to the development of an unstable basin margin and favouring MTC deposition at least since the Early–Middle Miocene. The width and relief of the canyons display a decreasing trend from west to east, which may be attributed to their relative distance from a possible drainage system in the vicinity of the Bosporus that might have acted as the major sediment supplier during this period.

The role of fluid migration in the occurrence of shallow gas and gas hydrates in the south of the Pearl River Mouth Basin, South China Sea

High-amplitude anomalies (HAAs), bottom-simulating reflectors (BSRs), vertical acoustic blanking features, polygonal faults, and mass transport complexes are observed in high-resolution 3D seismic data from the south of the Pearl River Mouth Basin, South China Sea. We have focused on the role of fluid migration in the occurrence of shallow gas and gas hydrates. HAAs are interpreted to be shallow gas accumulations or gas hydrates, depending on their polarity and the presence of BSRs. Acoustic blanking features are probably caused by the presence of gas chimneys and overlying free gas due to features including chaotic and weak seismic reflections, pull-downs, and long distances of vertical extension. These gas chimneys root into the underlying late Oligocene gas reservoir. There is a close spatial relationship among the gas reservoir, gas chimneys, shallow gas accumulations, and BSRs, suggesting a genetic link. We have developed a schematic model to explain the processes of fluid migration, which supplied gas to the shallow gas accumulations and the gas hydrate system. When the gas reservoir was critically pressured, the focused fluid migration pathways may be formed or be reactivated, and gas was transported vertically along these conduits to the shallow strata. Then, there were two destinies for the migrated gas at shallow depths. One is that gas was trapped below low-permeability seals, such as mudstone or gas hydrates, forming shallow gas accumulations or free gas zones. The other one is that gas entered the gas hydrate stability zone and formed gas hydrates. In addition, this research indicates that the shallow hydrocarbon resource in this area may not be very prolific probably due to limited fluid migration.