Submarine Mass-transport Deposits Research Articles

Seismic architecture of Yongle isolated carbonate platform in Xisha Archipelago, South China Sea

This study presented recently reprocessed multi-channel seismic data and multi-beam bathymetric map to reveal the geomorphology and stratigraphic architecture of the Yongle isolated carbonate platform in the Xisha Archipelago, northwestern South China Sea. Our results show that the upper slope angles of Yongle carbonate platform exceed 10° and even reach to ∼32.5° whereas the lower slope angles vary from .5° to 5.3°. The variations of slope angles show that margins of Yongle Atoll belong to escarpment (bypass) margins to erosional (escarpment) margins. The interior of carbonate platform is characterized by sub-parallel to parallel, semi-continuous to continuous reflectors with medium-to high-amplitude and low-to medium-frequency. The platform shows a sub-flat to flat-topped shape in its geometry with aggradation and backstepping occurring on the platform margins. According to our seismic-well correlation, the isolated carbonate platform started forming in Early Miocene, grew during Early to Middle Miocene, and subsequently underwent drowning in Late Miocene, Pliocene and Quaternary. Large-scale submarine mass transport deposits are observed in the southeastern and southern slopes of Yongle Atoll to reshape the slopes since Late Miocene. The magmatism and hydrothermal fluid flow pipes around the Yongle Atoll have been active during 10.5–2.6 Ma. Their activity might intensify dolomitization of the Xisha isolated carbonate platforms during Late Miocene to Pliocene. Our results further suggest that the Yongle carbonate platform is situated upon a pre-existing fault-bounded block with a flat pre-Cenozoic basement rather than a large scale volcano as previously known and the depth of the basement likely reached to 1400 m, which is deeper than the well CK-2 suggested.

Assessing the sealing quality of submarine mass transport complexes and deposits

Mass transport complexes and their associated mass transport deposits—both referred to here as submarine failure deposits—are virtually ubiquitous in the modern and ancient sedimentary record of many deepwater basins. As exploration expands to new frontiers, submarine failure deposits are being identified as components of the petroleum system in numerous prospects. Although such deposits were historically considered facies to avoid from a viewpoint of petroleum exploration, it is now recognized that they can act as source, reservoir, or seal elements. In this paper, we set out to investigate the role of submarine failure deposits as effective seals in the petroleum system using published data and propose a methodology to risk some first-order factors at a macro-, meso-, and micro-scale that influence the seal quality of these submarine failure deposits. We accomplish this by discussing the properties intrinsic to submarine failure deposits that affect the seal quality at different scales. Based on published literature, at least six offshore fields from the Gulf of Mexico and NW Borneo are reported to have a submarine failure deposit as an effective seal. These fields combined account for ∼0.9 billion barrels of oil equivalent of cumulative discovered reserves globally and prove the potential of submarine failure deposits as effective seals and a future undervalued play concept. We use three case studies to illustrate our methodology. Our methodology can be further customized using datasets from industry and public records. The seal risk matrix presented here is based on more than a decade of research and has been already used by several exploration companies with encouraging feedback. We acknowledge the limitations of the methodology, but future interdisciplinary research and integration of new datasets and results will improve the de-risking of submarine failure deposits in exploration. Additionally, this methodology can potentially be applied to assessing seal potential of submarine failure deposits for carbon capture sequestration and storage projects.

Large-scale mass-transport deposits recording the collapse of an evaporitic platform during the Messinian salinity crisis (Caltanissetta basin, Sicily)

The Messinian primary gypsum sequence, formed during the first stage of the salinity crisis in the Caltanissetta basin (Sicily), is characterized by a high degree of disruption and deformation. The primary gypsum occurs as huge (up to 3 km large and up to 250 m high) tilted blocks, detached from their stratigraphic base, floating within a matrix of folded shales and clastic gypsum. This association represents a giant chaotic unit lying unconformably on the Gela Nappe, the youngest structural unit of the Maghrebian fold-and-thrust belt. The origin of the deformations affecting the Primary Lower Gypsum unit is still debated; in the past they have been related to intra-Messinian tectonics or to the collapse of primary gypsum platforms due dissolution of interbedded to halite. Our study on the distribution, size and bedding of the gypsum blocks reveals a pattern similar to that of modern giant submarine mass-transport deposits, which may have been triggered by a combination of active tectonics and sea-level change, and favored by the strong mechanical contrasts of the involved units. The chaotic unit shows a downslope evolution from the south, characterized by elongated blocks with their main axes parallel to the inferred slide headwall (NW-SE oriented), to intermediate and distal areas, where the blocks become progressively smaller in size with their main axes parallel to the flow (SW-NE). We identify the source area as either a shallow evaporitic basin above the Gela Nappe or on the Pelagian foreland ramp. A general implication for the Messinian salinity crisis is that no primary gypsum was deposited in deep water basins during the first stage; instead, during the second stage, large gypsum blocks collapsed from the basin margin and were mass-transported into a deeper water setting, where halite was depositing.

Non-evaporitic gypsum formed in marine sediments due to sulfate-methane transition zone fluctuations and mass transport deposits in the northern South China Sea

Gypsum (CaSO4·2H2O) of non-evaporitic origins has been observed in marine sedimentary environments over the past decade. Our understanding of related diagenetic processes has improved by analyzing the sulfur and oxygen isotopic compositions of gypsum, but, a comprehensive study of the isotopic composition of authigenic gypsum precipitated in methane-rich marine environments has not been performed to elucidate their implications for the sulfur cycle (e.g., sulfide oxidation). Furthermore, there are no studies of authigenic gypsum associated with submarine mass transport deposits, which are common sedimentary features on continental slopes. In this study, we analyzed the sulfur and oxygen isotopic compositions of gypsum and the sulfur isotopic composition of pyrite in drill cores collected from methane hydrate-bearing sites GMGS2–08, GMGS2–16, and GMGS4-W02B in the northern South China Sea to determine the formation mechanisms of authigenic gypsum. The stable sulfur isotopic analyses of gypsum and co-existing pyrite revealed that sulfate produced by pyrite oxidation contributed to gypsum precipitation. The low oxygen isotopic composition values of gypsum (−2.6 to 4.5‰ Vienna Standard Mean Ocean Water) suggest that pyrite is oxidized by metal oxides under anaerobic conditions, incorporating water oxygen, whose isotopic composition was modified by gas hydrate formation, into the produced sulfate. As pyrite oxidation also releases protons (H+) into the porewater, calcium concentrations may be elevated by the dissolution of carbonate minerals. At sites GMGS2–08 and GMGS2–16, authigenic gypsum was precipitated within paleo sulfate-methane transition zones, indicating that gypsum formation is probably associated with the downward migration of the sulfate-methane transition zone, which causes anaerobic pyrite oxidation at its original site of formation. However, at site GMGS4-W02B, most of the gypsum was distributed in a mass transport deposit characterized by a high abundance of reworked foraminiferas between the two paleo sulfate-methane transition zones. Given that seawater sulfate contributes more (>85%) than pyrite oxidation to the gypsum sulfate at site GMGS4-W02B, it is believed that authigenic gypsum formation is associated with mass transport deposits, which can rapidly trap overlying seawater sulfate within the pore space of the newly deposited sediments, combined with the sulfate and calcium from pyrite oxidation, leading to a local gypsum supersaturation.

Mid-Eocene giant slope failure (sedimentary mélanges) in the Ligurian accretionary wedge (NW Italy) and relationships with tectonics, global climate change and the dissociation of gas hydrates

Upper Lutetian–Bartonian sedimentary mélanges, corresponding to ancient mud-rich submarine mass transport deposits, are widely distributed over an area c. 300 km long and tens of kilometres wide along the exhumed outer part of the External Ligurian accretionary wedge in the Northern Apennines. The occurrence of methane-derived carbonate concretions (septarians) in a specific tectonostratigraphic position below these sedimentary mélanges allows us to document the relationships among a significant period of regional-scale slope failure, climate change (the Early and Mid-Eocene Optimum stages), the dissociation of gas hydrates and accretionary tectonics during the Ligurian Tectonic Phase (early–mid-Lutetian). The distribution of septarians at the core of thrust-related anticlines suggests that the dissociation of gas hydrates was triggered by accretionary tectonics rather than climate change. The different ages of slope failure emplacement and the formation of the septarians support the view that the dissociation of gas hydrates was not the most important trigger for slope failure. The latter occurred during a tectonic quiescence stage associated with a regressive depositional trend, and probably minor residual tectonic pulses, which followed the Ligurian Tectonic Phase, favouring the dynamic re-equilibrium of the External Ligurian accretionary wedge. Our findings provide useful information for a better understanding of the factors controlling giant slope failure events in modern accretionary settings, where they may cause tsunamis.

Sediment properties in submarine mass-transport deposits using seismic and rock-physics off NW Barents Sea

Submarine mass movements may have profound effects on the morphology and stratigraphic architecture of continental margins. Furthermore, they can represent a threat to human life and infrastructures, and also have implications for hydrocarbon exploration/production. In polar regions, they are one of the predominant sedimentary processes on the continental slope. Exploration seismology has been widely employed to study mass-transport deposits (landslides, glacigenic debris flows, mass flows, etc.), which are usually characterized by chaotic reflections. In this study, we analyse a seismic profile acquired in the southern part of the Storfjorden trough mouth fan (NW Barents Sea margin), showing the presence of two submarine mass-transport deposits (MTDs). A giant MTD (PLS-1) is located on the lower continental slope at 2.6–3km depth, while a more recent MTD (PLS-2) occurs at 1.9–2.4km depth. Velocity and attenuation seismic tomography, seismic attributes analyses and rock-physics models reveal distinct petrophysical properties for PLS-1 and PLS-2. Despite the known influence of burial depth(s), fluid flow content, and compaction on the internal character of MTDs, the two deposits studied here, in fact, show distinct petrophysical characteristics that reflect lithological variations - more than to any other control. These results suggest different source areas for the two MTDs. The inferred coarser sediment in PLS-1 indicates provenance from areas with abundant glacigenic debris flows, (such as the Bjørnøya Trough-Mouth Fan). Conversely, the finer, relatively fluid-rich sediments of PLS-2 that underwent little translation could have an origin in the area between trough-mouth fans. Here, slow-moving ice resulted in a relatively scarce release of subglacial debris at the shelf edge and the continental slope was subject to enhanced erosion and degradation with a comparatively higher production of relatively fine-grained turbidite flows.

Micropaleontological and taphonomic characteristics of mass transport deposits in the northern Gulf of Eilat/Aqaba, Red Sea

Submarine mass transport deposits (MTDs) are a well-known phenomenon in tectonically active regions. Evidence for such deposits is commonly found in the continental slope sedimentary records, as distinct units with coarser grain size compared to the usual and continuous pelagic sedimentation. The Gulf of Eilat/Aqaba is located between the southernmost end of the Dead Sea transform and the spreading center of the Red Sea, and is considered as an active tectonic region.In this study, an innovative approach using symbiont-bearing Larger Benthic Foraminifera (LBF) to identify MTDs in the Gulf of Eilat/Aqaba (GEA) sedimentary record is presented. The abundance, size and preservation state of LBF shells were analyzed in two radiocarbon dated sediment cores collected at different deposition environments, at water depth of 532m and 316m.The microfaunal and taphonomic results show that the coarse units are characterized by a generally higher numerical abundance of LBF, dominated by Operculina ammonoides, Amphistegina papillosa and Amphistegina bicirculata. These benthic assemblages are found in deeper depths than their original habitat, ranging between 50 and 120m, in accordance with their symbionts light requirements. In the coarse units, LBF >1mm appear in high frequency, up to 161 specimens per g sediment, and poorly preserved shells are also abundant, containing up to 247 specimens per g sediment. In addition, these units also contain high numbers of yellowish and blackish colored LBF shells, as opposed to null in the non-disturbed units, and unlike their natural white color.The large shell size indicates that high energy is involved in the displacement of the sediments. The poor state of preservation also suggests a turbulent flow during transportation, which requires a high-energy triggering mechanism. The color alteration is probably associated with a diagenetic process related to increasing burial time/depth, also supported by the stratigraphic older ages of the MTDs, suggesting a long burial before the sediments were displaced. In addition, according to the dating of the record, some units correlate with historical and pre-historical earthquakes, reinforcing LBF species as a reliable proxy for mass transport events.

Sedimentary characteristics and processes of submarine mass-transport deposits in the Ulleung Basin and their relations to seismic and sediment physical properties

Three successive Pliocene MTD units (M2, M3, and M4 in ascending order) in the Ulleung Basin are characterized by continuous strong-amplitude negative basal reflections with transparent to chaotic internal seismic facies. Mapping of these MTD units in a dense 2-D grid of seismic reflection data reveals that each MTD unit generally thickens upslope to a maximum of 113m and wedges out downslope against a structurally uplifted pre-existing surface. Although the source failure scars cannot be traced on the slope, the minimum runout distances of the MTD units from the base of the upper slope near the 1000m isobath are estimated to be >120km.Each MTD unit is characterized in the logging-while-drilling (LWD) data by a gradual increase in bulk density, P-wave velocity, and resistivity values at the basal part and an abrupt decrease at the lower boundary. Nine sedimentary facies are identified in the MTD units, which indicate a variety of mass-transport processes such as sliding with brittle to plastic deformations and high-to low-viscosity debris flows. Vertical distribution of sedimentary facies shows a repeated generalized pattern in all MTD units. The pattern is interpreted to represent initial deep failures involving sand-prone consolidated sediments near the shelf break and successive shallow failures involving soft heterogeneous muddy sediments that were deposited in a deeper downslope area. The presence of sand-prone mass-transport deposits accounts for the basal densification and strong-amplitude negative basal reflections. Absence of such sand-prone basal parts in MTDs at other locales implies unpredictability of lithologic characteristics in MTDs with similar seismic reflection signatures.

Multi-event oceanic island landslides: New onshore-offshore insights from El Hierro Island, Canary Archipelago

This paper presents a combined onshore-offshore morpho-structural characterization of the El Golfo giant landslide, island of El Hierro (Canary Archipelago, Spain). Offshore data from multibeam echosounders, chirp sub-bottom profiles and multichannel seismic reflection data and onshore data coming from water wells and galleries have been analyzed. The geomorphology and the internal architecture of this giant flank collapse are defined in order to determine its multi-event nature. The subaerial headscarp shows a non-continuous arcuate profile formed by two nested semi-circular amphitheaters that extend offshore along a smooth chute, suggesting the occurrence of at least two large retrogressive events. Channels/gullies and escarpments developed along the submarine sector of the scar also indicate smaller-scale events and predominance of sediment bypass. In the lower slope, two subunits of submarine mass transport deposits (MTDs), debris avalanche, are identified on multichannel seismic reflection profiles with a total estimated volume of 318km3: ~84km3 and ~234km3, for the younger and older MTDs respectively. Data from wells and galleries show abrasion platforms with beach deposits at sea-level (0masl) formed after the landslide scar and buried by the post-collapse El Golfo lavas infill, suggesting an age at least older than 23.5–82.5Ka for the landslide.

Stratigraphy of the Neoproterozoic Damara Sequence in northwest Namibia: Slope to basin sub-marine mass-transport deposits and olistolith fields

The Neoproterozoic Damara Sequence (>1000m thick) is composed of siliciclastic and carbonate rocks that crop out in the Damara Belt, Namibia. In Damaraland (including the Vrede, Bethanis, Austerlitz and Toekoms farms), these rocks were deformed and metamorphosed under greenschist facies (biotite zone) conditions during the Damara Orogeny. The stratigraphy and paleoenvironments of the Damara Sequence rocks are debated by the scientific community. We use field data, including detailed 1:25,000 geological mapping, elaboration of stratigraphic profiles and observation of preserved primary structures, textures and composition, to identify lithofacies and lithofacies associations, and to interpret lateral stratigraphic relations.The Nosib and Otavi/Swakop groups of the Damara Sequence in the study area are herein interpreted as gravitational deposits, including up to 150m thick slide blocks, related to the opening of the Outjo Basin. Debrites of the Nabis Formation (Nosib Group) are basement derived, and were developed in alluvial fan setting. Siliciclastic, carbonate and mixed successions of the Otavi/Swakop groups reflect slope to basin deposits and can be grouped in two tectonic sequences. The lower sequence consists mainly of slope and proximal submarine fan successions; medium to distal fan deposits characterize the upper sequence. A U–Pb SHRIMP zircon dating of a dacite layer yielded a 757±5Ma age for the base of the lower sequence. In the study area these sequences are limited by an unconformity. This unconformity records basin margin onlap and was later partially reactivated as a thrust fault during Pan-African compression.The Chuos and Ghaub formations, considered by several authors as Sturtian and Marinoan glaciogenic deposits, are interpreted alternatively as gravitational subaqueous mass flows. Rudstone beds similar to those of the Ghaub Formation occur in the Berg Aukas and Karibib formations and reinforce this interpretation. Large blocks, locally containing stromatolites are interpreted as olistoliths and isolated blocks (pebble to boulder size), are interpreted as sieved from gravitational flows, leaving a non glacial interpretation for these formations as a distinct possibility.

Mass-transport deposits within basinal carbonates from southern Italy

A detailed characterisation of submarine mass-transport deposits (MTDs), in terms of both emplacement processes and internal architecture of depositional products, is crucial to define the hydraulic properties of slope-to-basinal deposits.The Late Jurassic-Early Cretaceous basinal Maiolica Formation exposed in the Gargano Promontory (southern Italy) represents an ideal natural laboratory to study the complex stratigraphic architecture of ancient MTDs. This formation consists of undisturbed intervals of flat-lying, thin-bedded, cherty micritic limestone inter-bedded with intervals of lithologically similar, but chaotic strata that are characterized by significant internal distortion. The stratigraphic thickness of these deformed units, which are interpreted to represent several types of mass movements (e.g., slumps and, to a lesser extent, slides and debris flows), varies from several decimetres to tens of metres. The internal deformation features comprise down-slope verging folds, together with both normal and reverse faults. In several places, the studied MTDs exhibit signs of reworking, as demonstrated by reactivation of the slump-related faults resulting in deformation of beds directly overlying the MTDs. Structural features within MTDs, provide information about the direction of the mass movement, and hence the orientation of the paleoslope. Measurements in the eastern and north-eastern part of Gargano Promontory suggest flow is directed towards the E and N respectively. The internal architecture of studied MTDs is discussed in the context of triggering mechanisms related to the characteristics of the Cretaceous paleoslope of the Apulian Platform.

Three-dimensional fault meshes and multi-layer shear in mass-transport blocks: Implications for fluid flow on continental margins

Submarine mass-transport deposits (MTDs) frequently include blocks of strata with variable deformation styles. In this paper, a 3D seismic volume from SE Brazil is compared with outcrop information from SE Crete to investigate multi-scale deformation patterns within mass-transport blocks. Block morphology in the two study areas relates to the orientation of internal faults and fractures, with wide chasms separating block fragments that opened along these planar features. Two families of sub-perpendicular faults associated with intra-block extension are observed at multiple scales. On seismic data, we show new evidence of intra-block horizontal shearing and thrusting. At outcrop, downslope verging folds, load casts and injectites are observed along poorly consolidated sandy intervals that are less than 1m thick. Documented cross-cutting geometries suggest a syn-kinematic origin for these structures. By integrating the observations made at different scales we suggest the development of 3D meshes in mass-transport blocks, which are characterised by the intersection of faults and fractures with bed-parallel shear planes. These meshes greatly increase the permeability of individual blocks. The results in this work have implications for the analysis of fluid flow paths on continental margins, and show that mass-transport blocks can comprise important fluid bypass points when buried in low permeability strata.

Multistage, progressive slope failure in the Pleistocene pro-deltaic slope of the West Nile Delta (Eastern Mediterranean)

Abstract The vast majority of submarine mass transport deposits (MTDs) have been attributed to retrogressive slope failure processes whereby an initial rupture zone migrates in a generally upslope direction towards the position of the ultimate headwall. Here we use 3D seismic data from the prodelta slope of the Western Nile delta to describe a series of volumetrically extensive MTDs (termed MTDs A, B and C) that are part of a linked sequence that failed progressively but with retrogressive modifications of the lateral and headwall margins. The MTDs have c. 500–1000 ms of relief on their marginal scarps with a minimum total volume of remobilised sediments of 750 km3. By comparing their motion histories, and by correlating their basal surfaces, we demonstrate that MTDs B and C are remnants of a single original body that was later cut by MTD A. This sequence is confirmed by cross-cutting relationships at the lateral boundaries between the three MTDs and the absence of any significant infill and burial of residual topography at the tops of MTDs B and C prior to the incision of MTD A. This implies that these two major submarine failures (MTD B/C, and then MTD A) were closely grouped in time. We suggest a mechanistic model whereby rapid load redistribution resulting from the initial failure led to localization of a deeper cutting failure, and the unloading in the headwall region then led to expansion of the deeper failure in an upslope direction until a new merged headwall was formed. Correlation with MTDs identified downslope in previous studies suggests a date between 117 and 105 ka for this giant slope failure. This was a period of relative sea level fall in the Nile region which may have contributed to increased pore pressure. Slope failure on this scale was probably preconditioned by high sedimentation rates and under-compaction in a mud-rich succession, leading to local increases of pore pressure.

Anatomy of a subduction complex: architecture of the Franciscan Complex, California, at multiple length and time scales

The Franciscan Complex of California records over 150 million years of continuous E-dipping subduction that terminated with conversion to a dextral transform plate boundary. The Franciscan comprises mélange and coherent units forming a stack of thrust nappes, with significant along-strike variability, and downward-decreasing metamorphic grade and accretion ages. The Franciscan records progressive subduction, accretion, metamorphism, and exhumation, spanning the extended period of subduction, rather than events superimposed on pre-existing stratigraphy. High-pressure (HP) metamorphic rocks lack a thermal overprint, indicating continuity of subduction from subduction initiation at ca. 165 Ma to termination at ca. 25 Ma. Accretionary periods may have alternated with episodes of subduction erosion that removed some previously accreted material, but the complex collectively reflects a net addition of material to the upper plate. Mélanges (serpentinite and siliciclastic matrix) with exotic blocks have sedimentary origins as submarine mass transport deposits, whereas mélanges formed by tectonism comprise disrupted ocean plate stratigraphy and lack exotic blocks. The former are interbedded with and grade into coherent siliciclastic units. Palaeomegathrust horizons, separating nappes accreted at different times, appear restricted to narrow zones of <100 m thickness. Exhumation of Franciscan units, both coherent and mélange, was accommodated by significant extension of the hanging wall and cross-sectional extrusion. The amount of total exhumation, as well as exhumation since subduction termination, needs to be considered when comparing Franciscan architecture to modern and ancient subduction complexes. Equal dextral separation of folded Franciscan nappes and late Cenozoic (post-subduction) units across strands of the (post-subduction) San Andreas fault system shows that the folding of nappes took place prior to subduction termination. Dextral separation of similar clastic sedimentary suites in the Franciscan and the coeval Great Valley Group forearc basin is approximately that of the San Andreas fault system, precluding major syn-subduction strike-slip displacement within the Franciscan.

Submarine mass-transport deposits in the Semantan Formation (Middle-Upper Triassic), central Peninsular Malaysia

Relatively fresh exposures of the Semantan Formation along the East-Coast Highway (Lebuhraya Pantai Timur - Fasa 1) between Karak and Kuantan, central Peninsular Malaysia, have given new insights into the sedimentary processes in the Triassic flysch basin that once separated west and east Malaya. An eastward change from distal to proximal facies between Karak and Maran indicates a west-facing, active continental shelf to slope sedimentation. Outcrops between Karak and Temerloh, east of the Late Triassic-Early Jurassic Bentong-Raub collisional suture, are generally characterized by classical flysch-like, thinly-bedded sandstone-mudstone facies. Further east of Temerloh towards Maran, and nearer to the paleo-shelf and slope, more sandy and thick-bedded turbidite facies occur. A proximal deep- marine facies association in the Semantan Formation is exposed at the Chenor Junction (Exit 821), kilometre 139 along the highway. South- and north-facing cuts on either sides of the highway reveal large gravity-slide blocks (megaclasts), slumps, debris flow deposits, and associated syn-sedimentary thrust faults and glide surfaces. These features are strongly indicative of large-scale submarine mass-transport processes on the palaeo-slope of the Triassic active margin. The Chenor mass-transport complex is made up of zones of incoherent slump deposits intercalated with well-bedded turbidite/debrite facies. In the lower part of the succession, there are megaclasts of sandstone-mudstone facies, measuring several metres in size, encased in a plastically deformed silty matrix. The megaclasts are highly deformed internally by numerous meso- scale normal faults, probably due to gravitationally-induced extension. Along with other smaller sandstone blocks, these megaclasts are interpreted as slide blocks due to slope failure up-dip. There are other gravity-induced structural features such as rotational slumps, glide surfaces, thrust faults and associated soft-sediment folds. The slump folds and thrusts show vergence to the west, as opposed to the generally eastward tectonic vergence. A few of the well-stratified units show strongly inclined stratal surfaces which may be attributed to lateral accretion of turbidite fan lobes. Several sets of these inclined surfaces are bounded by erosional surfaces which could have resulted from different episodes of turbidity flow. The association of incoherent mass-flow units with the more well-stratified deposits reflects the close spatial and temporal relationship between submarine mass-transport events and turbidity flows on the Triassic active slope and basin plain.