Abstract Pleistocene shelf-margin deltas (SMDs) of the northern Gulf of Mexico (Mobile, Mississippi, Brazos-Trinity, Colorado, Rio Grande) and the corresponding shelf-slope transition illustrate some of the conceptual and procedural issues pertaining to sequence stratigraphy of continental margins. As one approaches the shelf margin from the landward side, it is customary to pick the sequence boundary (SB) at the erosional base of channelized fluvial deposits, typically cut into marine clinoforms. Near the shelf margin, the sequence boundary could be picked at the erosional base of a submarine canyon cut into a SMD, or alternatively at the base of the SMD, especially if no submarine canyon is present. Thus, the SMD can be placed either below the SB (making the SMD part of the HST) or above the SB (making the SMD part of the LST). In practice, the latter is seldom done, because there is rarely a distinctive surface or break in stratal geometry to uniquely mark the change from shelf-phase delta to SMD. Therefore, SMDs are usually considered part of the HST. In some cases the SMD is characterized by submarine landslide deposits within the clinoforms, resulting in hummocky or chaotic clinoforms, which can grade downdip into massively chaotic, sand-bearing deposits on the upper slope. Even in these cases, the change from shelf-phase delta to SMD is typically gradational, without a single distinctive surface to uniquely define the SB or to place the SMD in the LST. In general, there is no consistent rule as to where the SB should occur relative to a SMD, or where the SMD should fit into the systems tract classification. As one approaches the shelf margin from the basinward side, where mini-basins are present on the upper slope, the SB is typically picked at a major onlap surface, which in late Pleistocene deposits can often be correlated across saddles between mini-basins with little ambiguity. In the Brazos-Trinity deposystem, the SB defined by this onlap surface is clearly different from (and stratigraphically below) the SB defined by the erosional base of fluvial deposits landward of the shelf margin. In the Mississippi deposystem, SBs defined by such onlap surfaces are also clearly different from SBs defined by erosional bases of submarine canyons. Onlap surfaces (and immediately underlying MFS shales) are useful for correlation along strike, especially on the upper slope. In contrast, submarine canyon surfaces are useful for long-distance correlation in the dip direction from shelf to basin plain, but are of very limited extent in the strike direction. The basin-floor fan (BFF) phase of slope deposition typically occurs just above the onlap surface, whereas the slope fan (SF) phase occurs at and above the submarine canyons. A composite framework of onlap surfaces and submarine canyons is useful for establishing temporal relationships within the Mississippi depositional province, although this framework does not fit readily within standard systems tract nomenclature. In concept, sequence boundaries are isochronous surfaces which separate deposits that are less closely genetically related while grouping deposits that are more closely related. Two difficulties are recognized with this concept. First, sequence boundaries picked at erosional surfaces are subject to regional diachroneity, such that some fluvial deposits above the SB may be coeval with some marine deltaic deposits farther downdip below the same SB. Secondly, the SB typically groups slope deposits with immediately younger transgressive deposits while separating them from immediately older deltaic deposits. However, in map view, Pleistocene deposystems of the northern Gulf of Mexico consistently show a close paleogeographic relationship between slope systems and immediately older deltaic systems. Conversely, the paleogeographic relationship between slope systems and immediately younger transgressive and highstand systems is typically much more distant. From studying a variety of Quaternary deposystems associated with Gulf Coast rivers, we recognize a composite succession, although not all phases will necessarily be present in any one deposystem: Incised valleys are filled by a combination of estuarine transgression, fluvial aggradation, and deltaic progradation. Once the incised valley is filled, the alluvial plain continues to aggrade without the lateral confinement of valley walls. This aggradation may or may not be accompanied by deltaic progradation. The delta progrades across the shelf, probably punctuated by minor transgressions and lobe switching. The net progradation is typically forced by sea-level fall, but may also occur purely by sedimentary progradation, as in the Holocene Mississippi delta. Forced regression of the delta is typically accompanied by valley incision farther updip. As the delta approaches the shelf margin, the deltaic depocenter becomes thicker and smaller in areal extent, while the prodelta becomes steeper and increasingly prone to slope failure. Slope failures may be manifested in a variety of ways, such as a single slide complex which is healed by subsequent clinoform progradation, or as repeated slides during progradation, resulting in chaotic clinoforms. Also during this phase, turbidity currents may be generated at or near river mouths, which generate sinuous slope channels without significant incision of the shelf margin. Alternatively, the SMD can remain gravitationally stable, with minimal generation of sediment gravity flows. The SMD is incised by a submarine canyon, typically connected to an incised valley. After a phase of sediment bypass to the slope and basin plain, the canyon is typically filled or healed by clinoform progradation. Regional transgression resets the paleogeography, and the next depositional succession is likely to be offset along strike from the previous, due to large-scale lobe switching. Overall, this depositional succession is controlled by eustacy. However, sea level and transgressive-regressive cycles are not necessarily in phase, and these phase relationships may vary from one one river to the next, and from one cycle to the next. In addition, the stratigraphic expression of a given eustatic cycle can be present in one locality and absent or cryptic in another. Therefore, inference of eustacy from a local stratigraphic record, or from a single dip section or corridor through one or more SMDs is likely to yield a sea-level history that is incomplete or otherwise inaccurate. From an operational standpoint, we prefer a descriptive classification of major surfaces. The relationships of various kinds of stratigraphic surfaces define the stratigraphic framework. For sequence boundaries we use those surfaces that are most robust for regional correlation.

Abstract A buried Quaternary channel–levee system (CLS) with unique architectural characteristics was identified and studied through 3-D seismic-reflection data on the lower continental slope of the NW Niger Delta. The CLS runs in an elongated topographic depression, created by the edges of two thick (100 m) mass-transport deposits (MTDs). Because of this confinement, the CLS is forced to run over a ridge 20–70 m tall. More than 3.5 km updip from the ridge, the CLS is ∼ 4 km wide, and displays well-developed outer levees and multiple, strongly meandering channel forms (up to 100 m deep). The depth of the channel forms suggests a minimum thickness of 100 m for the largest flows. An erosional valley 3.5 km long and 2 km wide that cuts 50 m down to the substrate develops immediately updip and downdip from the ridge. A plateau 6 km long and 2 km wide is observed at the top of the ridge. On the plateau, the CLS is ∼ 4 km wide, and consists of thin levees and multiple, strongly meandering channel forms. Packets of high-amplitude reflections (HARs) are widespread at the base of these channel forms, and much thicker (up to 80 m) compared to the HAR packets observed in the CLS updip and downdip from the ridge (up to 40 m thick). Gamma-ray logs indicate that these HARs represent thick channel-sand deposits. These observations indicate that updip from the ridge, turbidity currents went through a hydraulic jump and developed a turbulent bore, which prevented deposition and enhanced erosion. The flows were thinned and spread on the plateau, resulting in extensive sand deposition. On the steep downdip flanks of the ridge, the flows accelerated and became erosional. This interpretation is consistent with flume studies showing that flows are able to surmount and transfer their suspended material over an obstruction with relief less than half the height of the flows.

Abstract Introduction The significance of submarine mass movement on most continental margins is now well established in the scientific literature. The resultant sedimentary deposits have been called by many names, but will be hereafter termed mass-transport deposits (MTDs) for this publication. Such deposits are distinctive in deepwater depositional systems, most commonly due to their large size, distinctive morphology, and chaotic internal character. Recently, regional overviews have identified a number of margins around the world where MTDs are commonly observed on or near the seafloor across much of the continental slope (e.g., Weaver et al., 2000 ; Evan et al., 2005 ; Posamentier and Walker, 2006 , Huhnerbach et al., 2008; Lee, 2009 ; Twichell et al. 2009 ; Boyd et al., 2010 ). Equally as common, but documented mostly on an areally smaller scale, MTDs have been reported to be a substantial component of the near-surface stratigraphic record in at least a few basins (e.g., Newton et al., 2004 ; Moscardelli et al., 2006 ; Posamentier and Walker, 2006 ; Giles et al., 2010 ).

Abstract Extensive deep-water mass-transport deposits are observed in both slope and basin-floor settings. A detailed understanding of mass-transport deposits, in terms of emplacement processes, depositional products, and their stratigraphic and geographic distribution, is vital because they can constitute a significant portion of the stratigraphic section in deep-water settings. In addition, mass-transport deposits can play a significant role in hydrocarbon exploration, inasmuch as they can constitute seal, reservoir, and possibly source facies under the right circumstances. Different data types bring to light different aspects of mass-transport deposits. This paper focuses on insights derived from seismic and outcrop data. Overall geometries and architecture of mass-transport deposits are readily observable in 3D seismic data; however, features below seismic resolution that are vital for process and lithologic understanding need to be observed in outcrop. Integrating observations across a broad range of scales by linking seismic and outcrop observations constitutes an effective way of improving our understanding of when and where mass-transport deposits are likely to form. In addition, this linkage sheds light on details of internal architecture that commonly characterizes these deposits. Mass-transport deposits can comprise sheets, lobes, and channel fills, and can reach 150 m or more in thickness. Greater thicknesses are observed where successive flows are amalgamated. This paper documents both internal architectural or stratigraphic as well as external geomorphic attributes of such deposits, as expressed in outcrop and imaged by 3D seismic data. Recognition of mass-transport deposits in outcrop is based on identification of bedding deformed by synsedimentary processes, with deformation ranging from minimal redistribution of large slide blocks to complete disaggregation typical of debris-flow deposits. On seismic data, mass-transport deposits can be recognized by certain geomorphologic as well as stratigraphic distinguishing characteristics: basal linear grooved and scoured surfaces, hummocky relief at the top, and internal chaotic to transparent seismic facies, with internal thrust faulting common.

Abstract The volume and interplay of mass-transport (MTD) and turbidite-system deposits varies on different continental margins depending on local and external controls such as active-margin or passive-margin tectonic setting and climatic and/or sea-level change. Erosion and breaching of local grabens at the shelf edge of the southern Bering Sea produce giant, gullied canyons and MTD sheets that dominate the basin-floor deposition and disrupt development of turbidite systems. In contrast, external controls of great earthquakes (> 8 M ) along the Pacific active tectonic continental margins of Cascadia and northern California cause seismic strengthening of the sediment, which results in minor MTDs compared to turbidite-system deposits. Messinian desiccation of the Mediterranean Sea caused a deeply eroded Ebro subaerial canyon and an unstable central segment with an MTD sheet, whereas other stable Ebro margin segments have only turbidite systems. In the northern Gulf of Mexico, the delta-fed Mississippi Fan and intraslope mini-basins contain MTDs and turbidites that are equally intermixed from the largest scales with MTD sheets hundreds of kilometers long to the smallest scales with beds centimeters thick. In the Antarctic Wilkes Land margin, global climate cooling caused a late Oligocene to middle Miocene time of temperate continental ice sheets that resulted in massive deposition of MTDs on the margin, whereas later polar ice sheets favored development of turbidite systems. Our case studies provide the following new insights: (1) MTDs can dominate entire margins, dominate segments of a margin, be equally mixed with turbidites, or dominate a margin during some geologic times and not others; (2) on active tectonic margins with great earthquakes, the maximum run-out distances of MTD sheets across abyssal-basin floors are an order of magnitude less (~ 100 km) than on passive-margin settings (~ 1000 km), and the volumes of MTDs are limited on the abyssal sea floor along active margins; (3) where the most precise radiocarbon ages are available, major MTD episodes of deposition are correlated with the most rapid falls or rises of sea level; (4) gullied canyons feeding MTD sheets have irregular and steep axial gradients (5-9°), whereas canyons feeding turbidite systems have a regular graded profile and less steep gradients (1 to 5°). Our examples of MTD and turbidite systems provide analogues to help interpret ancient systems.

Abstract The present study provides an overview of recent sedimentation patterns on the central Algerian continental margin. Recent sedimentation patterns were assessed from morphological analysis, which is based on swath bathymetry and echo-facies mapping. It appears that sedimentation along the Algerian margin is controlled by two processes: (1) gravity-induced processes, including both mass-transport deposits and turbidity currents, and (2) hemipelagic sedimentation. Mass-transport deposits occur on the Algerian margin at the canyon heads and flanks, in the interfluve areas between canyons, along the seafloor escarpments, and on the flanks of salt diapirs. Mass-transport deposits (MTDs) sampled by coring consist of a variety of soft and hard mud-clast conglomerate and turbidite deposits. MTDs are mostly localized at the toes of steep slopes, where thrust faults were previously identified and mapped. Analysis of the spatial distribution of MTDs and their recurrence in time help reconstruct the main predisposing factors and triggering mechanisms, and evaluate their impact on evolution of the Algerian margin.

Abstract Abstract: Deepwater mass-transport deposits (MTDs) are associated with Upper Quaternary seafloor leveed-channel complexes at the mouth of a large canyon at the base of slope of the offshore Niger delta. They make excellent analogs for interpreting older subsurface features and reservoirs, and for geohazard analysis. These leveed-channel complexes and mass-transport deposits are assessed within a 3D seismic survey, using detailed images of seafloor maps and stratal surfaces, artificially digitally colored, and vertically exaggerated to create optimal imaging. A large canyon head incises the present-day shelf margin of the Niger delta and traverses down the upper and lower slope for 45 km towards the southeast. At the canyon mouth, a large apron of leveed-channel complexes covers the slope and basin plain for a distance of 30 km within the seismic survey. Large sediment waves occur on outer levees of channel bends, attaining heights of 200 m. In some areas, synclinal limbs of individual sediment waves have been deformed by numerous rotated blocks along small listric faults to form small mappable MTDs. Other mass-transport deposits occur associated with and above the leveed-channel complexes. Lengths of the MTDs range from 1 km to over 16 km, and thicknesses commonly range from 100 m to 200 m. Headward escarpments are well imaged in both map and cross-section views. Proximal facies of the MTDs includes rotated blocks and large angular glide blocks. These pass distally into smaller glide blocks and chaotic seismic facies inferred to be debrites. Intermediate parts of the MTDs have longitudinal linear features parallel to inferred flow direction. Distal patterns consist of transverse compressive ridges. Other MTDs too large to be completely imaged within the 3D survey show internal facies, consisting of large angular glide blocks in a matrix of seismically visible smaller blocks and chaotic facies inferred to be debrites. Multiple causes of MDTs in this area are probable. In possible order of importance, these include tilting and oversteepening of sediments because of tectonic uplift, high sedimentation rates at the mouth of the canyon, and eustatic falls of sea level.

Abstract Abstract: The characteristics, evolutionary history, and triggering mechanisms of successive siliciclastic mass-transport deposits (MTDs) of late Cenozoic age on the northwestern South China Sea margin were studied using borehole and 2D/3D reflection seismic data. Multiple mass-transport deposits of various scales and morphologies formed from Pliocene to Holocene time in high-slope-gradient and high-sedimentation-rate parts of the Qiongdongnan and Yinggehai basins. In plan view, MTDs documented by 3D seismic data, deposited between 3 and 2 Ma, are 1 to 11 km wide and 4 to 29 km long. Two seismic geomorphologic characteristics of a typical MTD comprise a basal surface and displaced masses of sediments. Internal seismic facies of the displaced mass consist of extensional wedge facies in upslope areas, thrusted facies in intermediate areas, and chaotic or mounded facies in distal downslope areas. These MTDs likely were triggered by a combination of mechanisms. Seafloor oversteepening, rapid accumulation of thick sedimentary deposits, overpressure, and a tectonically active basin setting provide a background favoring formation of MTDs. Additionally, seismicity, abrupt increase of sedimentation rates, rapid slope progradation, and release of gas contributed to triggering mass-transport deposition in the study area.

Abstract Abstract: The stratigraphic evolution of the Quaternary mass-transport deposits (MTDs) in the Mensa and Thunder Horse intraslope basins, Mississippi Canyon, northern deep Gulf of Mexico, was interpreted based on based on 378 square miles (970 square km) of 3-D seismic data in water depths ranging from 5300 to 6500 feet (1617 to 1983 m). Seven depositional sequences were defined in the study area between 1.3 Ma to the present. Allochthonous salt systems had bathymetric expression and influenced sediment thickness and location of depositional systems. Six MTDs are present in five of the depositional sequences. MTDs overlie erosional boundaries—up to 30 m of the underlying section has been eroded at the base of the deposits. These deposits consist primarily of chaotic, rotated, and thrusted seismic reflections. They vary in size and areal distribution from elongated to more equidimensional. The oldest MTD is in sequence 1, overlies the 1.3 Ma condensed section, and underlies a series of five east-trending channels. This MTD has an easterly trend and represents the initial deposition after a major reorganization of the slope system. In the underlying Miocene-lower Pleistocene sequences, channels trended from the northwest to southeast. Sequences 2 and 3 consist of seven additional channels that trend primarily from west to east. The second MTD is present in sequence 3, trends to the southeast, and truncates four channels. A series of stacked condensed sections (ca. 0.6 to 0.08 Ma) form a thin unit and separate sequences 3 and 4. Sequence 4 consists primarily of hemipelagic and overbank deposits. Four MTDs are present in the sequences 5-7. Multiple sets of these deposits have channelized into and stacked on one another. These MTDs appear to have been sourced primarily from the west, similar to the channels in the underlying sequences. This case study illustrates the many variations in MTDs that are present in the same intraslope setting. These variations can occur in their size, shape, thickness, seismic facies, the amount of erosion at their base, and their timing of formation within different positions of sea level.

Abstract A large submarine-slide deposit from the western Scotian Slope off eastern Canada was imaged on a 3D seismic reflection dataset in the Barrington exploration block. The mass-transport deposit (MTD) forms a tongue-shaped body that is 25 km long and 8 km wide, with a run-out distance from the headscarp of 41.5 km and a total volume of 12.5 km 3 . In profile, it consists of a chaotic seismic facies. This facies forms a highly rugose top surface morphology, suggesting that the flow consisted of an abundance of intact angular blocks. Its base reveals evidence of erosion typical of submarine MTDs, with linear downslope-trending gouges and excavation of a pit 50-m-deep. The source area and headscarp of the Barrington MTD are somewhat obscured by postdepositional erosion. Additionally, high-resolution seismic profiles show that the deposit is draped by approximately 30 m of late Pleistocene and Holocene sediment, providing an age estimate of 30 ka for the failure. Despite this drape, the modern seafloor above the MTD still has a highly rugose morphology, echoing the top surface of the deposit. Seismic profile data show a series of stacked MTDs underlying the Barrington MTD, suggesting that mass-failure recurrence is common on geologic time scales. Although it is difficult to attribute mass-failure triggering mechanisms, high sedimentation rates due to proximal shelf glaciers and intense erosion causing oversteepening, and likely established preconditions for instability. Local seismicity, possibly a result of glacial rebound, is the most probable initiating factor.

Abstract Sediment mass transport in the Pacific margin of the Antarctic Peninsula is strongly influenced by its peculiar tectonic and sedimentary evolution. Analysis of swath bathymetry and multichannel seismic reflection data shows that this setting reflects the passage from an active to a passive margin, and the transition from river-dominated to glacier-dominated sedimentation. Only contouritic sedimentation persisted throughout the late Neogene on the continental rise, while rapid progradation of steep wedges composed of glacial diamicton occurs on the slope. Gravitational instability and mass-transport processes, which occur on the continental rise, appear to relate to physical properties of contourite sediments deposited in this high-latitude setting. Other than minor erosional gullies on the upper slope, there is no evidence of major incisions such as channels, canyons, or slide scars on a steep continental slope (averages 13°). This situation results from high shear strength of the slope-forming diamicton delivered by grounded ice sheets. Short-run-out mass failures were the main sediment transport process to the slope. Turbidity currents, most likely originated by downslope evolution of mass flows, were able to generate large deep-sea channel systems at the base of the continental slope. On the continental rise, relatively good sorting and a high accumulation rate of sediments forming sediment drifts favored slope failure even on gentle slopes. Coalescent headscarps that form the drift crest were produced by undercutting of steeper flanks of drifts. This process formed the walls of turbidity-current channels, flowing in low-relief areas between drifts. Failure along stratal weak layers on the gentle sides of sediment drifts produced either relatively small, concave slide scars in the margin-proximal drift or long, rectilinear scars in distal locations.

Abstract This paper uses three-dimensional seismic data to investigate the typologies, genetics, and mechanisms of soft-sediment deformational processes on the Ebro Continental Margin (offshore northeastern Spain). The study focuses on the two major types of soft-sediment deformation in the region: slope failure and fluid-escape structures. Such processes have operated almost continuously throughout the post-Pleistocene history of the Ebro Continental Margin, and have played a critical role in its overall evolution and construction. This study shows that vertical stacking patterns of submarine canyons create preferential pathways for fluid migration and slope failure. In these areas, three-dimensional seismic analysis reveals a potential cause-and-effect relationship between focused fluid migration and repeated slope failure. The proposed model is that focused fluid flow from sands within stacked submarine canyons leads to overpressure generation and reduction of sediment shear strength, making sediment susceptible to failure. The presence of a widespread region of fluid-escape structures and slope failures on the Ebro Continental Margin has important implications for offshore facilities. The relatively high resolution provided by the seismic data has been sufficient to be used for a geohazard assessment study, aimed at exploratory well design and field development. The results from this study have led to a detailed program of seafloor and near-surface evaluation over a proposed area in the area.

Abstract Abstract: Offshore of northwest Borneo, the occurrence of distinct submarine mass failures on the upper continental slope poses a substantial challenge to deepwater operations for the energy industry. These features are part of a complex of mass-transport deposits (MTDs) that occur in the near-surface interval across most of the upper continental slope, including a large area undergoing field development for hydrocarbon production. In the study area, the shallowest and most prominent feature discernible on conventional 3D seismic data is MTD 1, which has a profound influence on the present-day seafloor topography. This feature has a distinctive fan-like outline in plan view, a maximum strike dimension of approximately 6 mi (10 km), a dip extent up to 24 mi (40 km), and a maximum thickness up to 570 ft (176 m). The fan-like external form and the presence of a dip-oriented erosional keel suggest that the depositional process was a less coherent debris flow, with little to no original internal stratigraphy preserved. The less coherent nature of this feature is further supported by a key observation that this MTD overran an area of substantial high bathymetric relief, which is located in the area considered for a field development. Locally overlying MTD 1 are a series of younger near-seafloor features, termed “canyon-to-fairway” corridors that display a confined updip to less confined downdip plan-view morphology. These unique features locally erode and smooth the rugose top surface of the near-surface MTD 1 and can be interbedded with the lower intervals of the usually overlying draped sediments. Development of these late Pleistocene canyon-to-fairway corridors suggests that these features probably formed during a period of sea-level fall or at a lowstand. A blanket of three distinct intervals of draped sediments cap this entire sequence, composed mostly of muddy turbidites grading upward into hemipelagic deposits. The present hummocky seafloor topography mimics the rugose top surface of the shallowly buried MTD 1, except along its northeast lateral margin and where smoothed by canyon-to-fairway corridors. Internally within MTD 1, physical properties probably vary substantially both laterally and vertically, because draped sediments, turbidites, and occasional channelized sediments were incorporated in the failed matrix of this feature. Some of the geohazards, potentially affecting a field development, are a direct a result of the ubiquitous occurrence of MTD 1 in the study area. These potential geohazards include local steep slopes, seafloor scarps, and variable near-seafloor soil conditions. Understanding the impact of each of these potential geohazards, caused primarily by the presence of MTD 1, on a field development is vital input for selection of production well-site locations and placement of subsea infrastructure.

Abstract Abstract: Deep-marine strata of the Windermere Supergroup, which currently are exposed in an area over 35,000 km 2 in the southern Canadian Cordillera, were deposited on the passive margin of Neoproterozoic western North America. In the Isaac Formation at the Castle Creek study area, stratigraphic evidence of slope instability occurs as mass-movement (slump and slide) and cohesive-debris-flow deposits that crop out locally through the 1.5-km-thick succession. These deposits are particularly common in a mass-transport deposit (MTD) up to 110 m thick that occurs sandwiched between two major channel complexes. Interstratified within these deposits are common coarse-grained channel fills that preferentially infilled irregular topography on the seafloor. In many instances, this irregular topography was most probably related to earlier emplacement of debris-flow and slump and slide deposits. Important stratigraphic characteristics in this succession suggest that this particular MTD represents a major change in the nature of sediment supply and transport and depositional processes within the basin. These changes are interpreted to be controlled principally by changes of relative sea level, which had a first-order control on sediment supply, sediment caliber, and sediment composition to the slope and more distal basin floor.

Abstract Abstract: The Williams Ranch Member of the upper Cutoff Formation in the Guadalupe and Delaware Mountains, west Texas, U.S.A., consists of six offlapping lithologic units. The deposits formed during carbonate turbidite deposition across a drowned Early Permian carbonate platform. They have an areal extent of more than 20,000 km 2 and reach a maximum thickness of at least 113 m. At the terminal margin of the older platform, the carbonate turbidites were partially redistributed by mass-transport events (MTEs) onto the slope and basin floor. Deposits formed during individual mass-transport events (MTE bodies) comprise the bulk of the Williams Ranch Member basinward from the drowned margin for at least 28 km along a transect oblique to depositional dip. MTE bodies are interbedded with undeformed carbonate turbidites and contain soft-sediment folds, faults, and extensional and shortening lineations, as well as termination surfaces (beds terminated from above and/or below). Turbidite deposition and subsequent mass transport caused general basinward thickening of the Williams Ranch Member from the drowned margin, where the Cutoff Formation is missing, to the basin floor. Deposition responded to, and modified, inherited bathymetric relief. Compared to isopach thins, isopach thicks formed in bathymetric lows and locally formed bathymetric highs. Isopach thicks contain more undeformed strata and show more soft-sediment folds. These relationships suggest better preservation of strata in structurally controlled inherited bathymetric lows. In general, MTE bodies are preferentially deposited in these paleobathymetric lows. A minimum of six vertically stacked MTE bodies are recognized in the main study area with thicknesses ranging from less than one to tens of meters. MTE bodies show a general S-to-SSE paleotransport direction, with significant local variation, reflecting either underlying bathymetric relief and/or different source locations. Repeated MTEs resulted in a reduction of the overall basin gradient and created local positive bathymetry. Sand fairways and ponded sheet deposits in the overlying Brushy Canyon Formation are focused in bathymetric lows, and sands thin and onlap onto bathymetric highs.

Abstract Abstract: The Paleocene and Eocene Chicontepec Formation crops out along the western margin of the Tampico-Misantla basin, located in northeastern Mexico in the states of Veracruz, Hidalgo, and San Luis Potosi. This succession records deposition in a deep-marine, foreland basin between the Cretaceous Golden Lane Atoll and the Tertiary Sierra Madre Oriental. In the northern part of this outcrop belt, slope deposition is recorded primarily by deformed and undeformed thin-bedded turbidites with occasional sand-rich lobes, channel fills, and debrites. Sediment transport and slumping direction in this area was to the east and southeast. The slumped, thin-bedded turbidites show the complete spectrum of deformation, including coherent slumps, semi-coherent, faulted, boudinaged, and chaotic slumps. More than one of these types of slumps can occur in a single outcrop. All the slumps have extremely flat upper surfaces, indicating that the tops of the slumps were probably eroded during subsequent bypass sedimentation. An unusual, flat-topped toe thrust is also preserved at one of the outcrops. A large, spectacular 26-m-thick debrite has faulting at its erosional margin, pressure ridges on its top, and strong evidence that the debris flow did not create the void that it occupies. These spectacular outcrops provide a unique opportunity to study the detailed internal characteristics and allow evaluation of the reservoir quality in terms of continuity and connectivity. Evidence from these outcrops indicates that sub-seismic-scale coherent, semi-coherent, and faulted slumps can be extremely complex, and hence, difficult to identify with typical oil-industry technology, such as 3D seismic, core, and image logs.

Abstract Abstract: Analysis of a deep-sea mass-transport deposit exposed as a nearly 1.6 km continuous outcrop reveals heterogeneous internal structures and existence of a compressional stress field during transportation and deposition. Deposit of gravelly mudstone, containing large deformed sedimentary blocks (long axis from tens of centimeters up to 100 m), occurs in the Upper Cretaceous (Maastrichtian) to Paleocene Akkeshi Formation, Hokkaido Island, northern Japan. The outcrop was photographed and sketched, and clast sizes were measured to study quantitatively the internal structure of this mass-transport deposit. The size distribution of sedimentary blocks exhibits a power-law distribution, but mean size and concentration of blocks exhibit highly variable, local fluctuations. This mass-transport exposure exhibits three facies, based on size and spatial arrangement of accumulated blocks. Facies A consists of relatively small blocks (long axes approximately 1 to 10 m), supported by a gravelly mudstone matrix. Facies B consists of clast-supported moderate blocks (long axes & 30 m). Generally, blocks in Facies B are deformed significantly. Facies C comprises mainly large blocks with long axes up to 100 m. Considering the evidence of turbidites in blocks of Facies C, these blocks not only slid but also rotated both horizontally and vertically. In some cases, original stratigraphy found within these blocks is inverted. Facies A and B alternate downcurrent, while Facies C occurs only at the more distal end of the exposure. Usually, long axes of blocks are oriented parallel to the bedding surface, suggesting a laminar state of flow. In addition, application of the multiple inverse method to mesoscale faults observed in the blocks reveals possible internal paleostress fields that existed before deposition. This analysis suggested two different stress fields: (1) a uniaxial compressional stress field, where the a 1 axis is oriented normal to bedding surface, and (2) a triaxial compressional stress field, where the a 1 axis is oriented parallel to the paleocurrent direction. This mass-transport deposit apparently experienced the first stress field when it moved downslope, thereby expanding its surface area. It then experienced the second stress field as it decelerated, because of compression parallel to paleocurrent direction. A heterogeneous nature of internal structures and compressional stress fields appear to be common features of mass-transport facies of deposits.

Abstract Abstract: Mass-transport deposits may act as barriers or baffles to fluid flow in the subsurface, or may conduct fluids via internal structures or lithological connectivity. Conventional seismic and borehole data present radically different scales of observation to assess the likely fluid-flow behavior of mass-transport deposits. Seismic-scale outcrops and high-resolution seismic data bridge this scale gap. Exceptional outcrops of large mass-transport deposits are used to develop strategies to relate core- and seismic-scale observations for the purposes of subsurface prediction of reservoir, baffle, or seal potential, and for prediction of fluid flow through mass-transport deposits in the subsurface. We present here an outline of our approach, and some preliminary results based on two systems of contrasting styles. One is a > 120-m-thick debrite of Carboniferous age in northwest Argentina; the other is an approximately 300-m-thick slide complex of Jurassic-Cretaceous age in Antarctica. Differences in these two systems are assessed by evaluating the internal structure and seismic expression of the deposits, based on forward modeling of the outcrop architecture. Topography on the top surface of mass-transport deposits is defined by very localized (a few meters wavelength and amplitude), localized (a few tens of meters wavelength, a few meters to ~ 10 m amplitude), and subregional (kilometers in wavelength, tens of meters in amplitude) “ponding” or partial confinement of turbidite beds immediately above the mass-transport deposits. Strain histories and strain distributions are complex and variable within deposits, implying that inferences based on limited well data are likely to yield incorrect conclusions regarding direction of movement and slope orientation. This observation is clearly illustrated by the non-coaxial deformation, which is visible in high-resolution seismic data.

Abstract In north Taranaki, New Zealand, spectacular examples of deep-water mass-transport deposits (MTDs) are exposed in coastal cliffs and imaged in nearby offshore seismic reflection data. The MTDs are Late Miocene (Tortonian) in age, and lie within successively overlying successions of volcaniclastic sandstone-dominated basin-floor turbidites (Mohakatino Formation), epiclastic sandstone- and siltstone-dominated basin-floor turbidites (Mount Messenger Formation), and siltstone-dominated slope deposits (Urenui Formation). Seismic-scale MTDs are exposed north of Awakino River and also between the Mohakatino and Tongaporutu river mouths. These MTDs incorporate a range of original deep-water lithofacies that subsequently have been dramatically deformed into highly chaotic packages during re-emplacement in mid-bathyal to lower bathyal water depths near to or beyond the base of slope. There does not appear to be any stratigraphic or lithologic control on the types of beds incorporated into these particular MTDs, and the deformed strata are potentially derived from several original submarine-fan depositional units. These seismic-scale MTDs are in places at least 50 m thick and appear to extend for a few kilometers, although lateral stratigraphic correlation is made difficult by the complex MTD deformation and by postdepositional normal faulting. It is possible that two different MTDs are represented in the outcrop section between Mohakatino and Tongaporutu, and together they may form a larger composite MTD or mass-transport complex. At least two large MTD successions are revealed on seismic reflection profiles located offshore, relatively near to the coastal outcrop section. They extend for several tens of kilometers in length, and are up to 330 m (250 ms two-way travel time (TWTT) thick. These MTDs are in approximately the same stratigraphic interval as those in outcrop (Mohakatino and lower Mount Messenger Formations). At least one of these MTDs extrapolates up structural dip to where an MTD is exposed onshore, but an absolute correlation between the two MTDs is precluded by the lack of seismic data in the intervening coastal transition zone. MTD intervals equivalent to those exposed are also imaged in high-resolution behind-outcrop seismic reflection lines, and are recorded by anomalous biostratigraphic signatures in the nearby Pukearuhe-1 exploration well. Siltstone-dominated MTDs of subseismic scale (up to 15 m thick) are evident in central parts of the outcrop section, within fine-grained intervals near the tops of inner-fan depositional cycles. They are also present in southern parts of the section (retrogradational and progradational slope), within fine-grained intervals that lie stratigraphically close beneath incised slope channels infilled with sandstone. Whilst these MTDs are only 5-15 m thick, they could thicken away from the outcrop transect. The magnitude and styles of deformation within the various exposed MTDs may reflect differing transportation processes or triggering mechanisms. The larger, seismic-scale MTDs appear to be the product of massive slope failure and downslope translation of strata, or relatively local tectonic movement of the sea bed. The ordered stratigraphic position of the sub-seismic-scale MTDs near the tops of inferred depositional cycles suggests that changes in relative base level may have controlled their development.