Downslope Creep Research Articles

Large‐scale elongated gas blowouts along the U.S. Atlantic margin

In May 2000 we surveyed a series of en echelon, asymmetric depressions along the outer shelf off Virginia and North Carolina using high‐resolution chirp and side‐scan sonar. The features, which are elongated parallel to the shelf edge and have steep landward walls, are ∼4 km long, 1 km wide, and up to 50 m deep. On the basis of internal stratal geometry interpreted from chirp profiles, the depressions do not appear to result from simple, down‐to‐the‐east, normal displacement along deep‐seated faults or structure. Rather, the depressions seem to have been excavated primarily by gas expulsion, creating large‐scale asymmetric gas escape structures that have been termed “gas blowouts.” Gas appears to have been trapped beneath a shelf edge delta that is a few tens of meters thick and exhibits internal soft sediment deformation suggestive of progressive downslope (seaward) creep. These new data suggest the blowouts occurred when thin‐skinned deformation and creep of the surficial deltaic sediment layers combined with updip/upslope gas migration, ultimately leading to gas pressure in excess of the overburden. The location of the expulsion craters along the shelf edge and their elongated, asymmetric shapes strongly suggests a causal relation between the downslope creep of the delta and the expulsion event. We suggest a positive feedback between upward migration of gas‐rich fluids through the low‐stand delta and the downslope creep processes. While the complex interplay between differential permeability, overpressure, and upslope fluid migration remains poorly understood, we suggest such interactions may play an important role in controlling slope stability.

Creeping soil

Research Article| February 01, 2002 Creeping soil Arjun M. Heimsath; Arjun M. Heimsath 1Department of Earth Sciences, Dartmouth College, Hanover, New Hampshire 03755, USA Search for other works by this author on: GSW Google Scholar John Chappell; John Chappell 2Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia Search for other works by this author on: GSW Google Scholar Nigel A. Spooner; Nigel A. Spooner 2Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia Search for other works by this author on: GSW Google Scholar Danièle G. Questiaux Danièle G. Questiaux 2Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia Search for other works by this author on: GSW Google Scholar Geology (2002) 30 (2): 111–114. https://doi.org/10.1130/0091-7613(2002)030<0111:CS>2.0.CO;2 Article history received: 05 Jul 2001 rev-recd: 17 Oct 2001 accepted: 22 Oct 2001 first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Arjun M. Heimsath, John Chappell, Nigel A. Spooner, Danièle G. Questiaux; Creeping soil. Geology 2002;; 30 (2): 111–114. doi: https://doi.org/10.1130/0091-7613(2002)030<0111:CS>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Soil creep is the most widespread and perhaps the least understood process of erosion on soil-mantled hillslopes. Soil is slowly “stirred” by burrowing creatures, and particles are displaced in wetting-drying cycles. These actions can cause downslope creep by processes analogous to particle diffusion. Other possible transport mechanisms include shear and viscous-like creep, such that precise characterization of the entire process appears to require the tracing of labeled soil grains. Here we use natural quartz grains in a mature soil to determine grain movements from the time elapsed since each grain last visited the ground surface, measured by single-grain optical dating. Downslope flux is calculated from soil production by rock weathering at the soil base, measured with in situ–produced cosmogenic nuclide concentrations (10Be and 26Al). Results show that grains throughout the soil profile repeatedly visit the surface, and give the first quantitative characterization of grain-scale transport processes within creeping soil. These unique field data are interpreted with a Monte Carlo simulation to suggest that soil creep involves independent movements of mineral grains throughout the soil body and that grains are reburied or eroded by overland flow upon reaching the surface. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

The mountains of Io: Global and geological perspectives from Voyager and Galileo

To search for local and global scale geologic associations that may be related to the internal dynamics of Io, we have completed a global catalog of all mountains and volcanic centers. We have identified 115 mountain structures (covering ∼3% of the surface) and 541 volcanic centers, including paterae (calderas and dark spots) and shield volcanoes. The average length of an Ionian mountain is 157 km, with the longest being 570 km. The mean height of Ionian mountains is 6.3 km, and the highest known mountain is Boösaule Montes (17.5±3 km). Five basic morphologic types of mountains have been identified; mesa, plateau, peak, ridge, and massif. Very few mountains bear any physical similarity to classic volcanic landforms, but many resemble flatiron mountains on Earth and are interpreted as tilted crustal blocks. This would be consistent with the hypothesis that most mountains are thrust blocks formed as a result of compressive stresses built up in the lower crust due to the global subsidence of volcanic layers as they are buried over time [Schenk and Bulmer, 1998]. More than one mechanism may be responsible for all Ionian mountains, however. The proximity of some mountains to paterae may indicate a direct link between some mountains and volcanism, although it is not always clear which came first. In contrast to earlier studies, a pronounced bimodal pattern is observed in the global distribution of both mountains and volcanic centers. The regions of highest areal densities of volcanic centers are near the sub‐ and anti‐Jovian regions, but are offset roughly 90° in longitude from the two regions of greatest concentration of mountains. This anticorrelation may indicate the overprinting of a second stress field on the global compressive stresses due to subsidence. The bimodal distribution of volcanic centers and mountains is consistent with models of asthenospheric tidal heating and internal convection developed by Tackley et al. [2001]. Over regions of mantle upwelling, compressive stresses in the lower crust induced by global subsidence might be reduced, encouraging volcanism and discouraging mountain building. In regions of mantle downwelling, these compressive stresses in the lower crust might be increased, discouraging volcanism and encouraging mountain building. Alternatively, the global pattern may be related to possible (but undocumented) nonsynchronous rotation of Io, which would produce two regions each of compression and extension in the crust. Evidence of layering and of mass wasting, including landslides, block sliding, debris aprons and downslope creep, on Ionian mountains suggests that the crust of Io is essentially a layered stack of partially consolidated volcanic lavas and plume deposits, becoming more consolidated with depth. The lower crust especially may also be ductily deformed, punctuated by volcanic intrusions and faulting at paterae, and broken into blocks, some of which have been uplifted to form mountains.

A side-scan sonar image of a glacially-overdeepened sea loch, upper Loch Etive, Argyll

Synopsis Loch Etive in Argyll on the west coast of Scotland, is a 30 km long, up to 145 m deep sea loch of glacial origin. A unique side-scan sonar image with new accompanying bathymetric data of the upper basin of the loch has been obtained, covering an area of 19.5 km 2 . These data illustrate the morphology of the loch floor and the depositional processes that have been active in the loch since the end of the last (Younger Dryas stadial) glaciation, approximately 10 ka bp . The new bathymetry reveals the complex physiography of the loch with the deepest (&gt; 100 m) regions confined to the SW with slope angles of 5–15°. Towards the head of the loch water depths are shallower, with &lt; 50 m common and the loch floor is also smoother with slope angles of 2–5°. Using the side-scan image, areas of high and low acoustic backscatter have been identified. The high backscatter areas correspond to regions of exposed outcrops of in situ granite, with localized glacial erratics on the loch floor, and the low backscatter areas to deeper-water basins containing fine-grained sediments. Other features observed are submarine cliffs, downslope creep of water-laden sediments on the steeper loch slopes, and outwash submarine fans from rivers.

Photometric Properties of Phobos Surface Materials From Viking Images

Clear-filter Viking images, and an accurate numerical model of the shape of Phobos, have been used to determine this satellite's photometric properties. A global-average Hapke function derived from disk-resolved data confirms previous indications that Phobos has a strong opposition surge. Photometrically corrected images were mosaicked into an albedo map; most of the resulting normal reflectances are in the range 0.06–0.10, and the brightest region on Phobos is the northeast rim of the crater Stickney, the portion of that rim with the highest concentration of grooves. Globally, there are three albedo classes, reasonably separated geographically: (1) Bright material is to the east and south of Stickney, corresponding approximately to the locations on Phobos having the highest, “bluest” visible/near-IR ratio (Murchieet al. 1991). (2) The darkest material is to Stickney's west, correlating with material having an intermediate visible/NIR ratio (Murchieet al.'s “bluish gray” unit). (3) Intermediate-albedo material dominates the anti-Stickney hemisphere, corresponding to material with a lower visible/NIR ratio (Murchieet al.'s “reddish gray” unit).A search for variations in phase behavior across Phobos' surface shows few such effects overall, limited to isolated areas: (1) Stickney's floor darkens with increasing phase faster than average Phobos. This crater floor is both slightly more backscattering and significantly rougher than the global average; the latter effect may be related to the slumping hinted at in low-resolution images of Stickney. (2) We confirm that in many cases, the contrast between the bright rims of small craters and grooves and their surroundings drops noticeably with increasing phase (phase angles in use = 10°–50°). However, these bright rims, overall, display a diversity of photometric behavior and are the most heterogeneous areas on Phobos in terms of regolith properties. (3) We confirm that dark deposits in the floors of smaller craters darken faster with increasing phase than their surroundings (cf., Goguenet al. 1978) and find that these deposits are more backscattering than average Phobos.Isolated regions with unusual phase behavior occur on Phobos but not on Deimos (Thomaset al. 1996). The variable photometric properties of Phobos' isolated craters and grooves, and the association of global albedo features with Stickney, reinforce interpretations that Phobos' regolith is emplaced and modified by discrete cratering events and is not mixed horizontally by extensive downslope creep as is apparently the case on Deimos.

Mechanical development of vein structures due to the passage of earthquake waves through poorly-consolidated sediments

Vein structures are typically the earliest expression of brittle deformation within sediments. These mud-filled veins, which characteristically occur regularly spaced within bed-parallel arrays, form in sediments that possess a strong interlocking particle framework. Downslope creep has been proposed to explain the origins of vein structures, however, a recent suggestion that they are generated by the passage of earthquake shear waves through sediments explains aspects of their morphology, and their dominant occurrence at active convergent margins. Their coexistence with less disruptive “ghost veins” in Peru margin sediments, and their almost normal attitude to bedding, however, suggests that vein structures were formed by processes more complex than downslope creep, or seismically induced shearing alone. Experimental earthquake simulation was undertaken by laterally shaking a box containing crushed diatomite. Fractures were induced almost normal to the horizontal shaking direction, and to a lesser extent as antithetic Riedel shears, both of which closely resembled vein structures. The fracturing process during shaking may be viewed as a progressive fragmentation of the diatomite, in which new fractures form half-way between pre-existing ones. Thus fracture spacings are progressively halved. Shear zones oriented at a low angle to the shaking direction were also generated, combining with the high-angle fractures to form structures very similar to those observed in Peru margin sediments. When shaken, fines added to the diatomite segregated into planar zones that resembled ghost veins, half-way between fractures. The alternating pattern of fractures and fines indicated that a standing pressure wave had been created within the box during shaking. The fractures were created by alternating compression and extension at the antinodes, while the fines concentrated in zones of minimum grain movement around the nodal planes. This suggests that vein structures are initiated by the combined action of shear and pressure waves within a sediment. The strain waves may be seismic in origin, or may also form in downslope movement system.

Creep in pelagic sediments and potential for morphologic dating of marine fault scarps

Cores from the flanks of the Galapagos spreading centre show that the top 25 cm of pelagic sediments is highly bioturbated. Kenyan and Turcotte [1985] have proposed that bioturbation may cause down‐slope creep of the sediment surface and bioturbation may therefore smooth sediment topography in a similar way to hillslope creep on land. Assuming this produces movement consistent with the diffusion transport model, sediment topography across scarp crests, digitised from Deep Tow profiler records, is modelled with a simple analytic solution to the diffusion equation and with a mean diffusion constant k of ∼0.007 m²/yr. This small value suggests creep rates are modest, for example, 2 cm/yr for a 30° slope, and is an upper bound since sediment avalanching and bottom currents probably also remove sediment from scarp crests. Although requiring further work, these results suggest that morphologic dating methods may be applicable to marine fault scarps and could be useful in relatively sediment‐free areas, such as mid‐ocean ridges, where it is commonly not possible to use datable offset seismic horizons.

Quantification of soil production and downslope creep rates from cosmogenic 10Be accumulations on a hillslope profile

Research Article| April 01, 1993 Quantification of soil production and downslope creep rates from cosmogenic 10Be accumulations on a hillslope profile James A. McKean; James A. McKean 1Department of Geology and Geophysics, University of California, Berkeley, California 94720 Search for other works by this author on: GSW Google Scholar William E. Dietrich; William E. Dietrich 1Department of Geology and Geophysics, University of California, Berkeley, California 94720 Search for other works by this author on: GSW Google Scholar Robert C. Finkel; Robert C. Finkel 2Center for Accelerator Mass Spectrometry, Lawrence Livermore National Laboratory, Livermore, California 94550 Search for other works by this author on: GSW Google Scholar John R. Southon; John R. Southon 2Center for Accelerator Mass Spectrometry, Lawrence Livermore National Laboratory, Livermore, California 94550 Search for other works by this author on: GSW Google Scholar Marc W. Caffee Marc W. Caffee 2Center for Accelerator Mass Spectrometry, Lawrence Livermore National Laboratory, Livermore, California 94550 Search for other works by this author on: GSW Google Scholar Geology (1993) 21 (4): 343–346. https://doi.org/10.1130/0091-7613(1993)021<0343:QOSPAD>2.3.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation James A. McKean, William E. Dietrich, Robert C. Finkel, John R. Southon, Marc W. Caffee; Quantification of soil production and downslope creep rates from cosmogenic 10Be accumulations on a hillslope profile. Geology 1993;; 21 (4): 343–346. doi: https://doi.org/10.1130/0091-7613(1993)021<0343:QOSPAD>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Average soil transport rates over a period of ∼3500 yr on a convex soil-mantled hillslope have been quantified using a mass-balance model that incorporates the soil concentration of the cosmogenic isotope 10Be. The 10Be model results support the assumption used in most geomorphic models that the soil creep rate is proportional to surface gradient. The predicted diffusion coefficient is 360 ±55 cm3 ⋅ yr-1 ⋅ cm-1 contour length and the average rate of soil production is 0.026 ±0.007 cm/yr. Within the uncertainty of this technique, the data do not reject G. K. Gilbert's hypothesis that some hillslopes may exist in a condition of dynamic equilibrium with a uniform soil production rate. However, the model does not require an assumption of dynamic equilibrium and may be an approach that uniquely allows the quantification of a local soil-production rate law. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Recent gravitative mass movements in a highly tectonically active arc system: The Hellenic Arc

The Hellenic Arc system is located within one of the world's most seismically active areas and has experienced extreme tectonism throughout Tertiary and Quaternary times. This paper discusses the various types of gravitative mass movements that have been detected and examines their causative factors in environments of high tectonic and seismic activity. The areas surveyed are the Sporadhes and Corinth grabens in the back-arc basin, the Zante-Cephalonia shelf on the inner slope of the Hellenic trench, and the Kythera ridge in the outer island arc. The various types of gravitative mass movements that have been documented include: (i) sliding of large masses with no or only slight internal deformation, (ii) sliding of masses with disintegration of the sediment fabric to debris flows and (iii) slow downslope creep. The mass movements are associated with fault escarpments, delta fronts and well-stratified slopes with slope gradients ranging from 2 to 40°. Earthquakes seem to be the main mechanism initiating mass movement, but other mechanisms such as increasing topographic gradient, gas in the sediment pores and heavy rain, can also independently trigger and/or contribute to the initiation of mass movements.

Depositional and Deformational History of the Franciscan Complex, Northernmost California: ABSTRACT

Pervasive extensional shear fractures and curvilinear arrays of clay and silt-filled veins in Franciscan Complex melanges and turbidites formed when Franciscan sediments were unlithified. Sandstone dikes both crosscut and follow fractures. Several scales of extensional faulting account for the juxtaposition of turbidites of different facies and/or with varying degrees of stratal disruption, the formation of sandstone lozenges and pinch-and-swell structures, and the formation of scaly foliation within the matrix of melange units. Within turbidites, the upper laminated portions of beds commonly contain abundant listric microfaults and the more massive lower portions of beds contain sediment-filled vein arrays. Veining and faulting occurred concurrently and resulted in differential extension of upper verses lower portions of beds. The finer sediment in veins reflects both cataclasis and filtering in of clay and silt from vein walls. Most Franciscan rocks record an early pervasive, layer-parallel flattening strain, which may be related to the gravitational collapse of late Mesozoic Franciscan inner trench slope sediments that accompanied accretionary prism expansion resulting from underplating. However, some turbidites record noncoaxial extension that resulted from downslope creep of sediments. At Crescent City, sediment creep resulted in oversteepening of the Franciscan inner trench slope, which, in turn, may have triggered large-scalemore » failure of slope materials resulting in the emplacement of the Crescent City olistostrome. The olistostrome crops out for 12 km along the coast, is up to 600 m thick, is in depositional contact with turbidites, and contains chiefly sandstone, greenstone, chert olistoliths up to 200 m across, and zones of slump-folded turbidites.« less

Drained deformation and failure due to cyclic pore pressures in soft natural clay at low stresses

Slow, recurring downslope movements in northern climates are frequently referred to as "creep movements," and are usually related to outwards freezing followed by vertical thawing movements. An alternative mechanism is examined in the reported test data.Undisturbed block samples of proglacial clay from a slope near yellowknife, N.W.T., have been tested by cyclically varying the pore-water pressure in triaxial specimens by an amount Δu, and measuring the resulting strains per cycle. The specimens were initially anisotropically consolidated with normal and shear stresses corresponding to those in the moving mantle. Drainage was permitted throughout the testing. This procedure represents changes that can occur in a natural slope from (a) seasonal groundwater level changes and (b) elevated pore-water pressures that accompany thawing. After 60–100 cycles, the pore-water pressure was systematically increased to the value Δuf at which the samples failed. This occurred on a steep, low-stress envelope, approximately c′ = 4 KPa, [Formula: see text]. The envelope is probably controlled by the nuggety macrostructure of the clay and appears to be slightly to the left of the [Formula: see text] locus.The strains per cycle were approximately linear in the range 30–100 cycles. As a first approximation they have been modelled as varying linearly with the ratio Δu/Δuf almost up to failure at Δu/Δuf = 1.0. Key words: downslope creep, solifluction, slope stability, clay, pore-water pressure, cyclic loading, low-stress failure.

Climatic warming and basal melting of large ice sheets: Possible implications for east Antarctica

Climatic warming is shown to be capable of inducing shear heating instability and basal melting in a model ice sheet that is creeping slowly downslope. Growth times of the instability are calculated from a nonlinear analysis of temperature and flow in the model ice sheet whose surface undergoes a prescribed increase of temperature. The source of instability lies in the decrease of maximum ice thickness for steady downslope creep with increasing surface temperature. A surface temperature increase of 5 to 10 K can cause instability on a 104 year time scale for realistic ice rheology. The instability occurs suddenly after a prolonged period of dormancy. The instability might be relevant to the East Antarctic ice sheet. Warming associated with the Holocene interglacial epoch that heralded the end of the last ice age may have set the East Antarctic ice sheet on a course toward widespread instability some 104 years later. The present CO2‐induced climate warming is also a potential trigger for instability and basal melting of the East Antarctic ice sheet.

Synsedimentary submarine slope failure and tectonic deformation in deep-water carbonates, Cow Head Group, western Newfoundland

The Cow Head Group, interpreted as a southeast-dipping base-of-slope carbonate apron, contains intraformational truncation surfaces and slide masses. Synsedimentary shear zones are formed (1) below intraformational truncation surfaces; (2) in the basal parts of slide masses; and (3) in the shallow subsurface because of downslope creep. Shear zones are characterized by a variety of synsedimentary deformation structures. Limestones are subject to folding, brecciation, rotation of fragmented beds, and the development of fitted-lenticular bedding. In the interbedded shales, there is both disruption of fine laminations and small-scale isoclinal folding and faulting. Outcrops characterized by these features and the lack of truncation surfaces or slide masses may reflect minor downslope creep. The presence of truncation surfaces, slide masses, and shear zones indicates deposition on an unstable sloping surface.The recognition of intraformational truncation surfaces and slide masses usually requires extensive strike exposure, which when lacking, (e.g., drill cores), limits the potential of these large-scale features as useful indicators of slope deposition. In the Cow Head Group, the recognition and proper interpretation of the common, small-scale deformation structures of synsedimentary shear zones provides evidence for slope deposition that is independent of other sedimentologic, stratigraphic, and regional data.In some parts of the Cow Head Group, "wrinkled" limestones characterized by a prominent dome-and-basin morphology reflect layer-parallel shortening related to tectonic deformation. The deformation of these limestones was previously considered to be synsedimentary, but their association with late-diagenetic precipitates and tectonic stylolites, in conjunction with their continuity and regularity, distinguishes these folds from those produced during synsedimentary deformation.

Explosive Growth of Shear-Heating Instabilities in the Down-Slope Creep of Ice Sheets

AbstractThe time-scale for the onset of the explosive growth of a finite-amplitude shear-heating instability in the down-slope creep of a thick ice sheet is determined by integrating the equation for the temporal evolution of the temperature-depth profile subsequent to a sudden change in ice thickness. All instabilities eventually grow explosively after a prolonged period of simmering or relatively slow monotonic growth. Though times for explosive growth depend on initial and final ice thicknesses, surface temperature, accumulation rate, basal heat flux, and ice rheological parameters, the explosion times are extremely sensitive to the activation energy and the pre-exponential constant of the ice-creep law. Sudden increases in ice-sheet thickness of 1–2 km due to a rapid climatic deterioration can lead to explosive instability and melting of the basal shear layer in only thousands of years if ice-creep activation energies are lower than about 60 kJ mol-1.

Explosive Growth of Shear-Heating Instabilities in the Down-Slope Creep of Ice Sheets

AbstractThe time-scale for the onset of the explosive growth of a finite-amplitude shear-heating instability in the down-slope creep of a thick ice sheet is determined by integrating the equation for the temporal evolution of the temperature-depth profile subsequent to a sudden change in ice thickness. All instabilities eventually grow explosively after a prolonged period of simmering or relatively slow monotonic growth. Though times for explosive growth depend on initial and final ice thicknesses, surface temperature, accumulation rate, basal heat flux, and ice rheological parameters, the explosion times are extremely sensitive to the activation energy and the pre-exponential constant of the ice-creep law. Sudden increases in ice-sheet thickness of 1–2 km due to a rapid climatic deterioration can lead to explosive instability and melting of the basal shear layer in only thousands of years if ice-creep activation energies are lower than about 60 kJ mol-1.

Intertidal drainage and mass-movement processes in the Severn Estuary: rills and creeks (pills)

Mass-movement processes acting at up to three scales influence the intertidal forms of drainage in the Severn Estuary and its bordering estuarine alluvium. The open mudflats are marked by delicate drainage rills into which some freshly deposited silt is transferred at low tide by downslope creep, as shown by widespread systems of small extensional fractures. The pills traverse the areas of reclaimed alluvium and the saltings. They retain a relatively symmetrical cross-sectional profile, even where tortuous, and are remarkably stable in both general position and detailed form. The pills are maintained by large rotational slips accompanied in many cases by channelled mudflows formed from much smaller failures. A profile of long-term sediment accumulation that favours the loading of the upper sides of the pills, irrespective of local channel curvature, appears to be a significant factor in the maintenance of these drainage lines.

Mechanisms of sediment transport and dispersion in a tectonically active submarine valley/canyon system: Zakynthos Straits, NW Hellenic Trench

Physical oceanographic and geophysical data sets are used to examine sediment transport processes. Currents were directed generally along the axis of the valley/canyon system and, in an area of low tidal range, are attributed to regional weather patterns. Near-bed and near-surface currents are opposed, both in terms of instantaneous observations and net (Lagrangian) drift. Near-bed down-canyon water movement provides a mechanism for the offshore movement of fine-grained sediment. Slope failures have been identified in the geophysical records and are considered to be controlled by earthquakes and salt diapirism, in a tectonically active area. These failures, as mass displacement or slow down-slope creep, result in debris infill and the generation of turbidity currents. Fine-grained sediments, resuspended by the turbidity currents, would be transported down-canyon by the slowly moving near-bed residual water movement. Hence, the dominant transport mechanisms in the valley/canyon system are: (1) frequent mass sediment failure; and (2) nearly continuous down-channel fine-grained suspended sediment movement.

Seismic Stratigraphy of Veracruz Tongue, Deep Southwestern Gulf of Mexico

The Veracruz Tongue is a region of continental slope and rise sediments bounded bathymetrically by the Mexican Ridges fold belt to the west and the Campeche Knolls salt province to the east. Study of two multichannel seismic lines and single-fold sparker data enables five post-early Miocene seismic stratigraphic sequences to be distinguished in the tongue. Sedimentary processes responsible for deposition of each sequence are inferred on the basis of external geometry, internal reflection configuration, and, where possible, core data. The five sequences are each interpreted to consist of turbidites grading laterally into hemipelagites. These primary deposits may be modified later by downslope creep or sliding and slumping. Because bathymetry is a major control on location of turbidity-current flows, slides, and slumps, the depositional history of the Veracruz Tongue provides indirect evidence of tectonic evolution of the Mexican Ridges fold belt and the Campeche Knolls salt province. Salt domes first formed a barrier to turbidity-current flow from the east at the close of the middle Miocene. The Mexican Ridges fold belt evolved gradually until it formed a complete barrier to turbidity-current flow from the west at the close of the Pliocene. As a result of shift in direction of turbidity-current flow through time, potentially sand-prone Miocene turbidites are overlain by hemipelagic clays and clayey oozes--a situation favorable to hydrocarbon entrapment.

Gravity-induced deformation on the north flank and floor of the sporadhes basin of the North Aegean sea trough

PDR and continuous seismic profiling surveys in the Sporadhes basin (the western part of the North Aegean sea trough) reveal active gravity-induced deformation of the postorogenic sedimentary sequence. Different deformational patterns in the uppermost sedimentary layers of the flanks and floor of the basin suggest that the layers are deformed by several gravity-induced processes including slumping, sliding and creep contemporaneous with sediment deposition. Antithetic faulting on the northern flank of the basin (the Thermaicos slope) results from downslope creep of surface layers over deeper tilted bedding planes. This process is a consequence of progressive slope development associated with the evolution of the Sporadhes basin. The morphology of the sea floor and the deformation of the upper layers clearly reflect synsedimentary tectonics in a subsiding basin environment.

Deltaic Influence on Shelf-Edge Instability Processes: ABSTRACT

Large river systems deliver significant quantities of fine-grained sediment to continental shelf regions. In specific areas off deltas, deposition rates are rapid and the sediment may be involved in a variety of mass movement processes on the subaqueous slopes (slumps and slides, debris flows, and mudflows) causing rapid sediment accumulation at shelf-edge depths and resulting in active progradation of the shelf edge. Seismically, the deposits appear as large-scale foresets and are commonly composed of in-situ deep-water deposits alternating with shallow-water sediments transported by mass movement. On electric logs, sands within these units are sporadic display sharp basal planes and blocky shapes. Progradation of the shelf-edge deposits is generally accompanied by overs eepening and large-scale instability of the upper self-edge slopes. Deep-seated and shallow rotational slides move large volumes of sediments and deposit them on the adjacent slopes and upper rise. Extensive contemporaneous faults commonly form at the shelf-edge. Continuous addition of sediment to the fault scarps, particularly by mass movement from nearby delta-front instability, causes large volumes of shallow-water sediment to accumulate on the downthrown sides of the faults, mostly forming large-scale rollover structures. Continued movement along the concave-upward shear planes commonly results in compressional folds and diapiric structures. Contemporaneous accumulation of shallow-water mass movement deposits may occur in association with these structures. Massive retrogressive, arcuate-shaped landslide scars and canyons or trenches can also form at the shelf edge owing to slumping and other mass-movement processes. Such canyons and trenches can attain widths of 10 to 20 km, depths of 800 m, and lengths of 80 to 100 km. The creation of such features by shelf-edge instability results in exceptionally large volumes of shallow-water sediment yielded to the deep basins in the form of massive submarine fans. The infilling of depressions by deltaic progradation is rapid, forming large foresets near the canyon heads. The low strength of the rapidly infilled, underconsolidated sediments causes downslope creep or reactivation of failure mechanisms, resulting in multiple episodes of filling and evacuation. End_of_Article - Last_Page 912------------