Large-scale Slumping Research Articles

Slumps, canyons and related features on the continental margin off East London, SE Africa (SW Indian Ocean)

Bathymetric and seismic reflection surveys in the Natal Valley east of East London show that the most important factors in the Neogene development of the outer continental margin have been large-scale slumping and penecontemporaneous canyon formation. Slope wasting and allochthonous mass movements have shifted the foot of the continental slope up to 25 km basinward and have constructed extensive continental rise lobes. Shelf break breaching by canyon heads is considered an important factor in transferring sediment entrained in the Agulhas Current from the continental shelf to the deep ocean basin. A preliminary sediment budget suggests that the continental rise lobes could have been constructed during the last 2.69 m.y. following an episode of massive slumping.

Unifite muds in intraslope basins, northwest gulf of Mexico

Uniformly structureless clayey muds, very much like those termed unifites or homogenites in the Mediterranean and other basins, occur in intraslope basins in the northwest Gulf of Mexico. Their organic carbon and carbonate contents indicate a terrigenous source. Their age (about 17,000 B.P.) approximates the time when large-scale slumping of terrigenous delta fronts formed the Mississippi Canyon (Trough). Their compositional dissimiliarity to nearby hemiplagic mud precludes a homogenite-like origin involving a tsunami. However, it is uncertain whether they are end-products of bypassing (unifites) or entrapments of entire flows of sandless clays from deltaic facies of the same composition.

Late Mesozoic and Tertiary sediment supply to the eastern Cape Basin (SE Atlantic) and palaeo-drainage systems in Southwestern Africa

Isopach asymmetry, and sediment component changes in DSDP cores from the SE Atlantic (Orange Basin) support the hypothesis of major drainage system changes in SW Africa during late Cretaceous—Cenozoic time. This involved alternations in the use of the 28°S (modern Orange River) and 31°S (modern Olifants River) exit points across the western escarpment by rivers carrying run-off from the Upper Orange/Vaal catchment areas, as well as radical re-organizations of internal drainage geometry. It is postulated that during late Cretaceous times the 28°S exit was used, with the Middle Orange River following a course in the interior well to the south (up to 150 km) of its modern channel. Sediment discharge rates from this river were relatively high (at least 10 × 10 6 m 3 yr −1), and resulted in rapid advancement of the continental margin sediment prism west of the mouth by large-scale slumping. The Palaeogene Orange/Vaal river exit was via the 31°S escarpment crossing, and during the later part of this period, the Cape Canyon was cut across the continental shelf and slope. A significant reduction in sediment discharge (to 2.0 × 10 6 m 3 yr −1) suggests that the Lower Tertiary climate for SW Africa was drier than that of late Cretaceous times. However, aridity did not commence until late Miocene times, when the Orange/Vaal discharge had switched back to the 28°S exit. Modern sediment discharge rates (6.5 × 10 6 m 3 yr −1) are relatively high and reflect soil erosion caused by agricultural activity. The two major alterations in exit point of the Orange/Vaal (late Cretaceous—early Tertiary, and late Oligocene—early Miocene) are related to periods of low sea level, which promoted river capture adjacent to the western escarpment. An additional factor in the first course change may have been the disruption of the Middle Orange channel by late Cretaceous igneous intrusions. Less important internal reorganizations of the drainage system are postulated in late Miocene—Pleistocene times. Economic implications for offshore diamond distribution are briefly mentioned.

Large-scale submarine rotational slumps and growth faults in the Eastern Mediterranean

Submarine slumps with areas of 1000 km 2 and more disturb the continental margin of Israel and nearby areas. In them the Plio-Quaternary silty-clayey sediments, up to 1.5–2 km thick, are broken by shore-parallel growth faults into 5–10 blocks which are generally rotated and tilted landward. The faults flatten downward and become rooted in ridges formed by deformation of the underlying mobile Messinian evaporite-bearing series. Deformation is thin-skinned and does not visibly extend to deeper levels. Slumping occurred only where a sufficiently thick overburden accumulated over a sufficiently well developed mobile substrate. Then gravitational instability developed and led to basinward sliding of the continental margin sediments over the Messinian series which acted as a lubricant and a level of decollement. The mobility of this series is probably caused by overpressured shale beds that are confined between the impervious evaporites. This mechanism explains all the features of the slumps. Hitherto, faults directly related to the slumps of the Eastern Mediterranean were not identified, but since the region is tectonically active it is not possible to exclude the possibility that basinward sliding was triggered either by motion of minor underlying faults or by seismic shaking. However, it seems that even if this were the case, slumping could be triggered only because the continental margin was anyhow gravitationally unstable or close to such a state. Therefore, the gravitational instability was the main factor that controlled the deformation of the Pliocene—Quaternary sediments. The slumps of the Eastern Mediterranean resemble large-scale slumps that are known from other passive continental margins in the world.

Mesozoic and late tertiary submarine fan sequences and their tectonic significance, Sumba, Indonesia

The Indonesian island of Sumba occupies a critical zone which separates a “normal” subduction complex (Java Trench System) from a continent-island arc collision zone (Banda Arc). Sumba is considered by some authors to be a microcontinental fragment, although in this context its pre-fragmentation origin is uncertain. Detailed studies of the stratigraphy of Sumba, however, throw light on the origin and subsequent geological history of the island and surrounding areas. On available evidence, the oldest outcropping rocks are of Cretaceous age and locally contain a tropical Tethyan marine fauna. They comprise dark-coloured, sometimes carbonaceous and often volcanogenic mudstones, sandstones, gravels and diamictites, pervasively intruded by andesitic and dacitic dykes and locally intruded by granodiorite plutons. These sediments are siliclastic and appear to have had a continental provenance. The Cretaceous rocks in many areas compromise part of a major submarine fan complex which, at least in south-central Sumba, prograded towards the southwest and south. Shallow marine and non-marine Paleogene sediments and volcanic agglomerates lie unconformably above the uplifted, tilted and eroded Cretaceous strata. These are overlain in turn by the widely outcropping Miocene Sumba Formation, an oceanic sequence with island-arc affinities. Accessible outcrops of the Sumba Formation comprise platform carbonates with abundant reef developments in west Sumba, and a major submarine fan—pelagic chalk complex in east Sumba. The fan complex, which extends beneath the southern Savu Sea north of Sumba, is dominated by large-scale slumps, olistostromes and thin- and thick-bedded turbidites. Turbidite sands and gravels invariably are volcaniclastic, containing components derived from a typical inner (volcanic) arc complex. Paleocurrent indicators are difficult to find in the Sumba Formation, thus it is uncertain at this stage whether the volcaniclastic turbidites were derived from the present volcanic arc or from an undetermined source which may since have subsided. The portion of the fan complex which underlies east Sumba, however, has undergone vertical uplift since its emplacement of between one and several kilometres. Uplift is actively in progress at present, possibly due to underthrusting by northward-moving Australian continental crust. In this context the island of Sumba comprises an uplifted, north-tilted and subaerially exposed part of the forearc basin of the Indonesian Arc. This uplift can be related to initial stages of continental collision.

Late Quaternary Depositional Systems and Sea Level Change--Santa Monica and San Pedro Basins, California Continental Borderland

A suite of seismic reflection data that provides different degrees of resolution and penetration was used to map the depositional systems that have developed in Santa Monica and San Pedro basins during the late Quaternary. Submarine fan growth, particularly at the mouths of Hueneme and Redondo Canyons, has been the dominant mode of basin filling. Mass movement processes, ranging from creep to large-scale catastrophic slumping, have been important locally. In general, large-scale fan growth fits Normark's model in which the suprafan is the primary locus of coarse sediment deposition. Smaller scale morphologic and depositional patterns on the Hueneme and Redondo fans (e.g., distributary channels and coarse sediment concentrations basinward of the inner suprafan) suggest that a significant amount of coarse sediment presently bypasses the suprafans, however. Long-distance coarse sediment transport was particularly pronounced during late Wisconsinan lowstand of sea level and resulted in progradation of lower mid-fan and lower fan deposits. As sea level rose rapidly between 18,000 y.B.P. and 10,000 y.B.P., shelves trapped much of the sediment supply. Consequently, rates of basin sedimentation decreased markedly and the area of fan developmen was restricted. As the rate of shelf sedimentation decreased concomitantly with a decrease in the rate of sea level rise 10,000 y.B.P., sediment transport to the basins was renewed and modern depositional patterns were established. Fan development also has been influenced by large-scale mass movement, extensive mid-fan overbank flow and the consequent development of sediment waves, deflection of turbidity currents by bathymetric obstacles, and local tectonism. Correlations between sand layer characteristics and high-resolution echo types for fan and basin-plain deposits show trends similar to some of those observed in deep-sea depositional environments. However, because Santa Monica and San Pedro basins are small, enclosed, tectonically active basins that receive sediment from relatively large drainage areas, proximal-to-distal changes in sand layer thickness, abundance, and frequency are less pronounced than those observed in deep-sea environments.

Large-scale slumps off southern India and Sri Lanka

Two suites of slumps from opposite margins of the Gulf of Mannar, between Sri Lanka and southern India, have met and coalesced. The “Eastern Comorin” Slump is the more coherent of the two with a length of 70 to 100 km. The “Colombo” side slump consists of two to four blocks 15 to 35 km in length. Both slump-suites decrease to the south. A paleoslump underlies the western toe of the East Comorin Slump at a depth of some 800 meters. To the south, an enlarging and deepening submarine canyon marks the area of slump coalescence.

Reconnaissance in DSRV Alvin of a “fluvial-like” meander system in Wilmington Canyon and slump features in South Wilmington Canyon

Three dives in DSRV Alvin on the Atlantic Continental Rise in Wilmington and South Wilmington Canyons, off the east coast of the United States, allowed examination and sampling of morphological features, in water depths of 2,300 to 2,400 m, that were observed in midrange sidescan sonar data. In Wilmington Canyon, a “fluvial-like” meandering system was confirmed. The meandering channel had steep undercut outer banks and gently sloping inner banks. Localized slumping is inferred from many steplike depressions on the steep outer banks. Although the meander system is as much as 500 m wide and has many characteristics typical of a fluvial system, extreme depth and evidence of episodic ongoing sedimentary processes preclude a true fluvial origin. Currents of unknown origin—that is, downcanyon turbidity flows—appear to be the only agent capable of sculpturing the observed features. A meander system was not observed in South Wilmington Canyon. Channel-floor features, including deformed and displaced sediment, support a previous suggestion of large-scale slumping in the area. These sediments include upturned clay beds, disaggregated gravels, loosely bound gravel conglomerates in a reddish-brown matrix, and a tubular structure, resembling a tree-root cast, within a thinly bedded, reddish-brown sandstone.

Sedimentologic Processes in Wilmington and South Wilmington Submarine Canyons as Indicated from DSRV Alvin: ABSTRACT

End_Page 1671------------------------------Three submersible dives in DSRV Alvin were made on the continental rise seaward of Atlantic City, New Jersey: two in Wilmington Canyon and one in South Wilmington Canyon. The dives, made on features observed in midrange sidescan sonar records reveal a sharp contrast in physiography between the two canyons and provide an insight to the sedimentologic processes associated with canyon development. On the upper rise, Wilmington Canyon is characterized by a well-developed meander pattern with alternating steep and gently sloping side walls. The steep channel walls correlate with the outside (concave) part of a meander bend and are segmented by numerous steep-walled, steplike depressions. Therefore, undercutting at the base of the channel wall, accompanied by localized slumping, is inferred. These depressions are missing on the gently sloping channel walls. Unconsolidated sediment, in excess of 1.3 m thick, is present in the channel axis but is generally missing in the depressions on the channel wall. The high quartz sand content (30%) in the channel sediments and the paucity of unconsolidated sediment in the depressions indicate recent activity. In South Wilmington Canyon various deformed and displaced(?) sediments support a previous suggestion of large-scale slumping. These features include upturned clay horizons at the base of the wall, stratigraphically overlain by disaggregated gravels, a loosely bound gravel conglomerate, and oxidized sandstone horizons. A tubular object 1 m long, tentatively interpreted as a tree root cast, was recovered from the sandstone. A meander pattern was not found in South Wilmington Canyon. End_of_Article - Last_Page 1672------------

Growth Faulting and Sedimentation in Upper Wilcox Delta System, South Texas: ABSTRACT

Well logs from the deep Wilcox trend of south Texas delineate the upper Wilcox Rosita delta system which comprises three locally developed delta complexes comparable in scale to deltas in the lower Wilcox Rockdale delta system of middle and upper Texas Gulf Coast. Each delta comprises several lobes, some of which can be traced across the deep zone where they thicken as much as tenfold, owing to progradation over a series of active growth faults. Characteristic upward-coarsening progradational units are interpreted to include prodelta, delta-front, distributary-channel, and interdistributary facies. There is an overall change from delta-plain to prodelta facies basinward across the growth-fault zone. Growth faults are large-scale slumps formed by basinward gravity gliding of huge masses of deltaic sediments. Faulting was initiated and maintained as sand and mud of the delta front prograded over previously deposited prodelta mud. The growth faults influenced the thickness of sediment which accumulated on each side of the fault by controlling the relative rates of subsidence (growth ratio). Constant growth ratios for a series of consecutive progradational sequences indicate that rates of subsidence were insensitive to local fluctuations in sedimentation. Log patterns suggest that sedimentary processes were continuous across fault traces; no important surface expression of faults is evident. End_of_Article - Last_Page 703------------

Slump Features on the Mississippi Fan, Northeastern Gulf of Mexico

During recent geologic time, the Mississippi River system has been the dominant contributor of terrigenous sediment to the northeastern Gulf Basin. A large mass of sediments has been produced on the continental slope seaward of the mouth of this system. This sedimentary feature has been referred to as the Mississippi Fan. During a recent geophysical cruise in the Gulf of Mexico, several crossings of the fan resulted in seismic records that led to the conclusion that large scale slumps were active on the fan in very recent time. Evidence presented here indicates one slump movement originated near the mouth of the Mississippi Trough and moved about 160 nm toward the southeast. A part of this slump flowed to the south and southwest, or a second slump developed at about the same time and flowed in this direction for a distance of about 130 nm. The southeasterly slump is about 70 nm in width, and the smaller slump, to the west, is nearly 38 nm wide. The maximum measured vertical distance from the point of origin of these features to the downslope limit is about 4000 ft.

Geology of Plancha L-12, Colombia, South America: A Reconnaissance

Plancha L-12 (quadrangle) of the Map of Colombia, covering 4800 sq km, includes part of the easternmost Andean chain of Colombia, Cordillera Oriental, its eastern foothills, and the adjacent flatlands. The exposed rocks have a cumulative thickness of about 7000 m and are divided by the author into ten major units. Pre-Devonian(?) granodiorite, phyllites, and metasandstones are unconformably overlain by 800 m of Middle to Upper Paleozoic argillites, sandstones, and limestones at the center of an anticlinorium which constitutes the core of Cordillera Oriental. A major reverse fault marks the eastern limit of Cordillera Oriental. The foothills display asymmetrical folds, thrust faults, and what appears to be large-scale slumping. The rocks exposed include 600 m of conglomerates, hales, and sandstones of Mesozoic age; 1700 m of sandstones and shales which range from Eocene through Oligocene or early Miocene; and about 3000 m of conglomerates, conglomeratic sandstones, and mudstones of Miocene age. In the eastern and southern half of Plancha L-12, the structures of the foothills disappear under Pliocene(?) to Recent alluvial cover in lowlands which make up 75 per cent of Plancha L-12. Some of the unconsolidated fluvial deposits of Pliocene(?) to Recent age have been tilted and folded. Analysis of the stream pattern of the lowlands suggests tectonic control of modern physiographic features in the eastern part of Plancha L-12. There is evidence of regional uplift and folding in Early Paleozoic, Jurassic(?), late Eocene or early Oligocene, Miocene, and Plio-Pleistocene time. The main uplift of Cordillera Oriental appears to have occurred in late Miocene time.

Permian palaeogeography of the northern Australia-Timor region

A summary of the character and disposition of the Permian rocks of Timor is adduced for revising the present views of Permian palaeogeography of the northern Australia-Timor region. It is shown that the autochthonous Permian rocks of Timor were derived as detritus (in part by large-scale slumping) from the Kimberley region of northern Australia. Bathymetry increased northwards from Australia, with Timor occupying a shallow shelf position, and the region north of Timor rapidly increasing to greater depths generally associated with the axial trough of geosynclines. The northern margin of the geosyncline was marked by the deposition of reef limestones and eruptive rocks. The geological evidence presented here indicates that the spatial relationships between northern Australia and Timor were much the same in Permian times as they are today. This conflicts with the suggestions of very large crustal dislocations immediately north of Australia recently advocated on the basis of regional palaeomagnetic studies, but there may be some support for such movements having taken place north of Timor.

ROLE OF FLUID PRESSURE IN MECHANICS OF OVERTHRUST FAULTING

Pressures of interstitial fluids significantly greater than the normal hydrostatic pressure are known in many parts of the world. Many occurrences are in thick sections of relatively young sediments; some are in areas that have been intensely deformed. Abnormal fluid pressures in the Gulf Coast region are associated with thick bodies of shale or mudstone, and with high hydraulic gradients across bedding. The rocks there have been buried rather rapidly and are evidently not yet fully compacted. The mechanism by which clay consolidates under pressure affords a quantitative relationship among the variables—depth, strength of clay, and fluid pressure—and this relationship indicates that the Gulf Coast examples agree fairly well with observations on depth and porosity in Paleozoic shales of Oklahoma and Tertiary shales of Venezuela. Critical data are lacking, but permeability clearly decreases tremendously as clay rocks are compacted. This decrease in permeability provides a self-sealing mechanism that greatly retards the escape of pore water from deeply buried clay rocks. The relationship between rate of compaction and the development of abnormal fluid pressures probably applies not only to clay rocks but also to carbonates and possibly to micaceous and chloritic metamorphic rocks. Conditions of geosynclinal deposition are, in general, those most favorable to the development of abnormal fluid pressures. The hypothesis that large-scale overthrusting is facilitated by abnormal fluid pressures which, in turn, are associated with geosynclinal deposition is applied to the overthrust belt of western Wyoming and adjacent States. This is a long curving belt of several bedding-plane faults which have an aggregate horizontal displacement across the belt of 50 miles or more. The sedimentary rocks that make up the belt were evidently deposited in a major geosyncline bordered by uplands not far to the west. At any given locality, the rate of deposition of the sediments increased continuously until the beginning of intense deformation and overthrusting. The geosynclinal axis and the bordering uplands probably migrated slowly eastward across the belt. Several lines of indirect evidence suggest that abnormal fluid pressures developed in this region during final stages of rapid geosynclinal sinking and that thick plates of Paleozoic and Mesozoic sedimentary rocks sheared off from the underlying rocks and moved slowly eastward. Rate of movement probably was controlled by erosion of upfolds that arose at the front of each moving plate. The fundamental cause of the lateral stresses that propelled the overthrusts is not known, but it may be examined instructively in the light of the fluid-pressure hypothesis. The thrust sheets might, for example, have slid by simple gravitation down the western limb of the geosyncline on reasonable slopes and not improbable fluid pressure-overburden ratios. Such large-scale slumping of thrust sheets, however, seems to require gaps at the rear of the thrust sheets. The long intermontane valleys of Idaho and Utah may possibly have originated as such gaps or rifts, but no proof has yet been recognized that they were formed in this manner. An alternative possibility, regional compression, requires concentration of the lateral stresses within the upper few miles of the earth9s crust; in this general region emplacement of the Idaho batholith seems the most likely source of such superficially concentrated stresses. However, this batholith is so far from the front edge of the overthrust belt that it would require extremely high fluid pressure-overburden ratios over a wide area. Perhaps some combination of the two forces—pushing wide thrust plates down a gentle slope—is the most likely explanation.