SLUMPING ON A CONTINENTAL SLOPE INCLINED AT 1°–4°
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ABSTRACT Continuous seismic profiles from the upper continental slope east of North Islands, New Zealand, show that surface sediment 10–50 m thick has slumped down bedding planes sloping at 1°–4°. There are four slumps, the Kidnappers Slump which has an area of 250 km 2 , the Paoanui Slump of 80 km 2 , a small slump of only several square kilometers and a slump of undetermined extent. All occurred during the last 20,000 years in Last Glacial Age sediments. A glide plane is exposed at the head of each slump and beds are thrust or contorted at the toe of some slumps. Slumping was probably caused by the failure of loosely packed sandy silt during major earthquakes.Keywords:
Slumping
Bed
Slope failure
Silt
Bedding
ABSTRACT Continuous seismic profiles from the upper continental slope east of North Islands, New Zealand, show that surface sediment 10–50 m thick has slumped down bedding planes sloping at 1°–4°. There are four slumps, the Kidnappers Slump which has an area of 250 km 2 , the Paoanui Slump of 80 km 2 , a small slump of only several square kilometers and a slump of undetermined extent. All occurred during the last 20,000 years in Last Glacial Age sediments. A glide plane is exposed at the head of each slump and beds are thrust or contorted at the toe of some slumps. Slumping was probably caused by the failure of loosely packed sandy silt during major earthquakes.
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Slope failure
Silt
Bedding
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ABSTRACT A core of a Yegua gas reservoir that is 45 ft (14 m) thick at a depth of 9940 ft (3030 m) shows highly disturbed bedding that is sheared and has increasing dip downward. The sandstone has some preserved ripple structure and abundant carbonaceous material that suggest the sands were deposited in an unstable, delta-front location and subjected to mass movement of sediments, similar to slumping on the modern Mississippi delta. Dip logs in four wells indicate normal faults at the base of the sandstone. The faults have variable orientations and divide the reservoir into small blocks. Slump-fault displacements are on the order of 50 ft (15 m), but their occurrence in all wells at the same level makes fault detection difficult without cores or dip logs. The multiple slump blocks are probably separated by fault zones that are barriers to fluid production.
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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------------
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Abstract The Waipoapoa Landslide (February 1976) in Southern Hawke's Bay is a complex failure in Upper Miocene weak alternating sandstone and mudstone (flysch). The landslide extends over approximately 18 ha and forms a reactivated portion of a larger ancient landslide complex. The Waipoapoa Landslide has a calculated volume of 8.35 × 106 m3. Morphologically the landslide is comprised of a head zone, front face, and debris-flow complex. Rock-mass defects have controlled slide block geometry. The crown escarpment of the head zone has propagated on nearly vertical intersecting fractures (joints and faults). The intersecting subhorizontal basal shear surface is coincident with bedding contact(s) in an alternating succession of friable porous sandstone and weakly consolidated slake-prone mudstone. Bedding attitude is inclined into the slope, with dips ranging up to 13°. Block movement has occurred against dip direction and has imparted a back-tilted geometry to the head zone depression. A multiple basal shear surface, and basal shear-gouge zone are inferred. The front face is postulated to have bulged outward during the main movement phase and successively collapsed, generating large, rapidly moving debris flows with an estimated volume of 1.75 × 106 m3. These flows stretch is excess of 900 m downvalley from the landslide toe. Failure mechanisms included both sliding and flowage. The failure involves several large blocks and is of multiple block geometry. Principal factors contributing to failure could have included: the presence of fractures (joints and faults) and other defects (e.g., bedding) along which block movement was facilitated; rapid ground-water infiltration along open fractures within the rock mass; high pore-water pressures in one or more porous sandstone beds; and the local steep relief. Keywords: Hawke's Baylandslidesblock slidedebris flowTertiaryflyschrock-mass defects
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