Three main kinds of syndepositional deformation are found in cross‐bedded sand‐grade sediments. The first type is characterized by simple recumbent folds of broadly parabolic form. The second is marked by series of folds, with or without overturning. The third type is much more complex, presenting a combination of faulting, folding, and the local destruction of bedding.
The type marked by recumbent folds is interpreted as due to the deformation of a liquefied (or perhaps fluidized) sand by current drag following an event in the majority of cases suspected to be an earthquake shock. By reference to empirical and theoretical studies of sedimenting systems, and the behaviour under small shear stresses of liquids of high viscosity, this physical model is developed analytically to yield equations describing the geometry of the deformations in terms of the thickness of the deformed bed, the settling velocity and concentration of particles in the liquefied sand, the viscosity of the liquefied sand, and the magnitude of the deforming force. The equations describe a fold surface that is a portion of a flat‐lying parabola, and show that the proposed circumstances of deformation are plausible in terms of what is known of the real situation. They further reveal that, under the assumptions made in the analysis, the vertical height of the fold hinge above the base of the bed is a function only of the initial shape of the deformed cross‐stratum, the shear rate in the liquefied materials, and the falling velocity and concentration of particles in the liquefied bed. The shapes of deformations calculated from the equations agree well with patterns observed from the geological record.
High quality seismic data recently acquired offshore Namibia have revealed the complex interplay between sedimentation and rifting that accompanied the breakup of Gondwanaland. Seismic sequence analysis has demonstrated a complex series of superimposed phases of rifting.
Jurassic early-rift sediments were deposited in a Basin & Range setting with marine embayments. Mid- to Late Jurassic extension probably culminated in acidic to intermediate volcanicity. Subsequently, late-rift fill sedimentary accumulations were terminated locally by a major period of plateau-forming effusive activity. This was coincident with the start of Early Cretaceous spreading in the South Atlantic. Shelf, slope and basinal facies are clearly defined in the post-rift succession.
Although virtually undrilled, the area is considered to have very considerable petroleum potential. Seismic stratigraphy will continue to play a vital role in defining the critical juxtapositions of source rock, reservoir and seal.
Flume experiments with medium sand confirm the increasing complexity in the shape of small‐scale current ripples with increasing flow velocity for constant depth. Experiments suggest that a measure of ripple shape (the ratio of wavelengths of transverse to streamwise features, λ‐x/λ‐z) has a more complex relationship with the flow property (Fr, H̄/d̄) (where Fr is Froude Number of the flow and H̄/d̄ is relative roughness), than was previously realized. It is suggested that hydraulic properties of the flow at the sediment water interface have a more general relationship with ripple form than do properties of the whole flow such as Froude and Reynolds Numbers. Properties of the whole flow tend to separate data into depthrelated curves at shallow flows where the free water surface influences the structure of the turbulence.
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