Aseismic controls on in situ soft‐sediment deformation processes and products in submarine slope deposits of the Karoo Basin, South Africa

Abstract

Intervals of soft‐sediment deformation features, including vertical fluid escape and load structures, are common and well‐exposed in Permian lower slope deposits of the Tanqua Depocentre, Karoo Basin. The structures mainly comprise elongated flames and load structures associated with ruptured sandstones and structureless siltstones, observed over a range of scales. The presence of an upper structureless siltstone layer linked to the flames, interpreted as a product of the debouching of fine‐grained material transported through the flame onto the palaeo‐seabed, together with the drag and upward folding of lower sandstone layers is evidence that the flames were formed in situ by upward movement of sediment‐rich fluids. Flames are oriented parallel to the deep‐water palaeoslope in lateral splay deposits between two major slope channel complexes. Statistical correlation and regression analyses of 180 flame structures from seven stratigraphic intervals suggest a common mechanism for the deformation and indicate the importance of fluidization as a deformation mechanism. Importantly, deformation occurred in an instantaneous and synchronous manner. Liquefaction and fluidization were triggered by incremental movement of sediment over steeper local gradients that were generated by deposition of a lateral splay on an inherited local north‐west‐facing slope. Seismic activity is not invoked as a trigger mechanism because of the restricted spatial occurrence of these features and the lack of indications of earthquakes during the time of deposition of the deep‐water succession. The driving mechanisms that resulted in the final configuration of the soft‐sediment deformation structures involved a combination of vertical shear stress caused by fluidization, development of an inverse density gradient and a downslope component of force associated with the local slope. Ground‐penetrating radar profiles confirm the overall north‐east orientation of the flame structures and provide a basis for recognition of potential larger‐scale examples of flames in seismic reflection data sets.

Distribution of soft-sediment deformation structures in clinoform successions of the Permian Ecca Group, Karoo Basin, South Africa

“…Facies association 3 comprises SSDS of the in situ (Oliveira et al, 2009) or load group (Dżułyński and Walton, 1965). These are related to vertical movement (orthogonal to the seabed) and include load structures (load casts, pseudonodules and ball-and-pillow), waterescape structures (Lowe, 1975), pillar, dish and flame structures and convolute lamination.…”

Extensive soft-sediment deformation structures in the early Darriwilian (Middle Ordovician) shallow marine siliciclastic sediments formed on the Baltoscandian carbonate ramp, northwestern Estonia

“…Rijsdjik 2007) -Slumps (e.g. Oliveira et al 2009) -Tsunamis (e.g. Meshram et al 2004, Matsumoto et al 2008) -Earthquake (e.g.…”

“…4), but they generally alternate with dome-, cusp-or spike-shaped flame structures (sensuOliveira et al (2009); Fig. 5A, B, C, D).…”

Types of soft-sediment deformation structures in a lacustrine Ploužnice member (Stephanian, Gzhelian, Pennsylvanian, Bohemian Massif), their timing, and possible trigger mechanism

“…For example, Lee et al (2014) studied bedding features and relations of coastal settings bearing SSDS and interpreted the origin of SSDS to be a consequence of seismic events related to active faults. Aseismic deformations were interpreted by many authors: Oliveira et al (2009), Tanner and Lucas (2010), Oliveira et al (2011) or Chen and Lee (2013), and seismic deformations by Ricci Lucchi (1995), Rossetti (1999), Rodrıǵuez-Pascua et al (2000), Rossetti and Góes (2000), Moretti et al (2002), Brandes andWinsemann (2013).…”