Bed-parallel slip: Identifying missing displacement in mass transport deposits

Abstract

Bed-parallel slip (BPS), where neighbouring beds slide past one another along bedding planes, is notoriously difficult to identify without reference to pre-existing features such as steep faults or dykes that act as markers to record BPS offset. While BPS is intuitively thought to operate during downslope sliding of mass transport deposits (MTDs) in sedimentary basins, there is a conspicuous lack of supporting outcrop or seismic data to corroborate this and BPS displacement may therefore have remained unaccounted for. This study addresses this gap in knowledge by investigating late-Pleistocene MTDs developed around the Dead Sea Basin and provides the first detailed analysis of BPS that pervades a gravity-driven setting. In particular, we examine the role of BPS that crosscuts earlier normal and reverse faults that act as markers to allow metre-scale patterns of horizontal displacement to be identified in MTDs. The studied BPS always forms with a consistent top-to-the east sense of offset that corresponds with gravity-driven downslope movement towards the depo-centre of the basin. BPS frequently develops adjacent to competent detrital-rich beds and forms discrete glide planes with little or no visible deformation in sediments on either margin, although detachment folds are occasionally developed above the slip plane and confirm directions of easterly movement. Early downslope-dipping normal faults that are cut by later BPS planes results in older over younger stratigraphic relationships across the BPS surface, together with 'sawtooth' patterns where multiple BPS planes have developed. Conversely, early upslope-dipping normal faults that are cut by BPS create younger over older stratigraphic relationships combined with missing section and 'staircase' patterns where multiple BPS planes exist. As BPS in subhorizontal sequences does not have a vertical component of displacement, it may be examined in terms of horizontal 'heave'. BPS increases heave in upslope-dipping normal faults, whereas it reduces heave in downslope-dipping normal faults and may even become negative where sections are repeated across sawtooth profiles. In addition, BPS increases heave in upslope-dipping reverse/thrust faults and reduces heave across downslope dipping 'backthrusts'. Although individual BPS planes may have limited displacement, the net consequence of multiple planes of BPS that form in the shallow-subsurface is to distort patterns and estimates of extension and contraction across fault zones in MTDs.

Microstructures of bedding-parallel faults under multistage deformation: Examples from the Southeast Basin of France

“…Microstructural and outcrop observations indicate that shearing (and eventually opening) occurred along the BPFs during normal faulting and before folding. This is in line with the results of recent publications that describe BPFs associated with normal faults in shallow dipping sequences (<10°; Delogkos et al, 2017;Alsop et al, 2020). Shearing in such conditions appear mechanically unfavourable and can be related to several driven mechanisms.…”

“…The shear movements during the extensional regime confirm other publications showing that BPFs may form in such context for a limited local tilting of the bedding (<10°) (Smart et al, 2009;Delogkos et al, 2018;Alsop et al, 2020). The movements on the BPFs could be attributed to a flexural slip mechanism due to normal fault-related folding (Watterson et al, 1998;Ferrill et al, 2007;Smart et al, 2009;Delogkos et al, 2018) or to gravity-driven downslope deformations (e.g., Alsop et al, 2020). The latter is unlikely in the studied areas, considering that the rocks have been deeply buried before the formation of the BPF.…”

“…However, this is not always straightforward because the BPFs can have various origins. They may occur, for example, with flexural-slip folding due to layer-parallel shortening or thrusting (e.g., Tanner, 1989;Fowler, 1996, Koehn andPasschier, 2000), in the rock volume surrounding normal faults due to fault-related folding (e.g., Watterson et al, 1998;Ferrill et al, 2007;Smart et al, 2009;Delogkos et al, 2018) and during gravity-driven deformations (e.g., Alsop et al, 2020).This diversity of context of formation raises critical questions concerning (i) the origin of BPFs in areas of superimposed deformation stages; (ii) the relative timing of BPFs and more steeply dipping faults; and (iii) the evolution of the BPF internal structures during deformation history.…”

A Paleoseismic Record Spanning 2‐Myr Reveals Episodic Late Pliocene Deformation in the Western Qaidam Basin, NE Tibet

“…The coring site is located close to the top of the Jianshan Anticline and we here interpret detachment surfaces preserved in the core as failure surfaces formed toward the head scarp of seismic-induced slumps that translated downslope from the anticlinal crest. Similar gravity-driven bedding-parallel detachment surfaces generated by the downslope-directed failure of sediments have been recently recorded in outcrops from the Dead Sea (Alsop et al, 2020).…”

Detachment fold duplexes within gravity-driven fold and thrust systems

“…We have previously suggested that detrital marker beds act as barriers or baffles to fluid flow, thereby forming seals to overpressured sediment that fails directly beneath it and locally fluidizes to create injected gouge (Alsop et al, 2018a). Mechanical heterogeneity linked to alternating detrital and aragonite layers, combined with variations in fluid pressure are thought to be the likely controls on positioning of both the basal and upper detachments, and bed-parallel slip planes in general (Alsop et al, 2020d). Thus, we interpret the aragonite-rich sediment above the uppermost (blue) detrital as fluid rich and weak due to being non-compacted and close to the sediment surface, while the aragonite-rich layers below the marker were overpressured and failed.…”