Characterising bed-parallel slip during gravity-driven deformation

Abstract

Despite the recognition that bed-parallel slip (BPS) must operate during gravity-driven deformation of basinal sediments, there is a general paucity of detailed outcrop-based observations to characterise and detect such a process. We therefore present detailed timing relationships between BPS and steeper dip-slip faults that were both created during seismically-triggered downslope-directed movement of sediments. Using the late Pleistocene Lisan Formation that was deposited around the Dead Sea Basin as our case study, we show that ‘sub-seismic’ decametric scale BPS planes may pre-date, post-date, or operate coevally with steeper faults generated as sediments slip downslope towards the depocentre. Older BPS can be recognised by sediment injections and minor folds and fractures, whereas younger BPS displaces marker faults downslope towards the basin. BPS operating coevally with steeper faults results in complex overprinting and development of fault-bound lenses. BPS that forms along single surfaces in the footwall of normal faults becomes separated into two distinct planes in the downthrown hangingwall block, indicating broadly coeval development. Adjacent BPS planes that operate synchronously result in synthetic and antithetic faults that ‘hard-link’ and transfer displacement between BPS planes. Attenuated bedding between segments of BPS that overlap and terminate next to one another suggests that ‘soft-linkage’ also forms between coeval BPS planes. Displacement-length relationships of measured BPS planes plot in the same range as recorded for normal faults, although BPS with larger displacements have relatively ‘short’ lengths, suggesting that complete BPS planes are missing due to limitations of outcrop size. Although the lack of displaced bedding across BPS makes it largely invisible on seismic sections across large-scale gravity-driven systems, it does potentially contribute towards the apparent inbalance between net extension and contraction observed in many sections across mass transport deposits. In addition, the realisation that BPS interacts with dip-slip faults to create repeated and missing sections that are particularly focussed along earlier deformed horizons and turbidites, has implications for palaeoseismic studies that assume broadly continuous stratigraphy.