This paper describes in detail hydroplastic structures, which are ‘odd’ kinematic indicators in the basal part of the Eocene Middle Rus Formation. Such structures were previously ignored or falsely interpreted. These hydroplastic structures are found in the massive limestone exposures on the King Fahd University of Petroleum and Minerals (KFUPM) campus. They occur in relation to a principal displacement zone along the boundary/interface between the Lower/Middle Rus, which is referred to as the Rus soft-sediment detachment. The structures are fist-sized vugs associated with carrot- or comet-trail imprints (VCT structures) which were previously translated calcite geodes that have been weathered out. VCT structures show transport/slip towards the NNW (345°) and are found on flat to low-dipping surfaces classified as Y, R and P shears with respect to the orientation of the Rus detachment. Palaeostress analysis indicates an Andersonian transtension stress regime, though it does not facilitate the activation of the Rus soft-sediment detachment. Detachment activity occurred due to the negative effective principal stress σ 3′ and the abnormally low frictional coefficient caused by fluid pressure. The soft-sediment Rus detachment can be considered a ‘sensitive stress sensor’ for the Zagros collision since it indicates the Arabian platform’s instability in the wider area of the Dammam Dome during the Late Eocene. This instability is attributed to the inception of the Zagros collision, which was previously considered to occur during the Oligocene based on the well-established pre-Neogene unconformity.

In the Ediacaran marine succession of the Cerro Negro Formation (Tandilia System, NE Argentina), abundant microbially induced sedimentary structures indicate general conditions of substrate biostabilization. Numerous discoidal structures in this succession were previously interpreted as moulds of soft-tissue holdfasts of sessile organisms, within the form genus Aspidella. In this study, we performed a detailed re-analysis of some of these features and discuss two alternative hypotheses to explain their genesis: (1) as the result of soft-sediment deformation and fluid injection structures; and (2) as structures of active animal–sediment interaction (i.e. trace fossils). We show that the dome-shaped discs are internally laminated, with a cylindrical to a funnel-shaped vertical tube at their central region. The presence of these downwards vertical extensions and other intricate internal arrangements cannot be explained under the taphonomic spectrum of discoidal fossils, but shows striking similarities to Intrites-like structures and other sand-volcano-like pseudofossils (e.g. Astropolithon). However, some structures are hard to distinguish from vertical dwelling burrows with funnel-shaped apertures and thick-lined walls, commonly produced by suspension- and detritus-feeding invertebrates (e.g. Skolithos isp., Monocraterion isp. and, less likely, Rosselia isp.). Since reliable age constraints are unavailable, and further investigation concerning other palaeobiological indicators is needed, the most parsimonious hypothesis is that of a structure derived from fluid-escape processes. Our study demonstrates the importance of detailed investigation on discoidal structures in either upper Ediacaran or lower Cambrian strata.