Soft-sediment deformation structures in the Lower Cretaceous Robberg Formation, South Africa: distinguishing the effects of dinosaurs and earthquakes in an estuarine, rift basin setting

The Early Cretaceous Robberg Formation is a clastic, syn-tectonic unit deposited near the updip limit of the Pletmos Basin during initial separation of Africa and South America. Robberg sandstones were deposited in estuarine tidal channels whereas siltstones fill abandoned channels. Abundant soft-sediment deformation and dewatering structures have previously been attributed to earthquakes. Our re-examination of a part of the Robberg Formation confirmed the presence of load casts due to inverse density stratification, whereas sand dikes probably record seismically triggered dewatering. Layers of siltstone pervasively disrupted by microfaults are probably seismogenic and matrix-supported, intraclastic breccia is interpreted to record seismogenic liquefaction and channel-bank collapse. However, the most common types of soft-sediment deformation, seen mainly in vertical cross-section and spatially and stratigraphically isolated in undeformed sediment, are attributed to dinosaurs. Track preservation takes five main types: (1) Undeformed to obliquely stratified sandstone fills sharply defined track impressions in siltstone in which lamination is truncated and deformed downward. (2) Bowl-shaped depressions, 50 cm to >1 m in diameter, in heterolithic channel-plug facies are transmitted undertracks, probably produced by sauropods. (3) Dinosaurs walking on a thin sand layer on the bottom of a tidal channel punched the sand down into underlying channel-plug siltstone, the latter being highly deformed. (4) Dinosaurs walked or waded in the mud of abandoned channels, producing highly deformed, destratified and sheared sediment, surrounded by wholly undeformed sediment. (5) Dinosaurs walked on moist sandy channel point-bars producing locally deformed, brecciated, and de-stratified, bowl-shaped depressions, bordered by folds and thrusts. The majority of tracks appear to be broadly cylindrical, and lack obvious digits, which together with size, suggest a sauropod (or possibly other quadruped) origin. A few tracks, seen in plan, have three digits, suggesting a bipedal trackmaker. This track assemblage appears to be the first recognized in the Cretaceous of southern Africa.