Plate 86

V-shaped scours

Plate 86 demonstrates the difficulty of recognizing structures on amalgamation surfaces (sand on sand). A turbiditic sandstone bed shows the already seen vertical transition from plane-parallel to cross laminae, in conditions of apparently continuous sedimentation. The lower laminated interval is, however, interrupted by V-shaped scours; several of them are nested (a sort of cone-in-cone arrangement) beside and above the hammer head, and can be laterally traced in at least two amalgamation surfaces. Other scours are barely visible on the right-hand side. Note that they are absent from the cross-laminated interval.

It is not clear whether the scours were made by different currents or by velocity pulsations within the same flow.

Marnoso-arenacea Formation near Sarsina, northern Apennines.