Plate 139

Close-up of slide scars


A detailed view of the previous outcrop emphasizes the abrupt angular contact along one of the scars. The unconformable surface is marked by a more pronounced cementation of the mudstone (another one is visible near the top). It is possible that this lithification is not an after-burial diagenetic effect, but occurred when the sediment was exposed on the sea bottom, during a period of very slow or absent sedimentation. In other terms, it could be the initial stage of a hardground  (see plate 171), a form of submarine encrustation.


Although the distinction between a slide and an erosional episode is important, their effects are the same under the viewpoint of the stratigraphic record. Geometrically, they produce unconformable or discordant surfaces in the source areas, and "excess" thickness of sediments in accumulation areas and depocenters; temporally, they leave stratigraphic gaps, or hiatuses.  For this reason, sliding and erosion could be encompassed by the term denudation.

Photo: P. Clari 1992.


Sedimentographica