An exceptional rocky shore preserved during Oligocene (Late Rupelian) transgression in the Upper Rhine Graben (Mainz Basin, Germany)

The Early Oligocene (Late Rupelian) Alzey Formation (Mainz Basin, Upper Rhine Graben, Germany) records the development of a rocky coast depositional system during transgression. The formation unconformably overlies Permian bedrock across a composite transgressive ravinement surface. Exposure of the surface shows a succession of subplanar bedrock terraces, separated by near-vertical risers. Terraces show a broad staircase geometry and display wave-erosional features (notches, sea stacks, furrows). Detailed sedimentological and palaeoecological investigations reveal prograding beachface and shoreface depositional units that overlie terraces and are adjacent to risers.

Terraces are interpreted as wave-cut platforms, backed by palaeocliffs. The staircase architecture records the episodic landward migration of palaeoshorelines onto palaeotopographic highs during the Early Oligocene. Stacking patterns of gravelly beach and shoreface associated units (facies tracts) indicate successive episodes of terrace cutting, beach development, drowning and shoreline backstepping during an overall relative sea-level rise.

The exceptional preservation of the stair-cased rocky shore may be attributed to a highly jerky rising relative sea-level, as the result of the conjugated effects of rift-controlled tectonic subsidence and eustatic sea-level oscillations.