An accretionary distally steepened ramp at an intrashelf basin margin: an alternative explanation for the Upper Triassic Steinplatte “reef” (Northern Calcareous Alps, Austria)

Abstract

The Upper Triassic massive limestone (the Oberrhdtkalk) at the Steinplatte, Austria, formed along the edge of an intrashelf basin that was situated within a shallow-marine to supratidal shelf approximately 40 km wide. This transition from shelf to intrashelf basin is marked by a distinct slope break that has been widely cited as an example of a reef-rimmed shelf. It is, instead, a distally steepened ramp. The Oberrhdtkalk consists of the ramp sediments lying between the Dachsteinkalk of the shelf and the Kössen Formation of the intrashelf basin. The ramp interpretation is based on the geometry and lateral facies progression in the profile from the shallow-marine to peritidal shelf, across the ramp, and into the basin, and on the absence of reef or grainstone deposits at the slope break typical of a rimmed shelf. The Oberrhdtkalk at the Steinplatte is an important example of shelf-margin deposition because: 1. (1) It formed in a protected and low-energy intrashelf basin setting, thus expanding the range of distally steepened ramps that have been used to develop a general model. 2. (2) It differs from previously described examples of distally steepened ramps, largely from open-ocean shelf-margin settings, in that: (a) it evolved constructionally from a homoclinal ramp rather than forming at a drowned shelf edge or an antecedent tectonic structure; (b) the sediments of the distal slope are largely autochthonous; (c) slumping and breccia deposits on the distal slope are of minor importance; turbidites on the basin floor are absent; and (d) skeletal packstone and grainstone characterize the outer ramp and slope rather than mudstone and wackestone. 3. (3) It provides an alternative ramp model for other Upper Triassic shelf and platform margins in the Northern Calcareous Alps, which have commonly been interpreted as reef or sediment rimmed.