22 February, 2009

A Jurassic omission surface within the Hauptrogenstein-Formation

Before I leave for fieldwork a quick post about an easy to recognise omission or discontinuity surface.

The lower, massive layer in the fotos below is the "Oberer Hauptrogenstein s. str." and the upper layer are the "Movelier-Schichten". Both part of the Middle Jurassic "Hauptrogenstein Formation" in the foothill zone of the Upper Rhine Graben. The omission surface is apparent by the sharp contrast of lithology between the two layers. Ooid-Grainstones at the bottom, Bioclastic Float-Rudstones on the top. The omission surface itselfs is characterised by an undulating surface, partially iron-stained, with small pits and holes that can be interpreted as paleokarst that developed on an exposed hardground. Heim (1924/34) first used the term omission surface to designate a depositional hiatus and discontinuity surface of minor form.

Outcrop foto of omission suface on top of the "Oberer Hauptrogenstein s. str." (top 1.20 m exposed on foto)

Hammer stuck in the omission surface. Beds show secondary karst.

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