The Early Aptian Grünten Member: Description of a new lithostratigraphic unit of the helvetic Garschella Formation

Abstract

Early Aptian sediments mostly referred to as “upper Orbitolina beds” are known from a relatively small number of outcrop areas throughout the distal part of the Helvetic Zone of Switzerland, Austria and Germany. These sediments are here formally defined as a new, basal member of the Garschella Formation; the Grunten Member. Equivalent and correlatable sediments also exist in the Vercors region of the French Dauphinee Zone. The historical type section (holostratotype) of the Grunten Member is situated on the Grunten Mountain in southern Germany. A new type section (lectostratotype) for the Grunten Member is chosen in the better suited Bauen-Brisen area of Central Switzerland and an additional reference section (hypostratotype) is defined near the Rawil Pass in the Bernese Oberland of Switzerland. In relatively proximal settings, the Grunten Member overlies Early Aptian limestones of the Urgonian Carbonate Platform (Schrattenkalk Formation), documenting its demise and early “drowning”. In relatively distal settings, it overlies the contemporary hemipelagic sediments of the Drusberg and Mittagspitz Formations. In complete successions, the Grunten Member is in turn overlain by the basal, phosphoritic Luitere Bed of the Brisi Member (Garschella Formation) documenting the continuing “drowning” of the Urgonian Carbonate Platform in the Late Aptian. The Grunten Member essentially consists of a single stratigraphic sequence, beginning with a marly base and gradually passing to crinoidal limestones at its top. Rare ammonite finds as well as sequence stratigraphic correlations suggest a late Early Aptian age (parts of the Deshayesi and Furcata Ammonite Zones). In the relatively proximal reference section of Rawil, the Grunten Member contains two phosphoritic horizons. Phosphateenriched horizons are also known from other proximal sections of the Grunten Member.