«Mittlerer Grünsand»

Rappresentazione e statuto

Varianti storiche: Mittlerer Grünsand (Jeannet et al. 1935), calcaires glauconieux moyens et Couche à Pecten (Jeannet & Leupold 1935), Middle Greensand (Lihou 1995)

Descrizione: Nach einem Sedimentationsunterbruch und teilweiser Verkarstung des Liegenden stösst eine dritte Transgression der Euthal-Formation weiter gegen NNW vor und lagert einen glaukonitischen Sandstein bis Sandkalk ("mittlerer Grünsand"), lokal mit Austern, Pecten, selten Korallen, max. 3 Meter mächtig, ab.

Fossili

Discocyclinen

Unità di rango inferiore

Unità sovrastante

Paleogeografia

North Tethyan Shelf (Helv.) :
marge continentale européenne

Termini generici

Tipo di origine

Revisione: Lihou Joanne C. (1995) : A new look at the Blattengrat unit of eastern Switzerland: Early Tertiary foreland basin sediments from the south Helvetic realm Eclogae geol. Helv. 88/1, 91-114

p.104: North of these profiles, the basal unconformity is overlain by the Middle Greensand. The greensand east of Elm is 2-6 m thick below the first hardground (Fig. 4), and contains fragmented mollusc shells, usually pectens and oysters, becoming more brecciose upwards as foraminifera such as Discocyclina pratti, Operculina gigantea, Assilina placentula and Nummulites murchisoni appear, along with bryozoans which colonise firm or hard substrates; this becomes a well-developed hardground in the Upper Windegg profile. In Weisstannental, the greensand unit is less than 2 m thick and consists of a sterile glauconitic sandstone occasionally with phosphate nodules overlain by a hardground of pectens, large oysters and gastropods (Fig. 5). The development of a hardground at the top of the Middle Greensand indicates that there can have been little or no sediment input from the land at this time, or else a lack of accommodation space prevented sand deposition, and the sediment was by-passed further out onto the shelf. The main process operating was winnowing by currents or storm-related waves.