Episodic deposition of the Lias in the Medianes nappe (western Switzerland): a record from mineralized ammonite-bearing beds

Sinemurian and Pliensbachian depositional sequences from the starved distal northern continental margin of the Tethys are preserved in the Médianes Nappe (Western Swiss Prealps). They contain mineralized beds which include a variety of facies. The northern continental margin of the Tethys was broken into blocks, less than 50 km wide, during an extensional phase. The Sinemurian and Pliensbachian sediments in the study are vary in thickness from less than 50 m in the north to more than 200 m in the south. Although the upper Sinemurian to Pliensbachian mineralized beds are thin, between 0.10 and 0.50 m thick, they commonly contain several distinct faunal horizons stacked one upon another. The mineralized beds are localized in the northern and central parts of the Préalpes Médianes in crinoidal calcarenites or between crinoidal calcarenites and alternating siliceous limestone sand marls. Petrographical and geochemical analyses of these beds show multiple phases of mineralization within an early oxic to suboxic environment. Formation of hematite, phosphates, glauconite, and silica, locally interlayered with pyrite, occurred either during a bacterial oxidation zone or during a bacterial manganese and iron reduction zone. The beds may have been reworked as intraformational conglomerates. Some ammonites in the beds have corroded upper surfaces. Although these beds can be traced from the northern to the central part of this section of the northern continental margin of the Tethys, deposition was not uniform. The episodic occurrence of these mineralized beds in space and time may be explained by local changes in accomodation potential. These changes are governed by differential subsidence and eustasy of the third order cycle scale with an added effect of higher order frequency cycles, possibly climatic in origin. Thus, there is an interplay of supraregional (i.e. global effects: eustasy, regional intraplate stresses), regional (break up of Tethyan margin into moderate scale blocks) and local (locally-derived sediment flux, local structural patterns) parameters that have affected the deposition of the beds. The interplay of these factors leads to a highly complex and unique rock record.