Litho- and biostratigraphy of the 250 m-deep Mont Terri BDB-1 borehole through the Opalinus Clay and bounding formations, St Ursanne, Switzerland (original) (raw)

2016

Abstract

A 250 m-deep, inclined well, the Mont Terri BDB-1, was drilled through Jurassic Opalinus Clay and its bounding formations at the Mont Terri rock laboratory (NW Switzerland). A continuous section is thereby available for the first time for the Mont Terri area, from the topmost members of the Staffelegg Formation to the basal layers of the Hauptrogenstein Formation. The well core was studied extensively for lithostratigraphy and biostratigraphy (drawing upon three sections from the Mont Terri area). The macro- and micropaleontological as well as the palynostratigraphical data are complementary not only spatially but account further for almost all biozones embracing the Late Toarcian and the Early Bajocian. A suite of geophysical logs was compiled and this method permits detection of formational and intraformational boundaries in the BDB-1 well. In the framework of interdisciplinary study, analysis of the above-mentioned formations permitted straightforward processing to derive substantial new data for the Mont Terri area. Lithologic inventory, stratigraphic architecture, thickness variations and the biostratigraphic classification of the studied formations in part deviate considerably from occurrences in northern Switzerland that outcrop farther to the east. For the Passwang Formation, with the exception of the Sissach Member, no further lithostratigraphic subdivision is possible according to the members defined in the type region. Also noteworthy is that the approximately 130 mthick Opalinus Clay in drill core BDB-1 appears thinner by 20 m than that in the Mont Terri tunnel. The lowermost 38 m of the Opalinus Clay can be attributed chronostratigraphically solely to the Aalensis Zone (Late Toarcian). The accumulation of the Opalinus Clay, however, was also initiated at the same time farther east in northern Switzerland (Aalensis Subzone, Aalensis Zone), but contemporaneously in the Mont Terri area, there was a sedimentation rate two or three orders of magnitude higher.

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