Stratigraphic re-assessment of the Seewen Formation in the classic Helvetic key locality “An der Schanz” quarry, Burgberg (Bavarian Alps; Turonian, Coniacian): biostratigraphy and δ13C correlations

The “An der Schanz quarry” near Burgberg exhibits parts of the Helvetic Seewen Formation. Its base is dated as Cenomanian, its top as Late Santonian by planktonic foraminifera by previous authors. A re-sampling of the section for inoceramid and δ13C stratigraphy gave an Early Coniacian age for the uppermost parts of the Seewen Formation. A literature review suggests that the top of Seewen Formation of the Allgäu (Bavaria) and Vorarlberg (Austria) is never younger than the Middle or terminal Middle Coniacian. δ13C data confirm the biostratigraphic dating by inoceramids. Furthermore, although only 28 m thick, the section exhibits all important δ13C events for interbasinal correlation previously recognized in sections of England and northwestern Germany. The δ13C calibration of the LO of H. helvetica is shown to be diachronous and not of value for isochronous zonations. The Caburn δ13C event is suggested as a marker for the Middle-Upper Turonian boundary due to the concomitant FO of I. perplexus. The re-dating of the facies turnover from a calcareous biosedimentary system towards a marl dominated system (succeeding Amden Formation) is briefly compared with the situation observed in northwestern Germany, where comparable trends are mapable.