Oceanic response to Pliensbachian and Toarcian magmatic events: Implications from organic-rich basinal succession in the NW Tethys.

Stefan Neumeister, Reinhard Gratzer, T.J. Algeo, Achim Bechtel, Hans-Jürgen Gawlick, R.J. Newton, Reinhard Sachsenhofer

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The Bächental bituminous marls (Bächentaler Bitumenmergel) belonging to the Sachrang Member of the Lower JurassicMiddle Allgäu Formationwere investigated using a multidisciplinary approach to determine environmental controls on the formation of organic-rich deposits in a semi-restricted basin of theNWTethys during the Early Jurassic. The marls are subdivided into three units on the basis of mineralogical composition, source-rock parameters, redox conditions, salinity variations, and diagenetic processes. Redox proxies (e.g., pristane/phytane ratio; aryl isoprenoids; bioturbation; ternary plot of iron, total organic carbon, and sulphur) indicate varying suboxic to euxinic conditions during deposition of the Bächental section. Redox variations were mainly controlled by sea-level fluctuations with the tectonically complex bathymetry of the Bächental basin determining watermass exchange with the Tethys Ocean. Accordingly, strongest anoxia and highest total organic carbon content (up to 13%) occur in the middle part of the profile (upper tenuicostatum and lower falciferum zones), coincident with an increase in surface-water productivity during a period of relative sea-level lowstand that induced salinity stratification in a stagnant basin setting. This level corresponds to the time interval of the lower Toarcian oceanic anoxic event (T-OAE). However, the absence of the widely observed lower Toarcian negative carbon isotope excursion in the study section questions its unrestricted use as a global chemostratigraphic marker. Stratigraphic correlation of the thermally immature Bächental bituminous marls with the Posidonia Shale of SW Germany on the basis of C27/C29 sterane ratio profiles and ammonite data suggests that deposition of organic matterrich sediments in isolated basins in the Alpine realm commenced earlier (late Pliensbachian margaritatus
Zone) than in regionally proximal epicontinental seas (early Toarcian tenuicostatumZone). The late Pliensbachian
onset of reducing conditions in the Bächental basin coincided with an influx of volcaniclastic detritus that was
possibly connected to complex rifting processes of the Alpine Tethys and with a globally observed eruptioninduced
extinction event. The level ofmaximumorganic matter accumulation in the Bächental basin corresponds
to the main eruptive phase of the Karoo-Ferrar large igneous province (LIP), confirming its massive impact on
global climate and oceanic conditions during the Early Jurassic. The Bächental marl succession is thus a record
of the complex interaction of global (i.e., LIP) and local (e.g., redox and salinity variations, basin morphology)
factors that caused reducing conditions and organic matter enrichment in the Bächental basin. These
developments resulted in highly inhomogeneous environmental conditions in semi-restricted basins of the
NW Tethyan domain during late Pliensbachian and early Toarcian time.
Original languageEnglish
Pages (from-to)62 - 83
JournalGlobal and Planetary Change
Publication statusPublished - 2015

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