From OAE 2 anoxia to oxic CORBs in the Cenomanian to Santonian of the Ultrahelvetics (Austria)

Category Environmetal Geology
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Wagreich, Michael; Neuhuber, Stephanie
Holding Date 23 September 2008

Ultrahelvetic units of the Eastern Alps were deposited on the distal European continental margin of the (Alpine) Tethys. 2 sections have been investigated: the Rehkogelgraben section and the Buchberg section. The Rehkogelgraben section comprises a 5 m thick succession of Upper Cenomanian marl-limestone cycles overlain by a black shale interval composed of three black, organic-rich (ca. 5% TOC, kerogen type II) layers and carbonate-free claystones, followed by Lower Turonian white to light grey marly limestones with thin marl layers. Carbon isotope values of bulk rock carbonates display the well documented positive shift around the black shale interval, allowing correlation of the Rehkogelgraben section with other sections.
The Buchberg section displays the continuation with white to light grey marly upper- to middle bathyal carbonates of Early Turonian age towards red-colored carbonates (Cretaceous Oceanic Red Beds - CORBs) of the Middle Turonian, a time span of about 1.5 my for red colors to become dominant. The total organic carbon content is very low throughout the section, but benthic foraminifera associations, indicate repeated phases of enhanced organic matter flux and less aerated bottom waters during the transitional interval. Sedimentation of red layers was controlled by periods of well oxygenated bottom waters, reduced sedimentation rates and degradation of organic matter in the underlying sediments. Principal component analysis of carbonate chemical data showed that the development of red coloured pelagic sediments is accompanied by a shift towards highly oligotrophic conditions in the surface ocean as well as a decrease in hydrothermal activity. The general formation of red beds is most likely associated with a shift towards more oxic conditions.
Above the Cenomanian-Turonian interval, red limestone-marl cycles are present of Coniacian-Santonian age in the Rehkogelgraben section. Mineralogical data suggest a constant source area over the investigated interval. The preservation of organic carbon is very low in this environment. The relatively higher content of organic carbon in the Upper Santonian may indicate a regressive event in accordance with the global sea-level curve. Plagioclase and organic carbon correlate positively and might indicate enhanced input of nutrient- like trace metals during episodes of higher volcanic activity. In the Upper Santonian, terrigeneous elements (Al, Li, Rb, Be) decrease upwards. Iron speciation data for marl and limestone layers attest to oxic early diagenesis during marl deposition compared to limestone episodes.
Low sediment accumulation rates (2.5 mm/ka) are reconstructed. Geochemistry and stable isotope data indicate a highly oligotrophic environment with efficient recycling of organic matter and nutrients in the upper water column. The investigated Santonian CORBs were deposited above the CCD and show that nutrient availability varied and resulted in periods of higher primary production.