Palaeoceanography of Lower Cretaceous Alpine platform carbonates

Abstract

Shallow‐water carbonate platforms evolve in an interplay of local, regional and global dynamics, thereby generating a detailed record of environmental conditions. Platform sediments constitute important archives of past environmental change and, as such, they are increasingly analysed for the information they contain on wider‐scale patterns. The integration of this information with palaeoceanographic data obtained from deeper‐water sediments provides a powerful tool to develop a more diverse and complete image of past environmental change. In this contribution, the remains of the northern Tethyan carbonate platform of Early Cretaceous age cropping out in the western Swiss Jura and the Helvetic Alps are used to illustrate the advantage of a palaeoceanographic approach in their interpretation. The tools applied are biostratigraphy and chemostratigraphy (carbon and strontium isotopes) in order to obtain a precise age model for correlation with (hemi‐)pelagic sediments in adjacent basins, sequence stratigraphy as a means to reconstruct the impact of sea‐level change, phosphorus stratigraphy as a proxy for trophic levels, facies and microfacies analyses as a gateway to gain better insight into the carbonate factory, and the detailed analysis of platform‐drowning unconformities as an indicator of wider‐scale palaeoenvironmental change. It is shown that changes from periods of predominantly photozoan to predominantly heterozoan carbonate production near the Berriasian–Valanginian boundary and middle early Aptian coincided with substantial increases in phosphorus burial rates and major carbon‐isotope excursions. Furthermore, the majority of the platform‐drowning episodes occurred just prior to and during episodes of major palaeoenvironmental change and, last but not least, the platform itself may have influenced the chemistry of adjacent basins and the carbon‐isotope composition of associated carbonates by the export of periplatform ooze.

An Albian demise of the carbonate platform in the Manín Unit (Western Carpathians, Slovakia)

“…The δ 13 C isotope data suggest shallow water realm and more or less continuous diagenesis under marine conditions. Values are comparable with other platform carbonates, especially with Urgonian ones (Immenhauser et al 2003;Godet et al 2006;Föllmi & Godet 2013;Huck et al 2013). The trends in stable isotopes mirror the separation of the succession into three intervals to some degree (the lower and upper parts of the Podhorie Fm and the Manín Fm) (Fig.…”

“…2). In spite of diagenetic effect, isotope ratios of shallow-water carbonates have a potential to record climatic changes and changes in the burial of inorganic carbon in the platform realm (Godet et al 2006;Föllmi & Gainon 2008;Föllmi & Godet 2013). The δ 13 C values in the range +2.19 to +2.68 ‰ are typical of the lowermost part of the section (M190-M203) formed by bioclastic limestones with a relatively high content of micrite matrix (wackstones), (Fig.…”

“…The δ 13 C record may be influenced by different biological fractionation of different groups of calcareous plankton, different types of mineralogy of pelagic and benthic producers, by quantity of organic carbon recycled, and by diagenetic processes (Föllmi & Godet 2013). The δ 13 C and δ…”

“…Analyses of Urgonian carbonate platforms significantly contributed to the understanding of paleo geographical, paleoecological and paleoclimatic changes through out the Early Cretaceous (Godett et al 2006;Föllmi & Gainon 2008;Föllmi 2012;Föllmi & Godet 2013). Bio strati graphic, sedimentological and chemostratigraphical methods, in particular distribution of P and isotopic composition of C and O are used for characterization of nutrient support, C cycling and definition of paleoceanic proxies but also for anoxic conditions in basins, which acquired a global character during Oceanic anoxic events (OAEs, see Föllmi & Godet 2013;Huck et al 2013). …”

Deciphering the message of Early Cretaceous drowning surfaces from the Helvetic Alps: What can be learnt from platform to basin correlations?

“…The link between episodes of carbonate platform drowning and more widespread, profound environmental crises has long been postulated in the Helvetic realm (Föllmi et al ., ; Weissert et al ., ; Föllmi and Godet, ). In other parts of the world the precise study of drowning surfaces, or of discontinuities reflecting the cessation of carbonate platform activity, revealed that the interaction of subaerial exposures with submarine hardgrounds is rare (e.g.…”