Unlocking paleo-environmental information from Early Cretaceous shelf sediments in the Helvetic Alps: stratigraphy is the key!

The northern alpine Helvetic thrust-and-fold belt includes an Early Cretaceous shallow-water carbonate succession, which was part of an extensive carbonate platform rimming the northern Tethyan margin. The structural architecture of the Helvetic zone allows for the palinspastic reconstruction of proximal-distal transects across the former platform into the outer-shelf realm for distances surpassing 80 km. The Early Cretaceous platform sediments preserved therein provide, therefore, excellent insight into the spatial and temporal evolution of this platform. Furthermore, the presence of ammonites in marker horizons within the Helvetic succession is key to unprecedented time control. During the life span of the Helvetic platform, carbonate build up and build out occurred along two distinct pathways: we discern between a mode including oligotrophic photozoan communities (latest Tithonian-Late Berriasian; Late Barremian; Early Aptian) and a mode dominated by mesotrophic heterozoan communities (Valanginian-Early Barremian; earliest Aptian; late Early Aptian-Late Aptian). The heterozoan mode was frequently interrupted by incipient platform drowning episodes, which materialized either in an important erosive hiatus, or in the deposition of highly condensed, glauconite- and phosphate-rich intervals (Early Valanginian-Early Hauterivian; late Early-early Late Hauterivian; latest Hauterivian-latest Early Barremian; middle Late Barremian; late Early Aptian-early Late Aptian; and latest Aptian -Early Albian). The photozoan mode is interpreted as essentially oligotrophic, whereas the heterozoan and drowning phases were associated with the input of coarser-grained detrital sediments and a correspondingly increased nutrient load, which were both the consequence of intensified chemical weathering on the continent due to warmer and more humid climate conditions. Their onset is signaled by increases in oceanic phosphorus burial rates and major positive excursions in the stable carbon isotope record. Oceanic anoxic episodes occurred during these latter phases. The northern Tethyan platform was not only controlled by climatic, environmental and paleoceanographic change, but changes in platform morphology and the composition of carbonate-producing benthic communities also influenced the quality and quantity of dissolved and particulate material exported into adjacent basins.