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



Drowning unconformities are a frequent feature of carbonate platforms and generally express the incapacity of shallow-marine ecosystems to adapt to abrupt sea-level rise and/or palaeoenvironmental change. During the late Hauterivian and early Barremian, the Helvetic carbonate platform experienced a major drowning phase documented by the phosphate and glauconite-rich Altmann Member. It has been shown that a drowning unconformity in the form of a hardground at the base of the Altmann Member on the Pilatus summit (central Switzerland) results from a complex, polyphased diagenetic history that includes two main phases of phosphogenesis. Using carbon-isotope stratigraphy, biostratigraphy and sequence stratigraphy, the correlation of this drowning unconformity from the Helvetic domain with more distal and more complete sedimentary archives allows reconstruction of the sequence-stratigraphic context of the drowning surface, and also an estimation of the amount of time represented by the episodes of condensation. Unconformities associated with the Altmann Member drowning phase developed during transgressive episodes, when strong currents arrived onto the previously subaerially exposed platform, and periods of phosphogenesis may have lasted up to 2·8 Myr. Variations in trophic levels and hitherto less well-known intervening emersion phases played an essential triggering role for the Altmann drowning phase. High-resolution studies of drowning unconformities unravel the diversity of palaeoenvironmental parameters involved in the unfolding of such crises during the evolution of carbonate platforms.