A Late Barremian age for the onset of Urgonian-type facies in the Swiss Jura Mountains Reply to the discussion by Conrad et al. on 'Stratigraphic, sedimentological and palaeoenvironmental constraints on the rise of the Urgonian Platform in the western Swiss Jura' by Godet et al. (2010), Sedimentology, 57, 1088–1125

摘要

The age of the Urgonian Formation preserved in the western Swiss Jura and eastern France has been the subject of divergent and partly controversial interpretations for more than 20 years (Clavel et al., 1986; Arnaud et al., 1998). Good time control on the origin, evolution and disappearance of these unique and widespread platform- carbonate occurrences is important in that it affects our general understanding of changing palaeoenvironment, palaeoclimate and palaeoceanography during the late Early Cretaceous. More specifically, a better understanding is needed of the conditions favouring the development of this vast and oligotrophic reef belt in the tropical and sub-tropical marginal seas of the late Early Cretaceous-a period which otherwise shows the highest temporal density in so-called oceanic anoxic episodes (OAE) for the entire Mesozoic (for example, the latest Hauterivian Faraoni OAE and the Early Aptian Selli OAE). We are therefore pleased to take the opportunity provided by Conrad et al. (2011) to discuss our lines of evidence for a Late Barremian age for the onset of the Urgonian in the Jura Mountains. In a first step, we will consider the arguments proposed by Conrad et al. (2011). Next, we will develop our own arguments in favour of a Late Barremian age for the onset of Urgonian carbonate deposition in the Jura Mountains, consider age models for the Urgonian platform in other regions and prove the consistency of our age model. Finally, we will place the evolution of the Urgonian platform in the more general context of palaeoenvironmental evolution during the late Early Cretaceous. We will specifically demonstrate that: 1 The sequence-stratigraphic interpretation forwarded by Conrad et al. (2011) is not consistent with modern sequence-stratigraphic interpretation of platform carbonates. 2 The orbitolinid biostratigraphy fide Conrad et al. (2011) is questionable and may lead to erroneous age interpretations. 3 New ~(87)Sr/~(86)Sr age data obtained on rhynchonellid brachiopods and K/Ar data on selected glauconite grains corroborate a Late Barremian age for the onset of the Urgonian in the Swiss Jura Mountains. 4 Our age model is coherent with age models obtained for the Urgonian in other regions in France and Switzerland.5 Our age model is compatible with global palaeoenvironmental change during the late Early Cretaceous.