Sequence stratigraphy, sedimentology and geochemistry of the Urgonian Formation (Late Barremian to Early Aptian) in the Helvetic Alps

Urgonian platform carbonates are a common feature of subtropical and tropical shallow-water environments of late Early Cretaceous age. They include the remains of rudists, corals, chaetetids and stromatoporoids, which are interpreted as indicators of a predominantly photozoan, oligotrophic carbonate-producing ecosystem. The late Early Cretaceous is also marked by the occurrence of several oceanic anoxic episodes, such as the latest Hauterivian Faraoni and the Early Aptian Selli Events, which are both interpreted as the consequence of generalized eutrophic conditions. These observations imply that the late Early Cretaceous underwent larger fluctuations in nutrient supply, which may have interfered with the evolution of the widespread Urgonian platforms.

Our goal is to study the interactions between paleoceanographic and paleoclimatic change, and Urgonian carbonate buildup in the northern, Helvetic Alps. This unit remains understudied relative to its counterparts in eastern and central France. We will examine a selection of stratigraphic sections along lateral and proximal-distal transects for their facies and microfacies, biostratigraphy, sedimentology, and geochemistry (stable isotopes, phosphorus contents). We will also use high-resolution panoramas from a selection of mountain walls, in order to detect lateral changes in facies and geochemistry and establish a sequence-stratigraphic framework. We specifically intend to compare the Urgonian units of Late Barremian age and Early Aptian age, which are separated by the so-called “Lower Orbitolina Beds”. The Late Barremian was less affected by anoxia, whereas the Early Aptian witnessed progressive change in paleoceanographic conditions, which led up to the Selli Event.