Aptian to Cenomanian deeper-water hiatal stromatolites from the northern Tethyan margin

A suite of deeper-water hiatal (DWH) stromatolites has been identified in the phosphatic and glauconitic sediments of Aptian to Cenomanian age in the alpine Helvetic thrust-and-fold belt, which represents the former northern Tethyan margin. The most important occurrences date from the latest Early to Late Aptian, the late Early to early middle Albian, and the Early Cenomanian. They are invariably associated with condensed phosphatic beds and occur preferentially on top of hardgrounds or on reworked pebbles and fossils. The zone of optimal stromatolite growth and preservation coincides with the zone of maximal sedimentary condensation, in the deeper parts of phosphogenic areas. The DWH stromatolites show variable morphologies, ranging from isolated laminae ("films") to internally laminated columns and crusts. They reach thicknesses of maximal 10 cm and are either preserved in phosphate or micrite. In the latter case, they may show peripheral impregnations of phosphate or iron oxyhydroxides. The quasi-complete lack of macroscopic sessile organisms suggests that the DWH stromatolites grew close to the upper boundary of an oxygen-minimum zone. Electron-scanning microscopic images show that the Early Cenomanian examples preserved in micrite consist of filamentous structures, which form spaghetti-like assemblages. They are. interpreted as the remains of poikiloaerobic, heterotrophic microbes. Coeval DWH stromatolites are known from the entire European segment of the northern Tethyan margin, and shallow-water counterparts are commonplace on Tethyan carbonate platforms. This indicates that, in general, paleoceanographic and paleoenvironmental conditions were appropriate for stromatolite growth and preservation. The here-described DWH stromatolites proliferated especially in time windows, which followed upon the oceanic anoxic periods OAE la (Early Aptian), lb (latest Aptian and earliest Albian), and Id (latest Albian). They may represent pioneer ecosystems, which thrived during the recovery phases following the "mid"-Cretaceous OAEs.