Evolution of the Urgonian shallow-water carbonate platform on the Helvetic shelf during the late Early Cretaceous

Urgonian platform carbonate (Cretaceous; Barremian, Aptian) forms an important lithostratigraphic unit in the Helvetic fold- and thrust unit of the northern Swiss Alps. Its widespread distribution and ubiquity allow for an integrated high-resolution study of macro- and microfacies, benthic foraminiferal biostratigraphy, sequence stratigraphy, and carbon-isotope and phosphorus records. The resulting data confirm the importance of environmental forcing and sea level change on the style of carbonate production and accumulation along the margin of the northern Tethys during the late Early Cretaceous. Stratal geometries, the succession of microfacies, the identification of major emersion surfaces, and biostratigraphy observed and analysed in twelve sections through the inner, middle, and outer platform, and in the panorama of the Churfirsten range, permit subdivision of the analysed succession into eight depositional sequences. The succession starts with a phase of sedimentary condensation (Altmann Member (Mb); late Hauterivian – late early Barremian; sequences H7, H8, and B1), followed by the deposition of hemipelagic sediments (Drusberg Mb; restricted to sequence B2 of the late early Barremian on the inner platform; and covering the late early Barremian to the middle late Barremian on the outer platform, and to the early Aptian on the outer shelf, thereby showing an important diachroneity of its upper boundary related to the inception and progradation of the Urgonian platform), and the development of predominantly lagoonal carbonate (Schrattenkalk Formation (Fm); early late Barremian – early Aptian; sequences B3, B4, B5, and A1). The Schrattenkalk Fm documents important progradation and aggradation of the carbonate platform, and the change from a ramp-like to flat-topped geometry. The oldest, allochthonous remains of the shallow-water carbonate platform were identified intercalated in and on top of the lower Barremian Drusberg Mb (sequence B2). Sequence boundary (SB) B3 resulted from an important regressive phase near the early-late Barremian boundary, which led to the emersion of the hemipelagic sediments of the Drusberg Mb in the inner part of the shelf and to the deposition of a lowstand systems tract at the base of the Lower Schrattenkalk Mb (late Barremian; sequences B3–5) in intermediate and distal domains. Deposition of in-situ platform carbonate started during the following transgressive phase in the middle late Barremian, which flooded the entire inv