Evolution and demise of a mid-Cretaceous carbonate shelf: the Altamira Limestones (Cenomanian) of northern Cantabria (Spain)

Abstract

Abstract In northern Cantabria, the Cenomanian transgression is pulsatory in character and progressively onlaps onto deltaic siliciclastics of latest Albian to earliest Cenomanian age. This pronounced 2nd-order sea-level rise can be subdivided into an earlier (constructive) phase and a later (destructive) phase, separated by a significant low sea-level stand at the Early/Mid-Cenomanian boundary. In the first phase, the flooding of the north Iberian continental margin in the early Early Cenomanian caused the sources of terrigenous sediments (located on the Iberian Meseta) to retreat, thus promoting the development of a carbonate ramp. This ramp evolved into a shelf-attached platform (Altamira Platform) during the course of the Early Cenomanian. Pronounced sea-level fall in the Early/Mid-Cenomanian boundary interval exposed the platform, increased fluvial input and terminated the constructive phase. The destructive phase started in the early Mid-Cenomanian and lasted until the Late Cenomanian. Carbon and oxygen stable isotopes, microfacies analysis, palaeontological criteria, and sequence stratigraphy are used to elucidate the complex interplay of climatic changes, sea-level history, nutrient supply, and platform drowning. Three 3rd-order sea-level rises (early and late Mid-Cenomanian, early Late Cenomanian) resulted in a westward-backstepping and final drowning of the Altamira Platform. The drowning succession is accompanied by a negative shift of δ 18 O and a positive δ 13 C excursion interpreted to reflect progressive climatic warming and elevated nutrient levels. The resulting eutrophication is represented by the biotic community of the drowning succession and negatively influenced the carbonate budget of the depositional system. As a result, the stressed platform was readily drowned during the Mid- and earliest Late Cenomanian 3rd-order sea-level rises.