From carbonate–sulphate interbeds to carbonate breccias: The role of tectonic deformation and diagenetic processes (Cameros Basin, Lower Cretaceous, N Spain)

Interpreting the origin of carbonate breccias requires a detailed analysis because they may be the result of a wide variety of processes that produce similar features. This is the case of the Lower Cretaceous carbonate breccias of the Cameros Basin (previously interpreted as slump or collapse breccias), whose origin is interpreted after performing a detailed sedimentary, petrographic and tectonic study.The studied carbonate breccias consist of angular carbonate mudstone fragments floating in a matrix of calcite and quartz crystals. The breccias are interbedded with, and laterally associated to, alternating layers of carbonate mudstone and calcite and quartz pseudomorphs after gypsum, which show strong similirities to the breccia fragments and matrix, respectively, suggesting that the brecciated beds were originally composed of identical alternating carbonate mudstone and gypsum layers as the unbrecciated layers. The breccias are associated with frequently polyharmonic deformation structures, which are similarly oriented as the regional tectonic structures, indicating that they are related with the alpine contractional deformation of this area of the Cameros Basin. All these features suggest that the carbonate breccias were formed by tectonic deformation of alternating layers of carbonate mudstone and calcium sulphate, which have very different rheological behaviours. As a result, during tectonic deformation, sulphate flowed and carbonate layers were broken and displaced, producing a breccia of carbonate fragments within a sulphate groundmass. Afterwards, the sulphate groundmass was replaced by quartz and calcite, and the breccia acquired its final composition.