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

Carbonate breccias may be formed by a wide variety of processes, both syn- and postdepositional, which implies that interpreting the mechanism of formation requires a detailed study because different processes may produce similar features. This is the case of the Lower Cretaceous carbonate breccias of the Cameros Basin, which were previously interpreted as syn-depositional slump breccias and as post-depositional evaporite-solution collapse breccias. However, a detailed analysis suggests that these breccias were the result of a different process: tectonic flow of sulphate layers (now vanished) interbedded with carbonates. Similar tectonically-driven evaporite flow processes have been described in thrust faults, but the formation of carbonate breccias by tectonic sulphate flow may be overlooked if they are not related to thrusts, due to difficulties in their recognition. The analysis of the carbonate breccias of the Oncala Group provides useful criteria to recognize this type of brecciation. The studied carbonate breccias are interbedded with, and laterally associated to, alternating layers of carbonate mudstone and calcite and quartz pseudomorphs after gypsum. The carbonate breccias consist of angular carbonate mudstone fragments floating in a matrix made up of calcite and quartz crystals. The breccia fragments and the carbonate mudstone layers of the unbrecciated adjacent deposits show identical composition and features, and the matrix of the breccias has the same composition as the calcite and quartz pseudomorphs after gypsum of the unbrecciated adjacent layers, which suggests that the brecciated beds were originally composed of identical alternating carbonate mudstone and gypsum layers as the unbrecciated layers. The carbonate breccias are generally matrix-supported and their fragments are from very slightly displaced to chaotically arranged. The breccias are associated with deformation structures, and breccia fragments are commonly arranged describing frequently polyharmonic folds, whose axial planes strike between N114ºE and N168ºE. The deformation structures affecting the carbonate breccias are similarly oriented as the regional tectonic structures, which strike between N144ºE and N155ºE, suggesting that they are related with the alpine, contractional, tectonic deformation of this area of the Cameros Basin. All these features suggest that these carbonate breccias were formed by tectonic deformation of alternating layers of carbonate mudstone and calcium sulphate (anhydrite during burial), which have very different rheological behaviour. 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 pseudosparitic calcite, and the breccia acquired its final composition.