The Carboniferous of the Bodón Unit (Cantabrian Zone; NW Spain): a case study on partly dolomitized mud-dominated platform sequences in a Paleo-Tethys foreland fold-and-thrust belt.

Abstract

The Cantabrian Zone represents the Variscan foreland fold-and-thrust belt in the Iberian Peninsula. Its succession comprises a nearly complete Paleozoic stratigraphic section, ranging from the Cambrian up to the Carboniferous. During the Carboniferous, important microbial-dominated carbonate platforms developed in the evolving foreland basin bordering the Paleo-Tethys. During and after deposition, the succession was thrusted and faulted into a foreland fold-and-thrust belt.

The Carboniferous succession in the Bodón Unit of the Cantabrian Zone shows evidences of dolomitization over an area of more than 200 km². An identified Permian dolomitization event (Gasparrini et al., 2006) is linked to secondary oroclinal bending, responsible for the curved shape of the Cantabrian Zone. The general conditions of dolomitization (i.e. circulation of concentrated seawater through fractures and faults) have been constrained. However, the reservoir characteristics of the dolomitized lithofacies are still undefined. Preliminary research suggests that petrophysical properties are facies-dependant and influenced by the dolomitization events.

The objective of the current study is twofold. Firstly, the dolomitized platform carbonates will be used as a test case for new analytical approaches. Clumped isotope paleothermometry, Mg isotope geochemistry and Rock Eval pyrolysis should give more constraints on the burial history of the platform deposits and the hydrothermal nature of the dolomitization. The second objective is related to the reservoir characteristics of the dolomitized deposits. Some of the tight host rock lithofacies gained a higher porosity and permeability upon dolomitization (e.g. microbial mud mounds), while others have remained tight (e.g. interplatform and platform-top deposits). Due to recent advances in our understanding of the sedimentology, anatomy and growth dynamics of the platform carbonates (e.g. Samankassou et al., 2013), these differences in reservoir characteristics can now be better understood.

The partly dolomitized Carboniferous successions of the Cantabrian Zone provide an interesting test case in which burial dolomitization is fault- and fracture-bounded, while porosity development is controlled by lithofacies. Since the carbonate platforms are underlain by organic-rich black limestones and covered with shales, they provide an ideal analogue for similar unexposed foreland basins.

AAPG Datapages/Search and Discovery Article #90226 © 2015 European Regional Conference and Exhibition, Lisbon, Portugal, May 18-19, 2015