Karstifcation in dolomitized Waulsortian mudmounds (Belgium)

The Waulsortian mudmounds consist of massive limestones that developed from late Tournaisian to early Visean. Secondary dolomitization has locally affected these mudmounds conducting to the coexistence of dolostone and limestone patches. Numerous karst cavities are preferentially developed in dolomitic intervals and are geometrically related to one or several fracture directions. These cavities, called the "dolostone cavity type" can be partially filled with coarse calcite and are usually bounded to the host dolostone by a calcitic transition zone which has roughly a lenticular shape. Petrographic observations, especially under cathodoluminescence, show the following sequence starting from the micro-cavities/micro-fractures network in the transition zone: 1) calcite cement in cavities, 2) dedolomite fabric, 3) host dolomite. The observed microscopic features, as well as morphological data, lead to the development of a conceptual genetic model of karst formation at macro- and microscale. In this model, first a strong dissolution of the host dolostone occurred from the fracture network thereby creating karst cavities (at macroscale) and important pervasive porosity (at microscale). Dissolution was then followed by dedolomitization in one or two-steps of the already corroded dolospar. Afterwards cementation of the residual porosity and precipitation of coarse calcite in the karst cavities occur. Even if the geological conditions under which these processes took place are not fully unraveled yet, a model that could ft with the data is presented. A discussion about the paleo-environment and paleo-hydrological conditions is settled and we also propose that the development of karst in the dolomitized mudmounds is favored by the coexistence of dolostone and limestone patches given their initial difference in porosity and competence.