Dolomite nodules are widespread within the Tortonian marls of Fortuna and Lorca basins in southern Spain. They occur as large bodies of various forms (round, ovoid, tabular) that are parallel or secant relative to the stratification. They are massive and present sometimes internal conduits that are considered as drains used for the migration of fluids. This study brings new results on the mineralogy and elemental and isotopic geochemistry of these dolomite nodules to better describe the processes that were occurring during their formation. The diagenetic reactions that have driven authigenic dolomite precipitation within the sediments involved on the one hand methanogenesis that produced 13C-poor CH4 and 13C-rich CO2, on the other hand carbonate and silicate weathering by the CO2-rich solutions, that released respectively alkalinity and cations in pore solutions. Moreover, the distribution of the major elements (Si, Al, K, Na, Ca, Mg, Fe) in the marls and dolomite nodules indicates that these elements were redistributed within the sedimentary formation during the diagenetic reactions without external inputs by the circulating fluids. These observations thus confirm the link between authigenic dolomite formation and clay minerals diagenesis in sediments where methanogenesis was active.