Seepage carbonate mounds in Cenozoic sedimentary sequences from the Las Minas Basin, SE Spain

A number of carbonate mounds composed of indurate, strongly folded and/or brecciated calcite and dolomitebeds occur interstratified in Cenozoic sedimentary sequences from the Las Minas Basin. Part of the fabric of therock forming the carbonate mounds is composed of laminated to banded dolostone similar to the host rockbut showing contrasted lithification. Moreover, the carbonate deposits of the mounds display aggradingneomorphism of dolomite, partial replacement of dolomite by calcite, calcite cementation, and extensive silicification,locally resulting in box-work fabric. Eightmain lithofacies were distinguished in the carbonatemound deposits.In some lithofacies, chert is present as both microcrystalline to fibro-radial quartz and opal, the latteroccurring mainly as cement whereas the former replace the carbonate and infill voids. Yet one of the carbonatemounds shows distinctive petrography and geochemical features thus suggesting a distinctive growth pattern.The carbon isotope compositions of calcite from the mound samples range from −11.56 to −5.15 δ‰ whilst dolomiteis depleted in 13C, with values of −12.38 to 3.02 δ‰. Oxygen isotopic compositions vary from −9.42 to−4.64 δ‰for calcite and between −6.68 and 8.19 δ‰ for dolomite. Carbonate in the mounds shows significantenrichment in Co, Cr, Ni and Pb content, especially in the strongly deformed (F-2-2 lithofacies) and brecciatedcarbonate (F-4). The carbonate deposits show depletion in REE and Y in contrast to that determined in lutite.The formation of the carbonate mounds was related to local artesian seepage thermal water flows of moderateto relative high temperatures. Pressure differences between the low permeability host rock and the circulatingfluids accounted for dilational fracturing and brecciation of the host sediment packages, which combined withprecipitation of new carbonate and silica mineral phases. Locally, some carbonate mounds developed wheregroundwater intersected the lake floor, this resulting in bedded carbonate deposits composed of organicconstituents.