Petrographic and geochemical evidence for the formation of primary, bacterially induced lacustrine dolomite: La Roda 'white earth' (Pliocene, centralSpain)

Upper Pliocene dolomites ('white earth') from La Roda, Spain, offer a good opportunity to evaluate the process of dolomite formation in lakes. The rel atively young nature of the deposits could allow a link between dolomites p recipitated in modern lake systems and those present in older lacustrine fo rmations. The La Roda Mg-carbonates (dolomite unit) occur as a 3.5- to 4-m- thick package of poorly indurated, white, massive dolomite beds with interb edded thin deposits of porous carbonate displaying root and desiccation tra ces as well as local lenticular gypsum moulds. The massive dolomite beds co nsist mainly of loosely packed 1- to 2-mum-sized aggregates of dolomite cry stals exhibiting poorly developed faces, which usually results in a subroun ded morphology of the crystals. Minute rhombs of dolomite are sparse within the aggregates. Both knobbly textures and clumps of spherical bodies cover ing the crystal surfaces indicate that bacteria were involved in the format ion of the dolomites. In addition, aggregates of euhedral dolomite crystals are usually present in some more clayey (sepiolite) interbeds. The thin po rous carbonate (mostly dolomite) beds exhibit both euhedral and subrounded, bacterially induced dolomite crystals. The carbonate is mainly Ca-dolomite (51-54 mol% CaCO3), showing a low degree of ordering (degree of ordering r anges from 0.27 to 0.48). Calcite is present as a subordinate mineral in so me samples. Sr, Mn and Fe contents show very low correlation coefficients w ith Mg/Ca ratios, whereas SiO2 and K contents are highly correlated. delta O-18- and delta C-13-values in dolomites range from -3.07 parts per thousan d to 5.40 parts per thousand PDB (mean = 0.06, sigma = 1.75) and from -6.34 parts per thousand to -0.39 parts per thousand, PDB (mean = -3.55, sigma = 1.33) respectively. Samples containing significant amounts of both dolomit e and calcite do not in general show significant enrichment or depletion in O-18 and C-13 between the two minerals. The correlation coefficient betwee n delta O-18 and delta C-13 for dolomite is extremely low and negative (r = -0.05), whereas it is higher and positive (r = 0.47) for calcite. The lacu strine dolomite deposit from La Roda is interpreted mainly as a result of p rimary precipitation of dolomite in a shallow, hydrologically closed perenn ial lake. The lake was supplied by highly saturated HCO3-/CO32- groundwater that leached dolomitic Mesozoic formations. Precipitation of dolomite from alkaline lake waters took place under a semi-arid to and climate. However, according to our isotopic data, strong evaporative conditions were not req uired for the formation of the La Roda dolomite. A significant contribution by bacteria to the formation of the dolomites is assumed in view of both p etrographic and geochemical evidence.