Publication: Dolomitization and silicification in low-energy lacustrine
carbonates (Paleogene, Madrid Basin, Spain)
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2002
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Elsevier
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Abstract
Repetitive sequences of carbonate deposits, occurrence include in the lower part of the Paleogene Carbonate Unit (northeast
border of the Madrid Basin), have been studied, defining regressive lacustrine sequences and early diagenetic processes.
Binocular microscopic examination, scanning electron microscopic/energy dispersive X-ray observations, and X-ray diffraction
analyses joint to isotopic studies (d18O and d13C) have been used to characterise the facies and environments. The sequences
consist of a lower uncemented carbonate mud unit, calcitic or dolomitic in composition, and an upper carbonate unit (arenites,
bioclastic limestones, and microbial laminated limestones with cherts). Visual features (vertical prismatic structures, fissures,
massive nodulization, rhizoliths, brecciation) and microscopic features (micrite micronodules, vug porosity, circumgranular
cementation, gypsum lenticular crystals) outline pedogenic processes. These features, found locally in bioclastic and microbial
laminated limestones or in dolomite uncemented muds, define, respectively, palustrine limestones or dolocretes. Facies analysis
allows us to define several lacustrine sub-environments (basinal, littoral, eulittoral and supralittoral) and characterises different
types of shallowing upward lacustrine sequences, either with or without subaerial exposure. The isotopic values of the bioclastic
and microbial laminated limestone (from 5.77xto 6.78xfor d13C, and from 5.25xto 5.53xfor d18O) and those of
uncemented calcitic muds (from 5.80xto 7.01xfor d13C, and from 4.98xto 5.58xfor d18O) establish that both
types of carbonates precipitated in equilibrium with meteoric waters. In the palustrine carbonate deposits, the d13C values
suggest a strong organic contribution. The dolomitization that only affected the uncemented carbonate muds is early interpreted
because of the structural and compositional characteristics of the dolomite (micro-rhombic dolomicrite, nearly stoichiometric
and poorly ordered), and because the calculated average D18O dol–cal for calcitic and dolomitic uncemented muds is about
6x. The d18O isotopic values indicate that the dolomite precipitated from water that was slightly more enriched in heavy
isotopes than the calcite, because of an increase in evaporation rates. Nodules and nodular levels of cherts occur in the upper
units of littoral and eulittoral sequences, probably as a consequence of the existence of microbial mats that could include the
silica source. According to their mineralogy (Opal CT and quartz/moganite) and structures (double nodules, lamination and
bioturbation), three types of chert are described (TB, MB and WO). These types define three general stages of silicification
during the early diagenesis, from the recently buried to the postcompactation of carbonate deposits. The d18O values of quartz
show that the silicification and ageing of opaline phases occurred from meteoric waters, which were lighter than the calcite or dolomite precipitating waters. The cherts included in palustrine limestones show d18O values of quartz that record more
evaporated waters than those of the general stages of silicification.