RT Journal Article
T1 Dolomitization and silicification in low-energy lacustrinecarbonates (Paleogene, Madrid Basin, Spain)
A1 Bustillo Revuelta, María Ángeles
A1 Arribas Mocoroa, María Eugenia
A1 Bustillo Revuelta, Manuel
AB Repetitive sequences of carbonate deposits, occurrence include in the lower part of the Paleogene Carbonate Unit (northeastborder 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 diffractionanalyses joint to isotopic studies (d18O and d13C) have been used to characterise the facies and environments. The sequencesconsist 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, circumgranularcementation, gypsum lenticular crystals) outline pedogenic processes. These features, found locally in bioclastic and microbiallaminated limestones or in dolomite uncemented muds, define, respectively, palustrine limestones or dolocretes. Facies analysisallows us to define several lacustrine sub-environments (basinal, littoral, eulittoral and supralittoral) and characterises differenttypes of shallowing upward lacustrine sequences, either with or without subaerial exposure. The isotopic values of the bioclasticand microbial laminated limestone (from 5.77xto 6.78xfor d13C, and from 5.25xto 5.53xfor d18O) and those ofuncemented calcitic muds (from 5.80xto 7.01xfor d13C, and from 4.98xto 5.58xfor d18O) establish that bothtypes of carbonates precipitated in equilibrium with meteoric waters. In the palustrine carbonate deposits, the d13C valuessuggest a strong organic contribution. The dolomitization that only affected the uncemented carbonate muds is early interpretedbecause of the structural and compositional characteristics of the dolomite (micro-rhombic dolomicrite, nearly stoichiometricand poorly ordered), and because the calculated average D18O dol–cal for calcitic and dolomitic uncemented muds is about6x. The d18O isotopic values indicate that the dolomite precipitated from water that was slightly more enriched in heavyisotopes than the calcite, because of an increase in evaporation rates. Nodules and nodular levels of cherts occur in the upperunits of littoral and eulittoral sequences, probably as a consequence of the existence of microbial mats that could include thesilica source. According to their mineralogy (Opal CT and quartz/moganite) and structures (double nodules, lamination andbioturbation), three types of chert are described (TB, MB and WO). These types define three general stages of silicificationduring the early diagenesis, from the recently buried to the postcompactation of carbonate deposits. The d18O values of quartzshow 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 moreevaporated waters than those of the general stages of silicification.
PB Elsevier
SN 0037-0738
YR 2002
FD 2002
LK https://hdl.handle.net/20.500.14352/57741
UL https://hdl.handle.net/20.500.14352/57741
LA eng
DS Docta Complutense
RD 1 sept 2024