Dolomitization, gypsum calcitization and silicification in carbonate–evaporite shallow lacustrine deposits

Abstract

This paper describes and interprets the mineral and facies assemblages that occur in carbonate–evaporite shallow lacustrine deposits, considering the importance of the processes pathway (i.e. dolomitization, gypsum calcitization and silicification). The Palaeogene deposits of the Deza Formation (Almazán Basin, central‐northern Spain) are selected as a case study to determine the variety of physicochemical processes taking place in carbonate–evaporite shallow lakes and their resulting diagenetic features. Dolostones are the predominant lithology and are composed mainly of dolomite with variable amounts of secondary calcite (5 to 50%), which mainly mimic lenticular gypsum (pseudomorphs). Five morphological types of dolomite crystal were identified as follows: dolomite tubes, dolomite cylinders, rhombohedral dolomite, spheroidal and quasi‐rhombohedral dolomite, and cocoon‐shaped dolomite. The dolomite cylinders and tubes are interpreted as the dolomitized cells of a widespread microbial community. The sequence of diagenetic processes started with growth of microlenticular interstitial gypsum in a calcareous mud deposited on the playa margin mudflats, and that sometimes included microbial sediments. Immediately following growth of gypsum, dolomite replaced the original calcite (or possibly aragonite) muds, the microbial community and the gypsum. Partial or total replacement of gypsum by dolomite was related mainly to the biomineralization of endolithic microbial communities on gypsum crystals. Later calcitization took place under vadose, subaerial exposure conditions. The development of calcrete in distal alluvial settings favoured the release of silica and subsequent silicification on the playa margin mudflats. Stable isotope compositions of calcite range from −9·02 to −5·83‰ δ13CPDB and −7·10 to 1·22‰ δ18OPDB; for the dolomite, these values vary from −8·93 to −3·96‰ δ13CPDB and −5·53 to 2·4‰ δ18OPDB. Quartz from the cherts has δ18OSMOW values ranging from 27·1 to 31·1‰. Wide variation and relatively high δ18OSMOW values for dolomite indicate evaporitic and closed hydrological conditions; increased influx of meteoric waters reigned during the formation of secondary calcite spar.

Nucleation and stabilization of Eocene dolomite in evaporative lacustrine deposits from central Tibetan plateau

“…Although modern seawater is supersaturated with respect to dolomite, neither dolomite precipitation nor dolomitization has been observed to occur at Earth's surface temperature and pressure (Mazzullo, 2000;. Even though the kinetic and geochemical barriers to low temperature dolomite formation are still poorly understood, this precipitation process has been observed to occur in dolomitegypsiferous facies in deposits in shallow ephemeral lakes and playas (Wolfbauer & Surdam, 1974;Smoot, 1978;Sanz-Montero et al, 2008;Lindtke et al, 2011;Meister et al, 2011a;Bustillo et al, 2017). A range of thermodynamic (carbonate alkalinity, pH and sulphate concentration) and kinetic factors (cation binding, Mg 2+ hydration and dissolved SO 2À 4 ) have been suggested, in previous literature, to control the primary formation of dolomite at low temperature (e.g.…”

Diversity and origin of quartz cements in continental carbonates: Example from the Lower Cretaceous rift deposits of the South Atlantic margin

“…length-fast criteria is mainly based on their petrographic features and their frequent association with evaporitic minerals. A recent approach from Bustillo et al (2017) showed the predominance of length-slow forms in carbonate-evaporite precipitating lakes, and the predominance of length-fast forms in carbonate precipitating lakes. No studies have yet documented whether the isotopic composition of the different fibrous microquartz (δ 18 O quartz ) could potentially help to characterize their conditions of precipitation.…”

Carbon and oxygen isotopic analyses of calcite in calcite–dolomite mixtures: Optimization of selective acid extraction

“…Hand‐picking of calcite and dolomite is impracticable for cryptocrystalline carbonates. As an alternative, selective acid extraction is utilized for the isotopic analysis of coexisting calcite and dolomite . The technique, described initially by Epstein et al, is based on the different reaction rates of calcite and dolomite with phosphoric acid, with some studies demonstrating that the reaction rates are affected by grain size and reaction temperature .…”

“…As an alternative, selective acid extraction is utilized for the isotopic analysis of coexisting calcite and dolomite. [9][10][11][12][13][14][15] The technique, described initially by Epstein et al, 16 is based on the different reaction rates of calcite and dolomite with phosphoric acid, with some studies demonstrating that the reaction rates are affected by grain size and reaction temperature. 12,17,18 Modified procedures have been developed to separately collect CO 2 liberated from calcite and dolomite, [17][18][19][20][21] with extraction steps as follows: (1) calcite-dolomite mixtures are reacted with phosphoric acid at 25°C, and CO 2 liberated over periods of up to 2 h is collected for calcite isotopic analysis; (2) mixed-CO 2 liberated from calcite and dolomite in the next periods of up to 24 h is discarded; (3) the temperature is then kept at 25°C or increased up to 50°C with further acid treatment for complete sample dissolution, and the liberated CO 2 in this stage used for dolomite isotopic analysis.…”