Origin of fabric-selective dolomitization recognizable in the field: two case studies from Anisian carbonate rocks in the western Balkanides

Abstract

The origin of dolomitized burrows (Cruziana ichnofacies) in limestones and dolomitic layers in limestone-dolostone ribbon rocks was studied on the basis of petrography, X-ray diffractometry, geochemical data and isotope signatures of the dolomites. Selective dolomitization of the burrows with a local source of Mg occurred in a near-surface setting from non-evaporitic solutions with low Mg/Ca ratio. The low-temperature precipitation of non-stoichiometric (with Ca excess) dolomite was microbially mediated and controlled by bacterial sulphate reduction. The carbon for dolomite formation was largely derived from seawater and/or dissolution of precursor carbonate sediments, and partly derived from the decomposition of organic matter. Selective dolomitization of primarily argillaceous carbonate layers in the ribbon rocks was mainly associated with an autochthonous source of Mg. Clay mineral transformations (i.e., illitization of smectite) at intermediate burial depths supplied Mg ions for the formation of Ca-rich (locally ferroan) dolomite. However, differential diagenesis of the originally clay-rich and clay-poor layers may also have favoured early dolomite precipitation, whereupon contemporaneous seawater was the primary Mg source during shallow burial. The obtained results show that various factors control the formation of fine-grained, non-stoichiometric dolomite with macroscopically recognizable fabric-selective character of replacement.

High‐magnesium calcite skeletons provide magnesium for burrow‐selective dolomitization in Cretaceous carbonates

“…Petrographic and stratigraphic analyses have ruled out other potential sources of Mg for burrow dolomitization, such as surrounding evaporites (Zenger, 1996a), volcanic ash (Callen & Herrmann, 2019) and clay minerals (Chatalov, 2018), all of which are conspicuously absent from the Rumaila Formation and surrounding strata. Therefore, echinoderm fragments initially composed of HMC are the sole candidate for a potential Mg source.…”

“…15), could be used as the Mg content of unaltered echinoderm fragments with burrows. Then, an Mg content of 43% could be assigned for the non‐stoichiometric (with Ca excess) burrow dolomite clear rims (Baldermann et al ., 2015; Chatalov, 2018). Due to inherent elemental differences between rims and centres (Fig.…”

“…The proposed mechanisms to overcome barriers to dolomite nucleation at low‐temperature in burrows involved bacterial sulphate reduction (Corlett & Jones, 2012; Baniak et al ., 2014; Baldermann et al ., 2015; Chatalov, 2018) and methanogenesis (Gingras et al ., 2004b; Baniak et al ., 2014). Both can generate ammonium to raise the Ph, alkalinity and activity of carbonate ions, and lower the hydration of Mg and Ca in pore water (Berner, 1980; Slaughter & Hill, 1991; Mazzullo, 2000; Wright, 2000).…”

“…The source of Mg in burrow‐selective dolomitization continues to be a topic of debate. In addition to the Mg produced by burrow organisms and supplied by coeval seawater (Gingras et al ., 2004b; Rameil, 2008; Chatalov, 2018), several additional Mg sources and pump mechanisms have been proposed. The most prevalent source is dense saltwater with a high Mg concentration, pumped by shallow seepage reflux and/or evaporitic tidal pumping (Morrow, 1978; Saller & Yaremko, 1994; Zenger, 1996a; Baniak et al ., 2014; Baldermann et al ., 2015; Niu et al ., 2020; Rahim et al ., 2020; Alibrahim et al ., 2021; Ye et al ., 2021; Yang et al ., 2022a).…”

Constraints on the preservation of proxy data in carbonate archives – lessons from a marine limestone to marble transect, Latemar, Italy

“…(A) The isotopic composition of Middle/Late Triassic marine calcite is indicated in the solid purple and dashed purple boxes (from Korte et al ., 2005; Grossman, 2012; Saltzman & Thomas, 2012; Veizer & Prokoph, 2015). The range of seawater associated with early diagenetic Middle/Late Triassic dolostones is indicated in the light green box (from Geske et al ., 2012; Chatalov, 2018; Rieder et al ., 2019). Data from previous studies (intrusion: Ferry et al ., 2002; Latemar: Jacquemyn et al ., 2014) is marked in dark green (dolomite) and blue (calcite) boxes.…”