Cenozoic coastal carbonate deposits of Qatar: Evidence for dolomite preservation bias in highly‐arid systems

Abstract

In the ancient rock record, early replacement of metastable marine calcium carbonate deposits by dolomite has long been associated with evidence of arid depositional environments. Such associations led to the development of the seepage reflux dolomitization model, whereby magnesium-rich marine waters concentrated by evaporation descend into underlying sediments, replacing primary aragonite and calcite deposits with dolomite through rock-water interaction. In the modern arid coastal systems of Qatar, where marine waters concentrate to halite saturation, both aqueous geochemical measurements and mineralogical investigations show that replacement of calcium carbonate deposits by dolomite is not occurring to any consequential degree. At best dolomite formation is minor and localized. Instead, modern and mid-Holocene coastal deposits retain their original mineralogy, showing little evidence of carbonate precipitation reactions with the notable exception of beachrock formation. Pleistocene coastal deposits are mostly absent in comparison with both Qatar Holocene coastal deposits, and Pleistocene coastal deposits from more humid settings. The lack of onshore Pleistocene-aged carbonate outcrops in Qatar, as well as regionally, is interpreted to reflect coastal sediment denudation during emergence due to: (i) the absence of coastal marine lithification; and (ii) the absence of meteoric cementation and root stabilization in the highly-arid realm. In contrast, marine Palaeogene and Miocene carbonate deposits are preserved in outcrop in Qatar, having suffered marine lithification by dolomite, thus promoting retention of carbonate strata through subsequent lowstands. These older deposits formed during times of ocean acidification, which, based on natural system and laboratory investigations, is interpreted to promote metastable carbonate dissolution and dolomite formation. The highly-arid Holocene and Pleistocene coastal systems of Qatar represent limestone factories, but these deposits are not being retained to the rock record. Based on these observations, the association between the occurrence of dolomite and arid depositional settings may be to some degree a preservation bias.

Extensive recrystallization of Cenozoic dolomite during shallow burial: A case study from the Palaeocene–Eocene Umm er Radhuma formation and a global meta‐analysis

“…It is also possible that the relationship between dolomite abundance and depositional environment from Sun (1994) is due to a dolomite preservation bias, as reported in Rivers et al . (2021). If this is the case, using dolomite abundance data could lead to wrongful assertions about past conditions.…”

Early diagenetic constraints on Permian seawater chemistry from the Capitan Reef

“…Higher temperatures in the shallow waters overlying the shelf and reef facies would have particularly favored the precipitation of aragonite over calcite (Burton and Walter, 1987). However, on the shelf, higher rates of aerobic respiration might have lowered the aragonite saturation state below the sediment-water interface (Bergmann et al, 2013), opposing the preservation of aragonite and favoring early diagenetic dolomitization (Rivers et al, 2021).…”

The Dolomite Problem: A Matter of Time

“…A vast number of experimental works and calculations indicate that dolomite formation can occur in several stages of replacement and recrystallization. − In recent decades, some field and geochemical studies have also shown that the sedimentary and diagenetic formation of dolomite can be extended over thousands to millions of years. − Therefore, it is currently agreed that, after a relatively rapid initial dolomitization process, a subsequent increase in the Mg–Ca order in the dolomite crystals can occur via secondary recrystallization operating over geological periods of time. − This makes some researchers think that the systematic measurement of cation ordering of dolomites over time can provide relevant information on the kinetics of dolomite formation in nature.…”