Cenozoic coastal carbonate deposits of Qatar: Evidence for dolomite preservation bias in highly‐arid systems. Issue 2 (6th December 2020)

• Record Type: Journal Article
• Authors: Rivers, John M.; Yousif, Ruqaiya; Kaczmarek, Stephen E.; Al‐Shaikh, Ismail
• Editors: Hollis, Cathy
• Is Part Of: Sedimentology. Volume 68:Issue 2(2021)
• Page Start: 771
• Page End: 787
• Publication Date: 2020-12-06
• Subjects: Arid -- dolomite -- gypsum -- saturation -- seepage reflux; Sedimentology -- Periodicals; 552.5
• DOI: 10.1111/sed.12805
• Languages: English
• ISSNs: 0037-0746

Abstract

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.