Response of Mg isotopes to dolomitization during fluctuations in sea level: Constraints on the hydrological conditions of massive dolomitization systems

Zhongya Hu, Or M. Bialik, Simon V. Hohl, Zhiguang Xia, Nicolas D. Waldmann, Chuan Liu, Weiqiang Li

Research output: Contribution to journalArticlepeer-review

Abstract

Marine dolomitization processes are characterized by complex variations in hydrological conditions and pore-fluid chemistry. Deposition of massive dolomitized carbonates on the Comanche Platform of south Texas, USA, during the Albian coincided with multiple fluctuations in sea level, thereby providing an ideal setting to study the response of magnesium isotopes to dolomitization during eustatic sea-level change. In this study, we conducted Mg, C, and O isotope analyses and complimentary mineralogical and petrographic investigations of dolomitized massive carbonates of the Albian Edwards Group in the Comanche Platform. Petrographic observations indicate that the carbonate rocks in the studied units were not altered by deep burial diagenesis or hydrothermal fluids. Based on our petrographic observations and the trace element and C–O isotope data, we infer the dolomites were formed via syn-depositional dolomitization during a period of low sea level. The δ26Mg values of the dolomites increase rapidly from −2.5‰ to −1.8‰ in the basal part of the unit, reflecting a change in fluid chemistry caused by dolomitization in a restricted marine environment. Subsequently, δ26Mgdolomite values gradually decrease back to their initial value of approximately −2.5‰ due to seawater replenishment during transgression. The observed variability in dolomite δ26Mg values reflects changes in the connectivity of the platform with the open ocean during marine transgressions. However, the δ26Mgdolomite values do not vary with the high-frequency eustatic sea-level change recorded in the lithological variations, indicating uniform hydrologic conditions of the massive dolomitization system despite the hydrodynamic variations in the sedimentary environment. Therefore, we propose that massive dolomitization systems are mostly fluid-buffered and, as a result, Mg isotopes of dolomites can be used to trace changes in the paleo-marine environment, such as basin connectivity.

Original languageEnglish
Article number105922
JournalSedimentary Geology
Volume420
DOIs
StatePublished - Jul 2021

Bibliographical note

Funding Information:
We thank the Texas Bureau of Economic Geology repository for sampling assistance; Ms. Mor Arnon is thanked for sample handling. This study is supported by the National Science Foundation of China (grants 41473002, 41561144002, and 41622301) and the China Postdoctoral Science Foundation (2020M681379). Additional funding was provided by jointly ISF grant No. 2208/15 to NDW in ISF-NSFC matched scheme to grant No. 41561144002. OMB is supported by H2020 Marie Sk?odowska-Curie Actions Fellowship No. 101003394 (RhodoMalta).

Funding Information:
We thank the Texas Bureau of Economic Geology repository for sampling assistance; Ms. Mor Arnon is thanked for sample handling. This study is supported by the National Science Foundation of China (grants 41473002 , 41561144002 , and 41622301 ) and the China Postdoctoral Science Foundation ( 2020M681379 ). Additional funding was provided by jointly ISF grant No. 2208/15 to NDW in ISF-NSFC matched scheme to grant No. 41561144002. OMB is supported by H2020 Marie Skłodowska-Curie Actions Fellowship No. 101003394 (RhodoMalta).

Publisher Copyright:
© 2021 Elsevier B.V.

Keywords

  • Albian
  • Cretaceous seawater chemistry
  • Dolomites
  • Edwards group
  • Texas

ASJC Scopus subject areas

  • Geology
  • Stratigraphy

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