Petrography and geochemistry of Shahbazan Formation dolomites and investigation of its possible boundary with Asmari Formation from the elemental geochemistry point of view (Northeastern Kohdasht, South Lorestan)

Authors

1 Assist. Prof., Dept., of Geology, Khorramabad Branch, Islamic Azad University, Khorramabad, Iran

2 Ph. D. Student, Sedimentology and Sedimentary Rock, Bu Ali Sina University, Hamadan, Iran

Abstract

Shahbazan Formation (Middle to Late Eocene) consists of dolomite sequences, dolomite limestone and marl. In order to investigate the lithology and geochemistry of the deposits of this Formation and determine its possible boundary with the Asmari Formation, a 121-meter thick stratigraphic section was selected in the northeast of Kohdasht. In this section, the Shahbazan Formation with limestone and dolomite lithology is discontinuously placed on the Kashkan clastic Formation and its upper border is covered by the carbonates of the Asmari Formation as an erosional discontinuity. Field and petrographic studies were carried out on 90 sedimentary thin sections and geochemical studies on 30 samples from Shahbazan dolomite Formation and 10 samples from Asmari carbonate Formation using EDS and EPMA methods. According to petrographic and geochemical data, two groups of dolomites were identified. Primary dolomites (dolomicrites) were observed in sizes less than 10 microns and secondary dolomites (dolomicrosparites and dolosparites) in sizes greater than 10 microns. The presence of evidence such as bird's eye porosity, algal laminae and intraclasts and the absence of evaporite minerals in the studied samples showed that the primary dolomites were formed in a tidal model, then due to seepage of basin floor sediments in the carbonate platform of Shahbazan Formation, secondary dolomites are spread in a shallow to medium burial diagenesis model. Using elemental analysis, it was determined that from the primary dolomites to the secondary dolomites, there is an increase in the amount of Fe, Mg, and Mn elements, and at the same time, the amount of Sr, Ca, and Na elements decreases. In the possible boundary between Shahbazan and Asmari formations, the amount of Sr, Ca, Na, Sr/Mn and Ca/Mg increases, and the amount of Mn, Mg elements and the ratio of Mn/Ca decrease. Geochemical evidence showed that all the studied dolomites are non-stoichiometric (Ca/Mg ≥2) and high amounts of Na element with an average of 0.6% by weight indicate this issue.

Keywords

References

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New Findings in Applied Geology
Volume 16, Issue 32 - Serial Number 32
January 2023
Pages 200-223

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  • Receive Date: 18 May 2022
  • Revise Date: 29 June 2022
  • Accept Date: 31 August 2022

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