Silicification of the Eocene carbonate deposits in southeast of Nizar, Qom

Document Type : Original Article

Authors

1 Exploration Directorate, National Iranian Oil Company, Tehran, Iran

2 Department of Geology, Faculty of Science, Bu-Ali Sina University, Hamedan, Iran

3 Department of Geology, Faculty of Science, Islamic Azad University, North Tehran Branch, Tehran, Iran

Abstract

Introduction
The Eocene deposits in the stuady area composed of a thick succession of pyroclastic, volcanic, and sedimentary rocks (Hajian, 1970). This study is mainly focused on the E5 lithozone in Takht-e Chakab area which is composed of tuff, sedimentary rocks and rhyolite. Based on Hajian (1970), the Eocene deposits are subdivided into 6 informal lithozones (E1 to E6).  i.e., the E1 unit are attributed to Ypresian–Lutetian and the E6 unit is attributed to Priabonian.
 
Materials and Methods
In order to determine effective diageneic proceses in the studied succession, one stratigraphic section is selected which is located in Takht-e Chakab anticline, 35 km noth of Delijan city. 78 thin sections were prepared and analysed.
 
Results and Discussion
The component allochems identified in the carbonate E5-5 lithozone including skeletal grains especially benthic foraminifera (Nummulitidae and Discocyclinidae), planktonic foraminifera, bivalve, gastropod, serpulids, bryozoa, red algae and corals. Hybrid samples consist of in-situ carbonte particles, clastic components, ash and volcanic particles. Petrographic studies of the studied deposits indicate that these sediments severly affected by diagenetic processes, which led to constructive and destructive porosity. One of the diagentic processes that affected Eocene carbonte deposits in burial environments is compaction. The mechanical compaction led to grain packing, deformation and sometimes crushed of the bioclasts. The grains contacts suturing and stylolite features are evidence of chemical compaction. Silicification is the main diagenetic processes in the studied section. Based on petrographic study some types of diagenetic silicification including chalcedony, micro and megaquartz that occurred in the form of secondary and replacement. Both selective and non-selective silicification is also recorded in some samples. Selectively silica replacement in the shell fragments, and pores filling siliceous cements were mainly filled interparticle porosities. The silica replacement of chalcedony type in bioclasts ocured as spherulitic replacement and controlled ones.
The chalcedony fibers have radial pattern in the spherulitic replacement type and independent from microstructures of the test and its orientation. In contrast to spherulitic replacement, in the controlled type of replacement the position and configuration of chalcedony fibrous follow the microscopic structures of the test and showing preferntial orientation. The microcrystaline quartz with equant crystals is less than 20 µm (Maliva and Siever, 1988). In some of bioclasts microquartz is replaced in the form of silica. This process occurred in some bioclasts such as Assilina and Nummulites. Megaquartz occured as intraparticle and interparticle cements. While the interparticle cement involved higher frequently. The petrography of the studied succession reveals that the selective silicification is mostly take place in Assilina, Nummulites and, also ostera fragments. The effects of this process are not the same in different Nummulites species, as it is very common in larger shells rather than smaller ones. Silicification is more common in hyaline foraminifera, while it is absents in porcelaneous ones. Typically, this process is also different in hyaline foraminifera test (very developed in Assilina and Nummulites, it is, rare in Discocyclina, Actinocyclina and Asetrocyclina). The silica source for silicification is usually provided via the biogenic and volcanic activities (Robertson, 1977). Some authors considerd the dissolution of biogenic Opal and or volcanic glasses in as the sources of silica in connate waters, while others considered clay minerals alteration (Nobel and Van Stempvoort, 1989). Based on rare occurences of fossils with siliceous tests such as radiolarian and sponge spicules in the studied sections, the organic silica for this widespread silicification is not rational, so the volcanic materials are a valid source for silicification in these deposits (Okhravi and Mobasheri, 1997).
 
Conclusion
The carbonate deposits belong to lithozone 5 (E5) consist of limestone, tuffa limestone and marl. Hybrid limestones are also observed in some horizons. Silicification as the main diagenetic process determined as replacing silica and pore-filling siliceous cement that influenced the studied strata. Skeletal factors play vital roles in type and amounts of silicification. Based on petrographic analysis, perforate hyaline foraminifera have undergo more silicification process in compare with other present bioclasts.

Keywords

Main Subjects

References

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Researches in Earth Sciences
Volume 15, Issue 3 - Serial Number 59
September 2024
Pages 39-56

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