عنوان مقاله [English]
ke the same sedimentary conditions in the Lower Cretaceous, the Upper Cretaceous sediments of Iran do not have the same facies features. Therefore, in this study, we try to study the sedimentary environment and microfacies. Using data and geochemical methods of Upper Cretaceous sediments led to valuable information about the environment and conditions of formation and primary mineralogy of Upper Cretaceous sediments across Dombak-kuh.
materials and methods
In Dombak-kuh section, sampling has been done based on lithological and facies changes in the direction perpendicular to the layers and at distances of less than 0.5 m to more than 2.5 m. These sections were stained with red alizarin solution and potassium ferrocyanide to distinguish calcite mineralogy from dolomite as well as iron content and the amount of iron in it with Dickson method (Dickson, 1965). The classification and naming of rocks is based on Dunham's method (Dunham, 1962). In sample naming, an attempt has been made to include all major allocations in sample naming in order of frequency. The percentage of alluvial abundance was obtained ocularly by comparing with Baccelle and Bosellini (1965) comparison tables. The known facies have been compared with the Flugel (2010) facies belt. In presenting the sedimentary model, the terms used in Burchett and Wright (1992) have been used.
Results and discussion
Petrogeraphic studies of this sections revealed 11 microfacies (2 microfacies from tidal flat, 1 microfacies from lagoon, 1 microfacies from shoal and 7 microfacies from open marine). All of them belong to carbonate ramp: 1. Dolomicrites – Dolomicrosparaite, 2. Intraclast Grainstone, 3. Bioclact Wackestone, 4. Bioclast Pelloidal Grainston, 5. Oligosteginids Packstone, 6. Nezzazatinella, Dictyoconella Wackestone-Packston, 7. Lenticulina/Marginotruncana/ Oligosteginids Wackestone-Packstone, 8. Heterohelix /Globotruncana/Macroglobigerielloides Wackestone, 9. Oligosteginids/ Macroglobigerielloides / Heterohelix Packstone, 10. Heterohelix /Globotruncana/Marginotruncana packstone, 11. Globotruncana/ Heterohelix /Macroglobigerielloides Wackestone-packstone.
The characteristics of these deposits indicate that the sedimentary environment was a ramp (hemocline). The absence of calcite turbidite deposits, fall structures and large reef and oncoid and piezoidal dams confirms the carbonate ramp environment. The results of elemental analysis and plotting of these values against each other indicate that the Upper Cretaceous sediment limestones primarily had aragonite mineralogy. The plotting of Mn values against Sr / Ca indicates a semi-closed to open diagenetic environment with a high water/rock ratio.
According to the studies carried out in the Dombak-kuh region of the Upper Cretaceous sediments (Cenomanian-Campanian) they have a thickness of 327.7 m, the lower boundary of which is the Tizkuh Formation and is of a steep type. Its upper boundary is a fault which has been disconnected. Cenomanian sediments, which are 54.26 m thick, start with a progressive conglomerate. Then the thoronine sediments, which are 22.85 m thick, first start with thick-layered limestone and then with chert limestone, and then with Oligosthenic limestones which change the facies and settle on it. Kenyasin sediments with a thickness of 19.85 m were observed with a combination of oligoesterated limestones followed by Santonin sediments with a thickness of 182.80 m and then Campanian sediments with a thickness of 48.5 m form the Upper Cretaceous stratigraphic sequence. In these sediments, 11 micro-facies were identified, which include tidal zone with 2 micro-losses, lagoon with 1 micro-loss, dam with 1 micro-loss and open sea with 7 micro-losses. The characteristics of these deposits indicate that the sedimentary environment was a ramp (hemocline). The absence of calcite turbidite deposits, fall structures, and large reef and oncoid and piezoidal dams confirms the carbonate ramp environment. The results of elemental analysis and mapping these values together, indicate that the Upper Cretaceous sediments in this section has aragonite primary compound which is placed in a semi-closed to semi-open diagenetic region with high proportion of water to rock (W/R).