Stratigraphy, sedimentary environment and geochemistry of the Ruteh Formtaion in Qharkhotlou region, south-west of the Zanjan

The Ruteh Formation in the Qharkhotlou region (southwest of Zanjan) mainly consists of 52 m sandy limestones and cream to grey thin to medium-bedded limestones. In this area, the Routh Formation overlies the quartz-bearing sandstones of the Doroud Formation and is overlain by laterite-bauxite horizon. Based on fieldwork and microscopic studies, six carbonate microfacies is identified in the Routh Formation in the Qharkhotlou area. The interpretation of microfacies and the lack of coral great barrier reefs, absence of turbidite deposits as well as the presence of skeletal allochmes such as green algae, bivalve, brachiopoda, benthic foraminifera and echinoderm debris indicate that these microfacies possibly were deposited inside the shallow parts of a carbonate ramp . In order to study the geochemical characteristics of the Ruteh Formation and also to determinate the original carbonate mineralogy, the main (Mg and Ca) and trace (Sr, Na, Mn and Fe) elements were analyzed on 15 samples of these limestones (mainly micrite). Based on the two-variable diagrams such as Sr vs. Na, Mn vs. Sr/Na, Na vs. Mn and also Sr/Mn vs. Mn, it can be stated that the original carbonate mineralogy of these studied limestones in the Ruteh Formation are calcite-aragonite mixture which is consistent with the formation enviroment of these studied carbonates at the Palaeo-Tethys southern margin during the Permian.  

The Ruteh Formation is one of the most fossiliferous carbonate units in the Alborz Mountains. Lithostratigraphically, the Ruteh Formation in Alborz Mountains was correlated with the Jamal Formation in Central Iran Basin. This formation, defined in Alborz Mountains by Assereto (1963), displays a carbonate sequence relatively homogeneous of grey to dark limestones with intercalated marls. The type section of the Ruteh Formation is located in central Alborz near the village of Ruteh (North of the Tehran), where it has a thickness of 230 meters and consists of dark grey, medium-bedded to massive fossiliferous limestones (Assereto 1963). Lasemi (2001) characterized the sedimentary palaeoenvironments of the Ruteh Formation as equivalents of modern carbonate environments of the southern Persian Gulf with open sea, shoal, lagoon and tidal flat, respectively. The erosional lower boundary of the Ruteh Formation rests everywhere unconformably on the older lithological units (mostly Doroud Formation) and the upper boundary of the Ruteh Formation is regionally marked by a bauxite-laterite deposits (Aghanabati 2010), in the most areas of Central Iran. In this research for the first time depositional conditions and elemental geochemistry of the Ruteh Formation in the Zanjan province (Qharkhotlou section) have been evaluated.  

In this research to recognize the sedimentary environment and original carbonate mineralogy of the Ruteh Formation, we used one unique outcrop at the Qharkhotlou region located in the southwest of Zanjan. The section measured a total thickness of 52 m and consists of sandy limestone and cream to grey thin to medium-bedded limestones. In this area, the Routh Formation overlies the quartz-bearing sandstones of the Doroud Formation and is overlain by laterite-bauxite horizon. During the fieldwork studies, 35 rock samples from carbonate deposits have been taken for petrographic studies and geochemical analysis. In order to differentiate ferroan and non-ferroan calcite from ferroan and non-ferroan dolomite in thin sections, the staining method of Dickson (1965) was applied. Carbonate rocks were classified according to the schemes of Dunhum (1962). Flügel (2010) facies belts and sedimentary models were also used in this research. The composition of associated fauna and non-skeletal grains was considered. Sedimentologic texture and structure have been described in a semi-quantitative manner. Elemental geochemistry analyses (major and trace elements) were performed on 15 samples of these carbonates through the succession. The concentration of Ca, Mg, Fe, Mn and Sr of samples was measured at the Zarmazma Mineral Studies Company, Tehran.  

Based on the field and petrographic studies, the microfacies and depositional environment of the Ruteh Formation were recognized in the studied section. This formation has been made of six microfacies which deposited in a shallow open marine environment. These facies mainly consists of different kinds of benthic foraminifers with microgranular and porcelaneous shells (such as: miliolid), algae, echinoids, brachiopods and bivalve debris, along with some non-skeletal components (e.g., aggregates and intraclasts). These recognized microfacies from shallowest to deepest environments included as follow: (1) aggregate bioclast sandy wackestone, (2) peloid small benthic foraminifera wackestone, (3) bivalve green algae wackestone to packstone, (4) intraclast bioclast packstone to grainstone, (5) green algae brachiopoda packstone and finally (6) echinoderm brachiopoda wackestone. Gradual microfacies change, abundant micrites, the absence of calciturbidites and lack of extensive barrier reefs with considerable thickness, confirms a carbonate ramp for the studied carbonates succession. The microfacies mostly deposited in a distal inner ramp. The five microfacies (MF1–MF5), belong to distal inner ramp and just one is located in the proximal middle ramp (MF6: echinoderm brachiopoda wackestone). Whether carbonate ramps were distally steepened or homoclinal cannot be confirmed by the current study, since we are focusing on the shallowest environments. In the studied area, the boundary between Routeh and Shemshak formations is identified by thick laterite-bauxite layers with a thickness of about 20 m which clearly show an erosional surface forming during a warm and humid climatic condition. The geochemical results show that the samples are completely composed of limestones. Geochemical analysis of the limestones such as Ca, Mg, Sr (147–-582 ppm), Na (262–-974 ppm), Mn (101–-577 ppm) and Fe (400–-14100 ppm), and their bivariate plots (such as Sr, Sr/Na and Sr/Ca) indicate that the original carbonate mineralogy is calcite-aragonite mixture which is consistent with the formation of these studied carbonate deposits at the Palaeo-Tethys southern margin during the Permian. Geochemical studies also confirm that Ruteh carbonates were deposited in a shallow warm-water environment in the study area.