Petrography and geochemistry of Miocene red clastic sediments in Taleqan area, northwest of Karaj

Three main goals have driven studies of sandstone in the past ten years: Firstly, the academics motive to understand the tectonic setting, climatic situation, paleo-geographic position. Secondly, the economic motive to predict reservoir ability and porosity and permeability in hydrocarbon fields. Thirdly, the motion or stasis of pore fluids and the scale of mass-transport to form cements. This research involved the first one. Sedimentary rocks are principal sources of information concerning past conditions on the Earth’s surface. Clastic rocks may preserve detritus from long-eroded source rocks and may provide the only available clues to the composition and timing of exposure of source rocks. Geochemistry of sedimentary rocks may complement the petrographic data, especially when the latter are ambiguous. The geochemical composition of sedimentary rocks is a complex function of various variables such as source material, weathering, transportation, physical sorting, and diagenesis. Very few studies on Miocene clastic sediments have been conducted in the country, in zones other than the Central Iran. Hence there are many questions without response in this area. This research is trying to get some solution for these questions

In this research, clastic sediments in the central Alborz have studied using facies analysis, petrography, grains counting and geochemical methods. For this propose, from two sections in the Taleqan area fresh rock samples were collected from outcrops exposed in stream and road cuts and were washed thoroughly in distilled water to remove dust contamination. 52 samples were selected for detailed petrographic study. Also 24 thin sections selected for grain counting and modal analyses according to Dickinson method. Coarse grains classified based on Miall method.  In geochemical studies, we used XRF Philips 1480 for determination of major and minor elements oxides. After preparation, 15 samples were selected for analysis. Measurement accuracy ranges were 0.1 to 0.001. This sequence is mainly composed of marl, sandstone and locally intercalation of conglomerate with Pebble size particles. Studied outcrop is formed of fine grained sedimentary sequence (Marl), sandstones (Feldespatic litharenite and volcanic arenite) and conglomerate (poly-mictic ortho and Para-conglomerate) with 127.2 m thickness. The strata in this formation are composed of two gravelly (Gmm and Gcm) and three sandy (St, Sh and Sm) facies. The main components of these deposits are igneous rock fragments with poorly to moderate sorting and moderate to good roundness that are welded together with hematite dominant cement.

Roser & Korsch (1986) established a discrimination diagram using log (K2O/Na2O) versus SiO2 to determine the tectonic setting of terrigenous sedimentary rocks. These authors used CaO and LOI-free 100% adjusted data to determine their field boundaries. Both parameters (SiO2 and log (K2O/Na2O) values) increase from volcanic-arc to active-continental-margin to passive-margin settings.  Discrimination of tectonic settings on the basis of major-element data also was proposed by Bhatia (1983); it includes oceanic island arc, continental island arc, active continental margin, and passive margin. Most of our sandstone samples fall in the general area of passive margin and active-continental-margin fields of the TiO2 versus Fe2O3* 1 MgO plot, but mostly in the passive-margin field of the Al2O3/SiO2 versus Fe2O3* 1 MgO diagram. Petrographic data show that K-feldspar dominates over plagioclase, which may result from intense weathering in the source area or from diagenetic alteration. The latter can be ruled out by the presence of abundant carbonate cement that developed probably during early diagenesis. The intensity and duration of weathering in sedimentary rocks can be evaluated by examining the relationships among alkali and alkaline earth elements (Nesbitt & Young, 1982, 1984). Feldspars are by far the most abundant of the reactive minerals. Consequently, the dominant process during chemical weathering of the upper crust is the alteration of feldspars and the neo-formation of clay minerals. During weathering, calcium, sodium, and potassium are largely removed from feldspars (Nesbitt et al., 1997). The amount of these elements surviving in the soil profiles and in the associated sediments is a quantitative index of the intensity of weathering (Fedo et al., 1995; Nesbitt et al., 1997). A good measure of the degree of chemical weathering can be obtained by calculation of the chemical index of alteration (CIA; Nesbitt & Young, 1982) using the formula (molecular proportions). According to results, these sediments have felsic igneous source rock similar to upper continental crust, which has been deposited in the semi-arid dry climate with weak weathering. Final deposits were traveled relatively small distance and were deposited in the first sedimentation cycle with low degree of maturity, low chemical weathering, in active tectonic environment. Thus, the low CIA values of the Taleqan sandstones do not reflect the general chemical weathering conditions in the source region, which can be inferred from the petrographic observations. This is probably due to the sedimentary sorting effect. Physical sorting of sediment during transport and deposition led to concentration of quartz and feldspar with some heavy minerals in the coarse fraction and of secondary lighter and more weathered minerals in the suspended-load sediments.

This sedimentation happened in the alluvial braided river’s channel and floodplain near the origin with weak hydraulic separation and oceanic arc tectonic Setting.