Effective factors in formation of tafoni in the Neogene sandstones of Eshtehard and Mardabad areas (Alborz province)

IntroductionTafoni weathering refers to large cave-like holes ranging from a few decimeters to several meters wide. They develop often in the medium and coarse-grained silicate rocks such as sandstones, conglomerates and granites in various areas, under arid and semi-arid climate conditionMaterials and MethodsTo identify the tafoni forms, we measured the morphological features (such as the dimensions and extent) of the tafoni in the sandstone layers in the north of Eshtehard and Mardabad sections. A total of ten medium to coarse-grained sandstone samples were selected from the lens-liked channel deposits. Framework mineral composition (modal analysis) was quantified using the point-counting method (300-500 point) of Gazzi and Dickinson, as described by Ingersoll et al. (1984).Results and discussionField surveyTo calculate the dimensions of the tafoni in the sandstone layers, the width (W), height (H) and depth (D) of the ninety eight tofoni were measured in different parts of two sections. The higher average values of W than the mean values of H and D and also the higher W/D values than H/D indicate that the tafoni forms in the sandstone layers are more elliptical and semicircular (W> H> D).PetrographyBased on the petrographical and modal analysis, the mean grain size in the sandstone samples are about 0.5 mm with weak to moderate sorting and rounded to subrounded grains. Sandstone samples from the Eshtehard section include high amount of the igneous, sedimentary and metamorphic rock fragments (average 53.8%), and the feldspar (orthoclase and plagioclase) (average 35.4%), and low amount of mono-and poly-crystallin quartz grains (average 10.8%) with point, straight to concavo-convex grain contacts. In turn, the Mardabad sandstone samples show higher proportion of the igneous and sedimentary rock fragments (average 83%) and lower proportion of the feldspar (average 12%) and quartz (average 2.5%). The average amount of calcium carbonate in the Eshtehard and Mardabad sandstone samples are 6.4% and 10.4%, respectively.Dissolution of unstable grains such as feldspars (plagioclase and orthoclase) and carbonates (microcrystalline and fossil particles) and rock fragments due to the infiltration of acidic waters, promotes the development of porosity in the rock and reduces its resistance during chemical and physical weathering (Hamblin and Christiansen 2008). The Eshtehard sandstones include higher feldspar amount (Q11 F35 RF54) than that of the Mardabad sandstones (Q5 F12 RF83). According to microscopic studies, the Eshtehard sandstones have higher propotion of feldspar and in turn, higher chemical alteration. The development alteration in the feldspar grains can cause the weakness or loose texture and finally physical destruction of rock zones.Dissolution of calcite and dolomite cementLow proportion of calcite and dolomite cement due to dissolution have caused an increased amount of porosity of grains in the process of tofoni formation in the Eshtehard sandstones. The relatively high amounts of calcite cement and calcium carbonate rock fragments, low intergranular porosity and also point, straight to concavo-convex grain contacts created a relatively strong texture in the sandstones of the north of Mardabad section.ConclusionsThe presence of feldspar grains (in high amounts) and low amounts of calcite and dolomite cement, and consequently the increase of intergranular porosity (due to the alteration of feldspars and dissolution of calcite and dolomite cement) are important factors in the formation of tafoni in sandstone layers of the Eshtehard compared to the Mardabad sections. In general, the high thickness and extent of sandstone layers, chemical alteration of feldspars, and patch carbonate cement due to dissolution, control the formation and development of tafoni in the Eshtehard section.