Petrography and diagensis of Padeha Formation sandstones (Lower-Middle Devonian) at Bujhan section, Binalud Basin, NE Iran

The siliciclastic Padeha Strata (Middle Devonian), in the Binalud, 108 metres thick, in the Bujhan area, rests unconformably on Ordovisian basalts. This strata conformably overlain by carbonate rocks of the Sibzar Formation (Middle Devonian). The one stratigraphic section was logged graphically, and 98 fresh sandstone samples were systematically collected, from which 63 thin sections were made. Petrographic modal analyses were made using a Nikon E400 Pol microscope, with 500 point counts on 63 selected samples using the Gazzi-Dickinson method to identify grain and cement types and proportions. Porosity was estimated from counts of 500 points in each of 22 thin sections prepared separately with blue epoxy. Six polished thin sections were studied to determine the composition of mineral components. The Scanning Electron Microscope (SEM) used was a LEO 1450 VP at an acceleration voltage of 30.00 kv. Luminescence characteristics of the same sandstone suite were studied using a conventional hot-cathode cathodoluminescence (HCL) microscope (model HC4-LM).
Discussion and Based on field and Laboratory studies, 3 association facies, sandstone, dolostone and shale have been identified. The sandstones are fine- to medium-grained and grain-supported, with some coarse-grained and well-rounded components. Based on angularity, sorting, and matrix content, most sandstones are mature and submature. Detrital grains are quartz, predominantly monocrystalline quartz with subordinate polycrystalline quartz, K-feldspar and plagioclase, lithic grains, and accessory minerals and micas. Lithic grains are mainly metamorphic and sedimentary. Dense-minerals include opaques, zircon and tourmaline, dispersed. The sandstones have a compositional range from quartzarenite, subarkose and little bit of arkose.
The Padeha sandstones experienced diagenetic events that included cementation, alteration, compaction and fracturing, dissolution and replacement and porosity. The predominant cement is silica and carbonate (dolomite, calcite, ankerite and siderite), iron oxide, clay minerals (kaolinite, illite and chlorite), with minor authigenic minerals such as appetite. The silica is typically non-luminescent, and mainly occurs as syntaxial overgrowths on detrital quartz grains; reddish rims of very fine-grained material that probably include clay and iron oxides mark the contacts between authigenic and detrital quartz. Silica also forms pore-filling cement in primary pores. The cements occupy inter- and intragranular spaces, form veins and fill fractures, and vary from microcrystalline to coarsely crystalline in the case of calcite. Iron oxide cement is present throughout the Padeha sandstones as an alteration product and cement. Clay minerals are less than other type of cements, but illite and kaolinite are the main clay minerals cement in Padeha sandstones. Authigenic minerals mainly fill fracturs and pores. The sandstones show variable degrees of mechanical and chemical compaction, which is particularly prominent where early cements are lacking. Grain contacts include elongate and concavo-convex, point contacts in rare cases, and sutured contacts that indicate intergranular pressure solution and deformation at a more advanced stage. Quartz and feldspar grains have been intensively fractured but the fractures have been largely healed through silica cementation, allowing the grains to maintain their integrity. This was evident using SEM and CL techniques, which show that the majority of grains contain fractures. Dissolution is prominent in the sandstones. Detrital K-feldspar, quartzand carbonate cement all show evidence of partial to complete dissolution. In feldspars, the proportion of voids is variable, with dissolution prominent along cleavages and fractures.
Based on two-dimensional estimates from thin sections, the mean porosity is 4.7%, and maximum 14.4% for 22 samples from the formation as a whole, with little apparent upward change. The bulk of the porosity is secondary. Pores formed mainly through dissolution of K-feldspar and carbonate cement, and as open fractures within grains.
Based on petrological and geochemical studies, minor diagenetic events in the eodiagenetic stage include cementation (calcite, dolomite and iron oxide) and rarely fracturing. Mesodiagenetic events were dominated by cementation (silica, dolomite, calcite, iron oxide components, clay minerals), compaction, intra-grain microfractures, alteration of unstable clasts, dissolution and replacement, pressure solution, and rarely formation of apatite. Minor telodiagenetic events include dissolution and cementation (dolomite, ankerite, siderite, iron oxide and rarely kaolinite). The bulk of the porosity is secondary, with an average of 4.7%, which is the result of dissolution and fractures