چكيده لاتين :
Introduction:
The siliciclastic Dahu Strata (Early Cambrian), in the Central Iran, 280 metres thick, in the Gazouieyeh area, rests with an erosional surface on Protrozoic-Early Cambrian sedimentary rocks (Dezu Series). This strata disconformably overlain by Middle Cambrian-Late Cambrian marine carbonate rocks (Kouh-Banan Formation). The upper part of this strata contains monomict orthoconglomerate bed with cherty clasts and chertarenite sandstone that compare with upper sandstone (Top quartzite) unite of other parts. The bulk of the formation is red to reddish-brown, and mostly purple.
Matherial and methods:
The one stratigraphic section was logged graphically, and 200 fresh sandstone samples were systematically collected, from which 100 thin sections were made. Petrographic modal analyses were made using a Nikon Eclipse E400 Pol microscope, with 500 point counts on 33 samples. Five 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.
Discussion of results and conclusion:
Based on field and Laboratory studies, 3 association facies, shale-sandstone and conglomerate 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 (quartzite) and sedimentary (sandstone and chert), with a few volcanic grains. Dense-minerals include opaques, zircon and tourmaline, dispersed or in thin laminae. The sandstones have a wide compositional range from quartzarenite to arkose, feldspathic litharenite and rarely litharenite (chertarenite). According to plots of feldspar garins, total quartzose grains, and total unstable lithic fragments, they were derived from craton interior, transitional continental, and recycled orogen sources. The Dahu sandstones experienced diagenetic events that included compaction and pressure solution, cementation, grain fracturing, alteration of unstable grains, dissolution and replacement.
The predominant cement is silica, but some samples include considerable proportions of carbonate, iron oxides, clays, with minor authigenic barite. 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, and large volumes of cement lie along primary and secondary fractures.
Geochemical analysis indicates calcite and dolomite as predominant phases, with minor ankerite and siderite. 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 Dahu Formation as an alteration product and cement. The oxides are principally hematite with some magnetite. Clay minerals are less than other type of cements, but illite and chlorite are the main clay minerals cemnt in Dahu sandstones. Barite is present as a minor authigenic mineral, and locally filling small fractures.
Dissolution is prominent in the sandstones. Detrital K-feldspar, quartz, volcanic rock fragments, and 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. 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.
Based on petrological and geochemical studies, we interpreted the diagenetic history for the Dahu sandstones, which consists of early, deep burial and late stages. The above results are based on surface studies, but it might be changed during increasing the depth.
Key words: Central Iran, Early Cambrian, Dahu, diagenesis
