جستجوی مقالات مرتبط با کلیدواژه « lalun formation » در نشریات گروه « زمین شناسی »

Early to Middle Cambrian successions exposed at the Shirgesht area, Tabas Block of central Iran has yielded a high diversity and abundant ichnofossils with arthropod trackways. These trace fossils contain diverse ethological patterns, including furrowing or crawling (Cruziana), resting (Rusophycus), walking (Diplichnites), sideways grazing (Dimorphichnus) and swimming grazing (Monomorphichnus). The assemblage of trace fossils reported from the Early to Middle Cambrian of the Tabas Block, has a significant stratigraphic importance as they provide evidence regarding the developmental patterns during the early phase of life on earth. Stratigraphically, trace fossils studies is significant for defining the age of the unfossiliferous Cambrian successions of the Tabas Block from Central Iran where typical diagnostic trilobite fauna is absent. According to Cruziana ichnostratigraphy and comparsion the Lalun ichnofauna with other examples from similar age, the present assemblage could represent the late Early to Middle Cambrian. The distribution of trace fossils in the Early to Middle Cambrian successions reflects the paleoecological conditions including the availability of oxygen and nutrient influx. Hwever, the diversity and abundance of infauna, smaller trace fossil size, sporadic distributions of burrowing and lower intensities of bioturbation of trace fossil suite and the presences of evidence of predominance of trilobite trace fossils with morphologically simple represents a stressed expression of the ichnofacies and ichnologically stressed environment during deposition such as estuary.

  The siliciclastic Lalun Strata (Early Cambrian), in the Binalud zone, with thickness of 70 metres in the Chenar area, rests with an fault contact on the Early Cambrian sedimentary rocks (Soltaniyeh dolomite). This strata overlain with erosional surface by Eocene conglomerates. The upper part of this strata contains sandstone that compare with upper sandstone unit of other parts (Porsoltani et al., 2014; Poursoltani and Ghotbi Ravandi, 2015). This formation is red to reddish-brown in color, and mostly purple. The one stratigraphic section was logged graphically, and 67 fresh sandstone samples were systematically collected, and 50 thin sections were made. Petrographic modal analyses were done using a Nikon Eclipse E400 Pol microscope, with 500 point counts on 20 samples. Four 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. Based on field and laboratory studies, two facies association including sandstone 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 (quartzite) and sedimentary (sandstone and chert), with a few volcanic grains. Heavy minerals include opaques, zircon and tourmaline, scattered or present as a thin laminae. The sandstones have a compositional range from quartzarenite to subarkose and sublitharenite (Folk, 1980). The Lalun sandstones experienced diagenetic events that included compaction, fracturing and cementation. The predominant cement is silica, but some samples contain considerable proportions of carbonate, iron oxides and clays cements, with minor authigenic minerals. 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 (McBride, 1989; Friis et al., 2010). 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 Lalun Formation as an alteration product and cement. Clay minerals present less than other type of cements, but illite and kaolinite are the main clay minerals cement in Lalun sandstones (Ketzer et al., 2005). 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 (Mansurbeg et al., 2008). 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 (Milliken, 1989; Dickinson and Milliken, 1995). This was evident using SEM and CL techniques, which show that the majority of grains contain fractures. Based on petrological and geochemical studies, it is interpreted that diagenetic history of the Lalun sandstones can be related to early, deep burial and late stages (Abdel Wahab, 1998; Salem et al., 2005). Based on petrogragpic studies of upper sandstone units of the Lalun Formation, in Binalud Zone, compaction, fracturing and cementation (mainly silica) are the most important diagentic events of these sandstones. The cements occupy inter- and intragranular spaces, form veins and fill fractures, and vary from microcrystalline to coarsely crystalline. The sandstones show variable degrees of mechanical and chemical compaction, which is particularly prominent where early cements are lacking. 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. According to diagenetic events, diagenetic history for the Lalun sandstones, in study area, is related to early, deep burial and late stages. Keywords: Diagenesis; Lalun Formation; Early Cambrian; Binalud. Reference
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