جستجوی مقالات مرتبط با کلیدواژه « سازند لالون » 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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The Early Cambrian Lalun Formation in Poshte- Badam block has been studied in Bahmoo section with 550 m thickness, to illuminate its sedimentary environment and sea level changes. The field investigations, collection of 110 paleocurrent data, petrography, study of SEM images and XRD analysis of shale samples have been used for facies analysis, paleocurrent recognition and sedimentary environment reconstruction. The facies analyses imply 5 silisiclastic coarse grained (Gm), medium grained (Sp, St, Sh) and fine grained (Fl) lithofacies and 1 carbonate (dolomite) microfacies. The lithofacies, bipolar bimodal paleocurrent pattern and petrographic evidences such as the existence of glauconitic sandstones with high textural and compositional maturity imply deposition of these sediments in a Tidal Flat environment. Studying the relative sea level changes of Lalun deposits in this area imply two sequences which have been bounded between SB1 unconformities.

The Early Cambrian sandstones of Lalun Formation with 550 m thickness have been studied in Bahmoo section, in Poshte – Badam block, to illuminate their provenance and omitting the effect of diagenetic processes on the sandstone composition. These investigations have been accomplished by study of 170 thin sections, modal analysis of 50 appropriate samples, and use of SEM and EDX analyses. Eogenetic stage processes in these sandstones include: early calcite cementation, mechanical compaction, corrosion of grains by calcite cement and hematite pigmentation. Mesogenetic stage processes include: chemical compaction, quartz cementation, dolomitization of calcite cements, dedolomitization, albitization of feldspars and grow up authigenic quartz, illite and chlorite, and telogenetic stage processes include: fracturing and filling them by calcite cement and alteration of eogenetic and mesogenetic cements. After recognition of these processes and omitting their effect from the results of modal analysis, we studied tectonic provenance of these samples by using Qt86 F5 L9, Qm46 F5 Lt49, Qp90 Lvm5 Lsm5, and Lm12 Lv5 Ls83 diagrams. These studies reveal the quartzolithic facies with recycled orogen and rifted continental margin tectonic provenance witch changed to quartzose facies with cratonic tectonic provenance in top quartzite. Petrographic evidences and quartz type analyses (diamond diagrams), reveal that these sandstones have been derived from mixed provenance. The QFRF and log Qt/F vs. log Qp/F diagrams propose trend of climate change from semi arid to humid during deposition of the Lalun Formation.

The lower part of Lalun Formation in type section is mainly sandstone. Base on laboratory study and observition field, lower part of Lalun Formation is incluse sub-arkose, sub-sedarenite, feldespatic sedarenite and sedarenite facies group. Main grains of these facies are quartez, rock fragments and feldespare. Sedimentary environment of lower part of Lalun Formation is Meandery River. Deposites have been fining upward facies. Cross bedding, lamination and evaporite cast have seen on this sequence.

The Lower Cambrian Lalun Formation includes the lower Sandstone, middle Shale and upper Top Quartzite units. The contact of the Shale unit with that of the Sandstone unit is unconformable grading upward to Top Quartzite unit. Two bioherm horizons are present near the base of the shale unit. The Shale unit represents deposition in an estuary environment. The Top Quartzite of the Lalun Formation records deposition in a shoreface depositional setting. Facies and depositional environment of the Hawke Bay Quartzite of the Appalachian Mountains and the Zabriskie Quartzite of California are similar to the Top Quartzite of the Lalun Formation. The upper contact of the Hawke Bay and Zabriskie Quartzites is a regional unconformity signifying the Lower and Middle Cambrian boundary. These deposits, as well as, the Top Quartzite are correlated to similar deposits in Turkey, Oman, China, southwest Europe and south of Australia. They are unconformably overlain by Middle Cambrian rocks, too. Therefore, the unconformity of the top of the Top Quartzite unit is interregional, coinciding with the Lower and Middle Cambrian boundary.Two biostromal and biohermal thrombolite horizons present near the base of Member 1 (2 meters above the boundary) indicating the onset of Middle Cambrian transgression. This event resulted in the formation of the transgressive Mila succession (Members 1-3). Therefore, the Top Quartzite of the Lalun Formation does not belong to the Mila succession and cannot be considered as its Base Quartzite unit, as some geologists have previously suggested.The contact between the Shale and Top Quartzite units is gradational and the two units are bounded by two erosional unconformities. Therefore, the Top Quartzite cannot be considered as a separate lithostratigraphic unit.

The shale unit of the Lalun Formation in Tuyeh-Darvar area (Southwest of Damghan) is unconformably underlain by the sandstone unit and is gradationally overlain by Top Quartzite unit of the Lalun Formation. The shale unit of the Lalun Formation is composed of siliciclastic facies and two carbonate horizons (less than two meters thick) in the lower part of the shale unit. These carbonate horizons include dolomitized microbialite and ooide grainstone facies, which were deposited in a estuary environment of the northwestern passive margin of Gondwana, following late Early Cambrian mass extinction.