GEOSCIENCES
Year:2000 | Volume:9 | Issue:35-36|Papers Count:6

The behaviour of all reconstituted and natural clays is determined by their fabric and bonding. A new classification of clays is proposed in this paper on the basis of standard penetration test (SPT),the geological history and the one- dimensional compression of the clay in the I v - σ v plane. Although the eight clay types defined in the classification have different origins, fabric and bonding, they all have either a syn-sedimentation or a post- sedimentation structure. The definition of these clay types is taken as a starting point for the construction of a general framework of behaviour of clays. In this research results of laboratory and field investigations of a very stiff Tehran silty clay in the natural and reconstituted states including SPT, Oedometer, Scanning Electron Microscopy and polarizing microscopy are presented. The structure of the Tehran silty clay is strongly influenced by bonding, calcium carbonate content and weathering intensity. This soil is a very stiff to hard clay which geologically is over-consolidated. Consolidation curve of soils lies close to the ICL line. Hence the Tehran silty clay is a type 4 clay but, at certain depths where the yield stress ratio is greater than 1, the Tehran silty clay becomes a type 7 clay.      

Shear planes, containing slickenside lineations were analysed statistically in 28 localities. The sampling sites were selected on the basis of Post-Eocene fault activations. The sites cover an area of 750 square kilometers of the southern part of the Central Alborz. Different methods of shape factor determination were tested. The results demonstrated that plotting 3-D unsealed Mohrdiagram is the most reliable method. Variation in R-spectrum in the studied area proved to be of type II, i.e geometrically type flattened (0< R< 1). The R- value of equivalent ellipsoid represents the whole ellipsoids can be replaced by an ellipsoid similar to that causing conjugate faults. Variation in R- spectrum also supports the slip-tendency model to be low and high in Mobarak Abad and Tochal area respectively.      

Uromieh Lake, which forms a closed, intermontane basin, located in northwest of Iran and is the biggest lake in the country. The lake has 5000 square kilometers area, with 140 km length and 15 to 50 km widths. Based on interpretations upon tectonic evolution of the region, Uormieh Lake is considered as a remnant portion of Tethys Ocean. The present situation of the lake shows that, it is surrounded by a few faults and forms a tectonic depression. Saturated clay and silt deposits overlying on bed rocks show evidence of marine fossils and demonstrate that earlier deposited environment of the lake was not so much salty. The source of soft sediments in the lake which were transported through north and south runoffs and floods are mainly comes from playa deposits of Maragheh Formation, Certaceous and Neogene shale and sandstone, limestone of Qum Formation, and granite intrusive. Besides these silt and clay sediments, detrital sediments in form of small sandy grains and insitu chemical deposits in lesser amounts are also seen in the sediments. These detrital deposits which are colored and composed of quartz, mica, and calcite are more frequent. Clay minerals are illite, kaolinite, and chlorite. Aragonite composes most parts of chemical deposits. The amount of organic material in sediment of the lake is almost 5% but in salty lagoons of the lake this amount increase to 15%.The mean content of salt is 250 g/lit. Due to this salt content in water of the lake, except at the conjunction of rivers to the lake it takes time for suspended sediments to deposit. Based on direct observation and through core loging form boreholes which were drilled along Shahid Kalantary freeway, the soft sediments of the lake have fine bedding. In samples collected from core logging along the lake coast in Tabriz and Uormieh region, fine sandy layers with up to 2 millimeters in thickness can be seen. The existences of these layers can considerably effect engineering characteristics of the deposits. Grains smaller than 75 micron, can be seen down to 150 - meter depth and constitute 95% of the deposits, obviously show their fine - grained nature. Sediments in the middle parts of the lake mainly consist of fine clay (CL) to silt (ML). Their activities decrease from surface to depth. By increase in activity of sediments, the amounts of internal friction, permeability coefficient (k), and consolidation index (Cv) decrease, but compaction index (Cc) increase. In general, sensitivity of top layer sediments is more than bottom one. Detail analysis on results of vane shear experiments, SPT, and uniaxial compressive strength demonstrate that shear strength of the sediments considerably increase with depth and that their sensitivity decrease in 40-45 meters depth.      

Extensive field investigations, sampling and laboratory studies have been done on soil samples from Khuzestan province in order to document their dispersivity and the distribution of dispersive soils. Geological conditions in different parts of the province show that the high relief areas in the north, northeast and eastern parts of the area are eroded by rivers and form deposits in the Khuzestan plain. Due to special conditions, the existence of dispersive soils in different parts of the area is not avoidable. Based on results of field investigations and laboratory results, the studied area is divided into three zones, wilt high and moderate dispersivity potential, and a floodplain with nondispersive soil. In addition to the different laboratory tests for identification, the dispersive soils have been compared with each other and related to the field conditions.      

Darestan- Baghu gold mining area is exposed at about 100 km south of Damghan. The area is a part of Torud- Chahshirin volcano-plutonic belt that crops out as a horst with a NE-SW trend in the northern limit Dasht-e-Kavir. The outcrops in the area are Middle- Eocene volcano- pyroclastics with andesite to dacite in composition, which are intruded by a number of intrusives including granite to granodiorite bodies. Mineralization in Darestan gold occurrence consists of a copper (gold)- bearing silicic vein with 400 meter length occurring within a N-S trending normal fault in andesitic to dacitic- andesitic massive breccia tuft associated with silicic, argillic and propylitic alterations. Mineralization in the vein includes quartz, chalcopyrite, pyrite, gold, specularite, malachite, and iron- hydroxides. Gold minerlization in Baghu vein (at about 3 km NE of Darestan gold occurrence) is as a silicic vein with 700 meter length occurring within a granodiorite- granite subvolcanic with argillic alteration. Mineralization in Baghu vein consists of quartz, pyrite, chalcopyrite, gold, iron oxides and iron- hydroxides, malachite, chalcocite, covellite and tourmaline. Field observations indicate that considerable contents of gold in Darestan area mainly related to sulfide- bearing silicic veins are intruded within N-S normal faults that are generally associated with hydrothemal alteration of argillic, silicic and chloritic ( ± epidote) zones. Investigation on other mineralized zones of Darestan and Baghu areas suggests a clear genetic relationship between Darestan copper (gold) occurrence and gold-bearing silicicvein of Baghu. Field observations and laboratory studies show a close relationship between the copper (gold) mineralization and late- stage silicic solutions induced from subvolcanic intrusives with acidic to intermediate composition in Darestan and Baghu districts. Some of the most important lines of evidence are spatial relationship between ore-bearing silicic veins and the position of subvolcanic intrusives, In addition high correlation between Au and Cu and homogenization temperatures of primary fluid inclusions  improve genetic relationship.