نوع مقاله : مقاله کامل
1 دکتری ژئومورفولوژی، دانشکدة علوم زمین، دانشگاه شهید بهشتی تهران
2 کارشناس ارشد زمین شناسی، مهندسین مشاور زمین ریزکاوان
3 دانشیار دانشکدة علوم زمین، دانشگاه شهید بهشتی تهران
عنوان مقاله [English]
The importance of Quaternary period in morpho-climatic interpretations is due to severe and consecutive climate changes, erosional processes and their environmental and socio-economic consequences. In the study of Quaternary nature and statues, chronology of erosional processes and river sediments provides an appropriate framework to determine the environmental, tectonic, climate and anthropogenic changes. Rivers, as one of the features in terrestrial landscape, are sensitive to changes that regulate their forms against a wide range of internal and external forces over time. River sediments are significant and long, although disconnected, archives of earth landscapes evolution as well as many Quaternary sedimentary sequences that reflect the reactions of drainage systems to changes in sea level and past environments.
Material and methods
Saqqez River basin with an area of 865 square KMs is located in northwest margin of Sanandaj-Sirjan zone. From a morphological point, middle parts of the study basin are largely constituted of Cretaceous clastic and volcanic rocks. They have a mild topography accompanied by low-lying hills and flat erosion levels. In this study, the physical and chemical analyses have been used to investigate the characteristics of Paleo-sediments of Saqqez river basin. First, a profile was selected as a study profile; recognition of the sedimentary characteristics of this profile was possible in field research based on genus, color and position of layers. Buried paleosol, carbonate layer, fluvial sediment (conglomerate), unconsolidated sediment, flood-plain deposit, exhumed paleosol, and recent sediments are the most important layers in the profile. Samples were taken from each of sedimentary deposits and 20 samples were collected finally. Thin sections were prepared from the samples for the analyses of mineralogy, point counting, physically tracing of sediments and recognizing the type of the carbonate cement. In laboratory, the thin sections were investigated using polarizing microscope in two modes of Plain Polarized Light (PPL) and Cross Polarized light (XPL) for the purpose of determining the samples’ shape and type of sediments. Point counting of the samples was done through visual method and microscopic interpretation. Carbonated samples were chemically staining using Alizarin Red-S solution to distinguish between the types of carbonates. Isotopic compositions of oxygen were measured in isotopic laboratory of Geoscience School of Ottawa University (Canada). Generally, 10 samples were used to oxygen isotope analysis. Isotopic compositions of the samples have been reported according to common scale of δ as part per thousand. The measurements analyses precision has been reported according to vpdb standard as ±0.1 per thousand.
Results and discussion
Physical investigations about the components of sediments and identifying the source rock of clastic sediments revealed that sediments nature are including fluvial sediments affected by pedogenic (sandy carbonate mudstones, sandy mudstones, sandy gravely sediments, conglomerates and micro conglomerates), coarse-grained fluvial deposits (unconsolidated sediment), and flood-plain deposits (silty sand and gravely muddy sand). Volcanic, metamorphic (Schist) and sedimentary (Shale) rock fragments are formed as the main components of sandy carbonate mudstone. Clastic grains of the volcanic and metamorphic rock fragments, such as slit, are the major components of sandy mudstone. Given the sandy gravely sediments, the main components are coarse rock particles including a variety of volcanic and metamorphic rock fragments (such as different types of schist and slit). The sandy parts are made up of Quartz and Feldspar. The conglomerates are mainly composed of rock fragments and monocrystalline Quartzes. The results of point counting phase show that with the igneous fragments in the samples the frequency of volcanic grains are much higher; also in the metamorphic fragments the frequency of Schist is too higher in comparison with other metamorphic fragments. Chemically staining of carbonate samples using Alizarin Red-S revealed that calcite composition for carbonated matrix in the samples of the study profile is the Sparite carbonate. The crystals of Sparite calcite have usually fine zoning that is resulted from small changes in the amount of iron and manganese.
A glance at dispersion of the lithological units across Saqqez River basin indicates that rock unit of Schist is located either near main channels of rivers or downstream the basin where river terraces (conglomerates) is formed. Accordingly, the hydrological and topographical conditions of the basin are organized in a way that the transport of schist rock fragments to downstream might have occurred via the main channels, thus, increase the frequency of Schist; as it is proved in the present study by the point counting of conglomerate rock fragments. Using the zoning pattern in the calcite in a sequence and a regional scale it would be possible to create the cement stratification which, in turn, contributes to reconstruction of the basin hydrology in a major scale. Sparite calcite indicates a more negative amount of O18 than the string calcite which reflects its deposition in a higher temperature. Therefore, Sparite cement of the study profiles from Saqqez River could be indicative of the hot and humid weather at the time of their formation. Also, the results of oxygen isotope analysis prove this claim. The amounts of carbonates δO18carb has varied from -8.9 to -8.18 which shows the minor changes of these values. Given the low values of δO18carb in the analyzed samples, vaporization has relatively taken lower amounts of O16; thus, most probably the study profile carbonates have experienced a wet condition in Saqqez River.