view:24241 Last Update: 2023-1-10
Rahimi, S., Abbassi, N., Yousefi Yeganeh, B.
Stratigraphy and architecture elements of Quaternary sediments of west Zanjan
سنگ چينه نگاری نهشته های آواری کواترنری شمال باختری زنجان
Extended Abstract 1- Introduction There are well exposures of Quaternary sediments along besides of Zanjanrud river, west Zanjan, northwest Iran. The Zanjanrud river watershed is restricted by two northwest-southeast trend mountain ranges: The Soltanieh Mountains in south and Tarom Mountains in north. The Quaternary sediments were divided in terrace and alluvial units in the previous studies. Here, we survived these sediments in stratigraphic viewpoint for dividing sediments in lithostraigraphic units and their age determination. 2- Material and Methods Zanjanrud watershed is 120 km in long and 60-70 km in width, which terminate to Qezel Ozan river as main watershed, in northwest. Seventeen sections were surveyed for determination and evaluating of the lithostratigraphic units, sedimentological characteristics, paleontological features and architecture elements of the Quaternary deposits of the west Zanjan. 16 sections were survived besides of the Zanjan-Tabriz highways from toll station to Mahneshan cross-road, located in the 40 Km. The well exposures of sediments are available in the tranches in the both sides of the highway. The last section (seventeenth section) studied in the eighth kilometers Zanjan’s airport highway. Thickness of surveyed sections, differ from 5 m in section number 14 to 26.4 m in section number 12. The Quaternary deposits of the west of Zanjan overlay on the colored Plio-Pleistocene marls as erosional unconformity, and their upper boundary is erosional surface. Provenance of particles of sediments are considerable from Tarom and Soltanieh mountains. Tarom Mountains comprise volcaniclastic sediments of Karaj Formation, Eocene in age. Soltanieh Mountains, however includes diverse rock units from Precambrian to Eocene. The rock units of Soltanieh Mountains are Kahar, Bayandor formations (Precambrian), Soltanieh, Barut, Zagon, Lalun and Mila formations (Cambrian), Dorud and Ruteh formations (Permian). Mesozoic rock units are scarce as Shemshak, Dalichai and Lar formations (Jurassic). Fajan, Ziyarat and Karaj formations are rock units of Cenozoic and are Paleocene to Eocene in age. 3- Results and Discussion Five lithostratigraphic units have been identified which include coarsening upward sequence as following as: Upper boundary; erosion surface. Unit (e), eroded mud-supported gravel, cream to green in colure. Unit (d), light colored and thick-bedded coarse grained gravels with brown to purple mud intercalations, and finally. Unit (c), regular alternations of light brown to cream mud, sand and gravels. Unit (b), gray to violet and moderate to well sorted sand and fine-grained gravels with cross-bedding and sparse cream mud, sand and gravel intercalations. Unit (a), alternations of dark to light brown muds with thin sand and fine-grained gravel intercalations. Lower boundary; concordance with Plio-Pleistocene colored marls. Sedimentology studies of the 176 sampled specimens show particle mean size variations as 1.80 to 2.91 ɸ, with statistical analysis as very poorly sorted, strongly coarse-grained skewed, near symmetrical and fine skewed, and platykurtic to mesokurticin the textural parameters. Heavy minerals of the 16 sampled specimens include Apatite, Anatase, Epidote, Sphen, Ilmenite, Barite, Titanomagentite, Rutile, Zircon, Limonite, Amphibole group, Pyroxene group, Magnetite and Hematite as main heavy minerals. Sphalerite, Biotite, Pyrite, Chalcopyrite, Garnet, Jarosite and Leucoxene are secondary heavy minerals in these samples. Frequency of main heavy minrals show sinus variations in the scrolling distance and secondary heavy minerals frequency, however are linear in the distribution in this distance. Architectural elements of the studied sections include five gravel facies (Gm, Gms, Gp, Gt, Gh), four sandstone facies (Sm, Sp, St, Sh), two mudstone facies (Fm, Fl), and a calcareous facies (p). In the studied sections, any significant body fossils were found, and only single trace fossil, Skolithos, was observed. According to the stratigraphic position, the age of the studied sections attributable to Pleistocene to Holocene. 4-conclusion Quaternary sediments of Zanjanrud watershed in the west of Zanjan, include coarse grained gravel, sands, silt and clay deposits. The thickness of sediments varies from 5 to 24 meters. Five distinctive lithostratigraphic units are distinguishable as coarsening upward sequence. Lower part of sequence includes dark to light brown muds with thin sand and fine-grained gravel intercalations, gradually sand size sediments increase in upper parts. The alternations of mud-gravel layers are well developed in the mid parts of the studied sequence. Upper most layers of sequences comprise thick bedded coarse grained matrix-supported gravels. No macrofossils were found in the studied section, and they are Pleistocene to Holocene in age at all. It seems that studied sediments deposited in the Alluvial fan system, which was fault controlled environment between Soltanieh and Tarom mountains. Keywords: Zanjan, Lithostratigraphy, Quaternary, Alluvial fan, Stratigraphic unit.