Wyniki wyszukiwania - BazTech

Ograniczanie wyników

Znaleziono wyników: 2

Liczba wyników na stronie

Wyniki wyszukiwania

Wyszukiwano:
w słowach kluczowych: Shemshak Group

Sortuj według:

Ogranicz wyniki do:

Geochemistry of siliciclastic rocks from the Shemshak Group (Upper Triassic-Middle Jurassic), northeastern Alborz, northern Iran : implications for palaeoweathering, provenance, and tectonic setting

Taheri A., Jafarzadeh M., Armstrong-Altrin J. S., Mirbagheri S. R.

Geological Quarterly

2018

Vol. 62, No. 3

522--535

Combined petrographic and geochemical data of the siliciclastic sedimentary rocks from the Shemshak Group in the northeastern Alborz Mountains, north of Iran are described, together with their implications for palaeoweathering, their provenance, and tectonic setting. Based on field observations and modal composition, the sandstones are classified as litharenites. The chemical index of alteration (CIA) indicated that the source terrains underwent a moderate intensity of chemical weathering. The index of chemical variation (ICV) values indicated that the Shemshak Group rocks were immature and related to a source area with an active tectonic regime. Major, trace and rare earth element (REE) data suggested the domination of mixed sedimentary (recycled) and igneous rocks in the source area of the Shemshak Group. Petrographic and geochemical characteristics of Shemshak Group rocks suggest an active continental margin (ACM), which corresponds to the collision of the Iran plate with the Turan plate.

The Upper Triassic - Middle Jurassic Shemshak Group of Iran: foreland basin or rift basin?

Fürsich F., Wilmsen M., Seyed-Emami K., Majidifard M.

Volumina Jurassica

2006

Vol. 4, no. 1

41-41

The Shemshak Group is a predominantly siliciclastic rock unit occurring over much of the Iran Plate. The group is sandwiched between two unconformities: the lower one resulted from the late Middle Triassic Early Cimmerian Orogeny, which documents the collision of the Iran Plate with Eurasia, the upper one results from the still poorly understood Bajocian Mid-Cimmerian tectonic movements. At the base, the group overlies, with distinct stratigraphic gap documented by karst features and locally extensive bauxite deposits, the platform carbonates of the Shotori and Elikah formations. At the top, the group is overlain by the marls of the Dalichai Formation, part of a widespread carbonate system existing for most of the late Middle-Late Jurassic and locally extending into the Early Cretaceous. The Shemshak Group reaches enormous thicknesses (up to 4000 m) and consists largely of fluvial sediments with common intercalations of coal beds. At the base and at the top, the group is represented by marine sediments which include reef and lagoonal carbonates, deposits of the storm-influenced shelf, deltas, and basinal mudstones. Another characteristic feature are volcanic and volcanoclastic rocks, which are commonly intercalated at or near the base of the group. Conventionally, the Shemshak Formation has been interpreted as the fill of a foreland basin. However, detailed sedimentological and stratigraphic studies over much of the outcrop belt of the group show that such a simple interpretation can no longer be upheld. Instead, the Shemshak Group is thought to reflect the following major tectono-sedimentary settings: during the Late Ladinian - Carnian initial collision of the Iran Plate with Eurasia in northeastern Iran (Koppeh Dagh-Binalud area), the Triassic carbonate platforms of the Iran Plate suffered warping and extensive lateritic weathering. In the Alborz area, a remnant basin of the Palaeotethys persisted, in which basinal sediments were deposited. During the Norian-Raethian, distension of the Iran Plate in connection with the onset of subduction of the Neotethys led to formation of rapidly subsiding marine grabens which were filled with sediments of neighbouring horsts (Nayband Formation). In the Alborz area, the remnant basin of the Palaeotethys became gradually infilled with partly shallow marine and partly fluvial sediments. The major collision between the Iran Plate and Eurasia took place around the Triassic/Jurassic boundary. Concomitant with the rising Cimmerian Mountains, a foreland basin developed, which received a thick conglomeratic fill in the north which southwards graded into a fluvial system with widespread coal swamps. During the Toarcian-Aalenian, this foreland basin changed into a rift basin, which is documented by transgression and a rapid change to basinal deposits, indicative of high rates of subsidence. By the end of the Early Jurassic, the Cimmerian Mountains were more or less eroded. The following Mid-Cimmerian tectonic movements are seen as recording strong distensional tectonics in connection with the opening of the South Caspian Basin in the north.