Writers : Qomi.J
Refference : Geosciences Scientific Quarterly Journal,No.:58,P.:100
Publishing Year : 1385

Abstract :

To investigate Nessen formation sediments in Alborz, two stratigraphic sections were studied in Central Alborz region, Noshahr (Menjeer) and Amol.
The objective of the research was to determine the sedimentary environments and the sequences of Nessen formation, therefore, 380 thin sections were prepared. Petrographic studies of thin-bedded limstone, marl, shally limestone and Nessen formation shales have resulted in the recognition of two groups of detrital and carbonated facies. Detrital facies are related to deeper marine environment (basinal facies) and in some cases have skeletal fragments of the open sea. Investigation of limstones from Nessen formation resulted in recognation of seven microfacies of three groups: open marine environments, barrier environment, lagoonal to tidal flat environment.
Carbonate facies were deposited in marine environment and on carbonate ramps while detrital facies deposited in distal ramps regions. The carbonate sequence of Nessen formation consists of three upward shallowing megacycles. Each of these megacycles has been made of some regressional microcycles in metric scale (parasequences). Regressional cycles in million- year scale with extentional phases, regional subsequences and sea-level fluctuations are mostly related to the sea-surface fluctuations made by tectonic activities and through gradual up-coming (ascending) and sudden down-going (descending) of the sea-surface, while the microcycles (parasequences) have probably originated from cyclic subsidence made by sea-surface fluctuations due to glaciers or Milanchovich climatic systems and accompanied by gradual descending and sudden ascending of the relative sea-level.

Subject List : Facies