R Farokhnejad; B Esfandiari; A Motamed
Abstract
In this paper the Fajan Formation of Paleocene age with a thickness of 172.5 meters is studied in northwest Shahroud (Eastern Alborz). Main lithology of this formation, at the studied section, consists of conglomerate and sandstone. The formation disconforably overlain upper Cretaceous limestones ...
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In this paper the Fajan Formation of Paleocene age with a thickness of 172.5 meters is studied in northwest Shahroud (Eastern Alborz). Main lithology of this formation, at the studied section, consists of conglomerate and sandstone. The formation disconforably overlain upper Cretaceous limestones and its upper boundry with Ziarat limestone of Eocene age is of transitional type. Dominant lithology of mentioned formation, at the studied section, consists of polymodal and polymictic orthoconglomerate which contains rounded to subrounded grains. These clastic grains are generally oval to spherical in shape and originated from limestone, sandstone and chert materials. Paleocene Conglomerate is characterized by two distinc lithofacies: clast-supported massive gravel (Gcm) and clast-supported horizontally stratified gravel (Gh) which are formed in a braided river system. This subject is interpreted by evidances as condensed texture gravels, imbrication fabric that indicate on one hand paleocurrent, non-existence of fossil by reason of oxidation conditions of sedimentary environment, sandstone lenses that have through cross bedding (St) and existence of gravels that are mostly rounded to subrounded and denote distance from origin. Abundant bed load mostly rang in size from pebble to cobble and partially boulder, gravelly beds with long extended lateral spread and lack of flood plain muddy sediments, are reasons in distinguish between recognized sedimentary environment and meandering river. The studied Sandstone layers are described by ripple marked sand (Sr) and planar-cross-bedded sand (Sp) which contain sublitharenite and quartzarenite type are deposited in tide-dominated deltaic system.Interpretation criteria are propounded by means of interference ripple mark that prove direction change of current bearing clastic particles, rounded to subrounded sands that can present repeated reworking of grains after deposition in sedimentary environment, well sorting, high textural maturity, insignificant matrix as criterion in distinction specified delta from other deltaic environments and herringbone cross bedding which is the most important evidence of tidal currents.
