Sedimentary environments and sequence stratigraphy of the silicilastic deposits of the Lashkarak Formation in Central Alborze zone

Silisiclastic deposits of Member 5 of the Mila Group in Central Alborz that call Lashkarak Formation in this research, were studied in Shahmirzad, Tueh-Darvar, and Deh-Molla sections in order to reconstruct their depositional environments and sequence stratigraphic framework. The integration of sedimentological and ichnological characteristics of estuary and open marine deposits of the Lashkarak Formation has led to more reliable determinations of the relative degree of influence imposed by river influx, waves, and storms on the coastal regime. Relying on the facies characteristics and stratal geometries, the siliciclastic successions of the Lashkarak Formation are divided into two facies associationswave-dominated shoreface-offshore complex andwave-influenced estuary. High resolution sequence stratigraphic data have been used to subdivide the strata into two third-order sequence sets contains five fourth-order parasequence sets. Lowstand fluvial deposits of the first stage of the valley-system fill occur in sequence 1. These fluvial deposits are overlain by coastal-plain and wave-dominated estuarine strata during TST and HST. A drastic change to open-marine conditions is marked by a marine flooding surface during HST. Open-marine strata are dominatly lower-shoreface and offshore-transition deposits. Although the interpreted depositional systems and stratigraphic architecture of sequence 1 of the Lashkarak Formation resemble transgressive incised-valley-fill successions, the greater thickness and larger size of the Lashkarak valleys suggest valley development during a relative sea-level fall. The sequences 2 of the lashkarek Formation is composed of transgressive and highstand systems tract couplets interpreted as reflecting eustatic sea-level changes and the continuous creation of accommodation during eustatic sea-level changes. These sequences indicate the transgressive and regressive cycles coeval with eustatic sea-level fluctuations during Ordovician.