Sedimentary environments and sequence stratigraphy of tidal deposits of the Fasham Formation, Mila Group, Central Alborze

Four stratigraphic sections of the Middle Cambrian Fasham Formation sediments exposed in the Central Alborz are described and interpreted on the basis of field observations and facies analysis in order to reconstruct their depositional environments and sequence stratigraphic framework. Relying on the facies characteristics and stratal geometries, the size of the bedforms, the sedimentary structures and the complex arrangement, the erosional base, the predominantly sandy grain size, the significant height of the sandbodies together with the tidal action evidences such as extensive cross-stratified sandstone, tidal bundles, reactivation surfaces and mud drapes, the siliciclastic successions of the Fasham Formation are deposited in a tide-dominated open-mouthed estuarine environment. During the initial inundation of the present-day Alborz region in the early Middle Cambrian, siliciclastic deposits formed in open-mouthed estuarine. Basal sequence of Middle Cambrian sediments is composed of a transgressive systems tract (TST) comprising the basal siliciclastic-dominated succession of the Fasham Formation and a carbonate-dominated highstand systems tract (HST) with in the lower part of the Deh-Sufiyan Formation. It records the drowning of the terrigeneous sources and subsequent establishment of the carbonate system. Based on sequential infilling model for open estuaries during transgression and initial flooding, tidal flats were developed along the margins and sandy dunes formed along the central part of the estuary and their lower surface was interpreted as a subaerial unconformity formed due to lowstand erosion. The last stage of infilling involves the development of sand bars that gradually migrated towards the open sea until they wholly occupy the mouth of the open estuary.