A. H. Jalilian
Abstract
The Neyriz Formation comprises the lowermost Jurassic strata in Fars area and the last lithostratigraphc unit of the Kazerun Group in the Zagros fold-thrust belt. This study focuses on facies analysis, reconstruction of depositional environments and sequence stratigraphy analysis of the Lower Jurassic ...
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The Neyriz Formation comprises the lowermost Jurassic strata in Fars area and the last lithostratigraphc unit of the Kazerun Group in the Zagros fold-thrust belt. This study focuses on facies analysis, reconstruction of depositional environments and sequence stratigraphy analysis of the Lower Jurassic Neyriz Formation in its type locality with a thickness of 233m and Surmeh Mountain with a thickness of 68m in Fars region. Results indicate that the Neyriz Formation consists mainly of carbonates and shale with minor constituents of limestone conglomerates and glauconite-arenite in the middle part of the type section and calclithite sandstones in Surmeh section. Available evidences confirm that this carbonate-clastic succession deposited in various part of a confined and shallow sea (epicontinental) in south of the Main Zagros Reverse fault line. The lower part of the Neyriz Formation is composed of dolomudstone and shale facies with subordinate limestones deposited in a ramp platform due to the first transgression of Jurassic sea over the eastern part the Zagros fold-thrust belt. The subsequent sea-level fall resulted in increased clastic influx and led to development of coastal plain and restriction of carbonate production to the southern part of the platform. This drastic change of sedimentary regime led to the expanding of a tide-dominated estuary system in the High Zagros Belt and the deposition of clastic facies in the middle part of the Neyriz Formation. The clastic unit that pinches out towards the southwest is thought to have received its clastic input from the northeast and it may well be related to the Lower Jurassic continental siliciclastics (Shemshak equivalent) of the Sanandaj-Sirjan Zone. As evidenced by the limitation of clastic influx and reestablishment of the carbonate factory, during the subsequent Early Jurassic sea level had risen steadily and platform flooded again. Vertical facies variations and comparison with global sea level curve reveal that the Neyriz Formation encompasses a second-order depositional sequence that correlates with the lower part of Farhad Supersequence (Jurassic of the Zagros Basin). The given sequence by itself includes two unconformity bounded third-order sequences deposited in the middle Liassic time (Sinemurian-Pliensbachian).
