Tectonic and Eustatic Signatures in Depositional Sequences of a Foreland Basin: Example from the Oligo-Miocene Asmari Formation of Southwest Iran

Hadi Amin-Rasouli
Earth Sciences Department of University of Kurdistan, Sanandaj, Iran


Abstract

The Oligo-Miocene Asmari Formation (up to 500 m) and its time-
equivalents in the Persian Gulf foreland basin are one of the world’s most
important petroleum reservoirs in southwest Iran. It is unconformably
underlain by the basinal deposits of the Eocene-Oligocene Pabdeh
Formation and is unconformably overlain by the continental deposits of
the Middle Miocene Gachsaran Formation. Field and petrographic
investigations of 10 outcrop and subsurface sections of the Asmari
Formation in the northwest of the basin led to recognition of dominantly
shallow marine carbonate, siliciclastic and evaporate facies. These are
arranged into 7 depositional sequences that are correlated with the Oligo-
Miocene eustatic sea-level cycles. In the southwest distal sections, the
lower and middle Asmari sequences (Chattian-Aquitanian) consist of
mixed carbonate and siliciclastic facies, which change laterally to pure
carbonate or carbonate and evaporate facies in the proximal area to the
northeast. These sequences are absent towards the Zagros thrust belt,
where the dominantly carbonate facies of the upper Asmari (Burdigalian)
unconformably overlies the Eocene Shahbazan Formation.
Both eustatic sea-level changes and regional tectonics were responsible in
controlling lateral and vertical facies variations of the Asmari succession.
The predominantly carbonate facies and correlation of Asmari sequences
with global sea-level cycles suggest a strong eustatic control during
deposition. Siliciclastic intervals in the mixed siliciclastic-carbonate
deposits are interpreted to represent forebulge uplift as a result of tectonic
loading of the thrust sheets. The shallow marine pure carbonate/carbonate-
evaporate intervals that cover the proximal foredeep or the entire foreland
basin appear to be the consequence of both eustatic sea-level changes and
tectonic quiescence.

Key Words:
Asmari, Facies,
Sequence stratigraphy,
Tectonic event,
Depositioal
environment

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