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

Volume 1 , Issue 1 , January 2016 , Pages 81-109

Authors

¹ Earth Sciences Department of University of Kurdistan, Sanandaj, Iran

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.

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