Age, Correlation, Depositional Environments, and Sedimentary History of the Fat’ha Formation (Burdigalian, Lower Miocene) of Iraq
Grabowski, George J.*1; Liu, Chengjie 1
(1) ExxonMobil Exploration Company, Houston, TX.
The Fat’ha Formation is a heterolithic unit of evaporites, carbonates, and fine-grained siliciclastics that is the topseal for the Kirkuk Group. It is equivalent to the Lower Fars Formation in Syria and the Gachsaran Formation in Iran. It is thickest in the Zagros Foldbelt and adjacent eastern parts of the Mesopotamian Basin and thin to the SW.
The Fat’ha is considered Middle Miocene based on benthic foraminifera. Strontium-isotope ratios of 137 samples of anhydrite and limestone from 22 wells and 7 outcrops consistently yield dates of 15.6 to 18.5 Ma (lower Langhian to middle Burdigalian - Early Miocene).
The Fat’ha consists of cycles of terrestrial facies passing upward to shallow-marine and then evaporitic facies. Cycles are 2 to greater than 25 meters in thickness. The most common lithologies are (from top to base):
- anhydrite, sharp top and base (evaporitic marine)
- skeletal-peloidal-oolitic limestone/dolomite, typically argillaceous and gradational at base (shallow marine)
- blue-grey marl, gradational at base (marginal-marine)
- unfossiliferous red-brown siltstone (non-marine).
Evaporites are thickest in basinal settings and thin and lapout on the flanks of the depositional basin. Halite occurs in certain anhydrite beds in the most basinal settings. Carbonate beds are thin (mostly 1-6 m), minor components of cycles. Siltstones are a higher proportion of cycles on the basin margins.
Four members established by IPC correlate regionally across Iraq using limestone marker beds. The basal beds of the Transgressive Beds (Bur3 sequence) are in the basin center and lap onto basin margins and above the Basal Fars Conglomerate in North Iraq. Halite is present in basinal wells near Iran.
The overlying Saliferous Beds (Bur4 sequence) contain halite in basinal wells and are the focus of most structural deformation. Correlation within the Saliferous Beds is not possible, and huge thickness variations are caused by repetition and folding.
The overlying Seepage Beds (basal Bur5 sequence) are uniformly 25-35m thick and are widespread deposits. The Upper Red Beds (Bur5/Lan1 sequence) at the top record progradation of siliciclastics into an areally restricting basin.
Repeat and missing sections occur in all members, caused by reverse and normal faults in areas where halite is present. Folded and overturned beds occur in some units, mainly in the Saliferous Beds. However, the bedded cyclical nature is almost always preserved despite the deformation.
AAPG Search and Discovery Article #90141©2012, GEO-2012, 10th Middle East Geosciences Conference and Exhibition, 4-7 March 2012, Manama, Bahrain