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ABSTRACT

A regional sequence-stratigraphic model is presented for the Barremian to Lower Albian sedimentary systems of southwest Iran, based on seismic-scale outcrop sections, and constrained by new biostratigraphic and chemostratigraphic age dating (ammonites, planktonic forams, orbitolinids and carbon and oxygen isotope curves). This study proposes a fundamental revision of the geometrical relationships between classically used lithostratigraphic units, and demonstrates the relative influence of both eustatic sea-level fluctuations and regional tectonic control on sedimentation. These new insights have significant implications for both the understanding of the sedimentation patterns, as well as the architecture of the mid-Cretaceous petroleum systems in the studied region.

The following depositional sequences are defined:

  • (1) two third-order sequences of Barremian age, which have a flat-bedded stratigraphic architecure and consist of low-angle, mixed carbonate-siliciclastic ramp systems (Gadvan Formation). The maximum flooding surface of the Arabian Plate (AP) Bar 2 Sequence has been dated with the occurrence of the short-range index fossil Montseciella arabica.

  • (2) an Aptian second-order Supersequence which shows a geometrical evolution from a flat-bedded architecture to a carbonate platform to organic-rich intra-shelf basin topography and corresponds to part of the Dariyan Formation and the Kazhdumi Formation. This evolution can be subdivided in three phases which are related to the Aptian third-order sequences defined in the UAE and Oman: the early transgression (Apt 1 sequence), the late transgression (Apt 2) and the highstand (Apt 3 and 4). The maximum flooding occurred in, or just after, the D. deshayesi ammonite zone.

  • (3) an Upper Aptian – Lower Albian second-order Supersequence, represents the continuation of the carbonate platform to organic-rich intra-shelf basin system established in the previous supersequence. It corresponds to the Upper Dariyan and Kazhdumi formations and the ‘Burgan equivalent’ informal unit. During the initial lowstand sedimentation (Apt 5) was confined to the intra-shelf basin, and during the transgression and highstand (Apt 6, Alb 1-2) only part of the exposed carbonate platforms were flooded, with facies varying from orbitolinid limestones along the northern margin to condensed siliciclastics along the southern margin.

Sedimentation was controlled by the eustatic sea-level variations, the influx of siliciclastics and the activation of regional fault systems such as the Kazerun Fault and the Hendijan Fault. The fault activiation caused the differentiation between the southern coastal Fars area, with relatively less accommodation, and the northern Khuzestan and Lurestan area, which subsided more rapidly. Due to the high-resolution biostratigraphic control the sedimentation patterns in these two tectonically different-behaving areas could be compared and depositional sequences defined. The time control also allowed comparison with the sequences defined in Oman, Qatar and the UAE, which are for the Barremian and Aptian very similar. The Upper Aptian – Lower Albian Supersequence is well documented in Iran and in Qatar, whereas in the UAE and Oman only the lowstand of this system is documented.

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