Strontium Isotope Stratigraphy of the Asmari Formation (Oligocene ‐ Lower Miocene), Sw Iran

Abstract

The Asmari Formation has been studied in the subsurface at the Bibi Hakimeh, Marun and Ahwaz oilfields and in an outcrop section from the Khaviz anticline. It consists of approximately 400 m of cyclic platform limestones and dolostones with subordinate intervals of sandstone and shale. The method of Sr-isotope stratigraphy is well suited for dating these strata because of the rapid rate of change of marine 87 Sr/ 86 Sr during Asmari deposition (roughly 32-18 Ma) and the common presence of well-preserved macrofossils.Profiles of age against depth in the four areas show a decrease from higher stratigraphic accumulation rates in the lower Asmari to lower rates in the middle to upper part of the formation. There is also a trend towards less open-marine depositional conditions and increasing early dolomitization and anhydrite abundance above the lower part of the formation. These changes reflect the dynamics of platform progradation across the areas studied, from early deposition along relatively high accommodation margin to slope settings to later conditions of lower accommodation on the shelf top.Ages of sequence boundaries are estimated from the age-depth profiles at each locality, providing a framework for stratigraphic correlation. Asmari deposition began in early Rupelian time in the Bibi Hakimeh area, when the studied areas to the NW were accumulating basinal marl facies. Progradation of the platform across the Marun and Ahwaz areas took place in mid-Chattian time and somewhat later in the more basinward Khaviz area. Depositional sequences have durations of 1-3 Ma, whereas component cycles represent average time intervals of 100-300 Ky.Sr analyses of most dolomite, anhydrite and celestite samples plot close to or below the macrofossil age-depth trend for each locality, indicating formation from waters preserving seawater 87 Sr/ 86 Sr approximately contemporaneous with or slightly younger than the time of sediment deposition. Local deviations from this trend are interpreted as indicating episodes of seepagereflux and also a contribution of Sr from non-marine sources during formation of the Gachsaran cap rock anhydrite.

Sedimentary Environment and Sequence Stratigraphy of the Asmari Formation at Khaviz Anticline, Zagros Mountains, Southwest Iran

“…Its lower boundary is characterized by a type 2 sequence boundary (Figure 8). The boundary (Sb III) is comparable with sequence surface Ch30 and with surface III reported by Ehrenberg et al [3] and by Van Buchem et al [4], respectively ( Figure 10). A parasequence consists of Mf2 (open marine; FA1), and Mf5 (semi restricted marine; FA2) form TST of Figure 10.…”

“…4.5 of Hag et al [27], Figure 10). The boundary between this sequence and previous sequence (Sb I) is comparable with sequence surface Ru30/Ch10 proposed by Ehrenberg et al [3] and surface II reported by Van Buchem et al [4] (Figure 10). The transgressive systems tract (TST) of DS1 is marked by deposition of Mf1, Mf2 and Mf4.…”

“…The contact between DS1 and DS2 is of the SB II type (Figure 8). This sequence boundary (SB II) is comparable with sequence surface Ch20 proposed by Ehrenberg et al [3] (Figure 10). …”

Biostratigraphy and paleoecology of the Oligo-Miocene Asmari Formation in the Izeh zone (Zagros Basin, SW Iran)

“…These two reports were used by many authors (e.g., Seyrafian, 2000;Vaziri-Moghaddam et al, 2006;Amirshahkarami et al, 2007a, b). Recently, Ehrenberg et al (2007), Laursen et al (2009) and Van Buchem et al (2010) conducted strontium isotope dating to improve the biostratigraphic ages for the Asmari Formation across most parts of the Zagros basin (Izeh zone and Dezful Embayment) ( Table 2). This biozonation is used by many recent authors (e.g., Allahkarampour Dill et al, 2010;Van Buchem et al, 2010;Sooltanian et al, 2011;Rahmani et al, 2012).…”

Sedimentary Environment and Relative Sea Level Changes of the Asmari Formation Deposited in Lurestan Basin, Southwest of Zagros

“…Biostratigraphic characteristics of Asmari are revised and three assemblage zones and two subzones are introduced by [4]. [5] revised the biostratigraphy of the Asmari Foemation using strontium isotope. [6] also proposed new biozones for Asmari.…”