Magnetostratigraphy of Miocene–Pliocene Zagros foreland deposits ...

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406 S. Homke et al. / Earth and Planetary Science Letters 225 (2004) 397–410 4. Correlation to the geomagnetic polarity time scale (GPTS) The weak absolute age constraints in the studied magnetostratigraphic sections have lead to a mostly independent correlation with the GPTS, largely based on a best-fit solution. The correlation of the upper part of the local magnetic polarity sequence with the GPTS [32,33] is based on the pattern of magnetozones recorded between 1326 and 2214 m, which matches the sequence of reversals of the lower Pliocene Gilbert chron (C2Ar, C3n, C3r), the C3An chron, and the C3Ar chron (Fig. 9). This is coherent with biostratigraphic data, which indicates a Miocene to Pliocene age for the Agha Jari Fm. [20]. Above and below this interval, the correlation becomes less evident. But even if the detailed polarity pattern is not clearly represented, some Fig. 9. Correlation of the magnetic polarity sequences of the studied section to the GPTS. Sedimentation rates are given in cm/ka. Approximate positions of the base of the growth strata, calculated in different localities of the Changuleh region, are displayed.
S. Homke et al. / Earth and Planetary Science Letters 225 (2004) 397–410 407 intervals with dominant polarities can be correlated with the GPTS. Consequently, the reversed magnetozone at the top of the series should correspond to the C2r chron, and the normal polarity interval recorded around 850 m should correlate with chron C4An (Fig. 9). The reliability of this part of the correlation is further supported by the high quality of the sample demagnetization data. Only 3.2% of the samples yielded class 3 directions (Fig. 8), while 87.2% were of class 1. The lower part of the magnetic polarity sequence presents a large normal polarity interval, recorded over 265 m of sediments. Since biostratigraphic ages constrains the correlation to the late Miocene and the Pliocene [20], this large normal polarity interval must be correlated with chron C5n (Fig. 9). Consequently, the reverse polarity recorded around 800 m must be related to the uppermost sub-chron of the C4Ar chron. Four samples collected between 400 and 600 m present an intermediate magnetic component with reversed polarity. They could represent the record of cryptochrons occurring during this long normal period, i.e. the C5n.2n-1, C5n.2n-2 and C5n.2n-3 cryptochrons, and the C5n1r subchron. Despite the good quality of the samples from the Zarrinabad syncline (none of the sites is represented by only third-class samples) (Fig. 8), the correlation of the lowermost part of the section is uncertain. The assumed early Miocene age of the Gachsaran Fm. based on regional biostratigraphic data [20] suggests a hiatus of more than 3 My at the base of the section. If we assume, however, a continuous sedimentation, the normal polarity recorded between 115 and 178 m could be correlated with chron C5An (Fig. 9), providing the youngest possible correlation for the base of the section. The magnetic polarity sequence obtained in the Changuleh syncline does not provide better constraints. Within the assumption of a continuous sedimentation, the youngest possible correlation to the GPTS can only be based on the geometric relationship with the Changuleh anticline section (Fig. 9), because of the insufficient thickness of the sampled section and the poor quality of paleomagnetic analyses, 22.2% of analysed sites being represented by third-class samples (Fig. 8). Thus, except for the lowermost part of the studied stratigraphic column, the magnetostratigraphy study provides a very reliable dating for the Agha Jari Fm. 5. Results 5.1. Age of Agha Jari Fm. The age of the base of the fluvial Agha Jari Fm. is little constrained. If the sedimentation has been continue through the deposition of the top of the Gachsaran Fm. and the Agha Jari Fm., the transition between the two formations would be dated around 12.8 Ma in the Zarrinabad syncline and at 12.3 Ma in the Changuleh syncline (Fig. 9), both in the middle part of the Serravalian (late middle Miocene). The transition could however take place as soon as the early Miocene, as suggested by biostratigraphic age of the Gachsaran Fm. [20], implying the presence of a hiatus at the base of the series. The deposition of the Lahbari Mb. begins in the Changuleh region at 5.5 Ma, at the end of the Messinian (latest Miocene) (Fig. 9). This fine-grained member is followed by deposition of conglomerates of the Bakhtyari Fm., starting at 3 Ma. The last age control is located 50 m above the base of the Bakhtyari Fm., which is dated at 2.5 Ma (Fig. 9). The long-term sedimentation derived from magnetostratigraphy averages 26 cm/ky, which is well in the range of the Miocene alluvial sedimentation in the Himalayan foreland [34]. Short-term sedimentation trends are smooth and vary gradually during the deposition of the Agha Jari Fm. The sediment accumulation rate does not vary greatly during the deposition of the Agha Jari Fm (Fig. 9). At the base of the series, the deposition rate of the lowermost Agha Jari Fm. is about 19.5 cm/ka. The rate increased then regularly, reaching 30.5 cm/ka around 700 m, and slowed down to 19.5 cm/ka after the deposition of the lower part of the Agha Jari growth strata. The rate starts again to increase few tens of meters below the top of the Lower Agha Jari Fm., until reaching about 33 cm/ka in the upper part of the Lahbari Mb. and in the lower 50 m of the Bakhtyari Fm. (Fig. 9). In the Zarrinabad syncline section, sedimentation increases gradually from bottom (19.5 cm/ky) to top (30.5 cm/ky). If we assume constant rates of deposition through the Bakhtyari conglomerates, the extrapolated age for the youngest preserved strata in the Changuleh syncline is close to the Pliocene–Pleistocene boundary (Fig. 9).
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Magnetostratigraphy of Miocene–Pliocene Zagros foreland deposits ...

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