Calcareous nannofossil biostratigraphy of the upper part of the Garau Formation at the south west of Kabir-Kuh anticline

Introduction: Garau Formation has a good distribution and outcrops at the Zagros sedimentary basin in Iran. This formation has been studied from different aspects of sedimentology and paleontology (e.g., Vincent et al. 2010; Jamalian et al., 2011; Ezampanah et al., 2012). In the present study, 510 m from the upper part of the Garau Formation at the south west of the Kabir-Kuh anticline has been investigated. The Kabir-Kuh anticline is located at the south east of Ilam and north east of Qaleh-Darreh village. The studied interval consists mainly of green to gray marls, marly limestone, limestone, shale and marly shale. This formation overlies the Gotnia Formation and is overlain by Sarvak Formation disconformably at the studied interval. The studied interval is investigated with regard to the calcareous nannofossils for determining the exact age the studied interval.

Materials and Methods: A total of 226 samples with a sampling resolution of 1 to 4 m were collected from the upper part of the Garau Formation and were processed using the simple smear slide technique described in Perch-Nielsen (1985). The prepared slides were examined using an Olympus BH2 transmitting light microscope at x1000 magnification.

Discussion: In the present study, the standard calcareous nannofossil zonation in low-latitude sections of Sissingh (1977) emended by Perch-Nielsen (1979, 1985) and Applegate and Bergen (1988) and Roth (1978) emended by Bralower (1987) and Bralower et al. (1993, 1995) were applied. The studied interval spans from CC5 to CC7 biozone of Sissingh (1977) and from NC5C to NC7B biozone of Roth (1978). Regarding to calcareous nannofossils, ninety species from 43 genus and 15 families have been recognized at the studied interval. Six important calcareous nannofossil bioevents were identified within the studied interval. Calcicalathina oblongata is recorded from the first part of the studied interval and its last occurrence (LO) is the lowermost nannofossil datum observed at 28 m of the studied interval, which defines the top of CC5 and NC5C. The first occurrence (FO) of H. irregularis (75 m) is recorded after the FO of Braarudosphaera africana (48 m) and Flabellites oblongus (64 m) at the studied interval and marks the top of CC6 zone of Sissingh (1977) and NC5 zone of Roth (1978) emended by Bralower et al. (1993, 1995). At low latitudes, the FO of R. irregularis and H. irregularis are the most reliable bio-events for defining the Barremian/Aptian boundary (e.g., Applegate and Bergen, 1988). The next important bio-event at the studied interval is the FO of E. floralis (122 m), defining the top of NC6 and CC7a (Early Aptian). The other important bio-event at the studied interval is the last occurrence of Micrantholithus hoschulzii (370 m) defines the top of NC7A biozone of Roth (1978) emended by Bralower et al. (1993, 1995)