Microbiostratigraphy of the Tarbur Formation, Zagros Basin, Iran

INTRODUCTION

The Tarbur Formation is carbonate sediments that extends from southeastern to northwestern of Zagros basin along the western edge of the Imbricated Zagros zone and between Main Zagros reverse fault and High Zagros fault and east of Sabzpushan fault (Alavi, 2004). High Zagros fault consists of several part, north to south, Sefid kuh, Zard kuh, Dena and Bakhtegan faults (Fig. 1 ). Southern outcrops of Tarbur Formation are between Sabzpushan and high Zagros fault. Facies and thickness contour maps controls and field investigation revealed that the Sabzpushan fault was frequently active from Middle Cretaceous (Cenomanian) to recent time (Safari, 2006). At Campanian-Maastrichtian, facies changed of Gurpi shale and marl, west of Sabzpushan fault, to Tarbur limestone, east of this fault (Ghazban, 2007).

This study relates to foraminifera from four stratigraphic sections (Fig. 1 ), 10 km north of Khorram Abad, Robut section, 30 km east of Khorram Abad, Chenar section, 18 km southwest of Semiroum Balgar section and 39km southeast of Shiraz, Sarvestan section. During Late Cretaceous the Arabian plate was subjected to both compressional and extensional stresses (Sharland et al., 2001).

Fig. 1:	Location map of sections referred to in text, B: Balghar section, C: Chamsangr section, S: Sarvestan section, R: Robat section (without scale)

Fig. 2:	Schematic cross-section through Zagros basin in Campanian-Mastrichtian

In north margin of Arabian plate, compressional stresses illustrated by the uplift of main Zagros reverse fault and High Zagros fault. This compression commend in Iran, Iraq and Oman in Middle Turonian with the initiation of obduction of ophiolith complex. This resulted deep water foreland basin along northeast plate margin (Lippard et al., 1986).

Foreland basin was in turn separated from what remained of Neotethys by a NW-SE trending high which later formed the Zagros imbricated zone (Stoneley, 1974).

The Gurpi Formation was deposited during Santonian-Middle Paleocene along the main through in an upwelling and foredeep environment. Also, late Cretaceous orogenic phase (Laramid) caused uplift of imbricated zone. The products of this phase were turbidity deposits along the main thrust Zagros (Amiran Formation).

According to the eustatic curve of Haq et al. (1988) a eustatic transgressive occurred in Middle Campanian-Maastrichtian. During this time, an association of rudist-rich thrived sediment was formed along the south eastern Neotethys (Stubar and Loser, 2000). This rock unit is called Tarbur Formation in Zagros Mountains (Fig. 2).

James and Wynd (1965) gave the name Tarbur Formation to a carbonate sequence of Campanian-Maastrichian. Biostratigraphic criteria of the Tarbur Formation was Studied by Wynd (1965) that divided this formation into two biozones, Monolepidobis-Orbitpides Assemblage zone (Campanian in age) and Omphalucyclus-Loftusia Assemblage zone (Masstrichtian in age).

James and Wynd (1965), Kalantari (1976), Khosrow-Tehrani and Afghah (2004) and Maghfuri (2006) reviewed and described the lithological and micro faunal characterizes of the Tarbur Formation. Most investigation of this formation is based at interior Fars (southern interior imbricated zone). In this area, Tarbur Formation lies between Gurpi Formation at the base and Sachun Formation at the top. But toward northwest, in Balgar section, the sediments of the Tarbur Formation lie between Gurpi and Amiran Formations. In Robat and Chamsangar sections, this formation lies between Amiran Formation at the base and Kashkan Formation at the top.

LITHO-STRATIGRAPHY

Lithological characteristics of the Tarbur Formation at the Sarvestan section consist of 400 m limestone which divided in to two parts. Lower part consists of massive rudist bearing gray limestone, slightly dolomitized at basal part and medium rudist bearing white limestone (188 m) at top. Irons nodular are present at exposed layer of Tarbur Formation.

At Balgar section, Tarbur Formation consists of medium to massive organodetrial gray limestone (101 m).

The thickness of Tarbur Formation in Chamsangar section is 71 m and consists of thick light gray limestone. In Robat section, the thickness of Tarbur Formation is 61 m and contains of medium gray limestone with amount of rudist fragment.

BIOSTRATIGRAPHY

The following benthonic foraminifera are identified in limestone of the Tarbur Formation at Sarvestan (Fig. 3a):

Omphalocyclus macroporus, Loftusia coxi, L. minor, L. morgani, sidrolites calcitropoides, rotalia c.f trochidiformis, lepidorbitoides sp., Orbitoides apiculata, An-talyna korayil.

The following microfauna are identified in the sediments of the Tarbur Formation at Balgar section (Fig. 3b):

Omphalocyclus macroporous, Loftusia minor, Loftusia sp., Dicyclina sp. and Valvulammina sp.

In Chamsangar section, the following benthonic foraminifera are identified (Fig. 3c, 4a-h):

Omphalocyclus macroporous, Orbitides media, Loftusia minor, L. elongata, Sidrolites calcitropoides, Rotalia skorensis and Minoxia sp.

The following index microfauna are identified in Robat section (Fig. 3d):

Fig. 3:	Selected faunal distribution of the Tarbur Formation at: (a) Sarvestan, (b), Balgar, (c) Chamsangar and (d), Robat sections

Fig. 4:	Representative foraminifera of the Tarbur Formation, (a): Luftosia harisoni, Equatorial section (X17), Sample No. Tar-26; (b): Luftosia minor, Axial section (X17), Sample No. Tar-30; (c): Luftsia elongata, Axial section (X17), Sample No. Tar 26; (d): Dicyclina sp., Equatorial section (X17), Sample No. Tar-30; (e): Omaphalocyclus sp., Axial section (X17), Sample No. Tar 19; (f): Cuneolina sp., Axial section (X17), Sample No. Tar3; G: Obitoides media, Axial section (x40) Sample No. 21 and (h): Valvulammina sp., Axial section (X17), Sample No. Tar1 9

Loftusia elongata, L. minor. L. coxi, L. harisoni, Dicyclina sp., Minoxia sp. and Antalynia korayi.

Based on these assemblages of microfauna and comparison with other parts of Middle East (Al Sharhan and Nairn, 1990; Al Sharhan and Kendal, 1991; Henson, 1948) and with other distribution of Loftusia (Meric and Mojab, 1977) the outcrops of the Tarbur Formation at Sarvestan section is Early-Middle Maastrichtian and in other studied section is Middle Maastrichtian in age.