tion): cal car e ous nannofossil ev i dences. Geo log i cal Quar terly, 63 (2): 215–229, doi: 10.7306/gq.1464 As so ci ate Ed i tor – Wojciech Granoszewski
A di verse Late Oligocene to Early Mio cene cal car e ous nannofossil as sem blage was ex am ined from the Qom For ma tion in the Cen tral Iran Ba sin, and the Oligocene-Mio cene bound ary was iden ti fied based on the quan ti ta tive anal y sis of the as sem - blages in 303 smear slides. Eleven well-es tab lished cal car e ous nannofossil bio-events are de lin eated in the Up per Oligocene through Lower Mio cene. The re sults clearly show that the High est Oc cur rence (HO) of Sphenolithus delphix is the clos est bio-event to the bound ary as tra di tion ally de lin eated on the lithostratigraphic cri te ria, and pro vides a dis tinct biohorizon be low it. The Low est Oc cur rence (LO) of the spe cies Discoaster druggii is the old est Mio cene bio-event that is ob - served shortly af ter the HO of S. delphix, show ing that cal car e ous nannofossils are well suited for ap prox i mat ing the Oligocene-Mio cene bound ary in the Qom For ma tion. The Oligocene-Mio cene bound ary is placed in the up per part of Sub-mem ber “c1” in all three sec tions stud ied here and it is trace able through out the Cen tral Iran Ba sin, which makes a po - ten tially re li able marker ho ri zon for se quence strati graphic and hy dro car bon stud ies in the area.
Key words: Oligocene-Mio cene bound ary, cal car e ous nannofossils, biohorizons, Cen tral Iran Ba sin, Qom For ma tion, “c1”
Sub-mem ber.
INTRODUCTION
Iden ti fi ca tion of the bound ary be tween the Oligocene and Mio cene has been of ten dif fi cult in geo log i cal re cords (Shackle - ton et al., 2000). Berggren (1969) de fined an age of 22.5 Ma for the Oligocene-Mio cene bound ary and dem on strated that it could be cor re lated ap prox i mately to the magnetochrone C6An.
Then, Berggren et al. (1985) re viewed the bound ary cri te ria and de ter mined an age of 23.8 Ma for it based on the last ap pear - ance of the nannofossil spe cies of Reticulofenestra bisecta, Reticulofenestra scrippsae and Cyclicargolithus abisectus.
Steininger et al. (1997) as signed an age of 23.8 Ma to the bound ary placed at the base of the C6Cn.2n magnetozone to de fine the GSSP for the base of the Neo gene. Lourens et al.
(2004), Palike et al. (2006) and Gradstein et al. (2012) in their new syn the sis have pro posed an age of 23.03 Ma for the base of the C6Cn.2n magnetozone.
Sev eral cli ma tic changes have been re ported at the Oligocene -Mio cene tran si tion from var i ous lo ca tions in the Tethyan and equa to rial realms (Zachos et al., 2001; Allen and
Armstrong, 2008; Beddow et al., 2016). The main cause for such en vi ron men tal changes is mas sive tec tonic dis place ments which led to re duc tion in at mo spheric pCO2 (Pagani et al., 1999;
Pearson and Palmer, 2000), and changes in palaeo climate and palaeoenvironmental con di tions (Zachos et al., 2001).
The Ara bia-Eur asia col li sion and the clo sure of the Tethys ocean gate way is one of the global tec tonic events that span from the Late Eocene to Mio cene (Allen and Armstrong, 2008;
Sadr, 2017). Pre vi ous stud ies based on the palaeo ec ol ogy and palaeoclimatology of the Oligocene to Mio cene tran si tion sug - gested that fluc tu a tions in global ice vol ume and eustatic sea level, and tem per a ture vari a tions dur ing gla cial or inter gla cial pe ri ods have re sulted in the bi o log i cal-en vi ron men tal cri ses (Miller et al., 1991; Zachos et al., 2001; Billups et al., 2002).
More over, mul ti ple shifts in the value of 18O and 13C iso topes were re ported dur ing this in ter val from var i ous sites (Zachos et al., 2001; Pekar et al., 2002; Wade and Palike, 2004; Beddow et al., 2016). They are re ferred to as Oi- and Mi-events. These changes and the global cool ing trends have led to the bi o log i cal cri sis in the biota as sem blages of the Late Oligocene to Mio - cene (as shown for in stance in the Oligocene-Early Mio cene range charts in Perch-Niel sen, 1985). Dif fer ences in the oc cur - rences of these bi o log i cal events (re corded or not at dif fer ent sites), and more spe cif i cally dif fer ences in their chronostrati - graphic po si tion from one site to an other, have re sulted in prob - lems in pre cise de lin ea tion of the Oligocene-Mio cene bound - ary. Be cause of these prob lems, iden ti fi ca tion and in tro duc tion of ma jor bio-events, and chronostratigraphic dat ing of the
* Corresponding author, e-mail: fhadavi@um.ac.ir
Received: February 24, 2018; accepted: November 9, 2019; first published online: May 17, 2019
Oligocene-Mio cene bound ary have of ten been con sid ered as a chal lenge.
An ac cu rate in ter pre ta tion of stage bound aries is very im - por tance for stra tig ra phy (in clud ing se quence stra tig ra phy) which is the main tool for hy dro car bon ex plo ra tion stud ies (e.g., Catuneanu, 2013). This, how ever, can only be fa cil i tated by hav ing de tailed and valid biostratigraphic in for ma tion in the area tar geted for hy dro car bon ex plo ra tion. There fore, this study at tempts to con trib ute to de ter min ing the Oligocene-Mio cene (Chattian/Aquitanian) bound ary through a de tailed study of cal - car e ous nannofossil biohorizons at three out crop sec tions in the Cen tral Iran Ba sin.
STRATIGRAPHIC SETTING
The Qom Ba sin in the Cen tral Iran Zone (CIZ) (Fig. 1A) formed dur ing mid dle Al pine orogenic time (Paleogene to Early Mio cene; Berberian and King, 1981). Dur ing the Eocene orog - eny, ex ten sive vol ca nism oc curred in the CIZ in a NW–SE - -trending area known in the lit er a ture of Ira nian ge ol ogy as the Orumia-Dokhtar Mag matic Arc (e.g., Berberian and King, 1981;
Aghanabati, 2004; Shahabpour, 2007). The sub se quent Oligo - cene - Mio cene ma rine trans gres sion re sulted in the sprea d ing of ma rine car bon ate se quences along the NW mar gin of the Ara bian plate (Zagros Ba sin; Asmari For ma tion) and the SSW mar gin of the Ira nian plate (Qom Ba sin; Qom For ma tion) (Dozy, 1944, 1955; Bozorgnia, 1966; Berberian and King, 1981;
Aghanabati, 2004; Fig. 1B).
Fol low ing pe tro leum dis cov ery in the Serajeh and Alborz fields (Cen tral Iran Ba sin) in 1934 (Mostofi and Gansser, 1957;
Abaie et al., 1964; Rosenberg, 1975), the Qom For ma tion be - came a fo cus of biostratigraphic, palaeogeographic, palaeo eco - logic, se quence strati graphic, microfacies, and tec tonic stu d ies (e.g., Reuter et al., 2007; Daneshian and Ramezani Dana, 2007;
Hadavi et al., 2010; Yazdi-Moghadam, 2011; Mohammadi et al., 2013; Daneshian and Ghanbari, 2017; Daneshian and Rame - zani Dana, 2017; Parandavar and Hadavi, 2017).
The Qom For ma tion is widely dis trib uted in the Qom back- arc, arc, and fore-arc bas ins (Reuter et al., 2007; Fig. 2A). The first ma rine trans gres sion of the Qom Sea can be traced back to the Early Oligocene in the fore-arc ba sin and to the Late Oligocene in the back-arc ba sin (Reuter et al., 2007; Yazdi - -Moghadam, 2011). Be cause of the var i ous fa cies pres ent in the Qom For ma tion, no type sec tion has been in tro duced for it yet, al though the Qom area is de fined as its “type area”
(Stocklin and Setudehnia, 1991; Aghanabati, 2004). In the type area, the Qom For ma tion in as cend ing strati graphic or der is di - vided into six mem bers. They are: a – sandy lime stones, b – silty lime stones with al ter na tions of silty marlstone, c – al ter nat - ing marlstones and lime stones, d – evaporites, e – green marlstones, f – reefal lime stones (Furrer and Soder, 1955;
Soder, 1956; Abaie et al., 1964; Stocklin and Setudehnia, 1991) (Fig. 3). The c-Mem ber is sub di vided into four sub-mem bers by Soder (1959), in clud ing: c1 – marlstones with in ter ca la tions of lime stones, c2 – evaporites, c3 – shal low-wa ter lime stones, c4 – green marlstones. Reuter et al. (2007) have not sub di vided the c-Mem ber and used the term “c-Mem ber” as a thick ened in - ter val of the Qom For ma tion. Out side of the type area (Qom), most of the men tioned mem bers and sub-mem bers are not pres ent in the CIZ (Stocklin and Setudehnia, 1991; Aghanabati, 2004). There fore, this study was fo cused on the Qom, Kashan and Garmsar ar eas where a com plete Oligocene-Mio cene suc -
Fig. 1A – gen eral map of Iran show ing the sed i men tary bas ins and struc tural zones (mod i fied af ter Aghanabati, 2004);
B – dis tri bu tion of the Oligocene-Mio cene ma rine car bon ates of the Qom and Asmari for ma tions in Cen tral Iran Zone (CIZ) and Zagros Ba sin (mod i fied af ter Berberian and King, 1981)
ces sion of the Qom For ma tion was re ported (Rahaghi, 1973, 1976, 1980; Reuter et al., 2007; Mohammadi et al., 2011, 2013;
Parandavar, 2018). Pre vi ous biostratigraphic stud ies of the Qom For ma tion reached no com pre hen sive agree ment on the chronostratigraphy of the For ma tion, in par tic u lar with re gard to the place ment of the Oligocene-Mio cene bound ary (Reuter et al., 2007; Yazdi-Moghadam, 2011).
STUDY AREA
The best out crops of the Qom For ma tion are ex posed in the type area, south-east of the Kashan, south of the Garmsar and around Qom. In the pres ent study, three strati graphic sec tions lo cated at the Shurab (south of Qom), Navab Anticline (south -
-east of Kashan) and Siah-Kuh (south of Garmsar) were mea - sured and sam pled in de tail. In all three sec tions, the Oligocene to Mio cene in ter val oc curs be low the red gypsiferous marlsto - nes and/or gyp sum suc ces sions of Sub-mem ber “c2” and po si - tioned within the green to grey marlstones of Sub -mem ber “c1”.
There fore, the Oligocene-Mio cene in ter val does not show a clear change in li thol ogy or sedimentological char ac ter is tics.
SHURAB SECTION
The Shurab sec tion is the best out crop of the Qom For ma - tion in the type area, in clud ing all mem bers of the Qom For ma - tion, crop ping out south of the city of Qom (Stoecklin, 1959),
~35 km to the south of Qom within the Qom back-arc ba sin Fig. 2A – sed i men tary bas ins of the Qom For ma tion: back-arc and fore-arc bas ins (mod i fied af ter Reuter et al., 2007); red stars in di - cate lo ca tion of the stud ied sec tions in the Qom Ba sin (back-arc ba sin) of Cen tral Iran; B, C D – geo log i cal maps of the Shurab, Navab Anticline, and Siah-Kuh ar eas, show ing lo ca tion of the sec tions (mod i fied af ter Emami, 1992; Khalatbari and Alavi, 1996)
(34°25’N, 51°08’E; Fig. 2A, B). The to tal thick ness of this-sec - tion is 543 m and it ex poses the “a” to “f” mem bers of the Qom For ma tion. De tailed anal y sis of cal car e ous nannofossils was per formed on Sub-mem ber “c1” of the sec tion (from sam ple No. 140 to sam ple No. 250). Lithologically, Sub-mem ber “c1”
con sists of al ter nat ing green to grey marlstones and marly lime stones which overly the dark brown silty lime stones of the Mem ber “b”, and is over lain by the red gypsiferous marlstones and gyp sum beds of Sub-mem ber “c2” (see Fig. 4).
NAVAB ANTICLINE SECTION
The sec tion (411 m thick) is lo cated 25 km to the south-east of Kashan city (Qom back-arc ba sin) with the co or di nates of 33°51’N, 51°38’E (Fig. 2C). The Qom For ma tion in cludes mem bers “a” to “f” in this sec tion (Fig. 3). As in the Shurab sec - tion, this study fo cus ses on Sub-mem ber “c1” with a thick ness of 63 m (from sam ple No. 85 to sam ple No. 225). The mem ber con sists of green to grey marlstones with al ter na tion of ar gil la - ceous lime stones; it over lies the thick-bed ded silty-ar gil la ceous lime stones of Mem ber “b” and un der lies the red gypsiferous marlstones of Sub-mem ber “c2” (see Fig. 5).
SIAH-KUH SECTION
The Qom For ma tion is crop ping out ~40 km to the south of Garmsar city in the Siah-Kuh within the Qom back-arc ba sin (34°43’N, 52°15’E; Fig. 2D). The to tal thick ness of the Siah-Kuh sec tion is 493 m and it ex poses sub-mem bers “c1” to “f” of the Qom For ma tion. The 48 m thick Sub-mem ber “c1” (from sam - ple No. 1 to sam ple No. 38) is stud ied here for cal car e ous nannofossil biostratigraphy. It con sists of ar gil la ceous lime - stones and green to grey marlstones. Sub-mem ber “c1” rests un con form ably on the red sand stones of the Lower Red For ma - tion (LRF) and is con form ably over lain by the red gypsiferous marlstones of Sub-mem ber “c2” (Fig. 6).
MATERIALS AND METHODS
Through out the stud ied out crops, sam ples were col lected with an av er age spac ing of 50 cm, al though the sam ple res o lu - tion de creased to 20–30 cm in some in ter vals. A to tal of 303 sam ples were thus ob tained from the marlstone and marly lime - stone suc ces sion of Sub-mem ber “c1”. They were pre pared us - ing the smear slide tech nique (Bown and Young, 1998). For each sam ple, 5 mg of rock was weighed, dis persed in 0.5 ml (~10 drops) of dis tilled wa ter, disaggregated with the so dium hexa-metaphosphate pow der, and a few drops of the sus pen - sion were fi nally trans ferred on a glass slide. Af ter dry ing the sus pen sion on a heat ing plate, the slides and coverslips were mounted us ing Entellan glue. Slides were ex am ined us ing a light mi cro scope (Olym pus BX53) at 1250X mag ni fi ca tion. Im - ages of coccoliths were taken us ing an Olym pus DP73 cam era.
Var i ous cal car e ous nannofossil zonal schemes were es tab - lished by Mar tini (1971), Okada and Bukry (1980), Varol (1998), Young (1998), Backman et al. (2012) and Agnini et al. (2014) for the Up per Oligocene to Lower Mio cene at low and mid dle lat i tudes. The stan dard zonal scheme of Mar tini (1971; NP and NN zones) is used here. How ever, the zonal mark ers of Backman et al. (2012; CNM zones) and Agnini et al. (2014;
CNO zones) were also con sid ered. For the de ter mi na tion of cal car e ous nannofossil spe cies, we adopted the tax on omy pro -
posed by Aubry (1984, 1988, 1989, 1990), Perch-Niel sen (1985), Farinacci (1989), Varol (1998), Young (1998) and Howe (2016). The High est Oc cur rences (HO) of Sphenolithus cipero - ensis, Reticulofenestra bisecta, R. stavensis, Cyclicargolithus abisectus, Helicosphaera recta and Zygrhablithus bijugatus, the Low est Oc cur rences (LO) of Triquetrorhabdulus carinatus, LO and HO of Sphenolithus delphix, and LO of Discoaster druggii, have been use ful for a biozonal sub di vi sion in the pres - ent study. The biohorizons used in this re search are as fol lows:
the Low est Oc cur rence (LO), the High est Oc cur rence (HO) and the High est Com mon Oc cur rence (HCO) (Aubry, 2016).
Quan ti ta tive anal y sis was uti lized to es tab lish dis tri bu tion pat - terns of se lected cal car e ous nannofossil taxa. The abun dance of se lected spe cies was de ter mined by count ing the num ber of spec i mens in a pre fixed area of smear slides (N/mm2) fol low ing Backman and Shackle ton (1983). The pre fix area (to tal area of fields of view) was re lated to 50 Fields Of View (FOV). The re - sults of count ing anal y ses per formed on Sub-mem ber “c1" of the Qom For ma tion in Shurab, Navab Anticline and Siah-Kuh sur - face sec tions are shown in Fig ures 4–6.
TAXONOMIC NOTES
Reticulofenestra bisecta
(Hay, Mohler and Wade, 1966) Roth, 1970
Basionym: Syracosphaera bisecta Hay, Mohler and Wade, 1966
Vari ants: R. stavensis (Levin and Joerger, 1967) Varol, 1989; Dictyococcites scrippsae Bukry and Percival, 1971.
Two dif fer ent tax o nomic sub di vi sions have been used for R.
bisecta ac cord ing to nannotax da ta base:
1. Spec i mens <10 µm = D. scrippsae, spec i mens >10 µm = D. bisectus;
2. Spec i mens <10 µm = R. bisecta, spec i mens >10 µm = R. stavensis.
The lat ter di vi sion is fol lowed here. In the stud ied ma te ri als, the size of R. bisecta ranges from 7.6 to 8.4 µm, and of R.
stavensis from 13.6 to 15.4 µm (see Fig. 9).
RESULTS
APPLICATION OF MARTINI (1971) BIOSTRATIGRAPHIC SCHEME AND ADDITIONAL BIO-EVENTS
This study shows that the Zone NN1 of Mar tini (1971) is pres ent in Sub-mem ber “c1” of the Qom For ma tion. Fol low ing the lit er a ture (Mar tini, 1971; Perch-Niel sen, 1985), this biozone is de fined as the HO of H. recta and/or S. ciperoensis to the LO of D. druggii. As this zone in cludes sev eral bio-events which have been re ported by var i ous au thors (Okada and Bukry, 1980; Perch-Niel sen, 1985; Backman et al., 2012; Agnini et al., 2014), Sub-mem ber “c1” can be sub di vided into smaller in ter - vals by spe cific events (Figs. 4–6). Herein, in or der to de ter mine the Oligocene-Mio cene bound ary, we de scribed the iden ti fied bio-events in our study area and com pared them with the known bio-events from var i ous sites. In the fol low ing, these are listed in the as cend ing strati graphic or der (see also Figs. 4–7).
Mi cro scopic pho tos of in dex cal car e ous nannofossil as well as of sev eral synchronic taxa are pro vided in Fig ures 8 and 9.
A: HO of Sphenolithus ciperoensis. HO of S. ciperoensis is used to de fine the base of zones NN1, CN1a and CNO6 (Mar tini, 1971; Okada and Bukry, 1980; Agnini et al., 2014) and
it has been used to de lin eate the Oligocene-Mio cene bound ary in the past (Perch-Niel sen, 1985). Re cent stud ies (e.g., Rio et al., 1990; Raffi et al., 2006; Backman et al., 2012; Agnini et al., 2014) have con firmed ear lier ones that showed it to be an Oligocene bio-event. In the Shurab sec tion, the bio-event is re -
corded at 198 m (Fig. 4). It is placed at 114 m of Navab Anticline out crops of Sub-mem ber “c1” (Fig. 5). The event has not been ob served in the Siah-Kuh sec tion (Ta ble 1). There fore, the base of the zone can not be cor re lated with other sec tions in the Qom Ba sin.
Fig. 3A – view of the lower part of the Qom For ma tion (sub-/mem bers “a” to “c1”) in the Navab Anticline sec tion; the Qom For ma tion rests un con form ably on the Lower Red For ma tion; per son for scale (white cir cle); B – view of the up per parts of the for ma tion con - sist ing of sub-/mem bers “c1” to “f”; the Qom For ma tion un der lies un con form ably the Up per Red For ma tion
Fig. 4. Lithological char ac ter is tics of the Oligocene/Mio cene in ter val of the Shurab sec tion
Abun dance pat terns of se lected spe cies use ful for the Up per Oligocene-Lower Mio cene stan dard biozonation are shown (NP and NN – Mar - tini, 1971; CNO – Agnini et al., 2014; CNM – Backman et al., 2012); the X-axis val ues rep re sent the num ber of spec i mens in a pre fixed area (N/mm2) re lated to 50 FOV; dif fer ent dashed lines high light the LO or HO of spe cies; the light grey rect an gles show un stud ied in ter vals
B: HO of Reticulofenestra bisecta. Okada and Bukry (1980) em ployed this biohorizon to de fine the lower bound ary of Subzone CN1a. Agnini et al. (2014) as sume that the LO of R.
bisecta oc curred at a level which is slightly youn ger than S.
ciperoensis. In this study, spo radic oc cur rence of R. bisecta (<10 µm) is ob served above the HO of S. ciperoensis and within Zone NN1 of Mar tini (1971) (Figs. 4–7 and Ta ble 1).
C: HO of Reticulofenestra stavensis. The HO of R.
stavensis is ob served in all stud ied sec tions. The biohorizon oc - curs within Zone NN1 of Mar tini (1971) and above of the HO of S. ciperoensis (Figs. 4–7) in all sec tions. Varol (2017) em - ployed this bio-event to de fine Subzone NN1a. The sam ple num bers and strati graphic po si tion of the bio-event are pre - sented in Ta ble 1 for all sec tions. Herein, this event has been re corded above the HO of R. bisecta (Figs. 4–6).
D: HO of Zygrhablithus bijugatus. This HO is sim i lar to the bio-event re ported by Varol (2017), which is used to de fine Subzone NN1b. In the pres ent study, this biohorizon is re - corded in the lower part of Zone NN1 of Mar tini (1971) and in the mid dle part of Sub-mem ber “c1” (Figs. 4–6 and Ta ble 1).
E: HCO of Cyclicargolithus abisectus. The spe cies is com mon and abun dant in the cal car e ous nannofossil as sem - blages of the stud ied sec tions. The HCO of large spec i mens of C. abisectus lies within Zone NN1 in all sec tions (Figs. 4–7 and Ta ble 1), while, as yet, the Oligocene spe cies (such as H. recta)
can be seen in the cal car e ous nannofossil as sem blages af ter the re cord of this bio-event (Figs. 4–6). Based on its dis tinct abun dance pat terns in Rio et al. (1990) and Olafsson (1992), the large form of C. abisectus (>10 µm) is very rare in the Mio - cene and the HO of the taxon oc curs in Zone NN7 of Mar tini (1971), but its high est abun dant oc cur rence lies above of the LO of the S. ciperoensis bio-event in Zone NN1.
F: LO of Triquetrorhabdulus challengeri. Varol (2017) re ported the bio-event within the Oligocene cal car e ous nanno - fossil as sem blages and used the LO of T. challengeri to de fine Subzone NN1c. In this study, we were not able to de ter mine con fi dently this form in the stud ied sec tions. How ever, we iden - ti fied the Triquetrorhabdulus cf. challengeri taxon and in tro - duced it as a bio-event. The LO of T. cf challengeri lies in the lower part of Zone NN1 in all sec tions (Figs. 4–6 and Ta ble 1).
The spe cies is rare and spo radic in Zone NN1 and be comes more abun dant in the Mio cene suc ces sion.
G: HO of Helicosphaera recta. This is an im por tant bio - -event ob served in the mid dle part of Zone NN1 and it has been used to de fine the Oligocene-Mio cene bound ary by some sci en - tists (Martnini, 1971; Perch-Niel sen, 1985), while other re search - ers (e.g., Rio et al., 1990; Raffi et al., 2006; Backman et al., 2012) re ported its LO al ready within the Oligocene. We also con sis - tently ob served the HO of H. recta within the Oligocene as sem - blages in our ma te rial. The abun dance pat terns of this taxon in Fig. 5. Se lected cal car e ous nannofossil abun dance pat terns from the Oligocene/Mio cene in ter val of the Navab Anticline sec tion
The stan dard biozonation of Mar tini (1971; NP and NN), Agnini et al. (2014; CNO) and Backman et al. (2012; CNM) are used in the pres - ent study; the X-axis val ues rep re sent the num ber of spec i mens in a pre fixed area (N/mm2) re lated to 50 FOV; dashed lines high light the
LO or HO of spe cies; the light grey rect an gles show un stud ied in ter val
our sec tions are shown in Fig ures 4–6 and the strati graphic po si - tions of its HO are re corded in Ta ble 1 for each sec tion.
H: LO of Sphenolithus delphix. Lit er a ture data places the LO of S. delphix be low magnetozone C6Cn.2n (Rio et al., 1990;
Fornaciari and Rio, 1996; Steininger et al., 1997; Shackle ton et al., 2000; Raffi et al., 2006; Backman et al., 2012; Agnini et al., 2014; Albasrawi, 2016). In the stud ied sec tions and in agree - ment with pre vi ous re sults, this bio-event is ob served in the up - per part of Zone NN1 and be fore the first ap pear ance of Mio - cene spe cies (Figs. 4–7 and Ta ble 1).
I: LO of Sphenolithus capricornutus. The LO of S.
capricornutus was re ported in the GSSP of the Paleogene - -Neo gene bound ary by Steininger et al. (1997). Ac cord ing to the phylogenic chart by Perch-Niel sen (1985), S. capricornutus is the youn gest of all Paleogene spe cies. Herein, the LO of S.
capricornutus oc curs in the Up per Oligocene nannofossil as - sem blages and within the up per part of Zone NN1. The dis tri bu - tion pat terns and the po si tion of the LO in the three sec tions are shown in Fig ures 4–6 and Ta ble 1.
J: HO of Sphenolithus delphix and S. capricornutus.
These bio-events (HOs of S. delphix and S. capricornutus) are re corded in the up per part of the Sub-mem ber “c1”, at the top of the Oligocene cal car e ous nannofossil as sem blages and/or strata (Figs. 4–7 and Ta ble 1). Just above these events, the in - dex cal car e ous nannofossil as sem blages of the Mio cene are pres ent, in clud ing S. tintinnabulum, Reticulofenestra haqii, Hughesius gizoensis, H. youngii and H. carteri. Pre vi ously, the HO of S. delphix was used to de fine the zonal bound ary of Zone CNO6 by Agnini et al. (2014), and Zone CNM1 by Backman et
al. (2012) in the Paleogene and Neo gene strata, re spec tively.
Ac cord ing to these stud ies, the HO of S. delphix is re corded close to the Oligocene-Mio cene bound ary.
K: LO of Discoaster druggii. The LO of D. druggii oc curs above the HOs of the Late Oligocene in dex spe cies in the stud - ied sec tions (Figs. 4–6 and Ta ble 1), which is in agree ment with other stud ies. Mar tini (1971), Okada and Bukry (1980) and Perch-Niel sen (1985) used this bio-event to de fine Zones NN1 and CN1b/c. Backman et al. (2012) used the LO of Spheno - lithus disbelemnos to de ter mine the CNM1/CNM2 zonal bound - ary which lies at a strati graphic level youn ger than the LO of D.
druggii. Backman et al. (2012) re ported the D. druggii bio - horizon in the up per part of Zone CNM1 and at the base of the Aquitanian suc ces sion at low to mid dle lat i tude sites (Fig. 7). In the pres ent study, the LO of D. druggii is re corded from the up - per part of Zone NN1 (Fig. 7). The S. disbelemnos was not re - corded among the cal car e ous nannofossil as sem blages in Sub -mem ber “c1” (Figs. 4–6).
DISCUSSION
The cal car e ous nannofossil mark ers used for the iden ti fi ca - tion of the Oligocene-Mio cene bound ary have changed over time. Ac cord ing to the stan dard zonation of Mar tini (1971) and def i ni tion of Zone NN1, the bound ary has been placed at the HO of ei ther Helicosphaera recta or Sphenolithus ciperoensis (Fig. 7). Okada and Bukry (1980) have used the HOs of S.
Fig. 6. Abun dance pat terns of marker cal car e ous nannofossil spe cies from the Oligocene/Mio cene in ter val of the Siah-Kuh sec tion shown against the zonal schemes of Mar tini (1971; NP and NN), Agnini et al. (2014; CNO) and Backman et al. (2012; CNM); the X-axis val ues rep re sent the num ber of spec i mens in a pre fixed area (N/mm2) re lated to 50 FOV; dashed lines high light the LO or HO of spe - cies; the light grey rect an gles show un stud ied in ter vals
.7 .giFsenozoiBdna snozirohoibeht tsniaga nwohs noi ti snart ene coiM-enecogilO eht ni snoitanoz fo ,)1791( ini traMadakOdna yrkuB,)0891( labolg eht lissofonnan fo stneve loraV ,)7102( namkcaBdna )2102( .la te iningA)4102( .la te desab si elacs la c igo lo norhc eht ;snoi ta le rroc rieht dna , nonietsdarGtxet eht ni denial pxe era snoi t ai ver bba ;)2102( .la te
Fig. 8. In dex and com monly re corded cal car e ous nannofossils from the stud ied sec tions
A–D – Sphenolithus ciperoensis Bramlette and Wilcoxon, 1967 (A, C – XPL; B, D – GP; A, B – 0°; C, D – 45°); E–H – Sphenolithus delphix Bukry, 1973 (E, G – XPL; F, H – GP; E, F – 0°; G, H – 45°); I–L – Sphenolithus delphix Bukry 1973 (I, K – XPL; J, L – GP; I, J – 0°; K, L – 45°);
M–P – Sphenolithus capricornutus Bukry and Percival, 1971 (M, O – XPL; N, P – GP; M, N – 0°; O, P – 45°); Q–T – Sphenolithus conicus Bukry, 1971 (Q, S – XPL; R, T – GP; Q, R – 0°; S, T – 45°); scale bar is 2 µm in all im ages
Fig. 9. In dex and com mon cal car e ous nannofossils from the stud ied sec tions
A, B – Reticulofenestra bisecta (Hay, Mohler and Wade, 1966) Roth, 1970 (A, B – XPL); C, D – Triquetrorhabdulus cf. chal lenger Perch-Niel - sen, 1977 (C – XPL; D – QP); E, F – Triquetrorhabdulus cf. challengeri Perch-Niel sen, 1977 (E – XPL; F – GP; E, F – 45°); G, H – Zygrhablithus bijugatus Deflandre, 1959 (G, H – XPL; G – 0°; H – 45°); I, J – Reticulofenestra stavensis (Levin and Joerger, 1967) Varol, 1989 (I, J – XPL); K, L – Cyclicargolithus abisectus (Mul ler, 1970) Wise, 1973 (K – XPL; L – QP); M, N – Helicosphaera recta (Haq, 1966) Jafar and Mar tini, 1975 (M – XPL; N – GP); O, P – Helicosphaera recta (Haq, 1966) Jafar and Mar tini, 1975 (O – XPL; P – GP); Q, R – Discoaster druggii Bramlette and Wilcoxon, 1967 (Q, R – QP); S, T – Discoaster deflandrei Bramlette and Riedel, 1954 (S, T – QP); scale bar is 2 µm in im ages of A to H, and 3 µm in I to T
ciperoensis and Reticulofenestra bisecta, and the end of the acme in ter val of Cyclicargolithus abisectus to de fine subzone CN1a, with the lat ter event and the first oc cur rence of Disco - aster druggii to de fine subzone CN1b (Fig. 7). Rio et al. (1990), Backman et al. (2012) and Agnini et al. (2014) have pos tu lated that the S. ciperoensis, H. recta, D. druggii, T. carinatus, R.
bisecta and C. abisectus events ex hibit un cer tain dis tri bu tion pat terns (see Fig. 10). There fore, biohorizons and de fined biozones across the Oligocene-Mio cene bound ary are con sid - ered to be of lim ited value (Rio et al., 1990; Backman et al., 2012; Agnini et al., 2014). The old est event is the HO of S.
ciperoensis at the lower bound ary of Zone NN1 (or CN1a), fol - lowed by the HO of R. bisecta, the top of the acme zone of C.
abisectus, the LO of S. delphix, the HO of H. recta, and fi nally the HO of S. delphix, all within Zone NN1 (Agnini et al., 2014) (Fig. 7).
Mar tini (1971) and Perch-Niel sen (1985) thought that the NP25/NN1 zonal bound ary was cor re la tive to the Paleogene and Neo gene sys tem bound ary and de fined to the HO of S.
ciperoensis and/or the HO of H. recta spe cies. Rio et al. (1990) con firmed the ear lier find ing that, in fact, these events oc cur be - low the base of the Neo gene in its type area. They have re - ported the bio-events youn ger than the HOs of S. ciperoensis and H. recta, which in clude the LOs and then the LOs of S.
delphix and S. capricornutus. Steininger et al. (1997) have pub - lished the po si tion of the afore men tioned bio-events (FOs and LOs of S. delphix and S. capricornutus) in the GSSP study of the base of the Neo gene (Aquitanian stage). In ad di tion, Ogg et al. (2016) have ap plied these biohorizons to de fine the Chattian/Aquitanian stage bound ary (cor re spond ing to the base of mag netic po lar ity chron C6Cn.2n). Lourens et al. (2004) and Palike et al. (2006) have de ter mined an age of 23.03 Ma for the base of this magnetochrone (C6Cn.2n) at the base of the Aquitanian. There fore, the LO of S. delphix is the near est bio-event to the Oligocene-Mio cene bound ary and pro vides a dis tinct ho ri zon oc cur ring prior to the bound ary (see Agnini et al., 2014; Fig. 7). Fur ther more, there are many pub li ca tions from the var i ous sites, which uni formly show that the FO of D.
druggii is in the Early Mio cene (e.g., Mar tini, 1971; Perch-Niel -
sen, 1985; Rio et al., 1990; Fornaciari et al., 1990; Raffi et al., 2006; Backman et al., 2012). In the high lat i tude ar eas, the men tioned bio-events are dif fer ent from those of low to mid dle lat i tudes (Perch-Niel sen, 1985).
A re view of cur rent lit er a ture clearly shows that the bio- events used for the def i ni tion of biozonal bound aries in low lat i - tude ar eas of the Tethys re gion can hardly be rec og nized in the cen tral Paratethys, and/or when pres ent they are re corded in the youn ger ho ri zons (Fig. 10; see Holcova, 2005; Ozdinova and Sotak, 2014; Grunert et al., 2015). Gen er ally, the Para - tethys do main, as a chain-re stricted ba sin, is marked by poor to mod er ate pres er va tion of nannofossils, rare or scant oc cur - rences of their as sem blages, and in ap pro pri ate en vi ron men tal con di tions (Holcova, 2005; Grunert et al., 2015). Due to these re stric tions, com par i son of the Paratethys and Tethys re gions is com pli cated and im poses many un cer tain ties.
As shown in Fig ure 10, some bio-events (e.g., H. recta and R. bisecta) are re corded in dif fer ent biostratigraphic ranges within the Paratethys realm, and/or ab sent (such as S. delphix, S. capricornutus, T. challengeri). There fore, the bio-events of the Paratethys are not ex actly cor re lat able with the bio-events in the Tethyan re gion (Fig. 10).
Eleven biohorizons have been de ter mined be tween the LO of S. ciperoensis and the FO of D. druggii, pro vid ing a bio - stratigraphic frame work and im prov ing res o lu tion for de fin ing the Oligocene-Mio cene bound ary. The dis tin guished bio- events are in agree ment with other stud ied sites in the low and mid dle lat i tudes, and con sist of the above-de scribed LOs and FOs within Zone NN1 of Mar tini (1971). The HO of S. delphix is the lat est re corded bio-event within the Oligocene cal car e ous nannofossil as sem blage in the stud ied sec tions of the Qom For - ma tion (Figs. 4–6). Fol low ing this bio-event, the first ap pear - ance of D. druggii is the low est de ter mi na ble biohorizon within the Neo gene nannofossil as sem blages which is ob served in the thick ness of 259.6 m Shurab, 168 m in the Navab Anticline and 42 m in the Siah-Kuh sec tions.
Reuter et al. (2007) pre vi ously placed the Oligocene-Mio - cene bound ary within the C-Mem ber of the Qom For ma tion.
This mem ber in cludes a thick ened in ter val of strata (Qom For -
LCO of Cyclicargolithus abisectus 217 133 26
LO of Zygrhablithus bijugatus 213 126 20
LO of Reticulofenestra stavensis 209 119 15
LO of Reticulofenestra bisecta 203 117 –
LO of Sphenolithus ciperoensis 198 114 –
The strati graphic po si tion of the bio-events is re ported from the base of the sec tions
noi ti snart ene coiM-enecogilO fo noi ta le rroc evi ta tneT .01 .giFsenozoibdna snozirohoib ,sedu t ital el ddim ot wol mor
f syhtetaraP lar tneC eht ot yduts tn eserp dna sa era nayhteT aera edu t ital el ddim-wol dna nayhteT ehTsenozoibnwohs era su sreveht snoitanoz fo ,)1791( ini traMadakOdna yrkuB ,)0891( namkcaBdna )2102( .la te iningA;)4102( .la te fo esoht era stneve-oib syhtetaraP lar tneC eht avocloH ,)5002( avonidzOdna katoSdna ,)4102( trenurG)5102( .la te no desab si elacs emiteht ;nietsdarG)2102( .la te ; txet eht ni denial pxe era snoi t ai ver bba
sites by Backman et al. (2012), Agnini et al. (2014) and Varol (2017). Among the de scribed biohorizons, the HO of S. delphix is a use ful marker for the iden ti fi ca tion and track ing of the Oligo - cene -Mio cene bound ary that is placed in the up per part of Sub -
sity of Mashhad. The au thors greatly ap pre ci ate the re view ers:
G. Auer and an anon y mous one for their re views, which sig nif i - cantly im proved the manu script. Many thanks go to T.M. Peryt and W. Granoszewski (ed i tor) and re viewer (G. Auer) for the manu script ed it ing.
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