DASYCLADALES (GREEN ALGAE) AND SOME BENTHIC FORAMINIFERA FROM THE UPPER CRETACEOUS ILAM FORMATION (LATE CONIACIAN–SATONIAN), SW IRAN (ONSHORE AND OFFSHORE)

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IntroductIon
Upper Cretaceous shallow-water carbonates have a comparably wide distribution along the northern and eastern margins of the Arabian Plate (Ziegler 2001;Barrier & Vrielynck 2008;Sharland et al. 2001;Bromhead et al. 2022).The micropalaeontological content of these carbonates is characterized by mixed assemblages of benthic foraminifera (small-and large-sized) and calcareous algae (dasycladalean and udoteacean green algae).Among these, the larger benthic foraminifera have attracted much more interest than the algae caused, among others, by often higher diversities of the former (Cox 1937;Henson 1948;Sampò 1969;Whittaker et al. 1998;Simmons et al. 2000;Schlagintweit et al. 2023;Simmons & Bidgood 2023).In the illustrations-plates provided in the literature, algae are clearly underrepresented and often just treated as 'algae' or in open nomenclature (e.g., genus level).In fact, just a few taxonomic works are available from Upper Cretaceous carbonates of various lithostratigraphic units of the Arabian Plate (Ellliott 1959(Ellliott , 1968;;Radoičić 1979;Kuss & Conrad 1991;Kuss 1994;Okla 1992Okla , 1995a, b;, b;Rashidi & Schlagintweit 2019a, b).
From southwestern Iran (onshore and offshore Zagros Zone), shallow-water carbonates above the Cenomanian-Turonian Sarvak or Surgah formations are known as Ilam Formation (James and Wynd 1965;Omidvar et al. 2014;Mehrabi et al. 2014Mehrabi et al. , 2023;;Bagherpour et al. 2021).For this formation two 'biozones' or typical assemblages have been established by Wynd (1965): a lower 'Rotalia sp.22 -algae assemblage zone' (no.30) and an upper 'Archaecyclus mid-orientalis -Pseudedomia (Fabularia)' (no.31) for which a Coniacian-Campanian age was indicated (= Santonian-Campanian in James & Wynd, 1965; Coniacian-Santonian in Sharland et al. 2001).The algae have neither been treated taxonomically nor been further classified (green algae?red algae?) or illustrated in these seminal works.In later papers, they were treated as debris of green algae or often in open nomenclature as Salpingoporella sp., or Actinoporella sp.(Bagherpour et al. 2021;Mehrabi et al. 2023) but not figured as such.About 20 years ago, a taxonomic study was prepared by Mark Alex-Sanders as 'Santonian calcareous algae from the subsurface Ilam Formation, southwestern Iran' that has, for reasons unknown to us, most likely confidential reasons, never been published (schedulded for Journal of Micropaleontology; see 'Friends of Algae Newsletter' no. 12, 2003, paleopolis.rediris.es/FotAN/fotan-12.html#Alex).Our contribution therefore represents the first taxonomic work on the dasycladalean green algae of the Ilam Formation of southwestern Iran.It includes data from one offshore well and two onshore sections (Fig. 1A-B).The discussion of the stratigraphic relevance includes in addition the two benthic foraminifera Reticulinella? kaeveri Cherchi et al. and Orbitoides mid-orientalis (Eames & Smout).

GeoloGIcal SettInG
In the Late Cretaceous (Cenomanian-Santonian), the Persian Gulf formed part of the northeastern edge of the Arabian Plate positioned near the equator (Sharland et al. 2001;Stampfli and Borel 2002;Barrier & Vrielynck 2008;Barrier et al. 2018).Towards the end of the Cenomanian, a significant shift in tectonic setting occurred, with the transition from a passive to an active tectonic margin (e.g., Mehrabi et al. 2015a, b, c;Bromhead et al. 2022).This transformation was accompanied by the subduction of the Neo-Tethys oceanic plate beneath Central Iran (e.g., Alavi 2004;Moghadam et al. 2020;Moghadam & Stern 2021;Navidtalab et al. 2024).
In the Zagros Basin and the Persian Gulf, the interaction between porous and permeable reservoir units and various prolific source rocks played a pivotal role in the accumulation of substantial hydrocarbon reserves within the Cretaceous strata (e.g., Mehrabi et al. 2023).Additionally, the presence of numerous tectonic structures, including anticlines, salt domes, and diapirs, associated with effective cap rocks, created favorable conditions for the entrapment of hydrocarbons (Ghazban 2007;Mehrabi et al. 2015bMehrabi et al. , c, 2018)).The oilfields related to salt formations in the eastern Persian Gulf exhibit distinctive characteristics, such as (a) dome-shaped structures, (b) independent closures, and (c) parallel faults within the structures (Kabirzadeh et al. 2020;Orang and Gharabeigli 2020).
At a large scale, the Cretaceous strata are subdivided into two tectonostratigraphic mega-sequences known as AP8 and AP9 (Sharland et al. 2001).The complete Albian to Turonian interval of the Arabian Plate falls within the AP8 mega-sequence, delimited by maximum flooding surfaces of K140-K90 (Shar-land et al. 2001;Davies et al. 2002).The upper segment of the AP8 mega-sequence is encompassed by the Wasia Group (Ehrenberg et al. 2008;Alsharhan, 2014).Notably, the termination of this megasequence is marked by a significant mid-Turonian disconformity, representing the boundary between the Wasia and Aruma groups (Rahimpour-Bonab et al. 2013;Mehrabi et al. 2022a, b).This refers to the deposits enclosing the K 150 SB of the Arabian Plate (e.g., Sharland et al. 2001;Van Buchem et al. 2011;Bromhead et al. 2022;Schlagintweit et al. 2023).Along the paleo-platform margin, the Albian to Turonian interval is characterized by the presence of shallow-water carbonates (van Buchem et al. 1996(van Buchem et al. , 2002;;Mehrabi et al. 2020).Within the Persian Gulf, these sequences encompass the Mauddud, Khatiyah, and Mishrif formations, collectively referred to as the Sarvak Formation in Iran and representing the upper segment of the Wasia Group.In the Coniacian-Santonian interval, the sequences comprise the Laffan and Ilam (Halul) formations, which are attributed to the lower part of the Aruma Group (Fig. 1C).

Offshore (core data)
The present study is based on the core data from a well drilled in a hydrocarbon field (S-Field) in the eastern part of the Persian Gulf.It belongs to the material and thin-sections studied by Mehrabi et al. (2023) but here with focus on the occurrence of dasycladalean algae.In the examined area, the Ilam Formation has a thickness of ~113 meters and is characterized by white to tan microcrystalline limestones, which are partly chalky, fossiliferous, and contain pyrite and cherts.Its lower boundary is identified at the top of the shale facies of the Laffan Formation (Fig. 1B, 2).

Khormuj section
The studied material refers to thin-sections of the Ilam Formation from the Khormuj section exposed in the northeast of Khormuj city, located in Coastal Fars where the Ilam Formation attains a thickness of about 130 m (Fig. 3).This locality with the coordinates N28 o 39'21'' latitude and E51 o 24'14'' and longitude has been studied by various researchers (Vincent et al. 2015;Schlagintweit et al. 2023;Xu et al. 2023;Hosseini et al. 2024).

Farashband section
The outcrop of the Ilam Formation is situated about 92 km south of Shiraz close to the city of Farashband, Coastal Fars, named here as Farashband section.It is located along the road from Farashband to Firozabad.The Ilam Formation of this locality has been described by Hoseininezhad & Noroozpour (2018).The geographic coordinates for the section base are 28°48'13.09"N,52°10'29.74"E.
Here, just an overview sampling (sample prefix IFL) has been conducted, no log been prepared and no concrete age can be assigned.The material illustrated can be assigned to the assemblage zone 31 of Wynd (1965).

Core data
Altogether 249 thin-sections have been prepared from the carbonate rocks of the Ilam Formation and studied by Mehrabi et al. (2023) confidentiality reasons no depths/sample numbers are indicated, instead the distribution of the taxa is referring to the lower and middle-upper part of the Ilam Formation, respectively assemblage zones no.30 and 31 of Wynd (1965).

Khormuj Section
The studied thin-sections of the Khormuj section labeled DMS-, are stored in the slide collection of the National Iranian Oil Company Exploration Directorate, Tehran.

Farashband section
The studied thin-sections of the Farashband section labeled IFL-are stored in the collection of the Department of Geology, Yazd University (collection K. Rashidi).
According to Reitner (1987, fig. 110, and p. 210), T. tripolitanus is restricted to the reef flat areas, platform sands (shoals) and the open lagoon of the Albian-Cenomanian carbonate platforms of northern Spain.It ranges up to the Santonian-Campanian boundary (Barattolo 2002, tab.2) witnessing a reduced biostratigraphic relevance.T. tripolitanus ocuurs in the basal as well as the topmost parts of the Ilam Formation (= assemblage zones no.30 and 31 of Wynd 1965).In the former it may be associated with Salpingoporella ubaiydhi Radoičić (Fig. 5A-B).
Dimensions (in mm, except d/D and w).Remarks.This taxon is distinctly smaller then T. tripolitanus, usually exhibits a higher d/D ratio and has just two orders of laterals.Whereas in the former they are elongated-clubshaped (Fig. 4I), they are subspherical in D. ondulata (Fig. 4A).Both species co-occur in the North-African type-locality (Lybia) that has lead to some taxonomic dissent (Pia 1936;Radoičić et al. 2005;Granier et  Remarks.Radoičić (1997) described the species from subsurface strata of the Western Iraqi Desert in different strata ascribed to the Santonian, Campanian and early Maastrichtian.As the partly associated Orbitoides gensacicus (Leymerie, 1851) is an upper Maastrichtian species (Caus et al. 1996;Özcan et al. 2021), the range of S. ubaiydhi is here extended.Among the different Salpingoporella species (see Carras et al. 2006) it is distinct by having laterals arranged in linear lines along the thallus and by displaying laterals with rounded ends thereby not touching neighbouring laterals.
Family Polyphysaceae Kützing, 1843 Genus Clypeina (Michelin, 1845) Clypeina cf.dusanbrstinai Radoičić, 1997 Remarks.This species usually occurs as isolated verticil discs (e.g., Fig. 7A, D-F) or even individual branches dispersed in the micritic matrix accounting for some specific uncertainties (Fig. 7B,  D).This might be the first record of the species since its description from Serbia by Radoičić (1997).The type strata have been assigned to the Santonian, but a Coniacian age might also be possible by the occurrence of Accordiella conica Farinacci, 1962 (see Frijia et al., 2015).In the Ilam Formation C. cf.dusanbristinai has been observed in the core data and the Khormuj section ?Family Polyphysaceae Gen. et sp.indet.Remarks.One oblique transverse sections of a comparably large dasycladale (D = ~4.0mm, d = ~1.6 mm, w = ~20 to ~24) of unknown generic status (Praturlonella?, Falsolikanella?, Milanovicella?, Clypeina?).It shows similarities to equivalent sec-tions of Praturlonella jordanica illustrated by Kuss and Conrad (1991, fig. 3.5-3.6)from the Cenomanian of Jordan.The distinctly larger size and the lack of further specimens (e.g.longitudinal sections) however account for the uncertain status of the find from Iran.The individual section has been detected in the upper part of the Ilam Formation (Santonian) from the core in the Persian Gulf and refers to the assemblage zone no. 31 of Wynd (1965).

dIScuSSIon on StratIGraPhy
Generally (with some exceptions), the larger benthic foraminifera experience a much better biostratigraphic resolution.Within the Zagros stratigraphic successions, an often-used local biozonation scheme is that of Wynd (1965), and James & Wynd (1965).This zonation corresponds to facies characteristic microfossil assemblages and does therefore not refer to a classical biozonation (Schlagintweit & Simmons 2022).Nonetheless, it has been proven useful in the correlation and recognising stratigraphic intervals.The 'biozones' are numbered consecutively from older to younger strata.Mostly

right).
Therefore, it appears reasonable to assume a late Coniacian age for the base of the Ilam Formation.On the other hand, we must also admit that this conclusion is only provisional lacking an accurate verification of correlation of the Spanish and Iranian biostratigraphic data.In addition, the Coniacian-Santonian boundary is difficult to assess in neritic carbonates based on larger benthic foraminifera.
In recent times, also the inclusion of upper Turonian strata for the Ilam Formation has been suggested based on the occurrence of the larger benthic foraminifer Reticulinella? kaeveri Cherchi et al. (Hosseini et al. 2024).This species has been found primarily in the basal parts of the Khormuj section and the core section, but occasionally also in the middle part.The further discussion focuses on the eponymous taxa of 'biozone 31', Archaecyclus mid-orientalis Eames & Smout and R.? kaeveri.

Orbitoides mid-orientalis (Eames & Smout, 1956)
This species was first described by Eames & Smout (1956, p. 505) as Archaecyclus mid-orientalis from 'limestones of Campanian age in the Kuwait Oil Company's Umm Gudair Well 1'.This age assignment was due to its occurrence 'below beds of Maestrichtian age with Omphalocyclus'.Eames & Smout (1956) noted structural affinities to Orbitoides Douvillé, and it has subsequently been accepted by several workers that it should be treated as a primitive species of this genus displaying a distinct trochospirally coiled early stage ( Van Gorsel 1978;Albrich et al. 2014) (Fig. 8C-K).Whether or not this stage is strictly bi-  2008).In any case, O. mid-orientalis is here considered a valid species with a well-developed coiled stage and interio-marginal foramina differing from the other primitive species.O. hottingeri Van Hinte has a distinctly reduced early coil and orbitoidal foramina (= on both sides of the chamber) already in the third chamber (Caus et al. 1996;Albrich et al. 2014).Van Gorsel (1978, p. 37) envisaged a possible synonymy of O. mid-orientalis (Eames & Smout, 1956) with O. hottigeri Van Hinte, 1966.These early primitive forms of Orbitoides originated in the late Santonian (e.g., Van Gorsel Tab. 2 -Microfauna and microflora of upper Barremian-Maastrichtian shallow-water carbonates from southwestern Iran occurring in the assemblage zones of Wynd (1965) (see also the appendix).
1978; Caus et al. 1996) or even around the earlymiddle Santonian boundary (Boix et al. 2009; Fig. 7).In the Khormuj section, O. mid-orientalis occurs around 17 m above the last occurrence of R.? kaeveri (Fig. 3) and, as such, provides further evidence that the Turonian age for the Ilam Formation suggested by Hosseini et (2024) has to be rejected.

Reticulinella? kaeveri Cherchi, Radoičić & Schroeder, 1989
Based on data deriving from Apenninic-Dinaridic-Hellenidic mountains chains, R.? kaeveri has been considered an intra-Turonian marker taxon (Cherchi et al. 1989;Frijia & Parente 2008;Frijia et al. 2015;Schlagintweit et al. 2023).Together with Mangashtia viennoti Henson, it has been recorded recently from the Sarvak Formation of the Khormuj section of southwestern Iran in time-equivalent strata between the two subaerial exposure surfaces (Schlagintweit et al. 2023) that were ascribed to the early Turonian and the late Turonian-early Coniacian respectively.The supposedly lower Coniacian paleosols associated with the upper exposure surface can be considered as time-equivalent to the Laffan Formation (Fig. 1B).Hosseini et al. (2024) recorded the occurrence of R.? kaeveri in the basal parts of the Ilam Formation from the Khormuj section that is confirmed by our restudy.Based on the published data from the peri-Mediterranean realm (see above), Hosseini et al. (2024, p. 21) concluded that the 'lower part of the Ilam Formation is upper middle to upper Turonian' meaning a 'significant improvement in the chronostratigraphic calibration of sequence stratigraphy in the Cretaceous of the Zagros fold-and-thrust belt'.The statement of Hosseini et al. (2024, p. 21) that 'Reticulinella?kaeveri has a wider range than previously suggested' was therefore not referring to chronostratigraphic range but a wider distribution than just the upper Sarvak Formation.The lack of Mangashtia viennoti Henson, 1948 the typical companion of R.? kaeveri in the upper Sarvak Formation, in the Ilam Formation including the Khormuj section and several others (unpublished data), is worth mentioning here.The record of M. viennoti in the Ilam Formation of the Persian Gulf core data (Mehrabi et al. 2023, fig. 11O) is in fact to be referred to another species (not belonging to the genus Mangashtia Henson, 1948) that will be described by one of us (F.S.) from the Upper Cretaceous of Tibet.
At first, one could think that the reticulinellids observed in the Ilam Formation in fact belong to Reticulinella fleuryi Cvetko, Gušić & Schroeder, 1997 et al., a species that is known from the middle Coniacian to late Maastrichtian (Schlagintweit & Sanders 2008;Schlagintweit et al. 2024) (Fig. 9L-U).A comparison with type-material from the Campanian of the island of Brač shows that R.? kaeveri has a strictly globular morphology, may attain larger dimensions and more solid (thicker) partitions.In R. fleuryi, the wall and internal structures are more delicate and the morphology is different.In fact, a tendency to uncoiling is observable along with an increase of height and width of the chambers during ontogeny resulting in peneropliform morphologies (Fig. 9M-N).These apparently larger sized specimens might be referable to microspheric specimens.But also, the undoubted megalospheric forms show a slight tendency to uncoil (Fig. 9L, 9Q).In addition, axial sections are slightly compressed (Fig. 9T; height diameter ratio < 1) and the radial partitions are more close-set.For further differences see Cvetko et al. (1997).The original descriptions of R. kaeveri (Turonian strata) and R. fleuryi (Campanian strata) indicate similar number of whorls (up to three) and equivalent dimensions (up to 0.47 mm and up to 0.46 mm).The specimens of R.? kaeveri from the Ilam Formation may show larger dimensions (up to 0.96 mm, Fig. 9E) and a greater number of whorls (up to five) that are here considered as just an infraspecific evolution.Accepting that the specimens observed in the basal Ilam Formation belong to R.? kaeveri we come to a conclusion different from Hosseini et al. (2024), namely the extension of the stratigraphic range into the Coniacian -early Santonian as deduced from the distribution of Orbitoides mid-orientalis as discussed previously.In summary, the cited intra-Turonian range of R.? kaeveri was based on limited data from the Mediterranean region.In the Iranian Zagros, it may well be that it has a longer range (i.e., into the Coniacian and/or Santonian), rather than being a biostratigraphic tool to assign a Turonian age.This is supported by other fossil occurrences in the Ilam Formation.

concluSIonS
Micropalaeontological studies have been carried out on the poorly known assemblages of Dasycladalean green algae in the Upper Cretaceous Ilam Formation of southwestern Iran comprising subsurface and outcrop data.The microflora consists of just a few Late Cretaceous taxa that are not suited for precise biostratigraphic dating.The larger benthic foraminifera are much better suited for providing age informations.The recently proclaimed late Turonian age for the basal part of the Ilam Formation can be excluded; instead a late Coniacian age is here suggested.A marker taxon for fixing the Coniacian-Santonian boundary however appears not present.An update for the taxonomic inventory (dasycladales and larger benthic foraminifera) of the p.p. Lower and Upper Cretaceous formations cropping out in the Zagros zone and widely used in the zonation of Wynd (1965) is provided.

Fig. 6 -
Fig. 6 -Palaeogeographic base map for the Santonian in the wider area of the Arabian plate showing distribution of Salpingoporella ubaiydhi Radoičić based on the synonymy list provided in the systematic part (map provided courtesy of Halliburton from the Neftex product suite).