The Cenomanian heterozoan carbonates in the north-central Alborz, north-east Kelardasht, north Iran

The Cenomanian heterozoan car bon ates in the north-cen tral Alborz, north-east Kelardasht, north Iran

Mohammad Ali KAVOOSI

* and Neda EZOJI

1 Ex plo ra tion Di rec tor ate of Na tional Ira nian Oil Com pany (NIOCEXP), Khodami Street, NE Sheikh Bahaei Square, PO Box 19395-6669, Teh ran, Iran

2 Min is try of Ed u ca tion, School of Pro fes sor Hessabi, Sadeghieyeh Square, Teh ran, Iran

Kavoosi, M.A., Ezoji, N., 2021. The Cenomanian heterozoan car bon ates in the north-cen tral Alborz, north-east Kelardasht, north Iran. Geo log i cal Quar terly, 2021, 65: 37, doi: 10.7306/gq.1606

As so ci ate Ed i tor: Michał Zatoń

De tailed field sur veys, petrographic in ves ti ga tion and SEM and EDS anal y ses have been used to eval u ate Cenomanian glauconitic heterozoan car bon ates in north-east Kelardasht, north-cen tral Alborz, north Iran. Lithofacies and microfacies anal y ses led to rec og ni tion of six microfacies types re lated to the in ner-, mid- and outer-ramp fa cies belts of a car bon ate ramp. The heterozoan na ture of these car bon ates is in ferred from a pre dom i nance of echinoderms as so ci ated with calcispheres, plank tonic foraminifers, a lack of ooid grains, and a low car bon ate pro duc tion rate, to gether with a high con tent of glauconite grains and pre vail ing high-Mg cal cite min er al ogy. Petrographic and SEM stud ies re veal that glauconite fill ing skel e tal grains re tains the shape and mor phol ogy of host grains, sig ni fy ing an authigenic or i gin at low sed i men ta tion rates and slightly re duc ing con di tions. SEM im ages show cau li flower and ro sette struc tures as so ci ated with well-de vel oped lamellae in di cat ing an authigenic or i gin of evolved glauconite grains. Our find ings are com pat i ble with a nu tri ent-rich wa ters and palaeo eco logi cal stress re lated to rel a tive sea-level rise and eutrophic con di tions, which con trib uted to the gen er a tion of these heterozoan car bon ates de spite the hot green house con di tions dur ing the Cenomanian in the north-cen tral Alborz Moun tains.

Key words: heterozoan car bon ates, authigenic glauconite, Cenomanian, north-east Kelardasht, Iran.

INTRODUCTION

The Cenomanian (93.9–100.5 Ma) was a time in ter val as - so ci ated with rel a tive sea-level rise and green house con di tions (e.g., Huber et al., 2018), which led to the drown ing of car bon - ate plat forms and de po si tion of glauconitic sed i ments re lated to Oce anic Anoxic Event 2 (OAE 2, e.g., Rudmin et al., 2017;

Hairapetian et al., 2018).

Cenomanian glauconitic de pos its have been re ported from the Cen tral Iran Ba sin in the Esfahan area (Ken nedy et al., 1979; Hairapetian et al., 2018), the Kopeh Dagh Ba sin in north-east Iran (Jafarzadeh et al., 2020), in Devon, south-west Eng land (e.g., Car son and Crowley, 1993), in the Cre ta ceous de pos its of south-east Spain (Jimenez-Millan et al., 1998), and in the Lower Cenomanian Bahariya For ma tion in the West ern Desert of Egypt (Baioumy et al., 2021). Glauconite has been re -

ported from Cre ta ceous ma rine de pos its of the Tethyan belt (e.g., Bansal et al., 2020; Jafarzadeh et al., 2020).

The heterozoan car bon ates (James, 1997) com prise an as - so ci a tion of light-in de pend ent, sus pen sion-feed ing, and hetero - trophic fauna in clud ing echinoderms, calcispheres, plank tonic foraminifers, bryo zoans and pelecypods (foramol as so ci a tion).

Fac tors re spon si ble for the oc cur rence of heterozoan car bon - ates are cli mate, geotectonic set ting, tem per a ture, sa lin ity, wa - ter depth, trophic con di tions, ox y gen level, CO2 con cen tra tions, Mg/Ca ra tio of sea-wa ter, al ka lin ity, mor phol ogy and bathy - metry of the sea-floor, sub strate, and trans par ency of the sea wa ter (Tucker and Wright, 1990; Pomar et al., 2005; Westphal et al., 2010). Cli mate and geotectonic set ting con trol tem per a - ture, wa ter cir cu la tion pat terns, tur bu lence, as well as nu tri ent sup ply (Tucker and Wright, 1990).

The Cenomanian lithostratigraphic unit of the Kelardasht and Chalus coun ties (Figs. 1–3) is marked by thin- to me - dium-bed ded glauconitic lime stones com pris ing heterozoan car bon ates com posed mainly of echinoderms, red al gae, bryo - zoans, plank tonic foraminifers, glauconite, as so ci ated with a few sparse phos phate grains and sub or di nate quartz grains.

Silt-to sand-sized quartz and ba saltic lithoclasts within the suc - ces sion in di cate weath er ing of nearby lands and in flux of siliciclastic sed i ment into the depositional set ting.

* Corresponding author, e-mail:

kavoosi.mohammadali47@gmail.com

Received: February 1, 2021; accepted: June 14, 2021; first published online: 19 August, 2021

Pe lagic fa cies com pris ing glauconite grains among other in - di ca tors may sug gest de po si tion ac com pa nied by a sud den sea-level rise and an in cre ment of nu tri ent re lated to upwelling cur rents (e.g., Yilmaz et al., 2018). Rel a tive sea-level rise to - gether with a low sed i men ta tion rate pro vide suit able con di tions for glauconite to form (e.g., Nichols, 2009; Rudmin et al., 2017).

The study was car ried out in the north-cen tral Alborz Moun - tain range, in the north-east Kelardasht county, west of the Mazandaran Prov ince, north Iran (Figs. 1 and 2). To date, microfacies anal y sis and depositional en vi ron ments of the Ceno manian suc ces sion in the study area (Figs. 1 and 2) has re mained poorly known ex cept for Ezoji (2002) and Abbaspour - -Tehrani (2014).

Microfacies anal y sis was un der taken to in ter pret the Ceno - manian heterozoan car bon ates in the north-cen tral Alborz. An un der stand ing of the fac tors con trol ling the car bon ate con tri bu - tion and pro duc tion would be use ful to eval u ate the in ter ac tion of the depositional set ting and tec tonic, cli mate, and sea-level changes (Kavoosi and Ezoji, 2018). The study ex am ines the fac tors which con trolled the tem po ral and spa tial dis tri bu tion of these glauconitic lime stones dur ing the Cenomanian.

GEOLOGICAL SETTING

The Alborz is a 1500 km long moun tain range and an ac tive fold-thrust belt across north ern Iran, ex tend ing from the Lesser Cau ca sus of Ar me nia in the north-west to the Paropamisus Moun tains of North ern Af ghan i stan, flank ing in its cen tral part the south ern coast of the Cas pian Sea (Alavi, 1996; Allen et al., 2003; Guest et al., 2007; Zanchi et al., 2009; Yassaghi and Naeimi, 2010; Kangi et al., 2010), still doc u ment ing seis mic ac -

tiv ity (Mattei et al., 2017). The cen tral Alborz range un der went extensional move ments re lated to dis tal ef fects of the Cim mer - ian Orog eny in the South Cas pian back-arc ba sin (Fürsich et al., 2005, 2009a, b; Berra et al., 2007; Zanchi et al., 2009; Egan et al., 2009).

The var ied geo log i cal his tory of Alborz is re corded by coarse- grained siliciclastics and shal low-plat form to basinal car bon ate de pos its in ter rupted by al kali-ba salt to andesitic vol - ca nic rocks (Fig. 2), as well as by non-depositional pe ri ods to - gether with ex hu ma tion phases ac com pa nied by fold and thrust ac tiv i ties.

The Mid-Cim mer ian event re flects a break-up un con formity and an over all deep en ing-up wards trend re lated to extensional move ments of the South Cas pian Ba sin dur ing the Mid dle-Late Ju ras sic to the Late Cre ta ceous (Fürsich et al., 2009a; Taheri et al., 2009; Kavoosi and Ezoji, 2018). Extensional move ment is in ferred from the ba sin evo lu tion and mafic vol ca nism dur ing the Late Ju ras sic, Early and mid-Cre ta ceous in the study area (Figs. 2 and 3). The vol ca nic rocks (al kali-ba salt, an de site, and pil low lava) most likely in di cate a back-arc po si tion and early Cenomanian extensional move ments in the north-cen tral Alborz (Egan et al., 2009; Wilmsen et al., 2009).

The low er most suc ces sion of the Cre ta ceous has is cut by nor mal fault sys tems with an E–W to WNW–ESE trend as so ci - ated with magmatism re lated to the N–S to NNE–SSW trending extensional sys tems ac com pa nied by a re gional un con formity in north ern Iran (Shahidi et al., 2011).

The Cre ta ceous-Paleocene un con formity can be at trib uted to be gin ning of the fold- and thrust-belt ac tiv ity re lated to the Lara mide Orogenic phase, which af fected the south ern mar gin of the South Cas pian Ba sin (Egan et al., 2009; Shahidi et al., 2011; Kavoosi and Ezoji, 2018).

Lake

Fig. 1. Lo ca tion map of the ex posed sec tions stud ied, lo cated in the north-cen tral Alborz Moun tain Range, in the Chalus and Kelardasht coun ties, N Iran

MATERIAL AND METHODS

The cur rent study of the Cenomanian car bon ates in the north-cen tral Alborz (Figs. 2 and 3) is based on in te grated de - tailed field sur veys, microfacies anal y sis and scan ning elec - tronic mi cros copy (SEM) with en ergy dispersive spec tros copy (EDS). Three ex po sure sec tions, at Pol-e Zoghal, Petilingeh and Chalajur were logged and sam pled, lo cate north-east of Kelardasht and south of Chalus (Fig. 1). Lithostratigraphic log -

ging and sedimentological anal y sis were con ducted on very thin- to me dium-bed ded glauconitic lime stones (Fig. 4). Litho - facies were de fined based on bed ding pat terns and sed i men - tary tex tures to gether with skel e tal grains and non-car bon ate com po nents. The true thick ness of the sec tions was mea sured with a Jacobs Staff.

Microfacies anal y sis was per formed on 65 thin-sec tions to com ple ment the field ob ser va tions and to de ter mine depositio - nal fa cies. Dun ham’s no men cla ture (1962) was used for the microfacies clas si fi ca tion. The grains and ma trix per cent ages Fig. 2. Mod i fied geo log i cal map (1:100,000) of the Chalus re gion (Vahdati Daneshmand et al., 2001),

west ern Mazandaran Prov ince; Geo log i cal Sur vey of Iran, Teh ran

Three ex po sures, at Pol-e Zoghal, Petilingeh, and Chalajur, are lo cated on the north ern and south ern flanks of the Valasht Syncline; for in ter pre ta tion of the ref er ences to col our in this fig ure legend

were es ti mated us ing vi sual per cent age charts (Flügel, 2010).

Microfacies anal y sis was per formed us ing stan dard microfacies de scrip tions and the pro posed mod els of Wil son (1997) and Flügel (2010). The pre fixes “glauconitic” and “sandy” were added to the names of microfacies with a pro por tion of >10%

glauconite grains, quartz and extraclasts, re spec tively. In ad di - tion, char ac ter is tic fab rics and main com po nents were taken into ac count in the clas si fi ca tion of microfacies.

Microfacies types and their depositional en vi ron ments were de ter mined on the ba sis of field and petrographic stud - ies, lat eral and ver ti cal microfacies changes, and com par i son with an cient and re cent depositional en vi ron ments (e.g., Purser, 1973; Tucker and Wright, 1990; Flügel, 2010). The biostratigraphic frame work of the suc ces sions in ves ti gated is mainly based on plank tonic foraminifers iden ti fied in the thin- sec tions (Fig. 5).

The SEM and EDS anal y ses were per formed at the Razi Met al lur gic Re search Cen tre (RMRC) in Teh ran. SEM was used to pro vide de tailed high-res o lu tion im ages of se lected glauconite grains by rastering a fo cused elec tron beam across the sur face.

The EDS was car ried out to better un der stand the for ma tion and chem i cal char ac ter iza tion of the glauconite grains.

RESULTS

LITHOSTRATIGRAPHY

The lithostratigraphic unit en coun tered com prises Ceno - manian glauconitic car bon ates (Figs. 3 and 4). The for ma tion is ex posed in the Kelardasht and Chalus coun ties (Fig. 2). No for - Fig. 3. The newly pro posed strati graphic chart of the north-cen tral Alborz Moun tain Range

This strati graphic chart is con structed based on com pi la tion of pre vi ous work (Cartier, 1971;

Ezoji, 2002; Rob ert et al., 2014; Kavoosi and Ezoji, 2018), to gether with the Chalus geo log i cal map, field ob ser va tions and biostratigraphic dat ing

mal lithostratigraphic units have yet been pro posed for the Up - per Cre ta ceous suc ces sion of north-cen tral Alborz (Fig. 3).

Based on the In ter na tional Strati graphic Guide (Sal va dor, 1994), we de fine the Chalajur For ma tion for the rock unit for - merly named the Cenomanian glauconitic lime stone (Cartier, 1971). The type lo cal ity/type sec tion of the Chalajur For ma tion is lo cated at the north ern end of Chalajur vil lage, com pris ing 55 m of dom i nantly green ish-grey to grey, fine-grained, thin- to me dium-bed ded lime stone (Figs. 3 and 4).

The Chalajur For ma tion is nonconformably un der lain by vol ca nic (al kali bas alts) and volcanoclastic rocks of Mem ber 5 of the Chalus For ma tion (Fig. 4A), and disconformably over lain by the slightly weath ered Campanian-Maastrichtian ar gil la - ceous lime stone and marl (Figs. 3 and 4B, D).

The Chalajur For ma tion con sists of well-bed ded glauconitic lime stone with sub or di nate marl interbeds in dis tinct cy clic pack ages (Fig. 4C, D). The bed ding thick ness ranges from a few to twenty centi metres. One char ac ter is tic of the cy cles is gradational changes in thick ness and lithofacies of beds and bed sets (Fig. 4C, D) in the study area (Fig. 1). Glauconite grains are scat tered through out the for ma tion. The glauconite im parts an ol ive green to green ish grey col our to the Chalajur For ma tion (Fig. 4A–C).

BIOSTRATIGRAPHY

The biostratigraphic as sign ment and chronostratigraphic frame work are based on a de tailed micropalaeontological in - ves ti ga tion of plank tonic foraminifers in thin-sec tion. The Chala - jur For ma tion, poorly doc u mented from a biostratigraphic point of view (Ezoji, 2002), has been stud ied us ing ben thic and plank - tonic foraminifers. Re sults based on ex am i na tion of thin-sec - tions led to rec og ni tion of sev eral plank tonic foraminifers, calci - spheres, and heterohelcids (Fig. 5).

Iden ti fi ca tion of taxa is ham pered by wide spread recrystalli - zation and neomorphism. For age de ter mi na tion, plank tonic foraminifers were stud ied us ing the clas si fi ca tion schemes of Caron and Premoli Silva (2007) and Petrizzo et al. (2015).

The plank tonic foraminifers iden ti fied in clude Murico hed - ber gella rischi (Fig. 5A, B), Thalmanninella appeninica (Fig.

5C), Heterohelix reussi (Fig. 5D), Globotruncana lapparenti (Fig. 5E, F), Pithonella trejoi (Fig. 5H, I), and Whiteinella paradubia (Fig. 5G), as so ci ated with Thalmanninella micheli, Pithonella ovalis, Stomiosphaera sphaerica.

Stratigraphically, Thalmanninella brotzeni oc curs slightly ear lier than Thalmanninella globotruncanides (Caron and Premoli- Silva, 2007). We as signed the Cenomanian age based Fig. 4. Field photo of the Cenomanian Chalajur For ma tion

A – field photo of the Cenomanian Chalajur For ma tion that shows its lower non con for mity with the bas alts at the Chalajur sec tion; B – a pan orama of the Chalajur For ma tion and its up per con tact with the Campanian marl and ar gil la ceous lime stone; C – close-up view of glauconitic lime stones and interbedded marl; the red num bers are lo ca tions of sam ples taken for microfacies anal y sis; D – close-up view of glauconitic lime stones near the top of the Chalajur For ma tion and the up per con tact with the Campanian ar gil la ceous lime stone; note an over all thick - en ing to wards the up per part of the for ma tion

Fig. 5. Pho to mi cro graphs of some in dex microfossils of the Chalajur For ma tion of Cenomanian age The iplanktonic foraminifers iden ti fied in clude: A, B – Muricohedbergella rischi as so ci ated with glauconite grains and py rite,

C – Thalmanninella appeninica; D – Heterohelix reussi; E, F – Globotruncana laparenti, G – Whiteinella paradubia; H, I – Pithonella trejoi; scale bars val ues in mm

on the strati graphic ranges of microfossils such as Thalman - ninella globotruncanoides and Thalmanninella brotzeni of the Taxon Zone of Caron and Premoli-Silva (2007). Biostrati gra - phic in dex microfossils are rel a tively rare as re gards rec og niz - ing the Cenomanian zones in the study area.

MICROFACIES

In te grated field sur veys and petrographic in ves ti ga tions of the Cenomanian Chalajur For ma tion led to de scrip tion and in - ter pre ta tion of six microfacies types (Ta ble 1), that com prise dom i nantly echinoderms (Figs. 6 and 7). Authigenic min er als in clude glauconite, py rite (Fig. 5A, B), and sparse phos phate grains (Fig. 6C). Plank tonic and small ben thic foraminifers (Figs. 6F and 7D) and calcispheres (Fig. 7C), to gether with red al gae, bryo zoans, and pelecypods (Figs. 6 and 7), are sub sid - iary con stit u ents. The only non-skel e tal grains com prise intra - clasts (Fig. 6D); quartz grains and ba saltic and andesitic rock frag ments are among extraclasts in the silt- to very fine-grained sand-size frac tion (Fig. 7B). Ma jor diagenetic pro cesses and prod ucts in clude neomorphism (Fig. 5A–D), sili ci fi ca tion (Fig.

7A), glauconitization (Fig. 7B) and recrystallization. Sed i men - tary fab rics and struc tures in clude bioturbation (Fig. 7D), mi cro - scopic ero sional sur faces and faint nor mal grad ing (Fig. 7E, F).

Petrographic in ves ti ga tions re veal that some glauconite grains are skel e tal-fill ing grains dom i nantly in echinoderms and red al - gae par ti cles. The gen eral char ac ter is tics of the microfacies, num bered 1–6, are sum ma rized in Ta ble 1.

MORPHOLOGY AND COMPOSITION OF GLAUCONITE GRAINS

Mi cro scopic in ves ti ga tion of glauconite grains shows a wide range of yel low ish brown, brown, pale green to dark green colours. Scan ning elec tron mi cro scope (SEM) anal y sis as well as pho to mi cro graphs of glauconite grains show typ i cal cau li - flower (Fig. 8A) and ro sette struc tures (Fig. 8B), as well as well-de vel oped lamellae in the glauconite grains (Fig. 8C, D).

Grain sur faces show ro sette mor phol ogy of the crys tal habit of a glauconite mixed-layer min eral (Fig. 8B).

EDX anal y sis on four sam ples and twelve glauconite grains in the lower to mid dle parts of the Chalajur For ma tion at the Chalajur sec tion show that O2, Mg, Al, Si, Ca, Fe, and K are the ma jor con stit u ents (Ta ble 2 and Fig. 9). EDS anal y sis sug gests two ranges of K-con tent in glauconite grains (Ta ble 2, Figs. 8 and 9B–B2), which var ies from 3.15 to 6.94 wt.%. Based on EDS anal y sis, there is a di rect re la tion ship be tween K2O and Fe2O3 of glauconite grains (Ta ble 2 and Fig. 9).

DISCUSSION

DEPOSITIONAL ENVIRONMENTS AND DEPOSITIONAL MODEL

Eustatic sea-level changes and car bon ate pro duc tion rate seem to have played the main con trol ling fac tors in the ver ti cal

fa cies changes (Moosavizadeh et al., 2015); mean while, tec - ton ics strongly de ter mined the mor phol ogy of the car bon ate plat forms, and in flu enced the spa tial fa cies changes as well as thick ness vari a tions. Sed i ments ac cu mu lat ing un der tec toni - cally rel a tively qui es cent con di tions show more the in flu ence of cli mate, depositional en vi ron ments, and diagenesis (Nel son and Hume, 1987; Ando et al., 2015), while depositional ge om e - try is di rectly de pend ent on the car bon ate pro duc tion rate and changes in the car bon ate fac tory (e.g., Me rino-Tomé et al., 2012).

The data ob tained from microfacies anal y sis and the pro - por tion of car bon ate to non-car bon ate par ti cles sug gest tec - tonic qui es cence dur ing de po si tion. Based on mod est vari a - tions in thick ness (10 m) and lat er ally fa cies dis tri bu tion (Fig.

10), some dif fer en tial tec tonic sub si dence was likely dur ing de - po si tion. Ac cord ingly, we sug gest the Cenomanian Chalajur For ma tion of the north-east Kelardasht and Chalus coun ties (Fig. 1) was de pos ited dur ing a rel a tively sta ble tec tonic re gime.

We fol lowed the microfacies and fa cies belts from one ex - po sure to an other with a high de gree of con fi dence. Six micro - facies types were iden ti fied, us ing the stan dard microfacies of the Wil son (1997) and Flügel (2010) (Ta ble 1). Char ac ter iza tion of the microfacies and fau nal suc ces sion sug gest de po si tion on a car bon ate ramp as de scribed by Burchette and Wright (1992), com pris ing in ner-, mid- and outer-ramp fa cies belts (Ta ble 1) (Figs. 10 and 11).

The main com po nents of the outer-ramp fa cies belt com - prise poorly sorted bioclastic packstone microfacies in clud ing echinoderms ac com pa nied by calcispheres and plank tonic fora minifers, to gether with glauconite grains and py rite (Fig. 5A, B), sug gest ing de po si tion in calm and slightly re duc ing con di - tions in a low-en ergy depositional en vi ron ment be low storm wave-base (e.g., Tucker and Wright, 1990; Flügel, 2010; Zeller et al., 2015; Rikhtegarzadeh et al., 2016).

The lo cal oc cur rence of quartz and silt-sized to very fine- grained sand al kali-ba salt and/or an de site extraclasts within the suc ces sion stud ied points to an in flux of siliciclastic de bris un - der warm and hu mid cli ma tic con di tions; to gether with the gen - eral tec tonic sta bil ity, this led to mafic vol ca nic land mass un der - go ing vary ing de grees of chem i cal weath er ing in a Cenomanian green house palaeoclimate.

The con sis tent pres ence of grain-dom i nated microfacies with a pre dom i nance of echinoderm skel e tons as so ci ated with plank tonic foraminifers and calcispheres (Figs. 5 and 6) sug - gests de po si tion in a mid-ramp fa cies belt (e.g., Flügel, 2010;

Salah, 2017). Calcispheres as so ci ated with op por tu nis tic plank - tonic foraminifers such as hedbergellids, heterohelicids, white - inellids and small ben thic foraminifers re flect trop i cal to sub trop - i cal warm wa ters and eutrophic to mesotrophic con di tions in the mid dle ramp fa cies belt in which sug gest nu tri ent-en riched sur - face wa ters, in creased pri mary pro duc tiv ity, and stress ful con di - tions in the depositional en vi ron ment (Car son and Crowley, 1993; Dias-Brito, 2000; OmaÔa et al., 2014) led to a low car bon - ate pro duc tion rate.

The mid-ramp fa cies belt is marked by mixed ben thic and plank tonic foraminifers and calcispheres as so ci ated with a few intraclasts (Fig. 6C, D), echinoderms, pelecypods and bryo - zoans, to gether with quartz and sparse vol ca nic rocks as well as faint lam i na tion (Fig. 6E, F). Based on the con stit u ent grains and strati graphic po si tion, this microfacies type be longs the mid dle-ramp fa cies belt cor re spond ing to a ~30–50 m wa -

ter-depth (Ad ams et al., 1967; Fornos and Ahr, 2006; Smith et al., 2006; Baldermann et al., 2017). Ac com pa ny ing fine- to me - dium-grained intraclasts are com posed of calcispheres, to - gether with very fine-grained ba salt clasts (Figs. 5D and 6C), re - flect storm events. The scar city of red al gae as a sub or di nate com po nent sig ni fies a lack of hard sub strate, which en hanced

re work ing of un con sol i dated car bon ates by storms (e.g., West - phal et al., 2010).

We pro pose too an in ner-ramp fa cies belt based on the strati - graphic pat tern of the microfacies, the oc cur rence of ben thic foraminifers and bioclastic grainstone and/or bioclastic pack - stone com pris ing echinoderms, red al gae, and ben thic fora - minifers as well as a lack of glauconite grains (Figs. 9 and 10).

T a b l e 1 Microfacies types of the Cenomanian Chalajur For ma tion

Microfacies Type De scrip tion of microfacies and com po nents In ter pre ta tion

Microfacies 1:

Lithoclastic -bioclastic wackestone

This microfacies oc curs in thin- to me dium-bed ded green ish grey beds only in the base of the suc ces - sion, which over lies mafic vol ca nic rocks in the study area. The microfacies com prises mainly ba - salt clasts as so ci ated with ben thic foraminifers, red al gae, echinoderms, pelecypods, and sparse shell frag ments (Fig. 6A).

Oc cur rence of ba salt lithoclasts at the be gin ning of the suc ces sion sig ni fies a flood ing sur face above the non con for mity sur face. The ba salt extraclasts sug gest a sub strate com posed of mafic volcanics.

Un al tered al kali-ba salt rocks sug gest min i mal chem i cal weath er ing and short trans port (Kavoosi, 2013). Lithoclastic bioclastic wacke - stone sug gests re work ing of skel e tal grains and ba saltic sub strate dur ing rel a tive sea-level rise.

Microfacies 2:

Echinoderm-bioclastic grainstone

Echinoderm-bioclastic grainstone pre dom i nates near the up per part of the sed i men tary suc ces sion (Fig. 4D). Echinoderms are the dom i nant con stit u - ents, as well as small ben thic foraminifers and red al gae (Fig. 6B). The main diagenetic pro cesses and prod ucts are syntaxial cal cite ce ment over - growths on the echinoderm par ti cles (Fig. 6B).

The grainstone fa cies re flects de po si tion in a high-en ergy shoal en vi - ron ment. The low di ver sity of small ben thic foraminifers is at trib uted to stress in the depositional en vi ron ment re lated to physicochemical pa - ram e ters (Jasionowski et al., 2012). Col o ni za tion of the sea floor by fil - ter-feed ing or gan isms, such as echinoderms and bryo zoans most likely ex erted an in flu ence on sta bi li za tion of sed i ment due to rapid ce - men ta tion un der low pro duc tion rate (Della Porta et al., 2003). Lack of sed i men tary struc tures can be at trib uted to re stricted con di tions.

Microfacies 3:

Sandy glauconitic-bioclastic

packstone

Char ac ter ized by thin- to me dium-bed ded green ish grey lime stone and em bed ded marl. Auto chtho - nous par ti cles com prise glauconite and a few phos phatic grains (Figs. 6C and 7B). Quartz and ba salt to an de site extraclasts are in the silt- to very fine-grained sand frac tion (Figs. 6C and 7B).

Glauconite grains re tain the shape of host vol ca nic extraclasts and skel e tal grains (red al gae and echinoderms). The microfacies yields a rich as - sem blages of skel e tal grains com pris ing mainly echinoderms as so ci ated with bryo zoans and pele - cypods. Ma jor diagenetic pro cesses and prod ucts in clude sili ci fi ca tion, glauconitization (Fig. 7A, B) and neomorphism.

Echinoderms are an im por tant con stit u ent of the ma rine com mu nity in the Cenomanian of the study area. The pre dom i nance of echino - derms sug gest es tab lish ment of echinodermal bioherms and/or banks in the study area in ferred from echinodermal mound ge om e try at the Petilingeh sec tion (Figs. 1 and 2). The echinoderm par ti cles were bro ken and re worked be fore de po si tion. Lo cal oc cur rence of quartz and silt-sized to very fine-grained sand al kali ba salt and/or andesitic extraclasts points to an in flux of siliciclastics into the depo - sitional en vi ron ment dur ing sea-level rise and/or cli ma tic per tur ba - tions. In spite of a low car bon ate pro duc tion rate, the lack of micritic en ve lopes sug gests low and even ab sent bac te rial ac tiv ity in the depositional en vi ron ment (e.g., Olchowy and Krajewski, 2020). The as so ci a tion of echinoderms, plank tonic foraminifers and calci - spheres; to gether with glauconite grains and siliciclastics, are char - ac ter is tic of a stress ful en vi ron ment with low sed i men ta tion rate.

Microfacies 4:

Glauconitic bioclastic-intraclastic

packstone

This microfacies oc curs as com monly green ish grey thin-bed ded lime stones. Intraclasts and echino - derms are the pre dom i nant par ti cles (Fig. 6D). Sub - sid iary con stit u ents are plank tonic foraminifers, calci s pheres, red al gae, glauconite and sparse pho - s phate grains. Sed i men tary struc tures in clude ero - sional sur faces and faint nor mal grad ing (Fig.

7C–F). Syntaxial cal cite ce ment overgrowths are well-de vel oped on the echinoderm com po nents (Fig. 7C, E).

A mix ture of ben thic and plank tonic foraminifers sug gests de po si tion at depths of ~30–50 m (Smith et al., 2006; Baldermann et al., 2017).

Echinoderms typ i cally ac com pany the red al gae and bryo zoans in the mid-ramp fa cies belt (Ad ams et al., 1967; Fornos and Ahr, 2006).

Fine-grained intraclasts ac com pa nied by faint nor mal grad ing sug - gest dis tal storm de pos its. Authigenic glauconites cor re spond to de - po si tion dur ing a ma jor eustatic sea-level rise with low sed i men ta tion rate. Intraclasts com pris ing the calcispheres are ob served at the Pol-e Zoghal sec tion, which is com pat i ble with a deeper palaeogeo - graphic po si tion dur ing de po si tion.

Microfacies 5:

Glauconitic bioclastic packstone

This microfacies com prises green ish grey well- bed ded lime stone. Sev eral beds make in di vid ual pack ages sep a rated by interbedded marl. The fau - nal as sem blage is marked by a pre dom i nance of echinoderm frag ments (Fig. 6E). Other con stit u - ents are calcispheres, plank tonic foraminifers, bryo zoans, red al gae, and glauconite grains

The high-di ver sity fauna doc u ments nor mal sa lin ity. The con sis tent pres ence of a grain-dom i nated microfacies with gen er ally poor sort - ing of skel e tal grains in clud ing the plank tonic foraminifers, to gether with very rare plat form de rived grains, as well as glauconite grains, sug gest a low car bon ate pro duc tion rate and de po si tion in a low- to mod er ate-en ergy depositional en vi ron ment be low storm wave base (e.g., Flügel, 2010; Zeller et al., 2015).

Microfacies 6:

Echinoderm-bioclastic packstone

This con sists of thin- to me dium-bed ded lime stone with out ev i dence of high-en ergy sed i men tary stru - c tu res. Echinoderm skel e tons (Fig. 6F) as so ci ated with calcispheres and Heterohelix are the main com po nents. Plank tonic foraminifers (Fig. 6F) are sub sid iary skel e tal grains. Py rite and glauco nite have filled the cham bers of some plank tonic fora - minifers (Fig. 5A, B).

Co-oc cur rence of echinoderms and plank tonic foraminifers as well as calcispheres sug gests de po si tion in a low-en ergy en vi ron ment be low storm wave base on an outer ramp. Calcispheres re flect eutrophic to mesotrophic con di tions ac com pa nied by high nu tri ent sup plies as well as high pri mary pro duc tiv ity and sur face warm wa - ters (e.g., Car son and Crowley, 1993; Dias-Brito, 2000; Wendler et al., 2002), which re sulted in very low car bon ate pro duc tion rate. The oc cur rence of py rite and glauconite sug gest suboxic-anoxic con di - tions at the wa ter-sed i ment in ter face dur ing and/or af ter de po si tion

Based on the re gional geo log i cal con text, the suc ces sion stud ied has a lim ited dis tri bu tion in the South Cas pian Ba sin, sur rounded to the south and north-west by mafic vol ca nic is - lands (Bar rier et al., 2018). We in fer that de po si tion of the Chalajur For ma tion took place on a mafic vol ca nic rock sub - strate (Fig. 11). Oc cur rence of ba salt extraclasts in some of microfacies sug gest that the study area was sur rounded by vol - ca nic land masses (Fig. 11).

CONTROLLING FACTORS OF THE HETEROZOAN CARBONATES

This study re ports Cenomanian heterozoan car bon ates (James, 1997) in the north-cen tral Alborz Moun tains. We iden ti - fied a heterozoan as so ci a tion with a light-in de pend ent, sus pen - sion-feed ing, and heterotrophic fauna in clud ing echinoderms, Fig. 6. The microfacies iden ti fied in the suc ces sion stud ied

A – pho to mi cro graph of a lithoclastic bioclastic wackestone; ba salt extraclasts as so ci ated with echinoderm and pelecypod frag ments, and sparse glauconite grains in the base of the for ma tion, crossed polars (XPL); B – pho to - mi cro graph of echinoderm bioclastic grainstone com posed pre dom i nantly of echinoderms as so ci ated with small ben thic foraminifers and sparse red al gae, plane po lar ized light (PPL); C – pho to mi cro graph of sandy glauconitic bioclastic packstone that in cludes skel e tal and non-car bon ate grains such as glauconite grains, extraclasts and quartz; the skel e tal grains are com posed mainly of echinoderms with sparse pelecypods, XPL; D – glauconitic bioclastic intraclastic packstone microfacies com posed of echinoderms, pelecypods, intraclasts, extraclasts and glauconite grains, XPL; E – pho to mi cro graph of glauconitic bioclastic packstone with echinoderm and pelecypod com po nents and glauconite grains, XPL; F – echinoderm bioclastic packstone microfacies com posed mainly of echinoderms and plank tonic foraminifers, PPL

calcispheres, plank tonic foraminifers, bryo zoans and pele - cypods (foramol as so ci a tion), to gether with a lack of aragonitic skel e tal- and non-skel e tal grains such as cal car e ous green al - gae and ooid grains (Figs. 6, 7 and 10).

Heterozoan car bon ates de velop as the re sult of high nu tri - ent con tent, mo bile sub strate, en vi ron men tal and trophic con di - tions, sea-wa ter chem is try, and tem per a ture (e.g., Knöerich, 2005; Pomar et al., 2005; Lukasik and James, 2006; García-Hi - dalgo et al., 2012).

The echinoderm re mains have con sid er able sig nif i cance to the heterozoan car bon ates in the study area (Figs. 6, 7 and 10), and can be re lated to en vi ron men tal con di tions and cli mate.

Microfacies anal y sis in di cates that the echinoderms are ac com - pa nied by dif fer ent fos sils re lated to a wide range of depo - sitional en vi ron ments from the in ner-, mid- to outer-ramp fa cies belts (Figs. 10 and 11). The con sis tent pre dom i nance of echino derm com po nents sug gests no con sid er able changes in sea-wa ter sa lin ity (cf. Peryt et al., 2016). Pri mary fac tors in Fig. 7. Pho to mi cro graphs of some microfacies, sed i men tary fab rics, and diagenetic tex tures

of the Cenomanian Chalajur For ma tion

A – sili ci fi ca tion in a bioclastic packstone com posed of echinoderms and pelecypods, XPL; B – authigenic glauconite in a sandy glauconite bioclastic packstone; vol ca nic extraclasts were re placed by glauconite and the glauconite grains re tain the shape and mor phol ogy of the host grains, which sig ni fies an authigenic or i gin, XPL; C – intraclast in cludes calcispheres pro vid ing ev i dence of dis tal storm de pos its on a mid-ramp, XPL; D – bioturbation in a bioturbated bioclastic packstone com pris ing echinoderms, plank tonic foraminifers, calcispheres and a few glauconite grains, PPL; E, F – mi cro scopic ero sional sur faces and faint nor mal grad ing in glauconitic bioclastic intraclastic packstone and sandy glauconitic bioclastic packstone microfacies, XPL

echino derm taphonomy were palaeoenvironmental con di tions and disarticulation modes (e.g., Brett et al., 1997; Hunter and Under wood, 2009). The dis tri bu tion and pres er va tion po ten tial of echinoderms have been at trib uted to many fac tors in clud ing nu tri ent avail abil ity, sub strate type, and tur bu lence as well as wa ter depths of >50–70 m (Nebelsick, 1995, 1996; Smith et al., 2006).

The disarticulated and some what abraded na ture of the echinoderm skel e tons, to gether with plank tonic foraminifers and calcispheres, sug gest that they were trans ported and re de - pos ited in the depositional en vi ron ment. Our re sults are in line with the idea that echinoderms were both autochthonous and allochthonous (e.g., Krajewski et al., 2020). A lack of com plete echinoderms sug gests that all of them were trans ported to some de gree be fore de po si tion.

The as so ci a tion of calcispheres, echinoderms, plank tonic foraminifers, and op por tu nis tic Tethyan or gan isms, sug gests warm- and nor mal sa lin ity con di tions, nu tri ent-en riched, and CaCO3-rich sur face wa ters of mar ginal seas (Hart, 1991;

Dias-Brito, 2000). These heterozoan car bon ates, form ing in warm wa ters dur ing the hot green house Cenomanian, raise the ques tion of their spa tial and tem po ral con trols.

An en hanced rate of the sea-level rise ac com pa nied by large-scale ig ne ous prov inces across the globe, with high global tem per a tures, led to an in crease in weath er ing rate of mafic vol ca nic rocks, which in tro duced colder, fresh and nu tri - ent-rich wa ters into the depositional en vi ron ment, re sult ing in eutrophic to mesotrophic con di tions (Car son and Crowley, 1993; Charbonnier et al., 2018a; Percival et al., 2018; Banerjee et al., 2019; Jafarzadeh et al., 2020; Baioumy et al., 2021).

Fig. 8. High-res o lu tion SEM im ages of glauconite grains

A – scan ning elec tron pho to mi cro graph rep re sent ing the cau li flower mor phol ogy of the glauconite; B – sur faces of the grain show ro sette struc ture and/or mor phol ogy, which sug gest a mixed-layer min eral habit of the glauconite grain; C, D – well-de vel oped lamellae of glauconite grains in di cat ing authigenic or i gin; for lo ca tion of sam ples see Ta ble 2 and Fig ure 10 (in the col umn of sam ples and weath er ing pro file)

Mafic vol ca nic erup tions around the world were the main mech a nism plau si bly re spon si ble for high at mo spheric car bon di ox ide lev els then and a pri mary con trol on the global cli mate, which led to high nu tri ent lev els (e.g., Nichols, 2009). Abun dant mafic vol ca nic rocks and al kali bas alts re lated to extensional move ments have been re ported from the South Cas pian Ba sin

as well as in and around the In dian Ocean dur ing the Ceno - manian (Bitschene et al., 1992; Bar rier et al., 2018).

A com bi na tion of petrographic in ves ti ga tions (Figs. 7B and 8) and EDS ana lyses (Ta ble 2 and Fig. 9) sug gest an over all nu tri ent-de pend ent en vi ron ment in com bi na tion with rel a tive sea-level rise and warm-wa ter con trol on the end-mem ber Fig. 9. En ergy dispersive spec tros copy (EDS) re sults and scan ning elec tron pho to mi cro graphs of some glauconite grains;

EDX anal y sis of four sam ples and twelve glauconite grains of the Chalajur For ma tion from the Chalajur sec tion show that O2, Mg, Al, Si, Ca, Fe, and K are the ma jor con stit u ents

A – high K con tent of glauconite grain in di cates a high de gree of mat u ra tion; glauconite is com pat i ble with Fe- and K-rich, and Al-rich glauconite; A1– scan ning elec tron pho to mi cro graph of glauconite and its EDS, in di cat ing a Fe-rich glauconite grain; B1 – scan ning elec tron pho to mi cro graph of glauconite and its EDS, in di cat ing a Fe-rich glauconite grain; B, B2– high K con tent, as well as the microtexture, in di - cates an evolved and ma ture glauconite grain; C – EDS of a glauconite grain; for lo ca tion of sam ples see Ta ble 2 and Fig ure 10 (in the col - umn of sam ples and weath er ing pro file)

heterozoan car bon ates of the study area. Our con clu sion is sup ported by the na ture of the biogenic as sem blages as well as by authigenic py rite (Fig. 5A, B), glauconite and sparse phos - phate grains in the Cenomanian Chalajur For ma tion.

Microfacies anal y sis sug gests palaeo eco logi cal stress dur - ing de po si tion in ferred from stenohaline and eutrophic/eury - haline spe cies in clud ing echinoderms and calcispheres, in di - cat ing nu tri ent-rich warm wa ter (e.g., Hart, 1991; Car son and Crowley, 1993; Dias-Brito, 2000; Bensong et al., 2008; West - phal et al., 2010).

These con di tions re sulted in es tab lish ment eutrophic con di - tions, which led to a de crease in car bon ate pro duc tion rate on mid- to outer-ramp fa cies belts. Ac cord ingly, nu tri ent-rich and eutrophic con di tions con trib uted to the gen er a tion of hetero - zoan car bon ates, de spite the hot green house con di tions dur ing the Cenomanian.

GLAUCONITIZATION AND PRESERVATION

Two main hy poth e ses may be in voked to ex plain the for ma - tion and pres er va tion of glauconite grains in the suc ces sion stud ied:

–

–

A re work ing source is un likely, given the lack of abraded glauconite grains or ev i dence of their trans port (Figs. 5–7).

The na ture and fre quency dis tri bu tion of min er als in sed i - men tary suc ces sions is mainly con trolled by cli mate, tec ton ics, depositional en vi ron ment, and diagenesis (Nel son and Hume, 1987). Based on our petrographic and SEM stud ies, we pro - pose an authigenic or i gin for glauconite grains (Figs. 8 and 9).

Petrographic in ves ti ga tions show that the glauconite infills skel - e tal grains (dom i nantly echinoderms and red al gal par ti cles, Fig. 12), are mainly authigenic, and de vel oped con tem po ra ne - ously and/or very soon af ter de po si tion. Our stud ies show that

some of the echinoderm and red-al gal skel e tons as well as vol - ca nic extraclasts were re placed by glauconite, while glauconite grains re tain the shape and mor phol ogy of the host grains (Figs.

7B and 12). This sig ni fies an authigenic or i gin un der low sed i - men ta tion and slightly re duc ing con di tions (Fig. 5A, B).

There is a di rect re la tion ship be tween the oc cur rence and/or fre quency of glauconite grains and siliciclastic in flux (Figs. 6C, E and 7B, C). Glauconite grains are scat tered through out the Chalajur For ma tion. In some lay ers glauconite grains are gen er ally as so ci ated with a vary ing ad mix ture of quartz and mafic extraclasts (silt- to very-fine-grained sand size, Figs. 6C, E and 7B, C). There fore, the abun dance of glauconite grains may have re sulted from a com bi na tion of en - hanced weath er ing of nearby mafic vol ca nic land masses, which pro vided a suf fi cient Al, Fe, Si, Mg, and K cat ions sup ply (Ta ble 2), and the es tab lish ment of re duc ing con di tions. The state of ox i da tion, ac com pa nied by enough Fe sup ply in the depositional en vi ron ment, led to high-Mg cal cite com po nents such as echinoderms and red al gae which turned into ferroan cal cite, while pre serv ing the skel e tal struc ture (Fig. 12; Rich - ter, 1983).

Based on mor phol ogy, min er al ogy, microfacies, and fau nal as so ci a tion to gether with depositional en vi ron ment, the glauco - nite grains are autochthonous. Glauconite grains typ i cally form in con di tions of ma jor eustatic sea-level rise, low sed i men ta tion rate, and global warm ing as well as sub-oxic con di tions (Car son and Crowley, 1993; Jafarzadeh et al., 2020; Baioumy et al., 2021). Ox y gen-de fi ciency and ox y gen-de ple tion in the depo - sitional en vi ron ment fa cil i tate the mo bil ity and fix a tion of Fe into the glauconite struc ture at the wa ter-sed i ment in ter face (Banerjee et al., 2019).

Cre ta ceous pe lagic fa cies in SE Spain con tain large num - bers of glauconite grains that mimic the shape of the bioclasts (Jimenez-Millan et al., 1998), and a high con tent of glauconite grains has been re ported from the Up per Cre ta ceous suc ces - sion in the South Is land of New Zea land (McConchie and Lewis, 1980). Field and pub lished data (Jafarzadeh et al., 2020) sug -

T a b l e 2 Rep re sen ta tive EDS data for some glauconite grains from the Chalajur sec tion

Sam ples Grains El e ments [wt.%]

O2 Mg Al Si Ca Fe K Ti

JL-2 CH-2C 33.98 1.45 3.62 19.26 1.29 14.47 5.02

CH-2E 25.18 1.19 3.13 17.18 0.44 18.90 5.34

JL-3

CH-3B 29.40 1.55 4.12 22.93 1.08 18.90 6.15

CH-3C 34.59 1.51 3.86 21.00 2.90 15.15 5.52

CH-3E 36.40 1.68 4.19 22.24 0.99 16.32 6.44

JL-4

CH-4A 30.72 2.44 5.64 23.94 1 13.29 3.54

CH-4B 32.44 1.17 3.29 17.08 1.60 14.30 6.94 1.02

CH-4C 28.67 1.65 3.78 17.03 3.42 9.90 3.15

CH-4D 21.89 1.43 4.00 19.72 2.28 11.42 4.90

JL-5

CH-5A 32.34 1.04 3.82 23.28 2.3 17.81 6.15

CH-5B 24.04 0.73 3.61 18.82 7.22 17.55 6.03

CH-5B 16.60 2.04 5.20 25.72 1.80 17.03 5.12 0.41

Re sults show two ranges of po tas sium con tent of glauconite grains, vary ing from 3.15 to 6.94 wt.%

gest that the Cenomanian suc ces sion of the study area is con - tin u ous lat er ally and pos si bly cor re lates with the Cenomanian Aitamir For ma tion in the Kopeh Dagh Ba sin. In the north-cen tral Alborz, glauconitic lime stone per sist, while the Cenomanian Aitamir For ma tion is a glauconitic sand stone and shale. The mid-Up per Cenomanian glauconitic lime stone of the south-east Esfahan area in the Cen tral Ira nian Ba sin, with a to tal thick ness of ~4 m, has been in ter preted as a con densed zone (Kennedy et al., 1979; Hairapetian et al., 2018).

The ter res trial in flux from in tense chem i cal weath er ing of mafic vol ca nic land masses led to higher nu tri ent lev els and a sup ply of ions, es pe cially Fe, un fa vour able for car bon ate pro - duc tion but pro vid ing suit able con di tions for authigenic glauco - nites to form and grow.

Palaeo eco logi cal stress re lated to high-nu tri ent con di tions as so ci ated with a lower state of ox i da tion in the depositional en - vi ron ment led to low pro duc tion of skel e tal car bon ates, al low ing

time for authigenic min er als such as glauconite to grow. The as - so ci a tion of abun dant echinoderms, plank tonic foraminifers, and calcispheres in the glauconitic well-bed ded chert-bear ing lime stone of the Chalajur For ma tion (Figs. 10 and 12) is char ac - ter is tic of a stress ful en vi ron ment with low sed i men ta tion rate re lated to rel a tive sea-level rise, with fresh- and nu tri ent-en - riched wa ters, a semi-con fined microenvironment within the sub strate, and re duced ox y gen lev els (Odin and Mat ter, 1981;

Odin and Fullagar, 1988; Dias-Brito, 2000; Nichols, 2009; Oma - Ôa et al., 2014).

The cau li flower struc ture (Fig. 8A), boxwork and ro sette struc tures (Fig. 8B), and well-de vel oped lamellae of the evolved glauconite grains (Fig. 8C, D), in di cate an authigenic or i gin (e.g., Oddin and Mat ter, 1981). EDS anal y sis re vealed glauco - nite grains with two ranges of po tas sium con tent (Ta ble 2, Figs.

8 and 9B–B2), vary ing from 3.15 to 6.94 wt.%. Based on the EDS anal y sis, there is a di rect re la tion ship be tween K2O and Fig. 10. Gen eral view and a cor re la tion chart of the Cenomanian Chalajur For ma tion at the Chalajur, Petilingeh,

and Pol-e Zoghal sec tions

The cor re la tion chart in cludes age, sam ple num bers, thick ness, weath er ing pro file, li thol ogy, and sed i men tary struc tures (phys i cal and bi o log i cal), tex tures, depositional en vi ron ments as so ci ated with main car bon ate and non-car bon ate grains,

to gether with un der ly ing and over ly ing bound aries (see Figs. 1 and 2 for lo ca tion)

Fe₂O₃ (Ta ble 2 and Fig. 9). A high con tent of K (≥5 wt.%) of glauconite grains (Fig. 9), in di cates a high de gree of mat u ra - tion. The green and brown colours of glauconite grains are com pat i ble with Fe- and K-rich glauconites, re spec tively (Ta ble 2, Figs. 6C, E, 7B, C, F, 9 and 12). The high con tent of K, as well as the mi cro-tex ture (Figs. 8, 9B–B2, 12A, D), is in line with an evolved and ma ture glauconite. By con trast, light brown and pale green colours (Fig. 12B, C) com pris ing a low con tent of K (<5 wt.%; Ta ble 2), in di cates a low de gree mat u ra tion of glauconite grains.

Green glauconite grains are Fe- and K-rich and ma ture (Figs. 6C, E, 7B, C, F, 9 and 12), while Al-rich glauconite grains have brown col our (Ta ble 2). The ma tu rity de pends on the res i - dence time of grains near to the sed i ment-wa ter in ter face (Banerjee et al., 2012). A high ma tu rity of glauconite grains is com pat i ble with a long res i dence time for glauconite grains at the sed i ment-wa ter in ter face un der a low sed i men ta tion rate (Banerjee et al., 2016a, b).

CONCLUSIONS

Our de tailed field and petrographic in ves ti ga tions on the Cenomanian car bon ates have re sulted in the rec og ni tion of six microfacies, re lated to the in ner-, mid- and outer-ramp fa cies

belts of a car bon ate ramp. Heterozoan car bon ates were de pos - ited on a car bon ate ramp un der eutrophic con di tions. A pre - dom i nance of echinoderms as so ci ated with calcispheres, plank tonic foraminifers, a lack of cal car e ous green al gae or ooids, a low car bon ate pro duc tion rate to gether with a high con - tent of glauconite grains, and a prev a lent high-Mg cal cite min er - al ogy, char ac ter ize tem per ate heterozoan car bon ates.

Palaeo eco logi cal stress re lated to high-nu tri ent con di tions, as so ci ated with a lower state of ox i da tion in the depositional en - vi ron ment and a sup ply of re quired ions es pe cially Fe, led to low pro duc tion of skel e tal car bon ates while pro vid ing suit able con - di tions for authigenic glauconite grains to form and grow.

Our petrographic in ves ti ga tion shows a di rect re la tion ship be tween the pro por tion of glauconite grains and pe ri ods of sea-level rise in ferred from siliciclastic in flux into the depo - sitional en vi ron ment. Petrographic and SEM stud ies re veal that glauconite-fill ing skel e tal grains re tain the shape and mor phol - ogy of the host grains which sig ni fies an authigenic or i gin un der low sed i men ta tion and slightly re duc ing con di tions. The cau li - flower, boxwork, and ro sette struc tures as so ci ated with well-de - vel oped lamellae of evolved glauconite grains in di cate an authigenic or i gin. The EDS anal y sis re vealed glauco nite grains with two ranges of po tas sium con tent. High con tents of K (≥5 wt.%) as well as the mi cro-tex ture is in line with evolved and ma - ture glauconite grains.

Fig. 11. The pro posed depositional model of the Cenomanian Chalajur For ma tion in the study area The depositional model is a car bon ate ramp that com prises the in ner-, mid- and outer-ramp fa cies belts

in which car bon ate de pos ited on a ba salt sub strate

Our find ings are com pat i ble with an over all nu tri ent-rich con di tions and palaeo eco logi cal stress re lated to rel a tive sea-level rise and eutrophic con di tions, which con trib uted to gen er a tion of heterozoan car bon ates de spite the hot green - house con di tions dur ing the Cenomanian in the north-cen tral Alborz Moun tains.

Ac knowl edge ments. We wish to ex press our most sin cere thanks to M. Krajewski and an anon y mous re viewer for their crit i cal re views, valu able ad vice and com ments on the first ver - sion of the manu script. The au thors ac knowl edge A. Kavo osi and E. Fassihi for their will ing as sis tance dur ing field trips. We ap pre ci ate the help of Z. Nayebi and A.R. Moghad dassi in the iden ti fi ca tion of plank tonic foraminifers.

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