Introduction

The most significant tectono-sedimentary event of the NE margin of Arabian Plate (including the eastern Fars area) during the Late Cretaceous is characterized by development and migration of the foreland basin to the SW which was associated by changes in depocenters as well as basinal and platformal settings (Alavi 2007; van Buchem et al. 2006; Piryaei et al. 2010, 2011). This process could be due to stack thrusting and subsequent tectono-sedimentary loading in the northern side of the foreland basin. Salt tectonics also locally affected the sedimentary systems (Player 1967; Kent 1970; Jahani et al. 2007, 2009). These changes in depositional features can be traced in the Maastrichtian sequences of the eastern Fars area. This time interval includes the upper part of Gurpi, Tarbur and lower part of Sachun formations as well as obducted radiolarite and ophiolitic complex. In terms of stratigraphy and regional paleogeography, many works have been published by authors (e.g., James and Wynd 1965; Setudenia 1978; Koop and Stoneley 1982; Amiri Bakhtiar et al. 2011; Piryaei et al. 2010, 2011; Abyat et al. 2013, 2015; Asgari Pirbalouti et al. 2012, 2013; Afghah and Yaghmour 2014). Despite this good background, little is known about the palaeogeographic evolution of eastern Fars area during and in relation with the Late Cretaceous tectonic and structural evolutions. The main goal of this paper is to interpret and map the migration effects of this foreland basin. To investigate Maastrichtian paleogeographic evolution, an SW–NE trending regional transect has been selected perpendicular to the Zagros main thrust, passing through the Kuh-e Chadur, Kuh-e Gach, and Kuh-e Parak outcrop sections together with subsurface data from the Pishvar-1 and Bavush-1 wells (Fig. 1). High-resolution sequence stratigraphic framework is prepared to understand the regional and local tectonic influences on the Maastrichtian sedimentary systems of the study area.

Fig. 1
figure 1

Location map of the studied area showing the selected transect in the eastern part of the Fars area

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Materials and methods

A total of 400 thin sections have been studied from three outcrop sections and two exploration wells. Petrographic studies and microscopic image analyses were carried out on thin sections. The modified Dunham (1962) textural scheme of Embry and Klovan (1971) was used for facies’ classification. In addition, field observations have been done to follow the bedding patterns and geometries, facies and thickness variations, and sequence stratigraphic surfaces and system tracts. Well data include paleontological range charts and biozonation, lithological interpretations associated with gamma ray and sonic logs. Based on this information, microfacies, depositional environments, and sequence stratigraphic framework reconstructed for the Maastrichtian sedimentary sequences. A combination of field observations, microscopic results and well log interpretation is used to understand the tectono-sedimentary evolution of eastern Fars area during the studied time interval. Dating of depositional sequences and sequence surfaces is based either on the Wynd’s (1965) biozonation or log correlation in the dolomitic intervals or other barren units. Depositional environment was studied based on the standard facies’ models presented by Wilson (1975) and Flügel (2010) standard models and sequence stratigraphic approach of van Wagoner et al. (1988), Vail (1991), Tucker (1993) and Emery and Myers (1996) Catuneanu et al. (2011, 2012) methods.

Geological setting

The study area covers mostly the interior parts of the Fars area, an area between the Nezam Abad Fault to the west and Bandar Abbas region to the east (Fig. 1). The Fars area is a part of the Zagros Foreland Fold and Thrust Belt (FFTB), belonging to the northeastern margin of the Arabian Plate (Alavi 2004; van Buchem et al. 2002; Piryaei et al. 2010, 2011). In addition to plate-scale foreland basin formation and obduction along the NE Zagros FFTB, Fars area also hosts many halokinetic events that episodically influenced the accommodation space and subsequent facies and thickness variations. A transect selected for this study that passes across the interior and sub-coastal Fars areas (Fig. 1).

Salt diapirs and Nezam Abad, Razak, Hendurabi, and High Zagros faults are important tectonic elements of the Fars area. These tectonic features strongly affect the sedimentation patterns of this area (Hesami et al. 2001; Bahroudi and Koyi 2003; Sepehr and Cosgrove 2005; Piryaei et al. 2010, 2011).

Salt tectonics occurs throughout the southeastern Zagros, as salt plugs exposed at the surface (e.g., at the Korsia, Gach, and Khamir anticlines) and/or known from the subsurface (e.g., the Genow anticline). Salt in the diapirs originates from the Infracambrian Hormuz Series and the Miocene Gachsaran Formation (Harrison 1930; Motiei 1995; Talbot and Alavi 1996; Ghazban and Al-Aasm 2010; Motamedi et al. 2011), both of which act as detachment horizons throughout the Zagros (Kent 1958, 1979; Sherkati and Letouzey 2004; Jahani et al. 2007).

Stratigraphy

The main Maastrichtian lithostratigraphic units of the studied area are the upper part of Gurpi, Tarbur and the lower part of the Sachun formations and emplaced radiolarites which are briefly discussed in the following (Fig. 2).

Fig. 2
figure 2

Lithostratigraphic correlation chart of Zagros (after James and Wynd 1965; Motiei 1993). The studied interval includes the Maastrichtian Tarbur, Gurpi, and Sachun formations

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Gurpi Formation The Santonian-to-Maastrichtian Gurpi Formation is dominated by thin-bedded pelagic marls which locally prograded by time equivalent carbonate facies of the Tarbur Formation in its upper part. This basinal facies is resting unconformably on the Albian-to-Santonian Sarvak-to-Ilam formations. Depending on the tectono-sedimentary setting, thickness of the Gurpi Formation varies from a few meters to more than one thousand meter and wedges out toward the paleohighs. The Gurpi Formation is found almost throughout of southwestern Iran and extends from Fars region and parts of the Dezful Embayment area and from the Campanian to Paleocene in the Lurestan.

Tarbur Formation The Tarbur Formation consists of a thick carbonate sequences with Late Companian-to-Maastrichtian age. The basal contact of this formation in the northern part of the Zagros (Darab area) with radiolarites is sharp (Fig. 3a) and in other parts of the basin with the Gurpi Formation is conformable and transitional (Fig. 3b). The upper contact with the grey to green, ferruginous shales of the Sachun Formation is associated with ironstone nodules which may indicate an interruption in sedimentation process. Toward the southwest of area, the Tarbur Formation grades into shales of the Gurpi Formation (James and Wynd 1965; Ghazban 2009).

Fig. 3
figure 3

a Field photos of the Tarbur Formation with obducted radiolarites, Barfdun Anticline, Darab area. b Filed photos of transitional contact of Tarbur–Gurpi formations. Mozaffari Anticline, south Shiraz

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Sachun Formation The uppermost Maastrichtian-to-Paleocene Sachun Formation comprises a variety of lithologies including intercalation of dolomite and evaporite, dolomitic limestone, argillaceous dolomite, shale, marl, and clastics. Toward southwest, this formation changes to Pabdeh Formation. In the northern part of the Fars area (Kuh-e Ahmadi and Mozaffari), the Sachun Formation is divided to members including Qorban limestone and Sarvestan (Motiei 1993). The Sachun Formation is restricted to the northeastern interior Fars of the Zagros Fold-Trust Belt. It also presents in northwest of Bandar Abbas (Finu and Kuh-e Namak), east of Shiraz (Sarvestan) and Burkh, Bavush, and Darmadan (James and Wynd 1965; Motiei 1993). The thickness of this formation increased from southwest to southeast.

Regional sedimentary facies’ distribution

The sedimentary facies and diagenetic processes can be attributed to two different tectono-sedimentary settings; a migrating platformal setting related to the foreland basin bulge and salt-induced facies and diagenetic features. Maastrichtian deposits consist of large variety of skeletal and non-skeletal grains. Skeletal components include Omphalocyclus macroporus, Loftusia sp., echinoids, Siderolites calcitrapoides, Orbitoides sp., sponge spicule and planktonic foraminifera such as Globotruncana sp., Globotruncana stuarti, and Heterohelix sp. Non-skeletal grains mainly consist of peloids and intraclasts. Based on the microscopic results and field observations including the study of sedimentary textures and structures, faunal assemblage, diagenetic features, and bedding patterns, different microfacies have been identified in the studied interval which can be classified into six facies’ belts.

Tidal flat facies’ belt

Tidal flat facies are mainly distributed in the northern part of the study area (e.g., Kuh-e Chadur section), where open platform environments are turning to restricted and marginal environments (a transition from the Tarbur bioclast dominated facies to evaporitic Sachun facies). These facies include: A1-Coarse-grained intraclast rudstone associated with other non-skeletal grains such as peloid and oncoid, which is deposited in intertidal to supratidal and high energy environments and can be equivalent to SMF 24 of Flügel (2010) (Fig. 4a). A2-Dolomudstones (Fig. 4b) with fenestral fabric in which the size of dolomitic crystals reaches to about 2 micron and is equivalent to SMF 25 of Flügel (2010). A3-Ooid packstone (200 micron in size) with bioclastic cores and mostly superficial texture. This type of facies is interpreted to be deposited in tidal channel and shows an inversional texture partly dolomitization (Fig. 4c). A4-Stromatolite bindstone associated with mud cracks and tepee structures, mould of evaporitic minerals, silt-sized quartz grains, and algal filaments (Fig. 4d). The stromatolite laminae are planar and locally wavy and are attributed to supratidal to upper intertidal environment (Tucker and Wright 1990; Flügel 2010). This microfacies is equivalent to SMF 20 of Flügel (2010).

Fig. 4
figure 4

Photomicrographs of different microfacies. a Intraclast rudstone (A1). XPL. b Dolomudstone (A2). PPL. c Ooid packstone (A3), PPL. d Stromatolite bindstone (A4), ppl. e Peloid packstone (B1). PPL. f Dacyclad bioclast packstone (B2) PPL. g Peloid bioclast packstone (C1), PPL. h Bioclast rudist debris floatstone (D1), PPL. i Coral bafflestone (D2), PPL. jOmphalocyclus Loftusia Packstone (E1), PPL. k Calciturbidite (E2), and PPL. l Planktonic foraminifera packstone (F1), PPL

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Restricted lagoonal facies’ belt

Two microfacies have been identified in this facies belt: B1-Dolomitized bioclast peloid packstone and wackestone with micritized bioclasts (~20–40%) and common bioturbation (Fig. 4e) and B2-Dacyclad bioclast packstone with green algae, miliolids, textularids and 40–60% peloid. This microfacies is completely dolomitized (Fig. 4f). It is equivalent to SMF 18 of Flügel (2010). The presence of miliolids shows a very shallow water environment, with subsaline-to-hypersaline conditions. They live preferably in low-turbulence water, where abundant sediment fines occur (Hottinger 1997, 2007; Geel 2000).

The presence of porcelaneous benthic foraminifera such as miliolids and textularids in a muddy background, low diversity of benthic foraminifera, and the presence of dacyclad green algae, pervasive dolomitization, and association with tidal flat microfacies in vertical interval indicate that B1 and B2 were deposited in the restricted lagoon (Flügel 2010). Green algae are abundant in the upper Cretaceous deposits of the Zagros successions and representing shallow warm water with relatively high salinity (Riding 1991; Mosadegh and Shirazi 2009; Mehrabi et al. 2015).

Open lagoon facies’ belt

This group of facies is mainly packstone in texture dominated by benthic foraminifers such as Cuneolina sp., Nezzazata sp., Minouxia sp., Dicyclina shlumbergeri, Rotalia sp., Valvulina sp., Sirtina sp., Coskinolina sp., Nezzazatinella sp., Dictioconous sp., Loftusia sp, and Omphalocyclus sp. in combination with gastropods, ostracod, coral, rudist and dacyclad, as well as echinoderm and bivalve debris (Fig. 4g). Peloids (30–40%) are the main non-skeletal allochems of this microfacies. Micritization and bioturbation are common. High diversity benthic foraminifera indicate the normal salinity of the sea water and abundant microfauna indicates prolific conditions (Purser 1973; Palma et al. 2007; Jamalian et al. 2010).

Reefal facies’ belt

Reefal facies includes: D1-Rudistic facies which are organized in packstone, grainstone, floatstone, rudstone, and boundstone textures. Rudists are mainly radiolitidae and rarely hiporitidae types and their amount reach to 10–50%. In addition, lagoonal fauna such as Cuneolina sp., Nezzazata sp., Minouxia sp., Dicyclina shlumbergeri, Miliolide, Valvulina sp., Sirtina sp., Coskinolina sp., Nezzazatinella sp., and Dictioconous sp. are also present (Fig. 4h). Boring and neomorphism strongly affected the rudist shells. Internal sediment, micrite, and sparry calcite cement filled the internal pores of the rudists. This microfacies is equivalent to SMF 7 of Flügel (2010). D2-Coral bearing facies with bafflestone texture (Fig. 4i). The cavity of the corals filled with micrite or sparite. Benthic foraminifera and shell debris are subordinates. These microfacies were deposited in reef setting.

E-Open marine facies’ belt

This group of facies can be divided into three kinds of microfacies: E1-Benthic foraminiferal dominated facies which are organized mostly in packstone texture. The bioclast components include Omphalocyclus macroporous, Loftusia sp., Siderolites calcitrapoides, Orbitoides sp. as well as rudist, echinoderm, and bivalve debris associated with non-skeletal grain like peloids (Fig. 4j). Omphalocyclcus are index fossils of Tethyan realm (Ozcan 2007) and lived in the upper part of photic zone and mostly observed in the upper part of shallowing upward cycles (Hottinger 1983; Moro et al. 2002; Abramovich and Keller 2002). E2-Calciturbidites consist of mixed planktonic and benthic foraminifers. This microfacies is characterized by the presence of large intraclasts in basinal microfacies (fine-grained bioclast packstone). Intraclasts contain bioclasts of shallower parts including bioclasts of Siderolites calcitrapoides and Rotalia sp. (Fig. 4k).

F-Basinal facies’ belt

F1-Planktonic foraminiferal dominated facies containing Globotruncana sp., Globigerina sp., Oligostegina sp. (10%), sponge spicule (3%), Radiolaria sp., and echinoderm (15%) (Fig. 4l). This facies sometimes includes smaller planktonic foraminifera such as Globotruncana sp. and Globotruncana stuarti with pyritization in a muddy matrix. This microfacies is equivalent to SMF 1 of Flügel (2010).

Maastrichtian depositional environments

Six microfacies’ belts have been identified based on the types and percentage of allochems, vertical change of microfacies, and correlation to the standard microfacies of Wilson (1975) and Flügel (2010). The presence of calciturbidites, lens form rudist patch reefs and rapid changes of microfacies, led us to consider a shelf carbonate platform with marginal rudist patch reefs for these deposits (Fig. 5). Accordingly, belts A, B, and C were deposited in the inner part of shelf, rudist patch reefs facies’ belt D in the shelf margin, and facies’ belts E and F in outer shelf and basinal environments, respectively.

Fig. 5
figure 5

Photomicrographs and relative position of facies belts shown in a schematic depositional model of the Maastrichtian sequences in the eastern part of Fars area

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In the studied transect, in the northern part (i.e. Kuh-e Chadur), inner shelf and shelf margin deposits are dominated deposits, while toward the south (Ku-e Parak), the outer shelf deposits are dominated. In the south most section of the studied transect (Bavush-1), only basinal facies (Gurpi Formation) are present.

Salt tectonic effects on the sedimentary system

Among the studied sections, the Kuh-e Gach section is located near a salt plug, and consequently, the Maastrichtian deposits (Tarbur Formation) were affected by the presence of salt plug. It influenced locally the accommodation spaces during the Maastrichtian (indicated by decreasing thickness of Maastrichtian deposits). This can be documented by variations in thickness, facies and types and intensity of the diagenetic processes. The most significant diagenetic processes affected the Maastrichtian strata on salt plug are as below:

Dolomitization: Dolomitization is the most important diagenetic process which has taken place related to the salt plug. Two forms of dolomite are present:

  1. 1.

    Coarse xenotopic fabric-destructive dolomite (dolosparite) (Fig. 6a). This type of dolomite has mainly occurred in the lower part of Tarbur Formation (in TST of Maas-3 sequence) close to the salt plug. Poikilotopic anhydrite cement was also formed around the xenotopic dolomite crystals (Fig. 6b). The nearby salt plug was the source of hypersaline brines that injected the soluble anhydrite to adjacent layers.

  2. 2.

    Fine crystalline fabric-retentive dolomite (Fig. 6c, d). Toward the upper part of interval, far from the source of dolomitizing fluid (salt plug), the supersaturation of dolomitizing fluid decreased. Consequently, finer crystals of dolomites mimically replaced both matrix and grains of the precursor limestone.

Dissolution: Dissolution is another diagenetic process occurred in the Maastrichtian deposits. Undersaturated fluids with respect to unstable to metastable minerals (e.g., aragonite and HMC) originated from salt plug have resulted in the formation of moldic (Fig. 6e) and vuggy porosity (Fig. 6f) (in Maas-3 sequence). In a similar trend, in comparison with the dolomitization, the intensity of dissolution has considerably decreased away from the salt plug. This confirms that the both dolomitizing and dissolving fluids were originated from the salt plug (Fig. 7).

Fig. 6
figure 6

Salt plug-related diagenetic features in the Maastrichtian deposits. Diagenetic processes in the zone 1, near salt plug. a Coarse xenotopic fabric-destructive dolomite crystals, XPL. b Poikilotopic anhydrite cement around xenotopic dolomite crystals, XPL. c Vuggy porosity in zone 2, XPL. d Moldic porosity after Omphalocyclus macroporous. Ghosts of allochems are also observed. XPL. Diagenetic processes in zone 3 (far from the salt plug). As the figures show, the effects of dolomitization decreased and dolomitization is fine crystalline and fabric-retentive. e Fine-crystalline dolomite with fabric-retentive texture. Diverse benthic foraminifera can be identified. PPL. f Fabric-retentive dolomitization of Loftusia sp. PPL

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Accordingly, a generally zonation was identified around the salt plug (Fig. 7).

Fig. 7
figure 7

Diagenetic processes around the salt plug. As the figure shows, there is zonation around the salt plug. Zone 1-Near salt plug, coarse xenotopic fabric-destructive dolomitization along with anhydrite cement are present. Toward the upper part of the interval, zone-2 with ghosts of allochems and intense dissolution; zone-3 with fine crystalline fabric-retentive dolomitization can be observed

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Zone-1 This zone presents in adjacent to the salt plug that consists of coarse xenotopic fabric-destructive dolomite crystals along with anhydrite cement. This zone presented in the lower part of Tarbur Formation.

Zone-2 It is the transitional zone between 1 and 3. Ghosts of allochems and intense dissolution can be observed.

Zone-3 This zone is far from salt plug and consists of fine crystalline fabric-retentive dolomitization and allochems are well-preserved. This zone presented in the uppermost part of the Tarbur Formation.

Sequence stratigraphy

To stepping the tectono-sedimentary events through the Maastrichtian, a sequence stratigraphic approach has been used to explain the distribution of depositional facies, diagenetic features, thickness variation, and bedding pattern in the both local and regional scales through the time and space. In terms of dating, age of the Maastrichtian is based either on the presence of fore-reef assemblage zone in the shallow-water carbonate (Tarbur Formation) or on the presence of Globotruncana stuarti–Pseudotextularia varians assemblage zone [zone 37 of Wynd (1965)] in the basinal setting (Gurpi Formation). This dating is not precise enough for the high-resolution sequence stratigraphy in the Maastrichtian scale. In addition, many parts of the sections are barren and Maastrichtian interval cannot be easily defined. Therefore, the prepared sequence stratigraphic framework is mainly based on regional correlation. On the other hand, effects of salt plugs on the sedimentary system have resulted in intensive diagenetic influences and reducing dating resolution. In this case, recognizing some key levels on the gamma ray logs can be helpful to establish a sequence stratigraphic framework. Accordingly, five 4th-order sedimentary sequences have been defined through the Maastrichtian interval and their variation studied along a regional NE–SW trending transect (Figs. 8, 9 and 10). Plaeogeographic maps are also prepared for each sequence to reconstruct the sequence stratigraphic-based depositional history of the Maastrichtian sequences in the eastern Fars area (Fig. 11).

Fig. 8
figure 8

Petrographical and sedimentological logs of the Maastrichtian deposit in Kuh-e Chadur

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Fig. 9
figure 9

Petrographical and sedimentological logs of the Maastrichtian deposit in Kuh-e Gach section

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Fig. 10
figure 10

Petrographical and sedimentological logs of the Maastrichtian deposit in Kuh-e Parak section

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Fig. 11
figure 11figure 11

Palaeogeographic evolution of study area during Maas-1 to Maas-5 sequences for TST and HST systems tracts. For preparing these maps, the data of the adjacent area were also used. These maps show foreland basin migration from northwest toward southeast

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Sequence Maas-1

This sequence is observed only in northern part of the basin (Kuh-e Chadur; see Fig. 1 for location) and dies out toward the south. The lower sequence boundary lies above the radiolarites can be considered as a type-I sequence boundary. Transgressive systems tract (TST) deposits of this sequence are not totally recorded in Kuh-e Chadur section. Highstand systems tract (HST) consists of Loftusia Omphalocyclus packstone (E1) of the proximal open marine environment. It changes upward to massive layer of rudist debris packstone to floatstone of the fore reef (talus) and followed by coral bafflestone (D2) and bioclast packstone of back-reef lagoonal environment. The upper boundary (SB1) is marked by dissolution (Fig. 12a), dolomite recrystallization (Fig. 12b), neomorphism, meteoric calcite cementation, and Fe-staining. This sequence downlapped toward the south and did not present in the southern sections, such as Kuh-e Gach and Parak. The thickness of this sequence is about 70 m.

Fig. 12
figure 12

Photomicrographs of distinct diagenetic processes identified in the sequence boundaries. a Dissolution forming isolated vugs. XPL. b Dolomitization. XPL. c Connected vugs as a result of intense dissolution. XPL. d Meteoric spary calcite cement. e Fe-staining of the allochems and matrix. XPL. f Dedolomitization, PPL. g. Brecciation along with Fe-staining, PPL. h Dolomitization. XPL. i Neomorphism in bioclast debris, XPL

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Paleogeographic map (Fig. 11a) of this sequence shows that a carbonate platform was present in the northeast of the basin. Rudist patch reefs in association with corals were formed in this area.

Sequence Maas-2

This sequence is observed only in the northern part (i.e. Chadur section). The lower boundary of this sequence in northern part (Kuh-e Chadur section) is SB1. The thickness of this sequence is about 220 m. The TST consists of intercalation of marl and limestone with Omphalocyclus bioclast packstone of proximal open marine environment. Top of the marly unit (in 105 m) considered as MFS. The HST is composed of rudist debris packstone to floatstone, coral bafflestone, and bioclast floatstone of fore-reef (talus) depositional setting. They change upward to lagoon and tidal flat microfacies (A4 and A3). The upper boundary (SB3) has evidence of karstic features (Fig. 12c) such as calcite cements (Fig. 12d), recrystallization, brecciation, and Fe-staining (Fig. 12e) and pink to red color in the field view was identified as SB1.

Based on paleogeographic map (Fig. 11b, c), it can be concluded that a carbonate platform with rudist patch reef in its inner parts was dominated the study area, during deposition of this sequence. Depocenter of this sequence was located in the northern part, in the Chadur section.

Sequence Maas-3

The upper (SB4) and lower (SB3) boundaries of this sequence are type 1 in northern part of the basin (Kuh-e Chadur section). Intense dissolution and high amount of vuggy porosity, Fe-staining, dolomitization, neomorphism, and brecciation (Fig. 12g) are evidence of exposure. Toward the south, both of them grades to type-II sequence boundary. This is as a result of the formation in the foreland basin. Northern section (Kuh-e Chadur section) is located on the forebulge, while toward the south the other sections formed in the backbulge of the foreland basin. TST (in Kuh-e Chadur section) consists of fine-grained peloid bioclast echinoderm packstone of proximal open marine environment. Toward the south (Kuh-e Parak section), it grades to the pelagic marl of Gurpi Formation and consists of fine-grained bioclast planktonic foraminifera packstone of basinal environment with Globigerina sp., Globotruncana sp., sponge spicule, and other planktonic foraminifera.

In Pishvar-1 well, TST is composed of Oligostegina sp., Heterohelix sp., Pithonella sp., Hedbergella sp., spicule sponge and calcispherula. Maximum flooding surface includes the deepest facies characterized by the high GR log response. In HST, echinoderm, bryozoa debris, and benthic foraminifera can be observed.

Maximum flooding surface (MFS-3) is identified by the presence of Oligostegina sp. in E1 microfacies (in Kuh-e Chadur) which shows the most deepening and shows the highest values of GR on well logs (Pishvar-1). This surface can be correlated with the K180 of Sharland et al. (2001) in other parts of the Arabian Plate (Fig. 13).

Fig. 13
figure 13

Late Cretaceous–Early Maastrichtian Global and regional (Arabian Plate) mean sea-level changes (Haq et al. 1988; Sharland et al. 2001)

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HST consists of rudist zone with D1 and D2 microfacies of fore reef, reef, and back reef, respectively, in northern part (Kuh-e Chadur). Toward the southern part (Kuh- Parak), it grades to outer shelf environment and includes sponge spicule, echinoderm, Rotalia, planktonic foraminifera, and fecal pellets.

In Kuh-e Gach section around the salt plug, this sequence was also completely dolomitized. In TST, the ghost of rudist debris along with Loftusia sp., Omphalocyclus sp., echinoderm, and algae can be observed which are the evidence of deposition in the paleaohigh with a small rudist zone. HST consists of Loftusia, benthic foraminifera, rudist debris, echinoderm, and algae of inner shelf.

In general, the facies is prograded from NE to SW. In northeast, the TST consists of rudist zone of mid shelf which is grades to outer shelf deposits in Kuh-e Parak section and basinal deposits (Gurpi Formation). In northern part, the HST consists of inner/mid shelf deposits, which grades to outer shelf deposits in Kuh-e Parak and Pishvar-1 sections. Maximum flooding surface includes the deepest facies characterized by the high GR log response in subsurface section (Pishvar-1) and marly unit in surface section (Kuh-e Chadur).

Paleaogeographic map of this sequence is present in Fig. 11d, e. As these maps indicate, a carbonate platform with shelf marginal rudist patch reefs is present in the northern parts. Toward the south and southeast, outer shelf and basinal depositional setting are present.

Sequence Maas-4

The upper boundary (SB5) is type-II in Chadur section and identified by facies’ changes form lagoon/back reef to proximal open marine environment. In this section, the TST consists of rudist reef facies which progrades toward the south to basinal Globotruncana marl of the Gurpi Formation in Kuh-e Parak section. HST consists of inner shelf deposits including lagoon to back-reef setting in the northern part. Toward the south, it grades to outer shelf deposits. The lower boundary (SB4) is type-I in Kuh-e Chadur section and type-II toward the south. Palaeogeographic maps of this sequence are presented in Fig. 11f, g. As the figures shown in Kuh-e Chadur section, rudist patch reefs are present in the north and progrades to basin toward the south.

Sequence Maas-5

In the northern part (Kuh-e Chadur), this sequence covers the lower part of Sachun Formation. The lower boundary is placed at top of the Tarbur Formation. The upper boundary is bounded by a regional unconformity (SB6) which is present throughout the Zagros and Arabian Plate (Sharland et al. 2001) (see Fig. 8). In Kuh-e Parak and Gach sections, intense dissolution, dolomitization (Fig. 12h), meteoric calcite cementation (Fig. 9d, i), dedolomitization (Fig. 12e), brecciation, Fe-staining (Fig. 12g) and neomorphism (Fig. 12i) are the evidence of exposure during K/T unconformity. K/T unconformity is an important boundary in geological history that marks with planktonic bioevents (Farouk 2014), faunal change, benthic foraminiferal diversity and assemblages changes (Farouk and Jain 2016) or mass extinction (Keller 2012). In the studied sections, most of fuana such as Loftusia sp., Siderolites sp. Omphalocyclus sp., and Orbitoides sp. were extinct.

In Kuh-e Chadur, the TST consists of intercalation of marl and marly limestone with Siderolites sp., Loftusia sp. echinoid and lamelibranch which was deposited in the outer shelf environment. Toward the south, this sequence grades to top of the Tarbur Formation. Lithology of TST (Kuh-e Parak) is limestone with fine-grained oligostegina echinoderm bioclast packstone (F1). Sponge spicule, Rotalia sp. and Siderolites sp. are also present as subordinate grains. This systems tract was deposited in outer to mid shelf environment.

In Pishvar-1 section, the upper and lower boundaries are distinguished by high GR values. In this section, the TST consists of echinoid, Oligostegina sp., and planktonic foraminifera and Cibicides sp. of outer shelf.

The lower boundary in northern part is a type-I sequence boundary that characterized by Fe-staining, neomorphism and dissolution. Toward the south, it grades to a type-II boundary. In Pishvar-1, the lower boundary is distinguished by high GR values.

In northern part of the basin (Kuh-e Chadur), maximum flooding surface (MF-4) is located at top of a marly unit (in 974 m). In this section, the HST starts with carbonate layer and followed by dolomite and marly limestone. This part has poor fauna and only Valvulinds and Textularids are present. This systems tract was deposited in inner shelf environment in restricted lagoon. It finally ends with anhydrite layer of Cretaceous–Tertiary boundary.

In Kuh-e Parak section, HST consists of thick-bedded dolomite with skeletal Omphalocyclus packstone of lagoonal environment. Omphalocyclus are iron-stained and matrix is replaced by xenotopic dolomite. In Pishvar-1, HST consists of algae, shell fragments, and benthic foraminifera.

In general, from northeast toward the southwest thickness of this sequence decreased dramatically and in Bavush-1, this sequence is not recorded. There is an anomaly in thickness in Kuh-e Gach which is the result of salt plug. As shown in Figs. 10 and 12, the thickness of Maastrichtian deposits decreases dramatically in Kuh-e Gach section. Depositional environment is changed from inner shelf in north to outer shelf in the south and the facies progradationally migrate toward south. Palaeogeographic map of this sequence is presented in Fig. 11h, i.

Tectono-sedimentary evolution

To study tectono-sedimentary evolution of the Maastrichtian sequences, a perpendicular transect to the Zagros trend is selected (Fig. 14). In Kuh-e Chadur, north of the basin, Maastrichtian deposits are thick (1140 m) and include all five sequences. The sequences mostly consist of inner/mid shelf microfacies with rudist zone and proximal open marine environment. Depocenter of the basin was located in the northern part (Kuh-e Chadur). The presence of obducted radiolarites caused over thickening of the crust (Miall 2000) and additionally caused subsequent subsidence in the northern part of the basin (Kuh-e Chadur section). Toward the south (Kuh-e Parak, Pishvar-1 and Bavush-1), the thickness of sequences has decreased gradually. Based on the previous studies, the Maastrichtian deposits were deposited in a foreland basin (Piryaei et al. 2010, 2011). Therefore, the accommodation space decreased.

Fig. 14
figure 14

Cross section with high-resolution sequence stratigraphy through the Maastrichtian deposits

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Based on high-resolution sequence stratigraphy performed along this transect, sequence Maas-1 and Maas-2 present only in northern part of the basin (i.e., Kuh-e Chadur).

Sequence Maas-3 presents in most of the sections. Depocenter in Sequence Maas-3 was located in northeastern part (i.e., Kuh-e Chadur) and the thickness decreased toward the SW. The dominate facies in n northeastern part (i.e. Kuh-e Chadur) are proximal open marine and rudist reefs in this sequence. These facies belt progrades toward the SW and change to basinal facies.

Sequence Maas-4 consists of rudist reef in northeastern part of the basin (i.e. Kuh-e Chadur) which progrades toward the SW to basinal Globotruncana marls of the Gurpi Formation in Kuh-e Parak section. The HST of this sequence in northeastern part (i.e., Kuh-e Chadur) consists of inner shelf deposits including lagoon to back-reef setting in the northern part, while toward the south, it grades to outer shelf deposits.

Sequence Maas-5 in northeastern part (i.e., Kuh-e Chadur) consists of dolomites and evaporites (restricted lagoon) of the (Sachun Formation) and grades to inner shelf deposits (lagoonal deposits) toward the south. In addition, the sequence thickness has also decreased form NE to SW.

Based on facies and thickness variations (Fig. 16), it can be concluded that Maastrichtian deposits are formed in a foreland basin. Carbonate deposits of Tarbur Formation are formed in bulge/forebulge setting and basinal facies of Gurpi Formation formed in back-bulge setting. Migration of depositional environments (Fig. 15), during different phases indicates bulge migration through different phases, which documented by sequence stratigraphic analysis. Based on this data, a schematic tectono-sedimentary model is prepared for Maastrichtian deposits in the eastern part of Fars area (Fig. 16).

Fig. 15
figure 15

Migration of mid-bulge from Maas-1 to -5 sequences during Maastrichtian time interval

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Fig. 16
figure 16

Schematic tectono-sedimentary model of the Maastrichtian deposits in the eastern part of the Fars area

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Conclusions

To reconstruct tectono-sedimentary setting of the Maastrichtian deposits, five sections were selected perpendicular to the Zagros trend (NE–SW) in eastern part of the Fars area. As this transect revealed, thickness of Maastrichtian deposits decreased from NE toward SW. Facies’ variations also show that progradation of microfacies has been occurred from NE toward SW. For example, sequence 3 in NE mainly consists of rudist reef facies which is grade to basinal deposits of Gurpi Formation toward the SW. Sequence 4 is also consists of rudist reefs of mid-shelf deposits in TST which progrades over the basinal facies of Gurpi Formation toward the south. In highstand systems tract, it consists of inner shelf deposits in NE and grades to outer shelf deposits with Oligosteginids, sponge spicule, echinoderm, and Siderolites sp. toward southwest.

In Sequence Maas-5 in NE, HST deposits consist of dolomites and evaporites of restricted lagoonal setting (Sachun Formation) which progrades to outer shelf to inner shelf deposits (in Parak section).

Based on thickness and facies’ variation and depocenter migration, it can be concluded that Maastrichtian deposits in Fars area have been deposited in a foreland basin. In this basin, carbonates of Tarbur Formation have been deposited in forebulge/bulge, while basinal Globotruncana marls of the Gurpi Formation was formed in backbulge setting. Depositional model and paleogeographic maps during five sequences revealed that the depocenter migrates during the time.

In addition, salt tectonics was locally affected the Maastrichtian deposits. Salt tectonics caused formation of a paleohigh along the studied transect. It has resulted into decrease accommodation space and so sediment thickness decreased (e.g., in Kuh-e Gach section). Moreover, salt plug also caused different diagenetic effects including intense dolomitization, dissolution, and formation of anhydrite cements, mainly within the adjacent sediments. Types and intensity of such diagenetic processes play an important role particularly in recognizing the salt movements and related uplifted area.