Introduction

A significant interval of the Cretaceous Period corresponds to expansive epeiric seas that formed vast carbonate platforms (Tucker and Wright 1990; Sanders 1998; Gómez-Gras and Alonso-Zarza 2003; Coimbra et al. 2016, 2017; Wilmsen et al. 2018). Shallow water condition with apparently restricted water circulation is characteristic of epeiric seas. The epeiric platform carbonates provide valuable insights for the reconstruction of sea level and climate of past and by recording the subtle breaks in sedimentation within an otherwise continuous shallow marine sequence. The subaerial exposure associated with the depositional breaks in carbonate succession causes diagenetic modification of the metastable sediments. Diagenetic overprinting alters carbon and oxygen isotope ratios of these shallow marine carbonate successions significantly (Joachimski 1994; Buonocunto et al. 2002). A detailed investigation involving field, petrographical and geochemical approaches, is necessary to characterize the subaerial exposure surfaces within marine carbonates (Meyers and Lohmann 1985; Goldstein 1988; Saller and Moore 1989; Alonso-Zarza et al. 1998, 1999; Gómez-Gras and Alonso-Zarza 2003; Budd et al. 2002).

The Upper Cretaceous sedimentary succession of the Narmada Basin in central India retains the crucial records of the paleogeographic evolution and sea level cycles before the onset of the globally significant Deccan volcanic eruptions. The Bagh Group of the Narmada Basin in central India represents the deposits of the Late Cretaceous marine inundation of the Indian subcontinent. It has received considerable attention for its rich fossil assemblage, including echinoderm (Chiplonkar 1937), bivalve (Chiplonkar and Badve 1976), bryozoa (Guha 1975), ammonite (Kennedy et al. 2003; Gangopadhyay and Bardhan 2000; Jaitly and Ajane 2013 and many others). While sedimentological studies remain focused on the clastic sequence of the Narmada Basin (Singh and Dayal 1979; Bose and Das 1986; Bhattacharya and Jha 2014; Jha et al. 2017), the carbonate deposits are largely ignored. The main objective of this paper is to present depositional and diagenetic histories of the Bagh Group of carbonates in the western part of the Narmada Basin. Further, we investigate the relationship between the depositional environments, diagenesis and stable isotope ratios of carbonates. We present a detailed interpretation of facies and paleo-environment based on field and petrographic investigation and integrate the data with cathodoluminescence pattern and stable isotope ratios of carbonates for a better understanding of the diagenetic history.

Geological and stratigraphic background of Bagh Group

The mixed carbonate–siliciclastic sequence of the Bagh Group comprises Nimar Sandstone, Nodular Limestone and Bryozoan Limestone formations in ascending order of succession. The Bagh Group of rocks is exposed as isolated inliers surrounded by Deccan Traps in west-central India (Fig. 1). The Bagh Group represents the deposits in an easterly transgressing arm of the paleo-Tethys. It crops out at the western part of the ~ 1200 km long Narmada basin.

Fig. 1
figure 1

Geological map showing locations of the study area

Full size image

The Bagh Group unconformably overlies the Precambrian basement and it grades to fluvio-marine Lameta Formation (Tripathi 1995, 2006; Tripathi and Lahiri 2000). The carbonate deposit of the Bagh Group overlies the siliciclastic Nimar Formation and consists of two formations, the Nodular Limestone at the base and the Bryozoan Limestone at the top (Fig. 2). The upper contact of the Bagh Group is controversial. Some authors have recorded an unconformable contact between the marine originated Bagh Group and the continental deposits of the Lameta Formation (Tripathi 2006). However, in the western Narmada basin, the marine sediments of the Bagh Group grade upward to estuarine deposits of the Lameta Formation (Ahmad and Akhtar 1990; Tandon 2000; Bansal et al. 2018, 2019). Sedimentological, paleontological, and ichnological data obtained from the Bagh Group (Singh and Srivastava 1981; Jaitly and Ajane 2013; Bhattacharya and Jha 2014; Jha et al. 2017). The best exposures of the Bagh Group occur mainly on the northern flank of the river Narmada around Chakroad, Kasdana, Baria, Mohi, Karondia, Zeerabad, Phutlibaori, Sitapura and Rampura (Fig. 1). Biostratigraphic investigations provide Cenomanian to Turonian age for the Bagh Group (Kumar et al. 2018). Recently Kumar et al. (op. cit.) provided Cenomanian, Turonian and Coniacian ages for Nimar Sandstone, Nodular Limestone and Bryozoan Limestone, respectively based on ammonite biostratigraphy (Table 1).

Fig. 2
figure 2

Lithocolumn showing the general stratigraphy of the Bagh Group, constituent facies and fossils

Full size image
Table 1 Stable isotopic ratios for the Nodular Limestone Formation and the Bryozoan Limestone Formation
Full size table

Methodology

The current study involves a detailed field investigation of the Bagh Group in Dhar district, central India that covered sections around Man River, Zirabad, Chakroad, Mohi, Ratitalai, Rampura, Karondia, Kosdana, and Phutlibaori (Fig. 1). We collected around 40 samples from ten localities and recorded their precise stratigraphy. Samples were examined using Leica DM 4500P polarizing microscope connected to a Leica DFC420 camera. Thin sections were examined using an optical cathodoluminescence system (CL8200Mk5-2) at 392–400 μA and 17.2 kV in the Sedimentology Laboratory of Jadavpur University. Stable carbon and oxygen isotope ratios of the carbonates were measured in the Stable Isotope Laboratory of the Indian Institute of Science Education and Research, Kolkata. Powdered samples were obtained from fresh surfaces of micritic carbonates and different generations of spars using a microdrilling device capable of high-resolution milling (Micro-Mill) in the Stable Isotope Laboratory of the Indian Institute of Science Education and Research, Kolkata. Approximately 80–100 µg powdered carbonate samples were inserted in glass vials and reacted with 100% phosphoric acid at 80 °C using a KIEL IV online automatic carbonate preparation device connected to MAT 253 mass spectrometer in dual inlet mode. A calcite standard (NBS18) and an internal standard (Z-Carrara) were run to monitor the instrumental drift. All carbon and oxygen isotope ratios are reported using the delta (δ) notation in per mil (‰) relative to the Vienna Pee Dee Belemnite (VPDB). The analytical reproducibility of the standard is ± 0.3‰ (1σ) for δ 13C and ± 0.05‰ (1σ) for δ 18O.

Results

We present facies analysis, luminescence characteristics and stable isotope ratios of constituent carbonates of the Bagh Group in the following discussion.

Facies analysis

The Nodular Limestone Formation overlies the fluvio-marine siliciclastics of the Nimar Formation. Description and interpretation of constituent facies of the Nodular Limestone and Bryozoan Limestone are provided below.

Nodular Limestone

The average thickness of the Nodular Limestone is around 5 m. The Nodular Limestone Formation comprises three non-repetitive facies, wackestone-mudstone alternations, nodular wackestone and poorly bedded wackestone.

Wackestone-mudstone alternation

Overlying the Nimar Formation, the thickness of the wackestone-mudstone alternation varies from 1.7 to 3.1 m. The thickness of constituent beds varies from 3 to 7 cm, and color generally varies from dark gray to pinkish-gray (Fig. 3a). The beds show tabular geometry. The vertical sections of both mudstones and wackestones often exhibit desiccation cracks. A gastropod-rich bed of ~ 5 cm thick occurs at the top of the facies (Fig. 3b). Although primary sedimentary structures are poorly preserved, the beds may exhibit ripple laminae in places. The wackestone contains abundant cacispheres exhibiting small circular to elliptical spar-filled structures (Fig. 4a). The calcispheres are often impregnated by Fe-oxide. The diameters of calcispheres vary from 10 to 150 µm. Most of the bioclasts are dissolved, leaving moldic pores. The voids are mostly filled by geopetal sediments. The wackestone beds often contain peloid, pisoid, intraclast and relics of bioclasts such as gastropods, echinoderms, molluscs and forams.

Fig. 3
figure 3

Field photographs showing wackestone-mudstone alternation facies (a); a gastropod shell-rich bed (b); nodules within the nodular wackestone (c); desiccation cracks (arrows) in vertical section of the nodular wackestone (d, arrow) with root structure (e, arrow); poorly bedded wackestone facies (f) (hammer length = 38, pen length = 14 cm, coin diameter = 2.7 cm)

Full size image
Fig. 4
figure 4

Photomicrographs under plane polarized light: Thin section of wackestone-mudstonre showing intensively calichified biopelsparite consisting of peloids-pisoids, intraclasts and relics of bioclasts (arrows) (a); haphazardly oriented bioclasts and biomolds (b); thin section of nodular wackestone showing a gastropod shell (blue arrow) between nodules (red arrow) (c); rounded to irregular pisoids (red arrows), (d); thin section of poorly bedded wackestone showing gastropoda (yellow arrow) cut across by a root (red arrow), with alveolar texture (blue arrow), (e); hairy, wavy and bifurcated rootlets (blue arrows) with foram (red arrow) (f)

Full size image

The abundance of micrite, poor sorting of bioclasts as well as intact nature of bioclasts reflect a low-energy environment of deposition. The paucity of marine fossils and sparse bioturbation indicates a restricted to semi-restricted intertidal to supratidal environments (Colombié and Strasser 2005). Abundant calcispheres and rare small foraminifers in micritic groundmass corroborate a low-energy depositional setting (Tucker and Wright 1990; Flügel 2004; Sellwood 1993; Adabi et al. 2008; Racey et al. 2016). Overall, the facies indicate a low-energy, upper intertidal to supratidal depositional environment undergoing frequent exposures.

Nodular wackestone

This facies overlies gradationally the wackestone-mudstone alternation and consists predominantly of wackestone. The average thickness of the facies varies from 1.6 to 2.9 m. The facies exhibits distinct nodularity in wackestone beds, which locally grades into packstone. The nodules are primarily made up of lime mudstone. Although most nodules are broadly spherical (diameter 3–7 cm), some are ellipsoidal (Fig. 3c). The vertical sections, as well as bedding planes of the nodular laminated beds often exhibit desiccation cracks and root structures (Fig. 3d, e). These beds are often associated with thin mudstone layers showing irregularly developed, polygonally-cracked surfaces. A 20 cm-thick hardground surface occurs at the top of the nodular wackestone. Brachiopods, pelecypods, and gastropods encrust the hardground surface. It predominantly consists of micrite with abundant gastropods, echinoderms/echinoid spines, and molluscs. The bioclasts comprise both intact and broken shell fragments, which are commonly bored. Biomolds are surrounded by a thin halo of dog-tooth cement; the remaining space is filled with a mosaic of coarse-grained xenotopic cement (Fig. 4b). The main matrix is either micritic or micro-bioclastic. It is characterized by micro-nodular texture of a calichified matrix that has been sometimes cemented by radiaxial type. Small gastropod shells may occur within the matrix (Fig. 4c). Hairy and bifurcated rootlets impregnated by Fe-oxides occur in a few places. Sediments often exhibit in-situ brecciated rootlets and alveolar-septal texture of rhizoconcretions. Pisoid appears as more or less rounded grains, coated with micritic laminae (Fig. 4d). They vary in diameter from 2 to 8 mm. Pisoids appear darker than the surrounding micritic matrix and consist of irregular micrite laminae. The intergranular spaces between pisoids exhibit meniscus cement. Sheet cracks are filled with internal sediment.

The abundance of micrite, poor sorting of bioclasts, as well as good preservation state of bioclasts (echinoderms, gastropods) suggests a low-energy environment of deposition. The presence of desiccation cracks pisoids, rhizoconcretions, alveolar-septal texture, in-situ brecciation and micro-nodulation reflect long periods of subaerial exposure and pedogenesis (Calvet and Julia 1983; Flügel 2004; Pomoni-Papaioannou and Zampetakis-Lekkas 2009). The absence of primary sedimentary structures in nodular beds suggests pedogenic modification. The hardground surface, associated with borings and encrustations, and filled with silt- to sand-sized particles in a micritic to microsparitic matrix, reflects meteoric diagenesis (Zampetakis-Lekkas et al. 2007). Secondary dissolution cavities, principally biomolds filled with internal sediment (vadose silt, sensu Dunham 1969a; Dunham 1969b) and drusy/blocky cement corroborates the meteoric diagenetic modification of the original lime mudstone (Pomoni-Papaioannou and Zampetakis-Lekkas 2009; Pomoni-Papaioannou and Karakitsios 2016). Thus, the sediment surface was lithified rapidly, and colonized by boring and encrusting endofauna and epifauna, following the marine incursion.

Poorly bedded wackestone

Resting on a hardground surface, the topmost facies of the Nodular Limestone Formation consists of poorly bedded wackestones. The beds appear either crudely laminated or massive. Bioclasts are less abundant than those in the nodular wackestone. The thickness of this facies varies from 1.3 to 2.4 m (Fig. 3f). Root traces are often found in-situ. Vertical and horizontal burrows may occur in places. A 20 cm-thick iron-stained hardground develops at the top of the facies, which separate the Nodular Limestone below and the Bryozoan Limestone above. The top part of the Nodular Limestone Formation often contains a few broken fragments of bryozoan.

Petrographic examination reveals alveolar texture and intensive calichification. Rhizoids, which are red spots of Fe-oxides dispersed in the matrix, display alveolar texture and hairy, wavy and bifurcated rootlets (Fig. 4e, f). Pisoids are more or less rounded grains, coated with irregular micritic laminae. The coatings are made up of irregular, dark micrite laminae alternating with lighter micrite layers that may include small pisoids. Bioclasts, mainly gastropod, bivalve and rare foraminifera, occur in patches and are frequently impregnated by Fe-oxides. Bioclasts are often dissolved and their molds are filled with blocky calcite cement. Coarse crystalline spar occurs within the breccia. In some places, hairy desiccation cracks are found (Fig. 4f). The micrite is recrystallized in many places to form microspars. Some anastomosing recrystallized non-fabric selective veins are present with a medium-grained sparry fabric. In some places, micrite appears like vadose silt. The nodules are small and appear darker than the surrounding matrix. Bioclasts are dissolved and partially filled with micrite within the hardground. Some vadose silt may occur within the hardground. Broken fragments of bioclasts such as brachiopods, pelecypods, gastropods and rare foraminifera fill the borings. The abundance of lime mudstone indicates a low-energy environment of deposition (Tucker and Wright 1990; Spence and Tucker 1997; Wilson and Evans 2002; Flügel 2004; Fournier et al. 2004; Rasser et al. 2005; Banerjee et al. 2018). The subaerial exposure condition is evident by the brecciation, root-traces, pisoids, desiccation and recrystallization (Flügel 2004). The brecciation and recrystallization processes tend to be more active in the upper part of the soil horizon. The facies, therefore, indicates pedogenesis during long periods of subaerial exposure (Strasser 1991; Wright 1994; Sattler et al. 2005; Zampetakis-Lekkas et al. 2007; Pomoni-Papaioannou and Zampetakis-Lekkas 2009; Pomoni-Papaioannou and Karakitsios 2016). The deposition possibly took place in an upper intertidal to supratidal environment with profound development of paleosol (Martin-Chivelet and Giménez 1992; Gómez-Gras and Alonso-Zarza 2003).

Facies of the Bryozoan Limestone Formation

The overlying Bryozoan Limestone Formation begins with a cross-stratified rudstone consisting predominantly of broken shells of bryozoans, bivalves, gastropods, brachiopods and echinoids. The Bryozoan Limestone Formation (also known as Coralline Limestone, Barwah Bryozoan Limestone and Chirakhan Limestone) forms low ridges in the study area, sharply overlying the Nodular Limestone Formation. Although the Formation is usually overlain by the fluvio-marine siliciclastic Lameta Formation, in many places, it is capped by the Deccan trap volcanics. The Bryozoan Limestone Formation consists of three facies, viz. cross-stratified rudstone, planar laminated rudstone and faintly laminated packstone. The planar laminated rudstone at the top is glauconitic, with abundant clastics, including quartz, feldspars and mud fragments. Facies constituting the Bryozoan Limestone Formation are described as follows.

Cross-stratified rudstone

The cross-stratified fossiliferous rudstone facies rests on a hardground surface. It is well exposed around Karondia, Zeerabad, Baria, Chakrod, Kosdana, Rampura, Ratilatai and Mohi. The maximum thickness of the facies is ~ 3 m around Karondia. It consists of reddish-brown to whitish brown, thick- to thin-bedded rudstones and exhibits large-scale trough and planar cross-stratification (foreset thickness 1.1–1.5 m) passing upward to small-scale cross-stratification (foreset thickness between 10 and 20 cm). Current ripples may occur on top of the cross-stratified beds (Fig. 5a). Cross-strata show weakly bipolar flood-tide dominated paleocurrent pattern. The facies exhibits broken fragments of bryozoa besides echinoid, brachiopods, bivalve and gastropods. The rudstone is occasionally bioturbated by Thalassinoides. A 5–18 cm-thick, well-sorted, brachiopod-rich, dark gray to greenish-gray bed occurs at the base of the rudstone (Fig. 5b). Brachiopods are randomly distributed consisting of in-situ assemblages and also reworked forms that are highly Fe-oxide impregnated. The rudstone primarily consists of broken fragments of bryozoa (~ 80%), with a few pieces of echinoderm and echinoid spines, gastropods, brachiopods and mollusks (Fig. 6a). Fragments of coral, ostracod and foraminifera may occur locally. The length of bioclasts mostly ranges from 1 to 3 mm. Over-sized, 4–9 mm long bioclasts of echinoderms and brachiopods may occur locally. Most bioclasts, except brachiopod, are dissolved and are strongly impregnated by Fe-oxides (Fig. 5a). Dissolution and micritic coatings obscure the internal structures of bioclasts. Syntaxial overgrowth rims occur on most echinodermal grains. In places, siliciclastic grains are common and allochems are cemented by drusy calcite, enlarging towards the center of the cavities (Fig. 6b). Primary pore filling spar occurs within the chambers of foraminifera. Blocky neomorphic cement with micrite inclusions also occurs (Fig. 6b). Micrite may appear similar to vadose silt (geopetal structure), and alveolar texture and thin rhizoliths are abundant in some intervals.

Fig. 5
figure 5

Field photograph showing a vertical section of the cross-stratified rudstone (a) vertical section of a brachiopod-rich bed in outcrop (b); vertical section of glauconite-bearing planar laminated rudstone (c) bedding plane of oyster-rich bed at the top of the faintly laminated packstone (d); faintly laminated packstone showing sandwich between two bed (e); Thalassinoides on a bedding surface (f); (pen length in a, b, c = 14 cm; hammer length in a = 38 cm, coin diameter in d = 2.7 cm; width of outcrop in b = 23 cm)

Full size image
Fig. 6
figure 6

Photomicrographs of thin sections of cross-stratified rudstone under cross polars showing micritic coatings around bioclasts (red arrows) (a); Photomicrographs under plane polarized light: enlarged dissolution cavities (red arrow) (b); Allochems cemented by drusy spars (c); glauconite within zooids in planar laminated rudstone (d); micritized bioclasts in faintly laminated packstone marked by arrow (e) and borings in echinoderm filled by micrite (red arrow) (f)

Full size image

Good sorting of limestone, absence of micrite, and abundant current structures indicate a high-energy depositional environment. The shape and scale of the cross-stratification indicate the migration of bedforms by current actions. The thinning-up cross-strata, coarse-grained nature of sediments and bipolarity of current structures suggest deposition in tidal channels. The dense packing of shells indicates high-energy depositional condition. Most bioclasts in this facies are characteristic of the lower intertidal environment (Gangopadhyay and Halder 1996; Gangopadhyay and Bardhan 2000; Jaitly and Ajane 2013). The meso- and micro-scale features of the facies indicates deposition in intertidal channels (cf. Calvet and Julia 1983; Pomoni-Papaioannou and Kostopoulou 2008; Pomoni-Papaioannou and Zampetakis-Lekkas 2009). Intense secondary micritization of sediments indicates prolonged calichification.

Planar laminated rudstone

The planar laminated rudstone occurs below and above the faintly laminated rudstone, and is well exposed around Karondia, Ratitalai and Kosdana (Fig. 1). The topmost planar laminated rudstone appears greenish-gray because of the occurrence of glauconite (Fig. 5c). The thickness of the facies varies from 20 cm to 1.3 m. An oyster-rich bed, up to 1.1 m-thick, occurs locally at the top of the facies around Chakrod, Rotitalai, Kosdana, Baria and Karondia (Fig. 5d). Microscopic observation reveals abundant skeletal grains within the rudstone, including fragments of echinoderms, and echinoid spines, bryozoans, gastropods, ostracods, brachiopods mollusks, and foraminifera. The lengths of bioclasts vary in length from 0.5 to 2.5 mm. Most bioclasts are dissolved and coated by Fe-oxides and are strongly bored. Borings are filled with micrite and also exhibits strong micritization (Fig. 6c). The rudstone contains a moderate amount of feldspar and quartz. The content of glauconite grains varies from 10 to ~ 25%. Glauconite occurs predominantly as infillings within zooecial aperture of bryozoa (Fig. 6d) and rarely within pore spaces of echinoid spines and carapace of ostracoda. The average diameter of individual bryozoan infilling varies from 40 μm to 250 μm. Petrographic examination reveals frequent replacement of K-feldspar by glauconite.

The rudstone containing abundant clastics, including quartz, feldspar and lime mud rock fragments indicates supply of siliciclastics in adjacent coastal regions. Planar lamination indicates a high-energy depositional condition. The oyster bed at the top further suggests lower intertidal depositional setting. The presence of glauconite reflects stratigraphic condensation at the top of the Bryozoan Limestone Formation (Banerjee et al. 2016a). Recently Bansal et al. (2019) presented a detailed investigation on the formation of the glauconitic bed within the Bryozoan Limestone and concluded that similar shallow marine glauconite formed along the paleo-Tethyan belt, which is related to the Turonian eustatic sea level rise.

Faintly laminated packstone

This facies consists of poorly bioturbated fossiliferous packstone, sandwiched between two planar laminated rudstones (Fig. 5e). The thickness of the facies ranges from 1 m to 1.5 m. It locally crops out at Kosdana, Baria and Karondia. Thalassinoides may be found occasionally, with burrow diameter ranging from 1 to 2.5 cm (Fig. 5f). Body fossils include entire brachiopod shell and skeletal fragments. The facies is dominated by bioclastic packstones exhibiting grain-supported fabric, with well-sorted allochems (Fig. 6e). Bioclasts, including bryozoan, echinoderm, and bivalve comprise up to 80% of the rock. Most bioclasts are dissolved and filled with calcite spars. Shell fragments are either partly micritized or recrystallized (Fig. 6e, f). Secondary porosity is filled with micrite within shell cavities.

The bioturbated packstone facies represents moderate-energy depositional conditions. Petrographic and biotic attributes of the facies relate it to lower intertidal deposits (cf. Pomoni-Papaioannou and Zampetakis-Lekkas 2009). Thalassinoides burrows suggest near-shore environments with moderately high energy conditions (Singh et al. 1983). Abundant meniscus cement and biomolds further support meteoric vadose diagenetic modification (cf. Pomoni-Papaioannou and Zampetakis-Lekkas 2009).

Paleogeography and depositional sequence of the Bagh carbonates

Overlying the Nimar Formation the Bryozoan Limestone indicates the continuation of the marine transgression and curtailment of siliciclastics. The dominance of lime mudstone in facies constituting the Nodular Limestone Formation indicates a low-energy environment of deposition (Fig. 7). The Nodular Limestone Formation exhibits well-developed calichification due to long-lasting subaerial exposure. Nodule formation is attributed to pedogenic modification driven by root activities. Abundant pedogenic features including root-related structures (rhizoliths), vadose silt (geopetal texture) and pisoids indicate subaerial exposure of the depositional substrate (James and Choquette 1990; Wright 1994; Kraus and Hasiotis 2006). The paucity of marine fossils and sparse bioturbation corroborate deposition in supratidal and upper intertidal settings (Colombié and Strasser 2005; Pomoni-Papaioannou and Karakitsios 2016). The environmental interpretation agrees well with the paleontological data of the Nodular Limestone Formation (cf. Gangopadhyay and Halder 1996; Akhtar and Khan 1997; Gangopadhyay and Bardhan 2000).

Fig. 7
figure 7

Depositional model showing spatial distribution of constituent facies of the Nodular Limestone and the Bryozoan Limestone. While the Nodular Limestone was deposited in low-energy supratidal to upper intertidal environment, the deposition of Bryozoan Limestone took place in lower intertidal environment including tidal channels

Full size image

The Bryozoan Limestone Formation, beginning with a cross-stratified rudstone, suggests an increase in energy condition. Tide-originated sedimentary features like reactivation surface and weak bipolarity in cross-stratification orientation indicate deposition in flood-tide dominated shallow channels in the lower intertidal environment (Fig. 7). Occasional exposure of the sediments causes intense calichification, in-situ brecciation, rhizoliths, alveolar-septal texture and circumgranular cracks. The study of the constituent facies suggests a lower intertidal depositional condition (cf. Pomoni-Papaioannou and Kostopoulou 2008; Pomoni-Papaioannou and Zampetakis-Lekkas 2009). The planar laminated rudstone, containing abundant quartz and feldspar grains corroborates the shallowing of the depositional environment.

The Nimar Formation, consisting primarily of shallow marine sandstones exhibits a transgressive trend (Bose and Das 1986; Bhattacharya and Jha 2014). The transgressive trend continues within the Nodular Limestone and the overlying Bryozoan Limestone (Fig. 8). The deposition of the Nodular Limestone takes place in supratidal and upper intertidal environments undergoing repeated subaerial exposure. The cross-stratified rudstone facies of the Bryozoan Limestone Formation represents channel deposits within the lower intertidal environment. Subaerial exposure surfaces within the Nodular Limestone and Bryozoan Limestone bear subtle evidence of significant breaks in sedimentation. The glauconite bed within the Bryozoan Limestone represents the boundary between the overall transgressive Bagh Group and the prograding Lameta Formation (Fig. 8). Thicknesses of the constituent facies broadly increase towards the open ocean. A stratigraphic condensation near the top of the transgressive systems and the input of clastic supply has allowed the formation of glauconite (Banerjee et al. 2016a, b; Bansal et al. 2019).

Fig. 8
figure 8

Panel showing lateral correlation of Nodular Limestone and Bryozoan Limestone across different sections

Full size image

Diagenetic evolution of carbonates in Bagh Group

Luminescence characteristics

The most important diagenetic processes affecting the Bagh Group of carbonates are cementation, micritization, neomorphism, minor compaction and fracturing. Thin sections of selected samples were investigated for their luminescence patterns. These carbonates exhibit broadly five luminescence types for bioclasts, micrite and three different spars viz. C1, C2 and C3 (Fig. 9a–f). Isotope ratios of all five varieties of carbonates are presented in Figs. 10 and 11.

  • Bioclast: identified on the basis of primary fabric, mostly non-luminescent, bryozoan and oyster shells are found most unaltered

  • Micrite or microspar (M): tiny, non-luminescent to weakly luminescent

  • C1 spar: acicular shape, tiny, non-luminescent to weakly luminescent, immediately overlying grains

  • C2 spar: blocky morphology with drusy fabric, bright luminescence

  • C3 spar: blocky morphology with drusy fabric, non-luminescent to dark, often showing bright to dull zonation.

Fig. 9
figure 9

Photomicrographs under plane polarized light (a, c, d) and corresponding cathodoluminescence images (b, d, f), showing the same field of view (B-bioclast, M-micrite/microspar; C1-marine cement; C2-meteoric cement; C3-burial cement (detailed description in text)

Full size image
Fig. 10
figure 10

adapted from Hudson (1977). The straight line indicates burial diagenetic trend (adapted from Choquette and James, 1987)

Cross-plot between δ18O and δ13C different facies of micrite/microspar and bioclast in the Nodular Limestone Formation (a) and different cements (b). Fields of average marine limestone (1), fresh water limestone (2) and meteoric cements (3) are

Full size image
Fig. 11
figure 11

adapted from Hudson (1977). The straight line indicates burial diagenetic trend (adapted from Choquette and James, 1987)

Cross-plot between δ18O and δ13C different facies of micrite/microspar and bioclast in the Bryozoan Limestone Formation (a) and different cements (b). Fields of average marine limestone (1), fresh water limestone (2) and meteoric cements (3) are

Full size image

Morphology, as well as the luminescence pattern, indicates marine, meteoric and burial origin for C1, C2 and C3 spars, respectively (cf. Machel et al. 1991; Machel 2000; Major 1991; Boggs and Krinsley 2006; Brand et al. 2012; Sarkar et al. 2014). Morphology of cements and their luminescence characteristics reveal that the marine-originated C1 spar was succeeded by the meteoric cementation (C2 spar). This was followed by the pedogenesis, and finally, cementation took place in burial condition (C3 spar).

Isotopic ratios of Bagh carbonates

Carbon and oxygen isotopic ratios of carbonate samples were determined separately for micrite/microspar, bioclasts (bryozoa and oyster) C1, C2 and C3 spars using the Micro-Mill facility. Isotope ratios are provided in Table 1.

Isotope ratios of the Nodular Limestone Formation

The δ13C ratios of micrite to microspar and oyster and bryozoan shell samples of the Nodular Limestone Formation range from − 2.3‰ to + 2.9‰. Around 90% of data plot between − 2‰ and + 2‰. The δ18O ratios of micrite to microspar, as well as oyster and bryozoa samples of the Nodular Limestone mostly range between − 3‰ and − 7‰. The δ18O values of samples of poorly bedded wackestone appear most consistent, ranging between − 5.6‰ and − 6.6‰ (av. − 6.6‰). The δ13C and δ18O ratios of micrite/microspar samples of the Nodular Limestone Formation exhibit poor correlation (r2 = 0.3; p = 0.03; Fig. 10a). A cross plot between δ13C and δ18O ratios distinguishes three varieties of cements based on separate clusters of data (Fig. 10b). The δ13C and δ18O ratios of C1 cement range from 0.9‰ to − 0.8‰ and from − 4.8‰ to − 9.1‰ respectively. The δ13C and δ18O ratios for the C2 cement range from 2.0‰ to − 5.6‰ and from − 4.3‰ to − 10.8‰ respectively. The δ13C and δ18O ratios for the burial-originated C3 cement range from 1.1‰ to − 1.6‰ and from − 12.2‰ to − 20.2‰ respectively.

Isotope ratios of the Bryozoan Limestone Formation

The δ13C ratios of micrite/microspar samples as well as oyster and bryozoan shells of the Bryozoan Limestone Formation range from − 1.1‰ to − 4.0‰. The δ13C ratios of samples of different facies overlap partially. The δ18O ratios of samples of the Bryozoan Limestone range from − 5.3‰ to − 9.2‰. The δ18O and δ13C ratios reveal a moderate correlation between δ18O and δ13C (r2 = 0.6, p = 0.01) (Fig. 11a). The δ13C and δ18O ratios for C1 cement range from 0.2‰ to − 1.5‰ and from − 1.5‰ to − 4.4‰ respectively (Fig. 11b). The δ13C and δ18O ratios for the C2 cement range from − 0.9‰ to − 3.72‰ and − 5.8‰ to − 10.2‰ respectively. While the δ13C and δ18O ratios for the C3 cement range from − 1.0‰ to − 3.0‰ and − 9.5‰ to − 18‰ respectively.

Diagenetic implications of isotope ratios of Bagh carbonates

The δ13C ratios of marine carbonate sediments remain close to 0‰, whereas carbonates precipitating from freshwater exhibit variably depleted ratios (Hudson 1977; Tucker and Wright 1990; Moore 2001; Bowen and Wilkinson 2002; Banerjee et al. 2006; Blanchet et al. 2012; Henderson et al. 2018; Scorrer et al. 2019 and many others). The diagenetic alteration tends to decrease both δ13C and δ18O ratios of marine carbonates (Gilleaudeau et al. 2018 and references therein). Petrographic investigation and the relationship between δ13C‰ and δ18O‰ ratios provide an estimation of the diagenetic resetting (Hudson 1977; Burdett et al. 1990; Banner and Hanson 1990; Joachimski 1994; Armstrong-Altrin et al. 2009; Gilleaudeau et al. 2018). The δ13C ratios of carbonates may remain near primary at a low water/rock interaction ratio (Hudson 1977; Banner and Hanson 1990; Magaritz et al. 1991; Gilleaudeau et al. 2018). The Upper Cretaceous marine carbonate has typical δ13C ratios between 1 to 3 ‰ (Wendler 2013; Jarvis et al. 2015). The δ18O ratios are usually less than − 4‰ for the Upper Cretaceous pelagic carbonates (Jarvis et al. 2015). However, the δ18O ratios are sensitive to latitude and climate. Isotope ratios of cement in the Nodular Limestone and Bryozoan Limestone formations are compared with those of Hudson (1977) to recognize the extent of diagenetic alteration (Figs.10 and 11). Isotope ratios of Bagh carbonates provide the following information regarding the diagenesis of carbonates.

  1. (a)

    The δ13C ratios of none of the micrite/microspar samples of the Bryozoan Limestone Formation plot within the range of Late Cretaceous carbonates (cf. Wendler 2013; Jarvis et al. 2015). On the contrary, many of the micrite/microspar samples of the Nodular Limestone Formation bear the original isotopic signatures of the Late Cretaceous Ocean.

  2. (b)

    Isotope ratios of C1 spars in both Nodular Limestone and Bryozoan Limestone broadly overlap with those of micrite/microspar. All data plot within the domain of the average marine limestone of Hudson (1977) (Figs. 10b and 11b). However, C1 cement in the Bryozoan Limestone Formation exhibits depleted δ13C ratios compared to those of the Nodular Limestone Formation.

  3. (c)

    C2 spars in both Nodular Limestone and Bryozoan Limestone are characterized by depleted δ13C and δ18O ratios and bright luminescence. The data points of δ13C and δ18O ratios plot within the fields of meteoric diagenetic cement and freshwater limestone of Hudson (1977), reflecting the diagenetic resetting of both oxygen and carbon isotopes. Petrographic evidence like calichification, in-situ brecciation, rhizoconcretions, pisoids, alveolar-septal texture, and circumgranular cracks establish the soil formation beyond doubt in both Nodular Limestone Bryozoan Limestone.

  4. (d)

    C3 spars in the Nodular Limestone Formation is characterized by highly depleted δ18O ratios, varying from − 10‰ to − 18‰ and weakly luminescing cement (Fig. 9b). Both meteoric and burial fluids can impart δ18O values between − 15 and − 10‰ on diagenetic carbonate (e.g., Derry 2010; Gilleaudeau et al. 2018). However, C3 cement is depleted in δ18O ratios, with nearly unaltered δ13C ratios. The isotopic signature, as well as luminescence characteristics of cements indicates burial diagenetic resetting (Fig. 9b). The comparison of isotope ratios with those of Choquette and James (1987) corroborates this interpretation. Therefore, C3 spars in both Nodular Limestone and Bryozoan Limestone, indicate shallow burial diagenetic cement.

Discussion

Although previous workers inferred the shallow marine origin for the Bagh carbonates, the facies study remained completely ignored and exposure surfaces within the succession remained unnoticed. Bagh carbonates were investigated for stable isotope ratios (cf. Bhattacharya et al. 1997). However, the meteoric diagenetic alteration of the original marine signature was overlooked. The present investigation confirms the deposition of Bagh carbonates within a shallow marine environment undergoing periodic exposures. The Nodular Limestone Formation, exhibiting well developed pedogenic and vadose diagenetic features, including geopetal, vadose pisoids, and rhizoconcretions, represents upper intertidal to supratidal deposits (Fig. 7). While the Bryozoan Limestone Formation exhibits abundant current structures and consists of coarse bioclasts, implying a moderately high-energy, lower intertidal depositional setting. The presence of reactivation surfaces, as well as weakly bipolar paleocurrent pattern, indicate the deposition of the cross-stratified rudstone in flood-dominated tidal channels (Fig. 7). Periodic exposure of the sediments causes intense calichification, cavities filled with coarse-grained drusy and poikilotopic cement, in-situ brecciation, rhizoconcretions, alveolar-septal texture, and circumgranular cracks. Overall facies character reveals a lower intertidal depositional condition.

The δ13C and δ18O ratios of Cretaceous shallow marine carbonates are mostly discontinuous (Heba et al. 2009). Besides, the isotope data bears the combined influence of several factors, including depositional setting, diagenetic resetting and paleosalinity (Vincent et al. 2006; Heba et al. 2009). The δ13C and δ18O ratios and luminescence patterns of Bagh carbonates corroborate the periodic subaerial exposure and extensive meteoric diagenetic resetting of the shallow marine carbonate deposits. Although pervasive diagenetic resetting has obscured the original δ13C and δ18O ratios of micritic carbonates and C1 spars retain the least altered signatures.

Epeiric platforms do not have any modern analog. The interpretation of the facies sequence of the Late Cretaceous Bagh Group provides snapshots for ancient epicontinental platforms. The carbonate platform develops during the Cretaceous by the inundation of large parts of the continents in response to worldwide sea level highstand. The Upper Cretaceous carbonate deposits of Bagh beds, is comparable to Turonian-Coniacian marine transgressive records in Bireno and Douleb members in Zebels Barda (Jaballah and Negra 2016) and central and southern atlas of Tunisia (Camoin 1993). Platform carbonates of Mesozoic constitute important archives of past environmental change (Föllmi and Godet 2013). These deposits are influenced by processes operating at a local, regional and global scale, which are often difficult to distinguish. However, a multi-proxy approach involving thorough analysis of sedimentary sequences and investigation of samples for calcium isotopes, magnesium isotopes and ion microprobes is likely to disentangle more information regarding environmental implications of Cretaceous platform carbonates.

Conclusion

The study of the carbonate sequence of the Bagh Group leads to the following conclusions.

  1. (a)

    Facies study indicates shallow marine deposition of the Bagh Group of carbonates, which underwent frequent exposure. While the Nodular Limestone Formation represents deposits within the upper intertidal to supratidal depositional settings, the Bryozoan Limestone relates to deposition in the lower intertidal setting.

  2. (b)

    Petrographic evidence like calichification, in-situ brecciation, rhizoconcretions, pisoids, alveolar-septa texture, and circumgranular cracks confirm the pedogenesis in both Nodular Limestone and Bryozoan Limestone formations. The conspicuous nodularity within the Nodular Limestone Formation relates to the pedogenetic process.

  3. (c)

    The Bagh carbonate succession, comprising the Nodular Limestone and the Bryozoan Limestone, represents the upper part of a transgressive systems tract. The glauconite bed at the top of the Bryozoan Limestone Formation marks the boundary between overall transgressive Bagh Group and the prograding Lameta Formation.

  4. (d)

    Isotope ratios of carbonate constituents within the Bagh Group, including bioclast, micrite/microspar reveal a thorough diagenetic resetting. While the δ13C ratios of a few micrite samples in the Nodular Limestone Formation bear the original isotopic signatures of seawater during the Upper Cretaceous, all samples of the Bryozoan Limestone are thoroughly altered. Luminescence pattern and isotope ratios clearly distinguish between unaltered marine carbonates, and meteoric and burial diagenetic cements.