1 Introduction

The Cretaceous global relative sea-level rise caused expansive epeiric seas that formed vast carbonate platforms, preserving signatures of marine transgression and regression (Coimbra et al. 2016, 2017; Wilmsen et al. 2018; Ruidas et al. 2020).

In the eastern Tethyan Region of central India, platform sediments are represented by the Turonian–Coniacian Bagh Group carbonates. They are formed in a (sub)tropical climate at a paleolatitude of about 30oS (Barron et al. 1981), supposedly as an eastern arm of the Tethys Sea transgressed through an intracratonic rift basin (Agarwal 1986; Acharyya and Lahiri 1991; Acharyya and Roy 2000; Tripathi 2006; Kumar et al. 2018).

The Bagh Group carbonates are shallow marine carbonates like the Red Bar and Rock River deposits of Eastern Wyoming (Jacobs 2020), the late Cretaceous chalk of NW Europe (Voigt 1959; Zijlstra 1995), or the carbonate platforms of the Adriatic (Skelton et al. 2003).

This research concerns a detailed investigation of the Bagh Group exposures in the Dhar district of central India (Survey of India toposheet 1:50000, No 8. 46N/3L 46J/15). The ~10 m thick Bagh Group sediments are well exposed along the northern flank of the river Narmada near Chakrud, Kasdana, Baria, Mohi, Karondia, Zeerabad, Phutlibaori, Sitapuri, and Rampura (figure 1A, B).

Figure 1
figure 1

(A) Geological map showing outcrops of Bagh Group, Lameta Formation and locations of the study area in central India. (B) Paleogeographic map showing outcrops of the Bagh Group in central India during the Late Cretaceous (Turonian) (Singh 1981). Marine palaeo shoreline from west up to Jabalpur and beyond; Green patches are outcrops of Bagh sediments.

Full size image

The Bagh Group contains laterally continuous hardgrounds, which are cemented, bored and encrusted layers that formed during times of non-deposition and intense sea bottom lithification (Ruidas et al. 2018). They are prominent stratigraphic markers within this overall transgressive succession (Shitole et al. 2021). They have been reported elsewhere from deposits of shallow marine environments from the Turonian–Coniacian stages. Moreover, marine hardgrounds may represent the physical expression of pivotal time intervals in Earth's climatic and evolutionary history (Olszewska-Nejbert 2004; Christ et al. 2015).

The regular rhythmic bedding of the cemented Bagh Group carbonates suggests earth-orbital forcing of climate that influenced sedimentation rates and early diagenetic lithification. Studying the Bagh Group hardgrounds leads to a better understanding of the relationship between sedimentation rates and early diagenetic lithification of the carbonate sediment. Recognising bed-by-bed variations of sedimentation rate allows for a more detailed reconstruction of chronostratigraphy in support of biostratigraphy, where the hardgrounds provide excellent marker beds that may aid in recognition of eventual hiatus.

The Bagh Group hardgrounds have been investigated in detail, studying field outcrops, identifying petrographic microfacies in thin sections, and analysing stable carbon and oxygen isotope signature of carbonate micro-samples. The results are used to discuss the dynamics of the depositional environment, the timing of the hardground genesis, and the importance of the Bagh Group carbonates for the reconstruction of the Upper Cretaceous paleo-environment.

2 Lithostratigraphy

Within the lithostratigraphy of the Narmada basin (Shitole et al. 2021), our investigated composite succession of the Bagh Group (figure 2) shows the Nimar Formation, the Nodular Limestone Formation, and the Bryozoan Limestone Formation (Ruidas et al. 2018). The Cenomanian fluvio-marine siliciclastic Nimar Formation rests on the Precambrian crystalline rocks (Sarkar 1973). The siliciclastic deposits are overlain by marine carbonates of the Turonian Nodular Limestone Formation and the Coniacian Bryozoan Limestone Formation. The carbonate deposits are overlain by the Maastrichtian fluvio-marine, siliciclastic Lameta Formation or by flood basalts of the Deccan Traps (i.e., at Chirakhan).

Figure 2
figure 2

Stratigraphic log of the Bagh Group with detailed log of bedding as exposed in the Ratitalai section, indicated by vertical variation of horizontally averaged pixel colour (red, green, blue) values of darker and lighter layers, and the composite section as exposed around the Manawar area.

Full size image

The Nodular Limestone Formation can be further divided into three members (Ruidas et al. 2020). The lower wackestone–mudstone member (WMM) varies from three to five meters in thickness, is mainly planar bedded with the colour varying from dark to creamy white and grey to pinkish grey, and with individual bed thickness between 6 and 15 cm. It is essentially a wackestone–mudstone alternation, characterised by a paucity of fossils and a low degree of bioturbation. This lower member ends with a sharp boundary and at Rampura, this horizon occurs between a pink layer below and a few centimetre-thick clay layer above. The middle nodular wackestone member (NWM) is slightly lighter in colour and the beds are more nodular than the lower member. The middle member is rich in invertebrate fossils and is essentially a wackestone. The middle member ends with a planar erosion surface on top of an intensely burrowed layer that developed into a bored hardground. The upper poorly bedded wackestone member (PBWM) is characterised by a distinct nodularity, and it has a high proportion of argillaceous material in the form of relatively thick seams wrapping around the limestone nodules. The upper member is essentially a poorly bedded wackestone, ending with a hardground.

The overlying Bryozoan Limestone Formation (BLF) is more resistant to weathering and forms low ridges in outcrop. It consists of cross-bedded carbonates that locally end in firm- or hardgrounds.

3 Palaeobiogeography

The Tethys Sea of the Late Cretaceous was so wide that the epicontinental faunas of different areas were dissimilar and therefore, many workers preferred to subdivide the Tethyan Realm into faunal provinces (Kennedy et al. 2003). The spreading of the seafloor and the concomitant breakup of Gondwanaland created the Indian Ocean influencing the eastern Tethyan Sea during the Late Cretaceous. Characteristic fauna of, especially ammonites, occurs in Madagascar, southern India, and Japan, constituting the Indo-Pacific Faunal Realm sensu Matsumoto (1973). In the Bagh Group, various affinities of ammonite faunas are demonstrated by Placenticeratidae and ancillary Coilopoceratidae. Placenticeratidae from the Coniacian horizons in the Nodular Limestone Formation is represented by a complete population structure, which was also found in Madagascar and Zululand, South Africa. Besides, it also occurs sporadically in Angola and in the Alphard Group of the Cape Province, South Africa (Gangopadhyay and Bardhan 2000). Coilopoceras in the Bagh Group was excessively and subjectively split. A thorough systematic revision reveals the presence of only a highly variable dimorphic species of Coilopoceras, which was recorded from the base of the Nodular Limestone and was assigned a Middle–Late Turonian age (Kennedy et al. 2003).

4 Biostratigraphy

Kennedy et al. (2003) reported Prionocyclus germari (Reuss 1845), a marker ammonite of the latest Turonian occurring in the Bagh Group. The type species of Barrosiceras, i.e., B. onilahyense (Basse 1947), having a wide biogeographic distribution cutting across several faunal provinces, is found in the Middle Coniacian beds of the Bagh Group (Gangopadhyay and Bardhan 2000).

Recently, Kumar et al. (2018) proposed Cenomanian, Turonian, and Coniacian ages for the Nimar Sandstone Formation, the Nodular Limestone Formation, and the Bryozoan Limestone Formation, respectively (figure 2). Based on the timing of marine transgression and occurrence of the guide fossils ammonite Placenticeras kaffrarium (Etheridge 1904) and inoceramid Volviceramus involutus (Sowerby 1828), the age of the Nodular Limestone has been refined to Late-Middle or early Late Turonian (Kennedy et al. 2003) to Early-Middle Coniacian (Gangopadhyay and Bardhan 2000). The occurrence of Coniacian large inoceramids Platyceramus mantelli (de Mercey 1872), at the lower part, and the ammonite Barroisiceras onilahyense, at the upper part of the Bryozoan Limestone Formation, respectively, indicate that the Bagh Group sedimentation continued till the end of the Middle Coniacian (Ganguly and Bardhan 1993). Gangopadhyay and Bardhan (2000) also proposed a Middle Coniacian age for the Bryozoan Limestone based on a new collignoniceratid ammonite. Jaitly and Ajane (2013) reviewed the biostratigraphy and mentioned a Turonian age for the Nodular Limestone Formation and a Coniacian age for the Bryozoan Limestone Formation. Thus, the overall consideration of the biostratigraphic data points to a Turonian to Coniacian age range for the Bagh Group carbonates (figure 2, Ruidas et al. 2018).

5 Methodology

A detailed bed-by-bed analysis of exposures of the Bagh Group in the Narmada basin was carried out to prepare a graphic log (figure 2). Around 50 samples were collected from 10 localities around Man River, Zirabad, Chakrud, Ratitalai, Rampura, Karondia, Kosdana, and Phutlibaori (figure 1A).

Petrography of thin sections of samples was examined using a Leica DM 4500P polarising light microscope connected to a Leica DFC420 camera.

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 carbonates were obtained from fresh surfaces of micrite and different generations of cement using a micro-drilling device capable of high-resolution milling (Micro-Mill). Approximately 80–100 μg powdered carbonate samples were inserted in glass vials and allowed to react with 100% phosphoric acid at 80°C using a KIEL IV online automatic carbonate preparation device connected to a 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 stable carbon and oxygen isotope ratios are reported using the delta (δ) notation per mil (‰) relative to the Vienna Pee Dee Belemnite (VPDB). The analytical reproducibility of the standard is ± 0.3‰ (1σ) for δ13C‰ and ± 0.05‰ (1σ) for δ18O‰.

6 Results

6.1 The hardground between the middle and upper members of the Nodular Limestone Formation

The first and oldest hardground is well exposed in the Manawar area. It occurs at the top of the middle nodular wackestone member (NWM) and below the upper poorly bedded wackestone member (PBWM). It is characterised by a brownish or pinkish-yellow colour, extremely hard in the Karondia section, and extensively burrowed, bored, and encrusted. Boring and burrows are strongly impregnated by Fe-oxides. The diameter of the borings is 0.5–4.5 mm. Two types of borings are present, Gastrochaenolites and Trypanites (Taylor and Badve 1995). At Khod–Chikhali, the hardground is glauconitized and bored by Trypanites. The vertical sections, as well as bedding planes of the hardground, often exhibit desiccation cracks and root structures. In the Rampura section, the surface is strongly micritised and recrystallised. Dissolution vugs and fissures penetrate perpendicularly, 5–15 cm downwards into the bed. Fossil roots, root moulds, microscopic rhizoliths, and other fabrics characteristic of calcretic soil development, like soil pisoids and mottling of calcrete, are found (figure 3a). Some cavities are filled with sediment containing angular mm- to cm-sized fragments of the host rock and, rarely, yellow to brown marly-clayey material.

Figure 3
figure 3

Field photograph showing vertical section of the 1st hardground between middle and upper members of the Nodular Limestone Formation. (a) Vertical section of hardground in pedogenic limestones with root structures (arrows); (b) borings are strongly impregnated by Fe-oxide (marker pen length = 13 cm); (c) plan view showing boring (blue arrow) and encrusting oyster (red arrow) (coin diameter = 2.7 cm); and (d) contact between middle and upper nodular limestone members. Photomicroscopic views under plane-polarized light: (e) Boring (red arrow), hairy, wavy and bifurcated rootlets (yellow arrow) with micrite coating and boring of bioclasts (blue arrow); and (f) ferrugination by various Fe mineralization phases inside, alongside and outside of the boring, where a faintly equidimensional habit of opaque iron mineralization suggests oxidized pyrite.

Full size image

Petrographic examination reveals that the borings have been filled by vadose silt consisting of silt- to sand-sized particles in a micritic to microsparitic matrix (figure 3e, f). Bioclasts have been coated by Fe-oxides and are strongly bored. Benthic Foraminiferas are very rare and present inside brachiopods, pelecypods, and gastropods. The frequently bored bioclasts are composed of broken shell fragments and echinoderm skeletal elements. Thin desiccation cracks are very common.

6.2 The hardground between the Nodular Limestone Formation and the Bryozoan Limestone Formation

The second hardground occurs at the top of the Nodular Limestone Formation and below the Bryozoan Limestone Formation (figure 4a–d). It is a 20-cm thick layer with a rust-brown coloured surface that is strongly impregnated by Fe-oxides. Bioclasts are frequently bored with echinoderm and bryozoan remains (figure 4f). Burrows of the moderate to intense omission suite (Ruidas et al. 2020) are commonly filled with the same sediment as found in the overlying facies. The surface is encrusted with large and small oysters. Some of the oysters post-date and others pre-date the Fe mineralisation phase. At Zirabad, the hardground is bored by Gastrochaenolites, subsequently eroded and truncated, and encrusted by Chiplonkarina and oysters (Taylor and Badve 1995).

Figure 4
figure 4

(a) Field photograph showing vertical section of 2nd hardground which separates the Nodular Limestone Formation below (red arrow) from the Bryozoan Limestone Formation above (blue arrow); (b) plan view showing boring and Thalassinoides burrows (marker pen length = 13 cm); (c) vertical section of hardground (red arrow) overlain by bryozoan mat and underlain by nodular limestone (blue arrow); and (d) plan view showing boring and Thalassinoides burrows (marker pen length = 13 cm). Photomicroscopic views under plane-polarized light: (e) fenestral structures inside of borings; (f) intensely bored echinoid fragment with micritization (red arrow); (g) Ferrugination around boring cut across by a root and a desiccation crack (blue arrow); and (h) boring (red arrow) and surrounding materials.

Full size image

Petrographically, this hardground of highly bioturbated carbonate shows bioclasts that are dissolved (figure 4e) and partially filled with micrite. Some vadose silt may occur within the hardground. Brachiopods, pelecypods, gastropods, and rare foraminifera fill the borings that often contain a few fragments of bryozoans. Micrite and bioclasts have been impregnated with Fe-oxides (figure 4g). Ferrugination around a boring is cut across by a root and a desiccation crack (figure 4g).

6.3 The hardground within the Bryozoan Limestone Formation

The third hardground is present within the Bryozoan Limestone Formation (figures 5a–d). It occurs at the top of one of the cross-stratified units of bryozoan limestone that higher up in the succession-end with firm grounds. The carbonate is a strongly bioturbated wackestone and locally, a packstone. Thalassinoides are common burrows (figures 5a, 6a–b), filled with yellow to dark-coloured sediment. They consist of vertical or inclined, straight or branched, cylindrical shafts and horizontal tunnels with variable interlacements in Y-shape (figures 5a, 6a–b). The burrows are strongly impregnated by Fe-oxides. The diameter of the burrows ranges from 0.6 to 4.5 cm. The burrows are mostly represented by casts, and fragments of shafts are usually characterised by circular cross-sections, while tunnels exhibit oval or ellipsoidal sections depending on host rock lithology.

Figure 5
figure 5

(a) Field photograph showing bedding plane of thoroughly bioturbated packstone (pen length = 14 cm); (b) plan view showing boring in the hardground (red arrow); (c) details of the surface of the hardground in between bryozoan limestones, marked by a red arrow (hammer length = 38 cm); and (d) oysters look like pebbles in planar view. Photomicroscopic views under plane-polarized light: (e) borings filled by sparite (red arrow) and bryozoan that suffered strong micritization and boring; and (f) boring filled by bioclasts (red arrow) and crack filled by sparite (blue arrow).

Full size image
Figure 6
figure 6

(a) Surface of fractured bryozoan limestone running parallel to bedding and showing well-developed Thalassinoides burrows (red arrow) and (b) plan view of hardground surface showing bioturbation (red arrow); (c) details with many small Trypanites borings (arrow) and few large borings upper right of coin (coin diameter = 2.7 cm).

Full size image

Post-depositional authigenic minerals are represented by calcite cement and (oxidised) pyrite. Bioclasts are mainly from oysters and bivalves, pebble- to cobble-sized, appearing reworked and partially filling pre-lithification burrows (figure 5d). Numerous Trypanites borings occur on the surface of the hardground. The Trypanites borings have a somewhat patchy distribution, with up to 100 borings counted per 10 cm2. The diameter of the borings is 0.5–20.4 mm (figure 6c). The apertures are circular, in some cases slightly oval, and the apertures of some borings are merged.

Petrographically, the carbonate of this hardground shows signs of strong bioturbation, and bioclasts are dissolved and partially filled with micrite. Bioclast shell fragments are partly micritised and recrystallised. The fine-grained bioclast-bearing micrite fills about 30% of the volume. Most of the dissolved bioclasts are impregnated/coated by Fe-oxides and are strongly bored (figure 5e–f).

7 Stable carbon and oxygen isotopes

The carbonate isotope signature was measured for samples from the middle (NWM) and upper (PBWM) members of the Nodular Limestone Formation and from the Bryozoan Limestone Formation (BLF). Carbon and oxygen isotopes were measured in the carbonate of the micritic matrix (figure 7) and for the hardgrounds, also in the carbonate of the sparitic cement (table 1; figures 8–10).

Figure 7
figure 7

Cross-plot of stable carbon and oxygen isotope ratios of matrix from Bagh Group Limestones with hardgrounds; Nodular Wackestone Member (NWM), Poorly Bedded Wackestone Member (PBWM) and Bryozoan Limestone Formation (BLF), with hardground samples indicated (-H).

Full size image
Figure 8
figure 8

Cross-plot of stable carbon and oxygen isotope ratios of cement from Bagh Group hardgrounds; Nodular Wackestone Member (NWM), Poorly Bedded Wackestone Member (PBWM) and Bryozoan Limestone Formation (BLF), with hardground samples indicated (-H).

Full size image
Table 1 Stable isotope signature of Bagh Group micro-samples relative to PDB standard, from Nodular Limestone Formation's middle Nodular Wackestone Member (NWM), upper Poorly Bedded Wackestone Member (PBWM) and Bryozoan Limestone Formation (BLF), with hardground samples indicated (-H) and with distinction of micritic matrix or sparry cement.
Full size table
Figure 9
figure 9

Schematic depiction of the variation of stable carbon isotope values of samples in stratigraphic succession; Nodular Wackestone Member (NWM), Poorly Bedded Wackestone Member (PBWM) and Bryozoan Limestone Formation (BLF), with hardground samples indicated (-H).

Full size image
Figure 10
figure 10

Schematic depiction of the variation of stable oxygen isotope values of samples in stratigraphic succession; Nodular Wackestone Member (NWM), Poorly Bedded Wackestone Member (PBWM) and Bryozoan Limestone Formation (BLF), with hardground samples indicated (-H).

Full size image

The δ13C‰ of the matrix varies between –4 and 2, and it decreases from the lower to the upper units (figure 9). Contemporaneous sequences elsewhere also show a decrease of δ13C‰, however, from about 3 to 1. Hence the Bagh Group matrix carbonates are thus lighter in δ13C‰ (Jarvis et al. 2006; Li et al. 2006). The δ18O‰ of the matrix varies between –10 and –4, and it also may decrease slightly from lower to upper units (figure 10). Contemporaneous sequences elsewhere show less decrease or even increase from –6 to –5. The Bagh Group matrix carbonates are thus also lighter in δ18O‰.

The stable carbon and oxygen isotope signature of the matrix of hardgrounds and non-hardgrounds is similar (figure 7). The signature of the cement from the hardgrounds allows for a distinction between the two types of cement. Cement with δ13C‰ and δ18O‰ is like that of the matrix and cement with a stable isotope signature that is different from that of the matrix. The latter shows much more negative δ18O‰ down to –20, while the δ13C‰ values tend to concentrate between 0 and –1 as the lighter δ13C‰ values are absent for cement with very negative δ18O‰ (figure 8).

8 Discussion

Although the nodular to massive lithification of the Bagh Group carbonates appear to have been affected by late burial stylolitization (Kahsnitz and Willems 2017) with non-sutured seam solution, deformation, and compression of argillaceous limestone beds (Ruidas et al. 2018), the early diagenetic lithification is considered still sufficiently preserved to allow for use in the reconstruction of the depositional environment.

8.1 Environmental reconstruction of the Bagh Group hardgrounds

Ruidas et al. (2020) have argued that the carbonates of the Bagh Group are a transgressive succession of tropical marine sediments deposited in a shallow environment, deepening from predominantly supratidal to intertidal and finally subtidal conditions (figure 11). The changing facies of the successive hardgrounds in the Bagh Group appear to reflect this deepening trend.

Figure 11
figure 11

Depositional model showing spatial distribution of hardground surfaces in Bagh Group. Hardground-1 and 2 were deposited in low-energy supratidal to upper intertidal environment (A, B), the hardground-3 formed in the lower intertidal to subtidal environment with tidal channels (C) (Ruidas et al. 2020).

Full size image

Carbonate hardgrounds are defined as carbonate seafloors that hardened in-situ into firm grounds by the precipitation of carbonate cement in the primary pore spaces and that were subsequently eroded, exposed, encrusted, and bored (Wilson and Palmer 1992; Bodenbender et al. 1989; Vinn and Wilson 2010; Vinn and Toom 2015). In the Bagh Group, cyclic variation in cementation and hydrodynamics produced nodular lithification, firm grounds, and three carbonate hardgrounds. Presumably, amongst the various mechanisms proposed for hardground genesis, the lithification of chalk (Kennedy and Garrison 1975) with its typical omission surface ichnofacies (Bromley 1975) seems an appropriate analogue for the genesis of the hardgrounds in the muddy carbonates of the Bagh Group. Shared features are the borings, the overgrowth by oysters, the micritic nature, the abundance of Bryozoa, the mineralisation by glauconite and pyrite, and the great lateral continuity. Presumably, like the Bagh Group carbonates, the chalk may have been deposited in shallow sub- to supra-tidal environments (Zijlstra 1997).

The beds of the Bagh Group carbonates are tempestite cycles (Aigner 1982) and formed because of the orbital forcing of climate during precession periods (De Boer and Smith 1994). The hardgrounds formed during precession periods were characterised by an exceptionally strong increase in average storm energy (Zijlstra 1995). This was accompanied by an increasing depth of storm reworking, with the scouring and exposure of carbonates that tend to lithify in anoxic zones of sulphate reduction (Hendry 1993) and methane oxidation at some depth below the fair-weather seafloor surface (Zijlstra 1995; Molenaar and Zijlstra 1997).

The increase in water depth and hydrodynamic energy is relative to that of the preceding cycle. The exceptional decrease of deposition rate, increase of lithification, depth of reworking, exposure of lithified sediment and genesis of hardgrounds may occur simultaneously in the sub, inter-, as well as the supra-tidal environment.

The different Bagh Group hardgrounds may thus have formed under similarly strong relative variations of average water depth and hydrodynamic energy, while the average absolute water depth and hydrodynamic energy were quite different and increased from the supra- to inter- and finally shallow sub-tidal environment, as reflected by properties such as (bioclast) grain size and degree of erosion of the sediment in which mature hardgrounds formed.

The first hardground in the lower part of the transgressive succession, lacking sedimentary structures and with a dominance of micrite, formed in a relatively low-energy depositional environment. The depth of erosion during higher energy conditions was shallow and any sedimentary structures of the thin redeposited storm layers were obliterated by bioturbation. Long periods of subaerial exposure and pedogenesis are indicated by the desiccation cracks, in-situ brecciation, micro-nodulation, calichefication (Calvet and Julia 1983; Flügel and Munnecke 2010; Pomoni-Papaioannou and Zampetakis-Lekkas 2009; Ruidas et al. 2020), and vadose silt and root related structures (James and Choquette 1990; Wright 1994; Kraus and Hasiotis 2006). The repeated wetting and subsequent drying by evaporation, lead to dissolution and reprecipitation of the carbonate (Bathurst 1974, 1975; Walker and James 1992; Scholle and Ulmer-Scholle 2003).

During a gradual increase in average storm strength and hydrodynamic energy, deposition decreased and changed into erosion. Presumably, the carbonate sediment of this hardground first lithified at some depth below the seafloor into a firmground, while it experienced prolonged anoxic conditions of sulphate reduction and carbonate cement precipitation (Hendry 1993). Relatively light δ13C‰ values as found in the lithified Bagh Group carbonates have been considered a by-product of sulphate reduction (Dickson et al. 2008).

When this layer was subsequently eroded repeatedly and exposed temporarily to be covered again by sediment, its surface experienced sub-oxic conditions of iron reduction, causing mineralisation by glauconite and pyrite. Eventually, during times of highest storm strength and hydrodynamic energy conditions, the surface of the firmground became continuously exposed, got encrusted and bored, while reduced iron minerals oxidised to brown iron-oxide upon contact with oxygenated meteoric- or seawater.

A sediment surface that is thus stabilised, lithified, mineralised, and ultimately exposed is prone to rapid colonisation by boring and encrusting endofauna and epifauna, respectively, after drowning by seawater during a subsequent marine incursion (Christ et al. 2011).

The second hardground should differ from the first because it occurs higher in the transgressive succession and may have formed at greater water depth and higher hydrodynamic energy than the supratidal hardground below. The fenestral structures are characteristic features of upper intertidal to supratidal environments (Tucker and Wright 1990). However, this hardground below the Bryozoan Limestone Formation also has a well-developed soil-like nature, even more than the first hardground in the Nodular Limestone Formation below. During the precession cycle with exceptional insolation maximum and minimum (Laskar et al. 2011), deposition possibly took place in an upper intertidal to supratidal environment. A precession cycle was characterised by, respectively, alternating periods of exceptionally high and low average storm sea levels. During the latter period, prolonged emergence resulted in profound development of paleosol (Martin-Chivelet and Giménez 1992; Gómez-Gras and Alonso-Zarza 2003; Ruidas et al. 2020). The abundance of mudstone again indicates a low-energy environment of deposition (Tucker and Wright 1990; Spence and Tucker 1997; Wilson and Evans 2002; Flügel and Munnecke 2010; Fournier et al. 2004; Rasser et al. 2005; Banerjee et al. 2018).

The third hardground formed under the highest-hydrodynamic energy conditions and greatest water depth with the strongest (storm) wave action and with bioclasts that are mostly characteristic of organisms that lived in the lower intertidal to shallow subtidal environment (Ganguly and Halder 1996; Gangopadhyay and Bardhan 2000; Ruidas et al. 2020; figure 6). The common Thalassinoides burrows suggest a sublittoral to near-shore environment with moderate to high energy conditions (Kumar and Tandon 1979; Singh and Dayal 1979; Singh et al. 1983; Kundal and Sanganwar 2000; Srivastava and Mankar 2012; Patel et al. 2018). Also, the large-scale wavy bedding, reworked and transported large clasts of lithified sediment, as well as the dense packing and stacking of shells, indicate fluctuating higher water energy conditions.

8.2 The Bagh Group stable carbon and oxygen isotopes

The stable δ13C‰ carbon and δ18O‰ oxygen isotope values of the matrix and bioclasts of the Bagh Group limestones (Ruidas et al. 2020) are lighter than other marine carbonates of the same age (Jarvis et al. 2006; Li et al. 2006). They are more like that of Pleistocene Bermuda Limestones (Hudson 1977) and possibly of a secondary origin because of the presence of syn-sedimentary meteoric calcite cement (Saller and Moore 1991). Recrystallisation by meteoric water may also be the cause of the hardground cement with strongly negative δ18O‰ and slightly increased δ13C‰.

The relatively light stable carbon and oxygen isotope signature of the matrix and bioclasts may also have a primary origin, fitting a restricted marine environment that experienced a relatively low open marine influx of heavy isotopes, in contrast to a relatively high influx of light isotopes from photosynthesis and meteoric water precipitation and runoff (Diz et al. 2009).

Presumably, the stable carbon and oxygen isotope signature become lighter, moving upward in the stratigraphy of the transgressive succession (figures 9, 10) due to changes in the depositional and early diagenetic conditions. From supra- via inter- to subtidal conditions, an emerged calcretic environment that was dominated by evaporation and decrease of light isotopes changed into a flooded environment dominated by precipitation and increase of light isotopes (Salomons et al. 1978).

Further detailed investigation of stable carbon and oxygen isotopes is required to better understand the isotope signature of the Bagh Group carbonates as not only a result of syn-depositional conditions but also of subsequent diagenetic resetting (Christ et al. 2015).

8.3 The Bagh Group hardgrounds in global perspective

The Cretaceous environment was characterised by a greenhouse climate, warm temperatures, and regular relative sea-level fluctuations (Basilone 2021). Decreased solubility of CaCO3 favoured extensive sedimentation of shallow-water carbonates along the Tethyan margins (figure 12) and facilitated the development of hardgrounds (Taylor and Wilson 2003). Table 2 shows a non-exhaustive list of Middle-Upper Cretaceous hardgrounds and the depth of the depositional environment in which they formed.

Figure 12
figure 12

Paleogeographic distribution of Turonian–Coniacian shallow marine environments with hardgrounds (in light blue) (Viviers et al. 2000; Delicio et al. 2000; Bardhan et al. 2002; Piovesan et al. 2013, 2014a, b; Puckett et al. 2016).

Full size image
Table 2 List of a selection of Middle–Upper-Cretaceous hardgrounds.
Full size table

At least the formation of the second hardground encountered in the Bagh Group, occurring between the middle Nodular Limestone member with late Turonian Mytiloides sp. aff. labiatoidiformis (Tröger) and the upper Nodular Limestone member with Coniacian Volviceramus cf. involutus (Sowerby, 1828) (Ruidas et al. 2018), may coincide with a global event that was responsible for the Turonian–Coniacian biostratigraphic boundary (Reitner et al. 1995; Olszewska-Nejbert 2004; Chacón and Martín-Chivelet 2008). The hardground may coincide with a medium cycle boundary of a maximum onlap during a short-term (third order) sea level fluctuation around the Turonian–Coniacian boundary of about 89.9 million years ago (Haq 2014).

8.4 Chronostratigraphy of the Bagh Group hardgrounds

The regular, laterally continuous bedding of the Bagh Group carbonates is reminiscent of earth orbitally induced rhythmicity, as it was observed for similar successions elsewhere (Chen et al. 2015), allowing for a detailed reconstruction of chronostratigraphy.

Preliminary analysis of outcrop images of nearly 7 m of the middle and upper members of the Nodular Limestone and lower part of the Bryozoan Limestone formations of the Ratitalai section shows that this succession consists of about 80 beds of on average 8.75 cm thickness. The average deposition rate for the 20,000-year precession cycles would have been about 4.4 mm per 1000 years (Ka). The interval would be ~80 × 20,000 or 1.6 million years in duration. This conforms to the proposed shorter biostratigraphic interval of Middle or Late Turonian to Middle Coniacian for the Bagh Group carbonates (Ruidas et al. 2018). This contrasts with the previously proposed longer biostratigraphic interval of the entire Turonian (Taylor and Badve 1995) to middle Coniacian (Gangopadhyay and Bardhan 2000), which is equivalent to a much longer chronostratigraphic interval of about 5 million years (–93.5 to –88.5 Ma). In the latter case, the erosion and genesis of the hardgrounds in the Nodular Limestone and Bryozoan Limestone formations might represent considerable hiatus.

A detailed study of the bedding in correlated outcrops is needed to shed more light on the chronostratigraphy of the Bagh Group carbonates, so that the eventual hiatuses are better defined and the interplay of rhythmically varying cementation and deposition rates as a function of cyclic climate variations, transgressive–regressive movements and longer-term basin dynamics can be better understood.

9 Conclusion

The Turonian–Coniacian Bagh Group carbonates are part of an upper Cretaceous, tropical, shallow marine, transgressive-regressive succession, representing a deepening from supra- to shallow subtidal depositional conditions. The regular rhythmic bedding is defined by carbonate nodules that form below the seafloor. Eventually, they amalgamated into firm grounds. When eroded and exposed, they evolved into bored and encrusted hardgrounds. The hardgrounds occur at regular intervals throughout the transgressive succession and are formed during periods of exceptional relative increase of average water depth and hydrodynamic energy. This was accompanied by relatively deep storm reworking, non-deposition, intensified carbonate cementation and soil genesis. The stable carbon and oxygen isotope signature reflects the transgressive nature of the Bagh Group carbonates and the change from evaporation- to precipitation-dominated, supra- to subtidal environment, respectively. The Bagh Group carbonates might have been deposited at relatively low average deposition rates. The thin beds remind us of precession cycles, inviting the precise chronostratigraphic reconstruction of Indian intracratonic basin dynamics. This contributes to a better understanding of global events during the upper Cretaceous.