Biotic components of the studied material include dominant benthic foraminifera, both small and larger, in association with calcareous green algae. Some major skeletal components of the evaluated carbonates assignable to the Umlatdoh Limestone including the species of Alveolina are shown in Fig. 6, Fig. 7 and Fig. 8. Species of Alveolina–A. oblonga (Fig. 6a–c), A. schwageri (Fig. 6d), A. cf. ruetimeyeri (Fig. 6e), A. aff. haymanensis (Fig. 6f) and A. aff. varians (Fig. 7d) are the most widely distributed biotic components of the studied successions. Larger benthic foraminifera are common to abundant biogenic components in all the samples, except few (TOP 1-1, TOP 1-2, TOP 2-1, TOP 3-1, TOP 3-2 and TOP 3-3) rich in ooids, coated grains and smaller benthic foraminifera. Coralline red algae mainly represented by sporolithaceans (Fig. 8a–b), echinoid spines (Fig. 6g) and plates (Fig. 8h), gastropod and bivalve shells (Fig. 6g), bryozoans (Fig. 8g), encrusting foraminifera (acervulinids and planorbulinids), ostracods, serpulids and fragmented/abraded indeterminate bioclasts are also present across the sections but recorded only as secondary facies components. Ooids are present as the most abundant non-skeletal components of the studied sections. Small benthic foraminifera (SBF) include porcellaneous (miliolids), perforated (rotaliids) and agglutinated (textulariids) forms. The Umlatdoh Limestone comprises abundant small miliolids represented by Quinqueloculina, Triloculina, Biloculina, Spiroloculina and Periloculina. Rotaliids feature both planoconvex forms such as Rotalia and well-ornamented forms like Daviesina (Fig. 7a), Lockhartia (Fig. 8e) and Neorotalia (Fig. 8f). Agglutinated foraminifera are represented by rare to common well-preserved, seriate forms such as textulariids. LBF forms comprise porcellaneous taxa such as Alveolina, Glomalveolina and Orbitolites along with hyaline-lamellar taxa like Discocyclina and nummulitids (Nummulites, Assilina and Operculina). Larger orthophragminids like Discocyclina (Figs. 8c–e) are recorded in several samples. Calcareous green algae show considerable frequency in the studied material (Fig. 6 and Fig. 7), including segments and plates of Halimeda (H. tuna, Fig. 6h; H. fragilis, Figs. 7e–f) and Ovulites with several dasycladalean fragments. Coralline red algae are represented by Sporolithon thalli in form of warty protuberances (Fig. 8a) and unconsolidated fragments (Fig. 8b), and algal debris constituted by abraded and fragmented thalli of geniculate and non-geniculate forms. Very rare reworked Distichoplax thalli are also observed. Non-geniculates (non-articulated corallines) are also present as very small nodules and protuberances not discernible at the generic level. Coralline red algae are frequently abraded and/or bioeroded, showing the influence of both strong hydrodynamic energy as well as other benthic organisms. Geniculate or the articulated forms are represented by very rare Corallina and Jania. These taxa were commonly diffused during the Cenozoic into very shallow ramp environments indicating bathymetric levels of less than 10 m (Brandano et al., 2009, Quaranta et al., 2012 and Tomassetti et al., 2016).

In the evaluated Lumshnong sections, lower Eocene massive to cherty marly fossiliferous limestones are recorded, which correspond to the uppermost part of the Umlatdoh Limestone. These sections are characterized by abundant well-preserved benthic foraminifera that enable precise biostratigraphy. The occurrence of Alveolina oblonga (Figs. 6a–c) and of A. schwageri (Fig. 6d) with Assilina laxispira (Fig. 7c), Nummulites burdigalensis (Fig. 7g), Nummulites globulus and N. atacicus (Fig. 7h) assign an early Eocene (Cuisian) age corresponding to the SBZ10. Nummulites atacicus (SBZ 8) and N. globulus (SBZ 8-9) are both limited to earlier horizons of Eocene age according to Serra-Kiel et al. (1998), but other biostratigraphic studies have confirmed their range extension into deeper time even up to the Bartonian (Bieda, 1963, Ionesi, 1971, Massieux, 1973 and Matsumaru and Sarma, 2010). The early Eocene (Cuisian) has been described by Hottinger (1960) as the Alveolina oblonga zone that stands equivalent to SBZ10 of Serra-Kiel et al. (1998). A. oblonga and A. schwageri are recorded most consistently in the studied samples, showing the highest frequency and abundance among all the LBF species, and found in the lowermost (except TOP 1-1) and topmost samples of all the sections, which confirms that the successions have deposited completely during the early Eocene. The SBZs can be well correlated with standard planktonic foraminiferal and calcareous nannoplankton zones (Papazzoni et al., 2017, Serra-Kiel et al., 1998 and Zhang et al., 2018). This facilitates the construction of a high-resolution bio- and chronostratigraphy in carbonate sequences featuring LBF species. Owing to the complete absence of calcareous nannoplankton and planktonic foraminifera in the studied sections, any direct correlation with the recorded shallow benthic zone is constraining. However, based on literature survey and comparative studies, the currently studied sections assigned to the SBZ10 are correlated with calcareous nannoplankton zone NP12 and possibly planktonic foraminiferal zones P6b-P7 (Molina et al., 1992, Papazzoni et al., 2017, Pujalte et al., 2009, Schaub, 1981 and Serra-Kiel et al., 1998).

Matsumaru and Sarma (2010) have reported Umlatdoh Limestone LBF assemblages from ridge sections outcropping in Nongkhlieh village of Shnongrim area, Jaintia Hills, Meghalaya. The presence of Alveolina oblonga, A. schwageri, Assilina laxispira and A. placentula as the major LBF in association with Nummulites atacicus, N. burdigalensis, N. globulus and Alveolina globosa indicated a correlation with the Laki Limestone of the Laki Stage of Sind area and the underlying Meting Limestone described by Nagappa (1959). A partial correlation was also established with LFA-2 and LFA-3 in the Indian Basins (Govindan, 2003), which is equivalent to the Letter Stage Tertiary a2 (Govindan, 2003 and Matsumaru, 1996). The Umlatdoh Limestone assemblages as described by Matsumaru and Sarma (2010) have been ascribed to SBZs 7 to 11 (Fig. 9). An exception to this assignment is the presence of Orbitolites complanatus, which has a range from SBZ 12 (?) to SBZ 16 (?) and deserves further analysis. Sarkar (2016) has also reported larger benthic foraminifera from the lower part of the Umlatdoh Limestone assignable to SBZs 7-8 (middle Ilerdian) from a section located 2 km southeast of Lumshnong on the Jowai–Badarpur Road.

All the carbonate microfacies recorded from the presently analysed Umlatdoh Limestone sections can be ascribed to a shallow-marine ramp environment. A general conceptual palaeoenvironmental model representing the depositional zones in the ramp environment and also the distribution of the major facies types are shown in Fig. 10. Whether these carbonate ramps were homoclinal or distally steepened in nature cannot be interpreted, since we are focusing on the shallowest environments. The studied limestone samples display low microfacies diversity and a total of six major facies types (MFTs) were distinguished based on the carbonate texture and the dominant skeletal components. Most well-distributed, abundant components of the carbonate sedimentary samples are the larger foraminifers: alveolinids and nummulitids. Smaller miliolid, rotaliid and textulariid benthic foraminifera and diverse green algae (Halimeda with udotacean and dasycladalean forms) are found in association with the LBF species and show moderate abundance in the studied Umlatdoh Limestone microfacies. Abiotic ooids and coated grains are important non-skeletal components of the studied sections and are represented in variable abundance. Echinoderm plates and spines, partly broken and abraded gastropod and bivalve shells, small bryozoans fragments, coralline red algae mostly in form of small protuberances and debris, encrusting foraminifera (acervulinids and planorbulinids), ostracods, serpulids and fragmented/abraded indeterminate bioclasts occur across the different MFTs as subordinate facies components, but showing no considerable abundance in any of the samples/section.

MFT 1: Oolitic-smaller benthic foraminiferal-green algal grainstone-packstone (Fig. 7a–c)

This well-sorted microfacies varying in thickness from 0.35 to 1.0 m is recorded in the studied sections in form of oolitic bands and characterized by a high abundance of ooids and coated grains. The ooids feature concentric layering and possibly represent relicts of radial structures. They are predominantly round in shape, with size varying from 0.15 to 0.6 mm. These are associated with varying proportions of smaller miliolids, rotaliids and green algae represented by Halimeda spp. followed by udotacean and dasycladalean forms. Subordinate components in this microfacies are peloids, algal fragments, echinoderm fragments and debris, intraclasts and aggregates also acting as the primary nuclei of the ooids and components of the coated grains. Some smaller miliolids, rotaliids, and textulariids unidentifiable up to the genus level are also observed in this microfacies. Rare alveolinids are also occasionally observed. Some unidentified sparite-filled bioclasts are also present. The matrix is generally sparite including few relicts of micrite material. Drusy sparite and isopachous fibrous cement could be observed at several places around the ooids and coated grains.

Interpretation. This microfacies corresponds to a shoal environment (shoal-sand bar) influenced by tides and waves much above the normal wave base related to depths < 10 m. The concentric ooids indicate high energy and normal marine salinity conditions that are well associated with a shoal setting in a shelf-edge or bank margin facies (Blendinger and Blendinger, 1989 and Chablais et al., 2010). The near-total absence of mud and the occurrence of isopachous fibrous cement also indicate a high-energy depositional setting.

MFT 2: Smaller miliolid- Alveolina grainstone (Fig. 7d)

This poorly sorted microfacies with packstone matrix occurs only in the basal part of all the sections with thickness ranging from 0.8 to 2.5 m. MFT 2 is characterized by a high abundance of smaller miliolids (Quinqueloculina, Triloculina, Biloculina, Spiroloculina, and Periloculina species) in association with moderately abundant Alveolina (assemblages dominated by ovoid to slightly elongated A-forms–Alveolina oblonga, A. schwageri, A. aff. varians and several unidentifiable alveolinids). Some miliolid specimens showed resemblance to the genus Pyrgo, but the identification attempts were inconclusive. The grain size varies from 0.25 to 4.0 mm. The average diameter of Alveolina spp. in this microfacies is < 2 mm, but rare larger forms are recorded. Smaller nummulitids (Nummulites spp.), assilinids (Assilina laxispira and some indeterminate species) and rotaliids (Lockhartia, Neorotalia, Daviesina), textulariids (Textularia), orthophragminids, echinoid plates and spines, bivalve and gastropod shells, rare planorbulinid encrusting foraminifers, green algae, coralline red algal debris and Sporolithon thalli, serpulids and ooid grains are the subordinate facies components.

Interpretation. This MFT characterized by abundant smaller miliolids in association with larger Alveolina spp. indicates a restricted/protected lagoon to proximal inner ramp setting in a meso-oligotrophic nutrient regime with moderate to high hydrodynamic energy. The abundance of miliolids is an indication of warm, shallow-marine depositional environment. They commonly develop habitats in both normal and hypersaline environments (Brasier, 1975, Chan et al., 2017, Gonera, 2012 and Hottinger et al., 1993). Genera like Quinqueloculina and Triloculina are reported from multiple lagoonal environments in the Arabian Gulf (Amao et al., 2016 and Murray, 1991), and their prolific occurrence in the studied carbonates indicates the deposition in a hypersaline environment (37–70‰) with water temperatures in the range of 16–40 °C (Murray, 1991), and bathymetric zonation from 12 to 18 m (Murray, 2006 and Parker and Gischler, 2015).

The smaller miliolids are commonly categorized as r-strategist foraminifera showing short generation times, high fecundity, and rapid growth of populations (Reolid et al., 2014 and Sarkar, 2015). The abundance of these fauna indicates the possibility of adverse fluctuating environmental conditions where the limited amount of resources needs to be utilized as soon as they become available in the niche. Modern miliolids are euryhaline biota thriving on soft substrates corresponding to shallow restricted lagoon environments featuring low rates of turbulence (Zamagni et al., 2008). Alveolinids are very tolerant to a broad spectrum of temperature and salinity conditions, thereby facilitating their occurrence in various zones of shallow-marine carbonate platforms (Drobne et al., 2011 and Hadi et al., 2018). Likewise, in the present case study, Alveolina spp. occurs throughout the section showing moderate to high abundance, thereby demonstrating a good degree of adaptability to various depositional environments. However, due to their symbiotic associations with microalgae as evidenced by several records (Hallock, 1985, Hohenegger, 2004 and Hohenegger, 2009), the entire successions are interpreted to have been deposited in shallow to very shallow water depths corresponding to the upper photic zone.

MFT 3: Green algal-benthic foraminiferal grainstone (Fig. 7e–f)

This poor-to-moderately-sorted microfacies with a packstone-grainstone matrix is 0.5–1.0 m in thickness and is dominated by calcareous green algae including multiple species of Halimeda (H. fragilis, H. tuna), udotacean genus Ovulites (O. arabica, O. pyriformis), and also various dasycladalean fragments. The benthic foraminiferal component of the MFT 3 is represented by Alveolina, smaller miliolids and rotaliids, smaller Nummulites, Glomalveolina, Orbitolites, Discocyclina, and encrusting forms like Acervulina and Planorbulina, and agglutinated Textularia. The grain size varies from 0.15 to 3.5 mm. Halimeda species are recorded in the form of segments and plates. Several small ooids and coated grains are dispersed throughout the microfacies and observed with few instances of isopachous fibrous cement. Gastropod and bivalve shells with ostracod, echinoderm, and bryozoan fragments are the other secondary components.

Interpretation. Calcareous green algae including dasycladalean and halimedacean forms are common constituents in early Eocene shallow-marine limestone sequences showing moderate/locally high abundance in the Umlatdoh Limestone (Sarkar, 2016). Green algae represent typical photozoan components that thrive in oligotrophic, well-illuminated waters facilitating photoautotrophy and photosymbiosis (Brandano et al., 2015 and Hallock and Schlager, 1986). However, there are some rare examples showcasing proliferation of green algae under increased nutrient inputs (Wilson and Vecsei, 2005). Lees (1975) described green-algae-dominated chloralgal assemblages in tropical, high-salinity areas (Halimeda shows broad salinity tolerance; 20–45‰). In a sharp contrast to its well-known status as a photozoan component sensitive to low light conditions, Halimeda has also been reported from deeper domains having low to very low light availability, for example the Hawaiian Islands, where it forms extensive meadows between depths of 65–120 m (Webster et al., 2006) and the Brazilian continental shelf extensively covered by rhodolith beds, where Halimeda is recorded from the intertidal zone to the 166-m bathymetric level (Amado-Filho et al., 2012 and Bandeira-Pedrosa et al., 2004). The abundance of dasylcadalean forms with photosymbiotic LBF like Alveolina and Nummulites co-occurring with the Halimeda species strongly indicates the deposition of this microfacies in very shallow-water conditions (<20 m) related to the proximal inner ramp. A meso-oligotrophic nutrient regime is interpreted because nutrient-enriched conditions tend to reduce light penetration and decrease the availability of well-lit substrate expected to be a threshold requirement for this assemblage featuring calcareous green algae, smaller foraminifera and LBF.

MFT 4: Larger porcellaneous ( Alveolina ) grainstone-packstone (Fig. 6a–c, e–f)

This microfacies with grainstone-locally abundant packstone matrix shows a thickness of 2.5–3.0 m with unidirectional cross beddings and is characterized by moderate to poorly sorted large porcellaneous foraminifera mainly comprising subglobular to ovoid tests of Alveolina (> 40%; Alveolina oblonga, A. schwageri, A. aff. haymanensis, A. aff. varians) and several alveolinids not identifiable up to the species rank). The grain size ranges between 0.8 and 6.0 mm. The alveolinid tests show evidences of high hydrodynamic energy as the outer surface of youngest whorls is exfoliated, with truncated poles in numerous cases. Localized silt-sized sediments are also observed, dominated by calcisiltite. Subordinate biotic components of this microfacies are small nummulitids (A-form Nummulites), dasycladalean algae and Halimeda segments, non-geniculate algal protuberances (Sporolithon and mastophoroids), other small–medium-sized miliolids, orbitolids, smaller rotaliids, orthophragminids, acervulinids, echinoid remains and bryozoan fragments. Rare cortoids and peloids are also observed. Cement predominantly consists of drusy and blocky spar with isopachous to syntaxial cement. Several alveolinid tests are mildly abraded and rare cases of acervulinid encrustations on nummulitid tests are also recorded. A gently dipping planar to trough cross-bedded stratification can be observed in the case of this facies, with 1–2-m-thick individual units.

Interpretation. This grainstone-packstone facies is interpreted as representing bioclastic carbonate sandy shoals built in a shallow-marine, high-energy depositional setting influenced by wave processes in a nearshore inner ramp environment. Comparable unidirectional cross-bedding lithology has also been reported from numerous Cenozoic carbonate systems of Malta, Tunisia, and Italy (Brandano et al., 2009, Loucks et al., 1998, Pedley, 1998 and Tomassetti et al., 2016). The presence of dasycladalean and halimedacean green algae suggests a very shallow-water depositional setting (Wray, 1977). The occurrence of small biconvex A-forms of Nummulites tests along with subglobular to ovoidal alveolinids suggests a shallow-marine depositional setting corresponding to euphotic and oligotrophic conditions. Recent alveolinids have been reported from a plethora of marine habitats including deep lagoons to fore-reef sub-environments (Yordanova and Hohenegger, 2002). Living and fossil alveolinids are generally associated with shallow-marine settings, and their distribution depends primarily on light intensity and hydrodynamic conditions (Drobne et al., 2011 and Tomassetti et al., 2016). Living larger miliolids like Borelis and Alveolinella quoyi do not dwell at depths > 50 m (Hohenegger, 1994, Hohenegger, 2009 and Langer and Hottinger, 2000). Their distributions are independent with respect to the substrate, which might have been any kind of algae, seagrasses or others (Beavington-Penney et al., 2006, Langer and Hottinger, 2000 and Severin and Lipps, 1989). Thick carbonate successions encompassing rich populations of alveolinids were also interpreted to have deposited in lagoon or back-bank environments (Hottinger, 1977). Small, lens-shaped nummulitids have been envisaged to build habitats that share a common niche with alveolinids in the inner platform horizon (Geel, 2000). The abundance of A-form alveolinids with several tests presenting signs of abrasion in the outer walls, other porcelaneous forms (orbitolitids and miliolids), and also the presence of nummulitids and small rotalliids showing low quantitative numbers all indicate a deposition of this microfacies in the inner part of the ramp featuring high energy environments. A foraminiferal community dominated by Alveolina has been reported from Oman, which possibly thrived on the patchily vegetated muddy sands of a protected lagoon or embayment (Beavington-Penney et al., 2006). A fossil assemblage dominated by alveolinids, nummulitids, and orbitolitids from Turkey has been interpreted to have been deposited in a lagoon environment (Özgen-Erdem et al., 2005). Alveolina-dominated LBF assemblages have been linked to inner platform depositional environment, influenced by siliciclastic inputs in France (Rasser et al., 2005) and Spain (Scheibner et al., 2007).

MFT 5: Alveolina -nummulitid grainstone-rudstone (Fig. 6d)

This moderately sorted microfacies with packstone matrix is 1.5–2.0 m in thickness and is characterized by the predominance of Alveolina spp. (30–35%) including all the species recorded in the current study with small nummulitids with occasional larger Nummulites (15-20%) represented by Nummulites burdigalensis and N. atacicus. Subordinate components are tests of Assilina, Daviesina, Lockhartia, encrusting foraminifera (acervulinids and planorbulinids), udotacean green alga Ovulites and Halimeda spp., undetermined orthophragminids (Discocyclina?), textulariids, smaller miliolids and rotaliids, gastropod shells, echinoderm plates, and peloids.

Interpretation. This MFT is restricted to the proximal to distal inner ramp depositional setting and indicates a slightly deeper bathymetric level. MFT 5 actually represents a transitional phase between the very-shallow-water microfacies (MFTs 1 to 4) and comparatively deeper but overall shallow-water microfacies (MFT 6) where smaller miliolids and dasycladalean algae reduce drastically and larger foraminifers like Alveolina, Nummulites and others begin to show very high abundance. Beavington-Penney et al. (2006) reported a microfacies from the Eocene sediments of Oman showing a high degree of affinity with MFT 5, dominated by the species of Alveolina, Nummulites, and Assilina, corresponding to an inner ramp setting. The LBF Assilina is recorded in varying proportions throughout the presently studied succession, but never attains very high abundance levels. The nummulitid taxa are mostly observed in form of small, robust, broken/abraded A-form tests associated with medium-large sized Alveolina (A. oblonga, A. schwageri, A. cf. ruetimeyeri) within a packstone-locally grainstone matrix, that in combination with the factor of complete absence of dasycladalean green algae suggests deposition in the proximal to distal inner ramp with high hydrodynamic energy conditions. It is widely acknowledged that small and lenticular A-forms of Nummulites are more common in shallower inner ramp/shelf/platform settings (Beavington-Penney and Racey, 2004, Beavington-Penney et al., 2006, Hadi et al., 2018 and Racey, 2001). Several examples of edgewise contact imbrications biofabrics also can be inferred as signatures of a high-energy depositional environment (Hadi et al., 2016 and Hadi et al., 2018). Numerous studies associated with the living and ancient LBF evidenced that, similarly to the currently studied successions, B-forms are rare or even completely absent in shallower horizons, whereas they are abundant at deeper depths (<100 m; Beavington-Penney et al., 2005 and Hottinger, 1977).

MFT 6: Nummulitid grainstone-rudstone (Fig. 7g–h)

This microfacies is distinguished by a moderately sorted grainstone with the grain size ranging between 0.5 and 3.5 mm. The localized abundance of larger nummulitids (>2 mm) gives the impression of grainstones grading into rudstones at certain places. This facies is dominated by small to larger nummulitid taxa (Nummulites, Assilina and Operculina) with Nummulites (N. globulus, N. atacicus, N. burdigalensis and other indeterminate species) showing higher frequency. Additionally, encrusting acervulinids and planorbulinids, orthophragminids, smaller miliolids and rotaliids, Halimeda, coralline algal protuberances, echinoids, bryozoans, ostracods, rare serpulids and some larger alveolinids with syntaxial calcitic cement occur in a packstone-grainstone matrix. Small peloids and micritized cortoids are also observed. Among the nummulitids, the analysed assemblages are characterized by dominant A-forms. This microfacies does not display any clear sedimentary structure, with massive beds ranging from 1 to 1.5 m.

Interpretation. The nummulitid assemblages dominated by A-form Nummulites are interpreted as proximal inner-ramp palaeohighs impacted by winnowing and current processes. Various studies related to modern and ancient LBF indicate that the test morphology is controlled by a complex interaction of environmental factors such as light, depth, and water turbulence (Beavington-Penney and Racey, 2004, Eder et al., 2018, Hottinger, 1977, Hottinger, 1983 and Torres-Silva et al., 2019. Nummulitids are bottom-dwelling benthic fauna, and can be found at depths < 30 m, but usually dominant at mesophotic to oligophotic depths ranging at more than 30 m (Hohenegger, 2005 and Španiček et al., 2017). Based on the ecophenotypical correlative studies with their modern counterparts, the nummulitids are associated with water depths between 40 and 80 m (Goeting et al., 2018 and Hadi et al., 2016). However, in the current study, relative palaeobathymetric position of this microfacies is inferred to be much less than 40 m, at about 25–30 m due to the presence of green algae and smaller foraminifera with the abundant nummulitids. MFT 6 is relatively deeper in comparison to the other MFTs due to the prevalence of LBF tests and coralline algal protuberances with lesser fragmentation and abrasion (tests and algal thalli more compact in this microfacies), almost negligible isopachous fibrous cement and also an increase in the incidence of encrusting foraminifera (acervulinids and planorbulinids). Small and lenticular tests of Nummulites are more common in very shallow to shallow inner platform/shelf/ramp settings (Racey, 2001). The accumulations of A-form Nummulites-dominated assemblages with robust/ovate tests and rare B-forms suggest a shallow-marine inner ramp depositional environment affected by limiting conditions, for example, slightly higher nutrient levels, favouring the development of a community of r-strategists (Beavington-Penney and Racey, 2004, Beavington-Penney et al., 2005, Eder et al., 2016 and Hadi et al., 2016).