1. Introduction
The Quilon Limestone of the Kerala Basin preserves one of the most fossiliferous shallow-marine carbonate successions along the southwestern Indian margin and provides an important archive of early Miocene tropical marine ecosystems, carbonate sedimentation, and paleoenvironmental evolution. The succession contains diverse marine fossils, including corals, molluscs, echinoids, foraminifera, bryozoans, and extensive seagrass-associated faunal assemblages, which collectively document the development of a warm, shallow-marine passive-margin embayment during the early Miocene (Reuter et al. Reference Reuter, Piller, Harzhauser, Kroh, Rögl and Ćorić2011; Sarkar et al. Reference Sarkar, Singh and Verma2024). These deposits formed during a climatically dynamic interval preceding the Miocene Climatic Optimum (MCO) and were subsequently influenced by environmental changes associated with the Middle Miocene Climatic Transition (MMCT). Consequently, establishing a robust depositional age for the Quilon Limestone is essential for accurately constraining the timing of biological diversification, carbonate platform development, paleoenvironmental evolution, and regional paleogeographic correlations between southwestern India and neighboring basins, including northern Sri Lanka. Despite its geological and paleontological significance, the chronology of the Quilon Limestone remains inadequately constrained, relying primarily on biostratigraphic interpretations and indirect stratigraphic correlations, which limit precise temporal reconstruction of depositional and early diagenetic events.
Carbonate minerals provide an increasingly powerful archive for resolving such chronological uncertainties because they incorporate uranium into their crystal lattice while generally excluding non-radiogenic lead during precipitation (Rasbury and Cole, Reference Rasbury and Cole2009; Roberts et al. Reference Roberts, Drost, Horstwood, Condon, Chew, Drake, Milodowski, McLean, Smye, Walker, Haslam, Hodson, Imber and Beaudoin2020). This characteristic enables carbonates to preserve the timing of depositional and early diagenetic processes directly through U-Pb geochronology. However, natural carbonate successions are rarely monomineralic or temporally homogeneous. Instead, they typically comprise multiple generations of carbonate cements with distinct textures, compositions, and ages that formed during evolving depositional, diagenetic, and fluid-rock interaction histories (Wood et al. Reference Wood, Bowyer, Penny and Poulton2018; Wu et al. Reference Wu, Yang, Roberts, Yang, Wang, Lan, Xie, Li, Xu, Huang, Xie, Yang and Wu2022). Consequently, conventional bulk-sample U-Pb analyses often average these multiple generations and may fail to resolve discrete episodes of carbonate precipitation. High-spatial-resolution in situ analytical techniques are therefore required to target individual carbonate domains while avoiding detrital contamination and inherited isotopic components.
Laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS), integrated with detailed petrographic characterization, enables selective analysis of texturally preserved carbonate domains and provides robust U-Pb age constraints for carbonate precipitation, typically with precisions of approximately 3–10% (Roberts et al. Reference Roberts, Drost, Horstwood, Condon, Chew, Drake, Milodowski, McLean, Smye, Walker, Haslam, Hodson, Imber and Beaudoin2020; Wu et al. Reference Wu, Yang, Roberts, Yang, Wang, Lan, Xie, Li, Xu, Huang, Xie, Yang and Wu2022). Complementary in situ 87Sr/86Sr isotope analyses further distinguish seawater-derived carbonate from phases influenced by continental input or diagenetic fluids, thereby providing valuable constraints on fluid evolution, open-system behaviour, provenance, and paleoenvironmental conditions during carbonate formation (Banner and Hanson, Reference Banner and Hanson1990; Melezhik et al. Reference Melezhik, Pokrovsky, Fallick, Kuznetsov and Bujakaite2009; Chen et al. Reference Chen, Cao, Algeo, Li, Wei, Lin, Zhang, Xiang, Zhu, Xu, Fan and Zhang2024). Recent studies have demonstrated the effectiveness of combining carbonate U-Pb geochronology with Sr isotope geochemistry to constrain the timing of shallow-marine carbonate deposition, decipher early diagenetic cementation histories, evaluate fluid-rock interactions, and reconstruct sedimentary basin evolution and paleoclimate in diverse geological settings (Nuriel et al. Reference Nuriel, Weinberger, Kylander-Clark, Hacker and Craddock2017; Holdsworth et al. Reference Holdsworth, McCaffrey, Dempsey, Roberts, Hardman, Morton, Feely, Hunt, Conway and Robertson2019; Woodhead and Petrus, Reference Woodhead and Petrus2019; Montano et al. Reference Montano, Gasparrini, Gerdes, Della Porta and Albert2021; Roberts and Holdsworth, Reference Roberts and Holdsworth2022).
In this study, we apply integrated in situ carbonate U-Pb geochronology and 87Sr/86Sr isotope analyses to the Quilon Limestone of the Kerala Basin to establish its depositional age and evaluate the geochemical conditions under which the carbonates formed. By targeting early carbonate cement while avoiding detrital components, we aim to obtain robust depositional age constraints for this important fossil-bearing succession. The resulting chronological framework allows improved correlation of the Quilon Limestone with coeval carbonate successions in northern Sri Lanka and elsewhere in the northern Indian Ocean, thereby refining the timing of early Miocene carbonate deposition, biological evolution, paleoenvironmental change, and regional paleogeographic connectivity along the southwestern Indian margin.
2. Geology and stratigraphy
The Kerala Basin constitutes the southern sub-basin of the pericratonic Konkan-Kerala Basin developed along the western passive continental margin of India (Figure 1). The basin extends for ∼600 km in a north-south direction and covers an area of ∼5.8 × 105 km2. It is separated from the Konkan Basin to the north by the Tellicherry Arch, a prominent basement high, and is bounded to the east by the steep escarpment of the Western Ghats, which locally exceeds 2.6 km in elevation. Westward, the basin deepens offshore and merges with the deep-water domain adjoining the Laccadive-Chagos Ridge system (Campanile et al. Reference Campanile, Nambiar, Bishop, Widdowson and Brown2008 and references therein). The tectonic evolution of the Kerala Basin is closely linked to the breakup of the Gondwana supercontinent. Rifting between India and Madagascar during the Late Cretaceous established the first-order extensional framework of the western continental margin, followed by separation of the Seychelles-Mascarene plateau during the Early Paleogene, which further enhanced lithospheric thinning and passive-margin subsidence. The near-linear configuration of the west coast, locally expressed by the West Coast Fault (WCF) extending from Panvel to Kanyakumari, reflects this inherited tectonic grain and the cumulative effects of Jurassic-Cretaceous and Early Cenozoic rifting episodes (Owen, Reference Owen1976; Biswas, Reference Biswas1982; Plummer and Belle, Reference Plummer and Belle1995). Vertical movements along the WCF and associated fault systems during the Late Tertiary generated accommodation space for marine and continental sedimentation along the coastal tract (Raha et al. Reference Raha, Sinha Roy and Rajendran1983).
Map showing the simplified configuration of the Kerala-Konkan basin (modified after Campanile et al. 2007). The inset shows the generalised stratigraphy of the Kerala basin (Figure not scaled). Important Miocene limestone localities along the southern part of Kerala Konkan Basin (1 = Meesalai, 2 = Mannar, 3 = Aruwakkalu). Red star marks the present sampling location.
The Tertiary sedimentary succession of the onshore Kerala Basin (Figure 1 inset) rests unconformably on Precambrian crystalline basement and is dominated by siliciclastic sediments with frequent lignite seams. These deposits record sustained denudation of the Western Ghats rift shoulder and delivery of terrigenous material into marginal-marine and continental depocentres (Campanile et al. Reference Campanile, Nambiar, Bishop, Widdowson and Brown2008). Microfaunal and palynological assemblages indicate deposition in low-energy environments, including brackish lagoons, coastal swamps, and freshwater wetlands (Rao and Ramanujam, Reference Rao and Ramanujam1975; Rao, Reference Rao1995). The Tertiary sedimentary record of Kerala is represented by the lower Vaikom Formation, the middle Quilon Formation, and the upper Warkalli Formation, which together document a transition from low-energy marine to near-shore depositional systems (Soman, Reference Soman2002). The Viakom Formation forms the basal unit and is composed predominantly of fine-grained argillaceous sediments with minor calcareous components, indicating sediment accumulation under relatively quiet, low-energy conditions. This succession is overlain by the Warkalli Formation, which displays a markedly different lithological character, beginning with basal gravel and pebble horizons, passing upward into carbonaceous clays with lignite seams, and then into friable, current-bedded, variegated sandstones interstratified with plastic and mottled clays. The sedimentary architecture and facies associations of the Warkalli Formation suggest deposition in a shallow-water, shoreline to littoral environment (Poulose and Narayanaswami, Reference Poulose and Narayanaswami1968).
The Quilon Formation represents a carbonate succession extending discontinuously along the coast between Edava in the south and Alappuzha in the north and is preserved in both onshore outcrops and the offshore subsurface (Narayanan et al. Reference Narayanan, Anirudhan and Grottoli2007; Reuter et al. Reference Reuter, Piller, Harzhauser, Kroh, Rögl and Ćorić2011). Two principal carbonate units are recognised: a lower coral limestone and an upper Pseudotaberina dominated limestone. At the Padappakkara type locality (Figure 2a), the lower coral limestone is replaced by a reworked carbonate bed, whereas at Pozhikkara (Figure 2b), the formation is predominantly fossiliferous limestone (Seralathan and Padmalal, Reference Seralathan and Padmalal1991; Reuter et al. Reference Reuter, Piller, Harzhauser, Kroh, Rögl and Ćorić2011). The upper contact grades into carbonaceous clay and is capped by a thin lateritised duricrust, above which continental deposits of the Ambalappuzha Formation occur. The Quilon carbonates host a diverse shallow-marine fauna including foraminifers, molluscs, corals, echinoids, crustaceans, bryozoans, ostracods, and vertebrate remains such as shark teeth, making the formation a key archive for Miocene palaeobiogeographic reconstructions along the western Indian margin (Harzhauser et al. Reference Harzhauser, Kroh, Mandic, Piller, Göhlich, Reuter and Berning2007, Reference Harzhauser, Reuter, Piller, Berning, Kroh and Mandic2009; Briguglio, Reference Briguglio2018). The presence of Pseudotaberina malabarica allows correlation with the Jaffna Limestone of Sri Lanka and indicates deposition within a broadly connected Indo-West Pacific shallow-marine province during the Burdigalian (Eames, Reference Eames1950; Epa et al. Reference Epa, Perera, Manamendra-Arachchi and Meegaskumbara2012).
Field and hand specimen photographs of limestones from (a–c) Channakodi and (b–d) Pozhikkara sections.
Multiple lines of evidence indicate that the Quilon Formation developed within a down-faulted embayment characterised by warm tropical conditions, limited siliciclastic supply, and enhanced carbonate productivity. Variations in stable oxygen isotope values indicate elevated seawater temperatures during deposition (Narayanan et al. Reference Narayanan, Anirudhan and Grottoli2007). Borehole records reveal cyclic stacking of beds, interpreted as higher-order transgressive pulses superimposed on a longer-term sea-level rise (Menon, 1967a, 1967b; Ramanujam et al. Reference Ramanujam, Reddy and Rao1992). Biotic associations, including epiphytic foraminifers, grazing gastropods, and bryozoans encrusting plant remains, support interpretation of the Quilon carbonates as seagrass-dominated ecosystems (Reuter et al. Reference Reuter, Piller, Harzhauser, Kroh, Rögl and Ćorić2011). High-precision strontium isotope ratios from well-preserved aragonitic molluscs constrain deposition to 17.08–17.13 Ma, placing the Quilon Formation within the Burdigalian Stage and marking the onset of the Mid-MCO in the Kerala Basin (Carolin et al. Reference Carolin, Vadlamani and Bajpai2022).
3. Sampling and methods
Field investigations were conducted at two representative exposures of the Quilon Formation along the western coast of Kerala, namely Channakodi and Pozhikkara (Figure 2). The Channakodi section (8°58′36.6″ N; 76°38′08.4″ E) is exposed along the bank of Ashtamudi Lake near Padappakkara village, Kollam district (Figure 2a). The outcrop comprises a laterally continuous, ground-level exposure of approximately 1–2 m thickness, with the basal contact not exposed. Lithologically, the section consists of fine grained carbonates that grades upward into siliciclastic rich facies. The limestone is strongly lithified and contains abundant fragmented bioclasts, including broken shells and skeletal debris, dispersed randomly within the matrix (Figure 2c). The textural attributes and vertical facies transition suggest deposition under shallow-marine conditions (Reuter et al. Reference Reuter, Piller, Harzhauser, Kroh, Rögl and Ćorić2011).
The Pozhikkara locality (8°44′07.9″ N; 76°42′15.2″ E) is situated at the base of coastal cliffs near the Chillakal Main Juma Masjid, Paravur, Kollam district (Figure 2b). Here, the Quilon Formation occurs as in situ, ground-level outcrops restricted to the southern part of the cliff, with no significant accumulation of detached boulders, indicating limited mass wasting. The basal contact is not exposed, whereas the upper part of the section grades upward into a thick lateritic duricrust, locally interlayered with carbonaceous clay and lignite lenses or beads. The limestone (Figure 2d) is fine-grained and is notably rich in fragmented skeletal components, reflecting intense bioclastic reworking. The association with laterite, carbonaceous horizons, and lignite suggests subaerial exposure and pedogenic overprinting during post-depositional uplift and weathering, in agreement with earlier studies that document episodic regression, coastal swamp development, and lateritization.
In situ U-Pb dating was carried out in thin sections at the Isotope Geochemistry and Geochronology Facility, NCESS, using a Teledyne 213 nm Nd:YAG laser combined with an Agilent 7800 quadrupole ICPMS. The laser was operated at a spot size of 100–150 microns at a 10 Hz repetition rate in Q-switch mode. Each analysis took ∼100 seconds, with the first ∼40 seconds used for measuring the gas blank (background), followed by 40 seconds of ablation and 20 seconds (end pause) of washout time to allow the signal to drop back to the gas blank values. Standard-sample bracketing was used during the analysis, comprising a synthetic glass standard (NIST 614) and three calcite standards (WC-1, ASH-15D, and JTC-1). Data reduction was carried out using iolite v.4 with the integrated Visual Age package. The integration interval of the reference material and samples was adjusted to achieve a good spread along the isochron without compromising precision. Synthetic glass standard NIST 614 was used for normalizing 207Pb/206Pb ratios. The 206Pb/238U ratios calculated against the calibration of NIST 614 were normalized to account for matrix effect, element fractionations, and mass bias by multiplying the normalization ratio calculated using the measured/accepted ratio derived from the session-based drift-corrected mean of the primary WC-1 reference material (Roberts et al. Reference Roberts, Rasbury, Parrish, Smith, Horstwood and Condon2017; Guillong et al. Reference Guillong, Wotzlaw, Looser and Laurent2020). The age of the analysed samples was determined from unanchored lower intercepts on a Tera-Wasserburg (TW) plot using IsoplotR (Vermeesch, Reference Vermeesch2018). During the analytical session, WC-1 (254.4 ± 6.4 Ma; Robert et al. Reference Roberts, Rasbury, Parrish, Smith, Horstwood and Condon2017), ASH-15D (2.965 ± 0.011 Ma; Nuriel et al. Reference Nuriel, Weinberger, Kylander-Clark, Hacker and Craddock2017), and JTC-1 (13.797 ± 0.031 Ma; Guillong et al. Reference Guillong, Wotzlaw, Looser and Laurent2020) yielded a TW lower intercept age of 254.6 ± 2.4 Ma (N = 34; 207Pb/206Pb anchored at 0.85), 2.96 ± 0.21 Ma (N = 29; 207Pb/206Pb anchored at 0.832) and 13.40 ± 0.73 Ma (N = 26; 207Pb/206Pb anchored at 0.839), respectively. All uncertainties are quoted in 2σ confidence.
In situ Sr isotopic analysis was carried out in thin sections at the Isotope Geochemistry and Geochronology Facility, NCESS, using a Teledyne 213 nm Nd:YAG laser combined with a Nu Plasma III MC-ICPMS. The laser was operated at a spot size of 100–200 microns at a 10 Hz repetition rate in Q-switch mode. Each analysis took ∼100 seconds, with the first ∼40 seconds used to measure the gas blank (background), followed by 40 seconds of ablation and 20 seconds of washout (end pause). Standard-sample bracketing was used during the analysis, comprising three calcite standards (MNP, HZZ-2 and ASH-15D). Data reduction was carried out using iolite v.4 with the Sr Isotope Universal package (Mulder et al. Reference Mulder, Hagen-Peter, Ubide, Andreasen, Kooijman, Kielman-Schmitt, Feng, Paul, Karlsson, Tegner, Lesher and Costa2023). Data reduction involved correcting for instrumental background, mass fractionation, isobaric interferences from Rb and Kr, spectral interferences from Ca and Ar dimers (CaCa/CaAr) and doubly charged REE species (Mulder et al. Reference Mulder, Hagen-Peter, Ubide, Andreasen, Kooijman, Kielman-Schmitt, Feng, Paul, Karlsson, Tegner, Lesher and Costa2023). The measured 87Sr/86Sr ratio is internally normalized using an 86Sr/88Sr of 0.1194 using the exponential fractionation law to correct for instrumental mass bias. During the analytical session, MNP (87Sr/86Sr = 0.70617 ± 0.00005; Liang et al. Reference Liang, Bao, Yang, Zong, Chen, Zhang and Yuan2023), HZZ-2 (0.70941 ± 0.00001; Li et al. Reference Li, Hu, Zhang, Zhu, Xie, Wang, Xu, Xu, Guo and Wu2025) and ASH-15D (87Sr/86Sr = 0.70790 ± 0.000010; Vaks et al. Reference Vaks, Woodhead, Bar-Matthews, Ayalon, Cliff, Zilberman, Matthews and Frumkin2013) yielded a weighted mean 87Sr/86Sr ratio of 0.706169 ± 0.00002 (N = 6), 0.709419 ± 0.00003 (N = 6) and 0.707921 ± 0.00006 (N = 6), respectively. All uncertainties are quoted in 2σ confidence.
4. Results
Under transmitted light microscopy and BSE imaging, the fossiliferous limestone samples are characterized by abundant bioclasts, including fragmented shells, coral debris and foraminiferal tests, set within a fine- to locally medium-grained carbonate matrix (Figure 3). The matrix locally encloses rounded to subrounded opaque and silicic detrital grains, interpreted as non-carbonate clastic components. These detrital phases are readily under the microscope and BSE images relative to the surrounding carbonate (Figure 3a and d). The carbonate cement is translucent under plane-polarized/transmitted light and displaying uniform grey to dark tones (Figure 3c and f). For in situ U-Pb geochronological analysis, analytical spots were deliberately restricted to the carbonate cement, avoiding bioclasts and detrital grains to minimize the influence of inherited or detrital isotopic components and to ensure that the obtained ages reflect carbonate cement precipitation.
Reflected light (a–d), Transmitted light (b–d) and SEM-BSE (c–f) images of limestones from Channakodi and Pozhikkara sections.
The U-Pb and Sr isotopic data obtained for the studied samples are presented in Appendix 1. Carbonate cement from the Channakodi section limestone (QLN-1) shows U and Pb concentrations ranging between 226–25 ppm and 187–136 ppm, respectively. Forty-five spot analysis from QLN-1 yielded a TW lower intercept age of 17.7 ± 1.9 Ma (MSWD = 0.77) (Figure 4a). On the other hand, carbonate cement from the Pozhikkara section limestone (QLN-2) shows U and Pb concentrations ranging from 247 to 91 ppm and from 802 to 4 ppm, respectively. Forty-nine spot analysis from QLN-2 yielded a TW lower intercept age of 17.6 ± 1.9 Ma (MSWD = 1.8) (Figure 4b). The pooled U-Pb population from QLN-1 and QLN-2 yields a lower intercept age of 17.6 ± 1.3 Ma (MSWD = 1.3) (Figure 4c).
Terra-Wasserburg diagrams for (a) Channakodi section limestone, (b) Pozhikkara section limestone. (c) Terra-Wasserburg diagram showing the pooled U-Pb population from both samples. All uncertainties are in 2σ confidence.
In situ Sr isotope analyses of carbonate cement from Channakodi section limestone (QLN-1) yield measured 87Sr/86Sr ratios ranging from 0.708667 to 0.708797 (n = 15), yielding a weighted mean of 0.708731 ± 0.000023 (2σ) (N = 15; MSWD = 1.2) (Figure 5). Sr isotope analyses of carbonate cement from Pozhikkara section limestone (QLN-2) yield measured 87Sr/86Sr ratios ranging from 0.708823 to 0.709002 (n = 17), yielding a weighted mean of 0.708917 ± 0.000026 (2σ) (N = 17; MSWD = 2.5) (Figure 5).
Insitu 87Sr/86Sr isotope data of carbonate cement from Channakodi and Pozhikkara sections, showing in green and purple colours, respectively. The average 87Sr/86Sr ratio of molluscs shells are from Caroline et al. (Reference Carolin, Vadlamani and Bajpai2022).
5. Discussion
U-Pb ages obtained from the carbonate cement of limestones from the Channakodi and Pozhikkara sections define a tightly clustered age population, yielding a pooled age of 17.6 ± 1.3 Ma (Figure 4c). This firmly places limestone deposition within the Early Miocene Burdigalian Stage and robustly assigns a Burdigalian age to the Quilon Formation. The reported U-Pb ages are in excellent agreement with independently derived strontium isotope numerical ages of 17.08–17.13 Ma obtained from molluscan shells (Carolin et al. Reference Carolin, Vadlamani and Bajpai2022), as well as with biostratigraphic constraints based on the occurrence of Pseudotaberina malabarica (Eames, Reference Eames1950; Epa et al. Reference Epa, Perera, Manamendra-Arachchi and Meegaskumbara2012). The concordance among geochronological, chemostratigraphic, and biostratigraphic datasets strongly indicates that the U-Pb ages accurately reflect the original time of precipitation of the carbonate cement. However, the measured 87Sr/86Sr ratios for the carbonate cement are markedly different from the 87Sr/86Sr ratios reported for the molluscan shells from Channakodi and Pozhikkara sections (Figure 5). While the 87Sr/86Sr ratios for the molluscan shells range between 0.708671 and 0.708630 (Carolin et al. Reference Carolin, Vadlamani and Bajpai2022), corresponding to ∼17 Ma seawater Sr isotope ratio (McArthur et al. Reference McArthur, Howarth, Shields, Zhou, Gradstein, Ogg and Smith2020), 87Sr/86Sr ratios from carbonate cement in the studied sample are more radiogenic, with values ranging between 0.708731 and 0.708917. Although marine carbonates are expected to preserve primary marine 87Sr/86Sr signatures owing to their readily incorporation of Sr and minimal Rb resulting in very little radiogenic Sr production (Rasbury et al. Reference Rasbury, Ward, Hemming, Li, Dickson, Hanson and Major2004), the radiogenic Sr isotope ratios of carbonate cement from Channakodi and Pozhikkara limestones suggest the open-system behaviour of Sr isotopes in the studied samples. In such scenarios, the elevated 87Sr/86Sr isotope signature can be related either due to siliciclastic contamination by continental detrital input (Blum and Erel, Reference Blum and Erel1995; Montanez et al. Reference Montañez, Banner, Osleger, Borg and Bosserman1996; Chen et al. Reference Chen, Cao, Algeo, Li, Wei, Lin, Zhang, Xiang, Zhu, Xu, Fan and Zhang2024), diagenetic recrystallization (Brand and Veizer, Reference Brand and Veizer1980, Reference Brand and Veizer1981; Sheild and Veizer, Reference Shields and Veizer2002) or by the interaction with basinal or pore fluids (Brand and Veizer, Reference Brand and Veizer1980; Banner and Hanson, Reference Banner and Hanson1990; Banner, Reference Banner1995). The lack of neomorphic textures and the preservation of primary skeletal fabrics, together with consistent intra- and inter-sample Sr isotopic compositions, argue against significant diagenetic recrystallization (Figure 3a–f). Furthermore, field and petrographic examination provide no evidence for hydrothermal fluid infiltration or post-depositional fluid-mediated recrystallization by the absence of macroscopic fracture-hosted carbonate veins, vein-fill cements, hydrothermal alteration halos, nor microscale vein networks, replacive textures, pervasive neomorphism, stylolitization, or pressure-solution. The absence of these indicators argues against significant hydrothermal overprinting or externally derived fluid-rock interaction capable of resetting the primary Sr-isotopic composition. In contrast, the presence of detrital siliciclastic and opaque mineral phases, coupled with mildly elevated Rb/Sr ratios (0.0026–0.010), points to a contribution from continental sediment input during carbonate cement precipitation.
The recognition of continental sediment input during the Early Miocene carbonate precipitation provides an important constraint on the depositional and evolutionary history of the Tertiary formations of the Kerala Basin. The Early- to Mid-Miocene represents a critical interval along the western Indian continental margin, marked by a major marine transgression (Reuter et al. Reference Reuter, Piller, Harzhauser, Kroh, Rögl and Ćorić2011; Singh et al. Reference Singh, Vadlamani, Bajpai and Maurya2024) and the development of thick carbonate successions under elevated global sea-levels and warm climatic conditions (Clift et al. Reference Clift, Shimizu, Layne, Blusztajn, Gaedicke, Schluter, Clark and Amjad2001; Molnar Reference Molnar2004). This phase of carbonate accumulation was preceded by significant terrigenous sediment supply from the hinterland, linked to the development of active fluvial systems delivering sediments to the coastal basin (Singh et al. Reference Singh, Rawat and Chandra1999; Campanile et al. Reference Campanile, Nambiar, Bishop, Widdowson and Brown2008). This high sediment flux reflects flexural and isostatic adjustments at ∼20 Ma along the margin during the Western Ghats upliftment (Ajayakumar et al. Reference Ajayakumar, Rajendran and Mahadevan2017). The presence of continental detrital input during carbonate precipitation, together with the close agreement between U-Pb ages of carbonate cements and Sr-isotope stratigraphic ages derived from coeval mollusk shells (Carolin et al. Reference Carolin, Vadlamani and Bajpai2022), indicates that carbonate deposition, shallow burial, and Early post-depositional stabilization occurred over a relatively short timescale during the Miocene marine transgression. However, the 87Sr/86Sr ratios of the carbonate cements are significantly less radiogenic (Figure 5) than those of potential highland source rocks (Tomson et al. Reference Tomson, Rao, Kumar and Choudhary2013), implying that silicate-derived continental inputs were insufficient to substantially modify the primary marine Sr-isotopic signature. This observation indicates a progressive reduction in terrigenous sediment supply from the Early to Mid-Miocene, implying a concomitant decline in continental chemical weathering rates during this interval. Comparable decreases in sediment flux have been independently inferred from sedimentation-rate estimates in the Kerala-Konkan Basin (Campanile et al. 2007; Figure 1). Such a shift is plausibly related to warmer Mid-Miocene climatic conditions (Clift et al. Reference Clift, Shimizu, Layne, Blusztajn, Gaedicke, Schluter, Clark and Amjad2001; Molnar, Reference Molnar2004) coupled with a weakened paleo-monsoon system, commonly described as a proto-monsoon or weak South Asian Monsoon (Betzler et al. Reference Betzler, Eberli, Kroon, Wright, Swart, Nath, Alvarez-Zarikian, Alonso-García, Bialik, Blättler, Guo, Haffen, Horozal, Inoue, Jovane, Lanci, Yao and Young2016, Reference Betzler, Eberli, Lüdmann, Reolid, Kroon, Reijmer, Swart, Wright, Young, Alvarez-Zarikian, Alonso-García, Bialik, Blättler, Guo, Nath, Niino, Petruny, Pratiwi, Slagle, Sloss, Su and Yao2018; Reuter et al. Reference Reuter, Harzhauser and Piller2021). The U-Pb carbonate ages further overlap with the timing of the Monterey carbon isotope excursion (ca. 16.9–13.5 Ma; Hodell and Woodruff, Reference Hodell and Woodruff1994; Holbourn et al. Reference Holbourn, Kuhnt, Schulz, Flores and Andersen2007), which is independently recorded by elevated δ¹3C values in limestones from the Pozhikkara section (Narayana et al. Reference Narayanan, Anirudhan and Grottoli2007). This temporal convergence suggests that perturbations in the global carbon cycle, including reduced atmospheric CO2 levels during the Monterey event, may have additionally modulated continental weathering intensity (Brady, Reference Brady1991; Liu et al. Reference Liu, Dreybrodt and Liu2011; Zhang et al. Reference Zhang, Bai, Zhao, Tan, Luo, Wang, Li, Wu, Chen, Li, Deng, Yang and Xi2021) and sediment delivery to the coastal basin during the Miocene.
These results are also consistent with previous interpretations of the Quilon Formation as a seagrass-associated shallow-marine carbonate system (Reuter et al. Reference Reuter, Piller, Harzhauser, Kroh, Rögl and Ćorić2011; Sarkar et al. Reference Sarkar, Singh and Verma2024). The development of extensive seagrass meadows reflects deposition under shallow, well-oxygenated marine conditions where water clarity remained sufficiently high to permit photosynthesis, while moderate continental input supplied essential nutrients required for sustained seagrass productivity (Hemminga and Duarte, Reference Hemminga and Duarte2000; Perry and Beavington-Penney, Reference Perry and Beavington-Penney2005). Such environments commonly develop under a balanced sedimentary regime in which carbonate production alternates with periods of siliciclastic influx during transgressive-regressive cycles. Under these conditions, seagrass canopies efficiently baffle currents, trap fine-grained siliciclastic particles, stabilize the substrate, and promote in situ carbonate accumulation and early marine cementation (Harris, Reference Harris2009; Lan et al. Reference Lan, Chen, Li and Kaiho2013). The limited incorporation of continental detritus inferred from petrography and geochemistry is therefore compatible with the preservation of near-marine 87Sr/86Sr signatures in the carbonate cements, suggesting that siliciclastic input was sufficient to supply nutrients but insufficient to overwhelm the carbonate factory. Similar seagrass-associated carbonate systems have been documented from Miocene shallow-marine successions worldwide (Betzler et al. Reference Betzler, Eberli, Kroon, Wright, Swart, Nath, Alvarez-Zarikian, Alonso-García, Bialik, Blättler, Guo, Haffen, Horozal, Inoue, Jovane, Lanci, Yao and Young2016, Reference Betzler, Eberli, Lüdmann, Reolid, Kroon, Reijmer, Swart, Wright, Young, Alvarez-Zarikian, Alonso-García, Bialik, Blättler, Guo, Nath, Niino, Petruny, Pratiwi, Slagle, Sloss, Su and Yao2018; Sarkar et al. Reference Sarkar, Singh and Verma2024), where carbonate productivity was maintained despite episodic siliciclastic influx associated with fluctuations in sea level and sediment supply. Together, these observations support carbonate deposition during a phase of diminished terrigenous influx, rapid Early stabilization, and relatively quiescent hydrodynamic conditions during the Miocene transgression.
The age and sedimentary organization of the Quilon Limestone acquire broader significance when viewed against the biogeographic evolution of the Western Ghats-Sri Lanka hotspot (Figure 1). The Oligocene-Miocene transition marks a pivotal interval during which several highly specialized and endemic lineages underwent major diversification across this region (Schulte II et al. Reference Schulte, Macey, Pethiyagoda and Larson2002; Van Bocxlaer et al. Reference Van Bocxlaer, Biju, Loader and Bossuyt2009; Reuter et al. Reference Reuter, Harzhauser and Piller2021). Phylogenetic reconstructions indicate multiple, independent faunal exchanges between Peninsular India and Sri Lanka, with seven such dispersal events identified to date (Lajmi et al. Reference Lajmi, Bansal, Giri and Karanth2019). Notably, the two earliest dispersal phases are constrained to the Early and Middle Miocene, corresponding to the divergence of H. depressus (24.9–13.6 Ma) and H. hunae (17.8–8.7 Ma). These intervals coincide with episodes of modest eustatic sea-level lowering, during which even minor falls of approximately 5–10 m would have exposed transient land connections across the shallow continental shelf separating India and Sri Lanka along Palk Isthmus. Such short-lived land bridges, particularly those formed before the MCO (MMCO) and following the MMCT, provide a plausible physical mechanism linking carbonate deposition along the southwest Indian margin with contemporaneous biotic dispersal and isolation events (Reuter et al. Reference Reuter, Harzhauser and Piller2021 and references therein). The emergence of such a land barrier is well-characterized on the Sri Lankan side based on the presence of shallow-marine limestones (referred to as Jaffna Limestone) with a characteristic Pseudotaberina-strombid-dominated faunal assemblage from Meesalai and Aruwakkula regions (Reuter et al. Reference Reuter, Harzhauser and Piller2021) with 87Sr/86Sr and U-Pb ages of ∼16.5 Ma (Yiu et al. Reference Yiu2018). Although several Miocene marine limestone units have been reported from this corridor including those from the Mannar region (ca. 23.5 and 18 Ma) and the Jaffna Peninsula (ca. 16.0 and 15.5 Ma; Yiu et al. 2018), this particular unit (and occasionally the ∼18 Ma Mannar limestone) has commonly been correlated with the Quilon Limestone (Reuter et al. Reference Reuter, Harzhauser and Piller2021) based on similarities in faunal assemblages, shallow-marine depositional settings, and seagrass affinity (Figure 1). In the present study, the reported U-Pb age of 17.6 ± 1.3 Ma (Figure 4c) obtained from the Quilon Limestone provides the first robust numerical constraint on its timing of deposition. This age places carbonate sedimentation firmly within the Early Miocene (Burdigalian) and overlaps closely with the age of shallow-marine limestones reported from the Sri Lankan margin (∼16.5 Ma; Yiu et al. Reference Yiu2018). The new age constraint thus resolves the long-standing uncertainty surrounding the temporal correlation of these carbonate successions and supports their deposition under broadly synchronous paleoenvironmental and eustatic conditions across the southwest Indian margin and northern Sri Lanka. Importantly, the timing of Quilon Limestone deposition coincides with a phase of episodic sea-level lowering preceding the MCO, during which transient land bridges across the shallow continental shelf would have intermittently connected Peninsular India and Sri Lanka. This temporal overlap provides an independent stratigraphic framework linking shallow-marine carbonate accumulation, regional sea-level fluctuations, and Early Miocene biogeographic dispersal and isolation events within the Western Ghats-Sri Lanka hotspot. Although, the reported U-Pb age provides a robust chronostratigraphic constraint on the timing of Quilon Limestone deposition within the Early Miocene, the associated analytical uncertainty should be considered when assessing finer-scale temporal relationships. Hence the inferred coupling of carbonate deposition with regional paleoenvironmental and biogeographic evolution is therefore interpreted at the scale of the broader Early Miocene chronostratigraphic framework.
6. Conclusion
This study establishes the first direct in situ U-Pb geochronological constraint on the Quilon Limestone of the Kerala Basin, unequivocally placing its deposition within the Early Miocene (Burdigalian). Carbonate cement domains from the Channakodi and Pozhikkara sections define a tightly clustered pooled U-Pb age of 17.6 ± 1.3 Ma, in excellent concordance with independent 87Sr/86Sr stratigraphic ages obtained from coeval molluscan shells and with biostratigraphic constraints based on the presence of Pseudotaberina malabarica. In situ 87Sr/86Sr compositions of the carbonate cements are modestly more radiogenic than contemporaneous seawater, indicating limited open-system Sr behaviour during carbonate formation. The absence of pervasive neomorphic textures or fluid-flow features, coupled with the presence of dispersed siliciclastic detritus, suggests that this radiogenic enrichment reflects minor continental input during deposition rather than late diagenetic overprinting. Integrated geochronological, isotopic and sedimentological evidence indicates that the Quilon Limestone accumulated in warm, shallow-marine, seagrass-dominated settings during an Early Miocene marine transgression, under conditions of reduced siliciclastic flux, high carbonate productivity and rapid early cementation. The Burdigalian age further enables robust temporal correlation with coeval shallow-marine limestones of northern Sri Lanka and situates carbonate deposition within a regional framework of modest eustatic sea-level fluctuations. These fluctuations likely promoted episodic emergence of transient land connections between Peninsular India and Sri Lanka, thereby linking passive-margin carbonate sedimentation in the Kerala Basin with Early Miocene paleoenvironmental reorganization and regional biogeographic evolution along the southwest Indian margin.
Supplementary material
The supplementary material for this article can be found at https://doi.org/10.1017/S0016756826100910
Acknowledgements
The authors wish to thank the Director, NCESS for encouragement and support. The authors are grateful to Mr. Eldhose K., Mr. Nishant N., Dr. Deepchand V., Dr. Haritha A., Mr. Ananthu Anil, Mr. Akshay K., and Ms. Aisha T. H. for their valuable assistance during fieldwork, sample preparation, and analytical work. We also acknowledge Dr. K. S. Sajin Kumar, Dr. Rajesh Regunath, and Dr. Kannan J. Prakash of the University of Kerala for their support in feild work and sampling. The authors sincerely thank the researchers who generously shared reference standards in response to our email requests; their contributions were invaluable to this study and are appropriately acknowledged through the relevant citations. Finally, we express our sincere appreciation to Prof. Zhongwu Lan and an anonymous reviewer for their thoughtful and constructive comments, which significantly improved the manuscript, and to Prof. Peter Clift for editorial handling.
Open access
