Petrography and Geochemistry of the Middle Jurassic–Lower Cretaceous Limestones from the Mustafakemalpaşa Quarries, Bursa, Turkey: The Depositional Environmental and Diagenetic Processes

Abstract

Inatlar limestone, which is dated to the Middle Jurassic–Lower Cretaceous, is exposed between the villages of Kabulbaba and Söğütalan in the Mustafakemalpaşa district of Bursa, Turkey. This study investigates its mineralogical, petrographic, and geochemical characteristics, focusing on major, trace, and rare earth element (REEs) compositions to interpret the depositional environment, paleoenvironmental conditions, and diagenetic processes. Petrographic analysis identified four main limestone types: siliceous, micritic, fossiliferous, and dolomitic. REEs geochemistry indicates enrichment in heavy REEs (HREEs), depletion in light REEs (LREEs), and characteristic anomalies with negative Ce and Eu and positive La, suggesting an open marine depositional environment and early diagenesis. Trace element data point to deposition in settings ranging from continental margins to open marine environments. Ni and V concentrations reflect a spectrum of depositional conditions, including terrestrial, transitional (oxic–dysoxic), and marine anoxic settings. Z values support the theory that the limestones have a marine origin. δ¹³C and δ¹⁸O isotope values indicate deposition in both hydrothermal and typical marine carbonate environments. Y/Ho and Er/Nd ratios reveal the influence of terrestrial input, as well as diagenetic and detrital material. Furthermore, V/(V + Ni) ratios reflect fluctuating oxic to suboxic/anoxic conditions, while Ni/Co ratios indicate predominantly euxinic and, to a lesser extent, anoxic conditions. Altogether, these geochemical signatures suggest that the Inatlar limestone was deposited in a dynamic marine system characterized by variable redox states and salinity fluctuations.

1. Introduction

Carbonate rocks that formed in marine and terrestrial environments throughout different geological periods in Turkey play an important role in numerous industrial and scientific fields. These rocks, which are intensively quarried for limestone, are also valuable for cement production and other industrial applications [1,2,3,4]. Furthermore, their role as hydrocarbon reservoirs [5,6,7] and as host rocks for metallic and non-metallic mineralization [8,9,10,11,12] enhances their economic and geological importance. The economic potential and reservoir quality of carbonate rocks are largely dependent on as alteration, karstification, and diagenetic processes. These processes influence porosity and, consequently, reservoir quality, while also determining the suitability of the rock as a raw material for cement industry [13,14,15,16,17,18].

Geochemical analyses provide invaluable information for reconstructing the depositional environments and paleoclimatic conditions of carbonate rocks. In particular, elemental geochemistry is a critical tool for elucidating the depositional conditions and diagenetic processes these rocks have undergone. The distribution of major and trace elements is governed by factors such as sedimentary conditions, mineralogy, and key physicochemical parameters [19,20,21,22,23,24,25]. Variations in elemental ratios are influenced by a wide range of factors, from temperature to salinity and light intensity. Consequently, analyzing the elemental composition of carbonate rocks allows for a detailed understanding of their depositional environments [26,27].

Specific geochemical proxies, such as rare earth elements (REEs), are crucial in the paleoenvironmental analysis of sedimentary rocks like carbonates and cherts. Specifically, Ce and Eu anomalies can reveal key information about marine anoxia and paleo-redox conditions [25,26,27,28,29,30,31,32,33,34,35,36,37,38]. Furthermore, REEs concentrations in seawater are dynamic, varying in response to terrestrial inputs from continental weathering and hydrothermal activity, which in turn alters the overall seawater chemistry [39,40]. Beyond geochemistry, biological indicators are also vital; the diversity and distribution of benthic foraminifera serve as important proxies in the biostratigraphic and paleoecological analysis of carbonate environments [41,42,43].

The Middle Jurassic–Lower Cretaceous carbonate rocks of the Bilecik Carbonate Group, located in the Western Sakarya Zone in northwestern Turkey, define a region renowned for its commercial limestone production. Specifically, the outcrops between the villages of Kabulbaba and Söğütalan villages (Mustafakemalpaşa, Bursa) are economically significant due to the presence of numerous limestone quarries. Previous studies on the carbonate formations in this specific region have predominantly focused on the general geology and the mineralogical properties of commercial limestone [4,44,45,46,47,48,49]. This study aims to move beyond these foundational works by conducting a detailed investigation into the mineralogical–petrographic characteristics and comprehensive geochemistry (major, trace, and rare earth elements) of these carbonates. The primary objective is to use this integrated dataset to elucidate their depositional environments, diagenetic processes and paleo-environmental implications. Ultimately, this research will not only offer a more nuanced understanding of the region’s geological history but will also provide a robust scientific basis for developing more efficient strategies for the economic and industrial utilization of these valuable carbonate resources.

2. Geological Setting

The geological structure of Turkey is shaped by tectonic units derived from Gondwana and the evolutionary history of the Tethys Ocean. In the north, the Pontides mark the southern edge of Laurasia, while to the south, the Anatolide–Tauride Block—including the Menderes–Tauride Block and Kırşehir Massif—represents the northern margin of Gondwana [50,51,52]. Two major carbonate platforms were developed within this framework: the Sakarya Zone platform to the north and the Tauride platform to the south. The Sakarya Zone formed along Laurasia’s southern margin after the Triassic Karakaya Orogeny, and is separated from the Tauride platform by the İzmir–Ankara–Erzincan Suture [53,54,55,56,57,58,59,60,61,62,63]. It is underlain by the Paleozoic-aged Karakaya Complex and overlain by Upper Jurassic–Lower Cretaceous carbonates [51,64,65,66,67] (Figure 1). These carbonates, including the Bilecik Group, formed on a basement by the Variscan orogeny and Later Jurassic–Cretaceous subduction-related events [50,51,68,69,70,71,72,73]. Volcanic and volcaniclastic activity, along with regional metamorphism, were prominent during the Middle to Upper Jurassic in the Pontides [74,75,76]. The Bilecik Group, a major Jurassic–Lower Cretaceous carbonate succession, includes regionally significant formations such as the Inatlar, Bilecik, and Soğukçam limestones, which are found in areas like Mustafakemalpaşa, İznik-Uludağ, Göynük, and Gölpazarı [44,54,62,77]. These carbonates have been extensively studied using stratigraphic and paleontological data, including radiolarians, planktonic foraminifera, and nannofossils, to reconstruct the basin’s evolution and environmental conditions [57,58,78].

In the study area (Bursa Province, Sakarya Zone), the basement is formed by Permian–Triassic Kızıltepe Metamorphics (Uludağ Group), composed of phyllite, schist, gneiss, amphibolite, and calcschist, affected by greenschist facies metamorphism [44,56,79,80]. These are overlain by the Triassic Karakaya Complex (referred to as the Nilüfer Unit in this paper), which consists of weakly metamorphosed sandstones, mudstones, radiolarites, and spilitic volcanic rocks, along with black limestone blocks [56] (Figure 2a). These units are associated with the Karakaya Orogeny and represent key tectonic events [57,58]. Above this complex, the Inatlar limestone of the Bilecik Group is exposed north and northeast of Kabulbaba village (Figure 2a,b).

Reaching ~200 m in thickness, it displays chert nodules (Figure 3a), karst features (Figure 3b–d), beige-to-brown colors with dolomite (Figure 3e,f), and varying bedding structures due to folding. The unit strikes in a northeast–southwest direction and generally dips towards the northwest and southeast, although fold-related deformation causes significant variability in dip angles in the field. Based on ammonite fauna, ref. [44] assigned a Middle–Upper Jurassic to Lower Cretaceous age to the Bilecik Formation (Inatlar limestone). Similarly, ref. [56] reported an Upper Jurassic age for Bilecik limestone samples collected from the Biga Peninsula. In an analysis of the eroded upper stratigraphic sections, proposed an age corresponding to the Upper Jurassic (Cimmeridian–Tortonian) [57].

The carbonate platform succession, dated to the Middle–Upper Jurassic to Lower Cretaceous and observed in the Mustafakemalpaşa region, records a well-defined transgressive–regressive depositional history. This succession unconformably overlies the metamorphic and volcanic basement rocks of the Triassic Karakaya Complex across a distinct transgressive surface. The lowermost stratigraphic levels (Middle Jurassic) consist of thick-bedded, pinkish, oolitic, and sandy–clayey limestones, which are often rich in lamellibranchs and ammonites. These facies were deposited in wave-dominated shallow marine shelf environments during a major transgressive episode, marking the early phase of carbonate platform development. The overlying middle stratigraphic interval (Upper Jurassic to Lower Cretaceous) is characterized by thick-bedded biomicritic and dolomitic limestones with reefal structures, indicating a highstand systems tract marked by expanded carbonate production, platform progradation, and relative sea-level stability. The uppermost strata (Lower Cretaceous) are composed of medium- to thick-bedded micritic limestones with chert bands, pelagic limestones, and radiolarites, reflecting a deepening phase of the platform associated with relative sea-level fall and regressive conditions. Vertical facies transitions—such as karstification and dolomitization—further indicate exposure events and diagenetic overprinting linked to sea-level fluctuations. Overall, the stratigraphy of the Bilecik Limestone in this region captures a complete depositional cycle from transgression to regression, highlighting significant paleoenvironmental and sea-level changes across the Jurassic–Cretaceous boundary. Unconformably overlying this unit is the Upper Miocene Değirmendere Formation, beginning with a basal conglomerate, followed by lacustrine sediments (sandstone, marl, coal, and tuff). The youngest units in the study area are alluvial (fluvial) sediments (Figure 3).

3. Materials and Methods

This study is based on a suite of 27 limestone samples collected from various depths within several quarries. To characterize the rocks comprehensively, a multi-analytical approach was employed. Mineralogical and petrographic properties were investigated through thin-section analysis, X-ray diffraction (XRD; n = 9), and scanning electron microscopy (SEM-EDX; n = 9). Geochemical compositions, including major and trace elements, were determined for 20 samples. Additionally, carbon and oxygen isotope compositions were analyzed in a subset of eight samples. The integrated results from these analyses were used to reconstruct the depositional conditions and diagenetic history of the carbonate sequence.

3.1. Mineralogical–Petrographic Analysis

To evaluate the microscopic structure, mineral assemblages, and fossil content, thin sections were prepared from twenty rock samples stored at the Geological Engineering Department Laboratory of Süleyman Demirel University (Isparta-Turkey)to determine their microscopic structure, mineral assemblages, and fossil content. The thin sections were examined under a polarizing microscope equipped with a 5.1-megapixel camera, and images were processed using the Image Pro Plus 5.1v image analysis system. For bulk mineralogy, nine powdered samples were analyzed via X-ray diffraction (XRD) at the Afyon Kocatepe University (Afyon-Turkey) Technology Application and Research Center. Analyses were conducted on a Shimadzu XRD-6000 X-ray diffractometer (Shimadzu, Afyon, Turkey) with CuKα radiation (40 Kv, 30 mA); the 2θ range was scanned from 2 to 70°. For morphological and micro-textural analysis, nine samples were coated with a gold film that was approximately 250–300 Å thick using a sputter coater to ensure adequate conductivity. Scanning electron microscopy (SEM) observations were carried out using a LEO VP-1431 instrument (ZEISS, Afyon, Turkey) to capture high-resolution surface images and identify microstructural characteristics. In conjunction with SEM, semi-quantitative elemental analyses were performed using energy-dispersive X-ray spectroscopy (EDX) on the same samples. Both SEM imaging and EDX analyses were conducted at the same research facility under standard operating conditions.

3.2. Geochemical Analysis

Twenty samples were ground to 200 mesh at Bureau Veritas Mineral Laboratories (Canada) to determine their chemical composition. Major oxides and trace elements were analyzed using ICP-ES. To measure trace and rare earth elements, samples were digested in dilute nitric acid and then fused with lithium metaborate/tetraborate to dissolve the elements. These solutions were analyzed by ICP-MS. The detection limits were 0.002–0.01 wt% for major oxides and trace elements, 0.01–8 ppm for trace elements, and 0.05–0.3 ppm for rare earth elements. Calibration was performed according to several geochemical standards: STD SO-18, STD DS8, STD GS311-1, STD GS910-4, and STD OREAS45CA (http://acmelab.com/services/method-descriptions/soil-till-and-sediment/, accessed on 28 October 2025). The quality and accuracy were verified with a range of reference materials.

3.3. Isotope Analyses

The δ¹³C and δ¹⁸O isotope ratios were measured at the Iso-Analytical Limited laboratory (Cheshire-United Kingdom) by converting carbonate samples to CO₂ through a reaction with phosphoric acid at 100 °C. The generated CO₂ was then analyzed using an EA-IRMS mass spectrometer. All the raw analytical data were corrected using standard procedures and reported in standard delta notation, with deviations expressed in per mil (‰) relative to the Vienna-Standard Mean Ocean Water (VSMOW) and PeeDee Belemnit (VPDB) standards. The reference materials used during the analysis included Iso-Analytical working standards IA-R022 (calcium carbonate: δ¹³C V-PDB = −28.63‰, δ¹⁸O V-PDB = −22.69‰), NBS-18 (calcite: δ¹³C V-PDB = −5.00‰, δ¹⁸O V-PDB = −23.20), and NBS-19 (limestone: δ¹³C V-PDB = 1.95‰, δ¹⁸O V-PDB = −2.20‰) (http://www.iso-analytical.co.uk/, accessed on 28 October 2025).

4. Results

4.1. Petrographic Characteristics

4.1.1. Optical Microscopy and Diagenetic Features of Inatlar Limestone

Twenty thin-section samples of Inatlar limestone, collected from quarries in the study area, were prepared. Petrographic analysis was conducted using a polarizing microscope under both plane-polarized and cross-polarized light, and representative microphotographs were obtained. The primary components include micrite, sparry calcite, pellets, fossil fragments, dolomite, and localized silicification. Microscopically, the limestone exhibits predominantly micritic and oolitic textures, with varying degrees of sparry calcite recrystallization. Sparry calcite veins are commonly observed within pellets and fractures and in several cases, fractures are filled with both sparry calcite and dolomite (Figure 4a). Brecciated textures, containing calcite and subhedral to euhedral dolomite crystals, are evident in dolomitic limestone samples (Figure 4b,c). In siliceous limestone, silicification, fossil fragments, and stockwork sparry calcite veins are present (Figure 4d–f). Fossiliferous light beige limestone shows sparry calcite and fossil shell infillings (Figure 4g), while dark beige fossiliferous limestone often contains iron oxide fillings in fractures (Figure 4h).

Carbonate samples within the Inatlar carbonate platform exhibit 20%–80% dolomitization. Dolomite typically occurs in the form of fine- to medium-crystalline, subhedral crystals scattered within the micritic matrix (Figure 4c). Dolomitization is a diagenetic process that has affected the Middle Jurassic–Lower Cretaceous carbonates, particularly in the upper parts of the stratigraphic succession. It is associated with subaerial exposure, karstification, and relative sea-level changes, and is consistent with both reflux (evaporative) and mixing-zone dolomitization models [81,82]. These processes were particularly effective in porous and permeable facies, such as biomicritic and oolitic limestones. The fossil content in dolomitized zones is notably reduced, suggesting dissolution or masking during recrystallization. Karst-related cavities and fractures are commonly filled with secondary dolomite or calcite, indicating late-stage diagenetic modification.

Textural features such as capillary fractures, dissolution-enhanced pore networks, stylolites, and brecciated sparry calcite fillings reflect tectonic deformation and hydrothermal influence. The presence of iron oxide zones, opaque minerals, and recrystallized fossils in dolomitized zones is indicative of post-depositional alterations. The reduction or absence of fossil content in these areas supports the concept of dolomitization-associated dissolution and masking. Secondary dolomite and calcite fillings within karst cavities and fractures further confirm late-stage diagenetic modification. Collectively, these features indicate that the Inatlar limestone underwent complex diagenetic evolution influenced by tectonism, sea-level fluctuations, and hydrothermal processes.

4.1.2. X-Ray Diffraction (XRD) Analysis

The results of the XRD analysis reveal that some of the examined limestone samples are composed entirely of calcite (Figure 5a). Such limestones, predominantly consisting of pure calcite, are interpreted as preserved sections of primary sedimentary rocks and appear to have been relatively unaffected by post-depositional processes such as diagenesis. In contrast, some carbonate rock samples exhibit dominant dolomite peaks, reflecting a high dolomite mineral content and suggesting that these rocks have undergone an almost complete dolomitization process (Figure 5b). Siliceous and fossiliferous limestone samples, on the other hand, contain approximately 90% calcite and 10% dolomite. This suggests that calcite is the primary mineral phase, while dolomite has formed as a result of replacement processes. These mineralogical findings, along with the diversity and purity levels of the limestone samples, provide significant insights into the depositional environment, subsequent chemical modifications, and the influence of the regional tectonic processes affecting the rocks.

4.1.3. Scanning Electron Microscopy Investigations

Analyses performed using a scanning electron microscope (SEM) with energy-dispersive X-ray spectroscopy (EDX) revealed that calcite in the Inatlar limestone formed through two distinct phases. The first phase, designated as calcite-1 (micritic calcite), exhibits a sheet-like morphology with pronounced cleavage planes (Figure 6), suggesting that its crystallization occurred during primary sediment deposition or early diagenetic stages. The second phase, calcite-2 (sparry calcite), occurs as veins filling fractures within calcite-1 and appears in euhedral to subhedral crystal forms (Figure 6a,b), indicating precipitation from hydrothermal or meteoric fluids transported through fractures during later diagenesis. Micritic-textured pelagic limestone samples predominantly consist of platy calcite crystals (Figure 6b), whereas fossiliferous limestones demonstrate euhedral and subhedral calcite crystals intermixed with dolomite (Figure 6c), reflecting variations in depositional environments and subsequent diagenetic alterations. Dolomitic limestone samples exhibit platy crystal morphology with pronounced relief and show distinct cleavage in two directions (Figure 6d).

EDX analyses detected elevated concentrations of SiO₂, Al₂O₃, and K₂O in certain sample areas, indicating enrichment in feldspar or clay minerals. The high SiO₂ content is also indicative of the presence of siliceous phases such as chert.

4.2. Geochemistry

4.2.1. Major–Minor Oxides’ Contents

The chemical composition of limestone samples representing different facies within the study area (

Table 1. Major oxide (%), trace and rare earth elements (ppm) concentrations of Inatlar limestone samples in the study area.

		Siliceous Limestone				Micritic Limestone		Fossiliferous Limestone						Dolomitic Limestone
Major Oxides (wt.%)	DL	YE-3A	PA-1	TC-1	YE-3B	KP-1	AK-2	AK-3	KP-2	YB-1	YB-2	AK-1	YU-1	PA-2	BI-1
SiO2	0.01	1.87	0.97	0.99	0.48	0.89	0.79	0.09	0.11	0.04	0.38	0.13	0.48	0.25	0.35
Al2O3	0.01	0.42	0.08	0.31	0.15	0.24	0.23	0.02	0.02	0.01	0.02	0.03	0.05	0.05	0.12
Fe2O3	0.04	0.25	0.12	0.08	0.19	0.15	0.25	0.06	0.08	0.04	0.14	0.04	0.07	0.19	0.16
MgO	0.01	0.74	0.62	0.67	0.98	0.64	0.73	0.21	0.45	0.42	0.32	0.22	1.3	17.56	15.86
CaO	0.01	53.68	54.93	54.71	54.72	54.79	54.63	56.31	55.76	56.03	55.81	56.18	55.17	35.97	37.42
Na2O	0.01	0.03	0.02	0.02	<0.01	0.01	0.02	<0.01	<0.01	<0.01	<0.01	<0.01	0.03	0.04	0.03
K2O	0.01	0.19	0.04	0.05	0.04	0.1	0.08	<0.01	<0.01	<0.01	<0.01	<0.01	0.03	0.02	0.03
TiO2	0.01	0.02	<0.01	0.01	<0.01	0.01	0.01	<0.01	<0.01	<0.01	<0.01	<0.01	<0.01	<0.01	0.01
P2O5	0.01	<0.01	<0.01	<0.01	<0.01	<0.01	<0.01	<0.01	0.04	<0.01	<0.01	<0.01	<0.01	<0.01	<0.01
MnO	0.01	<0.01	<0.01	<0.01	0.02	0.01	<0.01	<0.01	<0.01	<0.01	<0.01	<0.01	<0.01	<0.01	<0.01
Cr2O3	0.002	0.002	0.002	0.002	0.002	0.002	0.002	<0.002	<0.002	<0.002	<0.002	<0.002	<0.002	<0.002	<0.002
TOT/C	0.02	12.13	12.28	12.33	12.3	12.19	12.14	12.3	12.36	12.37	12.36	12.3	12.39	13.1	12.98
TOT/S	0.02	0.07	0.04	0.02	0.08	<0.02	<0.02	<0.02	<0.02	<0.02	<0.02	<0.02	0.05	0.04	0.03
LOI	−5.1	42.8	43.1	43.2	43.4	43.1	43.3	43.4	43.5	43.5	43.3	43.4	42.8	45.7	45.8
Total	0.01	99.96	99.89	99.95	99.95	99.99	99.98	100.1	99.95	99.97	99.97	100	99.94	99.69	99.72
Trace element (ppm)
Ba	1	15	139	21	11	25	4	1	2	3	2	3	19	2	3
Sr	0.5	264.6	221.7	227.0	130.0	153.1		86.9	151.5	135.9	184.6	92.8	271.9	147.3	133.6
Rb	0.1	3.8	0.8	1.1	1.1	2.0	1.9	0.4	0.4	0.4	0.4	0.6	0.9	0.5	0.6
U	0.1	3.3	2.1	1.2	1.7	0.6	<0.1	0.3	<0.1	0.5	<0.1	0.3	2.8	2.0	1.4
V	8	41	17	13	31	20	22	17	16	9	<8	24	18	15	13
Zr	0.1	5.7	4.8	9	3.9	20.8	5.8	9	2.5	3.4	5.1	3	2.4	2.8	4.1
Y	0.1	4.1	3	2.2	4.4	4.6	3.2	0.4	1.3	0.5	0.6	0.6	1.4	2.2	2.6
Ni	0.1	3.8	1.7	2.6	1.9	1.1	2.7	2.2	2.3	2.8	2	2.2	3.6	1.9	2.2
Co	0.2	0.3	0.7	0.6	0.6	1.4	0.5	<0.2	0.7	<0.2	<0.2	0.5	0.7	<0.2	0.7
V/V + Ni		0.92	0.91	0.83	0.94	0.95	0.89	0.89	0.87	0.76	0.80	0.92	0.83	0.89	0.86
Ni/Co		12.67	2.43	4.33	3.17	0.79	5.40	11.00	3.29	14.00	10.00	4.40	5.14	9.50	3.14
V/Ni		10.79	10.00	5.00	16.32	18.188	8.15	7.73	6.96	3.21	4.00	10.91	5.00	7.89	5.91
Sr/Ba		17.64	1.59	10.81	11.82	6.12	44.75	86.9	75.75	45.30	92.30	90.93	14.31	73.65	44.53
Sr/Rb		69.63	277.13	206.36	118.18	76.55	94.21	217.25	378.75	339.75	461.50	154.67	302.11	294.60	202.42
Y/Ho		58.57	60.00	55.00	55.00	51.11	106.67	20.00	32.50	25.00	30.00	30.00	70.00	110.00	86.67
Rare Earth Element (ppm)	DL	Siliceous limestone				Micritic limestone		Fossiliferous limestone						Dolomitic limestone
La	0.1	3.2	4.7	2.2	2.3	3.8	2.4	0.8	1.4	0.7	0.8	1.1	1.8	2.0	2.6
Ce	0.1	1.8	5.4	1.6	1.9	1.8	1.6	0.3	0.5	0.3	0.2	0.4	0.8	0.8	1.3
Pr	0.02	0.14	0.38	0.02	0.03	0.11	0.02	0.02	0.02	0.02	0.02	0.02	0.02	0.02	0.02
Nd	0.3	0.4	1.7	0.3	0.3	1.4	0.3	0.3	0.3	0.3	0.3	0.3	0.3	0.3	0.3
Sm	0.05	0.1	0.26	0.05	0.08	0.16	0.06	0.05	0.05	0.05	0.05	0.05	0.05	0.05	0.05
Eu	0.02	0.02	0.06	0.03	0.04	0.05	0.03	0.02	0.02	0.02	0.02	0.02	0.02	0.02	0.02
Gd	0.05	0.32	0.35	0.18	0.22	0.3	0.13	0.05	0.05	0.05	0.05	0.05	0.05	0.09	0.26
Tb	0.01	0.04	0.03	0.01	0.04	0.05	0.03	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.02
Dy	0.05	0.28	0.25	0.25	0.36	0.36	0.06	0.05	0.05	0.05	0.05	0.14	0.07	0.05	0.29
Ho	0.02	0.07	0.05	0.04	0.08	0.09	0.03	0.02	0.04	0.02	0.02	0.02	0.02	0.02	0.03
Er	0.03	0.16	0.07	0.07	0.3	0.15	0.15	0.04	0.03	0.07	0.04	0.06	0.08	0.09	0.06
Tm	0.01	0.02	0.03	0.01	0.07	0.04	0.03	0.01	0.02	0.01	0.01	0.01	0.01	0.02	0.02
Yb	0.05	0.19	0.15	0.14	0.35	0.2	0.18	0.05	0.11	0.06	0.05	0.18	0.14	0.08	0.13
Lu	0.01	0.03	0.02	0.01	0.05	0.03	0.01	0.02	0.01	0.01	0.01	0.01	0.01	0.01	0.01
Ce/Ce*		0.39	0.73	0.55	0.61	0.39	0.50	0.27	0.26	0.31	0.18	0.27	0.33	0.30	0.38
Eu/Eu*		0.34	0.61	0.97	0.92	0.70	1.04	1.22	1.22	1.22	1.22	1.22	1.22	0.91	0.54
Pr/Pr*		0.79	0.65	0.13	0.18	0.39	0.13	0.37	0.29	0.37	0.43	0.32	0.21	0.21	0.15
(Pr/Yb)n		1.26	4.32	0.24	0.15	0.94	0.19	0.67	0.30	0.55	0.67	0.19	0.24	0.42	0.26
(La/Yb)n		11.35	21.12	10.59	4.43	12.81	8.99	1.79	8.58	7.87	10.79	4.12	8.67	16.85	13.48
(La/Sm)n		20.13	11.67	27.68	18.08	14.94	25.16	10.06	17.61	8.81	10.06	13.84	22.65	25.16	32.71
(Sm/Yb)n		0.56	1.85	0.39	0.25	0.90	0.36	1.08	0.49	0.90	1.08	0.30	0.39	0.68	0.42
(La/Ce)n		4.63	2.27	3.59	3.18	5.50	3.91	6.97	7.29	6.11	10.32	7.10	5.87	6.52	5.21
Er/Nd		0.40	0.04	0.23	1.00	0.11	0.50	0.13	0.10	0.23	0.20	0.27	0.30	0.20	0.09

) offers valuable insights into their mineralogical makeup and the processes governing their formation. The fossiliferous beige limestone samples exhibit generally high CaO values ranging from 55.17% to 56.31%, accompanied by low MgO content (0.21%–1.13%). These analyses confirm that calcite is the dominant mineral phase within these samples. Higher SiO₂ (0.79%–0.89%) and Fe₂O₃ (0.15%–0.25%) concentrations observed in brecciated and stylolitic limestone samples suggest that these rocks have undergone more complex diagenetic and geological processes. Such features may be attributed to phenomena such as dissolution, reworking, and redepositing, or enhanced silica and iron enrichment likely induced by tectonic activity. The regional tectonic dynamics and associated mechanical deformation likely influenced mineralogical alterations and enrichment within these rocks.

The SiO₂ content indicates the notable input of quartz or siliceous minerals during carbonate precipitation. Typically, silica is transported and accumulated through mineralogical and chemical processes, eventually leading to the formation of quartz or other siliceous phases. Limestones characterized by high MgO content (15.86%–17.56%)—notable those hosting calcite veins and dolomite (samples Bi-1, PA-2)—provide clear evidence of dolomite presence and the effects of dolomitization. These rocks generally form in magnesium-rich settings where calcite and dolomite coexist. Finally, siliceous limestone samples (YE-3A, PA-1, TC-1) exhibit elevated SiO₂ concentrations (0.97%–1.8%), reflecting enrichment in chert, quartz, and other siliceous minerals. Such compositions imply depositional environments and processes dominated by siliceous material input.

4.2.2. Trace and Rare Earth Element Composition

Trace element analysis reveals that strontium concentrations range between 272 and 238 ppm in fossiliferous and siliceous limestones, whereas other limestone types exhibit lower Sr values between 130 and 166 ppm. These results indicate that siliceous and fossiliferous limestones are significantly enriched in Sr relative to other limestone varieties. Similarly, Ba concentrations are elevated in siliceous (58.3 ppm) and fossiliferous (19 ppm) limestones compared to other types, which range between 2.2 and 14.5 ppm. Vanadium content is slightly higher in siliceous and dolomitic limestones (31 ppm, 23.7 ppm, and 21 ppm) relative to the baseline value of 20 ppm and other types of limestone types (16.5–18 ppm) (Table 1). The high Sr content in limestone is consistent with the chemical similarity of Sr and Ca ions, allowing substitution of calcium by strontium in the carbonate lattice. The relative enrichment of Sr and Ba, along with the low concentrations of Y, Zr, and Ni, may reflect the specific formation conditions of the carbonate rocks, including weathering, transport, deposition environment, and redox conditions (pH-Eh), as well as elemental migration processes.

Chondrite-normalized rare earth element (REEs) patterns display complex behavior; Eu exhibits both negative and positive anomalies, La shows a positive anomaly, and Ce and Pr exhibit negative anomalies (Figure 7). The trace element and REEs concentrations in the analyzed samples—including Sr, Ba, Rb, Zr, Ni, and U—fall within the typical ranges reported for limestones.

4.2.3. Carbon–Oxygen Isotope Compositions

Inatlar limestone samples from various facies were selected for stable isotope analyses to investigate their carbon and oxygen isotopic compositions. The mineralogical composition of these limestone rock samples primarily consists of calcite and dolomite. The average δ¹³C values of the Inatlar limestone samples range from 0.50‰ to 3.08‰, with a mean value of −1.34‰. The δ¹⁸O values vary between −3.96‰ and −0.21‰, with a mean value of −2.23‰. These isotopic values correspond well with previously reported δ¹³C values between 0 and 3‰ and δ¹⁸O values between −7 and 0‰, which are characteristic of Mesozoic seawater [85]. Similarly, ref. [20] documented δ¹³C values ranging from 1.81 to 2.63 and δ¹⁸O values generally between 1.39‰ and −0.23‰ for limestone breccia (Pietra di Billiemi) from northwestern Sicily, Italy. They noted that such values are indicative of well-preserved marine limestones that have not experienced significant recrystallization during burial or diagenesis involving meteoric waters. Based on these comparisons, the isotopic signatures of Inatlar limestone samples support their classification as marine limestone, reflecting seawater influence during the diagenetic history of the rocks.

5. Discussion

5.1. Depositional Environment and Diagenetic Processes

Rare earth elements (REEs) concentrations, along with Eu and Ce anomalies represent critical proxies for interpreting marine depositional environments [86]. Among REEs, cerium is recognized as a particularly sensitive indicator of paleo-ocean redox conditions [28,87,88,89]. However, the interpretation of sedimentary REEs signatures should be approached with caution, as diagenetic processes and burial can alter the original REEs distribution through post-depositional fractionation [29,90,91]. The mobility of REEs within the water column and sediment porewaters is governed by a combination of adsorption, complexation, and redox reactions, particularly those involving Ce and Eu [90,92,93]. Cerium exhibits unique redox-sensitive behavior [37], with the presence of pronounced Ce anomalies that are commonly observed in seawater. Modern seawater typically shows a characteristic Ce depletion [94].

Ce anomalies preserved in marine carbonates serve as effective indicators of paleo-redox conditions and can also reflect the extent of terrestrial material input during sediment deposition [28,39,88,95,96,97,98,99,100]. Negative Ce anomalies in carbonates typically reflect the incorporation of REEs under oxidative conditions, sourced directly from seawater or sediment porewater [101]. Moreover, the authors of [102] demonstrated that the redox state of overlying seawater exerts a strong control over Ce behavior in sediment porewaters, highlighting the sensitivity of Ce anomalies to local redox conditions.

Ce deficiency in seawater is primarily attributed to the oxidation of Ce³⁺ to Ce⁴⁺, which is subsequently removed from the water column by adsorption onto suspended particles [101]. In well-oxygenated environments, Ce³⁺ oxidation to Ce⁴⁺ leads to fractionation relative to other REEs, as Ce⁴ is preferentially scavenged from seawater [39]. Conversely, under suboxic conditions, Ce is partially remobilized and released back into the water column, producing a less pronounced negative or even positive Ce anomaly [103]. The preferential removal of Ce⁴⁺ from sediment results in reduced Ce concentrations relative to La and Pr in seawater, yielding a negative Ce anomaly characterized by Ce/Ce* ratios below 1.0. In anoxic environments, Ce⁺³ behaves similarly to other REEs, minimizing interelement fractionation and resulting in a weak or absent Ce anomaly (Ce/Ce* ~ 1.0). In the modern ocean, the magnitude of the negative Ce anomaly typically increases with depth due to decreasing Ce and increasing La and Nd concentrations [39]. The authors of [104] proposed that Ce/Ce* ratios of <0.5, ~0.6–0.9, and ~0.9–1.0 correspond to oxic, suboxic, and anoxic seawater, respectively. The Ce/Ce* values measured in Inatlar limestone samples ranged from 0.18 to 0.73 (Table 1), indicating deposition within oxic–suboxic marine environments.

Eu is unique among the REEs in exhibiting variable valence states within the near-surface environment, where Eu³⁺ can be reduced to Eu²⁺ under strongly reducing conditions. The redox potential controlling Eu/Eu* in aqueous solutions is primarily dependent on temperature, with lesser influences from pressure, pH, and REEs speciation [105]. This explains the common occurrence of positive Eu anomalies in acidic, reducing hydrothermal fluids [106]. Positive Eu anomalies may originate from eolian inputs [39] or hydrothermal fluids [107,108], with the latter possibly reflecting an increase in primary detrital feldspar content [100] or diagenetic alteration of carbonate/limestone [109]. The Inatlar limestone samples exhibit Eu/Eu* ratios ranging from 0.34 to 1.22. The positive Eu anomalies evident in the REEs patterns may be attributed to the influence of hydrothermal fluid, diagenetic alteration processes within the carbonate/limestone, or enrichment of feldspar and quartz minerals. This interpretation is supported by elevated Sr concentrations in the samples. Conversely, negative Eu anomalies suggest a terrestrial derived component with a felsic provenance.

(La/Yb)_n versus (La/Sm)_n ratios were plotted to assess REEs fractionation mechanisms, including absorption and substitution, as described in [110] (Figure 8). These ratios for ocean water are relatively homogeneous, ranging from 0.6 to 1.6 for (La/Sm)_n and 0.2 to 0.5 for (La/Yb)_n [111]. However, the Inatlar limestone samples show elevated values ranging from 4.12 to 32.71 (Table 1), consistent with early diagenesis sorption processes, as illustrated in Figure 8. Similarly, ref. [35] reported that Lower Cretaceous limestones from Gümüşhane also exhibit signatures indicative of early sorption. Such elevated normalized (La/Sm)_n and (La/Yb)_n ratios are challenging to reconcile solely with seawater composition, suggesting the influence of secular seawater chemistry changes or extensive REEs fractionation through adsorption during diagenesis [111,112].

La and Ce anomalies were calculated using the Ce/Ce* and Pr/Pr* ratios, following the methodology proposed by [105]. All analyzed samples display positive La anomalies (Figure 9), indicating that observed Ce anomalies may partially result from La interference. In modern seawater, Ce/Ce* ratios typically range from 0.1 to 0.4, with values approaching 1 indicative of increased pollution or anthropogenic input [94,97]. Positive Ce anomalies are commonly attributed to detrital input [31,99,100,113], diagenetic alteration [32], scavenging processes [114], and paleo-redox conditions [86]. The authors of [40] documented that carbonate layers from the Aptian-Albian Mural Formation in the Pitaycachi section of northeastern Sonora, Mexico, predominantly exhibit negative Ce and positive La anomalies, although some samples lacked negative Ce anomalies. Similarly, ref. [35] reported that negative Ce anomalies observed in Lower Cretaceous limestones may be remnants of La interference. Carbonates from the Paleocene Ewekoro Formation in southwestern Nigeria also show negative Ce and positive La anomalies [25], whereas ref. [38] observed positive Ce and La anomalies in Archaean carbonate sediments of the Tanwan Group, Bhilwara Supergroup, India.

The analyzed limestone samples exhibit a negative Ce anomaly in the plot of Pr/Yb_n values versus the Ce/Ce* ratio (Figure 10a). Furthermore, the graph of Pr/Yb_n values against Al₂O₃ (wt%) content (Figure 10b) reveals depletion of light rare earth elements (LREEs). Similar LREEs depletion in carbonates has been reported by [25,35]. Thus, the geochemical characteristics of the studied carbonates, consistent with previous studies, indicate both LREEs depletion and a negative Ce anomaly, providing valuable insights into the depositional environment and diagenetic processes of these limestones.

In the ternary Rb–Sr–Ba diagram (Figure 11a), siliceous, micritic, fossiliferous, and dolomitic limestone samples are predominantly plotted within the continental margin field, although some samples—excluding the dolomitic limestone—fall within the open ocean domain. Meanwhile, the Sr/Ba versus Sr/Rb diagram (Figure 11b) shows all samples confined to the continental margin setting. These findings are consistent with the depositional environment interpretations of [1], who classified Gimo limestone samples as having formed along continental margins.

The relationships among the Eu/Eu*, Ce/Ce*, (La/Ce)_n, and (Sm/Yb)_n ratios, used to distinguish two groups of limestone, are presented in Figure 12. According to these data, all limestone samples are plotted within the open ocean field.

Depositional environments of limestone can be predicted using the Z = a(δ¹³C + 50) + b(δ¹⁸O + 50) equation proposed by [117]. The a value is 2.048 and the b value is 0.498. In limestone classification, a Z value greater than 120 indicates a marine environment, whereas a value below 120 suggests a freshwater setting. In this study, all samples ranged between 126.35 and 133.26. Z values above 120 indicate that the limestones are of marine origin. A bivariate plot of δ¹³C–δ¹⁸O with generalized isotopic fields for carbonate components, sediments, limestones, cements, dolomites, and concretions was created by [118] and later modified by [119].

The authors [118] proposed a diagram using δ¹³C and δ¹⁸O isotope values without distinguishing the diagenetic environment. Figure 13a shows that the İnatlar limestone falls within the warm-water carbonate sediments area, according to δ¹³C and δ¹⁸O isotope values. In the diagram created by [120], which uses carbon and oxygen isotopic compositions, İnatlar limestones fall into the sedimentary marine carbonate area (Figure 13b).

Yttrium (Y³⁺) shares the same ionic charge and a comparable ionic radius (1.02 Å) with holmium (Ho³⁺) [121]. Because yttrium is less efficiently removed from seawater compared to holmium, the Y/Ho ratio becomes particularly valuable as a geochemical proxy [121,122,123,124]. Since Ho is removed from seawater at approximately twice the rate of Y, the Y/Ho ratios show distinct differences between marine and non-marine sediments [121,125]. Terrestrial materials and volcanic ashes have constant chondritic Y/Ho values of 28. Marine waters have higher Y/Ho values, ranging from 44 to 74 [121,125]. In the limestones in this study, Y/Ho ratios vary between 20 and 106.67 and show differences. These values may indicate that the limestone was probably influenced by terrestrial materials.

The Er/Nd value in typical seawater is ~0.27 [126]. The Er/Nd value in limestones can effectively reveal the seawater signature carried by marine carbonates. Both detrital materials and diagenetic processes can cause a preferential Nd concentration compared to Er and can also reduce Er/Nd values below 0.1 [28,86]. Er/Nd values in the studied limestones range from 0.04 to 1.0, indicating that these limestones are generally influenced by diagenesis or detrital material.

5.2. Paleo-Environmental Implications

5.2.1. Paleo-Redox Conditions

Certain trace elements, including Ni and V, are commonly employed as proxies to interpret paleo-redox conditions during sedimentation. Since Ni and V tend to be enriched differentially under oxic versus anoxic conditions—with V generally being more enriched in anoxic settings—they serve as valuable indicators of redox states during deposition [127,128,129,130]. The V/Ni ratios of the Inatlar limestone samples range from 5 to 18.18 (Table 1). Ratios exceeding 3 coupled with Ni concentrations typically below 90 ppm suggest that sedimentation occurred under anoxic conditions, and that the source rocks consist primarily of marine organic matter along with carbonate shale or limestone [130]. As illustrated in Figure 14, the Ni and V concentrations in all limestone samples indicate depositional environments spanning terrestrial, marine–terrestrial oxic to dioxic, and marine anoxic shale–carbonate settings, potentially reflecting a transitional depositional regime. Similarly, the authors of [7] reported that the Upper Jurassic–Lower Cretaceous limestone samples from Gümüşhane (NE Turkey) were deposited under comparable paleoenvironmental conditions.

The Ni/Co and V/(V + Ni) ratios are widely recognized as consistent proxies for interpreting redox conditions [131]. The plot of V(V + Ni) versus Ni/Co values (Figure 15) indicates that the majority of the Inatlar limestone samples fall within oxic to suboxic/anoxic environments based on their V/(V + Ni) values, whereas Ni/Co ratios classify most samples, particularly fossiliferous limestones, as having been deposited under anoxic conditions. Similarly, the authors of [38] reported that Archaean carbonate sediments from the Tanwan Group rocks of the Bhilwara Supergroup of India were deposited in a suboxic environment.

5.2.2. Paleosalinity

Paleosalinity serves as a critical indicator for reconstructing the salinity conditions of ancient depositional environments. The Sr/Ba ratio is widely employed as a geochemical proxy for paleosalinity due to the differing concentrations of these elements in freshwater versus marine settings [134,135,136,137]. It is well established that the concentrations of Sr and Ba are generally lower in freshwater systems compared to seawater [138,139]. Consequently, elevated Sr/Ba ratios typically reflect higher-salinity conditions and indicate a significant marine influence on sediment deposition [140]. Threshold values for the Sr/Ba ratio are commonly used to differentiate depositional environments; a ratio below 0.5 suggests freshwater conditions, values between 0.5 and 2 correspond to brackish water, and values exceeding 1 are indicative of marine or saline depositional settings [136,141].

In the present study, Sr/Ba ratios measure from the Inatlar limestone samples range from 1.59 to 17.64 in siliceous limestone, 6.12 to 44.75 in micritic limestone, 6.12 to 92.3 in fossiliferous limestone, and 44.53 to 73.65 in dolomitic limestone (Table 1). These elevated ratios strongly suggest that sedimentation occurred under saline, marine-influenced conditions. Furthermore, the graphical analysis using Sr/Ba versus V/Ni ratios analysis using Sr/Ba versus V/Ni ratios, following the methodology of [137] (Figure 16a), demonstrates that both salinity and redox (reducing) conditions played a significant role during limestone deposition. This dual influence highlights the complex environmental controls affecting depositional processes and supports the robustness of Sr/Ba as a paleosalinity proxy. Supporting studies, such as [7], also found salinity and reducing environments to be critical controls for limestone deposition in similar regional settings (Gümüşhane, NE Türkiye), reinforcing the broader applicability of this multi-proxy approach. The diagram drawn using Sr/Ba and Rb/Sr ratios in Figure 16b indicates low weathering and high salinity.

6. Conclusions

Mineralogical, petrographic and geochemical analyses of the Middle Jurassic–Lower Cretaceous Inatlar limestone, exposed between Kabulbaba and Söğütalan villages, provide detailed insights into its depositional environment, and diagenetic processes.

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Field observations show that the limestone varies from dark to light beige, locally yellowish to grayish, with medium-bedded, karstic features, fractures, fossil content, and chert nodules. This commercially significant limestone includes siliceous, micritic, fossiliferous, and dolomitic varieties, reflecting its complex mineralogical composition.

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The Rb-Sr-Ba ternary diagram and Sr/Ba and Sr/Rb ratios place the limestone environment between the continental margin and open ocean settings. Further REEs ratio analyses (Eu/Eu*, Ce/Ce*, (La/Ce)_n, and (Sm/Yb)_n) support an open ocean depositional environment. Negative Ce and Eu anomalies, along with positive La anomalies and LREEs depletion, indicate the influence of early diagenetic processes.

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İnatlar limestones are of marine origin according to their Z values. δ¹³C–δ¹⁸O isotopic values indicate that they fall within the areas of hot-water carbonate sediments and marine carbonate sediments.

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The Y/Ho and Er/Nd ratios indicate that the İnatlar Limestones are affected by terrestrial material and diagenetic or detrital material.

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Paleo-redox and paleosalinity conditions were evaluated through elemental anomalies and metal ratios. Variations in Ni and V concentrations suggest depositional environments ranging from terrestrial to marine–terrestrial oxic–dioxic, marine anoxic, and possible transitional conditions. V/(V + Ni) and Ni/Co ratios further reveal deposition under oxic to suboxic/anoxic and euxinic conditions, respectively, with salinity and reducing conditions exerting significant influence during limestone formation. According to V/Ni, Sr/Ba and Rb/Sr ratios, the İnatlar limestones reveal that they were in low-weathering and high-salinity paleoenvironmental conditions. These results collectively demonstrated the complex interplay of marine transgressions, redox fluctuations, and diagenesis of the Inatlar limestone.