The Upper Lower-Triassic Mahil Formation contains thickly bedded polymictic brecciated floatstones, which comprise eight identified microfacies types, including undifferentiated foraminifera brecciated floatstones and foraminifera undifferentiated brecciated floatstones. These lithofacies consist of various skeletal and non-skeletal grains, with foraminifera being the most predominant, along with observed bivalve and brachiopod fragments (Table 1; Figures 3–6).

The most dominant lithofacies in both logged sections are grain-dominated, primarily consisting of grainstone and packstone lithologies (Figure 7). Eight microfacies have been identified within these lithofacies, including oolitic packstone/grainstone, foraminiferal packstone/grainstone, and other microfacies types. These microfacies contain small benthic foraminifera, brachiopods, ostracod, bivalves, ooids, peloids, and intraclasts (Table 2).

The least dominant lithofacies in both logged sections are mud-dominated, primarily composed of wackestone. Only two microfacies have been identified: foraminiferal wackestone and undifferentiated skeletal wackestone. These mud-dominated lithofacies are matrix-supported and contain micrite (Figure 8). The skeletal components are varied but mainly consist of small benthic foraminifera (Table 3).

The low-energy, shallow subtidal landward side of the shoal’s crest is identifiable by the presence of polymictic breccia floatstone (Figure 10), foraminiferal peloidal grainstones (Figure 7A), and undifferentiated skeletal wackestone (Figure 8C) microfacies. This area is characterized by matrix-supported microfacies, the absence of subaerial exposure features, rare marine biota presence, and weak microbial lamination (Figure 6). The depositional environment suggests limited biological activity and a lack of exposure to air during formation, contributing to the development of specific sedimentary rock types in this geological setting.

The weakly developed oolitic shoal was recognized only in the Saiq Plateau, indicating high to moderate energy conditions. This lithofacies association is mainly formed because of the abundance of the well-sorting oolite pack/grainstone and intraclass (Figures 7C–E). Further, it consists of weakly developed crossbedding and well-developed planar lamination (Figure 11).

This LFA is dominant in both studied sections. This association is represented mainly by the foraminiferal packstone to grainstone microfacies abundance and occurrence of the open marine biota such as brachiopods, gastropods, ostracods, bryozoa, and sponges (Figures 7B, F; Figures 8A, B). Furthermore, local hummocky structures and parallel lamination (Figure 12) were observed, which indicate storm waves in the foreshore environment.

The Late Paleozoic-Early Mesozoic series display specific content and layer geometry, including breccias, variations in thickness, and slumps, which offer compelling evidence for the presence of an Early Triassic extensional regime in the hanging wall of an ancient normal fault. Several sealed normal faults are clear indicators of this regime (Figure 13A). To explore the fault-controlled deposits of the Mahil Formation after pre-folding, a comprehensive study was carried out in Wadi Sahtan. The researchers aimed to restore the bedding plane to its original horizontal orientation, which revealed significant sites affected by the faulting. Upon rotation to their original positions, the results demonstrate that these faults were normal before the compressional tilting (Figure 13B). Consequently, the findings from this pre-deformation analysis in the Lower Triassic fault unequivocally indicate normal faulting, thus confirming the presence of an extensional (normal faulting) tectonic regime during that time. Moreover, normal faulting gives rise to growth strata filled with syntectonic sequences linked to typical syntectonic breccias and soft-sediment sequences associated with the ancient normal faulting activity. These normal faults are observed in the Saiq Plateau, particularly concerning Lower Triassic growth strata.

The Bulk-rock carbon and oxygen isotopic values of the Lower Mahil carbonates from Saiq Plateau and Wadi Sahtan are plotted against thickness levels in the measured section, lithology, and microfacies (Figures 14, 15). The δ¹³C values of the Lower Mahil Formation in the Saiq Plateau section range from +1.5‰ to +3.6‰ VPDB (average 2.1‰). The isotopic trends plotted against height and lithologies in the sedimentological log (Figure 14) show marked variations: the δ¹³C values show minimum points as well as the highest point at the middle of the section. The δ¹³C values start with a slightly decreased trend from 2.4‰ to 1.9‰. Then they increase to a distinct peak of 3.1‰ at 10 m before presenting another increase to 3.3‰ at 14.5 m. After a new negative shift at 19 m, the δ¹³C curve increases until the maximum value of 3.6‰ at 22.5 m. Then the δ¹³C drops to 1.7‰ at 26.5 m before increasing to 2.5‰ at 31.5 m. Then, the curve exhibits relatively low values of δ¹³C, and the trend remains almost steady as the δ¹³C is maintained between 1.5‰ and 2.8‰ to the end of the section.

δ¹³C values slightly drop from 2.3‰ to 1.2‰ at 0.5–9.5 m, although a spike of the maximum values occurs at 3.5 m. In the following, a rise from 1.4‰ to 2.6‰ before a drop to 1.1‰ are revealed in the curve. Then the trend shows a slight decrease in the δ¹³C values until a remarkable negative carbon isotope excursion at the end of the section.

The δ¹⁸O values are lower than that of δ¹³C values in both sections. In the Saiq Plateau section, the δ¹⁸O range from −4.2‰ to −1.2‰ VPDB (average −2.5‰). All the microfacies generally maintained similar variations in their δ¹⁸O average values, ranging from −2.8‰ to −2.2‰. In comparison, the foraminiferal packstone and foraminiferal grainstone show the most depleted values (average −2.8‰ for the first one and −2.7‰ for the last one). When δ¹⁸O plotted relative to the height and lithofacies (Figure 14), the most notable feature is that the section begins with the maximum oxygen isotope value of −1.2‰ VPDB, localized in the lower part of the Lower Mahil. A sharp fall in δ¹⁸O values from −1.2‰ to −3.9‰ occurred at the height of 1 m and pronounced negative oxygen isotope excursion. After a positive oxygen isotope shift, the relatively steady fluctuation is interrupted by a spiky negative carbon isotope shift to the minimum value of −4.2‰. In the middle part of the section, the δ¹⁸O values exhibit a temporary increase and a sudden drop to −3.9‰ is observed before rising again to the second peak of −2.5‰. Afterward, the δ¹⁸O values return to −3.0‰ and display a negative shift. Finally, the trend shows a positive excursion to the end of the section. The variation of δ¹⁸O is less evident and less consistent than that of δ¹³C. However, in some intervals approximately correlated with δ¹³C but not generally correlated, few similar evolutions can be observed with a positive trend at 14.5 and 31.5 m, a negative one at 38 m, and a slightly decreasing trend from 14.5 to 19 m.

Figures 14, 15 are displayed at 49 and 40 m sections to overview the spectral gamma-ray signatures within the outcrops, including the identified lithologies and the twenty-four microfacies. Spectral gamma-ray data are arranged in Figures 13, 14, including the complete gamma-ray logs, each spectral curve (K, U, and Th), total gamma-ray (total GR), computed gamma-ray (CGR), Th/U ratio, and Th/K ratio for each section.

The Lower Mahil Formation (KS-1 Khuff-Equivalent) is categorized based on age, representing a one-third order sequence (third). This third order sequence has distinct phases of sea level transgression and regression. Further subdivisions are made within this third order sequence to identify five fourth-order sequences (fourth) based on factors such as facies, isotope, and gamma-ray analysis. The sequence stratigraphy in this context identifies five fourth-order sequences, namely, ⁴S1, ⁴S2, ⁴S3, ⁴S4, and one incomplete sequence, ⁴S5. Furthermore, each of these five fourth-order sequences can be further divided into fifth-order cycles.

Sequence 1 (S1): is located in the lower portion of the Lower Mahil Formation, with its lower boundary at the base of the Mahil KS-1 Formation. The transgressive system tract (TST) of S1 is distinguished by significant increases in carbon isotope, uranium, and total gamma-ray values, indicating an upward trend. Foraminifera wackestone, foraminifera packstone, and foraminifera grainstone microfacies characterize this TST. The maximum flooding surface is identified within a higher positive excursion, specifically within the foraminiferal packstone microfacies. Moving higher in the sequence, the high system tract (HST) of S1 comprises foraminiferal packstone, foraminiferal grainstone, and undifferentiated packstone microfacies. The overall trend within the HST shows a decreasing shift toward the sequence boundary (SB). This implies a gradual shallowing of the environment leading to the eventual termination of the sequence. The interplay of various microfacies and the changes in geochemical values provide valuable insights into the sedimentary processes and depositional history during the formation of Sequence 1 within the Lower Mahil Formation.

Sequence 2 (S2) is characterized by distinct sequence boundaries, which are identified by the most decreasing shifts in the U (uranium), GRtotal (total gamma-ray), and C isotope profiles. Various microfacies were developed within the transgressive system tract (TST) of S2, with foraminiferal grainstone being the most common. Some microfacies pinch out towards other sections, such as foraminiferal intraclastic grainstone and foraminiferal grainstone, which pinch towards the Wadi Sahtan logged section. Additionally, the foraminiferal packstone pinches out towards the Saiq Plateau logged section. The maximum flooding surface is found within the highest positive deviation of the trends passing through the foraminiferal grainstone and undifferentiated grainstone microfacies. During the high system tract (HST), the foraminiferal packstone further pinches towards the Saiq Plateau, while the foraminiferal grainstone remains the dominant microfacies. Notably, Sequence 2 exhibits a thinning trend towards the Wadi Sahtan logged section, indicating a reduction in sediment accumulation in that direction. Identifying different microfacies, pinch-outs, and variations in sediment thickness provide valuable information about the depositional environment and processes that shaped Sequence 2 in the studied area within the Lower Mahil Formation.

Sequence 3 (S3) is defined by its lower and upper boundaries, which are identified based on the lowest points in the selected log profiles (Figure 16). The transgressive system tract (TST) of S3 exhibits a variety of microfacies, with the foraminiferal grainstone being the most predominant. However, some microfacies pinch out towards the Wadi Sahtan logged section, including the foraminiferal intraclastic grainstone, foraminiferal peloidal grainstone, and foraminiferal packstone. Simultaneously, the foraminiferal packstone is prevalent in the Wadi Sahtan section but pinches towards the Saiq Plateau section. The maximum flooding surface (MFS) of S3 coincides with the MFZ (Maximum Flooding Zone) of the third-order sequence, comprising foraminiferal packstone microfacies deposited in the deepest side of the studied area, likely in a foreshore setting. During the high system tract (HST), the foraminiferal packstone and oolitic grainstone are prominent microfacies, and both pinch out towards the Wadi Sahtan section. In contrast, the foraminiferal packstone dominates, and the undifferentiated packstone pinches towards the Saiq Plateau section. The sequence is capped by the sequence boundary (SB), indicated by decreasing trend on the selected profiles. Remarkably, Sequence 3 stands out as the thickest sequence within the studied area, indicating a considerable amount of sediment accumulation during its formation. The diverse microfacies, pinch-outs, and changes in sediment thickness provide valuable insights into the complex depositional processes and environmental changes that influenced the development of Sequence 3 within the Lower Mahil Formation.

Sequence 4 (S4) commences at the sequence boundary (SB) where Sequence 3 ends. The presence of undifferentiated grainstone, undifferentiated packstone, and undifferentiated foraminiferal brecciated floatstone microfacies characterizes the transgressive system tract (TST) of S4. The deposition of undifferentiated foraminiferal and foraminiferal undifferentiated brecciated floatstone microfacies marks the maximum flooding surface (MFS). During the high system tract (HST) phase of S4, the microfacies assemblage includes undifferentiated foraminiferal, intraclastic peloidal, undifferentiated peloidal, and foraminiferal undifferentiated brecciated floatstone. These microfacies suggest specific depositional environments and sedimentary processes during the HST.

The termination of Sequence 4 is marked by another sequence boundary (SB) located on top of the undifferentiated peloidal and undifferentiated foraminiferal brecciated floatstone microfacies. This sequence boundary indicates a change in depositional conditions, potentially leading to a subsequent depositional sequence in the geological record. Various microfacies and their changes within Sequence 4 provide valuable information about the sedimentary processes and environmental dynamics during its formation within the Lower Mahil Formation. Understanding each sequence’s distinct characteristics helps interpret the studied area’s geological history and paleoenvironmental conditions.

Sequence 5 (S5) represents the last fourth-order sequence in the studied area, but it is incomplete as it comprises only the transgressive system tract (TST). The lower boundary of S5 coincides with the termination of Sequence 4 (S4). The TST of S5 contains various microfacies, with the undifferentiated foraminiferal brecciated floatstone being the most common one. Several microfacies, including peloidal skeletal, foraminiferal skeletal, foraminiferal peloidal, and brecciated floatstone, pinch out towards the Wadi Sahtan section during the TST. Conversely, the foraminiferal undifferentiated brecciated floatstone microfacies pinch out towards the Saiq Plateau section. These variations in microfacies distributions suggest changes in depositional conditions and environmental settings during the TST of Sequence 5.

As Sequence 5 is incomplete and consists only of the TST, it ends in both studied sections with the termination of the brecciated floatstone microfacies. The absence of higher-order system tracts (HST) and the termination of the sequence with the brecciated floatstone suggest a potentially rapid depositional event or environmental change that led to the incomplete formation of this sequence in the Lower Mahil Formation. Studying the characteristics and termination of Sequence 5 provides valuable insights into the sedimentary processes and paleoenvironmental conditions during its formation. It contributes to a comprehensive understanding of the geological history of the studied area.

The Middle Permian to Early Triassic period in northern Oman witnessed the opening of the Neo-Tethys Ocean, leading to a significant syn-to post-rift phase (Lee, 1990; Robertson et al., 1990; Pillevuit, 1993; Wender et al., 1998; Alsharhan, 2006; Koehrer et al., 2010; Koehrer et al., 2011; Bendias et al., 2013; Peyravi et al., 2015). Consequently, the passive continental margin of the newly formed Neo-Tethys Ocean experienced the deposition of well-bedded, widespread shallow-water carbonate deposits known as the Khuff Formation (Al-Jallal and Ibrahim, 1991; Al-Aswad, 1997; Maurer et al., 2009; Koehrer et al., 2010; Fontana et al., 2013; Beckert et al., 2015). The presence of abundant breccias at the top of the sections, along with variations in thickness and slumps, provides evidence for the Early Triassic extensional regime in the hanging wall of an ancient normal fault (Figures 13A, B). By restoring the bedding plane to its original horizontal orientation (Gharbi et al., 2022), Mahil Formation’s fault-controlled deposits was observed after pre-folding. This extensional phase occurred concurrently with the rifting in the Arabian plate in the north and the rapid subsidence in the passive margin basin of the Neo-Tethys in the south (Glennie, 2000; Sharland et al., 2001). The interpreted tilted block geometry accounts for the significant Lower Triassic thickening variation, likely associated with substantial volcanic activity (Sharland et al., 2001; Chauvet, 2007; Jabir and Ali, 2023).

Isotopic values in the Saiq Plateau and Wadi Sahtan sections are primarily controlled by fluctuations in global seawater geochemistry and diagenesis [80, 81]. The δ¹³C shows little variation between the two sections, with average values of 2.1‰ for Saiq Plateau and 1.7‰ for Wadi Sahtan, increasing upwards. Lower Mahil KS-1 Formation δ13C values are relatively low, as expected from the Triassic samples (Baud et al., 1996). This depletion is possibly linked to diagenetic activities and the Permian-Triassic boundary mass extinction (Peyravi et al., 2016). The drop in δ¹³C values indicates a shift in the carbon cycle from organic carbon (reduced phase) in Permian time to dissolved carbonate (oxidized phase) in Triassic time (Gruszczynski et al., 1989; Magaritz and Holser, 1991; Morante, 1996; Horacek et al., 2007a). Enrichment of δ¹³C could result from local variations in salinity, circulation, and productivity (Grossman et al., 1993; Moustafa et al., 2016). Instead, higher δ¹³C values may be attributed to aragonite-rich sediments, which have relatively positive δ¹³C values compared to calcitic sediments (Weber and Schmalz, 1968; Swart et al., 2009; Moustafa et al., 2016). Local minima in δ¹³C may indicate intervals of subaerial erosion and oxidation (Magaritz and Holser, 1991; Atudorei, 1999; Swart and Kennedy, 2012). The δ¹³C curves are associated with rising relative sea levels, while negative shifts indicate falling relative sea levels (Immenhauser et al., 2003; Swart and Eberli, 2005; Fanton and Holmden, 2007). The δ13C curves are associated with rising sea levels, while negative shifts indicate falling sea levels (Immenhauser et al., 2003; Swart and Eberli, 2005; Fanton and Holmden, 2007).

Diagenesis significantly affects δ¹⁸O values, with low δ¹⁸O values possibly resulting from subaerial exposure and interaction with meteoric waters, reducing salinity [(Allan and Matthews, 1990; Grossman et al., 1993; Swart and Kennedy, 2012]. High-temperature water can also shift δ¹⁸O values towards negative values (Allan and Richard K, 1990; Popp et al., 1986; Grossman et al., 1993; Grossman et al., 2008). Chemostratigraphic correlation in this study primarily relies on carbon (δ¹³C) data rather than oxygen (δ¹⁸O) to minimize diagenesis-related errors (Figure 16).

The interpretation of potassium (K), thorium (Th), and computed gamma-ray logs in carbonate sections reveals relatively low K and Th values in the Saiq and Wadi Sahtan sections, indicating the negligibility of K and Th’s insolubility in carbonates (Glover, 2000; Omidpour et al., 2021). The Th/U ratio suggests oxidizing conditions during regression phases or near sequence boundaries, while the Th/K ratio indicates sudden depositional changes and potential diagenetic alterations (Serra, 1984; Doveton, 1991; Glover, 2000). In both sections, K values are nearly zero due to the absence of evaporative conditions in the Lower Mahil Formation (Al Raqaishi et al., 2023).

The Saiq Plateau section shows some local K and Th peaks within the backshoal lithofacies association, attributed to unobserved features or textures resulting from dolomitization (Figure 14). Additionally, the Th/U ratio confirms intervals of subaerial exposure and high vertical lithofacies variation (Adams and Weaver, 1958; Doveton, 1991; Klaja and Dudek, 2016).

The Wadi Sahtan section exhibits a positive K and Th excursion near the conformity boundary, indicating subaerial exposure and clastic influx (Figure 15). The Th/U ratio in both sections is relatively low, suggesting marine-reducing settings and U enrichments (Adams and Weaver, 1958; Doveton, 1991; Doveton and Prensky, 1992; Klaja and Dudek, 2016). The Th/K ratio varies depending on depositional settings and diagenetic alterations, with high values indicating oxidizing conditions (Serra, 1984; Glover, 2000).

Using spectral gamma-ray, specifically uranium (U) and total gamma-ray (total GR), improves chemostratigraphic resolution, aiding in identifying fourth sequences described in Section 6 and more detailed fifth-order cycles. The interpreted uranium and total gamma-ray curves match closely, indicating that the total gamma-ray in the studied sections reflects U values rather than clay minerals. The Lower Mahil mainly consists of radiometric dolomite, making U an inappropriate shale indicator. Uranium and total gamma-ray readings in the Saiq Plateau section range between 0 and 2.3 ppm and 2–41.2 API, respectively, while in the Wadi Sahtan section, they range from 0 to 4.4 ppm for U and 0–38.8 API for total GR (Figures 14–16).

Elevated readings in uranium (U) and total gamma-ray (GR) measurements serve as indicators of maximum flooding surfaces, signifying phases of transgression and increased accommodation space. This phenomenon has been noted in various studies (Davies et al., 1996; Ehrenberg and Svana, 2001; Halgedahl et al., 2009; Farouk et al., 2022; Farouk et al., 2023). The presence of enriched U and total gamma-ray values is linked to the deposition of organic-rich marine sediments in deep-water environments. These conditions correspond to transgressive events and rapid sea-level rises, often accompanied by a landward shift in sedimentary facies.

Conversely, during regression within carbonate systems, carbonate production slows down, leading to an increased proportion of coarser materials. This shift results in negative decreasing deviations in uranium and total gamma-ray logs (Halgedahl et al., 2009; Kalvoda et al., 2011; Tavakoli, 2017; Farouk et al., 2022; Farouk et al., 2023). Consequently, uranium values in the Saiq Plateau section tend to be slightly lower than those in the Wadi Sahtan section. This might be attributed to the shallower location of the Saiq Plateau or the relatively reduced context at greater depths, where uranium values exhibit more variability. Another contributing factor could be the diminished supply of clay detritus to the distal area of the study site, the “Wadi Sahtan section,” resulting in lower sedimentation rates and higher uranium values (Serra, 1984; Klaja and Dudek, 2016; Omidpour et al., 2021).

Spikes in U and total GR readings recognize maximum flooding surfaces, indicating transgressive phases (Davies et al., 1996; Ehrenberg and Svana, 2001; Halgedahl et al., 2009). Enrichment in U and total gamma-ray points to the accumulation of organic-rich marine sediments under deep-water, reducing conditions, corresponding to transgressive events. On the other hand, regressions within carbonate systems result in decelerated carbonate production and increased coarser material, leading to negative excursions in uranium and total gamma-ray logs (Halgedahl et al., 2009; Kalvoda et al., 2011). The U data in the Saiq Plateau section is slightly lower than that of the Wadi Sahtan section, possibly due to the shallower location of the Saiq Plateau, as more variable U values are regularly found in deeper reduced contexts (Serra, 1984; Klaja and Dudek, 2016; Omidpour et al., 2021).

The studied sections exhibit highly variable uranium (U) and total gamma-ray (total GR) data, without any specific trend associated with lithofacies and microfacies textures (Figures 14–16). The dominance of grain-dominated lithofacies in the dolomitized Lower Mahil Formation could explain the variability and lack of clear data for the same textures (Al Raqaishi, 2022).

In the grain-dominated Lower Mahil Formation, the relationship between gamma-activity and grain size is not evident. Previous studies have shown that low gamma-readings are typical of coarse-grained sediments, while high gamma-activity characterizes finer-grained sediments (Berstad and Dypvik, 1981; Dypvik and Eriksen, 1983; Tavakoli, 2017; Farouk et al., 2022; Farouk et al., 2023). However, this pattern is not observed in this formation.

Brecciated floatstone microfacies within thick mud-supported beds with various fossils show some elevated U and total GR readings. This could be attributed to readings from the mud-matrix, unseen fractures filled with “uranium-rich” matter, minor siliciclastic inputs, presence of stylolite, fossils, and the duration of subaerial exposure, which affects U and total GR readings (Luczaj, 1998; Ehrenberg and Svana, 2001; Halgedahl et al., 2009; Tavakoli, 2017; Farouk et al., 2022; Farouk et al., 2023).

The stratigraphic sequences, as determined by (Haq et al., 1987), serve as indicators of relative sea-level fluctuations. Increasing shifts in the U, total GR, and δ¹³C curves are associated with rising relative sea levels, while negative shifts indicate falling relative sea levels (Immenhauser et al., 2003; Swart and Gregor, 2005 Fanton and Holmden, 2007). In our interpretation of the relative sea-level changes during the Lower Triassic, we have adopted the third-order and fourth-order sequences (Figure 19). Additionally, Figure 19 illustrates the first and second-order relative sea-level variations based on a previous study by (Alsharhan, 2006). The third-order relative sea-level trends identified in this study exhibit distinct patterns. These patterns align transgression with high gamma-ray and carbon isotope levels, while regression corresponds to low gamma-ray and carbon isotope levels. The analysis of spectral gamma-rays and carbon isotopes suggests a believable connection between relative sea levels and the global sea level. Moreover, the variations in relative sea levels are proposed to be linked with tectonic activities, including rifting and drifting (as discussed by Koehrer et al., 2010; Koehrer et al., 2012; Bendias et al., 2013). Consequently, the spectral gamma-ray and carbon isotope data from this study (Figures 16–19) could serve as proxies for monitoring sea level changes, enabling broader global correlations.