Datasets used in this study consist of more than 40 cores (10.02 m) from two wells, namely, well Y-3 and well T Ge-2, along with 50 thin section images, wireline well logs from 176 wells, and 2D seismic data covering a significant portion of the Termit basin. The cores from well Y-3, located adjacent to the Moul Sag, were obtained from the Yogou Slope (Figure 1). These cores had a diameter of 8 cm and a recovered length of 1.9 m. Similarly, the cores from well T Ge-2 were collected from the Trakes Slope, with a core diameter of 8 cm and a recovered length of 1.4 m. Although only two wells contained short core sections, they nonetheless served as a robust and valuable foundation for elucidating the sedimentary environment.

This study systematically analyzed the Late Cretaceous sedimentary environment and fossil contents of the basins surrounding the Termit Basin. The identification and classification of lithofacies were based on the analysis of core lithology, fabric, texture, sedimentary structures, and bioturbation. The degree of bioturbation was assessed using the method of Taylor and Goldring (1993). Sequence stratigraphic analysis followed the basic concepts and criteria proposed by Mitchum et al. (1977) and Catuneanu et al. (2009).

The Upper Cretaceous strata in the East Niger Basin, which includes the Termit Basin, can be classified into three groups: the Donga, Yogou, and Madama Formations (Figure 1C). From the northern part of Tenere Basin to the western and eastern parts of Termit Basin, the Donga-Yogou Formation encompassed a complete transgression and regression sedimentary cycle. Extensive thick mudstone layers have been deposited between the Donga and Yogou formations, which were extensively deposited during the process of marine incursion due to the influence of flood events. The Donga Formation is in the early stage of marine transgression, and the transition to the Yogou Formation corresponds to the development process of marine incursion, during which the water depth significantly deepens. (Figure 3). Studies have reported various paleontological findings in the Tenere Basin. Well Fa-1 yielded species from the Neotethys Ocean and the South Atlantic, well Sh-1 contained species from the Neotethys Ocean, well Hl-1 contained diverse marine algae, and well Fc-1 revealed numerous flagellates (Liu et al., 2011; Mao et al., 2016; Liu et al., 2020; Dou et al., 2022; Figure 3). These paleontological findings collectively indicate a similar marine environment.

Tenere Basin and Termit Basin exhibit comparable stratigraphic structures and sedimentary infill characteristics (Figure 3). This suggests that the two basins share a similar tectonic evolutionary process and sedimentary environment. By considering the regional transgression patterns and marine deposits, it can be inferred that these basins underwent a similar infill history.

The thickness of the Upper Cretaceous Yogou Formation in the Termit Basin exhibits a regular decrease only in the deep areas of the Dinga depression due to the influence of fault activities. Conversely, on the eastern and western terraces and platforms, the thickness distribution of the Yogou Formation remains consistent, as evidenced by stable seismic reflection characteristics (Figure 4A). Currently, the Termit Basin encompasses an area of approximately 30,000 km², primarily determined through block division and the extent of seismic data coverage (Mao et al., 2016). The distribution of the Upper Cretaceous strata confirms that sedimentation extended beyond the area covered by seismic data and was subsequently truncated by Paleogene and Neogene tectonic stratum (Figure 4B). It is evident that the proto basin size exceeds the area currently captured by seismic data.

A total of four lithofacies were identified in this study. Detailed descriptions and interpretations of each facies are presented in Table 1. Each lithofacies reflected specific sedimentary structure characteristics and depositional processes within tidal flat and channel systems.

The core of well Y-3 can be divided into three units (Figure 6). Unit I comprises predominantly fine sandstone and siltstone, with sporadic thin (cm) mudstone beds. Unit II is characterized by a dominance of silty mudstone and muddy siltstone in terms of lithology. Unit III consists mainly of fine sandstone, with scattered thin mudstone beds.

The logging curve of the core section exhibits a consistent coarsening upward cycle. The core primarily displays the characteristics of transgressive cycles, while regressive successions are often limited due to erosion. The core sections, as a whole, consist of a sequence of mixed flat, sand flat, mixed flat, sand flat, mixed flat, mud flat, and tidal channel from bottom to top (Figure 6). Within these sections, the inversely graded lithofacies, formed by the overlay of mud flat, mixed flat, and sand flat, represent the transgressive succession. These lithofacies exhibit bidirectional cross-beddings, double mud drapes, and tidal rhythmites. Additionally, the tidal channel microfacies contains abundant clay rip-up clasts, indicating active erosion and deposition processes. (Figure 6).

Placing the core sample under a microscope for observation reveals significant tidal features. The fine sandstone within the sand flat microfacies shows the development of dominant tides (dt) and subordinate tides (st). These layers also contain double mud drapes, contributing to the spring-neap cycle characterized by a “thick-thin-thick” rhythm (Figures 7A, B). Figure 7C displays similar features, with more pronounced coarsening-upward and transgression. In tidal flat deposits, flaser bedding, wavy bedding and lenticular Bedding are typical in micrographs (Figure 8).

Furthermore, the formation, transport, and settlement processes of fluid mud under tidal environments have been comprehensively demonstrated. Within the gray-black mudstone containing fine sand of the mud flat microfacies (Figure 9A), there are visible mudstone layers with siltstone lenses (Figure 9B). Instead of exhibiting a simple slackwater deposition, the mud displays cross-bedding structures. Bidirectional cross-bedding, scouring surfaces, and downlap interfaces are evident (Figures 9C–1, 9C-2), indicating the transportation and deposition of flocculent particles induced by tidal action. These characteristics suggest that the Y-3 well in the Upper Yogou Formation is located near a provenance with significant tidal energy. This core period is generally regressive, and the dominant facies consist of marine-terrigenous facies.

The Donga Formation represents the first third-order sequence in the Termit Basin during the Late Cretaceous transgression, underlaid the Yogou Formation (Yuan et al., 2023). Although no coring was conducted in the Donga Formation of the Termit Basin, thin slice grinding with coring data from borehole wall shows rich fossil contents under the microscope (Figure 10). Bioclast-rich beds, bioclastic sandy mudstone (Figures 10A, B), and calcispheres (Figure 10C1, C2) were found in the Donga Formation. The application of scanning electron microscopy unveiled a significant number of calcareous foraminiferal tests within the muddy bioclastic limestone, with clay (kaolinite) commonly filling the interiors of the tests (Figures 10D, E). According to the stratigraphic division, these wall coring locations are located in the upper part of the Donga Formation of well Dw N-1 (Figure 1A). Well-loggings in dozens of wellsindicate the presence of a thin set of carbonate rocks (less than 2 m thick) at the upper section of the Donga Formation. These carbonate intervals consist predominantly of limestone, occasionally supplemented by dolomite, and exhibit abundant bioclastics, implying a locally clean water environment during the sedimentary period. The majority of these wells are positioned in proximity to the basin depression, away from the influence of provenance. Conversely, wells near the margin of the basin typically lack carbonate rocks at the top of the Donga Formation.

In this paper, we conclude that the Late Cretaceous Termit prototype basin exhibited strong tidal conditions. Towards the south and southwest, the Termit prototype basin connects to the Atlantic Ocean through the Bornu-Benue Trough and Bida Basin (Figure 1A). The Bida Basin has revealed abundant Late Cretaceous marine or transgressive strata, with prominent marine and tidal facies indicators such as marine bivalves, dinoflagellates (Dinogymnium, Senegalinium and Spiniferites) and trace-fossil (Ophiomorpha and Thallasinoides), as well as herringbone cross-bedding, hummocky cross-bedding, and double mud drapes (Ojo and Akande, 2009).

In the Bornu-Benue Trough, the Kasade-1 well has discovered typical marine strata, containing ten types of planktonic foraminifera, echinoderms, fish fossils, and a large number of planktonic microbiomes (Ola et al., 2016). Additionally, numerous marine biomarkers have been identified (Adegoke et al., 2015). To the north, the Termit prototype basin is connected to Tethys by the Tenere basin and the extensive region of Algeria (Konaté et al., 2019). The size of Termit prototype basin is sufficiently large to generate strong tidal, wave, and current interactions, analogous to those observed in the North American “Cretaceous Western Seaway” (Allen and Posamentier, 1993; Varban and Guy Plint, 2008; Lin and Bhattacharya, 2020; Gardner and Dorsey, 2021).

Previous studies indicate that during the early Cenomanian, seawater in the eastern Niger (including the Termit Basin, Tenere Basin, etc.) originated from the northern New Tethys Ocean. Towards the end of the Cenomanian, a connection was established with the South Atlantic Ocean, leading to transgression from two directions (Guiraud et al., 2005; Liu et al., 2011). Based on the findings of this study, it is concluded that the member Ys3 of the Upper Yogou Formation represents a marine-terrigenous environment under a regressive setting. The upper part of the Yogou Formation, specifically the member Ys3, predominantly consists of sand-mud interbedded deposits (Figure 3), aligning with the characteristics observed in the core section of well Y-3. During the depositional period of the member Ys3, the Yogou slope existed within a coastal sedimentary environment, primarily characterized by a gentle slope zone (Yuan et al., 2018). Analysis of the well Y-3 cores (Figure 5) reveals frequent fluctuations of transgression and regression. The transgressive succession demonstrates a relatively complete record, while the regressive succession is comparatively less developed. The cores exhibit fine grain size, diverse vertical facies successions, abundant sedimentary structures (such as bidirectional cross-bedding, ripple bedding, and bioturbation structures), and are dominated by coarsening-upward bedding, indicating the influence of various hydrodynamic processes during tidal cycles. This period coincides with the initiation of a global sea level fall from a high state and the commencement of seawater withdrawal from the Sahara Seaway (Guiraud et al., 2005; Edegbai et al., 2019). The deposits of this period were characterized by the predominance of remarkably thick mudstone, suggesting a significant supply of terrigenous material and a substantial accommodation space.

This paper presents a novel analysis of the late Cretaceous tidal processes in the Termit Basin based on core descriptions. In addition to the Y-3 core, data from the 1979 exploration well and the most recent coring well in 2022 were also collected, revealing similar sedimentary characteristics (Figure 5). The sedimentary environment of the basin in the member Ys3 period was mainly coastal, and the double mud drapes and the presence of bidirectional cross-bedding confirms the presence of tidal deposits rather than pelagic deposits in the late Cretaceous Termit Basin. It can be inferred that during periods of relatively high sea levels, such as the middle and late stages of the Donga Formation and member one to two of the Yogou Formation, tidal features likely developed along the basin margin due to the establishment of a unified regional water level, similar to the western edge of the Iullemeden Basin. The Termit Basin, positioned in the middle of the prototype basin, predominantly consists of mudstone, exhibiting less noticeable tidal characteristics. According to the available IHS(Information Handling Services) data and earlier literature, the Eastern Niger region and the surrounding basin, commonly referred to as the Chad Basin, encompass an area exceeding one million km². Based on the outcrop study, Pascal et al. (1993) proposed that during the early and middle Late Cretaceous, the Iullemeden-East Niger Basin experienced a gradual deepening of its continental shelf slope in the eastern region. The western part of the Iullemeden Basin is a marine-terrigenous facies, which reflects the frequent alternation between marine and terrigenous deposits. In the eastern portion of the Iullemeden Basin, the proportion of clastic rocks progressively decreases, while the occurrence of glauconite, ammonite, and limestone beds increases. Moving further east, the deposition of clastic rocks is primarily observed in the Termit Basin. Consequently, it is imperative to redefine the extent of the prototype basin in the late Cretaceous Termit Basin.

In general, carbonate rocks tend to develop easily in transgressive systems tract and highstand systems tract. As mentioned earlier, analysis of logging data revealed the presence of a thin set of carbonate rocks, measuring 1–2 m in thickness, at the top of the Donga Formation in the Termit Basin and calcareous mudstone and calcareous siltstone dominate the middle and lower parts. Wells containing carbonate rocks are primarily situated near the basin depression, such as well Dw N-1 (Figures 1, 10), whereas wells with fewer carbonate rocks are relatively closer to the basin margin. This distribution pattern reflects the characteristics of the water environment and sediment supply in the following ways: firstly, there was no large-scale clean water environment formed in the Termit Basin during transgression; secondly, there was abundant sediment supply from various sources surrounding the basin; thirdly, the settlement and sedimentation rates were relatively high, precluding the development of extensive carbonate rocks. In addition, previous studies have shown that the South Tethyan Upper Cenomanian platforms developed in an arid climate, and unfavourable climatic conditions could prevent carbonate deposition, while also being influenced by hypersaline and anoxic events (Philip and Airaud-crumiere, 1991; Philip, 2003).

Previous studies on the Member Ys3 (the main source rock in the basin) indicated a terrestrial origin for its organic matter (Wan et al., 2014; Dou et al., 2022). This understanding has led to the discovery of several billion-ton oil field clusters in the Termit Basin. Facies analysis reveals that the sandstone percent near the basin margin will increase, giving rise to a greater occurrence of sandy interlayers within the extensive mudstone layers. Consequently, resemblances to member Ys3 will emerge in the lower part of these deposits. The widely distributed mudstone and sandstone sequences will become source rocks in a wider range, potentially giving rise to hydrocarbon reservoirs based on the material foundation of source rock development (Yuan et al., 2023). For instance, based on the recognition of the widespread Upper Cretaceous source rocks developed across the basin in this region, the future prospects of the eastern hydrocarbon enrichment zone of the Termit Basin remain highly promising (Yuan et al., 2023).