# The first definitive Albian ammonites from the Kribi-Campo sub-basin in Cameroon and biostratigraphic implications for the opening of the South Atlantic

Elie Fosso Menkem, Emmanuel Robert, Paul Gustave Fowe Kwetche, Marie-Joseph Ntamak Nida, Jeremy Martin

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Elie Fosso Menkem, Emmanuel Robert, Paul Gustave Fowe Kwetche, Marie-Joseph Ntamak Nida, Jeremy Martin. The first definitive Albian ammonites from the Kribi-Campo sub-basin in Cameroon and biostratigraphic implications for the opening of the South Atlantic. Journal of African Earth Sciences, 2025, 228, pp.105659. 10.1016/j.jafrearsci.2025.105659. hal-05066115

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# The first definitive Albian ammonites from the Kribi-Campo sub-basin in Cameroon and biostratigraphic implications for the opening of the South Atlantic

Elie Fosso Menkem<sup>a,\*</sup>, Emmanuel Robert<sup>b</sup>, Paul Gustave Fowe Kwetche<sup>c</sup>, Marie-Joseph Ntamak Nida<sup>c</sup>, Jeremy E. Martin<sup>b,\**</sup>

## ARTICLE INFO

Handling Editor: M Mapeo

**Keywords:**

South Atlantic Ocean
Kribi-Campo Sub-Basin
Mundeck Formation
Lower Albian
*Douilleiceras* sp. and Uhligellinae

## ABSTRACT

The timing of the opening of the Equatorial South Atlantic Ocean during the Cretaceous is poorly constrained. Despite previous research carried out along the West African continental margin, the biostratigraphic attribution of the Kribi-Campo sedimentary deposits in Cameroon remain poorly known leading to uncertainties in the ages of the formation and thus leading to incorrect interpretations and correlations of stratigraphic sequences. To date, the deposits are known to correspond to alluvial fan facies from the Lower Mundeck Formation and the assigned age is Barremian?-Aptian. We carried out a prospection in the sedimentary outcrops along the shoreline in the Kribi-Campo sub-Basin where poorly preserved but identifiable ammonite specimens were discovered and collected for analysis in order to determine the age of the formation. The marker bed containing the ammonite fossils consists of thin carbonaceous sandy shale. Our determinations revealed the following ammonites: *Douvilleiceras* sp. and Uhligellinae that characterize the basal Lower Albian (*Douvilleiceras leightonense* Zone) interval. This result allows us to suggest a reconsideration of the Kribi-Campo sub-basin deposits to be part of the upper Mundeck Formation. Our results confirm a diachronic separation of South America and Africa and a full connection between the North and South Atlantic Ocean during the Lower Albian.

## 1. Introduction

The first marine connection between the Central and South Atlantic Oceans is known to have started during the Mesozoic; however, the precise time and the sequence of events remain a constant debate which need additional data for clarifications. Several authors have attempted to model and reconstruct the South America and Africa evolution breakup and the rifting stages processes (Cainelli and Mohriak, 1999; Moulin et al., 2010; Heine et al., 2013; Basile, 2015; Granot and Dyment, 2015; Mercier de Lépinay et al., 2016; Scotese, 2021; Bonifacio et al., 2023; Suhail et al., 2023; Scotese et al., 2024). However, these plate reconstruction models present several inconsistencies on the age of the different phases of the opened of the South Atlantic. For example, during the Albian, the model of Granot and Dyment (2015) presents Africa and America that were still in place at 108 ± 1 Ma, and the Atlantic Ocean

opening only on its Southern part. In contrast, Heine et al. (2013) proposed a detailed model where Africa and America were completely separated during the Albian (110 Ma) and a full marine connection along the Ivory Coast/Ghana Ridge between the South Equatorial and North Atlantic. More recent work by Bonifacio et al. (2023) and Scotese et al. (2024) further refines these models, but discrepancies persist, particularly with the timing of the full marine connection. Pre-breakup kinematic reconstructions in West Africa and west of the Paraná Basin (South America), based on geological and magnetic data, offer a tighter fit and critical insights into passive margin genesis and early oceanic crust evolution (Moulin et al., 2010). Along the African margin, the Ivory Coast and Guinea basins reveal rift-related sedimentary sequences, faulting, and volcanic activity (Mercier de Lépinay et al., 2016), indicating early rifting as far back as the Albian. On the South American side, similar tectonic features in the Pernambuco Basin, including

<footnote><sup>a</sup> University of Buea, Department of Geology, P.O. Box 63, Buea, Cameroon</footnote>

<footnote><sup>b</sup> Université de Lyon, Université de Lyon 1, ENSL, CNRS, LGL-TPE, Villeurbanne, France</footnote>

<footnote><sup>c</sup> Université de Douala, Faculté des Sciences, Département des Sciences de la Terre, BP 24157, Douala, Cameroon</footnote>

<footnote>\* Corresponding author.</footnote>

<footnote>\*\* Corresponding author.</footnote>

<footnote>E-mail addresses: fosso.menkem@gmail.com (E.F. Menkem), jeremy.martin@cnrs.fr (J.E. Martin).</footnote>
syn-rift sediments and volcanic structures, support the idea of a more gradual rifting process (Cainelli and Mohriak, 1999).

Based on biostratigraphical markers, studies on ammonites and foraminifers from the lowermost marine beds in the Sergipe Basin in Northeastern Brazil have indicated a surface-water connection as early as late Aptian (Bengtson and Koutsoukos, 1992). Some ammonites preserved in the shales on the Pointe-à-Pierre Coast from Trinidad were described and indicated a Barremian (Early Cretaceous) age (Imlay, 1954). This proves the presence of a North and South opening of the Atlantic; but the evidence on the equatorial part of the Atlantic that can confirm a marine connection during the Lower Cretaceous epoch from Africa are (Barremian-Aptian) rare. Contrary to the South American block, very few ammonite specimens dated from the Aptian-Albian, Albian-Cenomanian and Santonian have been discovered along the African margin of the Atlantic, respectively at Kribi-Campo, Cameroon, Kwanza Basin, Angola and Sikoum in the Douala Basin-Cameroon (Andreeff, 1947; Cooper, 1982; Ntamak-Nida et al., 2006; Tavares et al., 2007, 2021; Meister et al., 2011). These ammonites are true evidence of a marine presence in the South and North Atlantic Ocean during the Lower Cretaceous; but the question of the precise date of this connection remains uncertain.

Despite previous studies in the Kribi-Campo sub-basin, the stratigraphic attribution of the Campo outcrop deposits using biostratigraphic markers remains poorly known leading to uncertainties in the ages of the sedimentary deposits. To date, only a few authors (Andreeff, 1947; Mboumbouo et al., 2001; Ntamak-Nida et al., 2008, 2010) have mentioned biostratigraphic markers in Campo such as ammonites, but none of those specimens have been described nor figured and their taxonomic attributions remain unsettled. Additionally, the ammonites reported by Andreeff (1947) could never be located by subsequent authors and their taxonomic identification remains questionable. Moreover, precise occurrences of ammonites and true location in the field have not been recorded.

In view of these shortcomings, the objectives of this article are to present the first description of newly collected ammonite specimens that are used as biostratigraphic markers from the Kribi-Campo sub-basin sedimentary deposits in the Kribi-Campo Mundeck Formation. Based on the taxonomic determinations, the age of the Campo Lower? Mundeck Formation is discussed in light of the paleogeographic evolution during the progressive separation of the Equatorial and South Atlantic margins.

## 2. Geological background

The study area is structurally located in the North-Western part of the Congo Craton in Cameroon at the junction of the Kribi-Campo Shear Zone (KCSZ) which is composed of metasedimentary and metavolcanic rocks units (Feybesse et al., 1998; Maurizot, 2000). Geographically, the Kribi-Campo sub-basin is located between 2°10' -3° 20'N and 9°-10°30' E, and extends over 6150 km² offshore and 45 km² in a triangular onshore area (Ntamak-Nida et al., 2010). Most of the Geological researches done in this region have focused on the assessments of the petroleum systems and their structural evolution (Dumort, 1968; Kjemperud et al., 1992; Ross and Hempstead, 1993; Meyers et al., 1996; Chierici, 1996; Dailly and Goh, 2000; Dailly et al., 2002; Brownfield and Charpentier, 2006a,b; Mbina Mounguengui and Guiraud, 2009; Ntamak-Nida et al., 2008, 2010; Fu et al., 2014; Loule et al., 2018; Zhao et al., 2019; Zhang et al., 2023; Iboum Kissaaka et al., 2012, 2024). The Mesozoic sedimentation along the Brazilian-African continental margin started with the breakup of Western Gondwana in the Late Jurassic/Early Cretaceous and advanced towards the north, reaching the equatorial margin by the Late Aptian/Early Albian (Matos, 1992; Cainelli and Mohriak, 1999; Scotese, 2001, 2021; Benkhelil et al., 2002; Wilson et al., 2003; Olsen and Morgan, 2006; Fairhead et al., 2013; Heine et al., 2013; Bonifacio et al., 2023; Scotese et al., 2024). Prior to rifting, Jurassic continental sediments were deposited in the Afro-Brazilian depression, an intra-continental rift basin that extended over the Douala and Gabon

Basins (Garcia, 1991; Meyers et al., 1996). According to Brownfield and Charpentier (2006a), the West-Central Coastal Province includes the Douala, Kribi-Campo, Rio Muni, Gabon, Congo, Kwanza, Benguela, and Namibe Basins which together form the Aptian salt basin of Equatorial West Africa located in the Southern part of the Gulf of Guinea. Sedimentation in North Gabon and Douala basins was controlled by: pre-rift lithologies and topography; rift propagation from south to north; lateral migration between rift-units within rift-zones; basin subsidence; migration of rift-flank uplifts; location of drainage divides, transfer structures and river systems; and palaeoclimate and palaeoceanography (Meyers et al., 1996). In the West African context, paleoclimate during the Cretaceous and early Cenozoic was marked by arid to semi-arid conditions, with periodic humid phases that influenced sediment supply and weathering patterns (Peters et al., 2005). Palaeoceanographic conditions, including the opening of the South Atlantic and the development of marine anoxic events, strongly affected organic matter deposition and sediment distribution in these basins (Weissert and Erba, 2004; Wagner and Pletsch, 1999). The early opening of the Equatorial Atlantic gateway, as discussed by Dummann et al. (2023), further contributed to the establishment of distinct oceanographic circulation patterns, enhancing sediment transport and deposition across West African basins. The Douala Basin, well known as Douala-Kribi-Campo, is subdivided in two sub-basins with: (1) the Douala Sub-basin in the north and (2) the Kribi-Campo Sub-basin in the south, which borders the Rio Muni Basin in the Atlantic (Benkhelil et al., 2002; Tadjou et al., 2008; Ntamak-Nida et al., 2008, 2010; Loule et al., 2018). The two sub-basins are limited by the salt deposition and the Kribi-Fault zone. The northern part of this fault may define the limit of the Douala and Kribi-Campo sub-basins (Ntamak-Nida et al., 2010). The syn-rift deposits are known to exist in the Rio Muni, Douala and Kribi-Campo Basins of Equatorial Guinea and Cameroon (Brownfield and Charpentier, 2006a). The syn-rift rocks in the Rio Muni Basin consist of Neocomian to Barremian fluvial and lacustrine sandstones and shales deposited in a broad low-relief basin. In the Campo sub-Basin in Cameroon, the syn-rift formation known as Mundeck Formation (Abbot et al., 1978) or Lower Mundeck (Ntamak-Nida et al., 2008, 2010) is identified as conglomerate and basal sandstones deposits unconformably overlie the Precambrian basement. In the Kribi-Campo sub-basin, the Lower Mundeck Formation was until now considered as Barremian? - Aptian (Nguene et al., 1992; Tamfu et al., 1995; Ntamak-Nida et al., 2010). Ngon-Ngon et al. (2016) summarized the different depositional units of the post-rift in the Douala-Kribi-Campo basin as follow: (1) the Logbadjeck Formation (Cenomanian-Campanian), (2) the Logbaba Formation (Maastrichtian); (3) the N'kapa Formation (Paleocene-Eocene); (4) the Souellaba Formation (Oligocene); (5) the Matanda Formation (Miocene) and (6) the Wouri Formation (Plio-Pleistocene). This study focuses on the Lower Mundeck? Formation of the Kribi-Campo Sub-Basin (Fig. 2) where new biostratigraphic markers were collected in the sandstone deposits.

## 3. Material and methods

For this work, we carried out a prospection of all the outcrops described in Benkhelil et al. (2002) and Ntamak-Nida et al. (2010) along the coastline from Bouandjo, Etonde Yassa, Nanga Bodoua, Etanda, Enyenguele Manga and Campo (Fig. 1a and b). We benefited of a very low sea tide, which allowed us to access outcrops that are most of the time underwater or hidden by algae in the coastal area off Campo where the ammonite specimens were collected.

The fossiliferous outcrops (Fig. 1b) are located on the shore off Campo where there is a good exposure of sedimentary outcrops from various geomorphic surfaces along the continental border (Fig. 1a). The study material is composed of ammonite specimens (Fig. 3) collected at the Campo field (Fig. 1c) located about 1000 m from the Sous-Prefecture after Malombe I. The GPS coordinate of the site of the ammonite collection is recorded with the curated specimens.

The relative poor state of preservation of the specimens and their
<figure><img src="image_4.png" /><figcaption>**Fig. 1.** a) Location map for the West-Central Coast of Africa, showing the approximate locations of the Niger Delta, Benue Trough, Mamfe, Douala, Kribi-Campo, Rio Muni, Gabon, Congo, Kwanza (Cuanza), Benguela, and Namibe Basins (Map modified and adapted from Brownfield and Charpentier, 2006a. b) General Location Map of studied area (extracted from Ntamack et al., 2010). c) Log Reconstruction of Campo-Mundeck Formation presenting the ammonites zone which are overlain by massive conglomerates. d) thin section presenting fragmented ammonite shell; this may be resulting from sediment compaction processes. e) Thin section presenting very fine quartz grains, sub-angular to sub-rounded and moderately sorted.</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>**Fig. 2.** Tectono-lithostratigraphy chart of the Kribi-Campo sub-basin (compiled from Nguene et al. (1992) and Lawrence et al. (2002) in Ntamak-Nida et al., 2010).</figcaption></figure>

limited number were not obstacles for the determinations. A description of the sedimentary facies of the hosting rock is presented on the basis of field observations, thin sections and observations under polarized light.

In addition, microfauna researches were carried out on the source sediments of the ammonites by proceeding with the washing extraction method; then the sediment smears were examined in thin sections.

# 4. Results

## 4.1. Sedimentological description

The study formation is made of matrix-supported conglomerates composed of angular to sub-angular quartz pebbles and clasts sub-
<figure><img src="image_6.png" /></figure>

A-C. *Douvilleiceras* sp. (primitive morphology), lateral and ventral views.

D-F. *Uhligellinae* indeterminate, lateral view, section and ventral views.

Abbreviations: vlt, divided ventrolateral tubercles; ut, umbilical tubercles; c, constrictions; fr, fine ribs. Scale x 1

**Fig. 3.** Campo-Cameroon, plate presenting ammonites species *Douvilleiceras* sp. and *Uhligellinae* indeterminate. A-C; *Douvilleiceras* sp. (primitive morphology), lateral and ventral views. D-F; *Uhligellinae* indeterminate, lateral view, section and ventral views. Abbreviations: vlt, divided ventrolateral tubercles, ut, umbilical tubercles, c, constrictions; fr, fine ribs. Scale x 1.

angular to sub-rounded from diverse origins (mainly of the basement, size >15 cm); below the conglomerate, there are laminated thin beds of sandstones-claystones grey to brown color. The marker bed containing ammonite fossils consists of thin carbonaceous sandy shale with a grey to brown coloration. It is embedded within a chaotic brown conglomerate made up of centimeter-sized clasts, mixed with a matrix-supported, poorly sorted fine sandstone composed of angular quartz and feldspar grains. Other non-marker traces fossils such as the seaurchins, gastropods, shells fragments, traces of unidentified bioturbations, fishes and plants debris were also observed. The conglomerate is common towards the sea area and observable on the riverbeds in the inner land lying unconformably on the Precambrian basement. This deposit may represent upper alluvial fan deposits in transition to the Lower alluvial fan delta described by Ntamak-Nida et al. (2008, 2010), but based on field evidences, the studied formation is lower in the lithostratigraphic series presented by the previous authors.

The thin sections (Fig. 1d) present very fine-quartz grained sub-angular to sub-rounded and moderately to well sorted composed of mixed sedimentation of argillaceous-siltstones, well compacted. Lenticular, thinly laminated organic matter is compacted within a deformed layered structure. Also observed is the presence of micro-porosity in parts, relatively developed along the original micro-fractures occasionally filled with calcite Cement. The framework is mainly grain-supported. The cement may originate from diagenetic clay and carbonaceous sources. The ammonite shell presents a fibrous texture and is fragmented; this may result from sediment compaction and dissolution processes. The distinction between diagenetic clay cement and matrix is difficult. However, the calcite cement and some framework grains are irregular, which may indicate a possible replacement of silica by calcium carbonate.

Sediment may derive from a primary felsic plutonic rock which could be a granite or associated rock such as a granodiorite and also from a gneissic basement. The sub-angular to sub-rounded shape of the quartz grains observed in thin sections may indicate an immature deposit with a possible moderate to short distance of transport from the source. The sediments may have mainly moved as lower part of a debris flow deposit controlled by slopes from the continent to the sea and deposited as an alluvial fan. This caused the removal of original feldspar by mechanical abrasion also through alteration during transportation and sediment reworking processes. The micro-fractures observed may be related to the compaction processes which would at the same time facilitate the precipitation of carbonates during periods of non-deposition and minor dissolution associated to the compaction. This may also be directly linked to the tectonic event related to the border fault activation.

## 4.2. Ammonite systematic palaeontology

The observed ammonites are poorly preserved but determinations were possible following preparation of the specimens. The UCBL-FSL acronym indicates the repository of the specimens mentioned in the text and text-figure caption: Palaeontological-Geological Collections, Laboratoire de Géologie de Lyon, Université Claude Bernard Lyon 1.

Class Cephalopoda Cuvier, 1795

Superorder Ammonoidea Haeckel, 1866.

Order Ammonitida Haeckel, 1866

Suborder Ammonitina Hyatt, 1889.

Superfamily Douvilleiceratoidea Parona and Bonarelli, 1897

Family Douvilleiceratidae Parona and Bonarelli, 1897

Subfamily Douvilleiceratinae Parona and Bonarelli, 1897

Genus *Douvilleiceras* Grossouvre, 1894

(= *Trinitoceras* Scott, 1940; = *Eodouvilleiceras* Casey, 1961)

**Type species:** *Ammonites mammillatus* Schlotheim (1813), designated by Casey (1954).

**Discussion:** Numerous systematic studies have been published for the genus *Douvilleiceras* since its introduction by de Grossouvre (1894). Many species and varieties have been introduced (Cooper, 1982) but criteria to discriminate intraspecific variation still debate (Latil, 2011; Amédro et al., 2014; Kennedy and Klinger, 2015; Futakami and Haggart, 2018; Bulot et al., 2022). Following Robert (2002) and Latil (2011), the genera *Trinitoceras* and *Eodouvilleiceras* are herein considered as subjective junior synonyms of *Douvilleiceras*.

The first representative of the genus comes from the lowermost Albian, within the *Douvilleiceras leightonense* Zone (Szives et al., 2023, 2024). Its radiation is complex to establish because the lithological successions are dominated by hiatuses, discontinuities or condensed series in the Aptian/Albian transition and the lowermost Albian. Moreover, specimens are often poorly preserved (Kennedy et al., 2000; Latil, 2011; Jaillard et al., 2019; Giraud et al., 2021).

It is established that the geographic distribution of the genus *Douvilleiceras* is worldwide, even if its abundance highly varies. It is described in Cameroon for the first time but was cited in Andreeff (1947, see section 4.2). It reinforces its presence in the Central Atlantic region and, more largely, in the Equatorial and South Atlantic areas: Brazil (White, 1887; Brito and Rodrigues, 1967; Zucon Ramos de Siquiera, 2005), Angola (Choffat and De Loriol, 1888; Cooper, 1982; Tavares et al., 2007), South Africa (Kennedy and Klinger, 2015). The specimen of
the Kribi-Campo sub-Basin confirms the presence of the genus in shallow
epicontinental seas and continental shelves environment and mixed
carbonate-clastic sedimentation (Jaillard et al., 2019; Bulot et al., 2022).

Douilleiceras sp.

Text-fig. 3A-C

**Material:** One specimen (UCBL-FSL 341541)

**Description:** One poorly preserved, incomplete and distorted internal mould. The diameter is estimated at 50 mm. The ribs are difficult to observe but seems simple and regular. Its bears umbilical and ventrolateral tubercles (lateral not visible). Umbilical tubercles are undivided and strong. Ventrolateral ones are low and divided in two spirally elongated ridges. Intercalate ribs seems missing. The ventral sulcus is narrow. The ventrolateral tubercle and ventral sulcus are characteristic of primitive morphologies of the genus.

**Discussion:** Andreeff (1947, p. 9) mentions "a large internal mould, with a diameter of 130 mm, that shows simple ribs, projected onward and bearing tubercles difficult to distinguished". The specimen was determinate as a *Douilleiceras* by the palaeontologist V. Hourcq. Because of this very succinct description, and in the absence of figuration, any revision of the determination would be very speculative.

Size, ribbing and lateral tubercles of our specimen shows resemblance with the holotype of *Douilleiceras horridum* described by Riedel (1938, pl. 6, Figs. 1 and 2). It differs from its ventrolateral tubercles, divided in two spiral ridges while *D. horridum* bears simple pointed ones. The species *horridum* was first retained by Casey (1961) as type-species of its genus *Eodouilleiceras*. However, we follow Latil (2011) in considering *horridum* as an "ontogenetic stage of earliest members" of the genus *Douilleiceras*.

Ventrolateral ridges and ventral sulcus also compare with holotypes of *Douilleiceras inaequinodum* (Quenstedt, 1849; pl. 10, Fig. 1a) and *D. orbignyi* Hyatt (1903), species considered as a synonym of the first, refigured by Kennedy and Klinger (2015, text-fig. 5).

Regarding the representatives of the genus from the Equatorial and South Atlantic areas, our specimen compares with *Douilleiceras inaequinodum* of South Africa figured by Kennedy and Klinger (2015, text-fig. 11F-G) but it differs in particular by the ventrolateral tubercles morphology. The specimen UCBL-FSL 341541 bears two spirally elongated ridges while the *D. inaequinodum* is divided in three or four ones. The ventrolateral tubercles and ventral sulcus of our specimen also match with *D. inaequinodum* of Brazil (Zucon Ramos de Siquiera, 2005, pl. 21, Fig. 2A-B) and Angola (Cooper, 1982, text-fig. 7A-C), but differs by the lack of simple intercalate untuberculate ribs. The division of the ridges on the specimen figured by Zucon Ramos de Siquiera (2005) is difficult to established from the illustration but it looks similar to our specimen. *D. inaequinodum* figured by Koutsoukos and Bengtson (2024, supplementary material Fig. S16) also shows intercalate untuberculate ribs not present on our specimen. The division of the ventrolateral tubercles is crushed and, here again, difficult to characterize.

Otherwise, our specimen highly differs from the mammillate ventral and lateral tuberculate specimens, belonging the *mammillatum* group, which are dominating the South Atlantic *Douilleiceras* populations in Angola (see Choffat and De Loriol, 1888; Cooper, 1982; Tavares et al., 2007), Brazil by (White, 1887; Zucon Ramos de Siquiera, 2005) and South Africa (Kennedy and Klinger, 2015).

**Occurrence.** Stratigraphic range of *Douilleiceras* comprises the Lower Albian, including the *Douilleiceras leightonense* Zone and the *D. mammillatum* Zone and the early Middle Albian (Owen, 1988; Bulot et al., 2022; Szives et al., 2023). Our specimen corresponds to a primitive morphology of the genus, only presents in the lower part of the *D. leightonense* Zone. *D. horridum*, showing affinities with our specimen, should be considered as Lower Albian even if with uncertainty (Latil, 2011). Etayo Serna (1979) mentions a specimen similar to *D. horridum* in its Lower Albian association of Colombia while Wright et al. (1996), following Casey (1961), consider it as Upper Aptian. In contrast,

*D. inaequinodum*, other morphology close to our specimen, is ascribed from upper Lower to lower Middle Albian (Kennedy and Klinger, 2015). Its synonym *D. orbignyi*, is originally common in the middle Lower Albian but extends down to the upper Lower Albian. The South African *D. orbignyi* has been found in level correlating with the middle part of the *D. mammillatum* Zone (Bulot et al., 2022). Specimens from Dombe Grande of Angola are considered as upper Lower Albian by Kennedy and Klinger (2015). *D. inaequinodum* from Sergipe, Brazil, is placed within the Lower Albian sensu lato (Zucon Ramos de Siquiera, 2005).

Superfamily Desmoceratoidea Zittel, 1895.

Family Desmoceratidae Zittel, 1895.

Subfamily: Uhligellinae Latil, Murphy and Rodda, 2023

Uhligellinae indeterminate

Text-fig. 3D-F

**Comments.** The subfamily Uhligellinae has been recently introduced by Latil et al. (2023). It is considered as probably polyphyletic and brings together 9 genera with ranges from Late Aptian to Late Albian. Our specimen (see description below) seems close to the genus *Zuercherella* placed to this subfamily. But its small dimension, corresponding to juvenile stages, and poorly preservation, force us to maintain an open nomenclature. It also shows resemblances with few Desmoceratidae of Angola and Brazil placed within Puzosiinae (genus *Puzosia*) by their authors but which are possible representatives of Uhligellinae. They still maintained in the Puzosiinae pending their revision.

**Material:** two specimens (UCBL-FSL 341542 and 341543)

**Description.** Two poorly preserved internal mould. The coiling is involute (U/D = 0.19), with a narrow umbilic. The maximum diameter of UCBL-FSL 341542 (text- Fig. 3D-F), best preserved, reach 24 mm. Its whorl section is high whorled and compressed (W/H = 0.7), with slightly convex to flat flanks and rounded umbilical shoulder. Both specimens show shallow constrictions, slightly projected onward on the ventral area. Fine ribs are present between constrictions on the ventral and ventrolateral areas, and are probably continuing on the flanks.

**Discussion.** Sculpture and section compare with the inner whorls of the *Zuercherella zuercheri* lectotype (Jacob, 1906, pl. II, Figs. 1-3; refigured by Latil et al., 2023, text-Fig. 2a and b).

UCBL-FSL 341541 shows resemblance with few Desmoceratidae from the Equatorial and South Atlantic areas. Section, flat flanks and constrictions, shallow and projected onward on the venter, compare with the lectotype of *Puzosia* (*Puzosia* bistricta (White, 1887), refigured by Cooper (1982, text-fig. 14A-C). Narrow umbilic, section, shallow constrictions and fine ribs of the ventral area match well with *Puzosia* (*Puzosia* garajauana Maury (1930) figured by Zucon Ramos de Siquiera (2005, pl. 4, Fig. 1a–d). But the Lower Albian fauna described by White (1887) and Cooper (1982) needs a revision and the assignment to the genus *Puzosia* is questionable.

**Occurrence.** The stratigraphic range of Uhligellinae extend from the Upper Aptian to the Upper Albian (Latil et al., 2023). However, our specimens have been collected in the same level as *Douilleiceras* sp., which primitive ornamentation is restricted to the Lower Albian *D. leightonense* Zone.

The closer genus *Zuercherella*, is restricted to the Upper Aptian in Europe (Kemper, 1964; Dutour, 2005), Morocco (Peybernès et al., 2013; Giraud et al., 2021), Venezuela (Renz, 1982) and California (Latil et al., 2023). As for *Douilleiceras*, *Puzosia* (P.) bistricta from Dombe Grande, Angola, is placed within the upper Lower Albian (Kennedy and Klinger, 2015). *Puzosia* (P.) garajauana of Sergipe, Brazil is considered as Lower Albian in age by Zucon Ramos de Siquiera (2005).

4.3. *Micro fauna analysis*

One single sample coming from the sediment surrounding the fossil
has been processed for studying calcareous nannofossils under optical
microscope. A smear slide has been prepared (technique described in Bown and Young, 1998), then observed under a ZEISS Axioplan polarizing optical light microscope at 1000× magnification. Four transverses of the slide have been scrutinized under the microscope (1 transverse: 200 µm of diameter, 32 mm of length). Such a studied surface is considered as sufficient to detect the presence or, conversely, the absence of nannofossils. The studied sample turned out to be completely barren in nannofossils. It is impossible to affirm if this absence is primary in origin (e.g., the environmental conditions were not compatible with the development of calcareous nannoplankton), or secondary, diagenetic in origin.

# 5. Discussion

## 5.1. Depositional environment and paleoenvironmental significance

The matrix-supported conglomerates are poorly sorted, and indicate rapid debris flow deposits that moved under the effect of traction force and topographic gradient. Such a composition may characterize the alluvial fans at the basin border area or a close proximity of the emerged land. The presence of angular to sub-rounded clasts originating from gneiss and granites basement indicates a high-energy of sedimentary transport and traction. The size of the clasts, sometimes >15 cm, indicates a relief with very high topography at the time of deposition. This corresponds to channel deposits or proximal debris flow deposits (Miall, 1978, 1996; Lindsey and Gaylord, 1990). The deposits consist of thickening-up and coarsening-up sequences with erosion and hard ground at the lower contact, which may characterize cycles of progradational sequence deposits. This seems to start from M1 a little earlier below the retrogradational M2 to M3 genetic sequences described by Ntamak-Nida et al. (2010). Geochemical investigations by Tchouatcha et al. (2021), Ashukem et al. (2022) and Tchouatcha and Ganno (2023) indicated that the source rocks of the Atlantic Basin sediments of Cameroon comprise metamorphic and magmatic rocks of the Yaounde Group, Adamawa-Yade shield, and Nyong and Ntem complexes. Ntamak-Nida et al. (2008) interpreted the Lower Mundeck Formation of Kribi-Campo as an alluvial fan/lacustrine system with possible marine invasion. This deposit may be part of the syn-rift sequence known as the oldest formation of the Kribi-Campo sub-basin (Pauken, 1992).

The palynomorph data from the neighboring basin of Mamfe (Fig. 1a), SW Cameroon, which originates from the same tectonic system as Douala-Kribi-Campo makes it possible to calibrate the paleoclimate of our study area. In the Mamfe Basin, Bilobe et al. (2021) based on the palynomorph assemblages, defined for the Lower Cretaceous time humid conditions prior to the Barremian coinciding with the opening of the Atlantic Ocean and arid to semi-arid conditions intermittent with short-lived humid phases from the Barremian to the Cenomanian. At the Eastern Edge of the Douala Basin, Njike Ngaha et al. (2014) reported a depositional environment that changed over time during the early to middle Cenomanian, passing from a narrow and steep-sided tectonic basin which later became a lacustrine to palustrine basin with marine incursions and a warm climate characterized by severe aridity periods. This arid to semi-arid conditions could justify the presence of thick salt deposits reported on the Kribi-Campo marine seismic sequences with an estimated age interval corresponding to the Aptian-Albian (Mienlam-Essi et al., 2023).

At the other end in South America, in Brazil, the upper Aptian succession of the Sergipe Basin is characterized by a transgressional trend, reflecting changes in environmental conditions over time; the related changes are indicated by the lithologic characteristics of the column and coincide with interpretations based on vertical distribution patterns of several dinocyst communities (Carvalho et al., 2016). According to these previous authors, the global Aptian transgression resulted in the initial connection of the central South Atlantic with the equatorial Atlantic Ocean, although Haq (2014) indicates that only the late Aptian is

transgressive, with a sharp sea-level drop occurring in early Aptian times.

## 5.2. Age assignment of the Campo deposits

Pollen grains attributed to *Tucanopolis crisopolensis* (Regali et al., 1974a,b; Regali, 1989) and representing one of the most primitive angiosperm records have been previously reported from the Barremian to early Aptian of Brazil and Argentina in South America; and Gabon, Congo and Egypt in Africa (Doyle et al., 1977; Regali, 1989; Herngreen et al., 1996; Schrank and Mahmoud, 2002; Vallati, 2013). In the Kribi-Campo sub-Basin, palynological studies (Nguene et al., 1992; Lawrence et al., 2002; Brownfield and Charpentier, 2006a) on borehole data from offshore drilling wells have attempted to calibrate lithostatigraphic units with ages determined for 5 units: Albian-Cenomanian, Paleogene, Early-Middle Miocene, Upper Miocene-Pliocene and Pleistocene. To date, in the Rio Muni and Kribi-Campo Basins; no wells have penetrated strata older than middle to Late Aptian (Pauken, 1992; Loule et al., 2018; Zhao et al., 2019). Additional marker fossil records would help strengthen our work; however, the coverages of fossils, sediment outcrops, coal, magnetite, hematite and apatite for evaluating global paleoshoreline are limited (Middleton and Schmidt, 1982; Moore et al., 1986; Heine et al., 2013).

Despite the average conservation of the ammonite specimens collected in the field off Campo, the relative dating carried out enabled us to refine and to assign the age to the Lower Albian to the deposits observed based on the identification of *Douvilleiceras* sp. and Uhligellinae. It is important to mention that the stratigraphic level where the ammonite specimens were collected at Campo is stratigraphically lower in the sequence and consequently older than all the outcrops described in the previous works of Ntamak-Nida et al. (2008, 2010); works of which all the outcrops were revisited during this study. Based on this result, the Campo deposits may rather belong to the Upper Mundeck formation. Benkhelil et al. (2002) qualified the Campo sedimentary outcrops as forming a poorly stratified sequence which onlaps directly on the Precambrian surface at several locations. Furthermore, Benkhelil et al. (2002) report that palynological and foraminiferal evidence indicates the early deposition of the Campo outcrops on a Precambrian basement, with an age dating to the Early Cretaceous, specifically the Albian.

## 5.3. Paleogeographic evolution of the South Atlantic

The age determined based on the presently described ammonites *Douvilleiceras* sp. and Uhligellinae from the Kribi-Campo sub-basin in Cameroon is Early Albian. This is a decisive result allowing a better understanding of the paleogeographic evolution of the West African coast, in relation to the opening of the South Atlantic Ocean (Fig. 4). In fact, the sinistral trans-tension deformation observed in the Benue Trough is related to the opening of the South Atlantic Ocean and the extension in the West African Rift System (WARS) and Central African Rift System (CARS) (Popoff, 1988; Benkhelil, 1989; Fairhead and Okereke, 1990; Genik, 1992, 1993; Guiraud et al., 2005). Moulin et al. (2010) propose a comprehensive kinematic model addressing the complex evolution of the South and Equatorial Atlantic. They divide the South Atlantic into three key segments: the Equatorial, Central, and Austral, emphasizing the unique constraints shaping each. A notable contribution is their integration of intraplate deformation in both Africa and South America to reconcile earlier misfits in plate reconstructions. For example, they highlight the Benue Trough in Africa, formed as an aborted rift, and the Paraná Basin in South America, where dextral strike-slip movements and volcanic activity contributed to crustal adjustment. Their model also provides a refined understanding of the Equatorial Atlantic's diachronous opening, driven by northward rift propagation, which aligns well with the geological and geophysical evidence. Heine et al. (2013) reported that the Mamfe Basin at the SW
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generated by geopy 07
(c) Christian Beutel, 2012

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**Fig. 4.** Illustration of South Atlantic Rift Kinematics Paleo Tectonic Maps; respectively at 115 Ma, 110 and 100 Ma representing the Atlantic opening in Cretaceous from the Upper Aptian to the Late Albian (Modified from Heine et al., 2013, for more details kindly refer to the authors original publication). The red Star in the Equatorial area indicates the position of the studied location.

Bottom colour scales indicate geological stages and magnetic reversals with red diamond indicating time of reconstruction; abbreviations here: Val. – Valanginian; Haut'n – Hauterivian; Barrem. – Barremian. Restored continental margin is indicated by dashed magenta-colored line. Rigid lithospheric blocks denoted by black labels: SAf – austral African Plate, BoP – NE Brazilian Borborema Province plate; Jos – Jos Plateau sub-plate; NEA – NE African Plate; NPM – North Patagonian Massif; PT – Pampean Terrane; RaB – Rawson Block; Sal – Salado Sub-plate; SAm – main South American Plate; SJuB – San Julian Block; SLC – Sao Luis Craton – block. Actively extending basins are indicated by red background in label; post-rift basins are indicated by light grey background in label; abbreviations: BeT – Benue Trough; CaR – Canelones rift; D/D B. – Doba and Doseo basins; ColB – Colorado Basin; IuB – Iullemmeden Basin; marB – Marajo Basin; MugB – Muglad Basin; NFB – North Falkland Basin; PotB – Potiguar Basin; TRJ – Reconavo, Jatoba and Tucano basins; SaB – Salado Basin. Other abbreviations: RdJ – Rio de Janeiro; B. – basin; GPI. – Guyana Plateau; DeR – Demerara Rise. Present-day hotspots are shown as volcano symbol and plotted with 400 km diameter (dashed magenta-colored circle with hachured fill), assuming that they are stationary over time: A – Ascension; B – Bouvet; Ch – Chad/Tibesti; Ca – Mount Cameroon; Cd – Cardno Seamount; D – Discovery; T – Tristan da Cunha. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

<figure><img src="image_11.png" /></figure>

end of the Benue Trough separates the Oban Highlands block from the Adamaoua Microplate in the East, it is conjugate to NE Brazil. Luft-Souza et al. (2022) studies on the Early Cretaceous marine microbiofacies from the South Atlantic Ocean (Sergipe-Alagoas Basins in Brazil) concluded on water connections between the South Atlantic and the North Atlantic/West Tethys Sea since the late Aptian. Also, several authors (Benkhelil et al., 2002; Lawrence et al., 2002; Brownfield and Charpentier, 2006a; Ntamak-Nida et al., 2010; Zhao et al., 2019) have reported that post-rift rocks in the West-Central Coastal Province range from Aptian to Holocene in age and represent the initial opening of the Atlantic Ocean in equatorial west Africa. Heine et al. (2013) on their models proposed a three-stage kinematic history (a. Phase I: Initial opening – Berriasian to late Hauterivian (140–126.57 Ma); b. Phase II: Equatorial Atlantic rupture (126.57–120.6 Ma); c. Phase III: Break-up – Aptian to late Albian (120.6–100 Ma)) that can account for most basin-forming events in the Early Cretaceous South Atlantic in South America, Equatorial Atlantic, Central African and West African Rift Systems. The kinematics of the South Atlantic rift proposed by Bullard et al. (1965), Heine et al. (2013) and Bonifacio et al. (2023) are presenting the most appropriate boundary reconstructions that match with the age determined in this study. According to these models, it appears that from the earliest Aptian onwards, diachronous lithospheric breakup occurred along the central South Atlantic rift, first in the Sergipe-Alagoas/Rio Muni margin segment in the northernmost South Atlantic; the final break-up between South America and Africa occurred in the conjugate Santos-Benguela margin segment at around 113 Ma and in the equatorial Atlantic domain between the Ghanaian Ridge and the Piauí-Ceará margin at 103 Ma. This chronology is supported by Dummann et al. (2023), who suggest that the early opening of the Equatorial Atlantic gateway played a pivotal role in the timing of the tectonic events. Dummann et al. (2023) study present a nuanced timeline for the early opening of the Equatorial Atlantic Gateway (EAG), with key stages defined by geochemical and micropaleontological data. They identify three major phases in the Aptian to Cenomanian interval: (1) Aptian restricted shallow marine conditions with localized weathering inputs, (2) the onset of marine connectivity around 113 Ma marked by the influx of Tethyan waters and increasing salinity, and (3) full bathyal connectivity by ~107 Ma. This early connection profoundly influenced ocean circulation and global carbon cycles, as enhanced water exchange reduced organic carbon burial and contributed to Cretaceous greenhouse warming. In the neighboring Mamfe basin, the biostratigraphic dating using palynological assemblage components confirmed the presence of ancient deposits with an age ranging from early Barremian to Cenomanian (Ajonina, 2016; Bilobe et al., 2021). Correlatively, to these results; Ferreira et al. (2020) reported in the Parnaíba Basin of northern Brazil deposits dated from the Albian-Cenomanian. On both sides of the Atlantic, this could represent the sequence of early deposits linked to the tectonic fractures. The events as described are correlated to the opening phases II and III models of Heine et al. (2013) which could correspond to the Break-up that took place during the Aptian to late Albian interval (120.6–100 Ma). This is in agreement with the age determined in the current study.

## 6. Conclusion

The present report of the ammonites *Douvilleiceras* sp. and Uhligellinae in the Kribi-Campo Sub-Basin, as well as their attribution to the Upper Mundeck Formation, suggests a post-rift phase deposition during the basal Lower Albian. According to previous authors, the Lower Mundeck Formation sits unconformably on the Precambrian basement which is consistent with the observed depositional patterns (Ntamak-Nida et al., 2010). However, this introduces two plausible interpretations.

(1) Diachronous Lower Mundeck Formation: If the outcrop belongs to the Lower Mundeck Formation, its age variations imply diachronicity, reflecting localized tectonic influences or sediment supply variations during deposition.

(2) Absence of Lower Mundeck Formation: Alternatively, the observed deposits might directly belong to the Upper Mundeck Formation, with the Lower Mundeck Formation absent due to erosion or non-deposition. This absence might signify a diachronous post-rift transgression phase, as suggested by transitions between depositional systems as noted by Ntamak et al. (2010).

Both interpretations emphasize the significance of diachronous depositional environments (Dumann et al., 2023; Ntamak-Nida et al., 2010), consistent with tectono-stratigraphic models of the Kribi-Campo Sub-Basin. Further sedimentological and biostratigraphic analyzes could clarify whether the studied outcrop represents a transitional phase or the onset of Upper Mundeck Formation deposition. Correlating this with nearby facies and stratigraphic units may provide better temporal resolution for interpreting these depositional environments. Our results suggest that during the basal Lower Albian (112–105 Ma), a shallow marine Equatorial Seaway linking the Central and South Atlantic was already established. This seaway indicates the presence of marine conditions in the region. However, the presence of ammonites does not necessarily confirm that the area was floored by oceanic crust. While ammonites serve as evidence for marine connectivity, determining the existence of oceanic crust requires additional geophysical data. In addition, future biostratigraphic data will certainly make it possible to identify and clarify the location of the Lower Mundeck Formation which may outcrops deeper off Kribi-Campo Sub-Basin.

## CRediT authorship contribution statement

Elie Fosso Menkem: Writing – review & editing, Writing – original draft, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. Emmanuel Robert: Writing – review & editing, Writing – original draft, Software, Methodology, Formal analysis, Data curation. Paul Gustave Fowe Kwetche: Writing – review & editing, Methodology, Investigation, Formal analysis, Data curation. Marie-Joseph Ntamak Nida: Writing – review & editing, Validation, Resources, Formal analysis. Jeremy E. Martin: Writing – review & editing, Writing
– original draft, Supervision, Resources, Project administration, Methodology, Funding acquisition, Data curation, Conceptualization.

## Data availability

All the data used in this manuscript are original and collected on the field. Fossils specimens are kept at Palaeontological-Geological Collections, Laboratoire de Géologie de Lyon, Université Claude Bernard Lyon 1.

## Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this manuscript.

## Acknowledgements

The fieldwork was supported by the National Geographic Society (grant No. NGS GEFNE 74-13).

We thank Professor Emanuela Mattioli (University of Lyon), for her support in taking the time to assist us technically in this work for the search of nannofossils. We are grateful to the editor and especially the reviewers Jean-Louis Latil and Etienne Jaillard, for their constructive comments and suggestions that have greatly improved the quality of this manuscript.

## Data availability

Data will be made available on request.

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