The Atlantic Jigsaw Puzzle and the geoheritage of Angola Open Access

The purpose of this contribution is to emphasize a well-known observation realized by authors even before Wegener (1966): the puzzle-like fit of the western coast of Africa with the eastern coast of South America. Our approach is to view the southern coast of Angola (Figs 1 & 2) as a unique region of geoheritage due to its remarkable documentation of the major phases of opening and growth of the central South Atlantic Ocean, defined originally by marine geophysical models and well-core analysis (Fig. 2). To our knowledge, the Angolan coast is the only place that preserves relatively easily visited onshore outcrops that reflect this geologically significant chapter in Earth history.

J. Tuzo Wilson (1963) popularized plate tectonics by using the descriptive and easily visualized term ‘jigsaw-puzzle fit’ to describe the Atlantic coasts of Africa and South America (Fig. 2). Angola is at the centre of this jigsaw puzzle, and the geology of Namibe Province in southern Angola has geohistorical and geoheritage significance. The study of geology and of fossils in Angola has been reinvigorated in recent years, following the end of the civil war. Fossils are portable elements of geoheritage, and therefore worthy of perpetual care, so we include them here for their evolutionary, environmental, ecological, and palaeobiogeographical geoheritage value as it relates to the opening of the South Atlantic.

A useful framework for the opening of the central South Atlantic was presented by Quirk et al. (2013). The sequence of events began with the eruption of huge amounts of lava while Africa and South America were still conjoined (Figs 2–4). These flood basalts are part of the Etendeka–Paraná Large Igneous Province (LIP) and date from about 134 to 131 Ma (Fig. 3; Valanginian–Hauterivian, Renne et al. 1996; Jerram et al. 2019). Volcanic outpouring, crustal extension, and rifting formed extensional fault basins filled with alluvial, fluvial, and lacustrine deposits interbedded with basalt. With the cessation of rifting at approximately 121 Ma (Quirk et al. 2013; Quirk and Rüpke 2018), faulting ceased and the nascent ocean basin isostatically sagged, creating an early Aptian subaerial unconformity (Figs 2–4). Shallow-water lacustrine carbonates and siliciclastics were deposited (Fig. 3). Later in the Aptian, ocean water began to leak in to fill the subsiding basin, and then evaporated in extensive saline lakes to leave behind a great thickness of salt (2 to more than 6 km; Figs 2–4; Tedeschi et al. 2017; Quirk and Rüpke 2018). Following the cessation of salt deposition, the development of more open ocean conditions began about 113 Ma (Norton et al. 2016). Seafloor spreading began at the nascent Mid-Atlantic Ridge (Fig. 4). As the ocean basin widened through seafloor spreading it became fully flooded. The final phase was the sundering of the Equatorial Atlantic Gateway (EAG), separating the easternmost portion of what is now Brazil from the Bight of Benin and Gulf of Guinea. New ocean currents formed in the now conjoined North and South Atlantic oceans, transforming the water column and its ecology.

This discussion of onshore exposures begins at Piambo, near the south end of the study area. The salient observations here are the basal volcanic complex (Bero Volcanic Complex, part of the Etendeka–Paraná LIP; Fig. 3). The eroded irregular surface of these volcanic rocks represents the 121 Ma subaerial unconformity of the central South Atlantic basin. It is locally covered by metre-scale domal carbonate build-ups (Fig. 3; Fiordalisi et al. 2021). These and similar carbonate buildups are common throughout the Namibe Basin (see Bero and Chapéu Armado localities below) and represent precipitation of carbonate around groundwater resurgences and springs (tufa and travertines; Pentecost 2005), typically within heavily faulted transtensional zones along the basin-bounding fault. Such carbonates are common in volcanic and rifted areas (e.g. East African Rift or Italy; e.g. Renaut et al. 2002, 2013), where elevated (volcanic) heat flow drives a system of hydrothermal water circulation and metal leaching in the subsurface. Reduced pressure and temperature at the surface cause mineral precipitation, which will mainly include carbonates and silica, but metal (iron and manganese) oxides, and sulfates (barite, BaSO₄, and celestine, SrSO₄) were also observed in Angola (Carvalho 1961).

Overlying fine-grained siliciclastics and dolomitic carbonates at Piambo represent the uppermost sag phase (Fig. 2). The sag succession thickens dramatically from Piambo westwards to the offshore and the syn-rift and lower sag are only preserved offshore (Fig. 2). These observations reflect increased accommodation space offshore due to tilting of the proximal margin and thermal subsidence (Moragas et al. 2023). The salt phase is represented by the Bambata Formation, which is in turn overlain by the Albian marginal-marine Binga Member and terrestrial Giraul Conglomerate (Figs 2 & 3). This is followed by the marine Salinas Formation and then the Ombe Formation, which includes volcanic flows dated at Piambo to 88 Ma (Coniacian; Fig. 3; Gindre-Chanu et al. 2015; Jerram et al. 2019). These units are faulted against crystalline basement, and capped by the Campanian–Maastrichtian fossiliferous marine siliciclastic and calcareous Baba and Mocuio formations, which overstep the fault on to granitic basement (Fig. 2). The last phase of activity on the fault is thus Late Cretaceous in age. The basin-bounding fault at Piambo can be traced northwards some 50 km to Bentiaba, and then another approximately 50 km to Lucira (Fig. 1). Along that combined stretch, on the seaward side of the fault, outcrops show the broader expression of rock units representing the opening of the South Atlantic to the end of the Cretaceous. The upper units at Piambo contain marine reptiles that are correlated with the Campanian–Maastrichtian marine reptile fauna farther north at Bentiaba.

In the Giraul and Bero valleys, 20 km east of Moçâmedes, Precambrian basement granitoids and metasediments (Pereira et al. 2011) are abruptly juxtaposed against the rift fill along the NNW–SSE-trending basin-bounding master fault and NE–SW-trending linking faults. In the Bero valley, the rift fill starts with the basal Bero Volcanic Complex, which is intersected by a dense network of NE–SW- and NW–SE-trending conjugate normal faults parallel to the linking fault. This fault network creates a series of tilted blocks that form <1 km wide half-graben. The half-graben are filled by sag siliciclastics and carbonates of the Tumbalunda Formation (Fiordalisi et al. 2021). The most striking observation is the occurrence of boulder conglomerates, with clasts up to the size of vehicles and small houses, that abut the faults and overlie the volcanics (Fig. 5a). Clasts are made up of volcanics and basement, and testify to the dramatic erosion of the tilted blocks, and the high-energy deposition in alluvial fans that started to infill the nascent rift basin. Conglomerates pass laterally and upwards into finer-grained sandstones and travertine and lacustrine carbonates, which reflect quieter, fluvio-lacustrine environments as well as mineral precipitation from carbonate-rich fluids circulating along the faults. The succession is capped by erosive remnants of the Bambata Formation gypsum (<50 m thick), which forms a distinctive hardcap. The Bero small half-graben are analogous to tilted blocks in the modern East Africa Rift, for example around Lakes Asal and Magadi in Djibouti and Kenya, respectively, and provide a snapshot of the geomorphology in early volcano-sedimentary rifts.

Sag-phase carbonates are exposed near Chapéu Armado (Rochelle-Bates et al. 2021; Fig. 5b). They rest directly on faulted granitic basement rocks. The carbonates form a series of travertines up to several tens of metres high (Fig. 5c). Fabrics include small circular vents precipitated around orifices, as well as a series of terraces and connecting cascades where water flowed over steps in the substrate. About 12 m of sag phase siliciclastics overlie and interfinger with the travertines at this locality.

Another striking feature at this locality is the nephelenite/basanite volcanic plug called Chapéu Armado itself (Fig. 5b), meaning ‘armoured hat’ due to its distinctive cap of weathered volcanic rocks. In contrast to the basal volcanic complex, it has been dated to 91–88 Ma (Jerram et al. 2019; Rochelle-Bates et al. 2021), and thus represents a distinct post-rift phase of alkaline volcanism in Angola. Near the plug are some shallow galleries that were dug manually into soft Albian sandstones and mudstones of the Binga Member. These adits contain a fine network of veins filled with bitumen. Possibly the bitumen was mined by artisans for local consumption in heating, lighting, and lining the base of fishing boats, indicating the recognition of its significance (Fig. 5d, e). The source of the bitumen is from the mudstone itself – these contain a high amount of organic matter that accumulated in saline lakes at the end of Bambata Formation deposition in the Aptian (Binga Member of Piambo Formation; Fig. 3). The eruption of Chapéu Armado volcano some 30 million years later then produced enough localized heat to transform this organic matter into the tar-like bitumen (Fiordalisi et al. 2023).

The Mariquita valley traverses a north–south-trending graben structure that is parallel to the rift axis. Pre-salt Bero Volcanics constitute the footwall to the east, whereas Albian to Campanian rocks form the western footwall (Strganac et al. 2014a; Fiordalisi et al. 2021). In the west, Santonian basanite volcanics of the Ombe Formation are intercalated (Fig. 5f; Jerram et al. 2019; Fiordalisi et al. 2021). These are equivalent to the volcanics at Chapéu Armado. Deposits inside the graben belong to the Santonian Ombe Formation (Strganac et al. 2014a), and contain a mix of lava flows, volcaniclastics, carbonates and mudstones (Fiordalisi et al. 2021; Fig. 5f). Closely associated with the bounding faults and minor faults are spring-related carbonates (tufa, travertines) that appear as metre- to decametre-scale buildups. They commonly have a central steep vent where carbonate precipitated around the orifice, surrounded by more gentle slopes where spring water drained off the main orifice and precipitated carbonate. The carbonates on the distal parts of the slope contain numerous gastropod and ostracod casts. Surrounding the build-ups are horizontally bedded lacustrine clays and carbonate muds, locally containing silicified tree stumps (Fig. 5g). The build-ups are eventually overlain by the same clays and carbonates. Modern erosion of these clays has exhumed the palaeo-landscape of a depression dotted with carbonate spring mounds, surrounded by what were likely small lakes where the spring water accumulated. The lower reaches of mounds and the lakes teemed with gastropods, bivalves and ostracods, while the shoreline was colonized by trees. Positions of spring mounds were controlled by fault-related water resurgences, with heat provided by the post-salt volcanics immediately underlying the succession. A possible connection of these continental systems to the sea is indicated by the presence of brackish/marine Teredolites shipworm borings in the silicified trees (Fiordalisi et al. 2021).

The section along sea cliffs in the region of Bentiaba from north of Salinas to the Bentiaba River begins with coarse, pebbly red sands, the finer-grained facies of the Albian Giraul Conglomerate (Fig. 6a). The overlying Salinas Formation is rich in invertebrates, indicating a late Cenomanian age (∼100 Ma) near its base, progressing through the Turonian. The Salinas Formation is capped by the Ombe Formation, correlative of the distinct Mariquita Member (84.6 Ma, Santonian; Fig. 6c; Strganac et al. 2014a). The Ombe is succeeded by the Campanian Baba Formation (Fig. 6c; Cooper 2003), overlain by the late Campanian and Maastrichtian Mocuio Formation. Both the Baba and Mocuio formations produce vertebrate fossils, with the Bench 19 vertebrate fauna from the Mocuio Formation being arguably the richest marine reptile locality in Sub-Saharan Africa (Fig. 6b; Jacobs et al. 2006, 2016; Strganac et al. 2014a, b, 2015; Mateus et al. 2019). In addition to sharks and bony fish, four genera of turtles, at least seven mosasaur taxa (Schulp et al. 2006, 2008, 2013; Polcyn et al. 2010, 2023), two plesiosaurs (Araújo et al. 2015a, b; Marx et al. 2021), two taxa of pterosaurs (Fernandes et al. 2022), and rare indeterminate dinosaur bones (Mateus et al. 2012, 2019) have been recognized. A magnetostratigraphic section anchored by the Ombe Formation constrains the age of the Bench 19 fauna to chron 32n.1n (71.4–71.64 Ma; Husson et al. 2011; Strganac et al. 2014a). Stable carbon and oxygen chemostratigraphy is consistent with magnetostratigraphy (Strganac et al. 2014a), and the δ¹⁸O palaeotemperature estimate at Bench 19 is around 18°C (Strganac et al. 2015). The Agostinho Neto University Geological Museum in Luanda (UANMG) displays some fossils from Bentiaba and other Angolan sites. A Smithsonian Institution exhibition developed around the Bench 19 fauna has been on display in the US National Museum of Natural History since 2018. It will ultimately return to its home in Angola.

The rich Cretaceous vertebrate fossil deposits at Bentiaba, dominated by diverse apex predators, are indicative of a highly productive ecosystem. Palaeolatitude derived from geomagnetism of the Ombe Formation, and corrected for minor variation among models of palaeolatitude estimation (Jacobs et al. 2009, 2011, 2016; van Hinsbergen et al. 2015; Scotese 2021) was between about 26° and 22°S, or by modern analogy, between the latitudes of Lüderitz and Walvis Bay, Namibia. Compared to its present latitude of 14°S this indicates northward drift of approximately 10° through latitudes expected to produce upwelling. Today, from far southern Angola through Namibia and into South Africa, nutrient-rich coastal waters are brought to the surface by the Benguela Upwelling System (BUS). The position of the BUS relative to the continent is based on trade winds generated by the descending limb of the atmospheric Hadley Cell and Earth's rotation. The palaeolatitudinal setting of Bench 19 suggests the possibility of coastal upwelling and ecosystem productivity that supported the Cretaceous vertebrate fauna. Turonian (91.1 Ma, early Late Cretaceous) and Maastrichtian (68 Ma, late Late Cretaceous) palaeoupwelling maps by Scotese and Moore (2014) show upwelling progressing towards the southern extent of Africa as the continent moved northwards. Wagner et al. (2013), in their study of geochemical distribution patterns and Hadley Cell dynamics, support upwelling at the appropriate latitudes during the early Late Cretaceous (Cenomanian and Turonian stages). Thus, the proto-Benguela Current has had a long history. Its more modern iteration was attained in the Neogene, likely initiated by Antarctic glaciations and enhanced by uplift of the African continent (Siesser 1980; Heinrich et al. 2011; Jung et al. 2014).

Sedimentary evidence for a proto-Benguela current along the coast of Angola in the Albian has been reported (Quesne et al. 2009), but there is no evidence of marine reptiles in the central South Atlantic until after the completion of the EAG. Sea turtles and plesiosaurs, which are known to have occurred in Jurassic and Early Cretaceous seas elsewhere, first occur in the central South Atlantic in the Late Cretaceous. Mosasaurs have their evolutionary origin at ∼100 Ma, probably in the Middle East (Polcyn et al. 1999, 2014). The first records of mosasaurs, plesiosaurs, and sea turtles in Angola are ∼88 Ma (Jacobs et al. 2006; Mateus et al. 2009, 2011, 2012). The two earliest genera of mosasaurs are related to taxa first seen in the northern hemisphere. By 71.5 Ma, the Bentiaba Bench 19 mosasaur fauna indicates dispersal from North African and Trans-Saharan Seaway localities. In contrast, the plesiosaurs are related to southern hemisphere forms (Polcyn et al. 2014; Araújo et al. 2015b).

Thus, it appears that South Atlantic occurrences and the later biogeography of mosasaurs and sea turtles is intimately related to the opening of the EAG. Prior to its complete opening, Setoyama and Kanungo (2020) documented Coniacan (90 Ma) anoxic to dysoxic waters between the Walvis Ridge in the south and the EAG in the north. The initiation of well-ventilated bottom waters in the central South Atlantic heralded the appearance of marine reptiles and likely encouraged their dispersal. Moreover, the completion of the EAG was an event with global reach. The widening and deepening of the gateway allowed the flow of intermediate and deep water. The result of this reorganization of ocean currents may have been global cooling and the decline of the Cretaceous greenhouse (Granot and Dyment 2015).

The Tumbalunda area is the type locality for the sag-phase Tumbalunda Formation, resting directly on basement (Sharp et al. 2012). It consists of an overall fining-upwards succession of siliciclastics and carbonates deposited in alluvial fans, braided rivers, deltas, and lakes (Sharp et al. 2012). The Tumbalunda clastics transition upwards through a marginal-marine and tidal sabkha succession of evaporites, algal carbonates, and siliciclastics into the Bambata Formation evaporites (Gindre-Chanu et al. 2015, 2016; Moragas et al. 2023). The evaporites are overlain by fine-grained marginal-marine facies of the Binga/Gaio Member before being overstepped by alluvial fan conglomerates of the Giraul Member (Piambo Formation). Several Albian alluvial fans of 40–200 km² in area can be mapped through their distinct radial weathering pattern. The fans were sourced through westward-flowing palaeovalleys that cut through the basin-bounding fault and can be traced west towards the basin edge. Near the coast, in the Inamagando Valley, the Albian conglomerates pass laterally to fluvio-deltaic siliciclastics that sit conformably above lagoonal micrites of the Binga Member. They are overlain by a distinctive, regionally extensive transgressive metre-thick carbonate interval (Fig. 5h; Inamagando beds, Cooper 1976). This carbonate comprises a basal bed of oyster- and gastropod-rich limestone and is overlain by pillar-like build-ups rich in red algae, oncoids and microbial structures (Fig. 5h; Schröder et al. 2016). These carbonates represent a short-lived marine transgression into the marginal-marine depositional system, establishing brackish-water lagoons (Schröder et al. 2016). Only a limited set of higher organisms and microbes were able to colonize these lagoons due to the variable salinity and continued siliciclastic input.

Serra da Leba is a kilometre-high escarpment about 120 km east from Moçâmedes on the Atlantic coast. The escarpment runs broadly north–south, parallel to a regional fracture and fault network, and is part of the much more extensive Great African Escarpment that separates the coastal plains from the central plateau of southern Africa. East of Moçâmedes, the coastal desert plain is dotted with basement granitoid inselbergs that record Eburnean (Paleoproterozoic) to Pan-African (Neoproterozoic) orogenic activity (∼2.1–0.5 Ga; Pereira et al. 2011). At Serra da Leba, the basement is disconformably overlain by flat-lying Mesoproterozoic quartzites and limestones of the Chela Group (∼1.9 Ga; Pereira et al. 2011). These sediments form a prominent cliff at the edge of the Humpata Plateau that provides spectacular views over the Namibe desert plain below. Caves in the stromatolitic limestone of the Chela Group contain Mesolithic archaeological assemblages (de Matos and Pereira 2020) and Plio-Pleistocene mammal fossils, including cercopithecid primate remains concentrated by eagle predation (Gilbert et al. 2009). The Serra da Leba escarpment, due to its scenery, preserved geological record, and local cultural significance, is a tourism hotspot in SW Angola, and is recognized as a potential geoheritage site (Henriques et al. 2013; Tavares et al. 2015).

The origins of the Great African Escarpment are still poorly understood. One hypothesis sees the escarpment as an erosional remnant of the Lower Cretaceous rift shoulder (King 1951), whereas alternative hypotheses postulate repeated uplift events of the margin through the Cretaceous to Neogene (e.g. Green and Machado 2017). Either way, the presence of such topography and its possible rejuvenation via uplift increased erosion rates and delivered sands to offshore regions, constituting potential petroleum reservoirs. Increased offshore subsidence could have aided in maturation of organic-rich source rocks (Green and Machado 2017). Thus, the dynamic history of the Angolan continental margin was a key component in developing the petroleum systems offshore Angola, which are important for Angola's economic development.

The Cretaceous section is unconformably truncated by flat-lying conglomerates, boulders, and marine shells. This is considered a Pliocene highstand terrace as elsewhere in South Africa (Hearty et al. 2020). Post-Pliocene sediments include Pleistocene marine terraces (Sessa et al. 2013) and palaeosols with occasional mammal fossils and ostrich shell fragments, topped by younger silt with local shell middens, stone tools, hand axes, and hut circles, dating back to the Paleolithic (de Matos et al. 2021). Strikingly, cherty tools such as axes and blades cluster near outcrops of silicified rift and sag-phase carbonates.

Henriques et al. (2013) outlined the legal framework for protecting geoheritage and biodiversity in Angola. In 2019, the Declaration of Antananarivo on Geological Heritage and its Conservation in Africa was issued (PanAfGeo 2019). Among the signatories was a representative of the Organization of African Geological Surveys, of which Instituto Geológico de Angola Igeo is a member. The Declaration presented six principles, including a fundamental link between geoheritage and biodiversity, and seven recommendations, which acknowledge that ‘the value of African geological heritage [should be] brought to the attention of the greatest number of persons’. Neto and Henriques (2022) reviewed the status of geoconservation across Africa, pointing out that most geoheritage studies in Africa reviewed by them involve inventory and assessments of geoheritage value of specific locations. Such inventories showcase the wonder and beauty of Earth at varying levels of detail. Its audience is all people, all ages. Its benefits accrue to all – residents, students, tourists, industry geologists, or scholars, through economic development, aesthetic appreciation, or learning. It is science for development. This report conforms to that characterization.

In terms of bringing attention of Angola's geoheritage to the greatest number of persons, fossils from the locality of Bentiaba form the core of a temporary museum exhibit, Sea Monsters Unearthed: Life in Angola's Ancient Seas, in the Smithsonian Institution's National Museum of Natural History in Washington, D.C. Since its opening on 18 November 2018 through 2023, the museum had 14 614 718 visits (Smithsonian Institution 2024). Ten percent of museum visitors live in countries other than the United States; thus, the exhibit provides a far-reaching educational experience based on Angolan geoheritage. As the exhibit returns to Angola, the planning, construction, and visitation of the exhibit at the Smithsonian can further inform decision makers in Angola as to possible applications and conservation of Angola's geoheritage.

Geoheritage is the appreciation of geological and geomorphological features and processes as natural heritage, including their role in ecosystem function and cultural heritage. We have alluded to each of these aspects in the context of the Atlantic Jigsaw Puzzle, and to Angola specifically. However, the Atlantic Jigsaw Puzzle is not simply an Angolan or South America–African, or even southern hemisphere example of geoheritage. It is geoheritage on a global scale because it is an example of a fundamental paradigm in geology and its meaning is recognized by young and old the world over. The significance of the Angolan geological record is that it is the first place where a relatively complete sequence of geological events leading to the Atlantic Jigsaw Puzzle has been recognized in outcrop.

An international palaeontological consortium referred to as Projecto PaleoAngola has conducted field research in Angola since 2005. Projecto PaleoAngola acknowledges the encouragement of ISEM and Southern Methodist University. Since the inception of Projecto PaleoAngola, Universidade Agostinho Neto in Luanda has been its host institution. Sincere thanks are due Maria Luísa Morais, A. Olímpio Gonçalves, André Buta Neto, Tatiana da Silva Tavares, and the numerous students and friends who worked with us in all phases of field work. Sonangol and Statoil (now Equinor) staff were involved in the 2010–14 onshore Namibe Basin mapping project. Sonangol EP supported museum exhibit development.

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 paper

LLJ: conceptualization (equal), investigation (equal), visualization (equal), writing – original draft (equal), writing – review & editing (equal); SS: conceptualization (equal), investigation (equal), visualization (equal), writing – original draft (equal), writing – review & editing (equal); NDS: conceptualization (supporting), writing – original draft (supporting), writing – review & editing (supporting); RD: investigation (equal), writing – review & editing (supporting); EF: investigation (equal), writing – review & editing (supporting); AM: investigation (supporting), writing – review & editing (supporting); OM: investigation (equal), visualization (equal), writing – original draft (equal), writing – review & editing (equal); PCN: project administration (equal), writing – review & editing (supporting); MJP: conceptualization (equal), investigation (equal), visualization (equal), writing – original draft (equal), writing – review & editing (equal); GDCRP: investigation (equal), writing – review & editing (supporting); NR-B: investigation (equal), writing – review & editing (supporting); ASS: conceptualization (equal), investigation (equal), visualization (equal), writing – original draft (equal), writing – review & editing (equal); CRS: visualization (supporting), writing – review & editing (supporting); IS: investigation (equal), writing – review & editing (supporting); CGS: investigation (supporting), writing – review & editing (supporting); RS: visualization (equal), writing – review & editing (supporting); DPV: project administration (equal), visualization (equal), writing – review & editing (supporting).

This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.

Data sharing is not applicable to this article as no datasets were generated or analysed during the current study.