The occurrence and geological setting of Cretaceous dinosaurs, mosasaurs, plesiosaurs, and turtles from Angola

Jacobs, L. L., Mateus O., Polcyn M. J., Schulp A. S., Antunes M. T., Morais M. L., & da Silva Tavares T. (2006).  The occurrence and geological setting of Cretaceous dinosaurs, mosasaurs, plesiosaurs, and turtles from Angola. Paleont. Soc. Korea. 22(1), 91-110.


Vertebrate-bearing fossiliferous outcrops of Cretaceous age in sub-Saharan Africa are rare because of younger superficial deposits, vegetation cover, and the widespread occurrence of Precambrian metamorphic plateau basement comprising much of the continent. However, one area of extensive marine and nonmarine
Cretaceous exposures is found between the plateau and the coast in Angola. The Angolan margin was formed in conjunction with the breakup of Gondwana and subsequent growth of the South Atlantic. Cretaceous deposits are constrained in age by the emplacement of oceanic crust, which began no later than magnetozone M3
(approximately 128 Ma, Barremian). Shallow marine facies are exposed in sea cliffs but equivalent facies become increasingly terrestrial inland. Few vertebrate fossils have been described from Angola aside from sharks.
Notable exceptions are the late Turonian mosasaurs Angolasaurus bocagei and Tylosaurus iembeensis from northern Angola. Those taxa are significant because they are among the earliest derived mosasaurs. Recent field work led to the discovery of a new skull of Angolasaursus as well as sharks, fish, plesiosaurs, the skull of a new taxon of turtle, additional mosasaurs, and the articulated forelimb of a sauropod dinosaur, the first reported dinosaur from Angola. In southern Angola, marine sediments spanning the Cretaceous-Paleogene boundary are found.

We have embarked on what we hope will be a long-term field-based exploration of the vertebrate paleontology of Angola. There are no better exposures of Cretaceous and Paleogene rocks in all of sub-Saharan Africa than those along the coast of Angola (8oS to 15oS). Sea cliffs and extensive outcrops, especially in the south, are visible between the structural zone separating the metamorphic plateau in the east from the coastal plain leading to the South Atlantic Ocean in the west (Fig. 1).
Outcrops are richly fossiliferous in both invertebrates and vertebrates and comprise a variety of sedimentary lithologies and volcanic rocks. Rocks range in age from late Early Cretaceous through the Cenozoic, with vertebrate fossils reported from most units, especially important for this paper, from the Turonian, Campanian, and Maastrichtian stages. Older and younger vertebrate fossils are known, most
notably from the Paleogene. There was even the report of a Miocene rhinoceros, but Cenozoic vertebrates are beyond the focus of the conference for which this report was prepared.

The most recent comprehensive study of the stratigraphy and paleontology of the Angolan coast was that of Antunes (1964). More recently, Antunes and Cappetta (2002) published a systematic description of sharks collected primarily during the 1960’s by Antunes. (The surname Antunes has been inconsistently and incorrectly cited as Telles-Antunes or Telles Antunes; e.g., Bardet et al., 2005; Bell, 1997a, but not 1997b; Bell and Polcyn, 2005, but not Polcyn and Bell, 2005; Lingham-Soliar, 1994).
Previous studies of outcrop between the coast and the interior plateau of Angola were completed prior to the general acceptance and full development of plate tectonic concepts, magnetostratigraphy, seismic and sequence stratigraphy, and high precision radiometric dating. Antunes (1964) predated the general use of phylogenetic systematic techniques, the recognition of bolide impact at the Cretaceous-Paleogene transition, the application of stable isotopes to chemostratigraphic, paleoenvironmental, and paleoecological problems, and the advent of such analytical and field tools as high resolution X-ray computed tomography or GIS/GPS positioning. Nor have collections been improved by methodical acquisition of new material of known or additional taxa since the work of Antunes in the early 1960’s.
In contrast, offshore basins of Angola are recognized as being extremely important economically and geologically for their petroleum resources and as a geological record of the opening of the South Atlantic Ocean during the breakup of Gondwana. Consequently, there is an important and growing body of seismic and core data from the South Atlantic relevant to a more thorough understanding of the coastal deposits of onshore Angola. This is not only with respect to the initial formation of the South Atlantic, but as an environmental record of changing climatic, atmospheric, and oceanic conditions as the sea floor widened during the Late Cretaceous and Cenozoic. Therefore, we begin this paper with a discussion of offshore geology as a background to understanding the geological history of vertebrate-bearing deposits of Angola, then we discuss the localities visited in our 2005 field season, including preliminary observations and conclusions regarding fossils obtained at that time.

The geographic significance of Angola from a plate tectonic perspective is that it is the eastern border of the South Atlantic Ocean, formed in the breakup of Gondwana during the Mesozoic Era. Rifting was heralded in the area destined to become the southernmost South Atlantic by regional uplift and volcanism during the Jurassic Period, which continued progressively northward through the Early Cretaceous (Emery and Uchupi, 1984; Nurnberg and Muller, 1991). Flooding of the South Atlantic Ocean after initial volcanism began south of Angola during Jurassic and Early Cretaceous time. Prior to the Barremian,

subsidence of intracratonic rift basins farther north, in the area that is now Angola, resulted in deposition of more than a kilometer of clastic detritus over pre-existing metamorphic basement rocks (Brice et al., 1982; Karner and Driscoll, 1999). Emplacement of oceanic crust along the Mid-Atlantic Ridge at the coast of Angola began no later than magnetozone M3, or approximately 128 Ma (Barremian; Ogg and Smith, 2004). Rifting appears to have been propagated from the Walvis Ridge hotspot (Fig. 1; Standlee et al., 1982).
Moulin et al. (2005) utilized deep penetration multi-channel reflection and wide-angle seismic data to document that Early Cretaceous strata are parallel to the base of Aptian evaporites, thereby indicating that pre-Aptian beds were little affected by tectonic deformation during initial rifting. They conclude that environments of deposition were shallow during that phase of rifting and that horizontal motion was less of a factor than vertical motion during basin formation (see also Reston et al., 1996). Following
Cretaceous rifting, Africa was uplifted in the Cenozoic relative to other continents (Bond, 1978; Sahagian, 1988; Lunde et al., 1992; Nyblade and Robinson, 1994). However, neither the rates nor the timing of either Cretaceous or Cenozoic regional or continental vertical motions are precisely known, although it does not seem unlikely that the tectonic style of the formation of the South Atlantic played a role in the initial expression of the continental metamorphic plateau east of the current Angolan coastal plain.
From north to south, the offshore basins of Angola that extend landward are referred to as the Lower Congo, Kwanza, Benguela, and Namibe (Fig. 1). Conjugate basins have been identified on the Brazilian margin of the South Atlantic (Cainelli and Mohriak, 1999), reflecting their common origin. Separating the offshore basins along the Angola passive margin are the Ambriz Arch between the Lower Congo and Kwanza basins (7° to 8°S; Standlee et al., 1992), an east-southeast trending chain of volcanic seamounts between the Kwanza and Benguela basins (11°S, Schollnberger, 2001), and the Walvis Ridge, which divides the Benguela from the Namibe Basin (approximately 18°S near the continent; Lehner and de Ruiter, 1977).
(“Kwanza” is the English spelling and is used consistently, but not exclusively, in that form in English scientific literature. The spelling in Romance languages is “Cuanza,” and it is used in that form in Portuguese scientific literature. We use the term Namibe Basin to conform with the town and the province as they are now called, as opposed to the older synonym, Moçâmedes Basin.)
Cretaceous deposition from Barremian onward is generally comparable among the Angolan offshore basins in terms of lithology and stratigraphic thickness. Above the initial synrift sequence are sandstones up to 200 meters thick, which represent the initiation of marine deposition along the Angola margin (Ala and Selley, 1997; Marton et al., 2000). Lower Aptian rocks are dominantly evaporites, which may signify restriction of the Angolan basins from open oceanic circulation by vertical motion along the Walvis Ridge and other topographic features of the South Atlantic seafloor. Above the evaporites are thick carbonate and clastic units (Brognon and Verrier, 1966a, b; Gerrard and Smith, 1982). Post-Aptian salt tectonism is extensive and exacerbated by regional westward tilt (Schollnberger, 2001; Hudec and Jackson, 

Schollnberger (2001) studied two-dimensional seismic profiles of the three northern Angola offshore
basins and correlated four unconformity-bounded, second-order (10-100 Ma) sequences, which span the
time from the end of the Aptian through the Cenozoic. In addition, Schollnberger (2001) recognized
widely varying numbers of third-order (1-10 Ma) and fourth-order (0.2-0.5 Ma) unconformity-bounded
sequences among the basins. The mechanisms that can create marine unconformities are sea-level fluctuations,
changing circulation patterns, and tectonic activity, including salt tectonism and source-area perturbations
that may have affected the distribution of depocenters or sediment provenance. Variation in
the number of unconformity-bounded, third- and fourth-order sequences among the Angolan basins suggests
basin-specific attributes, the causes of which remain undefined.
Biostratigraphic and lithostratigraphic control for the seismic study of Schollnberger (2001) was provided
by a core from DSDP site 364 in the Benguela Basin (Bolli and Ryan, 1978). More recent drilling
by the Ocean Drilling Project on ODP leg 208 in the South Atlantic on the northeastern flank of the
Walvis Ridge specifically focuses on data relevant to understanding the Cretaceous-Paleogene transition
and Paleogene climatic events (Shipboard Scientific Party, 2003). No relevant data are currently available
from onshore Africa for rigorous comparison with those data derived from the Walvis Ridge by the
ODP, but the most likely prospect for deriving relevant data from the mainland is from outcrops in
Angola where Cretaceous and Paleogene deposits are exposed.

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