Cretaceous Ammonite Distributions and the Opening of the South Atlantic

Cretaceous ammonite distributions and the opening of the South Atlantic

WILLIAM JAMES KENNEDY & MICHAEL COOPER

SUMMARY Analysis of distribution patterns of middle to absence of early Turonian marine deposits in late Cretaceous ammonites along the line of Southern Africa), but such evidence as is the opening S. Atlantic suggest that open available also suggests continued connection. marine connections, if not actual rifting, Palaeobiogeographic evidence is thus corn- developed during early upper Albian times, patible with sea floor spreading and palaeo- and extended continuously to the close of the magnetic data, rather than in conflict, as Cenomanian. Evidence of early Turonian previouslysuggested. distributions is equivocal (due largely to an

PALAEOMAONETIC and sea floor spreading data (Creer et al. I965; Amaral et al. I966; McDougaU & Ruegg I966; Saito et al. x966; Siedner & Miller x968; Maxwell et al. I97o; Valencio & Vilas I97o ) have suggested to recent authors that initial rifling between Africa and S. America began in early or mid- Cretaceous times, while the recovery of chalks and nannofossil oozes of Albian to Cenomanian date from the Walvis Ridge (Pastouret & Goslin I974) confirms this view. In contrast, studies of distribution patterns of Cretaceous marine invertebrates, notably ammonites, from coastal basins flanking the south Atlantic (Beurlen I96I , Reyment i969-72 , Reyment & Tait I972, Cooper x972-I974) are held to indicate that a continuous marine connection between N. and S. Atlantic areas was not established until the close of the early Turonian. We would suggest on the basis of recent work on mid to late Cretaceous faunas from Angola (Cooper I972-I974) , Zululand and Natal (South Africa) (Kennedy & Klinger I97I , in press) that faunal distributions rather confirm a far earlier open marine N.-S. connection, although this may have been no more than flooding of a downwarped area of continental crust rather than an actual oceanic rift. The S. African and Angolan ammonite faunas (Fig. i) discussed here may have reached these areas either via the shelf seas flanking the western Indian Ocean, via the line of the Atlantic, or via a so-called trans-Saharan seaway (Reyment & Tait I972 ). Reyment & Tait (i972) concluded that Africa and South America remained connected during early middle Albian times, on the basis of the geographic restriction of the oxytropiceratid subgenus Venezoliceras. We can confirm this view on the basis of rich Oxytropidoceras faunas of early middle Albian date from Dombe Grande, south of Benguella (Angola), which show little resemblance to the equally rich contemporaneous Oxytropidoceras faunas from Texas (Young x966 ). In contrast, the associated douvilliceratids D. mammillatum (Schlotheim) and D. orbigni Hyatt are forms well known from S. Africa and Madagascar

Jl geol. 8oc. Lond. vol. x3x, x975, PP. 283-288, x fig. Printed in Northern Ireland.

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(Collignon I963) , appearing to be migrants to Angola via the Cape Seaway (Dingle & Klinger i97i , Jones I972, Dingle x972). Evidence of a continued barrier between northern and southern proto Atlantic (latter term used in the sense of the opening Mesozoic ocean, rather than the Palaeozoic seaway of Tuzo Wilson et al.) areas during early upper Albian times may also be indicated by the distribution of the essentially endemic south Atlantic genera Elobiceras, Neokentroceras and Angolaites, well known from Angola, Brazil and Nigeria (Reyrnent & Tait I972; Howarth I965; Spath I922; Haas z942), with records from Madagascar (Collingnon I963) again suggesting an open Cape Seaway. These taxa occur, however, in upper Albian faunas (approximately H. varicosum subzone) from Lobito and Hanha, Angola (Cooper x973; Haas I942) which have long been considered to have close affinities with those of N. Africa, Europe and S. Africa. We would add that a whole series of mortoniceratids are also known to be common to areas of trans-Pecos Texas, Chihuahua, northern Coahuila (Mexico) and Angola at this time, e.g. Mortoniceras (Boeseites) roemeri (Haas), M. (B.) perarmata Haas, M. (B.) proteus (Haas), M. (B.) el. howelli (Haas), M. (B.) of. barbouri (Haas) and Prohysteroceras cf. hanhaense Haas (Young i968 ). Equally strong evidence of an open southern proto Atlantic seaway at a slightly

j olorado,~,~l A ~'~Texas

/ Venezuela Postulat~ Angola Barrier' FIG. I. Location of principal Cretaceous faunas discussed in text, and possible migration routes of South i:iiii Atlantic faunas. A: proto-Atlantic, B: trans-Saharan, C C: Cape.

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later date (approximately C. auritus subzone) is given by the strong similarities between faunas from the La Puya Member of the Penas Altas Formation in the Venezuelan Andes and contemporary deposits in West and South Africa and Madagascar (Renz 1968). The following species are common to Venezuela and E. Africa (Zululand, Mozambique, and Madagascar)" Mortoniceras (Mortoniceras) pricei (Spath), M. (M.) pricei var. intermedium Spath, M. (M.) africanum (Spath), M. (M.) arietiforme andranofotsyense (Collignon), M. (M.) cf. recticostatum (Spath), M. (Deiradoceras) prerostratum (Spath), M. (D.) devonense Spath, M. (D.) mokara- haense (Collignon), M. (D.) cf. exile (Van Hoepen), M. (Rusoceras) nothum (Van Hoepen), Hysteroceras carinatum Spath, H. orbignyi (Spath), Puzosia (Anapuzosia) saintoursi Collignon, Bhimaites aontzyensis Collignon, B. stoliczkai (Kossmat) and Oxytropidoceras (Venezoliceras) madagascariense Collignon. Species common to Vene- zuela and Angola include Hysteroceras orbignyi (Spath), H. orbignyi evolutum Haas, H. carinatum Spath, H. carinatum aft. robusteocostatum Haas, H. choffati Spath and Mortoniceras (Mortoniceras) arietiforme arietiforme (Spath). This leaves little doubt that there was free faunal migration between the E. coast of Africa and Venezuela during the low upper Albian. Uppermost Albian (dispar Zone) faunas are rather poorly known from Brazil and Nigeria (Reyment & Tait 1972, Reyment 1955) but we now have extensive faunas of this date from Egito in Angola and from Zululand. Occurrence ofhetero- morph species such as Anisoceras perarmatum Pictet & Campiche, A. arrogans (Giebel) (=A. campichei Spath) and A. jacobi Breistroffer (=A. picteti Spath) together with Mortoniceras (Durnovarites) from Texas, N. Mexico and Africa down to Angola and to Zululand confirm a continued seaway. Particularly convincing is the distribution of Stoliczkaia of the africana (Pervinqui6re) group, known from England, Texas, Switzerland, N. Africa (Algeria and Tunisia), Nigeria, Brazil (although said to be of Cenomanian age) (Reyment & Tait i972), Angola and Zululand. Absence of records of this species group from the Middle East, Mada- gascar and S. India strongly suggests migration along a southern proto Atlantic seaway. Cenomanian faunas recently recorded from Angola (Cooper 1972) again support an open connection" forms such as Stomoham#es aft. simplex (d'Orbigny), Anisoceras plicatile (J. Sowerby), Turrilites costatus Lamarck, T. acutus Passy, Forbesiceras obtectum (Sharpe), Calycoceras coleroonense percostata Collignon, C. annulatum Collignon, Euomphaloceras cunningtoni meridionale (Stoliczka) and Acan- thoceras cf. tunetana Pervinqui6re (Cooper x972) showing strong links with NW. European and N. African faunas in many respects, but also include Malagasy and Zululand species. Latest Cenomanian faunas from Angola, correlative with the N. American Sciponoceras gracile Zone and the European Metoicoceras gourdoni-geslinianum Zones, again support continued connection and free interchange with Europe and N. America. Ammonites from Salinas, Angola (Cooper 1972) including Metoico- ceras whitei Hyatt, Sciponocerasgracile (Shumard), Kanabiceras septemseriatum (Cragin) Calycoceras naviculare (Mantell) and Pseudocalycoceras of the angolaense (Spath)- dentonense (Moreman) group from an assemblage identical with that known in Texas (Moreman 1927, 1942) and the U.S. Western Interior (Cobban & Scott

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I972 ) and Europe (Kennedy i97i , Juignet et al. I973) , and also repre- sented by sparse assemblages from Nigeria (Reyment x955) and Morocco (Collignon x966 ) (Metoicocerus ornatum Moreman of these authors is a synonym of M. whitei). The species Austiniceras dibleyi Spath is in fact only known from S. England and Angola (Kennedy x97 I). Considerable weight has been placed upon early Turonian distributions, with a suggestion of a northern Selwynoceras association typifying northern S. America, Mexico, Texas and parts of W. Europe and with the area of the southern proto Atlantic typified by a vascoceratid association with links across the Sahara to the Middle East. The Angolan Turonian is poorly understood, although one of us (Cooper I972 ) has described a condensed Turonian to Coniacian sequence yielding the ammonite genera Mammites, Proplacenticeras, Subprionocyclus, Baculites, Mesopuzosia, Hauericeras, Puebloites, Gaudryceras, Anagaudryceras, Damesites, Kossmati- ceras, Subtissotia ? and Hypophylloceras. It is arguable that absence of such typically Nigerian genera as Fagesia, Bauchioceras, Nigericeras, Wrightoceras, Vascoceras and Kamerunoceras is suggestive of the development of a barrier across the proto Atlantic seaway at this date, but it is equally arguable that strata synchronous with those yielding the above taxa are absent in Angola. Equally, absence of these taxa from southern Africa cannot be used to support the idea of a barrier since Turonian strata are absent on land (Kennedy & Klinger I97x ). We would rather point to the early Turonian occurrences of Watinoceras and Mammites species throughout W. Europe, the Western Interior of the United States, Nigeria, Brazil, Morocco and Angola as indicators of a connection along the line of the Atlantic. The occurrence of Nigerian species such as Pachyvascoceras compressum (Barber) and B. bulbosum Reyment in abundance in Texas-northern Mexico (Powell i963a , b), their absence from the well known early Turonian faunas of Algeria (Pervinqui6re x9io), Tunisia (Pervinqui6re I9o7) and the Middle East

TABLE I, Summary of connections along the southern proto Atlantic during the mid to late Cretaceous

Atlantic connections to Connections to Zululand Europe and N. America Angola faunas via Cape

Barrier Early mid-Albian Douvilliceras - Oxytropidoceratid-fauna of Dombe Grande Open Seaway

Intermittent barrier ? Early Upper Albia n An~olaites - Elobiceras - Open Seaway Neokentroceras occurrences

Open Seaway Early Upper Albian 'varicosum' and auritus equivalents Open Seaway Open Seaway Latest Albian dispar Zone fauna of Egito Open Seaway ,. Open Seaway Early to middle Cenomanian Mantelliceras saxbii to Turrilites acutus faunas of Novo Redondo Open Seaway

Open Seaway Late Cenomanian Sciponoceras gracile - Metoicoceras Open Seaway whitei faunas of Sa]inas

Open Seaway from (no record in Zululand and Texas to Nigeria (lowermost Turonian - ? absent) Mozambique) Connections to Madagascar OPen Seaway MiddleTuronian to Coniacian of San Nicolau Open Seaway

(Freund & Raab 1969) may also suggest an open seaway in addition to a trans- Saharan connection, as suggested by Collignon & Lefranc (I974). In conclusion (Table I) we suggest that the distribution patterns of Cretaceous ammonites along the line of the opening southern Atlantic indicate marine connection (although not necessarily rifting) between the N. Atlantic and Cape beginning intermittently at least in the early upper Albian and extending continuously from the mid upper Albian onwards. Only during the early Turonian is the evidence equivocal, the positive evidence suggesting continued marine connection. Palaeobiogeographic, palaeomagnetic and sea floor spreading data thus appear to us to be compatible, rather than in conflict as suggested by previous workers.

ACKNOWLEDGEMENTS.We thank H. C. Klinger (Pretoria), W. A. Cobban (Denver) and W. S. McKerrow (Oxford) for discussion and criticism. Support from the Natural Environment Research Council for field work in N. Africa and N. America and from the Trustees of the Strakosh Bequest for field work in S. Africa is gratefully acknowledged.

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Received 2 September 1974. William James Kennedy, Department of Geology and Mineralogy, Parks Road, Oxford, England. Michael Robert Cooper, PO Box 6 x, Cape Town, Republic of South Africa.

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