55 kauffman & bengtson 1985 - cretaceous research

8/13/2019 55 Kauffman & Bengtson 1985 - Cretaceous Research

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Cret aceous Resear ch (1985) 6 3 1 -3 15

Mid-Cretaceous Inoceramids from Sergipe Brazil:a Progress Report

The Cenomanian-Coniacian Cotinguiba Formation of the Sergipe Basin innortheastern Brazil (Figure 1) contains the largest and most diverse as-semblage of mid-Cretaceous Inoceramidae (Bivalvia) yet known from theSouthern Hemisphere. Preliminary studies of material collected suggest thatat least 5 late Cenomanian, 25 Turonian and 29 early Coniacian species andsubspecies of this biostratigraphically important family are present in theformation. We present here a status report of a long-term project on theBrazilian inoceramids initiated in 1970 at Uppsala University. The project iscurrently focused on taxonomic (E.G.K.) and biostratigraphical (E.G.K.P.B.) studies of the Sergipe inoceramids; in addition, more detailed biostrati-graphical work based on bed-by-bed collecting from the Retiro area inSergipe, and revision of mid-Cretaceous inoceramid material in Brazilianinstitutions, are being carried out by M. H. R. Hessel as part of a Ph.D.project at Uppsala University. For details of the geology of the SergipeBasin, with notes on the faunas of the Cotinguiba Formation, the reader isreferred to Bengtson (1983).

The great majority of the inoceramids of the Sergipe Basin are cosmo-politan taxa that are best known from localities throughout the Euramerican

ergipe-Alagoas

mamu BasinAlmada Basin

Figure 1. Continental margin of northeastern Brazil (Nordeste), with Cretaceous sedimentary basins.Location of the Sergipe Basin is indicated by arrow. Abbreviations of state names: MA= MaranhBo,PI = Piaui, CE = Cearh, RN = Rio Grande do Norte, PB = Paraiba, PE = Pernambuco, AL = Alagoas,SE = Sergipe, BA = Bahia.

0195-6671/85/030311+05 03.00/O : 1985 Academic Press Inc. (London) Limited


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Region of the North Temperate Realm (sensu Kauffman, 1973; 1975), butwhich are now turning up in various areas of Africa and South America.They are less commonly mixed with typical Tethyan elements of theCaribbean and Indo-Mediterranean provinces. Many of these species rangeinto the Pacific (Kauffman, 1977a). The Sergipe collections also containsome new taxa, including a Sphenoceramus fauna from the early Turonian(Hessel, 1984; in press); this genus was previously known only fromConiacian-Santonian strata in the North Temperate Realm. There is anexceptional succession of the largely endemic inoceramid Sergipia Maury,depicting its evolution into the cosmopolitan latest Turonian-earlyConiacian bivalve Didymotis.

The Inoceramidae are becoming increasingly important in Cretaceousbiostratigraphy; they have many of the evolutionary and biogeographicalqualities of ammonites and planktonic foraminifers-the present bases formost global marine biostratigraphical systems. The occurrence of diverse,mainly cosmopolitan inoceramid assemblages, closely associated with am-monites in the Sergipe Basin (Bengtson, 1983), is of considerable biostrati-graphical importance. Together, these groups provide a basis for detailedbiostratigraphical zonation of the middle Cretaceous in the western SouthAtlantic. They further allow detailed regional correlation of geological andbiological events in northeastern Brazil with those in Euramerica and Africaand, to a lesser extent, in the Pacific. The Brazilian occurrence of these taxademonstrates the great dispersal potential of inoceramids at the species level.

Co-occurring species of inoceramids, representing one or more lineages,are characteristic of most levels in the Cotinguiba Formation. Lineages aredefined around groups of derived, closely similar species and subspecies.Thus, principal inoceramids of the upper Cenomanian are the I noceramus(I.) pictus J. de C. Sowerby and I . (I .) tenuistriatus Nagao Matsumotolineages. In the lower Turonian the M ytiloides mytil oides (Mantell), theM . hercynicus (Petrascheck), and the M. Zatus (Mantell, Sense Hattin, 1962)lineages predominate, co-occurring with largely endemic species of Sergipia.The presence of these inoceramids marks the peak of the globalCenomanian transgression (T6 of Kauffman, 19773) during M . mytiloides

Zone time.Both inoceramid and ammonite studies suggest depositional breaks in the

mid-Turonian and beginning of the late Turonian in the Sergipe Basin; thisis confirmed by the presence of numerous discontinuity surfaces throughoutthis part of the sequence. This interval of time represents a common level ofdisconformity and omission in Cretaceous sequences throughout the world.It seems to reflect global eustatic fall (cf. Hancock Kauffman, 1979, Figure5) and regression of epicontinental and shelf margin seas (R6 of Kauffmans(1973, 19773) eustatic curve) between global transgressive maxima in theearly Turonian (ibid., T6) and early Coniacian (ibid., T7), although there is

no conclusive evidence for regression with emergence in Sergipe. The rareoccurrence of M ytiloides hercynicus (Petrascheck) and early members of theI noceramus (I .) cuvieri J. Sowerby lineage suggests that basal middleTuronian strata (sensu Kauffman et al., 1978) are locally present in Sergipe.

Common upper Turonian inoceramid groups include the I noceramus(I.) apicalis Woods lineage, the I . perpl exus Whitfield-I. vancouverensis


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(sensu Troger, 1967; non Shumard) lineage, and the Mytiloides striatocon-centricus (Gumbel) lineage. The Turonian-Coniacian boundary beds

and the lower Coniacian show the highest diversity of inoceramid bivalves,including members of the M . stri atoconcentr i cus (Gumbel) lineage, the M .dresdensis (Troger) lineage, the M . egei (Troger) lineage, the M . Zusati ae(Andert) lineage, the I. vancouverensis and I. winkholdioides Andert lineages,the I. rotundatus Fiege lineage, and the Cremnoceram us? w al t ersdor fensi s(Andert) lineage. The bivalve Didymotis is a common associate. Additionalspecies identified from the Cotinguiba Formation are listed by Bengtson(1983, pp. 4447); to this can be added the early Turonian species Spheno-ceramus mauryae Hessel and S. alatus Hessel recently described (Hessel, inpress).

In most cases, the inoceramid material of the Sergipe Basin is wellpreserved as internal or composite moulds showing sculpture and evenmuscle insertion areas. Prismatic shell material is mainly preserved in thehinge area (ligamental plate); all aragonite has been lost or altered to calcite,probably early in diagenesis, to allow composite moulds to form duringsubsequent compaction. Good preservation, abundance of specimens atmost stratigraphical levels, and exceptional representation among certainlineages that are poorly known elsewhere in the world are all factors thatemphasise the importance of systematic treatment of the Brazilianinoceramid fauna. In particular, much can be learned about the evolutionaryhistory of the M yt i loi des fi egei (Troger), Cremnoceram us? w al t ersdor fensi s(Andert), and Sergipiu-Didymotis lineages. All of these are common at manylocalities, and are presumed to be continuously represented in the strati-graphical sequence of Sergipe; in contrast, disjunct stratigraphical repre-sentation characterizes occurrences of these lineages in many other parts ofthe world. Of the new taxa noted in the Sergipe collections, a few appear to beendemic to the area; most others are known elsewhere, but unfortunatelyremain undescribed.

The inoceramid assemblages have been matched against the currentammonite zonation for the Sergipe Basin based on morphologically distinc-tive genera (Bengtson, 1983) and placed in stratigraphical order, for thepurpose of deriving an inoceramid biostratigraphy and correlating it with thedetailed zonation of North America (Kauffman et al ., 1978) and other areas.Despite the fact that most of the narrowly distributed zonal inoceramids ofthe North American upper Cenomanian-lower Coniacian sequence arepresent in the Sergipe Basin, it is not yet possible to create an equally refinedbiostratigraphy based on the Brazilian inoceramids. The reason for this lies:(1) in the nature of the outcrops, where small quarries and road cuttingsnormally expose only up to a few metres of sections; (2) in the difficulty ofprecisely correlating these exposures due to lack of continuous outcrop,locally variable attitude of the strata, and relative lithological homogeneity of

the rocks; and (3) in initial collecting procedures, in which most sections haveyielded only a small number of specimens. Quarries, which have the highestbiostratigraphical potential, have in many cases yielded large collections ofinoceramids; however, since the material was collected chiefly by quarryworkers, the relative position in sequence of the specimens is not known.Understandably, many quarries in Sergipe yield an assemblage of


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inoceramids that represent two or more zones in North America. Neverthe-less, the assemblage zonation which is being worked out for Sergipe providesa useful biostratigraphical framework for correlation of this importantCretaceous area.

A logical next step in the biostratigraphical work will be to refine theexisting zonation through the study of successive inoceramid assemblagesobtained by careful bed-by-bed collecting of key sections that have beendistinguished during the course of the present work. For example, the thickupper Turonian-lowermost Coniacian sequence in Sergipe, without obviousbreaks or non-sequences, is a promising object for detailed biostratigraphicalstudies of the Turonian-Coniacian stage boundary. In comparison with therecently proposed international standard section in Lower Saxony (Woodet al., 1984), the Sergipe sequence has the advantage of yielding bothinoceramids and ammonites and is thus a potential reference section for thestage boundary in the South Temperate Realm.

cknowledgments

The work on the Sergipe inoceramids is part of a research programme (P.B.)financed by the Swedish Natural Science Research Council (NFR). Thiscommunication is a contribution to the IGCP project Mid-CretaceousEvents.

References

Bengtson, P. 1983. The Cenomanian-Coniacian of the Sergipe Basin, Brazil. Fossils and Strata 12, l-78,1 map.

Hancock, J. M Kauffman, E. G. 1979. The great transgressions of the Late Cretaceous.j rnal of heGeological Society (of L ondon) 136 (2), 175186.

Hattin, DrE. 1962. Stratigraphy of the Carlile Shale (Upper Cretaceous) in Kansas. Kansas GeologicalSurvey Bul letin 156, 1-155.

Hessel, M. H. R. 1984. Inoceramideos brasileiros: passado, presente e futuro. Anais do XXXI ZZCongress0 Brasileir o de Geologia [Rio de Janeiro, RJ] 2, 6066614. Sociedade Brasileira de Geologia,Sao Paulo, SP.

Hessel, M. H. R., in press. Alguns inoceramideos (Bivalvia) radialmente ondulados do Turoniano inferiorde Sergipe. VI I I Congress0 Br asileir o de Paleontologia [Rio de Janeiro, RJ, 19831, DepartamentoNational da Produ@o M ineral, Geologia-Segao Paleontologia e Estratigrafi a 2.

Kauffman, E. G. 1973. Cretaceous Bivalvia. In Atl as of Palaeobiogeography, (Ed. A. Hallam); Elsevier:Amsterdam, 3533383.

Kauffman, E. G. 1975. Dispersal and biostratigraphic potential of Cretaceous benthonic Bivalvia in theWestern Interior. In The Cretaceous System in the Western I nteri or of Nor th Ameri ca, (Ed. W. G. E.Caldwell) Geological Association of Canada, Special Paper 13, 1633194.

Kauffman, E. G. 1977a. Systematic, biostratigraphic, and biogeographic relationships between middleCretaceous Euramerican and North Pacific Inoceramidae. Palaeontol ogical Society ofJ apan, SpecialPapers 21,1699212.

Kauffman, E. G. 19773. Geological and biological overview: Western Interior Cretaceous Basin. InCretaceous facies, faun as, and paleoenvir onments across the Western nterior Basin, (Ed. E. G.Kauffman); M ountain Geologist 14 (3- ), 75-99.

Kauffman, E. G., Cobban, W. A. Either, D. L. 1978. Albian through lower Coniacian strata,biostratigraphy, and principal events, Western Interior United States. Annal es du M useum d H istoireNaturelle de Nice 4 [for 19761, XXIII.l-XXIII.52.

Troger, K.-A. 1967. Zur Palaontologie, Biostratigraphie und faziellen Ausbildung der unterenOberkreide (Cenoman bis Turon). Tel I: Paliontologie und Biostratigraphie der Inoceramen desCenomans bis Turons. Abhandlu ngen des Staatlichen M useums fur M ineralogie und Geologic zuDresden 12, 133207.


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Wood, C. J., Ernst, G. & Rasemann, G. 1984. The Turonian-Coniacian stage boundary in Lower Saxony(Germany) and adjacent areas: the Salzgitter-Salder Quarry as a proposed international standardsection. Bul letin of he Geological Society of Denmar k 33 (l-2), 225-238.

Erle G. KauffmanDepar tment of Geological Sciences

University of Color adoCampus Box 250

Boulder, CO 80309U.S.A.

Peter BengtsonPaleontologiska museet

Box 558S-751 22 Uppsala

Sweden

55 kauffman & bengtson 1985 - cretaceous research

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