Marta Cláudia Viviers

The southeastern area of the brazilian continental margim: its evolution during the Middle and Late Cretaceous as indicated by paleoecological data

Biostratigraphic and paleoecologic investigations of Alb ian through Maastrichtian sediments in the Santos and Campos basins made possible a preliminary reconstruction of the paleoenvironmental evolution of the southeastern Brazilian continental margin . Five major Albian-Maastrichtian depositional sequences have been recognized in the Campos and Santos basins, and their paleoenvironmental maps drawn. As result, it was possible to reconstruct the regional paleogeography of the area at the close of eac h sequence. This procedure allowed inferences concerning sealevel variations in the Southeastern Brazilian Continental Margi n. The oldest marine fossil reco rd in the Campos and Santos basins is found in thick LowerlMiddle Albian carbonate layers. These sediments were deposited in a shal low neritic environme nt with hypersaline and oxygen-rich bottom waters. The Upper Alb ian sequence consists on marls and calc ilutites deposited under deeper bathymetric conditions , mostly mid to deep neritic. Such relative sea-level rise is of global extent and contributed to lowering the salinity of the waters then covering the southeaste rn portion of the Brazilian continenta l margin. T he Uppe r Albian carbonatic seq uence is overlain by and Upper CenomanianlMidd le Turonian silicic lastic sequence with varied faunal content, interpreted as result of environme ntal fluctuations. This sequence records anox ic events and turbidite .deposition interlay in normal marine depos its, environmental conditions ranging from deep neritic to upper bathyal. The depos itional history of the two basins evo lved separa tely from the Late Turonian throu gh the Maastri chtian. T he San tos Basin was sub ject to strong sed imentary influx that led to the development o f marginal and shel f facies. In its tum, co ntinuous sinkage of the Campos Basin allowed oceanic cond itions to deve lop in the studied area during this time. T he sedimentary record for this phase includes two sequences , separa ted by an interven ing unconfonnity due to the lack ofCampanian deposits. RESUMO A AREA SUDESTE DA MARGEM CONTINENTAL BRA SILEIRA: SUA EVOLUl;:AO DURANTE 0 CRETAcEO MllDIO E O SUPE RIOR SEG UNDO INDlCAM OS DADOS PALEOECOL6GICOS. Investiga cces bioestretigrdfices e paleoecol6gicas Ievadas a efeito em sedimentos albianos-maastrichtianos das beclas de Cam pos e Santos pennitiram conhecer a evolucao paleoambiental de porc jo sudeste da margem continental brasileira . Cinco seqd enclas deposicio nais foram reconhecldas e seus distintos paleoambientes mapeedos. Com isso, foi posstvel reco nstruir a paleogeografi a da reg iao ao final de cada sequencia , propiciando inferencies quanta ao comportamento do nfvel do mar na Mea sudeste da margem continenta l brasil eira. Os primeiros f6sseis mari nhos des becias de Campos e San tos ocorrem no espesso pecote cerbonatico EolMesoAlbiano e sao pro prios de ambientes de plataforma rasa submetidos a condlcoes de fundo oxigenados e hipersalinos. A sequencia segu inte , de idade neoelbiane, consiste em margas e calcilutitos depo sitad os em ambientes mais profundos (Nertti co medic a profundo). Essa elevacdo relativa do nfvel do mar teve expressac global e contribuiu para diminui r a salinidade das aguas do Atlantico SuI. Sobrejecente aos carbonatos albianos ocorre a seqiiancia siliciclas tica cenomantana/meso ruroniana, exibindo urn conteudo microfossilf fero veriado, que expressa com nitidez a instabilidade ambie ntal daquele perfodo. Eve ntos an6xicos , epis6dios turbidfticos e perfodos de condit;Oes mari nhas normals ficaram registrados nas camadas dessa un idade cro noesrrat igrafica. A hist6ria deposicional das duas bacias evo luiu de fonna diferen te a partir do Neoturoniano a~ 0 Maastrichtia no. Neste perfodo, a Bacia de San tos foi submetida a urn forte infl uxo sedimentar, 0 que propicio u 0 desenvolvime nto e a expansjo de facies marginais e de plataforma. Enquanto isso, em Campos. desenvolviam-se condicoes betimetricas cada vez mais profundas em razio, principalmente, do baixo aporte sedimentar que lhe era destinado. INTRODUCTION The Campos and Santos basins lie along Ihe East coast of Brazil. They represe nt two examples of passive marginal continental basins. whose sedimentary columns allow the reconstruction of the aspects of installation and evolution of the South Atlantic during the Mesozoic Era . This work integrates the resul ts obtained from biostra tigraphic and paleoeco logic studies done by various authors (Koutsoukos 1984, 1987, Viviers 1986, 1987, Viviers et al. 1986, Dias-B rito & Azevedo 1986, Richter 1987). It presen ts a synthetized version of the evolution of the SE portion of the Brazilian continental margin. The paleogeographic interpretations are shown in the form of maps that indicate changes occurred in these two basins from the Albian to the Maastrichtian. LOCATION OF THE A REA AND METHODOLOG Y The investigated area is bounded by the parallels 21° and 28° South, and the meridians 40° and 48° West. Altogether 103 wells have been selected along a NE-SW belt , aproximately 200,000 km in area, that borders on the cont inental margin of the Campos and Santos basins (Fig. I). Most of wells are concentrated in the Campos Basin, although the Santos Basin is considerably more extensive. The biostratigraphic stud ies upon which this work is based involve plankto nic foraminifera, calcareous nannoplankton, calcispherulids -and palynomorphs recovered from well cuttings and cores. The paleoecological interpretations were mainly based on generic composition of the benthic foraminifera assemblages from each sample. The planktonic foraminifera, metazoans, radiolarians, palynomorphs, and calcareous algae furnished the secondary elements for this type of analysis. Once paleobathymet ric curves were constructed for each well, it was possible to obtain paleoenvironmental maps which represent the most important stra tigraphic horizons for the understanding of the evolution of the area. The integration of the zonations based upon marine and nonmarine organisms allowed more detailed investigation of the lateral variations of the biota in response to environmental changes. Interpretation of electric logs and seismostratigraphic • Centro de Pesquisa da Petrobras (Cenpes), Setor de Bioestratigrafia e Paleoecologia, Cidade Universitaria , Quadra 7, Predio 20, llha do Pundao, CE P 2 J9 1O, Rio de Janeiro , RJ, Brasil 292 Revista Brasueim de Geociencias, Volume 18, 1988 Figure J Campos and Santos basins. Location map From a chronostratigraphic viewpoint. a uniform detailed study of the stages was not possible. The Albian, Turonian. Coniacian, and Maastrichtian were rather finely sliced, but not the Cenomanian, Santonian, and Campanian. The obtained results show that distinctive hiatuses are present in section. These discontinuites were mainly indicated by absence of biozones, although in certain cases lithologic, seismic or paleoenvi ronmental evidence may be helpfu l in their detection. In this manner , for instance, the boundary betwee n the Lower/Middle Albian sequence and the upper Albian sequence is shown to be of diastemic nature. Anot her hiatus, apparently of regional exte nt, is indicated by the regional scarcity of planktonic foraminifera and nannofossils of Early to Middle Cenomanian age. Azevedo et al. (op . cit.} discussed the hypothesis of the non-development of these organisms being due to ecologic constraints. but finally concluded that this section is actually absent in the Campos Basin. This gap was recogn ized in the wells of the Deep Sea Drilling Project (DSDP) in the South Atlantic, as shown by Magniez-Janin & MUUer (1987). The Middle Cretaceous also includes a third discontinuity, which is restricted to the Campos Basin. and indicated by the scarcity of the Middle to Late Turonian sediments. It is only in the Santos Basin that species of planktonic foraminifera of this age are fairly common. such as Praeglobotruncana helvetica. P. gibba, Dicarinella hagni and D. algeriana . The Upper Cretaceous also displays important hiatuses determined by the rare occurre nce of Campanian deposits in the Campos Basin, and Late Maastri chtian sediments in the Santos Basin. This diachronism displayed by the grea t unconformities partial ly reflects the separate geologic history of the two basins after the Turonian. Erosive event s related to carving of canyons or to the uplift of specific areas have also given rise to discont inuites of varied magnitude and stratigraphic position in some parts of the two basins. PALEOENVIRONMENTAL EVOLUTION The investigated sedimentary section overlies the barren evapori tes of the Alagoas Stage (local stage of Early Cretaceo us age) which were deposited during early phases of the South Atlantic evolution. Evidence of oceanic expansion during the Middle to Late Cre taceous is provided by carbon atic and siliciclastic units. This sedimentary section, accumulated during approx imately 40 Ma, reveals marked paleoenvironmental alterations recognizable in micropaleontologic studies as well as in the form of variations in sedimentary facies and depos itional systems. In general, they reflect the evolution of an oceanic margin of passive type , subject to the influence of tectonics and relative variations of the sea-level of either local or global extent. World-wide biologic and geochemical events were also recorded in the section. The sedimentary column in the Campos and Santos basins is subdivided into five sequences: Lower/ Middle Albian, Upper Albian, Cenomanian/Middle Turonian, Upper Turonian/Lower Campanian , and Upper Campanian/ Maastri chtian . Each of these mcgasequences is bounded by widespread uncon formities, the corresponding hiatuses being estimated on basis of biozones absent in the section. The magni tude of the hiatus implied by any unconformity is not necessarily equal in both basins. This diachronism can be mainly ascribed to another geologic causes such as the

New age constraints on Aptian evaporites and carbonates from the South Atlantic: Implications for Oceanic Anoxic Event 1a

High-resolution carbon isotope (δ 13 C) profiles from shallow- and deep-water carbonates in the South Atlantic (Campos and Santos Basins) are here correlated to stratigraphically well calibrated Tethyan sections, constraining the end of major evaporite deposition in the South Atlantic to the early Aptian Oceanic Anoxic Event (OAE) 1a interval. The unusually extensive evaporite deposition would have reduced the global dissolved sulfate inventory, possibly increasing global preservation of organic matter by decreasing sulfate reduction; this could explain the coincidence in timing between OAE 1a and the dramatic negative sulfur isotope excursion over this interval. Therefore, in addition to the coeval eruption of the Ontong Java Plateau, the opening of the South Atlantic may have played an important role in the genesis and character of OAE 1a.