Late Aptianeearly Albian limestones from the eastern Brazilian continental margin record the early evolution of the South Atlantic Ocean. In Tethyan and North Atlantic domains, a planktic foraminiferal turnover and organic-rich deposits related to Oceanic Anoxic Event 1b (OAE 1b) point to major oceanclimate changes through this interval. Coeval organic-rich deposits of the South Atlantic Ocean have been interpreted as the product of restricted circulation rather than attributed to a global event. However, previous investigations of the early marine phase of South Atlantic lack data from more distal facies, making correlations to global events difficult. Here, we present C, O, and Sr isotopes, elemental geochemistry, TOC and pyrolysis data, as well as a microfacies analysis of an upper Aptianelower Albian distal section from the Campos Basin (southeastern Brazil). Our focus is on the paleoenvironmental characterization of and the possible association between organic-rich deposits and major perturbations related to AptianeAlbian transition. Five microfacies associations (MA) were identified in the informal units I and III, which were deposited in the neritic region on a carbonate ramp. Organic-rich deposits were described in unit III, composed of planktic-dominated wackestones interbedded with black shales, in a distal dysoxic to anoxic environment. The carbonates 87 Sr/ 86 Sr ratios showed a drastic increase (0.7072e0.7074), interpreted as enhanced chemical weathering, supported by the increase of continental input to the top of section. This trend was accompanied by a long-term d 13 C carb negative excursion, which were assigned to the latest late Aptianeearly Albian interval of the isotope reference curves, in accordance with the described occurrence of Colomiella recta. This scenario matches those proposed for the late Aptianeearly Albian transition and OAE 1b set, as an enhanced greenhouse stage, pointing to the influence of the referred ocean-climate changes on the deposition of organic-rich deposits of the early South Atlantic Ocean. This investigation gives more evidences that these perturbations were a widespread event, as a product of broad-scale disturbances in the global carbon cycle which also controlled organic deposition and preservation on restricted settings.

The Macal! Fonnation (Albia n. Lower Turonian , Campos Basin) is very significan t for und erstanding the early history of the South Atlantic north of the Walvis Ridge/Sao Paulo Plateau system. Deposition of the unit imm ediately succeeded the Apti an evapo ritic event. and preceeded the ocea nic phase that began in post-Ear ly Turonian time s. This is probably the best known Middle Cretaceo us carbonate record in the So uth Atlantic. It consists of the Lo wer Macae (Ear ly to Middle Alb ian), fonned dominantly

An accurate biostratigraphic framework of the Cretaceous marine section of Campos Basin was established based on studíes of foraminifera, nannofossils, and palynomorphs. With the research of 77 wells it was possible to subdivide the Albian-Maastrichtian section in 11 zones of foraminifera,6 zones of nannofossils, and 7 zones of palynomorphs (the study ofwhich was restricted to the Albian-Santonían stages). Some of these zones remain still informal. Three main regional unconformities were found in the biostratigraphic sequence. These gaps correspond to the Early-Middle Cenomanían, Middle-Late Turonlan and Campanían. Less ímportant hiatuses were also found in the area.

The Cretaceous foraminifera fauna of nearly 50 wells drilled in the Campos Basin has been searched for paleoecological purposes. The paleogeographical development of the area has been reconstructed by means of maps concerning each of the stratigraphical intervals recognized. Marine sedimentation ín the area began in the Early to Middle Albian with the deposítíon of carbonates in hypersaline, shallow neritic environrnent. Later, during the Late Albian/Early Turonian, the regional bathymetric conditions became somewhat deeper (deep neritic/upper bathyal), thus allowíng the system to attain normal saliníty. During the same period the basin was submitted to an anoxíc event of worldwide extent. ln the Middle to Late Turonian, the ecological and geographícal conditions were thoroughly modified by the settlement of bathyal envíronments over wide areas of the basin. Since then, during the whole Late Cretaceous, sediments were deposited under greater depths, though the water masses remained well cxygenated and allowed a textulariid benthíc fauna thrive in the area. l-RJS-ñ l_IIJS_30 '"

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 sea-level variations in the Southeastern Brazilian Continental Margi n. The o ldest 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 seque nce consists on marl s and calc ilutites deposited under deeper bathymetric conditions , mostly mid to deep neritic. Such relative sea-level rise is of globa l extent and contributed to lo wering 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 con tent, interpreted as result of environme ntal fluctuation s. This sequence records anox ic eve nts and tur bidite .depositio n interlay in normal marine depos its, environmental conditions ranging from deep neritic to upper bathyal. Th e depos itional histo ry of the two basins evo lved separa tely fro m the Late Turonian throu gh the Maastri chtian. T he San tos Basin was sub ject to stro ng sed imentary influx that led to the development o f marginal and shel f facies. In its tum, co ntin uous sinkage o f the Campos Basin allowed oceanic cond itions to deve lop in the stud ied area during this time. T he sedimentary reco rd for this phase includes two sequences , separa ted by an interven ing unconfonnity due to the lack of Campanian deposits. RESUMO A AR EA SUDESTE DA MAR GEM CONT INENTAL BRA SILEIRA: S UA EVO LUl;:AO DURANTE 0 CRETAcEO MllDIO E O S UPE RIOR S EG UNDO INDlCAM OS DADOS PALEOECOL6GICOS. Investiga cces bioestretigrdfices e paleoecol6 gicas Ievadas a efeito em sedimentos albianos-maastrichtianos das beclas de Cam pos e Santos pennitiram conhecer a evo lucao paleoamb iental de porc jo sudeste da mar gem continental brasileira. Cinco seqd enclas deposicio nais foram reconhecld as e seus distintos paleoambientes mapeedos. Co m isso, foi posstvel reco nstruir a paleogeografi a da reg iao ao final de cada sequencia , pro piciando inferencies quanta ao com porta mento do nfvel do mar na Mea sudeste da mar gem continenta l brasil eira. Os pri meiros f6sseis mari nhos des becias de Campos e San tos ocorrem no espesso pecote cerbonatico EolMes o-Albiano e sao pro prios de ambientes de plataforma rasa submetidos a condlcoes de fundo o xigenados e hipersalinos. A sequencia segu inte , de idade neoe...

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THE SEDIMENTARY CYCLES AND THEIR MEANING IN THE DEVELOPMENT OF THE ALBIAN -Different sedimentary cycle's scales detected in geophysical profiles helped to detail the stratigraphic and the genetic interpretation of Albian carbonates of the Campos Basin, offshore of Brazil. The study has defined ten Large Composed Sequence Scale (SCGE's), involving 1.4 to 0.3 My periods, and more than seventy Small Composed Sequence Scale (SCPE's) of shorter periods. The halocinetic control is shown as most effective factor in the creation of space for the third order cyclotems. On the order hand, orbital parameters constrain the modulation of the faciologic succession of the allostratigraphic units of higher frequencies, being short eccentricity orbital cycles (1OOka of duration) the best developed. Based on wire-line log patterns of the SCGE's, it was possible to distinguish variations in the cyclothems formed in flat carbonate platform environments from those of deeper waters. It was recognized a difference in one order of magnitude in the accumulation rates between these two depositional environments. As consequence of the high resolution stratigraphy obtained, the detection of hiatus in the succession became more obvious, this will help in improove the precision of the geologic record. *ka-lOOO yearsResumo A analise de ciclos de diferentes escalas, marcados nos perfis geofisicos, pode auxiliar no detalhamento estratigrafico e na interpretacao genetica do padrao estratal. Apreciacao desta natureza, realizada nos carbonatos albianos da Bacia de Campos, possibilitou a identificacao de dez sequencias compostas de grande-escala (SCGE's), envolvendo periodos de 1,4 a 0,3 Ma., e mais de sete dezenas de sequencias compostas de pequena-escala (SCPE's). 0 controle halocinetico, superimposto asubsidencia termica, apresenta-se como 0 fator mais relevante na criacao de espaco para a sedimentacao dos ciclotemas de terceira ordem. Os fenomenos orbitais cumpriram urn papel significativo na modulacao da sucessao faciologica das unidades aloestratigraficas de mais alta frequencia, destacando-se aqueles associados aexcentricidade curta (lOOka* de duracao). o comportamento das curvas geofisicas nas SCGE's permitiu distinguir variacoes entre os'ciclotemas formados em ambientes de plataforma carbonatica rasa e aqueles acumulados em locais mais profundos. Reconheceu-se a diferenca de aproximadamente uma ordem de grandeza nas taxas de acumulacees entre os dois ambientes deposicionais. Com 0 detalhamento estratigrafico, os hiatos tornaram-se mais evidentes, contribuindo para ampliar a precisao da correlacao geologica. *ka-lOOO anos INTRODU<;AOSequencias estratigraficas podem apresentar padroes repetitivos, resultantes de processos sedimentares locais, designados de autociclos, ou refletirem mudancas climaticas ou hidrograficas regionais, caracterizando os alociclos (Fischer, 1986). Desde 0 final do seculo XIX, Gilbert (apud Fischer, op. cit.) percebeu que unidades lito16gicas, compostas por alociclos regulares preservavam os sinais precessio...