The Macae Formation, Campos Basin, Brazil
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Revista Brasi leira de Geociencias 18(3):261. 272. setembro de 1988 TH E MACAE FORMATION, CAM POS BASIN, BRAZIL: ITS EVOLUTION IN THE CONTEXT OF THE INITIAL HISTORY OF THE SOUTH ATLANTIC ADALI RICARDO SPADINI', }'ERNANDO ROBERTO ESTEVES", DI MAS D1AS.BRITO·, RICARDO LATGE MILWARD AZEVEDO', and RENE RODRIGUES' ABSTRACT The Macal! Fonnation (Albian. Lower Turonian, Campos Basin) is very significan t for understanding the early history of the South Atlantic north of the Walvis Ridge/Sao Paulo Plateau system. Deposition of the unit imm ediately succeeded the Aptian evaporitic event. and preceeded the oceanic phase that began in post- Ear ly Turonian times. This is probably the best known Middle Cretaceo us carbonate record in the South Atlantic. It consists of the Lower Macae (Early to Middle Albian), fonned dominantly by limestones deposited mostly in shallow neriti c environment, and the Upper Maca6 (Late Albian-Early Turonian) with calcil utites overlain by marls and shales, deposited duri ng progressive sea level rise. A tidal flat system was devel oped in the southern portion of the basin during depos ition of the lowermost Maca6 Formation, with supratidal, intertidal, and lagoonal environments. Burrowed carbonate mudstones were deposited in shallow shef environments , and oo ids and peloids in a high energy offs hore shoal system. Hypersalinity prevai1led during that time, as suggested by the low biotic diversity and the carbon isotop e values. Very negative oxygen isotope values indicate that surface water temperatures were high. Oncolite limestones and subordinate calcilutites prevail in the middle and upper parts of the Low er Macae, chiefly deposited in shallow neriti c environment. Hypersalinity and high water temperature persisted, as indicated by paleoecological and isotopi c studies. The Maca6 sea was then a juvenile ecosystem, unstab le and poor in nutrients, with maximum depth values arou nd 50 m, Th e Upper Maca6 carbonate mudstone s. Late Albian in age. are rich in calcispberulids, Its abundant pelagic biota probably reflects desalinization and increase in nutrients of surface waters related to the South Atlantic expansion and consequent increase in water volume and circulation. Th e benthic foraminifera are suggestive ofdeep neriti c environments (100 ·200 m). "At the end of the latest Albia n, a rhythmic deposition or calcilutites and marl s was probably caused by climatic variations (dry/warm to warm/wet climate cycles). From the Cenomanian to the Turonian, already under a wannlwet climate. were deposited the fine- grained siliciclastics, mos tly in upper bathyal environment; episodic anoxi c conditio ns are represented by lamin ated black shales, freq uently rich in radio laria, and with a relatively high organic carbon content". RESUMO A FOR/o1Al;AO MACA~ , BACIA DE CAMPOS. BRASIL: SUA EVOLUl;AO NO CONT EXTO DA HISTORIA INICIAL DO ATLANTICO SUL. A Formacao Macae (Albiano-Ecruronia no, Bacia de Campos) 6 muito significativa para 0 entendimento da hist6ria inicial do Atlant ico Sui ao norte do sistema Walvis Ridge-Plate de Sao Paulo. Sua deposicao sucedeu imediatamente ao evento eveporltico ai?tiano e precedeu a rase oceanica, que comecou em tempos pcs-eoturonianos. Ela l!. prov avelmente, 0 re grstrc cerbonanco mesocretscico mars bern conhecid o em todo 0 Atl1ntico SuI. E composta pelo Macal! In ferior (Eo- a Mesoalbiano), constitufdo dominantemente de calcarios, depositados sobretudoem meicnerftico . rasa , e pelo Maca6 Superior (Neoalbiano-Boturoniano), com calcilutitos sobrepostos por mar~ e fclhe Ihos, acumul ados em resposta a eventos transgressivos. Durante a deposicao da parte mats inferior da Per malj3.o Macae , urn sistema de planicies de marl! se desenvolveu a sui da bacia, com ambientes de supramare, Intermare e lagunas. Calcilutitos bioturbados precncheram frens ealmas de ambiente nerftico rasa enquanto ocides e pel6ides se acumularam em sistemas de bancos de alta energia desenv olvidos em freas distais. A baixa diversldade bi6 tica e os valores de isotopes de carbono sugerem que, naquele tempo, 0 eorpo mari nho era hipe:rsalino. Valores mui to negatives de isotopes de oxi~en io indicam <J.ue as ternperaturas das dguas su perficiais eram alias. Calcarios oncolCticos, sobretudo, e calcilutitos predommam nas partes media e super ior do Macae In ferior, tendo sido depositados em ambiente nerCticoraso. 0 corpo d'agua continuava hipersalino e quente, como sugerem os estudo s paleoecol6gicos e isot6picos. 0 Mar Macae era entao urn eeossistema ju venil, instdvel e pobre em nutri entes, com profundidades maxlmas em torno de 50 m. As roch es carboaaticas do Macae Supenor consistem em calcilutitos rico s em calcisferu1fdeos. Sua abund ante biota pelagica pro ve velmente reflete a dessalini zacac e 0 incremento de nutriente s nas liguas superficiais do corpo marinho re sultantes de importante expansao do Atl1ntico Sui ocorrida no Neoalbi ano. Uma melhor ctrcuiecso de dguas passou a ccorrer na bacia, cuja batimetri a ml!dia oscilava entre 100 e 200 m," Ao final do Albiano terminal ccorreu uma deposicjo rftmica de calcilutitos e margas, causada, provavelmente, por varialjOescllmatices altemantes (cic1os quentelseco e quentelumido). A partir do Cenomaniano , j4 sob clima quente e dmido, Io ram acumulados sedimentos sillcicldsricos Ilnos, em ambientes batia l superior (200·500 m); condl czes epi s6dicas de ano xia ocorre ram oeste intervale de tempo, 0 que l!atestado pela presence de folhelhos negro s, laminados, njo biorurbados , frequentemente rices em radiolarios, e relativamente ricos emc erbonoorgjlnico", I NTRODUCTION The Albian carbonate sequences of hydrocarbons from structural-stratigraphic traps in six oil the eastern Brazilian margin contain the first typical records fields. of the South Atlantic Ocean. Among all these carbonate The main purpose of this work is to characterize the sequences, the Macae Formation of the Campos Basin is the depositional history of the Macae Fonnation in order to one that pre sents of greatest number of data, recovered from contribute to the global understanding of the initial con more than 300 wells, which provide valuable infonnation for text of the South Atlantic. The investigation was based the understanding of the South Atlantic evolution. The on sedimentological, biostratigraphic, paleoecological, and Macae Formation is economically important, as it produces geochemical data . The results summarizes the effort of many ... Centro de Pesquisas e Desenvol vimento da Petrob rds (Ccnpes), Cidade Unlverslrdria, llh a do Fundao, quadra 07, CEP 21910, Rio de Janeiro, RJ. Brasil •• Departamento de Exploracdc da Pctrobn'is (Depex), Av , Chile, 65, 132 andar, CEP 2003 1, Rio de Janeiro , RJ. Brasil 262 Revista Brasdeira de Geocunctas, Volume 18, J988 people in Petrob ras since the beginning of exploration in the Campos Basin. Previous detailed facies analysis of the Macae carbonates have been documented by several auth ors, following the pioneer discovery of Garoupa Field in 1974, the first oil field in the Campos Basin. Tessari & Tibana (1975) defin ed the general facies framework, establishing a generic depositional evolution . Carozzi et al. (1977) defined a diagenetic. depositional model for the Macae Formation. Falkenhcin (1981) concentrated on the general tecto-sedimentary setting, the identification of microfacies and environmental interpretation. Beltrami (1982), using wells and seismic data, established a region al distribution of the carbonate high energy facies along NE-SW structural trend . The siliciclastic section was studied by Guardado et al. (1983). Esteves et al. (1987) proposed a general sedimentary evolution model for the Lower Macae, Guardado & Spadini (1987) established a depositional model relating previous rift struc tures and salt tectonics with the facies distribution. Detailed reservoir studies were performed by several authors (Franke 1981, Baumgarten et al. 1982, Spadini & Paumer 1983, Baumgarten et al. 1984, Paumer 1984, Scuta 1984, Franz 1987). Strong efforts in the biostra tigraphic research have been made in the Campos Basin since the first oil discover y in the Macae carbonate reservoirs (Gomide 1976, Uesugui 1976, Figure I - Location map ofthe Campos Basin Dias -Brito 1982, Gomide 1982, Dias-Brito 1985a, Moura & Praca 1985, Azevedo et al. 1987a, Shimabukuro et al. 1985, Richter 1985, 1987, Shimabukuro 1988, Moura 1988, by Schaller (1973). It comprises alluvial-lacustrine sediments Antunes 1988). Gomide (1982), based on calcareous of the rift phase, the evaporites of the transitional phase, and nanoplankton, defined an Albian/Cenomanian age for the carbonates and siliciclastics of the pre -oceanic and oceanic entire Macae sequence, reco gnizing only one biozone phases (Fig. 2). The Macae Forma tion, Albian to Early {Nannoconus truitti}. Uesugui (op. cit.). based on Turonian in age, characte rizes the deposition during the palynomorphs, established a sequence of thr ee informal pre-oceanic phase (Fig. 3), overlying the evapo rites of the biounits: the a/fa interval defined as probably Early Albian in Lagoa Feia Forma tion. The relationship between the age; the beta inter val defined as Middle Albian in age; and the biochronostratigraphic and lithostratigraphic units of the ganvna interval as Late Albian/Cenomanian in age. Macae Formatio n is showed in the figure 4. Dies-Brito (1985a), studying calcispherulids of the The Macae Forma tion is informally divided into Lowe r fine- grained limestones, 'defined two informal biozones: