Facies Analysis of the Codó Formation (Late Aptian) in the Grajaú Area, Southern São Luís-Grajaú Basin
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Anais da Academia Brasileira de Ciências (2004) 76(4): 791-806 (Annals of the Brazilian Academy of Sciences) ISSN 0001-3765 www.scielo.br/aabc Facies analysis of the Codó Formation (Late Aptian) in the Grajaú Area, Southern São Luís-Grajaú Basin DILCE F. ROSSETTI1, JACKSON D.S. PAZ2 and ANA M. GÓES2 1Museu Paraense Emílio Goeldi, Coordenação de Pesquisa e Pós-Graduação Cx. Postal 399, Av. Perimetral, 1901, Terra Firme, 66077-530 Belém, PA, Brasil 2Universidade Federal do Pará, Centro de Geociências, Campus do Guamá s/n, 66075-900 Belém, PA, Brasil Manuscript received on September 26, 2003; accepted for publication on February 19, 2004; presented by Alcides N. Sial ABSTRACT Facies descriptions of the Codó Formation in the Grajaú area are provided for the first time, and its sedimentary characteristics compared to those from the Codó area to allow paleoenvironmental reconstructions. Deposits in the Grajaú area include evaporites, limestones and argillites bearing features indicative of a shallow, low energy, subaqueous, saline environment exposed to meteoric and/or capillary conditions. Flooding- evaporative concentration-desiccation cycles suggest a saline pan complex surrounded by extensive evaporitic mudflats. The location of the system, whether coastal or inland, is a matter open for debate. However, the later hypothesis is favored considering: 1. Sr isotopic data, with values higher than those expected for Late Aptian marine waters; 2. calcitic composition of limestones (instead of dolomitic and/or magnesitic as expected in coastal settings); and 3. presence of continental ostracods and lack of marine fauna. This interpretation is consistent with that proposed for UpperAptian deposits of the Codó area, but the depositional system there was one dominated by more stable, well-stratified, anoxic waters and evaporite precipitation in central lacustrine areas, while in the Grajaú area the salt pan was more oxygenated and ephemeral, with salt precipitation mainly in marginal areas or along surrounding mudflats. Key words: evaporite, paleoenvironment reconstruction, lacustrine, Late Aptian, facies analysis, São Luís- Grajaú Basin. INTRODUCTION Basin, but outcrops exist only in its eastern and southern margins. Facies and stratigraphy of strata The Codó Formation was deposited during an exposed in the eastern margin of this basin were important moment in the evolution of the Brazilian emphasized in recent publications (e.g., Paz and Equatorial Margin, representing the only exposed Rossetti 2001, Rossetti et al. 2001). However, this record of the Late Aptian early rifting stage. De- type of study is completely lacking with respect to spite its great scientific relevance for reconstructing the rocks that occur in the southern edge of the basin. Cretaceous events in South America, this unit has The Codó Formation is well exposed in the Grajaú been only locally studied. Upper Aptian deposits area (Fig. 1A), where it consists of deposits litho- occur in subsurface throughout the São Luís-Grajaú logically similar to those from the Codó area, be- Correspondence to: Dilce de Fátima Rossetti ing represented by evaporites, laminated argillites, E-mail: [email protected] An Acad Bras Cienc (2004) 76 (4) 792 DILCE F. ROSSETTI, JACKSON D.S. PAZ and ANA M. GÓES black shales and limestones. In addition, a prelim- bian, Early-Middle Albian/ and Middle Albian/Late inary study revealed facies in the Grajaú area that Cretaceous, respectively (Rossetti 2001). The low- have not been documented for the Codó Formation ermost sequence S1 contains the Codó Formation, in the Codó area yet. The evaporite succession from which is the objective of this paper, and represents a the Grajaú area is better developed and shows a succession up to 450 m thick of sandstones, evapor- set of well-preserved primary sedimentary features ites, shales and limestones. This sequence displays not recorded in association with evaporites from the a tripartite subdivision into systems tracts (Rossetti Codó area, and which are very helpful to improve 2001), with the lowstand deposits grading south- the characterization of the depositional system pro- ward progressively from shallow marine to conti- posed for this unit. Exposures in the Codó area dis- nental (i.e., fluvial, deltaic, and lacustrine). The low- play a dominant lacustrine nature, marked by several stand deposits are overlain by strata formed in the episodes of progradation recorded by upward cycles transgressive systems tract, and consist of a wedge with gradation from central to intermediate and then of richly fossiliferous shales that pinches out to the marginal lacustrine deposits formed in a dominantly south. The highstand systems tract consists of shal- hypersaline and stratified system with a closed hy- low marine to continental deposits displaying stratal drology time to time (Paz and Rossetti 2001). Sr patterns varying upward from aggradational to pro- and S isotope data support the existence of conti- gradational, as typical for this stage of sequence de- nental deposits in this area (Rossetti et al. 2000, Paz velopment. et al. 2003). The exposures of the Grajaú area are The maximum thickness of the Codó Forma- important to analyze the possible continuity of the tion in the Grajaú Basin is 150 m (Rezende and Pam- lake system southwestward in the basin, and precise plona 1970). Its paleontological content mostly in- when a full oceanic connection developed for the cludes pollen, continental ostracod, insect, and fish, first time along the Brazilian Equatorial Margin. which are all in agreement with a dominantly la- custrine interpretation for the depositional system. Additionally, pollen has been recovered from these GEOLOGICAL FRAMEWORK deposits and allowed the establishment of a pre- The São Luís-Grajaú Basin occupies an extensive cise Late Aptian age with basis on the presence of area in the Brazilian Equatorial Margin, and is Sergipea variverrucata (Batista 1992, Lima 1982, included in the Gurupi Graben System (Azevedo Rossetti et al. 2001). The Codó Formation either 1991), formed by simple shear and strike-slip mo- grades downward into fluvial and deltaic deposits of tions associated with the opening of the Equatorial the Grajaú Formation (e.g., Mesner and Wooldridge Atlantic Ocean during the Early Cretaceous. Sub- 1964, Aguiar 1971, Lima and Leite 1978) or sharply surface and surface data show that faulting was im- overlies an unconformity over older Paleozoic and portant particularly during the Early/Middle Albian, Triassic basement rocks (Aguiar 1971). Its upper when the sedimentary successions throughout this contact is an unconformity with Albian to Cenoma- basin experienced the greatest offsets (Góes and nian shallow marine, green to brownish-red mud- Rossetti 2001). The rift system became abandoned, stones interbedded with fine- to very fine-grained, resulting in an aborted intracontinental feature as cross-stratified sandstones of the Itapecuru Group the continental break up migrated northward. The (e.g., Rossetti and Truckenbrodt 1997, Rossetti et sedimentary filling of the São Luís-Grajaú Basin al. 2001). (Fig. 1B) reaches up to 4,000 m thick in the depocen- Surface studies in the Codó area led to the re- ters, and consists chiefly of Cretaceous deposits or- cognition of three lacustrine facies associations in ganized into three depositional sequences (S1, S2 the Codó Formation, which are arranged, according and S3) formed during the Late Aptian/Early Al- to Paz and Rossetti (2001), into shallowing-upward An Acad Bras Cienc (2004) 76 (4) FACIES FROM CODÓ FORMATION IN THE GRAJAÚ AREA, MA 793 A) 48° 46° 44° 42° 0° Precambrian basement ATLANTIC OCEAN N Lower Mesozoic rocks B) Belém 2° Alcântara Age Stratigraphy 1 São Luís 4° SÃO LUÍS-GRAJAÚ BASIN Codó Açailândia Teresina Grajaú 6° S1 150 Km Late Cretaceous 8° 4º Bacabal BR-135 late MA--006 BR-010 ENSE NH IA Arame RA OV Itapecuru Group A D RO Açailândia NSM TRA CRETACEOUS Albian Presidente Dutra middle S2 IMPERATRIZ 1 23 BR-226 Barra do Corda 4 5 Grajaú early -226 6º BR Porto Franco Codó/Grajaú BR-010 S3 Late MA-006 formations Aptian Escala 1:2.500.000 48º 46º 44º 1 Exposures of the Codó Formation Fig. 1 – A) Location map of the Grajaú area in the south of the São Luís-Grajaú Basin, where the studied exposures of the Codó Formation are located. B) Simplified stratigraphy of the São Luís-Grajaú Basin (after Rossetti 2001). cycles attributed to: 1. central lake deposits, consist- SEDIMENTARY FACIES ing of black bituminous shale and evaporite formed at the base of the cycles; 2. intermediate deposits, The Codó Formation is exposed along several open representing laminated argillite, lime-mudstone, mines in the Grajaú area that average 15 m thick and peloidal mudstone to packstone, and meso-crystal- are currently under exploration for gypsum. At some line limestone; and 3. marginal lake deposits, con- localities, the Codó Formation is in sharp contact sisting of a variety of shallow water to subaerially (sometimes by faulting) with underlying basalts of exposed facies including massive pelite with pe- the Parnaíba Basin. Despite the long distance (i.e., dogenetic features, fenestral calcarenite, ostracodal more than 250 km), these deposits show many as- wackestone to packstone, pisoidal packstone, gip- pects that are similar to the Aptian deposits exposed sarenite, tufa, rhythmite of limestone, argillite, and in the Codó area: a) the lithologies are dominated microbial mat. by evaporite, limestone, black shale and argillite; b) An Acad Bras Cienc (2004) 76 (4)