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.
Highlights
The Codó Formation was deposited during an important moment in the evolution of the Brazilian Equatorial Margin, representing the only exposed record of the Late Aptian early rifting stage
The Codó Formation is well exposed in the Grajaú area (Fig. 1A), where it consists of deposits lithologically similar to those from the Codó area, being represented by evaporites, laminated argillites, An Acad Bras Cienc (2004) 76 (4)
Together with the Sr isotope data, which is considered more reliable for paleoenvironmental interpretation, the nature of the deposits associated with the evaporites in the Grajaú area, in some aspects seems to favor the hypothesis of an inland saline pan complex
Summary
The Codó Formation was deposited during an important moment in the evolution of the Brazilian Equatorial Margin, representing the only exposed record of the Late Aptian early rifting stage. The lowermost sequence S1 contains the Codó Formation, which is the objective of this paper, and represents a succession up to 450 m thick of sandstones, evaporites, shales and limestones. This sequence displays a tripartite subdivision into systems tracts (Rossetti 2001), with the lowstand deposits grading southward progressively from shallow marine to continental (i.e., fluvial, deltaic, and lacustrine). Surface studies in the Codó area led to the recognition of three lacustrine facies associations in the Codó Formation, which are arranged, according to Paz and Rossetti (2001), into shallowing-upward
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