Stratigraphic Analysis and Depositional Evolution of the Neoproterozoic Maricل Formation (Southern Brazil): Constraints from Field Data and Sandstone Petrography

This paper describes the application of facies analysis and sequence stratigraphic concepts and methods to the Neoproterozoic Maricá Formation. This unit crops out in the western portion of the Sul-rio-grandense Shield, which records the Brasiliano/Pan-African cycle (900–540 Ma) in southern Brazil. The sedimentary package (ca. 2, 500 m-thick) of the Maricá Formation encompasses sandstones, siltstones, shales and subordinated conglomerates, deposited by fluvial to shallow marine systems, along a NE-SW trending coastal area. These deposits are arranged in three distinct stratigraphic units: (1) a lower succession, of fluvial origin, with southeastward paleocurrent pattern, derived from the erosion of a granite-gneissic basement uplift; (2) an intermediate succession, comprising a transgressive-regressive marine sequence; and (3) an upper succession, also of fluvial affiliation, with volcanic as well as granite-gneissic provenance. Volcanogenic beds and other volcanism-related features are present jointly with the sedimentary rocks, as well as abundant volcanogenic clasts, that form the framework of sandstones and conglomerates, specially upsection. The textural immaturity of the sandstones (very angular to subangular fragments) is remarkable, and contrasts with the very good rounding of pebbles and cobbles. These features suggest limited transportation of the sedimentary load and reworking of already-rounded beach gravel deposits. The depositional evolution envisaged here accounts for an overall coastal environment, adjacent to an uplifted granite-gneissic source area, with significant volcanic contribution. In terms of sequence stratigraphy, the Maricá Formation is considered a high-order depositional sequence (the “Maricá Basin“ fill), subdivided in two lower-order sequences.