The West Gondwana fragmentation during the Late Triassic and Jurassic times was marked by intense climatic changes, concomitant to the implantation of extensive desertic/lacustrine systems. These deposits succeeded the emplacement and extrusion of lava flows, related to the pre-rift phase and initial opening of the Equatorial Atlantic Ocean. Part of these events is recorded in Pastos Bons Formation, exposed mainly in southeastern portion of Parnaíba Basin, Northeastern Brazil. Sedimentary facies are grouped in two facies associations (FA), representative of a lacustrine system. Central lake facies association (FA1) is composed by laminated mudstone, sandstone/mudstone rhythmite and even-parallel laminated sandstone. Distal turbidites (FA2) consist in sandstones with supercritically climbing ripple cross-lamination, sandstones with even-parallel stratification, massive sandstones and sandstones with soft-sediment deformation structures. FA1 was deposited in the deepest portions of the lake, characterized by low energy, episodically disturbed by fluvial influx. FA2 suggests gravitational flow reworked by combined-flow, probably linked to a flysch-like delta front, fed by ephemeral streams, in hyperpycnal flows. The installation of Pastos Bons lakes was controlled by subsiding areas, probably restricted depocenters. Predominance of siliciclastic facies and the lateral contact with humid desert facies of Corda Formation evidence climatic attenuation, which enhanced biological activity. Petrographically, the sandstones of this unit are classified as subarkoses, fine to medium-grained, with subangulose grains, well to moderated-sorted. Diagenesis of these rocks included eo- and mesodiagenetic stages, with mechanical compaction, dolomite cementation, iron-oxide cementation and generation of secondary porosity. X-ray diffraction analyses evidence mainly smectite and high content of feldspars, which coadunate an arid/semiarid climate, with incipient chemical weathering. Heavy mineral assemblage is characterized by abundance to superabundance of stable minerals, which evidences the high mineralogical maturity of these deposits. Despite of the small thickness and restrict distribution, Pastos Bons Formation can be a stratigraphical guide, which indicates subsiding portions in the Western Gondwana supercontinent, prior to post-Jurassic continental breakup.
|
