Stratigraphic evolution of the Upper Carboniferous-Lower Triassic Sequence, Parnaíba Basin, Brazil

ABSTRACT:

The Upper Carboniferous-Lower Triassic Sequence represents the third major sedimentary cycle of the Parnaíba Basin in northeastern Brazil and corresponds to the Balsas Group. The sequence is composed by a thick pack of siliciclastic rocks whose sediments were deposited under complex conditions, varying from clastic/evaporitic shallow marine to lacustrine and desert environment. Aiming to clarify the sedimentary sequence evolution, this work conducted a stratigraphic analysis of the Upper Carboniferous-Lower Triassic sediments. Modern and classic concepts of sequence stratigraphy were applied on a database of wells and seismic sessions. We identified three main depositional sequences. Sequence 1 corresponds to rocks initially deposited by a fluvial system which evolved to shallow marine conditions related to a transgressive stage, later evolving to a deltaic system. Sequence 2 corresponds to sediments deposited under lacustrine/arid conditions, during an important regressive stage, that evolved to a dominantly desert environment recorded in Sequence 3. Seismic stratigraphic analyses allow to infer key stratigraphic surfaces, and three main sequences. Seismic facies related with such sequences are dominantly parallel and sub-parallel, with good lateral continuity, suggesting that the sedimentary rates were homogenous along the interval analyzed in this sector of Parnaíba Basin.

KEYWORDS:
Sequence Stratigraphy; Seismic Stratigraphy; Paleozoic Syneclise