Incised valley paleoenvironments interpreted by seismic stratigraphic approach in Patos Lagoon, Southern Brazil

ABSTRACT:

The Rio Grande do Sul (RS) coastal plain area (33,000 km² ) had its physiography modified several times through the Quaternary, responding to allogenic and autogenic forcings. The Patos Lagoon covers a significant area of RS coastal plain (10,000 km² ), where incised valleys were identified in previous works. About 1,000 km of high resolution (3.5 kHz) seismic profiles, radiocarbon datings, Standard Penetration Test (SPT) and gravity cores were analyzed to interpret the paleoenvironmental evolution as preserved in incised valley infills. Seismic facies were recognized by seismic parameters. The sediment cores were used to ground-truth the seismic interpretations and help in the paleoenvironmental identification. Key surfaces were established to detail the stratigraphical framework, and seismic facies were grouped into four seismic units, which one classified in respective system tracts within three depositional sequences. The oldest preserved deposits are predominantly fluvial and estuarine facies, representing the falling stage and lowstand system tracts. The Holocene transgressive records are dominated by muddy material, mainly represented by estuarine facies with local variations. The transgression culminated in Late Holocene deposits of Patos Lagoon, representing the highstand system tract. The depositional pattern of the vertical succession was controlled by eustatic variations, while the autogenic forcing (paleogeography and sediment supply) modulated the local facies variation.

KEYWORDS:
incised valley; seismic facies; Quaternary; system tracts