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 km 2) 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 km 2), 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.

Back-barrier evolution and along-strike variations in infilling of the Kosi Bay lake system, South Africa

“…These spits are formed by wind-driven sediment reworking of the shorelines that feed the process of waterbody segmentation (Zenkovich, 1959). Unit C1 progrades from the palaeo-highs of the valley interfluves into the nearest available accommodation space, like those documented in Patos Lagoon, Brazil (Bortolin et al, 2018). This indicates formation during lower than present lake levels, when the back-barrier lagoon margins were being modified by wind-generated waves in the early stages of segmentation, a process that was previously regarded (Wright et al,1997, Cooper et al, 2012 as having occurred mainly after Holocene sea levels reached the present.…”

“…Wright et al, 2000;Dladla et al, 2019Dladla et al, , 2021 as well as globally (e.g. Simms et al, 2010;Nutz et al, 2015;Bortolin et al, 2018). These spits are formed by wind-driven sediment reworking of the shorelines that feed the process of waterbody segmentation (Zenkovich, 1959).…”

“…Holocene highstand, and the resultant deposition was characterised by draping of fine sediments (cf. Bortolin et al, 2018).…”

Links between surface sediment composition, morphometry and hydrodynamics in a large shallow coastal lagoon

“…1). Both allogenic and autogenic forcing factors influenced the depositional sequence framework of the coastal zone (Bortolin et al, 2018), where sea-level changes play the allogenic control responsible for the regional geologic evolution (Tomazelli and Villwock, 2000;Bortolin et al, 2019).…”

“…The coastal plain was subaerially exposed and deeply fluvially incised during numerous sea-level rise and fall events of the Quaternary period (Weschenfelder et al, 2008(Weschenfelder et al, , 2010(Weschenfelder et al, , 2014(Weschenfelder et al, , 2016, and more clearly after the last interglacial maximum of 120 ky BP (Bortolin et al, 2018(Bortolin et al, , 2019Cooper et al, 2018). Fluvial dissection of the coastal relief during forced regression events led to the formation of a series of adjacent incised valleys, accommodating base-level fall and long lowstand periods (Weschenfelder and Corrêa, 2018), which were flooded during subsequent transgressive events (Weschenfelder et al, 2014;Santos-Fischer et al, 2016Bortolin et al, 2019;Scottá et al, 2019).…”