Holocene Evolution of Patos Lagoon, Brazil: The Role of Antecedent Topography

Abstract

Bortolin, E.; Weschenfelder, J., and Cooper, A., 2019. Holocene evolution of Patos Lagoon, Brazil: The role of antecedent topography. Journal of Coastal Research, 35(2), 357–368. Coconut Creek (Florida), ISSN 0749-0208.The Patos Lagoon in Rio Grande do Sul, Brazil, is part of the largest barrier lagoon system in the world. It is enclosed by a 400-km-long composite late Pleistocene/Holocene sandy barrier and has a single tidal inlet. The modern lagoon is shallow (average < 5 m) and is dominated by silt deposition. More than 1000 km of shallow seismic data (3.5 kHz) indicate that the lagoon is underlain by several shore-normal incised valleys separated by interfluves. Each incised valley existed as an individual estuary since its first flooding during the mid-Holocene. The infill of these valleys contains a basal fluvial unit, a central estuarine mud unit, and locally developed tidal sand bodies associated with former tidal inlets. The contemporary lagoonal sediments form a blanketing upper unit. Ultimately, the interfluves were drowned and the contemporary lagoon was formed by the coalescence of the incised valley estuarine systems in the late Holocene. This expansion of accommodation space coincided with a dramatic reduction in vertical sedimentation rates. Seismic profiling reveals the contemporary sandy spits, and their subaqueous extensions coincide with the location of former interfluves, indicating that inherited topography exerts major control over the location and development of lagoon-marginal spits.