Coastal landscape evolution on the western margin of the Bahía Blanca Estuary (Argentina) mirrors a non-uniform sea-level fall after the mid-Holocene highstand

PRATOLONGO, PAULA D.; PIOVAN, M. J.; CUADRADO D. G.; GÓMEZ E. A.

GEO-MARINE LETTERS

SPRINGER

Lugar: Berlin; Año: 2017 vol. 37 p. 12 - 24

0276-0460

Sedimentary descriptions and radiocarbon ages fromtwo cores obtained from coastal plains along the western marginof the Bahía Blanca Estuary (Argentina) were integrated withprevious information on landscape patterns and plant associationsto infer landscape evolution during the mid-to-lateHolocene. The study area comprises at least two marine terracesof different elevations. The old marine plain (OMP), at an averageelevation of 5 m above mean tidal level (MTL), is a nearlycontinuous flat surface. The Recentmarine plain (RMP), 2 to 3mabove MTL, is a mosaic of topographic highs and elongateddepressions that may correspond to former tidal channels.Mollusks at the base of the OMP core (site elevation 5.09 maboveMTL),with ages between 5,660 ± 30 and 5,470 ± 30 yearsBP, indicate a subtidal setting near the inland limits of the marineingression. The sandy bottom of the core is interpreted as the laststage of the transgressive phase, followed by a tight sequence ofdark laminated muds topped by a thick layer of massive graymuds. The RMP core (site elevation 1.80 m above MTL) has asimilar sedimentary sequence, but unconformities appear at lowerelevations and the massive mud deposits are less developed.The thickness of the grayish mud layer is a major differencebetween theOMPand RMPcores, but deeper layers have similarages, suggesting a common origin at the end of the transgressivephase. The overlying massive muds would correspond to rapidsedimentation during a high sea-level stillstand or slow regression.It is proposed that, after a rapid sea-level drop to about 3 mabove MTL, a flat and continuous surface corresponding to theOMP emerged, and more recent coastal dynamics shaped thedissected landscape of the RMP. For the Bahía Blanca Estuary,smooth regressive trends have been proposed after the mid-Holocene highstand, but also stepped curves. A stillstand orslowly dropping sea level was described around 3,850± 100 years BP, as well as negative relative sea-level oscillations.In this study, the differentiation between the OMP and the RMPsupports the occurrence of a stepped regressive trend that, at leastlocally, presented two different stages.