Morphological evidences of eustatic events in the last 14,000 years in a far-field site, East-Southeast Brazilian continental shelf

Abstract

The post-LGM transgression was marked by eustatic variability events, comprising periods of increased sea-level rise, triggered by meltwater pulses (MWP) on polar regions, preceded by periods of stillstand. Several studies highlight the influence of such events on the geomorphology of far-field continental margins worldwide by mapping drowned relict features. However, there is still a lack of information in the South Atlantic and a major discussion about the global distribution of MWP-1B. The present work aims to investigate morphological evidences in the east-southeast Brazilian continental shelf (Abrolhos and Espírito Santo shelf), by examining shelf morphology using regional bathymetric data and high-resolution seismic sections. The analytical strategy involved comparing depth signatures of interpreted relict features with a set of sea-level curves. The results suggested the predominance of depositional process in shoreline sectors under closer influence of fluvial discharges during the Younger Dryas (YD). During YD stillstand or low sea-level rise rates (~60 m deep), it was possible to recognize the occurrence of a lagoonal system with shallow water/shoreface features (sandbanks, sand ribbons, or other bedforms) associated with tidal channels in the Abrolhos Shelf Depression and erosive features such as coastal cliffs. In the Espírito Santo outer shelf, the YD coastal features were characterized by barrier island systems associated with incised valley mouths and possible fringing reefs, in a context of a marginal shallow marine environment. All these features tend to be preserved in the modern shelf morphology due to a fast increase in the accommodation space that defines the end of the YD due to the onset of MWP-1B. In general, morphological interpretation revealed that MWP-1B caused the overstepping or drowning of those coastal features. Erosive features, defining the YD period, were also identified far from the influence of fluvial input. In these areas, coastal erosive processes prevailed and are morphologically expressed as abrasion terraces developed since the YD onset ~70 m and features like a paleo-sea cliff. During MWP-1B, the presence of shallow water reefs in the Abrolhos shelf is hypothesized, experiencing backstepping until their complete submersion. Despite the lack of additional data on composition, age and 3D geometry of the features, the morphological approach proposed here strongly supports the concept that eustatic events, triggered mainly in the Northern Hemisphere, influenced the shelf geomorphology and sedimentary processes evolution in the South Atlantic, during the last deglaciation.