DEPOSITIONAL SIGNATURES ASSOCIATED WITH LATE-STAGE TRANSGRESSION AND THE MID-HOLOCENE SEA-LEVEL HIGHSTAND IN BRAZIL

Coastal morphologic features associated with past shoreline transgressions and sea-level (SL) highstands can provide insight into the rates and processes associated with coastal response to the modern global transgression. Along the Brazilian coast of South America, 6000 years of SL fall following a 1-4 m high SL highstand resulted in the accumulation of extensive strandplain deposits seaward of highstand features, thereby preserving late-stage transgressive and highstand deposits commonly several kilometers landward of the modern shoreline. In contains a moderately-well-developed highstand evidenced by several linear barrier ridges located 3.8–5.2 m above modern SL. RTK-GPS data, ground-penetrating radar, stratigraphy, and radiocarbon dating allowed for the identification of diverse highstand features preserved within the Navegantes Strandplain in southern Brazil. The best-preserved feature is a segmented, barrier ridge backed by an 80-m-wide lagoon. Elsewhere in this plain, development of highstand features was limited by the presence of shallow bedrock that forms the inland boundary of the strandplain. In these locations, highstand features were either confined to barrier ridges lacking accommodation for the development of backbarrier environments or were completely absent, with the highstand marked by a bedrock-cliff shoreline. Using the diversity of highstand signatures identified at Navegantes and comparisons with sites across Brazil, we broadly classify highstand features into four categories: exposed bedrock coasts, backbarrier deposits, transgressive barrier ridges, and barrier-island complexes, according to local conditions of upland migration potential, wave exposure, and sediment supply. These Brazilian systems present a paradigm for understanding future coastal response to climate change and accelerated SL rise: the recognition of a minimum threshold SL-rise rate of ~2 mm/yr above which transgression proceeded too rapidly for the formation of these stable accretionary shoreline features demonstrates the nonlinearity of coastal response to SL change, and the site specificity of conditions associated with the formation of each highstand deposit type, even within a single small embayment, demonstrates the non-uniformity of that response.