Evolution of the French Guiana coast from Late Pleistocene to Holocene based on chenier and beach sand dating

Résumé En

The 1500-km-long Guianas coast between the Amazon delta in Brazil and the Orinoco delta in Venezuela is characterized by alternations of muddy shoreline advance and retreat caused by large mud banks migrating alongshore from the mouths of the Amazon. In this dominantly muddy environment, wave reworking of sand and shells results in the formation of beaches, termed 'cheniers', that provide valuable information on coastal evolution, especially on past erosional phases. Twenty-eight depositional ages showing the long-term patterns of shoreline mobility in French Guiana were obtained from optically stimulated luminescence. Twenty-one ages younger than 7 ka define three clusters centred on 4.5 ka, 1.0-1.3 ka, and 0.30 ka. They indicate that chenier formation was relatively synchronous and significantly affected by alongshore diversions of river mouths and changes in river-mouth position over time under the influence of muddy shoreline advance. A prominent cluster at 1.0-1.3 ka reveals a clear hiatus after the 4.5 ka cluster, indicating that the present muddy coastal plain of French Guiana was largely formed and preserved after 1.0-1.3 ka. This cluster also implies either an episode of coastal retreat or no coastal advance around 1.0-1.3 ka. The remaining seven samples were derived from Late Pleistocene deposits of headland-bound beaches and probable cheniers capped by aeolian sand, suggesting similar conditions of coastal Amazon mud-dominated sedimentation. By informing on past patterns of shoreline mobility, these results have broader implications for coastal landuse planning and shoreline management between the mouths of the Amazon and Orinoco Rivers.