CONSTRAINING THE EVOLUTION OF THE STRAND-PLAIN OF THE USUMACINTA AND GRIJALVA RIVERS (SOUTHERN MEXICO) AFTER SEA LEVEL STABILIZATION

The strand-plain of the Usumacinta and Grijalva rivers is one of the largest strand-plains of the Gulf of Mexico. The strand-plain is located at southern Mexico and comprises flood plains, lagoons and a deltaic plain of more than 150 km wide that is composed by beach-dune ridges parallel and subparallel to the current coastal line. These ridges are still building since the last sea level stabilization of the Atlantic Ocean in the Gulf of Mexico ~4 ka ago. Since then, three major constructive phases of the strand-plain can be identified. Each phase implies modification of the deltaic system and the relocation of rivers’ mouth. We dated some beach-dunes ridges using OSL, obtaining a series of geomorphologically recent luminescence results. We relate major changes on the deltaic system to avulsion processes occurred after intense sedimentation in a focalized area and not to climatic forcing. This is supported on geochemical analysis of sediments along the beach-dune ridges that show similar element concentrations during the three phases of construction. However, the youngest sample taken at the mouth of the current delta lobe (0.14 ka), indicate that Ti/Al, Zr/Al concentrations increase notably in comparison to older sites. This can be interpreted as an increase in the catchment erosion in recent times what is highly likely due to intense deforestation around the basin during the last two centuries. Additionally, we recognized a fault formed across the beach-dune ridges of the strand-plain, which raises the hypothesis that a recent pulse of tectonic activity might be related to the opening of the last new distributary at the Samaria River, which is altered part of the Grijalva River drainage since 1932.