HOLOCENE EVOLUTION, SEA LEVEL CHANGES AND ACTIVE TECTONICS OF THE GUERRERO COAST, MEXICO

The Holocene evolution of the Guerrero coast has been marked by the interaction of sea-level changes and tectonic activity. The Guerrero coast extends along the active Pacific margin of southwest Mexico and parallels the trench where the Cocos Plate subducts beneath the North American Plate. The last major seismic events occurred in Guerrero in 1899, 1907, 1909, 1911, and 1957 earthquakes, but none have occurred since the major 1911 (Ms=7.6) earthquake in the northwest segment of the Guerrero seismic gap. The Guerrero gap is currently considered to be matured for a severe earthquake of estimated Mw=8.1 to 8.4. We present preliminary results of geomorphic field surveying and sediment coring on the coast of Guerrero that suggest that this coast has experienced relative sea-level changes. The Coyuca lagoon strip of the Guerrero coastal area consists of long barrier beaches, behind which extends a lagoon, beach ridges, extensive swamps, mangrove swamps, salt pans, floodplains, alluvial plains, fluvial terraces, and abandoned meanders. Abandoned meanders and fluvial terraces indicate that the Coyuca River has migrated to the southeast. These and changes in hill elevations near the coast suggest a southeast tilting of this coastal segment. Five cores up to 5.5 m depth were taken nearby the Mitla, Coyuca, Tres Palos and Tecomate lagoons. Preliminary observations on the sediment-stratigraphic sequence (alternations of clay sediments, containing organic material, silts and sand) and geomorphic evidence point to changes on terrestrial and marine environments. Two cores included sediments with archeological remains (pottery). The sediment cores included probable stratigraphic markers indicative of tsunami events. We consider that changes in marine and terrestrial environments were produced by fluctuations in relative sea level associated to long-term tectonic subsidence. These results agree with tide gauge measurements (1953-1999) and GPS data (1992-2000) indicative of subsidence rates of ~3 mm/yr (Kostoglodov et al., 2001). In progress radiocarbon dating of shells and charcoal, geochemical and micro-faunal analyses would reveal the timing and confirm the nature of these events.