Studies of the coastal sedimentary record have allowed both the reconstruction of relative sea-level changes and the determination of local rates and magnitudes of tectonic deformation, particularly in tectonically active areas. Despite their successful use elsewhere, studies of this type are much less common for the Mexican Pacific coast, which parallels the Cocos-North America subduction plate boundary. Stratigraphic, geochemical and microfossil data from sediments in Laguna Mitla, the Pacific coast of Guerrero, Mexico, document late-Holocene sea-level changes induced by tectonic activity in the Mexican subduction zone. Three major events are identified. First, the formation of the lagoon by c. 4630 yr BP, as indicated by a freshwater to brackish peat. Second, a relative sea-level rise, or land subsidence, as indicated by a shift from a freshwater marginal lagoon environment to a marine setting, preceded by a marine inundation represented by a sand unit (possibly a tsunami deposit), by c. 3400 yr BP. And finally, a return to lagoonal conditions indicating a drop in relative sea level or coastal uplift by c. 2300 yr BP. The Laguna Mitla stratigraphy indicates general coastal subsidence or relative sea level rise of c. 1 mm/yr. We argue that these relative sea-level (land-level) changes have been induced by tectonic activity associated with the Mexican megathrust. A plausible explanation for the 3400 yr BP marine inundation is probably a tsunami produced by a large seismic event accompanied by coastal subsidence. Discrete fining upward, fine to coarse, sand units with an erosional basal contact, medium to poor sorting, and clay/mud rip-up clasts; an increase in Na and Sr elemental concentrations, indicative of a marine origin; and the landward extent of the sands support a tsunamigenic source for these deposits. However, these apparent tsunami deposits require further study to determine their lateral extent and to assess whether they can be correlated from one site to another. This study demonstrates the applicability of a multiproxy sedimentary approach in interpreting relative sea-level (land-level) changes and to derive data on related earthquake and tsunami events in tropical coastal lagoons.
Petrographic and scanning electron microscopy (SEM) analyses of quartz grains from beach and dune sands were carried out in the western and eastern Northland coasts, New Zealand, to examine variations in durability and surface texture, which are controlled by mechanical and chemical processes, in profiles across beach and dune environments. This was done through point counts of quartz grain properties based on extinction angle and crystallinity. Variations in surface texture were assessed through SEM observations of mechanical features (conchoidal fractures, smooth surfaces, groove forms) and chemical features (solution pits, etching, silica deposits). Mechanically produced grooves are associated with beach sands affected by the high energy of the surf zone. Both mechanical and chemical processes occur in the eastern dune sands. They are associated with the greater abundance of angular grains in the eastern dune sands than the western dune sands. In addition, conchoidal fractures produced by the collision of grains in aeolian environments and linear and curved grooves produced by quartz grains from the beach support the mechanical processes taking place in the dunes. Solution pits, etching, and the presence of diatoms in the quartz grains are associated with pedogenesis and high silica precipitation in the eastern beach and dune sands. The durability of coarse-grained polycrystalline quartz relative to fine-grained polycrystalline quartz suggests that chemical abrasion exerts control over the distribution of quartz types in the dune sands.
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