Sedimentary record of late-Holocene relative sea-level change and tectonic deformation from the Guerrero Seismic Gap, Mexican Pacific Coast

Abstract

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.

Extreme wave deposits on the Pacific coast of Mexico: Tsunamis or storms? — A multi-proxy approach

“…The study of tsunami deposits in Mexico is a recent development (Ramírez-Herrera et al, 2007) but a multi-proxy approach has been adopted and tested. Like in other areas of the World, there are still difficulties with the preservation of tsunami deposits and in establishing a chronology of events, but this work confirms that a multi-proxy approach can be successfully applied to tropical coastal environments.…”

“…Hutchinson and McMillan, 1997;Goff et al, 2001Goff et al, , 2010bPinegina and Bourgeois, 2001;Chagué-Goff et al, 2002Gelfenbaum and Jaffe, 2003;Moore et al, 2005, in press;Kortekaas and Dawson, 2007;Morton et al, 2007;Jankaew et al, 2008;Monecke et al, 2008). Similar geological evidence for the entire Mexican Pacific coast however is limited (Ramírez-Herrera et al, 2007), but historical data on the occurrence of earthquakes and tsunamis extend back to the 1700s.…”

“…Thus, Sr, Ba and Ca have often been used as indicators of marine flooding (e.g. Goff et al, 2004b;Ramírez-Herrera et al, 2007Chagué-Goff, 2010;Chagué-Goff et al, 2011). Zr on the other hand has been reported as an indicator of high-energy deposition, often in association with Ti (e.g.…”

“…Multi-proxy data have proven to be an invaluable tool in determining the origin of the two sand deposits in this tropical environment. By considering a wider range of proxies (Ramírez-Herrera et al, 2007;Chagué-Goff et al, 2011) than the conventional suite of sediment grain size and microfossils (diatoms and/or foraminifera), most often used in temperate environments where these basic indicators tend to be well preserved, we have been able to demonstrate the likely tsunamigenic nature of the deposits. The correlation of proxy data with historical, interview, numerical modeling, and a posttsunami survey (Abe et al, 1986) information has further enhanced our interpretations.…”

“…palaeoecology (Morton et al, 2007;Ramírez-Herrera et al, 2007;Goff et al, 2010a;Chagué-Goff et al, 2011;Goff and Dominey-Howes, in press). However, only a few studies have used a large number of these proxies to not only identify tsunami deposits (Goff et al, 2004a(Goff et al, , 2004b(Goff et al, , 2010bRamírez-Herrera et al, 2007;Chagué-Goff et al, 2011) but to distinguish between tsunamis and other highenergy events, i.e. storms (Goff et al, 2001(Goff et al, , 2004aMorton et al, 2007).…”