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La Boquita Terrace, Nicaragua: Evidence for Uplift of the Central Pacific Coast Robyn Gerth 1 (gerth@es.ucsc.edu) Eli Silver 1 (831-459-2266; esilver@es.ucsc.edu) Jeff Marshall 2 (j\_marshall@fandm.edu) Manuel Duarte 3 (dgh@ibw.com.ni) Michael Carr 4 (carr@rci.rutgers.edu) Kirk McIntosh 5 (kirk@utig.ig.utexas.edu) Wilfried Strauch 6 (wil@ibw.com.ni) Suzanne Sweet 1 (ssweet@es.ucsc.edu) Stanley Kling 7 (skling@gs.ucsd.edu) 1Earth Sciences Dept., University of California, Santa Cruz, CA 95062-1077 2Geosciences Dept., Franklin and Marshall College, Lancaster, PA 17604 3INE, Managua, Nicaragua, 4Earth Sciences Dept., Rutgers University, , NY 08855-1179 5Institute for Geophysics, University of Texas, Austin, TX 78759-8500 6INETER, Managua, Nicaragua, 7Consulting Micropaleontology, 416 Shoreview Lane, Leucadia, CA 92024 Reconnaissance field work along the Pacific margin of Nicaragua reveals the presence of a coastal terrace, 17-20 m elevation, along a 40 km stretch of coast seaward of the 900-m-high Las Sierras de Mateares. The terrace, named here "La Boquita Terrace", is cut across steeply dipping Late Miocene (El Fraile Formation) strata in most places, but locally it is developed on ash fall tuffs and pyroclastic flows of the Pleistocene Las Sierras Formation. Walker et al. (J. Volc. and Geotherm. Res., 56:379-400, 1993) correlated the geochemically distinctive Las Sierras Formation with the offshore J1-layer ash, dated stratigraphically at 135 ka (Ledbetter, Geol. Soc. Am. Bull., 96:77-82, 1985). We model the terrace development as the combined result of crustal uplift and late Pleistocene sea level fluctuation. At an uplift rate of 0.1 m/ka, we expect to find one terrace at about 18 m elevation, assuming sea level at 120 ka (stage 5e) was 5-6 m higher than present. At significantly higher rates of uplift, additional high stand terraces would be expected, but were not observed in the field. Landward of the terrace, at 100-150 m elevation, the horizontally bedded El Salto Formation overlies steeply dipping Miocene strata along an angular unconformity. Analysis of macro and nannofossils indicates shallow marine deposition of El Salto beds during the Mio-Pliocene transition. Local uplift of post-Miocene sediments is also indicated by an offshore seismic profile (C. Ranero et al., Tectonics, in press). The cause of this late Cenozoic uplift may be related to tectono-thermal processes associated with Las Sierras de Mateares and Masaya caldera. |
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