Eos, Transactions, American Geophysical Union, v. 93, Fall Meeting Supplement, 2012.

Beachrock horizons of the Nicoya Peninsula, Costa Rica: Implications for coastal neotectonics and paleogeodesy

Jeff Marshall ¹, Stephen Osborn ¹, Shawn Morrish ¹, Andrew Barnhart ¹, Lilibeth Wenceslao ¹, Michael Landeros ¹, Amber Butcher ¹, Brent Ritzinger ¹, Kacie Wellington ¹, Marino Protti ², Jim Spotila ³

1. Geological Sciences Department, Cal Poly Pomona, Pomona, CA, 91768, USA

2. OVSICORI, Universidad Nacional, Heredia, Costa Rica

3. Department of Geosciences, Virginia Tech University, Blacksburg, VA 24061, USA

Beachrock deposits are a common feature of tropical coastlines, formed by precipitation of carbonate cements (calcite or aragonite) within intergranular pore spaces of beach sediments. In this study, we employ geomorphic, petrographic, and hydrochemical analyses to evaluate the formation mechanism and neotectonic significance of uplifted Holocene beachrock horizons on the Nicoya Peninsula, Costa Rica.

The Nicoya Peninsula forms a prominent forearc high along the southern Middle America convergent margin. This emergent coastal landmass overlies the seismogenic zone and is sensitive to vertical movements of the earthquake cycle. The last major rupture of the Nicoya megathrust (M7.7, 1950) produced up to 1.5 m of coseismic coastal uplift, followed by gradual interseismic subsidence that continues today. Net Quaternary emergence is recorded by uplifted Pleistocene marine terraces and Holocene shore deposits, including carbonate-cemented beachrock.

Along the rocky macro-tidal coastline of the Nicoya Peninsula, beachrock deposits occur in tabular seaward-dipping horizons (5-15^o) that are 0.1-0.5 m thick, 2-5 m wide, and extend laterally 10s to 100s of m along the beach. At some sites, multiple imbricate horizons step up the beach face beyond the high tide zone. Outcrops typically occur where coastal streams or wetlands provide abundant groundwater. Radiocarbon ages (25 samples, 12 field sites) range from 0.7-3.8 ka for deposits on the modern beach, and from 4.5-5.3 ka for deposits located up to 0.5 km inland.

Petrographic analyses reveal compositional and textural diversity among constituent sand grains and interstitial carbonate cements. Sands consist primarily of sub-angular to sub-rounded marine shell and coral fragments mixed with lithic and mineral grains derived from local outcrops of Cretaceous oceanic basement rock (basalt, olivine, plagioclase, pyroxene, vein quartz, and radiolarian chert). Interstitial cements occur in two distinct forms: 1) acicular circum-granular fibers of aragonite, and 2) micritic pore-filling crystals of calcite.

Field observations and cement mineralogy indicate that Nicoya beachrocks form in the saturated zone where saline marine water mixes with fresh groundwater draining from the beach at low tide. Carbonate precipitation may result from water mixing, as well as evaporation of CO₂ saturated groundwater on the beach face. Pending chemical analyses of marine, estuarine, and phreatic waters will provide additional constraints on precipitation mechanisms.

With each earthquake, the Nicoya beachrock horizons undergo intervals of uplift and intervening subsidence. Over multiple cycles, net uplift moves these deposits upward on the beach and eventually into the landscape beyond. With improved understanding of the geomorphology and origin of Nicoya beachrocks, we ideally can use these outcrops as paleo-geodetic timelines to better track vertical tectonic movements