GEOMORPHOLOGY AND PETROLOGY OF HOLOCENE BEACH DEPOSITS AND ADJACENT BASALTIC BEDROCK, NORTHERN NICOYA PENINSULA, COSTA RICA

The Nicoya Peninsula on the northern Pacific coast of Costa Rica occupies an emergent segment of the southern Middle America fore arc. This 4800 km2 peninsula lies 50-60 km inboard of the Middle America Trench where the Cocos oceanic plate subducts NE at 10 cm/yr beneath the Caribbean plate.

Coastal cliffs and wavecut platforms along the northern Nicoya coast expose seafloor basalts of the Cretaceous Nicoya Complex. Rapid Quaternary uplift along this convergent margin coast has produced a rugged morphology of abrupt cliffs, rocky headlands, and intermittent pocket beaches. Holocene beach deposits consist of unconsolidated shell-rich sands and well-indurated, seaward-dipping horizons of carbonate-cemented beachrock.

Field mapping, topographic surveying, and aerial photo analysis reveal the geomorphic relationship between Holocene beach sediments and adjacent basement outcrops of Nicoya Complex basalt. Data include: slope, thickness, and volume of beach deposits, longshore drift direction, wave refraction and run-up patterns, and the location of principal sediment sources at stream mouths and along eroding cliffs.

Samples of basalt bedrock, unconsolidated sand, and carbonate-cemented beachrock were collected from three beaches along the northern Nicoya coast (Playas Negra, Blanca, and Soccoro). Preliminary petrographic analysis of beach sediments and basaltic bedrock indicate that the beach sands are locally derived. These sands consist of lithic basalt grains, subrounded phenocrysts of pyroxene and plagioclase, and abundant shell and coral fragments. Beachrock samples share a similar composition, but are cemented by grain-coating micrite and circumgranular, acicular crystals of aragonite.

At Playa Soccoro, beachrock samples from were collected from a seaward dipping sequence of heterogenous strata. Calibrated radiocarbon ages were determined for three distinct horizons along a beach-normal transect: 840 +90/-110 ybp, 1230 +50/-90 ybp, and 1370 +130/-90 ybp. An additional age of 1880 +100/-70 ybp was obtained from beachrock at Playa Negra. These ages indicate net Holocene uplift rates of <0.5 m/k.y., consistent with longer-term uplift rates estimated for adjacent late Pleistocene marine terraces.