Rhodolith Stranding Event on a Pliocene Rocky Shore from Isla Cerralvo in the Lower Gulf of California (Mexico)

Abstract

JOHNSON, M.E.; PEREZ, D.M., and BAARLI, B.G., 2012. Rhodolith stranding event on a Pliocene rocky shore from Isla Cerralvo in the lower Gulf of California (Mexico).Controls on present-day sedimentation around Isla Cerralvo in the southern Gulf of California provide a model for restricted Pliocene limestone distribution. The 10.46-km2 island is elongate and roughly parallel to the direction of prevailing north winds. Debris washed from deeply dissected valleys build fan deltas of sand-to-boulder size igneous clasts. These are transported south by long-shore currents, but the fans also create leeward zones with less agitated water. Remnants of a large Pliocene fan are exposed during low tide at Los Carillos on the SE side of the island. Adjacent is an unconformity between granite and granite-derived conglomerate with Nerita scabricosta. This extant gastropod is typical of the high intertidal rocky shore. The conglomerate is capped by a sandstone ramp with Argopecten abietus as an offshore facies. Basalt dikes exhumed from the granite formed natural groins that captured sediments and shells in the sand ramp. Whole rhodoliths, mostly 3.5 cm in diameter and 15-cm deep, covered 150 m2 within the ramp, now partly exposed among boulders in the basal conglomerate. Many rhodoliths encrust pea-sized rock cores. Accretion occurred in shallow water protected from extreme agitation by the nearby fan delta. Stranding of rhodoliths on the rocky shore was a storm-induced event.