EMHOFF, K.F.; JOHNSON, M.E.; BACKUS, D.H., and LEDESMA-VÁZQUEZ, J., 2012. Pliocene stratigraphy at Paredones Blancos: Significance of a massive crushed-rhodolith deposit on Isla Cerralvo, Baja California Sur (Mexico).
A white blaze across coastal cliffs is the hallmark of Paredones Blancos on Isla Cerralvo, Mexico. Cliff-forming strata include three roughly 10-m-thick units with a lateral coherence of only 0.75 km. The middle unit is a massive deposit of crushed rhodoliths. The other two units consist of matrix-supported conglomerate with cobbles and boulders of granodiorite, basaltic andesite, and hornblende diorite. By volume, the matrix accounts for >80% of those units, mostly composed of grus from weathered granite. Thin sections were studied from samples collected at five levels through the middle rhodolith unit to determine carbonate purity as a ratio between organic CaCO3 and inorganic minerals. The bottom and top margins of the deposit show higher levels of mixed clastics, with 37% inorganic mineral content, as compared with 11% toward the middle. Original depositional environments are interpreted as a rhodolith bank adjacent to a large fan delta built seaward from a wide canyon mouth. The stratigraphic sequence records a rise in sea level that brought rhodolith debris to the flooded canyon mouth above the basal conglomerate, and a drop that emplaced another conglomerate above the rhodolith deposit. A Middle Pliocene age is based on co-occurrence of Clypeaster bowersi and Argopecten revellei within the carbonates.