Quaternary Intertidal Deposits Intercalated with Volcanic Rocks on Isla Sombrero Chino in the Galápagos Islands (Ecuador)

Abstract

A stratigraphic succession composed of limestone intercalated with volcanic ash and basalt capped by a conglomerate of mixed limestone and basalt cobbles was deposited in a trough-shaped depression approximately 25 m wide and 50 m long to a thickness of 1.62 m on the southwest side of Isla Sombrero Chino in the Galápagos Islands of Ecuador. Two layers of well-cemented calcarenite up to 20 cm thick accumulated as beach deposits with bioclasts of gastropods dominated by the Galápagos Periwinkle (Nodilittorina galapagiensis), a representative of the Beaded Hoofshell (Hipponix grayanus), broken crab fragments, and bird bones. Crustacean remains most likely belong to the Sally Lightfoot Crab (Graspus graspus). The bird bones are attributed to Audubon's Shearwater (Puffinus iherminieri). Distinctly intertidal in origin, such a mixed assemblage of invertebrates and vertebrates is unusual, and the association with basalt flows is seldom met in the rock record. The pristine state of the volcanic cone on Sombrero Chino is consistent with a 3He exposure age of 13 ± 0.8 ka. The age of the basalt-limestone sequence is unknown but must be younger than the 3He exposure age. The basalt-limestone sequence is elevated approximately 3 to 4 m above current sea level. This implies that the intertidal limestone was deposited during an interval of higher sea level or, more likely, was uplifted by magmatic inflation. Such intertidal deposits, in conjunction with more precise dating, have the potential to constrain the history of relative sea-level change during island growth and isostatic subsidence related to volcanism and lithospheric cooling. Intertidal deposits of the kind reported here also help to distinguish between monogenetic as opposed to polygenetic history for volcanic islands.