PLEISTOCENE MARINE PALEOENVIRONMENTS ON THE GALAPAGOS ISLANDS

Nine types of Pleistocene marine deposits occur on the Galapagos Islands, each containing different fossil associations of foraminifera, ostracodes, mollusks, bryozoans, corals, and barnacles. Minor faunal elements include diatoms, sponge spicules, urchin spines, decapod claws, and clionid sponge and spionid polychaete borings. A few vertebrate fossils (e.g., sea lion, bird, lizard) are also present. Sediments recovered from eight of the Galapagos Islands are sands and gravels, limestones, fine carbonate sands, and palagonite tuff. These are exposed on marine terraces as limestone interbedded with basalt flows and in the nearshore as erupted palagonite tuff cones. With the exception of the latest Pliocene limestones, all of the deposits are late Pleistocene.

The different and commonly rich fossil associations characterize marine environments shallower than 30 m, including exposed rocky coast, rocky supratidal, sandy subtidal, and protected embayment. The very well preserved fauna in uplifted carbonate sands and silt at Villamil on Isla Isabela includes a variety of pelagic gastropods and planktic foraminifera that indicate access to open ocean currents. Marine terraces on Santa Fe contain abraded fossils, evidence of deposition along an open coastline with much wave action. Limestones, resulting from diagenesis of calcareous sediments, contain mollusks preserved as casts. Many of the nearly 20 palagonite tuff cones known in the Galapagos incorporated fauna and beach rock as they emerged and erupted through shallow nearshore substrates.

A large proportion of the biota appears to be endemic; for example, one-third of the more than 500 species of benthic foraminifera identified from the Galapagos have not been recorded elsewhere. Nearshore species radiation was enhanced by the geographic isolation of the islands and the numerous environmental discontinuities within the shallow waters of the archipelago.