Geology of Culebra Island, Puerto Rico
Banks, Thomas H
De Bremaecker, Jean-Claude
Master of Arts
Culebra Island, located about twenty miles east of Puerto Rico, is composed of undated andesite and spilitic volcanics which are intruded by diorite plutons. On the basis of the ages of similar rocks on Puerto Rico and the Virgin Islands the age of the Culebran rocks is probably within the Albian - Middle Eocene interval. The oldest rocks in the area are the Culebra Andesites (2400 feet exposed) which consist of augite andesite pillow lavas and massive flows interbedded with coarse andesite breccia and andesite tuff. Conformably overlying the Culebra Andesites is the Cerro Balcon Formation (1300 feet exposed). This formation is composed of thick-bedded spilitic lapilli tuff with a basal unit of thin-bedded, fine grained tuff called the Playa Resaca Member. The Cayo Norte Formation (750 feet exposed) is composed of albitic, quartz-rich pillow lavas which are the youngest stratified rocks in the area. Intrusive igneous rocks include two diorite stocks, a diorite dike and several small spilite intrusives. The rocks of Culebra are unmetamorphosed except for contact metamorphism of tuffs adjacent to the diorite intrusives. Structurally, Culebra is a homocline dipping 15° to the northeast. At least three northwest trending high angle faults cut the area. Stratigraphic throws are from a few hundred to about a thousand feet. The Culebra Andesites are considered to be orogenic igneous rocks which are related to the high-alumina basalt described by Kuno (1960). Orogenic andesites may be derived from high-alumina basalt by fractional crystallization or may simply represent a slightly lower melting fraction of the ultramafic material of the mantle. High-alumina basalts and related andesites are considered to be primary magmas which are generated by partial melting of the ultramafic mantle in active orogenic areas.