The geology of the western section of the Sabana Grande quadrangle: Implications for the geological evolution of southwestern Puerto Rico

A geologic map was produced for the western section of the Sabana Grande quadrangle. Late Cretaceous to Early Tertiary volcano-sedimentary sequences overlie Jurassic-Early Cretaceous ultramafic rocks of the Bermeja Complex. Deposition of volcano-sedimentary sequences initiated with the Late Santonian to Early Campanian Yauco Formation basal conglomerates. Middle Campanian Yauco slope-basin deposits were interbedded and overlain by coarse grained subaerial to near shore volcanic sediments of the Sabana Grande Formation. Subaerial volcanism and associated relative sea-level changes in the Maastrichtian formed the overlying El Rayo Formation volcano-sedimentary sequences. The Late Paleocene-Eocene Jicara Formation overlie the El Rayo Formation. Discordant contacts between the formations are either tectonic or an angular unconformity. Structural trends of older sequences are N-NE and are rotated clockwise about a vertical axis. The younger sequences have NW structural trends rotated to pre-Oligocene NE-SW shortening. Emplacement of the ultramafic bodies may be synchronous with deformation of the younger sequences.