NOTES ON THE GEOLOGY OF THE BONGABONG AREA, ORIENTAL MINDORO, PHILIPPINES

Oscar C. Perez
Comexco, Incorporated
Ortigas Center, Pasig City, Metro Manila, Philippines

ABSTRACT

        The Bongabong area in Oriental Mindoro is underlain by ophiolitic basement rocks. The ophiolites in the study area consist of pyroxenites, gabbros, basalts and diabase. These ophiolitic rocks form the steep paleo-shelf and slope areas in this particular section of the East Mindoro basin. Carbonate deposition during the Miocene is dominantly confined in these relatively steep shelf and slope areas of the East Mindoro basin. Two (2) northwest-trending carbonate belts are inferred to have developed at different elevations of the shelf areas. Middle Miocene to Early Pliocene clastic deposits that has turbidite characteristics are observed in the steeper sections of the slope areas. The Early to Middle Miocene transcurrent faulting which resulted in the formation of the East Mindoro Fault was also responsible for en-echelon folds that developed along the flanks of northwest-trending wrench fault splays during the Late Pliocene regional uplift. Middle to Late Pliocene uplift centered along the basinal to bathyal sections of the East Mindoro basin was responsible for the present configuration of East Mindoro.

        The dominantly montmorillonite clay in the Miocene to Pliocene clastics suggests a volcanic source terrain. The relatively lean total organic carbon content of these clastics also precludes the provenance of sediments from the western section, which have impressive shale and mudstone deposits. This scenario suggests that the source of sediments for the Middle Miocene to Early Pliocene clastics in the paleo-slope areas could have come from uplifted platform deposits and volcaniclastics from Pliocene volcanic centers.