Abstract

The upper Malubog Formation in Central Cebu (central Philippines), which spans a period of about 3.5 Ma (Lower Miocene, NN2 to NN3 nannoplankton zones), has a total thickness of at least 620 m. This member is composed of interbedded mudstones, siltstones, sandstones, minor carbonates, and rarely, conglomerates and thin (few dm to 2.5 m thick) coal seams.

The upper Malubog Formation represents a single depositional sequence (sensu Posamentier et al., 1988) comprised of three systems tracts: shelf margin, transgressive, and highstand. The shelf margin systems tract has a basal shell-rich, bioturbated muddy sandstone followed by fluvial-dominated deltaic cycles. Economic coals cap the two topmost deltaic cycles. During the transgressive phase, a retrograding barrier island system developed on top of the deltaic deposits. The highstand is comprised of at least three shallowing-upward cycles; each cycle starts with semi-protected shelfal mudstones and sandstones which shoal upwards into coastal plain sandstones, mudstones, and minor thin coal lenses. Shallow ramp carbonates were preferentially deposited during the late highstand in offshore areas probably due to the trapping of siliciclastics in coastal areas.

The top of this sequence is bounded by a tectonically-enhanced (angular) unconformity which represents the NN4 hiatus in the Visayan Basin.