GEOLOGICAL SOCIETY OF THE PHILIPPINES
Landslide Occurrence in Eastern Rodriguez, Rizal Province:
A Geological Perspective
As a prelude to the conduct of the geohazard assessment in the region, a detailed stratigraphic work was made in eastern Rodriguez, Rizal Province. The new data suggest that the region is underlain by the Mesozoic Montalban Ophiolite Complex (MOC) thrusted eastward over the Early to Middle Miocene Angat Formation. The former is represented mainly by moderately to highly weathered and altered pillow basalt. A close look at the Angat Formation indicates that the unit consists of two members: a lower clastic member (sandstone-mudstone-conglomerate) and an upper reefal limestone member. The clastic member, previously mapped as part of the Madlum Formation, consists of submarine, channelized and unchannelized facies.
Landslide inventory suggests that most slope failures, commonly in the form of translational slides, occur mainly in areas underlain by the pillow basalt of the MOC. This may be attributed to the highly jointed, altered and sheared nature of the rocks. Such highly deformed nature of the basalts may be related to the onramping of the MOC, although later tectonic events (including movement along the East Valley Fault) are not being discounted. Dissolution of the limestone member of the Angat Formation also resulted to the development of vertical joints in the rock unit. This makes the areas bounded by limestone highly susceptible to rock fall.
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