Landslide Occurrence in
Olongapo City: Insights from Stratigraphic, Sedimentologic and
Petrographic Studies
Leovigildo
O. delos Reyes, Karlo L. Queaño, Marianne V. Fernandez and Gil C.
Cardiel
Mapua
Institute of Technology
Intramuros, Manila
Abstract
In recent
years, there has been a growing need for the conduct of landslide
hazard assessment as well as a heightened awareness of the
socio-economic impact of landslides. The clamor for such an
assessment rises particularly in highly urbanized areas, such as
Olongapo City, where the need for space forces new developments on
mountain or hill slopes. Field investigation in Olongapo City
revealed several old and recent landslides, particularly in Kalaklan
Ridge bounding the western part of the region. Despite the common
occurrence of these events, their causal and controlling factors
have yet to be investigated. This study, which presents the detailed
stratigraphy, sedimentology and petrography of rocks in Olongapo
City, offer insights on the cause of landslides and provides
baseline data for future landslide susceptibility studies.
Basaltic
and diabasic dikes, pillow basalts and pyroclastic deposits comprise
the stratigraphy of the region. The dikes occur mainly as, highly
weathered, well to extremely-jointed units and have chill margin
orientations that are moderately to steeply inclined (usu. ≥ 45o;
dip directions varying). Associated with these dikes are dark gray,
epidotized fine-grained pillow basalts. Deciphering the original
orientation of the dikes and pillows is difficult due to the
inconsistency of dike chill margin orientations, unclear draping
orientations of pillows as well as the complex tectonic disruption
of the rocks. These igneous bodies are possible extensions of the
Coto Block of the Zambales Ophiolite Complex (ZOC). Unconformably
overlying these rocks are horizontally oriented, generally thickly
to very thickly bedded, mostly clast- and matrix-supported
pyroclastic deposits. Many of these beds display normal and/or
reverse graded bedding, and have sharp, irregular erosive base
typical of channel fill deposits. Logged sections suggest vertical
stacking and/or lateral juxtaposition of these deposits. These rocks
are presumably young (i.e. of Quaternary age) given their relatively
undeformed, unjointed, and relatively fresh conditions.
Landslide
inventory conducted in Olongapo City revealed the presence of at
least fifteen landslide events (old and recent combined). These
events occurred mainly at the southwestern and southern part of the
Kalaklan Ridge underlain mainly by the units of the ZOC. Preliminary
data suggest the highly weathered and altered as well as highly
jointed conditions (as opposed to those of the pyroclastic deposits)
of the ZOC units as being the primary causal factors for the
landslide occurrence in the area. Jointing in these rocks is likely
a consequence of deformation associated with the amalgamation of the
ZOC with the Luzon arc as interpreted by previous authors. |