ROAD GEOLOGY AND A PRELIMINARY GEOHAZARD ASSESSMENT OF
BO. BALUYAN, MUNICIPALITY OF MALALAG, DAVAO DEL SUR
A Field Report submitted to the Faculty of the College of Engineering, University of
Southeastern Philippines BO. Obrero Davao City
In partial fulfillment of the requirements for the course Structural Geology 412 (2940)
Jerahmeel M. Abellanosa
El-Jedidiah O. Villacampa
March 2011
Davao City
TABLE OF CONTENTS
PAGE
I. INTRODUCTION
1
Purpose and Scope
1
Location and Accessibility
1
Topography and Drainage
2
Climate and Vegetation
2
II. GEOLOGY
2
General Geology
2
Tectonic Genesis
3
III. STRUCTURES
4
IV. DISCUSSION
4
Potential for Geohazard Occurrence
4
V. RECOMMENDATIONS
5
VI. CONCLUSION
6
VII. ACKNOWLEDGEMENTS
8
VIII. REFERENCES
9
a.
b.
c.
d.
e.
f.
Topographic Map
Climate Map
Location of the Studied Area
Geologic Map
Geographic Map
Documentations
10
11
12-13
14
15
16
I. INTRODUCTION
Purpose and Scope
BO. Baluyan, Municipality of Malalag, Davao del Sur, is within souhwestern
periphery of Padada-Malalag sub-basin. The lithology of the sub-basin area constitutes
primarily alluvium deposits which obscure the underlying bedrocks as basin fill.
Moreover, numerous landslides incidents happened at the particular area, interfering the
line of transportation and necessarily needs risk management and mitigations.
This preliminary geohazard assessment and a general geologic study were
conducted on the 6th of February 2011 along the highway. The particular partial
assessment and study are supported by 1:50,000 scale topographic map as a reference
map and with other aerial view photos as preview of the specific study area.
Location and Accessibility
The study area in BO. Baluyan municipality of Malalag, Davao del Sur lies on
the geographic coordinates of 6 deg 32’ 27.67” north longitude and 125 deg 22’ 12.9” east
latitude (Figure 1). The said area is southwest of Davao City. Travelling by commercial
buses or any land vehicle, it takes approximately two hours of travel through Davao
City-General Santos City National Road depending on traffic conditions. The specific
study area is about six kilometers from the Malalag-General Santos City junction road.
Topography and Drainage
The topography of Malalag area and its vicinity is typically plain, as an alluvial
deposit with moderate to steep gradient terrain classified as hummocky moraine . The
region is along the Padada-Malalag sub-basin wherein tributaries unload surface runoffs
and fluvial deposits towards Malalag valley (Figure 1).
Climate and Vegetation
Municipality of Malalag, Davao Del Sur have a Type IV climate experiences rainfall
that is
more or less distributed throughout the year (Figure 2). Vegetation consists
mainly of moderate cover of shrubs and bushes on the upland, and cogon grass,
coconuts, and other fruit-bearing and tropical trees within the low-cultivated lands (Figure
3).
II.
GEOLOGY
General Geology
The
lithology
of
the
area,
within the study area constitutes
predominantly
sedimentary rocks as a Quarternary alluvial deposit in the lower land. Though, igneous
intrusives are exposed with the tilted sequence of sedimentary strata along the road cuts
as an apophysis.
Intrusive andesite intrudes with in the said sequence of sandstone, siltstone,
and conglomerate in which layers are tilted practically vertical. Sandstone with in the area is
well compacted, moderately sorted and having angular grains (Grit). In fact, black fine grained
sandstone is observed at the area (see Geologic Map). It denotes the gradual decomposition of
organic materials due to deficiency of exposure to atmosphere and light. The appearance of a
residuum of organic compounds largely resistant to further breakdown. Humus is a mixture of
complex compounds which are variable in compositions and amorphous (I.D. White 1984).
Conglomerate having a cobble to boulder sizes is supported by fine grained sandstone flanked by
siltstone and sandstone layer. Sandstone covering the area is classified as feldspathic litharenite
sandstones. The sandstone which is said to be feldspathic litharenite is having quartz less
than 75%, quartz feldspar to rock fragment ratio greater than 1:3 but less than 1:1. In
some outcrop having intrusive contact between andesite and sandstone layer, minor
oxidations are observed. Pyritization and oxidation occur along the contact planes of
arenaceous and argillaceous rocks.
Tectonic Genesis
Fold mountains have been formed by large scale folding and subsequent uplift
and deep erosion of stratified rocks. Sedimentary rocks in the area are related to the
formation in Kiblawan
which dated and established at Upper Miocene to Pliocene
according to Milares (1981). Porphyritic andesite intrusive are much younger than the
sedimentary sequence of sandstone, siltstone, and conglomerate. The whole formation was
folded by a regional dynamic compressive tectonic movement due to thrust fault trending
northwest to southeast at the investigated area.
III. STRUCTURES
Andesite intrusive are jointed and filled with quartz and calcite minerals within
fractures. It subsist along the tilted sequence of sedimentary sequence of sandstone,
siltstone, and conglomerate unconformable with the andesite intrusive. As a whole, Baluyan
formation considered as fold mountain by which it have been formed by the large-scale
folding and consequent uplift caused by andesite intrusion and deep erosion of stratified
rocks. For as much, due to folding and uplifting, the degree of noticeable shearing of
rocks within the range of the fault . The presence of incohesive gouge and breccias are
indicating the significant displacement of planar fracture. Within the vicinity of the
formation, colluvial deposits of andesites and sandstones lies at the base of the
formation.
IV. DISCUSSION
Potential for Geohazard Occurrence
Baluyan fold mountain displays several oriented peaks trending northeast to
southwest with an elevation ranging from 200 to 250 meters above sea level. The said
area was principally covered with loose compacted materials. The fragmental and
unconsolidated rock materials overlies massive jointed porphyritic andesite intruding the
sedimentary sequence of well compacted sandstone and siltstone, and matrix supported
conglomerate. The andesite intrusive acts as a bedrock overlaid by the allogenic
materials. In that case, in homogenous materials, an arcuate failure plane is liable to
develop. The inherent factors which develop a potential geohazard occurrence are the
predispose slopes, the slope gradient, and the height of the slope which imposes high
shear stresses towards its base.
These factors are observed within the studied area. The slope gradient values
ranges from 700 to 850 wherein it exceeds to the maximum gradient at which loose
cohesionless materials remains stable. According to Billings (1954), sandstone has a shear
strength value of 5-15 MN/m2 and has a tensile strength value of 1-3 MN/m2 as to
excessive loading of the slope by prolonged rainfall, increasing the overburden pressure
and reducing the shear stress by raising the pore water pressure. Under these
circumstances, the mass movement is characterized by expansion normal to the surface
and subsequent contraction. In fact, removal of supporting materials at the base of the
slope is a predestined cause of instability with or without the influence of water.
V. RECOMMENDATIONS
Asian highways 26-Maharlika Highway lies along the contour of 200 to 250 m
above sea level. The specified road links two major cities, General Santos City and
Davao City including the suburban between the respective cities. In case of geohazard
occurrence interrupts the line of transport, an alternative route is via Kiblawan-Talisay
road. However, it takes approximately 30-35 km to reach the junction of Asian Highway
26-Maharlika Highway within the boundary of Banate.
In this case:

The government should empower the responsible sector (DPWH) to continuously
monitor the area by assigning necessary number of enforcers and machinery
equipments to take action, warn, and assist the public in case the geohazard tends
to occur.

Implementing structural mitigations such as gravity walls, cantilever walls, and
counterfort/buttressed walls will provide stability and disallow the mass to assume
its natural slope. These structures should be established as they was classified
based on their method in achieving stability or else they might not be effective.

Extracting masses at the base of the slope should not be tolerated. It could loosen
the friction at the base of the slope causing the loose materials on the top of the
slope to slide.
V. CONCLUSION
The lithology of BO. Baluyan, municipality of Malalag, Davao Del Sur constitutes
predominantly
sedimentary
rocks
classified as Quarternary alluvium deposit. The said
sequence was established at Upper Miocene to Pliocene Epoch. It intruded by porphyritic
andesite which was jointed and infilled with quartz and calcite materials. Slicken slides,
incohesive gouge and breccias are present on the area, depicting an evident shearing
along the planar fracture which bisects a part of the Baluyan formation as an auxiliary fault of
the primary thrust fault.
In a particular part of the formation where fault contact is observed between
intrusive
andesite
and
sandstone
layer,
oxidation
and
pyritization
is
apparent.
Chloritization of porphyritic andesite consorts the minor mineralization. Weathered andesites
instigates the presence of feldspar conveying sandstone namely feldspathic litharenite
sandstone which presently covers the said area. A particular layer of dark fine grained
sandstone depicts the humification and gradual decomposition of organic materials due to
lacustrian setting. The said layer id followed by siltstone and matrix supported conglomerate
showing the gradual marine regression and subsequent uplifting due to intrusion of porphyritic
andesite. Therefore the Baluyan formation is a fold mountain that has been formed by
folding and consequent uplift and erosion. It is one of the results of regional dynamic
tectonic movement through subduction westward by the Pacific Plate to the Philippine
Plate.
VI. ACKNOWLEDGEMENTS
The authors would convey their genuine gratitude to the following persons to
whom they indebt with and to who, in one way or another be part of the cause in the
accomplishment of this particular field assessment: to Mines and Geosciences Bureau
Region 11, with their doubtless support and data given; to the dean of the College of
Engineering, Engr. Ricardo Forbes Abear; to the chairperson of the Geology Department,
Engr. Florencio S. Chua; to our respective families, for their consent, financial and moral
support; to Mr. Jose D. Madrona for his persistent indefatigable efforts and assistance and
divulged knowledge; to the people of Sulop for their sincerity and serene reception; to
the second field party of your passion, interest, and enthusiasm making the assessment
successful; to Miss. Deanne De Asis and Mr. Allen June Buenavista for their coordination
and buttress for references and data foundation; and to the Almighty God whom always
gives His full supports and guidance.
Jerahmeel M. Abellanosa
El-Jedidiah O. Villacampa
VII. REFERENCES
Bates, L. R., Jackson, A. J., Dictionary of Geological Terms 3rd Ed.
Dott, R., Dott’s Classification of Sandstone. Modified after Dott 1964
Folk, R., Folks Classification of Sandstone. Modified from Folk 1980
Milanes, F. SJ., 1981, “Report on the Geological Survey at Malita and Kabayawa
Quadrangles, Davao del Sur”, internal report, Bureau of Mines and Geos-ciences, Surigao
City.
April, 1992, “Geology and Mineral Resources od Davao del Sur”, Geological Survey
Division, Mines and Geosciences Development Service, Region XI, Davao City
VIII. FIGURES
Figure 1.
Topographic Map
Figure 2.
Climate map
Type I. There are two pronounced seasons: The dry season (from November to April) and wet
season (rest of the year).
Type II. There is no dry season under this classification, with a very pronounced rainfall from
November to January.
Type III. Seasons are not very pronounced. It is relatively dry from November to April, and wet
during the rest of the year.
Type IV. Rainfall is more or less evenly distributed throughout the year under this classification.
Location of the Studied Area
Figure3: An Aerial photo of the studied location as was indicated. The existing location is
bounded by a geographical coordinates of N 06.54102, E 125.37027. The following coordinates
may correspond to the topographic map as was presented.
Figure 4: A setting of the studied area. The indicated point is the initial location where the
assessment in progress.
Figure 5.
Geographic Map
BO. Baluyan,
Sarangani Province
Documentations
Figure 6. A cumulative perspective of the actual studied area.
Figure 7. A fault contact flanked by the intrusive andesite (left) and sandstone
layer (right).
Figure 8. Brecciation and the occurrence of gouge materials, showing a
prominent shearing through the planar fracture.
Figure 9 Jointed and folded sequence of sedimentary and intrusive andesite a
substantiation of folding and uplifting by gradual tectonic movement.