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.