Philippine Journal of Science 138 (2): 191-204, December 2009 ISSN 0031 - 7683 Georesistivity Signature of Crystalline Rocks in the Romblon Island Group, Philippines Leo T. Armada 1,*, Carla B. Dimalanta 1, Graciano P. Yumul, Jr.1,2, and Rodolfo A. Tamayo, Jr.1 1Tectonics and Geodynamics Group, National Institute of Geological Sciences University of the Philippines, Diliman, Quezon City, Philippines 1101 2Department of Science and Technology, Bicutan, Taguig City, Philippines 1631 Georesistivity surveys were conducted in the tectonically complex Romblon Island Group, Philippines to assess the groundwater potential of the crystalline rocks found in the area. Vertical electrical sounding (VES) using Schlumberger array with a maximum spread (AB/2) of 300 meters was used during the survey; this array provided vertical images of depth up to 60 meters. The VES results show significantly lower resistivity values for the regolith (~10 to 250 ohm-meters) compared with the resistivity values of the parent units (i.e., ultramafic rocks: ~ 800 ohm-meters and metamorphic rocks: 1000 to 2000 ohm-meters). These resistivity values are attributed to the elevated groundwater content of the regolith compared with the unweathered parent rocks. Furthermore, thick regoliths were formed in areas adjacent to pre- existing faults and fracture zones in the area. The flow of groundwater through the fissures in the crystalline rocks possibly contributes to enhancing deeper levels of weathering to produce the low-resistivity regoliths observed. Also, the regoliths, with an average thickness of 35m, serve as zones of enhanced groundwater potential in the Romblon Island Group because of their relative thick overburden and low resistivity. Key Words: Crystalline rocks, georesistivity, Philippines, regolith, tectonics INTRODUCTION report the results of such investigations in the Philippines. Some works which used the electrical resistivity method In the Romblon Island Group, attempts to provide potable include the resistivity survey done in Rizal, Philippines water sources to the rural communities had been carried to constrain the thickness of sand and gravel deposits out under various programs. Some projects involved the (Abarquez 1969). In Paoay, Ilocos Norte, the thickness drilling of wells to address the scarcity of water in rural and configuration of the sand dune aquifers were communities. Unfortunately, the water wells were poorly delineated through several electrical sounding points located and scientific investigations were not done to (Stirling Edwards and Gonzales 1984). determine the proper sites for the wells. As a result, water extracted from the dug wells were of poor quality (e.g., Based on the groundwater availability map of the some showed fecal contamination and some wells went Mines and Geosciences Bureau (1997), rocks found dry during the summer) (Asian Development Bank 1999). in different parts of the Philippines are characterized in terms of their suitability as aquifers and their Although the resistivity method has been around for potential for storing groundwater. Due to the nature several decades and has been widely used in the search of the underlying rocks, only a small area in Tablas for groundwater, very few papers have been published to Island is underlain by fairly productive aquifers (e.g., *Corresponding author: [email protected] 191 Philippine Journal of Science Armada et al.: Georesistivity of Crytalline Rocks Vol. 138 No. 2, December 2009 in the Romblon Island Group, Philippines sedimentary units) with the rest of the areas in the this block collided with the Philippine Mobile Belt (Fig. three islands (Tablas, Romblon, and Sibuyan) being 1). The interaction between these two blocks is responsible composed of rocks with limited potential or without for several episodes of collision, subduction, and accretion any known significant groundwater. In 2004, the Local in central Philippines. These processes produced the belt Water Utilities Administration (LWUA) conducted of metamorphic rocks and ophiolitic units in Mindoro georesistivity surveys in coastal areas underlain by Island, Romblon Island Group, and northwest Panay which sedimentary and alluvial aquifers in Romblon Island to delimits the extent of the arc – continent collision zone identify additional water sources. The sounding data (Ramos et al. 2005; Yumul et al. 2003; 2005; 2008). revealed the presence of possible water-bearing layers which should be confirmed by subsequent drilling The Romblon Island Group in west central Philippines (LWUA 2004). consists of features that attest to the Early Miocene collision between the Palawan Micro-continental Block Crystalline rocks (particularly igneous and metamorphic and the arc-related Philippine Mobile Belt. The three big rocks) are generally poor groundwater aquifers due to islands that make up the Romblon Island Group - Tablas, their lack of primary porosity and permeability. This Romblon, Sibuyan - consist mainly of crystalline rocks makes groundwater exploration difficult in hard rock (i.e., ophiolitic units and metamorphic, volcanic and terrains, although groundwater accumulations may intrusive rocks). The sedimentary sequences are found occur in crystalline rocks having limited secondary mostly in Tablas Island (Fig. 2). The distribution of these porosity acquired due to faulting, jointing and lithologic units offers some constraints on the possible weathering (e.g., Owen et al. 2005; Dutta et al. 2006; occurrence of water-bearing units in the area. Yadav and Singh 2008). These localized concentrations of groundwater within a crystalline formation become The search for groundwater in the Romblon Island Group unconventional targets for water resource prospecting. is made difficult by the fact that the islands are dominantly Extraction of groundwater from the weathered and made up of crystalline bedrock (hard-rocks). Units of fractured portions of the crystalline bedrock is reported the Sibuyan Ophiolite Complex are exposed in Tablas by Taylor and Howard (2000). These types of aquifer and Sibuyan Islands. From bottom to top, the sequence usually occur in the permeable zone overlying the is made of harzburgites and dunites, layered pyroxenites, unweathered crystalline bedrock (Taylor and Howard layered and isotropic gabbros, diabase dike swarms, and 1999). This potential groundwater resource, if tapped, basaltic to andesitic pillow lavas and flow deposits (Fig. will provide an important fresh water resource for areas 3a-3b). Units of the ophiolite are seen as tectonic slices underlain by hard rocks (e.g. Adepelumi et al. 2006; bound by thrust faults which generally trend NE and dip Owen et al. 2007; Surrette et al. 2007). NW (Fig. 2 ). This paper discusses the results of the evaluation Aside from the ophiolitic units, there are other volcanic of hard-rocks within a collision zone as potential and intrusive units that are exposed in Tablas and aquifers using electrical resistivity method. There Sibuyan Islands (Fig. 3c-3d). Andesite outcrops which has been relatively little work done on ophiolitic and are fractured and weathered mark the eastern coastline of metamorphic rock aquifers in the Philippines. This Tablas Island. Diorite intrusions are also found in several is one of the few georesistivity investigations carried localities in Tablas and Sibuyan Islands (Fig. 2). The out in collision zones and over hard-rock targets outcrops are highly fractured and weathered especially for groundwater sources in the region. The results in the exposure found in northern Tablas. The fractures obtained from this work suggest that the success rate of generally trend NE, NW, and E-W with SE, NE, and N groundwater exploration in geologically complex areas dips, respectively. Different varieties of metamorphic such as collision zones and in crystalline, hard-rock rocks were mapped in all three islands. These consist of areas may be improved by conducting georesistivity mica, quartz-mica, quartzo-feldspathic, chlorite, and talc- surveys prior to drilling. chlorite schists, and limited exposures of phyllite (Fig. 3e-3f). Marble is found only in Romblon Island (Fig. 2). The metamorphic rocks are complexly folded in some Geologic Setting of the Romblon Island Group places but the general foliation trends are NW (dipping The Philippine Island arc system is an amalgamation of 10-80°NE) and NE (dipping 20-70°NW). blocks of continent- and arc-derivation. The continent- derived block, the Palawan Micro-continental block, is a Clastic sedimentary rocks are generally characterized as fragment of mainland Asia. This piece broke off during the good aquifers, hence, most groundwater exploration work opening of the South China Sea (Taylor and Hayes 1980; targets these rock types. In the Romblon Island Group, a Hsu et al. 2004). As it was being translated southward, significant portion of Tablas Island, especially the western 192 Philippine Journal of Science Armada et al.: Georesistivity of Crytalline Rocks Vol. 138 No. 2, December 2009 in the Romblon Island Group, Philippines Figure 1. The Romblon Island Group is situated within an arc-continent collision zone. Area encircled in red is the RIG = Romblon Island Group. Yellow shaded region = Palawan Microcontinental Block. Gray shaded region = Philippine Mobile Belt. 193 Philippine Journal of Science Armada et al.: Georesistivity of Crytalline Rocks Vol. 138 No. 2, December 2009 in the Romblon Island Group, Philippines Figure 2. The Romblon Island Group is a crystalline, hard rock area. Exposures