Geologic Mapping for the Coronado Point Quadrangle

By General and Economic Geology Section

The Mines and Geoscience Bureau Regional Office IX (MGB-IX) conducted systematic geological mapping in the Coronado Point Quadrangle (Sheet No. 3443-IV) in the Zamboanga Peninsula as part of the 2022 Quadrangle Geological Mapping Project. The purpose of the mapping program is to produce a 1:50,000 scale quadrangle map in Region IX which is one of the least mapped areas in Mindanao. Six (6) quadrangles are targeted for the year 2022 all of which have been unmapped (Figure 1). The Coronado Point Quadrangle is located in the west-central portion of the Zamboanga Peninsula and immediately adjacent to the Mamawan Quadrangle on the east. It covers a large portion of the Municipality of Siocon and Baliguian in Zamboanga del Norte (Figure 2).

Field activities consisted of an examination of rock exposures along with accessible tracks and in coastal areas, physical and megascopic description of outcrops and rock samples, sampling of representative rock units and favorable units that contain fossils, structural feature measurements of prevalent deformation and sedimentary structures, and photo documentation. The mapping utilized a 1:50,000 topographic base map of NAMRIA furnished through their website. The locations were obtained using the Global Positioning System (Garmin GPSMAP 64s), while the data gathered were plotted on a 1:10,000 topographic map generated from the Interferometric Synthetic Aperture Radar (IFSAR) 2015 of NAMRIA.

The output will be a technical report and GIS-based geologic maps which are expected to be extremely useful for many practical applications such as in mineral exploration, groundwater resource and vulnerability assessment, geohazard mapping, solid waste disposal site selection, land use planning, and other applications.

Figure 1. The Coronado Point Quadrangle (violet box), one of the 6 targets for 2022 Quadrangle Geologic Mapping in Region IX, is located within the west central portion of the Zamboanga Peninsula and covers two municipalities.

LOCATION AND ACCESSIBILITY

The study area is located in the Zamboanga Peninsula which lies roughly in the southern part of the Philippine Archipelago chiefly on the island of Mindanao on the western segment. It lies between Moro Gulf on the south and the Sulu Sea on the north. Along the coast of the peninsula are lavish bays and islands with varying extents. It is connected to the rest of Mainland Mindanao through an isthmus situated between Pagadian Bay and Panguil Bay. The peninsula falls under the jurisdiction of Western Mindanao (Region IX) consisting of three provinces (Zamboanga del Norte, Zamboanga Sibugay and Zamboanga del Sur), and the boundary between the peninsula and mainland is marked by the border between the provinces of Zamboanga del Sur and Lanao del Norte.

Figure 2. The municipalities that are covered by the Coronado Point Quadrangle.

GEOLOGIC SETTING

The Philippine archipelago is a complex system of colliding terranes, subduction zones involving continental crusts, island arcs, and oceanic crusts, which have been tectonically juxtaposed by strike-slip displacement (Hamilton, 1979; Cardwell et al., 1980, Yumul et al., 2001). A large part of the archipelago referred to as the Philippine Mobile Belt is under a tectonically active regime characterized by volcanism and seismicity. It comprises the central NNW-SSE lengthwise section of the archipelago, which is flanked on both sides by subduction zones (Gervasio, 1971; Rangin, 1991). Diagonally cutting through its NNW-SSE length runs the left-lateral strike-slip fault called the Philippine Fault (Figure 3). Its east side is bordered by the Philippine Trench, wherein the west-verging subduction of the Philippine Sea Plate occurs. Its western flank is defined by the Manila-Negros-Cotabato Trench system, along which the South China Sea, Sulu Sea, and the Celebes Sea undergo east-verging subduction. The crustal fragments west of the Philippine Mobile Belt are generally of continental character with affinity to the mainland Asia-Sundaland plates.

Figure 3. Tectonic Map of the Philippines showing the extent of Philippine Fault, the west-verging Philippine Trench in the east, and the generally east-verging trench systems in the west. (Besana et al., 2005)

The Zamboanga Peninsula represents the 250-km long narrow west-southwest extension of western Mindanao Island. It is bordered along the NW side by the Sulu Trench (Gervasio, 1971), and along the southern side by the Celebes Trench (Hall, 2002). The eastern section of the Peninsula is cut by a left-lateral NW-SE trending strike-slip fault called the Sindangan-Cotabato Fault (Besana, 2005). In the central section of the Peninsula runs the NE-SW trending Titay Fault which is of primary interest in this current work. Most of the previous studies (Yumul, 2004) agree that the Zamboanga Peninsula is almost entirely characterized by continental affinity which contrasts with the bigger portion of Mindanao Island.

Figure 4. Generalized Regional Geologic map of Zamboanga Peninsula (MGB-IX, 2010).

LOCAL STRATIGRAPHY

Stratigraphic Units in the Coronado Quadrangle

Tungauan Metamorphics

The metamorphic rocks widely occurring in the western section of the Zamboanga Peninsula was named as Tungauan Schist by Santos-Yñigo (1953). This unit incorporates other metamorphic rocks such as schist, marble, quartzite, gneiss, slate and phyllite. The occurrence and spatial relationships of these different metamorphic rocks are yet to be determined as their respective lithologic contacts have not been encountered. This unit was named after its exposure on the coast of Tungawan, but its occurrence stretches north to the coastal Siocon-Baliguian area, as far northeast as Labason-Liloy area and as far southwest as Pasonanca (Zamboanga City). In Querubin and others (1999) the metamorphic rock complex consisting of schist and amphibolites in the Mt. Dansalan area (Labason) was named as Dansalan Metamorphics. Age: Cretaceous

Figure 5. Stratigraphic column for the Central Zamboanga Peninsula section showing previous works (GOP, 2012) and the modified stratigraphy of this work.
Figure 6. Plotting of Observation (Station) points in the Coronado Point Quadrangle.

GEOLOGY OF THE CORONADO POINT QUADRANGLE

The Coronado Point Quadrangle covers the municipalities of Baliguian and a small portion of Gutalac, Province of Zamboanga del Norte. The discussion focuses on selected outcrops of mostly metamorphic rocks with varying metamorphic grades or facies. Numbered station points are used as location references, which are properly indicated in the location map (Figure 6) and the sampling points are plotted in Figure 7.

Lithologic Exposures

METAMORPHICS EXPOSURE

The Tungauan Metamorphics widely occur as multiple-facies metamorphics in a large portion of Tungawan Municipality. This name originates from Tungauan Schist of Santos-Yñigo (1953) but was modified to Tungauan Metamorphics due to its multiple types of rock such as schist, marble, quartzite, gneiss, slate and phyllite, making the “Schist” rather inappropriate. The boundaries among these different metamorphic rocks are also difficult to delineate and thus treated as one unit, adopting the name Tungauan Metamorphics instead of Tungawan Schist. In this quadrangle geologic mapping activities, however, only four quadrangles have been covered so far in the west Zamboanga Peninsula, wherein this unit is encountered. In the Coronado Point Quadrangle, the Tungauan Metamorphics can widely be observed in the quadrangle.

Tungauan Metamorphics

Barangay Pitawe, Gutalac

A roadcut at Station #3183 (N7° 57’ 51.0”; E122° 14’ 16.6”) going to the Pitawe proper appears an exposure of alternating schistose rocks. The schistose rocks are composed of alternating layers of quartz chlorite schist and mica quartz schist. The chlorite schist layer show intense crenulation and the muscovite quartz schist has a granoblastic texture that is usually coated with pyrite oxidation that is more commonly resistant to the chlorite schist. The general foliation trend is oriented at N50°E dipping 55°NW. Some of the fractures are oblique to the foliation that is filled with quartz which forms quartz lenses of thickness ranging from 10 to 150 cm. Its high resistance to weathering is manifested by a very thin soil horizon.

Photo 1. Alternating layers of chlorite schist and muscovite quartz schist are exposed along the slope cut in Barangay Pitawe, Gutalac at Station #3183 (N7° 57’ 51.0”; E122° 14’ 16.6”).
Photo 2. Another exposure of the schistose outcrop is noted further downslope in Barangay Pitawe, Gutalac at Station #3184 (N7° 57’ 55.2”; E122° 14’ 22.2”).

The Coronado point area has a rocky coastline that shows tabular blocks of detached metamorphic rocks at Station #3186 (N7° 58’ 25.1”; E122° 14’ 40.6”). The boulder outcrops are dominantly composed of alternating granular muscovite and quartz minerals that show a banded texture. The foliation trend is oriented at N40°E dipping 70°SE. Thin elongated quartzite lenses are observed parallel to the foliation trend with a thickness ranging from 1 to 5 cm. Pyrite oxidation is also noted along the fractures that have an orange to reddish coating.

Photo 3. The rocky Coronado point shows several outcrops of gray banded gneissic rocks in Barangay Pitawe, Gutalac at Station #3186 (N7° 58’ 25.1”; E122° 14’ 40.6”).

Barangay Malinao, Baliguian

Along the north coast of Barangay Malinao, Baliguian is a pocket beach with a village Sitio Bangaan. At the south end of the pocket beach at Station #3120 (N7° 56’ 13.8”; E122° 13’ 49.6”) appears a gneiss exhibiting alternating bands of brownish orange, gray and white colors. Rusty orange colors along fractures and foliation planes are traces of oxidation indicating the presence of sulfides. The gneiss has a granoblastic texture with poorly planar foliation but has wavy quartz lenses. The general foliation trend is oriented at N20°E dipping 55°NW. Further south, the outcrop appears to be highly crenulated with thick lenses of orange-coated quartzite. The joint sets are measured at N20°W 55°NW, EW 45°S and N20°E 70°SE.

Photo 4. In Sitio Bangaan, Barangay Malinao, Baliguian at Station #3120 (N7° 56’ 13.8”; E122° 13’ 49.6”): [A-B] Potruding metamorphic rocks are observed along the coast; [C] the outcrop shows a highly jointed oxidized gneiss; and [D] the surface/texture of the gneissic rock that is exhibiting banded layers with oxidized fractures.

Barangay Diculom, Baliguian

Along the NE-SW trending Diculom ridge, exposures of moss-coated metamorphic rocks are observed toppled along the roadside at an of elevation 180 masl. At Station #3115 (N7° 55’ 17.3”; E122° 14’ 55.6”) the fresh outcrop shows alternating bands of granoblastic amphibole (actinolite), plagioclase and quartz minerals that are identified as gneiss. The rock is very dense and resistant to weathering. The occurrence of pyrite minerals is observed disseminated along the vein-filled fractures that oxidized to goethite.

Photo 5. A coarse-grained gneiss outcrop is exposed along the roadcut along the NE-SW trending ridge in Barangay Diculom, Baliguian at Station #3115 (N7° 55’ 17.3”; E122° 14’ 55.6”).

Further downslope, the rock had changed to a low-degree metamorphic rock composed of schists. At Station #3116 (N7° 55’ 12.1”; E122° 14’ 43.4”) the outcrop is composed of a moderately to highly weathered and jointed brown to bluish green talc chlorite schist that has platy cleavage. Its foliation trend is measured at N60°W dipping 30°NE and is parallel to the slope, which appears to be daylighted and can pose a landslide hazard along the road.

Photo 6. Roadcut exposures of a highly jointed day-lighted talc chlorite schist in Barangay Diculom, Baliguian at Station #3116 (N7° 55’ 12.1”; E122° 14’ 43.4”).

Barangay Tamao, Baliguian

Along the coastal section of the barangay near the Tamao River exposes a steeply dipping metamorphic rock. At Station #3105 (N7° 53’ 18.2”; E122° 12’ 21.7”) the outcrop has a coarse-grained banded texture composed of hornblende, plagioclase and quartz minerals that forms a gneiss. The foliation is measured at N70°E dipping 60°NW wherein parallel quartzite lenses are observed showing a thickness ranging from 0.5 to 5 cm. Due to the high proportion of quartz in its composition, the outcrop is well indurated, erosion-resistant to strong wave action and it could be a source for quarry materials.

Photo 7. Coastal exposure of a steeply dipping gneissic rock with thick quartzite boudinage in Barangay Tamao, Baliguian at Station #3105 (N7° 53’ 18.2”; E122° 12’ 21.7”).

Barangay Alegria, Baliguian

Along the Alegria-Tamao coastal road, a slumped road section exposes a moderately highly weathered metamorphic rock. At Station #3125 (N7° 51’ 53.3”; E122° 10’ 56.0”) Its foliation texture is exhibited by the alignment of its component minerals such as amphibole, plagioclase and quartz. Its component minerals segregate into bands according to composition and generated parallel black and white bands; dark bands being composed of amphibole and the white bands composed of the felsic minerals with foliation trending N30°W dipping 30°NE. This gneissose texture featured by the black and white banding commonly manifests folding and faulting structures.

Photo 8. A highly jointed high grade metamorphic rock exposed along the Alegria-Tamao coastal road in Barangay Alegria, Baliguian at Station #3125 (N7° 51’ 53.3”; E122° 10’ 56.0”).

Table 1. Canvassing/Inventory of Metalliferous and Non-Metalliferous deposits in the Coronado Point Quadrangle.

Figure 7. Plotting of Sampling points in the Coronado Point Quadrangle.
Figure 8. Lithologic distribution map of the Coronado Point Quadrangle.
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