Progress Report: Geologic Mapping for the Siocon Quadrangle

By General and Economic Geology Section

The Mines and Geoscience Bureau Regional Office IX (MGB-IX) conducted systematic geological mapping in the Siocon Quadrangle (Sheet No. 3443-III) in the Zamboanga Peninsula is 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 Siocon Quadrangle is located in the west-central portion of the Zamboanga Peninsula and immediately adjacent to the Tupilac Peak Quadrangle on the east. It covers a large portion of the Municipalities 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 Siocon 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 at 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.

Siocon Quadrangle covers the latitude N7° 40’ 0” to 7° 50’ 0”, and longitude E122° 0’ 0” to 122° 15’ 0” and is approximately centered in the Municipality of Siocon in Zamboanga del Norte. From Zamboanga International Airport, the target municipalities covered by Siocon Quadrangle are easily accessible via an all-weathered national road which serves as the main access throughout the entire peninsula. Far-flung areas can be accessed using a motorbike (locally known as habal-habal).

Figure 2. The municipalities that are covered by the Siocon 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), 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 Siocon Quadrangle

Siocon Ophiolitic Complex

A separate ultramafic body consisting of peridotite appears northwest of the Tungauan Metamorphics. Its occurrence has been traced along the Siocon-Roseller T. Lim Road particularly in the section of Barangay Kilalaban, Baliguian where fresh roadcuts appear. Because of its separate distribution, opposite to the ultramafics of Titay with the metamorphic complex in between them, the ultramafics in the northwest are treated as a separate ultramafic unit of possibly older age. This is an ultramafic body of the West Zamboanga Block, which is considered to have a high affinity with the Palawan Ophiolite rather than the Polanco Ophiolite.

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

Sirawai Formation

The Sirawai Formation was named by Santos-Yñigo (953) after the thermally metamorphosed green clastic rocks were exposed in the Siocon-Sirawai area in western Zamboanga. This formation also outcrops along the east coast of Vitali Island, near the headwaters of Vitali River, and at Panubigan Island along the southern projection of the Linguisan-Vitali ridgeline. At its type locality, the formation attains a maximum thickness of about 2,000 m. It seems to lie unconformably over the Tungauan Schist (Tungauan Metamorphics) and is presumed to be of Eocene age, for lack of any fossils by which to date the unit.

The formation consists mainly of conglomerates and minor shales and sandstones. These clastic rocks seem to have been derived chiefly from schist terrain. The conglomerates are poorly sorted and contain angular to subangular, pebble- to boulder-sized clasts of schists and numerous quartz fragments. On the other hand, the shales and sandstones exhibit cross-bedding features. Both the fine-grained clastic rocks and the conglomerates are typically epidotized in the vicinity of diorite intrusive bodies.

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 Siocon Quadrangle.

GEOLOGY OF THE SIOCON QUADRANGLE

The Siocon Quadrangle covers the municipalities of Siocon and Baliguian, Province of Zamboanga del Norte. The succeeding discussion focuses on selected outcrops starting with the ultramafic rocks followed by the metamorphic rocks and the sandstone-conglomerate sequence. 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

Ultramafics Exposures

There are two ultramafic rock bodies observed in the Central Zamboanga, which we initially considered as separate entities. The ultramafic body exposed southeast of the Titay Fault has been considered part of the Polanco Ophiolite, which is treated as the basement rock of the Zamboanga Terrane. The ultramafic rock found within or northwest of the metamorphic region is treated as a separate block trapped within the Labason Terrane.

Barangay Kilalaban, Baliguian

A quarry area along the Siocon-Roseller T. Lim Road at Station #70 (N7° 45’ 22.5”; E122° 14’ 52.0”) in Barangay Kilalaban, Baliguian exposes an ultramafic rock body (Photo 7). A thin laterite layer of 2 to 5-meter thick is developed above the ultramafic body. It is incised by multiple fault systems as well as pervasive jointing to the point of appearing as a highly sheared mélange-like block. A prominent fault structure has been identified, trending N45°E and dipping 70°SE. Serpentinites commonly form along fractures or fault zones often Its contact with the adjacent talc chlorite schist of Sitio Canatuan is unknown due to lack of exposure.

Photo 1. A quarry area along the Siocon-Roseller T. Lim Road exposing the ultramafic rocks in Sitio Tanuman, Barangay Kilalaban, Baliguian at Station #70 (N7° 45’ 22.5”; E122° 14’ 52.0”).

Barangay Kilalaban, Baliguian

A roadcut exposure along the Siocon-Roseller T. Lim Road on the eastern portion barangay shows a highly sheared to weathered ultramafic rocks. At Station #64 (N7° 45’ 38.6”; E122° 12’ 59.8”) the greenish-black to silvery gray peridotite appears to be serpentinized marked by fibrous serpentine and platy chlorite minerals that are usually having a sub-vitreous appearance. The serpentinized ultramafic rock has a general shear trend of N45°E and dips at 45°SE which is transected by several fault splay structures that converted the ultramafic host rock to a schistose rock. Some portions of the outcrop show an epidote alteration along the quartz-sulfide vein-filled fractures with varying degrees of oxidation developing goethite coating.

Photo 2. A roadcut exposure along the Siocon-Roseller T. Lim Road showed a sheared ultramafic rock in Barangay Kilalaban, Baliguian at Station #64 (N7° 45’ 38.6”; E122° 12’ 59.8”).

Barangay Bulacan, Siocon

Another roadcut exposure along the Pisawak-Makiang barangay road on the southwestern portion barangay shows a highly brecciated ultramafic rock. At Station #3016 (N7° 43’ 25.3”; E122° 10’ 29.2”) the grayish-green peridotite has a very coarse-grained groundmass composed of pyroxene and olivine that appears to be leached out which is depleting in plagioclase minerals. The leached surface shows a vuggy texture with no visible manifestation of serpentinization. Several occurrences of the ultramafic blocks are observed in the barangay which are used in road fills because of their highly fractured nature that can easily be quarried out.

Photo 3. A roadcut exposure along the Pisawak-Makiang barangay road shows a brecciated residual peridotite in Barangay Bulacan, Siocon at Station #3016 (N7° 43’ 25.3”; E122° 10’ 29.2”).

Metamorphics Exposures

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 Siocon Quadrangle, the Tungauan Metamorphics can be widely observed in the north-eastern and southern portions of the quadrangle.

Tungauan Metamorphics

Barangay Kauswagan, Baliguian

A quarry site near the Baliguian proper shows a moderately weathered metamorphic rock. At Station #2991 (N7° 48’ 1.4”; E122° 8’ 17.8”) the exposure shows foliated chlorite schist which occasionally grades into the metamorphic facies of phyllite, as well as quartz talc schist and chlorite mica schist. Its color ranges from cream-colored in the talc schist section to green in the chlorite schist/ phyllite section. The general foliation trend is oriented at N40°W dipping 35°NE. The fracture/joint sets exhibit a blocky appearance with a structural measurement of N40°W 65°SW and N20°E 75°NW.

Photo 4. A mountain quarry exposes a highly jointed foliated talc chlorite schist observed in Barangay Kauswagan, Baliguian at Station #2991 (N7° 48’ 1.4”; E122° 8’ 17.8”).

Barangay Tibangao, Siocon

Along the winding section of the Siocon-Baliguian road, a well-indurated marble exposure is observed on the elongated ridge. At Station #3049 (N7° 44’ 17.7”; E122° 7’ 24.1”) the outcrop has a gray to white color with a coarse-grained matrix which also exhibits a banded texture of alternating white and gray impurities (Photo 5). In the northern portion of the quadrangle, most of the roadcuts reveal a talc quartz chlorite schist which is highly weathered that is having a reddish-brown to cream soil horizon. This marble block seems to extend westward as it has also been encountered along the coastal section of Barangay Matiag at Station #3054 (N7° 43’ 40.3”; E122° 6’ 37.9”), where the marble appears in contact with the hornfels and schistose blocks (Photo 6). At Station #3046 (N7° 44’ 24.0”; E122° 7’ 25.3”) a roadcut shows a dark gray granoblastic amphibolite schist that is having a gneissic banding of the felsic and mafic minerals with a foliation trend at N60°E dipping 30°NW (Photo 7).

Photo 5. A roadcut exposure of a banded marble block is observed along the Siocon-Baliguian road in Barangay Tibangao, Siocon at Station #3049 (N7° 44’ 17.7”; E122° 7’ 24.1”).
Photo 6. Subcrops of banded marble are observed along the coastal section of Barangay Matiag, Siocon at Station #3054 (N7° 43’ 40.3”; E122° 6’ 37.9”).
Photo 7. A roadcut exposure of dark gray amphibolite schist was observed in Barangay Tibangao, Siocon at Station #3046 (N7° 44’ 24.0”; E122° 7’ 25.3”).

Barangay Lumay, Baliguian

Along the Baliguian-Gutalac road, a roadcut exposes a moderately weathered metamorphic rock. At Station #3084 (N7° 47’ 52.6”; E122° 10’ 11.9”) the schist outcrop has a silvery gray to white color with a granoblastic texture composed of mostly quartz with large platy muscovite mica. The foliation trend is measured at N60°E and dips 20°NW. The highly weathered section of the outcrop shows a cream-colored sandy horizon with remnants of fine quartz grains with a speck of mica forming a glittering sand deposit along the road canal.

Photo 8. A roadcut exposure of a mica quartz schist was observed in Barangay Lumay, Baliguian at Station #3084 (N7° 47’ 52.6”; E122° 10’ 11.9”).

Barangay Balagonan, Siocon

Along the coastal section in Sitio Dolongin, exposures of metamorphic rock are observed. At Station #3056 (N7° 43’ 42.4”; E122° 6’ 35.0”) the schist outcrop has a gray to white color that is exhibiting a granoblastic texture composed of mostly quartz and minor muscovite mica. The quartz grains are usually elongated and parallel to the foliation trend-oriented at N30°W and dip at 30°NE. Elongated quartzite lenses are also parallel to the foliation that has a boudinage structure. The blocky/rectangular fractures/joints are measured at NS 80°W and EW 80°S which is usually perpendicular to the foliation.

Photo 9. Coastal exposure of a highly jointed mica quartz schist was observed in Barangay Balagonan, Siocon at Station #3056 (N7° 43’ 42.4”; E122° 6’ 35.0”).

Sandstone-Conglomerate Sequence

The formation consists mainly of conglomerates and minor shales and sandstones. These clastic rocks seem to have been derived chiefly from a metamorphic terrain. The conglomerates are poorly sorted and contain angular to subangular, pebble- to boulder-sized clasts of schists and numerous quartzite fragments. Most of the exposure are observed in barangay New Lituban, Dionisio Reconalla and Pangian of Siocon.

Sirawai Formation

Barangay Dionisio Reconalla, Siocon

Along the unpaved access road on the western portion of the barangay exposes a highly weathered sedimentary sequence. At Station #3024 (N7° 41’ 38.4”; E122° 10’ 27.6”) the outcrop is composed of a massive polymictic conglomerate and sandstone sequence. The conglomerate is clast-supported, poorly sorted, sub-angular to rounded of pebble to boulder-sized schist and quartzite clast that is set in a fine-grained sandstone matrix. The moderately indurated sandstone sequence exhibits normal grading composed of sand and quartz grains with minor lithic fragments. This formation overlies the metamorphic rocks in the area, with the bed oriented at N20°E and dipping 35°SE.

Photo 10. A road exposure of a sandstone-conglomerate sequence was observed in Barangay Dionisio Reconalla, Siocon at Station #3024 (N7° 41’ 38.4”; E122° 10’ 27.6”).

Barangay Pangian, Siocon

A sedimentary sequence of the Sirawai Formation is exposed by a slope cut along the edge of an isolated hill. At Station #3036 (N7° 41’ 56.1”; E122° 8’ 57.7”) the sequence is composed of a thickly bedded conglomerate, pebbly sandstone and fine-grained sandstone. The conglomerate is composed of pebble to cobble-sized sandstone and quartz fragments that is clast-supported. The pebbly sandstone is matrix-supported and is made up of rounded pebble clasts of quartz and schist embedded in a sandy matrix. The indurated sandstone bed is oriented at EW 35°N and grades to a fining upward sequence set in 50 to 200 cm thick strata.

Photo 11. Exposure of a sandstone-conglomerate sequence was observed in Barangay Pangian, Siocon at Station #3036 (N7° 41’ 56.1”; E122° 8’ 57.7”).
Table 1. Canvassing/Inventory of Metalliferous and Non-Metalliferous deposits in the Siocon Quadrangle.
Figure 7. Plotting of Sampling points in the Siocon Quadrangle.
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