Progress Report: Geologic Mapping for the Bagolibud Point Quadrangle

By General and Economic Geology Section

The Mines and Geoscience Bureau Regional Office IX (MGB-IX) conducted systematic geological mapping in the Bagolibud Point Quadrangle (Sheet No. 3442-I) 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 Bagolibud Point Quadrangle is located in the west-central portion of the Zamboanga Peninsula and immediately adjacent to the Mamawan Quadrangle on the south. It covers a small portion of the Municipalities of Roseller T. Lim in Zamboanga Sibugay; Sirawai and Siocon in Zamboanga del Norte on the northwestern and northeastern limit of the quadrangle. A large portion covers the Municipality of Tungawan in Zamboanga Sibugay (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 Bagolibud 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 four 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.

Bagolibud Point Quadrangle covers the latitude N7° 30’ 0” to 7° 40’ 0”, and longitude E122° 15’ 0” to 122° 30’ 0” and is approximately centered in the Municipality of Tungawan in Zamboanga Sibugay. From Zamboanga International Airport, the target municipalities covered by Bagolibud Point 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 Bagolibud 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), 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 Bagolibud Point 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

Bungiao Mélange

The Bungiao Melange was named by Yumul and others (2001) for the melange at Bungiao, consisting of schist and serpentinized harzburgite blocks with minor marble clasts in highly sheared serpentinized matrix. It is a tectonic melange usually thrusted against the Tungauan Metamorphics. Exposures were also noted in Pilar, Tarlago, Lubay, Lunday, Ludasal, Sapa Manok, Vitali, Siocon River and Sta. Maria in Zamboanga Peninsula. The clasts of the Melange may reach hill-sized proportions consisting of schists, phyllite, slate, marble, sedimentary rocks, harzburgite, and andesitic igneous rocks in a serpentinized matrix. In Vitali, the Melange is inferred to be unconformably overlain by a sedimentary clastic sequence belonging to the Anungan Formation. Age: Paleocene

Polanco Ophiolite

A contiguous occurrence of rock bodies that comprise an ophiolitic sequence occurs in the central section of the Zamboanga Peninsula (Figure 4). These include the peridotite/ serpentinized peridotite that stretches from Ipil (northern), Titay, Tampilisan to Liloy, Sindangan, Siayan, and Dumingag. Extensive exposures of the peridotite in the Titay-Tampilisan area and Sindangan-Siayan-Dumingag area show pervasive shearing as well as serpentinization as exposed in quarry sites. Their highly fractured texture makes them a good source for aggregate quarry. This was previously labeled by Querubin et al (1999) as ZNAC Ultramafics for its road exposures around the Zamboanga del Norte Agricultural College. Exposures of gabbro and layered gabbro have been encountered in the southern portion of Tampilisan and Dumingag; the sheeted dike complex appears in the Sindangan-Siayan-Dumingag-Midsalip area; the pillow basalt occurs in some areas of Tigbao, Sindangan, Siayan and Dumingag. Age: Paleocene

Zamboanga Formation

The name Zamboanga Formation as originally proposed by Antonio (1972) consists of three members namely, the Metavolcanic member, the Metasedimentary member, and the Limestone member. In this study, only the “Metasedimentary” member is considered, described by Antonio (1972) as consisting primarily of an interbedded sequence of thin- to medium-bedded sandstone and mudstone, including argillite, with thin lenses of a conglomerate. This study further treats this clastic sedimentary sequence as a turbidite deposit representing the mid to distal section of a turbidite sequence, which consists of interbedded conglomerate, quartz-rich sandstone, and shale. It is widely distributed in all three provinces of the Zamboanga Peninsula although it is also widely covered by younger units especially the Zamboanga Volcanic Complex. Querubin et al (1999) renamed this unit as Camanga Sediments, but in this work, the name Zamboanga Formation has been re-adopted for its extensive lithologic descriptions as a deep marine clastic sequence by Antonio (1972). The name Zamboanga Formation appears more appropriate for this unit being among those with the widest distributions. Age: Oligocene

Sibuguey Formation

The lithologic unit Sibuguey Formation was named by Brown (1950) for the limestone exposure along the Sibugay River Valley described as a fairly uniform and thin-bedded sequence of clastic rocks and coralline limestone. Additional exposures appear as detrital of reefal components, as well as, calcarenite. It has more extensive exposures in all the three provinces of the Zamboanga Peninsula but its more contiguous occurrence is in Ipil, Titay, and Naga. Other significant exposures are in Buug, Bayog, Dumalinao, Guipos, San Miguel, Godod, Sindangan, Siayan and Midsalip. Ibañez et al (1956) and Antonio (1972) described this unit to also include interbeds of mudstone, sandstone, shale, and andesite. However, these older descriptions may refer to those intermittently encountered mélange bodies that contain mixtures of limestone, clastic rocks, volcanic rocks, and ultramafic rocks with ages varying from Early Miocene by Ibañez et al. (1956) to Oligocene-Early Miocene by Antonio (1972). As the concept of mélange had not been introduced in those earlier times, it is necessary in this current work to reinterpret the character of this formational unit, which solely refers to the limestone sequence. Brown (1950) gave a maximum thickness of 170 m for the formation, whereas Ibañez and others (1956) estimate the thickness to be more than 385 m. Age: Early to Middle Miocene

Zamboanga Volcanic Complex

The Zamboanga Volcanic Complex covers a vast portion most of the central to the eastern Zamboanga Peninsula. This unit is variably represented by volcaniclastics, volcanic breccia/ agglomerate, andesite, basalt, dacite, and pyroclastic rocks. It is distributed around Ipil, Kabasalan, Alicia, Malangas, Buug, Bayog, Lakewood, Kumalarang, Margosatubig, Tigbao, Dinas, Dumalinao, Pagadian and Midsalip. Many of which still have remnant volcanic edifices although these are already extinct volcanic structures. These Pliocene – Pleistocene volcanic rocks were named as Zamboanga Volcanics by Antonio (1972), which include basalt-andesite flows and associated pyroclastic rocks, hornblende andesite plugs, and dacitic plugs and cinder cones.

Radiometric K-Ar dating of samples of volcanic flows from east-central Zamboanga shows that the products of recent arc volcanism in the area range from 2.58 Ma to 0.41 Ma (Sajona and others, 1997). Tabular andesitic flows sampled at Pagadian gave ages of 2.58 Ma and 1.91 Ma; a sample from Buug gave an age of 1.71 Ma; basaltic andesite and basalt at Mt. Kaladis were dated 1.21 Ma and 1.08 Ma, respectively. Radiometric K-Ar dating of dacites from Lakewood gave ages of 0.97 Ma and 0.82 Ma. Cinder cones and lava domes overlying Middle Miocene and Late Miocene sediments in east-central Zamboanga and the Plio-Pleistocene basalts on the northernmost outcrops are dated 1.0 – 0.7 Ma. The youngest K-Ar age (0.4 ± 0.05 Ma) is that of a basaltic andesite flow collected northwest of Ipil. The Zamboanga Volcanic Complex may be correlated to the Mt. Maria Volcanics in the Zamboanga Peninsula. In Zamboanga Sibugay, it usually lies on top of the Early to Middle Miocene limestone stratigraphic unit, which is the Sibuguey Formation. Age: Middle Miocene to Pleistocene

Tigpalay Conglomerate

The Tigpalay Conglomerate was named by Miranda and Paredes (1965) for the coarse clastic rocks exposed from Tagasilay to Tigpalay. Aside from the conglomerate, the rock unit also includes lenticular beds of sandstone and shale. The conglomerate is thickly bedded with pebble to boulder-sized clasts of schist, quartz and other metamorphic rocks set in a sandy matrix. The sandstone is arkosic and coarse-grained. Age: Pleistocene

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 Bagolibud Point Quadrangle.

GEOLOGY OF THE BAGOLIBUD POINT QUADRANGLE

The Bagolibud Point Quadrangle covers minor portions of the municipalities of Siocon and Sirawai, Province of Zamboanga del Norte; Roseller T. Lim and a large portion of Tungawan, Province of Zamboanga Sibugay. The succeeding discussion focuses on selected outcrops starting with the metamorphic rocks followed by the ultramafic rock, mudstone-siltstone-sandstone sequence, sandstone-conglomerate sequence, and volcanic sequence. Numbered station points are used as location references, which are properly indicated in the location map (Figure 6).

Lithologic Exposures

Metamorphics Exposures (Tungauan Metamorphics)

The Tungauan Metamorphics widely occurs 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 thus treated as one unit, adopting the name Tungawan 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 Bagolibud Point Quadrangle, the Tungauan Metamorphics can be widely observed in the west and south portions of the Tungawan Municipality.

Barangay Linguisan, Tungawan

A peninsula on the east coast of Tungawan consists of two barangays namely Linguisan and Looc Labuan. The peninsula is elongated and is oriented NE-SW, with ridges parallel to its elongation. Most areas in these two barangays are composed of foliated metamorphic rocks, which are in contact with mélange bodies occurring along the southeast coast. Along the main access, the road can be found roadcuts or road surfaces showing weathered talc chlorite schist with occasional quartzite lenses. At Station #2007 (N7° 32’ 54.2”; E122° 28’ 13.2”) in Sitio Banga, Purok 2 appears a talc-chlorite schist on a road surface whose foliation trends at N60°E dipping 60°SE. A quarry in its vicinity at Station #2008 (N7° 32’ 55.4”; E122° 28’ 13.6”) yields chloritic phyllite, which grades down to chloritized sandstone mélange.

Photo 1. Along the unpaved access road exposures of a highly weathered schistose rock is observed at Station #2007 (N7° 32’ 54.2”; E122° 28’ 13.2”) in Barangay Linguisan, Tungawan.

Most rock exposures along the southeast coast of Barangay Linguisan are mélange bodies that consist of sandstone, chert and ultramafics. In the 2M Beach Resort at Station #1763 (N7° 32’ 28.3”; E122° 20’ 43.2”), a mélange body composed of sandstone grades into the phyllite stage of metamorphism. It has a greenish-gray color indicating pervasive chloritization. The rock exposure exhibits strong foliation which further underwent syncline folding whose fold axis is N40°W. Photo 2A shows a limb of the fold whose foliation structure trends N50°E dipping 70°NW.

Photo 2. Coastal exposure of a steeply dipping foliated phyllite to talc chlorite schist at Station #1763 (N7° 32’ 28.3”; E122° 20’ 43.2”) in Sitio 2M, Barangay Linguisan, Tungawan.

Barangay Looc Labuan, Tungawan

Behind the Tungawan View deck in Sitio Marmol Barangay Looc Labuan, Tungawan at Station #1766 (N7° 33’ 23.7”; E122° 20’ 34.5”) is a roadcut exposing a foliated metamorphic rock (Photo 3). It shows a moderately weathered talc chlorite schist with occasional quartzite lenses. Its local name “Marmol” means marble referring to the quartzite lenses which were mistaken as a marble. Its general foliation is oriented N-S dipping 5°E. About a kilometer to the southwest at Station #1975 (N7° 33’ 54.2”; E122° 26’ 19.1”) is another roadcut showing a highly weathered and argillized talc chlorite schist with foliation oriented N50°E dipping 30°SE. The color varies from cream-white, pale yellow to pale orange. This is the last appearance of the Tungauan Metamorphics as the remaining exposures westward are volcanic breccia.

Photo 3. A roadcut exposure of a highly weathered talc chlorite schist with thin quartzite lenses at Station #1766 (N7° 33’ 23.7”; E122° 20’ 34.5”) in Barangay Looc Labuan, Tungawan.

The northeast section of Barangay Looc Labuan consists mostly of foliated metamorphic rocks that vary from talc chlorite schist, chlorite carbonate schist and graphite schist. Occasional blocks of highly sheared chert and sandstone appear randomly among these metamorphic exposures. In Sitio Luksural at Station #1960 (N7° 34’ 23.0”; E122° 30’ 2.2”) the exposure of the Tungauan Metamorphics shows a quartz chlorite graphite schist. It has strong foliation trending N30°W dipping 30°SW with occasional occurrences of crenulated thin quartzite lenses.

Photo 4. Coastal exposure of graphite schist with sandstone and quartzite lenses at Station #1960 (N7° 34’ 23.0”; E122° 30’ 2.2”) in Sitio Luksural Barangay Looc Labuan, Tungawan.

In the southern coastal section of Barangay Looc Labuan at Station #1968 (N7° 32’ 50.4”; E122° 29’ 16.9”) can be found a talc chlorite schist exposed by a roadcut for a new access to another coastal area. Its foliation trend is N40°E dipping 80°SW. The outcrop appears crumbly having strong foliation and dense jointing normal to the foliation plane. In its vicinity is a sheared sandstone block which may be contained within the mélange structure.

Photo 5. A slope cut exposure of a highly weathered talc chlorite schist at Station #1968 (N7° 32’ 50.4”; E122° 29’ 16.9”) in Sitio Gapas-gapas Barangay Looc Labuan, Tungawan.

In the Gapas-gapas Beach Resort of Barangay Looc Labuan can be found extensive exposures of foliated metamorphic rocks comprising its rocky section of the coast. The Tungauan Metamorphics in the area includes sections of talc-chlorite schist to chlorite-carbonate schist. At Station #1969 (N7° 32’ 50.4”; E122° 29’ 6.8”) occurs strongly foliated chlorite to carbonate schist with colors of alternating mint green and yellowish-brown. The rock moderately reacts with an acid solution, which indicates foliated rock consists of carbonate. Its foliation trend is N30°W dipping 40°NE. This section of Gapas-gapas is swarmed with blocks of the chlorite-carbonate schist rubbles detached from the upper slopes.

Photo 6. Coastal exposure of an alternating intensely crenulated carbonate schist and chlorite schist at Station #1969 (N7° 32’ 50.4”; E122° 29’ 6.8”) in Sitio Gapas-gapas Barangay Looc Labuan, Tungawan.

Barangay Upper Tungawan, Tungawan

Throughout the stretch of AH-26 across the Tungawan Municipality, it is only in Barangay Upper Tungawan where the Tungauan Metamorphics appears on the surface as it mostly covered by either the Tigpalay Conglomerate or the Zamboanga volcanic Complex. At Station #1735 (N7° 47’ 46.3”; E122° 28’ 25.6”) in Purok 3, Barangay Upper Tungawan, a roadcut exposes a moderately weathered quartz talc chlorite schist (Photo 7). It has a cream to gray color with a foliation plane trending N10°E dipping 25°SE.

Photo 7. A slope cut the exposure of highly weathered quartz chlorite was observed at Station #1735 (N7° 47’ 46.3”; E122° 28’ 25.6”) in Barangay Upper Tungawan, Tungawan.

At Station #1736 (N7° 31’ 53.3”; E122° 28’ 3.7”) in Purok 3, Barangay Upper Tungawan, a marble hill appears beside the AH-26 (Photo 8). Having several huge balete trees growing in the area, it can be mistaken as limestone block especially since it exhibits karst features like caverns. However, upon examining the sample it shows a crystalline and massive texture of a marble. This marble block seems to extend westward as it has also been encountered along the road to Barangay San Vicente at Station #1882 (N7° 30’ 50.9”; E122° 22’ 11.7”), where the marble appears right across the road surface.

Photo 8. A karstic hill along the national road shows a granular marble with develop cavern structures identified at Station #1736 (N7° 31’ 53.3”; E122° 28’ 3.7”) in Barangay Upper Tungawan, Tungawan.

Barangay Datu Tumanggong, Tungawan

Barangay Datu Tumanggong has the largest area in the Tungawan Municipality and about three-fourths of its municipal area appear to be underlain by the Tungawan Metamorphics. At Station #1836 (N7° 36’ 47.9”; E122° 24’ 19.1”) along upstream of Litoban River, appears a foliated metamorphic rock composed of quartz talc chlorite schist with a general foliation trend of E-W dipping 50°N (Photo 9). Along the channel can be observed boulder-sized quartzite floats, which likely originate from rock exposures further upstream.

Photo 9. Along the upstream segment of Guinacutan River intensely crenulate quartz chlorite schist is exposed at Station #1836 (N7° 36’ 47.9”; E122° 24’ 19.1”) in Barangay Datu Tumanggong, Tungawan.

The mountainous west portion of Barangay Datu Tumanggong can be accessed via Dacon Area. Traversing along the logging interior access road from Barangay Guban (Sirawai) to Camp Canada and Camp Jordan, all the roadcuts reveal that the Tungauan Metamorphics underlie the entire stretch of the road. At Station #1993 (N7° 35’ 45.7”; E122° 17’ 3.1”) a roadcut shows a robust exposure of talc chlorite schist with a foliation trend of N-S dipping 10°E. Having a high degree of induration, its foliation plane as seen from Photo 10-B shows has become the flat surface of the road.

Photo 10. A highly weathered talc quartz chlorite schist is observed along the unpaved road and slope cut at Station #1993 (N7° 35’ 45.7”; E122° 17’ 3.1”) in Dacon Area, Barangay Datu Tumanggong, Tungawan.

Barangay Batungan, Tungawan

The area of Barangay Batungan stretches from its east border with Barangay Cayamcam for about 12-aerial kilometers to the west at the Tungawan-Sirawai boundary, based on the DENR cadastral municipal survey map. It generally has a mountainous terrain and most of its western portion has been covered by the DENR-granted IFMA area of Dacon. Along the Barangay Batungan road, which is one of the main access roads to Barangay Guban, the road exposures are mostly of the Tungauan Metamorphics. Most of the Barangay Batungan areas are underlain by metamorphic rocks. It is only in the barangay proper where volcanic rocks appear which overlie the metamorphic rocks. At Station #1776 (N7° 31’ 35.6”; E122° 21’ 29.0”) a quarry area exposes a quartz talc chlorite schist a foliation trend of E-W dipping 50°N (Photo 11). In another roadcut at Station #1771 (N7° 33’ 0.1”; E122° 20’ 48.3”), this metamorphic unit is represented by muscovite schist (Photo 12-B). It exhibits silvery micaceous flakes with manganese veinlets occurring as fracture fills, and in some portions as manganese nodules. The foliation trend is N50°E dipping 35°NW. Within the IFMA area in Camp London at Station #1999 (N7° 41’ 48.5”; E122° 24’ 48.9”), a roadcut shows a chlorite carbonate schist. It has well-defined foliation planes indicating color variation of bluish-gray and silvery-white. Most of the foliation sheets fizzle in acid, indicating carbonate components while the chlorite minerals form thin sheets that serve to split the relatively thicker carbonate sheets (Photo 13).

Photo 11. A quarry exposure of a moderately to highly weathered talc chlorite schist at Station #1776 (N7° 31’ 35.6”; E122° 21’ 29.0”) in Barangay Batungan, Tungawan.
Photo 12. A slope cut showing a talc mica schist with patches of manganese along the fracture observed at Station #1771 (N7° 33’ 0.1”; E122° 20’ 48.3”) in Barangay Batungan, Tungawan.
Photo 13. A road cut showing a banded to foliated carbonate schist and chlorite schist observed at Station #1999 (N7° 41’ 48.5”; E122° 24’ 48.9”) in Barangay Batungan, Tungawan.

Barangay Tigpalay, Tungawan

The Barangay Tigpalay area has a generally hilly to mountainous terrain which is the prevailing topography in the southern Tungawan area. About two-thirds of its barangay area particularly the west portion is underlain by the Tungauan Metamorphics. At Station #1810 (N7° 30’ 4.2”; E122° 20’ 44.8”) in Purok 7, a hillside beside the road reveals a marble exposure. It is characterized by varying colors (white, bluish-gray, and cream), crystalline and massive texture (Photo 14). It appears to be in contact with schist by a fault structure-oriented N60°E dipping 50°NW. Some joint planes appear to exhibit some sheen of thin micaceous material-oriented N70°W dipping 30°NE. In the Dacon Area around Camp Guadalupe, most of the roadcuts reveal talc chlorite schist. At Station #1818 (N7° 37’ 37.4”; E122° 19’ 13.0”) a roadcut shows a quartz talc chlorite schist with foliation plane oriented N30°W dipping 20°NE (Photo 15).

Photo 14. Blocky exposures of a white to cream-colored granular marble at Station #1810 (N7° 30’ 4.2”; E122° 20’ 44.8”) in Barangay Tigpalay, Tungawan.
Photo 15. A slope cut exposure of a highly weathered talc chlorite schist with thin lenses of quartzite was observed at Station #1818 (N7° 37’ 37.4”; E122° 19’ 13.0”) in Dacon Area, Barangay Tigpalay, Tungawan.

Ultramafics Exposures

There are few ultramafic rock bodies observed in the Bagolibud Point Quadrangle, which we considered part of the Polanco Ophiolite Complex. These have been encountered in the coastal Barangay Linguisan and upper Barangay Cayamcam in the Tungawan Municipality. Both exposures occur adjacent to talc chlorite schist bodies but are considered part of the Zamboanga Terrane.

Polanco Ophiolite Complex

Barangay Linguisan, Tungawan

In the southwest portion of the Linguisan peninsula can be found ultramafic rocks exposed by a quarry site and some roadcuts. It appears bounded by a mélange body in the southeast and by a talc chlorite schist in the northwest. At Station #1753 (N7° 33’ 3.5”; E122° 19’ 33.9”) a quarry site along the road in Sitio Tubo-tubo, Barangay Linguisan, Tungawan exposes highly sheared ultramafic rocks. This exposure stretches for about 500 meters along the road showing ultramafic rocks identified as highly sheared, serpentinized peridotite with a shearing trend of N40°E dipping 45°NW (Photo 16). It can also be treated as an ultramafic mélange with occasional sandstone lenses. Its top has a thin laterite cover of less than 1-meter thick. In another site about 300-meters to the northwest at Station # 1756 (N7° 32’ 50.2”; E122° 20’ 1.2”), the ultramafic rock exhibits a lesser shearing structure.

Photo 16. A quarry site in Sitio Tubo-tubo Barangay Linguisan, Tungawan exposing the ultramafic rocks of the Polanco Ophiolite Complex at Station #1753 (N7° 33’ 3.5”; E122° 19’ 33.9”).

Barangay Tigbucay, Tungawan

A roadside quarry site appears near the coast of Barangay Tigbucay at Station #1862 (N7° 32’ 45.1”; E122° 24’ 41.7”). Its contact with the talc chlorite schist at the immediate north has not been exposed but it is assumed to be a thrust fault contact similar to other areas in southern Tungawan, wherein the ultramafic rock is thrusted over the schist. The ultramafic rock is serpentinized, mylonitic and highly sheared, with the occasional presence of sandstone clasts. This may already be considered a mélange. Its shearing is oriented N40°E dipping 45°NW. On top of the ultramafic rock is a thin laterite cover of less than a meter thick. At Station #1863 (N7° 32’ 53.5”; E122° 24’ 37.1”) the sheared rocks are composed of gabbro.

Photo 17. Sheared ultramafic rocks are exposed along the road in Barangay Tigbucay, Tungawan at Station #1862 (N7° 32’ 45.1”; E122° 24’ 41.7”).
Photo 18. Exposure of a coarse-grained gabbro observed in Barangay Tigbucay, Tungawan at Station #1863 (N7° 32’ 53.5”; E122° 24’ 37.1”).

Barangay Cayamcam, Tungawan

In the northern portion of the Barangay Cayamcam can be found ultramafic rocks exposed by several quarry sites and roadcuts. This ultramafic body appears thrusted over the talc chlorite schist. At Station #1787 (N7° 30’ 27.1”; E122° 18’ 8.3”) a quarry site exposes highly sheared serpentinized peridotite with occasional clasts of gabbro. This can also be regarded as a mélange body consisting of ophiolitic blocks. Its shearing trend ranges from N40° to 70°W dipping 45°NE. Clay alteration chloritization and epidotization are commonly observed. The epidotization likely results from alteration of feldspars and pyroxene by localized hydrothermal fluids penetrating through fractures or boundaries between mélange blocks.

Photo 19. Ultramafic rocks exposed along the quarry area in Barangay Cayamcam, Tungawan at Station #1787 (N7° 30’ 27.1”; E122° 18’ 8.3”).
Photo 20. Another quarry exposure of the sheared ultramafic rocks with epidote alteration exposed in Barangay Cayamcam, Tungawan at Station #1788 (N7° 30’ 23.0”; E122° 17’ 42.7”)

Cayamcam, Tungawan

The mountainous north portion of Barangay Cayamcam is mostly underlain by foliated metamorphic rocks with occasional appearances of ultramafic rock bodies that were exposed as quarry sites. The metamorphic exposures disappear across a WNW-ESE topographic lineament where the terrain changes from mountainous to rolling of the barangay proper area. At Station #1791 (N7° 30’ 42.5”; E122° 17’ 5.6”), appears a quartz talc chlorite schist with a foliation trend of E-W dipping 40N (Photo 21). The quartzite lenses thicken up to 20 cm. At Station #1794 (N7° 30’ 59.6”; E122° 16’ 56.2”), the quartz talc chlorite schist has been thrusted over by a highly sheared ultramafic rock body (Photo 22).

Photo 21. A metamorphic rock exposure along the slope cut was observed at Station #1791 (N7° 30’ 42.5”; E122° 17’ 5.6”) in Barangay Cayamcam, Tungawan.
Photo 22. A road cut showing a thrust fault structure between the ultramafic and metamorphic rock was identified at Station #1794 (N7° 30’ 59.6”; E122° 16’ 56.2”) in Barangay Cayamcam, Tungawan.

Mudstone-Sandstone Sequence Exposure

The Zamboanga Formation comprises the Oligocene to Early Miocene turbidite sequence that occurs widely in the eastern segment of the Zamboanga Peninsula. It consists mainly of quartz-rich sandstone or quartz arenite which occurs commonly either thinly bedded or thinly bedded. In some of its bedded outcrops, it shows interbedded coarse-grained and fine-grained sandstone. In other sections, it interbeds with a massive polymictic clast to matrix-supported conglomerate. In the Bagolibud quadrangle, most of the exposures are observed in the north-eastern part mostly in barangays Sisay, Timbabauan, Gaycon, Baluran in Tungawan. Other exposures are observed in Tilasan, Casacon and Tupilac in Roseller Lim.

Zamboanga Formation

Barangay Tilasan, Roseller T. Lim

Along the eastern portion of the barangay, exposure of an indurated flysh-type turbidite deposit is identified that is capped by the younger volcanic sequence composed of pyroclastic and basaltic-andesite lava flows. At Station #1937 (N7° 39’ 13.7”; E122° 26’ 34.6”) the deep marine sedimentary sequence is highly indurated composed of an interbed of medium-grained quartz lithic arenite, fine-grained siltstone and mudstone. The strata are oriented at NS dipping 10°W (Photo 21). Its bed thickness ranges from 1 to 10 cm. Its color varies between beds from gray to dark brown with the occasional presence of calcareous components which effervesced during an acid test.

Photo 23. A flysch-type mudstone-sandstone interbed exposed in Barangay Tilasan, Roseller T. Lim at Station #1937 (N7° 39’ 13.7”; E122° 26’ 34.6”).
Photo 24. Another turbidite deposit observed in the upstream segment of the Tupilac River shows a flysch-type mudstone sandstone interbed with a rectangular to cubic fracturing that is exposed in Barangay Tilasan, Roseller T. Lim at Station #1936 (N7° 39’ 18.3”; E122° 26’ 2.8”).

Barangay Sisay, Tungawan

A turbidite sequence identified as part of the Zamboanga Formation appears along a quarry site adjacent to the provincial road in the north-western part of the barangay. At Station #1845 (N7° 49’ 26.8”; E122° 29’ 35.4”) appears a tilted sequence of flysch-type bedded arkosic sandstone. The sequence consists of interbedded friable mudstone and sandstone. It is moderately weathered which transformed its color from light gray to brownish from its usually dark gray (Photo 23). It orients N80°W dipping 20°NE with bed thickness ranging from 2 to 50 cm.

Photo 25. A flysch-type mudstone-sandstone interbed exposed in Barangay Sisay, Tungawan at Station #1845 (N7° 49’ 26.8”; E122° 29’ 35.4”).

Barangay Gaycon, Tungawan

A small quarry site fronting the barangay hall exposes a bedded sedimentary sequence of a turbidite deposit. At Station #1847 (N7° 36’ 6.8”; E122° 25’ 0.5”) the flysch-type interbedded sandstone-mudstone with intact strata is oriented N30°W dipping 30°NE (Photo 24). Its bed thickness ranges from 1 to 10 cm with color variation between beds as cream, brown to dark gray with the occasional presence of calcareous components. The medium-grained sandstone shows a friable fracturing and the fine-grained mudstone (shale) fissile fracturing which aligns with the lamination orientation. Perpendicular to the bed is tight joint structures oriented at N50°W dipping 50°SW. The turbidite exposure extends eastward 9-kilometers towards barangay Baluran.

Photo 26. A flysch-type mudstone-sandstone interbed exposed in Barangay Gaycon, Tungawan at Station #1847 (N7° 36’ 6.8”; E122° 25’ 0.5”).

Mélanges Exposure

The Bungiao Mélange (Yumul et al., 2001) consists of schist and serpentinized harzburgite blocks with minor marble clasts in a highly sheared serpentinized matrix. It is a tectonic melange usually thrusted against the Tungauan Metamorphics. These are elongated rock bodies that consist of chaotic blocks of ophiolite materials, chert, turbidite sequence, and marble hosted by crushed materials of varied compositions similar to those of the larger blocks. Structural alignment of the Linguisan Point and Tigbucay Ridge marks the boundary between the thrusted ophiolitic rocks on the metamorphic block with a general trend of NE-SW. In the Bagolibud Point Quadrangle, the exposures are observed in Barangay Linguisan, Cayamcam and Looc Labuan in Tungawan.

Bungiao Mélange

Barangay Linguisan, Tungawan

Along the rocky coast in Sitio Tubo-tubo, exposure of a highly brecciated to sheared melange is identified. At Station #1755 (N7° 32’ 57.5”; E122° 19’ 58.8”) the outcrop is consist of massive blocks of ferruginous chert and chloritized sandstone set in a sheared ultramafic matrix with elongation or shearing trend at N55°E dipping 40°NW (Photo 25). The greenish sandstone blocks undergo dynamic metamorphism forming a slaty to schistic rock with elongated lenses of quartzite while the ophiolitic (peridotite and gabbro) rocks show intense serpentinization with fine magnesia veins. This exposure is extending more than a kilometer southwest towards the Bangaan Island.

Photo 27. A melange exposure identified along the coastal section in Barangay Gaycon, Tungawan at Station #1847 (N7° 36’ 6.8”; E122° 25’ 0.5”).
Photo 28. Another coastal melange exposure with thick quartz stockworks and shows epidote alteration in Barangay Linguisan, Tungawan at Station #1755 (N7° 32’ 57.5”; E122° 19’ 58.8”).

Conglomerate Exposure

The polymictic conglomerate is composed of sub-rounded to well-rounded pebble to boulder-sized clasts of schist, quartz, limestone, chert and other metamorphic rocks set in a sandy matrix. The coarse-grained sandstone exhibits an arkosic matrix.

Tigpalay Conglomerate

Barangay Libertad, Tungawan

A quarry exposure along the interior road in the barangay shows a conglomerate sequence is identified. At Station #1957 (N7° 34’ 24.2”; E122° 29’ 43.1”) the outcrop is composed of a moderately indurated polymictic matrix to clast-supported conglomerate composed of sub-angular to well-rounded pebble to cobble-sized clasts of sandstone, chert, andesite and schist that is set in a coarse-grained sandstone matrix. This sedimentary unit overlies the turbidite deposit of the Zamboanga Formation. The individual beds range from 10 to 100 cm with color variation from brown to gray and are oriented N20°E dipping 40°NW. Perpendicular to the beds is a normal fault structure with a 50 cm displacement oriented at N30°W dipping 70°NE.

Photo 29. A mountain quarry exposure of a polymictic conglomerate sequence was observed in Barangay Libertad, Tungawan at Station #1957 (N7° 34’ 24.2”; E122° 29’ 43.1”).

Barangay Upper Tungawan, Tungawan

A roadcut along the national road exposes a conglomerate sequence capping the schistose basement of the Tungauan Metamorphics. At Station #1741 (N7° 32’ 2.6”; E122°28’ 36.4”) the outcrop is composed of a moderately indurated polymictic matrix-supported conglomerate composed of sub-angular to elongated chlorite schist and well-rounded quartzite pebbles set in a coarse-grained sandstone matrix. It is normally graded with beds oriented at N60°W dipping 35°NE.

Photo 30. A roadcut exposure of a conglomerate sequence dominated by schist-clast observed in Barangay Libertad, Tungawan at Station #1741 (N7° 32’ 2.6”; E122°28’ 36.4”).

Volcanic Sequence Exposure

All volcano-derived rock deposits include volcaniclastics, volcanic breccia/ agglomerate, andesite, basalt, dacite and pyroclastic rocks. These different volcanic rocks are to be treated as interlayered as a thick volcanic sequence. These volcanic units were deposited on top of the older rock formations.

Zamboanga Volcanic Complex

Barangay Magsaysay, Roseller T. Lim

A quarry exposure of volcanic rock sequence is identified along the barangay road. At Station #1944 (N7° 32’ 2.6”; E122° 28’ 36.4”) the outcrop is composed of moderately weathered brown volcaniclastics and gray porphyritic andesite. The tuffaceous volcaniclastic rock has a coarse-grained matrix that overlies the andesitic rock that exhibits a spheroidal weathering due to perpendicular joint sets oriented at N20°E, 70°NW and N 70°W, 80°SW.

Photo 31. A quarry exposure of moderately weathered volcanic sequence observed in Barangay Magsaysay, Roseller T. Lim at Station #1944 (N7° 32’ 2.6”; E122°28’ 36.4”).

Barangay Taglibas, Tungawan

A mountain quarry along the Taglibas-Looc Labuan barangay road exposes a volcanic deposit from Mt. Cinital. At Station #1978 (N7° 33’ 59.8”; E122° 25’ 14.0”) the outcrop is composed of a moderately weathered to well-indurated agglomerate composed of sub- to well-rounded andesite-basalt clast with minor chert and quartzite fragments that is probably derived from the Tigpalay Conglomerate sequence on which it overlies. The topography of the area is hilly to undulating wherein several outcroppings of the volcanic rocks are observed and it eventually disappears towards the eastern section of the barangay.

Photo 32. A quarry exposure of moderately weathered agglomerate was observed in Barangay Taglibas, Tungawan at Station #1978 (N7° 33’ 59.8”; E122° 25’ 14.0”).
Table 1. Canvassing/Inventory of Metalliferous and Non-Metalliferous deposit in the Bagolibud Point Quadrangle.

Figure 7. Geomorphometric maps of the quadrangle. [A] The topographic map shows area of equal elevation; [B] The relief map shows a 3D view of the topographic map; [C] The slope map shows the steepness of the topography; [D] The aspect map shows the direction where the slopes face.
Advertisement

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.