Progress Report: Geologic Mapping for the Alicia Quadrangle

By General and Economic Geology Section

The Mines and Geoscience Bureau Regional Office IX (MGB-IX) conducted systematic geological mapping in the Alicia Quadrangle (Sheet No. 3542-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 Alicia Quadrangle is located in the south-central portion of the Zamboanga Peninsula and immediately adjacent to the Kabasalan Quadrangle to the north. It covers a small portion of the Municipality of Siay and a large portion covered by Payao, Malangas, Imelda and Alicia 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, and structural features 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 Alicia Quadrangle (violet box), one of the 6 targets for 2022 Quadrangle Geologic Mapping in Region IX, is located within the south-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 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 of 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.

The Alicia Quadrangle covers the latitude N7° 30’ 0” to 7° 40’ 0”, and longitude E122° 45’ 0” to 123° 0’ 0” is approximately centered along the eastern portion of the Payao Municipality. The target municipalities covered by Alicia Quadrangle are easily accessible along Asian Highway (AH26) from the Zamboanga International Airport via an all-weather national road which serves as the main access along the Zamboanga Sibugay Province. Far-flung areas can be accessed using a motorbike (locally known as habal-habal).

Figure 2. The municipalities that are covered by the Alicia 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 Alicia Quadrangle

Salug Chert

This unit refers to the well-bedded chert sequence distributed as patches of exposures in Tampilisan, Liloy, Salug, Sindangan, Siayan, Dumingag and Sominot. The occurrence of chert in the Zamboanga Peninsula is first reported in this current quadrangle geologic mapping campaign. Its wider windows of exposure have been encountered in Salug, Siayan, Dumingag and Sominot. However, the chert exposures in the Salug Municipality were earlier covered in this geologic mapping activity, thus, the unit is named after this Municipality. It is assigned an Eocene Age based on its stratigraphic relations with the underlying Polanco Ophiolite and the overlying Zamboanga Formation. Its best exposure is found along the Salug River in Brgy Pacuhan, Salug where it is estimated to have a thickness of 100 meters.

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 for 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 meters. Age: Early to Middle Miocene

Lumbog Formation

This sedimentary unit was introduced by Ibañez et al (1956) for the sequence of clastic and pyroclastic rocks with interbeds of coal in the Malangas-Kabasalan region. This sequence consists of sandstone, shale and mudstone with coal seams. Its mudstone-shale are commonly moderately indurated, characterized by thin beds that grade to lamination. It is occasionally subjected to hydrothermal alteration generating yellow to white clay particularly in some areas of Kabasalan and Titay. In general, such deposits may have been deposited in a lagoonal environment whereby lamination of mudstone develops. This unit also intertongues with the Zamboanga Volcanic Complex whose emplacement may have developed or affected the depositional environment of the Lumbog Formation.

Varying depositional environments created varying depositional materials exhibiting lithofacies changes in the sedimentary sequence. Hence, three members of the sequence were recognized by Ibañez et al (1956) namely, Lalat, Gotas, and Dumagok. The Lalat Member consists of mudstone, sandy shale and sandstone with interbeds of pyroclastic rocks, coal and limestone occurring in Imelda, Diplahan and Buug. The Gotas member consists of mudstones, shale and sandstone with thick interbeds of coarse pyroclastic rocks and has no coal beds. The Dumagok member consists mainly of sandstones, including medium-grained arkosic sandstone with few interbeds of mudstone, coal and pyroclastic rocks (GOP, 2004). Age: Late Miocene to Pliocene

Zamboanga Volcanic Complex

The Zamboanga Volcanic Complex covers a vast portion 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, 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

Olutanga Limestone

The Olutanga Limestone was named by Santos-Yñigo (1953) for the limestone at Olutanga Island, off Sibuguey Peninsula. It can be observed on the southern tip of the Sibugay and Baganian Peninsula. The Olutanga is characterized by alternating layers of thin calcareous tuffaceous rocks at the base that grades upward into coralline limestone and has an estimated maximum thickness of 50 meters. Age: Pliocene to Pleistocene

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

GEOLOGY OF THE ALICIA QUADRANGLE

The Alicia Quadrangle covers minor portion of the municipality of Siay and a large portion of Alicia, Malangas, Imelda and Payao, Province of Zamboanga Sibugay. The succeeding discussion focuses on selected outcrops starting with the chert sequence, siltstone-sandstone sequence, limestone sequence, shale-sandstone sequence, volcanic sequence, coralline limestone sequence and sedimentary breccia sequence. Numbered station points are used as location references, which are properly indicated in the location map (Figure 6).

Lithologic Exposures

Chert Exposures

The occurrence of chert in the Zamboanga Peninsula is first reported in this ongoing quadrangle geologic mapping MGB program. It was first extensively observed in the Palandok Quadrangle specifically in the Salug Municipality; hence it is referred to as Salug Chert. But this was even more widely distributed along the Siayan Accretionary Complex, particularly in the Mandih Quadrangle where it occurs as huge blocks. Chert as a sequence scattered adjacent to the multiple fault structures or as block components of mélange bodies. The chert exposures in the Alicia Quadrangle appear to be less where it appears either as subcrops or as component blocks of mélange bodies along the contact with the turbidite sandstone and limestone blocks.

Salug Chert

Barangay Katipunan, Payao

At Station #2793 (N7° 34’ 30.1”; E122° 49’ 21.3”) the protruding chert sequence appears purplish to brown that is highly fractured with signs of minor recrystallization filled with thin white quartz stringers. The beds range in thickness from 5 to 10 cm which is steeply dipping oriented at N10°E dipping at 80°NW which remains visible although most of the rock surface is are covered by moss. This oceanic sedimentary rock usually forms on top of an oceanic crust which is composed of an ophiolite sequence.

Photo 1. Exposures of a brecciated chert boulder were observed along the hilly section of Barangay Katipunan, Payao at Station #2793 (N7° 34’ 30.1”; E122° 49’ 21.3”).

Siltstone-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 thickly 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 Alicia quadrangle, most of the exposures are observed in Barangay Katipunan of Payao.

Zamboanga Formation

Barangay Katipunan, Payao

An abandoned quarry site in the southern portion of the barangay exposes a siltstone-sandstone sequence of a turbidite deposit. At Station #2788 (N7° 34’ 12.1”; E122° 49’ 52.5”) the lower section of the outcrop is generally thickly bedded with bed thickness of around 50 to 70 cm, while the upper section has thin beds at around 5 to 10 cm. The thicker beds in the lower section have has a chloritized coarse to medium-grained texture composed of mostly quartz and lithic fragments with intervals of laminated mudstone. The upper section is composed of fine-grained sandstone that alternates with the siltstone layer. When fresh, the rock appears gray and well-indurated, but when partly weathered, it shows some hematite components become brown and also become less indurated.

Photo 2. A thickly bedded siltstone-sandstone sequence was exposed in an abandoned quarry site on the Barangay Road in Barangay Katipunan, Payao at Station #2788 (N7° 34’ 12.1”; E122° 49’ 52.5”).

Barangay Katipunan, Payao

Along the unpaved barangay road going to the coastal purok of Barangay Katipunan, a flysch type sandstone-mudstone sequence appears right across the road at Station #2796 (N7° 34’ 26.8”; E122° 49’ 30.6”). Its bedding trend is N30°W 80°SW which is perpendicular to the road. The bed thickness of the gray-colored sandstone beds ranges from 1 to 5 cm, while the cream-colored mudstone is measured at 0.5 to 1 cm (sampled as ALI 10). The outcrop is also reactive to acid which indicates the presence of planktonic foraminiferas which could be used in dating the sample. This exposure is considered to represent the mid-fan section of a submarine fan deposit, and part of the Zamboanga Formation.

Photo 3. A flysch-type sandstone sequence was exposed along the road in Barangay Katipunan, Payao at Station #2796 (N7° 34’ 26.8”; E122° 49’ 30.6”).

Limestone Exposure

The Sibuguey Formation (Brown, 1950) consists of the Early to Middle Miocene limestone sequence occurring widely in the east section of the Zamboanga Peninsula. It includes the calcarenite, bedded detrital, and massively bedded coralline limestone. It has varied characteristics classified wackestone, packstone, calcilutite, calcarenite, calcirudite, and boundstone. Toward its base can be seen laminated calcarenite which serves as the transition from the underlying turbidite sequence named as the Zamboanga Formation In the Alicia Quadrangle, its distributions are occurring as isolated windows that are covered by younger rock formations in Barangay Katipunan and Bulacan in Payao and Barangay Kawayan in Alicia.

Sibuguey Formation

Barangay Katipunan, Payao

Along the unpaved road in the southwestern portion of the barangay, limestone rock exposures are identified. At Station #2794 (N7° 34’ 29.9”; E122° 49’ 22.3”) appears a massive limestone with an undetermined bed orientation. The limestone occurs as scattered subcrops protruding above the surrounding turbidite rocks. Its texture shows a considerable degree of crystallization, which has already obscured its fossils such as foraminiferas. It has become highly indurated with its sparitic/recrystallized texture.

Photo 4. Numerous outcroppings of a gray sparitic/recrystallized limestone were observed along the road in Barangay Katipunan, Payao at Station #2794 (N7° 34’ 29.9”; E122° 49’ 22.3”).

Another exposure of a limestone outcrop is exposed along the roadcut which shows a sparitic/recrystallized matrix with no visible large benthic foraminiferas. At Station #2799 (N7° 34’ 45.0”; E122° 49’ 44.3”) the limestone exposure is capped by a highly weathered volcanic sequence composed of volcanic breccia and volcaniclastic sequence.

Photo 5. Numerous outcroppings of a gray sparitic/recrystallized limestone were observed along the road in Barangay Katipunan, Payao at Station #2799 (N7° 34’ 45.0”; E122° 49’ 44.3”).

Barangay Kawayan, Alicia

The topographic configuration of the barangay is rolling to hilly wherein massive limestone subcrops are observed scattered along the road. At Station #2892 (N7° 32’ 54.0”; E122° 52’ 26.1”) the limestone has a white to gray color that shows recrystallization of the matrix with no visible detrital fossils. Overlying the limestone rock is composed of a weakly indurated bedded lacustrine deposit composed of organic-rich mudstone, sandstone and polymictic conglomerate.

Photo 6. A recrystallized limestone is exposed in Barangay Kawayan, Alicia at Station #2892 (N7° 32’ 54.0”; E122° 52’ 26.1”).

Shale-Sandstone Sequence

The Lumbog Formation is characterized by a mudstone-sandstone sequence of Late Miocene to Pliocene age. Since it commonly hosts coal seams, it is usually the target for coal exploration, which concentrates in the Province of Zamboanga Sibugay. Stratigraphically this unit overlies the Sibuguey Formation and intertongues with the volcanic sequence of the Zamboanga Volcanic Complex.

Barangay Kawayan, Alicia

At Station #2822 (N7° 31’ 45.4”; E122° 51’ 49.7”) along the Kawayan-Gulayon barangay road appears a sequence of interbedded mudstone, sandstone and conglomerate oriented at EW 5°-15°N. Its mudstone beds are laminated exhibiting thin layers of organic-rich laminae and tuffaceous laminae. Lamina thickness ranges from 1 to 5 mm. The sandstone beds usually have coarse-grained pebbly components having bed thicknesses of 10 to 50 cm. The conglomerate beds are 50 to 100 cm thick that are composed of rounded pebble to cobble clast of andesite, quartz and mudstone. Patches of rusting or oxidation are common, indicating the significant presence of iron-bearing materials such as goethite stain observed along the concrete canal. This sequence is recognized as part of the Lumbog Formation. Its interbedding of mudstone, sandstone and conglomerate results from varying conditions of its depositional environment as well as types of sediments.

Photo 7. An interbedded lacustrine deposit was exposed along a roadcut in Barangay Kawayan, Alicia at Station #2822 (N7° 31’ 45.4”; E122° 51’ 49.7”).

Barangay Lower Baluran, Imelda

Along the Poblacion-Lower Baluran barangay road, at Station #2848 (N7° 38’ 17.7”; E122° 56’ 35.9”) appears a thinly bedded organic-rich mudstone which is identified as Lumbog Formation. It is oriented N50°W dipping 20°SW with portions exhibiting undulating bedding character. It is laminated showing interlayers of cream, brown, gray and black laminae; the black laminae are the organic-rich layers that make the sequence lignitic. Such characteristics indicate the low energy depositional environment, which may be lacustrine or marshy areas.

Photo 8. A thinly bedded mudstone and sandstone sequence were exposed in Barangay Lower Baluran, Imelda at Station #2848 (N7° 38’ 17.7”; E122° 56’ 35.9”).

Volcanic Sequence

The Zamboanga Volcanic Complex is a widely distributed volcanic sequence consisting of pyroclastic materials, volcanic breccias, agglomerates, volcaniclastics and lava flows. These volcanic materials are products of Early Miocene to Pleistocene volcanism in the Zamboanga Peninsula. Since this volcanism generated a considerably large volume of volcanic materials distributed widely, most of the older rock formations have been covered. In the Alicia Quadrangle, the volcanic centers have scattered on the eastern portion of the quadrangle that is observed in the Municipalities of Payao, Malangas, Alicia and Imelda.

Zamboanga Volcanic Complex

Barangay Mountain View, Payao

Along the Nanan-Mountain View barangay road, ongoing road construction and widening projects exposed a volcanic rock sequence of the Zamboanga Volcanic Complex. At Station #2866 (N7° 38’ 13.8”; E122° 53’ 38.4”) the sequence is composed of an indurated thinly bedded coarse-grained gray andesitic volcaniclastic with visible fragmented plagioclase minerals. The bed thickness ranges from 2 to 7 cm that is oriented at N50°E dipping 45°NW. A highly weathered gray andesitic dike cuts across the volcaniclastic sequence which is having an argillic alteration and goethite oxidation.

Photo 9. A roadcut exposes a bedded volcaniclastic that is intruded by a gray andesitic dike in Barangay Mountain View, Payao at Station #2866 (N7° 38’ 13.8”; E122° 53’ 38.4”).

Barangay Poblacion, Payao

A mountain quarry site on the northern portion of the barangay exposes a highly jointed/fractured volcanic sequence. At station #2733 (N7° 35’ 45.2”; E122° 48’ 14.5”) the outcrop shows an indurated thinly bedded white to cream medium to a coarse-grained dacitic volcaniclastic sequence. The bed thickness ranges from 5 to 10 cm which is oriented at N50°E and dips at 30°NE. Oxidation halos are pervasive in the matrix that is derived from the corrosion of fine sulfide minerals.

Photo 10. A mountain quarry exposes a volcaniclastic sequence in Barangay Poblacion, Payao 2733 at Station #2541 (N7° 35’ 45.2”; E122° 48’ 14.5”).

Barangay Baluyan, Imelda

On-going road widening and concreting along the Baluyan barangay road exposes a volcanic sequence from Mt. Calades which is corresponding to the widespread volcanic centers in the Imelda-Alicia-Malangas area. At Station #2830 (N7° 35’ 18.4”; E122° 56’ 14.2”) the outcrop shows a volcanic sequence composed of massive moderately indurated gray polymictic clast-supported volcanic breccia hosted in a volcaniclastic matrix with angular clast of cobble to boulder-sized andesite, basalt and tuff. Overlying the breccia is a highly jointed thinly bedded brown to gray andesitic volcaniclastic. The volcaniclastic bed thickness is oriented at N60°E and dips 25°NW. Several normal fault structures cut across the outcrop with a displacement of 20-40 cm and have a structural measurement of EW 60°S.

Photo 11. A volcanic sequence deposit was observed in Barangay Baluyan, Imelda at Station #2830 (N7° 35’ 18.4”; E122° 56’ 14.2”).

Barangay Rebocon, Malangas

Along the upstream tributary of the Labakan River exposed a basaltic lava flow. At Station #2969 (N7° 38’ 15.5”; E122° 59’ 16.8”) the outcrop shows a dark gray to black porphyritic basalt flow that is exhibiting a columnar structure with visible plagioclase minerals. The groundmass shows partial silicification due to hydrothermal intrusion with the occurrence of fine quartz stringers. The joint systems are measured at N60°W 80°NE and N30°E 70°SE that is having a pentagon to hexagon-shaped fractures.

Photo 12. A columnar basalt flow was observed in the upstream tributary of Labakan River in Barangay Rebocon, Malangas at Station #2969 (N7° 38’ 15.5”; E122° 59’ 16.8”).

Coralline Limestone

The Olutanga Limestone is a young porous, coralline and cavernous limestone formation that occupies most of the coastal municipalities in the Baganian Peninsula. It forms an uplifted reef terrace that intertongues with the young volcanic sequence deposits. In the Alicia Quadrangle, most of the exposures are observed in Barangay San Roque, Mayabo and Dalama (Payao).

Barangay Dalama, Payao

A mountain quarry in the barangay shows a massive limestone sequence. At Station #2810 (N7° 30’ 31.0”; E122° 49’ 13.6”) appears a porous coralline limestone consisting predominantly of massive coral fossils, indicating a raised carbonate reef terrace. The strata range from 1 to 2 meters thick oriented at EW 5°S. Beds characterized by finer detrital bioclasts with finger corals are observed at the lower beds as well as at the upper beds. In the middle portion of the sequence occurs the 2-meter-thick bed consisting of massive corals. Such a sequence consisting of finer bioclasts at the base, massive coralline bioclasts in the mid-layer, and again finer bioclasts at the upper layer imply fluctuating sea levels that started from deep to shallow and deep again. This serves as evidence of cyclic sea level transgression and regression. Some fossils have been dissolved leaving molds while some have been replaced by calcite.

Photo 13. A porous coralline limestone sequence was observed in Barangay Dalama, Payao at Station #2810 (N7° 30’ 31.0”; E122° 49’ 13.6”).
Table 1. Canvassing/Inventory of Metalliferous and Non-Metalliferous deposits in the Alicia Quadrangle.
Figure 7. Plotting of Sampling points in the Alicia Quadrangle.
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