Progress Report: Geologic Mapping for the Diwait Point Quadrangle

By General and Economic Geology Section

The Mines and Geoscience Bureau Regional Office IX (MGB-IX) conducted systematic geological mapping in the Diwait Point Quadrangle (Sheet No. 3646-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 Diwait Point Quadrangle is located in the northeast portion of the Zamboanga Peninsula and immediately adjacent to the Madalag Point Quadrangle to the west. It covers a small portion of the Municipality of Sindangan and a large portion of Jose Dalman, Manukan and President Manuel Roxas 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, 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 Diwait Point Quadrangle (violet box), one of the 6 targets for 2022 Quadrangle Geologic Mapping in Region IX, is located within the northeastern 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 Diwait Point Quadrangle covers the latitude N8° 20’ 0” to 8° 30’ 0”, and longitude E123° 0’ 0” to 123° 15’ 0” is approximately centered along the southern portion of the Manukan Municipality. The target municipalities covered by Diwait Point Quadrangle are easily accessible along Highway 79 from the Dipolog Domestic Airports via an all-weather national road which serves as the main access along the Zamboanga del Norte Province. Far-flung areas can be accessed using a motorbike (locally known as habal-habal).

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

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

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 m. Age: Early to Middle Miocene

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

Timonan Formation

In the work of Antonio (1972) who named the Timonan Formation, it was described as consisting mainly of limestone and marl with minor shale, sandstone and conglomerate representing the Pliocene sedimentary sequence observed in Timonan area in Sindangan. In this work, the Timonan Formation is modified to mainly include the intercalation of calcarenite, marl and pebbly conglomerate, whereas the thick coralline limestone on top is to be separated as the Liloy Limestone. Based on the exposure in Timonan River Antonio (1972) described its lithology as sandstone with grains of quartz and ferromagnesian minerals as well as small rock fragments; the intercalated sandstone and shale are, in turn, underlain by poorly sorted, compacted conglomerate containing granule- to boulder-sized clasts of metavolcanic rock, diorite, amygdaloidal basalt, limestone, serpentinite and highly indurated clastic rocks. In the Liloy area, it is composed of calcarenite intercalated with the pebbly conglomerate. This sequence appears to grade its composition from calcareous to tuffaceous particularly from west to east of Liloy. 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.
Figure 6. Plotting of Observation (Station) points in the Diwait Point Quadrangle.

GEOLOGY OF THE DIWAIT POINT QUADRANGLE

The Diwait Point Quadrangle covers minor portion of the municipality of Sindangan and a large portion of Manukan, Jose Dalman and President Manuel Roxas, Province of Zamboanga del Norte. The succeeding discussion focuses on selected outcrops starting with the ultramafic rocks followed by the chert sequence, siltstone-sandstone sequence, limestone sequence, volcanic sequence, calcarenite-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

Ultramafics Exposures

There are few ultramafic rock bodies observed in the Diwait Point Quadrangle, which we considered part of the Polanco Ophiolite. These have been encountered in Barangays Tamil, Ponot, Tabon and Ilihan (Jose Dalman), Barangays Mate, Suisayan, Dipane, Gupot, Saluyong (Manukan). Exposures are only limited to peridotite, serpentinized peridotite, gabbro and diabase.

Polanco Ophiolite

Barangay Tamil, Jose Dalman

A mountain quarry slope cut in Barangay Tamil at Station #2511 (N8° 25’ 10.2”; E123° 00’ 27.0”) exposes a section of the ultramafic rock belonging to the Polanco Ophiolite. The outcrop is generally sheared resembling a mélange similar to most ultramafic rock exposures in the Zamboanga Peninsula. Its general shearing direction is N40°W dipping 60°NE, which appears to coincide with the regional NW-SE trend of the Sindangan-Cotabato Fault. Most of its shearing fragments are serpentinized and coated with greenish light gray-colored clay fragments. A mantle of dark red laterite covers the ultramafics at a thickness ranging from 20 to 50 cm. In some shear zones, the oxidation percolates deeper resulting in the thickening of the soil cover.

Photo 1. [A-C] A quarry site exposing a sheared ultramafic rock in Barangay Tamil, Jose Dalman at Station #2511 (N8° 25’ 10.2”; E123° 00’ 27.0”).

Barangay Suisayan, Manukan

A mountain quarry exposure of a highly sheared ultramafic rock was observed on the south-eastern section of the barangay. The terrain in Barangay Suisayan is generally rugged that is characterized by a thick surficial cover composed of a weakly indurated sedimentary breccia with several window exposure of the peridotite blocks. It commonly features terracettes as a form of soil creep that are dominant observed along the slope. Such ground characteristic appears in areas underlain by ultramafic rock. At station #2288 (N8° 27’ 28.0”; E123° 05’ 14.0”), the peridotite is commonly serpentinized and highly sheared, which progressed into a mélange along a general trend of N30°W dipping 40°NE to vertical (Photos 1 & 2). Its color ranges from dark gray in larger fragments to light gray in crushed or shear zones.

Photo 2. [A-B] Serpentinized and highly sheared peridotite exposed along quarry site in Barangay Suisayan, Manukan at Station #2288 (N8° 27’ 28.0”; E123° 05’ 14.0”).

Barangay Dipane, Manukan

A roadcut along the newly constructed Dipane-Gupot barangay road exposes a moderately weathered gabbro and diabase. At station #2550 (N8° 28’ 2.8”; E123° 5’ 37.1”) the gabbro has a dark gray to black coarse-grained groundmass with abundant biotite mineral marked by its flaky sheen. Several aphanitic diabase dikes intruded the gabbroic rock which has a different groundmass characteristic of having a finer groundmass and dominant plagioclase and pyroxene minerals. The dominant joint sets are measured at N50°W 40°SW and EW 70°W. A young porous detrital coralline limestone overlies the gabbro-diabase rock which forms a conical hilltop.

Photo 3. [A-B] A roadcut exposure of a highly jointed medium to coarse-grained gabbro was observed Barangay Dipane, Manukan at Station #2550 (N8° 28’ 2.8”; E123° 5’ 37.1”).

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 Diwait Point Quadrangle appear to be less where it appears either as subcrops or as component blocks of mélange bodies or as clasts of a younger conglomerate.

Salug Chert

Barangay Tabon, Jose Dalman

Along the road in Barangay Tabon, Jose Dalman at Station #2134 (N8° 26’ 49.8”; E123° 2’ 12.4”) a hilltop exposes several protruding chert subcrops. The chert appears as red-colored ribbon chert. Whether these are boulder floats left on the ground or as tips of larger chert blocks concealed below the ground remains to be known. Its beds are generally thin with a thickness of 0.5 to 2 cm.

Photo 4. [A-B] A chert subcrops exposed on the hilltop in Barangay Tabon, Jose Dalman at Station #2134 (N8° 26’ 49.8”; E123° 2’ 12.4”).

Barangay Mate, Manukan

A roadcut along the East Polacion-Mate barangay road exposes a thinly bedded chert sequence. At Station #2366 (N8° 28’ 46.7”; E123° 06’ 27.9”) the chert sequence appears pinkish red, brown to white that is highly fractured with signs of partial recrystallization. Its bedding has been obscured by the dense fractures although occasionally they are recognizable, it is oriented N30°W dipping 30°NE. Its beds range in thickness from 3 to 5 cm. Examined through a hand lens the chert chips contain abundant radiolarians, which will be further analyzed for their paleontology.

Photo 5. [A-B] A chert outcrop exposed along a roadcut in Barangay Mate, Manukan at Station #2366 (N8° 28’ 46.7”; E123° 06’ 27.9”).

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 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 Diwait Point quadrangle, most of the exposures are observed in the central and southern parts mostly in Barangays Sto. Rosario, Lapero, Labakid, Motibot and Upper Nippan in Sindangan. Other exposures are observed in Barangay Patagan, Pangandao, Meses, Serongan and Lingatongan in Manukan. The southern limit of President Manuel Roxas is showing significant exposures of the turbidite sequence that is partially covered by the younger volcanic sequence which can be observed in Barangays Panampalay, Balubo and Marupay.

Zamboanga Formation

Barangay Santo Rosario, Sindangan

A well-bedded turbidite sequence identified as the Zamboanga Formation appears at Station #15 (N8° 21’ 13.4”; E123° 0’ 15.2”) along the road in Barangay Rosario, Sindangan. The sequence consists of interbedded sandstone, mudstone and shale with an occasional calcareous component, described as a flysch-type deposit that represents the distal portion of a submarine turbidite fan. It generally appears as beige to brown color being composed of quartz, chert, magnetite, mafic minerals and plagioclase. Its beds range from less than 1 cm to 25 cm in thickness with a general orientation of N50°E dipping 50°SE. There is an observed cyclic repetition of sedimentation cycles from thicker beds with coarser components to thinner/laminated beds with finer grains for every 5-meters, which probably represent sets of Bouma Sequences.

Photo 6. A well bedded sandstone sequence exposed along the Barangay Road in Barangay Santo Rosario, Sindangan at Station #15 (N8° 21’ 13.4”; E123° 0’ 15.2”).

Barangay Upper Nipaan, Sindangan

At Station #2499 (N8° 21’ 13.5”; E123° 1’ 23.1”) in Barangay Upper Nipaan along the Motibot-Lapero Road, appears a well-bedded flysch-type sandstone-shale sequence exposure along a roadcut. The sequence consists of thinly bedded sandstone and shale with bed thickness ranging from 0.5 to 1 cm; oriented E-W dipping 40°N. There are also intermittent thickly bedded coarse-grained sandstone beds of about 20-cm thickness. The sequence appears beige which is the common color when moderately weathered. The weathered shale beds are commonly friable.

Photo 7. A flysch-type sandstone sequence was exposed along a roadcut in Barangay Upper Nipaan, Sindangan, at Station #2499 (N8° 21’ 13.5”; E123° 1’ 23.1”).

Barangay Lapero, Sindangan

Along a north-flowing creek in Barangay Lapero, Sindangan at Station #2489 (N8° 20’ 46.2”; E123° 3’ 15.3”) appears a well-bedded sandstone-shale sequence oriented N50°W 30°SW. Some shale intervals are identified based on their fissile character. Its bed thickness ranges from 1 to 20 cm; the thicker beds are pebbly coarse-grained sandstone while the thinner beds are the shale intervals. Its generally bluish color indicates a relatively fresh exposure.

Photo 8. A well-bedded sandstone sequence was exposed along the creek in Barangay Lapero, Sindangan, at Station #2489 (N8° 20’ 46.2”; E123° 3’ 15.3”).
Photo 9. A closer look at the sandstone-shale interbeds exposed along a creek in Barangay Lapero, Sindangan, at Station #2489 (N8° 20’ 46.2”; E123° 3’ 15.3”).

Barangay Suisayan, Manukan

In the south portion of Barangay Suisayan, Manukan shows exposures of the sandstone sequence of the Zamboanga Formation. At Station #2286 (N8° 27’ 17.9”; E123° 5’ 0.9”) along a descending creek with intermittent waterfalls appears the bedded sandstone sequence oriented N50°E dipping 20°NW. This site is located around the southeast foothills of Mt. Disakan and the said creek flows S30W toward the Disakan River. It shows a well-bedded sandstone sequence consisting of thick and thin beds. The thin beds are 2 to 20 cm in thickness while the thicker beds are 40 to 100 cm in thickness. The sandstone is generally arkosic in composition, occasionally laminated, well sorted and well indurated. By projection, this sandstone sequence appears to generally underlie the Mt. Disakan which is an elevated limestone sequence forming a Karst tower. In terms of stratigraphy, it is suggested that this that this sandstone stratigraphically underlies the raised limestone of Mt. Disakan.

Photo 10. A thinly bedded turbidite sequence exposed at Station #2286 (N8° 27’ 17.9”; E123° 5’ 0.9”) along the upstream tributary of the Disakan River was observed in Barangay Suisayan, Manukan.

Barangay Pangandao, Manukan

In front of the Pangandao Elementary School at Station #2330 (N8° 24’ 3.8”; E123° 6’ 47.6”) appears the Zamboanga Formation showing a well-bedded flysch-type section of the turbidite sequence. It consists of mostly thin sandstone and shale interbeds oriented N30°W dipping 15°NE. It is gray-colored, moderately indurated and moderately weathered. The bed thickness ranges from 0.5 to 3 cm and the overall thickness of the sedimentary exposure is about 10 meters.

Photo 11. A flysh-type turbidite deposit is observed along a roadcut at Station #2330 (N8° 24’ 3.8”; E123° 6’ 47.6”) in Barangay Pangandao, Manukan.

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, 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. It has a wide distribution in the Zamboanga Peninsula such as in Ipil, Naga, Titay, Kabasalan, Buug, Bayog, Dumalinao, Godod, Siayan and Sindangan. In the Diwait Point Quadrangle, its distributions are mostly in the Municipalities of Jose Dalman, Manukan and President Manuel Roxas.

Sibuguey Formation

Barangay Suisayan, Manukan

A twin conical-shaped hill observed along the western limit of the barangay shows a typical karst topography with vertical limestone cliff walls. This prominent peak is named as Mt. Disakan which has an elevation of 428 meters. At station #2297 (N8° 27’ 35.3”; E123° 4’ 31.7”) limestone rubbles are observed along the base of the hill which shows a cream to pinkish indurated sparitic/recrystallized detrital to coralline limestone. The rock chips taken from its rubbles have been fossiliferous showing Lepidocyclina sp. and Miogypsina sp. indicating an Early to Middle Miocene age, which correlates to the limestone sequence of the Sibuguey Formation.

Photo 12. [A-B] A fossiliferous limestone was exposed in Sitio Balatik Barangay Suisayan, Manukan at Station #2297 (N8° 27’ 35.3”; E123° 4’ 31.7”).

Barangay Dinasan, Jose Dalman

Along the western portion of the barangay, a highly vegetated hill is observed with towering sinkhole biomarker like Balete tree. At station #2054 (N8° 20’ 52.9”; E123° 4’ 36.8”) the limestone block is characterized by a highly karstified surface with well-developed speleothem features that can be observed near the cave entrance. The sparitic limestone has a gray to a white matrix with no visible foraminiferas which are obscured by intense recrystallization. The bed is oriented at N70°E dipping 5°SE.

Photo 13. [A-B] A recrystallized and brecciated limestone exposed in Barangay Dinasan, Jose Dalman at Station #2054 (N8° 20’ 52.9”; E123° 4’ 36.8”).

Calcarenite Sequence

Timonan Formation (Antonio, 1972) refers to the younger limestone and marl with minor shale, sandstone and conglomerate representing the Pliocene sedimentary sequence observed at Timonan area in Sindangan. In this work, the Timonan Formation is modified to mainly include the intercalation of calcarenite, marl and pebbly conglomerate, whereas the thick coralline limestone on top is to be separated as the Liloy Limestone. In the Diwait Point Quadrangle, it is composed mostly of calcilutite intercalated with clacarenite and pebbly conglomerate found in the north portion of the quadrangle.

Timonan Formation

Barangay Poblacion, Manukan

Along the Poblacion-Libuton barangay road, a weakly indurated calcareous mudstone is exposed along the roadcut. At station #2280 (N8° 30’ 19.6”; E123° 5’ 33.2”), the calcareous mudstone or calcilutite has a white color with traces of small planktonic foraminiferas. The bed thickness ranges from 30 to 50 cm which is oriented at EW dipping 20°N. Most of the exposure of the calcilutite-calcarenite sequence extends further eastward of the quadrangle.

Photo 14. A roadcut exposure of a thickly bedded calcareous mudstone was observed in Barangay Poblacion, Manukan at Station #2280 (N8° 30’ 19.6”; E123° 5’ 33.2”).

Barangay Lipakan, Roxas

A roadcut exposure along the barangay road shows an interbedded calcareous clastic which corresponds to the Timonan Formation. At Station #2582 (N8° 27’ 24.3”; E123° 12’ 16.0”) the outcrop shows an interbedded friable calcareous mudstone and calcarenite sequence. The calcilutite is weakly indurated which usually has a white matrix and tends to form a cubic fracture system. The calcarenite is moderately indurated and has a brown to beige matrix. The bed thickness ranges from 30 to 40 cm which is oriented at EW dipping 20°N. Further west, a porous coralline detrital limestone is overlying the calcilutite-calcarenite sequence that can be observed near the Gomez Cave.

Photo 15. Exposure of an interbedded calcarenite-calcilutite sequence exposed along the Lipakan-Capase road in Barangay Lipakan, Roxas at Station #2582 (N8° 27’ 24.3”; E123° 12’ 16.0”).

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 Diwait Quadrangle, the volcanic centers have scattered on the central portion of the quadrangle which usually covers older rock formations.

Zamboanga Volcanic Complex

Barangay Meses, Manukan

On-going road construction along the northern portion of the barangay exposes a dark volcanic rock sequence. At station #2386 (N8° 27’ 43.6”; E123° 7’ 32.8”), the outcrop shows a dark gray to black porphyritic basalt flow and flow breccia. The basaltic rock has a distinct vesiculous groundmass and large pyroxene minerals. The flow breccia is well-indurated and composed of cobble to boulder-sized basalt and andesite clast set in a basaltic groundmass. Some section of the outcrop shows intense oxidation. Jointed systems are measured at N20°W 30°NE and N50°E 70°NW.

Photo 16. A roacut exposes a dark gray basaltic rock in Barangay Meses in Manukan at Station #2386 (N8° 27’ 43.6”; E123° 7’ 32.8”).

Barangay Lupasang, Manukan

Along the upstream tributary of the Dohinob Dacu River exposure of massive boulders of indurated volcanic breccia is identified. At station #2541 (N8° 27’ 56.5”; E123° 8’ 29.9”) the volcanic breccia is clast-supported and composed of sub-angular to sub-rounded pebble to boulder-sized light gray andesite clast that is set in a volcaniclastic matrix.

Photo 17. [A-B] A volcanic breccia deposit occurring in Barangay Lupasang, Manukan at Station #2541 (N8° 27’ 56.5”; E123° 8’ 29.9”).
Table 1. Canvassing/Inventory of Metalliferous and Non-Metalliferous deposits in the Diwait Point Quadrangle.
Figure 7. Plotting of Sampling points in the Diwait Point Quadrangle.
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.