Initial Report on the Geologic Quadrangle Mapping for Mandih Quadrangle

By GEGS

The Mines and Geoscience Bureau Regional Office IX (MGB-IX) conducted systematic geological mapping in the Mandih Quadrangle (Sheet No. 3644-IV) in the Zamboanga Peninsula is part of the 2021 Quadrangle Geological Mapping Project. The purpose of the mapping program is to produce a 1:50,000 scale quadrangle map in Region IX which one of the least mapped areas in Mindanao. Four (4) quadrangles are targeted for the year 2021 all of which have been unmapped. The Mandih Quadrangle is located in the northeastern portion of the Zamboanga Peninsula and immediately adjacent to the Mt. Dapiak Quadrangle on the east. It covers a small portion of the Municipality of Dumingag in Zamboanga del Sur and a large portion of the Municipalities of Siayan and Sindangan in Zamboanga del Norte.

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.

The study area is located in the Zamboanga Peninsula which lies roughly at the southern part of the Philippine Archipelago chiefly in 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.

Mandih Quadrangle covers the latitude N8° 10’ 0” to 8° 20’ 0”, and longitude E123° 0’ 0” to 123° 15’ 0” and is approximately centered in the Municipality of Siayan in Zamboanga del Norte. From Zamboanga International Airport, the target municipalities covered by Mandih 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 1. Location map of the study area indicated by the black box.

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. 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 2. 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, representing the 250-km long narrow west-southwest extension of western Mindanao Island. It is bordered along the NW side by the Sulu Trench (Reference), 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 (references). 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 (references) agree that the Zamboanga Peninsula is almost entirely characterized by continental affinity which contrasts with the bigger portion of Mindanao Island like a lateral outline. Its width ranges from 100 meters to more than 1 kilometer and lengths of 1 to 5 kilometers. Its outcrops normally exhibit a chaotic array of different components but structurally align with the NW-SE regional trend. These commonly occur along topographic lineaments, linear river channels, lithologic boundaries of fault structures. These were probably formed during the major tectonic collision of the Palawan- Zamboanga plate with the Philippine Mobile Belt during the Middle to Late Miocene, thus, its age.

LOCAL STRATIGRAPHY

Stratigraphic Units in the Mandih Quadrangle

Polanco Ophiolite

A contiguous occurrence of rock bodies that comprise an ophiolitic sequence occurs in the central section of the Zamboanga Peninsula. These include the peridotite/ serpentinized peridotite that stretches from Ipil (northern), Titay, Tampilisan to Liloy. Extensive exposures of the peridotite in Titay and Tampilisan 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 gabbros have been encountered in the southern portion of Tampilisan Municipality. The sheeted dike complex appears in the northern portion of Tampilisan.  Based on the Ocean Plate Stratigraphy (OPS) hypothesis that the overriding sedimentary suites of the East Zamboanga Block have the same ophiolite unit from Aurora-Tukuran area to Ipil-Titay-Tampilisan-Liloy area, then the previously described Polanco Ophiolite Complex should extend from the east end of the Zamboanga Peninsula up to the Titay Fault area.

Salug Chert

This unit refers to the well-bedded chert sequence distributed as patches of exposures in the central to northern portions of Central Zamboanga Peninsula. It occurs in Brgy Balakan (Salug), Brgy Overview (Liloy), and in Brgys New Dapitan, Malila T and Tiningaan (Tampilisan). It is generally red-colored thinly bedded. Many of its occurrences coincide with nearby ultramafics such as in New Dapitan and Overview, implying a conformable stratigraphic relationship between the chert and the ophiolitic materials. In other exposures such as New Dapitan and Tiningaan, the chert sequence occurs together with deep-marine clastic deposit; although their stratigraphic relation is not clearly shown, it is assumed that the pelagic deposit is conformably overlain by a turbidite sequence. It was decided to recognize it as a separate unit instead of being a mere carapace of an oceanic crust because it serves as the oldest sedimentary unit in the region. 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 the conglomerate. In this study, this clastic sedimentary sequence is regarded as a turbidite deposit representing the proximal to the distal section of a turbidite sequence, which consists of a massive conglomerate, interbedded conglomerate, quartz-rich sandstone and shale. Although Querubin et al (1999) renamed this unit as Camanga Sediments, the name Zamboanga Formation is again re-adopted for its significant role being the among the widest distribution and the earliest rock formation formed under submarine environment. It was earlier assigned under Early Miocene age by Antonio (1972), but based on nannofossils, giving late Late Oligocene age, its age range is adjusted earlier to Oligocene to Early Miocene.

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. 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, Godod, Siayan, Guipos and San Miguel. The unit variably appears as coralline, detrital and calcarenite. Ibañez and others (1956) described its lower portion being composed of mudstones with interbedded sandstone; the middle portion is characterized by sandstones with interbedded mudstones and sandy shale; the upper portion is composed of sandy shale with interbeds of limestone, calcareous shale and sandstones. Antonio (1972) adopted the term to include the folded and thermally metamorphosed interbedded sequence of clastic rocks and andesites with lenses of irregular masses of marbleized limestone widely exposed west of Sibuguey River from Siogan in the south to Luanan in the north. An Early Miocene age was assigned by Ibañez and others (1956) for the rock unit, although Antonio (1972) extends its age down to Oligocene. 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.

Midsalip Diorite Complex

Several diorite intrusive bodies have been encountered in the geologic quadrangles surveyed in the mid-section of the Zamboanga Peninsula. These occur as stocks occurring in Bayog, Lakewood, Buug, Sindangan, Midsalip and Lison Valley (Pagadian) and was named as Sibuguey Diorite by Antonio (1972), which was renamed as the Midsalip Diorite in GMRP (1982). However, recognizing the different diorite variants identified, namely, the quartz monzonite, monzodiorite, tonalite (quartz diorite) and hornblende diorite with occasional cross-cutting relationship, these are treated as separate intrusives emplaced in multiple intrusions.

Gunyan Melange

The Gunyan Melange was named by Yumul and others (2000) for the chaotic mega blocks of igneous and sedimentary rocks set in a serpentinized and clayey matrix. The Melange is a combination of tectonic and sedimentary melange distributed in a linear manner near the center of the so-called Siayan-Sindangan Suture Zone (also known as Sindangan-Cotabato Fault) in Gunyan, Siayan. The tectonic melange consists mainly of ophiolite-derived blocks of harzburgite, gabbro, basalt and chert in a serpentinite matrix. The blocks range in size from tens of meters to kilometer-sized hills. The ophiolite-derived blocks even include chromitites enveloped in dunite at Gunyan and its vicinity. The sedimentary melange, on the other hand, consists of sandstones, andesites, schists, as well as limestone ranging in size from boulders to kilometer-sized blocks set in a clayey matrix. An Oligocene age determined for one of the limestone blocks suggests an Early Miocene age for the Suture Zone as well as for the Melange.

Zamboanga Volcanic Complex 

The Zamboanga Volcanic Complex covers a vast portion of 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 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. Kiladis 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.

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 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. 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. Pliocene Age?

Figure 3. Stratigraphic column for the Central Zamboanga Peninsula section showing previous works (GOP, 2012) and the modified stratigraphy of this work.
Figure 4. Relief map showing the terrain of the study area.
Figure 5. Plotting of Observation (Station) points in the Mandih Quadrangle.

GEOLOGY OF THE MANDIH QUADRANGLE

The Mandih Quadrangle covers minor portion of the Municipality of Dumingag, Province of Zamboanga del Sur and a large portion of the municipalities of Siayan and Sindangan, Province of Zamboanga del Norte. The succeeding discussion focuses on selected outcrops starting with the ultramafic rock, followed by the chert sequence, turbidite sequence, limestone-clastic sequence, diorite-monzonite complex, volcanic sequence and calcareous clastic sequence. Numbered station points are used as location references, which are properly indicated in the location map (Figure 5).

Lithologic Exposures

Ultramafics Exposures (Polanco Ophiolite)

The Polanco Ophiolite consists of residual peridotites, cumulate peridotites and gabbro, sheeted dike complex and basalts. In the geologic mapping program for the past two years covering the quadrangles of Liloy, Titay, Ipil, Palandok, Kabasalan River, Kabasalan, Sindangan River, Lakewood and Kumalarang, the ophiolite exposures encountered are those along the Ipil, Titay, Tampilisan, Godod, Salug, which have been traversed by the northeast-southwest trending Titay Fault; and those in Dumingag, Bayog and Tigbao, which are in the east side of the Zamboanga Peninsula. In the Mandih Quadrangle, the ophiolite exposures are composed of serpentinite, peridotite, gabbro, diabase to pillow basalt with most exposure observed in the Municipality of Sindangan and Siayan. It was exhumed probably due to the deformation along the Sindangan-Siayan-Daguma-Cotabato fault structure.

Barangay Gunyan, Siayan

An open-pit mine along the cogon-covered highly dissected terrain on the central part of the municipality showing a highly sheared to partly weathered ultramafic rocks. At Station #536 (N8° 11’ 29.9”; E123° 10’ 3.8”) the greenish-black peridotite rock appears to be serpentinized mark by a high presence of serpentine minerals in the form of asbestiform silicates. Lenses of massive chromite minerals are observed along the dunite rocks which have a metallic luster and dense weight. The rock is highly sheared trending at N50W 40°SW which is transected by a subsidiary fault structure of the Sindangan-Cotabato fault.

Photo [1-4]. A highly sheared to brecciated ultramafic exposure with lenses of chromite observed in Barangay Gunyan, Siayan at Station #536 (N8° 11’ 29.9”; E123° 10’ 3.8”).

Barangay Sto. Niño, Sindangan

A newly-opened road along the steep-sided section of the barangay showing a highly brecciated to partly weathered basaltic volcanic rock. At Station #811 (N8° 13’ 38.3”; E123° 04’ 7.7”) the amygdaloidal basalt has a gray porphyritic groundmass and having a pillow margin marked with oxidized ferromagnesian minerals which are formed from underwater volcanism. The vesicles are filled with zeolite minerals with euhedral or well-formed pyroxene crystals set in the fine to coarse-grained groundmass.

Photo [5-8]. A highly brecciated pillow basalt was observed along a roadcut in Barangay Sto. Nino, Sindangan at Station #811 (N8° 13’ 38.3”; E123° 04’ 7.7”).

Chert Exposures (Salug Chert)

The occurrence of chert in the Zamboanga Peninsula is first reported in this current quadrangle geologic mapping campaign. The chert exposures in Siayan and Sindangan Municipality within the Mandih Quadrangle appear as clustered mega-blocks and lenses set in a highly sheared matrix composed of ophiolitic bodies and sandstone blocks.

Barangay Poblacion, Siayan

Along the Sindangan-Dumingag (Road 965), subcrop exposures of reddish ferruginous cherts mega-blocks are noted in the ultramafic rock bodies. At Station #257 (N8° 15’ 9.8”; E123° 05’ 44.5”) chert subcrops appear to be sheared and partly recrystallized with slickensides feature. The chert blocks are interpreted to be exhumed and aligned with the fault structure occurring as a melange component.

Photo [9-12]. Subcrops of ferruginous chert were observed in Barangay Poblacion, Siayan at Station #257 (N8° 15’ 9.8”; E123° 05’ 44.5”).

Turbidite Sequence (Zamboanga Formation)

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 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 conglomerate. Toward its base can be seen bedded conglomerate consisting of chert and basalt clast components, which are probably basal conglomerate that indicates stratigraphic boundary with the chert/ophiolite basement.

Barangay Moyo, Siayan

Along the upstream segment of the Piao River, a bedded deep marine sedimentary sequence is observed. At Station #504 (N8° 16’ 44.3”; E123° 06’ 39.4”) the clastic rocks have a rhythmic sequence of polymictic pebble to boulder-sized matrix-supported conglomerate composed of limestone, chert, basalt, and sandstone clast followed by a coarse to fine-grained gray to brown interbedded lithic sandstone-mudstone. Its beds range in thickness from 1 to 50 cm which orient at N40W dipping 40°SW.

Photo [13-16]. A rhythmic sequence composed of an indurated mudstone-sandstone-conglomerate was observed along the upstream tributary of Piao River in Barangay Moyo, Siayan at Station #504 (N8° 16’ 44.3”; E123° 06’ 39.4”).

Limestone-Clastic Sequence (Sibuguey Formation)

The Sibuguey Formation consists of the Early to Middle Miocene limestone sequence occurring widely in the east segment 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. The Sibuguey Formation has wide distribution exposed significantly in Ipil, Naga, Kabasalan, Buug, Bayog, and Godod.

Barangay Disud, Sindangan

The northern side of the barangay is a limestone area where caves and sinkholes occur. At Station #390 (N8° 15’ 42.3”; E123° 00’ 30.2”) outcrop of an indurated highly brecciated white, sparitic, fossiliferous limestone is observed to be amalgamated indicating a fault structure traversing the area. The limestone is conformable with the mudstone-sandstone sequence which is also brecciated.

Photo [17-20]. Subcrops of amalgamated brecciated sparitic limestone is observed along the Sindangan-Siayan road in Barangay Balok, Sindangan at Station #390 (N8° 15’ 42.3”; E123° 00’ 30.2”).

Intrusives (Midsalip Diorite Complex)

Several exposures of diorite intrusive bodies have been encountered in the geologic quadrangles surveyed in the mid-section of the Zamboanga Peninsula. These were encountered in the municipalities of Bayog, Lakewood, Buug, Midsalip, Sindangan and in the west portion of Pagadian City. Previous works that identified these diorite intrusive bodies named some of them as Midsalip Diorite, Sibugay Diorite, and Tres Reyes Microdiorite. The diorite exposures observed in this survey appear to vary from quartz-rich, K-feldspar-rich and mafic-rich. In this work, which is of a regional geology level, these diorite intrusive bodies are grouped as the Midsalip Diorite Complex.

Barangay Moyo, Siayan

A roadcut exposure of weathered diorite intrusive was observed along the western portion of the barangay. At Station #708 (N8° 16’ 56.8”; E123° 04’ 58.2”) the outcrop of a highly jointed coarse-grained quartz diorite with quartz-sulfide stockwork. The occurrence of epithermal gold mineralization is marked by the argillic alteration of the host rock.

Photo [21-24]. A roadcut exposure of a highly weathered diorite intrusive observed in Barangay Moyo, Siayan at Station #708 (N8° 16’ 56.8”; E123° 04’ 58.2”).

Volcanic Sequence (Zamboanga Volcanic Complex)

The Zamboanga Volcanic Complex is the widely distributed volcanic sequence consisting of pyroclastic materials, volcanic breccias, agglomerates, volcaniclastics, lava flows, etc… 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 Quadrangle this unit has widely covered in the south-western portion of the Mandih Quadrangle.

Barangay Litolet, Siayan

Along a rugged section of the barangay, subcrops of vesiculous andesitic lava deposits are identified along the lake periphery. At Station #769 (N8° 14’ 2.3”; E123° 12’ 21.4”) the andesite rock has a bluish-gray color set in a fine-grained gaseous groundmass. The lake is a crater of a small ephemeral volcano that emerged along the Sindangan-Cotabato Fault. It is interpreted that some crustal fractures form along complex fault zones and serve as short-lived vents for magmatic materials to generate small volcanoes along the vicinity having a lesser geobaric pressure.

Photo [25-28]. Subcrops of vesiculous andesite boulders are noted along a hilltop in Barangay Litolet, Siayan at Station #769 (N8° 14’ 2.3”; E123° 12’ 21.4”).

Calcareous Clastic Sequence (Timonan Formation)

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.

Barangay Piao, Sindangan

Along the Sindangan-Siayan road, a bedded calcareous clastics were observed that extends eastward. Station #781 (N8° 14’ 9.7”; E123° 00’ 7.9”) the bedded coarse-grained calcareous sandstone or calcarenite has a cream to beige color and porous matrix. This sequence is deposited from a shallow marine environment and overlies the sparitic or recrystallized limestone.

Photo [29-32]. A roadcut exposure of a bedded calcareous clastic was observed in Barangay Piao, Sindangan at Station #781 (N8° 14’ 9.7”; E123° 00’ 7.9”).

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