Initial Report on the Geologic Quadrangle Mapping for Madalag Point Quadrangle

By GEGS

The Mines and Geoscience Bureau Regional Office IX (MGB-IX) conducted systematic geological mapping in the Madalag Point Quadrangle (Sheet No. 3545-II) in Zamboanga Peninsula as 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 Madalag Point Quadrangle is located in the northern portion of the Zamboanga Peninsula and immediately adjacent to the Diwait Point Quadrangle on the east. It covers small portions of the Municipalities of Sindangan and Jose Dalman 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.

Madalag Point Quadrangle covers the latitude N8° 20’ 0” to 8° 30’ 0”, and longitude E122° 45’ 0” to 123° 0’ 0” which has a total landmass of 5 percent and covers only the southeastern portion of the quadrangle. From Dipolog Domestic Airport, the target municipalities covered by Madalag Point Quadrangle are easily accessible via 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.
Figure 2. Relief map showing the terrain of the study area.

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 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, 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 Madalag Point 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 the Aurora-Tukuran area to the 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.

Figure 4. Stratigraphic column for the Central Zamboanga Peninsula section showing previous works (GOP, 2012) and the modified stratigraphy of this work.
Figure 5. Plotting of Observation (Station) points in the Madalag Point Quadrangle.

GEOLOGY OF THE MADALAG POINT QUADRANGLE

The Madalag Point Quadrangle covers minor portions of the municipalities of Sindangan and Jose Dalman, Province of Zamboanga del Norte. The succeeding discussion focuses on selected outcrops starting with the ultramafic rock, followed by the chert and turbidite 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 and Ipil, the ophiolite exposures encountered are those along the Ipil, Titay, Tampilisan, Godod and Salug which have been traversed by the northeast-southwest trending Titay Fault. In the Madalag Point Quadrangle, the ophiolite exposures are limited to peridotite, pyroxenite and gabbro exposures in Barangays Dona Josefa (Sindangan) and Tamil (Jose Dalman). It was exhumed probably due to the deformation along with the NW-SE Sindangan-Cotabato Fault structure.

Barangay Tamil, Jose Dalman

An aggregate quarry exposure of highly sheared serpentinized ultramafic rocks was observed along the Sindangan-Jose Dalman highway. Station #27 (N8° 25’ 16.2”; E123° 0’ 18.5”) the ultramafic rock is composed of serpentinite and peridotite which has a silvery-greenish-black color that had undergone serpentinization process displayed by the serpentine fabric and glassy fibrous minerals. A dominant shear structure trending at N20W 35°SW is marked with white waxy magnesite veins. A leached layer of a reddish highly oxidized thin laterite deposit was noted along the upper horizon capped by brown organic-rich topsoil.  

Photo [1-4]. A quarry exposure of highly sheared ultramafic rocks observed in Barangay Tamil, Jose Dalman at Station #27 (N8° 25’ 16.2”; E123° 0’ 18.5”).

Barangay Dona Josefa, Sindangan

Along the rocky coast on the northern portion of the barangay, exposures of highly brecciated and sheared ultramafic rocks composed of gabbros and peridotites are observed to be structurally influenced. Station #05 (N7° 22’ 48.9”; E122° 58’ 41.8”) the gabbroic rock has a coarse-grained groundmass with euhedral pyroxene and plagioclase minerals. It is observed to be amalgamated in a fine-grained olivine-rich peridotite that is interpreted by the groundmass contrast. An old fault splay structure trending N40W dipping at 70°NE coincides with the general trend of the Sindangan-Siayan fault segment. This is cut by several N30-50E 70°NW trending fault structures marked with quartz veins and stringers.

Photo [5-8]. A highly brecciated ultramafic exposure along the coast in Barangay Dona Josefa, Sindangan at Station #05 (N7° 22’ 48.9”; E122° 58’ 41.8”)

Another coastal exposure of a highly sheared and indurated melangenic rock composed of coarse-grained gabbro, chert and peridotite rocks. The fractures are filled with quartz minerals with a dominant trend of EW 75°S which is transected by an N30°E 75°SE fault structure.

Photo [9-12]. A highly brecciated ultramafic exposure with fragmented chert lenses along the coast in Barangay Dona Josefa, Sindangan at Station #12 (N8° 22’ 43.4”; E122° 58’ 45.3”).

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 the Sindangan Municipality within the Madalag Point Quadrangle appear as clustered blocks in Barangay Nipaan, Motibot and Dona Josefa, Sindangan.

Barangay Motibot, Sindangan

A sparsely vegetated hilly terrain with multiple roadcuts exposes a highly weathered chert sequence. Station #19 (N8° 21’ 23.6”; E122° 59’ 29.1”) the chert is hydrothermally altered thus having cream to beige color with an argillic alteration. The weathered surface on the thinly bedded chert sequence has a powdery texture with fine granules of chert with a visible bedding trend at NS 15°W.

Photo [13-16]. A highly weathered to brecciated chert exposure along the road in Barangay Motibot, Sindangan at Station #19 (N8° 21’ 23.6”; E122° 59’ 29.1”).

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 lithic sandstone or arkosic sandstone, which occurs commonly either thinly or thickly bedded. In some of its bedded outcrops, it shows interbedded coarse-grained and fine-grained sandstone. In other sections, the sandstone interbeds with a massive conglomerate. Toward its base can be seen bedded conglomerate consisting of chert and basalt clast components, which are probably the lower section of the turbidite sequence. The occurrence of siliceous mudstone representing its base indicates the stratigraphic boundary with the chert/ophiolite basement.

Barangay Sto. Rosario, Sindangan

Slope-cut exposures of a thinly bedded turbidite deposit of the Zamboanga Formation were observed along the Motibot-Sto. Rosario barangay road. Station #15 (N8° 21’ 13.4”; E123° 0’ 15.2”) the moderated indurated fine-grained siltstone has a brown color and less reactive to acid. The highly indurated medium-grained lithic sandstone has a gray color and exhibits spheroidal weathering due to the perpendicular fracture system. The gray lithic sandstone is very reactive to acid indicating a high yield in foraminiferas. The bedded siltstone-sandstone sequence is oriented at N50E 50°SE which overlies the chert sequence 300 meters away from Station #19.

Photo [17-18]. A thinly bedded siltstone-sandstone sequence of a turbidite deposit of the Zamboanga Formation in Barangay Sto. Rosario, Sindangan at Station #15 (N8° 21’ 13.4”; E123° 0’ 15.2”).

Barangay Madalag, Jose Dalman

Along the western coastal section of the barangay, exposures of a bedded siltstone-sandstone sequence were identified corresponding to the mid-fan section of a turbidite deposit. At Station #34 (N8° 24’ 56.3”; E122° 58’ 47.2”) the turbidite outcrop is composed rhythmic sequence of a normally graded coarse-grained pebbly sandstone, medium-grained sandstone and fine-grained siltstone. The pebbly sandstone has a dark gray color and is poorly sorted with chert and quartz fragments, it also exhibits a cavernous surface of honeycomb (tafoni) weathering. The medium to fine-grained sandstones are partly chloritized, well-sorted and are resistant to wave disintegration. The dipping bed is oriented at N50E 35°SE with perpendicular joint sets trending at N30E 45°NW & N40W 40°SW filled with quartz vein material.

Photo [19-22]. Coastal exposure of a thickly bedded normally graded turbidite deposit in Barangay Madalag, Jose Dalman at Station #34 (N8° 24’ 56.3”; E122° 58’ 47.2”).

Barangay Manawan, Jose Dalman

Further north, the turbidite exposure composed of a mudstone-siltstone-sandstone sequence of the Zamboanga Formation extends in the coastal section of the barangay. At Station #47 (N8° 25’ 11.9”; E122° 59’ 0.5”) the turbidite outcrop is composed of normally graded medium-grained sandstone, fine-grained siltstone and siliceous mudstone. The medium to fine-grained sandstones is partly chloritized and well-sorted. The siliceous mudstone indicates the border between the sandstone and chert sequence which is observed 2 kilometers away from Station #28 (N8° 25’ 19.5”; E123° 0’ 13.0”). The dipping sandstone bed is oriented at N70W 50°SW with tight joint sets trending at N20-50E 45-80°SE with quartz fracture fills. The strike and dip orientations observed in Station #34 and #47 indicate a tight folding structure.

Photo [23-26]. Coastal exposure of a thinly bedded normally graded turbidite deposit observed in Barangay Madalag, Jose Dalman at Station #47 (N8° 25’ 11.9”; E122° 59’ 0.5”).

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.