Karst Subsidence Hazard Assessment and Mapping in the Municipality of Godod, Zamboanga del Norte

By General and Economic Geology Section

The conduct of the 2022 Karst Subsidence Hazard Mapping target area commenced on August 24, 2022, in the Municipality of Godod, Zamboanga del Norte.  This annual program is part of the continuing Geohazards Assessment and Mapping Program of MGB that has now expanded to include subsidence hazards due to sinkhole collapse and karst subsidence. Sinkholes are present in areas characterized by karst topography and underlain by limestone or calcareous lithologies.

The work program generally aims to assess and locate the presence of ground subsidence, sinkhole collapse, and caves using geomorphological information and quantify subsurface cavities. The program will also determine the subsidence susceptibility of the study area as a result of sinkhole and cover collapse, particularly in selected local government infrastructure sites such as school building grounds and other priority areas identified by the Local Government Units (LGUs) which are commonly used as evacuation sites during natural disaster; and after carrying out the survey, conduct awareness campaign on sinkholes and other karst features, and issuance of threat advisories to LGUs to the hazard subsidence due to sinkhole collapse.

The methods to be employed during the field campaign includes inventory of pre-delineated sinkholes interpreted from the 2015 NAMRIA Interferometric Synthetic Aperture Radar (IfSAR) satellite image, gathering of anecdotal accounts, Information, Education and Communication (IEC) Campaign, and issuance of “Threat Advisories” on the concern barangays in the Municipality of Godod where sinkhole and ground subsidence can develop.

Godod is a fourth income class municipality in the central portion of Zamboanga del Norte Province and covers a total land area of 190 km2. It is bounded by the Municipalities of Salug to the north, Leon Postigo in the east, Tampilisan on the west and Kabasalan to the south. Based on the Philippine Statistic Authority 2020 population census, it has a total of 17,510 inhabitants distributed in the 17 barangays.

The municipality is approximately 127 km southwest of Dipolog City and can be accessed by any mode of land transportation via Ipil-Dipolog Highway (Highway 79) and passing through the Salug-Godod Road. The road network within the peninsula links Godod to other urbanized cities in the region such as Pagadian City and Zamboanga City.

The 17 barangay centers can be easily accessed via a series of mixed paved, unpaved, sheeted, and concreted access tracks from the national highway. Access to far-flung barangays can be hampered by seasonal rain and irregular public transport serving these areas.

Figure 1. Location map of the Municipality of Godod, Zamboanga del Norte.

GEOLOGICAL AND GEOMORPHOLOGICAL ASSESSMENT

During the first course of the activity, the team conducted an in-situ outcrop/subcrop rock identification and sampling, paleontologic and physical analysis which are vital information needed for the finalization of the technical report. Delineation of lithologic boundaries/geologic contacts and structures was also conducted within the municipality to delineate areas with high concentrations of carbonate rocks which are prone to Karst landform formation. As for the initial result, the lithological boundaries and structures from the existing Regional Geologic Map of Zamboanga Peninsula are expected to be modified based on ground validation/field observations conducted.

There are two tectonic terranes forming the Zamboanga Peninsula, namely the Labason Terrane and Zamboanga Terrane. The Labason Terrane is composed largely of a metamorphic complex of phyllite, schist, granulite and gneiss while the Zamboanga Terrane is composed of an Ocean Plate Stratigraphy (OPS) having an ophiolite complex as its basement that is overlain by chert, turbidites, limestone, volcanics, and organic-rich mudstone. The Titay Fault is a thrust structure forming a distinct topographic lineament that exhibits juxtaposed crustal blocks of a totally different composition. Along this fault line, the Zamboanga Terrane is being thrusted toward the northwest over the Labason Terrane. The Municipality of Godod is found within the Zamboanga Terrane.

The Zamboanga Terrane comprises of an OPS with an ophiolite complex as basement, referred to as the Polanco Ophiolite (Yumul et al, 2000). It is inferred to be of Cretaceous age and consists normally of the typical ophiolite sequence having ultramafic rocks (peridotite), gabbro, sheeted dike complex, and basalt. These rocks are exposed along the Titay Fault around the Titay-Tampilisan area but are projected to extend deep below the Zamboanga Terrane wherein some exposures appear in Bayog, Tigbao, Siayan, and Dumingag. In the Municipality of Godod, ophiolite exposures are found in barangays Sarawagan and Guisapong.

Photo 1. [A-C] A highly sheared and serpentinized peridotite exposed along barangay road in Barangay Sarawagan, Godod.

The Polanco Ophiolite is stratigraphically overlain by the Eocene Salug Chert (MGB-9, 2019) which occurs as dismembered minor blocks along the Titay Fault, particularly around the Titay-Tampilisan area. The presence of a chert sequence (Salug Chert) indicates an Ocean Plate Stratigraphy (OPS) reflecting an abyssal environment of deposition. There has been no encounter of chert exposure within the Godod but it is like occurs in the northwest section as chert has been observed in the adjacent barangays Balakan and Danao of Salug Municipality.

Chert deposition is followed by the turbidite sequence named the Zamboanga Formation (Antonio, 1972), which comprises the Oligocene to Early Miocene turbidite sequence. This is among the major lithostratigraphic units distributed in the eastern segment of the Zamboanga Peninsula. The exposures of the Zamboanga Formation in Godod Municipality have been observed in several barangays such as Guisapong, Rambon, Mauswagon, Baluno, Miampic, Binuangan, Limbunga, Raba, Dipopor, Sioran, and Poblacion. These usually occur as well bedded and interlayered sandstone and shale, well indurated with commonly high amounts of quartz grains in its clastic components. The lower section massive sandstone sequence appears bluish to dark-colored while the upper sections shale to calcareous mudstone section of this unit varies color in the shades of brown, cream to beige color.

Photo 2. Arkosic sandstone sequence exposed along upstream Salug River in Barangay Guisapong, Godod: [A] waterfall section showing a cross-section of the turbidite sequence and [B] the texture of the sandstone.
Photo 3. Exposures of the sandstone-shale interbeds along the Raba-Sioran road in Barangay Raba, Godod described as quartz-rich, cream-colored, well-bedded commonly laminated interpreted to be flysch-type deposit, which represent the distal section of submarine fan turbidite deposit.
Photo 4. [A-B] A quartz-rich sandstone exposed in an abandoned mountain quarry area in Barangay Baluno, Godod.

Photo 5. [A-B] A bluish-gray quartz-rich highly indurated sandstone exposed along the road in Barangay Mauswagon, Godod.

On top of the turbidite sequence is the limestone unit referred to as the Sibuguey Formation (Brown, 1950). This 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, boundstone calcilutite, calcarenite and calcirudite. Toward its base can be seen laminated calcarenite which serves as the transition from the underlying turbidite sequence. In the Godod Municipality, this limestone unit forms karst topography in Barangays Poblacion, Raba, Baluno, Dilucot and Bunawan.

Photo 6. [A-B] A fossiliferous bedded limestone (Sibuguey Formation) exposed along the main access road in Barangay Poblacion, Godod.
Photo 7. Exposure of spiritic limestone subcrops along the karst terrain in Barangay Bunawan, Godod.

The Zamboanga Volcanic Complex is the 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, a significantly large exposures of older rock formations in the Zamboanga Terrane have been covered. In the Municipality of Godod, the volcanic rocks occupy about half of its area, particularly the southeast side. A relatively large volcanic center covering the San Pedro-Sianan area generated volumes of volcanic materials and covered much of older rock formations in southeast Godod. A number of smaller volcanic cones have also been recognized in the Poblacion area in which road cuts reveal sections of the cone with the vent in the center. Barangay Mauswagon in the north side of the municipality also shows roadcuts with thick volcanic sequence of volcaniclastics, tuff layers, agglomerate and volcanic breccia.

Photo 8. [A-C] A volcanic sequence consisting of volcaniclastics and volcanic breccia occurs along the road in Barangay Mauswagon, Godod.
Photo 9. [A-C] A section of a volcano has been exposed along a roadcut showing a tuff sequence with a vent structure occurring in Sitio Rebe, Barangay Poblacion, Godod.
Figure 2. Lithologic distribution map of the Municipality of Godod, Zamboanga del Norte.

SINKHOLE INVENTORY AND ASSESSMENT

The IfSAR-DEM was utilized to segregate sink-depth range of 0.5 to 30 meters with aperture size of 25 – 1,800 meters. The wider range of sink depth and aperture size was considered in this assessment because of these factors, a) varied geomorphic units, b) presence of limestone unit and c) adjacent active lineaments due to tectonic events that may affect the study area. These factors were found to have greater influence on rate of dissolution process and subsidence.

Figure 3. Distribution of IFSAR generated sink depth in the Municipality of Godod, Zamboanga del Norte.

Table 1. Summary of the sinkhole inventory per barangay using field assessment and IFSAR-DEM and topographic map interpretation.

SINKHOLE/GROUND SUBSIDENCE INVENTORY

LocationTypeNo. of SinkholeAperture
Diameter
(m)
Depth (m)Observations
Baluno
Purok 4 (263)
N 8° 3’ 2.9”
E 122° 50’ 36.2”
Elev. 305 m
Cover Collapse/Cover Subsidence102 – 1001 – 30A complex sinkhole system is identified along the north-western section of the barangay that is characterized by hilly topography, the sinkholes have circular to irregular-shaped ponded depressions mostly vegetated with ferns, tall grasses and taro (gabi) plants. Sinkhole biomarkers are also present in the area.

A swallow hole is noted at the base of the elongated sinkhole where a surface creek flowing west drains into it and eventually connects to other subsurface cavities, it has a narrow opening/cavity that is frequently blocked with flood debris mostly composed of branches and coconut husk. Patches of ponded ground depressions are common.

Limestone outcrops are widespread along the base of the hill and exhibit intense surface dissolution that forms jagged pinnacles. The area is mostly covered with reddish soil that is derived from the weathering of the volcanic rock deposits that buried the karstic morphology of the area.
Purok 4 (264)
N 8° 3’ 5.1”
E 122° 50’ 35.9”
Elev. 314 m
Cover
Collapse
120 x 1010A deep oval-shaped sinkhole depression is identified near the community center, the central portion of the sinkhole is heavily vegetated with shrubs, tipolo, and coconut trees. Limestone protrusions on the ground show brecciation.

Pronounced terracette features or soil creeping is identified on the hilltop that is slumping towards the direction of the sinkhole.

An elongated ground-validated sinkhole depression oriented at NE-SW is identified in remote sensing data. Several houses are built along the perimeter of the sinkhole.

The slopes are converted into an agricultural field cultivated with bananas and upland rice.

A surface creek generally traverses the central portion of the sinkhole that eventually drains into a swallow hole that is bordered by a thick bamboo cover.
Purok 4 (265)
N 8° 3’ 7.3”
E 122° 50’ 35.1”
Elev. 324 m
Cover Subsidence1150 x 10030An elongated ground-validated sinkhole depression oriented at NE-SW is identified in remote sensing data. Several houses are built along the perimeter of the sinkhole.

The slopes are converted into an agricultural field cultivated with bananas and upland rice.

A surface creek generally traverses the central portion of the sinkhole that eventually drains into a swallow hole that is bordered by a thick bamboo cover.
Purok 4 (266-267)
N 8° 3’ 10.6”
E 122° 50’ 38.3”
Elev. 327 m
Cover Subsidence210 – 1005 – 10Numerous elongated ground depressions have been turned into rubber tree farms. The central portion is mostly covered with tall grasses, and frequent ponding is typical.

These are identified adjacent to the barangay road with minimal residential houses.
Purok 3 (282)
N 8° 2’ 43.8”
E 122° 51’ 25.3”
Elev. 391 m
Cover Collapse/Cover Subsidence510 – 401 – 10Multiple circular collapse sinkholes are identified adjacent to the concrete barangay road that shows signs of subsidence which emanated this year due to intense rainfall events.

These are covered by thick vines and shrubs with toppled coconut trees as evidence of the continuous slumping.

A 20-cm gap is visible below the concrete road as a result of the sinking ground.
The barangay officials posted warning markers to alert passersby to the possibility of sinkhole slumping in the road section.

Multiple linear cracks on the ground surface on the opposite side of the road shows evidence of the unstable ground condition.

Few residential houses are noted near the identified sinkholes.

Based on local anecdotes, multiple cave systems can be observed in the forested portion of the barangay which shows a highly karstified topography.
Photo 10. Barangay Baluno. [A] A narrow stream is noted draining into a swallow hole that is filled with mud and flood debris; [B-C] The identified swallow hole is located at the base of the complex sinkhole system that is heavily covered in shrubs and grasses; [D] Subcrops of gray recrystallized limestone rocks are scattered in the vicinity that is covered in reddish brown clayey soil (N 8° 3’ 2.9” E 122° 50’ 36.2”)
Photo 11. Barangay Baluno. [A] A well-vegetated circular collapse sinkhole was identified on the hilltop (N 8° 3’ 5.1” E 122° 50’ 35.9”); [B] Tension cracks are pervasive on the ground surface that is slumping toward the sinkhole; [C] The periphery of the elongated sinkhole depression is surrounded by houses, ponding at the center is noted (N 8° 3’ 7.3” E 122° 50’ 35.1”).
 
Photo 12. Barangay Baluno. [A-B] A ground depression converted into a rubber tree farm was identified near the school perimeter (N 8° 3’ 10.6” E 122° 50’ 38.3”); [C-D] Another ground depression was identified adjacent the barangay road that is covered in tall grasses and shrubs (N 8° 3’ 13.1” E 122° 50’ 38.3”).

SINKHOLE/GROUND SUBSIDENCE INVENTORY

LocationType of SinkholeNo. of SinkholeApertureDepth (m)Observations
Baluno
Purok 3 (285)
N 8° 2’ 28.6”
E 122° 51’ 30.6”
Elev. 348 m
Solution120 x 105An elongated ponded/flooded sinkhole depression is identified 80 meters northeast of the barangay road. The upper section is covered in thin brown clayey soil with vertical protrusions of limestone rocks along the edge of the sinkhole.

Coconut trees are observed leaning towards the center of the sinkhole.

Freshwater fish also thrives in this murky pond.
Purok 3 (286)
N 8° 2’ 29.1”
E 122° 51’ 34.2”
Elev. 340 m
Solution/Cover Collapse420 x 102 – 5A ground-validated sinkhole depression is observed along the hilly section of the barangay that is exhibiting conical hills.

The limestone hills show porous surfaces with deep dissolution cavities. A resurgent spring is identified flowing out of the limestone cavities. Underlying the limestone rock is the turbidite sandstone deposit noted at the base of a creek.
Purok 3 (287)
N 8° 2’ 26.3”
E 122° 51’ 30.3”
Elev. 364 m
Cover Subsidence1100 x 105 A rubber tree farm has been cultivated inside of a sinkhole depression that is covered in a thick layer of clayey reddish soil. Ponding at the center, which is covered in tall grasses, is common during the rainy season.
Purok 1 (288-289)
N 8° 2’ 13.0”
E 122° 51’ 26.5”
Elev. 359 m
Cover Subsidence2503A circular ground depression adjacent to the barangay road is identified. The central portion is covered in tall shrubs and grasses with patches of ponding.

Coconut trees border the perimeter of the sinkhole which shows partial tilting.
Purok 1 (290)
N 8° 1’ 58.9”
E 122° 51’ 8.0”
Elev. 325 m
Solution/Cover Collapse35 – 103Based on the sink depth analysis, this area is identified as part of an extensive sinkhole depression. Irregular to circular patterns can be delineated using a terrain model to better interpret the subsidence pattern.

Several circular collapse sinkholes are hidden by dense vegetation. At the base sinkhole collapse, a stream can be seen running out of narrow cavities that eventually link to other voids in the nearby sinkhole.

Abundant black manganese pellets are observed along the spring channel.
Purok 1 (291)
N 8° 2’ 2.0”
E 122° 51’ 6.5”
Elev. 313 m
Solution/Cover Subsidence2103Further northwest from Sinkhole Station 290, a well-vegetated bowl-shaped ground depression is identified. This typically ponds during the rainy season, since flood water is collected in the depression and it cannot drain into the swallow hole because of high sediment influx along the narrow opening.

The identified swallow hole connects to a cave chamber that is inhabited by birds and snakes testified by the landowner.
Purok 1 (292)
N 8° 1’ 55.6”
E 122° 50’ 47.2”
Elev. 294 m
Solution50.5 – 22 – 3Multiple circular sinkhole cavities are identified adjacent to a westward flowing creek that is part of the upstream tributary of Salug River.

At the base of the creek, an isolated water vortex is emanating from a narrow cavity or a swallow hole, this suggests that the water is draining into the void. Sinkhole biomarkers like huge balete trees are also present in the area.

The banks of the creek are bordered by well-vegetated limestone rocks that have deep fissure cavities that connect to a cave system.
Photo 13. Barangay Baluno. [A-B] A ponded sinkhole was observed along the hilly section of the barangay with coconut trees leaning towards the center of the depression (N 8° 2’ 28.6” E 122° 51’ 30.6”); [C] A spring percolates out of the cavernous limestone that is having deep dissolution fractures (8° 2’ 29.1” E 122° 51’ 34.2”); [D] An elongated ground depression was identified along the rubber tree farm that is usually ponded during intense rainfall (N 8° 2’ 26.3” E 122° 51’ 30.3”).
Photo 14. Barangay Baluno. [A] Multiple circular collapsed sinkholes are covered in shrubs (N 8° 1’ 58.9” E 122° 51’ 8.0”); [B] Scattered manganese pellets are observed at the cavity of the resurgent spring [C-D] A circular ground depression with a plugged swallow hole is identified at the edge of the limestone hill, ponding in the area is common (N 8° 2’ 2.0” E 122° 51’ 6.5”).
Photo 15. Barangay Baluno. [A-B] Along the bank of the narrow creek, a huge balete is noted near the identified swallow hole; [C] Narrow cavities along the limestone rock connects to a cave the is present in the area; [D] A sinkhole collapse that is filled with bamboo was observed along the creek (N 8° 1’ 55.6” E 122° 50’ 47.2”).

SINKHOLE/GROUND SUBSIDENCE INVENTORY

    LocationType of SinkholeNo. of SinkholeAperture Diameter (m)Depth (m)Observations
    Bunawan
    Purok 3 (446-448)
    N 7° 59’ 13.0”
    E 122° 52’ 2.1”
    Elev. 255 m

    N 7° 59’ 12.9”
    E 122° 52’ 1.6”
    Elev. 262 m

    N 7° 59’ 12.9”
    E 122° 52’ 1.3”
    Elev. 263 m
    Cover Collapse      30.5 – 52 -15Most areas in the barangay are underlain by a volcanic rock deposit that covers the old limestone rocks that can only be found on the northernmost limit in the barangay which is adjacent to the Palandok River.

    Based on the Landsat imagery, limestone rocks can be recognized as having white patches on the surface along the hills or ridges.

    In an upland agricultural field, there are several circular to oval-shaped sinkhole occurrences that trend in an E-W direction. The terrain is characterized by having abundant cobble to boulder-sized white to gray limestone rocks along the slopes that are sparsely vegetated.

    The sinkholes along the vicinity are easily recognizable since it has thick vegetation cover and abrupt depression on the surface.

    According to the landowner, the identified sinkholes pose a danger to passersby who are unfamiliar with the area and could fall into the deep cavities.
    Purok 3 (449)
    N 7° 59’ 13.1”
    E 122° 52’ 1.3”
    Elev. 265 m
    Cover Subsidence120.5A developing ground depression surrounded by limestone pinnacles is identified adjacent to Sinkhole Station 448.
    Purok 3 (450-453)
    N 7° 59’ 11.5”
    E 122° 52’ 0.5”
    Elev. 267 m

    N 7° 59’ 11.0”
    E 122° 51’ 57.7”
    Elev. 271 m
    Cover Collapse630 – 5015 – 20Several funnel-shaped collapse sinkholes are identified along the hilltop that is converted into an agricultural field planted with upland rice, bananas, coconuts and rubber trees. The sinkholes are clustered in the area which normally has a wide aperture and deep base.

    The periphery of the sinkholes is bordered by protruding limestone pinnacles that show an intense dissolution surface, limestone rubbles are noted along the base.
    Purok 3 (454-455)
    N 7° 59’ 11.9”
    E 122° 51’ 56.8”
    Elev. 278 m
     
    N 7° 59’ 12.1”
    E 122° 51’ 56.5”
    Elev. 282 m
    Cover Collapse20.5 – 120A cave with a narrow opening is identified at the base of a huge balete tree which is bordered by thick vines and limestone rubbles. This deep cylindrical cave has a muddy base that has multiple lateral passages that connect to spacious chambers that are inhabited by swiftlet birds and bats.

    The area is also infested with alingatong trees and stinging caladium plants.
    Purok 3 (456)
    N 7° 59’ 10.9”
    E 122° 51’ 56.9”
    Elev. 280 m
    Cover Subsidence1105A progressing sinkhole depression is concealed by thick vegetation of mainly ferns and shrubs. This sinkhole frequently ponds and eventually dries up the next day.
    Purok 3 (457)
    N 7° 59’ 10.9”
    E 122° 51’ 58.8”
    Elev. 263 m
    Cover Collapse11010Another funnel-shaped sinkhole collapse is identified adjacent to a huge balete tree, this sinkhole shows blocks of limestone rubbles at the base.

    A cavity was identified at the base of the sinkhole which is covered in shrubs and gabi plants. Several houses made of light materials are also present in the area.
    Purok 3 (458)
    N 7° 59’ 34.5”
    E 122° 52’ 16.3”
    Elev. 233 m
    Cover Subsidence120 x 101An elongated ground depression is identified near the bank of the Palandok River that trends in a NE-SW orientation.

    Ponding is evident at the center of the depression which is used by water buffalos as a mud puddle. Short grasses and shrubs surround the depression where limestone pinnacles can be seen along the edge.
    Photo 15. Barangay Bunawan. [A-B] A circular collapsed sinkhole is concealed by thick gabi plants (N 7° 59’ 13.0” E 122° 52’ 2.1”); [B-C] Another sinkhole collapsed is dumped with braches to prevent farmers falling in to the deep cavity (N 7° 59’ 12.9” E 122° 52’ 1.6”).
    Photo 16. Barangay Bunawan. [A] A funnel-shaped sinkhole collapse is converted into a agricultural field that is situated on the hilltop (N 7° 59’ 11.5” E 122° 52’ 0.5”); [B] Another funnel-shaped collapse sinkhole has protruding limestone rocks along the slope (N 7° 59’ 11.0” E 122° 51’ 58.9”); [C]A shrub covered sinkhole was identified at the periphery of the bigger sinkhole collapse (N 7° 59’ 10.8” E 122° 51’ 57.7”); [D] A deep cylindrical cave has a narrow opening that is vegetated with alingatong and vines (N 7° 59’ 11.9” E 122° 51’ 56.8”).

    SINKHOLE/GROUND SUBSIDENCE INVENTORY

    LocationType of SinkholeNo. of SinkholeAperture Diameter (m)Depth (m)Observations
    Dilocot
    Purok 6 (341)
    N 7° 58’ 52.4”
    E 122° 49’ 55.4”
    Elev. 324 m
    Solution10.2 – 0.52-3Narrow vertical cavities are observed along the protruding limestone pinnacles seen along a narrow creek. The upper part is mostly covered by thick weathered reddish volcanic rock deposits.
    Purok 6 (342-343)
    N 7° 58’ 53.9”
    E 122° 49’ 53.0”
    Elev. 315 m
     
    N 7° 58’ 55.4”
    E 122° 49’ 51.8”
    Elev. 317 m
    Cover Subsidence35 – 501Multiple circular to elongated ground depressions are identified along the clustered residential houses in the northern part of the barangay. Since the central part of the sinkhole forms patches of ponding, houses made of light materials can be seen along the edge of the depression.

    Another ground depression was discovered nearby that has been converted into an agricultural field as a result of thick soil accumulation at the center in comparison to the surrounding area where porous limestone pinnacle increases.
    Purok 6 (344-345)
    N 7° 59’ 1.7”
    E 122° 49’ 51.2”
    Elev. 322 m
     
    N 7° 59’ 3.1”
    E 122° 49’ 51.5”
    Elev. 323 m
    Cover Collapse21 – 32 -3Multiple circular collapses are observed along the well-vegetated limestone hill that is exhibiting intense karstification. The area is mostly covered in vines and balete trees.

    The surface of the limestone is mostly coated in moss, making it slippery to walk on and dangerous for locals to forage in the cavernous terrain
    Purok 6 (346)
    N 7° 59’ 2.3”
    E 122° 49’ 52.3”
    Elev. 325 m
    Cover Subsidence50.5 – 120An elongated marshy ground depression with multiple deep ponds is identified along the road. This sinkhole is observed at the periphery of the vegetated limestone hill.

    Residents nearby utilized some portion of the sinkhole as a fish pond and agricultural field planted with rice, corn and cassava.
    Purok 6 (347)
    N 7° 59’ 1.1”
    E 122° 49’ 54.1”
    Elev. 323 m
    Solution/Cover Collapse11.51A buried swallow hole is identified at the edge of the clustered limestone hills, farmers who are cultivating the area directed flood waters into the cavity of the sinkhole to prevent ponding on the agricultural field which is having a saddle feature.

    The opening of the sinkhole is filled with mud and branches that are surrounded by limestone rubbles.
    Purok 3 (349)
    N 7° 58’ 57.3”
    E 122° 49’ 57.6”
    Elev. 330 m
    Cover Collapse50.5 – 12 – 3Several irregular-shaped collapsed cavities are identified along the karstified hill that is exhibiting pointy and sharp limestone pinnacles that are covered in vines and balete roots.

    A horizontal and vertical dissolution pattern is observed that forms rectangular to blocky fractures on the limestone rock. 
    Photo 17. Barangay Dilocot. [A] A circular ground depression with protruding limestone pinnacle at the center (N 7° 58’ 53.9” E 122° 49’ 53.0”); [B] Another ground depression is surrounded by residential houses made up of light materials (N 7° 58’ 55.4” E 122° 49’ 51.8”); [C] A swampy sinkhole is covered in tall grasses with circular pools that is converted into a fish pond (N 7° 59’ 2.3” E 122° 49’ 52.3”); [D] An elongated sinkhole is converted into an agricultural field planted with rice and corn (N 7° 58’ 57.5” E 122° 49’ 55.6”).
    Photo 18. Barangay Dilocot. [A-B] A limestone hill densely vegetated with vines and balete has intense dissolution fissures/cavities that form narrow cave chambers (N 7° 58’ 53.9” E 122° 49’ 53.0”).

    SINKHOLE/GROUND SUBSIDENCE INVENTORY

    LocationType of SinkholeNo. of SinkholeAperture Diameter (m)Depth (m)Observations
    Limbunga
    Purok 6 (279-280)
    N 8° 0’ 2.6”
    E 122° 44’ 26.5”
    Elev. 118 m
     
    N 8° 0’ 2.9”
    E 122° 44’ 23.7”
    Elev. 117 m
    Solution/Cover Collapse101 – 205 – 15The limestone areas in the barangay are observed on the border of barangay Miampic and Raba which is located in the southern portion that is known to have extensive cave occurrences.

    The Sicopao Cave, a subterranean cave has a lens-shaped opening that is embellished with well-developed stalactite features that are about 1 to 2-meter length. Large limestone boulders are present at the base which suggest progressing ceiling collapse.

    This subterranean river meanders inside the cave and eventually connects to the main channel of the Mucas River that is flowing N60°E.

    The lower section of the cave is mostly covered with moss which indicates water percolating from the ceiling to the walls which favours the development of lichens making the base slippery.

    High deposition of sediment is noted on the cave floor which is mostly composed of fine sediments (mud & silt) and vandalism on the cave walls are noted.

    Multiple chamber galleries are identified which are inhabited by swiftlet birds, bats, fishes and pythons.
    Purok 6 (281)
    N 8° 0’ 28.3”
    E 122° 45’ 31.2”
    Elev. 270 m
    Cover Subsidence350 – 2002 – 30Ø  Multiple elongated to irregular-shaped ground depressions that are converted into a rubber tree farm are identified as sinkhole features in the sink depth analysis. Ground verification confirms the existence of the sinkholes which are mostly covered in tall shrubs and ferns with limestone outcropping.

    Along the slopes, trees are noted leaning towards the center of the depression.
    Photo 19. Barangay Limbunga. [A] A subterranean cave system that is identified at the upstream tributary of the Mucas River (N 8° 0’ 2.9” E 122° 44’ 23.7”); [B-D] The interior of the cave is embellished with speleothems that are moss covered.
    Figure 4.  Graphical representation of the inventoried sinkholes per barangay based on the Geological Assessment, Ground Validated IFSAR Delineation, NAMRIA Topographic Map and Satellite Imagery.
    Figure 5.  Location of the inventoried sinkholes in the Municipality of Godod, Zamboanga del Norte.
    Figure 6.  Inferred sinkholes identified using a Landsat Imagery in the Municipality of Godod, Zamboanga del Norte.
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