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Geology in one’s schoolyard: The story of the limestone under a school in Cebu, Philippines

As a geologist who works with a startup educational consultancy company, my job involves a lot of travel. I hop from school to school to teach Earth Sciences to students. At one of the schools I visited, the Compostela Science and Technology High School (which the locals refer to as ‘Saytek’) I came across a reef limestone on which one of the school buildings was constructed. The school is located in the town of Compostela (Cebu), Philippines. This essay concerns the geologic wonders of Cebu island and its nearby places, particularly focusing on its limestones.



Clockwise from left: Outcrop of Compostela Science and Technology High School’s reef limestone upon which stands a classroom building; view of the same limestone outcrop from the side, with author (1.63 m) as scale; hand specimen sample of the reef limestone with hand lens to scale.
Figure 1. Clockwise from left: Outcrop of Compostela Science and Technology High School’s reef limestone upon which stands a classroom building; view of the same limestone outcrop from the side, with author (1.63 m) as scale; hand specimen sample of the reef limestone with hand lens to scale.

The outcrop is mostly grey-colored although it shows its fresh yellow-white color when broken up. Conveniently for the students and teachers, its slopes form an improvised staircase which they use to climb up and down one of their buildings. On one side however, it is flanked by their septic tank facility.


I asked both the students and teachers of Saytek what did they think about the rocks on the hillside within their school. They remarked that it looked like corals. I explained to them that reef limestone is a type of rock that forms from the remains of corals, colonial marine animals belonging to the same group as jellyfish, which have long since died. I then asked them if they knew where corals live and form reefs, to which they replied "in the sea". Then I asked them again, "Compostela Saytek is not under the sea, so how is it possible that these rocks ended up existing far above the sea?". And that's when the students really started to think about what the rocks around them mean and the implication on the geological history of where their school is located.



The author explaining the significance of the limestone outcrop on his left side with some of the students of Compostela Science and Technology High School. Image credit: Ly Cunanan/Compostela Science and Technology High School
Figure 2. The author explaining the significance of the limestone outcrop on his left side with some of the students of Compostela Science and Technology High School. Image credit: Ly Cunanan/Compostela Science and Technology High School

As innocuous as they may seem to high school students, especially in a geologically active area like the Philippines, the limestones of Compostela have a story to tell (like all rocks on our planet!). The limestone of this city, as well as that of much of eastern Cebu, belongs to the Carcar limestone unit, a chronostratigraphic unit dated by the Mines and Geosciences Bureau (MGB) of the Philippines between the late Pliocene and Pleistocene (about 2.59 million to 11,500 years old). It unconformably overlies the Barili formation found in the northern areas of the town of Compostela. The Barili formation comprises of sandstone and siltstone interbeds as well as a limestone member of late Miocene age (no older than 7.25 million years).


The age range of the Carcar limestone roughly overlaps with that of the Siquijor limestone, which according to the MGB is Pliocene to Pleistocene in age, as well as the limestone member of the Maribojoc unit of rocks that forms the western and central sides of Bohol island. Some geologic reports call this limestone member the Cortes limestone, named after the town where it is found. The island of Bohol is itself a larger island found to the west of Cebu. It is also UNESCO-designated Global Geopark and home to the scenic Chocolate Hills karst landscape. I have also visited Siquijor and obtained samples of reef limestone from that province which I use to compare with the reef limestones from Saytek whenever I teach Earth Science in the schools I subsequently visited.


Schematic diagram comparing the stratigraphic columns of Northern-Central Cebu (where Saytek is located), Siquijor, and Bohol, particularly the ages of the relatively younger limestone units found in these areas. Adopted and modified from: Peña (2008) and Rodrigo et al. (2019)
Figure 3. Schematic diagram comparing the stratigraphic columns of Northern-Central Cebu (where Saytek is located), Siquijor, and Bohol, particularly the ages of the relatively younger limestone units found in these areas. Adopted and modified from: Peña (2008) and Rodrigo et al. (2019)

The island of Siquijor today is found southeast of Cebu island whereas Bohol is separated from Cebu by what is now the Camotes Sea. However, the overlapping ages of the limestones of Eastern Cebu, Siquijor, and Western Bohol island as well as their present-day geographic proximity suggest that these areas were once part of a contiguous shallow marine environment during the Pliocene to Pleistocene times.


I emphasize ‘shallow’ because most coral species can only live up to the photic zone or the depth of the sea that sunlight can reach. Corals eat zooxanthellae, a type of algae which relies on photosynthesis to make its food. Without sunlight, zooxanthellae can’t survive, and by extension, the corals that depend on them.


Uplift and subsequent crustal thickening associated with the convergence of the terranes of the Philippine Mobile Belt and that of the Philippine Sea Plate gradually caused these limestones to emerge from the sea. However, unlike in Siquijor, there was a time that limestone formation in Eastern Cebu and Western Bohol was interrupted as suggested by the presence of thick siltstone and sandstone interbeds in Cebu’s Barili and Bohol’s Maribojoc formations respectively, perhaps due to subsidence or sea level rise or both.


 Location map of the coeval limestone units found in Saytek and Siquijor (indicated by yellow pin icons) and the limestones of the Chocolate Hills in the province of Bohol (red pin icon). The map of the Philippines and the location of the capital of the Philippines, Manila (indicated by a star), is on the left for reference. Philippine map image credit: Hellerick/CC BY-SA 3.0; Location map image credit: Google Earth/Landsat/Copernicus
Figure 4. Location map of the coeval limestone units found in Saytek and Siquijor (indicated by yellow pin icons) and the limestones of the Chocolate Hills in the province of Bohol (red pin icon). The map of the Philippines and the location of the capital of the Philippines, Manila (indicated by a star), is on the left for reference. Philippine map image credit: Hellerick/CC BY-SA 3.0; Location map image credit: Google Earth/Landsat/Copernicus

Limestone deposition also ceased earlier in Western and Central Bohol as evidenced by the absence of limestone units younger than 1.81 million years old in the area. On the other hand, in Siquijor and Eastern Cebu, limestone deposition continued into the Pleistocene, which meant that Siquijor and Eastern Cebu likely remained below the sea for a longer time than western Bohol. It could have been the case because Western Bohol was uplifted earlier than Eastern Cebu or Siquijor. It is uncertain whether the rate of uplift in Bohol have been faster.


After explaining to my Saytek students the story that the limestone in their school tells them about the geology of their area, they became more involved in the discussion. They also asked me questions about how the age of the rocks is determined and the economic value of the Cebu limestones. My students will never look at rocks the same way again as they did before they met me. For answers to these questions, stay tuned for the next article in this series on the limestones of Compostela and those of its neighboring provinces.



References

  • Cohen, K.M., Finney, S.C., Gibbard, P.L. & Fan, J.-X. (2013; updated) “The ICS International Chronostratigraphic Chart.” Episodes. 36: 199-204.

  • Peña, R.E. (2008). Lexicon of Philippine Stratigraphy. Mines and Geosciences Bureau.

  • Rodrigo, J.D., Gabo-Ratio, J.A.S., Queaño, Fernando, A.G.S., de Silva, L.P. Jr., Yonezu, K., and Zhang, Y. (2019). “Geochemistry of the Late Cretaceous Pandan Formation in Cebu Island, Central Philippines: Sediment contributions from the Australian plate margin during the Mesozoic.” The Depositional Record. 6:309-330.


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