Geologic History of Bohol
The island of Bohol, sitting elegantly between several islands of Visayas and Mindanao, took different forms throughout its geologic history. Its parts did not come together in one fell swoop, but was pieced together geologically for long, long, long time. From its lowly undersea abyssal beginnings to its subaerial idyllic magnificence, its geologic history is marked by periods of tectonic turbulence and occasional quiescence spanning nearly a hundred and fifty million years in the making.
Its oldest rock, Alicia Schist, was forged in the depths of the sea between the Late Jurassic to Early Cretaceous (146 mya – 66 mya). It started as sediments that were deposited in an ocean basin far from its present location. The sediments were later metamorphosed into schist during its undersea lithospheric tectonic movements before converging with the Bohol Ophiolite. The Bohol Ophiolite is a slice of complete oceanic lithosphere that formed nowhere near its present spot. It was forged in the depths of an ancient sea as well. For a very protracted time during the Cretaceous to Paleocene (66 mya – 56 mya), it ramped onto the older Alicia Schist during their tectonic convergence, and thus shaped the basement rocks of Bohol on top of which all the other younger rocks were built upon.
Between the Paleocene to Late Eocene times (56 mya – 38 mya) there was a prolonged period of either an emergence from the sea, causing the erosion of rocks, or an absence of deposition, such that the rock record went missing altogether. However, a surge in undersea volcanism and deposition during the Late Eocene to Oligocene produced the Ubay Formation, with its volcanic rocks and interlayering sedimentaries. Meanwhile during the Early Oligocene (33 mya – 27 mya), the western part of the sea was favorably quiet and calm for the deposition of low energy Ilihan Shale.
Continuous magmatism below the surface of the rocks by the Late Oligocene time (27 mya – 23 mya) supplied molten material for the various igneous intrusions that were cutting through the older rocks. These now hardened rock intrusions such as Getafe Andesite, Talibon Diorite and Jagna Andesite, can be observed in either the northern part of Bohol or in the southeast.
The fledgling undersea platform of Bohol was beginning to take shape, mass and space, but was still well and truly submerged. In some places it had the right conditions for temperature and water depth to hold a niche for conquering minute organisms, such as forams, and invertebrates, such as corals, that make up some of the limestone from these early times. Orbitoid forams comprise the Wahig Limestone, which was formed in the Early to Middle Miocene (23 mya – 11 mya). A possibly brief period of falling sea or emerging land promoted erosion of the Wahig Limestone real estate and some older rocks. Another subsequent submergence provided excellent conditions for the planktic forams and nanoplanktons that later became part of the clastics, volcaniclastics and limestones of the Carmen Formation at the end of the Middle Miocene. The underwater environment in the continuously shallowing sea in the Late Miocene (11 mya – 5 mya) was suitable for the establishment of widespread coral reefs that formed the Sierra Bullones Limestone on top of the Carmen Formation, especially in the east. In the west, the growing undersea platform provided a niche for the extensive reef forming species which comprise the limestone member of the Maribojoc Formation by the Pliocene time (5 mya – 2 mya). Slow emergence of the land in the Late Pleistocene time (0.126 mya – 0.117 mya), due in part to tectonic movements and the eventual erosion of the land, contributed to the sculpting of the landscape of Bohol that we now see today.
Chocolate Hills
The unique karst landscape is composed of smooth, uniformly shaped conical isolated hills that cover a vast area in the central portion of the island. The site was once a platform of thick widespread buildup of coral reefs that thrived during the Pliocene, approximately 2-5 million years ago and later to form a sedimentary formation. Soon, this limestone formation was raised above the sea level and fractured. Rainwater, streams and groundwater dissolved the limestone, gradually forming the present landscape of cone karst.
Hinagdanan Cave
The Hinagdanan Cave is one of the many wondrous karst caves in the Province of Bohol. This cave was accidentally discovered by a farmer while clearing his land. Stumbling on a hole in the ground, curious, he dropped a stone and heard a splash. He built a ladder, “hagdan,” and went down to find the cave’s hidden beauty, hence the origin of its name Hinagdanan. Its boasts of several stalactite and stalagmite formations and a clear lagoon that is about 10 to 20 meters deep. Aside from its awe-inspiring natural beauty, Hinagdanan also played a significant role in the history of Bohol. The cave was used during Second World War as a hideout by locals to escape from the Japanese Imperial Army.
Loon Uplifted Marine Terrace (Coral Garden)
One of the shoreline areas uplifted during the earthquake of October 15, 2013 is the presently named Loon Coastal Geomorphic Conservation Park which covers an area of 417 hectares. This former intertidal zone, where the tide sometimes submerged or exposed the land, is now an uplifted terrace and completely on dry land. It was raised approximately 1.5 meters vertically and the shoreline shifted 50 meters seawards. The former barren land right after the earthquake is now teeming with sea purslane, a creeping succulent seagrass plant of the family Aizoaceae, that covers the land with a mat of red and orange color during sunny days, and a luxurious green color during rainy days. The locals call the area the Coral Garden, a popular new destination for tourists and residents alike.
Maribojoc Uplifted Marine Terrace
A 7.2-magnitude earthquake that rocked Bohol on October 15, 2013 brought devastation everywhere on the island and some nearby island provinces. The shaking was the result of a demonstration of nature’s power that has been changing the face of the Earth for millions of years: uplift of blocks of land during movement along a fault. This time the coastal areas of Loon-Maribojoc were pushed approximately 1.5 meters upwards when motion occured along a new fault in Inabanga. What was once a portion of the sea bottom and an underwater reef was now thrusted above sea level, creating a newly raised marine terrace and shifting the coastline 50 meters seaward. This is the Maribojoc Uplifted Marine Terrace. It is declared as the Loon-Maribojoc Geologic Monument by the Department of Environment and Natural Resources under the DENR Administrative Order No. 2015-08. The geological monument covers 1.37 kilometers (137 hectares) of uplifted coastline within the towns of Loon and Maribojoc.
Older marine terraces can also be found inland as part of the limestones of Maribojoc Formation. They too were a result of tectonic uplift in the not so distant geologic past, proof of the immense power of nature to create ever changing landscapes.
Can-Umantad Falls
Can-Umantad Falls is regarded as the tallest waterfall in the entire province of Bohol. From a height of 60 meters, its source water from Cadapdapan River falls down over a backdrop of horizontal to slightly dipping beds of calcareous and tuffaceous sediments of the clastic member of the Carmen Formation (Middle Miocene, 16 mya – 11 mya)
Canawa Cold Spring
Tucked within a lush green forest in the town of Candijay is a popular leisure area for locals known as the Canawa Cold Spring. Its name came from the local term “Cawa” which means “pan”, taking from the natural shape of the spring. The depth of the spring is still unknown; several attempts has been made to dive at the floor of the spring but it was said that the water is too deep no one has been able to reach the bottom. Even the actual source of the spring water is still unknown.
The spring is carved out of the Middle Miocene Carmen Formation comprised of 400 to 800 m thick calcareous sedimentary formation. Concrete walls and stairs were built on the spring entrance to be of help to those who cannot swim especially to the children, but the main part of the spring which is the circular pool, was kept in its natural state.
Baclayon Uplifted Marine Terrace
The uplifted marine terraces in Baclayon are a similar landform to the more recently uplifted marine terrace in Maribojoc. However, the terraces in Baclayon are much older. It was formed during the Pliocene-Pleistocene time (5 million years – 11,000 years ago). Fault movement along the coast caused the uplift of formerly underwater reefs raising it at a height above the sea and creating a marine terrace along the coast. Several other subsequent fault movements shaped the successive step-like characteristic of the marine terraces seen at Baclayon. The three-step terraces there stand at 20 meters, 40 meters and 60 meters above sea level signifying at least three episodes of progressive tectonic uplift in the area. The ancient marine terraces can be observed in the step-like quality of uphill roads that branches inland from the highway in Baclayon.
Fragments of large and thick shells (Tridacna gigas) and fragments of the gastropod group Strombaceae, have been collected in the area. Two terraces in Baclayon were tested using the Uranium-Thorium Analysis to identify the age and determine when the terraces were uplifted. The results point to an age of around 129,000 years old for the terrace with an altitude of 20 m, and approximately 310,000 years old for the terrace with an altitude of 60 m.
Danajon Bank Double Barrier Reef
Parallel to the northeast coast of Bohol is a very rare geological formation known as the Danajon Double Barrier Reef. It is one of the 6 double barrier reefs in the world, one of 3 in Indo-Pacific and is the only one of its kind in the Philippines and Southeast Asia. It is composed of numerous but diverse islets and reef patches joining together into an inner Calituban and outer Caubyan barrier reef. Because of its unique coral reef ecology, it is a major breeding ground and habitat of many different species of fishes, marine mammals, mollusks, and seagrasses.
Cagong-Cagong Cave System: Princess Manan-aw Cave
Princess Manan-aw Cave is just one of the more than 20 caves in the Cagongcagong Cave System located in Bgy. Cagongcagong in the town of Alicia. The caves are a result of the hundreds of thousands of years carving work of water, whether rain or groundwater, on the Sierra Bullones Limestone (Late Miocene, 11 mya – 5 mya). Princess Manan-aw Cave, which is the chosen geosite, is 195 meters long, 5.6 m high and 3.7 m wide at the mouth. Inside the cave are impressive and peculiar formations and shapes that were formed very slowly from the dripping of water containing calcium bicarbonate from the limestone rock. As the water drips from the ceiling of the cave and encounters the dry air flowing inside the chamber, calcium carbonate precipitates back into limestone, as shapes and structures such as stalactites and stalagmites, as well as other unique shapes which appear feminine in nature, hence the name Princess Manan-aw was derived.
Inabanga Rupture Site
The Magnitude 7.2 earthquake in October 15, 2013 in Bohol damaged billions worth of infrastructures and properties including several centuries-old churches. It was caused by a previously unmapped fault which manifested on the land surface as an elongated fissure on the ground and can be traced for 6 kilometers from Brgy. New Anonang in Buenavista up to Brgy. Napo in Inabanga, and is believed to extend even further offshore. The most prominent expression of the fault is in Bgy. Anonang, Inabanga, where, a formerly flat rice field was cut and the northern part raised to up to 3 meters high, forming a conspicuous wall called a fault scarp. The scarp stretches for up to 2 kilometers long. The movement along the fault wherein one side moved upwards with respect to its opposite side as manifested in the fault scarp can only be caused by a tremendous amount of energy which is released during the Magnitude 7.2 earthquake.
Alicia Schist
Alicia Schist is the oldest rock identified in Bohol. It was named after the town where it is located, the northeastern town of Alicia. These metamorphic rocks were formed between the Late Jurassic to Early Cretaceous (146 mya – 66 mya). It initially started from ocean floor sediments that were successively buried deep underneath layers and layers of sediments until it turned into sedimentary rock. Afterwards it was subjected to tremendous regional pressure and deformation causing its upward movement and eventual emergence from the sea. This caused the metamorphism of the rocks, changing them to a different type of rock from what they were originally: a schist, with repetitive thin layers or sheet-like planes that can be easily split. The sheet-like planes can be seen in the outcrop and hand samples, as well. The Alicia Schist outcrop is located at an exposed hill along a road in Brgy. Cayacay, Alicia, Bohol. Other exposures are also found in the town of Ubay.
The Alicia Panoramic Park
The Alicia Panoramic Park features the breathtaking landscape of Binabaje Hills of Bgy. Cambaol in Alicia Municipality. The hills, which rise to 400 meters above sea level, are mostly covered in tropical perennial grasses commonly known as Cogon. These tall green grasses wave gently as the breeze passes, giving a soft furry-like appearance to the hills, which is composed primarily of metamorphic rock – the Alicia Schist, the oldest rock in Bohol (Late Jurassic – Early Cretaceous, 146 mya – 66 mya). Many adventure-seekers enjoy trekking the ridge trails over the hills and the picture-perfect and stunning green scenery.
Lamanok Island
Found in the easternmost tip of Anda is the Lamanok Island, which is underlain by massive to rubbly limestone deposits of the Late Miocene Sierra Bullones Formation. Prominent tidal notches can be observed around the edge of the island facing Bohol Sea showing dynamic movement of the sea during the Neogene Period. Large fossils such as giant shells protrude out of the limestone outcrops and cave walls. This section of the Anda Peninsula is also considered the seat of Bohol’s civilization as archaeological artifacts and burial sites of ancient settlers are preserved.
Baclayon Church
The Baclayon Church is a Roman Catholic church in the municipality of Baclayon. It is formally called the Church of Immaculate Conception. Its construction started in 1717 elevated as a parish on the same year. It took 200 obras pias or native forced laborers to drag coral stones from the sea, and egg whites from millions of eggs used as cement to build the magnificent structure. The present coral stone church was completed in 1727.
The church was founded by the first Spanish missionaries in the region, Jesuit priests Juan Torres and Gabriel Sanchez, when they first settled in Baclayon in 1596. There was no proper establishment available to conduct their missionary activities when they landed. They were able to convince the locals to build a church. That church became the oldest Christian settlement in Bohol. After completion of the building in 1727, an adjoining but separate tower was started under the supervision of the Jesuits, but was completed during the administration of the recollects in 1777. A large bell was added in 1835. And in 19th century, the Augustinian Recollects added a modern facade and stone buildings that surround the church.
In October 2013, the church was heavily damaged by a magnitude 7.2 strong earthquake. The church’s facade collapsed to the ground, as well as the bell tower. The church has not been used for official church meetings after the destruction, but the government conducted rehabilitation to restore the old building. It was reopened to the church-goers on February 27, 2018 after being closed for more than 4 years.
Cave Pools of Anda
Aside from the fine white sand beaches in Quinale, the Cave Pools of Anda are becoming common destinations in the southeastern part of Bohol for adventure seekers. Due to the large percentage of land underlain by limestones, cave pools or basically limestones filled with spring water forming pools are common sights in Anda. Among the famous cave pools are Cabagnow and Combento, and the smaller Kaligoon Cave Pool, East Coast Cave Pool and Kalorenzo Cave Pool.
The area is underlain by the Middle Miocene Anda Limestone belonging to the Sierra Bullones Limestone Formation. Years of dissolution and weathering of this limestone body resulted in karst features like these caves.
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