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.