giant_clamshell_found Fossilized Giant Clam Shell found in Mountains of San Fernando, Cebu

Welcome to our forums Asiboy ! Thanks for highlighting the discovery of a fossilized Giant Clam Shell high up in the mountains of San Fernando in the island province of Cebu in the Philippines. Your attempt to explain the presence of the fossilized giant clam shell in the mountains of San Fernando in an article published in your blog, philippineblog.com is very much appreciated, and contains a wealth of information about the geography and geology of the Philippines and the biology and ecology of the Giant Clam, Tridacna gigas, whose natural home are the coral reefs in the waters of the South China Sea surrounding the Philippines and also the Great Barrier Reef in the waters of the Pacific off the coast of Queensland in northeast Australia.

The Philippine archipelago of over 7,000 islands is situated at the convergent boundary of two major tectonic plates, the Eurasian Plate and the Philippine Sea Plate. When the plates collide and one subducts, the geological events that follow are the formation of trenches, volcanic activity leading to the formation of volcanic mountains and the thrusting and folding (Orogenesis) of plates to form fold mountains. Though most of the mountains in the Philippines appear to be of volcanic origin, the mountains of the Cebu Island and particuarly the San Fernando mountains appear to be of Orogenic origin, formed by the thrusting and folding of convergent tectonic plates. During this process areas that were previously under the ocean would have been raised up to form mountains.This explains how the Giant Clam reached the mountain tops of San Fernando in the Island of Cebu, where its fossilized remains were discovered. The extensive limestone deposits in Cebu also seems to be related to the Orogenic activity that brought up these deposits formed in the ocean bed as sedimentary rocks by the accumulation of dead corals, shells of dead mollusks, and foraminifera. The limestone deposits of the Mananga Group of rock formations date back from the Cretaceous to Paleocene age, 145 million to 60 million years ago.Sedimentary limestone deposits of the Lutak Hill Formation, Cebu Formation and Malubog Formation date back from the late Eocene to Early Miocene periods, 38 million to 23 million years ago. The Luka Formation and the Uling Limestone Formation are middle Miocene rock formations dating back to around 15 million years ago. The conglomeratic limestone deposits of the Maingit Formation belong to the late Miocene period and are around 7-15 million years old. Hence, most of the hard limestone deposits in the Cebu Island are between 7-145 million years old. Limestone deposits more recent than this are coralline, porus and soft, poorly bedded and sandy.

The convergent tectonics that led to the orogenesis of the San Fernando mountains could have occurred any time between the Upper Cretaceous (70-100 million years ago) and Pleistocene (0.1-2.5 million year ago), the period during which complex tectonic processes were intermittently active. Hence, the period during which the orogenesis of San Fernando mountains took place can be estimated only if the age of the limestone deposits in these mountains are determined. If for instance, the age of the limestone deposits are found to be around 15 million years old, the orogeneisis of the mountains must have taken place after this, ie. anytime between 15 million and 0.1 million years ago.

However, it is important to note that the geology of the Philippines is more complex than the effects of just a convergent boundary between the Philippine Sea Plate and the Eurasian Plate. In fact in the tectonic boundary between the Eurasian Plate and the Philippine Sea Plate a complex zone exists known as the Philippine Mobile Belt, upon which most of the islands of the Philippine archipelago are situated. An interesting feature observed here is that both the Philippine Sea Plate to the East and the Sunda Plate (Extension of the Eurasian Plate) to the west, undergo subduction beneath the Philippine Mobile Belt, forming two subduction zones, the Philippine Trench to the east and the Manilla Trench to the west.The Philippine Mobile Belt situated between the two trenches is composed of a number of crustal blocks or microplates, which have been sheared off the adjoining major plates. The crustal blocks or microplates are strips running in the north-south direction and the boundaries or zones of convergence between them are the fault lines of the Philippines Fault System. It is the convergent tectonic forces generated by the two subduction zones situated east and west, responsible for compressing the tectonic plates of the Philippine Mobile Belt, lifting parts of the Philippines and causing extensive faulting, mainly in a north-south direction, and it is the same tectonic forces responsible for the earthquakes and volcanic activity of the Philippines.