A Geological Study of the Angat-Novaliches Region

Alvir, Antonio D.

     The study of the geology of the Angat-Novaliches region was undertaken primarily for the purpose of supplying the Metropolitan Water District with geologic cross-sections along the proposed tunnel lines between their Ipo dam site on Angat River and the Novaliches reservoir site in connection with the Angat water system project. . . . The present study is a detailed study of the geology of the region with special emphasis on the structure. The most violent earthquakes have been occasioned by sudden movements or slips between portions of the earth's crust along fault planes. It is necessary to make a detailed study of the structure of a region, with a careful mapping of every fault and a study of its probable directions of movement and activities, in order to find the probable centers and directions of the shocks of future tectonic earthquakes in that region. At the same time, some important points in Philippine geologic history have been cleared in this paper, since the Angat-Novaliches region is an unusually good representative of Philippine general geology.
     Significant findings of the study include the following: (1) Granite is the oldest known rock in the Philippines and is probably Paleozoic. (2) The basement complex rocks were intruded during the close of the Paleozoic giving the Philippine Islands their general delineaments under the sea, and this was contemporaneous with the depression of the basin of the South China Sea. (3) The third intrusion consisting of pyroxenites and serpentines occurred after the Jurassic and before the Eocene, that is, during the Cretaceous. (4) The Eocene was a period of wide-spread emergence when the Philippine Islands had the greatest land area in their history. This was the first time that the Philippine Islands were above the water. (5) The Oligocene was a period of submergence, which was the greatest since the Islands emerged. There was also slight volcanic activity. (6) The Binangonan limestone and the sediments conformably below and above it are of Oligocene age. (7) The greatest deformation suffered by the Philippines occurred at the close of the Oligocene and extended through the Miocene. (8) The compressive stresses of the Post Oligocene-Miocene deformation was due to a thrust from the west, producing in the Angat-Novaliches region, thrust faulting, and intense folding with axes tending in a general north-south direction; they also initiated the cross faults 2 and 3. These compressive stresses persisted to the end of the Miocene but were probably strongest at the outset. (9) The principal north-south faults were initiated during the settling after the compressive stresses were spent, probably during the Pliocene or Quaternary. The overburden of lava has added greatly to these settling adjustments which may persist to the present day. (10) Erosion to maturity occurred before the extrusion of the lavas when the Angat River may have flowed southward to the older and larger Manila Bay. (11) The Miocene was the age of volcanic extrusions resulting in a lava plateau. Intrusions also occurred just north of this area giving rise to the iron deposits of Bulacan. (12) The Pliocene was a period of submergence and of great volcanic explosions, which resulted in thick, stratified, tuff deposits. (13) There is a thin littoral conglomerate, with cross-bedding and local unconformities at the base of the Pliocene. (14) The Pleistocene was initiated by an upward movement which has continued with oscillations to the present time, amounting to an uplift of about 500 feet. (15) Fault 7 is a major structural feature and can be traced topographically from Batangas Bay to the northeastern Pacific coast of Luzon, and is responsible for the separation of Laguna de Bay from Manila Bay. (16) Mariquina Valley is a graben.