Cenozoic Evolution of the Southwestern Margin
of the Cagayan Valley Basin
Lea Soria, Katherine Hipol, Geol 170 Class of 2001, Leopoldo de Silva, Jr., Joseph Foronda,
Zenon Mateo, Fernando Siringan, and Maria Luisa Tejada
National Institute of Geological Sciences, University of the Philippines, Diliman, 1101 Quezon City
Field mapping focused on the lithologic and structural aspects of the southwestern margin of the Cagayan Valley Basin. The area covered the municipalities of Asipulo, Kiangan, and Lamut in Ifugao, and Bagabag, Solano, Bayombong, and Bambang in Nueva Vizcaya. Five formations were encountered, namely, the Dumatata Formation, Ibulao Limestone, Lubuagan Formation, Balbalan Formation, and the Tabuk Formation.
Alkalic to calc-alkalic volcanism from Late Oligocene to Early Miocene is evidenced by plutonic and volcanic rocks on the southmost margin of the area. During the Late Oligocene, the area may have been under a deep marine to terrestrial environment. The Late Oligocene Dumatata Formation is characterized by highly fractured and well indurated sandstone-mudstone interbeds and breccia inter-tonguing with volcanic breccia at the base and andesite flow on top. Unconformably overlying this formation is the Early Miocene Ibulao Limestone. It is composed of rudstones, floatstones, and packestones formed in a shallow marine environment. Bedded biostromal to biohermal limestones of this formation comprise the north-south trending ridges found at the western side of the area. It is generally massive to very thickly-bedded but becomes medium-bedded towards the top. The type locality is found in the Ibulao Gate. Conformably overlying the Ibulao Limestone is the Early Miocene Lubuagan Formation. This formation is composed mainly of sandstone-siltstone interbeds in the lower part with alternating sandstones and conglomerates increasing in the upper part. The sandstone-siltstone sequence shows structures typical of turbidites. The Lubuagan Formation is unconformably overlain by the Balbalan Formation. It is composed of andesite flows, fossiliferous sandstones, shales, conglomerates, alternating sandstones and conglomerates, and minor limestones. It is dated to be Middle Miocene based on calcareous nannofossils. Depositional environments could be from bathyal to shallow marine (as suggested by the presence of pelecypods, gastropods, and finger corals) to terrestrial (as suggested by the tree barks in the conglomerates, and leaf imprints in the sandstone). The early part of Middle Miocene is marked by active volcanism. The Late Middle Miocene to Pliocene period is marked by a break in the rock record as no rocks of these ages are seen in the area. Unconformably resting on the older formations is the Pleistocene Tabuk Formation. It consists of volcanic plugs in lower sections and tuffaceous sand deposits, autobreccia, lahar deposits, and terrace gravel deposits in upper sections. This formation is characteristic of active volcanism in a terrestrial environment. Recent colluvium blankets the low-lying areas.
During the Early Miocene, NE-SW compression and extension directions are inferred based on fault array analysis. Strike-slip and reverse faults indicate that during the Middle Miocene, the primary stress direction slightly changed to the NNE-SSW direction. During the Quaternary, a NW-SE compressive stress is occurring. This force is associated with the movements along the left lateral strike-slip Philippine fault.