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North of Lat. 16° N. the island of Luzon, Philippines, consists morphologically, from east to west, of: (1) the Sierra Madre, (2) an intermontane structural basin, the Cagayan Valley, (3) the Cordillera Central, and (4) a fragmentary exposure of a structural basin extending northward from the Central Valley basin of central Luzon, the west margin of this basin lying beneath the South China Sea.
Luzon developed between the sialic continental region of China and the basaltic Pacific Ocean basin. This position has given it an intermediate to basic framework that supplied a predominance of quartz-deficient clastics to idiogeosynclines formed within it. The basement foundation of Luzon is predominantly of early Tertiary age. Most of the sediments accumulated in Miocene and Pliocene time. In early Miocene time the Cagayan and Central valleys were one interconnected depositional site. After deposition of the Sicalao and Kennon limestones (in the Cagayan and Central valleys respectively), the Cordillera Central became a positive zone separating two depositional basins. The Cordillera Central was the dominantly active tectonic feature of northern Luzon, while the Sierra Madre was a m re stable or passive element.
The depositional environments in northern Luzon progressed from marine (Miocene) through marine-brackish (upper Miocene-Pliocene) to fluviatile (upper Pliocene-Pleistocene). Concurrent with this development of idiogeosynclines, filling of the basins, and over-all acceleration of uplift from Miocene to Pleistocene time, a volcanic cycle progressed from mafic to silicic (quartz-bearing tuffs in the Ilagan and Awidon Mesa formations of the Cagayan Valley) and a recent reversion to mafic extrusives (Mt. Cagua and Camiguin Island basalt cones).
Most of the anticlinal and synclinal trends visible at the surface, fault zones, and stratigraphic units have been named. Type sections are designated and described for the stratigraphic units discussed and mapped.
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