GEOLOGICAL SOCIETY OF THE PHILIPPINES

GEOLOGY OF THE WEST COAST (ZAMBOANGA CITY-MAYALAL, ZAMBOANGA DEL NORTE), ZAMBOANGA PENINSULA, MINDANAO, PHILIPPINES

 

J. G. Ammugauan1, M. G. S. Asio1, J. V. Asis1, C. F. Bastero1, D. N. D. Hilario1, K. L. T. Larase1, M. B. Muan II1, E. R. L. Oporto1, B. D. Payot1, B. R. G. Ramos1, N. T. Ramos1, T. A. Tam III1, E. S. Andal3, A. G. Digdigan3,, D. V. Faustino3, J. Hernandez2, M. Manjoorsa2, F. O. Olaguera3, C. Querubin3, B. Saani2,   D. K. B. Velasco2, G. P. Yumol, Jr. 3

1National Institute of geological Sciences, University of the Philippines, Diliman, Quezon City

2Mines and Geoscience Bureau – Region IX

3Rushurgent Working Group, National Institute of Geological Sciences, University of the Philippines, Diliman, Quezon City, 1101

 

A three–week fieldwork was conducted to map the lithologies on the west coast of the Zamboanga Peninsula. Field mapping led to the recognition of seven formational units; The cretatious tunguan Schist, Paleogene Bungiao mélange, early to middle Miocene Anungan Formation, Late Miocene Curuan formation, Plio – Pleistocene Mt. Maria Formation, pliestocene Limpapa mélange and Quaternary alluvium deposits. New stratigrphic units, the limpapa Melange and patalon Volcaniclastic member of the anungan formation, were established.

The Tungauan Schist is composed of metamorphic rocks, which include quartz Schist and quartz chlorite schists. Thrusted onto the Yanguan Schist is the Bunguiao Melenge. This unit is characterized by megaclasts of schists and harzburgites in a highly sheared  serpentinitematrix. The Bunguiao mélange is conformably overlain by the Anungan Formation, which is composed of four members: Pico clastics, mala Volcanics, Patalon Volcaniclastics, and theManichan limestone. These tertiary deposits are shell half in origin as shown by their fossil Formation, which is composed of the Pasonanca Clastic and dulian Volcanic members. Miocene Vonlcanism, uplift erosion, and shallowing of the peninsula formed these members  of the Anungan and Curuan Formations. These activities can be attributed to the early Miocene subduction of Sulu Sea along a Proto – Sulu Trench and the on ramping of the Philippine Mobile belt against Sundaland I the middle Miocene. This accretion of the PMB eventually caused the cassation of subduction along the proto – Sulu Trench. However, in the Pliocene unit is characterized by a series of volcanic plugs, intrusions and pyroclastic flows that cap the older formations. The Limpapa Melange includes sandstone – siltstone, andesite and limestone clasts in a sheared limestone matrix. Slumping, which occurred during the pliestocene, may have triggered the formation of this new feature. Quaternary alluvial deposits characterized the fringes of the peninsula.

 
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