Notes on the Geologic Reconstruction of the Surigao Valley

Claro Jose C. Manipon, Rikki Pamela Pineda, Kristine Sobrepena, Katrina Delos Angeles and Jerry Gatinao

Philex Mining Corporation



The Surigao Valley is a north-south trending topographic low in Surigao Del Norte bounded by the municipality of Kitcharao, Agusan Del Norte in the south and by Surigao City in the north.  To the west lies the Malimono Ridge and to the east lies the northern edge of the Mindanao Pacific Cordillera.  Lake Mainit, Mt. Mainayao Volcano Complex and the Surigao Segment of the Philippine Fault are prominent natural features of the valley. The lithology is composed of volcaniclastics of the Recent  Maniayao Volcanic Complex; fine to coarse clastics of the Pleistocene Tugunan Formation; limestone of the Pliocene Timamana Formation; the turbidites of the Mindanao Motherlode Formation and the sea-floor basalts and limestones of the Early Miocene Bacuag Formation. Hidden diorites puncture the pre-Pliocene rocks which could be barren or mineralized as in the case of the Boyongan and Bayugo Cu-Au  (BBC) deposits located near the fringes of the younger Mt. Maniayao Volcano Complex. Pre-Pliocene reconstruction of the valley revealed a positive landmass with several geomorphic features: (1) a centripetal drainage pattern with steep walls mimicking the occurrence of a deep circular lake; (2) prominent hills rich in copper minerals (BBC); and (3) another set of river systems draining towards the north with headwaters coming from the exposed and eroding BBC. Upwards in the stratigraphy, Pre-Maniayao reconstruction showed submergence with reefs near the eastern part of the “Surigao Bay”. This bay was later enclosed and evolved into an elongate lake system with isolated ponds.  A fresh fluvial system developed over some of the lakes but was in turn buried by massive influx of lahars and pyroclastic rocks from eruptions of Maniayao Volcano. The latter provides explanation on the occurrence of porphyry copper boulders in the valley which are now believed to be erosion remnants of paleo river deposits from the BBC and perhaps from some other hidden deposit.

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