Controversy arises when attempting to relate unconformities on a tectonically active margin to global changes in sea level. Seismic stratigraphy studies on active margins generally concentrate on defining large seismic packets and do not directly relate unconformities and their correlative conformities to global sea level changes. In the Central Valley of Luzon, we determined sequence boundaries in the basin and developed an age model that strongly suggests that sea level change is the major factor affecting shorter term (less than 5 m.y.) changes in sedimentation on this active margin.

The Central Valley is a Cenozoic fore-arc basin bounded by an arc complex and the left-lateral strike-slip Philippine fault on the east and by an east-dipping subduction zone adjacent to the Manila Trench on the west. Multichannel seismic reflection, well, and outcrop data were used to determine the depositional history of the basin. Because much of the 13 km (8 mi) thick basin fill consists of deep-water marine sediments, conventional criteria such as coastal onlap and erosional truncation could not always be used. Instead, evidence for pulses of submarine fan deposition during lowstands of sea level (suggested by Vail and Hardenbol in 1979, and by Shanmugan and Moiola in 1982) was used to identify some of the sequence boundaries. The ages of the major boundaries, predicted from comp risons with Cenozoic Sea level curves, agreed very well with established ages from well and outcrop data.

The supposed difficulty in determining sea level changes in active margins is that tectonic effects override and cloud the effects of global sea level changes. We agree that major regional tectonic events such as the initiation of subduction or strike-slip movement that creates or destroys basin morphologies clearly are the dominant factors in the overall stratigraphy of the basin. However, episodic tectonic events during continued basin evolution result in discrete changes in local basin morphology and sediment source areas which may lead to local unconformities or local increase or decrease in sediment influx. These effects are probably small compared to the basin-wide effects of global sea level changes. Such is the case in the Luzon Central Valley where the effects of global sea l vel changes can be seen throughout the basin.

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