| Title | Paleoreconstruction of Gayong Sedimentary Formation |
|---|---|
| Authors | Baby Nocelwin Ernestina A Santos, Julius John T Dimabayao |
| Year | 2011 |
| Conference | New Zealand Geothermal Workshop |
| Keywords | Bacman, paleoenvironment, lithofacies, stratigraphy |
| Abstract | A field-wide occurrence of Gayong Sedimentary Formation (GSF) was established to refine stratigraphy of the Bacman Geothermal Project (BGP), and to reconstruct paleoenvironment and geomorphology of the basin. Based on paleoanalysis, the GSF is the oldest stratigraphic unit encountered in BGP with an established Late Miocene to Early Pliocene age. It is predominantly composed of carbonaceous and fossiliferous limestones, reefal carbonates, calcareous sedimentary breccias and fine-grained clastic rocks. Downhole well data correlation results combined with paleoanalysis and lithofacies characterization indicate a paleoenvironment gradation – from shallow neritic to shallow open marine in Botong and Palayang Bayan sectors, and deepening towards the south in Tanawon, possibly reaching the abyssal plains of deep marine setting. Observed lateral and vertical changes in lithofacies of GSF are attributed to natural deepening of ocean basin morphology, historical sea level fluctuations, and tectonic activities. The GSF has widespread occurrence in BGP except in Cawayan sector. The wells drilled in Cawayan did not observe GSF probably because of either of the following: deep occurrence of formation due to downfaulting; obliteration of sedimentary units due to Plio-Pleistocene intensive volcanism; or non-deposition of sediments attributed to a paleo-high configuration of the region in pre-Miocene to Pliocene. In Tanawon sector, presence of non-calcareous laminated fine-grained sediments indicates that the area was once submerged in deep marine environment. Its present high elevation at ~912 m ASL, however, suggests that the area was uplifted after deposition of GSF. Accurate subsurface depiction and reconstruction of depositional environments of GSF aims to aid particularly in delineating permeable zones and in predicting areas that are prone to calcite scaling. |