Late Tertiary geologic history of the continental shelf off Northwestern Palawan, Philippines

Marine seismic and magnetometer surveys, plus the drilling of two offshore wells on the northwestern shelf of Palawan, Philippines have yielded valuable information on the geology and Late Tertiary geologic history of this region. These exploration activities have shown that the structural framework of the shelf consists essentially of a thick westward-dipping sequence of sediments of Neogene age on a basement of older rocks. Among the more common structural features of this framework are horst blocks and grabens, compressional and supratenuous folds, deltaic and associated cut and fill structures, reefal buildups and unconformities. Thus far, structural forms suggesting Late Tertiary igneous activity have not been recognized in any of the geophysical data. Integration of available drill hole data with seismic information indicates that since the deposition of several thousand meters of deep sea marine clastic sediments on a lower Miocene limestone depositional base in early Miocene time, the region has undergone two major periods of regression and marine transgression. These events are recorded as two prominent unconformities recognized on seismic profiles which subsequently have been identified from well information as intervening between the middle Miocene and upper Miocene, and between upper Miocene and Pliocene strata. Major uplift of the north Palawan block toward the end of middle Miocene time resulted in erosional truncation of the tilted lower and middle Miocene sequence. In the latter part of the Miocene, the shelf off northwestern Palawan was once more transgressed by shallow seas, in which were deposited detrital carbonate and other clastic sediments. Some of these were laid down on synchronous highs developed on the folded and faulted middle and lower Miocene sequence. During the latter part of the Miocene, parts of the area may have been temporarily emerged to cause partial removal of upper Miocene sediments. The beginning of Pliocene time saw another transgression characterized by the deposition of carbonate sediments over the shallow shelf continuing up to the present