The Binangonan Formation has been previously dated as Upper Oligocene (Smith, 1906;
Yabe & Hanzawa, 1929). Foronda (1987) reported that the unit may extend to Early Miocene on the basis of a larger foraminiferal assemblage. He further interpreted the Teresa Tuffaceous Siltstone member (lower member) as shallow water turbidite and the upper limestone member as constituting a typical reef complex. He concluded that the shallow water turbidites served as a carbonate platform upon which carbonate deposition associated with reef building initiated. However, no detailed biostratigraphic work has been done in estahl ishing the temporal relationship between the members and the different facies of the Binangonan Formation.

A series of samples taken from portions of the Teresa Tutlaceous Siltstone and the upper limestone member reveal a strong variation in foraminiferal assemblage indicating changing
environmental conditions at the time the sequence was deposited. Samples taken from the limestone interbeds have a faunule exclusively of larger foraminifera belonging to the genera
Lepidocyclina (Eulepidina, Nephrolepidina), Spirocyclypeus, and Miogypsina (Miogypsina),
with several genera of smaller benthonic foraminifera such as Amphistegina. On the other hand, the tuffaceous silstones have a faunule of planktonic foraminifera belonging to the genera Globigerina, Globigerinoides Globoquadrina, Globorotalia, Globoratoloides, and Catapsydrax with several genera of smaller benthonic foraminifera which include Melonis, Karreriella, Pleurostomella, Lenticulina, and Nodosaria. These faunules are of great interest as they all contain in varying generic combination the same genera and species of larger benthonic and planktonic foraminifera. This suggests a considerable mixing of fauna by contemporaneous transfer of larger foraminifera from shallow to deeper waters probably as a result of turbidity currents. In addition, the occurrence of both benthonic and planktonic foraminifera in the same sample within the same formation may be significant as these associations may provide the link between these two groups in establishing the age of Tertiary marine elements whose age determination differ based on one group in the absence of the other.

As far as the authors are aware, this is the first time that planktonic foraminifera have been found and studied in the Binangonan formation. Previous datings were based mainly on larger
foraminifera. However, dating using planktonics show equivalence with larger foraminiferal age assignments.