Unconformable over ophiolite; conformably overlain by the Sta. Cruz Formation
Middle Miocene – Late Miocene
Zambales Limestone and Conglomerate (Corby and others, 1951)
This formation was previously named Zambales Limestone and Conglomerate by Corby and others (1951) for the rocks exposed as an inverted S-shaped belt 5 km east of Naluo Point, Sta. Cruz, Zambales. It can be subdivided into a lower clastic member and an upper limestone member. Karig and others (1986) proposed to make the Cabaluan River section as a reference section, since the formation is well exposed and developed there. The formation was renamed Cabaluan Formation by MGB (2004), as Zambales is a non-specific locality in terms of geographic appellation.
Along Cabaluan River, the lower clastic member consists of a 130-m thick sequence of conglomerate, sandstone and siltstone. Its base lies unconformably over serpentinized harzburgite. Clasts in the basal conglomerate are made up almost entirely of pebbles and cobbles of serpentinized harzburgite. Sand components of the finer clastics also consist mainly of serpentinites. In places, the clastic sequence is carbonaceous and contains fossils, including plant remains, gastropods and coral fragments. Coquina and lignite lenses are interspersed within the sequence. The conglomerates in the upper portion have smaller pebble-sized clasts and sandstones become more dominant towards the top. The lower clastic member is massive to moderately bedded, in places showing cross-bedding. A littoral setting is indicated for the deposition of the lower clastic member (Karig and others, 1986).
The upper limestone member consists mainly of reefal limestone. The lower portion of the limestone member is a 20-30 m thick sequence of buff-colored, poorly bedded bioclastic limestone which grades into medium bedded bioturbated calcareous sandstone and then into silty marl. This clastic sequence is overlain by 100 m of the main reefal limestone. This is predominantly massive and forms prominent ridge crests. The limestone grades upward into coral boulder limestone with abundant shell and coral debris to interbedded bioclastic limestone and sandy marl to mudstone.
The thickness of the formation varies widely, but the Cabaluan River section is approximately 250 m thick (Karig and others, 1986). The foraminiferal assemblage of the upper limestone member includes Orbulina universa indicating an age no older than Middle Miocene (Zone N9). Karig and others (1986) also report that calcarenites at the top of the limestone member yielded a foraminiferal assemblage of late Late Miocene age (Zone N17/N18).