Numerical Analysis of Foraminiferal Assemblages: A Tool for Recognizing Depositional Facies in Lower Miocene Reef Complexes

Study of the calcareous foraminiferal assemblages in thin sections of lower Miocene cores from the Matinloc 2 and Libro 1 wells off the northwest coast of Palawan, Philippines, revealed similar biofacies in the two wells. Multivariate and morphometric analysis of the foraminiferal data distinguished three assemblages. These three assemblages helped to define four distinct facies that are found in both wells: 1) small foraminiferal grainstone-packstones characterized by small rotalines and miliolines, 2) coral boundstones with foraminiferal-algal packstones in which a small rotaliine-milioline assemblage is represented along with some larger rotaliine foraminifera, 3) algal-larger foraminiferal packstones characterized by Amphistegina and miogypsinids, and 4) larger foraminiferal wackestones in which large, flat, oriented lepidocyclinids and Cycloclypeus dominate in a micrite matrix that bears planktonic foraminifera. These four facies were interpreted to represent, respectively: 1) lagoonal sediments, 2) coral-rubble reef flat or reef margin, 3) algal-stabilized reef flat, and 4) drowned reef or slope sediments. The depositional history of the 104 m of Matinloc 2 core is interpreted as follows: lagoonal infill topped by shallow algal-stabilized reef flat is indicated in the lower 44 m of core. A slight increase in energy on the reef flat allowed for temporary colonization by coral before replacement by vegetated reef flat. Finally, a major transgressional sequence of reef flat to reef to drowned reef is evident in the upper 41 m of the core. The drowned-reef limestones are overlain by shales. The Libro 1 core, though incomplete, shows similar conditions of shallow lagoon and reef flat in the lower portion and progressive drowning of the reef in the upper portion. -Author