The Lower Cretaceous Tuburan Limestone of Cebu Island, Philippines: Microfacies, micropalaeontology, biostratigraphy, and palaeogeographic perspectives

The occurrence of orbitolinid-bearing shallow water limestone blocks (Tuburan Limestone) incorporated into the volcanic series from Cebu Island, Central Philippines, has been known since the 1950's. Taxonomic studies including solid biostratigraphic data however are lacking or not substantiated. Herein we report the occurrence of Mesorbitolina texana (ROEMER), transitional forms between M. texana and M. subconcava (LEYMERIE), Mesorbitolina birmanica (SAHNI), Palaeodictyoconus acti-nostoma ARNAUD-VANNEAU & SCHROEDER, Neorbitolinopsis conica (MATSUMARU), Paracoskinolina sp. and other benthic foraminifera (Akcaya, Praechrysalidina, Vercorsella) indicating a latest Aptian age. The previously accepted Late Albian age for the Tuburan Limestone based on Neorbitolinopsis conulus (DOUVILLE) is rejected herein and suggested as a misidentification with the recently revised Aptian - Lower Albian Neorbitolinopsis conica (MATSUMARU). The general poverty of Lower Cretaceous dictyoconids in the Tuburan Limestone is interpreted as being caused by the lack of suitable extensive lagoonal facies that is generally typical for carbonate platforms of passive continental margins. The observed microfacies types instead refer to external platform margin settings with corals, stromatoporoids, sclerosponges (e.g., Acanthochaetetes), and planktic foraminifera. The recovered (micro) fauna from Cebu Island shows striking similarities to assemblages reported from Western and Mid-Pacific guyots but with indicated younger ages (up to the late Albian) based on data that - in our opinion - do not stand up to close scrutiny. Finally, a model is proposed interpreting the Tuburan Limestone from Cebu Island as remnants of a former carbonate cover of a guyot that originated as a volcanic island in the Western-Central Pacific, and later became incorporated into an accretionary wedge/melange zone due to subduction-collisional processes.

Automated summary

In this study, the occurrence of orbitolinids in the Tuburan Limestone from Cebu Island, Central Philippines, was reported and their ages were determined more accurately through taxonomic analysis. The results showed that the Tuburan Limestone is of late Aptian age. Additionally, the study found a scarcity of Lower Cretaceous dictyoconids in the Tuburan Limestone, which may be attributed to the absence of suitable environmental conditions. A model was proposed to explain the formation process and geological history of the Tuburan Limestone.