Stratigraphic revision of the Neogene rocks of Saipan Island, Commonwealth of the Northern Mariana Islands

Abstract

Geologic mapping in 2006 and 2007, and calcareous nannofossil biostratigraphy combined with comparative analysis of other published data, have resulted in a reinterpretation and revision of the ages and stratigraphic relationships of the Neogene lithostratigraphic units exposed on Saipan Island. These units, chiefly limestone and admixed volcaniclastic rocks, form much of the bedrock of Saipan Island. Stratigraphic revisions include a redefinition of the Tagpochau Limestone, restricting it to rocks of the newly named I Etdot, Tipo Poli, and Marpi Members. Other rocks, formerly included in the Tagpochau Limestone (the Donni Sandstone and the Machegit Conglomerate Members, and an informal transitional facies), are herein assigned to the newly defined Gloria Formation. The Gloria Formation comprises three members: the Machegit Conglomerate Member, the new As Teo Member, and the Donni Sandstone Member. The Tagpochau Limestone contains larger foraminifera that indicate an early Miocene (Aquitanian through middle Burdigalian) age, or a numerical range of about 23 to 18.7 Ma. Overlying the Tagpochau Limestone, both the Fina-sisu and the Gloria Formations are recognized herein to be distinctly younger than the Tagpochau Limestone and not correlative with any parts of that formation as had been previously thought. Basaltic lava flows in the Fina-sisu have yielded radiometric ages that indicate a middle Miocene age, whereas planktonic foraminifera recovered from interbedded sedimentary rock indicate a latest early Miocene age (Langhian). Assemblages of planktonic foraminifera and calcareous nannofossils recovered from the Gloria Formation indicate a latest Miocene age (Messinian) and a numerical age range of about 5.8 to 5.5 Ma. Additional studies could help to constrain the ages of these units more precisely because none of the biostratigraphic samples were taken in the context of measured stratigraphic sections. On an island dependent on groundwater, the revised stratigraphic understanding can inform the development of groundwater models and strategies for identifying water-well sites and targeting drilling depths for potable water. The new stratigraphic understanding can inform future interpretations of the geologic structure of Saipan Island and the Neogene tectonic history of Saipan Island and the Mariana subduction zone.