STRATIGRAPHIC REVISION OF THE NEOGENE ROCKS OF THE ISLAND OF SAIPAN, COMMONWEALTH OF THE NORTHERN MARIANA ISLANDS

Recent geologic mapping and biostratigraphic analysis from the island of Saipan has resulted in revision of the stratigraphic relationships and relative ages of exposed Neogene lithostratigraphic units. These units, principally limestone and admixed volcaniclastics, form much of the axial uplands of Saipan. Revision of their stratigraphic relationships has a direct impact on interpretation of the geologic structure, tectonic history, and hydrogeologic framework of the island. Formal revision of the stratigraphic nomenclature, planned in the near future, will include redefinition of the Tagpochau Limestone and reassignment of several of its subunits: the Donni Sandstone Member, the Machegit Conglomerate Member, and the “transitional facies”, to a new formation. The Tagpochau Limestone, exclusive of those units, is given a lower Miocene age based on larger foraminifera biostratigraphy. The Fina-sisu Formation overlies the Tagpochau Limestone. Published data report a middle Miocene K/Ar age for basaltic lava flows in the Fina-sisu, but sparse planktonic foraminifera data from some of its sedimentary beds indicate a slightly older, latest early Miocene age. Sparse calcareous nannofossil data previously reported from one locality in the Fina-sisu Formation suggest an anomalously young (compared with the foraminifera from the same locality, and the radiometric ages), late Miocene through mid-Pliocene age. We suspect a sample labeling, or processing error. Beds formerly defined as the “transitional facies” and the Donni Sandstone Member of the Tagpochau Limestone overlie the Fina-sisu Formation. Reported assemblages of planktonic foraminifera recovered from the Donni interval and the “transitional facies” suggest an early Pliocene age (Globorotalia margaritae Zone) for those samples. Assemblages of calcareous nannofossils recovered from the Donni interval, including 3 new samples collected and analyzed by us, indicate a Late Miocene (NN9-NN10) age for those strata. Since complete sections of the Neogene units on Saipan are yet to be collected for microfossils, precise composite age ranges for these units remain undefined.