A NEW GEOLOGIC MAP OF THE ISLAND OF SAIPAN, COMMONWEALTH OF THE NORTHERN MARIANA ISLANDS

A new geologic map of Saipan is critical to understanding the ground water resources of the island. The geology of Saipan was previously documented in a geologic report and map published by the U.S. Geological Survey (USGS) in 1956. The information in that report provided the geologic framework for earth science studies, particularly hydrologic, on the island. Since 2006 geologic information generated by USGS fieldwork has significantly improved our understanding of the geology of Saipan, allowing compilation of a new and improved geologic map. The new, GIS –based map is compiled on a modern topographic base map at 1:25,000-scale. Improvements to the older map include a revised interpretation of the stratigraphic relationships of the Neogene rock sequence, and delineation of additional faults and structural blocks. Stratigraphic revisions include: 1) elimination of the “rubbly facies” of the Tagpochau Limestone as a lithostratigraphic unit (it is a tectonic breccia); 2) promotion of the Donni Sandstone Member of the Tagpochau Limestone to formational status; and, 3) reassignment of both the Machegit Conglomerate Member and the “transitional facies” of the Tagpochau Limestone to the new Donni Formation. A paper will be published to formalize these changes to the stratigraphic nomenclature. Both the Fina-sisu and Donni Formations are now known to be younger than the Tagpochau Limestone based on field relationships, published radiometric data, and biostratigraphy. The Tagpochau is constrained physically and biostratigraphicaly to an early Miocene age. Basalts in the overlying Fina-sisu Formation yield middle Miocene radiometric dates, but sparse planktic foraminifera data from its sedimentary beds suggest a slightly older, latest-early Miocene age. Assemblages of planktic foraminifera reported from the Donni Formation suggest an early Pliocene age. These Miocene and Pliocene rocks, chiefly limestones, form much of the uplands of Saipan and constitute a major part of the aquifer that provides drinking water for the island. Improvement of the geologic map and 3-dimensional geologic framework of the island enables better development and management of the ground water resource that serves a population of about 70,000 people.