THE GEOLOGY OF THE AMERICAN MEMORIAL PARK AND THE ISLAND OF SAIPAN, COMMONWEALTH OF THE NORTHERN MARIANA ISLANDS

It has been said (at least in anecdotal mythology) that some National Parks have “no geology, only scenery”. If one considers the parcel occupied by the American Memorial Park in Garapan, Saipan, this statement is almost true. The park's 133 acres comprise artificial fill and recently emerged limesands, one of the simplest geologic inventories in the National Park Service (NPS) system! The island of Saipan, part of the United States, Commonwealth of Northern Mariana Islands (CNMI), is located in the Pacific Ocean about 1500 miles (2400 Km) south and east of Japan and about 120 miles (190 Km) north-northeast of the island of Guam. The park serves as an interpretive center for the World War II battle that raged across the island and the neighboring island of Tinian in June, 1944. The geology and tectonic setting of the islands were integral to the development of the landscape as well as the strategy, tactics, and course of the battle. For example, a lack of hard stone natural aggregate limited Japanese efforts to construct concrete artillery positions. The island comprises a core of Eocene volcanic rocks overlain by chiefly reef limestones ranging from Eocene to Holocene in age. The physiography of the island is complex, having been affected by humid, tropical weathering and erosion, tectonic movements related to subduction along the Marianas Trench, and changes in relative sea-level. Much of the island surface is karst, with numerous sinkholes, caves, and springs. Some of the physiographic features earned names like Death Valley, Purple Heart Ridge, Harakiri Gulch, and Suicide Cliffs during the battle. The U.S. Geological Survey (USGS) is revising the geologic map of Saipan previously published by Cloud and others in 1958. Significant changes include: 1) Reinterpretation of the “rubbly facies” of the Tagpochau Limestone, originally thought to be a fore-reef deposit, as a tectonic breccia, 2) Identification of faulting not recognized in previous mapping, and 3) Construction of new cross-sections and a 3-dimensional structural model of the island in light of modern plate tectonic theory and new field evidence, and 4) Identification of new potential bedrock aquifers to address the islands' severe drinking water shortage.