The geology of the

Northern Mariana Islands The Northern Mariana Islands, officially the Commonwealth of the Northern Mariana Islands (CNMI; ch, Sankattan Siha Na Islas Mariånas; cal, Commonwealth Téél Falúw kka Efáng llól Marianas), is an unincorporated territory and commonw ...

began to form with undersea volcanic eruptions in the

Eocene The Eocene ( ) Epoch is a geological epoch (geology), epoch that lasted from about 56 to 33.9 million years ago (mya). It is the second epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), Era. The name ''Eocene' ...

. Islands such as Saipan show a variety of rock types including volcanic rocks, breccia, tuff, conglomerate, sandstone, clay and extensive limestones.

Geologic History, Stratigraphy & Tectonics

The bulk of geological research in the Northern Mariana Islands has focused on the large, populous island of

Saipan Saipan ( ch, Sa’ipan, cal, Seipél, formerly in es, Saipán, and in ja, 彩帆島, Saipan-tō) is the largest island of the Northern Mariana Islands, a Commonwealth (U.S. insular area), commonwealth of the United States in the western Pa ...

. The U.S. Geological Survey first mapped the island in 1956 and it was remapped again in 2007. The oldest rocks exposed at the surface are

volcanic rocks, belonging to the Izu-Bonin-Mariana (IBM) arc system. Saipan is on the frontal arc of the system and experienced periodic uplift and subsidence since then. Volcaniclastic and carbonate rocks overly the IBM volcanics. The Sankakuyama Formation is the oldest individual rock unit, which includes Eocene

rhyolite Rhyolite ( ) is the most silica-rich of volcanic rocks. It is generally glassy or fine-grained (aphanitic) in texture, but may be porphyritic, containing larger mineral crystals (phenocrysts) in an otherwise fine-grained groundmass. The mineral ...

found mainly on north-central part of the island. Radiometric dating gives an age of 45 million years ago. During mapping in 1956, it was subdivided into flow rock,

breccia Breccia () is a rock composed of large angular broken fragments of minerals or rocks cemented together by a fine-grained matrix. The word has its origins in the Italian language, in which it means "rubble". A breccia may have a variety of di ...

tuff Tuff is a type of rock made of volcanic ash ejected from a vent during a volcanic eruption. Following ejection and deposition, the ash is lithified into a solid rock. Rock that contains greater than 75% ash is considered tuff, while rock cont ...

and pyroclastic rocks. Originally called the Hagman andesite by R. Tayama during initial research efforts in 1938, the Hagman Formation is exposed as cliffs along the Hagman Peninsula and deposited during the early development of the Mariana fore arc. The andesite lava flows, conglomerate and

sandstone Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks. Most sandstone is composed of quartz or feldspar (both silicates) ...

of the unit date to between 26 and 28 million years ago, during the

Oligocene The Oligocene ( ) is a geologic epoch of the Paleogene Period and extends from about 33.9 million to 23 million years before the present ( to ). As with other older geologic periods, the rock beds that define the epoch are well identified but the ...

. The Densinyama Formation includes andesitic breccia, conglomerate, tuff sandstone and tuff limestone, centered around Mount Talafofo and discontinuously exposed by Achuguo Spring in the west. It overlies the Hagman Formation and is present close to Papago. The Matansa Limestone grades laterally to a pure shallow-marine limestone and like other units shows conflicting results when biostratigraphy and isotope dating are compared. The Tagpochau Limestone is the second most extensive rock unit near the surface on Saipan after the Mariana Limestone. It include a limestone, marl and tuff facies. Calcareous marine tuff and interbedded volcanic flow rocks near the village of As Lito and the Fina Sisu hills was named the Fina-sisu Formation in 1956. The tuff is well bedded with numerous plankton fossils. Geologists have inferred that the rock formed during the

and is overlain by the Tagpochau Limestone. The Donni Sandstone Member of the Tagpochau Limestone is informally known as the Donni formation. It is exposed near the village of I Donni and throughout the eastern end of the island. The formation overtops the

Miocene The Miocene ( ) is the first geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and means "less recen ...

Tagpochau rocks with an angular unconformity. Talus, marsh sediments and limestones such as the Mariana Limestone are common across the surface, resulting from deposition and mass-wasting during the

Pleistocene The Pleistocene ( , often referred to as the ''Ice age'') is the geological Epoch (geology), epoch that lasted from about 2,580,000 to 11,700 years ago, spanning the Earth's most recent period of repeated glaciations. Before a change was fina ...

and

Holocene The Holocene ( ) is the current geological epoch. It began approximately 11,650 cal years Before Present (), after the Last Glacial Period, which concluded with the Holocene glacial retreat. The Holocene and the preceding Pleistocene togethe ...

Structural geology

Normal faults are common on the island, particularly the Angingan fault which places Fina-sisu Formation volcanic rocks against younger limestones. Geologists have identified breccia near Mount Tapkpochao as the trace of a normal fault. The Angingan, Obyan and Dago faults may be part of a larger, anastomosing fault system, running on both sides of the central highlands and connecting with faults in the area of Suicide Cliffs and Fanonchuluyan Bay. Most faults trend north-south and even young rocks from the Holocene show signs of offset.

References

{{Geology of the United States by political division +Northern Mariana Islands Geography of the Northern Mariana Islands