Geology of Guam
Guam consists of two geologically distinct parts. Northern Guam is an undulating limestone plateau, and southern Guam is a rugged volcanic highland with some limestone outliers.
The volcanic rocks that form the island's foundation are best seen in southern Guam, where they dominate the land surface. The oldest exposed rocks on the island are of Eocene and Oligocene age and comprise the Facpi Formation and Alutom Formation. They crop out in the highlands from central to northern Guam and underlie all other exposed rock units (Tracey and others, 1964; Reagan and Meijer, 1984). They contain a series of pillow basalts and pyroclastic rocks of volcanic origin, and range from tuffaceous shale to coarse boulder conglomerate and breccia.
Separate volcanic rocks of Oligocene to late Miocene age comprise the Umatac Formation and lay on top of the Alutom Formation. They crop out principally in the south-central highlands and plateaus and contain reef and forereef limestone, tuff breccia and volcanic conglomerate, and basalt flows (Meijer and others, 1983; Reagan and Meijer, 1984).
Volcanic rocks of northern Guam are locally overlain by limestone. The top of the mountainous ridge and central basin are capped by old limestone units. They are Miocene to Pliocene age and are known as Bonya and Alifan Limestone. Eastern coast and Orote Peninsula comprise of younger limestone, Pliocene to Pleistocene age and called Mariana Limestone. This limestone is clay-rich in the vicinity of volcanic uplands.
In northern Guam, the volcanic basement rocks of Alutom Formation are almost entirely overlain by a limestone plateau that comprises the northern half of the island and contains the island's principal aquifer, the Northern Guam Lens Aquifer (NGLA). The plateau is separated from the uplifted volcanic highlands in the southern half of the island by the large NW-SE-trending Pago-Adelup Fault. Elsewhere, the plateau is fringed by cliffs that rise precipitously over the sea or by a narrow coastal terrace. The plateau has a generally modestly-sloping, concave-upward surface, and is tilted from an elevation of about 60 m at Amantes Point at the SW to 150 m elevation at Pati Point on its NE corner and 180 m at Ritidian Point, its NW extreme.
The basement of the limestone plateau is thought to be everywhere composed of the Alutom Formation, the second oldest (Oligocene) volcanic unit on the island. On top of the basement rock is the lowermost limestone unit of the plateau: the detrital Mio-Pliocene Barrigada Limestone, a well-lithified-to-friable, white, detrital limestone, deposited in relatively deep water. It comprises most of the bedrock in northern Guam and is the principal aquifer rock, but it is exposed on the plateau surface only in the interior of northern Guam, where it occupies 18% of the surface. Elsewhere, it grades laterally and upward into the Plio-Pleistocene Mariana Limestone, a complex of reef and lagoonal limestones, forming 77% of the exposed limestone surface of the plateau. At the southern end of the plateau, the surface is composed mostly of argillaceous limestone interpreted as Mariana Limestone laced with syndepositional clays derived from volcanic mountains of southern Guam (Tracey et al., 1964).
Finally, in both southern and northern halves of the island exist minor reef limestone, beach deposits, and alluvium of Holocene age. The beach deposits are composed of poorly consolidated gravel or volcanic sand in the south and biogenic calcareous sand in the north. Alluvial deposits fill stream valleys and cover parts of the coastal lowlands.
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