Geology of northern Guam

The northern Guam is the limestone plateau comprising the northern half of the island. 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).