Geology of Ulupau Head, Oahu

Abstract

Ulupau Head is a secondary tuff crater forming the eastern salient of Mokapu Peninsula, northeast coast of Oahu, Hawaii. Subsequent to its building from an offshore, submarine vent, the crater was deeply eroded by wave action on the north and east sides, the rim being largely destroyed on the east, and the sea entering the bowl. Later, at a stand of the sea at least 45 feet above the present stand, a bed of oyster-bearing limestone and an overlying calcareous beach sandstone were deposited on steeply sloping rock surfaces inside the crater. Unconformable on these beds is a thick series of alluvial, tuffaceous silts, and in one sector a remarkable giant breccia, carrying 30-foot blocks of tuff and probably due to an enormous rock fall from an oversteepened marine cliff. At this time the sea stood at least as low as at present. Later reef formations and accompanying beach conglomerates indicate two more stands of the sea, the first at 25 feet and the second at about 12 feet above the present. Incorporated in the reef limestone of the northwest shore are great blocks of tuff let down by marine erosion from the rock-fall breccia and forming a remarkable reef breccia. This in turn, subject to attack by water-level weathering on the tuff blocks and solution benching on the reef limestone matrix, furnishes a striking example of the contrasted features of marine benches developed by processes peculiar to the two types of rock.