Geology of the Salt Lake Area, Oahu, Hawaii

Abstract

ABSTRACT: A series of pyroclastic eruptions and one lava flow, all of melilitenephelinite composition, were produced in the environs of present-day Salt Lake, Oahu, Hawaii, beginning about 500,000 years ago and terminating more than 100,000 years ago. The age of the flow has been dated by a K40/ A40 method at 400,000 years to 470,000 years. The earliest eruptions took place at a time of higher sea levels than at present, and their products are water-laid. Later eruptions, including the major ones from Salt Lake Crater and Makalapa Crater, took place at a time of lower sea levels than at present, and their products are air-laid. Sufficient time elapsed between individual eruptions to allow for erosion of portions of the earlier cones, to deposit silt and gravel, or to develop a soil prior to a subsequent eruption. The pyroclastics are medium- to fine-grained, lithic-vitric tuffs and contain, in addition to juvenile material, a considerable quantity of particles of older rocks-volcanic and sedimentary, as well as ultramafic. The youngest deposits of tuff are unconformably truncated by reef and limestone which have been correlated with rocks dated by a Th23o/UZ38 method at 90,000 to 140,000 years.