EarthRef.org Reference Database (ERR) -- Winchell 1947

The Honolulu series comprises about 30 distinct, separate eruptions of nepheline basanite, nepheline basalt, and nepheline-melilite basalt which occurred in late Pleistocene and Recent time in the southeastern part of the Koolau Range of Oahu. The unconformity between the Honolulu series and the underlying, internally conformable Koolau series which probably forms more than 99 per cent of the bulk of the Koolau Range represents a long erosion interval, since a relief of about 2000 feet was developed. The work of Stearns and Vaksvik (1935) covering all Oahu is the starting point for detailed descriptions of the areal and structural geology of the Honolulu series. Eight new analyses supplement the petrographic description of the rocks. No theory of differentiation yet advanced seems to account fully for the origin of these lavas. In particular, limestone syntexis is shown to be out of the question. Mineralizers are present and were present in larger quantity during the differentiation but are too imperfectly understood for an adequate discussion. Crystallization differentiation offers difficulties unless the rather improbable separation of hypersthene is postulated instead of olivine, from the primitive olivine basalt magma typical of Hawaii. There is no evidence for crystallization differentiation in place following extrusion, and none was found to indicate systematic variations in successive flow units from a given vent. Table 2 presents a chronology of Oahu formations, derived from Stearns, supplemented by Winchell's field notes. Winchell relates all of the Honolulu series vents to the various stands of the sea defined by Stearns. Winchell struggles to understand the differentiation process that produced the Honolulu basalt. He recognizes the Honolulu series as representing a number of different magma systems because of their complex chemistry and their distribution in space and time. He postulates that the average Honolulu series nepheline basalt could be produced by removal of a silica-rich basalt (i.e., a Koolau-like magma containing hypersthene as a fractionating phase) from a more primitive melt. He suggests that the pressure needed to induce filter-pressing might come from subsidence of the islands.