The Newport Bay radiolaria occur within the Valmonte Diatomite Member of the Miocene Monterey Formation. Radical assemblage changes through the Newport Bay sections make it possible to distinguish 3 radiolarian faunules. The basal faunule occurs through 450 feet of strata exposed on the eastern side of the bay. It consists of a radiolarian assemblage limited to a few species of smaller, unornamented representatives of the suborders Spumellina and Nassellina. Nassellina are dominant. The second faunule, recognized through 550 feet of strata, contains a diverse and abundant assemblage of dominantly large, well-ornamented species of Spumellina. Progressively poorer radiolarian assemblages, comparable to those of the lowest faunule, occupy the upper 175 feet of strata of the Newport Bay sections.
The uniform lithology throughout this section shows that the terrigenous contribution to the area of deposition remained constant. Parts of the section representing thinly laminated sediments indicate a condition of low ambient oxygen concentration near the bottom at the time of deposition.
Deposition of this section began within a restricted, shallow, ocean basin. Streams flowing into this basin were small and few in number. The basin gradually deepened and free access to the open sea was attained. The top of the section indicates a return to the shallow, restricted conditions of the lower section.