Previous provincial California stage designations for the Iversen-Skooner Gulch-Gallaway-Point Arena sequence do not appear to be the correct geologic age for these rocks in the Point Arena area of northern California. Time-transgressive “pseudo-Saucesian” benthic foraminiferal assemblages show little change in their gross composition through much of the deep-water Gallaway-Point Arena sequence near Point Arena, which includes middle Miocene strata.
About 30 mid-Tertiary coccolith taxa were recovered from 120 localities near Point Arena from Iversen-Skooner Gulch-Gallaway-Point Arena strata. The first occurrence (biohorizon) of Sphenolithus heteromorphus marks the base of the Helicosphaera ampliaperta Zone within the middle Gallaway Formation and serves as an important datum for correlation with California stage stratotype localities in central and southern California.
The Iversen Basalt appears to span the Oligocene-Miocene boundary. Revised radiometric dates for the Iversen Basalt indicate an average age of 23.8 Ma which is younger than the world Oligocene-Miocene boundary estimate of 24.6 Ma. Fossiliferous interbeds believed to occur within the lower part of this volcanic unit at inland localities contain the highest occurrence of Dictyococcites bisectus and D. scrippsae.
The stratigraphic distribution of coccolith taxa and the recognition of the Sphenolithus heteromorphus datum suggest that the Skooner Gulch and lower to middle Gallaway sequence is the early Miocene equivalent of the type Saucesian of Los Sauces Creek; that the middle and upper Gallaway and lower Point Arena Formations are the late early Miocene equivalents of the type Relizian section at Reliz Canyon; and that part of the Point arena Formation is equivalent in age to the early middle Miocene type Luisian Stage.