AGE DIAGNOSTIC DIATOMS FROM THE UPPER PETALUMA FORMATION, SONOMA COUNTY, CALIFORNIA

Diatom samples from 15 localities of the Upper Petaluma Formation were collected in the following areas: Carriger Creek (2011, 2020, 2021, 2023 and 2297); Lichau Creek (2296); Pressley Road (2022); Sonoma Mountain Road (1906, 1921 and 2019); Donnell Ranch (JA070204A, JA070204B and JA070204C); and Crane Canyon (JA070204D and JA070204E).

The age of 2011 is defined by the 5.3 to 2.3 Ma range of Melosira ambigua v. robusta. In 2020 the maximum age (5.3 Ma) is indicated by the first occurrence (FO) of Stauroneis kriegeri and the minimum age, by the Navicula amphibola v. major extinction at 3 Ma. A 4 Ma maximum for 2021 is indicated by the FO of Pinnularia brebissonii v. intermedia and a 3 Ma minimum, by the extinction of P. saga v. undulata. The oldest age for 2023 is designated by the FO of Cyclotella ovata at 5.3 Ma and the youngest age, by the extinction at 4 Ma of C. transsylvanica v. elliptica. The oldest age for 2297 is 3.5 Ma because of the FO of Caloneis silicula v. minuta, and the youngest age is 2 Ma based on the extinction of Cyclostephanos undatus. A 4.8 Ma tuff was dated from this locality.

FO of Hantzschia amphioxys v. densistriata at 4 Ma gives a maximum for 2296 and the extinction of Melosira areolata at 2 Ma, the minimum. The extinction of Melosira praedistans, 2022 is no younger than 2.5 Ma and no older than 4 Ma, because of the FO of Nitzschia apiculata. Sample 1906 is 4 Ma because both the FO of Cyclotella subatomus and the extinction of C. kansasensis occur at that time. The age of 1921 is the 5.3 to 2.3 Ma range of Melosira ambigua v. robusta. Sample 2019 is 5.3 Ma: the FO of Cyclotella ovata and the extinction of Epithemia sorex v. directa are at 5.3 Ma.

In JA070204A the FO of Navicula radiosa v. acuta at 5.3 Ma indicates a maximum age and the extinction of Melosira praedistans, a minimum of 2.5 Ma. A 5.2 Ma tuff has been dated from this locality. JA070204B is 4 Ma because both the extinction of Cyclotella kansasensis and the FO of C. subatomus occur at that time. JA070204C is 3.5 Ma because both the FO of Pinnularia vollosoviczi and the extinction of P. huei occur at 3.5 Ma. JA070204D has an age no older than 5.3 Ma because of the FO of Stauroneis kriegeri and no younger than 3.5 Ma because of the extinction of Eunotia majuscula. A 5.3 Ma maximum for JA070204E is denoted by the FO of Cymbella hagermanensis and a minimum of 3 Ma, by the extinction of Rhopalodia gibba v. boryana.