Abstract:
A bed-by-bed biostratigraphical study of two key Californian outcrop sections of the JurassiceCretaceous transition interval is presented. The studied succession is characterized by the occurrence of abundant buchiid bivalves and a discontinuous ammonite record. Several Lower Cretaceous ammonite zones were introduced in the literature, but the precise stratigraphic position of key ammonite records remains
unclear. Consequently, the buchiid zonation established in the 1970s is still considered the prime biostratigraphic tool, and ammonites have supplementary value. The buchiid zonation is revised herein, and the nomenclature and boundaries are updated. The Buchia sublaevis zone (upper Valanginian (?) to lower Hauterivian) is for the first time recognised in California. Two further biostratigraphic units, i.e. ‘Buchia inflata beds’(between B. uncitoides and B. pacifica zones) at McCarty Creek and ‘Buchia aff. vol-
gensis-B. unschensis beds’(between B. piochii and B. okensis zones) at Grindstone Creek, are proposed. The positions of zonal boundaries are determined more precisely than previously, and are based on the first or mass occurrence of index species. As a result, the Panboreal correlation of Tithonian to Hauterivian buchiid zones is improved. Ammonite biohorizons are used as an additional correlative tool, and the following succession is proposed for the Upper Tithonian to earliest Hauterivian interval of the study area: Fierrites dilleri, Paradontoceras storssi, Proniceras maupinense, Neocosmoceras euchrense, Paskentites paskentaensis, Kilianella crassiplicata, Neotollia mutabilis, Sarasinella hyatti, Thurmanniceras stippi, T. jenkinsi and Jeannoticeras jeannoti. A new genus, Howarthiceras n. gen. is established for “Groebericeras” baylei Imlay and Jones
