THE ALBIAN-CENOMANIAN BOUNDARY IN NORTHERN CALIFORNIA

There is no internationally agreed-upon stratotype for the Albian-Ceno manian (Lower Cretaceous-Upper Cretaceous) boundary, Type sections in France for the Albian and Cenomanian Stages are inadequate for this pu rpose, The proposed boundary stratotype sections in North Africa and T exas that were discussed in Copenhagen at the 1983 Third International Symposium on the Cretaceous System (Birkelund et al., 1984) are inade quately documented, facies restricted, or both, The widespread occurre nce of the ammonites Stoliczkaia and Mortoniceras in the upper Albian and of Mantelliceras in the Cenomanian classically bracket the boundar y and must be used until the boundary is formally defined, Recent work in North Africa (Robaszynski et al., 1993) promises an Albian-Cenoman ian boundary definition based on lineages within these taxa. The ammon ite-based Albian-Cenomanian boundary in northern California is between the last occurrence of the typically Albian genera Mortoniceras and S toliczkaia and the entry of mantelliceratine juveniles (probably Grays onites) associated with Mariella, Pseudouhligella japonicum, heretofor e regarded as a Cenomanian indicator in Japan, Alaska, and California, occurs in California in upper Albian and lower Cenomanian faunas. The boundary sequence in California is best exposed along Dry Creek in no rthern Tehama County, where an angular relationship within the Budden Canyon Formation, resulting from channel cutting on deep-sea fans, sep arates upper Albian ammonite-bearing rocks of the Chickabally Member f rom Lower Cenomanian ammonite-bearing rocks of the Bald Hills Member, Although angular relationships and coarse-grained strata of the lower Bald Hills Member are present in the boundary interval, no known ammon ite zone is missing, These relationships are interpreted to mean that relatively continuous sedimentation occurred during the Albian-Cenoman ian transition and reflect rapid uplift and erosion of a sedimentary, volcanic, and plutonic terrane to the north and east and, perhaps, acc ompanied by rapid sea-level changes, Collation of the ammonite biostra tigraphy herein with previously established foraminiferal and radiolar ian biostratigraphies for the Dry Creek sequence suggests that it is a prime Pacific-rim reference section for faunas in the Albian-Cenomani an boundary interval and that previous correlation of the uppermost Ch ickabally Member with the Cenomanian is incorrect, as Albian and Cenom anian stages are currently understood. This affects correlation and ch anges the dating of some units in California in the Franciscan Complex from Cenomanian to Albian.