Abstract

There is no internationally agreed-upon stratotype for the Albian-Cenomanian (Lower Cretaceous–Upper Cretaceous) boundary. Type sections in France for the Albian and Cenomanian Stages are inadequate for this purpose. The proposed boundary stratotype sections in North Africa and Texas that were discussed in Copenhagen at the 1983 Third International Symposium on the Cretaceous System (Birkelund et al., 1984) are inadequately documented, facies restricted, or both. The widespread occurrence of the ammonites Stoliczkaia and Mortoniceras in the upper Albian and of Mantelliceras in the Cenomanian classically bracket the boundary and must be used until the boundary is formally defined. Recent work in North Africa (Robaszynski et al., 1993) promises an Albian-Cenomanian boundary definition based on lineages within these taxa.

The ammonite-based Albian-Cenomanian boundary in northern California is between the last occurrence of the typically Albian genera Mortoniceras and Stoliczkaia and the entry of mantelliceratine juveniles (probably Graysonites) associated with Mariella. Pseudouhligella japonicum, heretofore 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 northern Tehama County, where an angular relationship within the Budden Canyon Formation, resulting from channel cutting on deep-sea fans, separates upper Albian ammonite-bearing rocks of the Chickabally Member from 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 ammonite zone is missing. These relationships are interpreted to mean that relatively continuous sedimentation occurred during the Albian–Cenomanian transition and reflect rapid uplift and erosion of a sedimentary, volcanic, and plutonic terrane to the north and east and, perhaps, accompanied by rapid sea-level changes.

Collation of the ammonite biostratigraphy herein with previously established foraminiferal and radiolarian biostratigraphies for the Dry Creek sequence suggests that it is a prime Pacific-rim reference section for faunas in the Albian-Cenomanian boundary interval and that previous correlation of the uppermost Chickabally Member with the Cenomanian is incorrect, as Albian and Cenomanian stages are currently understood. This affects correlation and changes the dating of some units in California in the Franciscan Complex from Cenomanian to Albian.

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