Sisquoc Formation, Harris Grade section, California

ABSTRACE Lower Pliocene diatoms were studied from the Sisquoc Formation and lowermost Foxen Mudstone, exposed along Hams Grade north of Lompoc, California, to refine the diatom biostratigraphy of post-Montercy Formation sediments in California. Sixty-seven diatom taxa were identified in the 25 samples examined from the 790-m thick (2950-ft) section. The diatoms are assignable to the uppermost N~rzschia reinholdii Zone and Thalassros~ra oestrupi ~ Zone of Damn ( 1 98 11, and five tcntative subzones for local correlation are proposed. Regional correlations and taxon occurrence are discussed, and the base of the Aritzschia reinholdii Zone is redefined as at the last occurrence of Thalassionema schraderi