A magnetic polarity zonation has been developed for nine stratigraphic sections of the Upper Cretaceous portion of the Great Valley Group in the Sacramento Valley of California. Collectively, the sections include five biostratigraphic range zones, based on ammonites and other mollusks. The zones are all of Santonian or Campanian age, and each section contains at least two zones.

A total of 681 fully oriented samples was collected from 252 horizons, spanning more than 3 km of section. The samples were subjected to alternating-field demagnetization after it was found that the rocks were not amenable to thermal demagnetization. Rock magnetic studies were used to determine that the principal magnetic carrier is magnetite.

The paleomagnetic measurements reveal the existence of an interval of reversed polarity that is consistently associated with one of the zones, namely, the Baculites chicoensis Zone. Given the biostratigraphic control, the reversed interval can be unambiguously correlated with the first reversed interval following the Cretaceous long normal interval. On the Magnetic Polarity Time Scale, this reversed interval corresponds to marine magnetic anomaly 33R. The correlation of the re-versed interval implies that the base of the Chicoensis Zone is nearly coincident with the Santonian/Campanian boundary in California and that it is not significantly time-transgressive between the shallow-water facies on the east side of the Sacramento Valley and the deep-water facies on the west side. Furthermore, the magnetostratigraphy supports the conclusion that all of the zones underlying the Chicoensis Zone are pre-Campanian in age, a result which is consistent with other lines of evidence. The biostratigraphy of these sections in California can now be compared directly to that of the paleomagnetic stratotype section in Gubbio, Italy.

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