Marine magnetic anomaly 33-34 identified  in the Upper Cretaceous of the  Great Valley Sequence of California  ABSTRACT An interval of reversed magnetic polarity has been located in Upper Cretaceous rocks of the Great Valley Sequence of California. The interval has been found at four sites, two on each side of the Sacramento Valley. In each case, the reversed interval is associated with the same ammonite local range zone. Firm biostratigraphic correlation and the structure of the magnetic polarity time scale allow us to determine that the reversed interval corresponds to marine magnetic anomaly 33-34 and represents the lower part of the Campanian Stage. The reversed interval serves as a marker horizon linking the different faunal assemblages and sedimentary facies of the Great Valley Sequence. It also provides the first direct correlation of provincial biostratigraphic zones of the North Pacific province with the Cretaceous stages in Europe. INTRODUCTION We report here the first discovery of an  interval of reversed magnetic polarity in  Cretaceous strata of western North Amer- ica. Normally, the occurrence of a single  reversed interval in a sedimentary sequence  does not provide enough information to  permit an unambiguous correlation to the  magnetic polarity time scale. However, the  present situation is unique in that the bio- stratigraphic control allows us to make  such an identification.  Sequence was deposited in a forearc basin  during Late Jurassic and Cretaceous time  (Dickinson, 1974; Ingersoll, 1979). The  present structure is a large syncline with  outcrops on the western side (Coast  Ranges) and eastern side (Sierra foothills)  of the Great Valley. Sedimentological stud- ies have shown the eastern outcrops to be  shelf deposits and the western outcrops to  be composed largely of slope, basin plain,  and submarine fan deposits (Ojakangas,  Peter D. Ward  Kenneth L. Verosub  James W. Haggart  Department of Geology  University of California  Davis, California 95616  1968; Ingersoll, 1979). Biostratigraphic  correlation between the two outcrop belts  has been imprecise because of the very dif- ferent facies, which contain different  groups of fossils. The eastern outcrops are  composed of massive, cross-bedded sand- stones and siltstones bearing rich concen- trations of mollusks but few microfossils.  Biostratigraphic zonation is based on  ammonites, gastropods, and bivalves. In  contrast, outcrops on the western side of  the Great Valley are largely shale and  turbidites that rarely contain mollusks but  are rich in foraminifera and radiolaria.  Biostratigraphic zonation of western and  subsurface deposits is based on these  microfossils. Only recently have the  ammonite zones of the eastern outcrop belt  been recognized in the western deposits,  allowing integration of microfossil and  molluscan regional zones (Ward and  Haggart, 1981).  Previous sampling of the Great Valley  GEOLOGIC SETTING Sampling for this study was conducted  on the eastern and western outcrop belts of  the Great Valley Sequence of northern  California (Fig. 1). The Great Valley  A) Figure 1. Map showing outcrop area of Upper Cretaceous rocks (in black) of Great Valley Sequence and location of four creeks discussed. CHICO CREEK BUTTE CREEK SAND CREEK SALT CREEK I00 200 300  ALTERNATING FIELD (o.e.) Nh  350. 3001 250f ZOOf I50j >o/  B) 400  I00 200 300 400  ALTERNATING FIELD (oe.) D S  Figure 2. Typical behavior upon demagnetization of two samples from Chico Creek. Sample on left shows normal polarity; one on right shows reversed polarity. For each sample, upper dia- gram shows changes in intensity, and lower shows changes in directions. 90 GEOLOGY, v. II, p. 90-93. February I983