JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 85, NO. B7, PAGES 3597-3605,JULY 10, 1980 A Review of Magnetic Stratigraphy Investigations in Cretaceous PelagicCarbonate Rocks W. LOWRIE AND J. E. T. CHANNELL Institut fiir Geophysik, Ziirich, Switzerland W. ALVAREZ Departmentof Geology and Geophysics, University of California, Berkeley, California 94720 Pelagic carbonate rocks possess many suitable characteristics for paleomagneticand magneto- stratigraphic studies. Paleomagnetic resultsare summarizedfor sevenlengthy sections of pelagic lime- stonesand marls from Umbria and the southern Alps in Italy. Differences in apparent polar wander pathsfrom these two regions are interpreted in termsof tectonic rotation of allochthonous Umbria. The magneticstratigraphies of the paleontologically dated sections are independent of their tectonicdiffer- ences and are combined to form a continuous record of geomagnetic polarity for the Barremian through Maastrichtian stages of the Cretaceous. All but one of the reversals in these sections are confirmed by duplicationin at least one other section. Additional Cretaceous reversals have been reportedin other land sections and in DSDP (Deep Sea Drilling Project) and IPOD (International Program of Ocean Drilling) cores.Some of thesereversals are not defined well magnetically,and confirmationof others is cloudedby imprecise paleontological dates.If real, they are probably of short duration. The confirmed reversalsequence correlates well with the Cretaceous oceanicmagneticanomaly sequence. The agesof certain key anomalies are altered: Late Cretaceous anomalies29-34 are younger,and Early Cretaceous anomalies M0 and MI are older than previouslythought. The longer duration of the Cretaceous mag- netic quiet interval of normal polarity results in a reductionof corresponding seafloor spreading rates to about 70% of earlier values,but they are still appreciably higher than during formation of the preceding M sequence anomalies. INTRODUCTION The geomagnetic polarity sequence in the Cretaceoushas been determined from two sources: from paleomagnetic stud- ies on paleontologically or radiometrically dated rocks and from analysis of lineated oceanic magnetic anomalies. On the basis of a summary of all available paleomagnetic data from continentalrocks, Helsley and Steiner [1969] de- duced the existence of a long period of constantnormal geo- magneticpolarity during the Cretaceous from 85 to 110 m.y. The relative dating of isolatedCretaceous samplingsites is too imprecise, because of the limited resolution of radiometric and paleontologicaldating methods, to allow development of a magnetic stratigraphy for the mixed polarity intervals pre- ceding and following this period. As a sequence of normal and reversedpolarities is essentially a binary signal, correla- tion of land sections with each other and with the oceanic anomalies dependson the recognition of patterns of polarity reversals. Stratigraphicallycontinuous sedimentary sequences coveringlong periodsof time are desirablefor the recognition of correlatable patterns. The marine magneticrecordhasprovided our bestinforma- tion about the history of the geomagnetic field during the Cre- taceous(and also most of the Cenozoic). The pattern of oce- anic magnetic lineations defines three distinct episodes of differing character of the geomagneticfield during the Cre- taceous. The Late Cretaceous interval of mixed polaritiescor- responds to the oldestof the numbered sequence of anomalies that stretches through the Cenozoic to the present time [Heirtzler et al., 1968]. The Early Cretaceous is characterized by the youngest part of the Mesozoic anomaly sequence [Lar- son and Pitman, 1972]. The two magnetic anomaly sequences are separated in the oceans by the Cretaceous magnetic quiet Copyright ¸ 1980 by the American Geophysical Union. zone,within which are found anomalies whose amplitudes are generally subdued, and which cannot be correlated to form lineations. They correspond to the interval of constant geo- magnetic polarity inferred from the continental paleomag- netic record. In this paper a number of detailed land-based and DSDP (Deep Sea Drilling Project) paleomagnetic studies that have been publishedelsewhere are integrated and evaluated with the objective of summarizing the present state of knowledge of geomagnetic polarity during the Cretaceous. Magnetic stratigraphy in several sequences of pelagic carbonate rocks in Umbria and the southern Alps of northern Italy are used to correlate and paleontologically date much of the Cretaceous magnetic reversal sequence interpreted from oceanic magnetic anomalies. SEDIMENTOLOGICAL AND MAGNETIC PROPERTIES OF PELAGIC CARBONATE ROCKS The pelagiccarbonate rocksof northern Italy possess many suitable characteristics for magnetic stratigraphy studies. They were deposited in deep water on the southern margin of Tethys in generally tranquil conditions that persisted through most of the Cretaceous and early Tertiary. Today, strati- graphic thicknesses of several hundredmeters of continuously deposited limestones and marls are available for paleomag- netic sampling. The carbonate matrix of micritic coccolith material accu- mulated slowly and steadily and contained a varying influx of wind-borne and water-transported detrital particles. These consisted largely of clay minerals and a minisculefraction of ferrimagnetic minerals which acquired the paleomagnetic field direction by a mechanismof postdepositional alignment [Irving and Major, 1964; Kent, 1973]. Bioturbation is a characteristic feature of the limestones, Paper number 9B 1470. 0148-0227 / 80/009B- 1470501.00 3597