L. Tauxe $^a$, C. Constable, $^a$ and M. Cronin $^a$
$^a$ Scripps Institution of Oceanography, La Jolla, CA
A long period of uniformly normal polarity in the Cretaceous was first detected by Helsley and Steiner (1969), who suspected that it corresponded to vast stretches of relatively low amplitude behavior in the sea-floor magnetic anomaly profiles (known informally as the Cretaceous ``Quiet Zone''). Since these first studies, paleomagnetists have continued to explore what is now called the Cretaceous Normal Superchron (CNS). The CNS is thought to last from about 121 Ma to 84 Ma or nearly 40 million years. This period has intrigued many earth scientists because of the unusually long duration of apparent stability of the geomagnetic field.
Why the field stopped reversing and why it started again remain open and interesting questions. Clues regarding this unusual behavior could come from paleomagnetic studies including absolute paleointensity (was the field stronger than average?), or paleosecular variation (was the field more or less ``stable'' during the CNS?). The pelagic carbonates of the Scaglia Bianca Formation in the Umbrian-Marche area of Italy are ideal for paleomagnetic studies because of their stable remanence characteristics and their rock magnetic simplicity. Based on extensive paleomagnetic investigations over decades, it is well known that it retains an excellent record of a long sequence of the CNS. We have carried out a detailed paleomagnetic investigation of two stratigraphically overlapping sections from the Scaglia Bianca Formation ($\sim $ 85-89.5 Ma) in the Umbria--Marche area in central Italy. Sampling was conducted over 32 meter and 7 meter intervals at La Roccaccia and Furlo respectively. After elimination of rock magnetic noise in the Scaglia Bianca data sets, there is a high degree of agreement in direction and to a lesser extent relative intensity between correlative portions of the two sections. We therefore offer this data set as a robust record of geomagnetic field behavior during the 4.5 m.y. interval represented by the La Roccaccia section. A statistical analysis of the relative intensity observations suggests that this period of the Cretaceous Normal Superchron is characterized by a normalized variability in paleointensity (standard deviation about 28\% of the mean value) that is significantly lower than seen during the Oligocene over intervals in which reversals or tiny wiggles occur (typically about 50\%). The directional stability results in VGP dispersion compatible with that found in volcanic rocks from around the same latitude and ranging in age from 80--110~Ma. Our principal conclusion from this study is that paleosecular variation during the Cretaceous Normal Superchron was subdued in both direction and intensity compared to times during which the field was reversing.