IMPLICATIONS FOR GEOMAGNETIC FIELD VARIABILITY DURING THE CRETACEOUS NORMAL SUPERCHRON FROM THE PALEOMAGNETISM OF THE LATE CRETACEOUS FORT HAYS LIMESTONE

We assess the directional dispersion and relative paleointensity of the geomagnetic field during a roughly half-million-year-long interval during the Cretaceous Normal Superchron using paleomagnetic data from the Fort Hays Limestone Member of the Niobrara Formation outcropping in Kansas and Colorado. The Fort Hays, which was deposited in the latest Turonian through early Coniacian (89.3-89.8 Ma), consists mainly of medium to thick beds of chalky limestone interbedded with thin layers of shale. We sampled the limestone beds where they are flat-lying east of the front range of the Rockies and where they have sustained variable amounts of Laramide-age tilting within the front range, e.g., at sampling localities near Boulder and the Cañon City area. Typically, 6-10 specimens were collected at each of the 57 sites sampled, with each site being an individual limestone bed or several contiguous beds. All specimens were measured in a cryogenic magnetometer and subjected to either stepwise alternating field or thermal demagnetization. Although weak, a stable remanent magnetization direction is well resolved, with a majority of the specimens giving demagnetization paths that trend linearly toward the origin above about 20 mT or 200°C. Principal component analysis of these paths yields a characteristic remanent magnetization direction that differs significantly from the present field direction and that passes a paleomagnetic fold test, consistent with the characteristic magnetization having been acquired at or near the time of deposition. These results therefore provide insight into the behavior of the geomagnetic field during the Cretaceous Normal Superchron, including fluctuations in the strength of the field and low paleosecular variation. They also further refine the paleogeographic position of North America in the Late Cretaceous.