CRETACEOUS NORMAL SUPER CHRON (CNS) AS A MEAN TO TEST THE RELIABILITY OF THE REMANENT MAGNETIZATION OF THE SUNDANCE FORMATION OF BLACK HILLS

The magnetization complexity of the middle Jurassic sedimentary units of the Black Hills of South Dakota and Wyoming has mostly led to inconclusive paleomagnetic results. In addition, the coexistence of inconclusive fold tests and the magnetic reversal pattern recorded by these rocks failed to firmly determine their magnetization age.

Thorough rock-magnetic analysis and carefully designed experiments were used to examine the reliability of the reversed remanent magnetization, which backed up the reversal test for the Sundance Fm. The aim was to test if the reversed remanence signal was robust enough to tolerate ~40 Myr of the normal polarity of CNS and not be affected by it. Hematite (main remanence carrier here) blocking-temperature curves show that viscous remnant magnetization (VRM) of ~40 Myrs imposing by Earth’s magnetic field (~50 µT) at room temperature is approximately equal to soaking in the same field for 24 and 6 hours at 230 and 270 °C respectively (Pullaiah et al., 1975).

After examining that the heating/cooling cycles make no chemical alterations, two experiments were conducted on samples from Hulett and Lak members of the Sundance Fm. (~163 Ma) First, samples (n=12) were magnetized at 2.7T along their Z-axis and then demagnetized in the alternating field (AF) up to ~160mT and also thermally at 180°C to remove soft remanences and iron hydroxides signals respectively. In separate rounds, they were put in the oven with the external field of 50µT along their X-axis at 230 °C for 24 hours and 270°C for 6 hours. After each round VpTRM was measured along with progressive AF demagnetization up to 160mT. On average, magnetic intensities increased in X-directions (x8) and dropped by 25% in Z-direction even after the 230 °C round.

The second experiment was performed similar to the first (n=7) but without the initial magnetization step. After AF and thermal demagnetizations up to 160mT and 180 °C, samples were placed in the oven so that the external field would be at 90° with their remanent magnetization vectors for two rounds like the first experiment. After 230 °C level, on average, remanence vector angular change was >60° and the intensities were tripled. Results suggest that reversed magnetizations observed in the Sundance Fm. of the Black Hills could not withstand the CNS and the age of the magnetization is most likely not primary.