Paleomagnetic and rock magnetic properties of Santa Fe Group sediments in the 98th Street core hole and correlative surface exposures, Albuquerque Basin, New Mexico
Mark R. Hudson, Marlo Mikolas, Geissman, John W., and Bruce D. Allen
Paleomagnetic and rock-magnetic results of upper to middle parts of the Neogene Santa Fe Group retrieved in the 457-m 98th Street core, west Albuquerque, were used to assess their age of deposition and their possible contributions to observed magnetic anomalies. Potentially correlative surface exposures in Arroyo Ojito, 40 km north of Albuquerque, were sampled to complement studies on the core material. For the 98th Street core, over 330 cube specimens (one per stratigraphic interval) were subjected to alternating field and limited thermal demagnetization to identify magnetic components and interpret polarity. For most Santa Fe materials, a phase of moderate coercivity, probably magnetite, carries the principal component of magnetization. The polarity of the interval retrieved is mainly normal, but specimens of reverse polarity cluster over three main intervals. A total of 21 apparent polarity zones are defined by at least two adjacent specimens of the same polarity. In the absence of independent age control, comparison of core polarity zonation to the geomagnetic polarity time scale does not yield a unique correlation. The dominance of normal polarity may be consistent with the sampled interval being part of several, closely spaced normal polarity subchrons that spanned 1-2 m.y. in middle Miocene to Pliocene time. Variation of magnetic properties within the core gives insight into potential contrasts of total magnetizations that control the aeromagnetic expression. Within the 98th Street core, magnetic susceptibility (MS) varies greatly, both in logs of whole-core MS (8.9E-5 to 6.7E-2 SI) and in individual specimens extracted from the core (4.8E-5 to 1.5E-2 SI). Likewise, natural remanent magnetization (NRM) intensity varies from 8.4E-4 to 2.7E-1 Aim. Both MS and NRM generally correlate with sediment grain size; the coarsest grained rocks have highest susceptibility and NRM intensity. Plots of both initial NRM and anhysteretic remanent magnetization (ARM) intensity versus MS define essentially single, continuous trends, implying a relatively uniform type of magnetic mineralogy that varies principally in concentration. Values of total magnetization from NRM and MS data indicate that Santa Fe Group sediments are capable of producing moderate to weak magnetic anomalies that have been identified in recent high resolution aeromagnetic surveys of the Albuquerque basin. Coarse-grained rocks have the highest total magnetization and greatest potential for generation of aeromagnetic anomalies where juxtaposed with fine sediments along faults.
- Hudson, Mark R.; Marlo Mikolas, Geissman, John W.; Allen, Bruce D., 1999, Paleomagnetic and rock magnetic properties of Santa Fe Group sediments in the 98th Street core hole and correlative surface exposures, Albuquerque Basin, New Mexico, in: Albuquerque Country, Pazzaglia, Frank J.; Lucas, S. G.; Austin, G. S., New Mexico Geological Society, Guidebook, 50th Field Conference, pp. 355-361.