Paleomagnetic and rock magnetic studies of upper to middle parts of the rift-filling Santa Fe Group were conducted in the 1500 ft (457 m) 98th Street core from western Albuquerque to determine the age of the deposition and to assess their possible contribution to magnetic anomalies. A total of 336 samples distributed over the core length were subjected to alternating field demagnetization to identity magnetic components and to determine their polarity. A magnetic mineral having moderate coercivity, probably magnetite, carries the principal component of magnetization. Magnetic polarity is dominantly normal, but samples with reversed polarity cluster over three main depth intervals through the core section. Twenty-one alternating normal and reverse polarity zones are defined by at least two adjacent samples of the same polarity. Comparison of core polarity zonation to the geomagnetic polarity time scale does not yield a unique correlation, yet the dominance of normal polarity suggests that the sampled interval may correlate with one of several potential periods of closely spaced normal subchrons that spanned 1-2 m.y. in middle Miocene or younger time. Numerous linear anomalies identified in recent high-resolution aeromagnetic surveys of the Albuquerque Basin are probably related to fault offsets of sediments within the Santa Fe Group. Variation of magnetic properties within the core gives insight into potential contrasts of total magnetizations that control the aeromagnetic expression of these sediments. 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 samples extracted from the core (6.0E-6 to 2.4E-2 SI). Likewise, natural remanent magnetization (NKM) intensity varies from8.4E-4to2.7E-l A/m. Both MS and NRM generally correlate with sediment grain size such that the coarsest grained rocks have highest magnetic property values. Plots of NKM versus MS define into two discrete groups of magnetized sediments that probably reflect contrasting magnetic mineral properties or magnetization acquisition mechanisms. Total magnetizations calculated from the NRM and MS values indicate that Santa Fe Group sediments are capable of producing moderate to weak magnetic anomalies. The data suggest that coarse-grained aquifer rocks have highest total magnetization and thus greatest potential for generation of aeromagnetic anomalies where juxtaposed with fine sediments along faults. properties. G. S. Morgan kindly shared age information for vertebrate fauna within the Santa Fe Group. We received a valuable review from R.L. Reynolds. Methods Discrete samples were taken from the core in the form of 7 cc nonmagnetic plastic cubes. Fine-grained sediments were preferentially sampled. The samples were oriented vertically within the core segments but lack azimuthal orientation. Natural remanent magnetization (NRM) for the cube samples (336 total) was measured with a three-axis superconducting magnetometer in a magnetically shielded room at the Unive...

Multiply By To obtain inch 25.40 Millimeter foot 0.3048 Meter mile 1.609 Kilometer kPa 1.45X10'3 lbf/in2 Altitude in this report is based on the National Geodetic Vertical Datum of 1929 used on 7.5-minute topographic maps in the Albuquerque area. SYMBOLS AND TERMS CMR = combinable magnetic resonance K = average horizontal hydraulic conductivity (ft/day) Kv = average vertical hydraulic conductivity (ft/day) Ohmm = ohm-meter, a unit of resistivity or specific resistance gAPI = a unit of natural gamma radiation, calibrated in American Petroleum Institute (API) test facilities long-normal resistivity = (ohm-meterVmeter) short-normal resistivity = (ohm-meterVmeter) mmoh = moh-meter, a unit of conductivity kPa = kilopascals, a unit of pressure psi = pound-force per square inch, lbf/in2 IV