Magnetic properties of Santa Fe Group sediments in the 98th Street core hole, Albuquerque, New Mexico

Abstract

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...

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“…Magnetic susceptibilities measured for core samples of the Arroyo Ojito Formation from the west-central part of the Albuquerque basin (Hudson et al, 1999a) give similar results as the San Ysidro fault study. Reconnaissance measurements of magnetic susceptibility of Santa Fe Group sediments from other parts of the central Rio Grande rift confi rm that values are on the order of 1 × 10 −3 (SI) for the dominant sand fraction.…”

“…Detailed magnetic-property studies in two areas of the Albuquerque basin show that total magnetizations of the Santa Fe Group fall toward the high end of the range for sedimentary rock types and are dominantly a function of magnetic susceptibility (Hudson et al, 1999a;2008). The remanent component likely accounts for less than 25% of the magnitude of the total magnetization (Köeningsburger or Q ratios less than 0.25) and causes negligible deviation in the direction of the total magnetization from one that is parallel to Earth's magnetic fi eld (Hudson et al, 1999a;2008). For these reasons, and given most geologists' greater familiarity with magnetic susceptibility rather than magnetization, we simplify all following discussions by referring only to magnetic susceptibility.…”

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“…A high-resolution aeromagnetic survey from the Albuquerque Basin, New Mexico ( Figure 7) revealed many more faults in the shallow subsurface than previously known , some of which are demonstrably bounding areas of subsidence related to well pumping (Heywood et al, 2002). Moreover, ground-based investigations of sediments juxtaposed across these faults show that magnetic properties can be generally characterized relative to certain geologic characteristics, such as sediment provenance, depositional history, and grain size (Hudson et al, 1999;Hudson et al, 2007). In particular, a general correlation between coarser grain size and stronger magnetization indicates that aeromagnetic data can provide clues about the extent to which low-permeability fine-grained sediments are juxtaposed against high-permeability coarse-grained sediments at faults (Grauch and Hudson, 2007).…”

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“…7) revealed many more faults in the shallow subsurface than previously known (Grauch et al, 2001), some of which are demonstrably bounding areas of subsidence related to well pumping (Heywood et al, 2002). Moreover, ground-based investigations of sediments juxtaposed to these faults show a general correlation between coarser grain size and stronger magnetization (Hudson et al, 1999). This relation indicates that aeromagnetic data can provide clues about contrasting aquifer characteristics of sediments (Grauch, 2001), a subject of ongoing research at U.S.G.S.…”