HR: 1340h
AN: GP33D-1604    [Abstracts]
TI: Rock Magnetic and Geologic Characteristics of Faulted Sediments With Associated Aeromagnetic Anomalies in the Albuquerque Basin, Rio Grande Rift, New Mexico
AU: * Hudson, M R
EM: mhudson@usgs.gov
AF: U.S. Geological Survey, Box 25046, MS980, Denver, CO 80225, United States
AU: Grauch, V
EM: tien@usgs.gov
AF: U.S. Geological Survey, Box 25046, MS964, Denver, CO 80225, United States
AU: Minor, S A
EM: sminor@usgs.gov
AF: U.S. Geological Survey, Box 25046, MS980, Denver, CO 80225, United States
AB: The rock magnetic and geologic characteristics of basin sediments that generate aeromagnetic anomalies are little studied. Variations in rock magnetic properties are responsible for the many linear, short-wavelength, low- amplitude magnetic anomalies that are spatially associated with faults cutting Neogene basin sediments in the Rio Grande rift, including the San Ysidro normal fault that is well exposed in the northern part of the Albuquerque Basin. Magnetic susceptibility (MS) values from 310 sites distributed through a 1200-m-thick composite section of rift-filling sediments of Santa Fe Group and pre-rift sedimentary rocks juxtaposed by the San Ysidro fault have lognormal distributions with well-defined means that generally increase up section through eight map units: from 1.7 to 2.2E-4 in the pre-rift Cretaceous and Eocene rocks, from 9.9E-4 to 1.2E-3 in three members of the Miocene Zia Formation of the Santa Fe Group, and from 1.5E-3 to 3.5E-3 in three members of the Miocene-Pleistocene Arroyo Ojito Formation of the Santa Fe Group. Natural remanent magnetization measurements from oriented Santa Fe Group samples indicate Koenigsberger ratios are less than 0.3. Rock magnetic parameters (e.g., ARM/MS and S ratios) and petrography indicate that the amount of detrital magnetite and its variable oxidation to maghemite and hematite are the predominant controls of magnetic property variations within the Santa Fe Group sediments. Magnetite is present in rounded detrital grains that in reflected-light petrography include both homogeneous and subdivided types, indicating likely plutonic and volcanic provenances, respectively. Santa Fe Group sediments with highest magnetic susceptibility have greatest magnetic-grain size as indicated by lowest ARM/MS ratios. Magnetic susceptibility increases progressively with sediment grain size to pebbly sand within the Arroyo Ojito Formation (deposited in fluvial environments) but within the Zia Formation (deposited in mostly eolian environments) reaches highest values in fine to medium sands. Partial oxidation of detrital magnetite, decreasing MS, is spatially associated with calcite cementation in the Santa Fe Group; both oxidation and cementation probably reflect past flow of ground water through permeable horizons. Forward magnetic models of geologic cross sections that incorporate mean magnetic susceptibilities for the different stratigraphic units successfully mimic the aeromagnetic profiles across the San Ysidro fault. These models demonstrate that the stratigraphic juxtaposition of units having maximum magnetic contrast changes with different exposure levels into the fault. This study highlights several geologic factors such as sediment provenance, depositional facies, as well as post-depositional preservation and alteration of magnetic minerals as responsible for producing aeromagnetic anomalies in faulted basin sediments.
DE: 1517 Magnetic anomalies: modeling and interpretation
DE: 1527 Paleomagnetism applied to geologic processes
DE: 1540 Rock and mineral magnetism
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2007 Fall Meeting