Measurement of the paleomagnetism of Quaternary sediments does not yield a numerical age as do isotopic dating methods. In order to convert paleomagnetic data into an age, it must be correlated to known conditions of the geomagnetic field that have been dated by some other method. Paleomagnetic data useful for this purpose include magnetic polarity (normal or reversed), field declination and inclination, secular variation, and magnetic susceptibility.
Diamictons, such as till, glaciomarine drift, and mudflows, may carry a stable magnetic remanence if they contain enough silt and clay in the matrix of the deposit. Glaciomarine drifts provide good examples of diamictons which, although poorly sorted, retain stable and reliable magnetism.
Remanence and anisotropy of magnetic susceptibility in tills may have distinctly different orientations, indicating that remanence is not significantly affected by the preferred orientation of larger grains during shearing. Clay/silt-rich tills in Nebraska, Iowa, and Minnesota give reliable normal and reversed DRMs that record the earth’s magnetic field at the time of deposition, despite anisotropy of susceptibility measurements that show a microfabric from glacial shearing. Thus, some, but not necessarily all, tills may be suitable for reliable paleomagnetic measurements if they have enough fine-grained matrix.
Small magnetic grains in some silt/clay-rich tills orient themselves parallel to the earth’s magnetic field within water-filled pore spaces. Magnetic grains have sufficient freedom to rotate into alignment because hydrostatic pore pressure carries part of the glacial load and the pore fluid does not transmit shear stresses that might otherwise result in mechanical grain rotation (Easterbrook, 1983). The DRM is fixed in till when enough pore water is expelled to restrict grain rotation.
Supporting evidence is necessary to use polarity changes in sediment for age determination. Boundaries between specific magnetic polarity changes are not identifiable without supporting evidence, because: (1) the change in magnetic polarity may represent any one of several possible reversal boundaries, (2) the change in polarity may belong to one of the many excursions of the magnetic field, and (3) significant erosion of the lower polarity epoch may be followed by deposition of sediment much younger than the beginning of the polarity change.
The degree of magnetic foliation in a till may be determined by the principal axes of a susceptibility ellipsoid, which is significantly more spherical for glaciomarine drift than till. Lodgement till deposited in a water-saturated high-shear environment at the base of a glacier may contain a well-defined magnetic fabric that accurately reflects its petrofabric. Massive till-like glaciomarine drift, consisting of clastic particles dropped from floating ice in marine waters, contains elongate particles that are more randomly oriented than those of subglacial till because it lacks the pervasive shear associated with subglacial till.
An example of the use of combined paleomagnetic measurements and fission-track dating of Pleistocene deposits in the Central Plains indicates that: (1) “Nebraskan” till is not the oldest drift in the region, (2) “Nebraskan” tills at various classic sections are not the same age, (3) the oldest till is older than 2 Ma, and (4) the early Pleistocene glacial sequence in the region is considerably more complex than previously thought.
Figures & Tables
Dating Quaternary Sediments
- clastic sediments
- depositional remanent magnetization
- glacial environment
- glaciomarine environment
- Great Plains
- magnetic declination
- magnetic inclination
- magnetic susceptibility
- marine environment
- mathematical models
- natural remanent magnetization
- North America
- pore pressure
- remanent magnetization
- secular variations
- United States