Geology Reference
In-Depth Information
remanent magnetizations to samples in the laboratory and then, of course, the
measurement of the resulting magnetization. The use of environmental mag-
netism to study either recent or past changes in the environment or climate has
reached fairly sophisticated levels, but rock magnetic cyclostratigraphy has
kept to simpler applications of environmental magnetism by measuring
magnetic mineral concentration variations to construct a data sequence for
time series analysis.
In contrast, environmental magnetic studies can examine many different
environmental processes including modern soil development, particularly
the magnetic enhancement and growth of new magnetic minerals in the
topsoil, paleosol development in loess by the growth of secondary magnetic
minerals and their connection to paleo-precipitation, reductive diagenesis
in marine and lacustrine, organic-rich sediments, erosion and transport of
catchment soils and bedrock into either a lake or river, ancient lake level
fluctuations, population fluctuations of magnetotactic bacteria in lake or
marine sediments, and the relative contributions of riverine and eolian
sources for near-shore marine sediments. There are many more examples
that can be found in excellent summaries of the environmental magnetic
literature (Thompson & Oldfield 1986; Maher et al. 1999; Evans & Heller
2003; Liu et al. 2012).
One consideration is of utmost importance for any rock magnetic
cyclostratigraphic study. It is absolutely critical that the magnetic minerals
in the sedimentary rocks being studied are primary depositional minerals
indicating that their concentration or grain size variations reflect deposi-
tional processes. Therefore, before a comprehensive cyclostratigraphic study
is conducted, the age of the magnetic minerals should be studied. The best
way to determine the age of the magnetic minerals in a sedimentary rock is
to conduct a pilot study of the paleomagnetism carried by the magnetic
minerals. The age of the resulting paleomagnetic directions can be con-
strained by standard paleomagnetic tests. The fold test determines whether
the paleomagnetism of a sequence of folded sedimentary rocks is older or
younger than the time of folding. If the magnetization fails the fold test and
is younger than folding, the magnetic minerals are clearly secondary and
undoubtedly unsuited for a rock magnetic cyclostratigraphic study. However,
even if the paleomagnetism passes the fold test and is older than the folding,
further tests should be conducted to constrain the age of the magnetic min-
erals, since the folding may not have occurred soon enough after deposition
to ensure that the magnetic minerals are depositional in age. Passage of the
fold test shows, at least, that the magnetic minerals are ancient and older
than the folding. Other tests constraining the age of magnetization, and the
magnetic minerals carrying the magnetization, are the baked contact test
and the conglomerate test. These tests and the fold test are described in
more detail in standard paleomagnetism texts. Butler (1992) and Tauxe
(2010) provide excellent summaries of these tests.
One powerful demonstration of primary magnetic minerals in a sedi-
mentary sequence is the successful determination of a magnetostratigraphy,
