Geology Reference
In-Depth Information
example, might emerge if the sediments were deposited at low latitudes, where
precession-scale insolation variations are expected to drive the monsoon. For
icehouse periods of Earth's history, obliquity might be observed due to varia-
tions in sea level driven by changes in global ice volume because insolation
variations at the obliquity timescale are important at high latitudes. If higher
frequency astronomically -forced cycles emerge in the power spectrum after
tuning to 405 kyr long eccentricity, then the long eccentricity cycle was cor-
rectly identified and a high resolution chronostratigraphy can be generated.
Assigning time at a coarse scale can be accomplished by several methods.
One powerful way, covered in this topic, is to determine a magnetostratig-
raphy for the sedimentary sequence. At some points in Earth history, the
reversal rate of the geomagnetic field was as high as 4-5 reversals per million
years, already giving nearly 200kyr time resolution. At other times, the
reversal rate diminished to as low as no reversals per million years in
the Cretaceous (126-84 Ma), when magnetostratigraphy is not possible. The
geomagnetic field also remained one polarity (reversed) for tens of millions
of years during the Late Paleozoic Kiaman superchron, so it is important to
know the age of the sedimentary sequence being studied to determine if
magnetostratigraphy is even possible. When magnetostratigraphy is an
option, it is an excellent way of assigning coarse time to the sedimentary
sequence to identify orbitally forced cycles. Magnetostratigraphy was used
in this way for the Eocene Arguis Formation (Kodama et al. 2010) and the
Stirone River sediments (Gunderson et al. 2012).
Biostratigraphy can also be used to assign coarse resolution time to the sedi-
mentary record under study. Wu et al. (2012) used the conodont biostratigraphy
of the rocks, which had been correlated globally and calibrated radiogenically, in
this manner to identify the 405 kyr long eccentricity cycles. They also used their
coarse age model to identify higher frequency astronomically forced cycles and
tuned them to short eccentricity and to precession.
Olsen and Kent (1996) identified the ~400 kyr long eccentricity cycle in the
lithologic cyclostratigraphy of the Newark Basin lacustrine rocks by assigning
time at three different geologic time-scale boundaries. According to Olsen and
Kent (1996), there are three biostratigraphically defined boundaries in the
Newark Basin rocks: the Triassic-Jurassic boundary, the Norian-Rhaetian
boundary, and the Carnian-Norian boundary, mainly identified by pollen and
spore assemblages. Olsen and Kent (1996) recognize long and short lithologic
cycles that they interpret to indicate variations in the depth of the lake that
deposited the sediments. The longer McLaughlin cycle, which modulates the
shorter Van Houten cycles, is shown to have a duration of nearly 400kyr
(397.7 ± 58.5 kyr) based on the ages for the geologic time-scale boundaries. In
this manner, Olsen and Kent (1996) very nicely demonstrated that the
McLaughlin cycles were driven by long eccentricity and could calibrate their
lithologic cyclostratigraphy accordingly. A similar approach could be used to
identify long eccentricity in a rock magnetic cyclostratigraphy record.
Another approach is to use the fourth order sequence boundaries that
mark the change from transgression to regression in sequence stratigraphy.
