Geology Reference
In-Depth Information
cyclostratigraphic studies—the Cupido Formation platform carbonates and
the Arguis Formation pro-deltaic marls and siltstones covered in Chapter 6
are two good examples. Even continental, fluvial sediments such as the
Mississippian Mauch Chunk Formation discussed in Chapter 6 may be able
to provide records of short eccentricity and precession, but more rigorous
study of similar rocks should be conducted before all terrestrial, fluvial
rocks can be trusted to give good cyclostratigraphic results. The discon-
tinuous sedimentation expected in a fluvial environment and the other
processes (landsliding, channel avulsion) that could “shred” the climate
signal (Jerolmack & Paola 2010) must be considered when conducting
a rock magnetic cyclostratigraphic study on fluvial rocks. For this reason,
marine sedimentary sequences are preferred because of the greater likelihood
of nearly continuous sedimentation.
There should be some kind of independent time control for the strati-
graphic section being studied. This is critical for correct identification of
astronomically forced climate cycles in the rock magnetic data series. The
main power of the cyclostratigraphic technique is its ability to assign time at
very high resolution, nominally at the precession scale (~20 kyr), but some
time control at a coarser scale is important for a successful study.
Enough time control is needed to identify the longest Milankovitch cycle,
usually long eccentricity with a period of ~400 kyr, expected in the data.
Some examples of ways that coarse-scale time was assigned to identify astro-
nomically forced cycles in rock magnetic or lithologic cyclostratigraphy
studies are magnetostratigraphy (Eocene Arguis Formation), biostrati-
graphically defined, radiogenically calibrated geologic time scale bound-
aries (Triassic Daye Formation (Wu et al. 2012), Arguis Formation (Kodama
et al. 2010) or the Newark basin depth rank cycles (Olsen & Kent 1996)), or
sequence stratigraphy boundaries (Cretaceous Cupido Formation (Hinnov
et al. 2013)). Of course, radiogenic ages for ash beds within a stratigraphic
section can be an important way of assigning time, but the successful
identification of astronomically forced climate cycles will depend on the
stratigraphic spacing of the ashes and the errors of the ages. For instance, in
the Latemar massif, the ages determined by U-Pb geochronology on zircons
extracted from tuff beds had ages that overlapped within their errors, so
an estimate of sediment accumulation rate was poorly constrained (Mundil
et al. 2003).
Sampling interval and the thickness of section are the next important
points to consider when planning a rock magnetic cyclostratigraphy study.
The sampling interval is dictated by the Nyquist frequency, i.e., the highest
frequency that a cyclostratigraphic study can detect. It must be sampled at
least twice per period. If precession is the target for the highest frequency,
samples need to be collected at least once per ~10kyr. This sampling
interval is a bare minimum and it is better to sample the precessional cycle
at least three or four times or every 5-7 kyr. Given an average sediment
accumulation rate of ~10 cm/kyr for rock types that have yielded successful
rock magnetic cyclostratigraphies (Table 7.1) would suggest sampling
