Geology Reference
In-Depth Information
the record, and very short events may be missed because of either small
hiatuses in sedimentation or the sampling interval that was used. The strati-
graphic sampling interval also affects the resolution of the stratigraphic
position of the polarity interval boundaries and hence the calculation of
the sediment accumulation rate. One key piece of information that must be
established is an approximate age for the stratigraphic section to aid the
tie to the GPTS. Biostratigraphy is often used for this purpose. Of course,
geochronology of ash layers can also be an important way to determine the
age of the sedimentary section. In the Spahn et al. (2013) magnetostrati-
graphic study in the Dolomites (Figure 3.4), both biostratigraphy and
geochronology of ash layers provided a sound tie to the integrated GPTS for
the Triassic (Hounslow & Muttoni 2010).
Before about 170 Ma, the GPTS is not continuous and not as well devel-
oped or calibrated. The integrated GPTS for the Triassic (252.5-201.6 Ma)
developed by Hounslow and Muttoni (2010) is based on the biostratigraphic
correlation of various continental sections globally. Hounslow and Muttoni's
(2010) Triassic GPTS is at some odds with Kent et al.'s (1995) astronomically
tuned GPTS from Newark Basin drill cores for the Late Triassic, so Gradstein
et al. (2012) present two options for the Late Triassic GPTS, one based on the
astronomically tuned record (Kent et al. 1995) and one based on the bio-
stratigraphically correlated marine sections (Hounslow & Muttoni 2010).
The Triassic GPTS has a time resolution, at best, of 20-30 kyr for its magne-
tozones (polarity intervals). Kent and Olsen (2008) have been able to extend
the Newark Basin magnetostratigraphy from 202 to 199.4 Ma using astro-
nomically tuned reversals recorded in Hartford Basin rocks. The period
between the end of the seafloor anomaly-based GPTS at around 170 Ma and
the Hartford Basin magnetostratigraphy at 199.4Ma is based on assorted
biostratigraphically correlated continental sections from England, France,
Switzerland, Spain, and Austria (Gradstein et al. 2004). Gradstein et al. (2012)
have compiled a GPTS for the Permian. The field remained in the reversed
polarity state for much of the Permian from about 300 Ma until about 267 Ma
with perhaps two short normal events at about 286 and 297 Ma. Recent work
by Opdyke et al. (2014) suggests that the base of this long polarity interval
may be as old as 318 Ma. This long polarity interval is the Kiaman reversed
polarity superchron, and like the Cretaceous normal polarity, superchron
(84-126Ma) is a period in Earth history when magnetostratigraphy is
impossible. Recently, Pavlov and Gallet (2005) provide evidence that a third
superchron occurred in the Ordovician, between about 480 and 460 Ma. The
Carboniferous GPTS is fairly continuous and is based on biostratigraphically
correlated continental sections from 299 to 359 Ma (Gradstein et al. 2004,
2012), most notably the Late Mississippian (Serpukhovian, 323-331Ma)
magnetostratigraphy compiled from Appalachian Basins (e.g., Mauch Chunk
Fm., (DiVenere & Opdyke 1991)) or the Canadian Maritimes (Opdyke et al.
2000). G. Ogg's chart (Ogg 2012) that summarizes the Geologic Time Scale
2012 (Gradstein et al. 2012) provides a quick view of the status of the GPTS
for Phanerozoic rocks (Figure 3.5).
