Geology Reference
In-Depth Information
6.7 Mauch Chunk Formation: Mississippian
Red Beds, Pottsville, Pennsylvania
The magnetostratigraphy of the upper Mississippian (upper Visean ~328
Ma) red beds of the Mauch Chunk Formation has been studied extensively
(DiVenere & Opdyke 1991). These rocks were, therefore, a natural target for
a rock magnetic cyclostratigraphy study, since absolute time has already
been established for them. One of the goals of this cyclostratigraphic study
was to see if astronomically forced cycles can be detected rock magnetically
in the Paleozoic. Another goal was to determine if terrestrial rocks depos-
ited in a fluvial environment can record Milankovitch-scale climate cycles.
As Jerolmack and Paola (2010) pointed out, a fluvial system has the strong
potential for shredding environmental signals because of depositional and
transport processes. Two important factors that would limit the ability of a
fluvial depositional environment's ability to record climate cycles are the
discontinuous deposition in a river's floodplain and the effects of channel
migration across the floodplain.
A well-studied stratigraphic section of the Mauch Chunk Formation was
sampled near Pottsville, Pennsylvania. Levine and Slingerland (1987) inter-
preted this section to show that the Mauch Chunk was deposited in a
braided, alluvial plain in a semi-arid environment subject to catastrophic
floods. The Pottsville site has good stratigraphic control based on Levine
and Slingerland's (1987) detailed stratigraphic section and has 2-3 m thick
channel deposits with 20-30 m thick sections of floodplain (overbank)
deposits. DiVenere and Opdyke (1991) conducted a magnetostratigraphic
study of the Pottsville section and their results suggest that the stratigraphy
is continuous for about 1.2 myr.
A reconnaissance rock magnetic cyclostratigraphy study was conducted
using a hand-held, portable susceptibility meter (GF Instruments, model
SM20). A measurement was made by simply taking a background measurement
in the air and then placing the hand-held meter on the rock outcrop for the
rock susceptibility measurement. Susceptibility measurements were collected
over 63 m of section at a 50 cm sampling interval in only several hours.
Samples did not need to be collected from the outcrop or taken back to the
laboratory for preparation and measurement. The data could be processed
almost immediately. The susceptibility data set showed strong, hierarchical
cyclicity with meter-scale high-frequency cycles superimposed on longer ~10
m cycles (Figure 6.12).
MTM spectral analysis of the susceptibility data series shows three
significant spectral peaks (>99% confidence level) with wavelengths of 13.3,
10.3, and 1.7 m, and peaks that rise about the 95% confidence level at 2.6
and 2.1 m wavelength (Figure 6.13).
DiVenere and Opdyke (1991) recognize a normal polarity interval that is
100 m thick in the lower part of the Pottsville outcrop very close stratigraph-
ically to where the susceptibility readings were made. According to Opdyke
