From: Keith Miller (kbmill@ksu.edu)
Date: Wed Oct 15 2003 - 18:45:56 EDT
> Also, "the location of the boundary between the Manakach and Wescogami
> formations [a 14 million year paraconformity] can be difficult to
> determine,
> both from a distance and from a close range." (Blakey RS. 1990. Supai
> group and
> Hermit formations. in Grand Canyon Geology [see above] p 147-182).
I have personally examined this stratigraphic interval at its type
locality. The reason it is hard to trace is that it is based on river
channel deposits. There are large river channels with beautifully
displayed cross stratification from lateral channel migration.
However, the channel deposits are discontinuous in space just as they
are in modern landscapes. Since the conglomeratic deposits at the base
of the channel fills were used to define the boundary, these marker
units must be correlated. As it turns out, if you trace the edges of
the channels upward they merge into thick paleosols (fossil soils) that
then continue on across the outcrop. The paleosols that we saw were
very well-developed and represented an extended period of subaerial
exposure and weathering. This is precisely what one would expect - and
mirrors what is seen on landscapes today. Furthermore, the paleosols
contain considerable amounts of pedogenic carbonate nodules (caliche
nodules). These nodules were eroded and reworked by the stream
channels and are found concentrated in the conglomerate channel lags.
These surfaces recording extended times of subaerial exposure and
weathering provide the basis for correlation. In sequence stratigraphy
terminology, these are called "sequence boundaries."
Paleosols are ubiquitous within the stratigraphic record. They have
been the focus of much of my recent geological research. There are
dozens and dozens of stacked paleosols within the Pennsylvanian and
Permian sections of the midcontinent. In addition to their value in
correlation, they provide excellent data on changing climates. There
is a huge literature on paleosols and their interpretation.
Keith
Keith B. Miller
Research Assistant Professor
Dept of Geology, Kansas State University
Manhattan, KS 66506-3201
785-532-2250
http://www-personal.ksu.edu/~kbmill/
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