The
Dinkey Creek Roof Pendant is accessible by unpaved Forest Service
access roads. The roads do get pretty rough, so a high clearance
vehicle would be a very good idea. There is a large parking area
near the coordinates. There are a number of coordinates to visit
that are all within 0.2 of a mile, but are on relatively steep
slopes. Because you will be looking at the rocks, the EarthCache is
unavailable during the winter since the rocks will be covered with
snow.
The Dinkey Creek Roof Pendant includes, schist, quartzite,
hornfelds, calc-silicate rocks, and marble. These rocks are thought
to have been originally deposited sometime between the Paleozoic and
the Cretaceous
Period of the Mesozoic in a
shallow marine area. That age range would be anywhere between 524
to 146 million years ago.
Following deposition, the sediments underwent a series of
deformations. First they were folded at least 3 times. Then, with
the intrusion of the
Dinkey Dome Pluton, the sediments were subjected to contact
metamorphism becoming the meta-sediments we see today.
Then, the Sierra Nevada Mountains were uplifted and erosion
removed the overlying meta-sediments. Today almost all of the
meta-sediments have been eroded leaving the granite of the Dinkey
Dome Pluton exposed at the surface. Only the roof pendants, the
portions that stuck down into the Dinkey Dome Pluton, remain.
So what is a Roof Pendant? When the Dinkey Dome Pluton expanded
out horizontally, it flowed along a contact between an existing
pluton and the series of meta-sediments that overlied the older
plutonic body. Since the meta-sediments were on top of this magma
that would become the Dinkey Dome Pluton, the meta-sediments are
considered the roof of the intrusion. The roof was not completely
flat, so portions protruded down into the magma like a necklace
pendant. Putting the two descriptions together, we get a roof
pendant.
The following is a short description of the rocks in the Dinkey
Creek Roof Pendant (in order from oldest to youngest)(Kistler &
Bateman1966)
The
Marble (N37 09.125 W119 06.265)
This is a grayish rock that is very fine grained giving it a sandy
texture. Many veins of quartz run through it. Close contact with
instrusive magmas (the surrounding pluton) caused the metamorphosis
from the source carbonate rock to the marble seen in the area.
The Calc-Silicate Rocks (A - N37 09.010 W119 06.253 and B
- N37 09.072 W119 06.199)
This group of rocks is separated into a lower and upper member
separated by a sill of diorite. The lower
(older) member is made up of layers of marble and two types of
hornfels. The
marbles are grayish and the hornfels are reddish-brown. The upper
(younger) member also is made up of marble and hornfels. But the
marble is a light tan to gray and the hornfels are greenish. The
multiple episodes of folding can be seen throughout the
calc-silicate rocks
Biotite-Andalusite Hornfels (N37 09.055 W119
06.155)
Originally this rock was a shale or mudstone that was very uniform.
Now the individual beds are very difficult to discern so the rock
looks like a gray non-descript rock. You can find the folding when
looking carefully and from the fracturing.
White Quartzite
(further to the east)
This is a light gray to light tan rock. Crossbeds can still be
found within the metamorphosed sandstone. Schist (East of the
Quartzite) This rock composed of layers of dark schist and light
tan quartzite. Much of the layering in this rock has been affected
by the folding and contact metamorphism such that it is not readily
recognizable.
Logging requirements:
Send me a note with :
- The text "GC30XNN The Dinkey Creek Roof Pendant" on the first
line
- The number of people in your group (put in the log as
well).
- Describe the feature at the coordinates and the texture of the
marble.
- Is location Calc-Silicate Rocks A the upper or lower member of
the calc-silicate rocks?
- Which is location Calc-Silicate Rocks B?
The above information was compiled from the
following sources:
- Kistler R.W, and P.C. Bateman; Stratigraphy and
Structure of the Dinkey Creek Roof Pendant in the Central Sierra
Nevada, California; Geologic Survey Professional Paper 524-B;
includes foldout map; 1966
- Petford, N., Cruden, A.,
McCaffrey, K and Vigneresse, J-L., Granite magma formation,
transport and emplacement in the Earth's crust, Nature, V. 408, p.
669¬673, December 2000.