The Tallac Roof Pendant was likely deposited during the Jurassic. Originally they included sedimentary and volcanic rocks. The sedimentary rocks were likely formed in a shallow marine environment. A volcanic system formed in the general area depositing the volcanic rocks.
During the Cretaceous, the Sierra Nevada Batholith began to form. Granitic magma migrated up toward the surface along fractures in the overlying rock. Near the surface there was a horizontal plane of weakness between an existing pluton and sedimentary and volcanic rocks above it. The magma stopped its upward movement and instead began spreading out laterally along this horizontal contact. The heat of the magma and the pressure of the overlying rock metamorphosed the sedimentary and volcanic rocks. This diagam of how the Dinkey Creek Pluton a portion of the Sierra Nevada Batholith intruded shows the process.
image from Petford et. al. 2000
Continued uplift of the Sierra Nevada Mountains eroded all of the overlying metamorphic rocks until only the metamorphic rocks that protruded down into the Sierra Nevada Batholith remained. The rocks at this location are the volcanic portion of the Tallac Pendant. The dark gray rock is volcanic breccia containing a variety of volcanic clasts and a few sedimentary clasts. The sedimentary clasts have undergone contact metamorphism to epidote, the dark green portions of the rock. Epidote is commonly found in metamorphosed limestone.
Looking up the mountain to the south, you can see the granite of the Sierra Nevada Batholith. Other parts of the Tallac Pendant can be found to the west and northwest.
Logging requirements:
Send me a note with :
- The text "GC4F45E Tallac Pendant" on the first line
- The number of people in your group (put in the log as well).
- What is the general shape of the epidote clasts
- Are there any pieces of the Sierra Nevada Batholith within 200 feet of the coordinates?
The following sources were used to generate this cache:
- Geologic Map of the Mount Tallac Roof Pendant, El Dorado County, California. G. Reid Fisher, 1989
- 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.