First a word of caution: The best parking is on the east side of the road but the view of the rock in the west side of the road. Cross with extreme caution. Grab a small hand sample and then retreat to the safety of the turnout on the east side of the road.
When examining rocks a hand lenses (10x or so) is always helpful, but not required. As per the EarthCache rules you’ll need to answer the questions at the end to not have your log deleted. Unless you’re a geologist, you’ll need to thoroughly read the description. All good faith attempts at answering questions will be honored.
Grab a small hand sample to examine, anywhere from golf ball to soft ball size should suffice, there are plenty lying at the base of the road cut. What you are looking at is a chunk of the Yuba Rivers Pluton. Here you are about in the center of this rock unit, which runs about 20 miles (north-south axis) and about 4 miles wide (east-west axis). This pluton is named for the Yuba Rivers (plural) because it crosses the north, middle, and south forks of this river system. This is rock is from a pluton of granodiorite. First some definitions:
- Granitic Rocks is an igneous, light colored rock with a high silica content and a relatively lower iron content. The minerals a relatively large because it cooled slowly from molten rock at depth (magma).
- Pluton is a “blob” a molten rock that migrated upward an became emplaced nearer the earth surface. Think of the wax rising in a Lava Lamp as it moves upward, but then becomes emplaced below the surface. This blob of magma can then feed volcanoes at the surface.
- Intrusive rocks that cooled at depth from highest silica (lowest iron) content to lowest silica (highest iron) content are granite, granodiorite, diorite, gabbro, and peridotite.
Most of the Sierra Plutons are emplaced along the crest of the Sierra. The Yuba Rivers Pluton is an outlier that emplaced in the western flank of the Sierra, where metamorphic rocks are present. These metamorphic rocks were already in place when this blob of magma “went astray” and ended up west of the rest emplaced magma. While emplaced this magma (before cooling) may have feed over lying volcanoes. The emplacement of this particular magma was around 150 to 164 million years before present. Any overlying volcanoes and other rocks were eroded to expose this rock. Being at depth the magma cooled slowly allowing large mineral crystals to form. Extrusive volcanic rocks (lava) cool rapidly and large crystals don’t have time to form being therefore are very fined grained. Minerals have regular arrangement of atoms into matrices, and separate out into distinct types like oil and water separate; forming distinct minerals.
In a granodiorite the most common minerals are:
- Quartz light gray in this rock, look closely to distinguish from white
- Plagioclase Feldspar (aka Sodium Feldspar) is pretty stark white.
- Orthoclase Feldspar (aka Potasium Feldspar) is distinctly pink, but hmmm, is it in this rock? Maybe not. There needs to higher potassium concentrations in this rock for this mineral to form.
- Biotite Mica is black, platy, iron rich mineral. If you rotate your rock is flashes of reflection in the sunlight on a black mineral that likely biotite mica.
- Hornblende is another black, iron rich mineral; but will be less common in a granodiorite. Hornblende will be in rods. The crystal shape hornblende is monoclinic, but why to complex to describe in this EC.
- Titanite sometimes form if there is small amounts of titanium present. This mineral is typically equant in shape root beer brown to green, and crystals will be uncommon and tiny. This mineral may be present, but is very rare so it’s unlikely present, but maybe you’ll find one.
Log this cache the day you find it. However you are required to message the CO with the answers to the questions below. You don’t need to be right, I honor all good faith efforts
(1) Name and GC code of the cache.
(2) Do you best to guess the minerals present in your hand sample (rock).
(3) Guess the percentage of each mineral in the rock.
(4) Look at the different faces of your rock (x, y, and z axes ). In any particular orientation do the minerals appear to be elongated to others? This would be a indication of either flow or deformation of the rock. Do you think deformation/flow occurred?
(5) Not required, but I always love photos.
Concise Background on the Sierra Geology - Not required for the cache answers
This segment is (or will be) repeated in several of the Geotour Hwy 49 (GT49 caches). Going west to east the Sierra is a series of terranes (yes the correct spelling) that are juxtaposed. That is each of these terranes are a series of units that my vary widely in rock type but originated for a common geological process.
First a bit of background if you were to do a line of section west to east from the Central Valley to core of the Sierra (highway 49 starts about center of this line. You encounter sequences as follows:
- Jurassic Foothill Belt (JFB): The JFB is unconformably (there is a erosion and time gap between the two) adjacent to Jurassic-Triassic Arc Belt. The JFB is a 165-155 million year old volcanic arc metamorphosed to greenstone, and sand and silts metamorphosed to greywacke and slate, respectively. Within JFB but more along Highway 20 and 70 is the ocean crustal segment known as the Smartville Complex.
- Jurassic-Triassic Arc Belt (JTA): This is a complex jumble of rocks from about 200 million years ago. This an volcanic arc that formed over subducting ocean crust was shoved under the continental rocks. The Calaveras Complex are the oceanic materials shoved into the the subduction trench. As this ocean crust slid under the continent and melted, volcanoes formed above them.
- Calaveras Complex (CC): The Calaveras Complex are the oceanic crust and overlying sediment that were shoved into the subduction trench when the JTA was erupting inland from the melting subducted materials. In the process of subduction these different rocks (sedimentary, volcanic, and ocean crust) became a jumbled mess. As Alt & Hyndman (2016) described it, “The best that can be said of the Calaveras Complex rocks is that they are a unholy mess, difficult to decipher in the field and impossible to describe adequately on maps or in words. The rocks include a scrambled assortment of pieces of old oceanic crust and seiments tat were deposited on them, all torn up along faults and jammed into tight folds, and recrystalized into metamorphic rocks in the heat of the deep trench.”
- Feather River Peridotite Belt (FRPB): A narrow belt of Devonian-age (~420 to ~360 million years ago) ocean crust metamorphosed to serpentine rock, and is roughly time synchronous with the Shoo Fly Complex.
- Shoo Fly Complex (SFC): These set of rocks originated from sediments deposed about 800 to 700 million years ago, and among the oldest sedimentary rocks in California. These rocks were shoved into North America along a subduction zone during the Antler mountain building event about 300 million years ago. Found in the complex are metamorphosed sandstones, shales, schists, limestone, and chert.
- Sierra Nevada Batholith and associated overlying volcanic (SNB): From about 225 to about 80 million years ago, large blobs of magma welled up during the subduction that helped build the modern coast ranges. Over these blobs of magma were volcanoes that in many cases have been eroded away, especially in the high Sierra near Lake Tahoe and into the southern Sierra. Individual "blobs" magma that cooled to granitic rocks are plutons. The mass of all the consolidated pultons on a batholith.
- Outlier Plutons are individual plutons associated with the granitic rocks along the crest of the Sierran Batholith, but that emplaced in the rocks outside the batholith. See GC9WG31 - GT49 Yuba Rivers Pluton.
References:
Alt, D & Hyndman, D (2016). Roadside Geology of Northern and Central California
Durrell, Cordell (1987). Geological History of the Feather River Country, California.
Koningsmark, Ted (2011). Geological Field Trips: Sierra Nevada