First a comment on safety.  Orr Springs Road is little used, but there is periodic traffic.  For both locations there is sufficient parking is on the opposite side of the road from where you need to make the observations.  There is easily room for one vehicle, two vehicles would be pushing it.   There is a bit of a drop off and no guard rail, so take care parking and getting out of your car.  Orr Springs Road has infrequent traffic, but take care crossing the road and out of the lane when making your obs`ervations.

I prefer the older name Franciscan Mèlange, although Franciscan Complex or Franciscan Assemblage is now the more common term.  This is an awesome location to see multiple rock types associated with this “formation”.  During the mid to late Jurassic through mid Cretaceous, prior to the formation of the San Andreas Fault*, this portion of the west coast was a convergent margin, with the Pacific (ocean) Plate subducting under the North American Plate (Figure 1).   Overall the age range of the Franciscan Melange is 180/190 to 15 million years old, I could not find a specific age for this formation in Medocino county. As the oceanic crust subducted under the continental crust, sedimentary and oceanic volcanic rocks are scraped off.  Because of the way these rocks get scraped off, they tend to be a jumbled mixture rather than the neat layer-cakes.    Until the understanding of how the Franciscan Complex was form, geologist were at a loss to explain how these rocks formed their chaotic arrangement.   Later these rocks were uplifted resulting is deformation.

Most of the rocks undergo some level of metamorphosis from pressure and heat if they descent near the heat of the mantle.   The sedimentary rocks include shales, sandstones, graywacke, limestone, and chert.  Igneous and igneous-to-metamorphic rocks include greenstone, serpentine, and rare blueschists.   At these two stops you’ll see several types of these sedimentary rocks within the Franciscan Mèlange.

Figure 1.  Subduction Continental Margin

For all these stops many of the observations you will be greatly aided by use of a hand lens, although if you have decent eyes it is not strictly necessary. 

Stop #1:

First when you first cross the road at the lowest level of the road cut you’ll see dark brown chert .  This rock is flinty and hard.  This chert was originally deposited horizontal and flat laying.  Chert is formed in the ocean distant from shore where the primary sedimentary input is a “snow” of the skeletons of radiolarian plankton at rates of a few millimeters every 1,000 years or so.  For the answer below assume an accumulation rate of 4 millimeters per 1,000 years.  You will see that as part of the subduction and later uplift the layers of chert layers were lightly folded and fractured.  Observe:

• How thick are the individual layers of chert.
• What is the spacing of fractures within a single layer.
• How much folding/bending do you see in individual layers.

Next, step up to the next level of the road cut above this lower level, about 2 to 3 feet.  On the left side you’ll see a face of light brown sandstone, possibly graywacke (see below for definitions). 

• At the base of the cut you’ll find many small pieces of sandstone.  Pick up a piece of this sandstone/graywacke and closely examine the texture (grain size and grain size distribution). 

Finally, at this stop look to the white-ish rocks to the upper right.  This is limestone.  Again pick up a few pieces of this limestone at the base of the road cut.

• Look closely at a few pieces of the limestone on freshly broken faces and closely examine the texture.
• If your lucky you’ll find an original fracture with secondary recrystallization.  In one of these faces this surface will sparkle in the sunlight.     

Stop #2:

At this location there is a great outcrop of graywacke, it is best to observe what has fallen to the road level.  There a few very large blocks.

• At the base of the cut you’ll find many small and large pieces of graywacke.  Look closely examine the graywacke’s texture (grain size and grain size distribution). 

As with all EarthCaches you will need to send the Cache Owner (me) some answers to the questions below.  Go ahead and log you cache immediately, but send me the answers in a timely manner, or I’ll be forced to delete your log.  I honor all good faith attempts regardless of the accuracy of the answer.  Most answers or obtained by field observations, although there are a few that require reference to information in the text of the description.  I only reply if I need more information or I have doubts that you actually physically visited the cache site.

1. Name of the EC and the GC code
2. Stop #1:
   1. Chert
      1. How thick would you say the average layers of chert is?
      2. From your field observation and using the rate provided in the description above how long do you calculate it would take for this individual to deposit?  We’re ignoring any potential compaction.
      3. What is the spacing of fractures within a layer?
      4. Describe the bending/folding of the layers, spacing and magnitude of bends.  Smooth folds or sharp folds.
   2. Light-brown sandstone/graywacke
      1. Describe the grains size and range of grain size.
      2. Based on the definitions of sandstone and graywacke which would you name this light brown rock?
   3. White limestone
      1. Describe the texture of the limestone upon close examination.
      2. Did you see the sparkly recrystallize calcium carbonate.  Please describe.
3. Stop #2:
   1. 1. Graywacke:  Describe the grains size and range of grain size.
      2. Describe the color of he graywacke.
4. Photographs are not required but always welcome.  If you do take a photo please cut off a portion of the condenser so people can’t fake “find” this cache, yes that happens.  Thanks

Glossary:

Chert:                  A microcrystalline (not visible) silica rich rock.  Frequently well bedded and hard and flint-like.  In the Franciscan Complex typically associate deposits of plankton with silica shells.

Graywacke:        A very poorly sorted sandstone with a mixed grain size from very course sand to silt, and typically with 15% clay.

Greenstone:       A green-ish gray low-grade igneous rock.  Tends to be massive and very hard.

Limestone:         A sedimentary rock composed primarily of calcium carbonate.  Usually white, light gray, or yellow

Lithosphere:      The hard, outer rocky crust of the earth

Mantle:               The semi-liquid to liquid layer of the earth beneath the lithosphere.

Mèlange:            An mixed assemblage of different rock type.  From the French for mixture.

Right-Lateral Strike-Slip Fault (in footnote): A fault where the sense of motion is lateral, in this case when standing facing the fault the opposite side is moving to the right

Sandstone:         A sedimentary rock composed of sand size grains, typically with less than 15% clay/silt size grains.

Serpentine:        A magnesium-rich, low-grade metamorphic rock formed from original igneous oceanic crustal rocks.  Tends to be mottled and usually soft.

Subduction Zone:            Where to tectonic plates converge and one slides under the other.

Reference:

Harden, D. 1997. California Geology.  1^st Edition

Jenning, C.W. and Strand, R.G. (complied by) 1960.  Geological Map of California – Ukiah Sheet. U.S. Geological Survey.  Assembled from 12 separate maps.

* I’m not going to cover the formation of the San Andreas Fault (right-lateral strike-slip fault) from the earlier subduction system.  It’s too long an explanation and tangential to what this EC is about.