Echo Arch Gully Area

 

Stop 3 is the Echo Arch Gully Area.  There are great exposures of Monterey Formation (15 MYO) here.

 

The Monterey Formation is a very soft, well stratified, fine-grained formation composed of hemi-pelagic and pelagic muds.  It is economically very important both as a petroleum source rock in offshore California and it contains large diatomaceous earth deposits in the Lompoc (near Pt Conception) area.  It was formed as a deep ocean sediment which means that it must have been greatly uplifted to its present position. Because it is very soft, it slumps easily and these slumps form the dominant landform in this area. This block is showing backwards rotation of the slump-block. Note the white streaks in the exposure.

 

 

Notice how fine-grained and well stratified (finely layered) the Monterey Formation is at this location.

 

 

The Monterey Formation contains volcanic ash layers that are white and have been age-dated radiometrically to about 15 MYO.  Onshore volcanism resulting from regional subduction offshore is most likely responsible for the ash deposits in the Monterey Formation.

 

 

These unique rock fragments (a boulder here) are found in this canyon.  They have been eroded locally from San Onofre Mountain and deposited here. They have a blueish color to them and are called Blue Schist facies rocks and are formed in subduction zones from high pressure, low temperature metamorphism.  These rocks have obviously been uplifted dramatically to be exposed on the earth’s surface.

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