The Cristianitos Fault

 

This stop is why we have come to this location.  We have an excellent exposure of the Cristianitos Fault that allows us to accurately age-date the fault activity.

 

This is the fault plane of the Cristianitos Fault. It shows offset between the lowermost tan formation on the left and the brownish formation on the right.  Both formations are overlain by a marine boulder/cobble bed.  We are going to come back to this in a moment, as we must first describe the geology a bit more.

 

 

The tan formation at the base of the cliff is the San Mateo Formation. It is a well-cemented, hard, coarse-grained formation that was deposited in a beach setting.  It contains fossils (not at this location) that age-date it to about 5 MYO. Because it is so well-cemented, it can stand nearly vertical and it has a different type of mass wasting than softer sediments like the Monterey Formation.  The type of mass wasting found in the San Mateo Formation is called rock or cliff fall. Above the San Mateo Formation is the Marine Boulder Bed which is described below and on top of that are the alluvial sediments previously described.

 

 

This is a close-up of the San Mateo Formation. This shows the coarse-grained nature of this formation.

 

 

Above the San Mateo Formation is the Marine Boulder Bed. It is a detrital sedimentary unit with grain sizes ranging from silt to boulder-sized material.  It was formed on a wave-cut bench or platform and contains marine fossils which we will age-date later in this lab. Notice how round the cobbles/boulders are and compare this with how angular the rock fragments are in the alluvial sediments. This feature makes it easy to distinguish the two units from each other. 

 

This is a close-up shot of the Marine Boulder Bed. The keys in the picture give you a sense of scale.....its the number one way geologists lose their keys!

 

The Cristianitos Fault controls the stratigraphy found in the lower portion of the cliff.  This is looking north-west from the fault. Here we see the San Mateo Formation overlain by the Marine Boulder Bed, and finally the alluvial sediments are on top.

 

Again, the Cristianitos Fault controls the stratigraphy found in the lower portion of the cliff.  This is looking south-east from the fault. Here we see the Monterey Formation at the base and alluvial sediments on top of the cliff. The Marine Boulder Bed is present here but not exposed.  Notice how different the cliff-face looks at this location compared with the last picture.  The Monterey Formation is very soft and cannot stand vertically like the San Mateo Formation.  So, we have soft sediment slumping dominating the sea-cliffs south-east of the fault.

 

 

Here is the Cristianitos Fault offsetting the San Mateo (5 MYO) and the Monterey (15 MYO) Formations in the lower portion of the cliff.  What about the Marine Boulder Bed and the alluvial sediments?  Let’s take a closer look.

 

The Marine Boulder Bed appears to be continuous above the fault.  So by definition the alluvial sediments must be also be continuous.  Let’s look closer.

 

Again, there appears to be no offset on the Marine Boulder Bed. That means that by using relative age-dating techniques the fault has not moved since the Marine Boulder Bed was deposited.  So, the age of the Marine Boulder Bed is very important to determine the last possible movement on the Cristianitos Fault.  Remember that the fault must have moved once in the last 35,000 years to be considered active. Let’s have a look at how this formed.

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