Pico Canyon Geology
Pico Canyon is the most westernly of all the canyons in the Newhall Oil District. The Pico Anticline is spectacularly exposed everywhere.
All of the productive Pico wells were drilled on the north flank of the anticline. The early oil men noticed what they called a "line of break". This is probably part of the axis of the anticline. No productive wells were found south of it. The "break" is visible from the hairpin curve if you look east at the exposed cliff (see photo below). Winterer and Durham (1962) also map a north-south fault through this area just west of the exposed "break". This fault could explain why the creek was able to cut perpendicularly through the anticline here. The fault would have weakened the rocks allowing water to erode through them.
As you drive down Pico Canyon Road and pass Stevenson Ranch Parkway you are already in the Pico Formation. When you park in the Mentryville pay parking lot you are still in the Pico Formation. On the geologic time scale (see chart on right), it is middle to late Pliocene. It was deposited in a shallow marine environment and consists mainly of mostly light gray siltstone and claystone, sandstone (sometimes pebbly), and conglomerate. The Pico Formation was first named in 1924 by W.S.W. Kew based on exposures in Pico Canyon, hence the name. I have not found any documented megafossils (fossils visible to the un-aided eye) from the Pico Formation in Pico Canyon south of Mentryville.
On the hike out of Mentryville, just past the bakery, you enter the marine Towsley Formation of early Pliocene age. This formation consists of mostly light gray to tan sandstone, medium grained to locally gritty and pebbly. There is also some claystone and siltstone. All the Pico wells were spudded in the Towsley Formation. I have also not found any documented megafossils in the Towsley Formation in Pico Canyon south of Mentryville.
As you hike deeper into the canyon the Towsley formation rocks become older and older until about the hairpin curve. There the age remains about the same as you climb higher and higher. As you get to the top ridge, the axis of the anticline is below your feet. The axis contains the upper Miocene Modelo Shale, but it is not exposed in Pico Canyon.
First good view of north flank of the Pico anticline as you hike up the main road
This is still the north flank only the view is to the northeast on CSO Hill
More of the north flank only these beds were eroded in an interesting way. The sandstone beds have different degrees of hardness causing them to weather like this.
Same beds from farther away
View toward the west on CSO Hill showing the lower beds getting more contorted as they near the axis of the anticline
This view if from the top of CSO Hill looking southwest at the head of the next canyon (Salt Canyon). This must be the south flank of the Pico Anticline. Notice the offset of the beds near the bottom - a amall fault.
In this photo we are looking west from the road just before CSO Hill showing the overturned beds at the top of the ridges of the north flank of the anticline
Nearly the same view from 1905 (From "The Santa Clara Valley, Puente Hills and Los Angeles Oil Districts, Southern California", George Eldridge and Ralph Arnold, United States Geological Survey, Bulletin No. 309, 1907)
Looking west with the road far below. The jagged ridges are caused by differential weathering of the different beds of the Pico anticline. The beds are nearly vertical here in the north flank.
View toward the west of the north flank from further away and much higher up then the previous photo. Some of the beds are the same.
Here is the "break". CSO 2 was located in this area. No productive wells were found right (south) of here. You can see folding, faulting, and all sorts of contortions of the beds.
North flank with my black backpack on the bottom center. These beds were originally horizontal when they were deposited.
Closer view of beds. This is the Towsley Formation.
The beds on the left are alternating shale and sandstone. Suddenly, on the right, the beds change to a large unit of conglomerate. Does this mean the ocean regressed, causing a quiet, deeper, environment to become shallower and closer to the shoreline?
Unusual erosional surface in the Towsley Formation. Erosion can happen in the sea as sediments flow down submarine canyons.
Here are the same beds from a little further away
Up the road from the previous photo something unusual is happening here. We are beginning to hike out of the north flank and into the south flank. As you hike higher on the road, you should notice the change in direction of the slope of the beds.
Here is a fault where we start to hike in the south flank of the anticline. The fault runs diagonally through the photo crossing the road at my black backpack.
Finally, as we near well Odeen 1, we are in the south flank. This view is toward the east showing south flank beds.
This landslide is hard to miss as you hike up the canyon before you get to Johnson Park. As you are driving up to the parking lot after you turn off of Pico Canyon Road you can see a lot of slides in the steep canyons.
I previously mentioned that I saw no documented megafossils in Pico Canyon. However, here is part of what is probably part of a pecten that I found in a pebbly sandstone in the Towsley Formation within site of CSO 4. There were a few other shell fragments in this same piece. Unfortunately, the pecten is too incomplete to be positively identified.
In the same bed as the pecten, I found this vertebrate bone fragment. Many bones have been found in the Towsley Formation in Elsmere Canyon, so it would not be unreasonable to find one in Pico Canyon. It is probably from a whale but, like the pecten, there is probably not enough material left for identification.
Here is some fossil charcoal in sandstone I found next to the road. The charcoal must have washed out to sea after a fire indicating that at least this part of the Towley Formation was reasonable close to shore.