The red rocks that are found in this area are part of a
thick deposit of sedimentary rocks called the Sespe Formation.
The Sespe Formation is made up of conglomerate, sandstone, and
siltsone and can be found throughout Southern California all
the way north to at least Ventura County.
The Sespe is a nonmarine formation that was deposited during a
significant global drop in sea level. At this location, you will
find white to red sandstone. The red coloration tends to be higher
up the cliffs. Layering is easily visible in the sandstone and is
accentuated by common layers of 1 to 3 inch (or larger) rounded
cobble layers. You can get a good look at a series of these layers
on your way back out at N 33 42.135 W 117 30.960. The changes in
grain size, from sand to large cobble and back, represent changes
in amount of energy in the the environmental conditions of the area
at the time when the Sespe was forming (the depositional
environment).
Micro fossils have been found in the Sespe. Fragments of jaws
and teeth of small land vertebrate fossils were found in the Sespe
during the construction of the nearby Eastern Transportation
Corridor and the Bowerman Landfill.
The Sespe is thought to range in age from approximately 41 to 28
million years ago (from sometime in the Eocene to Early Miocene).
However, though detailed fossil analysis, it was determined that
there was no rock record in the Sespe for about 10 million of those
years Fossils characteristic of the Eocene and Early Miocene were
identified in the Sespe, but fossils indicative of the end of the
Eocene and much of the Oligocene (from about 40 to 29.5 million
years ago were not identified. This means that there is an
unconformity, a contact between rocks of different ages that
represents a period of geologic history with no rock record in the
middle of the Sespe.
Unconformities can further differentiated based on how the
layers above and below the unconformity relate to each other and
the shape of the unconformity. In this case the layers above and
below the unconformity are roughly parallel. However, since
unconformity is identified primarily by fossils, it is difficult to
determine location and shape of the unconformity just by looking at
it.
That leaves two possible types of unconformities, a
paraconformity and a disconformity. In a paraconformity the
unconformity is roughly parallel with the rock layers. In a
disconformity the unconformity is not parallel to layers, but
instead cuts across the layers or makes curves like a river. It is
possible that the type of unconformity could change across wide
distances.
Another type of unconformity is an angular unconformity. In this
type of unconformity, the layers of rock are at an angle to the
contact while the other layers are parallel to the contact.
The Sespe Formation ends with the return of a shallow sea and
the deposition of the shallow marine sands of the Vaqueros
Formation.
Logging requirements:
Send me a note with :
- The text "GCQ0ZN Red Rock Canyon - Whiting Ranch Wilderness
Park" on the first line
- The number of people in your group.
- If this canyon were to be filled by horizontal layers of mud
that later turned to rock, what kind of unconformity would be
observed between the layers of rock you see at the coordinates and
the hypothetical mudstone.
The following documents were used to generate
this cache:
- Orange County Archaeology and
Paleontology Guidelines, Procedures, and
Policies(http://www.ocparks.com/uploadgraphics/OverviewFinalReport2.pdf)
- Preliminary Digital Geological Map of the
30'X60' Santa Ana Quadrangle, southern California, Version 2.0
Compiled by D.M. Morton, Open File Report 99-172, Southern
California Aerial Mapping Project
- Lithostratigraphic, Biostratigraphic, and
Magnetostratigraphic Documentation of a Major Unconformity in the
Middle Eocene to Early Miocene Continental Sespe Formation,
Northern Santa Ana Mountains, Orange County, Southern California by
Calvano, Gino, E. Bruce Lander, David P. Whistler, Mark A. Roeder,
Joshua Ludtke, Donald R. Prothero, Stephen L. Walsh, and Hugh M.
Wagner; AAPG Pacific Section / SPE Western Regional; Joint Meeting;
May 19-24, 2003, Long Beach, California, U.S.A.
(http://www.searchanddiscovery.com/documents/abstracts/spe_aapg2003/calvano.pdf)
-
http://geology.er.usgs.gov/paleo/geotime.shtml