Trabuco Formation in the Santa Monica Mountains


The Trabuco formation is a land-laid deposit of mid-Cretaceous age (about 95 million years old). The environment was probably a series of alluvial fans formed at the base of mountains. This is a high-energy type of environment as shown by the large size of many of the clasts and the poor sorting. The maximum transportation distance from the source was probably about 5 miles. No fossils have yet been found and probably won't be due to the depositional environment.



In 1916, Earl L. Packard proposed the name Trabuco formation based on deposits exposed in Trabuco Canyon in the Santa Ana Mountains. The following is from the USGS Bulletin 1200, Lexicon of Geologic Names of the United States for 1936-1960, Part 3, P-Z, by Grace C. Keroher:



The formation has a typical reddish color to it. Many clasts in the conglomerate have a high sheen. This may have been caused when the matrix rubbed against the clasts when the formation was deformed - "tectonic polishing". Many of the clasts have been sheared or fractured and then re-cemented. Some look like someone cut them in sections and shifted the sections. Also, some clasts are both polished and sheared.

With thanks to paleontologist/geologist John Alderson, who introduced and explained this formation (and the whole Santa Monica Mountains) to me.


Map showing the age of the Trabuco (in yellow). The youngest formations are on top and the oldest on bottom. (From the "Santa Monica Mountains National Recreation Area Paleontological Survey" of 2004)


Typical reddish color. The red is due to oxidized iron (usually forming the mineral hematite) coating the individual grains of the rocks in an arid climate on the land. In other words, the rocks rust. Also, note the clast in the middle of the picture above my hammer with the high sheen.


This large clast had a section sheared off.


This small clast has been sheared. Note the small faults and fractures in the light-colored sandstone. The shear zone and one of the faults is on the same plane.


Hammer on red sandstone bed in lower part of picture. Above that is a conglomerate layer, a light colored sandstone layer, and another conglomerate layer. Layering in the Trabuco Formation is not common and the layers do not extend for long distances.


Reddish sandstone layer


The two clasts in the circle have been sheared about 2 inches due to a small vertical fault.


More layering


Trabuco Formation contact with overlaying Tuna Canyon Formation. The Tuna Canyon Formation is a marine deposit of late Cretaceous age (about 90 million years old). This location gets a lot of water run-off, which stains the rocks white. The Trabuco looks to be in fault contact with the Tuna Canyon formation in this area.


Closer view of above picture.


Sample polished clast. Note the flat sides. This piece has a cross-section of a trapezoid and all the sides are equally polished. A stream polishing explanation seems out of the question.


These two samples have been fractured and re-cemented. The piece on the right has also has a high sheen. The piece on the left looks like someone cut it into pieces and shifted the pieces.


Sample piece on left in previous picture. Note how the sections have been shifted.


In the Santa Monica Mountains, the Trabuco Formation only occurs in a thin slice adjoining the Santa Monica Slate on its east. The main road into the area is the south end of Reseda Blvd from the San Fernando Valley. The following is section of the Geologic Map of the Topanga and Canoga Park (South 1/2) Quadrangles (DF-35) by Thomas Dibblee, 1992. Dibblee used many sources besides himself to fill in his map, including John Alderson. Kcr is the Trabuco Formation. The Santa Monica Slate is sms. The other green color formations to the left (west) of the Trabuco are the Tuna Canyon Formation. The Modelo Formation are the light orange sections (Tm, Tmsi, Tmss, Tmcg). There are other formations on the map. Dibblee Geological Foundation maps can be purchased online here.