Local Geology

 

Local Geology

On this fieldtrip, we will observe three rock units ranging from Cretaceous to Miocene in age.  Each unit is described below in order of decreasing age.

 

 

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Geologic map showing the Calavera Hills Volcanic Plug. The Plug is about 1/2 mile across and is surrounded on the north, south and east by the Santiago Formation and on the west by the Green Valley Tonalite.

Volcanic Plug
 
Figure #2a: Geologic map of the Calavera Hills area. The Cretaceous Green Valley Tonalite  (Kg (gv)) is shown in redish-pink, Eocene Santiago Formation (Tsa) is shown in tan, and the Miocene Dacite Volcanic Plug (Tda) is red and in the middle of the figure (Tan, 1996).

 

Green Valley Tonalite (Cretaceous)(Larson 1948)

The first rock unit we will see is part of the Peninsular Ranges. It is an intrusive igneous unit that was cooled slowly at depth and subsequently uplifted, eroded and exposed. It is mapped as a tonalite, which is inbetween a granite and a diorite.  This unit has a phaneritic texture (medium to coarse-grained), which means you can see the grains with the naked eye.

 

This picture shows the realtively coarse grain size of the Green Valley Tonalite.

Above Picture:  Green Valley Tonalite exposed at Stop #1. Note the medium to coarse-grained (phaneritic) texture.

 

Santiago Formation (Eocene)(Tan 1986, 1987)

In the Calavera Hills area, the Santiago Formation is very soft and for this reason there are only a few outcrops in the area. The local extent of the formation is primarily based upon the light colored material it generates from weathering. The Santiago Formation was deposited as a beach sand. The sandstone is fine to medium grained, poorly cemented, poorly bedded, and is interbedded with siltstone and claystone that is locally prone to significant slumping. The Santiago Formation is not to be confused with the Santiago Peak Volcanics that are much older.

This formation was renamed from the Scripps Formation locally in the mid-1980s (Tan 1986, 1987). It is found regionally from Orange County to Northern San Diego County where it begins to interfinger to the south of Leucadia with the Torrey Sandstone.

TThis picture shows the white eroded material from the Santiago Formation surrounding the volcanic plug.

Above Picture:  : Looking to the south toward the east side of the volcanic plug (right). The Santiago Formation as seen in the Calavera Hills area. There are few outcrops in the area and the local extent is primarily based upon the light colored material it generates from weathering.

 

Volcanic Plug (Miocene)

This rock unit is “mouse gray” in color and has an aphanitic texture, which means that you cannot see the grains with the naked eye. The rocks shown here are from the volcanic neck and are dacitic in composition. A dacite is between a rhyolite and an andesite in composition.

This picture shows examples of the grey, fine grained volcanic rocks found at Calavera Hills.

Above Picture:  Fine-grained (aphanitic) dacitic rocks associated with the volcanic plug found at Calavera Hills.