Geology
Regional Geology
Current
Structural Setting
The current structural setting has the San Andreas Fault between the Pacific (San Diego) and North American (San Francisco) Plates (Figure 1a). There are currently no subduction zones located in offshore Southern California as there were in the past and discussed below. There is a divergent plate boundary to the south in the Gulf of Mexico. A transform plate boundary (The San Andreas Fault) extends over much of the State of California. Farther north, there is a convergent plate boundary with subduction off the coast and andesite volcanoes inland.
Past Structural Setting
The features at the site were
formed in very different structural setting (Figures 1b and 1c). Extensive
subduction occurred on the western edge of North America from Early Mesozoic to
Early Miocene time (approximately 200 to 20 million years ago MYA). A
transition from subduction (and plate convergence) to primarily a transform
boundary occurred during Miocene time (from approximately 20 to 5 MYA). This
transform boundary eventually became the San Andreas Fault (Atwater 1998). The
volcanic domes found in Northern San Diego County are thought to be a product
of this tectonic transition from subduction to lateral motion.
Figure 1a: The current plate tectonic setting of the West
Coast of North America. The current San Andreas transform fault.
Figures 1b and 1c: The evolution of the West Coast
of the United States and the transition from subduction to a transform plate
boundary, which becomes the San Andreas Fault. 2nd figure is 20 MYA, 3rd figure
is 40 MYA. Red are divergent boundaries/spreading centers, black sawtooth are
subduction and convergent boundaries, arrows pointing away across faults are
transform boundaries.
Volcanic Domes
Calavera Hills is
an exogenous volcanic dome and is a product of highly viscous, slowly moving
magma (El-Najjar and Camp 2016)(Adkins 2017). Imagine a body of magma being
slowly pushed to the surface and because of the high viscosity lava it’s being
prevented from moving/flowing very far from the center. Many volcanic domes
form during final stages of eruption in composite cone volcanos, where the
thick viscous lava plugs the main vent. Calavera Hills is not related to
composite cone activity.
Both photos are lava domes. Photo 1 is from Mt.
St. Helens in Washington State, its rhyolite in composition and erupted in
1980. Photo 2 is from Novarupta in Alaska which erupted in 1912. Its located on the Alaskan Peninsula about
290 miles southwest of Anchorage, on a slope of the Trident (composite cone)
volcano and composed of rhyolite. For scale this dome is 295 feet high and
1,180 feet wide. As both are rhyolitic
in composition, they both have more silica than Calavera Hills which is
andesite. In volcanic rocks and with increasing silica they go from andesite
(lowest), to dacite and then rhyolite (highest) in composition. The more silica
the more viscous the magma/lava.
These are the No Agua
Volcanic Hills, they are about 30 miles northwest of Taos, New Mexico. It’s a
rhyolite dome that was mined for perlite, it has since been reclaimed. It’s in
a volcanic field but unrelated to composite cone activity, but the mechanics
are similar to the Calavera Hills dome with the exception of its composition.
My first fulltime job after graduating from college was mapping the extent of
the perlite on the flanks of the dome to estimate the remaining ore volume at
the mine.
It was an interesting place, fighter jets would make low level runs above the speed of sound right by dome. It would be quiet, then a massive sonic boom, and many were so close you could actually see the pilots at eye level.
For scale the width of the dome is about 1 mile.
Local
Geology
On this
fieldtrip, we will observe three rock units ranging from Cretaceous to Miocene
in age. Each rock formation/unit is
described below in order of decreasing age.
Make sure you
read and understand the Field Notes, they are important for recognizing
the geologic units in the area.
Figure
1d 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 (Andesite) Volcanic Unit (Tda) is red and
in the middle of the figure (Tan and Kennedy, 1986)
Green Valley Tonalite
(Cretaceous)
The first rock
unit we will see is part of the Peninsular Ranges. It’s an intrusive igneous
unit that was cooled slowly at depth and subsequently uplifted, eroded and
exposed. It’s mapped as a tonalite, which is similar to a granite as its mainly
composed quartz and feldspar (Larson 1948). The difference is that granite is
richer in K spar and tonalite in plagioclase feldspar. This unit has a
phaneritic texture (medium to coarse-grained), which means you can see the
grains with the naked eye and an intermediate color and composition. It exhibits spheroidal weathering. This is
a common type of weathering for this rock type and it gives an “onion skin”
appearance to the exposures.
Field notes: Spheroidal
weathering and pink/red/orange hues on
the weathered outcrop.
Green
Valley Tonalite, this is a fresh surface coarse grained phaneritic texture.
Photo 2 shows spheroidal weathering and pink/red/orange
hues on the weathered outcrop.
Santiago Formation
(Eocene)
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 and Kennedy 1986, 1987, 2005). 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.
Field notes: The Santiago
Formation is white to tan in outcrop and its sediment on the trails.
Eocene
Santiago Formation. On the second photo you can see the very light colored
material on the left where its exposed in the roads. This distinct color makes
it easy to identify.
Calavera
Hills Volcanic Unit (Miocene)
This rock unit is gray in
color and has an aphanitic texture, which means that you cannot see the grains
with the naked eye. While originally mapped as dacite it's actually andesitic
in composition. Andesite is intermediate between a rhyolite and basalt in
composition (El-Najjar and Camp 2016)(Tan and Kennedy 1986, 1987, 2005).
Field notes: A very fine
grained aphanitic texture and medium gray color. Green lichen likes to grow on
these rocks for some reason.
.
Andesitic
rocks associated with the volcanic dome found at Calavera Hills.