Los Peñasquitos
Canyon is one of several canyons in the region where Jurassic
sedimentary rocks can be seen. When hiking along the floor of the
canyon a steeply tilted, Jurassic sedimentary strata will be
revealed. If you approach from Black Mountain Road you will come to
the lowest (oldest) exposed Jurassic strata beginning at a seasonal
waterfalls.
The cross section of sedimentary
rock layers contain a bed of coarsest grains (pebbles), which
settle out first, near the bottom (oldest) with progressively
finer (younger) material overlying it, such as sand and then
mud, forming a graded bed. The currents that transported these
materials deposited them when the energy of the current
subsides The overall changes in grain size within the Jurassic
sedimentary section tell of changes in the depositional
environment through time. The younger layer, such as mud,
deposited slowly from suspension in seawater during quite
conditions. This layer generally has evidence of sponge
spicules (silica), sea urchins spines, and oyster-shell
fragments which are evidence that the layer was deposited in
the marine environment.
The sediments are originally deposited in horizontal layers, the
San Diego Jurassic strata are no longer horizontal but now steeply
inclined due to ancient compressive forces (Law of Original
Horizontality). The strata in this canyon are tilted 48 degrees
toward the west. In a sequence of sedimentary rock layers, the
lower strata are older than the upper strata deposited on top of
them (Law of Superposition). Due to the tilt of the strata,
the lowest (oldest) rocks are encountered upstream at higher
elevations. As you go downstream, you "climb up" the sedimentary
section through progressively younger rock layers. So, you walk
downhill and, at the same time, climb up the sedimentary
section.
Near the waterfall you will notice thick beds of granule- to
boulder-sized conglomerate containing angular pieces of volcanic
rocks. A conglomerate containing angular pieces of rock is commonly
called a breccia. If you look carefully you can see rare belemnite
fossils (the white calcite skeletal cones that formed part of the
internal skeletons of an extinct squid-like).
The Jurassic strata found in San Diego were dated after Perry
Crampton discovered marine clam fossils in some of the sandstone
beds (Fife, Minch, and Crampton 1967).These fossils date from
latest Jurassic time, 151-142 millions years ago, and are some of
the oldest fossils found in the San Diego urban area.
The composition of San Diego's Jurassic sedimentary rocks consists
of small pieces of volcanic rocks (47%), plagioclase feldspar
(47%), volcanic quartz (5%), and potassium feldspar (1%). There is
little or no continental debris evident, suggesting that the
sediments came from erosion of volcanic islands rising out of the
sea (Balch, Bartling, and Abbott, 1984).
TO LOG THIS
CACHE:
1. Photograph yourself with an
example of nearby "breccia" or find and photograph some of the
white "belemnite fossils".
2. Tell me if the
distribution of fossils in these rocks are "rare" or "abundant".
How many can you find in one square foot?
To log this cache e-mail me
the
answer
HERE
.
Sources:
- Abbot, P.L. 1999. The Rise and Fall of
San Diego: 150 Million Years of History Recorded in Sedimentary
Rocks. Sunbelt Publications, San Diego. 231p..
- Balch, D.C., S. Hosken Bartling, and P.L.
Abbott, 1984. Volcaniclastic strata of the Upper Jurassic Santiago
Peak Volcanics, San Diego County, CA. In Tectonics and
sedimentation along the California margin, ed. J.K. Crouch and
S.B. Bachman, p.157-170. Pacific Section, Society of Economic
Paleontologists and Mineralogists.
- Fife, D.L., J.A. Minch, and P.J.
Crampton. 1967. Late Jurassic age of the Santiago Peak Volcanics,
California. Geological Society of America Bulletin, v.78,
p.299-304.
Law of Superposition: The law
states that strata that are younger will be deposited on top of
strata that are older, given normal conditions of deposition. This
law is the guiding principle of stratigraphy, or the study of
geological or soil layers. Stratigraphy is still the single best
method that archaeologists have for determining the relative ages
of archaeological materials. Law of Original Horizontality Most
sediments, when originally formed, were laid down horizontally.
However, many layered rocks are no longer horizontal. Because of
the Law of Original Horizontality, we know that sedimentary rocks
that are not horizontal either were formed in special ways or, more
often, were moved from their horizontal position by later events,
such as tilting during episodes of mountain
building.
