An unconformity is a contact between rocks of
different ages that represent a period of geologic history with no
rock record. This can either be from no rock being deposited or
from rock being eroded away before more rock forms on top of the
eroded surface.
Geologists classify unconformities based on the orientation of
layers of rock above and below the contact. There is
paraconformities where the layers of rock above and below the
contact are roughly parallel to the contact. In a disconformity the
layers above and below the contact are also roughly parallel, but
the contact itself is not parallel to layers, but instead cuts
across the layers or cuts down into the rock like a river channel.
However neither of these describes the contact at this
location.
Here the layers of rock below the contact, the Timms Point Silt
are tilted at an angle and are cut of horizontally by the contact.
Above the contact is the roughly horizontal Pales Verdes Sand. This
type of contact is called an angular unconformity.
This configuration of rocks began in the Lower (?) Pleistocene
(about 700,000 years ago, although the literature does show a wider
range) with the deposition of the Timms Point Silt in about 151 to
600 feet of water. These silts were then buried and turned into
rock. Tectonic forces then brought the layered rock to the surface
tilting the rock in the process and where it was eroded. Then the
ocean covered the area depositing the Pales Verdes Sand in the
Upper Pleistocene (about 86,000 to 130,000 years ago) on top of
what was left of the Timms Point Silt. The unconformity represents
the time between the deposition of the Timms Point Silt and the
deposition of the Pales Verdes Sand, some where around 570,000
years. The rock that was deposited during this time period, if any
was, was eroded away leaving no geologic record of what happened in
the area.
Logging requirements:
Send me a note with :
- The text "GCZF18 Urban Unconformity" on the first line
- The number of people in your group.
- Take a close look at the contact and see if the contact is flat
or if there are holes in it.
- suggest possible reasons for your observation
The above information was compiled from the
following sources:
- Late Pleistocene angular
unconformity at San Pedro, CA, J. Douglas Yule, and Donald H.
Zenger, Department of Geology, Pomona College, Clarement, CA 91711,
Geolgocal Society of America Centennial Field Guide –
Cordilleran Section, 1987
Additional Information: There are additional exposures in
the alleyway west of the site that show the Lomita Marl. The San
Pedro Sand was exposed to the west, but has been removed by
construction. The Pales Verdes Sand is marine terrace sediment
derived from a transgression that carved out a number of marine
terraces in the area. The dates referenced above have been
simplified. The literature provides a much wider range of dates
based on a variety of methods. A variety of fossils are present in
these outcrops
