Remember there is no container to find here!
Rattle snakes have been seen in the area (and around other, nearby caches), so be careful. Make noise as you walk and be careful as you poke around rocks and crevices. This is a natural preserve. All natural features are protected.
This EarthCache is at the Type Section of the Altamira Shale Member of the Monterey Formation. That sentence likely makes most non-geologists' eyes glaze over as it has quite a bit of jargon that makes it sound complicated. Fear not, the description will break these concepts down so hopefully you will be able to figure out some other EarthCaches.
The first concept is the Type Section. A Type Section or Type Locality is the outcrop where a named stratigraphic unit where it is well exposed and described to be used as a standard of reference for their identification. These outcrops have as complete a geologic record as possible for the unit and have most, if not all, of the unique characteristics of that unit. In essence, if you see a rock somewhere else, compare it to this outcrop to see if it should be called the same thing.
Now comes the Member and Formation terminology. These are just two of the groupings used to describe a cohesive set of rock units. The other typical names are:
- Group - two or more formations
- Formation - primary unit
- Member - named subdivision of a formation
- Bed - named distinctive layer in a member or formation
- Flow - smallest distinctive layer in a volcanic sequence
As you can see, the hierarchy of names goes from big to small. So in this case, the Altamira Shale is just one part of the Monterey Formation. If you came up to this exposure and called it the Monterey Formation, you would be just as correct as if you called it the Altamira Shale. It is analogous to biological classifications, such as calling a Douglas Fir a Pine or a Tree.
An additional term that I will use later is lithofacies. This just means a distinctive set of rocks within one of groupings above that can be distinguished from another set of rocks within the same grouping by the make of up the rock. For example, within a Formation, one lithofacies could be a coarse-grained sand while another lithofacies is fine-grained sand.
Now to describing the Members, starting with the road cut closest to the end of Crenshaw Blvd..
The first and second road cuts (near Waypoint R1) are the Valmonte Diatomite. This rock was deposited during the upper Miocene at the bottom of the ocean. Diatomite is a rock made up of the fossilized remains of diatoms. Diatoms are single-celled aquatic algae that use silica to build their skeleton. When they die, the silica skeleton falls to the bottom of the water body, in this case the ocean, and gets buried under more skeletons. This exposure has layers of white to gray, relatively soft diatomite with thin 2 to 10 cm thick layers of grayish brown hard chert. Chert is also made primarily of silica, but its density and crystalline structure are much harder.
The next exposures are different parts of the Altamira Shale.
Near the posted coordinates is alternating layers of diatomaceous siltstone and shale (siltstone or shale with a significant poriton of diatomite) and phosphatic mudstone (mudstone with lots of phosphate). Periodically fine-grained sandstone beds are interspersed among these layers. These sand layers may have come from the Point Fermin submarine canyon fan. This is the phosphatic lithofacies of the Altamira Shale.
Downhill from the coordinates (Waypoint R4) the beds are characterized by thin beds of chert and siliceous shale (shale with lots of silica in it). The beds are easy to see in this section with orange to black iron oxide staining. This portion is called the cherty lithofacies of the Altamira Shale.
The lowest exposures (Waypoint R5) along the road cuts exposes layers of porcelanite (diatomaceous sediments that are a mixture of silica and mud that look like unglazed porcelain), silty and sandy shale, basalt, and tuff. Some of the layers appear folded. This section is called the tuffaceous lithofacies of the Altamira Shale.
Having studied this Type Section, you should be able to identify the Altamira Shale anywhere else it is exposed by compare it to this one.
Logging requirements:
Send me a note with:
- The text "GC57P9N Altamira Shale Type Section" on the first line
- Which of the lithofacies of the Altamira Shale is nearest to the coordinates?
- Approximately how thick are the layers in the cherty lithofacies?
The following sources were used to generate this cache:
- International Commission on Stratigraphy, http://www.stratigraphy.org/upload/bak/strats.htm http://www.stratigraphy.org/upload/bak/litho.htm
- The Industrial Minerals Association - North America http://www.ima-na.org/?page=what_is_diatomite