GeoTrek IV-A

GEOTREK IV-A
WESTERN SIMI HILLS

Saturday, February 12, 2011

Topic: Mesozoic and Cenozoic sedimentary and structural history of the Simi Hills.

Participants: Bob Ableson, Carmen Caceres-Schnell, Abraham Chavez, Adriano Defreitas, Gene Fritsche, Doug Hammond, Roberta Harma, Pam Irvine, Dennis Keating, Kathleen Keating, Carolyn Milkovich, George Milkovich, Andy Modugno, Syrus Parvizian, Vicki Pedone, Rebecca Rodriguez, Jane Rollins, Jon Schwalbach, Leo Stricker, Mike Tacsik, Christa Wolfe, and Jeffrey Zane. See the first photo below (the group photo) to pick out these people.

Distance and Route: About 10.1 miles, mostly in the Cheeseboro/Palo Comado section of the Santa Monica Mountains National Recreation Area. From the Cheeseboro trailhead we went north on the Modelo Trail to the Palo Comado Trail and continued north to China Flat. From China Flat we went north to the Albertson Motorway and west on the Albertson Motorway to the trail to Wood Ranch. We finished at the Wood Ranch trailhead on the southern edge of Simi Valley. See Google Earth route map below.

Profile: On the profile below the start of the hike is on the right end. Elevation gain was 1,782 feet; elevation loss was 1,675 feet.

Geology: The hike began on the Dibblee Calabasas quadrangle (DF-37) and ended on the Dibblee Thousand Oaks quadrangle (DF-49). Below is the hike route drawn on the Dibblee maps that have been overlain on the Google Earth image. Formations seen included the Cretaceous Chatsworth Formation, the Paleocene Santa Susana Formation, the Eocene Llajas Formation, and the Miocene Calabasas(?) and Monterey Formations. Depositional environments, stratigraphic problems, and structural history of the area were discussed along the trail.

PHOTOS TAKEN IN THE WESTERN
SIMI HILLS ON GEOTREK IV-A
Photos taken by Gene Fritsche, Adriano Defreitas, and Mike Tacsik. To see more photos taken by Adriano go to http://www.edmar-co.com/adriano/field/geotrek4a/geotrek4a.html. To see more photos taken by Mike go to https://picasaweb.google.com/metacsik/GeoTrekIVA?feat=email.
Click on the small picture to see an enlargement.

GeoTrek IV-A across the Simi Hills drew 22 participants, the largest group to date ever to attend a single day GeoTrek hike. Here's the group at the Cheeseboro trailhead eager to get going on the trail.
Once underway uphill toward the rising sun, our group took up quite a piece of the trail.
At a good exposure of the Monterey Formation along the trail just a short distance from the parking lot Gene stopped to discuss the Monterey Formation with the group.
Trailcut exposure of Monterey Formation shale. This rock unit is an upper Miocene, thin-bedded clay shale that weathers light gray. It is exposed throughout a large portion of the coastal ranges of California. It is a prolific source rock for much of the oil in California and also serves as a reservoir rock when it is highly fractured.
Gene reveals the planar trend of the rocks exposed along the trail. The group was trying to locate the hinge of an anticline that crossed the trail in this vicinity.
Trail exposure of diatomite (a light-weight shale made mostly of microscopic plant fossils called diatoms rather than clay particles). Discussion here centered on whether this rock unit is still part of the Monterey Formation or belongs to a younger upper Miocene unit called the Sisquoc Formation.
The striped pattern in the grassland on this hillslope is the result of differences in the underlying soil that are a reflection of the underlying shale rocks. Such vegetation patterns reveal to geologists the structure of the underlying rocks.
The group stopped at this rockpile to practice identifying rocks. Several different kinds of rocks were found in the pile.
Gene points out the location of the group on the Tom Dibblee geologic map. This was done to compare the geologic interpretation of the group at this site with the interpretation of Tom Dibblee.
Basal conglomerate of the upper Miocene Calabasas Formation. Pebbles and cobbles in the conglomerate consist of sandstone (S) and mudstone (M) fragments eroded from the underlying Chatsworth Formation and quartzite (Q), granite (G), basalt (B), and other rock types eroded and reworked from conglomerate beds in the Chatsworth Formation and other older rock units.
Resistant sandstone exposure of the upper Cretaceous Chatsworth Formation. The Chatsworth Formation was deposited mostly by turbidity currents and debris flows in relatively deep ocean water on a submarine fan.
Hiking through exposures of the upper Cretaceous Chatsworth Formation. Most of the Chatsworth Formation, as shown here, consists of thick-bedded sandstone strata.
This exposure of the Chatsworth Formation reveals some of the thin-bedded sandstone and mudstone strata that occur in the formation. These beds contain microfossils that are used to determine the age of the formation.
As we strolled along the trail, unaware of any lurking danger, a Simi Rockadile suddenly lurched out of his/her lair and tried to swallow Gene. Fortunately Gene leaped out of the way in the knick of time.
At lunchtime a single oak tree provided shade for the group. After a nourishing lunch we were eager to be on our way again.
Ceanothus bushes were common and in full bloom along the trail.
Prickly Phlox was also a common flower along the trail.
Studying a typical resistant sandstone exposure of the upper Cretaceous Chatsworth Formation.
Gene and Jon Schwalbach search an outcrop of the upper Cretaceous Chatsworth Formation for evidence of deposition by turbidity currents.
Some convoluted bedding in the upper Cretaceous Chatsworth Formation.
Fossils were common in the Paleocene Santa Susana Formation that was deposited in a shallow-marine environment. Studying the fossils (from left to right) are Doug Hammond, Roberta Harma, Pam Irvine, Gene, and Jon Schwalbach.
Fossils in the Paleocene Santa Susana Formation consist of broken fragments of both marine clams (bivalves) and snails (gastropods).
Near the end of the hike we posed again for a second group photo with Lake Bard, the western Simi Valley, Big Mountain and Oak Ridge, and the Topatopa Mountains in the background.

Return to the GeoTrek IV index.
THIS PAGE LAST MODIFIED ON JUNE 9, 2011
If you have questions or comments, you may leave a message at:
geneandsuef@dslextreme.com

	GeoTrek IV-A across the Simi Hills drew 22 participants, the largest group to date ever to attend a single day GeoTrek hike. Here's the group at the Cheeseboro trailhead eager to get going on the trail.
	Once underway uphill toward the rising sun, our group took up quite a piece of the trail.
	At a good exposure of the Monterey Formation along the trail just a short distance from the parking lot Gene stopped to discuss the Monterey Formation with the group.
	Trailcut exposure of Monterey Formation shale. This rock unit is an upper Miocene, thin-bedded clay shale that weathers light gray. It is exposed throughout a large portion of the coastal ranges of California. It is a prolific source rock for much of the oil in California and also serves as a reservoir rock when it is highly fractured.
	Gene reveals the planar trend of the rocks exposed along the trail. The group was trying to locate the hinge of an anticline that crossed the trail in this vicinity.
	Trail exposure of diatomite (a light-weight shale made mostly of microscopic plant fossils called diatoms rather than clay particles). Discussion here centered on whether this rock unit is still part of the Monterey Formation or belongs to a younger upper Miocene unit called the Sisquoc Formation.
	The striped pattern in the grassland on this hillslope is the result of differences in the underlying soil that are a reflection of the underlying shale rocks. Such vegetation patterns reveal to geologists the structure of the underlying rocks.
	The group stopped at this rockpile to practice identifying rocks. Several different kinds of rocks were found in the pile.
	Gene points out the location of the group on the Tom Dibblee geologic map. This was done to compare the geologic interpretation of the group at this site with the interpretation of Tom Dibblee.
	Basal conglomerate of the upper Miocene Calabasas Formation. Pebbles and cobbles in the conglomerate consist of sandstone (S) and mudstone (M) fragments eroded from the underlying Chatsworth Formation and quartzite (Q), granite (G), basalt (B), and other rock types eroded and reworked from conglomerate beds in the Chatsworth Formation and other older rock units.
	Resistant sandstone exposure of the upper Cretaceous Chatsworth Formation. The Chatsworth Formation was deposited mostly by turbidity currents and debris flows in relatively deep ocean water on a submarine fan.
	Hiking through exposures of the upper Cretaceous Chatsworth Formation. Most of the Chatsworth Formation, as shown here, consists of thick-bedded sandstone strata.
	This exposure of the Chatsworth Formation reveals some of the thin-bedded sandstone and mudstone strata that occur in the formation. These beds contain microfossils that are used to determine the age of the formation.
	As we strolled along the trail, unaware of any lurking danger, a Simi Rockadile suddenly lurched out of his/her lair and tried to swallow Gene. Fortunately Gene leaped out of the way in the knick of time.
	At lunchtime a single oak tree provided shade for the group. After a nourishing lunch we were eager to be on our way again.
	Ceanothus bushes were common and in full bloom along the trail.
	Prickly Phlox was also a common flower along the trail.
	Studying a typical resistant sandstone exposure of the upper Cretaceous Chatsworth Formation.
	Gene and Jon Schwalbach search an outcrop of the upper Cretaceous Chatsworth Formation for evidence of deposition by turbidity currents.
	Some convoluted bedding in the upper Cretaceous Chatsworth Formation.
	Fossils were common in the Paleocene Santa Susana Formation that was deposited in a shallow-marine environment. Studying the fossils (from left to right) are Doug Hammond, Roberta Harma, Pam Irvine, Gene, and Jon Schwalbach.
	Fossils in the Paleocene Santa Susana Formation consist of broken fragments of both marine clams (bivalves) and snails (gastropods).
	Near the end of the hike we posed again for a second group photo with Lake Bard, the western Simi Valley, Big Mountain and Oak Ridge, and the Topatopa Mountains in the background.