GeoTrek IV-C

GEOTREK IV-C
CENTRAL SANTA MONICA MOUNTAINS

Saturday, February 26, 2011

Topic: Mesozoic and Paleogene sedimentary and structural history, plus Miocene magmatic stoping in the central Santa Monica Mountains.

Participants: Bob Ableson, Michael Bennett (Mike came from Menlo Park, California to attend), Gene Fritsche, Doug Hammond, Pam Irvine, John Klutke, Carolyn Milkovich, George Milkovich, Jonathan Miller, Syrus Parvizian, Vicki Pedone, Jane Rollins, Martha Sgriccia, Mike Tacsik, Nancy Van Wagoner and Steve Van Wagoner (Nancy and Steve came from Kamloops, British Columbia to attend), and Mike Werner (Mike came from Anchorage, Alaska to attend). See the first photo below (the group photo) to pick out these people.

Distance and Route: About 10.7 miles, mostly in Topanga State Park, along the following trails: Caballero Canyon Trail, Mulholland Drive, Bent Arrow Trail, Fire Road 30, Eagle Springs Fire Road, and East Topanga Fire Road. See Google Earth route map below.

Profile: On the profile below the start of the hike is on the left end. Elevation gain was 1,920 feet; elevation loss was 2,262 feet.

Geology: The hike was on the Dibblee Topanga & Canoga Park (South 1/2) quadrangles (DF-35). Formations seen included the Jurassic Santa Monica Formation, the Cretaceous Trabuco and Tuna Canyon Formations, the Paleocene Santa Susana Formation, the Oligocene Sespe Formation, Miocene intrusive diabase, and the Miocene Topanga Canyon Formation. Depositional environments, stratigraphic problems, and structural history of the area were reviewed, along with a discussion of magmatic stoping in the area.

PHOTOS TAKEN IN THE CENTRAL SANTA
MONICA MOUNTAINS ON GEOTREK IV-C
Photos taken by Michael Bennett, Gene Fritsche, and Mike Tacsik. To see more photos taken by Mike go to https://picasaweb.google.com/metacsik/GeoTrekIVC
Click on the small picture to see an enlargement.

Seventeen participants showed up for GeoTrek IV-C through Topanga State Park. We assembled near the south end of Reseda Blvd. at the Caballero Canyon Trailhead for a short introduction to the day's hike. Carolyn Milkovich was apparently taking a group photo of her own, because she is not in this picture. Compare the list of participants above with the photo to find your friends.
Gene compares features on the Dibblee geologic map on the right under his hand with similar features on the USGS map on the left. Observations made during the hike can be used to confirm the accuracy of the two maps.
Gene traces the route of the hike on the geologic map as the participants look on.
The participants give Gene their rapt attention, eager to learn as much as they can. Carolyn, on the far right, is wondering, "What have I gotten myself into?".
Off we go on the Caballero Trail from Reseda Blvd. to Mulholland Drive on the crest of the Santa Monica Mountains. Everyone is in fine form at this point.
Most of the Caballero Trail is in the Upper Cretaceous Trabuco Formation which is nonresistant and not very well exposed. The Trabuco Formation is a nonmarine, cobble to, in some places, boulder conglomerate that was deposited by strong currents in river and alluvial fan environments.
Along a small portion of the Caballero Trail the Upper Jurassic Santa Monica Formation is exposed. This rock unit began in the Jurassic as a marine, deep-water black shale, but was moderately metamorphosed during the Upper Cretaceous by intrusion of plutonic ignous rocks. The rock is now a dark gray, severely fractured, low-grade slate. The contact between the Santa Monica Formation and the overlying Trabuco Formation is an unconformity that represents several tens of millions of years of uplift and erosion of the Santa Monica Formation before deposition of the Trabuco Formation.
At the crest of the Santa Monica Mountains on Mulholland Drive we took a second group photo with the San Fernando Valley and the San Gabriel and Santa Susana Mountains in the background.
An overview of the eastern San Fernando Valley and the western San Gabriel Mountains from Mulholland Drive.
After our second group photo we headed off westward for a short distance on Mulholland Drive which in this area is a dirt and gravel road within the Santa Monica Mountains National Recreation Area.
At a location on Mulholland Drive overlooking an exposure of the Trabuco Formation, Gene uses the Dibblee map to show the evidence for deposition of the Trabuco Formation in a 400-meter-deep canyon.
Hiking on a shady portion of the Bent Arrow Trail between Mulholland Drive and Fire Road 30.
More hiking on the Bent Arrow Trail. The rocks in this area are conglomerate and sandstone belonging to the Upper Cretaceous Tuna Canyon Formation.
Hiking at the crest and south end of the Bent Arrow Trail near its junction with Fire Road 30. Everyone is still smiling.
Studying a roadcut exposure of the Upper Cretaceous Tuna Canyon Formation along Fire Road 30.
Closeup of the Upper Cretaceous Tuna Canyon Formation. The rock unit here is a cobble to boulder conglomerate. Rare marine fossils in the formation prove that the rock was deposited in shallow ocean water, and the coarseness of the material indicates deposition on a fan delta (an environment near the shoreline where a nonmarine alluvial fan enters the ocean and forms a delta in shallow ocean water).
A hawk pauses on a pole to give us a once over. It's too bad that we can't hover over the geology like it does when we make geologic maps.
Gene taking one of the photos used to record the events of GeoTrek IV-C.
Roadcut exposure of the Paleocene Santa Susana Formation on Fire Road 30 just north of Hub Junction. The Santa Susana Formation is mostly brown, marine mudstone that was deposited below wavebase in an offshore shelf environment. Other parts of the formation, like those seen on GeoTrek IV-A, consist of sandstone containing shallow-water fossils that were deposited closer to the beach.
The Santa Susana Formation contains thin lenses of hard, white limestone beds made of calcareous algal growths.
Closeup view of the algal limestone. The small, rounded white blobs in the rock are the algal growths. The gray material holding the algal growths together as a hard rock is calcareous mud.
Resistant exposure of the upper Eocene, Oligocene, and lower Miocene Sespe Formation. The Sespe Formation is a nonmarine unit that was deposited in river channels and floodplains. Vertebrate fossils in the formation are used to establish its age.
Group walking on the Eagle Springs Fire Road. The dark brown color of the road indicates that it has been cut through lower Miocene igneous rocks, either intrusive diabase or extrusive basalt. In this instance it is nonresistant diabase that was intruded into the overlying older rocks like the resistant sandstone exposures of the Topanga Canyon Formation in the background.
As preparation for explaining the process of magmatic stoping, Gene puts together a volcano made of play-doh. The green and orange layers are the older rock units and the pink mound on top is the volcanic rock that was intruded into and extruded through the older green and orange layers to form the volcano that used to exist in the Santa Monica Mountains about 17 million years ago.
As the hot, melted magma is intruded into the older green layer of the model in the previous photo, the green layer is partially melted and collapses into the melted magma chamber below it allowing the next youngest orange layer to settle down into the melted magma to take the place of where the green layer had been. This process is called magmatic stoping.
The yellow arrow on the left points to a spot on the magmatic stoping model that can be related directly to a spot on the geologic map, shown by the yellow arrow on the right, where magmatic stoping has likely taken place.
As lunchtime approached, the group picked a nice spot where they could see the rocks while they ate. The rocks are part of the lower Miocene Topanga Canyon Formation that was deposited in shallow ocean water.
Everyone is smiling, so either lunch is good, everyone is glad to get off their feet for awhile, or both.
The rocks on which the group is eating lunch belong to the Topanga Canyon Formation and consist of mostly sandstone with some interspersed conglomerate beds. Fossils of marine invertebrates, such as clams (bivalves) and snails (gastropods), are common and aid in proving that the formation was deposited in shallow ocean water during the early Miocene epoch.
Closeup view of the Topanga Canyon Formation. Evidence from fossils indicates that these rocks were deposited in shallow ocean water, most likely within the zone of wave action. Across the center of the photo is a boundary between a lower sandstone layer and an upper pebble conglomerate layer. This boundary indicates that during deposition of the two layers wave velocity increased sufficiently to bring small pebbles to the site while previously only sand was being deposited.
A typical exposure of the lower Miocene Topanga Canyon Formation as seen from the East Topanga Fire Road. This formation is the most resistant formation in the Santa Monica Mountains.
Strolling leisurely along the East Topanga Fire Road toward Trippet Ranch, location of Topanga State Park Headquarters.
A shady oak grove along East Topanga Fire Road provides a little relief from the sun.
The group pauses to check the geologic map for information on the surrounding geology.
This polished rock surface is along a fracture through the Topanga Canyon Formation. Such a polished surface is formed when adjacent rocks on the two sides of the fracture move past one another during faulting. This exposure is on the east side of the East Topanga Fire Road where it is crossed by the Topanga Fault.
Sometimes words are not enough to explain a geologic point. Here Gene draws a diagram on the road to help with the explanation.
The pattern of layering crossing the upper part of the photo is referred to as cross bedding. It was formed in shallow ocean water during deposition of the lower Miocene Topanga Canyon Formation. The current moving on the ocean bottom created moderate-size ripples or dunes that migrated along the bottom. The diagonal layers formed on the downcurrent side of the ripple as the sand washed over the top and down the front face.
The magmatic stoping process led to large volumes of intrusive diabase in the area. Here the group passes a long roadcut through some of the diabase which has the same composition as basalt, but contains larger crystals because it cooled underground.
Normally, diabase weathers very quickly into soft brown sandy clay when exposed to weathering elements at the earth's surface. This exposure along the fire road was cut deeply, however, and exposes between fractures, spheroidally weathered portions of the diabase that are only slightly weathered.
During a previous rain, runoff along the East Topanga Fire Road produced this miniature birdsfoot delta. The moderately sinuous channel is clear. Along the sides of the channel are bulbous levee deposits formed when the water and sand spilled over the sides of the channel. Beyond the levees are flat floodplain deposits formed earlier before the most recent channel and levees advanced over the older floodplain.
Diabase on the left side of the photo and red sandstone of the Sespe Formation on the right side. The contact between the two rock units is in the canyon between the two.
Reddish sandstone and conglomerate outcrop of the upper Eocene through lower Miocene Sespe Formation. The reddish color is the result of oxidation of iron minerals in the sandstone. The color formed because the rock was deposited in a river environment on the earth's surface where oxygen is abundant.
Closeup view of Sespe Formation conglomerate and sandstone. The cobbles were deposited in river channels during flood periods.
Near the end of the hike the group paused at this overlook for a view of the Los Angeles Basin. Santa Monica Bay is on the right, the town of Santa Monica is below the overlook, the Baldwin Hills are on the center horizon, and the Wilshire District buildings on the left.
As we moved into the homestretch the buildings of Westwood came into view. We still had a bit of downhill walking ahead of us before the end, however.
A final view of Santa Monica Bay with the Palos Verdes Hills across the bay in the background. We were blessed by beautiful and clear weather all day long.
When we reached the trailhead at the end of the hike, the road sign even told us where we were. Here we are waiting for Vicki Pedone to arrive with the shuttle van to take us back to our cars.

THIS PAGE LAST MODIFIED ON JUNE 26, 2011
If you have questions or comments, you may leave a message at:
geneandsuef@dslextreme.com

	Seventeen participants showed up for GeoTrek IV-C through Topanga State Park. We assembled near the south end of Reseda Blvd. at the Caballero Canyon Trailhead for a short introduction to the day's hike. Carolyn Milkovich was apparently taking a group photo of her own, because she is not in this picture. Compare the list of participants above with the photo to find your friends.
	Gene compares features on the Dibblee geologic map on the right under his hand with similar features on the USGS map on the left. Observations made during the hike can be used to confirm the accuracy of the two maps.
	Gene traces the route of the hike on the geologic map as the participants look on.
	The participants give Gene their rapt attention, eager to learn as much as they can. Carolyn, on the far right, is wondering, "What have I gotten myself into?".
	Off we go on the Caballero Trail from Reseda Blvd. to Mulholland Drive on the crest of the Santa Monica Mountains. Everyone is in fine form at this point.
	Most of the Caballero Trail is in the Upper Cretaceous Trabuco Formation which is nonresistant and not very well exposed. The Trabuco Formation is a nonmarine, cobble to, in some places, boulder conglomerate that was deposited by strong currents in river and alluvial fan environments.
	Along a small portion of the Caballero Trail the Upper Jurassic Santa Monica Formation is exposed. This rock unit began in the Jurassic as a marine, deep-water black shale, but was moderately metamorphosed during the Upper Cretaceous by intrusion of plutonic ignous rocks. The rock is now a dark gray, severely fractured, low-grade slate. The contact between the Santa Monica Formation and the overlying Trabuco Formation is an unconformity that represents several tens of millions of years of uplift and erosion of the Santa Monica Formation before deposition of the Trabuco Formation.
	At the crest of the Santa Monica Mountains on Mulholland Drive we took a second group photo with the San Fernando Valley and the San Gabriel and Santa Susana Mountains in the background.
	An overview of the eastern San Fernando Valley and the western San Gabriel Mountains from Mulholland Drive.
	After our second group photo we headed off westward for a short distance on Mulholland Drive which in this area is a dirt and gravel road within the Santa Monica Mountains National Recreation Area.
	At a location on Mulholland Drive overlooking an exposure of the Trabuco Formation, Gene uses the Dibblee map to show the evidence for deposition of the Trabuco Formation in a 400-meter-deep canyon.
	Hiking on a shady portion of the Bent Arrow Trail between Mulholland Drive and Fire Road 30.
	More hiking on the Bent Arrow Trail. The rocks in this area are conglomerate and sandstone belonging to the Upper Cretaceous Tuna Canyon Formation.
	Hiking at the crest and south end of the Bent Arrow Trail near its junction with Fire Road 30. Everyone is still smiling.
	Studying a roadcut exposure of the Upper Cretaceous Tuna Canyon Formation along Fire Road 30.
	Closeup of the Upper Cretaceous Tuna Canyon Formation. The rock unit here is a cobble to boulder conglomerate. Rare marine fossils in the formation prove that the rock was deposited in shallow ocean water, and the coarseness of the material indicates deposition on a fan delta (an environment near the shoreline where a nonmarine alluvial fan enters the ocean and forms a delta in shallow ocean water).
	A hawk pauses on a pole to give us a once over. It's too bad that we can't hover over the geology like it does when we make geologic maps.
	Gene taking one of the photos used to record the events of GeoTrek IV-C.
	Roadcut exposure of the Paleocene Santa Susana Formation on Fire Road 30 just north of Hub Junction. The Santa Susana Formation is mostly brown, marine mudstone that was deposited below wavebase in an offshore shelf environment. Other parts of the formation, like those seen on GeoTrek IV-A, consist of sandstone containing shallow-water fossils that were deposited closer to the beach.
	The Santa Susana Formation contains thin lenses of hard, white limestone beds made of calcareous algal growths.
	Closeup view of the algal limestone. The small, rounded white blobs in the rock are the algal growths. The gray material holding the algal growths together as a hard rock is calcareous mud.
	Resistant exposure of the upper Eocene, Oligocene, and lower Miocene Sespe Formation. The Sespe Formation is a nonmarine unit that was deposited in river channels and floodplains. Vertebrate fossils in the formation are used to establish its age.
	Group walking on the Eagle Springs Fire Road. The dark brown color of the road indicates that it has been cut through lower Miocene igneous rocks, either intrusive diabase or extrusive basalt. In this instance it is nonresistant diabase that was intruded into the overlying older rocks like the resistant sandstone exposures of the Topanga Canyon Formation in the background.
	As preparation for explaining the process of magmatic stoping, Gene puts together a volcano made of play-doh. The green and orange layers are the older rock units and the pink mound on top is the volcanic rock that was intruded into and extruded through the older green and orange layers to form the volcano that used to exist in the Santa Monica Mountains about 17 million years ago.
	As the hot, melted magma is intruded into the older green layer of the model in the previous photo, the green layer is partially melted and collapses into the melted magma chamber below it allowing the next youngest orange layer to settle down into the melted magma to take the place of where the green layer had been. This process is called magmatic stoping.
	The yellow arrow on the left points to a spot on the magmatic stoping model that can be related directly to a spot on the geologic map, shown by the yellow arrow on the right, where magmatic stoping has likely taken place.
	As lunchtime approached, the group picked a nice spot where they could see the rocks while they ate. The rocks are part of the lower Miocene Topanga Canyon Formation that was deposited in shallow ocean water.
	Everyone is smiling, so either lunch is good, everyone is glad to get off their feet for awhile, or both.
	The rocks on which the group is eating lunch belong to the Topanga Canyon Formation and consist of mostly sandstone with some interspersed conglomerate beds. Fossils of marine invertebrates, such as clams (bivalves) and snails (gastropods), are common and aid in proving that the formation was deposited in shallow ocean water during the early Miocene epoch.
	Closeup view of the Topanga Canyon Formation. Evidence from fossils indicates that these rocks were deposited in shallow ocean water, most likely within the zone of wave action. Across the center of the photo is a boundary between a lower sandstone layer and an upper pebble conglomerate layer. This boundary indicates that during deposition of the two layers wave velocity increased sufficiently to bring small pebbles to the site while previously only sand was being deposited.
	A typical exposure of the lower Miocene Topanga Canyon Formation as seen from the East Topanga Fire Road. This formation is the most resistant formation in the Santa Monica Mountains.
	Strolling leisurely along the East Topanga Fire Road toward Trippet Ranch, location of Topanga State Park Headquarters.
	A shady oak grove along East Topanga Fire Road provides a little relief from the sun.
	The group pauses to check the geologic map for information on the surrounding geology.
	This polished rock surface is along a fracture through the Topanga Canyon Formation. Such a polished surface is formed when adjacent rocks on the two sides of the fracture move past one another during faulting. This exposure is on the east side of the East Topanga Fire Road where it is crossed by the Topanga Fault.
	Sometimes words are not enough to explain a geologic point. Here Gene draws a diagram on the road to help with the explanation.
	The pattern of layering crossing the upper part of the photo is referred to as cross bedding. It was formed in shallow ocean water during deposition of the lower Miocene Topanga Canyon Formation. The current moving on the ocean bottom created moderate-size ripples or dunes that migrated along the bottom. The diagonal layers formed on the downcurrent side of the ripple as the sand washed over the top and down the front face.
	The magmatic stoping process led to large volumes of intrusive diabase in the area. Here the group passes a long roadcut through some of the diabase which has the same composition as basalt, but contains larger crystals because it cooled underground.
	Normally, diabase weathers very quickly into soft brown sandy clay when exposed to weathering elements at the earth's surface. This exposure along the fire road was cut deeply, however, and exposes between fractures, spheroidally weathered portions of the diabase that are only slightly weathered.
	During a previous rain, runoff along the East Topanga Fire Road produced this miniature birdsfoot delta. The moderately sinuous channel is clear. Along the sides of the channel are bulbous levee deposits formed when the water and sand spilled over the sides of the channel. Beyond the levees are flat floodplain deposits formed earlier before the most recent channel and levees advanced over the older floodplain.
	Diabase on the left side of the photo and red sandstone of the Sespe Formation on the right side. The contact between the two rock units is in the canyon between the two.
	Reddish sandstone and conglomerate outcrop of the upper Eocene through lower Miocene Sespe Formation. The reddish color is the result of oxidation of iron minerals in the sandstone. The color formed because the rock was deposited in a river environment on the earth's surface where oxygen is abundant.
	Closeup view of Sespe Formation conglomerate and sandstone. The cobbles were deposited in river channels during flood periods.
	Near the end of the hike the group paused at this overlook for a view of the Los Angeles Basin. Santa Monica Bay is on the right, the town of Santa Monica is below the overlook, the Baldwin Hills are on the center horizon, and the Wilshire District buildings on the left.
	As we moved into the homestretch the buildings of Westwood came into view. We still had a bit of downhill walking ahead of us before the end, however.
	A final view of Santa Monica Bay with the Palos Verdes Hills across the bay in the background. We were blessed by beautiful and clear weather all day long.
	When we reached the trailhead at the end of the hike, the road sign even told us where we were. Here we are waiting for Vicki Pedone to arrive with the shuttle van to take us back to our cars.