Geology and Tectonics of Santa Catalina Islandand the California Continental Borderland

South Coast Geological Society2004 Annual Field Trip

23-24 October, 2004

Aerial view of Santa Catalina Island looking northwest (Photo provided by Paul Davis).

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Santa Catalina IslandField Trip Road Log

Day 1 [October 23, 2004]

Begin – Catalina Express boat trip from San Pedro terminal to Avalon. The Palos Verdes Hills are to the northwest and the Palos Verdes fault zone passes beneaththe Vincent Thomas Bridge near the east abutment. The West Turning Basin, to the northof the Catalina Express Terminal, is considered to be situated over a small pull-apart basinwhere the Palos Verdes fault makes a short right step-over before bending to the left aroundthe northeast flank of the Palos Verdes Hills. From San Pedro to Avalon, the boat will crossa major deep Borderland basin, the San Pedro Basin and the fault zone with the same name.Oblique extension along the San Pedro Basin fault zone has created this elongate deepregion, and an active pull-apart basin lies almost halfway between the Palos Verdes Hills andSanta Catalina Island. As the boat approaches Avalon, note the rugged coastline of SantaCatalina Island, and the narrow Isthmus that appears to split the island into two pieces. Also,compare the skyline of Santa Catalina Island with that of the Palos Verdes Hills and note theabsence of well-defined marine terraces on the island.

Avalon – Disembark from Catalina Express and head to Bus Terminal (about one block)Compare the development of Avalon with the photograph of Avalon Harbor in 1910. Therehas been much growth in Avalon and some adjacent areas such as Hamilton Cove to thenorth. Many of the steep coastal bluffs look relatively unchanged since the 1910photograph, which might suggest that these rocks are very durable, sustaining the steepslopes for significant time. However, bluff failures and other forms of mass wasting are verycommon on the island, especially when the area is subjected to severe winter storms andwave erosion. Avalon is on the relatively sheltered side of Santa Catalina Island, althoughstrong “southeasters” moving north from Baja California with high winds, high seas, andheavy rains, are responsible for severe coastal erosion on this side of the island.

Stop #1 – East End Quarry (approx. 1 mile SE from Avalon)Boulders of “Breccia” from the quarry were used to build the Los Angeles/Long Beachbreakwater and may be seen in the Avalon breakwater. Although breccia at the quarry isassigned to the San Onofre Breccia unit, rarity of glaucophane schist clasts and abundanceof volcanic clasts suggests more similarity to the Blanca formation of Santa Cruz Islandaccording to Vedder et al (1979). Clast composition includes quartz schist, amphibolite,actinolite schist, tremolite(?) schist, talc schist, saussuritized gabbro, vein quartz, aphaniticto porphyritic siliceous metavolcanic rocks, porphyritic basalt, and feldspathic sandstone(Vedder et al, 1979). The breccia appears to be locally derived and includes increasingabundance upsection of dacite clasts from the nearby pluton, so that plutonism and tectonismwere synchronous. Vedder et al (1979) describe fossiliferous Upper Cretaceous (?) remnantforearc sedimentary rocks from the quarry, but today these may be all removed by quarryingactivity.

On the road into the interior from Avalon, we pass through quartz diorite porphyry and someandesite dike swarms on switchbacks going up the steep slopes. The pluton is a hypabyssaligneous body intruded during the early-middle Miocene rifting of the Inner Borderland. TheSanta Catalina Island pluton has been described as a bulbous igneous body that

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approximately doubled in size by intrusion of the dike swarm. The composition of the dikesand pluton is mainly biotite-augite-hornblende quartz diorite. The dikes generally trendnortheast, with northwest dips, although local variations may be significant. These dikeswarms show that the intrusive mass grew by inflation as extension proceeded and theCatalina Schist basement was exhumed, from about 19 Ma to 15 Ma based on K-Ar and Ar-Ar ages of igneous rock samples. The areal extent of the pluton is comparable to the size ofEmery Knoll and the central uplift of Catalina Crater, but does not show the sub-circularoutline of the latter features. Seismic imaging shows a deep high velocity root beneath SantaCatalina Island, but lower velocities beneath Emery Knoll (ten Brink et al, 2000).

East End Quarry in 1983 (Photo provided by Paul Davis)

Stop #2 – Toyon JunctionFrom the high vantage point at Toyon Junction, we will have an overview of ContinetalBorderland geology. On clear days, the mainland coast is visible to the north and east, whilean excellent view of much of the southeast end of Santa Catalina Island is usually availablein all directions. The overlook is still within the large quartz diorite porphyry pluton.Haypress Reservoir is located across the road, and Wrigley Reservoir was passed on the wayup from Avalon. The main water supply for Avalon comes from Middle Ranch Reservoir,behind Thompson Dam. The water is pumped up to Wrigley Reservoir where it flows by

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gravity into Baker Reservoir and then into the distribution system for Avalon. WrigleyReservoir had serious leakage problems into the fractured quartz diorite base rock, but wasredesigned and religned by Southern California Edison Company is in use today.

On the road from Toyon Junction to the Airport-in-the-Sky, we cross into a major Miocenevolcanic unit, composed of primarily of pyroxene andesite flows and flow breccias andsubordinate olivine basalt and rhyolite flows. Small basalt dikes, rhyolite dikes, and minordacite stocks and dikes are also present. Road cuts between the Middle Ranch junction andthe airport provide excellent exposure of some of these volcanic units. We will see moreMiocene volcanic rocks on the second day in the Fisherman’s Cove and Two Harbors area.

East End Quarry and Pebbly Beach area in 1985 (Photo provided by Paul Davis).

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Stop #3 – Garnet Amphibolite in Serpentinite Unit Located a few hundred meters north of the Airport-in-the-Sky, along an older secondaryroad, two peaks provide exposures of garnet amphibolite and serpentinite. This exposureis within the serpentinite unit of Catalina Schist, and consists of a mélange matrixsurrounding large blocks of garnet amphibolite, eclogite and serpentinite. The underlyingamphibolite unit consists largely of a mafic igneous protolith that was converted to mostlyhornblende-zoisite schist/gneiss, with minor overlying semipelitic schist and garnetquartzite. The amphibolite unit is in contact with the greenschist unit along the Airport fault,which is exposed south of the airport.

View looking northwest from amphibolite boulders near airport (Photo by Mark Legg)

Stop #4 – Lunch at Airport-in-the-SkyAt the Airport-in-the-Sky, an excellent vista to the mainland coast is available. Largesamples of the major rock types found on Santa Catalina Island are placed around thedriveway area. The Catalina Island Conservancy has an excellent set of exhibits displayingthe natural history, and geology, of the island. The airport runway is constructed on top ofa hill, providing landing excitement, especially if there is a cross-wind. DC-3 aircraft makedaily trips to deliver mail and supplies to the island.

Stop #5 – Steatite (Soapstone) Archaeological Site Native populations gathered soapstone to carve into cooking bowls, that were used or traded.The bowls were called “Ollas”. The soapstone is easily carved and heat resistant, importantqualities for hand-carved cooking pots. The steatite, “Rosette-structured talc rocks”asdescribed by S. Sorensen, occurs within the metaultramafic rocks of the amphibolite unit.

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On the road from the airport to Little Harbor, we will pass down through the majormetamorphic units of the Catalina Schist, starting with the upper mantle serpentinite unit,followed by the underplated(?) amphibolite unit, then the greenschist and finally theblueschist unit. This sequence represents an inverted metamorphic gradient, with the highesttemperature rocks at the top, and the low temperature, but still high pressure (about 20-30km depth), blueschists at the bottom. The contact between the serpentinite and amphiboliteunits is poorly exposed, but does not seem to be faulted. The contacts between the otherunits are generally faults, and inferred to be thrust faults due to the structural configurationof a former subduction zone. It is possible that some Neogene detachment faulting occurred,and high-angle normal faults juxtapose some metamorphic units near the airport. See thediscussion in the 1998 field trip guide by Francis, Bohannon and Legg in this volume. Thefault contacts are difficult to see, especially during the dry season when most outcrops arecovered with dust. You may notice, however, that the color of the dust changes from a morered-brown to a more yellow tan as some contacts are crossed.

About half-way to Little Harbor, we pass through Rancho Escondido, where Arabian horsesare raised and shown for sale at the arena next to the road. A few years ago, the ranch areashad “bump” gates to keep cattle in, but now most gates have been replaced by cattle grates.We also cross a relatively level ridge line, which may be a true marine terrace, poorlydefined, but possible? Some marine fossils have been found in this area, but if dry anddusty, they are difficult to spot.

Metagraywacke in the blueschist unit exposed at Little Harbor (Photo by Mark Legg)

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Stop #6 – Little HarborThe bedrock at Little Harbor is composed chiefly of Catalina Schist, (glaucophane schist orblue schist). Large rock outcrops in Little Harbor include many different varieties ofmetaconglomerate, metagraywacke, and greenstone in a mélange. The seacliffs provideexcellent exposure and access, depending on tides, to examine the different metamorphicfacies and effects of metasomatism. The rocks are very weak here, however, and subject tolarge-scale mass wasting. Numerous landslides are evident, and the relatively flat-lyingsurfaces have been interpreted as wave-cut terraces. Shells and shell debris found on thesesurfaces originally considered beach deposits are now recognized as midden deposits froma long history of human occupation. The motion picture industry has used the Little Harborare to film many sea island pictures. Santa Catalina was the island in the fog for the morerecent version of King Kong, with Jessica Lange and Richard Dreyfuss (and the PetroxExplorer was actually the Scripps ship R/V Melville).

Stop #7 – Little Harbor Overlook, about 0.8 miles southAn overlook to the south toward Ben Weston Beach and up Middle Canyon and LowerCottonwood Canyon provides spectacular views of active landsliding along the canyonflanks in the metamorphic rocks. The northwest, coast-parallel, trend of these canyons mayresult from right-lateral shear along this side of the island, aligned with a submarineescarpment to the north that links to the Santa Cruz-Catalina Ridge fault zone.

East End Quarry in 1997 (Photo by Mark Legg)

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Day 2 [24 October, 2004]

Stop #8 – Fishermans Cover, Wrigley Institute for Environmental StudiesHike around the southeast side of the Fisherman’s Cove area where andesite flows and flowbreccias, and a large dacite dome are exposed. At the waterfront, along the east side of thecove, well-bedded water-laid tuffs and tuff-breccias are exposed showing graded beddingand contact metamorphism (baked zone) against the overlying andesite flows.

Stop #9 – Two HarborsHike from the Wrigley/USC Marine Science Center to Two Harbors along the road whereoutcrops of blueschist, andesite, San Onofre Breccia, Miocene sedimentary rocks (Montereyformation, diatomite), and large landslides are visible. The andesite flows are mostlyunstratified volcaniclastic breccia that includes abundant angular clasts of banded dacite.These flows overlay a large dacite dome exposed near the Wrigley Center, and areinterbedded with sedimentary rocks. Large landslides occur in this area. One landslide toeforms the beach cliffs at Fisherman’s Cove by the Wrigley Center; the second cuts acrossthe campground above Two Harbors.

Aerial view of Catalina Harbor/Isthmus looking NE – Note Palos Verdes Hills in the background(Photo by Mark Legg)

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Water-laid tuff with graded bedding at Big Fisherman’s Cove (Photo by Mark Legg)

The Palisades, south side of Santa Catalina – Note the andesite dikes(Photo by Mark Legg)