Smartville Field Trip Explores a Jurassic Ophiolite Complex in the Northern Sierra Nevada Foothills
Eldridge Moores, UC Davis—September 18, 2010
By Dan Day, from the April 2012 NCGS newsletter, with edits by Andrew Alden
Download the field trip guidebook*
*Note: Guidebook photos for Stops 8 and 9 are transposed.
Frontispiece for the guidebook (Stop 2)
Over 80 geoscientists attended the September 18, 2010 “Geology and Tectonics of the Smartville Complex, Northwest Sierra Nevada, California” Field Trip led by University of California Davis Distinguished Professor Emeritus Dr. Eldridge M. Moores. Dr. Moores has led other field trips for the NCGS, and all been very well attended. This trip was blessed with superb weather conditions.
The Smartville Complex was one of the first ophiolite sequences in the northern Sierra Nevada to be studied in detail. The initial field work that characterized the lithology and structure of the terrane was conducted in the mid to late 1970’s. More detailed field and petrologic studies were pursued in the 1980’s. Most originated from U.C. Davis research activities. In the 1980’s, a joint venture was conducted by Dr. Jason Saleeby of the California Institute of Technology and Dr. Eldridge Moores to establish geochronological and isotopic constraints on the origins of three ophiolitic terranes in the northwestern Sierra Nevada. These terranes, from west to east, include the Smartville Complex, the Central Belt, and the Feather River Belt.
A fourth and the oldest accretionary terrane, the Eastern Belt, is a thick stratified sequence of Paleozoic rocks to the east of the other terranes. Dr. Saleeby and his graduate students have characterized the Kings River Ophiolite Belt in the southwestern Sierra Nevada near Kings Canyon and Sequoia National Parks. His work with U.C. Davis colleagues in the northern Sierra Nevada was an effort to unravel its complex accretionary history and seek broader scale relationships between these ophiolitic terranes in the north and the Kings River ophiolite to the south. U/Pb (uranium-lead) zircon analyses were used to date the formation of the individual terranes. Other radiogenic isotope systems were used to define the mantle/crust sources of the igneous ophiolite constituents. In general, these isotopic, petrologic, and structural studies indicate the three northern ophiolite terranes were accreted to the continental margin by convergent plate tectonic processes followed by the Sierran igneous plutonic activity that anchored the terranes to the North American continental margin. Prior to their final tectonic accretion to the North American continent, these terranes grew by basaltic igneous processes in island arc, marginal basin, or volcanically active transform fault systems along the continental margin. The ages of these terranes decrease from east to west, the Upper Jurassic Smartville Complex being the youngest.
The Smartville Complex has been described as a rifted island arc volcanic-plutonic sequence exhibiting major late-stage extension and intrusion by a sheeted dike swarm. A volcanic dacite tuff high in the stratigraphic column gives a zircon U/Pb age of 164±2 m.y. Plagiogranite screens and dikes from the sheeted dike complex give 162±1 m.y. ages and postdate the island arc andesitic-basaltic volcanism. The Smartville block was accreted as a nappe structure and was subsequently intruded, along with portions of the Central Belt, by the 159±2 m.y. old Yuba Rivers tonalite pluton. Contact relationships indicate the latter was intruded during the thrust emplacement of the Smartville Complex. More recent work by Dr. Howard Day at U.C. Davis and his colleagues imply that the Central Belt and the Smartville Complex are part of the same original ophiolitic terrane, and that the intervening fault has facilitated only minor displacement since the 160 m.y. accretionary event.
From top to bottom, the Smartville Complex consists of minor epiclastic sediments, a thick volcaniclastic sequence, pillow lavas, massive diabase, and mafic and silicic plutonic rocks. All these units have been intruded by a sheeted dike complex. Small isolated outcrops of serpentinized peridotite and layered gabbro occur near the base of the complex. As mentioned above, the northwestern Sierra Nevada foothills contain four tectonic belts, decreasing in age from east to west. All contain ophiolitic rocks and have been juxtaposed against one another along the steeply dipping subparallel faults of the Foothills fault system. The adjacent Central Belt is a complex lithologic assemblage dominated by a chert-argillite sequence containing rare Triassic and Permian fossils. Limestone blocks in the assemblage contain Permian and Carboniferous coal seams and fusilinids.
The three prominent ophiolites in the Central Belt are the Slate Creek, Lake Combie, and Lost Creek sequences. They typically display a top-to-bottom stratigraphy of volcaniclastic sediments, pillow lavas and massive flows, massive diabase, mafic to silicic plutonic rock, and serpentinized peridotite. In the northern extremity of this tectonic terrane, the Jarbo Gap ophiolitic sequence contains metavolcanics, metagabbro, and peridotite that suggest formation in a fracture zone. Radiometric ages indicate these ophiolitic sequences are somewhat older than the Smartville Complex.
The Feather River Belt is a 150-km-long terrane of partially serpentinized peridotite and associated mafic igneous rocks. Radiometric dates on the belt components are as old as 380 m.y. The terrane has an early mantle fabric overprinted by a crustal fabric. In places it exhibits a talc-tremolite-anthophyllite metamorphic assemblage. In the Devil’s Gate region, metagabbro and metavolcanics may be part of an ophiolite sequence. These are some of the most highly metamorphosed rocks in the northern Sierra Nevada.
The Eastern Belt is a deformed sequence of rocks beginning with basal lower Paleozoic quartz-rich sandstone and phyllite, the Shoo Fly complex, and assorted serpentinite and accompanying albitite. The source and history of this sequence is unknown. This sequence is overlain by a Devonian–Lower Carboniferous oceanic island arc assemblage of quartz keratophyre, basaltic andesite, and minor latite capped by a thin tuffaceous, manganiferous chert unit that spans an 80 m.y. interval between the Lower Carboniferous and Lower Permian. The next three lithologic packages going upward in the stratigraphic column are a Permian-Triassic island arc-sediment sequence apparently oceanic in composition, a younger Triassic sedimentary-volcanic sequence, and an Early to Middle Jurassic volcanic andesite-dacite sequence with abundant shallow water fossils. Structurally these four tectonic belts are separated by a series of steeply dipping faults and are, with the exception of the Smartville block, isoclinally folded.
The field trip had eight stops scattered across the central Smartville Complex, and a ninth stop further south in Auburn. The Bay Area attendees met at Sunrise Mall in Pleasant Hill and proceeded to Davis, where Dr. Moores and a second contingent of enthusiastic geoscientists boarded the bus. The bus drove north on Highway 113 toward Woodland and eventually worked its way to State Route 99, then north to Marysville-Yuba City. Eldridge pointed out the Coast Ranges to the west and the Rumsey Hills visible in the foreground as the bus left Davis and headed to Woodland. The Rumsey Hills are Cretaceous sediments thrust westward over Plio-Pleistocene fluvial gravels and interbedded sands and tuffs washed out of the Coast Ranges. Looking northward up the Great Valley, Eldridge noted that the valley floor from Sacramento to Redding has approximately the same slope as the flank of a midocean ridge. As the bus approached Marysville, the Sutter Buttes were visible to the northwest. The Buttes are 1 to 3 m.y. old volcanics thought by Dr. Brian Hausback of Cal State Sacramento to be the last Cascade subduction-related volcanism at this latitude. In Marysville the bus took State Route 20 east into the Sierra Foothills to the first field stop. Along the way the remnants of hydraulic mining were obvious in the Yuba River bed.
The first stop (Stop 2 in the Guidebook) was at the Highway 20 bridge across the Yuba River near the old mining town of Timbuctoo. Just north of the bridge in the river bed is an outstanding exposure of pillow lavas (see the Smartville Field Trip Guidebook cover). Unfortunately the slope down to the river is treacherous and the group was not able to access this location. However, other pillow lavas were exposed along the river road and on the north side of Highway 20 at a roadcut immediately east of the Yuba River Bridge. Excellent examples of pillow lavas are shown in Photos 1 and 2.
Photo 1 Pillows exposed in roadcut east of the Yuba River bridge on Highway 20. Red arrows mark the darker (finer crystalline) chilled margins of a pillow. [Stop 2]
Pillow lava morphology is shown in Photo 2, taken at an outcrop immediately east of the roadcut exposure illustrated in Photo 1. These units dip steeply west and are exposed in a quarry wall visible across the Yuba River north of Highway 20.
Photo 2 Lobe-shaped pillow lavas exposed in an outcrop east of the road cut shown in Photo 1. North side of Highway 20. Hammer (in circle) for scale. [Stop 2]
The bus returned to Highway 20 and traveled westward to Marysville Road, then headed northward on Marysville Road toward LaPorte and the second stop. Roadcuts along the way expose pillow lavas, and a quartz-rich intrusive plug lingers on the skyline at about 11 o’clock.
The second stop (Stop 3 in the Guidebook) was at Stanfield Hill immediately before the turnoff to Merle Collins Reservoir. Exposed here is a cross-section of sheeted dikes intruded into the overlying volcanic pyroclastics and flows (not shown at the outcrop). They indicate a later stage of tensional rifting in the preexisting island arc volcanic pile. This is a complex sequence of intrusive events beginning with half-dikes (partial dikes or dike fragments missing one chilled margin) of quartz albitite, diabase, and plagioclase-phyric basalt. A second intrusive phase consists of diabase, basaltic andesite, andesite, and dacite. This dike series is crosscut by complete diabase dikes (both chilled margins preserved). A later event intruded thinner, fine-grained, dark nonporphyritic dikes into the existing dike sequence. A large dacite dike cuts all other dikes in this outcrop. A portion of a dike containing a darker, finer crystalline inclusion is shown in Photo 3.
Photo 3 Inclusion in dike at Stanfield Hill near Collins Lake. Note the alteration border on the inclusion, the chilled dike margin (red arrows), and late stage hydrothermal vein alteration (blue arrow). [Stop 3]
The dike chilled margin shows late-stage hydrothermal alteration that deposited epidote and chlorite along the dike contact chilled margin and along brittle fracture veins. The entire suite of dikes has been metamorphosed to greenschist facies, and although most of the original igneous textures have been preserved, the chemistry of individual phases has been strongly altered by mobilization of CaO, TiO2, and other chemical species. Epidote and calcite veins frequently follow chilled dike contacts and fill crosscutting brittle fractures. Dikes such as these were used to date the sheeted dike complex and establish age relationships in the Smartville Complex.
The group traveled a short distance from Stanfield Hill to road cuts along Marysville Road where it abruptly turns east toward Oregon House and Dobbins. At Guidebook Stop 4 the field trip members examined diabase, andesite, and dacite dikes cutting plagiogranite screens (lens-shaped fragments of plutonic rock) and multi-textured gabbro. The screens are relics of gabbro, tonalite, and trondhjemite plutonic rocks or segregates thereof. The plagiogranite is a potassium-poor, sodium-rich siliceous plutonic rock type found in other ophiolite complexes. Its microstructure reveals rapakivi-style quartz and plagioclase “wormy” intergrowths that are also called myrmekite. Screens surrounded by dikes are depicted in Photos 4 and 5.
Photo 4 Whitish plagiogranite screen (red arrows) surrounded by darker intruded basaltic to andesitic dikes. Marysville Road to Oregon House at the north end of Merle Collins Reservoir. [Stop 4]
Photo 5 Detail of lighter-colored plagiogranite screen and adjacent darker dike rocks. Marysville Road to Oregon House. [Stop 4]
Driving eastward on Marysville road, the bus passed over massive diabase, tonalite, and gabbro. At Guidebook Stop 5 the group examined rhythmic layering in a two-pyroxene gabbro containing calcic plagioclase (see Photo 6). This outcrop is located about one mile east of Dobbins, CA.
Photo 6 Layered gabbro exposed east of Dobbins. Pocket knife for scale. [Stop 5]
Guidebook Stop 6, a quarry in epidotized massive diabase, was excluded from this trip. Instead, the group chose to spend more time at Guidebook Stop 7 at Bullards Bar Reservoir Dam. Here steeply dipping volcaniclastic sediments crosscut by diabase dikes are exposed in vertical outcrops. A large outcrop showing these relationships is exposed on the north side of the dam parking lot.
Photo 7 Dr. Moores discusses regional geological relationships and local lithologies exposed at the Bullards Bar Reservoir Dam site [Stop 7].
Dr. Moores discussed regional and local lithological relationships at this stop (Photo 7). Photos 8 and 9 show the rock outcrop at this site and deformation features such as mineralized en-echelon gash fractures.
Photo 8 Outcrop at Bullards Bar Dam. Steeply dipping volcaniclastic sediments cut by dikes. This is part of the Smartville Upper Volcanic Unit.
Photo 9 En echelon mineralized gash fractures in the Bullards Bar Dam outcrop. These features indicate shear deformation. Knife (blue arrow) for scale.
At Bullards Bar Dam the field trip stopped for lunch. The dam is an impressive structure located on the north fork of the Yuba River (Photo 10). The Wolfe Creek fault zone has been mapped just east of the dam between the Smartville Complex and the 159±2 m.y. Yuba Rivers pluton.
Photo 10 Bullards Bar Dam, a popular recreational site. Marysville Road traverses the dam crest.
After lunch, the trip resumed its itinerary, and traveled eastward on Marysville Road to Highway 49. The bus turned south on Highway 49 toward Guidebook Stop 8. Highway 49 travels through biotite-hornblende tonalite and granodiorite. Stop 8 is in [south of] North San Juan, a popular town during the gold rush hydraulic mining period of the late 19th century. Trip members explored isoclinally folded metasediments exposed along Highway 49 just south [north in newsletter] of North San Juan (Photo 11).
At North San Juan, ultramafic olivine cumulates are exposed as low outcrops hugging the ground surface [Photos 12 and 13]. These rocks are interpreted to be the lower part of the Smartville Complex.
Photo 11 Isoclinally folded metasediments along Highway 49. [Stop 8]
Guidebook Stop 9 is farther down Highway 49 west [newsletter says east] of Nevada City. Outcrops consist of highly altered and sheared serpentinized peridotite. The textures are highly altered, but appear to be mantle tectonites. Stop 10 is in Auburn at the American River Overlook.
Photo 12 Highly altered and sheared serpentinized peridotite interpreted to be highly altered mantle tectonite. This is the basement of the Smartville Complex. [Stop 9] Andrew Alden photo, 2024.
Photo 13 Geologists examine surface exposures of olivine cumulates thought to be the lower part of the Smartville Complex. [Stop 9]
The field trip bus traveled south on Highway 49 to I-80, and then turned south towards Auburn. Auburn was the site for the Army Corps of Engineers Auburn Dam. The project was abandoned when it was learned that a potentially active fault zone passed through the proposed dam site. The outcrops here expose foliated sheeted dikes of the Smartville Complex. The group peered over the canyon rim into the river valley. The overlook name was changed from the “Auburn Dam Overlook” to the more appropriate “American River Overlook.” Any dam failure by seismic activity at this site would have posed a serious flooding threat to communities downstream, particularly in Sacramento.
Photo 14 The former Auburn Dam Overlook, now renamed the American River Overlook. Interrupted excavation here has exposed foliated sheeted diabase dikes affiliated with the Smartville Complex. [Stop 10] Andrew Alden photo, 2010.
The field trip group assembled at Overlook Park in Auburn and celebrated the end of an outstanding field trip. Afterwards, the attendees boarded the bus and returned to Davis and the Bay Area. A group photo was taken by Ray Sullivan at Bullards Bar Dam (Photo 15).
Photo 15 The Smartville Field Trip’s motley crew, taken by Dr. Ray Sullivan at Bullards Bar Dam.
The NCGS expresses its sincere thanks to Dr. Eldridge Moores of the University of California, Davis, for leading another exceptional field trip. Dr. Moores’ trips have always been very well attended, and this was no exception. The NCGS appreciates the time and effort that Dr. Moores spent preparing for this trip. The attendance speaks for his popularity as a field trip guide and as an expert on the geological evolution of Northern California.
The NCGS recognizes the efforts of Field Trip Coordinator Tridib Guha and Assistant Coordinator Dan Day. Tridib and Dan were involved with the Guidebook preparation, editing, and printing. The Davis meeting site was adeptly coordinated by NCGS member Chris Higgins. Many thanks to past Field Trip Coordinator and current Membership Chair Rob Nelson, and to past Field Trip Coordinator John Christian for their assistance with the lunch and other logistical duties. Mark and Karel Detterman also assisted with important tasks throughout the day. We thank NCGS member Don Downey for videotaping portions of the trip and posting them on YouTube. Don also offered his assistance as needed.