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Abstract This field trip provides an overview of the stratigraphic and structural evolution of the northern Sierra terrane, which forms a significant part of the wall rocks on the western side of the late Mesozoic Sierra Nevada batholith in California. The terrane consists of a pre-Late Devonian subduction complex (Shoo Fly Complex) overlain by submarine arc-related deposits that record the evolution of three separate island-arc systems in the Late Devonian-Early Mississippian, Permian, and Late Triassic-Jurassic. The two Paleozoic arc packages and the underlying Shoo Fly Complex have an important bearing on plate-tectonic processes affecting the convergent margin outboard of the Paleozoic Cordilleran miogeocline, although their original paleogeographic relations to North America are controversial. The third arc package represents an overlap assemblage that ties the terrane to North America by the Late Triassic and helps constrain the nature and timing of Mesozoic orogenesis. Several of the fieldtrip stops examine the record of pre-Late Devonian subduction contained in the Shoo Fly Complex, as well as the paleovolcanology of the overlying Devonian to Jurassic arc rocks. Excellent glaciated exposures provide the opportunity to study a cross section through a tilted Devonian volcano-plutonic association. Additional stops focus on plutonic rocks emplaced during Middle Jurassic arc magmatism in the terrane, and during the main pulse of Cretaceous magmatism in the Sierra Nevada batholith to the east .

Pre-Cretaceous rocks in the northern Sierra Nevada are subdivided from west to east into the Smartville, central, Feather River peridotite, and eastern belts. Cretaceous and younger sedimentary rocks form the western boundary of the Smartville belt, but various reverse-fault segments of the Foothills fault system separate the other belts. The Foothills fault system and associated structures involve rocks as young as Kimmeridgian (Late Jurassic) and are truncated by Early Cretaceous plutons. This relationship is often cited as evidence for the Nevadan orogeny which is commonly viewed as a temporally restricted event involving deformation and metamorphism during the Late Jurassic. Recent work, however, suggests that some of the Mesozoic structural fabric in the northern Sierra Nevada may not have been produced during the Late Jurassic, but instead may have formed between Early and Middle Jurassic time. Thus, distinguishing Nevadan-age deformation from older Mesozoic deformation is now one of the more important problems facing geologists working in the northern Sierra Nevada. The Haypress Creek pluton crops out in the eastern belt and historically has been cited as a post-Nevadan pluton. It intrudes the Early to Middle Jurassic Sailor Canyon Formation that, together with the overlying Middle Jurassic Tuttle Lake Formation, contains a domainally developed, locally penetrative, northwest-striking cleavage (S 2 ). S 2 can be traced into the contact metamorphic aureole of the Emigrant Gap composite pluton, where structural and microtextural evidence indicates that it predates pluton intrusion. New U-Pb zircon data for the Haypress Creek pluton suggest an age of 166 ± 3 Ma and previously published U-Pb zircon data for the oldest phase of the Emigrant Gap composite pluton suggest an age of 168 ± 2 Ma. The fossiliferous Sailor Canyon Formation ranges in age from Early Jurassic (Sinemurian) in its lower parts to Middle Jurassic (Bathonian or Bajocian) in its upper parts. The overlying Tuttle Lake Formation contains S 2 , which formed prior to emplacement of the Emigrant Gap and Haypress Creek plutons at ca. 168–166 Ma. This relationship suggests that the Tuttle Lake Formation must have been deposited and deformed entirely within the Middle Jurassic. Thus, S 2 and associated structures within the eastern belt formed prior to Late Jurassic Nevadan deformation associated with the Foothills fault system. There are two end-member models used to explain the plate tectonic evolution of pre-Cretaceous rocks in the northern Sierra Nevada. These are referred to as the arc-continent collision and single, wide-arc models. Data discussed herein do not preclude either of these models for Early to Middle Jurassic time. However, regardless of which of these models is favored, both scenarios place the approximately 168 Ma and younger Jurassic volcanic and plutonic rocks of the Smartville, central, and eastern belts in a distinctly intra-arc setting and further imply that the Foothills fault system and related Late Jurassic structures are also of intra-arc character. We conclude that there is no evidence along 39°30′N latitude for arc-continent collision during the Nevadan orogeny.

Devonian and Lower Mississippian volcano-plutonic complexes and related basinal strata that extend from California to British Columbia lie outboard of coeval North American shelf rocks. Remarkable similarities between these complexes indicate that they form the remnants of a mid-Paleozoic east Pacific fringing arc system. Arc basement was composite. It is characterized in part by high initial Sr ratios, radiogenic Pb, and −ɛ Nd values, and in part by low initial Sr ratios, various initial Pb ratios, and +ɛ Nd values. These distinctive isotopic signatures, together with the presence of Proterozoic average ages for detrital zircon in ultimately continent-derived sedimentary rocks and Proterozoic upper-intercept (inheritance) ages on zircon from arc- and rift-related magmatic rocks, imply that basement was composed of both continental crust and transitional or oceanic crust overlain by continent-derived sedimentary rocks. These data suggest that the arc system formed adjacent to a continental landmass. The initial geometry of the arc has been highly modified by subsequent crustal thickening, extension, associated uplift, and transport along strike-slip faults.

Lower Paleozoic strata of the Roberts Mountains allochthon were derived both directly and indirectly from Precambrian basement rocks. The Upper Cambrian(?) Harmony Formation was derived directly from erosion of Proterozoic crystalline rocks along the western margin of North America. Zircon data from feldspar-bearing strata of the Vinini Formation (Lower Ordovician) suggest a provenance link with the Harmony Formation. Compositionally mature, Middle Ordovician eugeoclinal (Palmetto Formation) and miogeoclinal (Eureka Quartzite) quartzites exhibit similar isotopic signatures except that the miogeoclinal quartzite unequivocally contains an Archean component. Volcaniclastic sediment within the Slaven Chert (Middle Devonian) contains Precambrian zircon but no evidence of neovolcanic zircon. New U-Pb isotopic data for detrital zircon from the allochthon are most consistent with models of Antler orogenesis that do not require significant interaction (collision) of a fringing volcanic arc with the continental margin.

The Ashgillian (Upper Ordovician) Montgomery Limestone occurs as slide blocks in melange of the Shoo Fly Complex, northern Sierra Nevada, northern California. Brachiopods and sphinctozoan sponges from the Montgomery Limestone have closest biogeographic ties to coeval faunas of the eastern Klamath Mountains (Yreka terrane), and in the case of the brachiopods, to east-central Alaska (Jones Ridge). The latter was part of North America in the Ordovician. A small collection of Montgomery rugose corals yielded one species that is known elsewhere only in the Yreka terrane and in northern Maine. Montgomery tabulate corals have affinities with contemporaneous faunas of the Yreka terrane, northern Europe/Asia, Australia, and eastern North America. The apparent absence of similar tabulate taxa in western North America may be an artifact of incomplete collecting. As a whole, the biogeographic data indicate that the Montgomery Limestone was deposited close enough to Ordovician North America for faunal interchange to occur, and during its deposition was probably relatively near that continent. A comparison of lithologic units of the Shoo Fly Complex with those of the Yreka terrane indicates that some units in each area have no counterparts in the other (e.g., schist of Skookum Gulch in the Yreka terrane), and other units have general similarities in age (where known) and lithology, but differ in detail. The Yreka terrane has been interpreted as the remnants of an Early Cambrian arc and Ordovician-Devonian arc–fore-arc–accretionary prism, and the Shoo Fly Complex as a fragment of a Devonian or older accretionary wedge. Available biogeographic and stratigraphic data can be reasonably explained, as has been done by earlier authors, by a paleogeography in which the Yreka terrane and Shoo Fly Complex were parts of the same arc-trench system but were situated at different points along the strike of the arc. Lateral changes along strike in tectonic conditions and source areas could account for the observed disparities.

In the northern Sierra Nevada, California, the pre–Upper Devonian Shoo Fly Complex has been subdivided into the following, from structurally lowest to structurally highest: (1) Lang sequence, (2) Duncan Peak chert, (3) Culbertson Lake allochthon, and (4) Sierra City melange. Detailed studies have been completed recently on the rocks in the Culbertson Lake allochthon in the Bowman Lake/Culbertson Lake area. The results of geochemical, sedimentologic, stratigraphic, and structural studies of rocks in the Culbertson Lake allochthon suggest that: (1) the allochthon is composed of a lower fault-bounded succession of basaltic rock and a complexly imbricated upper sandstone–dominated sequence; (2) the structurally lowest unit in the allochthon contains alkalic basalts that formed in a seamount or ocean island setting; (3) sedimentologic and stratigraphic patterns in the structurally higher units may be the result of sedimentation and deformation in a trench setting; (4) the allochthon is composed of a complex of northeast-dipping, thrust-fault–bounded slices, and is an imbricate structure; and (5) the imbricate structure, in terms of present-day geographic coordinates, formed as a result of generally westward-directed translations (i.e., northwest, west, or southwest) prior to the Late Devonian and probably after the Cambrian. Data presented here generally support models that portray the Shoo Fly Complex as having formed in a subduction complex setting that developed near enough to a continental landmass to receive sand-sized detritus derived from it. However, current data do not uniquely constrain the location of the continental landmass that provided sand-sized detritus to the Shoo Fly Complex, nor do they rule out the possibility that the sand-rich portions of the Shoo Fly Complex were deposited as part of a continental margin submarine fan system that was subsequently accreted and imbricated within a subduction complex. The continental landmass supplying continental detritus to the Shoo Fly Complex may have been located in western North America, or it may have been located somewhere in Panthalassa.

The eastern Klamath belt contains the fault-bounded Yreka, Trinity, and eastern Klamath terranes. The Yreka terrane comprises Lower Cambrian to Middle Devonian or younger igneous, metamorphic, and sedimentary rocks. The Trinity terrane consists of the Trinity ultramafic-mafic complex of Ordovician to Silurian age and an amphibolitic gabbro unit of Early Cambrian age. The eastern Klamath terrane bears igneous and sedimentary rocks of Early Devonian to Middle Jurassic age. Stratigraphic and intrusive relations imply that the Yreka and Trinity terranes were amalgamated by Early to Middle? Devonian time and that the Trinity terrane was the basement on which the eastern Klamath terrane formed. The lower Paleozoic rocks of the Yreka and eastern Klamath terranes are interpreted to represent the remnants of an Early Cambrian arc overlain by part of an Ordovician to Devonian arc-trench complex that faced west to northwest (present coordinates) in Late Ordovician and Early to Middle Devonian time. The Trinity complex may have formed in a marginal or back-arc basin northeast to southeast of the Lovers Leap–Gregg Ranch portion of the arc in Ordovician to Silurian? time (prior to existence of the eastern Klamath terrane). The biogeographic affinities of eight groups of early Paleozoic fossils, taken as a whole, demonstrate that the eastern Klamath belt was close enough to North America in the Middle Ordovician to Middle Devonian for faunal communication to occur, in some cases at the species level. This evidence and the presence in the belt of some coarse-grained strata possibly derived from a continent indicate that the belt may have been relatively near to North America in Silurian or Devonian time, yet its location is obscure.

The Central Metamorphic Belt and Trinity terrane of the Klamath Mountains and the Feather River terrane of the northern Sierra Nevada share strong similarities in their protolith types, metamorphic parageneses, structures, ages of formation and metamorphism, and relations with surrounding units. These terranes consist of ductilely deformed ultramafic to mafic plutonic and volcanic rocks and minor oceanic sedimentary rocks interpreted as oceanic lithosphere. Ultramafic rocks in the Trinity terrane and the Feather River terrane were hydrated and metasomatized under lower amphibolite-facies conditions. Mafic and sedimentary rocks in the Central Metamorphic Belt and the Feather River terrane contain upper greenschist to amphibolite-facies parageneses formed during Devonian time. Phase assemblages and mineral chemistries indicate peak P-T conditions of 500° to 650 ± 50°C and 500 ± 300 MPa for both terranes. The similarities imply that the Klamath Mountains and Sierra Nevada share a common early to middle Paleozoic history. The Trinity terrane/Central Metamorphic Belt represents an arc basement/subduction zone couple; the Feather River terrane may also represent such a couple.

Through the work of a number of investigators, a tremendous amount of geochronological data with excellent geological control exists on the numerous ophiolitic assemblages of the Sierran-Klamath orogen. Distinct ophiolitic assemblages of latest Precambrian, Ordovician-Silurian, Carboniferous-Permian, Late Triassic–Early Jurassic and late Middle–Late Jurassic (Callovian-Oxfordian) ages are recognized. The geochronological data, in conjunction with structural and petrologic data, facilitate a relatively detailed analysis of the petrotectonic and structural-stratigraphic development of the orogen from the perspective of ophiolite basement geology. Insights into the plate tectonic settings of ophiolite genesis and emplacement are offered by an abundance of information that now exists on active oceanic island arcs and related environments. Remnants of abyssal lithosphere, boundary transform melange, extensional fore-arc igneous sequences, and interarc basin lithosphere are all preserved within the Sierran-Klamath assemblages. In general, the older assemblages or the older elements of polygenetic assemblages have greater affinities to abyssal lithosphere, whereas the younger assemblages or elements reflect suprasubduction zone environments of genesis. The upper Paleozoic through Jurassic assemblages occur in regional belts, and their tectonic, as well as petrogenetic, histories may be related in detail with some confidence to the tectonic development of the Cordilleran margin.

Details of the stratigraphy, depositional setting, and clastic petrography of the upper Paleozoic Schoonover sequence in the Independence Mountains, northern Nevada, provide the basis for a better understanding of the paleogeography of the continental margin of western North America during the late Paleozoic. The Schoonover sequence represents the northernmost exposures of the Golconda allochthon, which was thrust over the outer edge of the continental margin of western North America during the Permo-Triassic Sonoma orogeny. The Schoonover sequence, like the Havallah sequence and other units of the Golconda allochthon, is an imbricated assemblage of thrust-bound packages of radiolarian “ribbon” chert, basaltic greenstone, silty limestone turbidites, and siliciclastic sandstones that range in age from Late Devonian to Early Permian. Detailed mapping, and facies and petrographic analysis of lithostratigraphic units, combined with paleontologic control, indicate that the Schoonover sequence consists of a coherent stratigraphic succession of basinal deposits with paleogeographic ties to the autochthonous shelf margin and to a volcanic arc. Latest Devonian- to earliest Mississippian-age basaltic and andesitic greenstones and tuffaceous sedimentary rocks form the stratigraphic base of the sequence and are succeeded by Lower Mississippian siliciclastic sandstones. The petrography of these sandstones indicates that they consist of mixed detritus derived from volcanic and continental shelf source terranes, indicating the proximity of an arc to the continental margin. In addition, stratigraphic relations between autochthon and allochthon indicate that the Schoonover basin evolved adjacent to the continental shelf throughout the late Paleozoic, its history punctuated by periods of basaltic volcanism and pulses of continent-derived clastic input. The onset of the Sonoma orogeny resulted in closure of the basin and emplacement of its deposits on the continental margin.

The upper Paleozoic Havallah sequence of central Nevada is a folded and thrust-faulted association of greenstone, siliceous marine sedimentary rocks, and deep-water clastic rocks. Microfossil assemblages (radiolarians, sponge spicules, and conodonts) are used as tools to unravel the stratigraphy and to interpret the paleoenvironments of the siliceous sedimentary rocks. Nine radiolarian assemblages (Osagean to Guadalupian) are described and used for delineation and correlation of fault-bounded lithotectonic units. The biostratigraphic zonation reveals that the oldest rocks in each lithotectonic unit are progressively younger from the structurally highest to the lowest units, suggesting progressive west-to-east upsection stepping of the Golconda sole thrust with accretion of each unit. Analyses of the radiolarian and sponge spicule faunas permit lateral and temporal comparisons of depositional environments. The lower structural units are coarsening-upward sequences of hemipelagic slope deposits overlain by sponge spicule-rich turbidites derived from a shallow source. The uppermost structural unit is a coarsening-upward basinal sequence. Permian sponge spicules in turbidites of the slope sequences and redeposited fusulinids in the basin sequence are similar to those in adjacent autochthonous (North American) regions. Permian radiolarians and sponge spicules in hemipelagic siliceous argillite of the slope sequences are similar to those in the Northern Sierra terrane to the west; the Havallah basin and the Northern Sierra arc terrane were overlain, therefore, by a similar water mass and may have been in proximity during the Permian. Clastic dikes and sills containing volcanic, metamorphic, and sedimentary rock clasts are Leonardian or younger.

Biostratigraphic data, based mostly on radiolarian assemblages, establish synchronous deposition in the northern Sierra terrane and the Havallah basin beginning in the Late Devonian and extending into the early Late Permian. Lower Mississippian and mid-Permian arc-derived volcaniclastic debris was deposited in parts of the Havallah basin during episodes of arc volcanism in the northern Sierra terrane. Between these episodes of arc volcanism, from late Early Mississippian to at least Middle Pennsylvanian, the northern Sierra terrane collected siliceous pelagic deposits that correlate with dominantly chert-argillite sections in the Havallah sequence. These intermixed lithic assemblages suggest shared stratigraphic evolution and geographic proximity between the Sierran arc terrane and the Havallah basin during the late Paleozoic. During Late Devonian and Early Mississippian arc volcanism in the northern Sierra terrane, lower Paleozoic rocks of the Roberts Mountains allochthon were thrust over coeval deposits on the North American shelf. Chert-quartz-rich siliciclastic debris, derived from the Antler orogenic belt, is interbedded with Upper Devonian and Lower Mississippian distal volcanic rocks in the northern Sierra terrane and with Kinderhookian volcaniclastic rocks and chert in the Schoonover sequence. These quartzose-clastic deposits not only provide an independent lithologic link between the Sierran arc terrane and the Havallah basin, they also tie the arc terrane and basin to North America at the time of the Antler orogeny. Late Devonian and Early Mississippian arc volcanism in the northern Sierra terrane occurred in an extensional regime. Extensional tectonism began locally in the Havallah basin during the Famennian and continued into the early Meramecian. Contemporaneous extension in the arc and basin during emplacement of the Roberts Mountains allochthon is difficult to reconcile with existing arc-continent collision models for the Antler orogeny.

Upper Paleozoic rocks of the northern Sierra and eastern Klamath terranes provide detailed stratigraphic records of ensimatic arc-related sedimentation and magmatism. Comparison of Paleozoic stratigraphic relations between the two terranes, however, suggests certain contrasts in depositional environments and the nature, volume, and timing of volcanism for given time intervals. Some lithologic and provenance ties indicate a paleogeographic relation. Variations in stratigraphy between the two terranes and within terranes imply differences in geodynamic setting. These and regional geologic relations indicate an early and persistent paleogeographic tie between the two areas, and they further suggest that the eastern Klamath terrane may have lain outboard and trenchward of the northern Sierra terrane during much of their late Paleozoic evolution. Stratigraphic ties for mid-Permian and lower Mesozoic rocks imply a closer relation and more similar geodynamic setting during subsequent evolution.

Carboniferous and Permian strata of the eastern Klamath terrane, California, include sedimentary and volcanic facies deposited in an island-arc setting. Carboniferous arc volcanism is first recorded in Upper Visean to Namurian portions of the Baird Formation, in which volcaniclastic sediments of marine deltaic origin grade southward to fine-grained, more offshore shelf deposits. During the Westphalian and Stephanian, a volcanic center developed at the southern end of the terrane, and volcaniclastic dispersal aprons of delta-front origin were deposited to the north. Pauses in deltaic progradation are represented by carbonate bank deposits. The Wolfcampian to Lower Leonardian McCloud Limestone represents a period of volcanic quiescence. The McCloud formed as a series of diachronous carbonate platforms that developed on topographic highs of the Carboniferous arc; platforms were bordered by deeper water ramps and escarpments. Renewed arc volcanism in Late Permian time is represented by Lower Guadalupian volcaniclastic sediments of the Nosoni Formation. More proximal volcanic deposition is represented by the overlying Dekkas and Bully Hill formations, with volcanic centers located at the southern end of the terrane. The Carboniferous and Permian sequence of the eastern Klamath terrane resembles Cenozoic deposits of topographically complex intra-arc settings and represents only a small portion of a larger Late Paleozoic island arc.

The abundant and well-studied Early Permian faunas of the eastern Klamath terrane, which contain a high percentage of endemic species and genera, constitute part of a biotic province (the McCloud province) that is distinct from the better known central Cordilleran and Tethyan provinces. The McCloud province evidently fringed an island arc that was located within the tropical climatic zone of the Paleopacific Ocean during the Early Permian. The development of an endemic biota in this province required considerable isolation for long periods of time, and because the shallow-water biotic provinces of the eastern Klamath terrane, Tethys, and the central Cordillera lay along the same general latitudinal band and were open to waters of the same ocean basin, their separate development must have been the result of geographic barriers. We conclude that geographic isolation was produced by large distances of deep water that lay between these shallow-marine provinces in the Early Permian Paleopacific Ocean. By analogy with biotic distributions in modern ocean basins, these deep-water barriers must have been several thousand kilometers wide.

New geologic mapping and fossil data from the Pine Forest Range, Black Rock Desert, northwest Nevada, indicates that the range contains a structurally intact sequence of variably metamorphosed middle (and early?) Paleozoic through latest Triassic strata. The oldest rocks in the range include metamorphosed quartzo-feldspathic sedimentary rocks and mafic volcanic and volcaniclastic rocks of Mississippian and/or older age. Overlying fan facies chert/argillite/quartz-rich clastic rocks are of post–Late Devonian(?) and pre–Late Mississippian age, and are succeeded by shallower marine Upper Mississippian to Lower Pennsylvanian(?) volcanic rocks, volcanic-lithic–rich clastic rocks, and limestone. The remainder of Paleozoic time is characterized mostly by shallow marine conditions and the development of several unconformities. A thin sequence of shallow marine carbonates and clastic sediments, yielding early Late Permian fossils at the top, overlies Pennsylvanian(?) strata across an unconformity that may span early Pennsylvanian through Early Permian time. Upper Permian(?) chert and shale unconformably overlie older rocks and reflect some subsidence in Late Permian(?) time. A third unconformity separates Paleozoic and Triassic rocks and spans latest Permian(?) through Middle or Late Triassic time. Triassic strata in the Pine Forest Range record two distinct periods of deposition: (1) fan facies sedimentary-lithic–rich sediments and basinal carbonates were deposited from Ladinian or Carnian (late Middle or early Late Triassic) through early Norian (late late Triassic) time, and (2) mafic to intermediate composition lavas and associated volcanic-lithic– and crystal-rich fan facies sediments were deposited during most of the remainder of Norian time. Lavas exhibit the trace-element characteristics of volcanic arc magmas. Relatively deep marine conditions of deposition occurred throughout Middle(?) to Late Triassic time. The Paleozoic stratigraphic record in the Pine Forest Range shows important similarities to that of other Paleozoic arc sequences in the western U.S. Cordillera, including those in the northern Sierra Nevada and eastern Klamath Mountains (California), Blue Mountain province (Oregon), and Chilliwack terrane (Washington). These similarities support an interpretation of paleogeographic and tectonic ties between the Black Rock Desert and these other arc sequences in Mississippian (and early Paleozoic?) through Permian time. In addition, the presence of a Permo-Triassic unconformity in the Pine Forest Range represents new evidence that these Paleozoic arc sequences were characterized by uplift and erosion during the time of the Sonoma orogeny. Early Mesozoic strata in the Pine Forest Range provide a record of volcanism and sedimentation that is similar to that in other early Mesozoic volcanic arc sequences from the southwestern United States through northern California. These similarities support an interpretation that early Mesozoic arc sequences in northwest Nevada, as well as northern California, form the northern continuation of the west-facing early Mesozoic arc documented in the southwestern United States. In addition, the Triassic record in the Pine Forest Range suggests that extension-related intra-arc subsidence, inferred to have characterized the southwestern United States during early Mesozoic time, may also have affected early Mesozoic rocks of the Black Rock Desert.