Geochemistry of the Great Valley Group: An Integrated Provenance Record

Sedimentary geochemistry of fine-grained strata of the Great Valley Group (GVG) in California documents a provenance signal that may better represent unstable, mafic minerals and volcanic clasts within sediment source regions than the provenance signal documented in the petrofacies and detrital zircon analysis of coarser sedimentary fractions. Geochemistry of the GVG provides an overall provenance framework within which to interpret sandstone petrofacies and detrital zircon age signatures. The geochemical signature for all Sacramento Valley samples records an overall continental arc source, with significant variation but no clear spatial or temporal trends, indicating that the geochemical provenance signal remained relatively consistent and homogenized through deposition of Sacramento basin strata. The San Joaquin basin records a distinct geochemical provenance signature that shifted from Early to Late Cretaceous time, with Lower Cretaceous strata recording the most mafic trace element geochemical signature of any GVG samples, and Upper Cretaceous strata recording the most felsic geochemical signature. These provenance results suggest that the early San Joaquin basin received sediment from the southern Sierran foothills terranes and intruding plutons during the Early Cretaceous, with sediment sources shifting east as the southern Sierran batholith was exhumed and more deeply eroded during the Late Cretaceous. The GVG provenance record does not require sediment sources inboard of the arc at any time during GVG deposition, and even earliest Cretaceous drainage systems may not have traversed the arc to link the continental interior with the margin. Because the GVG provenance signature is entirely compatible with sediment sources within the Klamath Mountains, the northern and western Sierran foothills belt, and the main Cretaceous Sierran batholith, the Klamath-Sierran magmatic arc may have formed a high-standing topographic barrier throughout the Cretaceous period

Hornbrook Formation, Oregon and California: A Sedimentary Record of the Late Cretaceous Sierran Magmatic Flare-Up Event

Early Late Cretaceous time was characterized by a major magmatic flare-up event in the Sierra Nevada batholith and early phases of magmatism in the Idaho batholith, but the sedimentary record of this voluminous magmatism in the U.S. Cordillera is considerably less conspicuous. New detrital zircon U-Pb ages from the Hornbrook Formation in southern Oregon and northern California reveal a significant and sustained influx of 100–85 Ma detrital zircons into the broader Hornbrook region beginning ca. 90 Ma. Detrital zircon ages and hafnium isotopic compositions, combined with whole-rock geochemistry, suggest that sediment was largely derived from the Sierra Nevada, requiring uplift and erosion of the Sierra Nevada batholith during and immediately following the Late Cretaceous magmatic flare-up event. Sediment derived from the eroded arc may have been transported northward along the axis of the arc, between a western drainage divide along the arc crest and the rising Nevadaplano to the east. Although the Klamath Mountains and Blue Mountains Province present more proximal potential sources of Jurassic and Early Cretaceous detrital zircons in the Hornbrook Formation than the Sierra Nevada, Late Cretaceous deposition on the Klamath Mountains 80 km west of Hornbrook Formation outcrops, and Late Cretaceous deep-water deposition on the Blue Mountains in the Ochoco Basin suggest that these regions were the locus of subsidence and sedimentation, rather than erosion, during Late Cretaceous time. The limited outcrop extent of the Hornbrook Formation may represent only a sliver of a much larger Late Cretaceous Hornbrook basin system. Complete characterization of the episodic magmatic history of continental arcs requires integration of age distributions from the arc itself and from detrital zircons eroded from the arc; it is critical to recognize the potential of drainage systems to transport sediment to depocenters not directly linked to present-day arc exposures

Provenance of the Pythian Cave Conglomerate, Northern California: Implications for Mid-Cretaceous Paleogeography of the U.S. Cordillera

Provenance analysis of middle Cretaceous sedimentary rocks can help distinguish between disparate tectonic models of Cretaceous Cordilleran paleogeography by establishing links between sediment and source, as well as between currently separated basins. This study combines new detrital zircon age data and compositional data with existing provenance data for the Pythian Cave conglomerate, an informally-named unit deposited unconformably on the eastern Klamath Mountains, to test possible correlations between the Pythian Cave conglomerate and similar-age deposits in the Hornbrook Formation and the Great Valley Group. These provenance results indicate that restoring Late Cretaceous clockwise rotation of the Blue Mountains adds a significant sediment source for Cretaceous basins previously associated with only the Klamath Mountains (e.g., the Pythian Cave conglomerate and Hornbrook Formation) or a combined Klamath-Sierran source (e.g., Great Valley Group). Comparison of the Pythian Cave conglomerate with the Klamath River Conglomerate and the Lodoga petrofacies suggests that the Pythian Cave conglomerate system was separate from the nearby Hornbrook Formation and was probably related to the Lodoga petrofacies of the Great Valley Group

The Birth of a Forearc: The Basal Great Valley Group, California, USA

The Great Valley basin of California (USA) is an archetypal forearc basin, yet the timing, structural style, and location of basin development remain controversial. Eighteen of 20 detrital zircon samples (3711 new U-Pb ages) from basal strata of the Great Valley forearc basin contain Cretaceous grains, with nine samples yielding statistically robust Cretaceous maximum depositional ages (MDAs), two with MDAs that overlap the Jurassic-Cretaceous boundary, suggesting earliest Cretaceous deposition, and nine with Jurassic MDAs consistent with latest Jurassic deposition. In addition, the pre-Mesozoic age populations of our samples are consistent with central North America sources and do not require a southern provenance. We interpret that diachronous initiation of sedimentation reflects the growth of isolated depocenters, consistent with an extensional model for the early stages of forearc basin development

Provenance Analysis of the Ochoco Basin, Central Oregon: A Window Into the Late Cretaceous Paleogeography of the Northern U.S. Cordillera

Cretaceous forearc strata of the Ochoco basin in central Oregon may preserve a record of regional transpression, magmatism, and mountain building within the Late Cretaceous Cordillera. Given the volume of material that must have been eroded from the Sierra Nevada and Idaho batholith to result in modern exposures of mid- and deep-crustal rocks, Cretaceous forearc basins have the potential to preserve a record of arc magmatism no longer preserved within the arc, if forearc sediment can be confidently linked to sources. Paleogeographic models for mid-Cretaceous time indicate that the Blue Mountains and the Ochoco sedimentary overlap succession experienced postdepositional, coast-parallel, dextral translation of less than 400 km or as much as 1700 km. Our detailed provenance study of the Ochoco basin and comparison of Ochoco basin provenance with that of the Hornbrook Formation, Great Valley Group, and Methow basin test paleogeographic models and the potential extent of Cretaceous forearc deposition. Deposition of Ochoco strata was largely Late Cretaceous, from Albian through at least Santonian time (ca. 113–86 Ma and younger), rather than Albian–Cenomanian (ca. 113–94 Ma). Provenance characteristics of the Ochoco basin are consistent with northern U.S. Cordilleran sources, and Ochoco strata may represent the destination of much of the mid- to Late Cretaceous Idaho arc that was intruded and eroded during and following rapid transpression along the western Idaho shear zone. Our provenance results suggest that the Hornbrook Formation and Ochoco basin formed two sides of the same depositional system, which may have been linked to the Great Valley Group to the south by Coniacian time, but was not connected to the Methow basin. These results limit northward displacement of the Ochoco basin to less than 400 km relative to the North American craton, and suggest that the anomalously shallow paleomagnetic inclinations may result from significant inclination error, rather than deposition at low latitudes. Our results demonstrate that detailed provenance analysis of forearc strata complements the incomplete record of arc magmatism and tectonics preserved in bedrock exposures, and permits improved understanding of Late Cretaceous Cordilleran paleogeography

Understanding a Critical Basinal Link in Cretaceous Cordilleran Paleogeography: Detailed Provenance of the Hornbrook Formation, Oregon and California

The Hornbrook Formation is a Cretaceous overlap assemblage that rests unconformably on accreted terranes and plutons of the Klamath Mountains in southern Oregon and northern California. The combined results of sandstone petrography, detrital zircon U-Pb age and Hf isotopic systematics, and whole-rock Nd analysis document an abrupt change in sediment sources for the Hornbrook Formation during the Late Cretaceous. The lower members of the Hornbrook Formation record provenance in the Klamath Mountains and the Sierran Foothills belt that is characterized by detrital zircon age distributions with large Jurassic and Early Cretaceous peaks (170-130 Ma) and positive zircon Hf and whole-rock Nd values. In contrast, upper members of the Hornbrook Formation include abundant sediment derived from the Cretaceous Sierran Batholith that is characterized by large Cretaceous peaks (120-85 Ma) in detrital zircon age distributions and less positive zircon Hf and whole-rock Nd values. A similar Late Cretaceous provenance shift is present in the Great Valley Group of California, which likely formed the southern continuation of the Hornbrook basin during deposition of the upper Hornbrook members. These provenance results may reflect changing plate kinematics along the U.S. Cordilleran margin during the Late Cretaceous, including extension and subsidence in the Klamath Mountains and Blue Mountains regions followed by rapid uplift of the main Sierra Nevada Batholith. Thus, the detailed provenance signature for the Hornbrook Formation presented here records regional tectonic events in the mid- to Late Cretaceous U.S. Cordillera, and suggests that the Hornbrook Formation and Great Valley Group shared similar sources but remained separate basins until mid- to Late Cretaceous time

Facies Architecture and Provenance of a Boulder-Conglomerate Submarine Channel System, Panoche Formation, Great Valley Group: A Forearc Basin Response to Middle Cretaceous Tectonism in the California Convergent Margin

Tectonic reorganization induced by a rapid increase in plate motion ­obliquity and rate beginning at ca. 100 Ma affected California’s Andean-style convergent margin, with concomitant changes in the accretionary prism of the Franciscan Complex, the Great Valley forearc basin, and the Sierran continental arc. Using facies analysis and a combined provenance approach, we suggest that this ca. 100 Ma tectonic signal is preserved in a Cenomanian (Upper Cretaceous) boulder-conglomerate outcrop along the San Luis Reservoir (SLR) in the southern Great Valley, which represents the thickest and coarsest deep-water deposit ever described in the Great Valley Group (GVG). We document a 1.8-km-thick by 4-km-long depositional-dip profile of an interpreted SE-directed (axial) submarine channel system that is part of a conglomeratic package that stretches 20 km along the east-central Diablo Range. Our facies analysis of the SLR area documents five facies associations within four aggradational channel complex sets, followed by regional abandonment. Sandstone petrography and mudrock geochemical data suggest a dissected continental Sierra Nevadan arc source. Conglomerate clast counts show abundant ophiolitic-type clasts that may be derived from the Coast Range Ophiolite and/or the Western Sierra Nevada Metamorphic Belt. Detrital-­zircon geochronology data also indicate western and central Sierra Nevadan sources; however, we interpret an anomalous (relative to other Cenomanian localities) 105–95 Ma zircon population to indicate the initial erosional products from the volcanic carapace associated with the Late Cretaceous magmatic flare-up within the eastern Sierran arc. This flare-up has been linked to an increase in arc-parallel plate motion that induced deformation along shear zones in the eastern Sierra Nevada, allowing for widespread plutonism. Our provenance interpretation makes the SLR area the earliest Upper Cretaceous GVG locality to receive significant detritus from the flare-up, effectively linking tectonic plate motion changes and coarse-grained, deep-water forearc sedimentation

East-derived Strata in the Methow Basin Record Rapid Mid-Cretaceous Uplift of the Southern Coast Mountains Batholith

The Jurassic–Cretaceous Methow basin of northern Washington State and southern British Columbia forms an overlap sequence linking several small tectonostratigraphic terranes. Sandstone petrography, sandstone and mudrock geochemistry, and detrital zircon U–Pb age and Hf analysis of mid-Cretaceous, east-derived Methow strata together document a remarkably uniform provenance signature that suggests proximal, abundant, and unchanging sediment sources throughout deposition. The eastern belt of the Coast Mountains batholith, intruded into Stikine and related inboard terranes of the Intermontane superterrane, along with Jurassic and Cretaceous plutons of the westernmost Okanogan Range, provide the best match to the provenance signature of east-derived sediment in the Methow basin during the mid-Cretaceous. Furthermore, the Cretaceous and Jurassic plutons of the eastern Coast Mountains batholith and western Okanogan Range were rapidly uplifted to provide the substantial thickness of sediment in the Methow basin, and they must have acted as a topographic barrier that effectively prevented sediment derived from the continental interior from reaching the basin. This uplift of a proximal eastern source occurred during regional late Early Cretaceous sinistral transpression and resulted in subsidence of the Methow trough and rapid deposition of east-derived strata in the Methow basin. Because Methow sediment sources apparently did not include the North American interior, the extent of post-depositional large-scale translation relative to the North American craton of the Methow basin with its proximal, eastern sources cannot be unequivocally determined. Le bassin de Methow (Jurassique–Crétacé) du nord de l\u27État de Washington et du sud de la Colombie-Britannique forme une séquence de chevauchement reliant plusieurs petits terranes tectonostratigraphiques. La pétrographie des grès, la géochimie des grès et des pélites ainsi que les âges déterminés par U–Pb sur des zircons détritiques et des analyses Hf des strates de Methow provenant de l\u27est (Crétacé moyen) documentent ensemble une signature de provenance remarquablement uniforme, suggérant des sédiments de sources proximales, abondantes et inchangées durant toute la déposition. La ceinture est du batholite de la Chaîne côtière, introduite dans le terrane de Stikine et d\u27autres terranes intérieurs reliés du superterrane intermontagneux, et les plutons datant du Jurassique et du Crétacé de la partie la plus à l\u27ouest du chaînon Okanogan, fournissent la meilleure concordance pour une signature de provenance de sédiments de l\u27est vers le bassin de Methow au Crétacé moyen. De plus, les plutons datant du Crétacé et du Jurassique du batholite de l\u27est de la chaîne Côtière et de l\u27ouest du chaînon Okanagan ont été soulevés rapidement, fournissant l\u27importante épaisseur de sédiments dans le bassin de Methow, et ils ont dû agir de barrière topographique qui a effectivement empêché les sédiments provenant de l\u27intérieur du continent d\u27atteindre le bassin. Ce soulèvement d\u27une source proximale à l\u27est a eu lieu durant une transpression senestre régionale, à la fin du Crétacé précoce, et a conduit à la subsidence de la fosse de Methow et à la déposition rapide de strates provenant de l\u27est dans le bassin de Methow. Puisque les sources des sédiments du bassin de Methow n\u27incluaient apparemment pas l\u27intérieur de l\u27Amérique du Nord, l\u27étendue de la translation post-déposition à grande échelle du bassin de Methow avec ses sources proximales de l\u27est, par rapport au craton de l\u27Amérique du Nord, ne peut pas être déterminée de manière catégorique

Evolution and Stratigraphic Architecture of Marine Slope Gully Complexes: Monterey Formation (Miocene), Gaviota Beach, California

Three small headlands in the sea cliffs west of Gaviota Beach, California, are the remnant fill of three discrete submarine gullies incised into the late Miocene submarine slope environment. These promontories provide excellent, three-dimensional exposure of the gully fill in outcrop, permitting documentation of their complex internal stratigraphic architecture. Detailed study of these exposures elucidates the sedimentologic processes that occur in the filling of slope gullies, guides interpretation of the acoustic records of otherwise unsampled modern gully systems on continental slopes, and provides insight into the heterogeneity that may characterize slope gully petroleum reservoirs. We develop a comprehensive facies scheme to describe the variability within these intercalated coarse- and fine-grained deposits and use two-dimensional photopans to interpret the overall depositional system. Defined by internal bedforms, sedimentation units, and sediment size, each facies records sedimentation under different hydrodynamic conditions and can be genetically related to a discrete depositional mechanism. We differentiate channel axis, margin, and overbank sub-environments within the gully fill, which together define overall crudely braided, low-sinuosity channels within the gullies. Like many modern gully systems, the Gaviota gullies probably initiated through local slope oversteepening that led to slope failure, slumping, and initiation of sediment flows. Erosion and sedimentation from these high-density turbidity currents formed the primary depositional process for the Gaviota conglomerate units. Once initiated, the gullies acted as sediment conduits from the shelf and possibly onshore regions. Despite their prevalence on modern upper slopes and their pivotal role in shaping shelf margins and transporting sediment to deeper water, the details of submarine slope gully formation and filling remain obscure. The Gaviota gully complexes provide valuable insights into long-term gully fill attributes not easily obtained from modern slope gully systems and rarely preserved in the rock record

Does the Great Valley Group Contain Jurassic Strata? Reevaluation of the Age and Early Evolution of a Classic Forearc Basin

The presence of Cretaceous detrital zircon in Upper Jurassic strata of the Great Valley Group may require revision of the lower Great Valley Group chronostratigraphy, with significant implications for the Late Jurassic–Cretaceous evolution of the continental margin. Samples (n = 7) collected from 100 km along strike in the purported Tithonian strata of the Great Valley Group contain 20 Cretaceous detrital zircon grains, based on sensitive high-resolution ion microprobe age determinations. These results suggest that Great Valley Group deposition was largely Cretaceous, creating a discrepancy between biostratigraphy based on Buchia zones and chronostratigraphy based on radiometric age dates. These results extend the duration of the Great Valley Group basal unconformity, providing temporal separation between Great Valley forearc deposition and creation of the Coast Range Ophiolite. If Great Valley forearc deposition began in Cretaceous time, then sediment bypassed the developing forearc in the Late Jurassic, or the Franciscan subduction system did not fully develop until Cretaceous time. In addition to these constraints on the timing of deposition, pre-Mesozoic detrital zircon age signatures indicate that the Great Valley Group was linked to North America from its inception