Great Valley Group Research Articles

Age and provenance relationships between the basal Great Valley Group and its underlying basement: implications for initiation of the Great Valley forearc basin, California, U.S.A.

ABSTRACT The Great Valley forearc (GVf) basin, California, records deposition along the western margin of North America during active oceanic subduction from Jurassic through Paleogene time. Along the western GVf, its underlying basement, the Coast Range Ophiolite (CRO), is exposed as a narrow outcrop belt. CRO segments are overlain by the Great Valley Group (GVG), and locally, an ophiolitic breccia separates the CRO from basal GVG strata. New stratigraphic, petrographic, and geochronologic data (3865 detrital and 68 igneous zircon U-Pb ages) from the upper CRO, ophiolitic breccia, and basal GVG strata clarify temporal relationships among the three units, constrain maximum depositional ages (MDAs), and identify provenance signatures of the ophiolitic breccia and basal GVG strata. Gabbroic rocks from the upper CRO yield zircon U-Pb ages of 168.0 ± 1.3 Ma and 165.1 ± 1.2 Ma. Prominent detrital-zircon age populations of the ophiolitic breccia and GVG strata comprise Jurassic and Jurassic–Early Cretaceous ages, respectively, with pre-Mesozoic ages in both that are consistent with sources of North America affinity. Combined with petrographic modal analyses that show abundant volcanic grains (> 50%), we interpret the breccia to be mainly derived from the underlying CRO, with limited input from the hinterland of North America, and the basal GVG to be derived from Mesozoic igneous and volcanic rocks of the Sierra Nevada–Klamath magmatic arc and hinterland. Analysis of detrital-zircon grains from the lower and upper ophiolitic breccia yields MDAs of ∼ 166 Ma and ∼ 151 Ma, respectively. Along-strike variation in Jurassic and Cretaceous MDAs from basal GVG strata range from ∼ 148 to 141 Ma, which are interpreted to reflect diachronous deposition in segmented depocenters during early development of the forearc. The ophiolitic breccia was deposited in a forearc position proximal to North America < 4 Myr before the onset of GVG deposition. A new tectonic model for early development of the GVf highlights the role of forearc extension coeval with magmatic arc compression during the earliest stages of basin development.

Late Jurassic paleogeography of the U.S. Cordillera from detrital zircon age and hafnium analysis of the Galice Formation, Klamath Mountains, Oregon and California, USA

The Upper Jurassic Galice Formation, a metasedimentary unit in the Western Klamath Mountains, formed within an intra-arc basin prior to and during the Nevadan orogeny. New detrital zircon U-Pb age analyses (N = 11; n = 2792) yield maximum depositional ages (MDA) ranging from ca. 160 Ma to 151 Ma, which span Oxfordian to Kimmeridgian time and overlap Nevadan contractional deformation that began by ca. 157 Ma. Zircon ages indicate a significant North American continental provenance component that is consistent with tectonic models placing the Western Klamath terrane on the continental margin in Late Jurassic time. Hf isotopic analysis of Mesozoic detrital zircon (n = 603) from Galice samples reveals wide-ranging εHf values for Jurassic and Triassic grains, many of which cannot be explained by a proximal source in the Klamath Mountains, thus indicating a complex provenance. New U-Pb ages and Hf data from Jurassic plutons within the Klamath Mountains match some of the Galice Formation detrital zircon, but these data cannot account for the most non-radiogenic Jurassic detrital grains. In fact, the in situ Cordilleran arc record does not provide a clear match for the wide-ranging isotopic signature of Triassic and Jurassic grains. When compiled, Galice samples indicate sources in the Sierra Nevada pre-batholithic framework and retroarc region, older Klamath terranes, and possibly overlap strata from the Blue Mountains and the Insular superterrane. Detrital zircon age spectra from strata of the Upper Jurassic Great Valley Group and Mariposa Formation contain similar age modes, which suggests shared sediment sources. Inferred Galice provenance within the Klamath Mountains and more distal sources suggest that the Galice basin received siliciclastic turbidites fed by rivers that traversed the Klamath-Sierran arc from headwaters in the retroarc region. Thus, the Galice Formation contains a record of active Jurassic magmatism in the continental arc, with significant detrital input from continental sediment sources within and east of the active arc. These westward-flowing river systems remained active throughout the shift in Cordilleran arc tectonics from a transtensional system to the Nevadan contractional system, which is characterized by sediment sourced in uplifts within and east of the arc and the thrusting of older Galice sediments beneath older Klamath terranes to the east.

Petrofacies of Eocene sand injectites of the Tumey Giant Injection Complex, California (USA)

The forearc succession of Great Valley Group in Central California provides some of the best examples of giant sand injection complexes in the world and is therefore considered valuable analogues for injectite systems in the subsurface. Several sand injection complexes are well described in the outcrop and subsurface, however the petrographic characteristics of injectites are still poorly documented. In this paper, we present the results of an integrated study of field observations, quantitative and qualitative sandstone petrography, provenance, and petrofacies analysis of the Tumey Giant Injection Complex (TGIC) in order to understand its lithostratigraphy and petrological evolution, and its impacts on reservoir petrofacies characteristics and fluid migration. The TGIC intrudes into a 450 m thick deep-water succession of slope mudrocks and sandy channel-fills of the Kreyenhagen Shale (Eocene), forming an interconnected network of sandstone sills, dykes and injection breccias. The complex generated a horizontal and vertical plumbing system for fluid migration, connecting isolated sandy channel-fills among low-permeability mudrocks. The primary detrital composition, diagenetic products, microtextures, and provenance signatures allowed for the definition and discrimination of depositional and intrusive petrofacies and their genetic relations. Petrofacies associations confirm that the gypsum-cemented feldspathic litharenites from the Kreyenhagen Shale channel-fills are the only source for the injection complex. Eodiagenetic compaction and extensive gypsum cementation reduced the primary porosity of the complex, while telodiagenetic dissolution of autigenic constituents formed pervasive secondary porosity. The underlying calcite-cemented arkosic sandstones of the Lodo and Domengine formations acted as barriers for fluid flow, aiding lateral fluid migration and overpressure buildup within the overlying Kreyenhagen channel-fills. Intense grain microfracturing occurred during sand remobilization and injection along with erosion of the host mudrocks. This study has significance for the generation of reliable stratigraphic and petrological models for sand injection complex genesis and evolution that consequently can help the understanding and exploration of injectite complexes elsewhere.

The birth of a forearc: The basal Great Valley Group, California, USA: REPLY

The birth of a forearc: The basal Great Valley Group, California, USA: REPLY

The birth of a forearc: The basal Great Valley Group, California, USA: COMMENT

The birth of a forearc: The basal Great Valley Group, California, USA: COMMENT

The birth of a forearc: The basal Great Valley Group, California, USA

AbstractThe 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.

Sedimentary compared to tectonically-deformed serpentinites and tectonic serpentinite mélanges at outcrop to petrographic scales: Unambiguous and disputed examples from California

This paper compares features of unambiguous tectonic serpentinite mélanges (TSM) or serpentinite shear zones in the Coast Range ophiolite, Franciscan subduction complex, of coastal California and Sierra City Mélange of the northern Sierra Nevada of northeastern California with undisputed sedimentary serpentinite mélange (SSM) of the Great Valley Group (GVG) forearc basin deposits of coastal California, and with Franciscan serpentinite mélanges of disputed (sedimentary versus tectonic) origin. The GVG sedimentary serpentinite mélanges and disputed Franciscan serpentinite mélanges share strongly similar matrix textures and block-matrix relationships at scales from tens of meters or more to petrographic scale but differ significantly from serpentinite shear zones and TSM. This comparison suggests shared (non-diagnostic) and distinguishing features of TSM versus SSM. Internal bedding or foliation in blocks is oriented subparallel to mélange boundaries and matrix foliation for both TSM and SSM both may have strongly foliated matrix and both may feature localized shearing in matrix around block borders, especially if an SSM underwent significant post-depositional deformation. The same holds true for deformation and dismemberment of blocks, which is the block-forming and mixing mechanism in TSM but variably exhibited in SSM. In contrast only SSM have blocks or clasts whose internal foliation or bedding terminates abruptly along clast/block boundaries with a mismatch in mineralogy and/or lithology across such boundaries. Matrix foliation cuts blocks/clasts in TSM but not in SSM. SSM may show block/grain size grading but not TSM. SSM have exotic blocks and blocks may span a range of metamorphic grade, whereas TSM lack exotic blocks and blocks are isofacial.

Serpentinites and serpentinites: Variety of origins and emplacement mechanisms of serpentinite bodies in the California Cordillera

AbstractMost serpentinitized peridotite in orogenic belts is derived from oceanic lithosphere, but the emplacement mechanisms of these rocks vary greatly, as illustrated by the nature of these rock bodies and their contacts. The diverse emplacement mechanisms have important implications for connecting ophiolitic rock occurrences to large‐scale orogenic processes. In the California Cordillera, the largest bodies of ultramafic rocks are parts of ophiolite sheets, such as the Coast Range ophiolite (CRO), that were part of the upper plate of an oceanic subduction system. Such units differ from smaller bodies within subduction complexes such as the Franciscan Complex that were transferred from the subducting plate to the subduction complex during accretion. Some intra‐subduction complex ultramafic rocks occur as nearly block‐free sheets within the Franciscan Complex, and as a part of mafic–ultramafic imbricates or broken formations within the Shoo Fly Complex of the northern Sierra Nevada. Franciscan Complex serpentinite also occurs as sedimentary serpentinite mélange that was partly subducted after deposition in the trench via submarine sliding. Such mélanges include blocks that record older and higher grade metamorphism than the matrix. Sedimentary serpentinite mélange that includes high‐pressure metamorphic blocks is also found in the basal Great Valley Group forearc basin deposits depositionally overlie the CRO. Distinguishing the different serpentinite origins is difficult in the California Cordillera even though a terminal continental collision did not affect this orogenic belt. In more typical orogenic belts with greater post‐subduction disruption, distinction between the types of serpentinite occurrences presents a greater challenge.

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.

Heavy-Mineral Assemblages In Sandstone Intrusions: Panoche Giant Injection Complex, California, U.S.A.

Excellent exposure from part of the Panoche Giant Injection Complex in the San Joaquin Valley is used to examine provenance characteristics of sandstone intrusions with respect to two parent sandstone units that are known to feed the sand-injection complex. The succession is part of the upper Mesozoic to lower Tertiary Great Valley Group, and was deposited in a deepwater part of an evolving deep-water forearc basin. The section examined is mudstone-dominated, and the sand injection is constrained to have occurred in the Danian. Sandstones in the Dosados Member (Moreno Fm) are identified as the main parent unit on the basis of total heavy-mineral-assemblage compositions and varietal studies of selected minerals (tourmaline, garnet, titanite, apatite, and zircon). Fluidized sand is emplaced in turbulent flow conditions creating high-velocity inter-grain collisions. Evidence of comminution and diminution of minerals that are less hard than quartz is documented using indices for the relative hardness (TAH) and durability (TAD) of heavy minerals. Preferential settling of high-density zircon relative to lower-density tourmaline produces density-controlled variations of zircon:tourmaline upward through the injection complex. Heavy-mineral dissolution occurred in the most permeable sandstone intrusions and is believed to record the effects of mid-Eocene deep weathering, when subtropical climate prevailed in the study area. Detrital heavy-mineral assemblages, which are dominated by titanite and garnet, record erosion of the Sierran metamorphic terrane with mafic and alkaline plutonic rocks. Zircon with U/Pb ages of c. 140–160 Ma and c. 90–110 Ma, consistent with earlier independent analyses, record erosion of Sierran granitoids. On the paleo-seafloor, enrichment of Ca-amphibole and epidote is indicative of Sierran provenance concurrent with sand extrusion. The presence of Na-amphibole in the Uhalde Sandstone supports earlier work that suggested sediment input from obducted seafloor to the west.

Sedimentary serpentinite and chaotic units of the lower Great Valley Group forearc basin deposits, California: updates on distribution and characteristics

ABSTRACTSedimentary serpentinite and related siliciclastic-matrix mélanges in the latest Jurassic to Lower Cretaceous lower Great Valley Group (GVG) forearc basin strata of the California Coast Ranges reach thicknesses of over 1 km and include high-pressure (HP) metamorphic blocks. These units crop out over an area at least 300 km long by 50 km wide. The serpentinite also contains locally abundant blocks of antigorite mylonite. Antigorite mylonite and HP metamorphic blocks were exhumed from depth prior to deposition in the unmetamorphosed GVG, but the antigorite mylonite may be mistaken for metamorphosed serpentinite matrix in localities with limited exposure. These olistostrome horizons can be distinguished from intact slabs of serpentinized peridotite associated with the Coast Range Ophiolite (CRO) or serpentinite mélanges of the Franciscan subduction complex (FC) on the basis of internal sedimentary textures (absent in CRO), mixing/interbedding with unmetamorphosed siliciclastic matrix and blocks (differs from CRO and FC), and preserved basal sedimentary contacts over volcanic rocks of the CRO or shale, sandstone, and conglomerate of the GVG (differs from CRO and FC). Even in the relatively well-characterized Palaeo trench–forearc region of the California Coast Ranges the GVG deposits are difficult to distinguish from similar units in the FC and CRO. In typical orogenic belts that exhibit greater post-subduction disruption, distinguishing forearc basin olistostrome deposits, subduction complex, and opholite mantle sections is much more difficult. Forearc basin olistostromal deposits have probably been misidentified as one of the other trench–forearc lithologic associations. Such errors may lead to erroneous interpretations of the nature of large-scale material and fluid pathways in trench–forearc systems, as well as misinterpretations of tectonic processes associated with HP metamorphism and exhumation of the resultant rocks.

Application of actualistic sand petrofacies in hydrogeology: An example from the northern Sacramento Valley, California, USA

Diverse detrital source areas surrounding the Sacramento Valley of northern California and the abundance of deep water wells from which sand samples can be acquired make this an ideal natural laboratory in which to apply actualistic sand petrofacies methods in solving hydrogeologic subsurface problems. Discriminant analysis identifies four distinct compositional suites (petrofacies) representing four distinct sources: metamorphic (M), volcanic (V), mixed volcanic and metamorphic (VM), and mixed volcanic, metamorphic, and sedimentary (VMS). Proportions of aphanitic metamorphic, volcanic, and sedimentary lithic fragments (using the Gazzi-Dickinson point-counting method) are the primary distinguishing parameters. The combined surface and subsurface mapping of lithofacies and petrofacies confirms that the Tehama and Tuscan Formations interfinger in the subsurface near the center of the northern Sacramento Valley. The Pliocene–Pleistocene Tehama Formation along the west side predominantly consists of metamorphiclastic detritus derived from the Klamath Mountains and the Coast Ranges, including the Franciscan Complex and the Great Valley Group. The Pliocene–Pleistocene Tuscan Formation along the east side predominantly consists of volcaniclastic detritus derived from the southern end of the Cascade Range and the Modoc Plateau. These petrofacies results complement regionally integrated surface mapping and subsurface data in order to construct regional cross sections showing basin architecture. The general correspondence between lithofacies and petrofacies provides a powerful tool for further investigation, but the lack of correspondence in a few cases indicates where additional work is warranted. The simultaneous study of sand composition and three-dimensional geologic relations provides additional insights regarding the architecture of aquifers, knowledge of which improves water-resource development and management.

Review of Late Jurassic-early Miocene sedimentation and plate-tectonic evolution of northern California: illuminating example of an accretionary margin

Production of voluminous igneous arc rocks, high-pressure/low-temperature (HP/LT) metamafic rocks, westward relative migration of the Klamath Mountains province, and U–Pb ages of deposition, sediment sources, and spatial locations of Jurassic and younger, detrital zircon-bearing clastic rocks constrain geologic development of the northern California continental edge as follows: (1) At ~175 Ma, transpressive plate underflow began to generate an Andean-type Klamath-Sierran arc along the margin. (2) Oceanic crustal rocks were metamorphosed under HP/LT conditions in an inboard, east-inclined subduction zone from ~170-155 Ma. Except for the Red Ant blueschists, such lithologies remained stored at depth; most HP/LT mafic tectonic blocks returned surfaceward only during mid- and Late Cretaceous time, chiefly entrained in circulating, buoyant Franciscan mud-matrix melange. (3) By ~165 Ma and continuing to ~150-140 Ma, erosion supplied volcanogenic debris to proximal Mariposa-Galice ± Myrtle overlap strata. (4) At ~140, immediately prior to the onset of paired Franciscan and Great Valley Group (GVG) + Hornbrook deposition, the Klamath salient was deformed and displaced ~100–150 km westward relative to the Sierran arc, stranding pre-existing oceanic crust on the south as the Coast Range Ophiolite (CRO). (5) After the end-of-Jurassic seaward step-out of the Farallon-North American convergent plate junction, terrigineous debris began to be deposited in the outboard Franciscan trench and intervening Great Valley forearc. (6) Voluminous sedimentation and accretion of Franciscan Eastern + Central belts and GVG detritus took place during paroxysmal igneous activity and rapid, nearly orthogonal plate convergence at ~125-80 Ma. (7) Sierran arc volcanism-plutonism ceased by ~80 Ma in northern California, signaling a transition to shallow, nearly subhorizontal eastward plate underflow attending Laramide orogeny far to the east. (8) Presently exposed Paleogene-lower Miocene Franciscan Coastal Belt sedimentary strata were deposited in a tectonic realm unaffected by HP/LT subduction. (9) Grenville-age detrital zircons are absent from the post-120 Ma Franciscan section. (10) Judging from petrofacies and zircon U–Pb data, the Franciscan Eastern Belt contains debris derived principally from the Sierra Nevada and Klamath ranges; detritus from the Idaho Batholith as well as Sierra Nevada Batholith may be present in some Central Belt sandstones, whereas clasts from the Idaho Batholith, Challis volcanics, and Cascade Range appear in progressively younger Paleogene-lower Miocene Coastal Belt sediments. (11) Gradual NW dextral offset of the Franciscan trench deposits of as much as ~1,600 km may have occurred relative to the native GVG forearc and basement terranes of the American Southwest.

Anatomy of a subduction complex: architecture of the Franciscan Complex, California, at multiple length and time scales

The Franciscan Complex of California records over 150 million years of continuous E-dipping subduction that terminated with conversion to a dextral transform plate boundary. The Franciscan comprises mélange and coherent units forming a stack of thrust nappes, with significant along-strike variability, and downward-decreasing metamorphic grade and accretion ages. The Franciscan records progressive subduction, accretion, metamorphism, and exhumation, spanning the extended period of subduction, rather than events superimposed on pre-existing stratigraphy. High-pressure (HP) metamorphic rocks lack a thermal overprint, indicating continuity of subduction from subduction initiation at ca. 165 Ma to termination at ca. 25 Ma. Accretionary periods may have alternated with episodes of subduction erosion that removed some previously accreted material, but the complex collectively reflects a net addition of material to the upper plate. Mélanges (serpentinite and siliciclastic matrix) with exotic blocks have sedimentary origins as submarine mass transport deposits, whereas mélanges formed by tectonism comprise disrupted ocean plate stratigraphy and lack exotic blocks. The former are interbedded with and grade into coherent siliciclastic units. Palaeomegathrust horizons, separating nappes accreted at different times, appear restricted to narrow zones of <100 m thickness. Exhumation of Franciscan units, both coherent and mélange, was accommodated by significant extension of the hanging wall and cross-sectional extrusion. The amount of total exhumation, as well as exhumation since subduction termination, needs to be considered when comparing Franciscan architecture to modern and ancient subduction complexes. Equal dextral separation of folded Franciscan nappes and late Cenozoic (post-subduction) units across strands of the (post-subduction) San Andreas fault system shows that the folding of nappes took place prior to subduction termination. Dextral separation of similar clastic sedimentary suites in the Franciscan and the coeval Great Valley Group forearc basin is approximately that of the San Andreas fault system, precluding major syn-subduction strike-slip displacement within the Franciscan.

Using 87Sr/86Sr Ratios to Date Fossil Methane Seep Deposits: Methodological Requirements and an Example from the Great Valley Group, California

AbstractMethane seep carbonates preserve information about the history of methane seepage and of the fauna inhabiting these ecosystems. For this information to be useful, a reliable determination of the carbonates’ stratigraphic ages is required, but this is not always available. Here we investigate the using strontium isotope stratigraphy to date fossil methane seep carbonates via detailed petrographic and geochemical investigation of the different carbonate phases in biostratigraphically well-dated seep carbonates of Paleozoic, Mesozoic, and Cenozoic age. The best results are obtained from banded, botryoidal rim cements from carbonate phases showing a weak or no cathodoluminescence signal, an oxygen isotope signature close to that of seawater, and the lowest Mn concentrations. We then applied the method to a presumably late Jurassic seep carbonate from the Great Valley Group in California. Strontium isotope ratios of the least diagenetically altered carbonate phases indicate a Tithonian (late Jurassic) ...

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.

Implications of Franciscan Complex graywacke geochemistry for sediment transport, provenance determination, burial‐exposure duration, and fluid exchange with cosubducted metabasites

AbstractInterpretation of graywacke provenance has long been used to evaluate the record of tectonic process in orogenic belts. Our geochemical data from graywackes of the Franciscan subduction complex, California, show that the connection between sedimentary record and geologic processes may be more complex than previously believed. Trace elements and Nd‐Sr‐Pb isotopes of Franciscan graywackes indicate two sources types. One group lacking negative Eu anomaly (Eu/Eu* &gt; 0.9), shows slightly concave‐up heavy rare earth elements, arc‐like trace element patterns, and western Pacific island arc‐like Pb isotopes, reflecting derivation from older accreted oceanic‐arc terranes in the Sierra Nevada‐Klamath Mountains. The other group displays small negative Eu anomalies, with trace element patterns resembling post‐Archean Australian shale and Pb isotopes similar to Jurassic‐Cretaceous Sierran batholith. There is no systematic separation of these two groups by depositional ages. Thus, geochemistry of the graywackes may partly reflect variation in location of sediment delivery systems, rather than solely reflecting evolution of the neighboring arc. Variation of Nd‐Sr isotopes with stratigraphic‐age for the graywackes mimics the trends of the coeval Great Valley Group clastic‐rocks, suggesting that (1) they share the same sediment sources, (2) there are no “exotic” sediment sources that fed the Franciscan trench, and (3) burial‐exposure cycles for Franciscan clastic rocks were comparatively brief. Comparison of Franciscan graywacke and metabasite geochemistry corroborates earlier conclusions that metabasites had little or no chemical exchange with fluids from cosubducted graywacke. Detrital zircon age populations, major element chemistry, and detrital framework modes, when compared to our data suggest that the former three parameters underrepresent the mafic component of clastic sediment provenance.

Controls on forearc basin architecture from seismic and sequence stratigraphy of the Upper Cretaceous Great Valley Group, central Sacramento Basin, California

Seismic and sequence stratigraphic analysis of deep-marine forearc basin fill (Great Valley Group) in the central Sacramento Basin, California, reveals eight third-order sequence boundaries within the Cenomanian to mid-Campanian second-order sequences. The third-order sequence boundaries are of two types: Bevelling Type, a relationship between underlying strata and onlapping high-density turbidites; and Entrenching Type, a significantly incised surface marked by deep channels and canyons carved during sediment bypass down-slope. Condensed sections of hemipelagic strata draping bathymetric highs and onlapped by turbidites form a third important type of sequence-bounding element, Onlapped Drapes. Five tectonic and sedimentary processes explain this stratigraphic architecture: (1) subduction-related tectonic tilting and deformation of the basin; (2) avulsion of principal loci of submarine fan sedimentation in response to basin tilting; (3) deep incision and sediment bypass; (4) erosive grading and bevelling of tectonically modified topography by sand-rich, high-density turbidite systems; and (5) background hemipelagic sedimentation. The basin-fill architecture supports a model of subduction-related flexure as the principal driver of forearc subsidence and uplift during the Late Cretaceous. Subduction-related tilting of the forearc and growth of the accretionary wedge largely controlled whether and where the Great Valley turbiditic sediments accumulated in the basin. Deeply incised surfaces of erosion, including submarine canyons and channels, indicate periods of turbidity current bypass to deeper parts of the forearc basin or the trench. Fluctuations in sediment supply likely also played an important role in evolution of basin fill, but effects of eustatic fluctuations were overwhelmed by the impact of basin tectonics and sediment supply and capture. Eventual filling and shoaling of the Great Valley forearc during early Campanian time, coupled with dramatically reduced subsidence, correlate with a change in plate convergence, presumed flat-slab subduction, cessation of Sierran arc volcanism, and onset of Laramide orogeny in the retroarc.

Bedrock geology of the San Francisco Bay Area: A local sediment source for bay and coastal systems

Abstract Local watersheds may contribute over half of the sediment load coming into San Francisco Bay today. The bedrock underlying these watersheds is the ultimate source for most of this sediment. This paper outlines the geologic history of this bedrock, which records the complex tectonic history of the San Francisco Bay Area over the last 200 Ma. The Jurassic to Eocene Franciscan Complex in the Bay Area is the most widespread bedrock. The local Franciscan can be broken into nine tectonic terranes that represent pieces of seafloor that were accreted to the North American margin in over a 100 Myr period of subduction. Each terrane has a unique age range, sequence of seafloor rocks, and metamorphic history. The Franciscan rocks were thrust eastward under the Great Valley Complex. The Great Valley Complex reflects a forearc basin comprised of Jurassic ocean crust—the Coast Range ophiolite—and overlying Jurassic and Cretaceous turbidite-dominated sedimentary rocks of the Great Valley Group. By the beginning of the Cenozoic Era, shallowing of the angle of subduction along the continental margin led to uplift and infilling of the forearc basin to shelf depth. Recorded at this time are sedimentary sequences governed by eustatic sealevel changes and evidence of the first unroofing of Franciscan rocks. Marine sedimentary rocks dominate the region through the Miocene and unconformities within the Early Tertiary sedimentary record reflect periods of tectonic activity and uplift. Middle to late Miocene sedimentary rocks are extensively deposited throughout the Bay Area. In the late Miocene, the Mendocino triple junction passed northward through the area and transform tectonism commenced. A slab window developed behind the triple junction resulted in volcanic activity that produced the Sonoma Volcanics in the North Bay and volcanics of the East Bay Hills. By Pliocene time, terrestrial sedimentary rocks were being deposited in many parts of the Bay Area. Faulting associated with the San Andreas system led to the development of small basins, the displacement of local bedrock blocks, and the transport of the Salinian Complex from the southern Sierra to the western margin of the Bay Area. The modern topography of the region started emerging by about 6 Ma to 4 Ma, when compression across the San Andreas system increased and uplift of the Coast Ranges began. The first estuarine influence in San Francisco Bay is recorded in ~ 600 ka rocks, about the same time as the modern Sacramento River system started flowing through the San Francisco estuary/valley. Today's local San Francisco Bay watershed is comprised of numerous small stream systems around the Bay with headwaters in the surrounding hills and mountains. Active tectonism produces relief in these watersheds of typically 500 m or more, with the ridgelines typically supported by more resistant bedrock units. Steep topography leads to rapid bedrock erosion, particularly of the finer-grained sedimentary rocks. Landslides provide a significant portion of the sediment carried by the streams and several of the bedrock units are more prone to landslides, particularly the Central and Novato Quarry terranes of the Franciscan Complex, and finer-grained units of both the Great Valley Group and Tertiary rocks. Many of the larger streams have dammed reservoirs that trap much of their sediment load before it can reach the bay.

Earliest Cretaceous Pacificward offset of the Klamath Mountains salient, NW California–SW Oregon

Although Late Triassic igneous rocks are present, the Sierra Nevada–Klamath calc-alkaline arc began massive construction along the continental margin at ca. 170 Ma during oblique underflow of paleo-Pacific oceanic lithosphere; intense activity continued throughout the volcanic-plutonic belt until at least ca. 140 Ma. This volcanic-plutonic arc supplied detritus to the Mariposa-Galice proximal clastic sequence starting by ca. 165–160 Ma. After onset of uppermost Jurassic Myrtle overlap sedimentation on the western flank of the Klamath Mountains, but before Hornbrook and Valanginian Great Valley Group overlap deposition on the eastern and southeastern sides, the Klamath Mountains salient was displaced ∼200 km westward relative to the igneous arc. The orogen thus moved off the deep-seated magmagenic zone underlying the arc and did not participate in the massive Sierra Nevada igneous flare-up between ca. 125 and ca. 85 Ma. I suggest that, beginning at ca. 140 Ma, underflow of a young, thin oceanic slab beneath the Klamath Mountains slid beneath the gently east-dipping stack of thrust sheets without disturbing their inclinations. Subduction and collision of much thicker oceanic lithosphere on both the north and south caused contraction, eastward relative displacement of the continental margin arc, and ductility-enhanced rotation of the superjacent stack of allochthons into near-vertical dips. After a magmatic lull, heightened igneous activity in the Sierra Nevada recommenced at ca. 125 Ma. The earliest Cretaceous oceanward plate junction rollback lay directly offshore the Klamath imbricate orogen, but to the south trapped the ca. 165 Ma Coast Range ophiolite on the North American side of the suture. After ca. 140 Ma, first-cycle arc detritus began to accumulate on the mafic igneous basement flooring the Great Valley forearc, and turbiditic clastic material also was carried oceanward across the forearc into the coeval Franciscan trench.