Biogeochemistry of Mono Lake,California

Modern sediments of Mono Lake show marked variation in lipid composition with depositional environment. Constituents derived from the drainage basin, characterized by high molecular weight alkane hydrocarbons (C₂₅–C₃₁), and the steroids β-sitosterol and brassicasterol, predominate in near-shore environments. In the deepest part of the lake, sediments exhibit a combination of externally-derived constituents, and lipids derived from the lake biota; the latter characterized by low molecular-weight alkanes and alkenes (C₁₅–C₁₇), phytane, and the steroids ergost-7-en-3β-ol and 24-ethylcholest-7-en-3-β-ol. Steranes, 4-methylsteranes, and the C₁₈ and C₁₉ isoprenoids appear to be forming in the intensely reducing bottom sediments at the present time.The compositions of samples from the Pleistocene succession of Mono Basin suggest that sample-to-sample variation within the same stratum is negligible so long as unweathered samples from the same depositional environment are compared. Sediments having equivalent lithologies may or may not have similar compositions, but sediments having similar fossil contents do show similar lipid compositions. Subaerial weathering of sediments causes a marked decrease in the amount of extractable organic material, as well as distinct changes in its hydrocarbon composition. Specifically, weathered sediments exhibit a decrease in relative content of low molecular weight hydrocarbons and a relative increase in nC₂₂.Organic composition of sediments from the Pleistocene stratigraphie column cannot be correlated with depth of burial. Compositional changes with stratigraphie position are probably related to paleo-ecological factors such as population or productivity rather than depth of burial. Lithology and organic composition provide mutually-corroborating evidence regarding glacial advances in the adjacent Sierra Nevada Mountains. During glaciations, the lake sediments are rich in sandstones, and the organic composition shows a predominance of externally-derived debris, with no evidence for contributions from the lake biota.     

Serpentinization at Burro Mountain,California

The Burro Mountain ultramafic complex, Monterey County, California, consists of dunites and peridotites which are partially or wholly serpentinized. Primary minerals in both rock types are olivine, enstatite, diopside, and picotite which upon alteration yield chrysotile, lizardite, brucite, magnetite, talc, tremolite, and carbonate. Electron microprobe analyses show that enstatite, En_85.8 to En_90.8, alters to “bastite” composed only of lizardite (5.0–12.0 weight percent FeO), whereas olivine, Fo_90.8 to Fo_91.6, forms lizardite+chrysotile+brucite with or without magnetite. The chrysotile ranges from 3.0 to 5.0 weight percent FeO, the brucite from 16.0 to 43.0 weight percent FeO. As Serpentinization proceeds, the alteration products are enriched in FeO relative to MgO. Serpentinization probably originates in a changing \(P_{O_2 }\)-T environment by two different reactions:

1. (a)
   Olivine+enstatite+H₂O+O₂?Mg, Fe⁺² chrysotile+Mg, Fe⁺³, Fe⁺² lizardite with or without magnetite.     
   
   

Carbon-Nitrogen-Phosphorus Cyclingin Tomales Bay, California

Tomales Bay, California, has been the site oflong-term study of carbon-nitrogen-phospho-rus fluxes in the coastal zone. It has also one of severalcoastal sites being used for comparison of C-N-Pbiogeochemical fluxes. The site releases phosphorusand consumes dissolved nitrogen. It also producesdissolved inorganic carbon, mostly as elevatedalkalinity. The overall interpretation placed on thebiogeochemical fluxes is as follows. The system is netheterotrophic; that is, it consumes more organicmatter than it produces. A pathway of consumption thatis of particular importance to the nitrogen cycle isdenitrification. The combination of net heterotrophyand denitrification is the simultaneous release ofdissolved inorganic phosphorus and the uptake ofdissolved inorganic nitrogen. Much of the dissolvedinorganic carbon released during the net heterotrophyis bound in alkalinity, apparently the result ofsulfate reduction.Because this work can be traced by to the trainingthat one of the authors (SVS) received from KeithChave, it is appropriate to present a summary of theTomales Bay research in memory of Keith.     

Vineyard site suitability in Eastern California

An analysis of the physical characteristics of an established wine region in eastern California, USA was undertaken to discern if a unique combination of environmental parameters in Zinfandel vineyards could be quantitatively identified. Development of a geographic information system (GIS) method to assess land suitability for current and potential viticultural regions in a Mediterranean-type climate is the primary focus. The assessment is based on spatial frequency distributions of measurable environmental criteria derived from characteristics of existing vineyards. In fact, statistically significant differences were found at the 95% confidence interval for 6 variables, and at the 85% confidence interval for an additional 3 variables. Thus 9 of the 15 variables analyzed (53%) were discriminated statistically between the vineyard and non-vineyard areas. The findings of this study illustrate the potential of GIS techniques for viticultural land suitability analysis, while supporting and contributing to the literature on the significance of soil and topographic characteristics in viticulture.     

加利福尼亚弧-盆-沟体系的地质特征

加利福尼亚中生代弧-盆-沟系的构造和演化模式一向为世人所重视。作者应邀有机会对加州内华达岛弧地块、弗兰西斯科杂岩和大谷弧前盆地进行了实地地质考察。本文仅就这3个岩性-构造单元的一些独特地质特征和地质景观作一简介。     

Tidal flat macrofaunal communities and their associated environments in estuaries of southern California and northern Baja California, Mexico

Several tidal flats in both Estero de Punta Banda and Bahia de San Quintín, Baja California, and one in Mission Bay, southern California, were sampled for macrofaunal properties (taxonomic composition, density, species richness, and functional groups for animals ≥0.3 mm) and associated environmental variables (sediment properties, salinity, plant belowground biomass, and cover ofZostera marina) in order to establish a benchmark data set for these areas. The grouping of macrofauna into higher taxonomic or functional groups for these comparisons reduced variability and revealed stronger relationships. Each estuary had a fairly distinct macrofaunal assemblage, with that of Estero de Punta Banda being different from Bahia de San Quintín and Mission Bay primarily due to dominance by a capitellid polychaete, lower proportions of surface deposit feeders, and higher proportions of fauna with a planktonic stage. The flats in Mission Bay and Bahia de San Quintín were dominated by peracarid crustaceans, oligochaetes and polychaetes and had higher proportions of direct developers and macrofauna with mobile adult stages than did Estero de Punta Banda. There was an overlap of the environmental characteristics among estuaries, with more variability of sediment and vegetation properties within than among estuaries. Within Bahia de San Quintín, there was an oceanic to back-bay distribution gradient of macrofauna that was similar to that found in estuaries in wetter climates, despite the lack of a salinity gradient in San Quintín. A decoupling of the benthos and the assumed anthropogenic stresses was observed with the degraded site, Mission Bay, being most similar to the relatively pristine Bahia de San Quintín. Selection of reference sites and sampling variables should be made cautiously because effects of disturbance factors on the benthos may be site-dependent, scale-dependent, or negligible.     

Mercury deposits of western California: an overview

Summary Mercury deposits in western California are near a thrust fault that separates two groups of Mesozoic rocks. The Franciscan Assemblage, a metamorphosed melange with serpentine and graywacke, is structurally overlain by the Great Valley Sequence, a sedimentary series resting on oceanic crust. These Mesozoic rocks are partly covered by volcanic and sedimentary rocks of Cenozoic age. Cinnabar with silica minerals, dolomite, native mercury, and bituminous matter occurs around the fractured margins of serpentine bodies and around hot springs that emanate from the Franciscan Assemblage. Fluid inclusions and hot springs suggest that cinnabar precipitated from CO₂-H₂O fluids with <2 wt% chlorine at T<250 °C. Prograde metamorphism of Mesozoic sediments expulsed mercury-bearing fluids that migrated up serpentine-related fractures and exhaled onto the surface.     

Fossil mammoths from Santa Cruz Island, California

Mammoth remains on Santa Cruz Island, one of the four Northern Channel Islands of California, are very sparse, in marked contrast to those reported from Santa Rosa and San Miguel Islands of the same island group. A probable major reason for this scarcity is that Quaternary deposits are greatly restricted on Santa Cruz Island. It is proposed, contrary to popular opinion, that fossils found on Santa Cruz Island were derived from animals which died on the island, and were not transported there by humans. Reasons for this conclusion are that the size and geological context of the fossils are similar to those of the largest mammoth fossils of Santa Rosa Island, and that, in spite of extensive investigations by many persons, mammoth remains have not been found in middens, either on the islands or on the adjacent mainland.     

Temporal changes in P-wave velocity in Southern California

P-wave delays at Tinemaha, China Lake, Pasadena, Riverside, Hayfield and Barrett, stations of the CIT Southern California seismic network, are measured for three explosions in the Aleutians and six deep-focus earthquakes in the Marianas. Except at Riverside, no change in P-delays exceeding the experimental uncertainty, ± 0.2 sec, is found during the period from 1965 to 1971. At Riverside, however, P-delay in 1971 is at least 0.4 sec smaller than that in 1965, indicating a temporal P-velocity increase beneath Riverside from 1965 to 1971. Evidence supporting this result is obtained from the P-times at Riverside for quarry blasts at Corona (Δ ≈ 20 km). Precise travel-time measurements are made for eight blasts since 1949. The travel time changes as a function of time, ranging from 3.3 to 3.7 sec. This range of variation seems to be larger than the experimental uncertainty. The trend for the period from 1964 to 1969 is consistent with the temporal change in the teleseismic P-delays observed at Riverside. The observed change in P-delays is not related in any obvious way to past seismic activity; rather it might represent a large-scale fluctuation of the property of the crust caused possibly by change in the tectonic stress and fluid-vapor flow, and may be related to future earthquake activity.     

Fluid-pressure variations and fault creep in central California

Observations of the change of water level in wells near the San Andreas fault in centra California indicate that there are pore-pressure changes associated with the process of fault creep. The sense of water-level and pore-pressure changes may indicate the direction of creep propagation.     

Spatial radon anomalies on active faults in California

Radon emanation has been observed to be anomalously high along active faults in many parts of the world. We tested this relationship by conducting and repeating soil-air radon surveys with a portable radon meter across several faults in California. The results confirm the existence of fault-associated radon anomalies, which show characteristic features that may be related to fault structures but vary in time due to other environmental changes, such as rainfall. Across two creeping faults in San Juan Bautista and Hollister, the radon anomalies showed prominent double peaks straddling the fault-gouge zone during dry summers, but the peak-to-background ratios diminished after significant rain fall during winter. Across a locked segment of the San Andreas fault near Olema, the anomaly has a single peak located several meters southwest of the slip zone associated with the 1906 San Francisco earthquake. Across two fault segments that ruptured during the magnitude 7.5 Landers earthquake in 1992, anomalously high radon concentration was found in the fractures three weeks after the earthquake. We attribute the fault-related anomalies to a slow vertical gas flow in or near the fault zones. Radon generated locally in subsurface soil has a concentration profile that increases three orders of magnitude from the surface to a depth of several meters; thus an upward flow that brings up deeper and radon-richer soil air to the detection level can cause a significantly higher concentration reading. This explanation is consistent with concentrations of carbon dioxide and oxygen, measured in soil-air samples collected during one of the surveys.     

Large tufa mounds,Searles Lake,California

Mounds that have formed around spring vents occur in a variety of environmental settings, many at sites generally difficult or inaccessible for sampling. In contrast, over 500 tufa mounds occur in the dry bed of Searles Lake, California. The mounds range from minor features to 45 m in height; most are 5 to 12 m high. These mounds, composed of calcite and aragonite, formed associated with spring vents in the Pleistocene lake bottom. Thus, analyses of these mounds in Searles Lake provide a model with regard to the origin and architecture of tufa mounds. The mounds consist of four distinctive tufa facies. The initial deposits consist of porous tufa, including the innermost (porous 1) and the outermost (porous 2) deposits, followed by nodular tufa, then columnar tufa, and laminated crusts. There are two simple sequences of tufa deposition. The first sequence is from porous 1 to nodular to laminated crusts and, finally, to porous 2. A second sequence consists of: porous 1 to columnar to laminated crusts and, lastly, to porous 2. Facies changes are a response to changes in environmental conditions from deep water (porous 1 facies) to an essentially dry lake phase (during and after the formation of laminated crusts facies), to deep water (porous 2 facies) and, at the present time, totally dry. The primary constituents that comprise the tufa deposits include thin laminae, pisoids, spherulites, peloids and stromatolite‐like crusts. On the microscopic scale, these constituents dominantly make up nano‐spheres, micro‐rods and rod‐like crystals, as well as other calcified bodies. These constituents are interpreted to be the calcified remains of bacterial bodies. These findings suggest that microbial participation in the construct of other mounds should be a major concern of investigation, both for terrestrial and extraterrestrial spring‐fed mounds.     

Garnet Types from the Cazadero Area, California

This is the third in a series of papers on glaucophane schistsfrom the Franciscan Formation near Cazadero, California. Previouspapers describe three distinct types of glaucophane-bearingFranciscan metamorphic rocks near Cazadero. The purpose of this study is to investigate the garnets presentin metamorphic types III (bedrock schists) and IV (tectonicblocks) as defined by Coleman & Lee (1963). Twenty-fourgarnet analyses are presented. Sixteen of these are from (aragonite-bearing)type III glaucophane schists, and eight are from type IV glaucophaneschists. Type IV rocks include California eclogites. Type III rocks include metabasalt, metachert, metashale, meta-ironstone,and metacarbonate that were formed under high pressure and relativelylow temperature. These rocks contain garnets that display awide range of composition, but the dominant molecules representedare consistently almandine, spessartine, and grossular. Type IV rocks are mainly metabasalts that were probably formedunder higher temperatures and pressures than type III rocks.There is a distinct difference between garnets from type IIIrocks and those from type IV (including eclogites); the lattercontainless spessartine and more pyrope, and the dominant moleculesare almandine and grossular. The four analyses of garnets fromCalifornia eclogites have an average pyrope content of aboutten molecular per cent, and they extend the range of compositionreported for eclogite garnets. Quantitative spectrographic determinations of minor elementsare listed for each of the garnets described. The values determinedfor some of the minor elements have a wide range and a capriciousdistribution over a few feet of outcrop area. As a group, both the garnets from type III rocks and those fromtype IV are pyralspites with large contents (as much as 35 molecularper cent) of ugrandite. This unusual admixture of the pyralspiteand ugrandite garnet series may have resulted in part from theconditions (high pressures and relatively low temperatures)under which the enclosing rocks were recrystallized.     

Monitoring of secular gravity change in southern California

Repeat gravity measurements have been carried out over more than 1000 km of level lines in southern California. The initial surveys over a number of these lines were made in 1962. Repeat gravity survey lines are located primarily in the San Joaquin Valley, in the Los Angeles and Palmdale areas and in the Imperial Valley. Secular changes of gravity have been noted in a number of areas. In areas of subsidence, due to groundwater withdrawal, direct correlation between elevation and gravity change is found. In areas of vertical crustal motion along fault zones, such as in the Palmdale area, gravity changes are also noted. However, these are not, in general, directly related to elevation change. Apparently, subsurface mass redistributions are associated with the vertical motions noted in fault-zone areas before and after earthquakes. Several possible models to explain the gravity observations are being explored.     

Geology of the Lick Observatory Quadrangle,California

The Lick Observatory 7.5-minute quadrangle exposes evidence of geologic events that range from subduction of Mesozoic Franciscan Complex, through accumulation of marine Miocene porcellanite and clastics, to the development of the San Andreas fault system and deformation within it. The active Calaveras fault zone, with its linear valleys and subparallel strike-slip strands, transects the quadrangle and, northwest of San Filipe Valley, joins and incorporates the older Madrone Springs fault. The topography has formed in the past 1 to 2 million years and rises northeastward from the East Evergreen range-front thrust, across the Calaveras and several inferred mountain-building faults, to the 1280 m crest of Mt. Hamilton.

The stratigraphy includes coherent, variously schistose metagraywacke of the late Mesozoic Franciscan Complex; discordant zones of melange of sheared shale and blocks that include blueschist and eclogite; serpentine that may represent the Coast Range Ophiolite; relatively undeformed sandstone, shale, and conglomerate of the late Mesozoic Great Valley sequence; marine Miocene Claremont Porcellanite, mudstone, and Briones Sandstone; and deformed nonmarine gravels of the Pleistocene and Pliocene Santa Clara Formation.

The Franciscan sandstones are complexly deformed and discordantly transected by tectonically emplaced melange zones; a local chert mass marks the remnant of a discordantly overlying thrust sheet. Southwest of the Calaveras zone, folded Miocene rocks are faulted over the more strongly deformed Great Valley sequence. Those rocks, in turn, are thrust over small windows of Franciscan rock, and the entire mountain mass is thrust over Santa Clara gravels at the foot of the range. These latter structures postdate the 3.5 Ma imposition of compression across the plate margin suggested by plate tectonic reconstructions.     

Phanerozoic Tectonic Evolution of Central California and Environs

The use of actualistic analog models for paleotectonic reconstruction produces significant advances in our understanding of evolutionary continental tectonics. The sequential application of such models is possible in a cross section from the central California coast to Utah. This transect represents one of the best-understood Phanerozoic continental margins on Earth. Excellent exposure, detailed local studies, and regional syntheses all contribute to the choice of appropriate plate-tectonic models for successive intervals from the latest Neoproterozoic to the Quaternary. These models include craton, terrestrial rift, nascent ocean, intraplate continental margin, intraoceanic magmatic arc intraplate margin, Devonian-Mississippian Antler orogeny (arc-continent suturing), Mississippian-Pennsylvanian intraplate margin, Pennsylvanian-Permian Ancestral Rockies orogeny (Ouachita-Marathon continental suturing), Permian-Triassic Sonoma orogeny (arc-continent suturing), Triassic-Jurassic continental-margin magmatic arc, Late Jurassic Nevadan orogeny (arc-arc suturing), latest Jurassic-Late Cretaceous continental-margin magmatic arc, latest Cretaceous-Eocene Laramide orogeny, Oligocene ignimbrite flare-up, and Miocene-Holocene triple-junction migration, transform boundary, and regional extension of the Great Basin. The integrated result of sequential superposition of the actualistic models produces a reasonable representation of the complexity of the study area. The discipline of applying actualistic models to the evolution of this continental margin provides new insights and forces one to consider new implications of the models. The complexity of the tectonic history of this continental margin argues against simplistic general models for the growth of continental crust.     

A structural-geomorphological interpretation of Southern California shear zones

The neotectonic pop-up structures of Southern California were studied using the structural-geomorphological method, which included the analysis of topographic maps that were constructed on the basis of digital elevation models. The data we obtained are compared with the geological structure and the kinematic model of the block structure of the region based on the analysis of the current movements in accordance with the GPS data. The applicability of this method for neotectonic reconstructions of shear zones is considered. It is shown that the structures that are identified this way may be correlated with the kinematic model of the Southern California block structure that was developed using the GPS data.