Richard D. Jarrard

Origin of bottom-simulating reflectors: Geophysical evidence from the Cascadia accretionary prism

Vertical seismic profile (VSP) data from two drill sites on the Cascadia margin show low-velocity zones, indicative of free gas, be. neath a bottom-simulating reflector (BSR). Offshore Oregon, at Ocean Drilling Program (ODP) Site 892, velocities drop from an average of 1750 m/s above the BSR to less than 1250 m/s below it. Sonic logs confirm that seismic velocity in the sediments adjacent to the borehole is less than that of water for at least 50 m beneath the depth of the BSR at this site. Similarly, at ODP Site 889 offshore Vancouver, velocities range from 1700 to 1900 m/s in the 100 m above the BSR and drop abruptly to 1520 m/s in the 15 m just beneath it. The low velocities observed beneath the BSR are strong evidence for the presence of 1%-5% free gas (by volume). The BSR at these two sites results from the contact between gas-free sediments containing a small quantity of hydrate above the BSR and a low-velocity free-gas zone beneath it. Although the BSR is associated with the base of the hydrate stability field, hydrate appears to account for relatively little of the velocity contrast that produces the BSR. Velocity above the BSR at Site 889 is only about 100 m/s greater than that expected for sediments of similar porosity. Sediments above the BSR at Site 892 appear to have normal velocity for their porosity and may, contain little hydrate.

Exceptionally Preserved Jellyfishes from the Middle Cambrian

Cnidarians represent an early diverging animal group and thus insight into their origin and diversification is key to understanding metazoan evolution. Further, cnidarian jellyfish comprise an important component of modern marine planktonic ecosystems. Here we report on exceptionally preserved cnidarian jellyfish fossils from the Middle Cambrian (∼505 million years old) Marjum Formation of Utah. These are the first described Cambrian jellyfish fossils to display exquisite preservation of soft part anatomy including detailed features of structures interpreted as trailing tentacles and subumbrellar and exumbrellar surfaces. If the interpretation of these preserved characters is correct, their presence is diagnostic of modern jellyfish taxa. These new discoveries may provide insight into the scope of cnidarian diversity shortly after the Cambrian radiation, and would reinforce the notion that important taxonomic components of the modern planktonic realm were in place by the Cambrian period.

Velocity‐porosity relationships for water‐saturated siliciclastic sediments

Seeking a global empirical relationship between compressional wave velocity and porosity for siliciclastic sediments, we have brought together an extensive suite of both new and published log- and core-based data. We undertook a detailed statistical analysis of Ocean Drilling Program data from Amazon Fan to examine variables affecting compressional velocity in shallow, unconsolidated sediments. We identify three dominant variables (porosity, shale fraction, and consolidation history) and present two empirically determined boundary curves (one for normally consolidated sediments and a second for highly consolidated environments (e.g., accretionary prisms)). These two empirical relationships predict the compressional velocity of siliciclastic sedimentary rocks with water-filled pores as a function of porosity and clay content for the full range of observed porosities. Velocities of siliciclastic sedimentary rocks decrease rapidly with both increasing porosity and increasing clay content. At fractional porosities higher than about 0.4, fluid dominates the elastic properties, and velocity exhibits a subtle dependence on porosity. Remarkably, the Amazon Fan data show that clay content has no direct influence on velocity at high porosities. Both clay content and sorting do indirectly affect velocity, through their control of porosity. Burial affects velocity not only by compaction-related porosity decrease but also by pressure-induced increase of intergrain coupling. Because of the sensitivity of velocity to consolidation history, particularly at intermediate fractional porosities of about 0.30–0.40, no single velocity-porosity relationship can apply to all high-porosity sediments. The two proposed relationships fit the majority of published and new data. They are applicable, however, only for normally pressured, in situ conditions and water-filled pores.

Middle Cambrian Arthropods from Utah

Abstract The Middle Cambrian Spence Shale Member (Langston Formation) and Wheeler and Marjum Formations of Utah are known to contain a diverse soft-bodied fauna, but important new paleontological material continues to be uncovered from these strata. New specimens of anomalocaridids include the largest and smallest near complete examples yet reported from Utah. New material of stem group arthropods includes two new genera and species of arachnomorphs: Nettapezoura basilika and Dicranocaris guntherorum. Other new arachnomorph material includes a new species of Leanchoilia comparable to L. protogonia Simonetta, 1970; Leanchoilia superlata? Walcott, 1912; Sidneyia Walcott, 1911a; and Mollisonia symmetrica Walcott, 1912. L. protogonia from the Burgess Shale is confirmed as a separate species and is not a composite fossil. The first example of the trilobite Elrathia kingii preserving traces of the appendages is described. In addition, new material of the bivalved arthropods Canadaspis Novozhilov in Orlov, 1960; Branchiocaris Briggs, 1976; Waptia Walcott, 1912; and Isoxys Walcott, 1890 is described.

Low-temperature behavior of single-domain through multidomain magnetite

Abstract We have investigated the low-temperature behavior of a suite of ‘grown’ synthetic and natural magnetites that span single-domain (SD) and multidomain (MD) behavior. Synthetic samples had been grown in the laboratory either in an aqueous medium or in glass. Natural samples included SD magnetites occurring in plagioclase and truly MD magnetites in the form of large octahedra. In all experiments a sample was first given a saturation remanence at room temperature; next, moment was measured continuously during cooling and warming between 230 K and 60 K. Similar to results reported earlier by other workers, magnetic memory is large in SD samples, whereas truly MD samples are almost completely demagnetized by cycling between room temperature and 60 K. Pseudo-single-domain samples exhibit behavior that is intermediate with respect to that of the SD and truly MD states. When data from this study are combined with data obtained by Hartstra [10] from sized, natural magnetites, it is found that the percentage of total remanence that survives cycling between room temperature and 60 K decreases linearly with the logarithm of grain size and, thus, with increasing number of domains. This relation suggests that memory can provide a reasonable estimate of grain size in those magnetite-bearing rocks for which these samples provide good analogues. Remarkably, some of the large natural octahedra provide a magnified view of MD response to low temperatures and thus reveal two surprising and intriguing types of behavior. First, below approximately 180 K these octahedra demagnetize through a series of large Barkhausen jumps. Second, near 117 K these same octahedra exhibit a ‘wild zone’, where magnetic moment executes large, random excursions. We interpret these two phenomena as direct evidence for the unpinning and irreversible displacement of domain walls in response to the drop in coercivity and, possibly, the broadening of domain walls as temperatures drop toward the isotropic point. One implication of this behavior is that cooling to progressively lower temperatures could provide an effective method for stepwise removal of paleomagnetic components carried by MD grains, even without passage through the isotropic point of magnetite.

Highly permeable and layered Jurassic oceanic crust in the western Pacific

Bulk permeability was determined from drillstring packer measurements in ODP Hole 801C in Jurassic oceanic crust in the western Pacific. The values average 8 × 10 14 m2 over 93 m of open hole, or 4 × 10 -13 m2 if the permeable interval is confined to an 18 m thick hydrothermal zone within oceanic basement. These values are about 1-10 times higher than those reported for the upper sections of Holes 395A, 504B and 735B in 3.7, 5.9 and 11 Ma old oceanic crust respectively. The discovery that 160 Ma old oceanic crust contains a zone with extremely high permeability is not predicted by any model of ocean crustal evolution. The high permeability interval appears to be associated with the burial by off-ridge volcanism of a zone of hydrothermal precipitates and altered basalts. The generality of this result may depend on the generality of off-ridge volcanism necessary to create and preserve such zones of high permeability within the ocean crustal section. Although the crustal and permeability structures detected at Hole 801C may not fit easily into currently accepted models of crustal accretion and aging, the generality of off-ridge volcanism suggests that these structures could actually be typical of large portions of the worlds oceanic crust.

Porosity/formation‐factor relationships for high‐porosity siliciclastic sediments from Amazon Fan

The electrical resistivity of siliciclastic rocks is a function both of pore-fluid resistivity and of formation factor (FF), an intrinsic rock property. For low-porosity rocks, FF depends on clay conduction and porosity. In contrast, we find that FF of high-porosity sediments (fractional porosities of 0.3–0.6) from Amazon Fan is controlled primarily by porosity and pore geometry; clay conduction is a minor effect. Porosity vs. formation factor (FF) plots for Amazon Fan well logs demonstrate two distinct trends which are dependent on the amount of shale present. Muds, with more than about 0.4 shale fraction, follow a trend that increases to much higher FF as porosity decreases, compared to sands and slightly muddy sands. This separation is of opposite sign to the separation between low-porosity sandstones and shales attributable to clay conduction. We conclude that in high-porosity muds, presence of clays reduces formation conductivity by increasing the tortuosity of pores.

Eocene biogenic silica accumulation rates at the Pacific equatorial divergence zone

[2] The Eocene biosiliceous oozes of the tropical Pacific are an easily recognizable sediment type having a reddish brown color and sugary texture. These sediments are dominated by radiolarian tests, with traces of terrigenous debris, hydrothermal oxides, a few diatoms and little if any carbonate. The paucity of carbonate is a result of the very shallow Pacific calcite compensation depth (CCD) of the Eocene (3300 m) [Rea and Lyle, 2005, and references therein]. The dominance of radiolarian tests over those of diatoms is an enigma in that diatoms usually dominate siliceous-rich sediments of the Oligocene and Neogene. The opal-rich character of these Eocene sediments is commonly associated with the development of cherts within the sections. [3] These unusual characteristics of Eocene radiolarian oozes have prompted some authors to puzzle over how so much silica could have been delivered to the sea floor during

A simple method for orienting drill core by correlating features in whole-core scans and oriented borehole-wall imagery

Abstract Assessing the regional significance of fractures in drill cores requires the collection of oriented core intervals. Direct orientation of cores during drilling is possible, but is commonly precluded because of expense and time requirements. A simple and accurate method of core reorientation is presented where high-resolution imagery of drill cores (whole-core scans) are directly compared with oriented borehole imagery. Core intervals are reoriented by aligning features (i.e. fractures, bedding, and clasts) in whole-core scans with correlative, oriented features in borehole-wall imagery. Unlike other core orientation techniques, the direct side-by-side comparison of core scans and borehole-wall imagery can identify core segments that were mismatched due to undetected rotation between two portions of core. The combined analysis of core-based fracture data, whole-core scans and borehole imagery in this method optimizes data integration to improve structural interpretations.

Sediment mineralogy based on visible and near-infrared reflectance spectroscopy

Abstract Visible and near-infrared spectroscopy (VNIS) can be used to measure reflectance spectra (wavelength 350–2500 nm) for sediment cores and samples. A local ground-truth calibration of spectral features to mineral percentages is calculated by measuring reflectance spectra for a suite of samples of known mineralogy. This approach has been tested on powders, core plugs and split cores, and we conclude that it works well on all three, unless pore water is present. Initial VNIS studies have concentrated on determination of relative proportions of carbonate, opal, smectite and illite in equatorial Pacific sediments. Shipboard VNIS-based determination of these four components was demonstrated on Ocean Drilling Program Leg 199.