Cathodoluminescence of shocked quartz at the Cretaceous-Tertiary boundary

Empirical studies have documented an association between rock type and the cathodoluminescence color of constituent quartz grains. Quartz from extrusive igneous sources luminesces uniform pale blue. Quartz from intrusive igneous and high-grade metamorphic rocks generally luminesces darker purple-blue, whereas quartz recrystallized under low-grade metamorphic conditions luminesces reddish-brown. Quartz grains in most sandstones luminesce a heterogeneous mixture of these colors because the grains were derived from a variety of ultimate source rocks. If shocked quartz found at the Cretaceous-Tertiary (K-T) boundary is volcanic in origin, its cathodoluminescence should be predominantly pale blue. Alternatively, quartz grains derived from bolide impact upon, and ejection of, mixed igneous, metamorphic, and sedimentary rocks should luminesce a variety of colors. Grain mounts of sand collected at the K-T boundary horizon from the Clear Creek North site in the Raton Basin, Colorado were examined. Shocked quartz luminesced a variety of colors and very few grains luminesced the pale blue color that is typical of volcanic quartz. It was concluded that the shocked quartz was derived from a petrologically diverse source region without substantial volcanic contribution. Most shocked grains apparently were derived from low-grade metamorphic rocks, with a slightly smaller contribution from high-grade metamorphic and intrusive igneous rocks. Rare quartz grains with brown-luminescing rims reflect a minor addition from detrital sedimentary sources. The apparent relative abundances of intrusive (and rare extrusive) igneous, metamorphic, and sedimentary ultimate source rocks suggested by CL colors of shock-deformed quartz at the K-T boundary is consistent with a crustal/supracrustal origin for the grains

On the inverse image of pattern classes under bubble sort

Let B be the operation of re-ordering a sequence by one pass of bubble sort. We completely answer the question of when the inverse image of a principal pattern class under B is a pattern class.Comment: 11 page

Is the Sevier Desert Reflection of West-Central Utah a Normal Fault?

A prominent west-dipping reflection that can be traced in seismic-reflection profiles over an area of 7000 km2 beneath the Sevier Desert basin of west-central Utah is generally referred to as the Sevier Desert detachment and is widely regarded as one of the best examples of an upper-crustal low-angle normal fault. The absence of evidence for fault-related deformation in drill cuttings and core from two industry boreholes that intersect this feature casts doubt on the fault interpretation. The existing interpretation is based mainly on the observation that high-angle normal faulting is restricted largely to Tertiary sedimentary and volcanic rocks above the reflection. An alternative explanation is that the high-angle faults are related to the withdrawal of early deposited lacustrine salt, which even today is as much as 2 km thick. Reevaluation of the seismic data suggests that the Sevier Desert reflection consists of two spatially and genetically distinct segments: a shallow segment here interpreted as an unconformity between Paleozoic and Tertiary strata and a fortuitously aligned deeper segment that is traceable to mid- and lower-crustal levels and that appears to represent a thrust fault related to the Cretaceous Sevier orogeny

Pea-barley intercrop N dynamics in farmers fields

Knowledge about crop performances in farmers’ fields provides a link between on-farm practice and re-search. Thereby scientists may improve their ability to understand and suggest solutions for the problems facing those who have the responsibility of making sound agricultural decisions. Nitrogen (N) availability is known to be highly heterogeneous in terrestrial plant communities (Stevenson and van Kessel, 1997), a heterogeneity that in natural systems is often associated with variation in the distri-bution of plant species. In intercropping systems the relative proportion of component crops is influenced by the distribution of growth factors such as N in both time and space (Jensen, 1996). In pea-barley intercrops, an increase in the N supply promotes the growth of barley thereby decreasing the N accumulation of pea and giving rise to changes in the relative proportions of the intercropped components (Jensen, 1996). The pres-sure of weeds may, however, significantly change the dynamics in intercrops (Hauggaard-Nielsen et al., 2001). Data from farmers’ fields may provide direct, spatially explicit information for evaluating the poten-tials of improving the utilisation of field variability by intercrops

Microfractures: A review

Microfractures are small, high-aspect-ratio cracks in rock that result from application of differential stresses. Although the term has been used to refer to larger features in the petroleum engineering and geophysics literature, in geologic parlance the term refers to fractures visible only under magnification, having lengths of millimeters or less and widths generally less than 0.1 mm. Nevertheless, populations of these structures typically encompass a wide size range and in some cases they form the small-size fraction of fracture arrays that include much larger factures. In geologic settings, microfractures commonly form as Mode I (opening) fractures where the minimum principal stress exceeds the elastic tensile strength creating a narrow opening displacement; in isotropic rocks such fractures mark the plane perpendicular to the least compressive principal stress during fracture growth. These planar or curviplanar openings provide an opportunity for fluids and/or gases to enter the created cavity. Cement deposits or crack closure may trap fluids or gases, leaving mineral precipitates and a track of enclosed fluids and gases. In transmitted light these precipitates frequently manifest as fluid-inclusion planes (FIPs). Cathodoluminescence (CL) images show that many are cement-filled microveins. Microfractures can be used to assess the paleostress history or fluid movement history of a rock body. Also, because sudden opening produces acoustic emissions, microfractures created in the laboratory can be used to assess the rock-failure process. Here we review recent discoveries made using microfractures, including fracture patterns, strain, fracture growth and size-scaling, evolution of stresses around propagating faults (process zones), far-field tectonic stresses, and insights into the state of stress leading to earthquakes

Cominco American Well: Implications for the Reconstruction of the Sevier Orogen and Basin and Range Extension in West-Central Utah

Re-evaluation of acoustic, gamma ray and dip meter logs from the Cominco American Federal No. 2 well in the Sevier Desert basin of west-central Utah sheds new light on the interpretation of Neoproterozoic and Cambrian stratigraphy and Mesozoic structure in a region that has been influential in the development of ideas about crustal shortening and extension. The most prominent of several major thrust faults (the Canyon Range and Pavant thrusts) have been interpreted by DeCelles and Coogan (2006) [Regional structure and kinematic history of the Sevier fold-and-thrust belt, central Utah: Geological Society of America Bulletin, v. 118, n. 7-8, p. 841-864, http://dx.doi.org/10.1130/B25759.1] as having been cut and in part re-activated between late Oligocene and Holocene time by as much as 47 km of displacement on the gently west-dipping Sevier Desert detachment. This interpretation, which is based upon a combination of outcrop, seismic reflection and well data, depends critically on the Canyon Range thrust intersecting the Cominco well at a depth of 2,551 to 2,557 m (8,370-8,389 ft.), and terminating downwards against a re-activated Pavant thrust. Our work suggests that the fault at 2,551 m (8,370 ft.) is a strand of the Pavant thrust, and that the Canyon Range thrust cuts the well at a depth of 1,222 m (4,010 ft.). This alternative interpretation depends in turn on identification of the section between the two faults as terminal Neoproterozoic to middle Cambrian Prospect Mountain Quartzite through Chisholm Formation rather than Neoproterozoic "Pocatello Formation," "Blackrock Canyon Limestone" and lower Caddy Canyon Quartzite. To support this interpretation we present evidence for stratigraphic repetition and for deformation at the 1,222 m (4,010 ft.) level. Use of the lithostratigraphic terms "Pocatello" and "Blackrock Canyon" in west-central Utah is shown to be inappropriate, and among the reasons that the critical interval in the Cominco well has been misinterpreted by some authors. If the Canyon Range and Pavant thrusts are both found in the Cominco well, as we suggest, then they cannot be used as a piercing point for the estimation of displacement on the Sevier Desert detachment or as justification for the existence of the detachment. Published estimates of extension across the Sevier Desert basin therefore need to be reduced, potentially to as little as ∼ 10 km

Pattern of Mesozoic Thrust Surfaces and Tertiary Normal Faults in the Sevier Desert Subsurface, West-Central Utah

Most tectonic models for the Sevier Desert basin, west-central Utah, envision it as the result of large-magnitude, normal-sense slip on a regional detachment fault. That interpretation, based principally on seismic reflection data, has helped shape views on the tectonics of the northeastern Great Basin area and, in a larger sense, the historical development of ideas about low-angle normal faulting. In recent years, however, several researchers have suggested, based on rock-mechanical, field, and subsurface evidence, that the hypothesized detachment fault does not exist and that the basin must have another explanation. Even among proponents of the detachment model, opinion has differed regarding the hypothesized extensional fault’s total displacement, estimates for which vary widely; the timing of detachment slip; and whether the hypothesized fault represents a “new” Tertiary extensional structure or an extensional reactivation of the Mesozoic Pavant thrust. A comprehensive reinterpretation of available subsurface data for the basin, including several previously unpublished seismic profiles, suggests: (1) that slip on the hypothesized detachment must have ceased by the Miocene in the southern Sevier Desert; (2) that estimates of large-magnitude offset on the hypothesized detachment are essentially unconstrained by structural data and need to be reevaluated; and (3) that models that view the hypothesized detachment as a Tertiary extensional reactivation of a Mesozoic thrust are likely incorrect. The newly available seismic data demonstrate that reflections from the Pavant thrust do indeed closely align with reflections from the Paleozoic/Tertiary contact in many parts of the northern Sevier Desert basin; however, to the south these same thrust fault reflections are directly traceable to a position well above the down dip projection of the presumed detachment within the Paleozoic Cricket Mountains block. Erosional truncation of the thrust faults and the absence in the south of other reflections aligned with the Paleozoic/Tertiary contact preclude extensional back-sliding on a Mesozoic thrust fault. These interpretations, if correct, are incompatible with the detachment hypothesis and necessitate alternative explanations for the basin’s origins

Possible world-wide middle miocene iridium anomaly and its relationship to periodicity of impacts and extinctions

In a study of one million years of Middle Miocene sediment deposition in ODP Hole 689B in the Weddell Sea near Antarctica, a single iridium (Ir) anomaly of 44 (+ or - 10) x 10 to the 12th gram Ir per gram rock (ppt) was observed in core 6H, section 3, 50 to 60 cm, after background contributions associated with manganese precipitates and clay are subtracted. The ODP Hole 689B is 10,000 km away from another site, DSDP Hole 588B in the Tasman Sea north of New Zealand, where a single Ir anomaly of 144 + or - 7 ppt over a background of 11 ppt was found in an earlier study of 3 million years of deposition. From chemical measurements the latter deposition was thought to be impact-related. Ir measurements were made, following neutron activation, with the Iridium Coincidence Spectrometer. The age vs depth calibration curves given in the DSDP and ODP preliminary reports indicate the ages of the Iranomalies are identical, 11.7 million years, but the absolute and relative uncertainties in the curves are not known. Based on the newest age data the age estimate is 10 million years. As the Ir was deposited at the two sites at about the same time and they are one quarter of the way around the world from each other it seems likely that the deposition was world-wide. The impact of a large asteroid or comet could produce the wide distribution, and this data is supportive of the impact relationship deduced for Deep Sea Drilling Project (DSDP) 588B from the chemical evidence. If the surface densities of Ir at the two sites are representative of the world-wide average, the diameter of a Cl type asteroid containing the necessary Ir would be 3 + or - 1 km, which is large enough to cause world-wide darkness and hence extinctions although the latter point is disputed

Distinguishing between Rooted and Rootless Detachments: A Case Study from the Mormon Mountains of Southeastern Nevada

Rooted detachment faults and detachments beneath rootless slide blocks exhibit many similar structural characteristics. However, while rooted detachments are thought to penetrate into the midcrust and to accommodate significant crustal extension, rootless detachments break to the surface downdip and are not directly involved in such extension. Distinguishing between these two mechanically different kinds of structure is central to the assessment of extension magnitude. Here we examine deformation along the Mormon Peak detachment, a feature that has been cited as an example of both a rooted and a rootless structure. Located in the Mormon Mountains of southeastern Nevada, this detachment has been interpreted as one of three low‐angle normal faults of regional scale that together are thought to have accommodated more than 50 km of Basin and Range extension. For the most part, however, the Mormon Peak detachment is expressed as a series of isolated exposures where Paleozoic rocks are in brittle fault contact with nonmylonitized underlying rocks. Individual blocks contain high‐angle normal faults that terminate downward at their respective detachment surfaces, yielding a geometry common to both modes of emplacement. In order to test between these competing interpretations, we studied deformational characteristics close to the detachment surface, reasoning that a seismogenic fault ought to differ fundamentally from a surficial slide block, particularly if the slide block was emplaced in a single event rather than by protracted or episodic creep. An examination of the contact mapped as the Mormon Peak detachment reveals that the character of deformation is indistinguishable from that of known gravity‐driven slide blocks and is fundamentally different from that associated with seismically cycled faults. Moreover, the orientation of kinematic indicators observed at detachment surfaces is consistently close to the downdip direction, which in many places diverges strongly from the expected direction of movement in the rooted detachment model. We conclude that outcrops of the inferred upper plate of the Mormon Peak detachment represent an assemblage of individual rootless gravity‐driven slide blocks and not the erosional remnants of a formerly contiguous extensional allochthon. If similar misidentifications have been made elsewhere in the Basin and Range Province, total Cenozoic extension may have been significantly overestimated. Implications for the interpretation of extensional geology in general are far‐reaching

Stratigraphic Controls on a Salt-Withdrawal Intraslope Minibasin, North-Central Green Canyon, Gulf of Mexico: Implications for Misinterpreting Sea Level Change

Three-dimensional seismic data from the Fuji basin, a salt-controlled intraslope minibasin in north-central Green Canyon, Gulf of Mexico, reveal complex interactions between gravity- and suspension-driven sedimentation. Seismic volumes for late Pleistocene (sim470 ka) to Holocene fill within the Fuji basin consist of approximately 45% mass transport complexes (MTCs), 5% channelized sandy turbidites, and 50% hemipelagites and muddy turbidites. At least ten MTCs within the Fuji basin flowed radially toward its depocenter, either from basin flanks (i.e., intrabasinal) or as a result of larger-scale salt motion (i.e., extrabasinal). Sediment transport directions are inferred on the basis of elongate basal incisions and smaller-scale scours, head scarps, fold orientation within the complexes, and stratigraphic thinning trends at downdip margins. An amalgamated set of three channelized sandy turbidite complexes less than 350 m (1148 ft) thick and 3 km (1.8 mi) across represents the main sand delivery pathway into the Fuji basin. These deposits are thought to be due to shelf bypass, and possibly, to proximity to the Pleistocene shoreline. Hemipelagites and muddy turbidites are homogeneous, and their thickness is relatively consistent at basin scale. This facies represents background sedimentation. A process-driven model has been developed involving halokinetic autocyclicity as the primary control on sedimentation in the Fuji basin. Passive salt motion accounts better for both the directions of sediment transport and the frequency of late Pleistocene–Holocene MTCs than currently popular eustatic and steady-state bathymetric models. The conclusion is significant in casting doubt on the generally assumed importance of eustasy in controlling off-shelf lowstand sedimentation and in implying marked variations in stratigraphic details at length scales of less than 10 km (6.2 mi)