Distinguishing salt welds from shale detachments on the inner Texas shelf, western Gulf of Mexico

Detachment surfaces have important implications for structural restoration, burial-history and thermal modeling, hydrocarbon migration, and diagenesis. We present criteria to distinguish salt welds from shale detachments based on geophysical data from the inner Texas shelf. Here, the Paleogene detachment has been variously interpreted as salt or shale by different people. A newly reprocessed 8200 km² (3200 mi²) 3D seismic volume provides excellent imaging of this detachment, which separates growth-faulted Oligocene–Miocene strata from the underlying, gently folded Cretaceous–Eocene section. Key criteria to evaluate detachment origins include seismic amplitude response, geometry, and relationship to supradetachment and subdetachment reflections. We argue that the detachment is a salt weld because (a) it is imaged as a high-amplitude, discrete reflection; (b) it has a ramp-flat geometry, cutting across underlying reflections; (c) it locally forms bowl-shaped depotroughs interpreted as former diapiric salt feeders; (d) it is overlain by seismically incoherent pods having high-amplitude tops and bases interpreted as remnant salt; and (e) in the depotroughs associated with former diapiric salt feeders the detachment has hints of upturned strata just beneath (possible halokinetic sequences). The inferred weld represents the evacuated remains of a patchy salt canopy emplaced across the study area during the Late Eocene to Early Oligocene. Preliminary examination beyond our study area suggests that this discontinuous canopy may have extended across most of the modern Texas shelf. Most of the salt was expelled from the canopy by loading from prograding Oligo–Miocene deltaic deposits.     

Glacier reconstruction and mass‐balance modelling as a geomorphic and palaeoclimatic tool

The reconstruction of former mountain glaciers has long been used to examine the implications of rapid climate shifts, for example at the last glacial–interglacial transition, and for evaluating asynchronous behaviour of mountain glaciers compared with mid‐latitude ice sheets during the Late Quaternary. Glacier reconstruction has also been used as a source of palaeoclimatic information, based on the recognition of empirical relationships between glaciers and climate. This paper reviews the application and implications of a recently revised method of glacier reconstruction (Carr and Coleman, 2007 ), based around glaciological principles of mass‐balance. This study examines how this approach can be used to test geomorphological interpretations of former mountain glaciation and also to infer precipitation fields at sites of former glaciation. Sites of Younger Dryas niche and icefield glaciation in the British Isles demonstrate how this method can verify interpretations of marginal glaciation and begin to understand the different behaviour of outlet glaciers within the same environmental regime. Examination of a site of former niche glaciation in Southern Africa demonstrates how glacier reconstruction may be used to infer annual and seasonal precipitation values and strongly supports the idea that winter precipitation in Lesotho and SE South Africa was substantially greater than present‐day values during the last glacial cycle. Copyright © 2010 John Wiley & Sons, Ltd.     

Preliminary results of optical searches of IPN3 localizations

We present preliminary results from deep optical searches of small (5 arcmin²) GRB error boxes determined using the Third Interplanetary Network (IPN³). Two of these fields also have been found to have historical OT events located within the IPN³ error boxes. We compare the preliminary results of these searches to those reported for the larger IPN¹ error boxes. The small size of the IPN³ error boxes should allow a test of the hypothesis suggested by the IPN¹ study that there are excess QSOs associated with the GRB fields.     

Search for GRB counterparts in the optical

The history and recent results of searches for optical counterparts to gamma ray burst sources in all three categories (i.e. flaring, fading and quiet) are outlined. The related problems such as background triggers and advantages and drawbacks of previous and recent approaches and investigations are discussed. Although there are some promising results especially in the appearance of optical transients i.e. short-lived star-like objects found in or close to the error boxes of some GRB, there is a need for more and better optical data, especially taken at or close to the times of GRB. Based on recent results, a strategy for further works in this direction is suggested with emphasis on obtaining of simultaneous (wide-field CCD experiments) and quasisimultaneous (robotic telescopes automatically pointed to the positions of newly detected GRB by Internet link) optical data.     

Operational testing of the SBIG ST-6 CCD camera

The SBIG Model ST-6 CCD Imaging Camera was tested and found suitable for various kinds of astronomical observations on telescopes with apertures from 6 mm to 0.6 m. We discuss different noise sources, linearity, sensitivity, and other detector characteristics. Limits, which they set to the observations and accuracy of data evaluation process, are also discussed. Detection, accurate astrometry, and VR photometry limiting magnitudes are about 21, 20, and 17.5, respectively, with the 0.6-m telescope and 10-min integration time. Detection of 5th magnitude 1-second optical transients is possible with the 6-mm wide-field lens.     

Ca2Mg(NO3)6×12H2O–structural investigations on a new compound retrieved from chimney deposits of a combined heat and power plant

Phase analysis of incrustations retrieved from chimney deposits of a combined heat and power plant in Malchow/Germany by X-ray powder diffraction gave evidence for the existence of a previously unknown hydrous calcium magnesium nitrate. Optical investigations of the sample showed the presence of colorless platy crystals with a maximum diameter of about 250 μm embedded in a partly polycrystalline and partly glassy matrix. Aided by EDX-analysis and Raman spectroscopy, a single-crystal diffraction study performed at ambient conditions demonstrated that the material represents a phase with composition Ca₂Mg(NO₃)₆×12H₂O. Basic crystallographic data are as follows: trigonal symmetry, space group type R \( \overline{3} \) , a?=?10.5583(5) Å, c?=?19.5351(10) Å, V?=?1885.97(16) ų, Z?=?3, (R(|F|) = 0.0248). The magnesium ions are coordinated by water molecules to form distorted Mg(H₂O)₆-octahedra. The calcium atoms are surrounded by nine ligands. The resulting CaO₉ tricapped trigonal prisms involve oxygen atoms from additional water moieties as well as from three different bidentate nitrate groups, respectively. Hydrogen bonds link one octahedron with two adjacent prisms into trimers. The trimers in turn are stacked in columns running parallel to [001]. Further hydrogen bonding between neighboring columns results in the formation of a three-dimensional network. To our best knowledge, Ca₂Mg(NO₃)₆×12H₂O represents a new structure type. However, column-like topologies with rods consisting of different types of polyhedra have been also observed in other trigonal hydrous nitrates. The structural relationships between these compounds are discussed. It is interesting to note that in previous phase equilibrium studies on the ternary system Ca(NO₃)₂-Mg(NO₃)₂-H₂O no other hydrous double salt has been described. Finally, the results of the structure analysis allowed a qualitative and quantitative phase analysis of the crystalline part of the chimney deposit by the Rietveld method.     

A simplified stress analysis of rising salt domes

We use a simple analytical model to estimate the stress field in density‐driven, rising salt domes and adjacent sediments, and to describe the evolution of these domes. We show that the pressure exerted by the salt pushing out against its wall rocks (the salt pressure) decreases linearly up the flank of the dome, but is always greater than the overburden stress. In fact, the salt pressure normal to the dome boundary is everywhere the maximum principal stress, whereas the hoop stress parallel to the circumference of the dome is the minimum stress. In addition, we quantitatively describe the critical stages of salt dome evolution (initiation, upbuilding, and downbuilding), relating these stages to sedimentation rate and basin thickness. This analysis also shows that even the highest sedimentation rates are unlikely to accumulate enough sediments to bury downbuilding domes as long as the salt supply is unrestricted. Despite the simplicity of the model, its predictions are in good agreement with field observations near salt domes. Overall, our analytical model can provide critical insight into the stress field perturbation in and near rising salt domes and can be used to assess the accuracy of numerical models and field measurements near these domes.     

Growth mechanism of snowball garnets from the Lukmanier Pass area (Central Alps,Switzerland): a combined μCT/EPMA/EBSD study

For two decades, considerable efforts have been made to explain the formation of snowball garnets by either the rotational or non‐rotational models. On the basis of morphological, chemical and crystallographic evidence, this paper presents new data on snowball garnets showing that the formation of these microstructures can be explained by the combination of the two previously proposed mechanisms operating consecutively during garnet growth. The crystallization sequence of garnet revealed by Mn contouring and the distribution of crystallographic orientations within the spiral indicate that the final stages of garnet growth are controlled by post‐kinematic crystallization. However, some microstructural arguments plead for a rotational contribution during the first stages of growth. In this view, the overall spiral geometry is thought to overestimate the true amount of rotation experienced by the garnets. Results also reveal the existence of complex snowball garnets consisting of several grains formed from distinct nucleation sites.     

Hydrothermal alteration in the core of the Yaxcopoil‐1 borehole,Chicxulub impact structure,Mexico

Abstract Petrographic, electron microprobe, and Raman spectrometric analyses of Yaxcopoil‐1 core samples from the Chicxulub crater indicate that the impact generated a hydrothermal system. Relative textural and vein crosscutting relations and systematic distribution of alteration products reveal a progression of the hydrothermal event in space and time and provide constraints on the nature of the fluids. The earliest calcite, halite, and gaylussite suggest that the impactite sequence was initially permeated by a low temperature saline brine. Subsequent development of a higher temperature hydrothermal regime is indicated by thermal metamorphic diopside‐hedenbergite (Aeg₃Fs_18‐33En_32‐11Wo_47‐53) after primary augite and widespread Na‐K for Ca metasomatic alkali exchange in plagioclase. Hydrothermal sphene, apatite, magnetite + (bornite), as well as early calcite (combined 3 to 8 vol%) were introduced with metasomatic feldspar. A lower temperature regime characterized by smectite after probable primary glass, secondary chlorite, and other pre‐existing mafic minerals, as well as very abundant calcite veins and open‐space fillings, extensively overprinted the early hydrothermal stage. The composition of early and late hydrothermal minerals show that the solution was chlorine‐rich (Cl/F >10) and that its Fe/Mg ratio and oxidation state increased substantially (4 to 5 logfO₂ units) as temperature decreased through time. The most altered zone in the impactite sequence occurs 30 m above the impact melt. The lack of mineralogical zoning about the impact melt and convective modeling constraints suggest that this unit was too thin at Yaxcopoil‐1 to provide the necessary heat to drive fluids and implies that the hydrothermal system resulted from the combined effects of a pre‐existing saline brine and heat that traveled to the Yaxcopoil‐1 site from adjacent areas where the melt sheet was thicker. Limonite after iron oxides is more common toward the top of the sequence and suggests that the impactite section was subjected to weathering before deposition of the Tertiary marine cover. In addition, scarce latest anatase stringers, chalcopyrite, and barite in vugs, francolite after apatite, and recrystallized halite are the likely products of limited post‐hydrothermal ambient‐temperature diagenesis, or ocean and/or meteoric water circulation.