Fluidization of buried mass-wasting deposits in lake sediments and its relevance for paleoseismology: Results from a reflection seismic study of lakes Villarrica and Calafquén (South-Central Chile)

A dense grid of very-high resolution seismic profiles on Lake Villarrica provides a quasi-3D view on intercalated lenses of low-amplitude reflections, which are connected by acoustic wipe-out patches and fractures to an underlying voluminous mass-wasting deposit. The lenses are interpreted as being created by earthquake-triggered liquefaction in this buried mass-wasting deposit and subsequent sediment fluidization and extrusion at the paleo-lake bottom. These sediment volcanoes are mapped in detail. They have a rather uniform circular geometry and show a linear relationship between apparent width and maximum thickness on a seismic section. The largest sediment volcanoes are up to 80 m wide and 1.9 m thick. Their slope angles designate a syn- to post-depositional sagging of most sediment volcanoes. Sediment volcano detection and mapping from nearby Lake Calafquén further strengthen the revealed geometrical relationships. Locally, some of the sediment/fluid escape structures extend to a higher position in the stratigraphy, which points to a polyphase escape process associated with multiple multi-century spaced strong earthquakes. Thickness and morphology of the source layer seem to exert a dominant control in the production of sediment/fluid extrusions. This study shows that reflection seismic profiling allowed recognizing 4 different seismic events in the studied stratigraphic interval, which are evidenced by mass-wasting deposits and/or fluidization features.     

An attempt to determine long-periodic variations in the drift of horizontal pendulums

Résumé On donne les résultats de l'analyse de la dérive des pendules horizontaux de la station Píbram-Bezové Hory au cours des années 1927–1938. Différemment aux résultats publiés déjà antérieurement[2], on a tout d'abord séparé par la méthode des moindres carrés la dérive linéaire et le terme à période de durée d'un an exactement et ce n'est que le reste qu'on a analysé par la méthode de l'analyse spectrale spéciale[4, 6]. On a trouvé dans les deux éléments une période de 222, respectivement 223 jours, non considérée jusqu'alors et ne pouvant pas être interprétée jusqu'à présent du point de vue physique. Les autres résultats sont en bon accord avec ceux reçus à l'origine. On peut conclure de la comparaison des phases des composantes annuelles des pendules horizontaux et du mouvement du pôle[5] que les deux événements sont du même origine.     

Biogeographic implications of a packrat midden sequence from the Sacramento Mountains, south-central New Mexico

Thirteen packrat (Neotoma spp.) and two porcupine (Erethizon dorsatum) middens from 1555 to 1690 m elevation from the Sacramento Mountains, New Mexico, provide an 18,000-yr vegetation record in the northern Chiuahuan Desert. The vegetation sequence is a mesic, Wisconsin fullglacial (18,000–16,000 yr B.P.) pinyon-juniper-oak woodland; a xeric, early Holocene (ca. 11,000–8000 yr B.P.) juniper-oak woodland; a middle Holocene (ca. 8000-4000 yr B.P.) desert-grassland; and a late Holocene (ca. 4000 yr B.P. to present) Chihuahuan desertscrub. The frequency of spring freezes and summer droughts in the late Wisconsin probably set the northern limits of Pinus edulis and Juniperus monosperma at about 34°N, or 6° south of today's limit. Rising summer tempratures in the early Holocene eliminated pinyon and other mesic woodland plants from the desert lowlands and allowed the woodland to move upslope. At this time pinyon-juniper woodland and pine forest dominated by Pinus ponderosa probably began their spectacular Holocene expansions to the north. Continued warming in the middle Holocene led to very warm summers with strong monsoons, relatively dry, cold winters, and widespread desert-grasslands. Desertscrub communities in the northern Chihuahuan Desert did not develop until the late Holocene when the biseasonal rainfall shifted slightly back toward the winter, catastrophic winter freezes decreased, and droughts in all seasons increased. The creosote bush desertscrub corridor across the Continental Divide between the Chihuahuan and Sonoran deserts was probably connected for the first time since the last interglaciation.     

Evaluation of ferrous and ferric Mössbauer fractions

The Mössbauer fractions f for various ferrous- and/or ferric-containing oxides and oxyhydroxides, silicates and carbonates were evaluated from the experimental temperature dependence of their center shifts, using the Debye approximation for the second-order Doppler shift. It is concluded that ferrous ions exhibit a lower fraction as compared to ferric ions. Using standard mixtures of -Fe₂O₃ with selected Fe²⁺ or Fe³⁺ compounds, it is found that the calculated Fe³⁺ f values are somewhat overestimated with respect to those of Fe²⁺. Possible explanations for this shortcoming are discussed and it is suggested that a different temperature dependence of the intrinsic isomer shift is the most likely reason. This suggestion is corroborated by analyses of hematite and hedenbergite data which are available for temperatures up to 900 K and 800 K respectively.     

Defect structure of the low temperature α-cristobalite phase and the cristobalite ⇆ tridymite transformation in (Si-Ge)O2

Using minimum exposure techniques, it is feasible to perform high resolution electron microscopy on the α-cristobalite phase of (Si_0.9 Ge_0.1)O₂, which is extremely radiation sensitive. Such images reveal atomic scale information of twins and tridymite-like stacking faults on (1 1 1)_β planes, as well as of domain boundaries resulting from the β→α transition. Polytype structures are formed in certain cases. Morphological features suggest that the phase transformation cristobalite → tridymite proceeds by means of a zonal dislocation mediated synchro-shear process on (1 1 1)_β planes; the geometry of this process is analyzed. Received: 13 June 1999 / Accepted: 30 October 1999     

Early dismissals in public schools on potential severe weather days

Survey responses from 76 public school districts in Mississippi and Alabama were analyzed along with county-level demographics, tornado-path data, and tornado warning data to investigate the emerging trend of early dismissals. No two districts had identical responses, but the concern of safety was consistently mentioned. The highest number of reported dismissals would have resulted in a loss of <2 % of total annual class time. The rates of dismissals reported by districts were compared with tornado warning days to identify potential over-dismissal, which was found in 51 % of districts. Rates of dismissal and over-dismissal were analyzed with variables from survey responses, tornado warning data, tornado-path data, and county census data to determine variables that influence differences among districts. Various stepwise models indicate that districts with higher poverty levels could be less likely to dismiss. Permutation tests and stepwise models show that early dismissal and over-dismissal are more likely when a district: (1) has a higher number of killer tornadoes in the last 5 years, (2) has a higher number of recent injury-causing tornadoes, (3) has a higher number of total killer tornadoes since 1950, (4) is in a smaller county, (5) has dismissal decisions made by a team, (6) is a county district rather than a city district, and (7) has a lower level of poverty. The inverse relationship with poverty suggests that schools in poorer areas may be holding students in school because they consider them to be safer there than at home, or it could be due to a lack of resources at the school. All of these are based on a district’s location and plans, things that can be determined before a potential dismissal. These indicators can be used to highlight districts most likely to dismiss and possibly reduce rates of over-dismissal by alerting districts to their predisposed tendencies.     

Hydrogeochemical Features of Thermal Waters of South Trungbo (Central Vietnam)

The results of studying the features of the hydrogeological structure and chemical and isotope composition of thermal waters from the central part of Vietnam that are characterized by intense manifestations of intrusive magmatism are presented. It is established that low–and high–thermal waters with temperature varying within 30–85°C are developed in the area under study. The value of total mineralization of the hydrotherms ranges from 0.05 to 10.05 g/dm³. It is assumed that the circulation of thermal waters that are different in temperature and chemical composition occurs at two levels. The regular change of the hydrotherm composition in the direction from mineralized chloride sodium, including with increased Ca content, to fresh sodium bicarbonate is revealed. The ratio of δ¹⁸O–δ²H isotopes indicates that the water component is based on meteoric water. In the coastal areas, there is an isotope shift towards the ocean waters, which is also confirmed by the hydrogeochemical data. The key factors for forming the chemical composition of the thermal waters in South Trungbo are their genetic type, the interaction processes in the “water–rock–gas–organic substance” system, and their equilibrium–nonequilibrium state.     

Hyperpycnal-fed turbidite lobe architecture and recent sedimentary processes: A case study from the Al Batha turbidite system, Oman margin

The main sediment depocenter along the Oman margin is the Al Batha turbidite system that develops in the Gulf of Oman basin. It is directly connected to the wadi Al Batha, and forms a typical sand and mud rich point source system that acts as regional sediment conduit and feeds a ~ 1000 km² sandy lobe.The Al Batha lobe depositional architecture has been investigated in detail using very high-resolution seismic, multibeam echosounder data and sediment cores. Several scales of depositional architecture can be observed. The Al Batha lobe is composed of several depositional units, made of stacked elementary sediment bodies (thinner than 5 m) that are each related to a single flow event. The lobe is connected to the feeder system through a channel-lobe transition zone (CLTZ) that extends on more than 25 km. The lobe can be divided into proximal, middle and distal lobe areas. The proximal lobe is an area of erosion and by-pass with small axial feeder channels that rapidly splay into several small distributaries. They disappear in the mid-lobe area where deposits consist of vertically stacked tabular to lens-shaped sediment bodies, with a lateral continuity that can exceed 10 km. The distal lobe fringe shows a classical facies transition towards thin-bedded basin plain deposits.Sub-surface deposits consist of sandy turbidites and hyperpycnites, interbedded with fine-grained deposits (thin turbidites, hyperpycnites, or hemipelagites). Although these distal deposits are mainly related to flow transformations and concentration evolution, they highlight the importance of flooding of the wadi Al Batha on the sediment transfer to the deep basin. The thick sandy hyperpycnites recovered in such a distal area are also possibly related to the initial properties of gravity flows, in relation to the flooding characteristics of mountainous desert streams.Finally, the Al Batha lobe depositional architecture is typical of sand-rich lobes found within “small”, sand and mud rich turbidite systems fed by mountainous “dirty” rivers. Turbidite sedimentation in the Al Batha system appears to be primarily controlled by the strong climatic and geomorphic forcing parameters (i.e. semi-arid environment with ephemeral, mountainous rivers subjected to flash-flooding).