Subsidence prediction in shallow room and pillar mines

Summary A mathematical model has been developed utilizing the relative flexural strength of the strata overlying a coal seam to predict the vulnerability of shallow room and pillar mined areas to subsidence. The model assumes the failure of the immediate roof as the precursor of a subsidence event in shallow room and pillar mines. After the roof fails, either a sink hole subsidence event develops if the unconsolidated material is thin and dry; or a subsidence trough forms if the unconsolidated material is thick and wet. The model relates the Missavage number (Mn), which is dependent only on the stratigraphy and rock strength, to the extraction number, which is dependent only on the extraction ratio and maximum span of the opening. A high correlation coefficient (r=0.78) betweenMn and the extraction number for 27 subsidence events in a southern Illinois mine showed potential for using this model to delineate areas more vulnerable to subsidence. The developed and validated model was then subjected to a blind test on a 12.9 square kilometer area of an Illinois Coal Basin mine. The model successfully predicted 10 out of 12 subsidence events in the blind half of the study area and two of three additional subsidence events in the known half of the study area.     

Composition of olivine,silica activity and oxygen fugacity in kimberlite

Two generations of primary olivine are present in kimberlite, rounded phenocrysts (Fo₉₄Fo₉₁) and euhedral groundmass olivines (Fo₉₁Fo_88-5). Rounded phenocrysts less magnesian than Fo₈₈ are considered to be mantle derived xenocrysts; such crystals comprise up to 40% of the phenocrysts material in kimberlite. Calculated silica activity ranges from 10^?1,5 at 1200 °C, 50 kbs, to between 10^?1,6 and 10^?2.4 at 600 °C, 0.5–1.0 kb. Silica buffers involving olivines, pyroxenes and garnet are considered. Oxygen fugacities on the order of 10^?20 bars indicate that kimberlite magmas are highly reduced at the time of groundmass formation.     

Un type de minéralisations épigénétiques familières: les filons à antimoine du Massif Central français. Hypothèse de la sécrétion latérale

Résumé Les filons antimonifères étudiés sont encaissés dans les terrains métamorphiques du socle hercynien (district cévenol, district de Brioude-Massiac), ou dans les grès autuniens formant la base de la couverture sédimentaire (Buzeins). La plupart sont contrôlés par des failles de direction NE. Ils sont aussi sous la dépendance de métallotectes lithostratigraphiques plus ou moins stricts. D'autre part, le socle et les dépôts détritiques de base de la couverture sédimentaire (Stéphanien, Autunien) sont caractérisés par une forte anomalie géochimique antimonifère. Les roches encaissant les filons représentent la source possible de l'antimoine, mais aussi du soufre et de tous les éléments des minéraux de gangue. Ces résultats tendent à montrer la familiarité des concentrations filoniennes par rapport à leur encaissant. L'hypothèse d'une concentration par «sécrétion latérale» est étayée par les profils de distribution de l'antimoine en traces à proximité des filons (mise en évidence de zones de drainage probables). Le contenu des filons antimonifères serait donc hérité, par remobilisations successives, d'un stock métallique déjà présent dans les roches avant le métamorphisme.

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The antimoniferous lodes studied in this paper are formed either in the metamorphic rocks of the Hercynian basement (Cévennes and Brioude-Massiac districts), or in the Autunian sandstones which form the lowest part of the sedimentary cover (Buzeins). Most of them are controlled by faults of a NE trend. They also depend upon more or less strict lithostratigraphic metallotects. Moreover, a high antimoniferous geochemical anomaly characterises the rocks of the basement and of the lowest detrital deposits of the sedimentary cover (Stephanian, Autunian). The host rocks of the lodes are a possible source for antimony, and also for sulfur, as for all the elements of the gangue minerals. These results tend to show the consanguinity of the lode concentrations with their enclosing rocks. The hypothesis of a concentration by lateral secretion is supported by the curves of Sb distribution in the rocks near the lodes (revealing probable areas of leaching). The contents of Sb lodes would consequently be inherited, by successive remobilizations, from the metal contents which were already included in the rocks before metamorphism.

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Cette publication résume une Thèse d'Etat, dirigée par M. le Professeur P. Routhier, présentée le 2 Avril 1971 à la Faculté des Sciences de l'Université de Paris VI (11 quai Saint-Bernard, Paris 5°).     

The roles of bryozoans in modern coral reefs

Contrary to impressions gained from the literature, collecting in the living reefs of Bermuda, Eniwetok, and Eleuthera implies that bryozoans are common and diversified in at least some modern coral reefs.In all the reefs examined, encrusting cheilostomes are the most abundant and diversified bryozoans. Tuft-like cheilostomes, and lichenoporid and idmoneid cyclostomes, frequently are moderately important. Reteporid and aeteid cheilostomes, and crisiid cyclostomes, are also occasionally encountered.Never the principal frame-builders, the bryozoans do reinforce the reef mass by encrusting the undersides of coral heads, rock ledges, and rock fragments, and by locally partially filling cavities deep within the reef. These animals do not form significant amounts of loose carbonate sediment, nor do they trap or bind such sediment. A few bryozoans drift epiplanktonically through the surface waters around some reefs. Some cyclostome bryozoans may participate in the cryptic sclerosponge-brachiopod community of modern reefs. Correlation of bryozoan morphologic variations with environmental-factor fluctuations may eventually be possible.Of interest to students of living reefs and bryozoans, clarification of the roles played by bryozoans in modern reefs will also be valuable for paleontologists studying both fossil reefs and bryozoan evolutionary history.

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Zusammenfassung Sammlungen an lebenden Korallenriffen in der Nähe Bermudas, Eniwetoks und Eleutheras weisen darauf hin, daß Bryozoen hier häufig und differenziert in den Korallenriffen vorkommen, was bisher in der Literatur wenig beachtet wurde.In all den hier untersuchten Riffen sind die inkrustierenden Cheilostomen die häufigsten und vielfältigsten Bryozoen. Büschel-ähnliche Cheilostome sowie lichenoporide und idmoneide Cyclostome besitzen häufig einige Bedeutung. Außerdem werden dort ab und zu auch reteporide und aeteide Cheilostome und crisiide Cyclostome gefunden.Obwohl sie niemals das primäre Riffgerüst bilden, unterstützen die Bryozoen den Aufbau des Riffes erheblich dadurch, daß sie die Unterseite der Korallenstöcke sowie Felsenkanten und -fragmente inkrustieren. Teilweise füllen die Bryozoen auch Hohlräume, die tief im Riff eingelagert sind. Diese Tiere bilden keine bedeutenden Mengen von Karbonatablagerungen; sie können auch kein loses Sediment einfangen und binden. Manche Bryozoen treiben epiplanktonisch in den oberen Wasserschichten in der Umgebung von Riffen. Einige cyclostome Bryozoen sind in geschützten Bereichen zusammen mit Sclerospongien und Brachiopoden vergesellschaftet. Wahrscheinlich lassen sich einmal Beziehungen zwischen der Morphologie der Bryozoen und der Veränderungen im Biotop herstellen.Die Klärung der Rolle, welche Bryozoen in rezenten Riffen spielen, wird nicht nur das Interesse der Forscher wecken, die die lebenden Riffe und Bryozoen untersuchen, sondern es wird auch den Paläontologen anziehen, der sich mit fossilen Riffen oder der Bryozoen-Evolution befaßt.

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Résumé Contrairement aux impressions données par la littérature, il apparait d'après des collections faites dans les récifs vivants de Bermuda, Eniwetok, et Eleuthera que les bryozoaires s'y présentent souvent et de façons diverses.Dans tous les récifs examinés, les cheilostomes incrustés sont des bryozoaires se présentant le plus souvent et sous des aspects les plus différents. Les cheilostomes comme des touffes, les cyclostomes lichenoporides et idmoneides sont fréquemment de quelque importance. Par ailleurs les cheilostomes rétéporides et aétéides, et les cyclostomes crisides n'y sont rencontrées qu'occasionnellement.Bien qu'ils n'aient jamais bâti le cadre du récif, les bryozoaires en renforcent la masse en s'incrustant au-dessous des têtes coralliennes, des bords et fragments de rochers; ils remplissent aussi en partie des cavités profondes du récif. Ces animaux ne forment pas de quantités significatives de carbonate et ils ne peuvent ni capter ni lier des sédiments détachés. Quelques bryozoaires flottent epiplanktoniquement dans les eaux de surface de quelques récifs. Quelques bryozoaires cyclostomes participent à la communauté cryptique des sclérosponges et brachiopodes dans les récifs modernes. Des corrélations entre les variations morphologiques des bryozoaires et les fluctuations environnantes seront éventuellement possibles.Intéressante pour les chercheurs étudiant les récifs vivants et les bryozoaires, la clarification des rôles joués par les bryozoaires dans les récifs modernes aura aussi de la valeur pour le paléontologue s'occupant de récifs fossiles et d'évolution des bryozoaires.

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Bermuda, Eniwetok Eleuthera , , . ( Cheilostoma, Lichenoporida Idmoneida — Cyclostomata -, Reteporida Aeteida — Cheilostomata -, Cyclostomata. , , , . , . , . . . . , Cyclostomata, Sclerospongia chiopoda. , . - , , , .

     

Nine Fallacies of Floods

A number of important misconceptions or fallacies stand in the way of a better understanding of the nation's flood problem. The fallacies are not universal, with many flood experts, decision makers, and sectors of the public escaping their seductive logic. But enough people do fall prey to these fallacies of floods so as to create obstacles to improved utilization of the lessons of experience. This paper uses three of these lessons to organize presentation of the nine fallacies:We know the wrong things about the nature of the problem.Fallacy 1: Flood frequencies are well understood.Fallacy 2: Damaging flooding in recent years is unprecedented because of global warming.Fallacy 3: Levees prevent damages.Fallacy 4: Flood forecasts are universally available.We don't know enough about why and with what intensity we should act.Fallacy 5: Societal vulnerability to floods is well understood.Fallacy 6: Data on flood casualties is a proxy for flood risk.Fallacy 7: Data on flood damages is a proxy for flood risk.We know enough about what might be done. Fallacy 8: Knowledge leads to action.Fallacy 9: The U.S. flood problem can be addressed without Federal leadership and support.The purpose of raising the fallacies in this paper is to contribute to a systematic definition of the nation's flood problem.     

Lake Manly(?) Shorelines in the Eastern Mojave Desert, California

Near Mesquite Spring on the southern edge of the Soda Lake basin in the Mojave Desert, there is a shoreline of an ancient lake at an elevation of 340 m above sea level. At present, Soda Lake would overflow at 280 m; a lake surface at 340 m would extend 240 km northward, to the northern end of Death Valley. Shorelines and lacustrine deposits near the Salt Spring and Saddle Peak Hills, 75 km north of Mesquite Spring, are at 180 m; a lake surface at this elevation today would also extend to the northern end of Death Valley. The most prominent shoreline of the pluvial lake that occupied Death Valley during the Pleistocene, Lake Manly, is that of the Blackwelder stand which ended 120,000 yr ago. This shoreline is 90 m above sea level. The Mesquite Spring and Salt Spring Hills shorelines were probably formed by the Blackwelder stand and subsequently displaced with respect to one another, tectonically, due to transpression in the northeastern Mojave Desert and NW–SE extension across Death Valley. This tectonic regime would result in subsidence of Death Valley and the Salt Spring Hills relative to Mesquite Spring. A reconstruction suggests that the topography at the time of the Blackwelder stand would have had a sill near the level of the highest lake, and also one 20 m lower, corresponding to the next most prominent shoreline in Death Valley. Expansion of the lake over these sills would have increased evaporation, thus possibly stabilizing the lake level.     

A contemporary appraisal of some key Fiskian concepts with emphasis on Holocene meander belt formation and morphology

Looking back five decades and reflecting on the contributions of Harold Fisk to Lower Mississippi Valley (LMV) geological knowledge, it is apparent that he was highly successful in making engineers aware of the critical role of glacial advance and retreat, sea level changes, valley slope and river gradient, and time in affecting geomorphic processes, sedimentary patterns, and river regimes. Perhaps his greatest contribution to river engineering was in recognizing and elaborating on the effects of bed and bank materials on stream meandering. Fisk, however, could only work within the prevailing geological theory and method of his time with regard to glacial chronology and stratigraphy and such matters as global structure and tectonics. Major advances in these areas with new concepts and techniques have shown his LMV glacial response model to be oversimplified and invalid in some respects. More cycles of valley filling and cutting are now known to be represented in the alluvial sequence and his concept of deep valley entrenchment as a direct response to sea level changes is untenable in light of substantial new data. The presence of a regional, rectilinear fracture pattern with a corresponding network of major fault zones has not been substantiated by subsurface data. The most widely known aspect of Fisk's work is his detailed interpretation and colorful depiction of the complex pattern of overlapping, cross cutting, and discontinuous Holocene meander belts and their hundreds of abandoned channels. However, it is now recognized that Fisk's elaborate chronological reconstruction is largely invalid both because of some incorrect basic assumptions and his lack of radiocarbon and other numerical dating techniques for precise control. A newly emerging model of meander belt formation is more generalized and relies heavily on archaeological evidence. It places emphasis on episodes of multiple active river courses (partial flow channels), frequent formation of distributary channels, and the significant influence of Mississippi River tributaries and minor basin drainage. Interesting regional variations over time in meander belt configurations and morphology may reflect climate changes, but at present broad-scale variations in the lithology of the Quaternary alluvial sequence appear to be the dominant control.